A novel simultaneous abatement of bromate and diphenyl phosphate using the freezing process

The purification of bromate (BrO₃^-)-contaminated water has become a challenge because of its persistence and adverse effects. Furthermore, there has been concern over the release of byproducts, such as diphenyl phosphate (DPHP), from flame retardants in wastewater treatment plant (WWTP). In this study, we designed the water treatment system for the oxidation of DPHP accompanied by bromate (BrO₃^-) reduction via freezing the solution. A sample containing 10 μM DPHP, 100 μM Br^-, and 50 μM BrO₃^-, with a pH of 3 was frozen at -20^oC, approximately 25 μM BrO₃^- was reduced, and DPHP was fully eliminated after a 0.5 h reaction time. Conversely, these reactions did not advance in water at 20^oC. This increase in the rate of chemical reaction in ice is the consequence of the freeze concentration effect, which refers to the extraction of dissolved chemical species into the liquid-like regions of the polycrystalline ice micro-structure during the freezing of the solution. The redox reactions among DPHP, Br^-, and BrO₃^- become thermodynamically favorable due to the distinctive environment in the liquid brine in ice. The efficiency of the DPHP oxidation significantly increased with an increase in BrO₃^- concentration, and vice versa. The Br^-/BrO₃^--induced HOBr production is proposed as a primary oxidant for DPHP degradation. The proton activity (pH) has a significant influence on the reaction efficiency. The low freezing temperature accelerated the reaction kinetics of DPHP degradation and BrO₃^- reduction. The results of this study indicate the possibility of utilizing ice chemistry for the BrO₃^- reduction that concomitantly removes DPHP for water treatment. This environmentally friendly water treatment method can be considered to implement in regions with a cold climate.