Bromate control during ozonation by ammonia-chlorine and chlorine-ammonia pretreatment: Roles of bromine-containing haloamines

Journal Pre-proofs Bromate control during ozonation by ammonia-chlorine and chlorine-ammonia pretreatment: Roles of bromine-containing haloamines Li Ling, Zhuo Deng, Jingyun Fang, Chii Shang PII: DOI: Reference:

S1385-8947(19)32860-8 https://doi.org/10.1016/j.cej.2019.123447 CEJ 123447

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

16 August 2019 20 October 2019 8 November 2019

Please cite this article as: L. Ling, Z. Deng, J. Fang, C. Shang, Bromate control during ozonation by ammoniachlorine and chlorine-ammonia pretreatment: Roles of bromine-containing haloamines, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123447

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Bromate

control

chlorine-ammonia

during

ozonation

pretreatment:

by

Roles

ammonia-chlorine of

and

bromine-containing

haloamines

Submitted to: Chemical Engineering Journal

Li Linga, Zhuo Denga, Jingyun Fangb*, Chii Shanga,c* a

Department of Civil and Environmental Engineering, the Hong Kong University of Science and

Technology, Clear Water Bay, Kowloon, Hong Kong. b

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation

Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China c

Hong Kong Branch of Chinese National Engineering Research Center for Control & Treatment of

Heavy Metal Pollution, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong *

Address correspondence to either author. Phone: +852 2358 7885 (C. Shang); + 86 20 8411 0692 (J.

Fang).

Fax:

+852

2358

[email protected]

1534

(C.

Shang).

E-mail: (J.

[email protected]

(C.

Shang); Fang).

1

Graphical abstract

Highlights ► Bromide is mainly masked as NHBrCl in the NH3-Cl2 strategy. ► Bromide is partially masked as NHBr2 in the NH3-Cl2 and Cl2-NH3 strategies. ► NHBrCl and NHBr2 are more stable than NH2Br in ozonation. ► The Cl2-NH3 strategy controls the BrO3- formation better at acidic pH levels. ► The NH3-Cl2 strategy is less pH-dependent and better at low bromide concentrations.

2

Abstract The ammonia-chlorine (NH3-Cl2) and chlorine-ammonia (Cl2-NH3) pretreatment strategies are commonly used for controlling bromate formation during the ozonation of bromide-containing waters. In this study, the roles of bromine-containing haloamines in ozonation after both strategies are identified at different pHs and initial bromide dosages. Bromide is primarily masked as bromochloramine (NHBrCl) and monobromamine (NH2Br) in the NH3-Cl2 and Cl2-NH3 strategies, respectively, and partially masked as dibromamine (NHBr2) with similar concentrations (p > 0.05) in both strategies. NHBrCl and NHBr2 are 4 times more stable than NH2Br in the presence of ozone, while they are all recalcitrant to HO•, and thus make the NH3-Cl2 strategy control the bromate formation better than the Cl2-NH3 strategy. At pH 6, the Cl2-NH3 strategy reduces 15–36% of the bromate formation comparing with those at pHs 7 and 8, because more NH2Br is transformed to stable NHBr2, while the NH3-Cl2 strategy is less pH-dependent, because the NHBrCl concentrations remain the same at different pHs (p > 0.05). At initial bromide concentrations of 6.25 to 18.75 µM, the Cl2-NH3 strategy shows no difference in controlling the bromate formation (p > 0.05). However, ~ 2.5-time more bromate is formed at the bromide concentration of 18.75 µM comparing with those at 6.25 and 12.5 µM, in the NH3-Cl2 strategy, because the 6-min NH3-Cl2 pretreatment is insufficient to mask all bromide at high concentrations as NHBrCl. This study demonstrates that the masking of larger amount of bromide as NHBrCl and NHBr2 in ozonation after either strategy is essential in controlling the bromate formation.

Keywords:

bromate

formation,

ozonation,

chlorine-ammonia,

ammonia-chlorine,

bromine-containing haloamines

3

1. Introduction Ozonation is practiced worldwide for water disinfection, oxidative destruction of micropollutants, and control of tastes and odors. The coupling of ozonation with biological activated carbon (ozone/BAC) for the potable reuse of municipal wastewater effluents becomes an attractive alternative advanced treatment process to those based on membrane and advanced oxidation processes [1]. However, the potable water produced with the ozone/BAC process contains elevated bromide (Br-) concentrations, because the process does not remove Br-. Ozonation of Br--containing waters produces bromate (BrO3-) either through the production of hypobromous acid/hypobromite ions (HOBr/OBr-) by ozone directly or bromine oxide radicals (BrO•) initiated by hydroxyl radicals (HO•) (Eqs. 1–9, Table 1) [2], [3], [4], [5]. BrO3- is a class 2B carcinogen regulated by WHO and USEPA in drinking water at a maximum contaminant level of 10 μg/L [6]. The situation becomes even worse with the additional Br- discharges from the chemical industry and municipal waste incinerators into rivers [7]. Controlling the BrO3- formation is thus important in the ozone/BAC process for the potable reuse of municipal wastewater effluents. Several strategies, such as pH depression, ammonia (NH3) addition, and chlorine-ammonia (Cl2-NH3) and ammonia-chlorine (NH3-Cl2) pretreatment, have been proposed [2], [5], [8], [9], [10], [11]. At lower pH, ozone decomposes more slowly to form less HO•, and the HOBr/OBr- equilibrium moves toward HOBr. HOBr only reacts with HO• to form BrO3-, so less BrO3- is formed [3]. The NH3 addition allows NH3 to react preferentially with the key intermediate HOBr/OBr- to form monobromamine (NH2Br) during ozonation [2]. These two strategies can reduce the BrO3- formation by roughly 50% [2], [12]. However, the BrO3- formation in ozonation may not be well-controlled by the pH depression and NH3 addition, if high ozone dosages or long contact times are needed [3].

