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Transformation of Sulfamethazine by Manganese Oxide in Aqueous Solution

Gao, Juan, Hedman, Curtis, Liu, Cun, Guo, Tan, Pedersen, Joel A.
Environmental Science & Technology 2012 v.46 no.5 pp. 2642-2651
anti-infective agents, aqueous solutions, carbon, dissolved oxygen, extrusion, free radicals, isotope labeling, manganese oxides, oxygen, sediments, sodium, soil pH, stable isotopes, sulfur dioxide, tandem mass spectrometry
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.