# Bromate Formation Characteristics of UV Irradiation, Hydrogen Peroxide Addition, Ozonation, and Their Combination Processes

Authors: Naoyuki Kishimoto; Eri Nakamura
Journal: International Journal of Photoenergy
 (2012)

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## Abstract

Bromate formation characteristics of six-physicochemical oxidation processes, UV irradiation, single addition of hydrogen peroxide, ozonation, UV irradiation with hydrogen peroxide addition (UV/H2O2), ozonation with hydrogen peroxide addition (O3/H2O2), and ozonation with UV irradiation (O3/UV) were investigated using 1.88 μM of potassium bromide solution with or without 6.4 μM of 4-chlorobenzoic acid. Bromate was not detected during UV irradiation, single addition of H2O2, and UV/H2O2, whereas ozone-based treatments produced BrO3-. Hydroxyl radicals played more important role in bromate formation than molecular ozone. Acidification and addition of radical scavengers such as 4-chlorobenzoic acid were effective in inhibiting bromate formation during the ozone-based treatments because of inhibition of hydroxyl radical generation and consumption of hydroxyl radicals, respectively. The H2O2 addition was unable to decompose 4-chlorobenzoic acid, though O3/UV and O3/H2O2 showed the rapid degradation, and UV irradiation and UV/H2O2 showed the slow degradation. Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H2O2 are applicable to organic contaminants removal without bromate formation. However, if the concentration of organic contaminants is high, O3/H2O2 and O3/UV should be discussed as advanced oxidation processes because of their high organic removal efficiency and low bromate formation potential at the optimum condition.


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## Body

## 1. Introduction

Nowadays, the world demand for water is growing because of the rapid population growth. Furthermore, pollution of freshwater resources proceeds in all over the world. For instance, China encounters severe water pollution caused by industrial chemicals, heavy metals, and algal toxin with an extraordinary economic growth [1]. Gadgil [2] reported that about half the population in the developing world is suffering from one or more of the six main diseases, diarrhea, ascaris dracunculisis, hookworm, schistosomiasis, and trachoma, which are associated with water supply and sanitation. In the industrialized countries, micropollutants like pharmaceuticals gather much concern as potential contaminants in drinking water [3] and surface water [4]. As a result of this situation, water supply section has made efforts to supply a plenty of safe drinking water. In this context, various advanced water treatment like UV disinfection, ozonation, and adsorption processes [5–7] have been introduced to water purification plants.UV irradiation and ozone-based chemical oxidation are widely used as advanced water purification processes. These processes can achieve higher level of disinfection and organic pollutants removal [9, 10]. However, bromate (BrO3-) formation in these chemical oxidation processes may bring a potential health risk, because BrO3- is a possibly carcinogenic to human [11]. Therefore, it is important to understand BrO3- formation potential in these processes.Various knowledge of theBrO3- formation during UV and ozone-based chemical oxidation processes has been accumulated for past a few decades. For instance, ozonation of bromide-containing water produces BrO3- via ozone and hydroxyl radical pathways [8], but pH depression [12] and ammonia addition [12, 13] successfully decrease BrO3- formation. The pH depression decreased 50–63% in BrO3- formation per a decline in one-pH-unit [12] because of a depression of hydroxyl radical generation and a decrease in hypobromite (BrO-), which is a key intermediate of BrO3- formation. The inhibition effect of ammonia addition on BrO3- formation is caused by bromamines formation from the reaction of HOBr with ammonia [14]. The effective ammonia dose for BrO3- depression was limited to 200 μg/L and further increase in ammonia addition did not enhance the BrO3- minimization [14]. Effect of hydrogen peroxide (H2O2) in ozonation on BrO3- formation is complicated. As H2O2 can reduce BrO- into bromide (Br-) [15], it seems to be useful to depress the BrO3- formation. However, ozone reacts with hydroperoxide anion (HO2-) and produces hydroxyl radicals (OH•) [16], which promotes oxidation of Br- [8]. Ozekin et al. [13] reported that an increase in H2O2 dose in ozonized water enhanced BrO3- formation at pH 6.5, though the BrO3- formation at pH 8.5 did not depended on the H2O2 dose and was smaller than that of ozone alone. Kim et al. [17] pointed out the importance of molar ratio of H2O2/O3 for BrO3- formation during ozonation with H2O2 addition; the molar ratio of H2O2/O3 above 0.5 and the dissolved ozone concentration below 0.1 mg/L successfully depress the BrO3- formation. The inconsistent results between Ozekin et al. [13] and Kim et al. [17] may be caused by the difference in their experimental designs: H2O2 was injected during ozonation [17] or after ozonation [13]. But, both research groups suggested that the OH• generation under a certain concentration of dissolved ozone enhanced the BrO3- formation during ozone/H2O2 treatment [13, 17]. UV light is known to decompose BrO3- into BrO- and/or Br- [18]. As the result, Collivignarelli and Sorlini [19] reported that BrO3- formation during ozonation with UV 254 nm irradiation (O3/UV) was about 40% lower than that during conventional ozonation. However, Ratpukdi et al. [20] showed that the BrO3- formation potential of O3/UV was similar to ozonation alone, though ozonation combined with vacuum UV irradiation could decrease the BrO3- formation. Thus, the BrO3- formation mechanisms of UV and ozone-based chemical oxidation processes have been explored extensively. However, each research was performed using different reactors, different procedures, and different water matrices. Therefore, it is not easy to judge which process should be selected for BrO3- control.In this study,BrO3- formation in UV irradiation, H2O2 addition, ozonation, and their combination processes, UV irradiation with H2O2 addition (UV/H2O2), ozonation with H2O2 addition (O3/H2O2), and ozonation with UV irradiation (O3/UV), were discussed using the same reactor and the same water matrix to provide comparable information of their features of BrO3- formation and its control.


