Feature Article

Speciation of Arsenic(V) and Arsenic(III) in Soil Extracts byCathodic Stripping VoltammetryJoanna Kowalska,* Katarzyna Cha¯ko, Ewa Stryjewska

Warsaw University, Chemistry Department, 02-093 Warsaw, Pasteura St.l, Poland; e-mail:askow@chem.uw.edu.pl

Received: November 1, 2001Final version: February 15, 2002

AbstractCathodic stripping voltammetry with a hanging mercury drop electrode was applied for the determination of As(III)and As(V) in soil extracts. The content of inorganic arsenic(III) and arsenic(V) compounds was determined afterchemical reduction of arsenic(V) by KI and ascorbic acid solution, while arsenic(III) was determined in the solutionwithout the reduction. Organic compounds were determined after UV irradiation of the samples. For arseniccompounds extraction three different extractants were tested: water/methanol, ortophosphoric acid and mixture ofHCl/HNO3. As different arsenic forms have different toxicity, the ability to discriminate between them is a distinctadvantage of the proposed method.

Keywords: Arsenic, Speciation, Voltammetry, Extraction, Soils

1. lntroduction

Arsenic may accumulate in soils due to the use of arsenicpesticides, application of fertilizers, irrigation, oxidation ofvolatile arsine in air, dust from the burning of fossil fuels anddisposal of industrial, municipal and animal wastes. In soilsarsenic may exist in different chemical forms such asarsenite, arsenate or organometalic compounds: dimethy-larsonic acid and monomethylarsonic acid. In case of arsenicthe toxicity strongly depends upon the chemical form inwhich this element is present in the environment. So toassess environmental impact and health risk of this elementchemical speciation is necessary.

During the past few years different analytical techniques,such as the combination of high-performance liquidchromatography with on-line detection by inductivelycoupled plasma mass spectrometry [1 ± 4] or hydridegeneration atomic absorption spectrometry [5] have beendeveloped for arsenic speciation. Henze et al. [6] describedthe cathodic stripping voltammetry with D-mannitol forAs(III) and As(V) speciation in fresh water samples.

To determine different arsenic species in solid environ-mental samples a very careful sample pretreatment isrequired. For this purpose solid-liquid extractions bymethanol/water (1 :1) [1, 7 ± 10], methanol/chloroform(2 : 1) [2, 8], methanol/water (9 : 1) [3, 11], methanol/water(1 : 9) [12] were applied.

In this article extraction of arsenic compounds from soilsamples by three different extractants (HCl/HNO3, H3PO4,methanol/water) is described. The extraction efficiency andthe suitability of applying investigated extractants forspeciation analysis is discussed. For arsenic determinationin soil extracts cathodic stripping voltammetry on hangingmercury drop electrode according to the procedure descri-

bed in our previous article [13] was used. The proposedmethod lets one to discriminate between inorganic As(III)and As(V) compounds. Additional UV irradiation of soilextracts makes determination of whole extractable arsenic-compounds possible.

2. Experimental

2.1. Apparatus

Princeton Applied Research 264 A Polarographic Analyzerequipped with an X ± Y recorder (RE 0150 PAR) and anelectrode system: HMDE as a working electrode, saturatedAg/AgCI as a reference electrode and the Pt-wire as anauxiliary electrode.

Ultrasonic bath: Trassonic T-570 ELMA (Poland); cen-trifuge: MPW-340 (Poland); UV digestion system: R-6(250 W) Mineral (Poland).

2.2. Reagents

HNO3 (d� 1.40 g mL�1), HCI (d� 1.15 g mL�1), H2SO4

(d� 1.84 g mL�1), Suprapur (Merck); Standard solutionsof As(III), As(V), Cu(II) containing 1 mg mL�1 wereprepared from ampules Titrisol (Merck);

KI, ascorbic acid, H2O2, methanol, H3PO4 (d�1.70 gmL�1) were of analytical grade (POCh Gliwice, Poland).

All solutions were prepared using deionized water fromMilli-Q-Water-System, Millipore, (USA).

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2.3. Sample and Sample Pretreatment

Two soil samples were collected and prepared (air-dried andsieved through a 1-mm sieve) for the analysis according tothe procedure outlined in our previous article [13].

2.4. Extraction Procedure

The arsenic compounds were extracted from soil samplesusing three different extractant: HCl/HNO3 (2 :1), 0.3 molL�1 H3PO4 (pH 1.3) and methanol/water (1 :1).

1 g of soil sample was weighted into polyethylene tube,10 mL of extractant solution was added and sample wassonicated for 12 minutes. Then centrifugation at 4000 rpmduring 15 min was carried out. The extraction of solidresidue was repeated twice. The three extracts, afterfiltration through paper filter under low pressure, werecombined in the polyethylene bottles. For voltammetricdetermination about 0.2 ± 1 mL of the solution was taken.