4

The Cl2-NH3 and NH3-Cl2 pretreatment strategies have been developed for better BrO3- control [5], [13]. The Cl2-NH3 strategy has been reported to lead to a 4-fold decrease in the BrO3- formation compared with the NH3 addition during the ozonation of Br--containing waters at alkaline conditions [5]. In the Cl2-NH3 strategy during the pretreatment stage, Br- will first react with the added free chlorine (HOCl/OCl-) to form HOBr/OBr-, which then reacts with the subsequently added NH3 to form NH2Br. The reaction rate constant between NH2Br with ozone (Eq. 10) is 4 times lower than that of Br- (Eq. 1), so less BrO3- will be produced [5]. In the NH3-Cl2 strategy, prior to ozonation, NH3 is added or exists as naturally occurring NH3 in a range of 0.2 to 12 mg/L [14], and is followed by the addition of HOCl/OCl- to form monochloramine (NH2Cl) (Eq. 11). The formation of NH2Cl in the pretreatment stage quenches HO• in ozonation, which controls the BrO3- formation [5], [15]. However, the good control of the BrO3- formation in ozonation after the Cl2-NH3 and NH3-Cl2 strategies cannot be satisfactorily explained by the masking of Br- as NH2Br and the quenching of HO• by NH2Cl. We hypothesize that the formation of dibromamine (NHBr2) and bromochloramine (NHBrCl) in the two pretreatment strategies contributes to the better control. The roles of NHBr2 and NHBrCl are thus worth investigating. The objectives of this study are thus to 1) identify the roles of the bromine-containing haloamines in ozonation after the two pretreatment strategies, 2) compare the effectiveness of the BrO3- control by the two strategies at different pHs and Br- concentrations, and 3) determine the mechanisms of two pretreatment strategies for the BrO3- control.

2. Materials and methods 2.1 Chemicals and Solution Preparations All solutions, unless specified otherwise, were prepared from reagent grade chemicals. Dilution to the target concentrations was with pure water (18.2 MΩ/cm) prepared using a NanopureTM Diamond 5

purifier system (Barnstead). A stock of hypochlorite solution at 1500–2000 mg/L as Cl2 was prepared by diluting sodium hypochlorite (mass conc. > 4%) (from Allied Signal). A concentrated ozone stock solution (about 15–20 mg/L) was produced by continuously bubbling ozone-containing oxygen gas into a 500-mL flask of pure water through a glass diffuser. A 10-mM sodium bromide (NaBr) stock solution was used as the Br- source. A 50 mM sodium sulfite stock solution was prepared and used within 3 days to quench the oxidants, including ozone and haloamines, in the samples. The stock solutions of the phosphate buffer at pH 6 and 7 and the borate buffer at pH 8 were used to adjust and maintain the solution pH. Tert-butanol (t-BuOH) of HPLC grade was obtained from Sigma-Aldrich. The detailed procedures to prepare standards of NH2Cl, NH2Br, NHBr2 and NHBrCl are available in Text S1 of the supplementary material.

2.2 Experimental procedures All experiments were performed at 20°C using half-liter amber glass bottles, each capped with a dispenser. They were filled with 400 mL of the test solution spiked with 6.25–18.75 μM Br- and buffered at a certain pH (phosphate buffer or borate buffer, 2 mM). 1-mM t-BuOH was added in some cases to scavenge HO• [16]. In the Cl2-NH3 strategy, the solutions were dosed with 50-µM chlorine for 5 min, followed by the addition of 100-µM NH3 for 1 min, following Buffle et al. (2004) [5]. In the NH3-Cl2 strategy, 50-µM chlorine was added to an NH3-containing water (at 100 µM) to form NH2Cl in-situ, while other haloamines were formed during the 6-min (unless specified otherwise) pretreatment. The reasons for selecting such a pretreatment time and chlorine to NH3 dosages are justified in Text S3. In all cases, two samples were taken after the 6-min pretreatment. One was analyzed instantly for the haloamine concentrations (without quenching), and the other was quenched by sodium sulfite and analyzed for BrO3-. After the sampling, ozone (104.2 µM) was added immediately, with stirring. After ozonation for 30 min, two samples were also taken and 6

subjected to the analyses of haloamine concentrations (without quenching) and BrO3- concentrations (after quenching by sodium sulfite). Another test was conducted in the same manner in solutions containing 5 mg/L (as DOC) Suwannee River humic acid and buffered at pH 7 to compare the formation of halogenated byproducts from the Cl2-NH3 and the NH3-Cl2 processes (details shown in Text S4). All tests were duplicated. Error bars in the figures represent the maximum and minimum values of the duplicated test results. Statistical significance (p < 0.05) of the data was evaluated by one-way analysis of variance (ANOVA).

2.3 Analytical methods The stock solution of the free chlorine was periodically standardized by the DPD/FAS titration [17]. Ozone concentrations in the ozone stock solution were measured by the indigo method according to Standard Method 4500-O3 B [18]. BrO3- concentrations in the water samples were determined using a reagent-free ion chromatograph (ICS-3000, Dionex) equipped with an anionic column (IonPac® AS19, Dionex). The detection limit of BrO3- was 5 μg/L, with an injection volume of 250 μL. Solution pHs were measured using a benchtop Orion 3-star plus pH meter from Thermo Scientific. UV–vis spectroscopy was used to estimate haloamine concentrations [19]. According to Luh and Mariñas (2014), the spectra of NH2Cl, NHBrCl, NH2Br, NHBr2 and tribromamine (NBr3) partially overlap (Figure S1), and five wavelengths at 220, 232, 245, 258 and 278 nm were all used to measure the concentrations of the five haloamine species using a temperature-controlled 1-cm path length cell with the spectrophotometer (Lambda25, PerkinElmer) scanning from 220 to 350 nm. The molar absorptivities of NH2Cl at the five wavelengths were determined by dividing the measured absorbance values at the five wavelengths by the concentration of NH2Cl determined by the DPD/FAS titration. The molar absorptivities of NH2Br and NHBr2 at the five wavelengths were determined according to [19], [20], [21], with simultaneous measurements of the concentration and 7

