Arsenic release and colloidal species characterization during microbial reduction of As-containing ferrihydrite

Ferrihydrite is a common iron hydroxide in mining environments and arsenic has high adsorption affinity with it. Biological anaerobic reduction of arsenical ferrihydrite represents an aqueous arsenic and colloids release process. Arsenical ferrihydrite was collected from Jiepaiyu As-(Au) Mine area and used to investigate aqueous arsenic (III and V) and the relationship with colloid fractions (Large: <10 μm, medium: <1 μm and small: <10 nm) under sulfate-reducing bacteria (Desulfovibrio vlugaris miyazaki). Different arsenical ferrihydrite solid to solution ratios were investigated by batch experiment. In anaerobic conditions, the highest concentrations of aqueous arsenic(III) and arsenic(V) released into solution per gram arsenical ferrihydrite were on days 2 and 3 for low ratio (9449 μg/L and 8035 μg/L, respectively) and high ratio conditions (13672 μg/L and 7760 μg/L, respectively). Arsenic(III) concentrations with colloids varies the same as for aqueous arsenic(III), increasing from 0 day to 3 days and then decreasing. These colloids contain total iron of 7–563 mg/L, arsenic(III) of 84–3534 μg/L and arsenic(V) of 5082–14520 μg/L. Ferrous (Fe2+) and sulfur (S2-) components are found in the colloids, which their mole ratio is 0.8–8.7. There is considerable difference among mobilization of various colloids. The transportation velocities of these colloids follow this sequence: large > medium > small. These results suggest that biological reduction changes arsenic species and colloid fractions, which affect geochemical mobilization of arsenic.