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Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation
Journal of Hazardous Materials 363 (2019) 428–438
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Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation
T
Jingxin Yanga, Zijun Dongc, Chengchun Jiangc, Hong Liua,b, , Ji Lid ⁎
a
Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou, 510006, China b Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, China c Department of Building and Environmental Engineering, Shenzhen Polytechnic, Shenzhen 518055, China d Shenzhen Key Laboratory of Water Resource Application and Environmental Pollution Control, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen, 518055, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Bromate Carbonate radicals (CO3%−) Carbonate/bicarbonate Ozonation Hydroxyl radical (%OH)
Bicarbonate scavenges %OH to form CO3%− that enhances the bromate formation by ozonation. However, the role of CO3%− in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO3%− based on the detection of CO3%−involved bromate and CO3%− exposure. Experiments demonstrated that the CO3%−-involved bromate was responsible for 33.7–69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO3%− exposure was two orders of magnitude higher than the corresponding %OH exposure during ozonation. These results demonstrate that CO3%− plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than %OH. A model was developed based on the ratio of bromine oxidized by CO3%−, which could predict the CO3%−-involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO3%−-involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of
⁎ Corresponding author at: Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou, 510006, China. E-mail address: [email protected] (H. Liu).
https://doi.org/10.1016/j.jhazmat.2018.10.013 Received 22 March 2018; Received in revised form 24 August 2018; Accepted 3 October 2018 Available online 09 October 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 363 (2019) 428–438
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humic acid led to an increase in this contribution during ozonation. The results of this study provide a more indepth understanding of the mechanism of bromate formation during ozonation.
1. Introduction
bromate formation. As mentioned above, bicarbonate plays a dual role in the bromate formation during ozonation, since it not only leads to the generation of CO3%-, which enhances the bromate formation, but also scavenges the same amount of %OH, which inhibits the bromate formation. This means that the CO3%–involved bromate formation includes the difference in the bromate formation in the presence and absence of bicarbonate and also the reduced bromate formation due to the scavenging %OH by bicarbonate. Thus, the method adopted in their study neglected the inhibiting effect of bicarbonate on the bromate formation via •OH scavenging. Therefore, a new and correct quantitative method is required to evaluate the CO3%–involved bromate formation. In addition, the effect of the water conditions on the CO3%– involved bromate formation remains unexplored. CO3%− is a moderately strong one-electron oxidant with a redox potential of 1.78 V at pH 7 [20]. It oxidizes many organic and inorganic contaminants, including bromide and HOBr/–OBr, with rate constants ranging from 103 to 109 M-1s-1 [21–24]. CO3%− plays a similar role as % OH in the bromate formation during ozonation. Thus, it could enhance the bromate formation by ozonation via two potential pathways. One is the oxidation of Br- by CO3%− to produce Br%, which can be further oxidized by ozone to eventually form bromate. The other is the oxidation of HOBr/-OBr to %OBr, which eventually transforms to bromate. To elucidate the mechanism of the CO3%−-involved bromate formation, the CO3%- exposure needs to be evaluated. Unfortunately, to the best of our knowledge, the CO3%- exposure has never been quantified in previous studies involving bromate formation during ozonation in the presence of bicarbonate. The major objectives of this study are (1) to quantify the contribution of pathways involving CO3%− to the bromate formation during ozonation in the presence of bicarbonate; (2) to determine the CO3•exposure and elucidate the mechanism of CO3%−-involved bromate formation; (3) to develop a model to predict CO3%−-involved bromate; and (4) to evaluate the effects of pH, bromide, ozone dosage and humic acid on the CO3%−-involved bromate formation.
Ozone is globally applied as a disinfectant and/or oxidant in water and wastewater treatment [1–3]. The application of ozone, however, may be limited when the targeted water contains bromide with a concentration greater than 50 μg/L [4]. These levels of bromide readily lead to excessive bromate formation during ozonation. Due to its potential carcinogenicity, bromate is typically regulated at a maximum contaminant level of 10 μg/L in drinking water in many countries [5,6]. The kinetics and mechanisms of bromate formation during ozonation have been intensively studied [4,7–9]. As has been reported previously, bromate is produced from the oxidation of bromide by ozone and/or hydroxyl radicals (%OH) in series and in parallel during ozonation [10]. Its formation mechanism is divided into three pathways, including the direct pathway (Br− + O3 → HOBr/−OBr + O3→ … → BrO3−), the direct-indirect pathway (Br− + O3 → HOBr/−OBr + %OH → … → BrO3−) and the indirect-direct pathway (Br− + %OH → Br% + O3→ … → BrO3−) [11–13]. The detailed reactions of each pathway were listed in support information Table S1. The formation of bromate via the direct pathway only proceeds via the oxidation by ozone. It typically occurs at alkaline pH when the equilibrium of HOBr/–OBr shifts towards –OBr, due to the extremely low reactivity of ozone towards HOBr. In comparison, the formation of bromate requires the oxidation by %OH in other two pathways. Besides oxidizing both HOBr and –OBr with similar rate constants, %OH is capable of oxidizing Br− to Br%. As a result, Br− can be still be transformed into bromate even in the presence of HOBr/–OBr scavengers such like H2O2 and NH2OH via the indirect-direct pathway, which is the only pathway that requires no HOBr/–OBr as an intermediate product. Accordingly, %OH plays an essential role in the formation of bromate during ozonation. Based on this formation mechanism, several options, such as %OH scavenging, have been proposed to control bromate formation during ozonation [14,15]. Bicarbonate is a common %OH scavenger and is typically detected in surface and ground water with concentrations ranging from 1 mM to 5 mM [16]. It has been found to have a negative effect on the degradation of refractory organic contaminants during ozone-based advanced oxidation processes [17,18]. Similarly, it was thought that bicarbonate would decrease the bromate formation by inhibiting the pathways that involve %OH (including the direct-indirect pathway and the indirect-direct pathway) during ozonation. However, several previous studies have shown that the addition of bicarbonate enhances the formation of bromate during ozonation [9,10,19]. They ascribed this enhancement effect of bicarbonate addition to (a) CO3%− produced by the reaction of %OH with H2CO3/HCO3-/CO32- acting as a secondary oxidant to oxidize OBr- to %OBr; (b) a higher steady-state concentration of CO3%− compared to %OH, due to its higher selectivity for the oxidation of contaminants in aqueous solution; and (c) a lower ozone decomposition due to the inhibition of %OH chain reactions. Thus, the reasons for the above mentioned enhanced bromate formation in the presence of bicarbonate are strongly related to CO3%−. The CO3%−-involved bromate formation must therefore contribute substantially to the total bromate formation during ozonation in the presence of bicarbonate. However, to the best of our knowledge, only one prior study has ever quantified the CO3%−-involved bromate formation during the ozonation of bromide-containing water in the presence of bicarbonate [11]. In that study, the CO3%−-involved bromate formation was obtained via subtracting the bromate formation in the absence of bicarbonate from that in the presence of bicarbonate. However, this quantitative method probably underestimates the CO3%–involved
2. Material and methods 2.1. Material All the solutions in present work were prepared with ultrapure water produced from the Millipore system (18.2 MΩ cm). Potassium indigotrisulfonate and humic acid were purchased from Sigma–Aldrich. Atraizine (ATZ), 2,6-dichlorophenol (99.0%), potassium bromide, potassium bromte, tert-butyl alcohol (t-BuOH, ≥99.8%) and sodium sulfite (98.0%) were obtained from Aladdin Industrial Corporation. Nitrobenzene (NB, ≥99.0%), sodium bicarbonate (≥99.5%), Sodium hydroxide (≥96.0%), sulfuric acid (95.0–98.0%), sodium phosphate dibasic (≥99.0%) and sodium dihydrogen phosphate (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ozone stock solution was prepared by passing an oxygen/ozone mixture gas produced from an ozone generator into 4 °C ultrapure water cooled by an ice-cooling system. The concentration of dissolved ozone typically achieved 1.1 mM in the stock solution. 2.2. Experimental procedure The ozonation experiments were conducted in a 600 mL glass bottle with a bottle-top dispenser (purchased from Brand, Germany) for sampling. The bottle was performed at 20 °C in a water bath. Synthetic water, prepared using ultrapure water, was buffered with 5 mM of 429
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phosphate, and the pH value was adjusted to the desired value with sulfuric acid (0.1 M) and sodium hydroxide (0.1 M). Atrazine (ATZ) and nitrobenzene (NB), as probe compounds to quantify the radical exposure, were both spiked with concentrations of 0.5 μM before the experiments. A certain volume of ozone stock solution was rapidly added to the experimental solution with a glass syringe to achieve the desired concentration of ozone and mixed with the prepared synthetic water by a magnetic stirrer, and the reaction time was simultaneously marked. The stirring was stopped after 20 s to minimize the loss of dissolved ozone by stripping. Samples were withdrawn at the predetermined time intervals via the bottle-topped dispenser, and the residual dissolved ozone was removed by N2 purging for one minute. The samples were then divided into four portions to measure the BrO3−, HOBr/−OBr, Br− and the probe compounds. All the experiments were conducted at least twice. The results represent the mean values with standard deviations.
