Experimental and theoretical insights into kinetics and mechanisms of hydroxyl and sulfate radicals-mediated degradation of sulfamethoxazole: Similarities and differences

Hydroxyl radical ((OH)-O-center dot)- and sulfate radical (SO4 center dot-)-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)-O-center dot and (SO4 center dot-) using an experimental and theoretical approach. First, the second-order rate constants (k) of SMX with (OH)-O-center dot and SO4 center dot- were determined to be (7.27 +/- 0.43) x 10(9) and (2.98 +/- 0.32) x 10(9) 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)-O-center dot and SO4 center dot- , but that 80:1 exhibits higher energy barriers and selectivity than (OH)-O-center dot 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)-O-center dot-, and SO4 center dot- -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)-O-center dot/SO4 center dot- at the molecular level and can help improve antibiotics removal using radical based AOTs in WWTPs. (C) 2019 Elsevier Ltd. All rights reserved.