Performance and mechanism of simultaneous removal of chromium and arsenate by Fe(II) from contaminated groundwater

Abstract

The feasibility and the mechanisms of simultaneous removal of chromium and arsenate by Fe(II) were investigated. In the absence of arsenate, chromium removal by Fe(II) increased to the maximum with increasing pH from ∼4 to ∼7 and then decreased with further increase in pH. Chromium removal by Fe(II) was controlled by the rate of Cr(VI) reduction by Fe(II) and the solubility of Fe 0.75Cr 0.25(OH) 3 at pH ≤ 7, but by the extent of Cr(VI) reduction under alkaline conditions. The presence of arsenate resulted in a decrease in chromium removal by Fe(II) under neutral and alkaline conditions as a result of the depression in the magnitude of Cr(VI) reduction by Fe(II) and sequestration of the Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitation by HAsO 4 2 - . Arsenate removal by Fe(II) alone was trivial but was improved significantly at pH 4–9 due to the presence of 10–30 μmol L −1 chromate. It was the oxidative property of chromate that resulted in the oxidization of Fe(II) to Fe(III) concomitantly facilitating the removal of arsenate. Arsenate was removed by both adsorption and co-precipitation with Fe 0.75Cr 0.25(OH) 3 and FeOOH precipitates. EXAFS results revealed that arsenate mainly coordinated with Fe(III) rather than Cr(III) and arsenate formed bidentate-binuclear complexes with FeO(OH) octahydra as evidenced by an average Fe–As(V) bond distance of 3.25–3.26 Å.