AWWA WQTC58979

This study presents kinetics and degradation pathways for the reactions between sodium hypochlorite (free available chlorine - FAC) or chloramines (combined chlorine - CC) and substrate compounds representative of three commonly prescribed and environmentally prevalent classes of antibiotics: fluoroquinolones, sulfonamides, and dihydrofolate reductase (DHFR) inhibitors. Pseudo-first-order kinetics was observed for oxidation of the fluoroquinolone antibiotic enrofloxacin (EF), sulfonamide antibiotic sulfamethoxazole (SMX), and DHFR inhibitor trimethoprim (TMP) by FAC. Second-order rate constants for reactions involving EF, SMX, and TMP were calculated from observed pseudo-first-order constants, on the assumption that concentration of oxidant remained essentially constant throughout the monitoring periods of each kinetic experiment. Solution pH exhibited a marked influence on reaction rates for all three antibiotic classes. Consideration of predominant antibiotic species and corresponding reaction rates at specific pH values allowed a preliminary determination of reactive sites within the antibiotics. Compounds representing the hypothesized reactive and non-reactive portions of each antibiotic class were utilized to verify the proposed location(s) of reactivity. LC/MS and H1-NMR were used where applicable to identify reaction products and to assess product evolution during each reaction time course. Product characterization of CF reaction mixtures indicates the formation of a number of products, represented primarily by four major degradates corresponding to m/z 263, 297, 306, and 340 (corresponding to full or partial dealkylation of the piperazine ring and, in two cases, substitution of Cl on the quinolone structure's aromatic ring). The relatively rapid oxidation of SMX is accompanied by what appears to be a unique radical-chain cleavage of the S-N sulfonamide bond to yield 3-amino-5-methylisoxazole and an unknown product. S-N bond cleavage, combined with N-chlorination of the aniline functional group, also appears to lead to the formation of relatively stable dimers and a number of lower mass products. Chlorination of TMP yields primarily stable, multiply-halogenated products, with the parent compound undergoing relatively minor structural modification. The results of this investigation indicate that representative members of these three antibiotic classes are substantially degraded under conditions simulating chlorination of water supplies during disinfection processes, yielding a wide variety of lower and higher mass degradates. Includes 32 references, figures.