Applied Environmental Measurement Techniques

Performance of a passive sampler system for nitrate and phosphate

monitoring CLAUDIA RIVERA

Water Environment Transport CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden December, 2003.

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Abstract Nitrate and phosphate monitoring is important to assess water quality, and its potential

for eutrophication. Spot sampling is the most common technique to monitor water.

Since spot sampling gives information of the water condition only at the sampling

time, the necessity of developing new techniques able to give information over longer

periods of time has raised.

Passive samplers have been developed in order to assess water quality over the period

of time they have been deployed and also to simplify sampling and sample

preparation processes. Passive samplers can be left unattended over long periods of

time, help to avoid changes in the sample composition while transported to the

laboratory and also to reduce analysis costs.

The passive sampler studied in this thesis is based on the Fick´s Law and the principle

is the diffusion of nitrate and phosphate from the sampled to the collecting medium.

The concentration of nitrate and phosphate is obtained as a time-weighted average

(TWA). Environmental factors influence on the diffusion of nitrate and phosphate

were studied. Temperature affects the diffusion of both nitrate and phosphate while

pH just affects phosphate. Turbulence does not affect nitrate and phosphate diffusion.

Sampling rates of nitrate and phosphate were obtained (148 and 75 ml/day

respectively) and the comparison between passive sampler and spot sampling nitrate

and phosphate determination was made. The passive sampler determined between 30-

50% nitrate and between 12-50% phosphate concentrations that spot samples did.

Keywords: water monitoring, passive sampler, nitrate, phosphate, eutrophication,

experimental design.

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ABSTRACT __________________________________________________2

1. INTRODUCTION ____________________________________________5

2. OBJECTIVES _______________________________________________6

3. BACKGROUND _____________________________________________7

3.1 Nitrate __________________________________________________________7

3.2 Phosphate________________________________________________________8

3.3 Passive Sampler___________________________________________________9 3.3.1 Fick´s Law _________________________________________________________ 12

3.4 Ion Chromatography _____________________________________________13

3.5 Inductively Coupled Plasma – Mass Spectrometry _____________________14

3.6 Experimental Design: multiple-level experiments ______________________15

4. METHODOLOGY ___________________________________________17

4.1 Sampler preparation______________________________________________17

4.2 Elution procedure and samples preparation prior analysis ______________17

4.3 Bulk concentration experiment _____________________________________18

4.4 Elution experiment _______________________________________________18

4.5 Environmental factors ____________________________________________19

4.6 Calibration curve ________________________________________________20

4.7 Field test (Rya Water Treatment Plant) ______________________________20

5. RESULTS AND DISCUSSION _________________________________21

5.1 Bulk concentration experiment _____________________________________21

5.2 Elution experiment _______________________________________________22

5.3 Environmental factors ____________________________________________23

5.4 Calibration curve ________________________________________________25

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5.5 Field test (Rya Water Treatment Plant) ______________________________26

6. CONCLUSIONS ____________________________________________28

7. RECOMMENDATIONS ______________________________________28

8. ACKNOWLEDGMENTS______________________________________29

9. REFERENCES _____________________________________________30

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1. Introduction

Water is a major resource and is essential for life. It is naturally recycled through a

process called hydrological cycle. The demand of water over the world has been

increasing as population and industrialization does, while available drinking water has

been deteriorating due to pollution (Schindler, 2001).

Nitrogen and phosphorous are considered major nutrients because they are essential to

the growth of plants (Metcalf, 1991). There are different forms of nitrogen and

phosphorous found in water, in this case the forms that will be studied are the nitrate

(NO3-) and phosphate (HPO4

2-) ions.

The presence of nutrients in waters increases the growth of plants and this means food

for higher organisms, increasing fish population and biological diversity. Excessive

concentration of nitrate and phosphate will cause an imbalance or instability in the

ecosystem, causing eutrophication in the water (Conrath et al, 1995), especially in

areas with poor or slower water renovation (Tovar et al, 2002; Schindler, 2001).

Eutrophication has been recognised as a major water quality problem worldwide

(Maher et al, 2002), it leads to an increase of biomass and primary productivity within

aquatic communities (Hanrahan et al, 2001), causing anoxic waters and/or toxic algal

blooms and finally habitat destruction, making the water unable for human

consumption. Nitrogen and phosphorous then have the potential to pollute water.

Monitoring nitrate and phosphate gives indication of water quality (Connolly & Paull,

2001), and it is also possible to evaluate the potential for eutrophication (Galhardo &

Masini, 2001). A typical way of monitoring nitrate and phosphate is through spot

sampling; this means to take water samples at a certain time and transport them to the

laboratory in order to analyse them.

