Degradation of propranolol by UV-activated persulfate oxidation: Reaction kinetics, mechanisms, reactive sites, transformation pathways and Gaussian calculation

Abstract

Contamination with β-blockers such as propranolol (PRO) poses a potential threat to human health and ecological system. The present study investigated the kinetics and mechanisms of PRO degradation by UV-activated persulfate (UV/PS) oxidation. Here, the experimental results showed that the degradation of PRO followed pseudo-first-order reaction kinetics, the degradation rate constant (kobs) was increased dramatically with increasing PS dosage or decreasing initial PRO concentration. And increasing the initial solution pH could also enhance the degradation efficiency of PRO. Radical scavenging experiments demonstrated that the main radical species was sulfate radicals (SO4•−), with hydroxyl radicals (HO·) playing a less important role. Meanwhile, the second-order rate constants of PRO degradation with SO4•− and HO· were determined to be 1.94 × 1010 M−1 s−1 and 6.77 × 109 M−1 s−1, respectively. In addition, the presence of natural organic matter (NOM) and nitrate anion (NO3−) showed inhibitory effect on PRO degradation, whereas bicarbonate anion (HCO3−) and chlorine anion (Cl−) greatly enhanced the degradation of PRO. Moreover, the transformation products of PRO were identified by applying ultra performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF-MS) technique. Molecular orbital calculations were used to estimate the reaction site of PRO with radicals, simultaneously. Hence, the transformation pathways including hydroxylation, dehydration, naphthalene ring opening, and the cleavage of aldehyde groups were proposed. This work enriches the mechanism of PRO degradation under UV/PS system on the basis of results obtained by experimental characterization and Gaussian theoretical calculation.