[32]. The derivative product (4-bromo-2,6-dichlorophenol) of the reaction was then monitored using HPLC, equipped with a Waters XBridge™ C18 column (150 mm × 4.6 mm, 3.5 μm particle size), an auto-sampler (Waters 717) and a dual λ detector, with an eluent containing 0.5% acetic acid and methanol (60:40, v/v %) at λ = 252 nm and a retention time of 3.5 min. This method was applied to quantify the concentrations of HOBr/-OBr [33]. ATZ was quantified using HPLC with an eluent containing 0.5% acetic acid and methanol (40:60, v/v %) at λ = 221 nm and a retention time of 5 min. NB was also quantified using HPLC with the same eluent as for ATZ but at λ = 268 nm and a retention time of 3.2 min. 3. Results and discussion 3.1. Quantification of the CO3%−-involved bromate formation Fig. 1(a) shows the time-dependent bromate formation via different processes at an ozone dosage of 2 mg/L and pH 8. Compared to the control, the concentration of bromate formed increased by 103.9% due to the addition of 10 mM of bicarbonate and decreased by 59.7% due to the addition of 168 μM of t-BuOH. This indicates the bromate formation was enhanced by the bicarbonate addition but inhibited by the t-BuOH addition during ozonation, which is consistent with previous studies [11]. As Fig. 1(b) shows, the decomposition rate of the dissolved ozone in the presence of 10 mM of bicarbonate was lower than in the control but was similar to the rate in the presence of 168 μM of t-BuOH. Meanwhile, the %OH exposure in the presence of 10 mM of bicarbonate was lower than in the control but was very similar to the %OH exposure in the presence of 168 μM of t-BuOH (see Fig. 1(c)). These results suggest that both bicarbonate and t-BuOH inhibit the %OH chain reactions and thus lead to higher ozone exposure and lower %OH exposure. Most importantly, the effect of the bicarbonate addition on the ozone decomposition and the %OH exposure can be approximated by adding tBuOH. This can be mainly attributed to the fact that the same %OH scavenging ability is obtained via the addition of 10 mM of bicarbonate or 168 μM of t-BuOH in the experimental solutions at pH 8. In spite of the quite similar ozone and %OH exposure, the bromate formation varied greatly in these two cases, since the bicarbonate scavenged %OH to generate a secondary oxidant, CO3%−, while t-BuOH scavenged %OH to terminate the radical chain reactions during ozonation. These results confirm the significant role of CO3%− in the bromate formation during ozonation in the presence of bicarbonate. The enhancement effect of bicarbonate on the bromate formation during ozonation, via the production of CO3%−, overwhelmed its inhibiting effect via the scavenging of %OH. Due to the similar behavior of both ozone and %OH in the presence of bicarbonate and t-BuOH with the same %OH scavenging ability, the CO3%−-involved bromate formation can be approximated by subtracting the bromate formed in the presence of t-BuOH from that formed in the presence of bicarbonate with the same %OH scavenging ability. According to the results shown in Fig. 1(a), approximately 80% of the total formed bromate was produced via the CO3%−-involved pathways during ozonation in the presence of 10 mM of bicarbonate. In addition, bicarbonate addition still led to an increase of 104% in
2.3. Quantification of CO3%− exposure In this study, we used nitrobenzene (NB) and atrazine (ATZ) as probe compounds to evaluate the %OH exposure and CO3%− exposure. Trace amounts of NB and ATZ have a negligible effect on the bromate formation during ozonation. Since the two probe compounds are degraded by molecular ozone, %OH and CO3%− during ozonation in the presence of bicarbonate, the degradation rate of NB and ATZ can be described using Eqs. (1) and (2), respectively. NB reacts with CO3%− with a second-order rate constant of 1.4 × 104 M-1s-1, which is far less than its reaction rate with %OH (k = 3.9 × 109 M-1s-1). Thus, in this case, its degradation by CO3%- can be neglected. Accordingly, Eq. (1) can be simplified to Eq. (3). Upon integration of Eqs. (3) and (2), the % OH exposure and CO3%- exposure can be obtained from Eq. (5) and Eq. (6).
d [NB ] = k (OH , NB) [OH ][NB] + k (O3, NB ) [O3][NB] + k (CO3 dt
, NB ) [CO3
][NB ] (1)
d [ATZ ] = k ( OH , ATZ ) [ OH ][ATZ ] + k (O3, ATZ ) [O3 ][ATZ ] dt + k (CO3
, ATZ ) [CO3
(2)
][ATZ ]
d [NB ] = k ( OH , NB ) [ OH ][NB] + k (O3, NB) [O3 ][NB] dt [ OH ] dt = (ln
[CO3 ] dt = (ln
[NB]0 [NB]t [ATZ ]0 [ATZ ]t
/ k (CO3
k (O3, NB )
[O3 ] dt )/ k ( OH , NB )
k (O3, ATZ )
[O3 ] dt k ( OH , ATZ )
(3) (4)
[ OH ] dt ) (5)
, ATZ )
where ∫ [O3]dt, ∫ [ OH]dt and ∫ represent the ozone exposure, %OH exposure and CO3%- exposure, respectively. The secondorder rate constants in the equations above are listed in Table 1. %
[CO3%−]dt
2.4. Analytical methods
Table 1 Second-order rate constants for the two probe compounds with O3, %OH and CO3%−.
Ozone in the stock solution is directly measured at 258 nm (ε = 3000 M−1 cm−1). The dissolved ozone in the experiments is determined with indigo agent at 600 nm (ε = 20,000 M−1 cm−1) [30]. Bromate was analyzed using an ion chromatography method combined with a post-column reaction [31]. Bromide was determined using an ion chromatography system (ISC-3000, Dionex) with an eluent containing 20 mM of KOH. HOBr/-OBr reacts with 2,6-dichlorophenol at a moderate rate to form 4-bromo-2,6-dichlorophenol (pKa = 6.97, k(2,6-di4 M−1s−1, k(2,6-dichlorophenolate,HOBr) = 4.5 chlorophenol,HOBr) = 3.5 × 10 −1 −1 × 105 M s , and k(2,6-dichlorophenolate,-OBr) = 1.6 × 104 M−1s−1)
Probe compound
kO3 (M−1s−1)
Nitrobenzene Atrazine
0.09 6d
a b c d e
430
[25]. [26]. [27]. [28]. [29].
a
k%OH (M−1s−1)
kCO3%- (M−1s−1)
3.9 × 109 3 × 109 d
1.4 × 104 4 × 106 e
b
c
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Fig. 1. Time-dependent formed bromate (a), dissolved ozone residuals (b), %OH exposures (c), CO3%− exposures (d) in the different processes. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 10 mM, [t-BuOH] = 168 μM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C.
bromate formation, even in the presence of t-BuOH. This indicates that CO3%− is negligibly scavenged by t-BuOH compared to %OH, due to its higher selectivity for the latter radical. The second-order rate constants are 9.6 × 104 M-1s-1 and 6.0 × 108 M-1s-1 for the reactions of t-BuOH with CO3%− and %OH, respectively.
of t-BuOH, a portion of the %OH was scavenged by t-BuOH, and as a result, there was less %OH available for the production of CO3%−. It should be noted that the CO3%− exposure (10-8 M s) is two orders of magnitude larger than the corresponding %OH exposure (10-10 M s). For the reaction with Br- to form Br•, the second-order rate constant with CO3%− is 1.0 × 105 M-1s-1, which is four orders of magnitude smaller than with %OH (k(%OH,Br-) = 1.1 × 109 M-1s-1). Accordingly, the oxidation of bromide by CO3%- is negligible compared to its oxidation by %OH. The transformation of Br- to Br% by the total radical content (%OH and CO3%−) is largely inhibited in the presence of bicarbonate. For the reaction with -OBr to form BrO%, the second-order rate constant with CO3%- is 4.3 × 107 M-1s-1, while the second-order rate constant with % OH is 4.5 × 109 M-1s-1. This suggests that the oxidation of -OBr to BrO% by CO3%− would be comparable or even more pronounced than the oxidation by %OH. In addition, the presence of bicarbonate reduces the buildup of HOBr/-OBr during ozonation due to greater transformation of -OBr to BrO% by CO3%−, as presented in Fig. S1. Accordingly, it seems likely that CO3%− primarily enhances the bromate formation during ozonation via enhancing the oxidation of -OBr to BrO%. To further confirm this assumption, we spiked the experimental solutions with excess ammonia and investigated its impact on the bromate formation in the presence of bicarbonate. Ammonia effectively inhibits the bromate formation by scavenging bromine but is largely ineffective at
3.2. Elucidation of the mechanism of the CO3%−-involved bromate formation As mentioned above, CO3%− is a strong oxidant that can oxidize both bromide and HOBr/-OBr, which is favorable for bromate formation (see Eqs. (6) and (7)). To elucidate the mechanism of the CO3%−involved bromate formation, we evaluated the corresponding CO3%exposure during the ozonation of bromide-containing water in the presence of bicarbonate. Br− + O3 → HOBr/-OBr + CO3%- →…→ BrO3−
Br
+
CO3%-
→ Br + O3→…→ BrO3%
(6) (7)
Fig. 1(d) shows the CO3 exposure obtained from different processes of degradation of ATZ and NB, as illustrated in Eqs. (1)–(5). The CO3%- exposure was observed to be much higher in the absence of tBuOH than in the presence of t-BuOH during ozonation. In the presence %−
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inhibiting the bromate formation by scavenging Br% [15]. As shown in Fig. S2, the amount of bromate formed decreased by more than 90% in the presence of ammonia. This suggests that ammonia almost completely inhibited the bromate formation in the presence of bicarbonate. Thus, HOBr/-OBr is a critical intermediate in the bromate formation via pathways that involve CO3%−. In conclusion, in the pathways involving CO3%−, bromide is initially oxidized by ozone to bromine, which is further oxidized by CO3%- to BrO% and eventually converted to bromate, as in Eq. (6).