## 2. Experimental

### 2.1. Material and Experimental Conditions

A low-pressure mercury vapor lamp (20W, UVL20PH-6, Sen Lights, Japan) was used as a UV light source. Ozone gas was generated from analytical grade oxygen gas with a silent discharge ozonizer (ED-OG-R3Lt, Eco Design, Japan). Hydrogen peroxide was purchased from Nacalai Tesque, Japan as about 35% aqueous solution (extra pure grade) and used without further purification. The accurate H2O2 concentration was checked just before an experiment and final concentration was set at 10, 100, or 1,000 μM. Figure 1 shows the experimental setup. The reactor was made of glass with a volume of 1.9 L. The UV lamp in a duplex quartz jacket was installed in the center of the reactor. Ozone was injected through two gas diffusers made of glass at the injection rate of about 20 mg/min. Inlet and outlet ozone gas concentration was monitored with two ozone monitors (EG-600, Ebara Jitsugyo, Japan). The exhaust ozone gas was dried with a gas dryer (DH106-1, Komatsu Electronics, Japan) before ozone monitoring, because water vapor biases the ozone concentration. Oxygen gas flow rate was regulated with a mass flow controller (CMQ9200, Yamatake, Japan) at 500 mL/min. Test solution was 1.9 L of 1.88 μM potassium bromide (KBr, Nacalai Tesque, Japan) solution with or without 6.4 μM of 4-chlorobenzoic acid (4-CBA, Wako Chemicals, Japan). The 4-CBA was used as a model compound of organic scavengers of hydroxyl radical (OH•), because it was unreactive with ozone [21]. The solution pH was adjusted by addition of sulfuric acid or sodium hydroxide at around 2.5 or 7. An experimental run continued for 10 or 30 minutes and solution in the reactor was sampled every two or five minutes for chemical analyses of BrO3-, bromide ion (Br-), dissolved ozone, H2O2, 4-CBA, and pH.Figure 1
Experimental setup.


### 2.2. Chemical Analysis

TheBr- concentration was analyzed using an ion chromatography system with a conductivity detector (DX-500, Dionex, USA). Analytical conditions were as follows. Column: Dionex IonPac AS12A with a suppressor (Dionex ASRS-ULTRA 4 mm); mobile phase: aqueous solution with 2.7 mM sodium carbonate and 0.3 mM sodium bicarbonate; flow rate: 1.0 mL/min; sample injection volume: 100 μL; oven temperature: 40°C. The BrO3- concentration was determined by the ion chromatography coupled with a postcolumn system (Dionex BRS-500) [22]. Reaction conditions were as follows, reactant A: 1.5 M potassium bromide and 1.0 M sulfuric acid; reactant B: 1.2 mM sodium nitrite; flow rate: 0.4 mL/min for reactant A and 0.2 mL/min for reactant B; reaction temperature: 40°C; detection: absorbance at 268 nm. The determination limit was estimated to be 0.050 μM. Dissolved ozone and H2O2 were analyzed by indigo-colorimetric method [23] and DMP method [24], respectively. The 4-CBA concentration was determined by the high-performance liquid chromatography (LV-10ADVP, Shimadzu, Japan) [21]. Analytical conditions were as follows: column: ODS-80TM (4.6×250 mm, Tosoh, Japan); mobile phase: acetonitrile (70%) and 0.1% phosphoric acid (30%); flow rate: 1.0 mL/min; sample injection volume: 200 μL; oven temperature: 40°C; detection: absorbance at 234 nm. The solution pH was measured with a pH meter (Twin pH B-212, Horiba, Japan).


