Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part II: Evidence from sediment incubations

Freshly collected sediments from Bangladesh were incubated for two months to study the release of arsenic to groundwater in a low-oxygen environment. The aquifer sands, diluted to concentrations of 3 to 22 g/L with anoxic well water, covered a range of redox conditions, as indicated by gray to orange color. Suspensions amended with either acetate or a respiratory inhibitor were incubated in parallel with unamended suspensions. Two batches of unamended gray sediment gradually released the equivalent of 0.5 to 1.0 mug/g As to the dissolved phase in the presence of some dissolved oxygen (similar to1 mg/L) and without any detectable increase in dissolved Fe. The quantity of As released accounted for 30 to 80% of the initial As fraction that could be leached from the particles in hot acid. A suspension of gray sediment amended with fresh antibiotic did not release As. The addition of acetate to the gray sediment resulted in the release under anaerobic conditions of only 1 to 3% of the acid-leachable Fe fraction; the quantity of As released during these incubations was comparable to that released from the unamended suspensions. Amendment with acetate was required to release acid-leachable As from the suspensions containing some or mostly orange Fe(III) oxyhydroxides. These observations suggest that mobilizable As is distributed heterogeneously in the Fe phases that coat aquifer sands. Truly anoxic conditions may not be required for the release of As from reducing (gray) sediment in Bangladesh. Copyright (C) 2004 Elsevier Ltd