Rates of microbially mediated arsenate reduction and solubilization

Reduction of arsenate [As(V)] to arsenite[As(III)] influences the mobility and toxicity of arsenic (As), yet the mechanisms controlling the rate: of r eduction in soils and natural waters are poorly understood. The goal of thi s study was to determine processes affecting reduction rates of both aqueou s and sorbed phase As(V). Reduction experiments were conducted anaerobicall y in serum bottles with a range of glucose and As(V) concentrations. Serum bottles were inoculated with microorganisms extracted directly from an agri cultural soil having naturally elevated concentrations of As (unenriched po pulation), or with a pure culture isolate obtained from the same soil after enrichment for As(V) reduction. At As(V) concentrations ranging from 6 to 600 mu M, the rate of As(V) reduction by the soil isolate was first order w ith respect to both As(V) concentration and microbial biomass. Reduction ra tes of As(V) with the soil isolate were 2 to 10 fold greater than in the un enriched population, suggesting As(V) reducers represented only a subset of the unenriched population. Compiled data indicated that the pure culture i solate was fermenting glucose, and potentially reducing As(V) as a detoxifi cation mechanism. In a parallel study, reduction rates of As(V) with the un enriched population were evaluated in the presence of goethite or ferrihydr ite. When redox potential decreased from 500 to near 0 mV, aqueous As conce ntrations decreased by approximately 30% in a goethite suspension,vith a hi gh As surface coverage, Set increased by seven fold in a goethite suspensio n with a low As surface coverage. In a ferrihydrite suspension, aqueous As concentrations during reduction increased approximately 100 fold faster tha n in a goethite suspension at similar initial aqueous As(V) concentrations, corresponding to differences in Fe oxide surface areas and reductive disso lution rates. The results indicate that rates of As mobilization during red uction in soils are highly dependent on oxide surface area and As surface c overage.