## 2.1. Material and Experimental Conditions

A low-pressure mercury vapor lamp (20W, UVL20PH-6, Sen Lights, Japan) was used as a UV light source. Ozone gas was generated from analytical grade oxygen gas with a silent discharge ozonizer (ED-OG-R3Lt, Eco Design, Japan). Hydrogen peroxide was purchased from Nacalai Tesque, Japan as about 35% aqueous solution (extra pure grade) and used without further purification. The accurate H2O2 concentration was checked just before an experiment and final concentration was set at 10, 100, or 1,000 μM. Figure 1 shows the experimental setup. The reactor was made of glass with a volume of 1.9 L. The UV lamp in a duplex quartz jacket was installed in the center of the reactor. Ozone was injected through two gas diffusers made of glass at the injection rate of about 20 mg/min. Inlet and outlet ozone gas concentration was monitored with two ozone monitors (EG-600, Ebara Jitsugyo, Japan). The exhaust ozone gas was dried with a gas dryer (DH106-1, Komatsu Electronics, Japan) before ozone monitoring, because water vapor biases the ozone concentration. Oxygen gas flow rate was regulated with a mass flow controller (CMQ9200, Yamatake, Japan) at 500 mL/min. Test solution was 1.9 L of 1.88 μM potassium bromide (KBr, Nacalai Tesque, Japan) solution with or without 6.4 μM of 4-chlorobenzoic acid (4-CBA, Wako Chemicals, Japan). The 4-CBA was used as a model compound of organic scavengers of hydroxyl radical (OH•), because it was unreactive with ozone [21]. The solution pH was adjusted by addition of sulfuric acid or sodium hydroxide at around 2.5 or 7. An experimental run continued for 10 or 30 minutes and solution in the reactor was sampled every two or five minutes for chemical analyses of BrO3-, bromide ion (Br-), dissolved ozone, H2O2, 4-CBA, and pH.Figure 1
Experimental setup.


## 2.2. Chemical Analysis

TheBr- concentration was analyzed using an ion chromatography system with a conductivity detector (DX-500, Dionex, USA). Analytical conditions were as follows. Column: Dionex IonPac AS12A with a suppressor (Dionex ASRS-ULTRA 4 mm); mobile phase: aqueous solution with 2.7 mM sodium carbonate and 0.3 mM sodium bicarbonate; flow rate: 1.0 mL/min; sample injection volume: 100 μL; oven temperature: 40°C. The BrO3- concentration was determined by the ion chromatography coupled with a postcolumn system (Dionex BRS-500) [22]. Reaction conditions were as follows, reactant A: 1.5 M potassium bromide and 1.0 M sulfuric acid; reactant B: 1.2 mM sodium nitrite; flow rate: 0.4 mL/min for reactant A and 0.2 mL/min for reactant B; reaction temperature: 40°C; detection: absorbance at 268 nm. The determination limit was estimated to be 0.050 μM. Dissolved ozone and H2O2 were analyzed by indigo-colorimetric method [23] and DMP method [24], respectively. The 4-CBA concentration was determined by the high-performance liquid chromatography (LV-10ADVP, Shimadzu, Japan) [21]. Analytical conditions were as follows: column: ODS-80TM (4.6×250 mm, Tosoh, Japan); mobile phase: acetonitrile (70%) and 0.1% phosphoric acid (30%); flow rate: 1.0 mL/min; sample injection volume: 200 μL; oven temperature: 40°C; detection: absorbance at 234 nm. The solution pH was measured with a pH meter (Twin pH B-212, Horiba, Japan).


## 3. Results and Discussion

### 3.1. UV Irradiation, H2O2 Addition, and UV/H2O2 Process

The H2O2 addition did not change concentrations of BrO3-, Br−, and 4-CBA under both acid and neutral pH conditions (data not shown). Although H2O2 is an oxidant, H2O2 is nucleophilic too. Therefore, H2O2 can oxidize Br- into bromine (Br2), but Br2 is reduced into Br- by H2O2 as follows [25]:
(1)2Br-+H2O2+2H+→Br2+2H2OBr2+H2O2→2H++2Br-+O2
Accordingly, Br- concentration did not change because of the catalytic behavior of Br- as shown in reactions (1). On the reactivity of 4-CBA with H2O2, Dionysiou et al. [26] also observed that H2O2 did not decompose 4-CBA under the dark condition. Since the standard electrode potential of H2O2 (1.736 V versus standard hydrogen electrode (SHE)) is lower than that of ozone (2.07 V versus SHE) and OH• (2.38 V versus SHE) [25], the low oxidation potential of H2O2 may be responsible for the low reactivity with 4-CBA.Figure2 shows the time-course changes in Br-, BrO3-, and 4-CBA concentrations during UV irradiation and UV/H2O2 at neutral pH. The concentration changes at acidic condition were almost the same at neutral pH, though the H2O2 accumulation was enhanced at acidic condition. The low-pressure mercury vapor lamp emits vacuum UV light of 185 nm, which can photolyze water molecules into hydrogen atoms and OH• [27]. Therefore, H2O2 accumulation was caused by H2O2 production via the combination of two OH• [28]. The concentrations of Br- and 4-CBA declined during the UV irradiation and UV/H2O2, though BrO3- was not generated (Figure 2). No BrO3- formation during UV irradiation and UV/H2O2 was also reported by Kruithof et al. [29]. The H2O2 concentration in the both treatment increased with the passage of time, and the final concentration in UV irradiation reached over 10 μM, which was the initial concentration in UV/H2O2. The 4-CBA degradation in UV irradiation slightly delayed in comparison with that in UV/H2O2, but the degradation was enhanced with the accumulation of H2O2. Accordingly, OH• generation via UV photolysis of H2O2 [27] was believed to contribute to the 4-CBA degradation during the UV irradiation and UV/H2O2. The decline in Br- concentration without BrO3- accumulation indicates the formation of intermediates. Von Gunten and Oliveras [8] reported that ozone and OH• oxidized Br- to BrO3- (Figure 3). In this mechanism, hypobromite ion (BrO−) and bromite ion (BrO2-) are the critical intermediates, which participate in all BrO3- formation pathways. Accordingly, UV irradiation and UV/H2O2 were thought to produce BrO- and/or BrO2-. The H2O2 can reduce hypobromous acid (HOBr) and BrO- as follows [15]:
(2)BrO-+H2O2→Br-+H2O+O2k=1.3×106M-1s-1,HOBr+HO2-→Br-+H2O+O2k=7.6×108M-1s-1,HOBr+H2O2→Br-+H++H2O+O2k=1.5×104M-1s-1.
Therefore, the accumulation of H2O2 was inferred to contribute partly to the prevention of BrO3- formation in UV irradiation and UV/H2O2. Phillip et al. [30] reported that low-pressure mercury vapor lamps decayed free bromine into Br- (major) and BrO3- (minor). Thus, the photo-degradation of HOBr/BrO- might conduce to the prevention of BrO3- formation too.Changes in concentrations ofBr-, BrO3-, H2O2, and 4-CBA during UV irradiation (UV) and UV/H2O2 treatment at neutral pH. The initial H2O2 concentration for UV/H2O2 was 10 μM.
(a)
(b)Figure 3
Br
O
3
- formation pathways illustrated on the basis of Von Gunten and Oliveras [8].


