Naturally occurring redox transformations of organic pollutants involving manganese species, can significantly affect the chemical fate and transport of the pollutants in the environment. Under anoxic condition the reductive dissolution of manganese oxides by bacteria or organic acids produces Mn(III) species, which can be stabilized by forming complexes with organic or inorganic ligands. Such relatively stable Mn(III) complexes have been observed at high concentrations in suboxic water and soil. Mn(III) complexes, especially with carboxylated ligands have been shown to be capable of facilitating many redox reactions of organics, however, there is lack of information regarding the detailed pathways of such transformation processes and their contributions to the observed bulk degradations of organic pollutants in the environment. We investigated the role of Mn(III) in presence of oxalate ligand on the reductive degradations of quinoxaline-di-N-oxide antibiotics, by using a combination of quantum mechanical calculations and EPR spectroscopy. The reaction occurred primarily at the quinoxaline-di-N-oxide moiety resulting in the removal of one O from N1-oxide and formation of desoxy- products. The critical step in the reaction was the formation of a quinoxaline-di-N-oxide/Mn(III)/oxalate ternary complex in which Mn(III) functioned as the central complexing cation and electron conduit in which the arrangement of ligands facilitated electron transfer from oxalate to quinoxaline-di-N-oxide. In the complex, the C-C bond in oxalate was cleaved to create CO2-� radicals, followed by electron transfer to quinoxaline-di-N-oxide through the Mn(III) center. This reaction mechanism is extensively characterized by the reaction kinetics, EPR spectroscopy and quantum mechanical calculations. The results obtained from this study suggest that naturally occurring Mn(III)-oxalate complexes could reductively decompose certain organic compounds in the environment.

Computational and EPR studies on the degradation mechanisms of organic pollutants by Mn(III)-oxalate

247th National Meeting (2014)

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