Experimental and theoretical insights into kinetics and mechanisms of hydroxyl and sulfate radicals-mediated degradation of sulfamethoxazole: Similarities and differences
2020
Gao, Lingwei | Mao, Qiming | Luo, Shuang | Cao, Linying | Xie, Xiande | Yang, Yuan | Deng, Yunfeng | Wei, Zongsu
Hydroxyl radical (•OH)- and sulfate radical (▪)-based advanced oxidation technologies (AOTs) have been proven an effective method to remove antibiotics in wastewater treatment plants (WWTPs). This study aims to gain insights into kinetics and mechanisms of neutral sulfamethoxazole (SMX) degradation, a representative antibiotic, by •OH and ▪ using an experimental and theoretical approach. First, the second-order rate constants (k) of SMX with •OH and ▪ were determined to be (7.27 ± 0.43) × 10⁹ and (2.98 ± 0.32) × 10⁹ M⁻¹ s⁻¹ in UV/H₂O₂ and UV/persulfate (UV/PS) systems, respectively. The following theoretical calculations at the M06–2X level of theory revealed that addition of radicals to the benzene ring is the most favorable first-step reaction for both •OH and ▪, but that ▪ exhibits higher energy barriers and selectivity than •OH due to steric hindrance. We further analyzed subsequent reactions and, interestingly, our findings closely corroborated HOMO/LUMO distributions of SMX to the oxidation pathways. Finally, the estimation of energy consumption for UV alone, •OH–, and ▪–mediated oxidation processes was compared. These comparative results, for the first time, provide insights into the similarities and differences of degradation of SMX by •OH/▪ at the molecular level and can help improve antibiotics removal using radical based AOTs in WWTPs.
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