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 (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 pK_a 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 pK_a 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.