### 3.2. Ozonation

In ozonation,BrO3- formation was correspondent to a decrement in Br- at neutral pH without 4-CBA. However, the BrO3- formation was much lower than Br- removal at acidic pH or coexistence of 4-CBA (Figure 4). Although both ozone and OH• promote the oxidation of Br- to BrO3- via BrO− and BrO2- (Figure 3), our experimental results shown in Figure 4 indicated that contribution of OH• to BrO3- evolution was relatively large. Because acidic pH restrains OH• generation via self-decomposition of ozone [31], and 4-CBA is a OH• radical scavenger with low reactivity with ozone [21]. Since HOBr has a pKa of 8.8–9.0 [32], acidification decreases the percentage of BrO-. The decrease in BrO- at acidic pH also contributed to the decrease in BrO3- formation [33]. In addition, the discordance of a decrement in Br- and an increment in BrO3- at acidic pH in Figure 4 suggested the accumulation of HOBr.Figure 4
Changes in concentrations ofBr-, BrO3-, and 4-CBA during ozonation at neutral pH without 4-CBA (N-1), ozonation at neutral pH with 4-CBA (N-2), ozonation at acidic pH without 4-CBA (A-1), and ozonation at acidic pH with 4-CBA (A-2).


### 3.3. O3/H2O2

Figure5 shows the changes in concentrations of Br-, BrO3-, and 4-CBA during O3/H2O2 at various H2O2 doses and pHs. The O3/H2O2 processes showed additional effect on lowering BrO3- formation, especially at higher H2O2 dose. Bromate ion was not detected in O3/H2O2 at the H2O2 dose of 1,000 μM. When the H2O2 dose was lowered to 100 or 10 μM, rapid BrO3- formation was observed (Figure 5). As H2O2 can reduce HOBr/BrO- into Br-, the excess H2O2 was believed to restrain BrO3- formation. The behavior of BrO3- formation in O3/H2O2 at the H2O2 dose of 10 μM (Figure 5) was similar to that in ozonation (Figure 4), since H2O2 concentration in ozonation increased to around 10 μM, which was nearly equal to H2O2 concentration in O3/H2O2 at the H2O2 dose of 10 μM (Figure 6). Thus, the initial H2O2 dose of 10 μM was too low to restrain the BrO3- formation in this study.Figure 5
Changes in concentrations ofBr-, BrO3-, and 4-CBA during O3/H2O2 at various H2O2 doses. The concentrations in the legend mean the H2O2 dose. The symbols in the bracket mean O3/H2O2 at neutral pH without 4-CBA (N-1), O3/H2O2 at neutral pH with 4-CBA (N-2), O3/H2O2 at acidic pH without 4-CBA (A-1), and O3/H2O2 at acidic pH with 4-CBA (A-2).Changes in H2O2 concentrations during O3/H2O2 and ozonation without 4-CBA. The concentrations in the legend mean the H2O2 dose in O3/H2O2.
(a)
(b)When acidification was applied,BrO3- was not detected in O3/H2O2 at the H2O2 dose of 100 μM or higher. This effect was caused by both the HOBr/BrO- reduction by H2O2 and inhibition of OH• generation in O3/H2O2. Because OH• generation in O3/H2O2 is expressed by the following reactions [16] and the acidification inhibits the dissociation of H2O2 and HO2 (reactions (3) and (5)) as follows:
(3)H2O2⟷H++HO2-(4)O3+HO2-→O3•-+HO2•(5)HO2•⟷H++O2•-(6)O3+O2•-→O3•-+O2(7)O3•-+H+⟷HO3→O2+OH•
The inhibition of OH• generation in O3/H2O2 was also confirmed by a slow decrease in 4-CBA at acidic pH (Figure 5).


