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Mechanistic insight into chromium(VI) reduction by oxalic acid in the presence of manganese(II)

Wrobel, Katarzyna, Corrales Escobosa, Alma Rosa, Gonzalez Ibarra, Alan Alexander, Mendez Garcia, Manuel, Yanez Barrientos, Eunice, Wrobel, Kazimierz
Journal of hazardous materials 2015 v.300 pp. 144-152
acidification, atomic absorption spectrometry, chromium, high performance liquid chromatography, manganese, manganese sulfate, mass spectrometry, nitrogen, oxalic acid, pH, potassium, remediation
Over the past few decades, reduction of hexavalent chromium (Cr(VI)) has been studied in many physicochemical contexts. In this research, we reveal the mechanism underlying the favorable effect of Mn(II) observed during Cr(VI) reduction by oxalic acid using liquid chromatography with spectrophotometric diode array detector (HPLC–DAD), nitrogen microwave plasma atomic emission spectrometry (HPLC–MP-AES), and high resolution mass spectrometry (ESI–QTOFMS). Both reaction mixtures contained potassium dichromate (0.67mM Cr(VI)) and oxalic acid (13.3mM), pH 3, one reaction mixture contained manganese sulfate (0.33mM Mn(II)). In the absence of Mn(II) only trace amounts of reaction intermediates were generated, most likely in the following pathways: (1) Cr(VI)→Cr(IV) and (2) Cr(VI)+Cr(IV)→2Cr(V). In the presence of Mn(II), the active reducing species appeared to be Mn(II) bis-oxalato complex (J); the proposed reaction mechanism involves a one-electron transfer from J to any chromium compound containing CrO bond, which is reduced to CrOH, and the generation of Mn(III) bis-oxalato complex (K). Conversion of K to J was observed, confirming the catalytic role of Mn(II). Since no additional acidification was required, the results obtained in this study may be helpful in designing a new, environmentally friendly strategy for the remediation of environments contaminated with Cr(VI).