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 (OH) in UV/H2O2 and sulfate radical (SO4•-) in UV/peroxydisulfate (PS) systems for the first time. The second-order rate constants (k) of protonated cationic CLD with OH and SO4•- were measured to be (2.15 ± 0.07) × 109 M-1 s-1 and (1.12 ± 0.03) × 109 M-1 s-1, respectively. We also calculated the pKa value of CLD and thermodynamic behaviors for reactions of CLD/HCLD+ with OH and SO4•- at M05-2X/6-311++G**//M05-2X/6-31+G** level with SMD solvation model. The pKa value was calculated to be 8.14, confirming the literature value. H atom abstraction pathway was the most favorable pathway for both OH and SO4•-, while single electron transfer pathway was thermodynamically feasible only for SO4•- 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 CLD and proposed the possibilities of the elimination of pharmaceuticals by applying AOTs during wastewater treatment processes.
Keywords: Advanced oxidation technologies; Clonidine; Density functional theory; Hydroxyl radical; Sulfate radical.
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