2.5. Voltammetric Determinations

Arsenic was determined as As(III) by cathodic strippingvoltammetry on hanging mercury drop electrode under thecondition established in our previous experiments [13 ± 15].For the described method the determination limit calculatedfor soil samples was 0.6 �g g�1.

To determine inorganic compounds of As(III) and As(V)an aliquot of the sample solution was pipetted into avoltammetric cell where 200 �L of H2SO4 (conc.) wereadded. It was heated to the temperature reaching about150�C to remove nitric acid. After cooling down the residuewas completed with water to about 800 �L then 200 �L ofthe solution containing 0.6 mol L�1 potassium iodide and0.6 mol L�1ascorbic acid were added and the mixture wasleft for 10 min at ambient temperature. Then 8 mL of

water, 50 �L of 1 g mL�1 copper(II) solution and 1 mL ofHCI (conc.) were added. The solution was purged withargon gas for 10 min. The preconcentration of arsenic asintermetallic compound As-Cu was carried out at thepotential of �0.45 V in a stirred solution for 120 s. Afterthe resting time of 15 s the voltammetric curves of cathodicreduction of arsenic were recorded in the potential range of�0.45 to �0.90 V using differential pulse technique withscan rate 10 mV S�1 and amplitude 50 mV. For quantitativedetermination the double standard addition method wasused.

To determined inorganic As(III) compounds the chemicalreduction by KI and ascorbic acid was omitted.

Before determination of total extractable arsenic com-pounds in soil extracts the samples with addition of H2O2

were UV irradiated. In both cases contents of inorganicAs(V) and organic arsenic compounds were calculated bysubtraction.

3. Results and Discussions

To check the usefulness of CSV for speciation analysis ofinorganic As(III) and As(V) experiments with standardsolution were carried out. Obtained results (Table 1)confirm the possibility of inorganic As(III) determinationin the presence of inorganic As(V) and total inorganic Asdetermination after chemical reduction with KI andascorbic acid solution. Stability of inorganic arsenic speciesunder the extraction condition was also examined.

For this purpose known amounts of As(III) and As(V)standard solutions were added into extractants. Then theextraction procedure and electrochemical determinationwere carried out. Obtained results (Table 2) illustrate, thatduring extraction with methanol/water and phosphoric acidAs(III) is not oxidized to As(V). So applying those twosolutions for extraction step and determining As by CSV letone to use the proposed procedure for discriminationbetween inorganic As(III) and As(V) compounds.

During experiments two soil samples were investigated.The total arsenic content in soils was determined aftermicrowave digestion of the samples. About 250 mg of soilsamples was digested in CEM Teflon vessels (13.8 bar) withmixture of: 2 mL HNO3, 1.5 mL HClO4 with or without0.2 mL HF). Heating of the samples was carried out inseveral cycles increasing time and power of microwaveenergy (2 min. 255 W, 4 min. 425 W, 8 min. 595 W, 4 min.425 W, 2 min. 850 W, 4 min. 425 W, 8 min. 595 W, 4 min.

Table 1. Results of As determination in standard solution withand without chemical reduction. Results are expressed as a mean� RSD for n�6, ��0.5.

Added amount(ng)

Chemicalreduction

Found(ng)

Recovery(%)

20.0 As(III)20.0 As(III), 20.0 As(V)20.0 As(III) 20.0 As(V)

���

19.9� 1.120.4� 0.738.5� 1.2

99.510296

Table 2. The stability studies of inorganic As(III) during the extraction procedure with methanol/water (1 : 1) and 0.3 mol L�1

phosphoric acid, (without chemical reduction). Results are expressed as a mean � RSD for n�6, �� 0.5.

Added amount (ng) Methanol/water 0.3 mol L�1 H3PO4 (pH 1.3)

Found (ng) Recovery (%) Found (ng) Recovery (%)

20.0 As(III)20.0 As(III)20.0 As(V)

19.5� 0.820.0� 0.7

98100

20.7� 0.919.8� 0.4

10399

1509Speciation of Arsenic(V) and Arsenic(III) in Soil Extracts

Electroanalysis 2002, 14, No. 21

425 W, 2 min. 850 W, 2 min. 255 W). Arsenic content wasdetermined by CSV and GF AAS and the results werepublished in the previous work [13].

For arsenic extraction from soils besides methanol/waterand phosphoric acid also mixture of HCl/HNO3 was appliedalthough in this case differentiation between inorganicarsenic species is not possible owing to the oxidation ofinorganic As(III) to As(V). Nevertheless extraction withHCl/HNO3 gives the highest efficiency and allows, accord-ing to the literature data [16], differentiation betweenorganic and inorganic arsenic species.

