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

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.


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][6][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 (BrO 3 − ) formation in these chemical oxidation processes may bring a potential health risk, because BrO 3 − is a possibly carcinogenic to human [11]. Therefore, it is important to understand BrO 3 − formation potential in these processes. Various knowledge of the BrO 3 − 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 BrO 3 − via ozone and 2 International Journal of Photoenergy hydroxyl radical pathways [8], but pH depression [12] and ammonia addition [12,13] successfully decrease BrO 3 − formation. The pH depression decreased 50-63% in BrO 3 − 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  formation. The inhibition effect of ammonia addition on BrO 3 − formation is caused by bromamines formation from the reaction of HOBr with ammonia [14]. The effective ammonia dose for BrO 3 − depression was limited to 200 µg/L and further increase in ammonia addition did not enhance the BrO 3 − minimization [14]. Effect of hydrogen peroxide (H 2 O 2 ) in ozonation on BrO 3 − formation is complicated. As H 2 O 2 can reduce BrO − into bromide (Br − ) [15], it seems to be useful to depress the BrO 3 − formation. However, ozone reacts with hydroperoxide anion (HO 2 − ) and produces hydroxyl radicals ( • OH) [16], which promotes oxidation of Br − [8]. Ozekin et al. [13] reported that an increase in H 2 O 2 dose in ozonized water enhanced BrO 3 − formation at pH 6.5, though the BrO 3 − formation at pH 8.5 did not depended on the H 2 O 2 dose and was smaller than that of ozone alone. Kim et al. [17] pointed out the importance of molar ratio of H 2 O 2 /O 3 for BrO 3 − formation during ozonation with H 2 O 2 addition; the molar ratio of H 2 O 2 /O 3 above 0.5 and the dissolved ozone concentration below 0.1 mg/L successfully depress the BrO 3 − formation. The inconsistent results between Ozekin et al. [13] and Kim et al. [17] may be caused by the difference in their experimental designs: H 2 O 2 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 BrO 3 − formation during ozone/H 2 O 2 treatment [13,17]. UV light is known to decompose BrO 3 − into BrO − and/ or Br − [18]. As the result, Collivignarelli and Sorlini [19] reported that BrO 3 − formation during ozonation with UV 254 nm irradiation (O 3 /UV) was about 40% lower than that during conventional ozonation. However, Ratpukdi et al. [20] showed that the BrO 3 − formation potential of O 3 /UV was similar to ozonation alone, though ozonation combined with vacuum UV irradiation could decrease the BrO 3 − formation. Thus, the BrO 3 − 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 BrO 3 − control. In this study, BrO 3 − formation in UV irradiation, H 2 O 2 addition, ozonation, and their combination processes, UV irradiation with H 2 O 2 addition (UV/H 2 O 2 ), ozonation with H 2 O 2 addition (O 3 /H 2 O 2 ), and ozonation with UV irradiation (O 3 /UV), were discussed using the same reactor and the same water matrix to provide comparable information of their features of BrO 3 − formation and its control.

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 H 2 O 2 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 BrO 3 − , bromide ion (Br − ), dissolved ozone, H 2 O 2 , 4-CBA, and pH.

Chemical
Analysis. The Br − 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 BrO 3 − 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 H 2 O 2 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).   [25], the low oxidation potential of H 2 O 2 may be responsible for the low reactivity with 4-CBA. Figure 2 shows the time-course changes in Br − , BrO 3 − , and 4-CBA concentrations during UV irradiation and UV/ H 2 O 2 at neutral pH. The concentration changes at acidic condition were almost the same at neutral pH, though the H 2 O 2 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, H 2 O 2 accumulation was caused by H 2 O 2 production via the combination of two • OH [28]. The concentrations of Br − and 4-CBA declined during the UV irradiation and UV/H 2 O 2 , though BrO 3 − was not generated (Figure 2). No BrO 3 − formation during UV irradiation and UV/H 2 O 2 was also reported by Kruithof et al. [29]. The H 2 O 2 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/H 2

Results and Discussion
Therefore, the accumulation of H 2 O 2 was inferred to contribute partly to the prevention of BrO 3 − formation in UV irradiation and UV/H 2 O 2 . Phillip et al. [30] reported that low-pressure mercury vapor lamps decayed free bromine into Br − (major) and BrO 3 − (minor). Thus, the photo-degradation of HOBr/BrO − might conduce to the prevention of BrO 3 − formation too.

