Abiotic transformations of sulfamethoxazole by hydroxylamine, nitrite and nitric oxide during wastewater treatment: Kinetics, mechanisms and pH effects.

Hydroxylamine (NH2OH), nitrite (NO2-) and nitric oxide (NO), intermediates enzymatically formed during biological nitrogen removal processes, can engage in chemical (abiotic) transformations of antibiotics. This study determined the kinetics, mechanisms and pathways of abiotic transformations of the antibiotic sulfamethoxazole (SMX) by NH2OH, NO2- and NO in a series of batch tests under different pH and oxygen conditions. While NH2OH was not able to directly transform SMX, NO2- (with HNO2 as the actual reactant) and NO can chemically transform SMX primarily through hydroxylation, nitration, deamination, nitrosation, cleavage of S-N, N-C and C-S bonds, and coupling reactions. There were substantial overlaps in transformation product formations during abiotic transformations by HNO2- and NO. The second order rate constants of SMX with NO2- and NO were determined in the range of 1.5 × 10-1 - 4.8 × 103 M-1 s-1 and 1.0 × 102 - 3.1 × 104 M-1 s-1, respectively, under varying pH (4 - 9) and anoxic or oxic conditions. Acidic pH significantly enhanced abiotic transformation kinetics, and facilitated nitration, nitrosation, and cleavage of S-N and N-C bonds. The findings advance our understanding of the fate of antibiotics during biological nitrogen removal, and highlight the role of enzymatically formed reactive nitrogen species in the antibiotic degradation.