Microbial reduction of arsenate in the presence of ferrihydrite

Increased mobilization of As under anaerobic conditions is of great concern in As contaminated soils and sediments. The identification of important release mechanisms may assist in designing safe and effective remediation strategies. In this study we investigated the effect of microbial reduction of aqueous arsenate (As(V)) on the solubilization of As(V) sorbed to ferrihydrite, in the absence of reductive dissolution of the Fe(III)-oxide solid phase. The addition of 0.1, 1.0, and 5.0 mM As(V) to serum bottles containing 10 mmol L^-1 Fe(III) as ferrihydrite resulted in the sorption of 98, 75, and 20% of the applied As(V), respectively. Inoculation with an As(V) reducing, glucose fermenting microorganism (CN8) was followed by complete reduction of aqueous As(V) to As(III) at nontoxic As concentrations (up to 1.0 mM), but no reduction or dissolution of the Fe(III) solid phase was observed. Despite rapid reduction of aqueous As(V) to As(III), sorbed phase As remained primarily as As(V), and desorption of As(V) was too slow to cause a significant increase in aqueous As concentration over the 24-day experiment. Our study suggests that the reduction of aqueous As(V) may play a relatively minor role in the solubilization of As(V) sorbed to Fe (hydr)oxide. Arsenic release from contaminated soils and sediments may proceed considerably faster under conditions favoring dissimilatory Fe(III) reduction leading to the dissolution of sorbing phases.