Effect of pH and molar ratio of pollutant to oxidant on a photochemical advanced oxidation process using hypochlorite

Abstract

Ultra violet (UV)-photolysis of hydrogen peroxide (H2O2) is a conventional advanced oxidation process (AOP) and is advantageous in its simplicity, although H2O2 is costly. Accordingly, we tried to substitute H2O2 by hypochlorite in the photochemical AOP, and discussed the effect of pH and the molar ratio of a pollutant to hypochlorite on the process using 1,4-dioxane as a model pollutant. The photochemical treatment of hypochlorite solutions at a wavelength of 254 nm under various pH values revealed that the UV-photolysis of hypochlorous acid (HOCl) species mainly contributed to hydroxyl radical (•OH) production. The reaction efficiency, as defined by the molar ratio of removed 1,4-dioxane to consumed hypochlorite, deteriorated under higher pH levels due to the stronger radical scavenging effect of hypochlorite ion (ClO(-)) as compared to that of HOCl. The optimal pH for the UV-photolysis of hypochlorite as an AOP was found to be in the range of 3-6. The reaction efficiency at a high molar ratio of initial 1,4-dioxane to initial hypochlorite exceeded 100%, which was caused by the regeneration of HOCl from photochemically generated chlorine radicals (•Cl). Finally, the overall reaction of the UV-photolysis of HOCl was proposed on the basis of the radical reactions that were related to chlorine species, which suggested that the UV-photolysis of 1 mmol of HOCl stoichiometrically produced 2 mmol of •OH. Since the use of liquid chlorine is more economical than that of H2O2, the substitution of HOCl for H2O2 in the photochemical AOP was concluded to be feasible from the viewpoints of both stoichiometry and chemical costs.

Removal of selected antibiotics and antiretroviral drugs during post‐treatment of municipal wastewater with UV, UV/chlorine and UV/hydrogen peroxide

“…Recently, the use of aqueous chlorine and UV as an AOP has been investigated and suggested to be effective in the degradation of recalcitrant organic micropollutants (Watts et al ., 2007; Watts and Linden, 2007; Sichel et al ., 2011; Watts et al ., 2012; Boal et al ., 2015). The UV/Cl 2 process has been suggested as a possible alternative to the UV/H 2 O 2 in the removal of organic micropollutants for several reasons including; (i) the higher UV absorbance and the lower scavenging of HOCl as compared to H 2 O 2 resulting in higher quantum yield (ɸ) during photolysis of aqueous chlorine (HOCl ɸ = 1.4 mol Es −1 and H 2 O 2 ɸ = 1.0 mol Es −1 at 254 nm) (Watts and Linden, 2007; Jin et al ., 2011; Rosenfeldt et al ., 2013; Fang et al ., 2014; Kishimoto and Nishimura, 2015). (ii) The UV/Cl 2 AOP produces three reactive radicals including hydroxyl, chlorine and oxygen radicals (Feng et al ., 2007; Jin et al ., 2011).…”

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

“…He also found that light irradiation to chlorine water produced oxygen and hydrochloric acid [4]. Figure 2 shows absorption spectra of hypochlorous acid (HOCl) and hypochlorite ion (ClO − ) [5]. Absorption bands of both species are in the range of UV region, but are quite different each other; HOCl has two absorption peaks at 237 nm with molar extinction coefficient (ε) of 102 M −1 cm −1 and 289 nm with ε of 36.1 M −1 cm −1 , whereas ClO − has a strong absorption peak at 292 nm with ε of 378 M −1 cm −1 and does not show strong absorption around 230 nm.…”

“…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]. Since reaction rate constants of ClO − with HO˙, Cl˙, and Cl 2˙− are higher than those of HOCl (Table 1), the radical scavenging effect of free chlorine is stronger in basic pH than in acidic pH.…”

“…O˙− + H 2 O ↔ HO˙ + OH − (6) O( 1 D) + H 2 O → H 2 O 2 (7) Since hydrogen peroxide (H 2 O 2 ) is further photolyzed and produces two HO˙ [13], the photolysis of ClO − through reactions (4) and (5) is proved to produce two active radicals; 2HO˙ or HO˙ + Cl˙. According to these chain reactions, the reaction (2) is proved to be an overall reaction of reactions (4) and (6).…”

“…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.…”