A Novel Approach to Continuous Sampling

and Measurement of Uranium Containing

Particulate Matter

A Thesis Submitted to the Committee on Graduate Studies

in Partial Fulfillment of the Requirements for the Degree of Doctor of

Philosophy in the Faculty of Arts and Science

TRENT UNIVERSITY

Peterborough, Ontario, Canada

© Copyright by Kashif Imtiaz Choudhry, 2011

Environmental and Life Sciences Graduate Ph.D. Program (ENLS)

January 2012

This work is dedicated

to my

Parents

Their prayers are always behind me

i

Abstract

A Novel Approach to Continuous Sampling and Measurement of

Uranium Containing Particulate Matter

Kashif Imtiaz Choudhry

ii

Abstract

Continuous monitoring of industrial heavy metals release into the environment is

important for emission control and compliance with standards. In this research, a

method for continuous monitoring of uranium-containing particles in industrial

emissions was developed. A particle-into-liquid sampler (PILS) was found to be a

suitable instrument for the continuous collection of uranium dioxide (UO2) and

uranium tetrafluoride (UF4) at a rate of 1-5 mg h-1

into the transporting solution. The

efficiency of various solutions (as sample transport media), including water and a

sodium carbonate/hydrogen peroxide (Na2CO3-H2O2) solution for the collection of

particles was evaluated. The sodium carbonate/hydrogen peroxide solution was found

to be preferable to water for the collection of UO2 and UF4 because particle build-up

on the impaction surface and blockages in liquid transport lines were eliminated.

The data collected in experiments show that a sample air flow rate (l min-1

) has a

significant effect on particle collection efficiency. The combination of a sample air

flow rate of 10 l min-1

(for UO2) or 16.7 l min-1

(for UF4), a steam flow rate of 1.5 ml

min-1

and a sample transport solution flow rate of 0.5 ml min-1

demonstrated greater

than 89% recovery of the particle mass of UO2 and greater than 92% recovery for UF4

in the sample plus impactor drain lines.

A comparison was also made between uranium concentrations in particles collected

from a traditional high volume sampler (filter) with aerosols collected by the PILS.

Results showed that U in particles collected with the high volume air sampler using

filters was consistently higher than in aerosols collected with the PILS. The PILS and

filter results show a good correlation (R2

= 0.98); on average the PILS collected 80%

of uranium found in the filter samples.

iii

Significant quantities of rare earth elements (REE) are found in tailings of uranium

ore. Therefore, a microwave digestion method was also developed for six commonly

used rare earth oxides using 2% (v/v) nitric acid that can be used with PILS for

continuous monitoring of rare earth elements in ambient air. Results show that using

20 ml of nitric acid (2 % v/v) and closed vessel microwave heating at 100oC for 60

minutes yields greater than 90±5% recoveries of all six REEs. The PILS is an

effective instrument for aerosol collection into a liquid; it is very reproducible, it is

easy to use, it offers a better understanding of aerosol composition and provides time-

dependent information.

Keywords: Continuous emission monitoring, uranium dioxide, uranium tetrafluoride,

microwave digestion, particle-into-liquid sampler, rare earth oxide, high volume air

sampler.

iv

Preface

This thesis is submitted as partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Environmental and Life Sciences (ENLS). The work was

carried out at Trent University, Peterborough, ON, Canada and Cameco Corporation,

Port Hope, ON, Canada on the topic of “A novel approach to continuous sampling

and measurement of uranium containing particulate matter”.

v

Acknowledgement

I would like to give my sincere thanks to my thesis advisors, Dr. R. Douglas Evans,

Dr. R. Jack Cornett and Dr. Qianli Xie, because they provided me with incredible

support, encouragement, and guidance during the course of my thesis. I am thankful

to Dr. Huan Zhong for his assistance for the sample analysis. A very special thank to

Cameco Corporation, Port Hope, Ontario for providing some of the samples and

facilities to complete the work. Dr. Simon Reid, Dr. Katerina Kyyst and Jacques

Gauthier (Cameco Corporation, Port Hope, ON) also have my complete gratitude.

Their experience and previous work played a very important role and made my thesis

task much easier.

The financial support from the Trent University and Natural Science and Engineering

research Council of Canada (NSERC) is gratefully acknowledged. Many thanks to Dr.

Hayla Evans for her editorial work. I also thank Linda Cardwell, (Environmental and

Life Sciences graduate study office) and Laurie Denise Kryshka (Evans lab manager)

for their help with logistics during my time of study.

Finally, I take this opportunity to express my profound gratitude to my beloved

parents, my brothers and sisters, my wife and daughter for their moral support and

patience during my study at Trent University, Peterborough, Ontario, Canada.

At last, I want to devote this thesis to my Almighty God. It would not have been

possible to complete this degree Doctor of Philosophy in Environmental and Life

Sciences (ENLS) without His grace.

vi

TABLE OF CONTENTS

ABSTRACT…..................................................................................................... i

PREFACE…...................................................................................................... iv

ACKNOWLEDGMENT ......................................................................................... v

TABLE OF CONTENTS …………………………………………………………. vi

LIST OF FIGURES ……………………………………………………………. x

LIST OF TABLES ……………………………………………………………… xiii

CHAPTER 1 ……………………………………………………………. 1

INTRODUCTION ………………………………………………..…………… 1

1.1 Introduction ………………………..………………………………. 2

1.2 Aerosol chemical composition measurements …………………….. 3

1.2.1 Sample collection methods ………………….…………….. 3

1.2.1.1 Off-line measurements ……….……………. 4

1.2.1.2 Continuous or semi-continuous measurements. 7

1.2.2 Sample preparation ………………………….……………… 9

1.2.3 Sample analysis ……..……………………….……………… 11

1.3 Proposed research ……………………………………………………. 13

1.4 References ……………………………………………………………. 15

CHAPTER 2 ……………………………………………………………… 21

DISSOLUTION OF URANIUM DIOXIDE (UO2) AND URANIUM TETRAFLUORIDE (UF4)

PARTICLES IN A Na2CO3-H2O2 SOLUTION ……………………………………… 21

2.1 Abstract ………………………..………………………………………. 22

2.2 Introduction ………………………..…………………………………... 23

2.3 Methods ……………………………………………………………….. 25

2.3.1 Reagents and standards ………………….……………………… 25

2.3.2 Dissolution experiments ………………………….…………… 25

2.3.3 X-ray fluorescence (XRF) analysis …………………………… 26

2.4 Results and discussion ………………………………………………… 26

2.5 Conclusion ………………………………………………...…………... 33

2.6 References ……………………………………………………………... 34

vii

CHAPTER 3 ………………………………………………………………… 36

DETERMINATION OF TRACE LEVEL RARE EARTH ELEMENTS USING MICROWAVE

DIGESTION AND INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY ......... 36

3.1 Abstract ………………………..……………………………………….. 37

3.2 Introduction ………………………..…………………………………… 38

3.3 Materials and Methods …………………………………………………. 41

3.3.1 Chemicals and standards ………………….……………………. 41

3.3.2 Sample preparation ………………………….………………….. 42

3.3.3 Determination of REEs by ICP-MS ……………………………. 43

3.4 Results and discussion ………………………………………………….. 44

3.4.1 Effect of sample size – Microwave digestion …………………... 44

3.4.2 Effect of acid concentration – Microwave digestion …………… 45

3.4.3 Effect of digestion time – Microwave and hotplate digestion ….. 48

3.4.4 Microwave versus hotplate digestion …………………………... 50

3.5 Conclusion ………….…………………………………………………... 52

3.6 References ……………………………………………………………… 53

CHAPTER 4 ……………………………………………………………….. 57

PARTICLE-INTO-LIQUID SAMPLER (PILS) OPTIMIZATION FOR THE CONTINUOUS

MONITORING OF URANIUM DIOXIDE (UO2) AND URANIUM TETRAFLUORIDE (UF4)

PARTICLES IN HIGH PARTICLE CONCENTRATION ENVIRONMENTS …….......... 57

4.1 Abstract ………………………..……………………………………… 58

4.2 Introduction ………………………..…………………………………. 59

4.3 Experiments …………………………………………………………… 64

4.3.1 Reagents and standards ………………….……………………... 64

4.3.2 Aerosol generator ………………………….…………………... 64

4.3.3 Particle-into-liquid sampler (PILS) ………….………………… 67

4.3.4 Pre-treatment of PILS samples ………...…..…………………... 69

4.3.5 ED-XRF analysis ………………………………………………. 69

4.4 Results and discussion …………………………………………………. 70

4.4.1 Water as the transport liquid ………………….………………... 70

4.4.2 Optimization of PILS system for UO2 and UF4………………. 73

4.4.2.1 PILS – Sample line plus impactor drain ………... 75

4.4.2.2 PILS – Growth chamber drain ………...…..…… 83

4.4.2.3 PILS – Filter plus cold trap ………...…..………. 85

4.5 Conclusion ………………………………………………...…………… 85

4.6 References ……………………………………………………………... 87

viii

CHAPTER 5 ……………………………………………………………….. 91

DETERMINATION OF URANIUM (U) IN ATMOSPHERIC AEROSOLS USING A

PARTICLE-INTO-LIQUID SAMPLER (PILS)…………………………………. 91

5.1 Abstract ………………………..………………………………………. 92

5.2 Introduction ………………………..…………………………………… 93

5.3 Materials and Methods …………………………………………………. 95

5.3.1 Sampling location ………………….…………………………… 95

5.3.2 Aerosol sampling ………………………….…………………… 96

5.3.2.1 Particle-into-liquid sampler (PILS) …………….. 96

5.3.2.2 High volume air sampler ………...…..…………. 98

5.3.3 Reagents and standards …………………………………………. 99

5.3.4 Pre-treatment of the PILS and filter samples ……………..……. 99

5.3.4.1 Filter samples …………………………............... 99

5.3.4.2 PILS samples …………………………............... 100

5.3.5 Inductively coupled plasma mass spectrometric (ICP-MS)

analysis ……………………………………………………….... 100

5.4 Results and discussion …………………………………………………. 101

5.4.1 Optimization of PILS system ………………….………………. 101

5.4.2 Uranium concentration in atmospheric particles …………… 102

5.4.3 Monitoring of uranium in the air ………….………………….. 104

5.5 References ……………………………………………………………… 110

CHAPTER 6 ………………………………………………………………… 112

GENERAL DISSCUSION AND CONCLUSIONS …..……..………………. 112

6.1 General discussion and conclusions…..………………………………… 113

6.2 References ……………………………………………………………… 122

APPENDICES ………………………………………………………………… 123

Appendix 1 …………………………………..………………………………… 124

Appendix 2 …………………………………..………………………………… 125

Appendix 3 …………………………………..………………………………… 126

Appendix 4 …………………………………..………………………………… 129

Appendix 5 …………………………………..………………………………… 130

Appendix 6 …………………………………..………………………………… 131

ix

Appendix 7 …………………………………..………………………………… 132

Appendix 8 …………………………………..………………………………… 134

Appendix 9 …………………………………..………………………………… 135

Appendix 10 …………………………………..………………………………… 136

Appendix 11 …………………………………..………………………………… 138

x

LIST OF FIGURES

Figure 2.1 UO2 dissolution (%) in sodium carbonate (0 - 7 wt %) /

hydrogen peroxide (0.15 wt %) as a function of time.

……. 29

Figure 2.2 UF4 dissolution (%) in sodium carbonate (0 - 5 wt %) /

hydrogen peroxide (0.01 wt %) as a function of time.

……. 29

Figure 2.3 UO2 dissolution (%) in sodium carbonate (5 wt %) /

hydrogen peroxide (0 - 0.3 wt %) as a function of time.

……. 30

Figure 2.4 UF4 dissolution (%) in sodium carbonate (5 wt %) /

hydrogen peroxide (0 - 0.3 wt %) as a function of time.

……. 30

Figure 3.1 Effect of sample size (20, 40, 60 and 80 mg) on

recovery, 20 ml (50% HNO3), 100oC, 15 min

microwave digestion. For each element, the same

letters on different bars indicate no significant

difference (p > 0.05) between them, whereas different

letters on different bars indicate a significant difference

(p < 0.05) between them.

……. 45

Figure 3.2 Effect of acid concentration (2, 5 or 20% v/v) on

recovery. 20 mg sample, 20 ml HNO3, 100oC

microwave digestion. For each element, the same

letters on different bars indicate no significant

difference (p > 0.05) between them, whereas different

letters on different bars indicate a significant difference

(p < 0.05) between them.

……. 47

Figure 3.3 Effect of digestion time (30, 45 or 60 min) on recovery.

20 mg sample, 20 ml (2% HNO3), 100oC, microwave

digestion. For each element, the same letters on

different bars indicate no significant difference (p >

0.05) between them, whereas different letters on

different bars indicate a significant difference (p <

0.05) between them.

……. 49

Figure 3.4 Effect of digestion time (30, 60 or 90 min) on recovery.

20 mg sample, 20 ml (2% HNO3), 100oC, Hot plate

digestion. For each element, the same letters on

different bars indicate no significant difference (p >

0.05) between them, whereas different letters on

different bars indicate a significant difference (p <

0.05) between them.

……. 49

xi

Figure 3.5 A comparison of microwave vs hot plate digestion. 20

mg sample, 20 ml (2% HNO3), 100oC.

……. 50

Figure 4.1 TSI fluidized bed aerosol generator with particle

removal setup to control aerosol output rate to 1-5 mg

h-1

.

……. 65

Figure 4.2 Schematic diagram of the PILS; for the continuous

collection of UO2 and UF4 particles into the liquid

stream.

……. 66

Figure 4.3 PILS running with water as sample transport liquid:

UO2 particle mass recovery (%) in different lines

during four successive experimental run at the sample

air flow rate of 10 l min-1

, water flow rate for steam

generation of 1.5 ml min-1

and transport liquid flow rate

of 0.4 ml min-1

.

……. 71

Figure 4.4 PILS running with water: particle build-up on

impaction surface and blockage in liquid collection

lines.

……. 73

Figure 4.5 UO2 particle mass collection efficiency (%) in sample

line at different sample air flow rates and transport

liquid flow rates.

……. 77

Figure 4.6 UF4 particle mass collection efficiency (%) in the

sample line at different sample air flow rates and

transport liquid flow rates.

……. 77

Figure 4.7 UO2 particle mass collection efficiency (%) in the

impactor drain at different sample air flow rates and

transport liquid flow rates.

……. 78

Figure 4.8 UF4 particle mass collection efficiency (%) in the

impactor drain at different sample air flow rates and

transport liquid flow rates.

……. 78

Figure 4.9 UO2 particle mass recovery (%) in the PILS unit’s

different lines at different sample air flow rates and

transport liquid flow rates (Note: UO2 percentage

recovery in filter + cold trap is too small to see clearly

on the graph).

……. 81

Figure 4.10 UF4 particle mass recovery (%) in the PILS unit’s

different lines at different sample air flow rates and

transport liquid flow rates.

……. 82

xii

Figure 5.1 Schematic diagram of the PILS for the continuous

collection of particle into the liquid stream.

……. 97

Figure 5.2 Uranium concentrations during July and August 2010. ……. 103

Figure 5.3 A comparison of uranium concentrations in

atmospheric aerosols collected using high volume air

sampler vs the PILS.

……. 104

Figure 5.4 Uranium concentrations measured using the PILS (with

digestion vs without digestion).

……. 106

xiii

LIST OF TABLES

Table 2.1 UO2 and UF4 particle size distribution. ………. 31

Table 2.2 UO2 - initial dissolution rate and solution pH as a

function of various combinations of sodium carbonate

and hydrogen peroxide solution.

………. 31

Table 2.3 UF4 - initial dissolution rate and solution pH as a

function of various combinations of sodium carbonate

and hydrogen peroxide solution.

………. 31

Table 3.1 Instrumental operating conditions and data acquisition

parameters.

………. 43

Table 4.1 UO2 and UF4 particle size distribution. ………. 66

Table 5.1 Instrumental operating conditions and data acquisition

parameters.

………. 101

1

Chapter 1

Introduction

2

1.1 Introduction

The effects of poor air quality on human health are well-known and principally affect

the body’s respiratory system and the cardiovascular system [1]. The pollutant species

most often of concern are carbon dioxide (CO2), carbon monoxide (CO),

hydrocarbons (HC), ozone (O3), nitrogen oxides (NOx), particulate matter (PM),

sulphur dioxide (SO2), and lead (Pb).

Particulate matter (PM) consists of a complex mixture of organic and inorganic

substances depending on the source. Particulate matter is classified on the basis of its

aerodynamic diameter (i.e. the diameter of a sphere of unit density) or particle size.

The most commonly used fractions are

• TSP : Total suspended particles, comprises all particles

• PM10 : Particles with an aerodynamic diameter less than 10 µm

• PM2.5 : Particles with an aerodynamic diameter less than 2.5 µm

• Coarse fraction: Aerodynamic diameter between 2.5 and 10 µm

Particles emitted directly into the air are known as direct or primary particulate matter,

while other particles formed indirectly in the atmosphere from chemical reactions of

gaseous pollutants are known as secondary particulate matter. Particulate matter can

remain in the air from a few hours to several days and can be transported over long

distances.

3

There is increasing interest in determining the concentration of contaminants in

aerosols (defined as solid and liquid particles suspended in air) in order to assess their

impact on the environment and human health. It is also very important to continuously

monitor particulate matter releases to the environment from industry in order to

control them and be compliant with regulatory standards. Due to the health effects of

PM, many standards have been set by various governments for the maximum amount

of pollutant emissions released from industry to the environment. Most of the national

ambient air quality standards (NAAQS) for aerosols are based on particles smaller

than 10 µm in aerodynamic diameter (PM10 and PM2.5) [2]. Atmospheric aerosol

measurements were extensively reviewed by McMurry [3] and Chow [4]. Particulate

matter chemical composition measurement frequently requires the collection of

representative samples, sample pretreatment / dissolution prior to chemical analysis.

1.2 Aerosol chemical composition measurements

1.2.1 Sample collection methods

Sampling of atmospheric particulates is a first step towards the continuous monitoring

of heavy metal releases from industry to the environment. Aerosol chemical

composition measurement can be performed using offline or continuous sampling

methods. Atmospheric particulate uranium and rare earth elements are normally

collected using offline techniques such as filters and impactors.

4

1.2.1.1 Off-line measurements

To date, measurements of particle composition are typically performed off-line by

collecting particles on filters or other media, for a period of time ranging from hours

to days, depending on the sample air flow rate and particle concentration. A filter is

weighed before and after sampling and the concentration of PM is determined from

the increase in filter mass divided by the volume of air sampled. The US

Environmental Protection Agency (EPA) has well-established reference methods to

obtain the best results when sampling particulate matter on filters using high-volume

and low-volume samplers for different concentrations and particle sizes [5]. Different

types of filters including fiber, membrane and granular bed filters have been used to

collect aerosols [4-7]. The material of choice depends on several factors such as

mechanical, chemical and temperature stability of the filter, blank concentration, flow

resistance and loading capacity [4].

The major disadvantage with traditional sample collection techniques is the usually

long sampling time, depending on the particle concentrations in the air. Other

problems that may be associated with filter measurements include: adsorption of

vapors onto the substrate (positive artifact) [8-13], evaporation losses of semi-volatile

compounds during and after sampling (negative artifact) [3, 4, 14-19], contamination

of filters during filter loading in the field [4] and reactions between collected particles,

gases and the filter substrate [16]. Loss of samples or contamination can also occur

due to particle removal from the gas, the chemical composition of the aerosol, the

filter material, changes in temperature, and pressure changes within the sampler

5

during sampling. Combined, these problems during filter preparation, sampling,

sample collection, transportation and storage before analysis can lead to significant

uncertainties in the results.

Atmospheric particulate uranium is normally collected on filters. Most of the

previously reported concentrations of particulate uranium in air are based on this

method, usually with long sampling times. For example, an atmospheric uranium

concentration as high as 200 ng m-3

has been reported near a nuclear fuel production

plant (based on 1 week sampling time) [20-22]. In another study [23], atmospheric

particulate uranium was collected on filters over a 24 hour period using a high volume

air sampler and over a one week period using a low air flow rate air sampler; the

reported average particulate uranium concentration in the air at urban and rural

locations within New York State, ranged from 0.10 to 1.47 ng m-3

.

Thus, atmospheric particulate matter collection using filtration techniques requires a

long period of time, from 24 hours to a week, to collect the required minimum sample

mass for analysis and therefore cannot provide time-dependent information (variation

over a short time period). In addition, filter based sampling is labor intensive because

the filter needs to be conditioned before sampling, weighed before and after the

sampling, and installed and removed from the filter holder and sampling instrument.

Results may not be available in a timely fashion because of the long sampling period,

the time elapsed between sample collection, sample preparation and analysis.

Therefore, atmospheric particle collection on filters using a high volume sampler is

not suitable for investigating short time frame variations in atmospheric trace metal

concentrations since it takes a long time to collect enough sample mass for analysis.

In the case of heavy metal release from industry, continuous emission monitoring is

important for either continual compliance or to determine exceedances from

6

regulatory standards. Therefore it is important to develop a continuous monitoring

method that can provide information on variation in atmospheric trace metal

concentrations.

A common alternative type of aerosol sampler is the impactor. An impactor is an

instrument in which particle impaction in a non-rotating flow is the basic mechanism

of particle capture. In a conventional impactor, air carrying the particles passes

through a single jet nozzle and particles are collected by impaction onto a flat plate.

Only particles larger than the cut-off size of the impactor are collected on the plate;

smaller particles follow the streamline and leave the system or enter a series of

impactor stages (in the case of a cascade impactor), each with a successively smaller

cut-off size in order to collect particles with differing size ranges. Generally, inertia

impactors can be used over a wide range of impactor cut-off sizes from 0.005 µm to

approximately 50 µm [24]. The MOUDI cascade impactor (MSP Corporation,

Minnesota, USA) is used regularly to collect size-fractionated particles (0.032 – 18

µm, with as many as 10 impactor stages) in many size intervals at the same time.

Particle bounce is an inherent problem with impactors; however coated (e.g. with

greases and oils) substrates largely eliminate bounce and are commonly used for

atmospheric sampling [3].

Cyclones are another type of off-line instrument that can separate particles according

to their aerodynamic diameter. In cyclone samplers, a jet of air containing particles

enters tangentially into the cylinder / conical chamber and then swirls downwards.

Particles with sufficient inertia cannot follow the air streamline and deposit on the

7

inner walls of the cyclone or collect on the bottom of the cyclone, while finer particles

remain in the air streamline and leave the system through the outlet at the top.

