Validation of an arsenic sequential extraction method for evaluating mobility in sediments

Arsenic (As) mobility and transport in the environment are strongly influen ced by arsenic's associations with solid phases in soil and sediment. We ha ve tested a sequential extraction procedure intended to differentiate the f ollowing pools of solid phase arsenic: loosely and strongly adsorbed As; As coprecipitated with metal oxides or amorphous monosulfides; As coprecipita ted with crystalline iron (oxyhydr)oxides; As oxides; As coprecipitated wit h pyrite; and As sulfides. Additions of As-bearing phases to wetland and ri verbed sediment subsamples were quantitatively recovered by the following e xtractants of the sequential extraction procedure: As adsorbed on goethite, 1 M NaH2PO4; arsenic trioxide (As2O3), 10 M HF; arsenopyrite (FeAsS), 16 N HNO3; amorphous As sulfide, 1 N HCl, 50 mM Ti-citrate-EDTA, and 16 N HNO3; and orpiment (As2S3), hot concentrated HNO3/H2O2 Wet sediment subsamples f rom both highly contaminated wetland peat and less As-rich sandy riverbed s ediment were used to test the extraction procedure for intra-method reprodu cibility. The proportional distribution of As among extractant pools was co nsistent for subsamples of the wetland and for subsamples of the riverbed s ediments. In addition, intermethod variability between the sequential extra ction procedure and a single-step hot concentrated HNO3/H2O2 acid digestion was investigated. The sum of the As recovered in the different extractant pools was not significantly different than results for the acid digestion.