### 3.4. O3/UV

Figure7 shows changes in Br-, BrO3-, and 4-CBA concentrations during O3/UV. The O3/UV increased BrO3- concentration rapidly, even at the acidic pH. However, the addition of 4-CBA successfully decreased the BrO3- formation regardless of the pH condition. Collivignarelli and Sorlini [19] also observed lower BrO3- formation in O3/UV than that in ozonation. As mentioned in the Section 3.2, the acidification decreases BrO3- formation by the inhibition of OH• generation via the self-decomposition of ozone. Accordingly, it was thought that OH• generated by the self-decomposition of ozone did not contribute to BrO3- formation very much in O3/UV. This discussion was supported by the lower concentration of dissolved ozone in O3/UV (Figure 8). The low dissolved ozone concentration also brought the negligible contribution of molecular ozone to BrO3- formation. The decrease in BrO3- formation by the addition of 4-CBA indicated the contribution of OH• to BrO3- formation. Accordingly, it is suggested that the main oxidant in O3/UV was OH•, which mainly generated via UV photolysis of ozone [16]. The first step of OH• generation in O3/UV is the production of H2O2 [16]. Then the H2O2 generates OH• through UV photolysis [16] and the same reactions as O3/H2O2 (reactions (3)–(7)). As the coexistence of dissolved ozone and OH• favors BrO3- formation [13, 17], low dissolved ozone concentration in O3/UV was thought to be advantageous to the depression of BrO3- formation. Moreover, strong H2O2 accumulation was observed during O3/UV (Figure 9). Therefore, the reduction of intermediates by H2O2 [15] and UV photolysis [30] was also inferred to contribute to the decline in the BrO3- formation potential.Figure 7
Changes in concentrations ofBr-, BrO3-, and 4-CBA during O3/UV at neutral pH without 4-CBA (N-1), O3/UV at neutral pH with 4-CBA (N-2), O3/UV at acidic pH without 4-CBA (A-1), and O3/UV at acidic pH with 4-CBA (A-2).Figure 8
Changes in dissolved ozone concentrations during ozonation, O3/H2O2 (the H2O2 dose of 100 μM), and O3/UV with the addition of 4-CBA.Figure 9
Changes in H2O2 concentrations during O3/UV with 4-CBA.


### 3.5. Strategy for Organic Contaminants Removal with PreventingBrO3- Formation

The aim of advanced oxidation processes is organic contaminants removal from a water stream. Therefore, it is important to remove organic contaminants withoutBrO3- formation. In this context, H2O2 addition is inapplicable to advanced water treatment, because it is not effective to degrade refractory organic matters like 4-CBA. Ozonation is also difficult to apply to the organic contaminants removal, because it has higher BrO3- formation potential at the neutral pH than at the acidic pH as shown in Figure 4. Although the acidification successfully decreases the BrO3- formation potential of ozonation, it decreases the removal rate of organic contaminants too.Contrary to H2O2 addition and ozonation, UV-based processes and O3/H2O2 are potentially applicable to advanced water treatment with inhibiting BrO3- formation. The UV irradiation and UV/H2O2 successfully decompose organic contaminants without BrO3- formation. But their degradation rate of organic contaminants is not high, and H2O2 dose of 10 μM is too low to enhance the degradation rate of UV irradiation. The O3/H2O2 is characterized by rapid degradation of organic contaminants and low BrO3- formation rate at high H2O2 dose. Although the acidification effectuates further decrease in BrO3- formation, it spoils the degradation of organic contaminants. Therefore, the acidification should not apply to O3/H2O2. The feature of O3/UV is rapid degradation of organic contaminants, low dissolved ozone concentration, and much H2O2 accumulation. As the BrO3- formation in O3/UV is restrained under the coexistence of organic contaminants, the O3/UV is applicable, if water contains sufficient quantity of organic contaminants. Figure 10 shows the relationship between BrO3- concentration and cumulative ozone consumption. Figure 10 demonstrates that O3/UV and O3/H2O2 at the H2O2 doses of 100 and 1,000 μM were significantly decreased BrO3- formation per ozone consumption in comparison with ozonation and O3/H2O2 at the H2O2 dose of 10 μM. Kim et al. [17] reported that BrO3- concentration in O3/H2O2 remained less than 10 μg/L, when the molar ratio of H2O2 to ozone was above 0.5. In our study of O3/H2O2 at the H2O2 dose of 100 μM, BrO3- was not produced at ozone consumption less than 250 μM. Thus, our result approximately accorded with the research by Kim et al. [17].Figure 10
Relationship between cumulative ozone consumption andBrO3- accumulation during each treatment with 4-CBA addition at neutral pH. Each treatment continued 30 minutes.Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H2O2 are applicable to organic contaminants removal without BrO3- formation, though it is necessary to beware nitrite formation during the treatment [34]. However, if the concentration of organic contaminants is high, O3/H2O2 and O3/UV should be discussed as advanced oxidation processes. When O3/H2O2 is applied, the H2O2 dose should be more than the half of ozone consumption, because low H2O2 dose is ineffective for BrO3- control. When O3/UV is applied, the reaction time should be optimized, because extended reaction time increases the BrO3- formation potential. The real-time monitoring of UV absorbance of organic contaminants in water [35] may be effective in the optimum control of the reaction time of O3/UV without BrO3- formation.