Same authors suggest that the extraction with HCl/HNO3

can be applied instead of total digestion of soils [17 ± 18].Since under conditions of voltammetric determination

only inorganic As(III) is electrochemically active theexperiments were carried out in a few steps:

1) in an aliquot of extract solution without chemicalreduction inorganic As(III) was determined;

2) in an aliquot of extract solution reduction by KI andascorbic acid was done and then both inorganic As(III)and As(V) were determined;

3) an aliquot of extract solution with addition of H2O2

(10 � L H2O2 for 10 mL solution) was UV irradiated for3 hours, then reduction by KI and ascorbic acid wasdone and all extractable arsenic compounds ± organicand inorganic ± were determined.

Obtained results are presented in Table 3.The most often recommended in the literature for

speciation analysis extractant ± methanol/water ± was notsuitable for arsenic compounds extraction from soils. Theextraction efficiency was extremely low ± below 1%.Moreover voltammetric determinations were possible justafter additional UV irradiation. In the other case the matrixinfluences were observed, background current was high andany voltammetric peak corresponding to arsenic reductionwas recorded. Orthophosphoric acid seems to be useful forspeciation analysis. First of all it gives possibility ofextraction of inorganic As(III) and As(V) species withoutchanging its oxidizing stage. The voltammetric determina-tion of As could be carried out in the extract solutionwithout UV irradiation, so the discrimination between

inorganic and organic arsenic compounds is possible. Theefficiency of the extraction above 60% for soil 1 and 25% forsoil B was achieved, which are considerably higher thanarsenic bioavailability from the investigated soils (25% and15% respectively [13]). Bioavailable arsenic contents in thesoils were determined after the extraction with 0.43 mol L�1

CH3COOH according to the BCR procedure [19 ± 20].The highest extraction yields were achieved when HCl/

HNO3 mixture was applied (nearly 100% for soil 1 and 50%for soil 2). But as it was demonstrated such extraction couldnot always be used instead of total digestion of soil samples.Nevertheless the obtained data can be sufficient forestimation of the potential hazardous influence of arseniccompounds for the environment. The results of all experi-ments showed that in any case contents of As in extractsobtained after UV irradiation were not higher then inextacts without irradiation. This indicates that arsenic instudied soil extracts was present as inorganic species, mainlyas inorganic As(III), one of the most toxic arsenic form.

Described investigation confirmed that the proposedanalytical procedure ± extraction with H3PO4 and Asdetermination by CSV on HMDE - is a useful tool inspeciation analysis of this element and gives the possibilityto assess the potential danger of arsenic for environment.

4. References

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1995, 351, 420.[9] X. C. Le, M. Ma, N. A. Wong, Anal. Chem. 1996, 68, 4501.

Table 3. Results of determination of extractable arsenic compounds in soil extracts (�g g�1). Results are expressed as a mean � RSDfor n�6, �� 0.5.

Soil sample Extractant

HCl/HNO3 (2 : 1) H3PO4 Methanol/water (1 : 1)

Inorganic Ascompounds

Inorganic andorganic Ascompounds

InorganicAs(III)compounds

Inorganic Ascompounds

Inorganicand organic Ascompounds

Inorganic Ascompounds

Inorganicand organic Ascompounds

Soil 1total As content56 �g g�1

52.0� 5.7 50.3� 4.9 1.7� 0.3 35.0� 2.5 33.7� 2.4 ± 0.62� 0.04

Soil 2total As content235 �g g�1

119� 14 111� 9 4.2� 0.4 61.5� 11.7 65.3� 9.1 ± 0.90� 0.10

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Electroanalysis 2002, 14, No. 21

[10] Z. Slejkovec, J. T. van Elteren, A. R. Byrne, Talanta 1999, 49,619.

[11] Z. Slejkovec, J. T. van Elteren, A. R. Byrne, Anal. Chim.Acta 1998, 358, 51.

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2001, 13, 872.[14] J. Kowalska, J. Golimowski, Electroanalysis 1998, 10, 857.[15] J. Kowalska, E. Stryjewska, P. Szyman¬ ski, J. Golimowski,

Electroanalysis 1999, 11, 1301.

[16] C. Demesmay, M. Olle, Fresenius J. Anal. Chem. 1997, 357,1116.

[17] M. L. Berrow, W. M. Stein, Analyst 1983, 108, 277.[18] R. T. T. Rantala, D. H. Loring, Anal. Chim. Acta 1989, 220,

263.[19] P. Quevauviller, G. Rauret, B. Griepnik, Intern.J. Environ.

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Sanchez, H. Fiedler, H. Muntau, Mikrochim. Acta 1995, 120,289.

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