absorbance. The molar absorptivity for NBr3 has been reported in Galal-Gorchev and Morris (1965), and it was used to confirm that the concentrations of NBr3 were below the detection limit under all experimental conditions tested in this study [21], [22]. The molar absorptivity values of NHBrCl at the five wavelengths were calculated assuming single-atom stoichiometry [20]: [NHBrCl] = [apparent bromine]/2 [NH2Cl] = ([total oxidant] - 2[apparent bromine])/2 where the “apparent bromine” concentration arose from the rapidly formed red oxidation product in the first step of the DPD/FAS titration. The “total oxidant” concentration was the continued titration result after adding one gram of KI. The absence of dichloramine (NHCl2) and NHBr2 in the NHBrCl standard solution was confirmed following Valentine (1986) [20]. Table 2 lists the molar absorptivities of the five haloamines determined and used in this study. They are comparable to published values (shown in parentheses). It should be noted that the concentrations of NH2Cl in the testing solutions were determined by the DPD/FAS titration because they were much higher than those of the other haloamines. In addition, as aforementioned, the concentrations of NBr3 were negligible in all cases. The spectroscopic method, coupled with multi-variable linear regression analyses at wavelengths of 220, 232 and 278 nm, was used to estimate the concentrations of NHBrCl, NHBr2 and NH2Br. The detection limits were determined to be 0.35 μM for NHBr2, 0.33 μM for NH2Br, and 0.35 μM for NHBrCl (details of the determination can be found in Text S2 and Table S1). The high values of the detection limits restricted our investigation at more realistic Br- concentrations. Thus, the initial concentrations of Br- of 6.25–18.75 μM were used. It should be noted that the spectroscopic method was used in solutions based on pure water in the absence of ozone only, as there was interference from ozone. It should also be noted that HOBr and OBr- display maximum molar absorptivities at 284 nm and 329 nm,

8

respectively [23], [24], and their presence under the test conditions contributed less than 7% to the haloamine measurements.

2.4 Computer Simulations The haloamine formation during the pretreatment stages was simulated using Kintecus® modeling software [25]. The simulations simultaneously solved the coupled reactions at an assigned pH by using the initial concentrations of the reactants and the corresponding set of rate constants reported in the literature or experimentally determined from supplementary tests in the current study, details of which are shown in Table 1. It should be noticed that the model is only valid in predicting the haloamine formation during the pretreatment stages.

3. Results and discussion 3.1 BrO3- formation and bromine-containing haloamine concentrations Figure 1 illustrates the concentrations of BrO3- after 30 min of ozonation of the Br--containing (12.5 µM, 1 mg/L) water pretreated with either the NH3-Cl2 or Cl2-NH3 process at chlorine and NH3 dosages of 50 and 100 µM, respectively, in the presence and absence of t-BuOH at 20 oC and pH 8. It also shows the concentrations of bromine-containing haloamines after the pretreatment and after the 30-min ozonation. As shown in Figure 1a, the bromate concentration after 30 min ozonation in the absence of t-BuOH was 1.4 times higher after the Cl2-NH3 pretreatment compared with that after the NH3-Cl2 pretreatment. Meanwhile, the NH3-Cl2 and Cl2-NH3 pretreatment strategies produced similar total bromine-containing haloamine concentrations after the 6-min pretreatment stage (p > 0.05, Table S2), suggesting that the two strategies were able to mask Br- as haloamines to a similar extent. However, the much reduced total bromine-containing haloamine concentrations and the higher BrO3- formation after the Cl2-NH3 pretreatment strategy reveals that ozonation diminished the 9

masking more, releasing more Br- to be further oxidized to BrO3- during ozonation. It should be noted that the ozone exposures in ozonation after the two pretreatment strategies were similar (Figure S2). (1) Haloamine species during the pretreatment stage The NH3-Cl2 strategy was better in controlling the BrO3- formation compared to the Cl2-NH3 strategy, which can be attributable to the formation of different bromine-containing haloamine species during the two pretreatment strategies. After the Cl2-NH3 pretreatment strategy, the concentration of NH2Br was about 7 times higher than those of NHBr2 and NHBrCl, while after the NH3-Cl2 pretreatment strategy, the concentration of NHBrCl was about twice and four times those of NH2Br and NHBr2, respectively (Figure 1b). The formation of bromine-containing haloamines during the Cl2-NH3 and NH3-Cl2 pretreatment strategies at pH 8 was also kinetically modeled using the reactions in Table 1 and shown in Figure 2. The simulation results after the 6-min pretreatment are in reasonable agreement with the corresponding experimental results. In the Cl2-NH3 pretreatment strategy, the simulation demonstrates that Br- is fully oxidized by HOCl/ClO- to HOBr/BrO- in the 5-min pre-chlorination via Eq. 12 in Table 1. The subsequent NH3 addition rapidly (< 1 s) transformed HOCl/ClO- to NH2Cl (36.8 μM) via Eq. 13. The unreacted NH3 and the formed NH2Cl react with HOBr/BrO- to form NH2Br (11.8 μM) via Eq. 14 and NHBrCl (0.68 μM) via Eq. 15, respectively [26], [27]. The higher NH2Br concentration compared to that of NHBrCl is mainly due to the faster reaction rate constant of Eq. 14 than that of Eq. 15. The formed NH2Br then gradually decomposed to NHBr2 through the disproportionation reaction via Eq. 16 [21], [28]. Therefore, the major bromine-containing haloamine after the Cl2-NH3 pretreatment strategy is NH2Br with the presence of lower concentrations of NHBr2 and NHBrCl. Unlike the Cl2-NH3 pretreatment strategy, the added chlorine in the NH3-Cl2 strategy reacts rapidly with NH3 but not Brto form NH2Cl, which reacts with Br- to produce NHBrCl and then NHBr2 (Eq. 17 in Table 1) [29]. 10

The concentration of NHBrCl and NHBr2 reached about 5.0 and 2.5 μM, respectively, after 6 min. In addition, the formation of NH2Br in the process was primarily from the disproportionation of NHBr2 (Eq. 16). Therefore, the major bromine-containing haloamine after the NH3-Cl2 pretreatment strategy is NHBrCl with the presence of lower concentrations of NHBr2 and NH2Br. (2) Reactivity of haloamines during ozonation Higher stabilities of the haloamine species in ozonation is another reason attributable to the better control of the BrO3- formation in the NH3-Cl2 strategy. Upon the completion of the Cl2-NH3 and NH3-Cl2 pretreatment, ozone was added to the solution and allowed to react for 30 min, both in the absence and presence of t-BuOH. As shown in Figure 1b, the concentration of NH2Br was reduced by more than 30%, while the concentrations of NHBrCl and NHBr2 were not affected after 30-min ozonation regardless of the absence or presence of t-BuOH. The large decrease in NH2Br concentration is primarily attributed to the oxidation of NH2Br by ozone at the rate constant of 40 M-1 s-1(Eq. 10) [30], [31], but not to its autonomous disproportionation (Eq. 16), because the rate of Eq. 16 is one order less than the rate of Eq. 10 under the experimental condition (detailed calculation can be found in Text S5). The large decrease in NH2Br concentration is not attributed to HO•, since the presence of t-BuOH did not affect the total bromine-containing haloamine concentrations or their species distribution after ozonation (Figure 1). Upon the decomposition of NH2Br by ozone to Br-, it will be oxidized by ozone to HOBr/BrO-, which is then partially quenched by NH3 and NH2Cl as NH2Br and NHBrCl, respectively, or further oxidized by ozone to BrO3-. Br- can also be oxidized to Br• by HO•, then to BrO• by ozone, and finally to BrO3- [5]. The addition of 1 mM t-BuOH to the reaction system can quench HO•, but it can only inhibit about 50% of the BrO3- formation, suggesting that the BrO3- formation in ozonation after the Cl2-NH3 and NH3-Cl2 pretreatment strategies can be attributed to the oxidation of the regenerated Br- by ozone and HO• together.

