Influence of the Anions and Oxalic Acid on the Electrochemical Reduction of Cr(VI) on Au and Magnetite Surfaces

Hexavalent chromium, Cr(VI), is a highly toxic carcinogen occurring in natural and industrial environments. Pathways to economical reduction to the more benign trivalent form, Cr(III), are necessary for treatment of contaminated groundwater. Magnetite’s (Fe 3 O 4 ) mixture of Fe(II) and Fe(III) make it a promising material for remediation. This study investigated the mechanisms for reduction of Cr(VI) catalyzed by Fe 3 O 4 as a redox mediator in the presence of oxalic acid in HClO 4 and SO 4 2− solutions, a system where the interactions among these species are not fully understood. The reduction of Cr(VI) in different anion environments is first measured on an Au rotating disk electrode. SO 4 2− inhibits the formation of a passivation layer and Cl - partially inhibits passivation. The reduction of Cr(VI) on Fe 3 O 4 is limited by the availability of Fe(II) surface sites. Addition of oxalic acid works synergistically through liberation of Fe(II)-oxalate and soluble Cr(III)-oxalate products. A combination of Fe 3 O 4 activated by exposure to oxalic acid and use of an oxalic acid solution as a medium for reduction of Cr(VI) produces over 97% removal of Cr(VI). These results provide relevant insights regarding interactions of Fe 3 O 4 with organic acids and the anion environment which lead to the effective reduction of Cr(VI).