The disadvantage of spot sampling is the possibility of not having a representative

value of the analytes over a period of time because it gives only instantaneous

pollutant concentrations (Rauch et al, 2003). If one wants to have the analyte

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concentration, which represents the entire waterbody over a period of time, then it is

necessary to take several water samples and then obtain the average of them.

A passive sampler has been developed to overcome this disadvantage. This passive

sampler has the ability to give the concentration of the analyte over a period of time. It

is inexpensive compared to on-line determination techniques and can help to reduce

changes in the sample composition which might occur during transportation or

storage.

The development of the passive sampler in order to monitor nitrate and phosphate in

wastewater has been studied and evaluated in this study. An evaluation of the

influence of environmental conditions, such as pH, temperature and turbulence, on

nitrate and phosphate uptake by the passive sampler was performed.

Additionally an accumulation factor of nitrate and phosphate on the passive sampler

was obtained based on the uptake of these nutrients by the passive sampler over time

and the analyte concentration in the water. Finally the passive sampler was deployed

in the outlet of Rya water treatment plant and the results obtained were compared with

spot samples analysed daily by the water treatment plant.

2. Objectives

The objectives of this study were:

1. Get a better understanding about how the passive sampler works and develops

in laboratory and external environments.

2. Find the optimal elution procedure in order to obtain 100% of the analytes

(nitrate and phosphate) from the passive sampler once it has been deployed.

3. Evaluate the influence of environmental parameters such as pH, temperature

and turbulence on nitrate and phosphate uptake by the passive sampler.

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4. Compare nitrate and phosphate measurements obtained from the passive

sampler and spot sampling.

3. Background

3.1 Nitrate

Nitrate is a stable negatively charged ion highly soluble in water (Luk & Au-Yeung,

2002), it is considered as the most oxidized form of nitrogen that can be found in

water and the most abundant form of inorganic nitrogen (Karlsson et al, 1995). It is

difficult to remove nitrate from water due to its low potential for co-precipitation or

adsorption (Luk & Au-Yeung, 2002).

Some of the anthropogenic sources of nitrate include fertilizers, septic tanks effluent,

leaching, commercial or industrial wastewater and acid rain (Bleifuss, 2003). Several

drinking water supply wells have been closed due to exceeding concentrations of

nitrate found in them, and therefore making them not suitable for human

consumption. Unfortunately an increasing rate of nitrate concentration in groundwater

has been observed in many countries (Luk & Au-Yeung, 2002; Levallois et al, 1998).

Nitrate is considered one of the most common contaminants in surface and

groundwater, and when water with high nitrate concentration is used for human

consumption, some adverse health effects have been reported (Luk & Au-Yeung,

2002; McLay et al, 2001). Nitrate consumption by young children can cause

methaemoglobinaemia or blue baby syndrome in which nitrate is converted to nitrite

once arrives to the stomach, then nitrite combines with haemoglobin and forms

methemoglobin, decreasing the ability of the blood to carry oxygen (McCasland et al,

1998).

Severe methemoglobin can cause brain damage or even death (Luk & Au-Yeung,

2002). Some studies also indicate a positive correlation between gastric cancer and

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nitrate consumption in drinking water (McCasland et al , 1998; Kiely, 1997). Damages

in the reproductive system in animal studies have also been linked to high levels of

nitrate in drinking water (Luk & Au-Yeung, 2002). Finally, chronic exposure of adults

to nitrate could lead to deleterious effects (Levallois et al, 1998). Nitrate can be found

in natural river waters in the concentration range of 0 to 25 mg/l (Reeve, 1994).

3.2 Phosphate

Phosphate is a negatively charged ion, considered as the growth limiting nutrient for

plants and microbial growth (Muñoz et al, 1997) and responsible of controlling

freshwater and terrestrial ecosystems productivity (Zhang et al, 2998).

Phosphate sources include run-off from agricultural sites, fertilizers, sewage and

atmospheric deposition (Kiely, 1997). Phosphates are widely used in cleaning

products, toothpaste, fire extinguishers, textile processing and food (Weiner et al,

2001). It is found in natural river waters in the concentration range of 0 to 1 mg/l

(Reeve, 1994), and when excessive, can lead to eutrophication in water bodies.

Due to inorganic phosphates salts are used as food ingredients (Weiner et al, 2001),

several toxicological reviews of inorganic phosphates have been made. Weiner et al

(2001) performed a toxicological review of inorganic phosphates in order to study

acute toxicity by oral, dermal and inhalation routes in animals.