accounted for more than half of the total bromate formation when the bicarbonate concentration was greater than 1 mM. In comparison, the non−CO3%−-involved bromate decreased with increasing bicarbonate concentration. These results are mainly due to the greater transformation of %OH into CO3%− with increasing bicarbonate concentration, which could be observed from the evolutions of CO3%- exposure and % OH exposure. As shown in Fig. 2(b), the CO3%- exposure increased from 2.71 × 10-8 M s to 5.66 × 10-8 M s with increasing bicarbonate concentration from 0.5 mM to 4 mM. Simultaneously, the corresponding % OH exposure declined from 3.40 × 10-10 M s to 8.67 × 10-11 M s (see Fig. 2(c)). Since the ratio of the CO3%- exposure to the %OH exposure increased from 121 to 657 as the bicarbonate concentration increased from 0.5 mM to 4 mM, the oxidation of bromine is primarily performed by CO3%− rather than %OH. As demonstrated above, the formation of HOBr/-OBr is required for the bromate formation pathways that involve CO3%−. HOBr/-OBr is primarily produced by the oxidation of bromide by molecular ozone. Accordingly, the decomposition of ozone is important for the CO3%−-involved bromate formation. As shown in Fig. 2(d), the presence of bicarbonate slows down the decomposition of ozone. The decomposition rate of the dissolved ozone declined from 0.0373 min-1 to 0.0310 min-1 with increasing bicarbonate concentrations from 0.5 mM to 4 mM. This resulted in an increase in the ozone exposure from 0.0377 M s to 0.0423 M s. Similar trends during
3.3. The CO3%−-involved bromate formation and CO3%− exposure with different bicarbonate dosages Fig. 2(a) shows the concentrations of bromate formed during ozonation with an initial bromide concentration of 5 μM and different bicarbonate concentrations (0.5–4 mM) at pH 8. The total bromate formed during ozonation in the presence of bicarbonate was divided into two parts: the CO3%−-involved bromate and the non−CO3%−-involved bromate. Both the total bromate and the CO3%−-involved bromate increased with increasing bicarbonate concentration. The contribution of the CO3%−-involved bromate accounted for 33.7% to 69.9% of the total bromate formation as the bicarbonate concentration increased from 0.5 mM to 4 mM. In particular, this contribution
Fig. 2. Bromate formation (a), CO3%− exposures (b), %OH exposures (c) and dissolved ozone decay (d) at different bicarbonate concentrations. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 432
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ozonation in the presence of bicarbonate have been commonly reported by many previous studies and are attributed to the inhibition of %OH chain reactions by the presence of bicarbonate [34].
[CO3%−] to [O3] at any time during the reaction, as shown in Eq. (11), while R ct , OH represents the ratio of [%OH] to [O3], as in Eq. (12). The substitution of Eqs. (11) and (12) into Eq. (8) yields Eq. (13). The CO3%−-involved bromate can be obtained using Eq. (14).
3.4. Modeling the CO3%−-involved bromate formation
fCO3• =
As demonstrated above, the bromine scavenger ammonia fully inhibits bromate formation in the presence of bicarbonate. This suggests that HOBr/−OBr is a required intermediate for the formation of bromate in this case, while the indirect-direct pathway, a bromine-free pathway for bromate formation during ozonation, contributes negligibly to the total bromate formation. Accordingly, we constructed a model to predict the CO3%–involved bromate formation based on the oxidation of bromine. The formed bromine is mainly oxidized by three oxidants, including ozone, %OH and CO3%- and is eventually converted to bromate. The ratio of the CO3%−-involved bromate to the total bromate ( fCO3 ) is given by Eq. (8). To simplify the equation, we define a term R ct , CO3 that describes the ratio of the CO3%− exposure to the ozone exposure, as in Eq. (9). This is similar to a commonly used term, R ct , OH , which refers to the ratio of the %OH exposure to the ozone exposure, as in Eq. (10). As presented in Fig. S3, R ct , CO3 and R ct , OH can be shown to be constant over the entire reaction time, suggesting that the two terms are independent of the reaction time. Thus, R ct , CO3 represents the ratio of
[BrO3 ]CO3• [BrO3 ]total kCO3• [CO3• ][HOBr ]t
=
kCO3• [CO3•
][HOBr ]t + k•OH [•OH ][HOBr ]t + kO3 [O3 ][HOBr ]t (8)
R ct , CO3 = R ct ,•OH =
[CO3• ] dt / [•OH ] dt /
[O3 ] dt [O3 ] dt
(9) (10)
R ct , CO3 = [CO3• ]/[O3]
(11)
R ct ,•OH = [•OH ]/[O3]
(12)
fCO•3 =
kCO•3 R ct,CO•3 kCO•3
R ct,CO•3 + k•OH Rct,•OH + kO3
(13)
Fig. 3. Effect of pH on bromate formed concentrations (a), CO3•− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 6–8, 20 °C. 433
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[BrO3 ]CO•3 = fCO•3 [BrO3 ]total = [BrO3 ]total
pH dependence of the CO3%–involved and non−CO3%–involved bromate are explained by the pH-dependent CO3%− exposure and %OH exposure. As shown in Fig. 3(b) and (c), the CO3%- exposure and %OH exposure increased by 474–813% and 317–453%, respectively, as the pH increased from 6 to 8 for all the bicarbonate concentrations. The production of %OH by the reaction of ozone with OH- is enhanced at higher pH. Since CO3%- is produced by the reaction of %OH with H2CO3/HCO3-/ CO32-, higher %OH exposure results in higher CO3%− exposure. Moreover, the pH affects the H2CO3/HCO3-/CO32- equilibrium, as shown in Eqs. (15) and (16). The three species scavenge %OH with different rate constants (Eqs. (17), (18) and (19)) [26]. Thus, the rate constants for the reaction of H2CO3/HCO3-/CO32- with %OH vary with pH. As shown in Fig. S5, increasing the pH from 6 to 8 shifts the H2CO3/HCO3equilibrium toward HCO3-, and thus, the second-order rate constant for the reaction of %OH with H2CO3/HCO3- increases from 3.26 × 106 M-1s1 to 1.01 × 107 M-1s-1. According to Eq. (20), the protonation of CO3%− is negligible at pH 6–8. In addition, the pH dependence of HOBr is also partially responsible for the enhanced bromate formation during ozonation in the presence of bicarbonate as the pH increases. Increasing the pH shifts the HOBr/-OBr equilibrium toward –OBr, which is more readily oxidized than its protonated form.
kCO•3 R ct,CO•3 kCO•3
Rct,CO•3
+ k•OH R ct, OH + kO3 •
(14)
3.5. Effect of pH The solution pH is a significant factor in the bromate formation during ozonation. As has been reported previously, decreasing the pH is typically employed as an effective method to reduce bromate formation [8,35]. However, our previous study found that pH depression unexpectedly enhanced the bromate formation during ozonation in the presence of hydroxylamine via the indirect-direct pathway, due to a higher Br• formation rate at lower pH [36]. Thus, it is necessary to investigate the effect of pH on the CO3%−-involved bromate formation during ozonation. Fig. 3 shows the pH-dependent bromate formation, CO3%− exposure, % OH exposure and ozone exposure in the presence of bicarbonate, with an initial bromide concentration of 5 μM and an ozone dosage of 41.7 μM. In Fig. 3(a), the concentrations of both the CO3%–involved and non−CO3%–involved bromate markedly increased with increasing pH from 6 to 8 for all the bicarbonate concentrations. The model results for the CO3%–involved bromate are generally consistent with the corresponding experimental results at different pH values (see Fig. S4). The
H2CO3 ↔ HCO3− + H+, pKa1 = 6.37
(15)
Fig. 4. Effect of bromide on bromate formation (a), CO3%− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 2.5–10 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 434
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HCO3− ↔ CO32- + H+, pKa2 = 10.33 H2CO3 + OH →
CO3%−
%
HCO3− CO3
2−
HCO3%
+ OH → %
+ OH → %
→
CO3%−
CO3%CO3%-
+
6
-1 -1
+ H2O + H , k = 1.0 × 10 M s 6
-1 -1
+ H2O, k = 8.5 × 10 M s 8
-1 -1
+ HO- k = 3.9 × 10 M s
+
+ H , pKa < 0
−
(17)
HOBr + OH → BrO + H2O, k = 2 × 10 M
(18)
−
OBr + OH → BrO + OH-, k = 4.5 × 10 M s
(19)
−
OBr + CO3 - → BrO + CO3 , k = 4.3 × 10 M s
%
%
%
−1 −1
s
9
%
%
(22)
9
%
2-
-1 -1
7
-1 -1
(23) (24) (25)
(20)
It should be noted, however, that the contribution of the CO3%−involved bromate to the total bromate formation decreases with increasing pH during ozonation in the presence of bicarbonate. For example, the contribution of the CO3%−-involved bromate accounted for approximately 84.4%, 76.0% and 69.9% of the total bromate formed in the presence of 4 mM of bicarbonate at pH 6, pH 7 and pH 8, respectively. This result can be mainly attributed to the pH dependence of the bromine oxidation. Bromine is oxidized by ozone, %OH and CO3%during ozonation, as shown in reactions (21)–(25) [10]. The oxidation of bromine by ozone is more sensitive to pH than the oxidation of bromine by radicals. Thus, the direct pathway for bromate formation is enhanced by increasing the pH more greatly than the pathways involving radicals. HOBr + O3 → BrO2− + O2 + H+, k = 0.01 M-1s-1
OBr + O3 → BrO2− + O2, k = 100 M-1s-1
(16)
3.6. Effect of bromide Fig. 4 shows the bromate formation, CO3%− exposure, %OH exposure and ozone exposure at bromide concentrations of 2.5 μM, 5 μM and 10 μM during ozonation in the presence of different concentrations of bicarbonate. The experimental CO3%–involved bromate increased by 91.7–108.4% with increasing bromide concentrations, which is consistent with the modeling results (see Fig. S6). The total bromate increased by 122.2–135.8% as the bromide concentration increased from 2.5 μM to 10 μM. However, the contribution of the CO3%–involved pathway to the total bromate formed decreased with increasing bromide concentration. Bromide competes for %OH with bicarbonate to lower the CO3%− production, and thereby leads to lower contributions of CO3%–involved bromate to the total bromate with increasing ratio of bromide to bicarbonate during ozonation in the presence of
(21)
Fig. 5. Effect of ozone dosage on bromate formation (a), CO3%− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 5 μM, [O3] = 0.5–4 mg/L (10.4–83.3 μM), [HCO3-] = 4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 435
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bicarbonate. As shown in Fig. 4(b), the CO3%− exposure decreased by 26.6–40.9% as the bromide concentration increased from 2.5 μM to 10 μM. An increase in the bromide concentration leads to an increase in the total %OH scavenging capacity of the system and thus a lower %OH exposure, as presented in Fig. 4(c). Moreover, the %OH scavenged by bicarbonate decreased with increasing bromide concentration. As shown in Fig. S7, the %OH scavenging capacity of bicarbonate decreased by 15.0–42.3% as the bromide concentration increased from 2.5 μM to 10 μM at different bicarbonate concentrations. In addition, according to Fig. 4(d), the ozone exposure decreased with increasing bromide concentration. Lower ozone exposure also results in a lower contribution of the CO3%–involved bromate to the total bromate.
