Experimental and theoretical insight into hydroxyl and sulfate radicals-mediated degradation of carbamazepine

Carbamazepine (CBZ), a widely detected pharmaceutical in wastewaters, cannot currently be treated by conventional activated sludge technologies, as it is highly resistant to biodegradation. In this study, the degradation kinetics and reaction mechanisms of CBZ by hydroxyl radical ((OH)-O-center dot) and sulfate radical (SO4 center dot-)-based advanced oxidation processes (AOPs) were investigated with a combined experimental/theoretical approach. We first measured the UV absorption spectrum of CBZ and compared it to the theoretical spectrum. The agreement of two spectra reveals an extended pi-conjugation system on CBZ molecular structure. The second-order rate constants of (OH)-O-center dot and SO4 center dot- with CBZ, measured by competition kinetics method, were (4.63 +/- 0.01) x 10(9) M-1 s(-1) and (8.27 +/- 0.01) x 10(8) M-1 s(-1), respectively at pH 3. The energetics of the initial steps of CBZ reaction with (OH)-O-center dot and SO4 center dot- were also calculated by density functional theory (DFT) at SMD/M05-2X/6-311++G**//M05-2X/6-31 + G**level. Our results reveal that radical addition is the dominant pathway for both (OH)-O-center dot and SO4 center dot-. Further, compared to the positive Delta G(R)(0) value for the single electron transfer (SET) reaction pathway between CBZ and (OH)-O-center dot, the Delta G(R)(0) value for SET reaction between CBZ and SO4 center dot- is negative, showing that this reaction route is thermodynamically favorable. Our results demonstrated the remarkable advantages of AOPs for the removal of refractory organic contaminants during wastewater treatment processes. The elucidation of the pathways for the reaction of (OH)-O-center dot and SO4 center dot- with CBZ are beneficial to predict byproducts formation and assess associated ecotoxicity, providing an evaluation mean for the feasibility of AOPs application. (C) 2019 Elsevier Ltd. All rights reserved.