They found that inorganic phosphates are safe for both food and commercial

applications and are practically non-toxic regarding acute oral, dermal and inhalation

exposures. Eye and skin irritation varied depending on the inorganic orthophosphate

tested. Orthophosphoric acid was tested on human skin, having no skin sensitisation

effects. The phosphate ion was found not to be genotoxic or mutagenic in standard

test systems (Weiner et al, 2001).

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Weiner et al (2001) also studied subchronic and chronic toxicity of inorganic

phosphates in animals, finding that they could cause renal effects, particularly

calcification of the kidneys at high doses. They concluded that inorganic phosphates

are not a risk to human health when consumed daily as part of a normal diet (Weiner

et al, 2001).

3.3 Passive Sampler

About 70-90% of analysis time during research is used to do sampling and sample

preparation, being sampling then one of the most important steps for analytical

procedures (Górecki & Namieśnik, 2002). Spot sampling is considered as the most

prevalent technique for pollutants measurement (Prest et al, 1995). It represents

conditions only at the time of sampling, giving an instant concentration, which may

not be representative of the sampled waterbody over a period of time. From the

necessity to simplify the process of sampling and sample preparation and to reduce

analysis costs, passive samplers have been developed.

Passive samplers can be used to monitor gas, liquid and solids. They were first used in

1973 for the semi-quantitative determination of CO and later for the determination of

NO2 and SO2 in air (Górecki & Namieśnik, 2002; De Santis et al, 1997). In 1987

Södergren used the passive sampling system for non-polar aqueous contaminant

monitoring. It consisted of a hydrophilic dialysis bag filled with hexane (Kot et al,

2000).

The passive sampling system is based on the Fick´s Law, and can be defined as a

sampling technique that relies on the flow of analyte from the sampled medium to the

collecting medium (Górecki & Namieśnik, 2002). Passive samplers have been

deployed in various environments such as stormwater, porewater, freshwater,

effluents and marine environments (Björklund, 2002) for metals and organic pollutant

determination.

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The amount of the collected analyte principally depends on the deployment time and

the concentration of the analyte in the sampled medium, but some environmental

conditions (pH, temperature, turbulence) can also affect.

A barrier (membrane) located between the sampled and the collected medium defines

the analytes collection rate. The concentration of the analyte is obtained as a time-

weighted average (TWA), representing the average concentration of the analyte in the

sampled medium during the exposure time (Górecki & Namieśnik, 2002).

The passive sampler (figure 1) used in this study is made of polypropylene and

consists of three parts, which screw together in order to hold a receiving phase and a

limiting membrane. The receiving phase is an Anion-SR (EmporeTM) disk (90%

sorbent particle, 10% PTFE) and the diffusion limiting membrane is a Sartorius

cellulose acetate (CA) with pore size of 0.45µm.

Figure 1 Passive Sampler layout. 1 Diffusion limiting membrane, 2 Receiving phase

(Anion-SR (EmporeTM), 3 Polypropylene supporting disk, 4 Polypropylene body, 5

Polypropylene screw lid.

The passive sampler suits particularly for the determination of TWA analyte

concentrations because it is just necessary the use of one sampler in order to obtain

the TWA concentration over the desired period of time (Górecki & Namieśnik, 2002).

If one wants to obtain the same information using spot sampling, it is necessary to

collect several samples, which implicates increased labour and costs.

On the other hand it is well known that changes in the sample composition might

occur when it is transported to the laboratory or stored prior to its analysis (Zhang et

1

2

4

3

5

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al, 1998; Michalke, 2003), and the use of passive samplers can help to reduce these

changes. Passive samplers are less sensitive to accidental extreme variations and are

able to give better information of contaminants concentrations over a long period of

time (Kot et al, 2000).

Passive samplers are also useful for screening applications and long-term integrative

sampling (Brumbaugh et al, 2000). They can also be left unattended over long periods

of time. Therefore passive samplers can be considered ideal when one wants to reduce

risks regarding change of sample composition and when it is necessary to do

monitoring over a long period of time, even in remote locations.

Sequential injection analysis systems (SIA) represent an alternative way of

monitoring over a long period of time and have been previously used to determine

nitrate and phosphate in natural and waste water (Galhardo & Masini, 2001; Cerdà et

al, 1998; Muñoz et al, 1997). Flow injection analysis (FIA) methods have also been

already implemented for on-line phosphate and nitrate monitoring (Hanrahan et al,

2001; Gabriel et al, 1998).

The use SIA and FIA for nitrate and phosphate monitoring allows the detection of

short time concentration changes and also represents that storing and preservation of

samples is not necessary and therefore errors due to chemical modifications during

sample handling and storage are minimized. Nevertheless these advantages, SIA and

FIA are sometimes extremely expensive to adopt due to increased equipment and

human labour requirements. Additionally field calibration is necessary to obtain

accurate results (Gibson et al, 1997).

