Catalysis of manganese(II/III) on chromium(VI) reduction by oxalic acid
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Hexavalent chromium (Cr(VI)) compounds are important raw materials in the modern industry but it is also known that chromate chemicals can have severe negative health effects to human. Previous studies have shown that oxalic acid could reduce chromate to less toxic trivalent species in the presence of divalent manganese; however, the mechanism of this process is poorly documented. In this study, we reported the rates of Cr(VI) reduction under various concentrations of oxalic acid and Mn(II), initial pH, and light irradiation. A very low concentration (5micro-mol/L) of Mn(II) was observed to be able to catalyze the reduction of Cr(VI) by oxalic acid. LC-MS analysis indicated the formation of ([Mn(C2O4)2]2-) complex and chromium hydroxide during the reaction. Speciation calculations using the conditions relevant to the experimental system suggested that the formation of the oxalate complexes could lower the reduction potential of Mn(III/II) half reaction to below the Cr(VI/III) half reaction, which enabled the Cr(VI) reduction by Mn(II) with concurrent formation of Mn(III). Subsequent reduction of Mn(III) by oxalic acid regenerate Mn(II). Initial experimental results also suggested the importance of Mn(II)-oxalate complexation kinetics to the overall Cr(VI) reduction rate in the system.