Efficiency of pre-oxidation of natural organic matter for the mitigation of disinfection byproducts: Electron donating capacity and UV absorbance as surrogate parameters
Graphical abstract
Introduction
During water treatment, a large fraction of chlorine (Free Available Chlorine, FAC), the most commonly used disinfectant, reacts with natural organic matter (NOM), partially leading to the formation of disinfection byproducts (DBPs) (Richardson et al., 2007; Sedlak and von Gunten 2011; von Gunten 2018). Pre-oxidation is a possible treatment option to reduce NOM reactivity towards chlorine prior to disinfection and to mitigate DBP formation (Gallard and von Gunten 2002; Gan et al., 2015; Yang et al., 2013b). Depending on the type of oxidant, the water characteristics and the type of DBPs, differences in DBP mitigation efficiency have been observed during pre-oxidation treatment (de Vera et al. 2015; Gan et al., 2015; Jiang et al., 2016c; Yang et al., 2013a; Yang et al., 2013b). In previous studies, the efficiency of oxidants has been compared based on their concentrations/doses (Jiang et al., 2016b; Xie et al., 2013). In other studies, virus inactivation efficiency (Selbes et al., 2014), or the concentration of oxidants commonly used in drinking water plants guided the choice of their doses (Jones et al., 2012; Xie et al., 2013; Yang et al., 2013b). However, each oxidant used for pre-oxidation has its own characteristics. Chlorine dioxide (ClO2) reacts with NOM moieties (e.g., activated aromatic moieties) through a one-electron transfer to form chlorite (ClO2–). Recent studies have also highlighted the importance of oxygen transfer mechanisms releasing FAC (Rougé et al., 2018; Terhalle et al., 2018). FAC released by ClO2 can then react through a two-electron oxidation or electrophilic aromatic substitution (Criquet et al., 2015). Ozone (O3) is particularly reactive towards olefins, leading to cleavage of C=C bonds through the Criegee mechanism, activated aromatic moieties, or neutral amines (Lim et al., 2019; von Sonntag and von Gunten 2012). O3 reacts mostly by oxygen or electron transfer associated with the possible release of hydroxyl radical (•OH), singlet oxygen (1O2), superoxide radical (O2•–) or hydrogen peroxide (H2O2) (von Gunten 2003). •OH released by O3 exhibits a very high reactivity towards a wide range of moieties, mostly through addition or hydrogen abstraction (von Sonntag 2007). Permanganate (Mn(VII)) and ferrate (Fe(VI)) can react through electrophilic attack on double bonds (olefins), or electron transfer, notably with phenolic compounds and neutral amines (Perez-Benito 2009; Shin and Lee 2016; Waldemer and Tratnyek 2006). Furthermore, the products of these oxidants, Mn(VI), Mn(V) and Fe(V), are also highly reactive (Rush and Bielski 1995; Rush et al., 1995; Simándi and Záhonyi-Budó 1998; Záhonyi-Budó and Simándi 1996).
Considering the large array of reaction mechanisms, it is difficult to compare the impact of each pre-oxidation treatment on the mitigation of DBPs based on a specific oxidant dose. Comparing the reactivity of oxidants based on their impact on NOM characteristics may be an interesting alternative. The absorbance at 254 nm (UV254), or the SUVA254 (UV absorbance at 254 nm divided by the concentration of the dissolved organic carbon, DOC), have been widely used as proxies for NOM aromaticity and its reactivity towards chlorine (Croué et al., 2000; Reckhow et al., 1990; Weishaar et al., 2003). The capacity of other spectrophotometric indicators, e.g., the absorbance at 272 nm or spectral slopes, to characterize organic matter has also been investigated (Helms et al., 2008; Korshin et al., 1997b; Wenk et al., 2013). These spectrophotometric indicators were shown to correlate with the formation of DBPs such as trihalomethanes (THMs), haloacetic acids or adsordable organic halogen (AOX) in some studies (Amy et al., 1987; Archer and Singer 2006; Chen and Valentine 2008; Croué et al., 2000; Edzwald et al., 1985; Korshin et al., 1996, 1997a; Korshin et al., 2002). However, poor correlations between SUVA254 or UV254 and DBP formation have been reported in other investigations (Ates et al., 2007; Weishaar et al., 2003). Recently, the measurement of the electron donating capacity (EDC) was developed, based on a one-electron transfer from NOM to a radical, ABTS•+ (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (Aeschbacher et al., 2012; Önnby et al., 2018). Monitoring the ABTS•+, either by spectrophotometry or chronoamperometry (Chon et al., 2015; Walpen et al., 2016), allows for the estimation of the electrons availability in NOM for its reactions with oxidants. This method has been successfully applied to characterize the NOM reactivity with oxidants (de Vera et al. 2017; Önnby et al., 2018a; Remucal et al., 2020; Walpen et al., 2020), and used as a proxy for activated aromatic moieties in NOM (Aeschbacher et al., 2012; Walpen et al., 2016), which are important precursors of halogenated DBPs. Furthermore, the use of EDC combined with UV254 allowed to better understand the reaction mechanisms of ClO2, O3 and FAC with NOM (Önnby et al., 2018a; Wenk et al., 2013). Notably, the oxidation of phenolic-type moieties to quinone-type moieties leads to a higher EDC abatement (quinone-type moieties are poor electron-donating compounds) compared to the UV254 abatement (quinone-type moieties retain some absorbance). It has been demonstrated that ClO2 oxidation leads to a limited EDC abatement compared to UV254, which was explained by the formation of quinone-type moieties (Wenk et al., 2013). In comparison to ClO2, O3 abated more UV254 due to, in part, the opening of aromatic rings (Wenk et al., 2013). However, at near neutral pH (in presence of an •OH quencher) the relative abatement of EDC was still more pronounced than for UV254, suggesting that quinone-type moieties were also formed with O3 under these conditions (Önnby et al., 2018a).
The aims of this study are (i) to compare the impact of increasing doses of ClO2, O3, Fe(VI) and Mn(VII) on NOM properties, i.e. EDC and UV254 abatement, based on their known reaction mechanisms with model compounds and (ii) to explore the possibility of using the EDC and UV254 as complementary pre-oxidant-independent surrogates for predicting the chlorine demand and the formation of DBP during post-disinfection. The impact of different pre-oxidant doses of ClO2, O3, Fe(VI) and Mn(VII) on Suwannee River NOM (SRNOM) extract characteristics was monitored using both the EDC and UV254. The pre-oxidized samples were then chlorinated and the chlorine consumption, the AOX, THM and haloacetonitrile (HAN) formation were compared to the measured NOM extract characteristics (EDC and UV254).
Section snippets
Chemicals and reagents
Sodium chlorite (80%) and a sodium hypochlorite solution (10–15%) were purchased from Sigma-Aldrich, with impurities being mostly chloride and traces of chlorate (< 0.01 µM of ClO3– per µM of NaOCl or NaClO2). All other chemicals were of analytical grade quality (≥ 98%). Solutions were prepared with ultrapure water (Purelab Ultra, Elga, UK). SRNOM extract was purchased from the International Humic Substances Society (Cat. No. 2R101N).
Preparation of oxidant solutions
Chlorine stock solutions were prepared from a sodium
Impact of oxidation on SRNOM properties
The relative EDC and UV254 abatements (%), were measured after treatment of 3 mgC L−1 of SRNOM at pH 8 with different specific doses of ClO2, O3 (with or without t-BuOH), Fe(VI) or Mn(VII) (Figs. 1a and b). Both the relative EDC and UV254 decreased with increasing doses of oxidants and the extent of abatement depended on the type of oxidant.
For a specific pre-oxidant dose of 4 µM ox mgC−1, the relative EDC abatement was highest for Mn(VII), with ~ 70–75% EDC abatement, similar for ClO2, Fe(VI)
Conclusions
An efficient control of the pre-oxidant dose is a prerequisite for an optimized disinfectant demand, to mitigate the formation of potentially toxic DBPs while maintaining a disinfectant residual. For pre-oxidation with ClO2, Mn(VII) or Fe(VI), a minimum EDC abatement (50% under our experimental conditions) is required to abate FAC-reactive moieties efficiently. Compared to the EDC, the relative UV254 abatement occurs to a much lower extent, which leads to a significantly smaller measurement
Notes
The authors declare no competing financial interest.
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.
Acknowledgments
We acknowledge Peter Hopper for his assistance with GC–MS. Curtin University (Curtin International postgraduate Research Scholarship) and Curtin Water Quality Research centre are also acknowledged for providing financial support for V.R.
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