The use of a passive sampler is therefore a good alternative due to its reduced cost,

simplicity and potential for large-scale measurements (Gibson et al, 1997; De Santis

et al, 1997), even when through its use is not possible to detect short time

concentration changes.

Recently, measurement techniques based on passive sampling have been developed.

In 1995 Dasika and Atwater used a multi-layer sampler (previously developed by

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Ronen et al in 1986) based on a dialysis membrane technique to passively sample

nitrate in groundwater (Dasika & Atwater, 1995).

Additionally, phosphorous has been previously measured in water and sediments by

means of diffusive gradients in thin films (DGT) by Zhang et al (1998). DGT gives

the time-averaged mean concentration during deployment and it is also based in the

Fick´s first law of diffusion. It promotes the phosphorous diffusion through a

polyacrylamide gel layer and then bind to ferrihydrite, which is embedded in a further

layer of gel (Zhang et al 1998).

3.3.1 Fick´s Law

Fick´s first law of diffusion (equation 1) is the base on which the passive sampler

works (Björklund, 2002). It is assumed that a concentration gradient will be created

between the bulk aqueous phase and the receiving membrane; the difference in

concentration between them will be the driving force for the analytes diffusion.

dxdCDm −= Equation 1

Where, m (gcm-2s-1) is the analyte mass flux, D (cm2s-1) is the diffusion coefficient

and the negative sign indicates the direction in which the concentration gradient goes,

dxdC represents the concentration gradient across distance x, where C (gcm-3) is the

analyte concentration and x (cm) the distance (Björklund, 2002). In this study the

bulk concentration Cb (mg/l) was calculated using equation 2,

tRsMCb = Equation 2

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Where, M (mg) is the mass present in the sampler during the deployment time (t), Rs

(ml/h) is the sampling rate of the analyte. The sampling rate (accumulation factor) is

obtained from the slope of the graph ml vs. time (section 5.4) (Knutsson, pers. comn.

2003).

3.4 Ion Chromatography

Ion Chromatography is a technique that works based on the chromatographic

separation of the ions. The separation of the ions is done by an anion exchange

column packed with porous polymethacrilate-based low-capacity anion exchange

resin, with a buffer solution of sodium carbonate/hydrogen carbonate as eluent

(Reeve, 1994).

The determination of the nitrate ion was done by direct ultraviolet (UV) absorption

since nitrate ions present significant sensitivity and selectivity by UV detection at

200-220 nm, even in the presence of chloride excess. Therefore UV detection gives

better sensitivity than conductivity detection in this particular case of nitrate analyses

(Waters, 1989). The detection of nitrate using ultraviolet absorbance is based on the

Beer-Lambert Law (Reeve, 1994) and it has been a common method used for nitrate

determination in several studies (Al-Momani, 2003; Brodnjak-Vončina et al, 2002;

Connolly & Paull, 2001).

In this method an aqueous sample is introduced into the eluent stream and then

transported to the anion exchange column. Then the analytes of interest are separated

and determined by a system comprised of a guard column, analytical column and an

UV detector, which has been already adjusted to a 214nm wavelength. In figure 2, the

components of a Waters ion chromatograph used for measuring nitrate are shown

(Waters, 1989).

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Figure 2 Waters ion chromatograph components (Modified from Waters 1989).

3.5 Inductively Coupled Plasma – Mass Spectrometry

Inductively coupled plasma – mass spectrometry (ICP-MS) is considered a precise

and sensitive analytical technique and is able to analyse and determine (generally at

ng l-1 range) most elements in the periodic table (Rauch, 2001). ICP-MS has become

one of the most important methods in analytical chemistry for elemental speciation

analysis due to its versatility, robustness and multi-elemental capabilities (Becker &

Dietze, 2003, Montes-Bayon et al, 2003). One of the advantages of ICP-MS is the

fast calibration procedure due to the atmospheric pressure working conditions as well

as the excellent detection limits for elements abundant in nature which would be more

difficult to determine at ng l-1 range by other techniques (Becker & Dietze, 2003).

The ICP-MS is a plasma ion source, which operates at atmospheric pressure and high

temperature (5000-10000 K). A cross section of a quadrupole-ICP-MS instrument is

presented in figure 3. A quartz torch generates a plasma of argon. Argon is provided

to the plasma by the main gas flow and the secondary gas flow helps to centre and

stabilize the plasma and at the same time, to cooling the torch and preventing it from

melting (Kannamkumarath et al, 2002). An electric discharge ignites the plasma and

then an electromagnetic field maintains the plasma ignited (Rauch, 2001). In the

Sodium Hydroxide eluent

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electromagnetic field, electrons collide with other argon atoms producing higher

thermal energy (Kannamkumarath et al, 2002).

