COMPARISON BETWEEN EXTRACTION AND ACID DIGESTION ... · PDF fileCOMPARISON BETWEEN EXTRACTION...

American Journal of Engineering and Technology Research Vol. 15, No.2, 2015

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COMPARISON BETWEEN EXTRACTION AND

ACID DIGESTION TECHNIQUES TO REMOVE

HEAVY METAL FROM SOIL

Haifa El-Sadi1, Maria Elektorowicz

2 , Ammar Badawieh

3

1Wentworth Institute of technology, Mechanical Engineering and technology, Boston, MA, USA, 02115

2,3 Concordia University, Building, Civil and Environmental Engineering, Montreal, QC, Canada

Correspondent author email: [email protected]

Abstract:

A series of laboratory tests were conducted in order to compare between two techniques supercritical extraction and acid

digestion and the effect of shaking time on the extraction efficiency of heavy metals from clayey soil. Supercritical fluid

extraction and acid digestion were used for the analysis of heavy metals concentrations after the completion of Electro-kinetic

experimentation. Supercritical fluid (carbon dioxide) extraction is a new technique used to extract the heavy metal (lead, Nickel,

Calcium and Potassium) from clayey soil. The comparison between supercritical extraction and acid digestion of different metals

was carried out. Supercritical fluid extraction, using Ethylenediaminetetraacetic acid (EDTA) as a modifier, proved to be efficient

and safer technique than acid digestion technique in extracting metals from clayey soil.

Mixing time of soil with EDTA before extracting heavy metals from clayey soil was investigated. The optimum and most

practical shaking time for the extraction of lead, Nickel, calcium and Potassium was two hours.

Keywords: clay soil, heavy metals, supercritical fluid extraction, acid digestion

1. Introduction

From an engineering standpoint, natural soil presents a medium that is complex in structure, physic-chemical

properties and general behavior. Knowledge of the soil horizon forms the initial foundation for the study of soil

composition. The efficiency of any remediation effort is predicated on a knowledge of soil composition, since this

presents an idea related to the fate (transport, transformation and remediation). Soil consists of a myriad of

components that can be subdivided based on their physical structure and their chemical behavior related to

contaminant fate. The soil has been used in this study is illites.

Illites consist of repeated layers of alumina sheets between two silica sheets with oxygen atoms shared. Each layer

is bonded by potassium ions which fit exactly into the hexagonal gaps provided by the silica sheets. As a result of

the presence of these potassium ions, a positive charge results, which is dissipated by the substitution of aluminum

for silicon. The structure and physical properties of illite are favorable to the retardation of chemicals, particularly

heavy metals. Illite is characterized by a small particle diameter, less substitution of aluminum for silicon, and more

sites for exchangeable cations than kaolinite and chlorite [3].

Metals play an important role in environment. Heavy metal contamination develops from accidental spills, fume

emission, electroplating operations etc. metals are universal and cause the toxicity of plants and humans. Also, the

clay involves of tight structure and very small pores which affect difficulty on clay remediation. The fate of metals

in the subsurface, which includes their physical transport and chemical transformation is crucial to understand which

type of remediation technique can be used. Since the subsurface is heterogeneous and anisotropic, the metal content

is subject to spatial variability [1]. Heavy metals contain different kind of metals such as Lead, Nickel, Potassium

and Calcium. The retention of nickel is primarily due to the presence of organic matter, precipitation and due to the

tendency of nickel for hydrolysis. Harter [2] established that nickel adsorption in soil primarily follows a Langmuir

isotherm and is not as strongly nor as quickly adsorbed by a given soil as is lead. In addition, retention of lead by

insoluble humic and soluble fulvic acid has been reported by Yong, et al. and Harter [2,3]. Fergusson, [4]


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demonstrated that lead compounds are enriched in mosses, podzolic soils, ombrotropic and in other environmental

targets in southernmost Norway.

there are different techniques can be utilized to remediate the soil from heavy metal such as soil washing which is

expensive from a standpoint of capital costs. However, Electrokinetics (EK) soil remediation has confirmed to be an

effective on a pilot scale for remediation of lead-spiked Georgia Kaolinite [4]. On the other hand, the transport of

ionic species and pore water between cathode and anode, via electroosmosis and electrolytic migration is more

effective than standard hydraulic gradients [6]. Two techniques, acid digestion and supercritical fluid extraction have

been used after the completion of Electrokinetic experimentation to analyze the heavy metal concentrations.

