Oxidation of 2,4-dichlorophenol in saline water by unactivated peroxymonosulfate: Mechanism, kinetics and implication for in situ chemical oxidation

Inorganic and organic pollutants present a hazard to surface and groundwater resources. Peroxymonosulfate (PMS, HSO₅ ⁻) has received increasing attention for in situ chemical oxidation (ISCO) capable of remediating contaminated sites. Considering that saline waters occur widely in natural environments, it is desirable to evaluate the effect of Cl⁻ on the PMS oxidation of organic compounds. In this study, 2,4-dichlorophenol (2,4-DCP) was used as a model pollutant. At a PMS concentration of 2.0 mM, Cl⁻ concentration of 50 mM, and solution pH of 7.0, 2,4-DCP was completely degraded by PMS in the presence of Cl⁻ (PMS/Cl⁻ system), while PMS alone exhibited almost no reactivity with 2,4-DCP. The degradation of 2,4-DCP was optimized at a solution pH of 8.4 and high concentrations of PMS and Cl⁻. Quenching experiments and degradation pathway analyses indicated that HClO was responsible for 2,4-DCP oxidation, and HClO was mainly generated by the interaction of Cl⁻ with HSO₅ ⁻, rather than SO₅ ²⁻. Consequently, the transformation from HSO₅ ⁻ to HClO appeared under a solution pH of 10.0 and was favored in an acidic solution. Given the ambient pH and Cl⁻ concentrations of saline waters, a considerable amount of HClO may be produced by the interaction of PMS with Cl⁻ in the oxidant delivery stage of ISCO processes. Interestingly, H₂O₂ and peroxydisulfate did not exhibit reactions similar to those of PMS. This research indicated that caution must be exercised when choosing an oxidant for ISCO processes in saline waters.