## 3.1. UV Irradiation, H2O2 Addition, and UV/H2O2 Process

The H2O2 addition did not change concentrations of BrO3-, Br−, and 4-CBA under both acid and neutral pH conditions (data not shown). Although H2O2 is an oxidant, H2O2 is nucleophilic too. Therefore, H2O2 can oxidize Br- into bromine (Br2), but Br2 is reduced into Br- by H2O2 as follows [25]:
(1)2Br-+H2O2+2H+→Br2+2H2OBr2+H2O2→2H++2Br-+O2
Accordingly, Br- concentration did not change because of the catalytic behavior of Br- as shown in reactions (1). On the reactivity of 4-CBA with H2O2, Dionysiou et al. [26] also observed that H2O2 did not decompose 4-CBA under the dark condition. Since the standard electrode potential of H2O2 (1.736 V versus standard hydrogen electrode (SHE)) is lower than that of ozone (2.07 V versus SHE) and OH• (2.38 V versus SHE) [25], the low oxidation potential of H2O2 may be responsible for the low reactivity with 4-CBA.Figure2 shows the time-course changes in Br-, BrO3-, and 4-CBA concentrations during UV irradiation and UV/H2O2 at neutral pH. The concentration changes at acidic condition were almost the same at neutral pH, though the H2O2 accumulation was enhanced at acidic condition. The low-pressure mercury vapor lamp emits vacuum UV light of 185 nm, which can photolyze water molecules into hydrogen atoms and OH• [27]. Therefore, H2O2 accumulation was caused by H2O2 production via the combination of two OH• [28]. The concentrations of Br- and 4-CBA declined during the UV irradiation and UV/H2O2, though BrO3- was not generated (Figure 2). No BrO3- formation during UV irradiation and UV/H2O2 was also reported by Kruithof et al. [29]. The H2O2 concentration in the both treatment increased with the passage of time, and the final concentration in UV irradiation reached over 10 μM, which was the initial concentration in UV/H2O2. The 4-CBA degradation in UV irradiation slightly delayed in comparison with that in UV/H2O2, but the degradation was enhanced with the accumulation of H2O2. Accordingly, OH• generation via UV photolysis of H2O2 [27] was believed to contribute to the 4-CBA degradation during the UV irradiation and UV/H2O2. The decline in Br- concentration without BrO3- accumulation indicates the formation of intermediates. Von Gunten and Oliveras [8] reported that ozone and OH• oxidized Br- to BrO3- (Figure 3). In this mechanism, hypobromite ion (BrO−) and bromite ion (BrO2-) are the critical intermediates, which participate in all BrO3- formation pathways. Accordingly, UV irradiation and UV/H2O2 were thought to produce BrO- and/or BrO2-. The H2O2 can reduce hypobromous acid (HOBr) and BrO- as follows [15]:
(2)BrO-+H2O2→Br-+H2O+O2k=1.3×106M-1s-1,HOBr+HO2-→Br-+H2O+O2k=7.6×108M-1s-1,HOBr+H2O2→Br-+H++H2O+O2k=1.5×104M-1s-1.
Therefore, the accumulation of H2O2 was inferred to contribute partly to the prevention of BrO3- formation in UV irradiation and UV/H2O2. Phillip et al. [30] reported that low-pressure mercury vapor lamps decayed free bromine into Br- (major) and BrO3- (minor). Thus, the photo-degradation of HOBr/BrO- might conduce to the prevention of BrO3- formation too.Changes in concentrations ofBr-, BrO3-, H2O2, and 4-CBA during UV irradiation (UV) and UV/H2O2 treatment at neutral pH. The initial H2O2 concentration for UV/H2O2 was 10 μM.
(a)
(b)Figure 3
Br
O
3
- formation pathways illustrated on the basis of Von Gunten and Oliveras [8].