Soil constituents and the properties have a significant influence on the fate of contaminants. A contaminant

present in the subsurface is transported to other locations in the soil by pore water potential, advection, dispersion

and diffusion. In clay soils, molecular diffusion is the typical transport mechanism due to the low permeability of

clay. While the contaminant is transported through the media, retardation occurs which aids in removing these

contaminants from the liquid phase and onto the solid phase, thereby ceasing their mobility. Retardation of cationic

substance, particularly metals, is of particular importance to remediation engineering. In any remediation process,

accessibility of metals to transport (i.e. electro kinetics) is paramount. Heavy metals have the ability to accumulate

in living organisms and can therefore cause toxicity within the food chain. It is generally understood that the degree

of mobility and activity of metals is influenced by temperature, cation exchange capacity of the solid phase, complex

metal and soil composition and the concentration of metal in the soil solution. In natural clay soil, the use of EDTA

has the potential to keep more heavy metals in the solution phase, thereby allowing Electrokinetic method to

transport an increasingly higher amount of metals.

Ethylenediaminetetraacetic (EDTA) has been used to extract heavy metals from contaminated soil, through

solubility and mobility [11]. It is very soluble in water and dissociates into a wide variety of species such: H3Y-,

H2T-2

, HY-3

and Y-4

. The pKa value are shown in table 1.

Table 1. Ethylenediaminetetraacetic acid (EDTA) species and pKa values (Yeung and Menon, 1996)

Ionic species pKa

H3Y- 2

H2T-2

2.76

HY-3

6.16

Y-

10.26

Each EDTA ion can coordinate bond to a metal at six different sites, namely, each of the four acetate sites and the

two nitrogen sites, which have free electron pairs available to coordinate bond formation. The structure and

configuration of the metal-EDTA complex is shown in Figure 1. The numerous coordination sites inherent to EDTA

ions creates complexes that are highly stable and favorable to chelation technology.

Figure 1. Structure and location of metal-EDTA complication sites [11]

Supercritical fluid extraction (SFE) is a technique used for the analysis of heavy metal concentrations after the

completion of electrokinetic experimentation. SFE requires a supercritical fluid such as carbon dioxide and a


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modifier if needed [7, 8] extracted metals from liquid and solid materials using lithium bis-triflouroethyl-

dithiocarbamate (FDDC) as a modifier. This modifier revealed high solubility in supercritical fluid carbon dioxide.

Wang and Marshall [12] performed a series of SFE tests in order to improve the solubility of the metals in

supercritical fluid CO2 by using tetrabutylammonium dibutyldithiocarbamate as a modifier.

The objectives of this research are to determine the extraction efficiency of SFE technique versus acid digestion

and to study the most optimum and practical shaking time for extraction of metals.

2. Materials and Methods

Tests were conducted on laboratory-prepared contaminated samples to assess SFE extraction efficiency, acid

digestion and Atomic absorption spectrometry (AAS) of heavy metals from clayey soil.

2.1 Materials

Four metals were analyzed versus shaking time for samples employing EDTA and supercritical extraction as

shown in table 1. Ethylenediaminetetraacetic acid (EDTA) is a highly branched and high-molecular weight acidic

compound. EDTA has been extensively used to extract heavy metals from contaminated soil, through solubility and

mobility enhancement [9]. Table 2 shows the soil characterization which has been used for the experiments.

Table 2. Soil characterization

Parameter value

Mineral composition

Cation exchange capacity 21meq/100g

pH 7.6

Organic matter content 1.3%

Carbonate content 4.5%

Specific gravity 2.753

Total Kjeldahl nitrogen 0.042%

Sulfate test 0.6 ppm as SO4-2

Metal content Fe 273mg/kgdry soil

K 176mg/kg dry soil

Ca 250mg/kgdry soil

Ni 31 mg/kg dry soil

Pb 9.3 mg/kg dry soil

1.2 Supercritical Fluid extraction equipment and procedure

All extraction experiments were performed by the SFX 220 extraction System, which consists of an SFX 220

Extractor and SFX 200 Controller, and 100 ml syringe pumps (ISCO Model 100DX). Both the pumps and the

extractor and connected to a SFX 200 Controller, which controls all pumping and extraction operations by changing

the parameters such as temperature and pressure (Fig. 2). The SFX 220 Extractor is a bench top, dual chamber

(cartridge filter with a 5/8-inch diameter filter element), which fits on the top of an SFX 200 Controller. The

extractor incorporates six motor-actuated valves, which are controlled by the SFX 200 controller. The fluid source

for the extractor is supplied by a D-series pump and the other pump for the modifier. Fused-silica tubing with an

inner diameter of 50 µm, 30 cm long, was used as an outlet restrictor, allowing analysts to be conveniently collected

in the test tubes. A vent valve allows rapid depressurization of the chamber after the extraction is completed.

