Arsenic removal and activity of a sulfate reducing bacteria-enriched anaerobic sludge using zero valent iron as electron donor

•Sulfate reduction by anaerobic sludge was better in the presence of AsIII than AsV.•The co-existence of S, As and Fe was demonstrated in solids from assays with As.•Fe-As-S type minerals appear to be formed during As removal.•As declined to safe levels under sulfidogenic conditions with ZVI as electron donor.•As removal was mainly mediated by biological activity of SRB-enriched sludge. Arsenic (As) removal from water, subject to sulfate-reducing conditions has been shown to result in safe As levels. We evaluated sulfate-reducing activity and arsenic removal by an anaerobic sludge enriched with sulfate-reducing bacteria (SRB), using zero valent iron (ZVI) as electron donor and different concentrations of AsV or AsIII (up to 5 mg/L). Sulfate and As removal were monitored in aqueous samples of batch assays. Likewise, precipitates resulting from As removal were characterized in solids. Sulfate-reducing activity on the part of anaerobic sludge was slightly decreased by AsIII and it was 50% decreased, particularly at 5 mg/L AsV, for which arsenic removal equaled 98%. At all other As concentrations assayed, 100% As was removed. The co-existence of S, As and Fe in solids from assays with As, was demonstrated by scanning electron microscopy (SEM-EDS) and by micro-X-ray fluorescence, corroborating the possible formation of Fe-As-S type minerals for As precipitation. Pharmacosiderite and scorodite minerals were identified by micro-X-ray absorption near edge structure and confirmed by extended X-ray adsorption fine structure, and these were related to the oxidation of arsenopyrite during analysis. Results indicate the suitability of the anaerobic sludge for bioremediating arsenic-contaminated groundwater under sulfidogenic conditions with ZVI as electron donor.