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Sequential Extraction Method for Speciation of Arsenate and Arsenite in Mineral Soils Jen-How Huang* and Ruben Kretzschmar Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland A novel sequential extraction method for the speciation of AsIII and AsV in oxic and anoxic mineral soils was developed and tested. The procedure consists of seven extraction steps targeting various As pools ranging from weakly adsorbed to well-crystalline species. Each step was specifically designed to preserve the AsIII and AsV redox states, e.g., by complexation of AsIII with diethyldithiocarbamate or pyrrolidinedithiocarbamate, using mild reductive (NH2OH · HCl) or oxidative (hot HNO3) extractions, and complexing (Fe3+ with Cl-, acetate, and oxalate) or precipitating (S2- with Hg2+) matrix elements, which may cause As redox transformations. Using high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) for the quantification of dissolved AsIII and AsV in the extracts, the detection limit for each step was in the range of 1.0-75 ng As/g, depending on the extraction matrix. Thus, the procedure is also wellsuited for As speciation in soils or sediments with low As concentrations, where analyses by X-ray absorption spectroscopy (XAS) may be difficult. The entire extraction sequence can be performed under normal atmosphere, which greatly simplifies sample handling. The proposed method was tested using model minerals spiked with AsIII or AsV, two strongly As-polluted soil previously characterized for As speciation by XAS, and three less-polluted soils. Arsenic (As) is a toxic trace element that is naturally present in all terrestrial and aquatic environments, but it is also an important environmental pollutant. There is increasing evidence of cancer risk associated with chronic exposure to low levels of arsenic, e.g., through drinking water. Therefore, elevated As concentrations in groundwater and soils represent a major environmental and health problem for millions of people worldwide, including Bangladesh, China, and the United States.1 Determination of the total As concentrations in soils and sediments does not provide any information regarding its chemical form, potential mobility, and bioavailability.2 Depending on the redox conditions, arsenate (AsV) or arsenite (AsIII) predominate in the environment. The distribution of AsV and AsIII is * Author to whom correspondence should be addressed. Tel.: +41 44 632 88 19. Fax: +41 44 633 11 18. E-mail:
[email protected]. (1) Smedley, P. L.; Kinniburgh, D. G. Appl. Geochem. 2002, 17, 517–568. (2) Silveira, M. L.; Alleoni, L. R. F.; O’Connor, G. A.; Chang, A. C. Chemosphere 2006, 64, 1929–1938.
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especially relevant, because of the elevated toxicity and mobility of AsIII, compared to AsV, under most environmental conditions.3 Organic As species found in soils are usually less toxic and also less abundant than inorganic As species.3 X-ray absorption spectroscopy (XAS) can provide detailed information on the speciation and coordination environment of As in soils, provided that the concentrations are sufficiently high.4,5 However, in many soils and sediments, total As concentrations are too low for high-quality XAS measurements. Sequential extraction methods are frequently used to assess operationally defined pools of trace elements, characterizing their lability on the basis of the dissolution behavior of the target binding phases in soils and sediments.4 Sequential extraction methods can be performed in most chemical laboratories and have sufficient sensitivity at low As concentrations.4,6,7 Published sequential extraction methods quantify only total As in various fractions.7 Redox speciation of As during sequential extraction was not possible with these procedures, since they included strongly reducing or oxidizing reagents, which change the redox state of As. In addition, redox transformations of As during extraction may be caused by exposure to air and matrix elements released, e.g., Fe3+ 8 and S2-.9 