11

The minor change in the NHBrCl and NHBr2 concentrations before and after 30 min of ozonation was due to their stability to ozone and HO•. The decomposition rates of NHBrCl and NHBr2 are low in the presence of ozone (Figure S3). The rate constant of the reaction between NHBr2 and ozone (Eq. 18) was reported as 10 M-1 s-1 [30], which gives NHBr2 a half-life of 10–15 h at the NHBr2 concentrations anticipated during ozonation (in the 1-5 mg/L range) [28], which is 4 times longer than that of NH2Br (Eq. 10). The reaction rate between NHBrCl and ozone (Eq. 19) was estimated to be 10 M-1 s-1 in our supplementary test (Figure S3), and the NHBrCl decay was as slow as that of NHBr2 (Figure S3). As for the reactivity of haloamines with HO•, the little or no difference (p > 0.05, Table S2) in the concentrations of each bromine-containing haloamine after 30-min ozonation in the presence and absence of t-BuOH (Figure 1b) indicates that NH2Br, NHBrCl and NHBr2 are inert to HO•. In summary, Br- was gradually masked as NHBrCl as the major species, and NHBr2 and NH2Br as minor species in the NH3-Cl2 pretreatment strategy at pH 8, while Br- was oxidized to free bromine and then fast masked as NH2Br as the major species, and NHBr2 and NHBrCl as minor species in the Cl2-NH3 pretreatment strategy. During ozonation, NHBrCl and NHBr2 are more recalcitrant than NH2Br to ozone to release Br- and then to BrO3-. It should be noted that HO• cannot decompose bromine-containing haloamines to regenerate Br-, but it contributes to around 50% of BrO3- formation in both processes at pH 8.

3.2. Effects of pH Figure 3 shows the concentrations of bromine-containing haloamines after 6 min of the NH3-Cl2 and Cl2-NH3 pretreatment strategies and after 30 min of ozonation and the concentrations of BrO3formed after 30 min of ozonation at pHs 6, 7 and 8 in the absence and presence of t-BuOH. At pH 6 without

t-BuOH,

the

two

control

strategies

yielded

similar

concentrations

of

total 12

bromine-containing haloamines and similar concentrations of BrO3-, albeit with different species distributions (p > 0.05, Table S2). After the NH3-Cl2 pretreatment strategy, NHBrCl is the major species formed and is recalcitrant to ozone, while after the Cl2-NH3 pretreatment strategy, NHBr2, rather than NH2Br, is the major species formed mainly due to the acid-assisted disproportionation of NH2Br to NHBr2. However, in the subsequent ozonation after both pretreatment strategies, a similar amount of BrO3- was formed. This is mainly because a similar amount of NH2Br is decomposed and oxidized to BrO3- in ozonation. In the presence of t-BuOH at pH 6, about 40% of NH2Br (1.13 μM) was decomposed after ozonation, which was similar to the 50% of NH2Br decomposition (1.44 μM) without t-BuOH. Meanwhile, the BrO3- formation was reduced to half. This result confirmed our previous hypothesis that the BrO3- formation after the Cl2-NH3 and NH3-Cl2 pretreatment strategies is attributed to the oxidation of the regenerated Br- by ozone and HO• together. With increasing pH from 6 to 8, the total concentrations of bromine-containing haloamines decreased and the BrO3- formation increased significantly in the Cl2-NH3 pretreatment strategy without the t-BuOH addition. The increasing pH shifts the bromine-containing haloamine species from mainly NHBrCl and NHBr2 towards more NH2Br, and the latter can be decomposed by ozone to regenerate Br- at a faster reaction rate constant and then form BrO3-. The increasing pH also increased the steady state concentrations of HO•, which can enhance the BrO3- formation in ozonation [3]. In addition, increasing pH increased the fraction of BrO-, which reacts faster with ozone than with NH3 or NH2Cl [3]. The higher NH2Br concentrations, higher steady state HO• concentrations, and higher fraction of BrO- result in more BrO3- formation (increase from 0.45 μM at pH 6 to 0.70 μM at pH 8) after the Cl2-NH3 pretreatment strategy. On the other hand, the decreasing trend of the total concentrations of bromine-containing haloamines and the increasing trend of the BrO3- formation with increasing pH after the NH3-Cl2 pretreatment strategy are much less significant compared with those after the Cl2-NH3 pretreatment strategy, even at similar ozone exposures 13

(Figure S2). The slight changes were mainly due to NHBrCl being stable and did not transform to NH2Br even at higher pH (Figure S4) [24]. Since less NH2Br and more NHBr2/NHBrCl was formed at higher pH after the NH3-Cl2 strategy compared with that after the Cl2-NH3 strategy, the increase of the BrO3- formation with increasing pHs in the NH3-Cl2 strategy (from 0.46 μM at pH 6 to 0.53 μM at pH 8) is less significant than that in the Cl2-NH3 strategy. In addition, as shown in Figure 3, the addition of t-BuOH decreased the BrO3- formation and reduced the pH-dependency of the BrO3formation after both pretreatment strategies, but it did not change the total concentrations of the bromine-containing haloamines in the 30-min ozonation after either pretreatment strategy. It further indicates that the different pH-dependent performance of the two pretreatment strategies in controlling the BrO3- formation are mainly attributed to the pH-dependent haloamine species distributions and their stabilities. In addition, the increases of the BrO3- formation with increasing pHs after the Cl2-NH3 and NH3-Cl2 pretreatment strategies were about 55% and 15%, respectively, which are less significant than the increase of the BrO3- formation with increasing pHs (100%) without any pretreatment strategy [2]. Modeling of the haloamine concentrations after the two 6-min pretreatment processes at pH 7 and 8 were also conducted. The experimental and modeling results are in good agreement (Table S3), which reinforces the discussion above. It should be noted that the simulation cannot be done at pH 6, because the rate constant of Eq. 14 is not available at pH 6.

