The role of chlorine oxide radical (ClO^•) in the degradation of polychoro-1,3-butadienes in UV/chlorine treatment: kinetics and mechanisms

Polychoro-1,3-butadienes (CBDs) were widely found in aqueous environment and resistant to conventional water treatment. In this study, the abatement of CBDs during UV/chlorine treatment was investigated. In comparison to UV irradiation alone, free chlorine addition brought benefits for the reduction of tetra-CBDs (TCBDs), but to lesser extent for penta-CBDs (PCBDs), and virtually no benefit for hexa-CBD (HCBD). At a UV dose of 128 mJ cm^-2 and a chlorine dose of 10 mg L^-1, about 71.7-97.8% CBDs were degraded by UV/chlorine treatment within 10 min. UV irradiation contributed 32.8%-97.6%, HO^• contributed 2.6%-14.4%, and reactive chlorine species (RCS) contributed less than 0.5%-42.3% to CBDs degradation. The percentages of RCS contribution generally followed the order of TCBDs (except (Z,Z)-1,2,3,4-TCBD) > PCBDs > HCBD. The chlorine oxide radical (ClO^•) was the dominant RCS contributing to the degradation of CBDs. The second-order reaction rate constants of ClO^• with CBDs ( [Formula: see text] ) were at ∼ 10⁷ M^-1s^-1 except (Z,Z)-1,2,3,4-TCBD and HCBD (<10⁶ M^-1s^-1). [Formula: see text] generally decreased with increasing numbers of chlorine atoms and was also affected by the positions of chlorine atoms in CBDs. A distinct reaction pathway of ClO^•, with (Z)-1,1,2,3,4-PCBD as a representative CBD, was proposed. Photoisomers of CBDs from Z or E configuration were observed at lower concentrations in UV/chlorine treatment than under UV irradiation alone due to the radical-involved oxidation, but more organic acids including oxalic acid were observed. In a natural water sample, UV/chlorine treatment also exhibited a good performance in abatement of TCBDs and PCBDs, but not in abatement of HCBD.