Factors affecting the roles of reactive species in the degradation of micropollutants by the UV/chlorine process

State of the Art of UV/Chlorine Advanced Oxidation Processes: Their Mechanism, Byproducts Formation, Process Variation, and Applications

“…Thus, it is expected that the relative contribution of HO˙ decreases at acidic pH, if pollutants are reactive with RCS. However, some researches show that contributions of both HO˙ and RCS increase with a decrease in pH [31,37,40,79]. This discrepancy can be explained by the radical scavenger effect of ClO − [5,16,40,77,79].…”

“…Therefore, it is important to elucidate the reactivity of each radical species with chemical compounds for understanding the efficacy of UV/chlorine AOPs. Table 2 summarizes reaction rate constants of HO˙, Cl˙, ClO˙, and Cl 2˙− with some chemicals reported in literatures [18,[24][25][26][27][28][29][30][31][32]. HO˙ has second-order reaction rate constants with approximately 10 9 -order level regardless of saturated compounds or benzene derivatives.…”

“…Boxes depict primary radicals produced by photolysis of chlorine. [24] 6.0×10 8 [24] 1.0×10 9 [32] 1.7×10 9 [32] 1.5-3.2×10 9 [30] 6.2×10 8 [32] ---1.3×10 7 [31] 3.5×10 3 [25] 4.5×10 4 [25] 1.2-1.9×10 5 [25] 0 [26] Aldehyde formaldehyde [24] 5.9×10 9 [24] 5.0×10 9 [24] 7.0×10 9 [24] 7.0×10 9 [24] 6-12×10 9 [28] 0 that RCS show relatively high reactivity with olefins and benzene derivatives and electron-donating groups promote the attack of benzene derivatives [23]. HO˙ is known as a non-selective electrophilic reagent.…”

“…Some researchers applied this method for UV/chlorine AOPs to estimate the contribution of reactive species [20,23,31,37,40]. However, the addition of chemical probes affects both the degradation rate of pollutant and the steadystate concentration of reactive species through competitive consumption of reactive species.…”

“…Many researchers have observed that acidic pH is better for UV/chlorine AOPs on removing organic pollutants such as amitriptyline hydrochloride [75], benzalkonium chloride [53], caffeine [31], chloramphenicol [22], diethyltoluamide [31], 1,4-dioxane [5], iodoform [76], methyl salicylate [75], metronidazole [31], nalidixic acid [31], naproxen [40], neonicotinoid insecticides [74], nitrobenzene [77], 2-phenoxyethanol [75], ronidazole [78], and taste and odor compounds [79]. Three mechanisms, namely a change in quantum yield of chlorine photolysis, equilibrium shifts of radical species, and radical scavenger effects of HOCl and ClO − , are mainly responsible for the pH dependence of the performance of UV/chlorine AOPs.…”