Water Research 122 (2017) 172e182
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Performance characterization and kinetic modeling of ozonation using a new method: ROH,O3 concept Minhwan Kwon a, b, c, Homin Kye a, Youmi Jung a, Yeojoon Yoon d, Joon-Wun Kang a, * a
Department of Environmental Engineering (YIEST), Yonsei University, 234 Maeji, Heungup, Wonju, 220-710, South Korea Trojan Technologies, 3020 Gore Rd., London, Ontario N5V 4T7, Canada c Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada d Water Resources and Environmental Research Division, Korea Institute of Construction Technology, 2311, Deawha, Ilsan, Goyang, Gyeonggi 411-712, Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2017 Received in revised form 25 May 2017 Accepted 28 May 2017 Available online 31 May 2017
Ozonation is an effective treatment for removing various organic pollutants from aquatic systems. The Rct concept, which is defined as the ratio of OH exposure to O3 exposure, has been widely used to predict the removal efficiency of target compounds, but it has significant variations by water temperature and initial O3 dose which are crucial parameters in drinking water plant. The ROH,O3 concept, which is defined as the OH exposure by O3 consumption, was proposed as a kinetic parameter for characterization and kinetic modeling for ozonation. The ROH,O3 concept is independent of temperature and initial O3 dose. A higher ROH,O3 value indicates a higher OH formation when the same amount of O3 is consumed in different water samples; therefore, the OH yield from O3 decomposition of the water samples can be compared using the ROH,O3 values. The ROH,O3 concept can also be used to characterize and model the initial ozone demand phase, and it is more convenient method compared to Rct concept. Using the ROH,O3 concept, the dynamic O3 and OH kinetics and the removal efficiencies of iopromide and ibuprofen were well predicted (R2 ¼ 0.98) over a wide range of experimental conditions (n ¼ 124). © 2017 Elsevier Ltd. All rights reserved.
Keywords: Advanced oxidation process Hydroxyl radical Kinetics Ozone Rct ROH,O3
1. Introduction Ozonation is widely used for inactivation of microorganisms and oxidation of organic pollutants (e.g., taste, odor, color, and micropollutants) (Dodd et al., 2006; Huber et al., 2003; Jung et al., 2012). Ozone (O3) is a strong oxidant (E ¼ 2.07 V), but it has selective reactivity toward organic pollutants. In contrast, the hydroxyl radical (OH), which is formed by O3 decomposition, is a nonselective oxidant and highly reactive (E ¼ 2.8 V) with organic pollutants. Although both O3 and OH are the main oxidants in ozonation, OH alone reacts with certain O3-resistant micropollutants (Elovitz and von Gunten, 1999). In the ozonation of natural water, OH can be produced by the reaction of O3 with OH and functional groups such as phenols and amines in natural organic matter (NOM) (Buffle and Von Gunten, 2006; von Gunten, 2003). To predict the removal efficiencies of organic pollutants, determination of the O3 and OH concentrations in solution is required.
* Corresponding author. E-mail address:
[email protected] (J.-W. Kang). http://dx.doi.org/10.1016/j.watres.2017.05.062 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
Concentration of O3 can be determined using electrochemical, optical, or colorimetric techniques. However, direct measurement of OH concentration is difficult because of its extremely low steadystate concentration (1012 M), resulting from the high reactivity of OH toward the water matrix (von Gunten, 2003). To solve this and Badera (1979) suggested using a OH probe problem, Hoigne compound to quantify the potential degradation of O3-resistant micropollutants by OH oxidation. They suggested an oxidationcompetition value, U, to evaluate the OH oxidation potential of natural water:
kS;i MS;i D½O3 t ¼ h k0OH;M ln ½Mt ½M0
P
UM ¼
(1)
P where kS;i ½MS;i is the total inhibition rate of OH (s1), h is the radical formation yield, k0OH;M is the second order rate constant between OH and micropollutant ‘M’ (M1s1), D½O3 t is consumed O3 f reaction time ‘t (s)’ (M), and ½M0 and ½Mt are concentration of micropollutant ‘M’ at time ‘0’ and ‘t’, respectively. The U value can be used to determine the required O3 dose to achieve 37% removal of micropollutant from a given water sample, but it has its
M. Kwon et al. / Water Research 122 (2017) 172e182