Extracted analyte was collected out-side of the oven at room temperature by placing the outlet end of the restrictor

into a 30-ml vial containing 7-10 ml of solvent.


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Figure 2: Dual syringe pump system for modifier SFE

The different extraction sequences can be described as follows: (1) 1 g of soil and 9 mL of 0.1 M EDTA are

placed into a 2.5 ml cartridge body. Valves closed, the cartridge is loaded into extraction chamber. (2) Filters are

placed at the top and bottom of the cartridge. (3) The supply valve is opened and the supercritical fluid is supplied

by two 100DX syringe pumps. The fluid flows into the extraction chamber, filling the cartridge upwards. The

samples were placed into the supercritical fluid extractor for 15 min dynamic extraction at 90C and 5000 psi. The

restrictor temperatures were maintained at 90 C. The samples were shaken at 30 minutes, 1 hour, 2 hours, 4 hours

and 8 hours (shaking time) before were placed into the cartridge of extraction chamber. The recoveries of the metals

from the collection were determined by using Atomic absorption spectrometry (AAS).

2.3 Acid Digestion experiment and procedure

Acid digestion is applying a very strong acid to the matrix in order to dissolve certain elements (metals)

that could become environmentally available, and prepare the collected samples for analysis by atomic absorption

spectrometry. American Environmental Protection Agency EPA, generated a standard procedure for acid digestion

[10]. The samples were prepared by transferring a 1-2 g sample (wet weight) and 1 g sample (dry weight) to a

digestion vessel. The sample is used as long as digestion is completed. Then 10 mL of 1:1 HNO3 was added to the

sample, mix the slurry, and cover with a watch glass or vapor recovery device. The sample was heated to 95oC ± 5

oC

and reflux for 10 to 15 minutes without boiling. Then the sample was allowed to cool, then 5 mL of concentrated

HNO3 was added, the cover and reflux were replaced for 30 minutes. If brown fumes are generated indicating

oxidation of the sample by HNO3, repeat this step (addition of 5 mL of conc. HNO3) over and over until no brown

fumes are given off by the sample indicating the complete reaction with HNO3. A ribbed watch glass or vapor

recovery system were allowed either allows the solution to evaporate to approximately 5 mL without boiling or heat

at 95oC ± 5

oC without boiling for two hours. Maintain a covering of solution over the bottom of the vessel at all

times. After the sample has been cooled, 2 ml of water was added and 3 ml of 30% H2O2 was added with warming

until the effervescence is minimal or until the general sample appearance is unchanged. Heat was continued until the

volume has been reduced to approximately 5 mL without boiling for two hours. Maintain a covering of solution over

the bottom of the vessel at all times. 10 ml concentrated HCl was added to the sample digest and cover with a watch

glass or vapor recovery device and Placed in the heating source for 15 minutes. After adjusting to the required

volume, sample is ready for atomic absorption spectrometry for metal detection.

2.4 Atomic absorption spectrometry (AAS)

The recoveries of the metals from the collection were determined by using Atomic absorption spectrometry

(AAS) during all the experiments. AAS is an analytical technique that measures the concentrations of elements.

Atomic absorption is so sensitive that it can measure down to parts per billion of a gram (µg dm–3

) in a sample. The

technique makes use of the wavelengths of light specifically absorbed by an element. They correspond to the

energies needed to promote electrons from one energy level to another, higher, energy level. AAS is used to analyze

a sample to see if it contains a particular element means using light from that element, since atoms of different


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elements absorb characteristic wavelengths of light. Sample preparation for AAS is often simple, and the chemical

form of the element is usually unimportant. This is because atomization converts the sample into free atoms

irrespective of its initial state. The sample is weighed and made into a solution by suitable dilution. Elements in

biological fluids such as urine and blood are often measured simply after a dilution of the original sample. When

making reference solutions of the element under analysis, for calibration, the chemical environment of the sample

should be matched as closely as possible where the analyte should be in the same compound and the same solvent.

Teflon containers may be used when analyzing very dilute solutions because elements such as lead are sometimes

leached out of glass vessels and can affect the results.

