Although antibiotics are typically observed at trace concentrations (ng/L) in municipal water and wastewater systems, these contaminants of emerging concern pose adverse ecological and biological hazards. The purpose of this study was to investigate the fate of select sulfonamide antibiotics by direct photolysis at 253.7 nm. These compounds were hypothesized to undergo antibiotic-to-antibiotic transformations due to their shared characteristic base structure. Antibiotic samples were prepared at 5-50 mg/L at seven pH values between 4 and 10. These samples were irradiated with low-pressure bulbs in a batch UV reactor. Sample aliquots were collected at predetermined times. Each sample was analyzed using LC-MS/MS to quantify the parent sulfonamide and other known sulfonamides generated during irradiation. From these data, fluence-based rate constants were determined at near neutral pH to be 1.28 (± 0.10) × 10-4 cm2/mJ for sulfamethazine, 4.85 (± 0.35) × 10-5 cm2/mJ for sulfamerazine, and 4.12 (± 0.39) × 10-5 cm2/mJ for sulfadiazine. At pH 4.34 and 8.46, the apparent fluence-based rate constants for sulfamethazine were 1.01 (± 0.04) × 10-4 cm2/mJ and 1.48 (± 0.22) × 10-4 cm2/mJ, respectively. These results indicate that solution pH exerts a clear impact on reaction kinetics during UV treatment. Non-target analysis of transformation products was conducted using LC-MS, and structures were proposed through validation of the corresponding MS/MS spectra. Transformation products generated in the fluence range of 0-400 mJ/cm2 were of particular interest, as these will be formed in disinfection systems. The chemical similarity of transformation products to the sulfonamide pharmacophore suggests that these products retain antimicrobial activity. Residual antimicrobial activity was assessed using growth inhibition of Escherichia coli and Pseudomonas fluorescens. Understanding antibiotic transformation, and the corresponding change in antimicrobial activity, in treated wastewater is an important step to protecting human and ecological health.

UV-driven antibiotic-to-antibiotic transformation pathways and kinetics of sulfonamides

253rd National Meeting (2017)

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