limitation on determination of both the O3 and OH kinetics because O3 exposure is not considered in the concept. To improve the model, Elovitz and von Gunten (1999) proposed the Rct concept; this is a dynamic kinetic-based method that enables differentiation R between total OH exposure (i.e., ½OHdt ) and total O3 exposure R (i.e., ½O3 dt ):
Z Rct ¼ Z
½OHdt ½O3 dt
¼
ln ½pCBAt ½pCBA0 Z kOH;pCBA
1
(2)
½O3 dt
The Rct concept can be used to determine both the O3 exposure and the OH exposure by measuring the degradation rate of O3 and a OH probe compound. p-Chlorobenzoic acid (pCBA) is commonly used as the probe compound in ozonation because it has high reactivity with OH [kOH,pCBA ¼ 5 109 M1 s1 (Neta and Dorfman, 1968)] and low reactivity with O3 [kO3,pCBA ¼ 0.15 M1 s1 (Yao and Haag, 1991)]. The Rct concept has been widely used to predict the removal of organic pollutants because it enables an unknown term, i.e. concentration of OH, to be removed from the kinetic equation. Yong and Lin (2012) found a new interpretation that the Rct value is not only the ratio of OH exposure to O3 exposure but also the ratio of the total initiation capacity to the total inhibition capacity in a system; therefore, the Rct can also be used to quantify the initiation and inhibition capacities. However, kinetic modeling of ozonation is still a challenge for the following reasons: i) the variation of Rct value between initial phase (before 100 s) and second phase (after 100 s) during ozonation (Elovitz and von Gunten, 1999; Shin et al., 2016) and ii) the variation of Rct value with initial O3 dose, temperature, pH, alkalinity, and concentration/composition of NOM (Elovitz et al., 2000; Westerhoff et al., 1999; Yong and Lin, 2013). To explain the different removal rate of a target compound between initial and second phase, two Rct values were used in previous studies (Elovitz and von Gunten, 1999; Shin et al., 2016; Westerhoff et al., 2006), but the two different Rct values imply its limitation for performance characterization during the entire ozonation time. To cover the variations of Rct by the water characteristic and operational factors, a linear correlation between Rct and the O3 decomposition rate (kO3) [Rct/kO3] has been used to derive changes in Rct from kO3 (Elovitz et al., 2000; Kaiser et al., 2013). To determine the linear correlation coefficient of [Rct/kO3] for a water sample, several sets of Rct data [pCBA vs. O3] were needed as function of kO3 by adjusting key parameters such as temperature and initial O3 dose. In this study, we suggest a rearranged equation R (½ ½OHdt =D½O3 t , Equation (3)), which allows to use one set of data [pCBA vs. O3], to simplify the determination of the correlation [Rct/ kO3].
Z Rct ¼Z kO3 Z ¼
Z
½OHdt ½O3 dt kO3 ½OHdt
D½O3 t
Z ½OHdt ½OHdt ¼ ¼ ½O3 0 ½O3 t ½O3 0 1 ekO3 t
¼ ROH;O3
(3)
where kO3 is pseudo first order decomposition rate of O3 (s1). Equation (3) shows that the correlation [Rct/kO3] indicates the OH R exposure by consumed O3 ½ ½OHdt =D½O3 t , which is contrasted with Rct concept indicating the OH exposure by O3 residual
R
173
R
exposure ½ ½OHdt = ½O3 dt . Using the rearranged equation, we developed a new method to predict the performance of ozonation process during the entire ozonation time which includes initial ozone demand phase. Interestingly, the U values have an inverse 1 relationship with [Rct/kO3] (i.e. Rct =kO3 ¼ UpCBA kOH;pCBA ), which indicates the key modeling parameters have connections. To prevent confusion with other parameters, the determined method R using ½ ½OHdt =D½O3 t was named as ROH,O3 concept. The main objective of this study is to determine kinetic parameters that allow the performance characterization and kinetic modeling during entire ozonation time. Using the ROH,O3 concept, the performance of ozonation was characterized in four different water sources. Furthermore, a new method using the ROH,O3 value during the entire ozonation time was proposed for performance characterization and kinetic model. Finally, the removal efficiencies of micropollutants were predicted based on the measured ROH,O3 and the proposed kinetic equation. 