Figure 3 Cross section of a quadrupole-ICP-MS instrument (Rauch, 2001).

The ions formed in the plasma enter the mass spectrometer through the sampler and

skimmer cones and then they are focused by ion lenses into the mass analyser where

they get separated depending on their mass-to-charge ration (m/z) and finally detected

(Rauch, 2001; Kannamkumarath et al, 2002). Accurately phosphorous determination

at very low levels is very difficult to perform by most traditional analytical

techniques.

Recently ICP-MS has been used for phosphorous determination. Phosphorous has just

one isotope at m/z 31 (Hughes et al, 1998), however NOH ions, which are produced

in the plasma from water and atmospheric gases, could produce interference when

phosphorous is determined (Hughes et al, 1997). In this study phosphate was

determined using inductively coupled plasma – mass spectrometry. Jiang and Houk

(1987) as well as Divjak et al (1999) have previously determined phosphate and other

ions using ICP-MS due to unique advantages (element specificity, wide dynamic

linear range, low detection limits) that ICP-MS offers (Divjak et al 1999).

3.6 Experimental Design: multiple-level experiments

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Experiments were designed in order to find the optimal elution procedure for the

passive sampler for nitrate and phosphate monitoring and also for the evaluation of

some environmental factors (pH, temperature, turbulence) influence in the sampler

development.

When one wants to test several factors at one time it is important to design a balanced

experiment in order to evaluate the factors independently of one another. In

experimental design this is called orthogonality, where the effect of one factor does

not affect the estimation of the effect of another factor (Ross, 1996). For this study the

Taguchi design of experiment process was followed. An orthogonal array L9 (table I)

was chosen in order to test parameters at three different levels.

Table I L9 Standard Array (Ross, 1996).

Factors to testTrialA B C D

1 1 1 1 1 2 1 2 2 2 3 1 3 3 3 4 2 1 2 3 5 2 2 3 1 6 2 3 1 2 7 3 1 3 2 8 3 2 1 3 9 3 3 2 1

After the experiments were performed and the results obtained an analysis of variance

(ANOVA) was performed. ANOVA is a statistically based, objective decision-making

tool, which is used to detect differences in average performance of different tested

groups. The decision takes variation between tested groups into account (Ross, 1996).

The tool, which provides a statistically based decision at a certain confidence level as

to whether the variations are significantly different, is called the F test. The F test

consists on a ratio of sample variances. When the ratio is large enough (according to

specific tables related to a confidence level) then the two sample variances are

considered as being unequal at some confidence level (Ross, 1996). When this

happens, the tested parameter is expected to influence in a significant way the

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performance of the passive sampler. In this study all the decisions as whether a factor

affected or not were done at 99% of confidence.

In order to assess the relative power of a factor, a percent contribution was calculated.

The percent contribution reflects the portion of the total variation observed in an

experiment attributed to each significant factor, indicating that the factors with

substantial percent contributions are the most important items affecting the sampler.

4. Methodology

4.1 Sampler preparation

In order to condition the Anion-SR disks (receiving phase) they were first mounted on

the glass support base of a vacuum apparatus, they were then covered by a glass

funnel and secured by means of a clamp. HPLC-grade methanol (15 ml) was added

and the disks were left soak for 60 seconds. Mild vacuum was applied until methanol

was almost drawn through the disk, at this time 15 ml of grade I laboratory reagent

water were added, the addition of reagent water was repeated one more time.

The conditioned Anion-SR disk was placed over the supporting disk and a diffusion

limiting membrane was placed over the conditioned disk. The supporting disk was

placed on the sampler body and finally screwed to the lid. The prepared sampler was

left in nanopure water until deployed.

4.2 Elution procedure and samples preparation prior analysis

Elution of the Anion-SR disks was performed according to the results obtained in the

elution experiment (section 5.2). The eluted solution was placed in polyethylene

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bottles and frozen until analysed, this procedure has been found satisfactory for the

storage of samples up to 6 months (Maher et al, 2002).

For nitrate determination the samples were diluted 100 times before IC analysis to

reduce the acid strength to enter the column. In the case of phosphate determination

the samples were diluted 5 times, rhodium was added as internal standard and HCl

when samples did not already contain acid.