Cyclones are very popular for aerosol sampling to separate larger particles from

smaller particles (e.g. coarse fractions from PM2.5). Most of the cyclones used are

single stage, examples of which include, the Cyclone PM1.0, Cyclone PM2.5 and

Cyclone PM10 (URG Corporation, Chapel Hills NC, USA and BGI Incorporated.

Waltham, MA, USA). A cascade version of the cyclone sampler, designed for gas

emissions stack sampling, consists of 5 stages with cut-off sizes ranging from 0.32 to

5.4 µm at a sampling flow rate of 28.3 l min-1

[25]. Cyclone samplers are not easily

subjected to errors due to particle bounce and they have a large capacity for particle

loading.

1.2.1.2 Continuous or semi-continuous measurements

To overcome artifacts associated with off-line aerosol measurements, several real-

time or near real-time methods for the measurement of aerosol mass concentrations

have been explored. Buhr et al. (1995) [26] developed an automated method that

collects particulate matter on a frit surface, which is continuously flushed with de-

ionized water. Anders et al. [27] developed a method where the particles were directly

impacted on flowing liquid and then analysed. In the system devised by Shaorong et

al. [28], particles are charged and collected on the surface of an electrode, which is

periodically washed with de-ionized water for sample collection and analysis. The

Aerosol-to-Liquid Particle Extraction System (ALPES), developed by the Savannah

River Technology Center (SRTC) and Oak Ridges National Laboratory (ORNL), is

8

another useful device for the monitoring of particulate matter [29]. APLES is a dual

mechanism collection system in which ambient air is drawn in at an air flow rate of

300 l min-1

, then an ionization section is used to ionize the particles and finally a

liquid collection medium is employed for capturing the particles.

It is known that very small particles are difficult to collect by impaction [30]. To

overcome this problem, the concept of growing the aerosol particle size with steam

prior to particle collection was introduced. The particle-into-liquid sampler (PILS)

was developed by Weber et al. [31] based on the work of Simon and Dasgupta [32].

A sample of air is introduced into the PILS system using a vacuum pump and the flow

rate is controlled by a critical orifice, which is placed at the exit of the PILS system.

At the entrance of the PILS the air is mixed with steam to obtain a supersaturated

environment in which particles grow. This is followed by collection of the particles

by inertial impaction onto a quartz plate (the impaction plate), which is continuously

washed with a steady stream of water. Originally the PILS was coupled only to two

ion chromatographs for separate anion and cation analysis, but it has the ability to

connect with other analytical instruments for online sample analysis. The PILS

collection efficiency for particle diameters between 0.03 to 10 µm is greater than 97%

and can be coupled to an analytical technique for the continuous measurement of

aerosol composition [33].

9

1.2.2 Sample preparation

Sample preparation/digestion is a critical step in continuous monitoring of heavy

metals in air samples. For most analytical techniques involving the measurement of

particulates, it is first necessary to dissolve the particle samples prior to elemental or

isotopic analysis. Samples can be prepared using different techniques such as particle

dissolution in alkaline solutions or acid digestion in an open, closed or flow through

digestion system using a conventional or microwave heat source. Commonly used

acids include nitric acid, sulfuric acid, hydrogen peroxide, perchloric acid,

hydrochloric acid and hydrofluoric acid or any combination of all these acids. Open

vessel acid digestion using a hot plate is the oldest and simplest method of sample

preparation. This is an inexpensive system that operates at atmospheric pressure and

so does not suffer from problems associated with pressure build-up; however open

vessel acid digestion using a hotplate is limited by a low maximum digestion

temperature that cannot exceed the boiling point of the acid at ambient pressure. Other

disadvantages are that it is relatively slow, it may create a temperature gradient within

the sample, there is the risk of contamination, a large amount of acid is required and

there is the danger of loss of trace elements.

Abu-Samra et al. [34] first used microwaves as a heating source for digestion in 1975.

Since 1975, much research has been carried out using microwave digestion to

improve the technique’s efficiency for different types of environmental samples [35].

The closed vessel digestion method involves placing the sample in a closed vessel to

achieve a reaction temperature above the atmospheric boiling point of the acid. Thus

10

the reaction rate is increased but the reaction time is decreased. However, excessive

pressure build-up during the digestion of samples with high organic content, can lead

to rupturing of the sealed vessel. The move to a microwave digestion approach offers

many advantages over conventional methods including a reduction in digestion time

and the ability to digest difficult matrices; however sample digestion remains multi-

step and labor intensive. These problems can be overcome or controlled by adopting a

continuous-flow microwave digestion system. The concept of online continuous flow

microwave digestion at higher temperatures and pressures has been discussed in the

literature and has resulted in significant time savings compared with batch microwave

digestion [36, 37].

This study focuses on the dissolution of uranium dioxide (UO2) and uranium

tetrafluoride (UF4) particles because of their use in the nuclear fuel processing

industry. A microwave digestion method was also developed for the determination of

trace level rare earth elements (REE) because significant quantities of rare earth

oxides are found in tailings of uranium ore and now are widely used in different

industries. Unfortunately, there is limited information regarding the dissolution of

uranium tetrafluoride, although it is known to be slightly soluble in water and more

soluble in concentrated acids and alkalis [38-40]. There have been many studies

investigating the dissolution of UO2 powder / pellets in acidic media, such as

phosphoric acid (H3PO4) [41, 42] and sulphuric acid (H2SO4) [43-45], with the most

commonly used being nitric acid [46-52].

11

In addition, the dissolution of UO2 using supercritical carbon dioxide (CO2) [53, 54]

and also alkaline solutions [55, 56] has been extensively reviewed. Carbonate

solutions, in particular, exhibit a high capacity for dissolved uranium. Peper et al. [57]

studied the dissolution kinetics of UO2 powder at room temperature using various

oxidants (K2SO4, NaOCl, H2O2) in alkaline solutions. They found that hydrogen

peroxide (H2O2) provided the most rapid initial dissolution rate; as well, the initial

dissolution rate of UO2 increased with increasing hydrogen peroxide concentration.

1.2.3 Sample Analysis

Chemical analysis of collected samples is the final step towards aerosol chemical

composition measurements. Several analytical techniques, including for example,

instrumental neutron activation analysis (INAA) [58-60], atomic absorption

spectrometry (AAS) [61, 62], X-ray fluorescence (XRF) [63-66] and inductively

coupled plasma mass spectrometry (ICP-MS) [67-77] have been used for the

determination of uranium and REEs in different environmental samples (see [78-80]

for operating principles and more details on the application of these analytical

methods). In this study, ICP-MS and x-ray fluorescence (XRF) spectrometry were

used for the measurement of uranium or rare earth element concentrations in the

samples. Specifically, XRF was used to analyze liquid samples collected from the

particle-into-liquid-sampler system because the particle concentrations were relatively

high. For ambient air samples, ICP-MS was used instead due to its low detection

limits.

12

XRF is a nondestructive analysis that can be used to determine the total concentration

of elements including uranium and REEs in liquid, solid and powder samples. The

detection limit of XRF is in the parts per million (ppm) range and analysis cannot

distinguish variation among isotopes of an element. X-ray fluorescence (XRF) has

been used often to determine REEs [65, 66], uranium and other actinides [81-83] in a

variety of matrices because it has advantages such as rapid sample analysis, minimum

sample handling, relatively low cost and it is non-destructive. Inductively coupled

plasma mass spectrometry (ICP-MS) is a powerful analytical technique, which allows

the simultaneous detection of almost all elements. The advantages of ICP-MS include

quick and accurate sample analysis, low detection limits as well as the determination

of multiple elements and isotopes simultaneously.

Atmospheric particulate uranium typically collected on air filters requires long

sampling time (depending on the flow rate and particle concentration in the air),

followed by sample preparation and then analysis by various methods, including ICP-

MS, alpha spectrometry, or X-ray fluorescence (XRF). Previously, most reported

concentrations of particulate uranium in the air were based on one week air particulate

sampling on filters and analysis by ultraviolet fluorometry, fission track analysis or

mass spectrometry [21, 23, 84]. Querol et al. [85], and Singh and Wrenn [86]

collected air particulate samples on cellulose filters and analyzed them with

instrumental neutron activation analysis (INNA) (detection limit 0.03 µg per filter)

and with alpha spectrometry (detection limit of 0.02 dpm/L for 238

U; 27 ng l-1

)

respectively. In another study, Boomer and Powell [67] collected air particulate

samples on glass fiber filters, digested them in nitric acid and finally analyzed them

using ICP-MS and found a detection limit of 0.1 µg l-1

in the final solution. Uchida et

13

al. [87] measured uranium (238

U) at a detection limit of less than 0.1 ppt using ICP-

MS, making it a suitable instrument for the measurement of uranium concentrations in

environmental samples.

1.3 Proposed research

The main purpose of the present study is to further our understanding of continuous

emission monitoring using a particle-into-liquid-sampler system. The information

gained in this study could help improve the continuous monitoring of particulate

contaminants, especially uranium.

Specifically, three questions are addressed in this study: (1) Is it possible to use a

particle-into-liquid sampler (PILS) instead of filters for the continuous collection of

uranium-containing particulates? (2) Is it possible to dissolve uranium containing

particulates into the PILS system or digest particles using low acid concentrations

after collection in liquid? (3) Can ICP-MS be combined with the PILS for the

continuous measurement of ambient particulate uranium concentrations in the air?

For the nuclear industry it is very important to continuously monitor uranium release

into the environment for control purposes and compliance with emission standards. A

fluidized bed aerosol generator was used to generate high purity UO2 and UF4

aerosols at a rate of 1 – 5 mg h-1

to simulate stack conditions. Uranium dioxide (UO2)

and uranium tetrafluoride (UF4) particles that were generated were used for the

14

optimization of the PILS for the continuous collection of particles into liquid.. An

aqueous carbonate solution used as a sample transport liquid in the PILS to dissolve

UO2 and UF4 particles within the system and finally elemental analysis was carried

out using XRF or ICP-MS.

Significant quantities of rare earth oxides are found in tailings of uranium ore and

now widely use in other industries. Therefore, a microwave digestion method was

also developed for the digestion of six commonly used rare earth oxides, including

praseodymium oxide (Pr6O11), neodymium oxide (Nd2O3), samarium oxide (Sm2O3),

gadolinium oxide (Gd2O3), dysprosium oxide (Dy2O3) and ytterbium oxide (Yb2O3)

using 2% (v/v) nitric acid. Because particles of these rare earth oxides have different

physical and chemical characteristics (including density, solubility, and stability), it is

anticipated that in the future, the PILS can be optimized for these particles as well.

15

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17

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49. Asano, Y.; Kataoka, M.; Ikeda, Y.; Hasegawa, S.; Takashima, Y.; Tomiyasu,

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18

50. Ikeda, Y.; Yasuika, Y.; Nishimura, K.; Hasegawa, S.; Takashima, Y., Kinetics

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21

Chapter 2

Dissolution of uranium dioxide (UO2)

and uranium tetrafluoride (UF4)

particles in a Na2CO3-H2O2 solution

22

2.1 Abstract

The dissolution of uranium dioxide (UO2) and uranium tetrafluoride (UF4) particles in

a sodium carbonate (Na2CO3) - hydrogen peroxide (H2O2) solution at room

temperature was studied. It was found that the UO2 dissolution increases with an

increase in carbonate concentration in the solution with maximum dissolution at 5

wt % of sodium carbonate concentration in the solution. Using the optimum

concentration of Na2CO3 for the dissolution of UO2 and UF4 (i.e. 5 wt %), further

experiments were carried out with varying concentrations of H2O2 (0 - 0.3 wt %) to

determine the optimum concentration of H2O2 for the dissolution of UO2 and UF4

particles. It was found that the UO2 and UF4 dissolution rate increases with an

increase in H2O2 concentration, with the maximum initial dissolution rate at 0.15

wt % of H2O2 in the solution. UF4 particles were dissolved more rapidly i.e., higher

initial dissolution rate than UO2 particles. Within 3 minutes, complete dissolution of

100 mg of UF4 was achieved in 200 ml of 5 wt % Na2CO3 and 0.15 wt % of H2O2 at

room temperature. A sodium carbonate (5 wt %) / hydrogen peroxide (0.15 wt %)

solution was found to be the most favorable combination for the dissolution of UO2

and UF4 particles.

Keywords: Uranium dioxide, uranium tetrafluoride, sodium carbonate, hydrogen

peroxide, dissolution.

23

2.2 Introduction

Uranium is the heaviest naturally occurring radioactive element. Uranium isotopes

emit mainly alpha particles, which have little penetrating ability and so they cannot

make their way into human body tissue through skin. Thus, uranium is a health

hazard only when uranium compounds are ingested or inhaled [1]. Exposure to

uranium can cause lung cancer, kidney damage, respiratory diseases, etc [2]. Uranium

is used mainly for nuclear power plants as a fuel to generate electricity. Global

production of uranium is increasing every year [3]. Therefore, it is important to

continuously monitor uranium release into the environment.

For most analytical techniques involving the measurement of particulates, it is first

necessary to dissolve the particle samples prior to elemental or isotopic analysis.

Unfortunately, there is limited information available regarding the dissolution of

uranium tetrafluoride, although it is known to be slightly soluble in water and more

soluble in concentrated acids and alkalis [4-6]. There have been many studies

investigating the dissolution of UO2 powder / pellets in acidic media, such as

phosphoric acid (H3PO4) [7, 8] and sulphuric acid (H2SO4) [9-11], with the most

commonly used being nitric acid [12-18].

In addition, the dissolution of UO2 using supercritical carbon dioxide (CO2) [19, 20]

and also alkaline solutions [21, 22] has been extensively reviewed. Carbonate

solutions, in particular, exhibit a high capacity for dissolved uranium. Peper et al. [23]

24

studied the dissolution kinetics of UO2 powder at room temperature using various

oxidants (K2SO4, NaOCl, H2O2) in alkaline solutions. Hydrogen peroxide (H2O2)

showed the most rapid initial dissolution rate; as well, the initial dissolution rate of

UO2 increased with the increasing hydrogen peroxide concentration. Pierce et al. [24]

studied the dissolution kinetics of UO2 using carbonate, as a function of solution pH

and temperature. They reported that the dissolution rate of UO2 increased with

increasing pH and that the rate of UO2 dissolution increased by an order of magnitude

with a 30oC increase in temperature.

In the present study the dissolution of uranium dioxide (UO2) and uranium

tetrafluoride (UF4) particles was investigated. The objective was to optimize uranium

dioxide (UO2) and uranium tetrafluoride (UF4) powder dissolution in a sodium

carbonate (Na2CO3) / hydrogen peroxide (H2O2) solution at room temperature. The

overall goal was to determine experimentally the best combination of sodium

carbonate (wt %) and hydrogen peroxide (wt %) for the dissolution of high purity

uranium dioxide and uranium tetrafluoride focusing on a high initial dissolution rate.

The optimal carbonate (Na2CO3)/hydrogen peroxide (H2O2) solution then can be used

in a particle-into-liquid sampler (PILS) as a sample transport liquid for the continuous

monitoring of particulate uranium.

25

2.3 Methods

2.3.1 Reagents and standards

High purity uranium dioxide and uranium tetrafluoride particles with particle sizes

smaller than 100 µm (Table 2.1) (Cameco Corporation, Port Hope, Ontario Canada)

were used for the dissolution study. Hydrogen peroxide (H2O2) (30% W/W ACS

grade; EMD Chemicals Inc, Gibbstown, NJ, USA) and sodium carbonate (Na2CO3)

(Mallinkrodt Baker, Phillipsburg, NJ, USA) were used to dissolve the uranium

dioxide and uranium fluoride particles. Distilled de-ionized water (18.2 MΩ) was

used for the preparation of all solutions.

2.3.2 Dissolution experiments

Various concentrations of sodium carbonate / hydrogen peroxide solutions were

prepared (Na2CO3 = 0 - 7 wt % and H2O2 = 0 - 0.3 wt %). A beaker filled with 200 ml

of sodium carbonate / hydrogen peroxide solution was placed on a stir plate. 100 mg

of uranium dioxide or uranium tetrafluoride particles were weighed and added to the

sodium carbonate / hydrogen peroxide solutions. The solution was continuously

stirred and small samples (~ 5ml) were withdrawn at regular intervals (1, 3, 5, 10, 20,

30 and 60 minutes). Samples were filtered to remove any undigested material from

the solution using a 0.45 µm pore size syringe filter (VWR International).

26

2.3.3 X-ray fluorescence (XRF) analysis

An X-ray fluorescence spectrometer (Innov-X System, inc, Woburn, MA, USA) was

used in this study for the measurement of uranium concentrations in the liquid

samples. Calibration curves for uranium dioxide (0-600 mg UO2 l-1

) and uranium

tetrafluoride (0-600 mg UF4 l-1

) were created. The calibration curves were then used

to determine UO2 and UF4 particle mass dissolution in each sample.

2.4 Results and discussion

UO2 is insoluble and UF4 is slightly soluble in water. Dissolution of UO2 (Appendix 1

and Table 2.2) and UF4 (Appendix 2 and Table 2.3) was measured in solutions having

different sodium carbonate - hydrogen peroxide concentrations (using nine different

Na2CO3 - H2O2 concentration combinations for UO2 and ten combinations for UF4)

as a function of time. The experiments were carried out at room temperature.

Carbonates have been used primarily as leaching agents for the recovery of uranium

from soils and ores [25]. Carbonates form highly soluble anionic carbonate uranyl

species, uranyl tricarbonate (UO2(CO3)34-

) [25]. First, the effect of sodium carbonate

concentration (0 - 7 wt %) was studied on the dissolution of UO2 (Appendix 1 and

Figure 2.1). In the absence of sodium carbonate i.e., 0 wt %, UO2 (with 0.15 wt %

H2O2 solution) did not significantly dissolve in solution but at Na2CO3 concentrations

of 1 to 5 wt %, UO2 dissolution increased (Figure 2.1). When only H2O2 solution is

used some compounds of UO4.xH2O2 may precipitate [26], which supports our results.

27

Results indicate a significant difference (p < 0.05) in UO2 particle dissolution (%)

between 1, 3, and 5 wt % carbonate concentrations in the solution. It was also

observed that UO2 does not significantly dissolve in carbonate solution in the absence

of the oxidant (Figure 2.3). It has been previously reported that UO2 partially

dissolves in carbonate solutions and that the highest solubility occurs when Na2CO3

and H2O2 are used in combination [26]. Therefore, H2O2 was used with carbonate to

oxidize U(IV) to U(VI) prior to the formation of uranium carbonate complexes. The

general reaction (equation 2.1) for the dissolution of UO2 in the carbonate and

hydrogen peroxide solution [23] is;

____________ (2.1)

Where M+ denotes an alkali metal cation and x and y are the molar stoichiometries of

H2O2 and CO32-

, respectively. This equation suggests that various compounds of

UO2(O2)x(CO3)y2-2x-2y

may coexist with different combinations of Na2CO3 and H2O2

concentrations. Results indicate a significant difference (p < 0.05) in UO2 particle

dissolution (%) between 0.025, 0.05, and 0.15 wt % but no significant difference (p >

0.05) between 0.15 and 0.3 wt% H2O2 concentration (with 5 wt % carbonate

concentrations) in the solution (Figure 2.3). The highest initial dissolution rate was

observed at 5 wt % sodium carbonate (at H2O2 concentrations of 0.15 or 0.30 wt %;

Table 2.2) with nearly 60% of the UO2 (100 mg) being dissolved within 20 minutes;

~95% of the UO2 particles were dissolved after one hour (Figure 2.1).

UF4 particles were more readily dissolved, even in the absence of sodium carbonate

(Figure 2.2). Approximately 80 % of the UF4 was dissolved after 60 minutes even in

28

sodium carbonate free solution with only a very low concentration (0.01 wt %) of

H2O2 in the solution (Appendix 2). Results indicate a significant difference (p < 0.05)

in UF4 particle dissolution (%) between zero and 1 wt % but no significant difference

(p > 0.05) between 1, 3 and 5 wt % sodium carbonate concentrations (with 0.01 wt%

hydrogen peroxide concentrations) in the solution. Compared to UO2, rapid

dissolution of UF4 was observed with nearly 50% of the UF4 (100 mg) being

dissolved within the first 3 minutes. The highest initial dissolution rate for UF4 was

observed at 5 wt % sodium carbonate concentration, similar to the UO2 dissolution

results (Table 2.3). Therefore, a concentration of 5 wt % Na2CO3 was selected and

various concentrations of H2O2 were studied to obtain the solution concentration of

Na2CO3-H2O2 needed for maximum dissolution of UO2 and UF4 particles.

The dissolution results for 100 mg of UO2 or UF4 in 200 ml of 5 wt % of Na2CO3 and

various concentrations of H2O2 are shown in Figure 2.3 and Figure 2.4, respectively.

In the solution containing only sodium carbonate (i.e. H2O2 = 0 wt %) there was no

dissolution of UO2 particles (Appendix 1); however UO2 dissolution appears to

increase with an increase in hydrogen peroxide concentration (Figure 2.3). At H2O2

concentrations of 0.15 and 0.3 wt % there is the same initial dissolution rate of the

UO2 particles (Table 2.2, Figure 2.3). UF4 particles were dissolved more rapidly i.e.,

higher initial dissolution rate (Table 2.3, Figure 2.4) than UO2 particles. Within 3

minutes, complete dissolution of 100 mg of UF4 was achieved in 200 ml of 5 wt %

Na2CO3 and 0.15 wt % of H2O2 at room temperature (Figure 2.4 and Appendix 2).

29

Table 2.2 and Table 2.3 also show pH values of the Na2CO3-H2O2 solutions. The

variation in the solution pHs used in this study were too small to determine the effect

of solution pH on the dissolution of UF4 particles, but Pierce et al. [24] reported that

the rate of UO2 dissolution increased with an increase in solution pH.

Dissolution Time (minutes)

0 10 20 30 40 50 60 70

UO

2 Dissolution (%)

0

20

40

60

80

100

120

Na2CO3 = 0 wt %

Na2CO3 = 1 wt %

Na2CO3 = 3 wt %

Na2CO3 = 5 wt %

Na2CO3 = 7 wt %

H2O2 = 0.15 wt %

Figure 2.1: UO2 dissolution (%) in sodium carbonate (0 - 7 wt %) / hydrogen peroxide

(0.15 wt %) as a function of time.

