The multiple roles of chlorite on the concentrations of radicals and ozone and formation of chlorate during UV photolysis of free chlorine
Graphical Abstract
Introduction
Advanced oxidation processes (AOPs), which generate highly reactive species, are becoming increasingly necessary in drinking water treatment for abatement of micropollutants, in view of the potential hazards that micropollutants can cause to human health (Boczkaj and Fernandes, 2017; Kishimoto, 2019). The UV/chlorine AOP (i.e., UV photolysis of free chlorine including hypochlorous acid and hypochlorite ions), which generates hydroxyl radicals (HO·), reactive chlorine species (RCS), and ozone via chlorine photolysis, has stood out of numerous AOPs for micropollutant abatement, due to the easy installation of UV lamps in the current water treatment units and the relatively low chemical and energy demand of this process (Cheng et al., 2019; Fang et al., 2014; Wu et al., 2019; Yin et al., 2018a). The UV/chlorine AOP is often placed at the end of the drinking water treatment train to further purify the treated water and thus is affected by the water quality and the prior treatment processes (e.g., pre-oxidation, disinfection, and/or pH adjustment). For example, the chloride ions (Cl−) and bicarbonate/carbonate ions (HCO3−/CO32−) that are derived from prior treatment processes (e.g., pre-oxidation, disinfection, and/or pH adjustment) have been reported to affect the efficacies of micropollutant degradation by the UV/chlorine AOP via radical scavenging and transformation (Fang et al., 2014; Wu et al., 2017).
In pre-oxidation and/or disinfection practices, chlorine dioxide (ClO2) has emerged as a common alternative to free chlorine because of its low yields of trihalomethanes (THMs) and haloacetic acids (HAAs) and high biocidal efficacy in water (Al-Otoum et al., 2016; Jia et al., 2017; Richardson et al., 1994; Zhong et al., 2019). It has been used as the primary disinfectant in drinking water treatment in some countries, including China, Italy, and USA (Gan et al., 2020; De Luca et al., 2008; Richardson et al., 1994; Zhong et al., 2019). In the case when ClO2 is applied prior to the UV/chlorine AOP, for either disinfection or pre-oxidation, ClO2− is formed inevitably from ClO2 (with a molar yield of ~70%) in the treated water and will be brought to the UV/chlorine AOP (Jia et al., 2017; Gan et al., 2019). However, the effects of ClO2− on the UV/chlorine AOP in the drinking water context are unclear: Would the photolysis of ClO2− generate reactive species? Would ClO2− react with radicals/ozone that are generated in the UV/chlorine process and reduce their concentrations? Would the reactions transform ClO2− to other products of health concern (e.g., chlorate and perchlorate)?
UV photolysis of ClO2− has been considered and studied as an AOP for simultaneous removal of elemental mercury (Hg0), nitric oxide (NO), and sulfur dioxide (SO2) from flue gases (Hao et al., 2019a, Hao et al., 2019b). Primary radicals (e.g., ClO· and O·−) are generated from UV photolysis of ClO2− via Eq. 1 (Buxton and Subhani, 1972). Secondary radicals (e.g., HO· and Cl·) can also be formed via a series of chain reactions (Chuang et al., 2017; Guo et al., 2017). However, the ClO2− doses (e.g., 10.0 g·L−1 as NaClO2) in these studies were high and irrelevant to drinking water treatment, which is limited by the regulation-limited concentration of ClO2− aforementioned. Moreover, the concentrations of various radicals that were generated from ClO2− photolysis were not evaluated in these studies (Hao et al., 2019a, Hao et al., 2019b). Photolysis of ClO2− has also been reported to generate triplet oxygen atom (O3P), which is an ozone precursor (Buxton and Subhani, 1972). Whether ozone can be generated from ClO2− photolysis in aqueous solutions and the corresponding concentration relevant to drinking water scenarios remain unclear.ClO2−, on the other hand, may decrease the radical/ozone concentrations in the UV/chlorine AOP because it competes for UV photons with chlorine, as an inner filter, and scavenges radicals/ozone that are generated from chlorine photolysis (Buxton and Subhani, 1972; Guo et al., 2017). The molar absorption coefficient of ClO2− (-ClO2−) at 250 nm has been reported to be 140 M−1·cm−1 (Hong and Rapson, 1968), which is 2–3 times higher than the molar absorption coefficients of hypochlorous acid (-HOCl = 60 M−1·cm−1) and hypochlorite (-OCl− = 58 M−1·cm−1) at 254 nm (Kwon et al., 2018). As for radical scavenging, ClO2− exhibits high reactivities towards HO·, Cl·, ClO·, and ozone, with second order rate constants of 7.00 × 109, 7.90 × 109, 9.40 × 108, and 4.00 × 106 M−1·s−1, respectively (Bulman et al., 2019; Guo et al., 2017). However, the extent of inner filtering and the scavenging towards different radicals and ozone by ClO2− relevant to treating ClO2−-containing drinking water by the UV/chlorine AOP remain unknown.