4.3 Bulk concentration experiment

In order to confirm the diffusion theory and have a better understanding about the

development of the passive sampler, an experiment was performed. Three 2-litre

beakers with different nitrate and phosphate concentrations: 10ppm beaker 1, 20ppm

beaker 2 and 3; were prepared. A prepared sampler was placed on each of beakers 1

and 2. On beaker 3 a sampler was also placed but it had a glass fibre membrane (pore

size 1,2µm) as diffusion limiting membrane.

The beakers had constant stirring and were left for 5 days. Water samples were taken

at regular intervals over the experiment period. The three samplers were eluted three

times with 50ml 0,1M HCl and nitrate was determined by ion chromatography. The

results are presented in section 5.1

4.4 Elution experiment

As already mentioned in section 3.6, a multi-level experiment was designed in order

to find the optimal elution procedure for the passive sampler for nitrate and phosphate

monitoring. In this case the amount and concentration of acid were identified as the

two principal factors that could affect the elution results. For each factor three levels

were chosen (table II).

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Table II Factors tested for elution experiment.

Level Factors 1 2 3

Acid amount (ml) 25 40 50Acid concentration (M) 0,1 0,5 1

Nine trials were performed in order to test the two chosen factors at three different

levels (section 3.6 for details). For each trial a receiving membrane was prepared and

placed on a vacuum apparatus. Additionally a 1-litre solution containing 10ppm of

nitrate was prepared and drawn through the disk. Once all the solution was drawn, the

disk was eluted three times according to the defined parameters of elution for each

trial. The initial and final (after passing through the disk) concentration of the solution

as well as the three elutions for each trial, were analysed using standard ion

chromatography (UV detection).

4.5 Environmental factors

The uptake of the passive sampler is expected to be affected by environmental factors

such as pH, temperature and turbulence and additionally by the concentration of the

analysed substance as well as time. A robust experiment was designed in order to test

and quantify the influence of these factors. The nitrate and phosphate concentration of

the solution was maintained at 10ppm; pH, temperature and turbulence were tested at

three different levels (table III).

Table III Environmental factors and levels tested.

Level Factors 1 2 3

pH 5 7 9 Temperature (°C) 7 14 21 Turbulence (rpm) 50 200 400

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The experiment consisted on nine trials in which the levels of the different parameters

were tested, analysed and quantified (section 3.6). Each trial was run for three days

maintaining all the parameters constant during this time as well as a constant flow of a

solution of 10ppm nitrate and phosphate, creating a dynamic system where nitrate and

phosphate concentrations were continually replenished at a 10ml/min rate. The pH

was adjusted with the addition of 1 M NaOH, and the temperature was maintained at

any selected level with help of a water bath as previously has been done by Luk and

Au-Yeung (Luk and Au-Yeung, 2002). A digital stirring device controlled the

turbulence. In each trial three passive samplers were deployed.

4.6 Calibration curve

Two calibration curve trials were performed. A vessel was prepared in which a

constant flow (10 ml/min for the first trial, 0,5 ml/min for the second trial) was

created. Nitrate (2ppm) and phosphate (1ppm) concentration were continually

replenished by means of this dynamic system. Temperature was established at 14°C

with help of a water bath and pH level was 7±0,2 adjusted with the addition of 1 M

NaOH. A stirring device maintained the turbulence at 200 rpm for the first trial and 30

rpm for the second. In this vessel the samplers were placed (14 first trail, 10 second

trial) and one of them was taken every day. Membranes were eluted and nitrate and

phosphate determined by IC and ICP-MS respectively.

4.7 Field test (Rya Water Treatment Plant)

Nine samplers were placed in the outlet of Rya water treatment plant in 3-sampler

batches. The samplers were left for one week on the following dates: 1st batch August

18th-25th; 2nd batch September 9th-16th, 3rd batch September 16th-23rd. During the

deployment time the samplers experienced biofouling from the 4th day of exposure.

After one week the samplers were brought back to the laboratory, eluted and nitrate

21

and phosphate amounts in the samplers were determined and the measured

concentration by the passive sampler was determined using data from the calibration

curve results. Results were also adjusted according to average temperature during the

deployment time.

5. Results and discussion

5.1 Bulk concentration experiment

The bulk concentration experiment confirmed the diffusion of nitrate from the bulk

solution to the passive sampler by the decreasing nitrate concentration on the beakers

over time (figure 4). The percentage of total collected analyte, related to the total

analyte present in the solution, by sampler 2 (24%) was twice as sampler 1 (12%),

confirming the assumption that the amount of collected analyte by the sampler

depends on the concentration of the analyte in the sampled medium.

Bulk concentration experiment

y = -0,0394x + 18,976R2 = 0,9796

y = -0,0103x + 9,299R2 = 0,9533

0

5

10

15

20

0 20 40 60 80 100 120 140ho urs

beaker 1 beaker 2 beaker 3

Linear (beaker 2) Linear (beaker 1)

Figure 4 Bulk concentration experiment.

