Mobilization of arsenite by dissimilatory reduction of adsorbed arsenate

Sulfurospirillum barnesii is capable of anaerobic growth using ferric iron or arsenate as electron accepters. Cell suspensions of S. barnesii were abl e to reduce arsenate to arsenite when the former oxyanion was dissolved in solution, or when it was adsorbed onto the surface of ferrihydrite, a commo n soil mineral, by a variety of mechanisms (e.g., coprecipitation, presorpt ion). Reduction of Fe(lll) in ferrihydrite to soluble Fe(ll) also occurred, but dissolution of ferrihydrite was not required in order for adsorbed ars enate reduction to be achieved. This was illustrated by bacterial reduction of arsenate coprecipitated with aluminum hydroxide, a mineral that does no t undergo reductive dissolution, The rate of arsenate reduction was influen ced by the method in which arsenate became associated with the mineral phas es and may have been strongly coupled with arsenate desorption rates. The e xtent of release of arsenite into solution was governed by adsorption of ar senite onto the ferrihydrite or alumina phases. The results of these experi ments have interpretive significance to the mobilization of arsenic in larg e alluvial aquifers, such as those of the Ganges in India and Bangladesh, a nd in the hyporheic zones of contaminated streams.