Enhanced Radical Generation in an Ultraviolet/Chlorine System through the Addition of TiO₂

Ultraviolet (UV)/chlorine draws increasing attention for the abatement of recalcitrant organic pollutants. Herein, it was found that TiO₂would significantly promote the degradation of dimethyl phthalate (DMP) in the UV/chlorine system (from 19 to 84%). Hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) were the dominant reactive species for DMP degradation in the UV/chlorine/TiO₂system. Chlorine decayed much faster in UV/chlorine/TiO₂compared with UV/chlorine, which is possibly because photogenerated electrons (e_cb^-) and superoxide radicals (O₂^•-) have high reactivity with chlorine. As a result, the recombination of photogenerated holes (h_vb⁺) and e_cb^-was inhibited and the accumulation of HO^•and Cl^•was facilitated. A kinetic model was established to simulate the reaction process, and it was found that the concentrations of HO^•and Cl^•were several times to dozens of times higher in UV/chlorine/TiO₂than that in UV/chlorine. The contributions of HO^•and Cl^•to DMP degradation were 70.3 and 29.7% by model simulation, respectively, and were close to the probe experiment result. In the UV/chlorine/TiO₂system, the degradation of DMP did not follow pseudo-first-order kinetics but the degradation of benzoate fitted well with pseudo-first-order kinetics. This phenomenon was elucidated by the structure of the pollutant and TiO₂and further tested by calculating the adsorption energy (E_ads)/binding energy (E_b) with density functional theory. Due to faster decay of chlorine, lower amounts of disinfection byproducts formed in UV/chlorine/TiO₂compared with UV/chlorine. Adding TiO₂into the UV/chlorine system can promote the degradation of recalcitrant organic pollutants in an aqueous environment.