Based on the literature discussed above, ClO2− was hypothesized to exhibit multiple roles in the UV/chlorine AOP since ClO2− can simultaneously generate radicals/ozone, compete for UV photons, and scavenge radicals/ozone. It is thus necessary to differentiate the multiple roles of ClO2− and evaluate the significance of each role, in order to improve the fundamental understanding that are essential to the design and operation of the UV/chlorine AOP for abatement of micropollutants of different reactivities towards various reactive species in ClO2−-containing drinking water.
Another hypothetical concern is the formation of chlorate (ClO3−) and perchlorate (ClO4−) from the oxidation of ClO2− in the UV/chlorine AOP. ClO3− potentially leads to goitrogens and methemoglobinemia through ingestion and inhalation, while ClO4− is a thyroid hormone disruptor (Alfredo et al., 2015; Rao et al., 2012). A health reference level of 210 μg·L−1 for ClO3− and a proposed MCL of 56 μg·L−1 for ClO4− were reported by US EPA (US, 2016, US, 2016). UV photolysis of chlorine produces ClO3− with a molar yield of 2.0–30.0%, and ClO4− with a molar yield of 1.2 × 10−3% (Rao et al., 2012). The high reactivities of the reactive species (e.g., O3P, ozone, HO·, and ClO·) towards ClO2− suggest that ClO2− is oxidized in the UV/chlorine AOP, though the oxidized byproducts remain unrevealed and could be radical-specific (Bulman et al., 2019; Guo et al., 2017).
This study was thus designed to fill the knowledge gaps and verify the hypotheses made above. The effects of ClO2− on radical/ozone concentrations and ClO3−/ClO4− formation in the UV/chlorine AOP under drinking water relevant conditions were investigated. The generation of reactive species from ClO2− photolysis, the inner filter effect and the radical/ozone scavenging effect of ClO2− were investigated individually and their contributions to the overall effect of ClO2− on radical/ozone concentrations in the UV/chlorine AOP were differentiated both experimentally and numerically. The yields of ClO3−/ClO4− from ClO2− oxidation and the contributing reactive species in the UV/chlorine AOP were reported.
Section snippets
Chemicals
Sodium chlorite (NaClO2), sodium hypochlorite (NaClO), sodium chlorate (NaClO3), sodium perchlorate (NaClO4), sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), sodium thiosulfate (Na2S2O3), nitrobenzene (NB), benzoic acid (BA), 1, 4-dimethoxybenzene (DMOB), cinnamic acid, ascorbic acid, and benzaldehyde were purchased from Sigma-Aldrich. The stock solutions were prepared by dissolving or diluting these chemicals in the deionized water (18.2 MΩ·cm). Solutions were
Effect of ClO2− on radical and ozone concentrations in the UV/chlorine process
The concentrations of radicals that were generated in the UV/chlorine process in the absence and presence of ClO2− were experimentally determined using competition kinetics based on the degradation rate constants of the selected probe compounds (Table S2) (Guo et al., 2017; Yin et al., 2020). NB reacts rapidly with HO· but barely reacts with RCS (Table S2) (Bulman et al., 2019). It was used as a probe compound to determine the concentration of HO· only. BA is highly reactive towards HO· and Cl·
Conclusions
ClO2−, at drinking water relevant concentrations of 0.1–1.0 mg·L−1 as NaClO2, decreased the concentrations of radicals (i.e., HO·, Cl·, and ClO·) and ozone in the UV/chlorine process and the extent of reduction followed the order of ClO· > HO· > ozone > Cl·. Radical/ozone scavenging was the predominant role of ClO2− in the UV/chlorine process and the inner filter effect of ClO2− was minor. UV photolysis of ClO2− generated HO·, but the generation of Cl·, ClO·, and ozone was negligible. HO·
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the Hong Kong Innovation and Technology Fund (grant number GHP/010/18GD), Guangdong Province Science and Technology Planning Project (2019A050503006), and the Hong Kong Research Grants Council (grant number T21-604/19-R).
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