The mutual promotion of photolysis and laccase-catalysis on removal of dichlorophen from water under simulated sunlight irradiation

The combined removal of dichlorophen (DDM) by photolysis and laccase-catalysis in aqueous solution under simulated sunlight irradiation was investigated. It was found that DDM could be effectively degraded under simulated sunlight irradiation, which was triggered primarily by singlet/triplet state ((DDM)-D-1*/(DDM)-D-3*). The photolysis process of DDM is highly pH-dependent, mainly because of the change in electronic density distribution on the aromatic ring of DDM in its varied ionic speciation under different pH. The significant role of singlet oxygen (O-1(2)) and (DDM)-D-3* in DDM photolysis was verified by experimental measurements and density functional theory (DFT) calculation. The combination of photolysis and laccase-catalysis processes can promote the removal of DDM synergistically. Product identification revealed that photolysis and laccase-catalysis attacked DDM molecule at different moieties. Single aromatic ring products were formed via photo-oxidation of DDM by O-1(2). Meanwhile, photolysis can efficiently remove the oxidative coupling products of DDM formed in laccase-catalysis process, which are considered to be potentially high toxic products. In addition, this study also provided first evidence of the laccase inactivation mechanisms induced by simulated sunlight, namely, the destruction of copper catalytic centers and secondary structure.