3.3. Effects of Br- Concentration Figure 4 illustrates the effects of the Br- concentrations from 6.25 to 18.75 µM (0.5–1.5 mg/L) on the total bromine-containing haloamine concentrations and the BrO3- formation after the Cl2-NH3 and NH3-Cl2 pretreatment strategies followed by ozonation at pH 8. In general, the BrO3- formation increased with increasing initial Br- concentrations, mainly because more NH2Br was formed at 14

higher initial Br- concentrations. NH2Br can be decomposed during the 30-min ozonation to regenerate Br-, which caused the increase of the BrO3- formation. Figure 4 also shows that the NH3-Cl2 strategy controlled the BrO3- formation better than the Cl2-NH3 strategy at low Brconcentrations (6.25 and 12.5 µM), but the opposite trend was observed at a higher Br- concentration of 18.75 µM. At Br- concentrations of 6.25 and 12.5 µM, over 92% of Br- is masked as bromine-containing haloamines after both Cl2-NH3 and NH3-Cl2 pretreatment strategies. The NH2Br concentrations after the NH3-Cl2 strategy are about 1/3 and 5/7 compared with those after the Cl2-NH3 strategy at the Br- concentrations of 6.25 and 12.5 µM, respectively. The experimental results are in good agreement with the simulation results (Table S2 and Figure S5a). However, at a Br- concentration of 18.75 µM, over 92% of Br- can still be masked after the Cl2-NH3 pretreatment, however, only 84% of Br- can be masked after the NH3-Cl2 pretreatment, due to the relatively slow reaction rate constant between NH2Cl and Br-. The unmasked Br- then contribute to the BrO3formation during ozonation and makes the NH3-Cl2 strategy much less effective in controlling the BrO3- formation than the Cl2-NH3 strategy at high Br- concentrations. In fact, more than 95% of Brcan be masked after the NH3-Cl2 pretreatment, if the pretreatment time can be increased to 15 min as found in the experiments and predicted by the kinetic modeling of the NH3-Cl2 strategy at a Brconcentration of 18.75 µM (Figure S6). On the other hand, the Br- concentrations in fresh water are about 1.25–5 µM, which are masked as bromine-containing haloamines by the NH3-Cl2 pretreatment strategy as shown in the simulation (Figure S7).

3.4. Mechanisms of Controlling the BrO3- Formation The mechanisms of controlling the BrO3- formation by different bromine-containing haloamines formed after the Cl2-NH3 and NH3-Cl2 pretreatment strategies are shown in Scheme 1. In the Cl2-NH3 strategy, NH2Br is the dominant species masking Br-. However, the decomposition of 15

NH2Br will regenerate Br-, which can be oxidized by ozone and HO• to BrO3-. At acidic pH, some ozone-recalcitrant NHBr2 will be formed after the Cl2-NH3 strategy to improve the controlling performance of the BrO3- formation, but such improvement will disappear at the alkaline conditions due to the disproportionation of NHBr2 to NH2Br. On the other hand, in the NH3-Cl2 strategy, NHBrCl is the dominant species masking Br-. NHBrCl is 4 times more stable than NH2Br, and thus reduces the overall BrO3- formation in the NH3-Cl2 strategy. It should be noticed that the reaction rate constant between Br- and NH2Cl is low, suggesting that a sufficient amount of pretreatment time and chlorine dosage be provided to mask Br- as NHBrCl. For example, at initial concentrations of Br- and NH3 of 1.1 and 18 μM, respectively, the concentrations of Br- masked as haloamines increased from 0.2 to 0.9 μM with the increasing initial chlorine dosages from 5 to 15 µM (Figure S8). In addition, with an initial chlorine dosage of 15 µM, the concentrations of Br- masked as haloamines increased from 25% to 79% with increasing reaction times from 6 to 30 min, given that excessive NH2Cl is available (Figure S8c). In these cases, with up to 15 µM initial chlorine concentration, Br- is easier to be oxidized to BrO3-, but if more chlorine is added to yield an initial NH2Cl concentration of 50 μM (3.5 mg/L as Cl2), over 92% of Br- at initial concentrations of 1.25 to 5 µM can be masked as haloamines during the 6 min of pretreatment (Figure S7). In these cases with an initial NH2Cl concentration of 50 μM and initial Br- concentrations of 1.25 to 5 µM, masking Bras haloamines becomes the major mechanism. In addition, in the presence of SRHA at 5 mg/L as TOC, the BrO3- formation in ozonation after the Cl2-NH3 and NH3-Cl2 pretreatment strategies decreased from 42.93 μg/L (0.53 μM) to 25.5 μg/L and from 38.07 μg/L (0.47 μM) to 11 μg/L, respectively (Figure S9a). The NH3-Cl2 strategy out-performed the Cl2-NH3 strategy in controlling the BrO3- formation even in the presence of SRHA. The decreased BrO3- formation after both pretreatment strategies was mainly due to the formation of the Br-containing byproducts, such as brominated disinfection byproducts, from the reaction between HOBr/BrO- and SRHA. The 16

formation of tribromomethane (CH3Br) and dibromoacetonitrile (DBAN) in ozonation after the Cl2-NH3 strategy was higher than that after the NH3-Cl2 strategy (Figure S9b), mainly because HOCl/ClO- added in the Cl2-NH3 strategy oxidizes Br- to HOBr/BrO- in the pretreatment stage and prolongs the reaction period between HOBr/BrO- and SRHA [5], [15]. Scheme 1. Reaction schemes in controlling BrO3- formation in the Cl2-NH3 and Cl2-NH3 processes before and during ozonation of Br--containing water.

