Effect of arsenic concentration on microbial iron reductionand arsenic speciation in an iron-rich freshwater sediment

摘要

Depth profiles in the sediment porewaters of the Chattahoochee River (Georgia, USA) show that iron oxides scavengearsenate in the water column and settle to the sediment—water interface (SWI) where they are reduced by iron-reducing bac-teria. During their reduction, these particles seem to release arsenic to the porewaters in the form of arsenate only. Sedimentslurry incubations were conducted to determine the effect of low concentrations of arsenic (iu10μM) on biogeochemical pro-cesses in these sediments. Experiments confirm that any arsenate (As(V)) added to these sediments is immediately adsorbed inoxic conditions and released in anoxic conditions during the microbial reduction of authigenic iron oxides. Incubations in thepresence ofiu 1 μMAs(V) reveal that arsenate is released but not concomitantly reduced during this process. Simultaneously,microbial iron reduction is enhanced significantly, spurring the simultaneous release of arsenate into porewaters and second- ary formation of crystalline iron oxides. Above 1 As(V), however, the microbial reductive dissolution of iron oxidesappears inhibited by arsenate, and arsenite is produced in excess in the porewaters. These incubations show that even lowinputs of arsenic to riverine sediments may affect microbial processes, the stability of iron oxides and, indirectly, the cyclingof arsenic. Possible mechanisms for such effects on iron reduction are proposed.