Investigation of the mechanisms controlling chromate and arsenate removal from water using zerovalent iron media

Abstract

This research investigated the mechanisms controlling chromate and arsenate removal by zerovalent iron media. The removal kinetics of aqueous Cr(VI) and As(V) were studied in batch experiments for initial concentrations ranging from 100 to 10,000 μg/L. Removal kinetics were also studied in columns packed with zerovalent iron filings over this same concentration range. Electrochemical analyses were used to investigate the electron transfer reactions occurring on the iron surface, and to determine the effect of chromate and arsenate on the iron corrosion behavior. The removal mechanism for chromate involved reduction to Cr(III) and the formation of hydroxide precipitates. Increasing chromate concentrations resulted in decreasing removal rates due to iron surface passivation. Even at low concentrations, chromate acts as a corrosion inhibitor and decreases iron corrosion rates. The condition of the iron surface prior to exposure to chromate determined the chromium removal kinetics. Air-formed oxides significantly inhibited chromate removal, whereas oxides formed in anaerobic, chromate-free water resulted in higher removal rates. Although direct reduction of chromate at cathodic sites on the iron surface was observed at early elapsed times, chromate removal eventually became limited by the rate at which Fe²⁺ could be generated at anodic sites. The removal mechanism for arsenate did not involve reduction and was due to the formation of inner-sphere, bidentate complexes with iron corrosion products. At low arsenate concentrations the rate of arsenate removal was limited by diffusion to adsorption sites. At high concentrations the rate of arsenate removal was limited by the rate of adsorption site generation resulting from iron corrosion. Adsorbed arsenate blocked electroactive sites on the iron surface and decreased iron corrosion rates. Arsenate is expected to remain as the principal adsorbed species in iron filter media because electrochemical reduction of As(V) to As(III) is not favorable under the conditions relevant to freely corroding iron.