Significance of nucleophilic substitution for ultraviolet dechlorination of pentachlorophenol in the presence of hydrogen sulfide in aqueous solution

Pentachlorophenol (PCP), a highly toxic and persistent agrochemical, is urgent to be dechlorinated for the disintoxication and degradation. Thus, this study aimed to investigate the dechlorination of PCP through ultraviolet (UV) photolysis in the presence of hydrogen sulfide (H₂S) in the water, using density functional theory calculations. The theoretical results suggest that 78.2 and 87.6 kcal·mol⁻¹ were required for the dechlorination of PCP and pentachlorophenolate (PCP⁻) by the homolysis of C-Cl bonds under UV irradiation. The UV photolysis of PCP⁻ could eject hydrated electron (e_aq⁻) and pentachlorophenoxyl radical (PR) in the water. The absorption of e_aq⁻ by PCP⁻ enhanced the dechlorination by releasing Cl⁻. Additionally, the dechlorination could also occur through the nucleophilic reactions of OH⁻, HS⁻ and S²⁻ with PR, which might be responsible for the enhancement of dechlorination from pH 6.2 to 7.7, and their respective forward energy barriers were 0.5, 6.6 and 21.7 kcal·mol⁻¹ at the para-position. Furthermore, the dechlorination of major quinoid metabolite of PCP (tetrachloro-p-benzoquinone) and its monohydroxylated product under the UV irradiation was facilitated by the nucleophilic attacks on the first triplet states, with the forward energy barriers of 20.4 and 21.7 kcal·mol⁻¹, respectively. Based on the findings in this work, the nucleophilicity of additives may play a dominant role in the dechlorination of PCP, and the combination of H₂S and PCP may be helpful for not only the dechlorination of PCP but also the elimination of H₂S.