Freezing facilitates the non-radical activation of peroxydisulfate by β-MnO₂ for contaminants degradation

In cold regions, the freezing process typically presents difficulties for the remediation of contaminated soil and groundwater. However, in this work we report a freeze-assisted activation of peroxydisulphate (PDS) by β-MnO₂ for the degradation of phenolic compounds. The results showed that 60.8 % of 2,4-dichlorophenol (2,4-DCP) could be abated in frozen solutions at an initial pH of 6.5 within 2 h, whereas only 12.4 % of 2,4-DCP disappeared in aqueous solutions under other identical conditions. This enhancing effect was highly associated with the well-known freeze concentration effect, which could decrease the solution pH at the ice grain boundary by 2.4 units. Electron spin resonance (ESR) spectroscopic analysis and experiments with quenching agents and D₂O suggested that radicals and singlet oxygen were not the major reactive species involved in the β-MnO₂/PDS system. Instead, Raman analysis indicated that PDS may be bound to the surface of β-MnO₂ to form a reactive inner-sphere complex of MnO₂-PDS*, which is proposed to account for the oxidative degradation of 2,4-DCP in aqueous and frozen β-MnO₂/PDS systems. Chronoamperometric experiments further showed the redox potential of the β-MnO₂/PDS system was as high as 1.2 V. This work highlights the possibility of applying the freezing effect to accelerate PDS activation by minerals abundant in environment and thus offers a promising strategy for pollution remediation in cold regions.