## 3.2. Ozonation

In ozonation,BrO3- formation was correspondent to a decrement in Br- at neutral pH without 4-CBA. However, the BrO3- formation was much lower than Br- removal at acidic pH or coexistence of 4-CBA (Figure 4). Although both ozone and OH• promote the oxidation of Br- to BrO3- via BrO− and BrO2- (Figure 3), our experimental results shown in Figure 4 indicated that contribution of OH• to BrO3- evolution was relatively large. Because acidic pH restrains OH• generation via self-decomposition of ozone [31], and 4-CBA is a OH• radical scavenger with low reactivity with ozone [21]. Since HOBr has a pKa of 8.8–9.0 [32], acidification decreases the percentage of BrO-. The decrease in BrO- at acidic pH also contributed to the decrease in BrO3- formation [33]. In addition, the discordance of a decrement in Br- and an increment in BrO3- at acidic pH in Figure 4 suggested the accumulation of HOBr.Figure 4
Changes in concentrations ofBr-, BrO3-, and 4-CBA during ozonation at neutral pH without 4-CBA (N-1), ozonation at neutral pH with 4-CBA (N-2), ozonation at acidic pH without 4-CBA (A-1), and ozonation at acidic pH with 4-CBA (A-2).


## 3.3. O3/H2O2

Figure5 shows the changes in concentrations of Br-, BrO3-, and 4-CBA during O3/H2O2 at various H2O2 doses and pHs. The O3/H2O2 processes showed additional effect on lowering BrO3- formation, especially at higher H2O2 dose. Bromate ion was not detected in O3/H2O2 at the H2O2 dose of 1,000 μM. When the H2O2 dose was lowered to 100 or 10 μM, rapid BrO3- formation was observed (Figure 5). As H2O2 can reduce HOBr/BrO- into Br-, the excess H2O2 was believed to restrain BrO3- formation. The behavior of BrO3- formation in O3/H2O2 at the H2O2 dose of 10 μM (Figure 5) was similar to that in ozonation (Figure 4), since H2O2 concentration in ozonation increased to around 10 μM, which was nearly equal to H2O2 concentration in O3/H2O2 at the H2O2 dose of 10 μM (Figure 6). Thus, the initial H2O2 dose of 10 μM was too low to restrain the BrO3- formation in this study.Figure 5
Changes in concentrations ofBr-, BrO3-, and 4-CBA during O3/H2O2 at various H2O2 doses. The concentrations in the legend mean the H2O2 dose. The symbols in the bracket mean O3/H2O2 at neutral pH without 4-CBA (N-1), O3/H2O2 at neutral pH with 4-CBA (N-2), O3/H2O2 at acidic pH without 4-CBA (A-1), and O3/H2O2 at acidic pH with 4-CBA (A-2).Changes in H2O2 concentrations during O3/H2O2 and ozonation without 4-CBA. The concentrations in the legend mean the H2O2 dose in O3/H2O2.
(a)
(b)When acidification was applied,BrO3- was not detected in O3/H2O2 at the H2O2 dose of 100 μM or higher. This effect was caused by both the HOBr/BrO- reduction by H2O2 and inhibition of OH• generation in O3/H2O2. Because OH• generation in O3/H2O2 is expressed by the following reactions [16] and the acidification inhibits the dissociation of H2O2 and HO2 (reactions (3) and (5)) as follows:
(3)H2O2⟷H++HO2-(4)O3+HO2-→O3•-+HO2•(5)HO2•⟷H++O2•-(6)O3+O2•-→O3•-+O2(7)O3•-+H+⟷HO3→O2+OH•
The inhibition of OH• generation in O3/H2O2 was also confirmed by a slow decrease in 4-CBA at acidic pH (Figure 5).


## 3.4. O3/UV

Figure7 shows changes in Br-, BrO3-, and 4-CBA concentrations during O3/UV. The O3/UV increased BrO3- concentration rapidly, even at the acidic pH. However, the addition of 4-CBA successfully decreased the BrO3- formation regardless of the pH condition. Collivignarelli and Sorlini [19] also observed lower BrO3- formation in O3/UV than that in ozonation. As mentioned in the Section 3.2, the acidification decreases BrO3- formation by the inhibition of OH• generation via the self-decomposition of ozone. Accordingly, it was thought that OH• generated by the self-decomposition of ozone did not contribute to BrO3- formation very much in O3/UV. This discussion was supported by the lower concentration of dissolved ozone in O3/UV (Figure 8). The low dissolved ozone concentration also brought the negligible contribution of molecular ozone to BrO3- formation. The decrease in BrO3- formation by the addition of 4-CBA indicated the contribution of OH• to BrO3- formation. Accordingly, it is suggested that the main oxidant in O3/UV was OH•, which mainly generated via UV photolysis of ozone [16]. The first step of OH• generation in O3/UV is the production of H2O2 [16]. Then the H2O2 generates OH• through UV photolysis [16] and the same reactions as O3/H2O2 (reactions (3)–(7)). As the coexistence of dissolved ozone and OH• favors BrO3- formation [13, 17], low dissolved ozone concentration in O3/UV was thought to be advantageous to the depression of BrO3- formation. Moreover, strong H2O2 accumulation was observed during O3/UV (Figure 9). Therefore, the reduction of intermediates by H2O2 [15] and UV photolysis [30] was also inferred to contribute to the decline in the BrO3- formation potential.Figure 7
Changes in concentrations ofBr-, BrO3-, and 4-CBA during O3/UV at neutral pH without 4-CBA (N-1), O3/UV at neutral pH with 4-CBA (N-2), O3/UV at acidic pH without 4-CBA (A-1), and O3/UV at acidic pH with 4-CBA (A-2).Figure 8
Changes in dissolved ozone concentrations during ozonation, O3/H2O2 (the H2O2 dose of 100 μM), and O3/UV with the addition of 4-CBA.Figure 9
Changes in H2O2 concentrations during O3/UV with 4-CBA.