4. Conclusions and Engineering Implications The water shortage and quality deterioration around the world highlight the increasing need in using less pristine water and even reclaimed water as the source of drinking water. A recent survey revealed that Br- concentrations in major Swiss rivers ranged from < 0.05 mg L-1 to about 50 mg L−1 [7]. Br- also gets accumulated in drinking water when the ozone/BAC process is implemented for the potable reuse of municipal wastewater effluents [1]. The elevated Br- concentrations bring new challenges to drinking water utilities in controlling the BrO3- formation when using ozonation, because previous studies of the Cl2-NH3 and NH3-Cl2 strategies focus mainly on controlling the 17

BrO3- formation at low Br- dosages. This study shows the important roles of bromine-containing haloamines formed in the Cl2-NH3 and NH3-Cl2 pretreatment strategies in controlling the BrO3formation in ozonation, especially at the elevated Br- concentrations. The recalcitrant nature of NHBr2 and NHBrCl towards ozone makes them more stable and releases less Br- than NH2Br in ozonation. In the Cl2-NH3 pretreatment strategy, over 60% of Br- is masked as NH2Br at pH 8, which is easier to be oxidized back to Br- by ozone, while in the NH3-Cl2 pretreatment strategy, over 50% of Br- is masked as NHBrCl, which is stable in the presence of ozone. In general, the NH3-Cl2 pretreatment strategy controls the BrO3- formation better than the Cl2-NH3 pretreatment strategy. At pH 6, the Cl2-NH3 pretreatment strategy controls the BrO3- formation better than at pHs 7 and 8, because NH2Br is mostly transformed to NHBr2. However, pH does not affect the NH3-Cl2 pretreatment strategy, because the strategy mainly forms NHBrCl, whose concentration is less pH-dependent. At different initial Br- concentrations, the Cl2-NH3 pretreatment strategy can control the BrO3- formation well. However, the NH3-Cl2 pretreatment strategy can only control the BrO3formation at Br- concentrations lower than 5 µM in 6 min, because NH2Cl formed in the NH3-Cl2 strategy is insufficient to mask all Br- as NHBrCl and NHBr2 in 6 min. By understanding the roles of bromine-containing haloamines formed after the two strategies in controlling the BrO3- formation in ozonation, we provide some suggestions to utility owners in selecting better controlling strategies or changing operating parameters to reduce the BrO3formation in ozonation depending on their source water qualities. For facilities treating waters with naturally occurring NH3, the NH3-Cl2 pretreatment strategy has to be conducted. In practice, increasing the chlorine dosage to mask Br- as haloamines is not feasible, because extra chlorine preferentially converts NH2Cl to NHCl2 but does not mask Br- as haloamines and more DBPs might be formed. Therefore, adding chlorine earlier to allow enough reaction times between NH2Cl and Bris suggested to mask more Br- as haloamines. In this case, the control of the BrO3- formation is less 18

sensitive to the pH levels of the source water, as the NHBrCl formation is comparable at different pHs. For water without naturally occurring NH3, the selection of the pretreatment strategy depends on the source water pH, Br- concentration, and the time available for pretreatment. For source water with acidic or neutral pHs, the Cl2-NH3 strategy is likely to be a better choice, because it masks Brmainly as more ozone-stable NHBr2 regardless of the Br- concentrations or operation times, but it will generate more halogenated DBPs. However, at alkaline pHs, the NH3-Cl2 strategy may be more suitable if the Br- concentration is low or a longer pretreatment time can be allowed to mask all Brto haloamines. Otherwise, the Cl2-NH3 strategy is still preferred. Acknowledgements This work was supported by Hong Kong’s Research Grants Council under grant number 618312. Appendix A. Supplementary material Supplementary material associated with this article can be found in the online version.

19

REFERENCES [1] Chuang, Y.H., Szczuka, A., Mitch, W.A., 2019. Comparison of toxicity-weighted disinfection byproduct concentrations in potable reuse waters and conventional drinking waters as a new approach to assessing the quality of advanced treatment train waters. Environ. Sci. Technol. 53(7), 3729–3738. [2] Pinkernell, U., von Gunten, U., 2001. Bromate minimization during ozonation: Mechanistic considerations. Environ. Sci. Technol. 35, 2525–2531. [3] von Gunten, U., 2003. Ozonation of drinking water Part I: Oxidation kinetics and product formation. Water Res. 37, 1443–1467. [4] von Gunten, U., 2003. Ozonation of drinking water Part II: Disinfection and by-product formation in presence of bromide, iodide or chlorine. Water Res. 37, 1469–1487. [5] Buffle, M., Galli, S., von Gunten, U., 2004. Enhanced bromate control during ozonation: The chlorine-ammonia process. Environ. Sci. Technol. 38, 5187–5195. [6] WHO, 2011. Guidelines for Drinking-Water Quality 4th ed. World Health Organization. [7] Soltermann, F., Abegglen, C., Götz, C., and von Gunten, U., 2016. Bromide sources and loads in swiss surface waters and their relevance for bromate formation during wastewater ozonation. Environ. Sci. Technol. 50 (18), 9825–9834. [8] Glaze, W. H., Weinberg, H. S., Cavanagh, J. E., 1993. Evaluating the formation of brominated DBPs during ozonation. J. AWWA. 85, 96–103. [9] Hofmann, R., Andrews, R.C., 2007. Potential side effects of using ammonia to inhibit bromate formation during the ozonation of drinking water. J. Environ. Eng. Sci., 6 (6), 739–743. [10] Krasner, S.W., Yates, R., Gabelich, C. J., Liang, S., 2007. Evaluation of alternative bromate control strategies. Proceedings of AWWA Annual Conference. [11] Wert, E.C., Neemann, J.J., Johnson, D., Rexing, D., Zegers, R., 2007. Pilot-scale and full-scale evaluation of the chlorine-ammonia process for bromate control during ozonation. Ozone Sci. Eng. 29 (5), 2007. [12] von Gunten, U., Pinkernell, U., 2000. Ozonation of bromide-containing drinking waters: A delicate balance between disinfection and bromate formation. Water Sci. Technol. 41(7), 53–59. [13] Ling, L., Li, Z.B., Fang, J.Y., Shang, C., 2018. Controlling bromate formation in the Co (II)/peroxymonosulfate process by ammonia, chlorine-ammonia and ammonia-chlorine pretreatment strategies. Water Res. 139, 220–227. 20

