Freezing-induced activation of the binary chloride-Oxone system to free chlorine and its application in water treatment

Accelerated chemical reactions in frozen solutions can be applied in the degradation of organic pollutants in water. Herein we propose a novel freezing system that enables the degradation of various organic compounds in a frozen solution. Although the degradation of 4-chlorophenol (4-CP) by Oxone as the sole primary oxidant did not take place in aqueous solutions (25 °C) regardless of the presence of chloride ions (Cl−, micromolar levels) and only took place to a minor extent in frozen solutions (−20 °C) in the absence of Cl−, the addition of Cl− (micromolar levels) to the freezing/Oxone system significantly accelerated the degradation of 4-CP. Various analytical characterizations and pH measurements of the frozen solution suggested that the enhanced degradation of 4-CP in the freezing/Oxone/Cl− system could be because Cl−, Oxone, and protons are concentrated in the liquid brine upon freezing. This process subsequently facilitates the formation of hypochlorous acid (HOCl) as a secondary oxidant. The positive effect of Cl− was observed under widely varying conditions (i.e., [Cl−] = 25–1000 μM, pHi = 3–11, and freezing temperature = from −10 to −30 °C), and the freezing/Oxone/Cl− system described herein successfully degraded all 12 tested organic pollutants. In addition, outdoor freezing experiments carried out on winter days confirmed the successful performance of the freezing/Oxone/Cl− system without the requirement for electrical energy.