non−CO3%−-involved bromate formation pathways during ozonation. These results again demonstrate that the enhancement of the ozone exposure by bicarbonate addition is partially responsible for the enhanced bromate formation. In the CO3%−-involved bromate formation pathways, ozone is mainly responsible for the oxidation of bromide to yield HOBr/-OBr and for the conversion of BrO2- to BrO2%. Moreover, the higher ozone dosage not only leads to higher ozone exposure but also to higher CO3%- exposure and %OH exposure (see Fig. 5(b–d)). 3.8. Effect of humic acid Humic acid is ubiquitously found in natural water. It can act both as an initiator for the reaction with ozone to generate %OH and as a scavenger of the formed %OH and HOBr/−OBr [37]. Thus, a set of experiments was conducted in the presence of 0.2 mg-C/L humic acid to investigate its effect on the bromate formation during ozonation in the presence of bicarbonate. As shown in Fig. 6, both the total bromate and the CO3%−-involved bromate increased with increasing bicarbonate concentrations from 0.5 mM to 4 mM by 52.2% and 188.1%, respectively, in the presence of humic acid. The modeled CO3%−-involved bromate concentrations agreed closely with the experimental values (see Fig. S9). This suggests
3.7. Effect of ozone dosage Fig. 5 shows the bromate formation at ozone dosages of 0.5–4 mg/L in the presence of 4 mM of bicarbonate. The total bromate formed increased 30-fold with increasing ozone dosage from 0.5 mg/L to 4 mg/L, while the CO3%−-involved bromate and the non−CO3%−-involved bromate increased 36-fold and 23-fold, respectively. The experimental and the modeled CO3%−-involved bromate agreed well (see Fig. S8). This indicates that ozone plays a critical role in both the CO3%−-involved and
Fig. 6. Bromate formation (a), CO3%− exposures (b), %OH exposures (c), ozone exposures (d) in the presence of humic acid (HA) at different bicarbonate concentrations. Conditions: [HA] = 0.2 mg/L, [Br-] = 5 μM, [O3] = 2 mg/L (41.7 μM), [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 436
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that the bicarbonate addition enhances the bromate formation even in the presence of humic acid via oxidation by CO3%−. Moreover, both the total bromate and the CO3%−-involved bromate in the presence of humic acid were observed to be slightly lower than in pure water at the corresponding bicarbonate levels. This can be ascribed to the relative high special ozone dose (10 mgO3/mgDOC) in this experiment. As reported in previous literature, bromate formation was still pronounced at a special ozone dose > 0.25 mgO3/mgDOC during ozonation, where electron-rich moieties have be completely degraded [38,39]. It should be noted that the presence of humic acid enhances the contribution of the CO3%−-involved bromate to the total bromate. This trend may be due to the higher CO3%- exposure in the presence of humic acid than in pure water. The reactions of ozone with the electron-rich compounds from humic acid accelerated the production of %OH and thus enhanced the resulting CO3%- exposure.
disinfection, bromate formation and oxidation, Chem. Eng. J. 328 (2017) 207–214. [4] U. von Gunten, Ozonation of drinking water: part II. Disinfection and by-product formation in presence of bromide, iodide or chlorine, Water Res. 37 (2003) 1469–1487. [5] USEPA, United States environmental protection (USEPA), national primary drinking water regulations: stage 2 disinfectants and disinfection byproducts rule, Fed. Regist. 71 (2006) 387–493. [6] World Health Organization (WHO), fourth edition, Guidelines for Drinking-Water Quality 38 (2011), pp. 104–108. [7] B. Legube, B. Parinet, K. Gelinet, F. Berne, J. Croue, Modeling of bromate formation by ozonation of surface waters in drinking water treatment, Water Res. 38 (2004) 2185–2195. [8] M. Moslemi, S.H. Davies, S.J. Masten, Bromate formation in a hybrid ozonationceramic membrane filtration system, Water Res. 45 (2011) 5529–5534. [9] R. Song, C. Donohoe, R. Minear, P. Westerhoff, K. Ozekin, G. Amy, Empirical modeling of bromate formation during ozonation of bromide-containing waters, Water Res. 30 (1996) 1161–1168. [10] U. von Gunten, J. Hoigné, Bromate formation during ozonization of bromide-containing waters: interaction of ozone and hydroxyl radical reactions, Environ. Sci. Technol. 28 (1994) 1234–1242. [11] K. Ozekin, P. Westerhoff, G.L. Amy, M. Siddiqui, Molecular ozone and radical pathways of bromate formation during ozonation, J. Environ. Eng. 124 (1998) 456–462. [12] U. von Gunten, Y. Oliveras, Advanced oxidation of bromide-containing waters: bromate formation mechanisms, Environ. Sci. Technol. 32 (1998) 63–70. [13] A. Fischbacher, K. Löppenberg, C. von Sonntag, T.C. Schmidt, A New reaction pathway for bromite to bromate in the ozonation of bromide, Environ. Sci. Technol. 49 (2015) 11714–11720. [14] U. Pinkernell, U. von Gunten, Bromate minimization during ozonation: mechanistic considerations, Environ. Sci. Technol. 35 (2001) 2525–2531. [15] R. Hofmann, R.C. Andrews, Impact of H 2 O 2 and (bi) carbonate alkalinity on ammonia’s inhibition of bromate formation, Water Res. 40 (2006) 3343–3348. [16] M.J. Stiff, Copper/bicarbonate equilibria in solutions of bicarbonate ion at concentrations similar to those found in natural water, Water Res. 5 (1971) 171–176. [17] J. Yang, J. Li, W. Dong, J. Ma, J. Cao, T. Li, J. Li, J. Gu, P. Liu, Study on enhanced degradation of atrazine by ozonation in the presence of hydroxylamine, J. Hazard. Mater. 316 (2016) 110–121. [18] J. Li, H. Liu, X. Zhao, J. Qu, R. Liu, J. Ru, Effect of preozonation on the characteristic transformation of fulvic acid and its subsequent trichloromethane formation potential: presence or absence of bicarbonate, Chemosphere 71 (2008) 1639–1645. [19] J.P. CrouÃ, B.K. Koudjonou, B. Legube, Parameters affecting the formation of bromate ion during ozonation, Ozone:Sci. Eng. 18 (1996) 1–18. [20] V.W. Cope, S. Chen, M.Z. Hoffman, Intermediates in the photochemistry of of carbonato-amine complexes of cobalt(III). Carbonate(-) radicals and the aquocarbonato complex, J. Am. Chem. Soc. 95 (1973) 3116–3121. [21] S.N. Chen, V.W. Cope, M.Z. Hoffman, Behavior of carbon trioxide (-) radicals generated in the flash photolysis of carbonatoamine complexes of cobalt(III) in aqueous solution, J. Phys. Chem. (1973) 1111–1116. [22] M. Patrick, B. Céciel, D. Anne, D.L. Joseph, A comparison of fenuron degradation by hydroxyl and carbonate radicals in aqueous solution, Water Res. 41 (2007) 4585–4594. [23] M.L. Dell’Arciprete, J.M. Soler, L. Santosjuanes, A. Arques, D.O. Mártire, J.P. Furlong, M.C. Gonzalez, Reactivity of neonicotinoid insecticides with carbonate radicals, Water Res. 46 (2012) 3479–3489. [24] F. Orellana-García, M.A. Álvarez, M.V. López-Ramón, J. Rivera-Utrilla, M. SánchezPolo, Effect of HO•, SO4•− and CO3•−/HCO3• radicals on the photodegradation of the herbicide amitrole by UV radiation in aqueous solution, Chem. Eng. J. 267 (2015) 182–190. [25] J. Hoigné, H. Bader, Rate constants of reactions of ozone with organic and inorganic compounds in water—II: dissociating organic compounds, Water Res. 17 (1983) 185–194. [26] G.V. Buxton, C.L. Greenstock, W.P. Helman, A.B. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH/•O− in aqueous solution, J. Phys. Chem. Ref. Data 17 (1988) 513–886. [27] T. Umschlag, H. Herrmann, The carbonate radical (HCO3•/CO3•−) as a reactive intermediate in Water chemistry: kinetics and modelling, Acta Hydrochim. Hydrobiol. 27 (1999) 214–222. [28] J.L. Acero, K. Stemmler, U. von Gunten, Degradation kinetics of atrazine and its degradation products with ozone and OH radicals: a predictive tool for drinking water treatment, Environ. Sci. 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4. Conclusion The present work proposed for the first time a quantitative method for determining the CO3%−-involved bromate and CO3%− exposure during ozonation in the presence of bicarbonate. The results showed that ozone and %OH behave quite similarly in the presence of bicarbonate and in the presence of t-BuOH with the same %OH scavenging capacities. Accordingly, the CO3%−-involved bromate was evaluated by subtracting the bromate formed in the presence of t-BuOH from that formed in the presence of bicarbonate with the same %OH scavenging capacity. The CO3%− exposure was obtained from the degradation of NB and ATZ. Based on these methods, the CO3%− exposure was determined to be two orders of magnitude greater than the corresponding %OH exposure. Moreover, the bromine scavenger ammonia fully inhibited the CO3%−-involved bromate formation. Accordingly, the primary role of CO3%- in the bromate formation is the oxidation of bromine, while its role in the oxidation of bromide to Br• is negligible. The CO3%−-involved bromate contributed a large fraction of the total bromate formed when the concentration of bicarbonate was greater than 1 mM. A model based on the ratios of the bromine oxidation by ozone, %OH and CO3%− was developed to predict the CO3%−-involved bromate. Both the experimental and the model results showed that the amount of CO3%−-involved bromate is greater at higher bicarbonate concentrations, pH values, initial bromide concentrations and ozone dosages but lower in the presence of humic acid. As a result, the bromate formation should be carefully examined during the ozonation of bromide-containing water with a high concentration of bicarbonate. Further studies are required to determine the role of CO3%− in the formation of brominated organic products. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51525805, 51708563, 51608330) and the Shenzhen Science and Technology Project (JCYJ20150403161923535). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.10.013. References [1] C. von Sonntag, U. von Gunten, Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications, IWA publishing, 2012. [2] A. Tekle-Röttering, C. von Sonntag, E. Reisz, C. Vom Eyser, H.V. Lutze, J. Türk, S. Naumov, W. Schmidt, T.C. Schmidt, Ozonation of anilines: kinetics, stoichiometry, product identification and elucidation of pathways, Water Res. 98 (2016) 147–159. [3] J. Yang, J. Li, J. Zhu, Z. Dong, F. Luo, Y. Wang, H. Liu, C. Jiang, H. Yuan, A novel design for an ozone contact reactor and its performance on hydrodynamics,
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J. Yang et al. [35] N. Lu, X. Wu, J. Zhou, X. Huang, G. Ding, Bromate oxidized from bromide during sonolytic ozonation, Ultrason. Sonochem. 22 (2015) 139–143. [36] J. Yang, J. Li, W. Dong, J. Ma, Y. Yang, J. Li, Z. Yang, X. Zhang, J. Gu, W. Xie, Enhancement of bromate formation by pH depression during ozonation of bromidecontaining water in the presence of hydroxylamine, Water Res. 109 (2017) 135–143. [37] M.B. Heeb, J. Criquet, S.G. Zimmermann-Steffens, U. von Gunten, Oxidative treatment of bromide-containing waters: formation of bromine and its reactions with inorganic and organic compounds — a critical review, Water Res. 48
