An experimental and theoretical study on the degradation of clonidine by hydroxyl and sulfate radicals

Emerging contaminants such as pharmaceuticals that cannot be completely removed by traditional biological treatments are ubiquitously present in water bodies with detected concentrations ranging from ng L-1 to mg L-1. Advanced oxidation technologies (AOTs) are promising, efficient, and environmentally friendly for the removal of these pharmaceuticals. In this study, we investigated the degradation kinetics of a model pharmaceutical, clonidine (CLD), via hydroxyl radical (center dot OH) in UV/H2O2 and sulfate radical (SO4 center dot-) in UV/peroxydisulfate (PS) systems for the first time. The second-order rate constants (k) of protonated cationic CID with center dot OH and SO4 center dot- were measured to be (2.15 +/- 0.07) x 10(9) M-1 s(-l) and (1.12 +/- 0.03) x 10(9) M-1 s(-1), respectively. We also calculated the plc value of CLD and thermodynamic behaviors for reactions of CLD/HOLD - with center dot OH and SO4 center dot- at M05-2X/6-311++G**//M05-2X/6-31+G** level with SMD solvation model. The pK(a) value was calculated to be 8.14, confirming the literature value. H atom abstraction pathway was the most favorable pathway for both center dot OH and SO4 center dot-, while single electron transfer pathway was thermodynamically feasible only for SO4 center dot- for CLD but not for HCLD+. In addition, the reactivities of both tautomeric forms of CLD (i.e., amino and imino CLD) with both radicals were also investigated. This study contributed to a better understanding on the degradation mechanisms of CID and proposed the possibilities of the elimination of pharmaceuticals by applying AOTs during wastewater treatment processes. (C) 2020 Elsevier B.V. All rights reserved.