Therefore, a new sequential extraction method that also allows redox speciation of As is needed. Here, we present the first attempt to develop a sequential extraction method specifically for the speciation of AsIII and AsV in soils, sediments, or aquifer materials. The extraction steps were either newly developed or adapted from previous studies, but modified to preserve AsIII and AsV during extraction. The proposed extraction sequence can be performed under normal atmosphere and is suitable for polluted and unpolluted mineral soils. The procedure was validated using model minerals spiked with AsIII and/or AsV, two strongly As-polluted soils that were previously characterized for As speciation by XAS, and three less-polluted soils. (3) Cullen, W. R.; Reimer, K. J. Chem. Rev. 1989, 89, 713–764. (4) Keon, N. E.; Swartz, C. H.; Brabander, D. J.; Harvey, C.; Hemond, H. F. Environ. Sci. Technol. 2001, 35, 2778–2784. (5) Voegelin, A.; Weber, F.-A.; Kretzschmar, R. Geochim. Cosmochim. Acta 2007, 71, 5804–5820. (6) Wenzel, W. W.; Kirchbaumer, N.; Prohaska, T.; Stingeder, G.; Lombi, E.; Adriano, D. C. Anal. Chim. Acta 2001, 436, 309–323. (7) Hudson-Edwards, K. A.; Houghton, S. L.; Osborn, A. Trends Anal. Chem. 2004, 23, 745–751. (8) Oliveira, V.; Sarmiento, A. M.; Go´mez-Ariza, J. L.; Nieto, J. M.; Sa´nchezRodas, D. Talanta 2006, 69, 1182–1189. (9) Rochette, E. A.; Bostick, E. C.; Li, G.; Fendorf, S. Environ. Sci. Technol. 2000, 34, 4717–4720. 10.1021/ac100415b 2010 American Chemical Society Published on Web 06/04/2010
EXPERIMENTAL SECTION Chemicals and Reference Materials. The following mineral phases were used for testing AsIII and AsV stability in each of the proposed extraction steps: arsenic sulfide (As2S3; Alfa Aesar, Karlsruhe, Germany), ferrous sulfide (FeS; Fluka, Buchs, Switzerland), manganese oxide (MnO2, pyrolusite) and aluminum hydroxide (Al(OH)3) (both from Merck, Darmstadt, Germany), boehmite (γ-AlOOH; Sasol, Houston, TX), freshly prepared 2-line ferrihydrite (2-LF), goethite (R-FeOOH), and birnessite (Na0.25MnO2.07 · 0.66H2O) with and without AsIII or AsV coprecipitation (see the Supporting Information for details). Based on powder X-ray diffraction (XRD) analysis (Bruker Model AXS D4 Endeavor, Karlsruhe, Germany), 2-LF, As2S3, and γ-AlOOH were poorly crystalline phases and FeS, MnO2, Al(OH)3, and R-FeOOH were well-crystalline phases. Sodium diethyldithiocarbamate trihydrate (NaDDTC; Merck, Darmstadt, Germany) and pyrrolidinedithiocarbamate ammonium salt (APDC; Fluka, Buchs, Switzerland) were used as complexing agents for AsIII. The proposed sequential extraction method was tested on four different soil materials. A permanently anoxic wetland soil (Fibric Histosol, mineral subsoil, 50-60 cm) and a well-drained forest soil (Haplic Podzol, subsoil, 12-30 cm) were sampled in the Lehstenbach catchment in northeast Bavaria, Germany.10 A paddy soil (Hydragric Anthrosol, surface soil, 30-35 cm) irrigated for more than 10 years with As-rich groundwater was sampled after rice harvest in the Munshiganj district, which is ∼30 km south of Dhaka, Bangladesh.11 A strongly contaminated soil (Gleyic Fluvisol, 0-15 and 20-40 cm) was sampled in the floodplain of the Mulde River in southeast Saxony-Anhalt, Germany, during a nonflooded period. The contamination at this site is mainly a result of past mining activities ∼150 km upstream in the Erzgebirge (Ore Mountains), where sulfide ores have been mined for centuries for silver, lead, cadmium, zinc, and other elements. The floodplain soil was air-dried and stored at room temperature. The other soil samples were freeze-dried and stored at -20 °C. In addition, airdried floodplain soil (0-15 cm) was submerged with synthetic river water (0.6 mM CaSO4, 0.6 mM NaCl, and 0.3 mM Mg(NO3)2) in microcosms, following the procedure of Weber et al. and incubated for 28 days at 23 °C in the dark.12 The soil was then freeze-dried and stored at -20 °C under N2 atmosphere before analysis.12 For analysis, all soil samples were passed through a 2-mm sieve and subsequently homogenized and ground to a size of
