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Transformation of Sulfamethazine by Manganese Oxide in Aqueous Solution

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journal contribution
posted on 06.03.2012, 00:00 by Juan Gao, Curtis Hedman, Cun Liu, Tan Guo, Joel A. Pedersen
The transformation of the sulfonamide antimicrobial sulfamethazine (SMZ) by a synthetic analogue of the birnessite-family mineral vernadite (δ-MnO2) was studied. The observed pseudo-first-order reaction constants (kobs) decreased as the pH increased from 4.0 to 5.6, consistent with the decline in δ-MnO2 reduction potential with increasing pH. Molecular oxygen accelerated SMZ transformation by δ-MnO2 and influenced the transformation product distribution. Increases in the Na+ concentration produced declines in kobs. Transformation products identified by tandem mass spectrometry and the use of 13C-labeled SMZ included an azo dimer self-coupling product and SO2 extrusion products. Product analysis and density functional theory calculations are consistent with surface precursor complex formation followed by single-electron transfer from SMZ to δ-MnO2 to produce SMZ radical species. Sulfamethazine radicals undergo further transformation by at least two pathways: radical–radical self-coupling or a Smiles-type rearrangement with O addition and then extrusion of SO3. Experiments conducted in H218O or in the presence of 18O2(aq) demonstrated that oxygen both from the lattice of as-synthesized δ-MnO2 and initially present as dissolved oxygen reacted with SMZ. The study results suggest that the oxic state and pH of soil and sediment environments can be expected to influence manganese oxide-mediated transformation of sulfonamide antimicrobials.