Substituent structure variances alter degradation pathways of sulfonamides in UV/PAA system: Insights from...

Sulfonamides (SAs) are one of the major emerging contaminants of concern, but comparative studies on the degradation of different types of SAs are still limited. This work comprehensively compared the degradation of sulfadiazine (SDZ), sulfamethoxazole (SMX) and sulfathiazole (STZ) under UV light in peracetic acid (PAA) from both experimental and theoretical aspects, as they represent two structural classes based on substituent differences. The two SAs with five-membered heterocyclic substituents (SMX, STZ) generally decomposed at faster rates, with SMX degrading up to 10 times faster than SDZ (pH = 3; PAA dosage = 80 mg/L). For all three SAs, the degradation efficiency stayed high across pH levels, peaking at pH 9 for SDZ and STZ, and at pH 3 for SMX. Free radical scavenging experiments and EPR tests proved that the degradation of SAs involved various free radicals, among which ³SA*s and •OH played a major role. Fukui functions indicated N(7) in aniline groups of SDZ and SMX had the highest reactivity, while in STZ, S(23) in the thiazole moiety was the most active site. The degradation pathways were proposed and compared and the cleavage of S-C bond was observed in all three SAs and hydroxylation was the most common reaction. This study elucidated the impacts of different substituent structures on the degradation of three sulfonamide drugs in the UV/PAA system, contributing to a better understanding of the degradation behavior of various types of sulfonamide drugs in water treatment.