## 3.5. Strategy for Organic Contaminants Removal with PreventingBrO3- Formation

The aim of advanced oxidation processes is organic contaminants removal from a water stream. Therefore, it is important to remove organic contaminants withoutBrO3- formation. In this context, H2O2 addition is inapplicable to advanced water treatment, because it is not effective to degrade refractory organic matters like 4-CBA. Ozonation is also difficult to apply to the organic contaminants removal, because it has higher BrO3- formation potential at the neutral pH than at the acidic pH as shown in Figure 4. Although the acidification successfully decreases the BrO3- formation potential of ozonation, it decreases the removal rate of organic contaminants too.Contrary to H2O2 addition and ozonation, UV-based processes and O3/H2O2 are potentially applicable to advanced water treatment with inhibiting BrO3- formation. The UV irradiation and UV/H2O2 successfully decompose organic contaminants without BrO3- formation. But their degradation rate of organic contaminants is not high, and H2O2 dose of 10 μM is too low to enhance the degradation rate of UV irradiation. The O3/H2O2 is characterized by rapid degradation of organic contaminants and low BrO3- formation rate at high H2O2 dose. Although the acidification effectuates further decrease in BrO3- formation, it spoils the degradation of organic contaminants. Therefore, the acidification should not apply to O3/H2O2. The feature of O3/UV is rapid degradation of organic contaminants, low dissolved ozone concentration, and much H2O2 accumulation. As the BrO3- formation in O3/UV is restrained under the coexistence of organic contaminants, the O3/UV is applicable, if water contains sufficient quantity of organic contaminants. Figure 10 shows the relationship between BrO3- concentration and cumulative ozone consumption. Figure 10 demonstrates that O3/UV and O3/H2O2 at the H2O2 doses of 100 and 1,000 μM were significantly decreased BrO3- formation per ozone consumption in comparison with ozonation and O3/H2O2 at the H2O2 dose of 10 μM. Kim et al. [17] reported that BrO3- concentration in O3/H2O2 remained less than 10 μg/L, when the molar ratio of H2O2 to ozone was above 0.5. In our study of O3/H2O2 at the H2O2 dose of 100 μM, BrO3- was not produced at ozone consumption less than 250 μM. Thus, our result approximately accorded with the research by Kim et al. [17].Figure 10
Relationship between cumulative ozone consumption andBrO3- accumulation during each treatment with 4-CBA addition at neutral pH. Each treatment continued 30 minutes.Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H2O2 are applicable to organic contaminants removal without BrO3- formation, though it is necessary to beware nitrite formation during the treatment [34]. However, if the concentration of organic contaminants is high, O3/H2O2 and O3/UV should be discussed as advanced oxidation processes. When O3/H2O2 is applied, the H2O2 dose should be more than the half of ozone consumption, because low H2O2 dose is ineffective for BrO3- control. When O3/UV is applied, the reaction time should be optimized, because extended reaction time increases the BrO3- formation potential. The real-time monitoring of UV absorbance of organic contaminants in water [35] may be effective in the optimum control of the reaction time of O3/UV without BrO3- formation.


## 4. Conclusion

In this research,BrO3- formation potential of UV irradiation, H2O2 addition, ozonation, UV/H2O2, O3/H2O2, and O3/UV treatment were discussed for organic contaminants removal with restraining BrO3- formation using KBr solution with 4-CBA as a model refractory organic contaminant.The UV irradiation, H2O2 addition, and UV/H2O2 prevented BrO3- formation completely. However, H2O2 addition was inapplicable as advanced water treatment because of its weak oxidation ability. The UV irradiation and UV/H2O2 could decompose the organic contaminant moderately. Ozonation produced the most BrO3- at neutral pH. Although acidification could decrease the BrO3- formation, it also deteriorated the oxidation ability of ozonation. Therefore, it was thought to be difficult to apply ozonation to organic contaminants removal with restraining BrO3- formation. The O3/H2O2 successfully decreased BrO3- formation at the H2O2 doses of 100 μM or higher. The degradation rate of 4-CBA was larger than the UV irradiation and UV/H2O2. However, the behavior of BrO3- formation in the O3/H2O2 at the H2O2 dose of 10 μM was similar to that in ozonation because of a deficiency of H2O2. The O3/UV also showed rapid degradation of 4-CBA. Although it produced much BrO3- under the absence of 4-CBA, the BrO3- formation was strongly inhibited by the coexistence of 4-CBA. Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H2O2 are applicable to organic contaminants removal without BrO3- formation. However, if the concentration of organic contaminants is high, O3/H2O2 and O3/UV should be discussed as advanced oxidation processes because of their higher organic removal efficiency.



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*Source: 107293-2012-09-25.xml*