The use of a limiting membrane with two different pore sizes was in order to evaluate

the effect of the limiting membrane in the sampler development. The sampler with a

limiting membrane with bigger pore size (beaker/sampler 3) showed quicker uptake

than the sampler with a limiting membrane of smaller pore size (beaker/sampler 2),

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confirming that the limiting membrane defines the analyte collection rate by the

sampler.

When the percentage of recovered analyte after each elution was analysed, it was

observed that after the first elution just about 65% of the expected analyte was

recovered, and just after the third elution between 80-90% of the analyte was

obtained. From these results raised the need of finding an optimal elution procedure

for the membranes.

5.2 Elution experiment

The results of each trial were analysed according to the percentage of nitrate

recovered in each elution. The elution experiment showed (figure 5) that the optimum

elution of the disk would be obtained using 25ml of 0,5M hydrochloric acid (HCl).

Acid amount and concentration effect on elution

80

82

84

86

88

90

92

94

96

Level 1 Level 2 Level 3

amount concentration

Figure 5 Acid amount and concentration effect on elution.

These optimal elution parameters were used for the development of the whole project.

In order to assure the recovery of the 100% of the analytes, a second elution is

necessary. The final elution procedure was therefore established as 2 times 25 ml of

0,5M HCl.

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5.3 Environmental factors

The results of each trial were analysed according to the quantity (mg) of nitrate and

phosphate attached to the receiving phase of the sampler. The experiment for

environmental factors evaluation arrived to different conclusions for nitrate and

phosphate.

In the case of nitrate just temperature was identified as a significant factor (table IV,

figure 6). Temperature is calculated as responsible for 46% of the variation of nitrate

uptake by the sampler; pH and turbulence were just responsible for very little

variation, which is not considered statistically significant at 99% confidence level.

However more than 50% of the variation is due to other factors not accounted in the

experiment.

Table IV ANOVA Table Nitrate

Factor Degrees of freedom

Sum of squares

Variance F Percent

pH [2]a Temperature 2 11,4148 5,7074 12,1943 46,27 Turbulence [2]a

Error 24 11,2329 0,4680 53,73 Total 26 22,6477 100,00

a Values in brackets []indicate factors which have been pooled (added to error due to

being considered statistically insignificant).

Temperature influence on nitrate uptake

y = 0,113x + 2,2337R2 = 0,9869

0

2

4

6

0 5 10 15 20 25

temperature

temperature Linear (temperature)

Figure 6 Temperature influence on nitrate uptake.

24

As can be seen in figure 6, the amount of nitrate in the disk increases as temperature

increases, suggesting a direct proportional relation between nitrate uptake and

temperature.

Phosphate uptake by the sampler is affected by pH and temperature changes, and

these two factors are considered as significant (table V, figures 7 and 8). Temperature

is expected to contribute with about 34% of phosphate uptake variation and pH is

responsible for about 40%. Turbulence is just responsible for very little variation,

which is not considered statistically significant at the 99% confidence level. The

chosen parameters were responsible for around 73% of the total variation of the

passive sampler phosphate uptake.

Table V ANOVA Table Phosphate

Factor Degrees of freedom

Sum of squares

Variance F Percent

pH 2 2,4596 1,2298 20,2736 39,59 Temperature 2 2,1122 1,0561 17,4105 33,71 Turbulence [2] a

error 22 1,3345 0,0607 26,70 Total 26 5,9063 100,00

a Values in brackets []indicate factors which have been pooled (added to error due to

being considered statistically insignificant).

pH influence on phosphate uptake

y = 0,1792x + 0,161R2 = 0,9397

0,0

0,5

1,0

1,5

2,0

0 2 4 6 8 10pH

pH Linear (pH)

Figure 7 pH influence on phosphate uptake.

25

Temperature influence on phosphate uptake

y = 0,0487x + 0,733R2 = 0,9916

0,0

0,5

1,0

1,5

2,0

0 5 10 15 20 25temperature

temperature Linear (temperature)

Figure 8 Temperature influence on phosphate uptake.

Temperature and pH influence on phosphate uptake show the same variation trend as

temperature shows for nitrate uptake. The different compounds phosphate takes at

different pH levels can explain the effect of pH on phosphate uptake. When the pH is

low, phosphate compounds are uncharged and less soluble in water, when pH

increases phosphate compounds become charged and their solubility in water

increases (Josefson, 2002). It can be concluded that temperature and pH have a direct

proportional effect on nitrate and/or phosphate uptake by the passive sampler.