3. RESULTS AND DISCUSSION

3.1 SFE versus acid digestion

Extraction efficiency and feasibility of SFE technique versus acid digestion was studied. The concentration of

various metals measured in the extract of SFE samples and acid digestion samples was compared. A summary of

results for various metals is shown in table 3.

Table 3: Comparison of SFE technique versus acid digestion

Metal

type

Concentration of

Metals in Extract:

SFE Technique

(ppm)

Concentration of Metals in

Extract:

Acid Digestion

(ppm)

Lead 9.3 below detectable limits

Nickel 31.1 below detectable limits

Calcium 249.8 56.0

Potassium 61.8 63.8

Iron 200 150

Table 3 shows that the utilization of SFE techniques with EDTA was significantly higher in the extraction of lead,

nickel and calcium. The extraction from acid digestion produced lead and nickel concentrations that were below

detectable limit during atomic absorption spectrophotometry. However, lead and nickel concentrations were detected

in the extract from SFE. The calcium concentration in the SFE extract was 346% higher than that of the acid

digestion extract. Therefore, supercritical fluid extraction with EDTA represents a viable and more efficient method

for analyzing metals in clay soils, as is the case in the analysis of soil after the termination of electrokinetic

treatment.

3.2 SFE tests: obtaining optimum shaking time

SFE extraction with EDTA was more efficient in the extraction of heavy metals, particularly lead and nickel,

establishment of the optimum shaking time of contaminated soil of heavy metal with EDTA acid was crucial. The

samples were shaken at 60 rpm for 30 minutes, 1 hour, 2 hours, 4 hours and 8 hours using an AROS orbital shaker. It

should be noted that four samples for each shaking time were used. The metals (lead, nickel, calcium and potassium)

were analyzed versus shaking time, for samples employing EDTA and SFE extraction and EDTA without SFE

extraction. All metals show a significant improvement in extract concentration when SFE is applied as shown in

table 4.

Table 4: Average percent increase in extracted Metals (SFE versus No SFE)


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Metal % increase in Metal Extracted when

utilizing SFE technique

Lead

Nickel

Calcium

Potassium

Iron

213

117

144

124

24%

Figure 3: Metal extracted versus shaking time without SFE for

(a) Calcium, iron and potassium and

(b) Lead and nickel.

0

50

100

150

200

0 5 10

Me

tal E

xtra

cte

d

(pp

m)

Mixing Time (hours) (a)

Calcium

Iron

potassium

Metal extracted with no SFE

0

5

10

15

20

25

0 5 10Me

tal e

xtra

cte

d

(p

pm

)

Mixing time (hours) (b)

Lead

Nickel


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Figure 4. Metal extracted versus shaking time with SFE for

(a) calcium and iron, Lead,

(b) Nickel and potassium

Figures 3 (a and b), 4 (a and b) show a general increasing trend in metal concentration versus shaking time, for all

metals. The highest percent of metal in the extract occur from 0.5 to 2 hours of shaking. After 2 hours, the

concentration of metals in the extract increase slightly. Table 5 shows the percent increase in metals extracted within

each time interval.

0

50

100

150

200

250

300

350

400

0 5 10M

eta

l Ext

ract

ed

(p

pm

)

Mixing Time (hours) (a)

Calcium

Iron

Metal extracted with SFE

0

20

40

60

80

100

0 5 10

Me

tal e

xtra

cte

d

(pp

m)

Mixing time (hours) (b)

Lead Nickel potassium


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Table 5. Metal Extraction: percent increase versus shaking time

Metal SHAKING INTERVAL

0.5-1.0 hrs. 1-2 hrs. 2-4 hrs. 4-8 hrs.

Lead 6.7% 23.1% 2.5% 11.1%

Nickel 4.3% 17.6% 2.2% 4.9%

Calcium 10.3% 5.2% 1.7% -0.1%

Potassium 0.7% 6.2% 1.7% 2.3%

Iron 3.5% 1% -1% 2.2%

3.3 Correlation between Extraction and shaking time

In spite of the fact that the effectiveness of extraction for different metals is difficult to predict due to the

soil complex and metal structure, an attempt was conducted in order to correlate the extraction efficiency of different

metal with different shaking time. Nonlinear regression was considered to model extraction process and shaking

time:

𝑐 (𝑡, 𝑎0, 𝑎1 ) = 𝑎0 ∗ 𝐿𝑛(𝑡) + 𝑎1

Where c is the extracted metal concentration, a0 and a1 are constants and depend on metal structure and t is the

shaking time. The recoveries of the metals from the collection were determined by using Atomic absorption

spectrometry (AAS) during all the experiments to analyze the concentrations of extracted metals in collection vials

of SFE. Extraction efficiency (E) was estimated as follows:

%100XCV

CVE

imetal

t

Where Vt is trapped volume, Vmetal is volume of metal, C is extracted concentration, Ci is initial concentration of

metals in soil. Figure 6 shows that the calcium has the highest extraction efficiency, However, potassium has the

lowest recovery because potassium has high bivalent ions.

