Comparison of four extraction procedures to assess arsenate and arsenite species in contaminated soils

Inorganic arsenic in soils poses an important environmental concern. Several studies reported an oxidation of arsenite to arsenate during its extraction from soils. The objectives of this study were to (1) identify, among published procedures, an extraction method which preserves the oxidation state of arsenic and (2) to assess the influence of soil physicochemical properties on the performance of these methods. Four extraction strategies were compared: 1) 10 M HCl, 2) 15% (v/v) H3PO4, 3) 10 mM phosphate + 0.5% (w/v) NaDDC, and, 4) 1 M H3PO4 + 0.5 M ascorbic acid (C6H8O6). Separation and analysis of As species was performed by HPLC-ICP/MS. Oxidation of As(III) into As(V) during extraction was more important in soils with high content of Mn oxides. Extraction of arsenic from soils with 1 M H3PO4 + 0.5 M C6H8O6 under microwaves was the best strategy to extract the majority of As while minimizing conversion of As(III) into As(V).