(2014) 15. [38] K. Chon, E. Salhi, U. von Gunten, Combination of UV absorbance and electron donating capacity to assess degradation of micropollutants and formation of bromate during ozonation of wastewater effluents, Water Res. 81 (2015) 388–397. [39] W.T. Li, M.J. Cao, T. Young, B. Ruffino, M. Dodd, A.M. Li, G. Korshin, Application of UV absorbance and fluorescence indicators to assess the formation of biodegradable dissolved organic carbon and bromate during ozonation, Water Res. 111 (2017) 154–162.
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Quantitatively assessing the role played by carbonate radicals in bromate formation by ozonation
T
Jingxin Yanga, Zijun Dongc, Chengchun Jiangc, Hong Liua,b, , Ji Lid ⁎
a
Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou, 510006, China b Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, China c Department of Building and Environmental Engineering, Shenzhen Polytechnic, Shenzhen 518055, China d Shenzhen Key Laboratory of Water Resource Application and Environmental Pollution Control, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen, 518055, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Bromate Carbonate radicals (CO3%−) Carbonate/bicarbonate Ozonation Hydroxyl radical (%OH)
Bicarbonate scavenges %OH to form CO3%− that enhances the bromate formation by ozonation. However, the role of CO3%− in the bromate formation during ozonation has never been quantitatively investigated. Herein, we establish a quantitative approach for evaluating the role played by CO3%− based on the detection of CO3%−involved bromate and CO3%− exposure. Experiments demonstrated that the CO3%−-involved bromate was responsible for 33.7–69.9% of the total bromate formed with bicarbonate concentrations from 0.5 mM to 4 mM. The CO3%− exposure was two orders of magnitude higher than the corresponding %OH exposure during ozonation. These results demonstrate that CO3%− plays a comparable or even more pronounced role in the oxidation of bromine during bromate formation than %OH. A model was developed based on the ratio of bromine oxidized by CO3%−, which could predict the CO3%−-involved bromate formation well. Modeled and experimental results illustrated that the contribution of the CO3%−-involved bromate to the total bromate decreased with increasing pH or initial bromide, but almost remained unchanged at different ozone dosages. Moreover, the presence of
⁎ Corresponding author at: Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou University, Guangzhou, 510006, China. E-mail address: [email protected] (H. Liu).
https://doi.org/10.1016/j.jhazmat.2018.10.013 Received 22 March 2018; Received in revised form 24 August 2018; Accepted 3 October 2018 Available online 09 October 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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humic acid led to an increase in this contribution during ozonation. The results of this study provide a more indepth understanding of the mechanism of bromate formation during ozonation.
1. Introduction
bromate formation. As mentioned above, bicarbonate plays a dual role in the bromate formation during ozonation, since it not only leads to the generation of CO3%-, which enhances the bromate formation, but also scavenges the same amount of %OH, which inhibits the bromate formation. This means that the CO3%–involved bromate formation includes the difference in the bromate formation in the presence and absence of bicarbonate and also the reduced bromate formation due to the scavenging %OH by bicarbonate. Thus, the method adopted in their study neglected the inhibiting effect of bicarbonate on the bromate formation via •OH scavenging. Therefore, a new and correct quantitative method is required to evaluate the CO3%–involved bromate formation. In addition, the effect of the water conditions on the CO3%– involved bromate formation remains unexplored. CO3%− is a moderately strong one-electron oxidant with a redox potential of 1.78 V at pH 7 [20]. It oxidizes many organic and inorganic contaminants, including bromide and HOBr/–OBr, with rate constants ranging from 103 to 109 M-1s-1 [21–24]. CO3%− plays a similar role as % OH in the bromate formation during ozonation. Thus, it could enhance the bromate formation by ozonation via two potential pathways. One is the oxidation of Br- by CO3%− to produce Br%, which can be further oxidized by ozone to eventually form bromate. The other is the oxidation of HOBr/-OBr to %OBr, which eventually transforms to bromate. To elucidate the mechanism of the CO3%−-involved bromate formation, the CO3%- exposure needs to be evaluated. Unfortunately, to the best of our knowledge, the CO3%- exposure has never been quantified in previous studies involving bromate formation during ozonation in the presence of bicarbonate. The major objectives of this study are (1) to quantify the contribution of pathways involving CO3%− to the bromate formation during ozonation in the presence of bicarbonate; (2) to determine the CO3•exposure and elucidate the mechanism of CO3%−-involved bromate formation; (3) to develop a model to predict CO3%−-involved bromate; and (4) to evaluate the effects of pH, bromide, ozone dosage and humic acid on the CO3%−-involved bromate formation.
Ozone is globally applied as a disinfectant and/or oxidant in water and wastewater treatment [1–3]. The application of ozone, however, may be limited when the targeted water contains bromide with a concentration greater than 50 μg/L [4]. These levels of bromide readily lead to excessive bromate formation during ozonation. Due to its potential carcinogenicity, bromate is typically regulated at a maximum contaminant level of 10 μg/L in drinking water in many countries [5,6]. The kinetics and mechanisms of bromate formation during ozonation have been intensively studied [4,7–9]. As has been reported previously, bromate is produced from the oxidation of bromide by ozone and/or hydroxyl radicals (%OH) in series and in parallel during ozonation [10]. Its formation mechanism is divided into three pathways, including the direct pathway (Br− + O3 → HOBr/−OBr + O3→ … → BrO3−), the direct-indirect pathway (Br− + O3 → HOBr/−OBr + %OH → … → BrO3−) and the indirect-direct pathway (Br− + %OH → Br% + O3→ … → BrO3−) [11–13]. The detailed reactions of each pathway were listed in support information Table S1. The formation of bromate via the direct pathway only proceeds via the oxidation by ozone. It typically occurs at alkaline pH when the equilibrium of HOBr/–OBr shifts towards –OBr, due to the extremely low reactivity of ozone towards HOBr. In comparison, the formation of bromate requires the oxidation by %OH in other two pathways. Besides oxidizing both HOBr and –OBr with similar rate constants, %OH is capable of oxidizing Br− to Br%. As a result, Br− can be still be transformed into bromate even in the presence of HOBr/–OBr scavengers such like H2O2 and NH2OH via the indirect-direct pathway, which is the only pathway that requires no HOBr/–OBr as an intermediate product. Accordingly, %OH plays an essential role in the formation of bromate during ozonation. Based on this formation mechanism, several options, such as %OH scavenging, have been proposed to control bromate formation during ozonation [14,15]. Bicarbonate is a common %OH scavenger and is typically detected in surface and ground water with concentrations ranging from 1 mM to 5 mM [16]. It has been found to have a negative effect on the degradation of refractory organic contaminants during ozone-based advanced oxidation processes [17,18]. Similarly, it was thought that bicarbonate would decrease the bromate formation by inhibiting the pathways that involve %OH (including the direct-indirect pathway and the indirect-direct pathway) during ozonation. However, several previous studies have shown that the addition of bicarbonate enhances the formation of bromate during ozonation [9,10,19]. They ascribed this enhancement effect of bicarbonate addition to (a) CO3%− produced by the reaction of %OH with H2CO3/HCO3-/CO32- acting as a secondary oxidant to oxidize OBr- to %OBr; (b) a higher steady-state concentration of CO3%− compared to %OH, due to its higher selectivity for the oxidation of contaminants in aqueous solution; and (c) a lower ozone decomposition due to the inhibition of %OH chain reactions. Thus, the reasons for the above mentioned enhanced bromate formation in the presence of bicarbonate are strongly related to CO3%−. The CO3%−-involved bromate formation must therefore contribute substantially to the total bromate formation during ozonation in the presence of bicarbonate. However, to the best of our knowledge, only one prior study has ever quantified the CO3%−-involved bromate formation during the ozonation of bromide-containing water in the presence of bicarbonate [11]. In that study, the CO3%−-involved bromate formation was obtained via subtracting the bromate formation in the absence of bicarbonate from that in the presence of bicarbonate. However, this quantitative method probably underestimates the CO3%–involved
2. Material and methods 2.1. Material All the solutions in present work were prepared with ultrapure water produced from the Millipore system (18.2 MΩ cm). Potassium indigotrisulfonate and humic acid were purchased from Sigma–Aldrich. Atraizine (ATZ), 2,6-dichlorophenol (99.0%), potassium bromide, potassium bromte, tert-butyl alcohol (t-BuOH, ≥99.8%) and sodium sulfite (98.0%) were obtained from Aladdin Industrial Corporation. Nitrobenzene (NB, ≥99.0%), sodium bicarbonate (≥99.5%), Sodium hydroxide (≥96.0%), sulfuric acid (95.0–98.0%), sodium phosphate dibasic (≥99.0%) and sodium dihydrogen phosphate (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ozone stock solution was prepared by passing an oxygen/ozone mixture gas produced from an ozone generator into 4 °C ultrapure water cooled by an ice-cooling system. The concentration of dissolved ozone typically achieved 1.1 mM in the stock solution. 2.2. Experimental procedure The ozonation experiments were conducted in a 600 mL glass bottle with a bottle-top dispenser (purchased from Brand, Germany) for sampling. The bottle was performed at 20 °C in a water bath. Synthetic water, prepared using ultrapure water, was buffered with 5 mM of 429
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phosphate, and the pH value was adjusted to the desired value with sulfuric acid (0.1 M) and sodium hydroxide (0.1 M). Atrazine (ATZ) and nitrobenzene (NB), as probe compounds to quantify the radical exposure, were both spiked with concentrations of 0.5 μM before the experiments. A certain volume of ozone stock solution was rapidly added to the experimental solution with a glass syringe to achieve the desired concentration of ozone and mixed with the prepared synthetic water by a magnetic stirrer, and the reaction time was simultaneously marked. The stirring was stopped after 20 s to minimize the loss of dissolved ozone by stripping. Samples were withdrawn at the predetermined time intervals via the bottle-topped dispenser, and the residual dissolved ozone was removed by N2 purging for one minute. The samples were then divided into four portions to measure the BrO3−, HOBr/−OBr, Br− and the probe compounds. All the experiments were conducted at least twice. The results represent the mean values with standard deviations.