Dissolution Time (minutes)

0 10 20 30 40 50 60 70

UF4 Dissolution (%)

0

20

40

60

80

100

120

Na2CO3 = 0 wt %

Na2CO

3 = 1 wt %

Na2CO3 = 3 wt %

Na2CO3 = 5 wt %

H2O2 = 0.01 wt %

Figure 2.2: UF4 dissolution (%) in sodium carbonate (0 - 5 wt %) / hydrogen peroxide

(0.01 wt %) as a function of time.

30

Dissolution Time (minutes)

0 10 20 30 40 50 60 70

UO

2 Dissolution (%)

0

20

40

60

80

100H2O2 = 0.000 wt %

H2O2 = 0.025 wt %

H2O2 = 0.050 wt %

H2O2 = 0.150 wt %

H2O2 = 0.300 wt %

Na2CO3 = 5.0 wt %

Figure 2.3: UO2 dissolution (%) in sodium carbonate (5 wt %) / hydrogen peroxide (0

- 0.3 wt %) as a function of time.

Dissolution Time (minutes)

0 10 20 30 40 50 60 70

UF

4 D

issolu

tion (%

)

0

20

40

60

80

100

120

H2O2 = 0.000 wt %

H2O2 = 0.005 wt %

H2O2 = 0.010 wt %

H2O2 = 0.025 wt %

H2O2 = 0.050 wt %

H2O2 = 0.150 wt %

H2O2 = 0.300 wt %

Na2CO3 = 5.0 wt %

Figure 2.4: UF4 dissolution (%) in sodium carbonate (5 wt %) / hydrogen peroxide (0

- 0.3 wt %) as a function of time.

31

Table 2.1. UO2 and UF4 particle size distribution.

Diameter (µm) UO2

Mass finer (%)

UF4

Mass finer (%)

60.0 94.4 97.5

40.0 92.2 96.1

20.0 79.4 95.2

10.0 56.3 93.9

8.0 49.0 92.1

6.0 40.9 89.9

4.0 31.5 89.0

2.0 20.1 65.5

1.0 12.5 28.3

0.5 3.5 5.0

Table 2.2. UO2 - initial dissolution rate and solution pH as a function of various

combinations of sodium carbonate and hydrogen peroxide solution.

Na2CO3-H2O2 solution UO2 - Initial dissolution

ratea

Na2CO3 conc. (wt %) H2O2 conc. (wt %) Solution pH % dissolved / minute

0 0.150 05.55 0.0

1 0.150 10.74 2.3

3 0.150 10.87 2.3

5 0.000 11.29 0.0

5 0.025 11.16 0.5

5 0.050 11.09 1.5

5 0.150 10.93 3.6

5 0.300 10.80 3.6

7 0.150 10.98 1.9 a= dissolution during the first minute.

Table 2.3. UF4 - initial dissolution rate and solution pH as a function of various

combinations of sodium carbonate and hydrogen peroxide solution.

Na2CO3-H2O2 solution UF4 - Initial dissolution

ratea

Na2CO3 conc. (wt %) H2O2 conc. (wt %) Solution pH % dissolved / minute

0 0.010 05.68 06.13

1 0.010 11.15 22.5

3 0.010 11.21 28.8

5 0.000 11.29 06.8

5 0.005 11.25 23.3

5 0.010 11.22 32.6

5 0.025 11.16 41.7

5 0.050 11.09 63.7

5 0.150 10.93 86.9

5 0.300 10.80 90.6 a= dissolution during the first minute.

32

In the present study, the dissolution of UO2 (Figure 2.1) is faster within the first 20

minutes for all the concentrations of Na2CO3 and then slows down through time, most

likely due to the decreasing concentration of free carbonate in solution, which appears

to be rate limiting at this point [23]. The initial dissolution rate of UO2 with 5 wt % of

Na2CO3 is the highest while it is the lowest for the 7 wt % of Na2CO3. Similarly,

Peper et al. [23] reported that the dissolution rate of UO2 decreases above 5 wt % of

Na2CO3, and suggest that this might be as a result of increased ionic strength or an

increased rate of H2O2 degradation.

UF4 particles were dissolved more rapidly i.e., higher initial dissolution rate than UO2

particles. Complete dissolution of 100 mg of UF4 particles was achieved (in 3 minutes)

compared to ~95% of the UO2 particles (in one hour) in 200 ml of 5 wt % Na2CO3 and

0.15 wt % of H2O2 at room temperature. The solution (5 wt % Na2CO3 and 0.15 wt %

of H2O2) was used with the PILS system as sample transport liquid. The particle-into-

liquid sampler system restricted our study for the higher concentration of H2O2 in

Na2CO3-H2O2 solution. A high concentration of H2O2 in the transport solution under a

high temperature steam may result in corrosion of the mesh and other parts of the

PILS system. In this study, UO2 particles show low initial dissolution rate at room

temperature but Pierce et al. [24] reported that the rate of UO2 dissolution increased

by an order of magnitude with a 30oC increase in temperature. The UO2 and UF4

particle diameter (particle size distribution; Table 2.1) used in this dissolution study

were much bigger than those used with the PILS optimization (smaller than 10 µm).

The smaller the particle (larger surface area) the faster the dissolution rate because

dissolution takes place at the surface. The PILS operates with steam (~100oC), so it

was anticipated that Na2CO3-H2O2 solution would dissolve uranium tetrafluoride and

33

uranium dioxide particle within the system as demonstrated by the elimination of

deposits and blockages in the PILS (Figure 4.4). Thus the Na2CO3-H2O2 solution can

be used for the sample transport liquid in the particle-into-liquid sampler for

continuous emission monitoring.

2.5 Conclusions

The dissolution of UO2 and UF4 particles in Na2CO3-H2O2 solutions was studied. The

sodium carbonate and hydrogen peroxide concentrations were optimized for higher

initial dissolution rates and maximum dissolution of UO2 and UF4 particles. UF4

particles had higher initial dissolution rates and high solubility (completely dissolved

within 3 minutes) in Na2CO3-H2O2 solutions compared to UO2 particles. A sodium

carbonate (5 wt %) / hydrogen peroxide (0.15 wt %) solution was found to be the

most favorable combination for the dissolution of UO2 and UF4 particles at room

temperature.

34

2.6 References

1. Li, W.; Skinner, R.; Megna, K.; Chen, J.; Perera, S.; Murimboh, J.; Waller, E.;

Erhardt, L.; Cornett, R. J., In vitro dissolution study of uranium dioxide and

uranium ore with different particle sizes in simulated lung fluid. Journal of

Radioanalytical and Nuclear Chemistry 2009, 279, 209-218.

2. Bleise, A.; Danesi, P. R.; Burkart, W., Properties, uses and health effects of

depleted uranium (DU): a general overview Journal of Environmental

Radioactivity 2003, 64, 93-112.

3. WNA Fact and Figures - Uranium Production Figures: 1999 – 2009.

http://www.world-nuclear.org/infomap.aspx]

4. Zavodska, L.; Kosorinova, E.; Scerbakova, L.; Lesny, J., Environmental

chemistry of uranium In Hungarian Electronic Journal of Sceicne - HU ISSN

1418-7108: HEJ Manuscript no.: ENV-081221-A, 2008.

5. Wise, W. M.; Soehnlin, H. R.; McBride, C. H., An improved method for

dissolution of uranium tetrafluoride. Analytical Chemistry 1962, 34, 1035.

6. Lukyanychev, Y. A.; Nikolaev, N. S., The solubility of uranium tetrafluoride in

aqueous solutions of acids. Atomic Energy 1963, 15, 1184-1187.

7. Stromatt, R. W.; Connally, R. E., Determination of the stoichiometry of uranium

dioxide by controlled potential coulometry. Analytical Chemistry 1961, 33, 345-

346.

8. Takeishi, H.; Muto, H.; Aoyagi, H.; Adachi, T.; Izawa, K.; Yoshida, Z.;

Kawamura, H., Determination of oxygen/uranium ratio in irradiated uranium

dioxide based on dissolution with strong phosphoric acid. Analytical Chemistry

1986, 58, 458-462.

9. Habashi, F.; Thurston, G. A., Kinetics and mechanism of the dissolution of

uranium dioxide. Energia Nuclearel 1967, 14, 238-244.

10. Shying, M. E., Oxide dissolution mechanisms – III: Surface activation in the

system uranium dioxide – sulphuric acid. Journal of Inorganic and Nuclear

Chemistry 1973, 35, 3299-3305.

11. Murty, B. N.; Yadav, R. B.; Ramamurthy, C. K.; Syamundar, S.,

Spectrophotometric determination of the oxygen to uranium ratio in uranium

oxides based on dissolution in sulphuric acid. Talanta 1991, 38, 1335-1340.

12. Shabbir, M.; Robins, R. G., Kinetics of the dissolution of uranium dioxide in

Nitric acid - I. Journal of Applied Chemistry 1968, 18, 129-134.

13. Shabbir, M.; Robins, R. G., Kinetics of the dissolution of uranium dioxide in

Nitric acid - II. Journal of Applied Chemistry 1969, 19, 52-56.

14. Ikeda, Y.; Yasuike, Y.; Takashima, Y.; Park, Y.-Y.; Asano, Y.; Tomiyasu, H., 17

O NMR study on dissolution reaction of UO2 in nitric acid mechanism of

electron transfer. Journal of Nuclear Science and Technology 1993, 30, 962-964.

15. Asano, Y.; Kataoka, M.; Ikeda, Y.; Hasegawa, S.; Takashima, Y.; Tomiyasu, H.,

New method for dissolving UO2 using ozone. Progress in Nuclear Energy 1995,

29, 243-249.

16. Ikeda, Y.; Yasuika, Y.; Nishimura, K.; Hasegawa, S.; Takashima, Y., Kinetics

study on dissolution of UO2 powders in nitric acid. Journal of Nuclear Materials

1995, 224, 266-272.

17. Sasaki, S.; Wada, Y.; Tomiyasu, H., Basic study of photochemistry for

application to advanced nuclear fuel cycle technology. Progress in Nuclear

Energy 1998, 32, 403-410.

35

18. Kim, E.-H.; Hwang, D.-S.; Yoo, J.-H., Dissolution mechanism of UO2 in nitric

acid solution by photochemical reaction. Journal of Radioanalytical and

Nuclear Chemistry 2000, 245, 567-570.

19. Tomioka, O.; Meguro, Y.; Enokida, Y.; Yamamoto, I.; Yoshida, Z., Dissolution

behavior of uranium oxide with supercritical CO2 using HNO3-TBP complex as

a reactant. Journal of Nuclear Science and Technology 2001, 328, 1097-1102.

20. Samsonov, M. D.; Wai, C. M.; Lee, S.-C.; Kulyako, Y.; Smart, N. G.,

Dissolution of uranium dioxide in supercritical fluid carbon dioxide. Chemical

Communications 2001, 18, 1868-1869.

21. Grambow, B. Spent fuel, dissolution and oxidation: An evaluation of literature

data; Svensk Kӓrnbrӓnslehantering AB (Swedish Nuclear fuel and waste

management Co) - Report SKB TR 89-13 - Sweden: 1989.

22. McKenzie, W. F. UO2 dissolution rates: A review; Lawrence Livermore

National Laboratory Report UCRL-ID-111663: 1992.

23. Peper, S. M.; Brodnax, L. F.; Field, S. E.; Zehnder, R. A.; Valdez, S. N.; Runde,

W. H., Kinetic study of oxidative dissolution of UO2 in aqueous carbonate

media. Industrial and Engineering Chemistry Research 2004, 43, 8188-8193.

24. Pierce, E. M.; Icenhower, J. P.; Serne, R. J.; Catalano, J. G., Experimental

determination of UO2(cr) dissolution kinetics: effects of solution saturation state

and pH. Journal of Nuclear Materials 2005, 345, 206-218.

25. Turney, W. R.; Mason, C. F. V.; Longmire, P.; Dander, D. C.; York, D. A.;

Chisholm-Brause, C. J.; Thomson, B. M., Carbonate heap leach of uranium-

contaminated soil. Radioactive Waste Management and Environmental

Restoration 1994, 3, 2087-2090.

26. Lee, E. H.; Lim, J. K.; Chung, D. Y.; Yang, H. B.; Yoo, J. H.; Kim, K. W., The

oxidative-dissolution behavior of fission products in a Na2CO3-H2O2 solution.

Journal of Radioanalytical and Nuclear Chemistry 2009, 281, 339-346.

36

Chapter 3

Determination of trace level rare earth

elements using microwave digestion

and inductively coupled plasma mass

spectrometry

37

3.1 Abstract

A closed vessel microwave digestion method was developed for the digestion of six

rare earth oxide powders (praseodymium oxide, neodymium oxide, samarium oxide,

gadolinium oxide, dysprosium oxide and ytterbium oxide). Twenty mg of rare earth

oxide samples were digested at 100oC for 30, 45, 60 and 90 minutes in 20 ml of nitric

acid (2 % v/v). The recoveries of the REEs in digested samples using microwave and

hot plate digestion were compared. Statistically significant differences (p < 0.05) were

observed between the microwave and hot plate methods. A combination of closed

vessel with microwave heating, 20 ml of nitric acid (2 % v/v) at 100oC for 60 minutes

yields greater than 90±5% recoveries of all six REEs. The results indicate that

microwave heating is preferable to the hot plate for the digestion of rare earth oxide

powder samples.

Keywords: High purity earth oxides, microwave digestion, hot plate digestion,

inductively coupled plasma mass spectrometry (ICP-MS), nitric acid.

38

3.2 Introduction

Air quality and human exposure to airborne pollutants has gained much attention. It is

of special concern to those living in cities where there is the potential for exposure to

greater amounts of pollutants. Atmospheric particulate matter (PM) is one of the

major pollutants of concern in ambient air. Exposure to PM has been shown to cause

adverse effects on human health in cities throughout the world in both developed and

developing countries [1]. In particular, PM with an aerodynamic size less than 10 µm

(PM10) can be inhaled by humans and cause lung damage. Previous studies have

found that morbidity and mortality rates were positively related to high PM10 levels

[2].

Rare earth elements (REEs) consist of 17 elements, including the15 lanthanides (Z=

57 - 71), and scandium (Sc) and yttrium (Y). The elemental forms of REEs are iron-

grey to silvery metals; they are soft, malleable and ductile, and they react with oxygen

to form rare earth oxides when exposed to air [3, 4]. Since the 1980s, REEs have been

used in industry as high-efficiency magnetic materials, solid metal hydrides, and high-

intensity luminescence materials [5]. Nowadays, rare earth elements are widely used,

for example, in magnets, NiMH batteries, auto catalysts, fluid cracking catalysts,

polishing powders and glass additives, colored plastics, oxygen sensors, capacitors,

cathodes, photography equipment, and in the nuclear industry [6-12]. As a result,

industrial demand for REEs has increased rapidly; global production of rare earth

elements increased from 41,047 to 94,381 metric tons from 1985 to 2001 [7].

Furthermore, large quantities of REEs are emitted into the atmosphere from industry

39

and pose potential risks to human health [13-16]. For example, significant quantities

of rare earth oxides are found in tailings of uranium ore, shale and loparite mining

[17]. Those REE particles could be suspended and transported in the air, which is

found to be the major route of REE transportation [18, 19]. Along with a worldwide

use of REEs in different industries, their emissions into the atmosphere have been

increasing. Since humans are always breathing air this may cause serious problems

for human health; therefore, it is of importance to continuously monitor REE

concentrations in air [20-22] as well as their release from the industrial stacks for

emission control purposes. Thus, we needed to develop a microwave flow digestion

method for the analysis of rare earth elements in collected samples.

To date, measurements of atmospheric PM composition are typically performed off-

line by collecting particles onto filters using a low or high volume air sampler, for a

long period of time depending on the sample air flow rate and particle concentration.

Sample analyses are conducted off-site in distant laboratories leading to transportation,

handling and time-related constraints. To overcome these constraints, continuous

emission monitoring is an important tool and would more rapidly confirm compliance

or non-compliance with regulatory permit limits. These measurements can provide

real-time feedback to control the industrial emissions of pollutants. The particle-into-

liquid sampler (PILS) [23] is a useful instrument for continuous monitoring of particle

composition, that continuously collects particles into a small volume of liquid;

however collected sample requires continuous dissolution prior to analysis.

40

Rare earth elements are usually present in the air at trace levels. Several analytical

techniques, including neutron activation analysis (NAA) [24, 25], isotope dilution

mass spectrometry (ID-MS) [26], X-ray fluorescence (XRF) [27, 28], atomic

absorption spectrometry (AAS) [29], and inductively coupled plasma atomic emission

spectrometry (ICP-AES) [30, 31] have been used for the determination of rare earth

elements. However, NAA has high cost, slow analysis time, poor precision, and

several interferences, while ID-MS cannot be used for the determination of Pr, Tb, Ho,

and Tm because these elements do not have multiple stable isotopes [30]. In addition,

other techniques such as ICP-AES and XRF are not appropriate for rare REEs

determination due to errors induced by the spectral interferences [32], and high

detection limits and poor sensitivity [33], respectively. Consequently, inductively

coupled plasma mass spectrometry (ICP-MS) is considered to be the most effective

analytical technique for the determination of REEs [8-12, 34-38]. ICP-MS has several

advantages over other techniques, including wide dynamic range, low detection limits,

isotopic information and higher sample throughput.

Most analytical techniques including ICP-MS require injection of liquid samples; thus

a dissolution or digestion step is needed for solid samples, such as atmospheric

particulate REEs. Due to their low solubility [18, 19], dissolution of REEs should be

done before analysis by ICP-MS. Normally, the dissolution procedure for rare earth

oxide samples used in most ICP-MS analysis involves open vessel digestion using

conventional heating (hot plate / oven) with concentrated nitric acid. However, high

acid concentrations in samples may decrease signal intensity and damage the cones

inside the mass spectrometer. For continuous monitoring of atmospheric REEs, it is

preferable to use low or moderate acid concentrations for sample digestion, so that

41

digested liquid samples can be introduced directly into the ICP-MS without further

dilution. Abu-Samra and co-workers [39] first reported the use of microwaves as a

heating source for sample digestion. Microwave heating offers several advantages

over conventional heating, e.g., increased temperature control, no heat waste, shorter

digestion time, minimal sample contamination and less acid required for sample

digestion.

The main purpose of this study is to develop a microwave digestion method for six

commonly used rare earth oxides, including praseodymium oxide (Pr6O11),

neodymium oxide (Nd2O3), samarium oxide (Sm2O3), gadolinium oxide (Gd2O3),

dysprosium oxide (Dy2O3) and ytterbium oxide (Yb2O3), for analysis by ICP-MS.

Recoveries of REEs using microwave digestion versus hot plate digestion are

compared as well.

3.3 Materials and Methods

3.3.1 Chemicals and standards

Milli-Q® water (18.2 MΩ.cm) was used in the preparation of all solutions. All bottles,

containers and volumetric flasks used in preparing solutions were soaked in 10%

nitric acid (HNO3) overnight and rinsed with Milli-Q water prior to use. Double

distilled nitric acid, prepared from 67 – 70% trace metal grade nitric acid (VWR,

42

West Chester, PA, USA) was used in sample preparation and dilution. Single element

calibration standards, 1000 µg ml-1

; (PlasmaCAL, SCP Science) of praseodymium

(Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), dysprosium (Dy) and

ytterbium (Yb)were used to prepare working standard solutions. High purity rare

earth oxides powders of praseodymium oxide (Pr6O11), neodymium oxide (Nd2O3),

samarium oxide (Sm2O3), gadolinium oxide (Gd2O3), dysprosium oxide (Dy2O3) and

ytterbium oxide (Yb2O3) were purchased from Hefa Rare Earth Canada Co. Ltd

(Richmond, British Columbia Canada).

3.3.2 Sample preparation

The Teflon® vessels used in microwave digestion (Ethos PRO, Milestone Inc.,

Sorisole (BG)) were washed with soap and Milli-Q water, soaked in 10% nitric acid

overnight, rinsed with Milli-Q water and air dried before use. To test various

digestion conditions and optimize the microwave digestion procedure, the REE oxide

powder sample sizes, acid concentrations and digestion time were modified. Different

amounts of rare earth oxide powder (80, 60, 40 or 20 mg) were put into the Teflon

digestion vessels. Twenty ml of nitric acid (50, 20, 15, 10, 5, and 2 % v/v) were added

to the digestion vessels and the samples digested for 15, 30, 45, 60 minutes at 100oC.

After microwave digestion, the Teflon vessels were removed from the microwave

oven and allowed to cool in a fume hood for 30 minutes. All digested REE solutions

were then diluted (if needed) until the nitric acid concentrations were lowered to 2%

and stored prior to analysis by ICP-MS.

43

For samples digested using conventional heating, twenty mg of rare earth oxide

powder samples (Pr6O11, Nd2O3, Sm2O3, Gd2O3, Dy2O3 and Yb2O3) were weighed

and transferred into Teflon tubes. Samples were then dissolved with 20 ml of 2% (v/v)

nitric acid and heated on a hotplate at 100oC for 30, 60 or 90 minutes.

3.3.3 Determination of REEs by ICP-MS

Following microwave and/or hot plate digestion, prepared samples were analyzed for

REE concentrations using a PerkinElmer (ELAN DRC II) ICP-MS. Prior to use, the

machine was optimized using a tuning solution. The operating conditions of the ICP-

MS are summarized in Table 3.1. Standard solutions for calibration curves were

prepared from the 1000 µg ml-1

stock solutions by diluting them with 2% nitric acid.

Table 3.1:

Instrumental operating conditions and data acquisition parameters.

Parameters / Conditions Results

RF Power (W) 1225

Plasma gas flow rate (l min-1

) 15

Auxiliary gas flow rate (l min-1

) 1.2

Nebulizer gas flow rate (l min-1

) 0.86

Sampler and skimmer cone composition Nickel

Scan mode Peak Hopping

Dwell time (ms) 50

Sample replicates 5

44

3.4 Results and discussion

Sample digestion is a critical step in any analytical processes. Elemental or isotopic

analysis of a majority of organic and inorganic matrices requires complete sample

digestion prior to instrumental analysis. Sample digestion can be performed in open,

closed or flow-through digestion systems using a conventional, ultraviolet or

microwave heating source. Frequently, hot plate digestion with nitric acid is used to

digest rare earth oxide samples for trace metal analysis [8-12]. We have tested the

effects of variables including sample size, digestion time and different acid

concentrations on the recovery of REEs using microwave and hot plate digestion

methods.

