ROS reevaluation for degradation of 4-chloro-3,5-dimethylphenol (PCMX) by UV and UV/persulfate processes in the water: Kinetics, mechanism, DFT studies and toxicity evolution

Abstract 4-Chloro-3,5-dimethylphenol (PCMX), as a typical antimicrobial compound, has been widely used in medical products and cosmetics and has been detected in municipal wastewater. Significant concerns about its persistence and potential toxicity has been paid to in the aquatic environment. In this study, we explored UV and UV/persulfate (UV/PS) advanced oxidation process as potential options to remove PCMX for the first time. A steady-state kinetic model was established to simulate the experimental results and to predict the concentration of reactive oxidant species (ROS) under different reaction conditions. The second-order rate constants for PCMX reacting with SO4− and HO were determined to be (1.75 ± 0.04) × 109 M−1 s−1 and (2.74 ± 0.14) × 109 M−1 s−1, respectively, with SO4− as the dominant species in PCMX decay by UV/PS. Interestingly, the pseudo-first-order rate constant of PCMX changed slightly in a broad pH range of 3.0 – 11.0, but was significantly increased when pH > 14.0. Natural organic matter (up to 5 mg/L as C) hindered the degradation rate of PCMX from 3.85 × 10−4 s−1 to 1.43 × 10−4 s−1 due to the effects of radical scavenging and the “inner filter” effect. Furthermore, the potential reactive sites (i.e., certain carbons in the benzene ring and phenolic hydroxyl group), in PCMX were identified based on the electronic structure of the molecule, using frontier molecular orbital theory, natural population analysis (NPA), and density functional theory (DFT) calculation. A variety of byproducts were identified using HPLC/MS/MS, which, in combination with the reactive site analysis, led to six main degradation pathways. Quantitative Structure Activity Relationship (QSAR) method of Toxicity Estimation Software Tool was applied to estimate the developmental toxicity of the byproducts.