[32]. The derivative product (4-bromo-2,6-dichlorophenol) of the reaction was then monitored using HPLC, equipped with a Waters XBridge™ C18 column (150 mm × 4.6 mm, 3.5 μm particle size), an auto-sampler (Waters 717) and a dual λ detector, with an eluent containing 0.5% acetic acid and methanol (60:40, v/v %) at λ = 252 nm and a retention time of 3.5 min. This method was applied to quantify the concentrations of HOBr/-OBr [33]. ATZ was quantified using HPLC with an eluent containing 0.5% acetic acid and methanol (40:60, v/v %) at λ = 221 nm and a retention time of 5 min. NB was also quantified using HPLC with the same eluent as for ATZ but at λ = 268 nm and a retention time of 3.2 min. 3. Results and discussion 3.1. Quantification of the CO3%−-involved bromate formation Fig. 1(a) shows the time-dependent bromate formation via different processes at an ozone dosage of 2 mg/L and pH 8. Compared to the control, the concentration of bromate formed increased by 103.9% due to the addition of 10 mM of bicarbonate and decreased by 59.7% due to the addition of 168 μM of t-BuOH. This indicates the bromate formation was enhanced by the bicarbonate addition but inhibited by the t-BuOH addition during ozonation, which is consistent with previous studies [11]. As Fig. 1(b) shows, the decomposition rate of the dissolved ozone in the presence of 10 mM of bicarbonate was lower than in the control but was similar to the rate in the presence of 168 μM of t-BuOH. Meanwhile, the %OH exposure in the presence of 10 mM of bicarbonate was lower than in the control but was very similar to the %OH exposure in the presence of 168 μM of t-BuOH (see Fig. 1(c)). These results suggest that both bicarbonate and t-BuOH inhibit the %OH chain reactions and thus lead to higher ozone exposure and lower %OH exposure. Most importantly, the effect of the bicarbonate addition on the ozone decomposition and the %OH exposure can be approximated by adding tBuOH. This can be mainly attributed to the fact that the same %OH scavenging ability is obtained via the addition of 10 mM of bicarbonate or 168 μM of t-BuOH in the experimental solutions at pH 8. In spite of the quite similar ozone and %OH exposure, the bromate formation varied greatly in these two cases, since the bicarbonate scavenged %OH to generate a secondary oxidant, CO3%−, while t-BuOH scavenged %OH to terminate the radical chain reactions during ozonation. These results confirm the significant role of CO3%− in the bromate formation during ozonation in the presence of bicarbonate. The enhancement effect of bicarbonate on the bromate formation during ozonation, via the production of CO3%−, overwhelmed its inhibiting effect via the scavenging of %OH. Due to the similar behavior of both ozone and %OH in the presence of bicarbonate and t-BuOH with the same %OH scavenging ability, the CO3%−-involved bromate formation can be approximated by subtracting the bromate formed in the presence of t-BuOH from that formed in the presence of bicarbonate with the same %OH scavenging ability. According to the results shown in Fig. 1(a), approximately 80% of the total formed bromate was produced via the CO3%−-involved pathways during ozonation in the presence of 10 mM of bicarbonate. In addition, bicarbonate addition still led to an increase of 104% in
2.3. Quantification of CO3%− exposure In this study, we used nitrobenzene (NB) and atrazine (ATZ) as probe compounds to evaluate the %OH exposure and CO3%− exposure. Trace amounts of NB and ATZ have a negligible effect on the bromate formation during ozonation. Since the two probe compounds are degraded by molecular ozone, %OH and CO3%− during ozonation in the presence of bicarbonate, the degradation rate of NB and ATZ can be described using Eqs. (1) and (2), respectively. NB reacts with CO3%− with a second-order rate constant of 1.4 × 104 M-1s-1, which is far less than its reaction rate with %OH (k = 3.9 × 109 M-1s-1). Thus, in this case, its degradation by CO3%- can be neglected. Accordingly, Eq. (1) can be simplified to Eq. (3). Upon integration of Eqs. (3) and (2), the % OH exposure and CO3%- exposure can be obtained from Eq. (5) and Eq. (6).
d [NB ] = k (OH , NB) [OH ][NB] + k (O3, NB ) [O3][NB] + k (CO3 dt
, NB ) [CO3
][NB ] (1)
d [ATZ ] = k ( OH , ATZ ) [ OH ][ATZ ] + k (O3, ATZ ) [O3 ][ATZ ] dt + k (CO3
, ATZ ) [CO3
(2)
][ATZ ]
d [NB ] = k ( OH , NB ) [ OH ][NB] + k (O3, NB) [O3 ][NB] dt [ OH ] dt = (ln
[CO3 ] dt = (ln
[NB]0 [NB]t [ATZ ]0 [ATZ ]t
/ k (CO3
k (O3, NB )
[O3 ] dt )/ k ( OH , NB )
k (O3, ATZ )
[O3 ] dt k ( OH , ATZ )
(3) (4)
[ OH ] dt ) (5)
, ATZ )
where ∫ [O3]dt, ∫ [ OH]dt and ∫ represent the ozone exposure, %OH exposure and CO3%- exposure, respectively. The secondorder rate constants in the equations above are listed in Table 1. %
[CO3%−]dt
2.4. Analytical methods
Table 1 Second-order rate constants for the two probe compounds with O3, %OH and CO3%−.
Ozone in the stock solution is directly measured at 258 nm (ε = 3000 M−1 cm−1). The dissolved ozone in the experiments is determined with indigo agent at 600 nm (ε = 20,000 M−1 cm−1) [30]. Bromate was analyzed using an ion chromatography method combined with a post-column reaction [31]. Bromide was determined using an ion chromatography system (ISC-3000, Dionex) with an eluent containing 20 mM of KOH. HOBr/-OBr reacts with 2,6-dichlorophenol at a moderate rate to form 4-bromo-2,6-dichlorophenol (pKa = 6.97, k(2,6-di4 M−1s−1, k(2,6-dichlorophenolate,HOBr) = 4.5 chlorophenol,HOBr) = 3.5 × 10 −1 −1 × 105 M s , and k(2,6-dichlorophenolate,-OBr) = 1.6 × 104 M−1s−1)
Probe compound
kO3 (M−1s−1)
Nitrobenzene Atrazine
0.09 6d
a b c d e
430
[25]. [26]. [27]. [28]. [29].
a
k%OH (M−1s−1)
kCO3%- (M−1s−1)
3.9 × 109 3 × 109 d
1.4 × 104 4 × 106 e
b
c
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Fig. 1. Time-dependent formed bromate (a), dissolved ozone residuals (b), %OH exposures (c), CO3%− exposures (d) in the different processes. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 10 mM, [t-BuOH] = 168 μM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C.
bromate formation, even in the presence of t-BuOH. This indicates that CO3%− is negligibly scavenged by t-BuOH compared to %OH, due to its higher selectivity for the latter radical. The second-order rate constants are 9.6 × 104 M-1s-1 and 6.0 × 108 M-1s-1 for the reactions of t-BuOH with CO3%− and %OH, respectively.