3.4.1 Effect of sample size - Microwave digestion:

The effect of sample size was studied by applying the microwave-digestion procedure

to various masses of sample. Sample size was varied between 20 mg and 80 mg (5

replicates for each mass) and digested using 20 ml of 50 % (v/v) nitric acid at 100oC

for 15 minutes. Statistical significance for REE elements between the different sample

sizes was performed using t-tests (Figure 3.1). For each element, the same letters on

different bars indicate no significant difference (p > 0.05) between them, whereas

different letters on different bars indicate a significant difference (p < 0.05) between

them. Recovery of REEs increased as the sample size decreased (Figure 3.1). Highest

REE recoveries were obtained when the sample weight was as low as 20 mg and

decreased as the sample weight was increased (Appendix 3). This could be due to the

45

limited volume of nitric acid used [40]. Highest recovery was obtained for Pr

(97.9±1.1%; based on 5 replicates) and lowest for the Nd (60.6±1.4%; based on 5

replicates) (Appendix 3).

Rare earth elements

Pr Nd Sm Gd Dy Yb

Recovery (%)

40

50

60

70

80

90

10020 mg

40 mg

60 mg

80 mg

a b c c

a b b b

a b a c

a a b b

a a b b a b b b

Figure 3.1: Effect of sample size (20, 40, 60 and 80 mg) on recovery, 20 ml (50% HNO3), 100oC,

15 min microwave digestion. For each element, the same letters on different bars indicate no

significant difference (p > 0.05) between them, whereas different letters on different bars indicate

a significant difference (p < 0.05) between them.

3.4.2 Effect of acid concentration - Microwave digestion:

The effect of nitric acid concentration was also studied on the recovery of rare earth

elements. The effect of nitric acid concentration (2, 5 and 20% v/v) on the recovery of

REEs for 45 minutes digestion time (Figure 3.2a) indicates that the recovery of REEs

increases with an increase in nitric acid concentration (based on 5 replicates for each

46

concentration); the highest recovery was obtained with 20 % nitric acid. This suggests

that increasing the nitric acid concentration increases the dissolution of the rare earth

oxide powders. Samples prepared with the higher acid concentrations, had to be either

diluted with water (in order to reduce the acid concentration to 2%), or evaporated to

dryness and then re-dissolved in weak nitric acid, before ICP-MS analysis. This is

because samples containing nitric acid concentrations above 5%, can decrease ICP-

MS signal intensity and accelerate corrosion of the cones [41].

Thus for the continuous monitoring of trace quantities of REEs, it is useful to digest

samples in dilute nitric acid, so instrumental analysis can be done without further

sample dilution. In addition, water is an excellent dipole molecule with the ability to

convert microwave energy efficiently into heat and the use of dilute solutions thus

improves the efficiency of microwave heating [42]. Rare earth element recoveries at

lower acid concentrations (2 and 5%) and a longer digestion time (60 minutes; Figure

3.2b) indicate no significant difference in REE recoveries (p > 0.05) except for

neodymium at 2 % HNO3 versus 5 % HNO3. Highest recovery (Appendix 3) was

obtained for Dy (97.7±2.7%) and lowest for Nd (90.2±1.4%). There was less than 4%

variation in recovery of the REEs when using 2% versus 5 % HNO3.

47

Recovery (%)

60

70

80

90

100 2%

5%

20%

45 min

Rare earth elements

Pr Nd Sm Gd Dy Yb

60

70

80

90

10060 min

2a

2b

a a a ba a

a a

a a a a

a a a

a b ba a a

a a a

a b b a b b

Figure 3.2: Effect of acid concentration (2, 5 or 20% v/v) on recovery. 20 mg sample, 20 ml

HNO3, 100oC microwave digestion. For each element, the same letters on different bars indicate

no significant difference (p > 0.05) between them, whereas different letters on different bars

indicate a significant difference (p < 0.05) between them.

48

3.4.3 Effect of digestion time - Microwave and hotplate digestions:

Digestion time is very important in online monitoring of REEs. Twenty mg of rare

earth oxide sample was mixed with 20 ml of 2% nitric acid and the digestion time was

varied between 30 and 90 minutes. Using microwave digestion (Figure 3.3), recovery

of the six REEs (5 replicates for each digestion time) ranged between ~85% and 95%

(Appendix 3) and there was a trend towards increasing recovery with increasing

digestion time. While there is no significant difference (p > 0.05) in REE recoveries

with digestion time (between 30 and 45 minutes) (Figure 3.3) except praseodymium

(p < 0.05), the best rare earth element recoveries appear to be obtained after a 60

minute digestion. The maximum variation in recovery is only 7% (for gadolinium)

among the various REEs for digestion times of 30 and 60 minutes. Similarly, using

hot plate digestion (Figure 3.4) there is no difference in REE recoveries between

digestion times of 30, 60 and 90 minutes except for ytterbium. Recoveries range

between ~74% and 88% (Appendix 3). These results indicate that rare earth oxide

samples digested with microwave heating for only 30 minutes yield higher REEs

recoveries than using hot plate digestion for 90 minutes (Appendix 3), suggesting that

microwave digestion could be more appropriate for online monitoring of REEs.

49

Rare earth elements

Pr Nd Sm Gd D

yYb

Recovery (%)

60

70

80

90

10030 min

45 min

60 min

a b b

a a b

a a,b b

a a,b b

a a a a a,b b

Figure 3.3: Effect of digestion time (30, 45 or 60 min) on recovery. 20 mg sample, 20 ml (2%

HNO3), 100oC, microwave digestion. For each element, the same letters on different bars indicate

no significant difference (p > 0.05) between them, whereas different letters on different bars

indicate a significant difference (p < 0.05) between them.

Rare earth elements

Pr Nd

Sm Gd D

yYb

Recovery (%)

60

70

80

90

10030 min

60 min

90 mina a a

a a a

a a a

a a a

a a aa a,b b

Figure 3.4: Effect of digestion time (30, 60 or 90 min) on recovery. 20 mg sample, 20 ml (2%

HNO3), 100oC, Hot plate digestion. For each element, the same letters on different bars indicate

no significant difference (p > 0.05) between them, whereas different letters on different bars

indicate a significant difference (p < 0.05) between them.

50

3.4.4 Microwave versus hot plate digestion:

A comparison of the REEs recoveries determined from microwave digestion and hot

plate digestion (Figure 3.5) indicates that microwave digestion yields consistently

higher recoveries than hot plate digestion. The increase in sample digestion time from

30 to 60 minutes leads to an increase in REE recoveries in the digested samples using

microwave heating (Figure 3.5 and Appendix 3).

Using microwave heating, gadolinium recovery in the digested sample averages

85.6±3.9% at a sample digestion time of 30 minutes but it increases to 92.7±4.4% at a

sample digestion time of 60 minutes (Appendix 3). However using hot plate digestion,

there was a very small change (less than 2%) in the gadolinium recovery when the

sample digestion time was increased from 30 to 60 minutes.

Recovery by Microwave (%)

70 75 80 85 90 95 100

Recovery by Hot plate (%)

70

75

80

85

90

95

100 30 min 60 min

Pr

Nd

Sm

Gd

Dy

Yb

Pr

Figure 3.5: A comparison of microwave vs. hot plate digestion. 20 mg sample, 20 ml (2% HNO3),

100oC.

51

Results indicate a significant difference (p < 0.05) in REE recoveries at different

digestion times between microwave and hot plate digestion. Hot plates conductively

heat the digestion tubes and then transfer the heat to the solution. This indirect heating

technique is relatively slow. However, microwave radiation passes through the

digestion tube walls and causes the solution molecules to oscillate, generating heat.

Microwave heating is more efficient than conventional heating because heating takes

place within the solution and no heat is lost to the environment during digestion

process.

Microwave heating was found to be preferable to hot plate digestion for the rare earth

oxide powder samples (Figure 3.5 and Appendix 3). A flow through digestion system,

the key part of which is a microwave heating section, could be used for sample

digestion, in order to continuously monitor atmospheric REEs. A flow through

digestion system such as this would continuously uptake samples (e.g., from the PILS

system), and automatically mix the sample with acid. Sample/acid mixtures would

then flow into the microwave heating section, be digested for the set residence time

and temperature, and then continuously flow out. Those digested samples could

subsequently be introduced into the ICP-MS to determine real-time REE

concentrations. According to our results, microwave digestion of REEs by nitric acid

(2 % v/v) at 100oC for 60 min would guarantee higher than 90% recoveries for all six

rare earth elements. Thus we may expect that a similar setup for a continuous flow

digestion system would also result in high recoveries of REEs, which may be used for

the continuous monitoring of REEs released from industry to control industrial

emissions and conform with compliance regulations.

52

Atmospheric rare earth element concentrations in ambient air are between 0.22 to 36

ng m-3

[20, 43]. If ambient air is sampled (e.g. REE concentration = 0.22 ng m-3

) for

10 minutes using the PILS with a transport liquid flow rate of 0.3 ml min-1

, the

particulate REE concentrations in the liquid sample will be higher than the ICP-MS

detection limits [35]. Therefore a sample collection time of 10 minutes would be

sufficient to collect enough particles into a sample volume of liquid using the PILS

system. The collected sample would be ready for ICP-MS analysis after sample

digestion (for 1hour) using 2 % nitric acid.

3.5 Conclusions

A relatively simple, effective and rapid microwave digestion method was developed

for high purity rare earth oxide powders. A combination of microwave heating at

100oC for 60 minutes, using 20 ml of nitric acid (2 % v/v) yields greater than 90%

recoveries of all six REEs. The results indicate that microwave heating is preferable to

hot plate digestion for these rare earth oxide powder samples. This method can be

used for the continuous monitoring of trace quantities of REEs without further sample

dilution because the digestions were conducted using 2% (v/v) nitric acid.

53

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d.; Boaventura, G. R.; Pimentel, M. M., Trace amounts of rare earth elements

in high purity samarium oxide by sector field inductively coupled plasma mass

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19. Chiarenzelli, J.; Aspler, L.; Dunn, C.; Cousens, B.; Ozarko, D.; Powis, K.,

Multi-element and rare earth element composition of lichens, mosses, and

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20. Wang, C.; Zhu, W.; Wang, Z.; Guicherit, R., Rare earth elements and other

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Water, Air, and Soil Pollution 2000, 121, 109-118.

21. Wang, C. X.; Zhu, W.; Peng, A.; Guichreit, R., Comparative studies on the

concentration of rare earth elements and heavy metals in the atmospheric

particulate matter in Beijing, China, and in Delft, the Netherlands.

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929-935.

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Particle-into-Liquid collector for rapid measurement of aerosol bulk chemical

composition. Aerosol Science and Technology 2001, 35, 718-727.

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impurities in high purity lanthanum oxide determined by neutron activation

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earth impurities in ultrapure europium oxide by inductively-coupled plasma

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38. He, M.; Hu, B.; Zeng, Y.; Jiang, Z., ICP-MS direct determination of trace

amounts of rare earth impurities in various rare earth oxides with only one

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41. Kulkarni, P.; Chellam, S.; Flanagan, J. B.; Jayanty, R. K. M., Microwave

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57

Chapter 4

Particle-into-liquid sampler (PILS)

optimization for the continuous

monitoring of uranium dioxide (UO2)

and uranium tetrafluoride (UF4)

particles in high particle concentration

environments

58

4.1 Abstract

The need to accurately and continuously monitor the release of industrial heavy

metals into the environment for control and compliance with emission standards

cannot be over-emphasized. A particle-into-liquid sampler (PILS) was used for

continuous monitoring of particulate uranium emissions (UO2 or UF4) in a high

particle concentration environment. A fluidized bed aerosol generator was used to

generate high purity uranium dioxide or uranium tetrafluoride aerosols at a rate of 1-5

mg h-1

to simulate stack conditions. Sodium carbonate (5 wt %) / hydrogen peroxide

(0.15 wt %) solution was found to be preferable to water as the sample transport

liquid to collect UO2 particles using the PILS. The sodium carbonate / hydrogen

peroxide solution enhances UO2 or UF4 recovery while the particle build-up on the

impaction surface and blockage in liquid transport lines were eliminated. The data

collected in experiments show that the sample air flow rate (l min-1

) has a significant

effect on particle collection efficiency. The combination of a sample air flow rate of

10 l min-1

(for UO2) or 16.7 l min-1

(for UF4), a steam flow rate of 1.5 ml min-1

and a

sample transport solution flow rate of 0.5 ml min-1

demonstrated greater than 89%

recovery of the particle mass of UO2 and greater than 92% recovery for UF4 in the

sample plus impactor drain lines. The results demonstrate that the particle-into-liquid

sampler (PILS) is a suitable technology for UO2 and UF4 particle sampling in high

particle concentration environments.

Keywords: Particle-into-liquid sampler; uranium tetrafluoride; uranium dioxide;

continuous monitoring; aerosol

59

4.2 Introduction

Air quality control plays a crucial role in the protection of the environment. It is of

special concern to those living in proximity to industrial and manufacturing facilities,

where there is potential for exposure to a greater amount of pollutants. Particulate

matter (PM) is the term for particulates found in the air. Exposure to particulate

matter consistently shows adverse effects on human health in cities throughout the

world in both developed and developing countries [1].

Electricity consumption is increasing rapidly in the world. Electricity generation

increased from 6116 TWh to 19771 TWh from 1973 to 2007. In 2007, nuclear power

contributed ~14 % of the total world’s electricity production [2]. Canada is one of the

leading electricity-consuming countries in the world and every year its electricity

demand is increasing. Most of the electricity in Canada is generated by hydroelectric,

natural gas, coal and nuclear power. Nuclear power contributes ~14.7 % of the total

electricity supply for Canada. Uranium production is increasing too for the

preparation of nuclear fuel for electricity generation. In 1999, the world’s uranium

production was ~31065 tonnes, which increased to ~50772 tonnes in 2009 (~ 63 %

increase); the forecasted uranium requirement for the year 2010 is 68646 tonnes [3].

Uranium is the heavy, silvery-white, metallic, naturally radioactive element with

atomic number 92; it belongs to the actinides group and is found generally as an oxide.

Natural uranium is composed of three isotopes 234

U (0.005%), 235

U(0.711%) and 238

U

(99.284%) [4]. Uranium is used mainly for nuclear power plants as fuel, where the

60

energy released from a controlled nuclear fission reaction is used to generate

electricity. Tri-uranium octoxide (U3O8, i.e., yellowcake) is refined into uranium

trioxide (UO3) and then converted into uranium dioxide (UO2) for use in heavy water

reactors. In the case of light water reactors, first purified uranium trioxide (UO3) is

converted into uranium dioxide (UO2) and then to uranium tetrafluoride (UF4). The

uranium tetrafluoride is then converted into uranium hexafluoride (UF6) using

elemental fluorine; then it undergoes 235

U enrichment and subsequent conversion into

enriched uranium dioxide (UO2).

Almost all energy production methods involve the possibility of some form of

contamination to the environment; from fossil fuel to nuclear energy, pollution occurs

in different ways and to different degrees. Exposure to uranium can cause lung cancer,

kidney damage, and respiratory diseases [5]. The nuclear regulatory commission

(NRC) in the USA has set atmospheric uranium release limits at 0.09µg m-3

[6] and

the Canadian Nuclear Regulatory Commission (CNSC) has a Naturally Occurring

Radioactive Material (NORM) release limit for 238

U series isotopes of 0.24µg m-3

(0.003 Bq m-3

) [7]. For the nuclear industry it is very important to continuously

monitor uranium release into the environment for control and compliance with these

emission standards.

The most commonly used technique for measuring PM involves filtration. Filters are

weighed before and after sampling and the PM concentration is determined from the

increase in filter mass divided by the volume of air sampled. Different types of filters

including fiber, membrane and granular bed filters made from a variety of materials

61

are used to collect aerosols [8-10]. The material of choice depends on different factors

such as mechanical, chemical and temperature stability, blank concentration, flow

resistance and loading capacity [8]. Artifacts associated with filter measurements

include: adsorption of vapors on the substrate (positive artifact) [11-16], evaporation

losses of semi-volatile compounds during and after sampling (negative artifact) [8,

17-23], contamination of filters during filter loading in the field [8] and reactions

between collected particles, gases and the filter substrate [20].

To overcome these artifacts several real-time or near real-time methods for the

measurement of aerosol mass concentrations have been investigated. Buhr et al. [24]

developed an automated method that collects PM on a frit surface, which is

continuously flushed with de-ionized water. Anders et al. [25] developed a method

where the water-soluble particles were directly impacted on flowing liquid. In

addition, aerosols can be charged and collected on the surface of an electrode and

periodically washed with de-ionized water for sample collection and analysis [26].

The Aerosol-to-Liquid Particle Extraction System (ALPES), developed by the

Savannah River National Laboratory (SRNL) is another promising device for

continuous PM monitoring [27].

It is known that very small particles are difficult to collect by impaction [28]. To

overcome this problem, the concept of growing aerosol particle size with steam prior

to particle collection was introduced. Simon and Dasgupta [29, 30] introduced an

aerosol particle collection system in which aerosol particles were grown with steam

under supersaturation, followed by condensation of the water vapors with a

62

thermoelectric cooler in a stainless steel maze. The condensed water-containing

particles from the stainless steel maze were collected and separated from the air using

an air/liquid separator. The design was later improved and simplified by removing the

thermoelectric cooler from the maze [31]. At the same time Khlystov et al. [32, 33]

developed a technique to collect droplets using a cyclone while Zellweger et al. [34,

35] and Löflund et al. [36] used a mist chamber / cooling device and air liquid

separator to collect droplets.

More recently, the particle-into-liquid sampler (PILS), was developed by Weber et al.

[37], which is a modified form of the particle size magnifier (PSM) design of

Okuyama et al. [38]. Later the particle-into-liquid sampler (PILS) was improved for

particle collection at higher air sample flow rates and for both ground and airborne

measurements of water-soluble aerosol composition [39]. Sample air is introduced

into the PILS system using a vacuum pump and the flow rate is controlled by a critical

orifice, which is placed at the exit of PILS system. At the entrance of the PILS the air

is mixed with steam to obtain a supersaturated environment in which particles grow.

This is followed by collection of particles by inertial impaction on a quartz plate, (the

impaction plate), which is continuously washed with a steady stream of water.

Originally the PILS was coupled only to two ion chromatographs for separate anion

and cation analysis, but it has the ability to connect with other analytical instruments

for online analyte concentration determination. Analysis of a given element can be

accomplished using nondestructive methods such as x-ray fluorescence [40-44] and

instrumental neutron activation analysis [45] or by destructive techniques such as

inductively coupled plasma mass spectrometry [46].

63

The particle-into-liquid sampler was designed for continuous measurement of ambient

aerosol composition. The PILS has proven successful for the measurements of water

soluble aerosols at high air sample flow rates. There is no information available about

the PILS particle collection efficiency in high particle concentration environments,

especially for particles with high particle density, or those that are hygroscopic / non-

hygroscopic and insoluble in water, such as UF4 and UO2. In the PILS, the impacted

liquid sample on the impaction plate is wicked away using a 62 µm stainless steel

mesh. UF4 and UO2 are insoluble in water and could cause clogging in a stainless

steel mesh and the liquid sample transport line when running in a high particle

concentration environment.

In this study, the utility of the PILS was assessed and optimized for the collection of

UF4 and UO2 particles into a liquid stream at higher particle concentrations. A

fluidized bed aerosol generator was used to generate high purity UF4 or UO2 aerosols

and to simulate stack conditions. UF4 and UO2 particles were captured into a liquid

stream using the PILS and collected samples were analyzed off-line using ED-XRF.

The PILS operating parameters, such as sample air flow rate, steam flow rate,

transport liquid flow rate and sample transport solution were optimized for maximum

particle mass recovery in the sample line and minimum recovery in the growth

chamber drain as a percentage of the total mass of particles collected.

64

4.3 Experiments

4.3.1 Reagents and standards

Hydrogen peroxide (H2O2) (30% W/W ACS grade; EMD chemicals Inc, Gibbstown,

NJ, USA) and sodium carbonate (Na2CO3) (Mallinkrodt Baker, Phillipsburg, NJ, USA)

were used to prepare all the samples and uranium dioxide (UO2) and uranium

tetrafluoride (UF4) standards. For the preparation of solutions, distilled de-ionized

water (18.2 MΩ) was used. High purity UO2 and UF4 powder with particle sizes

smaller than 10 µm (Table 4.1) (Cameco Corporation, Port Hope, Ontario Canada)

were used to generate aerosols.

4.3.2 Aerosol generator

A fluidized bed aerosol generator (Model-3400A, TSI Corporation, MN, USA) was

used to generate UF4 and UO2 aerosols (Figure 4.1). The instrument is designed for an

aerosol output range of 10 - 100 mg m-3

with a powder feed rate of 3 to 30 mm3 min

-1

i.e., a feed rate of 180 - 1800 mg h-1

assuming a powder with unit density. UF4

(density = 6.7 g cm-3

) or UO2 (density = 10.96 g cm-3

) powder was placed in the

powder reservoir and then transported into the fluidized bed chamber using a bead

chain. The fluidized bed consists of 100 µm bronze beads supported by a porous plate.

The clean and dry air is introduced from the bottom of the porous plate to create a

boiling action, which de-agglomerates the powder moving upward with the airflow

through the elutriator. Aerosols were generated using a low air flow rate to minimize

65

aerosol output rate. A filter, pressure gauge, flow meter and pressure relief valve

were attached at the outlet of the aerosol generator to achieve different aerosol

particle rates (1 - 5 mg h-1

).

Figure 4.1. TSI fluidized bed aerosol generator with particle removal setup to control

aerosol output rate to 1-5 mg h-1

.

66

Figure 4.2. Schematic diagram of the PILS for the continuous collection of UO2 and

UF4 particles into the liquid stream.

Table 4.1. UO2 and UF4 particle size distribution.

Diameter (µm) UO2

Mass finer (%)

UF4

Mass finer (%)

10.0 100.0 100.0

8.0 93.3 99.2

6.0 85.2 98.4

4.0 75.3 97.9

2.0 60.0 83.0

1.0 42.9 42.6

0.5 12.9 7.8

67

4.3.3 Particle-into-liquid sampler (PILS)

The particle-into-liquid sampler (Model: ADI 2081, Applikon Analytical BV,

Schiedam, The Netherlands) was used to capture UF4 and UO2 particles into a liquid

stream. The PILS unit was coupled to the TSI fluidized bed aerosol generator, as well

as three separate peristaltic pumps and a vacuum pump (Figure 4.2). Separate

peristaltic pumps were used for better control over liquid flow rates. Pump 1 was used

to control water flow rate for steam generation and peristaltic pump 2 was used to

collect liquids from the growth chamber drain and impactor drain. The third peristaltic

pump was used to control the flow rate of the transport liquid over the impaction plate

and the flow rate of sample collection out of the PILS impactor cavity. Generated

aerosols were introduced to the PILS using a vacuum pump. Inside the PILS, the

aerosol stream is mixed with steam as a result of which the particles grow larger,

followed by collection of the particles by inertial impaction onto the impaction plate.