Ozonation.
In ozonation, BrO 3 − formation was correspondent to a decrement in Br − at neutral pH without 4-CBA. However, the BrO 3 − 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 BrO 3 − via BrO − and BrO 2 − (Figure 3), our experimental results shown in Figure 4 indicated that contribution of • OH to BrO 3 − 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.    acidification decreases the percentage of BrO − . The decrease in BrO − at acidic pH also contributed to the decrease in BrO 3 − formation [33]. In addition, the discordance of a decrement in Br − and an increment in BrO 3 − at acidic pH in Figure 4 suggested the accumulation of HOBr. Figure 5 (3) and (5)) as follows:

O 3 /H
The inhibition of • OH generation in O 3 /H 2 O 2 was also confirmed by a slow decrease in 4-CBA at acidic pH ( Figure 5). Figure 7 shows changes in Br − , BrO 3 − , and 4-CBA concentrations during O 3 /UV. The O 3 /UV increased BrO 3 − concentration rapidly, even at the acidic pH. However, the addition of 4-CBA successfully decreased the BrO 3 − formation regardless of the pH condition. Collivignarelli and Sorlini [19] also observed lower BrO 3 − formation in O 3 / UV than that in ozonation. As mentioned in the Section 3.2, the acidification decreases BrO 3 − 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 BrO 3 − formation very much in O 3 /UV. This discussion was supported by the lower concentration of dissolved ozone in O 3 / UV (Figure 8). The low dissolved ozone concentration also brought the negligible contribution of molecular ozone to BrO 3 − formation. The decrease in BrO 3 − formation by the addition of 4-CBA indicated the contribution of • OH to BrO 3 − formation. Accordingly, it is suggested that the main oxidant in O 3 /UV was • OH, which mainly generated via UV photolysis of ozone [16]. The first step of • OH generation in O 3 /UV is the production of H 2 O 2 [16]. Then the H 2 O 2 generates • OH through UV photolysis [16] and the same reactions as O 3 /H 2 O 2 (reactions (3)- (7)). As the coexistence of dissolved ozone and • OH favors BrO 3 − formation [13,17], low dissolved ozone concentration in O 3 /UV was thought to be advantageous to the depression of BrO 3 − formation. Moreover, strong H 2 O 2 accumulation was observed during O 3 /UV (Figure 9). Therefore, the reduction of intermediates by H 2 O 2 [15] and UV photolysis [30] was also inferred to contribute to the decline in the BrO 3 − formation potential. 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 BrO 3 − formation potential at the neutral pH than at the acidic pH as shown in Figure 4. Although the acidification successfully decreases the BrO 3 − formation potential of ozonation, it decreases the removal rate of organic contaminants too.

Strategy for Organic Contaminants Removal with Preventing
Contrary  Figure 10 shows the relationship between BrO 3 − concentration and cumulative ozone consumption. Figure 10 [17].
Consequently, if the concentration of organic contaminants is low, the UV irradiation and/or UV/H 2 O 2 are applicable to organic contaminants removal without BrO 3

Conclusion
In this research, BrO 3 − formation potential of UV irradiation, H 2 O 2 addition, ozonation, UV/H 2 O 2 , O 3 /H 2 O 2 , and O 3 /UV treatment were discussed for organic contaminants The UV irradiation, H 2 O 2 addition, and UV/H 2 O 2 prevented BrO 3 − formation completely. However, H 2 O 2 addition was inapplicable as advanced water treatment because of its weak oxidation ability. The UV irradiation and UV/ H 2 O 2 could decompose the organic contaminant moderately. Ozonation produced the most BrO 3 − at neutral pH. Although acidification could decrease the BrO 3 − 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 BrO 3 − formation.