of t-BuOH, a portion of the %OH was scavenged by t-BuOH, and as a result, there was less %OH available for the production of CO3%−. It should be noted that the CO3%− exposure (10-8 M s) is two orders of magnitude larger than the corresponding %OH exposure (10-10 M s). For the reaction with Br- to form Br•, the second-order rate constant with CO3%− is 1.0 × 105 M-1s-1, which is four orders of magnitude smaller than with %OH (k(%OH,Br-) = 1.1 × 109 M-1s-1). Accordingly, the oxidation of bromide by CO3%- is negligible compared to its oxidation by %OH. The transformation of Br- to Br% by the total radical content (%OH and CO3%−) is largely inhibited in the presence of bicarbonate. For the reaction with -OBr to form BrO%, the second-order rate constant with CO3%- is 4.3 × 107 M-1s-1, while the second-order rate constant with % OH is 4.5 × 109 M-1s-1. This suggests that the oxidation of -OBr to BrO% by CO3%− would be comparable or even more pronounced than the oxidation by %OH. In addition, the presence of bicarbonate reduces the buildup of HOBr/-OBr during ozonation due to greater transformation of -OBr to BrO% by CO3%−, as presented in Fig. S1. Accordingly, it seems likely that CO3%− primarily enhances the bromate formation during ozonation via enhancing the oxidation of -OBr to BrO%. To further confirm this assumption, we spiked the experimental solutions with excess ammonia and investigated its impact on the bromate formation in the presence of bicarbonate. Ammonia effectively inhibits the bromate formation by scavenging bromine but is largely ineffective at
3.2. Elucidation of the mechanism of the CO3%−-involved bromate formation As mentioned above, CO3%− is a strong oxidant that can oxidize both bromide and HOBr/-OBr, which is favorable for bromate formation (see Eqs. (6) and (7)). To elucidate the mechanism of the CO3%−involved bromate formation, we evaluated the corresponding CO3%exposure during the ozonation of bromide-containing water in the presence of bicarbonate. Br− + O3 → HOBr/-OBr + CO3%- →…→ BrO3−
Br
+
CO3%-
→ Br + O3→…→ BrO3%
(6) (7)
Fig. 1(d) shows the CO3 exposure obtained from different processes of degradation of ATZ and NB, as illustrated in Eqs. (1)–(5). The CO3%- exposure was observed to be much higher in the absence of tBuOH than in the presence of t-BuOH during ozonation. In the presence %−
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inhibiting the bromate formation by scavenging Br% [15]. As shown in Fig. S2, the amount of bromate formed decreased by more than 90% in the presence of ammonia. This suggests that ammonia almost completely inhibited the bromate formation in the presence of bicarbonate. Thus, HOBr/-OBr is a critical intermediate in the bromate formation via pathways that involve CO3%−. In conclusion, in the pathways involving CO3%−, bromide is initially oxidized by ozone to bromine, which is further oxidized by CO3%- to BrO% and eventually converted to bromate, as in Eq. (6).
accounted for more than half of the total bromate formation when the bicarbonate concentration was greater than 1 mM. In comparison, the non−CO3%−-involved bromate decreased with increasing bicarbonate concentration. These results are mainly due to the greater transformation of %OH into CO3%− with increasing bicarbonate concentration, which could be observed from the evolutions of CO3%- exposure and % OH exposure. As shown in Fig. 2(b), the CO3%- exposure increased from 2.71 × 10-8 M s to 5.66 × 10-8 M s with increasing bicarbonate concentration from 0.5 mM to 4 mM. Simultaneously, the corresponding % OH exposure declined from 3.40 × 10-10 M s to 8.67 × 10-11 M s (see Fig. 2(c)). Since the ratio of the CO3%- exposure to the %OH exposure increased from 121 to 657 as the bicarbonate concentration increased from 0.5 mM to 4 mM, the oxidation of bromine is primarily performed by CO3%− rather than %OH. As demonstrated above, the formation of HOBr/-OBr is required for the bromate formation pathways that involve CO3%−. HOBr/-OBr is primarily produced by the oxidation of bromide by molecular ozone. Accordingly, the decomposition of ozone is important for the CO3%−-involved bromate formation. As shown in Fig. 2(d), the presence of bicarbonate slows down the decomposition of ozone. The decomposition rate of the dissolved ozone declined from 0.0373 min-1 to 0.0310 min-1 with increasing bicarbonate concentrations from 0.5 mM to 4 mM. This resulted in an increase in the ozone exposure from 0.0377 M s to 0.0423 M s. Similar trends during
3.3. The CO3%−-involved bromate formation and CO3%− exposure with different bicarbonate dosages Fig. 2(a) shows the concentrations of bromate formed during ozonation with an initial bromide concentration of 5 μM and different bicarbonate concentrations (0.5–4 mM) at pH 8. The total bromate formed during ozonation in the presence of bicarbonate was divided into two parts: the CO3%−-involved bromate and the non−CO3%−-involved bromate. Both the total bromate and the CO3%−-involved bromate increased with increasing bicarbonate concentration. The contribution of the CO3%−-involved bromate accounted for 33.7% to 69.9% of the total bromate formation as the bicarbonate concentration increased from 0.5 mM to 4 mM. In particular, this contribution
Fig. 2. Bromate formation (a), CO3%− exposures (b), %OH exposures (c) and dissolved ozone decay (d) at different bicarbonate concentrations. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 432
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ozonation in the presence of bicarbonate have been commonly reported by many previous studies and are attributed to the inhibition of %OH chain reactions by the presence of bicarbonate [34].
[CO3%−] to [O3] at any time during the reaction, as shown in Eq. (11), while R ct , OH represents the ratio of [%OH] to [O3], as in Eq. (12). The substitution of Eqs. (11) and (12) into Eq. (8) yields Eq. (13). The CO3%−-involved bromate can be obtained using Eq. (14).
3.4. Modeling the CO3%−-involved bromate formation
fCO3• =
As demonstrated above, the bromine scavenger ammonia fully inhibits bromate formation in the presence of bicarbonate. This suggests that HOBr/−OBr is a required intermediate for the formation of bromate in this case, while the indirect-direct pathway, a bromine-free pathway for bromate formation during ozonation, contributes negligibly to the total bromate formation. Accordingly, we constructed a model to predict the CO3%–involved bromate formation based on the oxidation of bromine. The formed bromine is mainly oxidized by three oxidants, including ozone, %OH and CO3%- and is eventually converted to bromate. The ratio of the CO3%−-involved bromate to the total bromate ( fCO3 ) is given by Eq. (8). To simplify the equation, we define a term R ct , CO3 that describes the ratio of the CO3%− exposure to the ozone exposure, as in Eq. (9). This is similar to a commonly used term, R ct , OH , which refers to the ratio of the %OH exposure to the ozone exposure, as in Eq. (10). As presented in Fig. S3, R ct , CO3 and R ct , OH can be shown to be constant over the entire reaction time, suggesting that the two terms are independent of the reaction time. Thus, R ct , CO3 represents the ratio of
[BrO3 ]CO3• [BrO3 ]total kCO3• [CO3• ][HOBr ]t
=
kCO3• [CO3•
][HOBr ]t + k•OH [•OH ][HOBr ]t + kO3 [O3 ][HOBr ]t (8)
R ct , CO3 = R ct ,•OH =
[CO3• ] dt / [•OH ] dt /
[O3 ] dt [O3 ] dt
(9) (10)
R ct , CO3 = [CO3• ]/[O3]
(11)
R ct ,•OH = [•OH ]/[O3]
(12)
fCO•3 =
kCO•3 R ct,CO•3 kCO•3
R ct,CO•3 + k•OH Rct,•OH + kO3
(13)
Fig. 3. Effect of pH on bromate formed concentrations (a), CO3•− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 5 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 6–8, 20 °C. 433
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[BrO3 ]CO•3 = fCO•3 [BrO3 ]total = [BrO3 ]total
pH dependence of the CO3%–involved and non−CO3%–involved bromate are explained by the pH-dependent CO3%− exposure and %OH exposure. As shown in Fig. 3(b) and (c), the CO3%- exposure and %OH exposure increased by 474–813% and 317–453%, respectively, as the pH increased from 6 to 8 for all the bicarbonate concentrations. The production of %OH by the reaction of ozone with OH- is enhanced at higher pH. Since CO3%- is produced by the reaction of %OH with H2CO3/HCO3-/ CO32-, higher %OH exposure results in higher CO3%− exposure. Moreover, the pH affects the H2CO3/HCO3-/CO32- equilibrium, as shown in Eqs. (15) and (16). The three species scavenge %OH with different rate constants (Eqs. (17), (18) and (19)) [26]. Thus, the rate constants for the reaction of H2CO3/HCO3-/CO32- with %OH vary with pH. As shown in Fig. S5, increasing the pH from 6 to 8 shifts the H2CO3/HCO3equilibrium toward HCO3-, and thus, the second-order rate constant for the reaction of %OH with H2CO3/HCO3- increases from 3.26 × 106 M-1s1 to 1.01 × 107 M-1s-1. According to Eq. (20), the protonation of CO3%− is negligible at pH 6–8. In addition, the pH dependence of HOBr is also partially responsible for the enhanced bromate formation during ozonation in the presence of bicarbonate as the pH increases. Increasing the pH shifts the HOBr/-OBr equilibrium toward –OBr, which is more readily oxidized than its protonated form.