Particles too small for collection on the inertial impactor leave the system with the

exhaust air flow; those particles were collected using a cold trap and filter installed

between the critical orifice and the vacuum pump.

The sample transport liquid (water or sodium carbonate/ hydrogen peroxide solution)

is continuously pumped over the impaction plate to collect particles into the liquid

stream. Different transport liquid flow rates (speeds of peristaltic pumps) were tested

in order to optimize the particle-into-liquid sampler for the UO2 / UF4 aerosols. Liquid

flow rates were maintained using two different sizes of tubing. Transport liquid flow

rates (0.5 and 0.7 ml min-1

) were controlled using 0.89 mm (I.D) tubing. Tubing with

68

an inner diameter of 1.42 mm was used to collect samples from the PILS impactor

cavity. Because of droplet and condensation effects on the impactor plate, larger inner

diameter tubing was used to ensure that all the liquid sample was removed from the

impactor plate base. Flow rates for the growth chamber drain (1.0 ml min-1

), impactor

drain (1.0 ml min-1

) and steam flow (1.5 ml min-1

) were controlled using 1.42 mm

(I.D) peristaltic pump tubing.

Different sample air flow rates were tested in order to optimize the particle-into-liquid

sampler for the UO2 aerosol. The Applikon particle-into-liquid sampler (Model ADI

2081) was optimized (from the supplier) for an air flow rate of 16.7 l min-1

with a

critical orifice placed at the outlet of the PILS (downstream of the sampling device,

PILS and upstream of the sampling vacuum pump). To achieve other sampled air flow

rates with the PILS, the critical orifice was removed from the outlet and a flow

controller was installed to control different sample air flow rates. The PILS’s

condensation chamber was kept at a slight angle to collect wall condensate and UF4

and UO2 particles lost from the growth chamber drain line.

The PILS was optimized using aerosol particle rates between 1 - 5 mg h-1

, two levels

of sample air flow rates (16.7 l min-1

and 10.0 l min-1

), two different transport liquid

flow rates (0.5 ml min-1

and 0.7 ml min-1

) and different solutions for the sample

transport liquid.

69

4.3.4 Pre-treatment of PILS samples

The PILS samples were collected from the growth chamber drain, the impactor drain,

the cold trap, the filter and from the sample line. Initially the PILS was operated with

water as the sample transport liquid and liquid collected from the growth chamber,

impactor drain, cold trap and sample line, as well as particles collected on the filter

needed pretreatment prior to XRF analysis. The sodium carbonate (Na2CO3) /

hydrogen peroxide (H2O2) solution was added to all liquid samples to make a final

solution concentration of 5 wt % Na2CO3 and 0.15 wt % H2O2 to dissolve uranium

particles. Filters were soaked in 10 ml of the same Na2CO3/H2O2 solution before

sample analysis.

4.3.5 ED-XRF analysis

The Innov- X-50 mobile XRF spectrometer has the capability to analyze liquid,

powder and solid samples combined with simultaneous measurements of up to 30

elements from phosphorous (Z=15) to uranium (Z=92). UF4 and UO2 standards (0 –

200 mg UO2 l-1

and 0 – 200 mg UF4 l-1

) were prepared to generate standard curves.

The XRF measures uranium concentration in weight percentage; it was then

converted to parts per million (mg U l-1

) to generate the calibration curve. All the

liquid samples collected from the sample line, impactor drain, growth chamber, cold

trap and filter were analyzed for uranium concentrations using XRF and the

concentrations were recorded in weight percent. They were then converted into mg U

70

l-1

and compared with the UF4 and UO2 calibration curves to calculate UF4 and UO2

particle mass recovery in each line.

4.4 Results and discussion

4.4.1 Water as the transport liquid

In these laboratory experiments the first step was to modify and calibrate the fluidized

bed aerosol generator for the UF4 and UO2 powder. As outlined in the Methods

(above), powder was fed into the sample powder chamber and a chain conveyor (ball

chain) transported the powder from the sample chamber to the fluidized bed at a

constant rate (Figure 4.1). The instrument should run at a minimum chain speed of 20

(arbitrary units of the instrument control) or higher (necessary for stirring action of

the rake inside the sample chamber) to insure that the chain does not create a channel

in the powder chamber. This would result in depletion of the fluidized bed and lead to

unstable/reduced aerosol output concentration over time. At a chain speed of 20,

there is uniform transfer of powder to the fluidized bed resulting in a constant aerosol

output concentration. Fluidized bed aerosol generator disperses powders at a feed rate

range of 180 to 1800 mg h-1

(assuming unit density). A filter holder, pressure gauge,

flow meter and pressure relief valve were attached at the outlet of the aerosol

generator to achieve different aerosol particle rates. Aerosol output rate was reduced

to obtain three different levels (low, medium and high) by removing a fraction of the

particles from the air flow using a series of filter papers. For the low level, particles

were removed by three separate filter papers. The remaining particles were collected

71

on the fourth filter paper and subsequently weighed on an analytical balance. For the

medium level, particles were removed by two separate filter papers. The remaining

particles were collected on the third filter and subsequently weighed on an analytical

balance. Finally for the high particle rate, particles were removed using a single filter

paper. The remaining particles were collected on a second filter and subsequently

weighed on an analytical balance. The filter paper also helped to remove all the larger

particles (> 10 µm) from the aerosol stream. Fluidizing action of the bronze beads de-

agglomerates the powder and generates charged aerosols due to mechanical friction

within the fluidized bed; aerosol charge increases with increasing fluidization air flow

rate and also changes with the length of operating time [47, 48]. To minimize aerosol

losses in the aerosol transport line, a conductive tube was used to transport the aerosol

from the fluidized bed generator to the PILS.

Experimental Run

0 1 2 3 4

UO2 distribution (%)

0

25

50

75

100Impactor drain

Impactor wash

Filter + Cold trap + Growth chamber

Sample line

Figure 4.3. PILS running with water as sample transport liquid: UO2 particle mass

recovery (%) in different lines during four successive experimental run at the sample

air flow rate of 10 l min-1

, water flow rate for steam generation of 1.5 ml min-1

and

transport liquid flow rate of 0.4 ml min-1

.

72

Initially the PILS was operated with water as the sample transport liquid for the

collection of the uranium dioxide particles, followed by the dissolution of particles

prior to sample off-line analysis using XRF. Initial tests (Appendix 4) were carried

out with a transport liquid flow rate of 0.4 ml min-1

and a water flow rate for steam

generation of 1.5 ml min-1

. Results (Figure 4.3) show a low percentage uranium

dioxide particle mass recovery in the sample line. The maximum percentage of

uranium dioxide particle mass recovery (68%) from sample line was achieved in a

sample collected from experimental run 1 but it is reduced to 39 % in experimental

run 4. After each experimental run the impaction plate was removed from the PILS

unit and washed to collect all the particles for a particle mass balance; we found that

for each successive experimental run the percentage of uranium dioxide particle mass

recovered in the sample line decreased whereas it increased in all the drain lines as

well as on filter. The PILS ran continuously and during operation, particle build-up

was clearly observed on the impaction plate and blockage was evident in drain lines

(Figure 4.4). Thus water was ineffective as a liquid for the transport of uranium

dioxide particles out of the sampler, at concentrations typical of normal stack

operations.

73

Figure 4.4. PILS running with water: particle build-up on impaction surface and

blockage in liquid collection lines.

4.4.2 Optimization of PILS system for UO2 and UF4

Uranium dioxide is insoluble in water but dissolves in aqueous carbonate media;

hydrogen peroxide increases the rate of uranium oxidation [49]. Therefore, to improve

the particle mass collection efficiency of uranium in the sample line, a sodium

carbonate (5 wt %) and hydrogen peroxide (0.15 wt %) solution was used as the

sample transport liquid. It was anticipated that this solution would dissolve uranium

tetrafluoride and uranium dioxide particles within the system because uranium forms

carbonate complexes in solutions. This transport liquid solution also reduces chances

of particle build-up on the impaction surface and blockages in the liquid lines. To our

knowledge, this is the first time that a liquid other than water has been used as a

74

sample transport liquid in a particle-into-liquid sampler. The particle-into-liquid

sampler tip temperature was optimized at a temperature of approximately 90oC

because at this temperature steam flow transitions from spitting to a steady steam jet

with no water drops coming out of steam injection tip. The PILS tip temperature was

maintained at approximately 90oC for all experiments, which resulted in good steam

generation for proper particle-into-liquid sampler operation.

Two separate pumps were used to control the water flow rate for steam generation and

sample transport. During the PILS-UO2 experiments, the average collected volume

from the sample line (Pump 3) was 0.51±0.06 ml min-1

and 0.72±0.04 ml min-1

for a

liquid transport flow rate of 0.5 and 0.7 ml min-1

, respectively. During the PILS-UF4

experiments, the average collected volume from the sample line (Pump 3) was

0.54±0.05 ml min-1

and 0.75±0.05 ml min-1

for a liquid transport flow rate of 0.5 and

0.7 ml min-1

, respectively. The sample was collected at a higher flow rate than the rate

at which transport liquid was introduced at the top of impaction surface. A tube with a

larger inner diameter was necessary to collect the sample at the bottom of impaction

surface (Figure 4.2) because the total flow is the sum of transport liquid flow,

collected water droplets and condensed water vapor in the impactor cavity. The

volumes of the collected samples were significantly different because of the tubing

used in the peristaltic pumps. The peristaltic pumps did not provide a constant flow

of liquid due to deformation in the tubing, which takes place over time, but also

because air temperature and supersaturated conditions in the PILS can affect the

liquid volumes collected. When doing on-line analysis, it is very important to know

the exact sample flow rate in order to accurately calculate the concentration of the

element of interest in ambient air. Considering variations in sample flow rates, Weber

75

et al. [37, 39] added a known quantity of lithium fluoride to the sample flow to correct

for differences in sample volume during on-line analysis. In our study, analyses of

samples were done off-line, and so the addition of a calibrant was not required

because final volumes of the samples were measured.

4.4.2.1 PILS – Sample line plus impactor drain

Several experiments were performed (Appendix 5 and Appendix 8) to determine the

influence of the variation in particle mass concentration (low, medium and high),

transport liquid flow rate (0.5 and 0.7 ml min-1

), and sample air flow rate (10 and 16.7

l min-1

) on the particle mass collection efficiency of the PILS. Experiments to

optimize the PILS for uranium dioxide and uranium tetrafluoride particles were

carried out at the industrial project partner’s research lab (Cameco Corporation, Port

Hope, Ontario Canada). It was decided to optimize the PILS at three different aerosol

flow rates (low, medium and high) for UO2 (total of 24 experimental runs - Appendix

5) and two different aerosol flow rates (low and high) for UF4 (total of 16

experimental runs - Appendix 8) because of time and budgetary constraints. We

performed eight different experiments for each aerosol flow rate (for both of UO2 and

UF4). For the aerosol flow rate experiments, statistical analyses for UO2 results

(Appendix 7) show an average particle mass collection recovery of 85.4 % in the

sample plus impactor drain, with a variance of 168.7. For the high aerosol flow rate,

statistical results show an average particle mass collection recovery of 85.9 % in the

sample plus impactor drain, with a variance of 114.0. Statistical analysis for UF4

results (Appendix 10) show a mean particle mass collection recovery of 89.2 % in the

sample plus impactor drain, with a variance of 14.4 for the low aerosol flow rate, and

76

a mean particle mass collection recovery of 91.3%, with a variance of 7.2 for the high

aerosol particle flow rate in the sample plus impactor drain. The results (Appendix 7

and Appendix 10) show no significant difference (p > 0.05) in uranium dioxide or

uranium tetrafluoride particle mass collection efficiency in the sample line plus

impactor drain at different aerosol flow rates (low, medium and high) that were

introduced into the system. Based on these findings, the three aerosol flow rates were

considered to be the same (1-5 mg h-1

).

Figure 4.5 shows UO2 mean particle mass recovery in the sample line for individual

variables such as sample air flow rate of (10 and 16.7 l min-1

), transport liquid flow

rate (0.5 and 0.7 ml min-1

) and for the combination of sample air flow rate and

transport liquid flow rate (for maximum and minimum uranium particle mass

collection efficiency in the sample line). The mean particle mass collection

efficiencies for the individual variables such as sample air flow rate of 10 l min-1

(based on 12 experimental runs; Appendix 5) and a transport liquid flow rate of 0.5 ml

min-1

(based on 12 experimental runs; Appendix 5) are higher than for a sample air

flow rate of 16.7 l min-1

and transport liquid flow rate of 0.7 ml min-1

. A UO2 particle

mass recovery of 86±6.7% (based on 6 experimental runs; Appendix 5) was achieved

for the combination of a sample air flow rate of 10 l min-1

and a transport liquid flow

rate of 0.5 ml min-1

, but it is reduced to 68.8±20% at a sample air flow rate of 16.7 l

min-1

and a transport liquid flow rate of 0.7 ml min-1

with a large variation in mass

recovery.

77

10 16.7 0.5 0.7 10 & 0.5 16.7 & 0.7

Particle mass collection efficiency (%)

0

20

40

60

80

100

Sample air flow rate

(l min-1)

Transport liquid flow rate

(ml min-1)

Sample air flow rate (l min-1)

and

Transport liquid flow rate (ml min-1)

Sample Line

Figure 4.5. UO2 particle mass collection efficiency (%) in the sample line at different

sample air flow rates and transport liquid flow rates.

10 16.7 0.5 0.7 10 & 0.7 16.7 & 0.5

Particle mass collection efficiency (%)

50

60

70

80

90

100

Sample air flow rate

(l min-1)

Transport liquid flow rate

(ml min-1)

Sample air flow rate (l min-1)

and

Transport liquid flow rate (ml min-1)

Sample Line

Figure 4.6. UF4 particle mass collection efficiency (%) in the sample line at different

sample air flow rates and transport liquid flow rates.

78

Sample air flow rate

(l min-1)

Sample air flow rate (l min-1)

and

Transport liquid flow rate (ml min-1)

Impactor drain

Transport liquid flow rate

(ml min-1)

10 16.7 0.5 0.7 10 & 0.5 16.7 & 0.7

Particle mass collection efficiency (%)

0

5

10

15

20

25

Figure 4.7. UO2 particle mass collection efficiency (%) in the impactor drain at

different sample air flow rates and transport liquid flow rates.

Sample air flow rate

(l min-1)

Sample air flow rate (l min-1)

and

Transport liquid flow rate (ml min-1)

Impactor Drain

Transport liquid flow rate

(ml min-1)

10 16.7 0.5 0.7 10 & 0.7 16.7 & 0.5

Particle mass collection efficiency (%)

0

1

2

3

4

5

Figure 4.8. UF4 particle mass collection efficiency (%) in the impactor drain at

different sample air flow rates and transport liquid flow rates.

79

The UF4 particle mass collection efficiencies for the individual variables (8

experimental runs for each individual variable) in the sample line are higher at a

sample air flow rate of 16.7 l min-1

and a transport liquid flow rate of 0.5 ml min-1

than for the sample air flow rate of 10 l min-1

and transport liquid flow rate of 0.7 ml

min-1

(Figure 4.6 and Appendix 9). The data collected in these experiments show that

sample air flow rate (l min-1

) has a significant effect (p < 0.05) (Appendix 10) on UF4

particle collection efficiency. Using the PILS, UF4 particle collection efficiency in the

sample line averages 91.1±1.6% using a sample air flow rate of 16.7 l min-1

with the

combination of a transport liquid flow rate of 0.5 ml min-1

, but it is reduced to

84±6.6% at a sample air flow rate of 10 l min-1

and a transport liquid flow rate of 0.7

ml min-1

(Figure 4.6 and Appendix 9).

Figure 4.7 demonstrates mean uranium dioxide (UO2) particle mass recovery in the

impactor drain line for individual variables such as sample air flow rate (l min-1

) and

transport liquid flow rate (ml min-1

). The sample air flow rate and transport liquid

flow rate should be optimized to minimize the loss of particles into the impactor drain.

The mean particle mass collection efficiency in the impactor drain for a sample air

flow rate of 10 l min-1

and transport liquid flow rate of 0.5 ml min-1

is lower than that

measured for a sample air flow rate of 16.7 l min-1

and transport liquid flow rate of

0.7 ml min-1

(Figure 4.7). UO2 particle collection efficiency in the impactor drain is

3.8±2.4% at a sample air flow rate of 10 l min-1

and a transport liquid flow rate of 0.5

ml min-1

but it is increased to 12.6±9 % at a sample air flow rate of 16.7 l min-1

and a

transport liquid flow rate of 0.7 ml min-1

. Results show that the mean particle mass

losses in the impactor drain increase (also large standard deviation) (Figure 4.7) with

an increase in sample air flow rate. UF4 particle mean collection efficiency (Figure

80

4.8) in the impactor drain is approximately 1.4±1% at a sample air flow rate of 16.7 l

min-1

and a transport liquid flow rate of 0.5 ml min-1

but it increases to 2.3±2.1% at a

sample air flow rate of 10 l min-1

and a transport liquid flow rate of 0.7 ml min-1

.

The UO2 particle mass collection is increased in the impactor drain (Figure 4.7) and

decreased in the sample line (Figure 4.5) with an increase in the sample air flow rate.

UF4 particle mass collection in the impactor drain is decreased with an increase in the

sample air flow rate (Figure 4.8) but particle mass collection in the sample line is

increased with the increase in sample air flow rate (Figure 4.6). It is important to

minimize particle mass collection in the impactor drain because too many particles in

the impactor drain can cause blockage in the drain line. Theoretically, all particles in

the impactor will be collected upon striking the collection surface but in practice this

is not always true. One of the reasons is that particles bounce off the particle

collection surface. Orsini et al. [39] explained that small liquid drops are produced

from larger droplets upon impaction on a collection surface and bounce back off as

small drops, reducing the collection efficiency in the sample line. Our system

operated at a sample air flow rate of 16.7 l min-1

and 10 l min-1

with corresponding

nozzle velocities. At controlled relative humidity, the fraction of particle bounce

increases with nozzle velocity [50]. This evidence supports our experimental results;

at high sample air flow rate, particle bounce back increases from the impactor, wall

losses increase and UO2 mass collection in the sample line is reduced [51], all of

which results in an increase in UO2 mass collection in the impactor drain line.

81

To investigate the possibility of joining the sample line and the impactor drain and

collecting both liquids together as a one sample, the volumes of liquid collected from

the impactor drain and the sample line were combined and analyzed. The results show

(Figure 4.9) that the UO2 mean combined collection efficiency for the impactor drain

plus sample line is greater than 89.7% at a sample air flow rate of 10 l min-1

and

liquid transport flow rate of 0.5 ml min-1

. This decreased to 81.4% at a sample air

flow rate of 16.7 l min-1

and a liquid transport flow rate of 0.7 ml min-1

(Figure 4.9).

Sample air flow rate (l min-1) &

Transport liquid flow rate (ml min-1)

10&0.5

10&0.7

16.7&0.5

16.7&0.7

UO2 distribution (%)

0

10

20

80

100

Growth chamber drain

Sample line + Impactor drain

Filter + Cold trap

89.7 ± 6.3

%

87.8 ± 13.4

%

83.8 ± 11.7

%

81.4 ± 14.6

%

Figure 4.9. UO2 particle mass recovery (%) in the PILS unit’s different lines at

different sample air flow rates and transport liquid flow rates (Note: UO2 percentage

recovery in filter + cold trap is too small to see clearly on the graph).

82

Sample air flow rate (l min-1) &

Transport liquid flow rate (ml min-1)

16.7&0.5

16.7&0.7

10&0.5

10&0.7

UF4 distribution (%)

0

10

20

80

100

Growth chamber drain

Sample line + Impactor drain

Filter + Cold trap

89.3 ± 1.0

%

86.8 ± 4.6

%92.5 ± 1.9

%

92.3 ± 1.2

%

Figure 4.10. UF4 particle mass recovery (%) in the PILS unit’s different lines at

different sample air flow rates and transport liquid flow rates.

The UF4 results show that the combined mean collection efficiency (Figure 4.10 and

Appendix 9) for the sample line plus impactor drain is 92 % at a sample air flow rate

of 16.7 l min-1

in combination with both transport liquid flow rates (0.5 and 0.7 ml

min-1

) but this is decreased to 86.8 % at a sample air flow rate of 10 l min-1

and a

liquid flow rate of 0.7 ml min-1

. By using the carbonate and hydrogen peroxide

solution as the sample transport liquid, particle build-up on the impaction surface and

blockage in the sample line or the impactor drain was eliminated.

83

4.4.2.2 PILS - Growth chamber drain

The UO2 particle mass collection efficiency in the growth chamber drain results

(Figure 4.9; Appendix 6) demonstrate higher particle losses at higher sample air flow

and transport liquid flow rates. UO2 particle loss in the growth chamber drain line is

18.3±14.7 % for a combination of a sample air flow rate of 16.7 l min-1

and a

transport liquid flow rate of 0.7 ml min-1

but it is reduced to 9.8±6.4% at a sample air

flow rate of 10 l min-1

and a transport liquid flow rate of 0.5 ml min-1

. The UF4

particle mean mass collection efficiency (Figure 4.10; Appendix 9) is increased in the

growth chamber with the decrease in the sample air flow rate. UF4 particle loss in the

growth chamber drain line is 11.2±4.1% for a combination of a sample air flow rate of

10 l min-1

and a transport liquid flow rate of 0.7 ml min-1

but it is reduced to

6.4±1.2 % at a sample air flow rate of 16.7 l min-1

and a transport liquid flow rate of

0.5 ml min-1

. Physical and chemical properties of the UO2 and UF4 particles also play

a crucial role in particle mass losses in the growth chamber corresponding to

operating parameters of the PILS. Sample air flow rate and size of the growth

chamber are important because they are related to particle residence time in the

growth chamber, whereas particle residence time and steam flow rate are related to

particle growth rate. Particle residence time, τ (in seconds), can be calculated based on

sample air flow rate (Qa in l min-1

), inside diameter (Di) and length (L) (in centimeters)

of growth chamber using equation 4.1 [52]:

a

i

Q

LD .4

2λ

τ = (4.1)

84

Possible reasons for particle mass losses in the growth chamber may include

gravitational settling, turbulent inertia deposition and losses in both conical sections at

the beginning and end of the growth chamber. The inside diameter and length of the

growth chamber are fixed for the Applikon ADI-2081 PILS; the only parameter that

can affect particle residence time is sample air flow rate. According to equation (4.1),

as sample air flow rate increases, residence time decreases. If the residence time is too

short, very small particles do not grow to a size that is big enough to be captured on

the impactor plate. However, if residence time is too long, particles grow to an

excessively large size resulting in droplet losses in the conical section leading to the

impactor jet. Steam flow rate and the particle-into-liquid sampler tip temperature also

affect particle growth; these parameters were optimized at a steam flow rate of 1.5 ml

min-1

and a tip temperature of ~90oC. UF4 particles are hygroscopic and require

shorter residence time in the growth chamber (higher sample air flow rate, i.e 16.7 l

min-1

) compared to non-hygroscopic UO2 particles (lower sample air flow rate, i.e 10

l min-1

) to grow particles to a size that is big enough to be captured on the impactor

plate and maximize the particle mass collection in the sample line (minimize the

particle mass collection in the growth chamber). There is a critical range in the sample

air flow rate (l min-1

) within which the PILS provides maximum particle mass

recovery. However, we do not know what the PILS operating range limits are (range

of sample air flow) because we used a sample air flow rate of 16.7 and 10 l min-1

to

optimize PILS for UO2 and UF4 particle collection. The results indicate that UO2 and

UF4 require different optimum sample air flow rates in order to minimize losses in the

growth chamber drain and maximize recovery in the sample line; i.e. 10 l min-1

for

UO2 and 16.7 l min-1

for UF4.

