Comparison of Removal Characteristics of Organophosphorus Flame Retardants Using Ozone and UV-Based Advanced Oxidation Processes in the Downstream of Nakdong River

Objectives : This study aimed to evaluate the removal efficiency of trace organic contaminants (TrOCs) in UVbased advanced oxidation processes (AOPs) as an alternative to the post-ozonation process in a drinking water treatment plant (DWTP) in the downstream of Nakdong River. The results are expected to be utilized as basic data for modernizing aging water treatment facilities.Methods : Eight organophosphorus flame retardants (OPFRs), known for their persistence and resistance to degradation, were selected as the target TrOCs for evaluating the efficiency of ozonation and UV-based AOPs. Experiments were conducted using sand-filtered water from a DWTP in the downstream of Nakdong River, with the OPFRs spiked into the sample matrix. Lab-scale experiments of ozonation and UV-based AOPs (UV, UV/H<sub>2</sub>O<sub>2</sub>, and UV/Cl<sub>2</sub>) were performed, and the concentrations of the OPFRs were analyzed using stir bar sorptive extraction (SBSE) system, followed by GC-MS/MS.Results and Discussion : The second-order rate constants (<i>k</i><sub>OH</sub>) for the reaction of OPFRs with OH radicals were estimated using the group contribution method and ranged from 1.4(±0.6)×10<sup>9</sup> to 1.8(±0.1)×10<sup>10</sup> M<sup>⁻1</sup> s<sup>⁻1</sup>, depending on their structural characteristics. In the ozonation process, the removal of Cl-containing aliphatic OPFRs was not effective, likely due to the electron-withdrawing effects of the -Cl groups, which inhibited their reactivity with both ozone and OH radicals. However, for non-Cl-containing aliphatic and aromatic OPFRs, more than 90% removal efficiency was achieved at a specific ozone dose of 1.5 mgO<sub>3</sub>/mgDOC. In the UV-based AOPs, the aliphatic OPFRs showed poor removal efficiency of 5~35% at a UV fluence of 1,000 mJ/cm<sup>2</sup> under direct UV photolysis. However, in the presence of oxidants (H<sub>2</sub>O<sub>2</sub> or free available chlorine), the UV/H<sub>2</sub>O<sub>2</sub> process showed up to an 89% increase in removal efficiency, and the UV/Cl<sub>2</sub> process exhibited up to a 49% increase. The difference in removal patterns between the UV/H<sub>2</sub>O<sub>2</sub> and UV/Cl<sub>2</sub> processes is likely attributed to the difference in reactivity between reactive chlorine species and aliphatic OPFRs. For aromatic OPFRs, direct UV photolysis achieved high enough removal efficiencies of 90~98% at a UV fluence of 1,000 mJ/cm<sup>2</sup>. These results demonstrate the potential of UV/H2O2 and UV/Cl<sub>2</sub> processes for the effective removal of OPFRs, particularly for Cl-containing aliphatic OPFRs, which exhibited low removal efficiency in the ozonation process.Conclusion : The comparative evaluation of removal efficiencies of OPFRs in ozone and UV-based AOPs confirmed the potential of UV processes for efficient removal of OPFRs. The UV/H<sub>2</sub>O<sub>2</sub> and UV/Cl<sub>2</sub> processes showed promise as alternative treatments to ozonation for the removal of OPFRs during drinking water treatment.