5.4 Calibration curve

The best results obtained from the two calibration curve trials were chosen and

presented in this paper. Due to some uncertainties during the elution of two samplers,

the values of them were suppressed from the nitrate calibration curve. Phosphate

(figure 9) and nitrate (figure 10) accumulation factor (sampling rate) over time were

obtained as explained in section 3.3.1 and analysed. Both phosphate and nitrate

accumulation factors present a quite linear accumulation over time (R2 phosphate=0,9369

and R2 nitrate=0,9248).

26

Accumulation factor phosphate

y = 3,1363x - 0,8652R2 = 0,9369

0

200

400

600

800

1000

1200

1400

1600

0 100 200 300 400 500

t ime (h)

Accumulation factor (ml) Linear (Accumulation factor (ml))

Figure 9 Phosphate accumulation factor over time.

Accumulation factor nitrate

y = 6,1774x + 344,08R2 = 0,9248

0

500

1000

1500

2000

2500

0 50 100 150 200 250 300 350

t ime (h)

accumulation factor Linear (accumulation factor)

Figure 10 Nitrate accumulation factor over time.

Sampling rates were calculated for both nitrate and phosphate, resulting in 148 and 75

ml/day respectively. Using these results the concentration measured by the samplers

placed in Rya water treatment plant was calculated.

5.5 Field test (Rya Water Treatment Plant)

The results obtained from the samplers were compared with the results measured by

Rya water treatment plant. Rya water treatment plant takes water samples every day

and determines temperature, pH and nitrate; phosphate is determined twice a week.

27

Nitrate and phosphate concentration is reported depending on flow. The temperature

during the sampled weeks ranged between 19-19,9ºC, and pH between 6,5-6,9.

The passive sampler (figures 11 and 12) determined between 30-50% nitrate and

between 12-50% phosphate concentrations that spot samples did. A higher variation is

found during the first batch of passive samplers (WTP1) than during the second

(WTP2) and third (WTP3) for both nitrate and phosphate.

Passive Sampler vs WTP data nitrate

0

1

2

3

4

WTP1 WTP2 WTP3

Sampling perio ds

Sampler Nitrate (mg/L) WTP Nitrate mg/L

Figure 11 Passive sampler vs WTP nitrate determination.

Passive Sampler vs WTP data phosphate

0,0

0,1

0,2

0,3

0,4

WTP1 WTP2 WTP3

Sampling perio ds

Sampler Phosphate (mg/L) WTP Phosphate mg/L

Figure 12 Passive sampler vs WTP phosphate determination.

28

6. Conclusions

It can be possible to determine nitrate and phosphate concentration in wastewater

using the passive sampler if its dependence on environmental factors is known. Due to

this it is important to take into account the variations that nitrate and phosphate uptake

may suffer depending on environmental factors such as temperature and pH.

In this study temperature is found to affect both nitrate and phosphate uptake by the

passive sampler, showing a higher uptake at higher temperature and vice versa.

Phosphate uptake by the passive sampler is also affected by pH changes, showing the

same behaviour as temperature does. On the other hand turbulence seems to have no

or minimal effect on the passive sampler, at least at the tested levels in this study (50,

200 and 400 rpm).

Since the nitrate and phosphate determination by the passive sampler obtained in this

study are not very accurate when compared with the Rya water treatment plant results

(maximum 50%) it is important to continue testing different parameters which could

be affecting the development of the passive sampler on the water treatment plant.

A possible problem for the passive sampler development is biofouling, which can be

easily found when it is exposed to wastewater. The uptake rate variation according to

temperature and pH is also a drawback, which could be possibly overcome by the use

of internal or external standards.

7. Recommendations

Further testing of the passive sampler in wastewater as well as in rivers, lakes, etc is

recommended in order to assess its performance and accuracy in these environments.

It is also desirable to find other possible factors, which affect the passive sampler

performance, especially for nitrate determination, since in this study just 46% of the

variation of this analyte was identified.

29

The determination of a feasible internal and/or external standard is certainly a

challenge for the future. It could be also interesting to test other kind of diffusion

limiting and/or receiving membranes in order to achieve reduction in variability and

dependence on environmental factors and make the sampler more robust.

8. Acknowledgments

I would like to acknowledge first Jesper Knutsson for all his support and guidance

over the development of this project. Greg Morrison is also acknowledged for his

support as supervisor and examiner. Mona Zanders is thanked for her help in the

laboratory. Britt-Marie Wilen is acknowledged for her help on Rya water treatment

plant and supply of data required to perform this thesis. Rya water treatment plant is

thanked for give me the possibility of testing the passive samplers and for the supply

of data.

30

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