Products distribution and contribution of (de)chlorination, hydroxylation and coupling reactions to 2,4-dichlorophenol removal in seven oxidation systems.

We systemically investigated the transformation behavior of 2,4-dichlorophenol (24-DCP) in seven different reaction systems including KMnO4, heat/PS, O3, UV, Fenton, NaClO and K2FeO4 treatment. The results revealed that complete removal of 24-DCP could be reached in minutes, especially for Fe(VI), KMnO4, NaClO, Fenton and O3 system. A total of 41 products were identified by LC-MS, and 10 of them were validated using commercial and self-synthesized standards. Hydroxyl substitution and coupling reactions were commonly observed in the studied systems. Meanwhile, extra routes such as sulfate substitution, (de)chlorination and direct oxidation were also involved for certain oxidation methods. Comparisons showed that a high degree of chlorination (>90%) occurred for NaClO system, while coupling products accounted for ~45% of the removed 24-DCP under PS oxidation. Moreover, low mineralization degree together with high aquatic toxicity was attributed to the occurrence of coupling reaction, which was possibly related to the redox potential of the main oxidative species. Considering the low abundance of coupling products and the gentle reaction condition, UV irradiation is a better option for 24-DCP removal in water and wastewaters. These findings can deepen our understanding on the transformation process of 24-DCP and provide some useful information for the environmental elimination of substituted phenols.