85

4.4.2.3 PILS - Filter plus cold trap

After the aerosol particles are grown in the PILS’s growth chamber they are focused

on to the impactor using a single nozzle. Droplets large enough are collected on the

impaction surface and droplets too small to be collected follow the airflow streamlines,

remaining in the air until they leave the system. Particles entrained in the venting air

were collected using a cold trap and filter at the exit of the particle-into-liquid sampler.

The percentage of UO2 particle mass recovery from the cold trap plus the filter was

less than 0.6% (for UO2 , Figure 4.9) and 2% (for UF4, Figure 4.10) of the total

particle mass collected from all other lines for both sample air flow rates (16.7 l min-1

and 10 l min-1

).

4.5 Conclusions

A particle-into-liquid sampler was optimized for the collection of UO2 and UF4

aerosols. The sampling set-up includes a fluidized bed aerosol generator. Sample air

flow rate, steam flow rate, liquid transport flow rate and sample transport solution

were optimized for the maximum recovery of UO2 and UF4 particles in the sample

line. The fluidized bed aerosol generator was optimized to generate aerosol at a rate of

1- 5 mg h-1

. A sodium carbonate/hydrogen peroxide solution was used as a sample

transport liquid to collect UO2 and UF4, and was found to be preferable to water for

the collection of UO2 and UF4; particle build-up on the impaction surface and

86

blockage in the liquid transport lines were eliminated. The combination of a sample

air flow rate of 10 l min-1

(for UO2) or 16.7 l min-1

(for UF4), a steam flow rate of 1.5

ml min-1

and a sample transport solution flow rate of 0.5 ml min-1

demonstrated

greater than 89% recovery of the particle mass of UO2 and greater than 92% recovery

for UF4 in the sample plus impactor drain lines. Our results demonstrate that the PILS

is suitable technology for UO2 and UF4 particle sampling in high particle

concentration environments.

We optimized the PILS system for a high particle concentration environment and for

water-insoluble particles such as uranium dioxide and uranium tetrafluoride thus

extending the range and application of the PILS system. An aqueous carbonate

medium was used successfully as a sample transport liquid for the collection of UO2

and UF4 particles. This opens the door for the collection of water-insoluble particles

by changing the chemistry of transport liquids. A PILS unit can be installed on an

industrial stack for continuous emission monitoring with the appropriate transport

solution, and by adjusting the transport liquid flow rate and sample air flow rate. It is

an effective instrument for aerosol collection into a liquid; it is very reproducible,

easy to use, and will provide a better understanding of aerosol composition including

time-dependent data.

87

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91

Chapter 5

Determination of uranium (U) in

atmospheric aerosols using a Particle-

Into-Liquid sampler (PILS)

92

5.1 Abstract

A particle-into-liquid sampler (PILS) and high volume air sampler (filter) were used

to collect aerosols and atmospheric particles, respectively, and samples were analyzed

for uranium concentrations. Comparisons between particulate uranium concentrations

in the air using the high volume air sampler and the PILS indicate that uranium

collection efficiency with the high volume air sampler was consistently higher than

the PILS. Digested PILS samples and filter results were correlated (R2

= 0.98); on

average the concentration of uranium obtained using the PILS was 80% of the

concentration measured using the high volume sampler. The PILS is a promising

device for the collection of atmospheric aerosols. In addition it can provide detailed

information on the variation in ambient levels of uranium, through time.

Keywords: Particulate matter; uranium; particle-into-liquid sampler; high volume air

sampler; continuous monitoring.

93

5.2 Introduction

Atmospheric particulate-associated contaminants pose a risk to human health,

especially in cities. Among those particles, those with an aerodynamic size of less

than 10 microns (PM10), which could be inhaled by humans and cause lung damage,

are of special concern. Thus, the capture or monitoring of atmospheric particles,

especially PM10 is critical in protecting human health.

There is a growing interest in determining heavy metal concentrations in atmospheric

particles due to the toxic effects of heavy metals to humans and other organisms.

Among these metals, uranium is of special concern due to its risk to human health.

Uranium in the air is mainly present as particulate matter. Approximately 70% of the

uranium is associated with particles greater than 2 µm and 40–50% greater than 7 µm

[1]. Particulate uranium is released into the atmosphere both from natural and

anthropogenic sources. Uranium is introduced into the atmosphere primarily by re-

suspension of soil, volcanic eruptions, coal-burning power plants [2-4], and during the

nuclear fuel cycle including mining, milling, refining, fuel fabrication and fuel

processing [5].

There is limited information available about ambient atmospheric uranium

concentrations in particles, mainly due to the low metal concentration in the air and

the detection limit of monitoring systems. Atmospheric uranium concentrations as

high as 200 ng m-3

have been reported near nuclear fuel production plants [5-7].

Average particulate uranium concentrations ranged from 0.10 to 1.47 ng m-3

found in

94

the air at urban and rural locations within New York State [8]. For the Atlantic Ocean

air sampled in Northern Hemisphere, a mean particulate uranium content of 4.1 pg m-

3 was reported [9].

At present, measurements of concentrations of metals in airborne particulates are

typically performed off-line by collecting particles onto filters. Due to the very low

concentration of uranium associated with airborne particles, a large volume of air is

normally required to collect a sufficient mass of particles onto the filter for metal

analysis. A more sophisticated system, the particle-into-liquid sampler (PILS)

developed by Weber et al. [10, 11], which was modified from the particle size

magnifier (PSM) designed by Okuyama et al. [12], was designed for continuous

measurement of aerosol composition. However, the PILS sampler has not been

applied to the analysis of particulate uranium in ambient air.

Analysis of many elements in particles can be accomplished using destructive and

nondestructive methods such as Neutron Activation Analysis (NAA), X-ray

fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS)

[13-18]. ICP-MS is able to detect trace elements at levels in the part per trillion (ppt)

ranges or lower. The analytical method of choice for the determination of uranium

concentration in the samples was ICP-MS due to its low detection limits, high

sensitivity and good measurement accuracy.

95

Atmospheric particle collection on filters using a high volume sampler is not suitable

for investigating variation in atmospheric trace metal concentrations since it takes a

long time to collect enough sample mass for analysis. However the PILS can provide

us with detailed, rapid information on the variation in ambient levels of particulate

uranium, because aerosols are collected into a small volume over a short period of

time. In this study, we compared measurements of uranium in particles collected from

a traditional high volume air sampler with those from the PILS. Our aim was to

evaluate the PILS as an easy and reliable analytical method for the continuous

monitoring of uranium in atmospheric aerosols.

5.3 Materials and Methods

5.3.1 Sampling location

Atmospheric particulate matter sampling was conducted on the roof of a three-story

building at Trent University, Peterborough, ON, Canada. The Trent University

campus in Peterborough (44o 21' 0" N, 78

o 18' 0" W) is located on the northern

boundary of a small city of 74,600 people, with limited heavy industry. The

immediate area of the campus is surrounded by agricultural land and forests. Wind

direction during the sampling period was approximately 60 % of the time from south

west and 40 % of the time from the north west. Peterborough, ON, is ~30 kilometers

north of the Port Hope Cameco Corporation nuclear fuel production facility. There are

96

no known point sources of particulate uranium in the immediate vicinity of

Peterborough.

5.3.2 Aerosol sampling

Atmospheric particulate matter samples were collected simultaneously using a high-

volume air sampler and PILS during days of no rain in July and August, 2010. The

weather during the sampling periods varied. It was generally hot and windy with a

minimum temperature of 18.2oC, maximum temperature of 29.5

oC, and relative

humidity between 37 - 65% during the sample collection period (0900-1700 hours).

5.3.2.1 Particle-into-liquid sampler (PILS)

A PILS (Model: ADI 2081, Applikon Analytical BV, Schiedam, The Netherlands)

was used to collect atmospheric particulate matter into a liquid stream. The PILS unit

was coupled to a PM10 inlet (BGI Incorporated, USA), as well as three separate

peristaltic pumps and a vacuum pump (Figure 5.1). Separate peristaltic pumps were

used for better control over liquid flow rates. Pump 1 was used to control the water

flow rate for steam generation, while peristaltic pump 2 was used to collect liquids

from the growth chamber drain and the impactor drain. The third peristaltic pump was

used to control the flow rate of the transport liquid over the impaction plate and thus,

the sample collection flow rate out of the PILS impactor cavity. Inside the PILS, the

aerosol stream is mixed with steam as a result of which the particles grow larger,

97

following which the particles are collected by inertial impaction onto the impaction

plate. Particles too small for collection on the inertial impactor leave the system with

the exhaust air flow; those particles were collected using a cold trap installed between

the critical orifice and the vacuum pump (Figure 5.1).

Figure 5.1. Schematic diagram of the PILS for the continuous collection of particles

into the liquid stream.

Milli-Q water was used in the PILS for steam generation and for sample transport.

The sample transport liquid is continuously pumped over the impaction plate to

collect particles into the flowing liquid stream. Liquid flow rates were maintained

using two different sizes of tubing. Transport liquid flow rate (0.3 ml min-1

) was

controlled using 0.89 mm (I.D) tubing. Tubing with an inner diameter of 1.42 mm

98

was used to collect samples from the PILS impactor cavity. The larger inner diameter

tubing was needed to ensure that all liquid sample was removed from the impactor

plate base because of droplet and condensation effects on the impactor plate. Flow

rates for the growth chamber drain (1.0 ml min-1

), impactor drain (1.0 ml min-1

) and

steam flow (1.5 ml min-1

) were controlled using 1.42 mm (I.D) peristaltic pump

tubing. The PILS’s condensation chamber was kept at a slight angle to collect wall

condensate and lost atmospheric particle mass from the growth chamber drain line.

5.3.2.2 High volume air sampler

A high-volume air sampler from HI-Q Environmental Products Company (Model

HVP-3300BRL) was used to collect atmospheric particulate matter onto glass-fiber

filters. The high-volume sampler holds a rectangular filter sized 8 x 10 inch (20.32 x

25.4 cm) supported by a wire mesh. This sampler entrains the air, using a flow

measurement control device, into the sampler through the filter by means of a blower,

so that particulate material collects on the filter surface. Particles with aerodynamic

diameter larger than 2.5 µm were collected on the filter during the 8-hour sampling

period on each sampling day. The sampler has a maximum entrainment rate of 50 ft3

min-1

(~84.95 m3 h

-1). However in this study, atmospheric particles were collected on

the filter at a sample air flow rate of 45 ft3

min-1

(~76.45 m3 h

-1) during the 8 hour

sampling period.

99

5.3.3 Reagents and standards

Milli-Q water (18.2 MΩ.cm) was used for preparation of all solutions and for the final

rinsing of containers. Doubled distilled nitric acid (prepared from 67 – 70 %, trace

metal grade, VWR, West Chester, PA, USA) and hydrochloric acid (prepared from 34

– 37%, trace metal grade, VWR, West Chester, PA, USA) were used for digestion of

the filter and PILS samples. A single element standard, 1000 mg U l-1

(Institute for

Reference Material and Measurement; IRMM-184) was diluted to prepare solutions

for the calibration curves.

5.3.4 Pre-treatment of the PILS and filter samples

5.3.4.1 Filter samples

Airborne particulate matter samples were collected on the filter paper in the high

volume air sampler. A piece (with an area of 20 in2) of filter paper loaded with

airborne particulates was cut from each filter and put into a Teflon tube. Particles

collected on the filter paper were dissolved by boiling in a mixture of 25% nitric acid

and 1% hydrochloric acid for 1 hour to ensure complete digestion of the particles on

the filter. An unused (clean) filter was treated in a similar manner to produce a

procedural blank. Afterwards, the digested samples were cooled and filtered through

a 0.45 µm pore size syringe filter (VWR International) to remove any undigested filter

100

material from the solution. Digested samples (in 25 % v/v nitric acid) were evaporated

and finally diluted with 2% nitric acid to 10 ml for ICP-MS analysis.

5.3.4.2 PILS samples

Samples were collected every thirty minutes and stored in screw-capped sample tubes

until analysis. The volume of the each sample collected was between 10 – 11.5 ml.

Each sample was diluted with Milli-Q water to a final volume of 12 ml. One half of

each sample was analyzed by ICP-MS for uranium concentration without any

pretreatment and the other half of the sample was digested with a mixture of 25 %

nitric acid and 1% hydrochloric acid using a laboratory microwave digestion system

(UltraCLAVE, Milestone Inc). Samples were put into Teflon digestion tubes and

digested by the microwave system using an initial ramping of heat for 20 minutes to

reach 160oC followed by digestion for 1 hour at 160

oC. After the digestion, samples

were removed from the Teflon tubes, evaporated, diluted with 2% nitric acid to 6 ml

and finally stored in screw-capped tubes until analysis.

5.3.5 Inductively coupled plasma mass spectrometric (ICP-MS) analysis

Prepared samples and blanks were analyzed for uranium concentration using a Varian

820 ICP-MS. The ICP-MS instrument was optimized using a 5 ppb tuning solution.

The operating conditions for the ICP-MS are summarized in Table 5.1.

101

Table 5.1:

Instrumental operating conditions and data acquisition parameters.

Parameters / Conditions Results

RF Power (KW) 1.40

Plasma gas flow rate (l min-1

) 18.00

Auxiliary gas flow rate (l min-1

) 1.80

Nebulizer gas flow rate (l min-1

) 0.95

Sampler and skimmer cone composition Nickel

Scan mode Peak hopping

Dwell time (µs) 10000

Sample replicates 5

Scan Segments 1 (238)

5.4 Results and discussion

5.4.1 Optimization of the PILS system

The PILS tip temperature was optimized at the temperature (approximately 90oC) at

which the steam flow transitioned from spitting to a steady steam jet i.e., with no

water drops coming out of steam injection tip. At 90oC, there was a clear star pattern

of the jet on the impactor plate indicating good steam generation for proper PILS

operation. Previously the PILS was optimized for the continuous monitoring of UO2

particles at concentrations typical of normal stack operations (Chapter 4). Results

showed that water was ineffective as a liquid for the transport of UO2 particles out of

the particle-into-liquid sampler system. Later, a sodium carbonate-hydrogen peroxide

solution was used for the PILS optimization for the continuous monitoring of UO2

particles. However, the concentration of particulate uranium is very low in the

ambient air compared to the concentrations typical of normal stack operations.

Therefore water was used as the sample transport liquid to collect all the particles out

102

of the PILS system and collected samples were digested using microwave digestion

method prior to analysis. When doing on-line elemental analysis using the PILS, it is

very important to know the exact sample flow rate in order to calculate the

concentration of the element of interest in ambient air. Weber et al. [10, 11] added a

known quantity of lithium fluoride to the sample flow to correct for differences in

sample volume during on-line analysis. In our study, analyses of samples were

conducted off-line, and so the addition of lithium fluoride was not required because

final volumes of the samples were measured directly.

5.4.2 Uranium concentrations in atmospheric particles

The concentration of uranium (Appendix 11) in digested samples varied from 6.7 pg

m-3

to 29.9 pg m-3

and from 3.1 to 12.7 pg m-3

in undigested samples (Figure 5.2),

when using the PILS during the sampling period (July- August 2010). The uranium

concentration varied from 9.9 pg m-3

to 36.7 pg m-3

in samples collected

simultaneously with filters (Appendix 11). For the Northern Hemisphere Atlantic

Ocean air a mean uranium content of 4.1 pg m-3

was reported [9] whereas at urban

and rural locations within New York State, average particulate uranium

concentrations in air ranged from 0.10 ng m-3

to 1.47 ng m-3

[8]. Atmospheric

uranium concentrations as high as 200 ng m-3

have been reported near nuclear fuel

production plants [5-7]. Uranium concentrations in our samples are thus lower than

the ambient uranium concentrations reported elsewhere.

103

Uranium ConcentrationJuly - August 2010

Sample Day

0 1 2 3 4 5 6 7 8 9

Uranium concentration (ug m

-3)

0.000001

0.000010

0.000100

0.100000

1.000000

10.000000PILS-digested samples

PILS- Un-digested samples

ATSDR MRL

WHO guidelines

Figure 5.2: Uranium concentrations during July and August 2010.

We collected three PILS samples (30 minutes for each sample, at 9 am, 12 am and 4

pm) over a period of 8 hours and used an average concentration of these samples to

compare with that of the high volume air sampler (one composite sample for 8 hours).

Total uranium concentrations in the air in all samples collected from Peterborough,

ON, Canada during July 2010 and August 2010 using the high volume air sampler

and the PILS (Figure 5.2) are well below World Health Organization (WHO)

guidelines (1 µg m-3

, [19]) and the U. S. Agency for Toxic Substances and Disease

Registry (ATSDR) minimum risk levels (MRLs; 0.3 µg m-3

, [3]) for uranium

exposure aimed to protect human health and aquatic life. The Ontario Ministry of

Environment (MOE) Canada has also identified the need to include uranium and

uranium compounds in air quality standards and has proposed a 24-hour average

standard of 0.03 µg m-3

in PM10 based on kidney toxicity associated with exposure in

air [20].

104

5.4.3 Monitoring of uranium in the air

The high volume air sampler is the most popular method for the collection of

suspended particulates in aerosols. In this study atmospheric particulate sampling was

carried out using both a high volume air sampler and the PILS. ICP-MS was used to

measure uranium concentrations in liquid samples.

High volume air sampler (pg m-3)

0 10 20 30 40

PILS after digestion (pg m

-3)

0

10

20

30

40

r2=0.98

y=0.816x-0.586

Figure 5.3: A comparison of uranium concentrations in atmospheric aerosols collected

using a high volume air sampler vs. the PILS

A comparison of uranium concentrations obtained from the PILS (after digestion) and

the filter (high volume air sampler) indicates a good relationship between the two

methods (R2

= 0.98; Figure 5.3). To our knowledge, this is the first time the PILS

(uranium) results have been compared directly with filter results. The uranium

measurements with the PILS (after digestion) were lower than those from the filter

(high volume air sampler). On average, the digested PILS uranium concentrations

were 80% of those collected using the high volume air sampler. This 20 % difference

105

maybe the result of several factors. First, particle sampling using a PM10 inlet is

designed to have a 50% cut-off point at 10 µm in wind speeds of up to 16 km h-1

, but

during the sampling period, wind velocity was as high as 20 km h-1

. Other possible

reasons could be passive deposition of windblown uranium containing dust particles

on the filter during filter installation and contamination of the filter during collection.

The comparison of the PILS and the filter results suggests that the PILS could be an

alternative method for monitoring uranium concentrations in atmospheric particles.

Until now, scientists have relied on the collection of atmospheric particles on filters

using a high volume air sampler over a long period of time. Filter sampling cannot

provide us with detailed information regarding the variation in atmospheric trace

metal concentrations in the short term (hours) because of long sampling times, sample

transport, sample storage and sample preparation before elemental analysis. However,

when using the PILS, the atmospheric particles can be continuously collected into a

small volume of liquid, and particulate elements can be rapidly monitored.

The collected particles on the filter using a high volume air sampler and also in the

small volume of liquid using the PILS, have to be dissolved in solution before

elemental analysis. Microwave digestion was not used for the digestion of the filter

paper because of the large size of the filter paper, which did not fit into the digestion

vessel. When the filter papers were cut into small pieces, which could be put into the

digestion vessels, the uranium concentrations were too low for measurement. In

contrast when using the PILS, particulate uranium was collected in a small volume of

liquid (10 – 11.5 ml per 30 minutes). This liquid could be digested with the

microwave system, making it possible for the accurate detection of uranium by ICP-

106

MS. This indicates the advantage of the PILS method over the filter method for

monitoring particulate uranium in air.

PILS without digestion (pg m-3)

0 10 20 30 40

PILS after digestion (pg m

-3)

0

10

20

30

40

Figure 5.4: Uranium concentrations measured using the PILS (with digestion vs.

without digestion)

Sample digestion is necessary to release uranium into the liquid prior to ICP-MS

analysis (Figure 5.4). There is no correlation between samples that were analyzed

without digestion versus those analyzed with digestion suggesting that only a small

fraction and constant amount of the uranium is easily extractable. Without digestion

uranium concentrations ranged between 2 and 12 pg m-3

representing 20 % - 96% of

the PILS digested samples (Appendix 11). Thus in this study 4 % - 80 % of uranium

in air is bound with particles. Therefore a sample digestion step is necessary to release

uranium into the liquid for analysis. In conclusion, considering that digested samples

107

from the PILS were well correlated with filter samples (Figure 5.3), we may infer that

our digestion process is appropriate to recover most of the particulate uranium.

For the inter-comparison study between the PILS and the high volume air sampler

(filter), all atmospheric particle sampling was carried out using 62 µm stainless steel

mesh (inside the PILS), A different mesh size (76µm; Appendix 11) also was tested

with the PILS to estimate the effect on particulate sampling efficiency. First of all, the

star pattern on the impactor, which is a good indicator of proper PILS operation, was

not as clear as it was with the 62 µm mesh. In addition, during sampling the sample

transport water passed through the mesh (rather than around the mesh) and was

unable to equally distribute transport liquid around the impactor plate. On average,

the PILS fitted with a mesh size of 76 µm collected only 46% of the uranium (in

digested samples) when compared to uranium collected using a high air volume

sampler.