2. Materials and methods 2.1. Solution preparation Four water samples were collected and tested in this study. Two water samples were collected from the Han River in Korea, one at a raw water intake station (HR-R) and one after sand filtration (HR-F) in a water treatment plant. From Nakdong-river in Korea, a filtrated water sample by micro-filtration membrane was collected from a water treatment plant (NR). A synthetic solution was prepared by Suwannee River RO isolated NOM (International Humic Substances Society, IHSS) into 5 mM phosphate buffer solution at pH 7. Table 1 lists the water quality parameters of the water samples. As target micropollutants, i.e. ibuprofen and iopromide were purchased from Sigma-Aldrich and U.S. Pharmacopeia, respectively. Iopromide is an iodinated X-ray contrast media compound (ICM) and was the major species among the identified 31 EDCs and PPCPs in the Han River of Korea (Yoon et al., 2010). Ibuprofen, a non-steroidal anti-inflammatory drug, influences the cyclooxygenase pathway, which could affect the regulators of reproduction in both vertebrates and invertebrates (Han et al., 2010). 2.2. Analytical methods The indigo method is used for O3 detection (Bader and Hoigne, 1981). O3 decolorizes the indigo solution, and the change at 600 nm is determined using a spectrophotometer. A highperformance liquid chromatography (HPLC) system (Gilson Inc., USA) equipped with a reverse-phase column (Xbridge™ C18 5.0 mm, 4.6 mm 250 mm), a degasser, an auto-sampler, and an ultraviolet (UV)/visible wavelength detector was used to determine the quantities of the tested organic compounds (iopromide, ibuprofen, and pCBA). The detector wavelengths were 222 mm (ibuprofen), 242 nm (iopromide), and 235 nm (pCBA). The mobile phases were 23% v/v A (5 mM phosphoric acid) and 77% v/v B (methanol) for ibuprofen, 85% v/v A and 15% v/v B for iopromide, and 60% v/v A and 40% v/v B for pCBA. The detection limits were 0.015 mM (ibuprofen), 0.015 mM (iopromide), and 0.020 mM (pCBA). The dissolved organic carbon (DOC) was determined using a total organic carbon analyzer (TOC-VCPH/CPN, Shimadzu Co., Japan). The alkalinity was measured by titration with 0.02 N H2SO4 to the methyl orange endpoint. The pH values were measured using a pH meter (Orion 3 Star, Thermo, USA) calibrated with pH 4, 7, and 10 standard buffer solutions (Orion, Thermo, USA). The UV absorption coefficient at 254 nm was determined using a UV spectrophotometer (Cary-50, Varian).
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Table 1 Water quality parameters for each water sample. Samples
pH
Alkalinity (mg as CaCO3/L)
UV254 (cm1)
DOC (mg/L)
SUVA (L/(mg$m))
HR-R HR-F NR SR
7.1 7.6 7.0 7.1
34 33 42 14
0.036 0.017 0.041 0.023
1.9 1.3 3.0 0.7
1.89 1.31 1.37 3.28
2.3. Ozonation experiments All ozonation experiments were performed in a 300-mL Pyrexglass reactor. A saturated O3 stock solution (60e70 mg L1) was prepared from high-purity oxygen using an O3 generator (Ozonia, Switzerland) at low temperature ( HR-R (0.54 mg/L) z SR (0.53 mg/L) > HR-F (0.24 mg/ L). The IOD is known to depend on several factors, including the composition and concentration of dissolved organic matter (DOM), turbidity, and Fe(II), Mn(II), and NO 2 contents (Cho et al., 2003; Park et al., 2001; von Gunten, 2003). The difference in IOD among the
P
kS;i ½MS;i (s1)
27,126 18,262 51,549 10,755
Turbidity (NTU) 1.58 0.69 0.30 0.16
water samples shows a positive correlation with the UV254 trend: NR (0.041 cm1) > HR-R (0.036 cm1) > SR (0.023 cm1) > HR-F (0.017 cm1). UV254 is an indicator for aromatic organic components, and it has been reported that O3 reacts directly and preferentially with the electron-rich aromatic components of DOM (Anderson et al., 1986; Cho et al., 2003). Beyond 60 s, the kO3 in the water samples were clearly different and increased in the following order: NR (0.0051 s1) > HR-R (0.0024 s1) > SR (0.0012 s1) z HR-F (0.0012 s1). The stability of O3 is primarily determined by i) the pH because of initiation of O3 decomposition by OH ions; ii) direct reactions between O3 and DOM; and iii) the indirect effects of OH inhibition by DOM and carbonate/bicarbonate reactions which lead to both inhibition and promotion of O3 decomposition (Sonntag and Gunten, 2012). Overall, the kO3 had a positive correlation with UV254, which is in agreement with a previous study (Elovitz and von Gunten, 1999). For the case of SR and HR-F, the kO3 of SR was similar to that of HR-F even though SR has higher UV254 than that of HR-F. This can be explained by the higher pH in HR-F than SR. Because OH initiate O3 reactions, the O3 decomposition rate could be increased as pH increases (Staehelin and Hoigne, 1985). Fig. 1c shows pCBA decomposition, which