Figure 6: Recovery versus different metals (lead, Nickel, calcium and Potassium)

4. Conclusion

A series of laboratory tests were conducted in order to compare between two techniques Supercritical fluid

extraction (SFE) and acid digestion techniques and to study the effect of shaking time on the extraction efficiency of

heavy metals from clayey soil. SFE and acid digestion have been used for the analysis of heavy metals after the

completion of electrokinetic experimentation. The results show that SFE is safer than acid digestion. SFE requires

supercritical CO2 which don’t pose a serious health threat. Acid digestion requires the use of high concentrations of

nitric acid, hydraulic acid, sulfuric acid and 30% of hydrogen peroxide. These chemicals are highly reactive. SFE is

0

50

100

Pb Ni Ca k

recovery %

metal


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faster, each SFE sample can be obtained in 15 -30 minutes, while acid digestion samples are typically obtained in 3-

6 hours. Also, the results show that SFE technique with the use of EDTA has higher extraction efficiency than acid

digestion for all metals tested.

The optimum and most practical shaking time for the extraction of lead, Nickel, calcium and Potassium was two

hours. Increasing the shaking time to four hours or eight hours has slight effect on the extraction efficiency. Also the

results show that SFE technique is more safe and efficient than acid digestion and it can be applied to any porous

medium. Atomic absorption spectrometry was used to analyze the concentration of the collect extract in 10 ml of

distilled water.

5. References

[1] Merian, P.D., Metals and their Compounds in the Environment: Occurrence, Analysis and Biological

Relevance, VCH Publishing, New York, 1438 pages, 1991.

[2] Harter, D.R., “Effect of soil pH on Adsorption of Lead, Copper, Zinc, and Nickel”, Soil Science Society

American Journal, Vol. 47, pp. 47-52, 1983

[3] Yong, R. N., A. M. O. Mohamed, B. P. Warkentin, 1992, Principles of Contaminant Transport in Soils,

Elsevier Publishing, New York, 327 pages, 1992.

[4] Acar, Y. B., A.N. Alshawabkeh, Electrokinetic Remediation I: Pilot-Scale Tests with Lead-Spiked

Kaolinite, Journal of Geotechnical Engineering, Vol. 122, No. 3, pp. 173, 1996.

[5] Ferguson, J. E., The heavy Elements: chemistry, Environmental Impact, and Health Effects, Pergamon

Press, New York, 614 pages, 1990.

[6] Hamed, J., Y. B. Acar, R. J. Gale, Pb(II) Removal from Kaolinite by Electrokinetics, Journal of

Geotechnical Engineering, Vol. 117, No. 2, pp. 214-267, 1991.

[7] Laintz, K. E., C. M. Wai, Extraction of Metal Ions from Liquid and Solid Materials by Supercritical Carbon

Dioxide”. Analytical Chemistry, Vol. 64, No. 22, pp. 2875-2878, 1992

[8] Lin, Y, R. D. Brauer, K. E. Laintz, C. M. Wai, supercritical Fluid extraction of Lanthanides and Actinides

from Solid Materials with A Fluorinated ß-Diketone, Analytical Chemistry, Vol. 65, No. 18, pp. 2549-2551,

1993

[9] Albert T. Yeung, Cheng-non Hsu, Rajendra M. Menon, Physicochemical soil-contaminant interactions

during electrokinetic extraction, Journal of Hazardous Materials, Vol. 55, pages 221-237, 1997.

[10] EPA, Acid Digestion of Sediments, Sludge, and Soil, Method 3050B, American Environmental Protection

Agency, 1996.

[11] Yeung, A. T., R. M. Menon, 1996, “EDTA-Enhanced Electrokinetic Extraction of Lead”, Journal of

Geotechnical Engineering, Vol. 122, No. 8, pp. 666-673.

[12] Wang, J., W. D. Marshall, 1994, “Metal Speciation by Supercritical Fluid Extraction with On-Line

Detection by Atomic Absorption Spectrometry”, Analytical Chemistry, Vol. 66, No. 22, pp. 3900-3907

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