Sequential extraction method for speciation of arsenate and arsenite in mineral soils

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 well-suited 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.