[14] WHO, 1986. Environmental Health Criteria: Ammonia. Vol. 54 World Health Organization. [15] Yun, T.I., Liang, S., Yates, R.S., Krasner, S.W., Dale, M., 2011. Full-scale evaluation of the “ammonia-chlorine” process as a bromate control strategy. Proceedings of AWWA Annual. [16] Ling, L., Wang, C., Ni, M.L., Shang, C., 2016. Enhanced photocatalytic activity of TiO2/single-walled carbon nanotube (SWCNT) composites under UV-A irradiation. Sep. Purif. Technol. 169, 273–278. [17] Yin, R., Ling, L., Shang, C., 2018. Wavelength-dependent chlorine photolysis and subsequent radical production using UV-LEDs as light sources. Water Res. 142, 452–458. [18] APHA-AWWA-WEF, 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed.; American Public Health Association/American Water Works Association/Water Environment Federation: Washington DC. [19] Luh, J., Mariñas, B.J., 2014. Kinetics of bromochloramine formation and decomposition. Environ. Sci. Technol., 48 (5), 2843–2852. [20] Valentine, R., 1986. Bromochloramine oxidation of N,N-Diethyl-p-phenylenediamine in the presence of monochloramine. Environ. Sci. Technol. 20, 166–170. [21] Lei, H., Marinas, B.J., Minear, R.A., 2004. Bromamine decomposition kinetics in aqueous solutions. Environ. Sci. Technol. 38 (7), 2111–2119. [22] Galal-Gorchev, H., Morris, J.C., 1965. Formation and stability of bromamide, bromimide, and nitrogen tribromide in aqueous solution. Inorg. Chem. 4(6): 899–905. [23] Ingham, T., Bauer, D., Landgraf, J., Crowley, J., 1998. Ultraviolet-visible absorption cross sections of gaseous HOBr. J. Phys. Chem. A. 102, 3293–3298. [24] Pope, P., 2006. Haloacetic acid formation during chloramination: Role of environmental conditions, kinetics, and haloamine chemistry. Ph.D. dissertation, the University of Texas at Austin. [25] Ianni, J.C., 2012. Kintecus Version 4.50. www.kintecus.com. [26] Wajon, J.E., Morris, J.C., 1982. Rates of formation of N-bromo amines in aqueous solution. Inorg. Chem. 21, 4258-4263. [27] Gazda, M., Dejarme, L., Choudhury, T., Cooks, G., Margerum, D., 1993. Mass spectrometric evidence for the formation of bromochloramine and N-Bromo-N-chloromethylamine in aqueous solution. Environ. Sci. Technol. 27, 557–561. [28] Hofmann, R., Andrews, R.C., 2001. Ammoniacal bromamines: A review of their influence on bromate formation during ozonation. Water Res. 35, 559–604. 21

[29] Valentine, R., 1998. Chloramine decomposition in distribution systems and model waters. AWWA Research Foundation, Denver. [30] Haag, W., Hoigné, J., Bader, H., 1984. Improved ammonia oxidation by ozone in the presence of bromide ion during water treatment. Water Res. 18, 1125–1128. [31] von Gunten, U., Hoigné, J., 1994. Bromate formation during ozonation of bromide-containing waters: Interaction of ozone and hydroxyl radical reactions. Environ. Sci. Technol. 28(7), 1234–1242.

22

Table 1. Molar absorptivities of haloamines at selected wavelengths (M-1 cm-1) λ (nm)

220

232

245

258

278

NH2Cl

162 ± 3 (162)a

312 ± 6

460 ± 8 (453)a

288 ± 8

77 ± 5

573 ± 25

170 ± 12

270 ± 18

400 ± 20

(273)b

(425)b

884 ± 38

710 ± 20

(2000)b

(884)b

(715)b

3810b

5000b

1400b

2100 ± 7 NHBrCl

710 ± 11 613 ± 16

(2100)a NH2Br

832 ± 45

(712)a 76 ± 15

(82)b

245 ± 15

1988 ± 22 NHBr2

2135 ± 42

NBr3 a

Valentine, 1986

b

Lei et al., 2004

1293 ± 33

23

Table 2. Reactions in the ozonation of bromide-containing water pretreated with the NH3-Cl2 and Cl2-NH3 strategies.

No.

Reaction

k+, k- or K

Ref.

1

Br- + O3 → HOBr + O2

160 M-1 s-1

Buffle et al., 2004

2

Br- + HO• → Br• + HO-

1.1 × 109 M-1 s-1

Buffle et al., 2004

3

Br- + Br• + H2O → HOBr + H+

1010 M-1 s-1

Buffle et al., 2004

4

HOBr ↔ OBr-

pKa = 9

5

HOBr/OBr- + HO• → BrO• + HO-

6

Br• + O3 → BrO• + O2

1.5 × 108 M-1 s-1

7

BrO• → BrO2-

5.0 × 109 M-1 s-1

8

HOBr/OBr-

9

BrO2- + O3 → BrO3- + O2

10

-

+ O3 → BrO2 + O2

NH2Br + 3O3 → NO3- + Br- + 3O2 + 2H+

2.0 × 109 M-1 s-1 (HOBr)

Buffle et al., 2004

Buffle et al., 2004

4.5 × 109 M-1 s-1 (OBr-)

0.01 M-1 s-1 (HOBr)

Buffle et al., 2004

Buffle et al., 2004

Buffle et al., 2004

100 M-1 s-1 (OBr-) 8.0 × 104 M-1 s-1

Buffle et al., 2004

Haag et al., 1984; 40 M-1 s-1

von

Gunten

and

Hoigné, 1994

11

NH3 + HOCl → NH2Cl+ H2O

4.2 × 106 M-1s-1

12

Br- + HOCl → HOBr + Cl-

1550 M-1 s-1

13

NH2Cl + Br- → NH2Br + Cl-

0.014 M-1 s-1

Qiang

and

Adam,

2004 Kumar

and

Margerum, 1987 Trofe et al., 1980

24

Table 2 (continue). Reactions in the ozonation of bromide-containing water pretreated with the NH3-Cl2 and Cl2-NH3 strategies. No.

Reaction

k+, k- or K

3.0 × 105 M-1 s-1 (pH=7) 14

HOBr + NH3 → NH2Br + H2O 2.0 × 106 M-1 s-1 (pH=8)

15

HOBr + NH2Cl → NHBrCl + H2O

2.86 × 105 M-1 s-1

Ref.

