IMPACT OF IRON OXIDE MINERALOGY ON ARSENIC MOBILITY IN THE PRESENCE OF BIOGENIC SULFIDE

Anaerobic microbial reduction of iron oxides is implicated among the causes of arsenic contamination in aquifers. Iron oxides can also be reductively dissolved by reaction with sulfide produced by sulfate-reducing bacteria (SRBs). The resulting ferrous iron can readsorb onto remaining oxides and catalyze transformations to more stable iron oxides or to magnetite, or it can precipitate with sulfide. The relative importance of these mineral transformations affects how arsenic partitions between aqueous species and mineral phases, both reduced and oxidized. Here we investigate the impact of initial iron oxide mineralogy on mineral transformations occurring during secondary reduction of iron by biogenic sulfide, and thus on arsenic mobility. We use both column experiments and incubations containing Desulfovibrio desulfuricans, a sulfate-reducing bacterium, with either arsenic-bearing ferrihydrite or goethite. The extent of arsenic release is a function of the kinetics of mineral dissolution and transformation, which is affected by the greater stability of goethite relative to ferrihydrite. Columns show relatively high arsenic mobility, with differing rates of sulfide reaction with the two oxide phases affecting steady-state sulfide activity, and thus the importance of sulfide mineral precipitation relative to oxide recrystallization. No incubation showed arsenic release into solution, although patterns of remineralization differed depending on initial mineralogy. The contrast between the static and flow systems confirms the kinetic control over the iron oxide-arsenic-SRB system. Furthermore, our results indicate that the mineralogical impact on reaction kinetics in systems of iron reduction by biogenic sulfide has implications for arsenic mobility during aging of iron-rich sediments.