The PILS is a very sensitive instrument and requires careful operation for proper

functioning. Steam flow rate is an important parameter for maintaining the PILS tip

temperature. Relative humidity is another factor that needs to be considered when

operating the PILS in different weather conditions; also the size of the stainless steel

mesh, which can affect the particle collection efficiency of the PILS. Another

important parameter of the PILS operation is ambient temperature. In our work we

installed the PILS on the roof enclosed in a small housing. On hot days (> 25oC), the

PILS heater overheated after a couple of hours of continuous use requiring that it be

turned off for short time (~30 min) and then started again for proper operation. For

108

continuous operation it is recommended that the PILS be installed in an air-

conditioned room (with controlled temperature and humidity).

Atmospheric particle collection on filters using a high volume sampler is not suitable

for investigating variations in atmospheric trace metal concentrations on shorter time

scales because of the long sampling time needed to collect enough sample for

measurement, sample transport, sample storage and sample preparation before sample

chemical analysis. However, the PILS collects aerosol particles from the airstream

and transfers them to a small volume of liquid that can be coupled with a flow

microwave digestion system to speed up the particle digestion process. Coupling it

with the analytical instrument eliminates problems in storing the samples and

guarantees contamination free analysis. Other advantages of the PILS system over

filter sampling include a reduction in sampling time and cost.

The PILS can be used for the continuous monitoring of metal emissions with the

appropriate transport solution, which is obtained by adjusting the transport liquid flow

rate and sample air flow rate. Assuming that ICP-MS measurements require a

minimal sample size of 3 ml (depending on pump intake rates of ICP-MS), the

minimal sampling time for the PILS could be as low as 10 minutes using a transport

liquid flow rate of 0.3 ml min-1

. Our results show that the lowest measured

concentration of particulate uranium in the ambient air was ~ 5.0 pg m-3

during the

sampling period. If the ambient air is sampled for 10 minutes using the PILS with a

transport liquid flow rate of 0.3 ml min-1

, the particulate uranium concentration in the

liquid sample will be higher than the detection limit of the ICP-MS [21]. Therefore a

109

sample collection time of 10 minutes would be sufficient to collect enough particles

into the liquid using the PILS system. The collected sample will be ready for ICP-MS

analysis after sample digestion (for one hour). It is thus an effective instrument for

aerosol collection into a liquid, very reproducible, and easy to use. When combined

with microwave flow digestion and ICP-MS, PILS can provide us with detailed

information on atmospheric metal concentrations.

110

5.5 References

1. CCME, Canadian soil quality guidelines for uranium: Environmental and

human health. In Canadian Council of Ministers of the Environment (CCME),

PN 1371, ISBN 978-1-896997-64-3 PDF.: 2007.

2. Kuroda, P. K.; Essien, I. O.; Sandoval, D.-N., Fallout of uranium isotopes

from the 1980 eruption of mount St. Helens. Journal of Radioanalytical and

Nuclear Chemistry 1984, 84, 23-32.

3. ATSDR, Toxicological profile of uranium. In Agency for toxic substances and

disease registry (ATSDR); U.S. Department of Health and Human Services,

Public Health Service: Atlanta, GA, 1999.

4. Tadmor, J., Atmosheric release of volatilized species of radioelements from

coal-fired plants. Health Physics 1986, 50, 270-273.

5. Tracy, B. L.; Meyerhof, D. P., Uranium concentration in air near a Canadian

uranium refinery. Atmospheric Environment 1987, 21, 165-172.

6. Ahier, B. A.; Tracy, B. L., Evaluating the radiological impact of uranium

emissions in Port Hope, Ontario – A comparison of monitoring and modeling.

Journal of Environmental Radioactivity 1997, 34, 187-205.

7. Al-Khayat, T. A. H.; Eygen, B. v.; Hewitt, C. N.; Kelly, M., Modelling and

measurement of the dispersion of radioactive emissions from a nuclear fuel

fabrication plant in the U.K. Atmospheric Environment 1992, 26A, 3079-3087.

8. McEachern, P.; Myers, W. G.; White, F. A., Uranium concentration in surface

air at rural and urban localities within New York State. Environmental Science

and Technology 1971, 5, 700-703.

9. Hamilton, E. I., The concentration of uranium in air from contrasted natural

environments. Health Physics 1970, 19, 511-520.

10. Weber, R. J.; Orsini, D.; Daun, Y.; Lee, Y.-N.; Klotz, P. J.; Brechtel, F., A

Particle-into-Liquid collector for rapid measurement of aerosol bulk chemical

composition. Aerosol Science and Technology 2001, 35, 718-727.

11. Orsini, D. A.; Ma, Y.; Sullivan, A.; Sierau, B.; Baumann, K.; Weber, R. J.,

Refinement to the particle-into-liquid sampler (PILS) for ground and airborne

measurements of water soluable aerosol composition. Atmospheric

Environment 2003, 37, 1243-1259.

12. Okuyama, K.; Kousaka, Y.; Motouchi, T., Condensation growth of ultrafine

aerosol particles in a new particle size magnifier. Aerosol Science and

Technology 1984, 3, 353-366.

13. Lange, P. W. D.; Venter, J. H.; Wet, W. J. D., Non-destructive neutron

activation analysis of gold and uranium in residue samples of different ore

bodies. Journal of Radioanalytical Chemistry 1969, 2, 219-228.

14. Dragnev, T. N.; Karamanova, J.; Damianov, B., Precise non-destructive x-ray

fluorescence method for uranium and thorium concentration measurements.

Journal of Radioanalytical Chemistry 1979, 52, 439-448.

15. Berdikov, V. V.; Grigor’EV, O. I.; Iokhin, B. S., X-ray fluorescence

determination of uranium and neighbouring elements in solution. Journal of

Radioanalytical Chemistry 1982, 68, 181-192.

16. Boomer, D. W.; Powell, M. J., Determination of uranium in environmental

samples using inductively coupled plasma mass spectrometry. Analytical

Chemistry 1987, 59, 2810-2813.

111

17. Bou-Rabee, F., Estimating the concentration of uranium in some

environmental samples in Kuwait after the 1991 Gulf War. Applied Radiation

and Isotopes 1995, 46, 217-220.

18. Santos, J. S.; Teixeira, L. S. G.; Santos, W. N. L. d.; Lemos, V. A.; Godoy, J.

M.; Ferreira, S. L. C., Uranium determination using atomic spectrometric

techniques: An overview. Analytica Chimica Acta 2010, 674, 143-156.

19. WHO, World Health Organization, Fact Sheet No. 257. In 2003.

20. MOE, Rationale for the development of Ontario air standards for uranium and

uranium compounds. In Ministry of Environment - Standards Development

Branch Ontario: 2009.

21. Becker, J. S., Inductively coupled plasma mass spectrometry (ICP-MS) and

laser ablation ICP-MS for isotope analysis of long-lived radionuclides.

International Journal of Mass Spectrometry 2005, 242, 183-195.

112

Chapter 6

General Discussion and Conclusions

113

6.1 General Discussion and Conclusions

Particulate matter (PM) is an air pollutant that is suspended in the air and represents a

complex mixture of organic and inorganic substances depending on the source. PM

can stay in the air from a few hours to several days and can be transported over long

distances. Exposure to particulate matter consistently shows adverse effects on human

health. For example, exposure to uranium and rare earths can cause lung cancer,

kidney damage, respiratory diseases, lower mental age and intelligence in children

and damage to the central nervous system; it also affects activities of some digestive

enzymes [1-4], etc.

Nowadays, uranium and rare earth elements are widely used in industry and as a

result, industrial demand has increased rapidly. Therefore global production of these

elements has increased every year. Along with worldwide use of these elements, large

quantities are emitted into the atmosphere from industry. Therefore, it is important to

continuously monitor the concentrations of these elements in ambient air, as well as

the release from industrial stacks for the emission control and compliance with

emission standards.

Most of the analytical instruments require sample dissolution before elemental

analysis. Therefore, dissolution of uranium dioxide (UO2) and uranium tetrafluoride

(UF4) particles was investigated in an aqueous carbonate medium at room temperature.

The goal was to determine experimentally the best combination of sodium carbonate

(wt %) and hydrogen peroxide (wt %) for the dissolution of uranium dioxide and

uranium tetrafluoride focusing on a high initial dissolution rate (considering the short

114

residence time for the transport of liquid in the PILS system). The optimal sodium

carbonate (Na2CO3) / hydrogen peroxide (H2O2) solution then can be used in the PILS

as a sample transport liquid for the continuous monitoring of uranium. Peper et al. [5]

studied the dissolution kinetics of UO2 powder in aqueous carbonate media at room

temperature using various oxidants and among those oxidants, hydrogen peroxide

(H2O2) showed the most rapid initial dissolution rate.

From dissolution experiments, it was found that UO2 dissolution increases with an

increase in carbonate concentration in the solution with maximum dissolution at 5

wt % of sodium carbonate concentration. It was also found that UO2 and UF4 particle

dissolution rates increase with an increase in H2O2 concentration, with a maximum

initial dissolution rate at 0.15 wt % of H2O2 in the solution. UF4 particles had higher

initial dissolution rates and high solubility (completely dissolved within 3 minutes) in

Na2CO3-H2O2 solutions compared to UO2 particles. A sodium carbonate (5 wt %) /

hydrogen peroxide (0.15 wt %) solution was found to be the most favorable

combination for the dissolution of UO2 and UF4 particles at room temperature.

A microwave digestion method was also developed for the digestion of six commonly

used rare earth oxides, including praseodymium oxide (Pr6O11), neodymium oxide

(Nd2O3), samarium oxide (Sm2O3), gadolinium oxide (Gd2O3), dysprosium oxide

(Dy2O3) and ytterbium oxide (Yb2O3) using nitric acid. Experimental results show

that a combination of a closed vessel with microwave heating and 2 % v/v nitric acid

at 100oC for 60 minutes yields greater than 90% recoveries of all six REEs.

A method for continuous monitoring of uranium-containing particles (UO2 and UF4)

in high particle concentration environments was developed for the control of

115

emissions and compliance with emission standards, and to provide timesaving cost

benefits, control of parameters and an improvement in process efficiency. A fluidized

bed aerosol generator was used to generate high purity UO2 and UF4 aerosols at a rate

of 1-5 mg h-1

to simulate stack conditions and the PILS was used to capture those

particles into the small volume of transport liquid. Initially the PILS was operated

with water as the sample transport liquid. However, during the PILS operation,

particle build-up was clearly observed on the impaction plate and blockage was

evident in drain lines, at concentrations typical of normal stack operations. This may

be due to the insolubility of uranium particles in water, a low transport liquid flow

rate (0.4 ml min-1

) and a higher particle concentration in sampled air. Particle build-up

and blockage in drain lines can be eliminated and the particle mass collection

efficiency in the PILS sample line can be improved by changing the chemistry of the

transport liquid or sample transport liquid flow. Therefore, it was decided to change

the transport liquid and transport liquid flow rate to eliminate blockage in the drain

line and improve particle mass collection in the sample line. Depending on the

composition of the particles, there are different possibilities for eliminating particle

build-up and blockage in drain lines including the use of acid and alkaline solutions as

transport liquids. The PILS body material restricted the use of acids or acid

combinations as transport liquids because they may result in high corrosion of the

mesh and other parts of the PILS system. Therefore, Na2CO3 (5 wt %) - H2O2 (0.15

wt %) solution was used as sample transport liquid to dissolve UO2 and UF4 particles

inside the PILS. The transport liquid flow rate was increased to 0.5 and 0.7ml min-1

to

reduce chances of particle blockage in transport lines and the PILS system was

optimized for maximum particle mass recovery in the sample line plus impactor drain.

116

The data collected during the PILS optimization experiments show that sample air

flow rate (l min-1

) has a significant effect on particle collection efficiency. The

combination of a sample air flow rate of 10 l min-1

(for UO2) or 16.7 l min-1

(for UF4),

a steam flow rate of 1.5 ml min-1

and a sample transport solution flow rate of 0.5 ml

min-1

demonstrated greater than 89% recovery of the particle mass of UO2 and greater

than 92% recovery for UF4 in the sample plus impactor drain lines. The sodium

carbonate / hydrogen peroxide solution was found to be preferable to water for the

collection of UO2 and UF4 because particle build-up on the impaction surface and

blockages in liquid transport lines were eliminated. Hygroscopic particles (UF4)

required a shorter residence time compared to non-hygroscopic particles (UO2) in the

growth chamber for the growth of smaller particle to a droplet size (> 1µm) necessary

to collect on the PILS impactor plate. Our results demonstrate that the PILS is suitable

technology for UO2 and UF4 particle sampling in high particle concentration

environments. A PILS unit can be installed on an industrial stack for the continuous

emission monitoring of different types of particles with the appropriate transport

solution, and by adjusting the transport liquid flow rate and sample air flow rate.

After optimization of the PILS for the continuous monitoring of UO2 and UF4

particles at concentrations typical of normal stack operations, the PILS system was

then used for continuous monitoring of atmospheric particulate uranium in ambient

air. Atmospheric particle collection also was carried out with a traditional high

volume air sampler (filter) and a comparison was made between particulate uranium

concentrations in particles collected by the PILS and the filter (high volume air

sampler). The concentration of uranium in digested samples varied from 6.7 pg m-3

to

29.8 pg m-3

, when samples were collected using the PILS during the sampling period

117

(July- August 2010) whereas the uranium concentration varied from 9.9 pg m-3

to

36.8 pg m-3

in samples collected with filters. Results showed that uranium

concentrations in particles collected with the high volume air sampler using filters

were consistently higher than in aerosols collected with the PILS. The PILS and filter

results show a good correlation (R2

= 0.98); on average the PILS collected 80% of

uranium found in the filter samples. However, the PILS system continuously collects

particles into a small volume of liquid and after microwave digestion, the sample can

be accurately analyzed for uranium concentration. This indicates the advantage of the

PILS method over the filter method for monitoring particulate uranium in air.

Initially, the PILS system was optimized for the monitoring of UO2 (density = 10.96 g

cm-3

, non-hygroscopic) and UF4 (density = 6.7 g cm-3

, hygroscopic). To test the

applicability of the PILS for continuous monitoring of different elements of interest in

ambient air or at an industrial stack, particles (rare earth oxides) with different

hygroscopic properties and densities were investigated. A microwave digestion

method using nitric acid was developed for the digestion of six commonly used rare

earth oxides, including Pr6O11, Nd2O3, Sm2O3, Gd2O3, Dy2O3 and Yb2O3. This sample

digestion method can be used for the continuous monitoring of trace quantities of

REEs using the PILS without further sample dilution because the digestions were

conducted using 2% (v/v) nitric acid. Future studies should investigate in greater

detail the effects of physical and chemical properties of the particles on the PILS

collection efficiency. Rare earth oxides powders, including Pr6O11, Nd2O3, Sm2O3,

Gd2O3, Dy2O3 and Yb2O3 with densities ranging from 6.7 – 9.2 g cm-3

(in between

UF4 and UO2 powder density) could be useful for the understanding collection

efficiency behavior of the PILS for different particles.

118

The PILS is a very sensitive instrument and requires careful operation for proper

functioning. Our experiments show that maintaining a constant liquid flow rate for

consistent steam generation is one of the key factors that affect particle growth.

Switching from peristaltic pumps to computer controlled syringe pumps will provide

better control over the liquid flow rates and improve measurement precision.

Changing the chemistry of the transport liquid or using warm transport liquid may

increase the solubility of particles in the system. Relative humidity is another factor

that needs to be considered when operating the PILS in different weather conditions.

Another important parameter of PILS operation is ambient temperature. In our work

we installed the PILS on the roof in a small wooden housing to minimize sample line

(transport losses). On hot days (> 25oC), the PILS heater overheated after a couple of

hours of continuous use requiring that it be turned off for short time (~30 min) and

restarted for proper operation. In future, for continuous operation, it is recommended

that the PILS be installed in an air-conditioned room (with controlled temperature and

humidity) in order to improve the PILS collection efficiency. Faster measurement is

more preferable and important for heavy metal releases from industry into the

environment for process control. The PILS can be directly coupled to an analytical

instrument to measure the miscible particles in aqueous and non aqueous media. In

the case of non-soluble particles, microwave flow digestion can be coupled between

the PILS and the analytical instrument for direct analysis of elements in the

environment, providing a better understanding and time-dependent information on

aerosol composition.

119

The Canadian Environmental Protection Act (CEPA) 1999 is an act for preventing

pollution and for the protection of the environment and human health. Each province

has its own environmental statutes. For example the Ontario Ministry of the

Environment (MOE), Ontario Regulation 127/01 (airborne contaminant discharge

monitoring and reporting) was introduced in 2001 under the Environmental Protection

Act for Ontario-based facilities that release certain quantities of specific substances,

requiring them to report their emissions to the government. This guideline also

addresses the installation and operation of continuous emission monitoring systems.

Another example, MOE Ontario Regulation 419/05, which came into effect on

November 2005 regulates industrial emissions of specific contaminants. In 2009, the

MOE proposed a amendment to Ontario regulation 419/05 and introduced new or

updated standards for 9 contaminants including uranium (Environmental Bill Rights;

EBR registry Number 010-7190); all the new or updated standards will be in effect as

of July 01, 2016. In 2009, the Ontario Toxic Reduction Act also was passed by the

Ontario legislature and will be phased in starting in 2012. Under this act, facilities

have to prepare a toxic substance reduction plan during phase one and have to track

and quantify toxic substances release into the environment during phase two.

Installation of an optimized PILS on industrial stacks for continuous emission

monitoring can help with emission control and compliance with the emission

standards imposed by regulatory agencies. Because the PILS can provide real-time

information about the concentrations of elements, it can flag potential non-compliance

situations or relate the effects of source emissions to ambient air quality. Continuous

emission monitoring can also help correlate industrial emission releases with process

variables, information that will also fulfill the Ontario Toxic Reduction Act 2009

requirements.

120

Continuous emission monitoring of toxic substances helps regulatory authorities

manage their impact on the environment. The continuous atmospheric monitoring of

elements of interest having very low concentrations in the environment requires an

instrument with very low detection limit with a fast response time. Assuming that

ICP-MS measurement requires a minimal sample size of 3 ml (depending on pump

intake rates of ICP-MS), the minimal sampling time for PILS could be as low as 10

minutes using a transport liquid flow rate of 0.3 ml min-1

. Our results show that

lowest measured concentration of particulate uranium in the ambient air was ~ 5.0 pg

m-3

during the sampling period. If ambient air is sampled for 10 minutes using the

PILS with a transport liquid flow rate of 0.3 ml min-1

, the particulate uranium

concentration in the liquid sample will be higher than ICP-MS detection limit.

Therefore a sample collection time of 10 minutes would be sufficient to collect

enough particles into the liquid using PILS system. This is not possible using a filter

system.

Collected samples need to be digested before sample analysis and this is the limiting

step towards fast and real-time quantification of aerosol composition. This can be

resolved by optimizing the appropriate transport solution for the dissolution of

particles within the PILS system, e.g. by changing the temperature of the transport

liquid and/or by using concentrated acid or a combination of both. The use of

concentrated acid in the PILS (as it is currently constructed) is not allowed, but by

changing the PILS body material, concentrated acid may be used to dissolve particles

within the system and thus provide real-time quantification of the aerosol composition

under ambient conditions. Upon successful installation of an optimized PILS unit at

an industrial stack or for PM measurements in ambient air, continuous online

121

monitoring of industrial wastes would result in time savings, cost benefits, greater

control of parameters, better evaluation of the maintenance needs of control

equipment and improved industrial process efficiency.

122

6.2 References

1. Bleise, A.; Danesi, P. R.; Burkart, W., Properties, uses and health effects of

depleted uranium (DU): a general overview Journal of Environmental

Radioactivity 2003, 64, 93-112.

2. Zhu, W. F.; Xu, S. Q.; Zhang, H.; Shao, P. P.; Wu, D. S.; Yang, W. J.; Feng, J.,

Investigation of children intelligence quotient in REE mining area: Bio-effect

study of REE mining area in South Jiangxi province. Chinese Science Bulletin

1996, 41, 914-916.

3. Zhu, W.; Xu, S.; Shao, P.; Zhang, H.; Wu, D.; Yang, W.; Feng, J.,

Bioelectrical activity of the central nervous system among populations in a

rare earth element area. Biological Trace Element Research 1997, 57, 71-77.

4. Zhu, W.; Xu, S.; Shao, P.; Zhang, H.; Wu, D.; Yang, W.; Feng, J.; Feng, L.,

Investigation on liver function among population in high background of rare

earth area in South China. Biological Trace Element Research 2005, 104, 1-7.

5. Peper, S. M.; Brodnax, L. F.; Field, S. E.; Zehnder, R. A.; Valdez, S. N.;

Runde, W. H., Kinetic study of oxidative dissolution of UO2 in aqueous

carbonate media. Industrial and Engineering Chemistry Research 2004, 43,

8188-8193.

123

Appendices

124

Appendix 1: Summary of UO2 dissolution tests – UO2 dissolution (%) in solutions of different sodium carbonate - hydrogen peroxide

concentrations as a function of time.

H2O2 = 0.15 wt%

Time (min) Na2CO3 = 0 wt% Na2CO3 = 1 wt% Na2CO3 = 3 wt% Na2CO3 = 5 wt% Na2CO3 = 7wt%

0 0.0 0.0 0.0 0.0 0.0

1 0.0 2.3 2.3 3.6 1.9

3 0.5 6.6 7.0 9.0 5.0

5 0.3 12.3 12.7 15.9 8.7

10 0.3 26.6 27.1 32.1 19.6

20 0.3 54.8 56.9 57.6 42.5

30 0.0 71.9 75.2 72.6 61.6

60 0.3 91.3 96.6 94.1 83.5

Na2CO3 = 5 wt%

Time (min) H2O2 = 0 wt% H2O2 = 0.025 wt% H2O2 = 0.05 wt% H2O2 = 0.15 wt% H2O2 = 0.3 wt%

0 0.0 0.0 0.0 0.0 0.0

1 0.0 0.5 1.5 3.6 3.6

3 0.0 1.9 9.0 9.7

5 0.5 2.7 4.6 15.9 17.4

10 0.3 5.3 9.5 32.1 36.9

20 0.0 11.7 21.2 57.6 62.7

30 0.5 18.7 33.0 72.6 74.4

60 0.4 36.0 57.8 94.1 93.7

125

Appendix 2: Summary of UF4 dissolution tests - UF4 dissolution (%) in solutions of different sodium carbonate - hydrogen peroxide

concentrations as a function of time.