Wajon and Morris, 1982

Gazda

and

Margerum, 1994

k+ = 0.5 M-1 s-1 + 5 × 108 M-2 s-1 [H+] + 290 M-2 s-1 [NH4+] + 16

2NH2Br ↔ NHBr2 + NH3

3.4 × 105 M-2 s-1 [H2PO4-] k- = 1 M-1 s-1 + 1 × 109 M-2 s-1

Lei et al., 2004

[H+] + 190 M-2 s-1 [NH4+] + 6.5 × 104 M-2 s-1 [H2PO4-] 3.54 × 106 M-2 s-1 (pH=7)

17

2NH2Cl + Br- → NHBrCl + Cl- + NH3

18

NHBr2 + O3 → NO3- + 2Br- + products 10 M-1 s-1

19

NHBrCl + O3 → NO3- + Br- + Cl- + products

2.92 × 106 M-2 s-1 (pH=8)

10 M-1 s-1

20

NHBrCl + Br- → NHBr2 + Cl-

565 M-1 s-1

21

NH2Br + Br- + H+ → NH3 + Br2

3.60 × 109 M-2 s-1

22

NH2Br + Br2 → NHBr2 + Br- + H+

4.60 × 106 M-1 s-1

23

NH2Cl + Br2 → NHBrCl + Br- + H+

4.18 × 108 M-1 s-1

Trofe et al., 1980

Haag et al., 1984

a Luh

and

Marinas,

and

Marinas,

and

Marinas,

and

Marinas,

2014 Luh 2014 Luh 2014 Luh 2014

25

Table 2 (continue). Reactions in the ozonation of bromide-containing water pretreated with the NH3-Cl2 and Cl2-NH3 strategies. No.

Reaction

k+, k- or K

24

NH2Cl + NH2Br + H+ = NHBrCl + NH3

3.10 × 107 M-2 s-1

25

NHBrCl + NH3 + H+ = NH2Cl + NH2Br

3.90 × 109 M-2 s-1

26

NHBrCl + Br- + H+ = NH2Cl + Br2

3.30 × 109 M-2 s-1

27

NH2Cl + NHBr2 = NHBrCl + NH2Br

0.19 M-1 s-1

a

Ref. Luh

and

Marinas,

and

Marinas,

and

Marinas,

and

Marinas,

2014 Luh 2014 Luh 2014 Luh 2014

Determined by experiment in this study.

26

a. Total haloamines and bromate formation Total bromine-containing haloamines

Bromate 0.7 0.6

11 0.5 10

0.4

9

0.3 0.2

8 1

Bromate concentration (M)

Haloamine concentration (N)

12

0.1

0 0.0 30-min + t-BuOH 30-min + t-BuOH6-min pretreatment 6-min pretreatment ozonation ozonation 66-min min30-min 30 30-min min ozonation 66-min min30-min 30 30-min min ozonation 30-min 30-min pretreatment ozonation

ozonation + t-BuOH

pretreatment ozonation

NH3-Cl2

Cl2-NH3

10

ozonation + t-BuOH

b. Haloamine species 6-min pretreatment 30-min ozonation 30-min ozonation + t-BuOH

Haloamine concentration (N)

8

6

4

2

0 NHBr2

NHBrCl

Cl2-NH3

NH2Br

NHBr2

NHBrCl

NH2Br

NH3-Cl2

Figure 1. (a) Bromate and total bromine-containing haloamine concentrations and (b) haloamine species distributions after 6 min of NH3-Cl2 or Cl2-NH3 pretreatment and 30 min of ozonation at 20oC and pH 8. [Br-]0 = 12.5 µM (1 mg/L), [HOCl]0 = 50 µM, [NH3]0 = 100 µM, and [O3]0 = 5 mg/L. The lines in Figure 1a are the initial bromide concentrations. Normality (μN) was used to express the haloamine concentrations to calculate the mass balance of bromine atom in pretreatment processes, because 1 M NHBr2 contains 2 N bromine.

27

(a) Cl2-NH3

(b) NH3-Cl2

Figure 2. Evolution of bromine containing compounds and monochloramine during (a) Cl2-NH3 and (b) NH3-Cl2 pretreatment strategies up to 15 min at 20oC and pH 8. [Br-] = 12.5 µM (1 mg/L), [HOCl] = 50 µM, and [NH3] = 100 µM. The lines show the simulated haloamine concentrations, and the dots display the measured data. 28

14

6-min pretreatment (left bar) 30-min ozonation (medium bar) 30-min ozonation w. t-BuOH (right bar)

NHBr2 NHBrCl NH2Br Bromate formation Bromate formation w. t-BuOH

.8

.6

10

8 .4 6

4

.2

Bromate concentrations (M)

Haloamine concentrations ()

12

2

0

0.0

pH 6

pH 7

Cl2-NH3

pH 8

pH 6

pH 7

pH 8

NH3-Cl2

Figure 3. Bromate and bromine-containing haloamine concentrations after 6 min of NH3-Cl2 or Cl2-NH3 pretreatment and 30 min of ozonation at 20oC and pH 6, 7 and 8, in the presence and absence of t-BuOH. [Br-] = 12.5 µM (1 mg/L), [HOCl] = 50 µM, [NH3] = 100 µM, [O3] = 5 mg/L, and [t-BuOH] = 1 mM. The lines define the initial bromide concentration.

29

20

6-min pretreatment

18

30-min ozonation

1.5

Bromate

14

1.0

12 10 8 0.5

6 4

Bromate concentration (M)

Haloamine concentration (N)

16

2 0

l2 Cl2-NH3NH3-Cl2 H3 -C H-3N C3l2 -CNl2 3N-H 2 l H C N

6.25

l2 Cl2-NH3NH3-Cl2 H3 -C H-3N C3l2 -CNl2 3N-H 2 l H C N

l2 Cl2-NH3NH3-Cl2 H3 -C H-3N C3l2 -CNl2 3N-H 2 l H C N

12.5 Bromide concentration (M)

0.0 --

18.75

Figure 4. Bromate and total bromine-containing haloamine concentrations after 6 min of NH3-Cl2 or Cl2-NH3 pretreatment and 30 min of ozonation at different initial bromide concentrations, 20oC and pH 8. [Br-] = 6.25–18.75 µM (0.5–1.5 mg/L), [HOCl] = 50 µM, [NH3] = 100 µM, and [O3] = 5 mg/L. The lines define the initial bromide concentration.

30

Declaration of interests ☐ 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.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

This work was supported by Hong Kong’s Research Grants Council under grant number 618312.

31