HIGH ARSENIC ACCUMULATION IN IRON PHASES IN NEAR-SURFACE SEDIMENTS FROM EASTERN BANGLADESH

Although it is accepted that arsenic (As) in Bangladesh ground water is derived from natural As-bearing solid phases in sediments, little direct characterization of the structure and reactivity of these phases has been reported. Here we describe As accumulation in thin (3-5 cm), laterally extensive, reddish-orange, iron (Fe)-rich subhorizontal layers in shallow (< 3 meters depth) sediments east of the Meghna River. Measured bulk Fe contents of the layers range to 20 wt.% (5x background) with a maximum As enrichment of 800 ppm (80x background). Samples of Fe bands from two locations and an Fe-oxyhydroxide-rich sediment from a third were selected for microanalysis (SEM, EPMA). Spot analyses of Fe phases in two of these samples show little correlation between As and other analytes with the highest As enrichments on the order of 0.2 wt%. However, in one Fe band As is positively correlated with P2O5 and enrichments of up to 2.5 wt % As were measured in a matrix of ca. 20 wt% P2O5 and 65-70 wt% Fe2O3. The high P2O5 content suggests this is an Fe-phosphate phase rather than a phosphate-rich Fe oxyhydroxide. Bulk x-ray absorption fine structure (XAFS) spectroscopy at the As and Fe K edges indicates that 90-100% of the total As in the Fe-rich layers is present as As(V), and that > 80% of the total Fe is present as Fe(III). The Fe-enriched bands are thought to form because of a steep redox gradient between the ground surface and the saturated zone. Fe(II) in pore water is oxidized resulting in the accumulation of Fe oxyhydroxide that scavenges As. Presently the mechanism and timing of As incorporation in the putative Fe phosphate phase is unknown. XAFS spectroscopy verifies that the beige-colored sediments above the saturated zone contain predominantly As(V) and Fe(III). In the saturated zone, sediments are blue-gray and contain substantial amounts of As(III) and Fe(II) with total As concentrations similar to those in oxidized sediments. Treatment of the sediments with various extracts shows that proportionally less As is released from the Fe bands than from the oxidized sediments above and below. The low extractability of As in the Fe bands may be consistent with a low-solubility Fe phosphate. Our results suggest that both As-bearing Fe phosphate and Fe oxyhydroxide phases should be considered when evaluating subsurface As mobility.