Degradation of pentachlorophenol in peroxymonosulfate/heat system: Kinetics, mechanism, and theoretical calculations

In this work, we systematically investigated the transformation of pentachlorophenol (PCP) in the heat-activated peroxymonosulfate (PMS) system. The degradation efficiency of PCP increased with initial PMS concentration, temperature and pH. The calculated % reaction stoichiometric efficiency (RSE) reached up to 92.2%. SO 4 2− was found to promote the degradation of PCP, because it significantly enhanced the generation of HO • and SO 4 •− as confirmed by EPR experiments. According to the radical quenching tests, SO 4 •− was the dominant reactive radical for PCP degradation. A total of 10 intermediates were identified, and hydroxylation, sulfate addition, dechlorination and oligomerization were speculated as main reaction pathways. Then, possible reactions between reactive radicals and substrate were calculated at the m062x/land2dz level to interpret the mechanisms of different reactions. It was found that ionic PCP had lower activation energy than molecular PCP, and hence showed higher reactivity towards SO 4 •− and HO • in the course of radical addition reaction. As for the coupling reaction, SO 4 •− more likely to react with ionic PCP to generate phenoxy radicals than HO • , due to the negative Gibbs free energy change. Meanwhile, the abstraction of an Cl atom by HO • was confirmed to be the first step for the formation of dechlorination products. In addition, toxicity evaluation by ECOSAR and TEST programs illustrated that the toxicity of reaction products was all lower than that of PCP. Findings of this work demonstrated the feasibility of this PMS/heat method for effective removal of PCP in contaminated waters.