Electrochemical and spectroscopic study of arsenate removal from water using zero-valent iran media

This study investigated the mechanisms involved in removing arsenate from d rinking water supplies using zero-valent iron media. Batch experiments util izing iron wires suspended in anaerobic arsenate solutions were performed t o determine arsenate removal rates as a function of the arsenate solution c oncentration. Corrosion rates of the iron wires were determined as a functi on of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreas ed iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained aft er approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the a rsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly o xidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iran corrosion p roducts released at anodic sites on the iron surfaces. ion chromatography a nalyses indicated that there was no measurable reduction of As(V) to As(III ). X-ray absorption spectroscopy analyses indicated that all arsenic associ ated with the zero-valent iron surfaces was in the +5 oxidation state. Inte ratomic arsenic-iron distances determined from EXAFS analyses were consiste nt with bidentate corner-sharing among arsenate tetrahedra and iron octahed ra. Results from this study show that under conditions applicable to drinki ng water treatment, arsenate removal by zero-valent iron media involves sur face complexation only and does not involve reduction to metallic arsenic.