title: Mechanistic insight into chromium(VI) reduction by oxalic acid in the presence of manganese(II).
abstract: 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 (0.67 mM Cr(VI)) and oxalic acid (13.3mM), pH 3, one reaction mixture contained manganese sulfate (0.33 mM 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).
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Wrobel
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico.
authors.forename: Alma Rosa
authors.initials: AR
authors.lastname: Corrales Escobosa
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico.
authors.forename: Alan Alexander
authors.initials: AA
authors.lastname: Gonzalez Ibarra
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico.
authors.forename: Manuel
authors.initials: M
authors.lastname: Mendez Garcia
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico.
authors.forename: Eunice
authors.initials: E
authors.lastname: Yanez Barrientos
authors.affiliation: Chemistry Department, Division of Natural and Exact Sciences, University of Guanajuato, L. de Retana 5, 36000 Guanajuato, Mexico. Electronic address: kazimier@ugto.mx.
authors.forename: Kazimierz
authors.initials: K
authors.lastname: Wrobel
keywords: Chromium(VI); High resolution molecular mass spectrometry (ESI–QTOFMS); Liquid chromatography–nitrogen microwave plasma-atomic emission spectrometry (HPLC–MP-AES); Manganese(II); Oxalic acid