Removal of arsenic from groundwater by zerovalent iron and the role of sulfide

Several recent investigations have shown encouraging potential for the removal of arsenic (As) from groundwater by granular zerovalent iron (Fe-0). In contrast to previous studies conducted, we have investigated the applicability of this method and the nature of As bonding under conditions with dissolved sulfide. Three column tests were performed over the period of 1 year using solutions with either As(V) or As(III) (2-200 mg/L) in the input solution. Arsenic outflow concentrations decreased from initially 30-100 mu g/L to concentrations of below 1 mu g/L with time. XANES (X-ray absorptions near edge structure) and EXAFS (expanded X-ray absorption fine structure) spectra indicated that As in the solid phase is not only directly coordinated with oxygen, as is the case in adsorbed or coprecipitated arsenite and arsenate. Samples with high sulfur content showed additional bonding, for which Fourier transformations of EXAFS data exhibited a peak between 2.2 and 2.4 angstrom. This bonding most likely originated from the direct coordination of sulfur or iron with As, which was incorporated in iron sulfides or from adsorbed thioarsenites. The formation of this sulfide bonding supports the removal of As by Fe-0 because sulfide production by microbial sulfate reduction is ubiquitous in permeable reactive barriers composed of Fe-0.