Effect of small-molecule phenolic compounds on the removal of triclosan (TCS) by heat/persulfate process: Formation of cross-coupling products and implications for wastewater treatment

In this study, the effect of three small molecule phenolic compounds (SMPCs) (Phenol (PHOH), p-hydroxybenzoic acid (PHBA) and 3,5-dihydroxytoluene (DHT)) on the transformation of triclosan (TCS) in heat activated persulfate (heat/PS) process was investigated. TCS in single and mixed solutions were almost completely removed after 120 min of heat/PS treatment. Compared with single TCS system, the generation of cross-coupling products in the mixed system was beneficial for the removal of total organic carbon (TOC). Overall cytotoxicity and genotoxicity tests on mammalian cells showed that the reaction products in mixed phenol system was less toxic than those in the single TCS system, which confirmed the low toxicity of cross-coupling products. Product analysis was combined with theoretical calculations to propose a mechanism for the cross-coupling of coexisting phenolic pollutants that hydrogen on the phenolic hydroxyl groups of different pollutants was extracted by SO4•- to form phenoxy radicals, which coupled with each other to form cross-coupling products. Lowering the pH of the reaction solution can promote the occurrence of cross-coupling reaction, favoring the removal of TOC and the reduction of toxicity. These results will provide insights into the transformation of phenolic pollutants in water treatment for further control of environmental risks.