kCO•3 R ct,CO•3 kCO•3
Rct,CO•3
+ k•OH R ct, OH + kO3 •
(14)
3.5. Effect of pH The solution pH is a significant factor in the bromate formation during ozonation. As has been reported previously, decreasing the pH is typically employed as an effective method to reduce bromate formation [8,35]. However, our previous study found that pH depression unexpectedly enhanced the bromate formation during ozonation in the presence of hydroxylamine via the indirect-direct pathway, due to a higher Br• formation rate at lower pH [36]. Thus, it is necessary to investigate the effect of pH on the CO3%−-involved bromate formation during ozonation. Fig. 3 shows the pH-dependent bromate formation, CO3%− exposure, % OH exposure and ozone exposure in the presence of bicarbonate, with an initial bromide concentration of 5 μM and an ozone dosage of 41.7 μM. In Fig. 3(a), the concentrations of both the CO3%–involved and non−CO3%–involved bromate markedly increased with increasing pH from 6 to 8 for all the bicarbonate concentrations. The model results for the CO3%–involved bromate are generally consistent with the corresponding experimental results at different pH values (see Fig. S4). The
H2CO3 ↔ HCO3− + H+, pKa1 = 6.37
(15)
Fig. 4. Effect of bromide on bromate formation (a), CO3%− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 2.5–10 μM, [O3] = 41.7 μM, [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 434
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HCO3− ↔ CO32- + H+, pKa2 = 10.33 H2CO3 + OH →
CO3%−
%
HCO3− CO3
2−
HCO3%
+ OH → %
+ OH → %
→
CO3%−
CO3%CO3%-
+
6
-1 -1
+ H2O + H , k = 1.0 × 10 M s 6
-1 -1
+ H2O, k = 8.5 × 10 M s 8
-1 -1
+ HO- k = 3.9 × 10 M s
+
+ H , pKa < 0
−
(17)
HOBr + OH → BrO + H2O, k = 2 × 10 M
(18)
−
OBr + OH → BrO + OH-, k = 4.5 × 10 M s
(19)
−
OBr + CO3 - → BrO + CO3 , k = 4.3 × 10 M s
%
%
%
−1 −1
s
9
%
%
(22)
9
%
2-
-1 -1
7
-1 -1
(23) (24) (25)
(20)
It should be noted, however, that the contribution of the CO3%−involved bromate to the total bromate formation decreases with increasing pH during ozonation in the presence of bicarbonate. For example, the contribution of the CO3%−-involved bromate accounted for approximately 84.4%, 76.0% and 69.9% of the total bromate formed in the presence of 4 mM of bicarbonate at pH 6, pH 7 and pH 8, respectively. This result can be mainly attributed to the pH dependence of the bromine oxidation. Bromine is oxidized by ozone, %OH and CO3%during ozonation, as shown in reactions (21)–(25) [10]. The oxidation of bromine by ozone is more sensitive to pH than the oxidation of bromine by radicals. Thus, the direct pathway for bromate formation is enhanced by increasing the pH more greatly than the pathways involving radicals. HOBr + O3 → BrO2− + O2 + H+, k = 0.01 M-1s-1
OBr + O3 → BrO2− + O2, k = 100 M-1s-1
(16)
3.6. Effect of bromide Fig. 4 shows the bromate formation, CO3%− exposure, %OH exposure and ozone exposure at bromide concentrations of 2.5 μM, 5 μM and 10 μM during ozonation in the presence of different concentrations of bicarbonate. The experimental CO3%–involved bromate increased by 91.7–108.4% with increasing bromide concentrations, which is consistent with the modeling results (see Fig. S6). The total bromate increased by 122.2–135.8% as the bromide concentration increased from 2.5 μM to 10 μM. However, the contribution of the CO3%–involved pathway to the total bromate formed decreased with increasing bromide concentration. Bromide competes for %OH with bicarbonate to lower the CO3%− production, and thereby leads to lower contributions of CO3%–involved bromate to the total bromate with increasing ratio of bromide to bicarbonate during ozonation in the presence of
(21)
Fig. 5. Effect of ozone dosage on bromate formation (a), CO3%− exposures (b), %OH exposures (c) and ozone exposures (d) during ozonation in the presence of bicarbonate with different levels. Conditions: [Br-] = 5 μM, [O3] = 0.5–4 mg/L (10.4–83.3 μM), [HCO3-] = 4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 435
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bicarbonate. As shown in Fig. 4(b), the CO3%− exposure decreased by 26.6–40.9% as the bromide concentration increased from 2.5 μM to 10 μM. An increase in the bromide concentration leads to an increase in the total %OH scavenging capacity of the system and thus a lower %OH exposure, as presented in Fig. 4(c). Moreover, the %OH scavenged by bicarbonate decreased with increasing bromide concentration. As shown in Fig. S7, the %OH scavenging capacity of bicarbonate decreased by 15.0–42.3% as the bromide concentration increased from 2.5 μM to 10 μM at different bicarbonate concentrations. In addition, according to Fig. 4(d), the ozone exposure decreased with increasing bromide concentration. Lower ozone exposure also results in a lower contribution of the CO3%–involved bromate to the total bromate.
non−CO3%−-involved bromate formation pathways during ozonation. These results again demonstrate that the enhancement of the ozone exposure by bicarbonate addition is partially responsible for the enhanced bromate formation. In the CO3%−-involved bromate formation pathways, ozone is mainly responsible for the oxidation of bromide to yield HOBr/-OBr and for the conversion of BrO2- to BrO2%. Moreover, the higher ozone dosage not only leads to higher ozone exposure but also to higher CO3%- exposure and %OH exposure (see Fig. 5(b–d)). 3.8. Effect of humic acid Humic acid is ubiquitously found in natural water. It can act both as an initiator for the reaction with ozone to generate %OH and as a scavenger of the formed %OH and HOBr/−OBr [37]. Thus, a set of experiments was conducted in the presence of 0.2 mg-C/L humic acid to investigate its effect on the bromate formation during ozonation in the presence of bicarbonate. As shown in Fig. 6, both the total bromate and the CO3%−-involved bromate increased with increasing bicarbonate concentrations from 0.5 mM to 4 mM by 52.2% and 188.1%, respectively, in the presence of humic acid. The modeled CO3%−-involved bromate concentrations agreed closely with the experimental values (see Fig. S9). This suggests
3.7. Effect of ozone dosage Fig. 5 shows the bromate formation at ozone dosages of 0.5–4 mg/L in the presence of 4 mM of bicarbonate. The total bromate formed increased 30-fold with increasing ozone dosage from 0.5 mg/L to 4 mg/L, while the CO3%−-involved bromate and the non−CO3%−-involved bromate increased 36-fold and 23-fold, respectively. The experimental and the modeled CO3%−-involved bromate agreed well (see Fig. S8). This indicates that ozone plays a critical role in both the CO3%−-involved and
Fig. 6. Bromate formation (a), CO3%− exposures (b), %OH exposures (c), ozone exposures (d) in the presence of humic acid (HA) at different bicarbonate concentrations. Conditions: [HA] = 0.2 mg/L, [Br-] = 5 μM, [O3] = 2 mg/L (41.7 μM), [HCO3-] = 0–4 mM, [ATZ] = [NB] = 0.5 μM, 5 mM phosphate buffer at pH 8, 20 °C. 436
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that the bicarbonate addition enhances the bromate formation even in the presence of humic acid via oxidation by CO3%−. Moreover, both the total bromate and the CO3%−-involved bromate in the presence of humic acid were observed to be slightly lower than in pure water at the corresponding bicarbonate levels. This can be ascribed to the relative high special ozone dose (10 mgO3/mgDOC) in this experiment. As reported in previous literature, bromate formation was still pronounced at a special ozone dose > 0.25 mgO3/mgDOC during ozonation, where electron-rich moieties have be completely degraded [38,39]. It should be noted that the presence of humic acid enhances the contribution of the CO3%−-involved bromate to the total bromate. This trend may be due to the higher CO3%- exposure in the presence of humic acid than in pure water. The reactions of ozone with the electron-rich compounds from humic acid accelerated the production of %OH and thus enhanced the resulting CO3%- exposure.
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4. Conclusion The present work proposed for the first time a quantitative method for determining the CO3%−-involved bromate and CO3%− exposure during ozonation in the presence of bicarbonate. The results showed that ozone and %OH behave quite similarly in the presence of bicarbonate and in the presence of t-BuOH with the same %OH scavenging capacities. Accordingly, the CO3%−-involved bromate was evaluated by subtracting the bromate formed in the presence of t-BuOH from that formed in the presence of bicarbonate with the same %OH scavenging capacity. The CO3%− exposure was obtained from the degradation of NB and ATZ. Based on these methods, the CO3%− exposure was determined to be two orders of magnitude greater than the corresponding %OH exposure. Moreover, the bromine scavenger ammonia fully inhibited the CO3%−-involved bromate formation. Accordingly, the primary role of CO3%- in the bromate formation is the oxidation of bromine, while its role in the oxidation of bromide to Br• is negligible. The CO3%−-involved bromate contributed a large fraction of the total bromate formed when the concentration of bicarbonate was greater than 1 mM. A model based on the ratios of the bromine oxidation by ozone, %OH and CO3%− was developed to predict the CO3%−-involved bromate. Both the experimental and the model results showed that the amount of CO3%−-involved bromate is greater at higher bicarbonate concentrations, pH values, initial bromide concentrations and ozone dosages but lower in the presence of humic acid. As a result, the bromate formation should be carefully examined during the ozonation of bromide-containing water with a high concentration of bicarbonate. Further studies are required to determine the role of CO3%− in the formation of brominated organic products. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51525805, 51708563, 51608330) and the Shenzhen Science and Technology Project (JCYJ20150403161923535). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.10.013. References [1] C. von Sonntag, U. von Gunten, Chemistry of Ozone in Water and Wastewater Treatment: From Basic Principles to Applications, IWA publishing, 2012. [2] A. Tekle-Röttering, C. von Sonntag, E. Reisz, C. Vom Eyser, H.V. Lutze, J. Türk, S. Naumov, W. Schmidt, T.C. Schmidt, Ozonation of anilines: kinetics, stoichiometry, product identification and elucidation of pathways, Water Res. 98 (2016) 147–159. [3] J. Yang, J. Li, J. Zhu, Z. Dong, F. Luo, Y. Wang, H. Liu, C. Jiang, H. Yuan, A novel design for an ozone contact reactor and its performance on hydrodynamics,
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