H2O2 = 0.01 wt%

Time

(min) Na2CO3 = 0 wt% Na2CO3 = 1 wt% Na2CO3 = 3 wt% Na2CO3 = 5 wt%

0 0.0 0.0 0.0 0.0

1 6.1 22.5 28.8 32.6

3 8.2 47.8 51.1 51.1

5 8.9 73.9 64.0 61.6

10 17.2 91.4 79.2 76.0

20 32.2 96.2 91.2 89.3

30 51.8 98.2 95.1 91.3

60 78.9 100.8 99.7 94.4

Na2CO3 = 5wt%

Time

(min)

H2O2 = 0

wt%

H2O2 = 0.005

wt%

H2O2 = 0.01

wt%

H2O2 = 0.025

wt%

H2O2 = 0.05

wt%

H2O2 = 0.15

wt%

H2O2 = 0.3

wt%

0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1 6.8 23.3 32.6 41.7 63.6 86.9 94.6

3 6.9 35.2 51.1 67.2 88.8 100.0 102.3

5 7.5 41.8 61.6 81.6 96.3 101.0 104.7

10 7.8 51.1 76.0 87.9 102.3 102.6 105.5

20 9.3 55.3 89.2 90.1 103.3 102.3 101.5

30 10.6 56.0 91.3 94.7 103.1 104.5 104.1

60 12.9 62.5 94.4 96.3 108.6 107.0 101.2

126

Appendix 3: Summary of rare earth oxides powder digestion tests.

Microwave digestion: Percentage recovery (%) of rare earth elements using sample sizes of 20, 40, 60 and 80 mg (5 replicates for each mass)

and digested using 20 ml of 50 % (v/v) nitric acid at 100oC for 15 minutes.

Microwave Digestion

20 ml (50% HNO3), 15 min, 100oC

Rare Earth Element 20mg 40mg 60mg 80 mg

Praseodymium (Pr) Average 97.9 93.3 84.8 83.6

SD 1.1 1.4 2.0 1.6

Neodymium (Nd) Average 60.6 58.4 58.1 57.7

SD 1.4 1.5 1.1 0.8

Samarium (Sm) Average 87.5 91.7 88.1 83.0

SD 2.2 2.1 1.3 1.2

Gadolinium (Gd) Average 84.6 85.6 80.8 79.8

SD 2.4 2.8 1.2 2.2

Dysprosium (Dy) Average 89.8 87.9 84.8 84.9

SD 1.7 3.0 1.4 1.3

Ytterbium (Yb) Average 91.2 86.9 86.9 85.2

SD 1.7 2.4 1.2 2.2

127

Microwave digestion: Percentage recovery (%) of rare earth elements for various nitric acid concentrations and digestion times. Nitric acid

concentration varied between 2 and 20% (v/v) (5 replicates for each acid concentration). Digestion time varied between 30 minutes and 60

minutes (5 replicates for each time). Samples (20 mg) were digested using 20 ml of nitric acid at 100oC.

Microwave Digestion

20 mg, 20 ml (HNO3), 100oC

Rare Earth Element 30 min 45min 60 min

2% 2% 5% 20% 2% 5%

Praseodymium (Pr) Average 90.0 92.8 93.4 94.7 93.7 91.4

SD 1.8 2.7 3.1 2.6 1.9 3.4

Neodymium (Nd) Average 87.0 86.1 90.4 92.2 90.2 94.6

SD 2.1 4.3 1.7 1.4 1.4 2.6

Samarium (Sm) Average 92.0 93.5 93.4 93.7 95.2 94.1

SD 3.2 4.2 3.2 3.7 0.8 4.6

Gadolinium (Gd) Average 85.6 88.8 90.5 89.7 92.7 93.2

SD 3.9 4.1 1.5 2.7 4.4 3.7

Dysprosium (Dy) Average 92.6 91.4 97.9 97.4 94.9 97.7

SD 2.1 2.4 2.6 3.0 4.3 2.7

Ytterbium (Yb) Average 90.5 91.2 96.1 96.5 93.9 95.7

SD 2.9 2.9 1.6 3.7 1.5 5.1

128

Hot Plate digestion: Percentage recovery (%) of rare earth elements. Digestion time varied between 30 minutes and 90 minutes (5 replicates for

each time). Samples (20 mg) were digested using 20 ml of 2 % (v/v) nitric acid at 100oC.

Hot plate

20 mg, 20 ml (2% HNO3), 100oC

Rare Earth Element 30 min 60 min 90 min

Praseodymium (Pr) Average 81.7 81.7 81.9

SD 2.2 1.8 1.8

Neodymium (Nd) Average 80.6 85.3 84.0

SD 3.0 2.5 2.9

Samarium (Sm) Average 87.3 87.3 87.7

SD 1.2 2.1 1.9

Gadolinium (Gd) Average 74.2 74.8 75.9

SD 3.1 2.7 2.8

Dysprosium (Dy) Average 84.8 84.5 85.5

SD 2.9 2.6 2.1

Ytterbium (Yb) Average 79.6 81.6 84.6

SD 3.0 2.6 1.7

129

Appendix 4: PILS-UO2 optimization - initial tests.

Percentage of Uranium Recovered from Different Parts of the PILS

Test Mass (mg) Sample Impactor Impactor Filter Growth Chamber Cold Filter + Growth Cahmber

Line Wash Drain Drain Trap Cold Trap

1 0.33 68.2 10.6 9.1 12.1 0.0 0.0 12.1

2 0.36 67.6 8.5 11.3 12.7 0.0 0.0 12.7

3 0.28 56.4 12.7 12.7 18.2 0.0 0.0 18.2

4 0.18 38.9 22.2 19.4 19.4 0.0 0.0 19.4

130

Appendix 5: PILS - UO2 particle recovery (%) in different parts of the PILS (sample line, impactor drain, growth chamber, cold trap and

filter) during different experimental runs (total of 24 experimental runs). For example, run 1 was performed on day 1, using a medium aerosol

rate (i.e. conc. of particles), a transport liquid flow rate of 0.5 ml min-1

and sample air flow rate of 10 l min-1

.

Conc. Liquid flow Air flow Mass Uranium dioxide recovery (%) in different parts of the PILS

Run Rate Rate collected Sample Impactor Growth Sample + Cold Filter Cold Trap +

Day Levels ml min-1 l min

-1 mg Line Drain Chamber Impactor Trap Filter

1 1 Medium 0.5 10.0 1.6538 89.38 2.97 7.66 92.34 0.00 0.00 0.00

2 1 Low 0.5 16.7 0.9882 97.24 0.58 0.00 97.82 2.18 0.00 2.18

3 1 High 0.5 16.7 4.0883 99.11 0.47 0.42 99.58 0.00 0.00 0.00

4 1 High 0.5 10.0 2.4399 79.09 1.98 18.95 81.05 0.00 0.00 0.00

5 1 Low 0.7 10.0 1.4702 58.30 3.10 38.25 61.40 0.34 0.00 0.34

6 1 High 0.7 16.7 4.1976 67.64 12.59 19.57 80.23 0.20 0.00 0.20

7 1 High 0.7 10.0 3.2536 86.56 0.62 12.49 87.17 0.34 0.00 0.34

8 1 Medium 0.5 16.7 1.5890 94.43 5.01 0.56 99.44 0.00 0.00 0.00

9 1 Low 0.5 10.0 1.2664 82.19 7.93 9.87 90.13 0.00 0.00 0.00

10 1 Low 0.7 16.7 1.2920 97.63 2.37 0.00 100.00 0.00 0.00 0.00

11 1 Medium 0.7 16.7 1.5177 84.99 13.58 0.78 98.57 0.78 0.66 1.44

12 1 Medium 0.7 10.0 2.6266 95.54 0.54 3.67 96.09 0.24 0.00 0.24

13 2 Low 0.5 10.0 2.0180 83.28 3.69 11.34 86.97 0.69 0.99 1.68

14 2 High 0.7 10.0 3.6781 93.55 1.45 5.00 95.00 0.00 0.00 0.00

15 2 High 0.7 16.7 4.2901 66.28 6.15 27.33 72.44 0.23 0.00 0.23

16 2 Medium 0.7 10.0 1.5707 94.19 1.75 4.06 95.94 0.00 0.00 0.00

17 2 Low 0.5 16.7 1.3585 68.49 14.72 16.05 83.21 0.00 0.74 0.74

18 2 Medium 0.5 10.0 1.3240 95.95 2.19 1.12 98.13 0.75 0.00 0.75

19 2 Low 0.7 16.7 2.0303 43.57 28.68 27.06 72.25 0.69 0.00 0.69

20 2 Medium 0.5 16.7 1.6377 44.07 4.95 50.40 49.03 0.57 0.00 0.57

21 2 Medium 0.7 16.7 2.1384 52.81 12.38 34.82 65.18 0.00 0.00 0.00

22 2 Low 0.7 10.0 1.1489 90.01 1.27 6.24 91.28 1.61 0.87 2.48

23 2 High 0.5 10.0 2.9665 65.60 32.33 2.07 97.93 0.00 0.00 0.00

24 2 High 0.5 16.7 4.8625 64.81 9.05 26.14 73.86 0.00 0.00 0.00

131

Appendix 6: Effects of individual variables such as sample air flow rate (10 and 16.7 l min-1

; 12 experimental runs for each sample air flow

rate), transport liquid flow rate (0.5 and 0.7 ml min-1

; 12 experimental runs for each transport liquid flow rate) and for their combinations (6

experimental runs for each combination) on PILS-UO2 particle recovery (%) in different parts of the PILS (sample line, impactor drain, growth

chamber, cold trap and filter).

Individual variables Mass Uranium dioxide recovery (%) in different parts of the PILS

And collected Sample Impactor Growth Sample+ Cold Filter Cold Trap +

Combinations Mg Line Drain Chamber Impactor Trap Filter

10 l min-1

Average 2.1 89.0 2.4 8.0 91.4 0.4 0.1 0.5

SD 0.9 6.0 2.2 5.3 5.2 0.5 0.4 0.9

16.7 l min-1

Average 3.3 73.4 9.2 16.9 82.6 0.4 0.1 0.5

SD 3.1 20.8 7.9 16.8 16.7 0.6 0.3 0.7

0.5 ml min-1

Average 3.0 81.6 4.9 13.0 86.5 0.4 0.1 0.5

SD 3.1 16.9 4.3 15.1 15.0 0.7 0.4 0.8

0.7 ml min-1

Average 2.4 77.6 7.0 14.9 84.6 0.4 0.1 0.5

SD 1.1 18.9 8.4 13.8 13.8 0.5 0.3 0.7

10 l min-1 &

0.5 ml min-1

Average 1.9 86.0 3.8 9.8 89.7 0.2 0.2 0.4

SD 0.7 6.7 2.4 6.4 6.3 0.4 0.4 0.7

16.7 l min-1 &

0.7 ml min-1

Average 2.6 68.8 12.6 18.3 81.4 0.3 0.1 0.4

SD 1.3 20.0 9.0 14.7 14.6 0.3 0.3 0.6

10 l min-1 &

0.7 ml min-1

Average 2.3 86.4 1.5 11.6 87.8 0.4 0.2 0.6

SD 1.0 14.1 0.9 13.4 13.4 0.6 0.4 1.0

16.7 l min-1 &

0.5 ml min-1

Average 4.1 78.0 5.8 15.6 83.8 0.5 0.1 0.6

SD 4.3 22.4 5.4 20.1 11.7 0.9 0.3 0.8

132

Appendix 7: PILS-UO2 optimization – t-test results for recovery in sample line plus impactor drain.

Results of eight different experiments for each aerosol rate (low, medium and high) show an average particle mass collection recovery of

85.4 % in the sample plus impactor drain, with a variance of 168.6 for the low rate. For the high aerosol rate, the average particle mass

collection recovery is 85.9 % in the sample plus impactor drain, with a variance of 114.0. There is no significant difference (p > 0.05) in

uranium dioxide particle collection efficiency in the sample line plus impactor drain at different aerosol flow rates (low, medium and high)

that were introduced into the system. There is also no significant difference (p > 0.05) in uranium dioxide particle collection efficiency in the

sample line plus impactor drain at different transport liquid flow rates (between 0.5 and 0.7 ml min-1

) and sample air flow rates (between 10

and 16.7 l min-1

).

t-Test: Two-Sample Assuming Equal Variances t-Test: Two-Sample Assuming Equal Variances

No significant difference No significant difference

Aerosol rate Low medium Aerosol rate Low high

Mean 85.3825 86.84 Mean 85.3825 85.9075

Variance 168.64925 360.1701714 Variance 168.64925 114.0129

Observations 8 8 Observations 8 8

Pooled Variance 264.4097107 Pooled Variance 141.3310786

Hypothesized Mean Difference 0 Hypothesized Mean Difference 0

Df 14 Df 14

t Stat -0.179266797 t Stat -0.088322319

P(T<=t) one-tail 0.430148399 P(T<=t) one-tail 0.465435738

t Critical one-tail 1.761310115 t Critical one-tail 1.761310115

P(T<=t) two-tail 0.860296798 P(T<=t) two-tail 0.930871476

t Critical two-tail 2.144786681 t Critical two-tail 2.144786681

133

t-Test: Two-Sample Assuming Equal Variances

No significant difference

Aerosol rate Medium High

Mean 86.84 85.9075

Variance 360.1701714 114.0129071

Observations 8 8

Pooled Variance 237.0915393

Hypothesized Mean Difference 0

Df 14

t Stat 0.121121379

P(T<=t) one-tail 0.452658066

t Critical one-tail 1.761310115

P(T<=t) two-tail 0.905316133

t Critical two-tail 2.144786681

t-Test: Two-Sample Assuming Equal Variances t-Test: Two-Sample Assuming Equal Variances

No significant difference No significant difference

Transport liquid flow rate 0.5 0.7 Air flow rate 10 16.7

Mean 87.4575 84.62916667 Mean 89.4525 82.63417

Variance 215.7611841 189.7431902 Variance 104.1036023 280.4061

Observations 12 12 Observations 12 12

Pooled Variance 202.7521871 Pooled Variance 192.2548598

Hypothesized Mean Difference 0 Hypothesized Mean Difference 0

Df 22 Df 22

t Stat 0.486545481 t Stat 1.20452326

P(T<=t) one-tail 0.31569614 P(T<=t) one-tail 0.120593141

t Critical one-tail 1.717144335 t Critical one-tail 1.717144335

P(T<=t) two-tail 0.631392279 P(T<=t) two-tail 0.241186281

t Critical two-tail 2.073873058 t Critical two-tail 2.073873058

134

Appendix 8: PILS – UF4 particle recovery (%) in different parts of the PILS (sample line, impactor drain, growth chamber, cold trap and

filter) during different experimental runs. For example, run 1 was performed on day 1 using a low aerosol rate (i.e. conc. of particles), a

transport liquid flow rate of 0.5 ml min-1

and sample air flow rate of 10 l min-1

.

Conc. Liquid flow Air flow Mass Uranium tetrafluoride recovery (%) in different parts of the PILS

Run Rate Rate Sample Impactor Growth Sample+ Cold Filter Cold Trap

+

Blocks Levels ml min-1 l min

-1 mg Line Drain Chamber Impactor Trap Filter

1 1 Low 0.5 10.0 3.6382 66.27 23.06 7.74 89.34 1.83 1.10 2.93

2 1 High 0.5 10.0 3.8796 89.72 0.69 8.82 90.41 0.00 0.77 0.77

3 1 Low 0.7 10.0 2.2407 75.03 5.39 16.59 80.42 1.21 1.79 3.00

4 1 High 0.7 10.0 4.1550 89.95 1.18 7.20 91.13 0.71 0.96 1.67

5 1 Low 0.5 16.7 2.3950 90.14 2.69 5.26 92.83 0.66 1.25 1.91

6 1 High 0.5 16.7 4.2322 91.47 0.55 7.03 92.02 0.00 0.95 0.95

7 1 Low 0.7 16.7 2.2137 88.95 2.18 6.67 91.13 0.85 1.36 2.21

8 1 High 0.7 16.7 4.8597 91.58 1.28 6.31 92.87 0.00 0.82 0.82

9 2 Low 0.5 10.0 1.7974 88.23 0.94 9.47 89.17 1.36 0.00 1.36

10 2 High 0.5 10.0 3.7959 87.72 0.61 9.68 88.33 1.20 0.79 1.99

11 2 Low 0.7 10.0 1.7953 87.62 1.21 8.94 88.83 0.00 2.23 2.23

12 2 High 0.7 10.0 3.6637 85.60 1.29 11.95 86.89 0.89 0.27 1.16

13 2 Low 0.5 16.7 1.9437 89.68 0.53 8.76 90.21 0.00 1.03 1.03

14 2 High 0.5 16.7 3.5790 93.17 1.66 4.72 94.82 0.46 0.00 0.46

15 2 Low 0.7 16.7 2.1234 89.93 1.68 6.50 91.61 0.00 1.88 1.88

16 2 High 0.7 16.7 3.1470 92.04 1.67 6.30 93.70 0.00 0.00 0.00

135

Appendix 9: Effects of individual variables such as sample air flow rate (10 and 16.7 l min-1

; 8 experimental runs for each sample air flow

rate), transport liquid flow rate (0.5 and 0.7 ml min-1

; 8 experimental runs for each transport liquid flow rate) and for their combinations (4

experimental runs for each combination) on PILS-UF4 particle recovery (%) in different parts of the PILS (sample line, impactor drain,

growth chamber, cold trap and filter).

Individual variables Mass Uranium tetrafluoride recovery (%) in different parts of the PILS

And collected Sample Impactor Growth Sample+ Cold Filter Cold Trap +

Combinations Mg Line Drain Chamber Impactor Trap Filter

10 l min-1

Average 3.0 86.3 1.6 10.4 87.9 0.8 1.0 1.7

SD 1.1 5.2 1.7 3.1 3.6 0.6 0.8 0.7

16.7 l min-1

Average 3.1 90.9 1.5 6.4 92.4 0.2 0.9 1.2

SD 1.1 1.4 0.7 1.2 1.5 0.4 0.6 0.8

0.5 ml min-1

Average 3.1 90.0 1.1 7.7 91.1 0.5 0.7 1.2

SD 1.0 1.9 0.8 2.0 2.2 0.6 0.5 0.6

0.7 ml min-1

Average 3.0 87.6 2.0 8.8 89.6 0.5 1.2 1.6

SD 1.1 5.5 1.4 3.7 4.3 0.5 0.8 0.9

10 l min-1 &

0.7 ml min-1

Average 3.0 84.6 2.3 11.2 86.8 0.7 1.3 2.0

SD 1.1 6.6 2.1 4.1 4.6 0.5 0.9 0.8

16.7 l min-1 &

0.5 ml min-1

Average 3.0 91.1 1.4 6.4 92.5 0.3 0.8 1.1

SD 1.1 1.6 1.0 1.8 1.9 0.3 0.6 0.6

10 l min-1 &

0.5 ml min-1

Average 3.2 88.6 0.7 9.3 89.3 0.9 0.5 1.4

SD 1.2 1.0 0.2 0.4 1.0 0.7 0.5 0.6

16.7 l min-1 &

0.7 ml min-1

Average 3.1 90.6 1.7 6.4 92.3 0.2 1.0 1.2

SD 1.3 1.4 0.4 0.2 1.2 0.4 0.8 1.0

136

Appendix 10: PILS-UF4 optimization – t-test results for recovery in sample line plus impactor drain

Results of eight different experiments for two aerosol flow rates (low and high) show a mean particle recovery of 89.2 % in the sample plus

impactor drain, with a variance of 14.4 for the low level, and a mean particle recovery of 91.3 %, with a variance of 7.2 for the high aerosol

particle rate in the sample plus impactor drain. There is no significant difference (p > 0.05) in uranium tetrafluoride particle collection

efficiency in the sample line plus impactor drain at the two different aerosol flow rates. Results also show no significant difference (p > 0.05)

in uranium tetrafluoride collection efficiency at different transport liquid flow rates (between 0.5 and 0.7 ml min-1

) but show significant

difference (p < 0.05) at different sample air flow rates (between 10 and 16.7 l min-1

) in the sample line plus impactor drain recoveries.

t-Test: Two-Sample Assuming Equal Variances t-Test: Two-Sample Assuming Equal Variances

No significant difference No significant difference

Aerosol rate Low High Transport liquid flow rate 0.5 0.7

Mean 89.1925 91.27125 Mean 90.89125 89.5725

Variance 14.42505 7.173869643 Variance 4.727526786 18.34688

Observations 8 8 Observations 8 8

Pooled Variance 10.79945982 Pooled Variance 11.53720268

Hypothesized Mean Difference 0 Hypothesized Mean Difference 0

Df 14 Df 14

t Stat -1.265118602 t Stat 0.776501336

P(T<=t) one-tail 0.113240679 P(T<=t) one-tail 0.225186834

t Critical one-tail 1.761310115 t Critical one-tail 1.761310115

P(T<=t) two-tail 0.226481357 P(T<=t) two-tail 0.450373667

t Critical two-tail 2.144786681 t Critical two-tail 2.144786681

137

t-Test: Two-Sample Assuming Equal Variances

Significant difference

Air flow rate 10 16.7

Mean 88.065 92.39875

Variance 11.17457143 2.161383929

Observations 8 8

Pooled Variance 6.667977679

Hypothesized Mean Difference 0

Df 14

t Stat -3.356578293

P(T<=t) one-tail 0.00235101

t Critical one-tail 1.761310115

P(T<=t) two-tail 0.00470202

t Critical two-tail 2.144786681

138

Appendix 11: Uranium concentrations in atmospheric aerosols using the PILS and a filter system.

Filter PILS Uranium

Date Digested Without Digestion Digested samples Digested Non-digested

Mean SD Mean SD

pg m-3

pg m-3

pg m-3

pg m-3

pg m-3

% %

22-Jul-10 9.88 8.14 2.40 6.74 1.05 68.21 82.38

26-Jul-10 10.66 8.47 1.87 8.84 1.09 82.90 79.43

27-Jul-10 19.69 12.68 2.66 16.18 1.24 82.17 64.40

29-Jul-10 36.69 6.89 3.35 29.95 5.30 81.64 18.78

30-Jul-10 20.32 3.11 0.90 15.78 7.34 77.67 15.31

3-Aug-10 26.04 6.77 1.82 21.70 2.40 83.33 26.00

5-Aug-10 32.26 8.17 3.02 24.17 7.60 74.92 25.32

6-Aug-10 14.96 2.62 0.80 11.09 3.54 74.13 17.51

17-Aug-10 15.89 1.19 0.28 7.34 0.96 46.19 7.49

mesh(76um)