Contrasting effects of dissimilatory iron (III) and arsenic (V) reduction on arsenic retention and transport.

Reduction of arsenate As(V) and As-bearing Fe (hydr)- oxides have been proposed as dominant pathways of As release within soils and aquifers. Here we examine As elution from columns loaded with ferrihydrite-coated sand presorbed with As(V) or As(III) at circumneutral pH upon Fe and/or As reduction; biotic stimulated reduction is then compared to abiotic elution. Columns were inoculated with Shewanella putrefaciens strain CN-32 or Sulfurospirillum barnesii strain SES-3, organisms capable of As (V) and Fe (III) reduction, or Bacillus benzoevorans strain HT-1, an organism capable of As(V) but not Fe(III) reduction. On the basis of equal surface coverages, As(III) elution from abiotic columns exceeded As(V) elution by a factor of 2; thus, As(III) is more readily released from ferrihydrite under the imposed reaction conditions. Biologically mediated Asreduction induced by B. benzoevorans enhances the release of total As relative to As (V) under abiotic conditions. However, under Fe reducing conditions invoked by either S. barnesii or S. putrefaciens, approximately three times more As (V or III) was retained within column solids relative to the abiotic experiments, despite appreciable decreases in surface area due to biotransformation of solid phases. Enhanced As sequestration upon ferrihydrite reduction is consistent with adsorption or incorporation of As into biotransformed solids. Our observations indicate that As retention and release from Fe (hydr)oxide(s) is controlled by complex pathways of Fe biotransformation and that reductive dissolution of As-bearing ferrihydrite can promote As sequestration rather than desorption under conditions examined here.