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ARSENIC SOLID-PHASE PARTITIONING IN REDUCING SEDIMENTS OF CONTAMINATED WETLAND
Citation:
Wilkin*, R T. AND R G. Ford*. ARSENIC SOLID-PHASE PARTITIONING IN REDUCING SEDIMENTS OF CONTAMINATED WETLAND. B. Bourdon, D. Dingwell, J. Fein, S.L. Goldstein, D. Rickard, R.L. Rudnick (ed.), CHEMICAL GEOLOGY. Elsevier Science Ltd, New York, NY, 228(1-3):156-174, (2006).
Impact/Purpose:
to present information
Description:
The geochemical partitioning of arsenic in organic-rich sediments from a contaminated wetland is examined using X-ray absorption spectroscopy and selective chemical extraction procedures, and evaluated in context to the anoxic diagenesis of iron and sulfur. The interaction between ground water and surface water has a significant influence on iron sulfide formation in the wetland sediments. Ground-water seeps supply concentrations of sulfate, dissolved hydrocarbons, ferrous iron, and arsenic. Sediments located near ground-water seeps are anomalously enriched in arsenic, reactive iron, and acid-volatile sulfides, and degree-of-sulfidation (DOS) values are high, ranging from 0.57 to 1.0. Pyrite (Fe2) formation is not limited by the abundance of any one reactant, e.g., organic carbon, sulfate, or reactive iron; instead, persistence of precursor iron monosulfides is attributed to slow pyrite formation kinetics due to low concentrations of reactive intermediate sulfur species or possibly due to high concentrations of arsenite, dissolved organic-carbon, or other solutes that adsorb to iron monosulfides surfaces and impede transformation reactions to pyrite. Greigite (Fe3S4) accounts for >80% of total reduced sulfur in sediments rich in acid-volatile sulfide and X-ray absorption spectroscopy data for magnetic separates provide direct evidence that As(III) is, at least in part, associated with reduced sulfur in the form of greigite. However, pyrite can only account for a small percentage, <20%, of the total arsenic budget in the reduced sediments. Although pyrite is the predicted stable endpoint for reactive iron and sulfur, it appears that within a 30 y time period pyrite is a relatively unimportant host for arsenic in the system investigated here. The abundance of reactive iron in the sediments prevents accumulation of dissolved sulfide and thus prevents formation of soluble thioarsenic species. X-ray absorption near-edge structure (XANES) spectroscopy demonstrates only the presence of As(III) in the reduced sediments. The spectroscopic results are consistent with sulfur- and/or oxygen-coordinated As(III) in association with iron monosulfides, or other ferrous-bearing carbonates and hydroxides.
