Surface Coordination and Radical Binding Induce Efficient Decomposition of Chlorinated Organophosphate Flame Retardants during Catalytic Ozonation

Chlorinated organophosphate flame retardants (Cl-OPFRs) make up a class of highly refractory pollutants that have raised wide environmental concerns. Conventional AOPs, e.g., the benchmark O3/H2O2 (peroxone), are not efficient in removing them from water/wastewater, because of their relatively low reaction rates with hydroxyl radical (•OH). We report here that Cl-OPFRs (i.e., TCEP, TCPP, and TDCPP) can be efficiently removed by catalytic ozonation with zinc oxide (O3/ZnO) through a novel reaction pathway. Based on in situ characterization with confocal Raman spectrum and fluorescence microscopy imaging, we confirmed that the ozone-ZnO interaction resulted in the formation of surface-bound •OH, with yields 50% higher than those of the peroxone system. Using TCEP as a model Cl-OPFR and ATR-FTIR characterization, we recognized the coordination between the phosphoryl oxygen and the surface Zn(II) of ZnO. The surface coordination improved the reactivity of TCEP toward the surface-bound •OH, which was confirmed with a competitive reaction on ZnO. The O3/ZnO was most effective at neutral pH for TCEP removal and can achieve partial mineralization and detoxification of the compound. This work highlights the importance of surface reaction in removing the highly refractory Cl-OPFRs.