Theoretical investigation on the degradation of sulfadiazine in water environments: Oxidation of •OH, SO4•−and CO3•− and reactivity of (TiO2)n clusters (n = 1–6)

This study reported the degradation of micro pollutants through reactive radicals like •OH, SO4•−and CO3•−. Sulfadiazine (SDZ) is widely used antibiotic and emerging water pollutant. The synergistic degradation of sulfadiazine by •OH, SO4•−and CO3•− was investigated through quantum chemistry methods. The degradation of SDZ by •OH radicals was dependent on radical adduct formation (RAF) reactions while degradation of SDZ by SO4•− radical was caused by RAF, hydrogen atom transfer (HAT) and also single electron transfer (SET). The degradation of SDZ by CO3•− radical mainly caused by RAF and HAT. The degradation rate constants of SDZ initiated through •OH, SO4•−and CO3•− radicals were calculated separately: kSDZ-SO4•− (5.29 × 1010 M−1 s−1) > kSDZ-•OH (1.38 × 1010 M−1 s−1) > kSDZ-CO3•− (1.28 × 108 M−1 s−1). The degradation mechanism of SDZ by (TiO2)n (n = 1–6) cluster was investigated. The conformation between benzene ring and heterocyclic ring of SDZ has a lower adsorption energy and the (TiO2)n cluster (n = 5) is the most stable adsorption conformation with SDZ. The toxicity assessment showed that most of the degradation products were less toxic and eco-friendly. However, O-P1, P4 and P9 exhibited more severe aquatic toxicity.