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 . 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.

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 [57] 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 () formation in these chemical oxidation processes may bring a potential health risk, because is a possibly carcinogenic to human [11]. Therefore, it is important to understand formation potential in these processes.

Various knowledge of the 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 via ozone and hydroxyl radical pathways [8], but pH depression [12] and ammonia addition [12, 13] successfully decrease formation. The pH depression decreased 50–63% in formation per a decline in one-pH-unit [12] because of a depression of hydroxyl radical generation and a decrease in hypobromite (), which is a key intermediate of formation. The inhibition effect of ammonia addition on formation is caused by bromamines formation from the reaction of HOBr with ammonia [14]. The effective ammonia dose for depression was limited to 200 μg/L and further increase in ammonia addition did not enhance the minimization [14]. Effect of hydrogen peroxide (H2O2) in ozonation on formation is complicated. As H2O2 can reduce into bromide () [15], it seems to be useful to depress the formation. However, ozone reacts with hydroperoxide anion () and produces hydroxyl radicals () [16], which promotes oxidation of [8]. Ozekin et al. [13] reported that an increase in H2O2 dose in ozonized water enhanced formation at pH 6.5, though the 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/ for formation during ozonation with H2O2 addition; the molar ratio of H2O2/ above 0.5 and the dissolved ozone concentration below 0.1 mg/L successfully depress the 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 generation under a certain concentration of dissolved ozone enhanced the formation during ozone/H2O2 treatment [13, 17]. UV light is known to decompose into and/or [18]. As the result, Collivignarelli and Sorlini [19] reported that formation during ozonation with UV 254 nm irradiation (/UV) was about 40% lower than that during conventional ozonation. However, Ratpukdi et al. [20] showed that the formation potential of /UV was similar to ozonation alone, though ozonation combined with vacuum UV irradiation could decrease the formation. Thus, the 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 control.

In this study, formation in UV irradiation, H2O2 addition, ozonation, and their combination processes, UV irradiation with H2O2 addition (UV/H2O2), ozonation with H2O2 addition (/H2O2), and ozonation with UV irradiation (/UV), were discussed using the same reactor and the same water matrix to provide comparable information of their features of 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 (), 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 , bromide ion (), dissolved ozone, H2O2, 4-CBA, and pH.

2.2. Chemical Analysis

The 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 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 ( 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 , 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 into bromine (Br2), but Br2 is reduced into by H2O2 as follows [25]: Accordingly, concentration did not change because of the catalytic behavior of 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 (2.38 V versus SHE) [25], the low oxidation potential of H2O2 may be responsible for the low reactivity with 4-CBA.

Figure 2 shows the time-course changes in , , 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 [27]. Therefore, H2O2 accumulation was caused by H2O2 production via the combination of two [28]. The concentrations of and 4-CBA declined during the UV irradiation and UV/H2O2, though was not generated (Figure 2). No 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, 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 concentration without accumulation indicates the formation of intermediates. Von Gunten and Oliveras [8] reported that ozone and oxidized to (Figure 3). In this mechanism, hypobromite ion (BrO) and bromite ion () are the critical intermediates, which participate in all formation pathways. Accordingly, UV irradiation and UV/H2O2 were thought to produce and/or . The H2O2 can reduce hypobromous acid (HOBr) and as follows [15]: Therefore, the accumulation of H2O2 was inferred to contribute partly to the prevention of formation in UV irradiation and UV/H2O2. Phillip et al. [30] reported that low-pressure mercury vapor lamps decayed free bromine into (major) and (minor). Thus, the photo-degradation of HOBr/ might conduce to the prevention of formation too.

3.2. Ozonation

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

3.3. O3/H2O2

Figure 5 shows the changes in concentrations of , , and 4-CBA during O3/H2O2 at various H2O2 doses and pHs. The O3/H2O2 processes showed additional effect on lowering 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 formation was observed (Figure 5). As H2O2 can reduce HOBr/ into , the excess H2O2 was believed to restrain formation. The behavior of 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 formation in this study.

When acidification was applied, was not detected in O3/H2O2 at the H2O2 dose of 100 μM or higher. This effect was caused by both the HOBr/ reduction by H2O2 and inhibition of generation in O3/H2O2. Because 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: The inhibition of generation in O3/H2O2 was also confirmed by a slow decrease in 4-CBA at acidic pH (Figure 5).

3.4. O3/UV

Figure 7 shows changes in , , and 4-CBA concentrations during O3/UV. The O3/UV increased concentration rapidly, even at the acidic pH. However, the addition of 4-CBA successfully decreased the formation regardless of the pH condition. Collivignarelli and Sorlini [19] also observed lower formation in O3/UV than that in ozonation. As mentioned in the Section 3.2, the acidification decreases formation by the inhibition of generation via the self-decomposition of ozone. Accordingly, it was thought that generated by the self-decomposition of ozone did not contribute to 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 formation. The decrease in formation by the addition of 4-CBA indicated the contribution of to formation. Accordingly, it is suggested that the main oxidant in O3/UV was , which mainly generated via UV photolysis of ozone [16]. The first step of generation in O3/UV is the production of H2O2 [16]. Then the H2O2 generates through UV photolysis [16] and the same reactions as O3/H2O2 (reactions (3)–(7)). As the coexistence of dissolved ozone and favors formation [13, 17], low dissolved ozone concentration in O3/UV was thought to be advantageous to the depression of 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 formation potential.

3.5. Strategy for Organic Contaminants Removal with Preventing Formation

The aim of advanced oxidation processes is organic contaminants removal from a water stream. Therefore, it is important to remove organic contaminants without 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 formation potential at the neutral pH than at the acidic pH as shown in Figure 4. Although the acidification successfully decreases the 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 formation. The UV irradiation and UV/H2O2 successfully decompose organic contaminants without 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 formation rate at high H2O2 dose. Although the acidification effectuates further decrease in 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 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 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 formation per ozone consumption in comparison with ozonation and O3/H2O2 at the H2O2 dose of 10 μM. Kim et al. [17] reported that 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, was not produced at ozone consumption less than 250 μM. Thus, our result approximately accorded with the research by Kim et al. [17].

Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H2O2 are applicable to organic contaminants removal without 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 control. When O3/UV is applied, the reaction time should be optimized, because extended reaction time increases the 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 formation.

4. Conclusion

In this research, formation potential of UV irradiation, H2O2 addition, ozonation, UV/H2O2, O3/H2O2, and O3/UV treatment were discussed for organic contaminants removal with restraining formation using KBr solution with 4-CBA as a model refractory organic contaminant.

The UV irradiation, H2O2 addition, and UV/H2O2 prevented 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 at neutral pH. Although acidification could decrease the 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 formation. The O3/H2O2 successfully decreased 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 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 under the absence of 4-CBA, the 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 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.