Precipitation of arsenic under sulfate reducing conditions and subsequent leaching under aerobic conditions

The stability of As precipitates formed by microbial SO4 reduction was examined via leaching of precipitate-laden sediments with aerobic groundwater. Sediment and groundwater collected from an As-contaminated aquifer were used to construct the flow-through columns utilized to demonstrate precipitate formation and stability. For 4 months, indigenous SO4-reducing bacteria in these columns were stimulated with injections of lactate, ethanol, FeCl2, (NH4)3PO4 and SO 4 2 - . Analyses, including 35S autoradiography, sequential extraction, SEM and XANES, of one of the columns revealed elevated As and 35S sulfide concentrations, and microcrystalline overgrowths comprised of FeAsS, AsS, S and phosphates restricted to “black” zones that had formed within the sediment. Two columns were then treated with a H2O2/K2HPO4 solution in order to produce Fe phosphate coatings that would theoretically protect sulfides from subsequent oxidation. Sediment from an untreated and a treated column were also packed into smaller syringe columns to record As depletion during aerobic leaching. Aerobic, As-bearing natural groundwater and artificial, As-free groundwater were passed through both untreated and treated columns and syringe columns for 4 months during which the As release rate from the columns and the removal of As from the syringe columns was observed. The As release rate from the untreated column and the rate of As removal from the syringe columns of untreated sediment correlated well and both indicated overall leaching of ∼2% of the microbially precipitated As-bearing sulfide. The As release rate from the treated sediments was 3–4 times greater than that of the untreated sediments, contrary to expectations that treatment would minimize As release. Sequential extraction, SEM and XANES analyses of the untreated column after aerobic leaching revealed that a significant portion of the remaining As was arsenate associated with Fe3+ hydroxides. A reactive transport model, which incorporated the oxidative dissolution of Fe and As sulfides and the adsorption of arsenate to Fe3+ hydroxides, reproduced the timing of As and sulfate release from this column and indicated that the release rate was limited by the O2 flux and Fe sulfide abundance. Scaling down the laboratory flow rate to that of the groundwater at a local field site suggests that microbially precipitated FeAsS produced in this manner should be stable over a decadal time scale.