indicates OH formation, in each water sample at same initial O3 dose, 1.3 mg/L. In ozonation, OH can be formed through several chain reactions of O3 with OH and functional groups of DOM. The trend in OH exposure at 660 s for the water samples [SR (6.97 1010 M s) [ HR-R (2.76 1010 M s) > HR-F (2.42 1010 M s) > NR (2.14 1010 M s)] differed significantly from the kO3 trend [NR > HR-R > SR z HR-F], although radical formation mostly occurs via O3 decomposition. For NR, the kO3 was 4.2-fold higher than that for SR, but the OH exposure for NR was 3.3-fold lower than that for SR. These results indicate that the radical formation/consumption characteristics differ depending on the water source. Among the natural water background substances, DOM not only generates OH through reaction with O3, but also scavenges OH (von Gunten, 2003). Nothe et al. (2009) reported that OH could be formed by side reactions of O3 with electron-rich compounds such as amines, phenols, and alkoxylated aromatics. Carbonate alkalinity and various anions also scavenge OH [carbonate ion (kOH,CO32 ¼ 3.9 108 M1 s1), bicarbonate ion (kOH,HCO3 ¼ 8.5 106 M1 s1), nitrite ion (kOH,NO2 ¼ 1.0 1010 M1 s1), bromide ion (kOH,Br ¼ 1.1 1010 M1 s1) (Buxton et al., 1988)]. The total OH P inhibition rates ( kS;i ½MS;i ) were determined experimentally in the different water sources. Details of the method for the total OH inhibition rate are provided in the Supplementary Material (Text S1 and Fig. S1). Table 1 lists the measured inhibition rates. The highest and lowest inhibition rates were observed in NR and SR, respectively. The high OH inhibition rate in NR (51,549 s1) could lead to low OH exposure in NR despite the high kO3 of NR, whereas higher OH exposure was achieved in SR because of much lower inhibition rate (10,755 s1) as compare with that in NR. To characterize and model the kinetics of O3 decomposition and OH formation, the Rct and ROH,O3 values were calculated and compared.
M. Kwon et al. / Water Research 122 (2017) 172e182
175
Fig. 1. Degradation of O3 and pCBA in four water samples: (a) O3 decomposition (C/C0), (b) pseudo-first order kinetics for O3 (ln(C/C0)), (c) pCBA decomposition (C/C0), and (d) pseudo-first order kinetics for pCBA (ln(C/C0)) ([O3]0 ¼ 1.3 mg/L and Temp. ¼ 15 C).
3.2. Comparison between Rct and ROH,O3 concepts 3.2.1. Determination of Rct and ROH,O3 values Using the same data sets for degradation of O3 and pCBA in Fig. 1, the Rct and ROH,O3 concepts were plotted by Equations (2) and (3), respectively (Fig. 2). In both cases, linear relationships were observed for all water samples, and the linear slopes were used to calculate the Rct and ROH,O3 values (Table 2). The Rct values were used to categorize water samples in terms of their total specific oxidation R R budgets [ ½OHdt = ½O3 dt ]; the values increased in the following order: NR (5.21 1010) z SR (5.18 1010) > HR-R (4.16 1010) > HR-F (2.02 1010). The ROH,O3 values represent the R exposure to OH produced by O3 consumption [ ½OHdt =D½O3 ]; the values increased in the following order: SR (4.21 105 s) » HR-F
(1.62 105 s) > HR-R (1.46 105 s) > NR (1.00 105 s). Overall, the ROH,O3 values increased with increasing O3 or OH exposure of the water samples (Table 2). For example, HR-R and HR-F have similar OH exposure, but ROH,O3 value in HR-F was higher because HR-F has higher O3 exposure from less consumption of O3 in comparison with HR-R. For the case of HR-F and SR, the ROH,O3 in SR was higher than that in HR-F because of higher O3 exposures and similar OH exposure in SR in comparison with HR-F. The oxidation efficiencies of the water samples can therefore be compared using the ROH,O3 values because a higher ROH,O3 value represents a higher OH formation when the same amount of O3 is consumed in different water samples. However, a comparison among the Rct values of different water samples is meaningless in terms of oxidation efficiency because the Rct values increased with
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M. Kwon et al. / Water Research 122 (2017) 172e182
(Fig. 1), which means that the oxidation efficiencies by both O3 and OH in SR should be higher than those in NR. The ROH,O3 value of SR was higher than that of NR, whereas the Rct values for SR and NR were similar because of the relatively high O3 exposure for SR. Equation (4) shows that the radical formation yield (h) from O3 decomposition could be determined by ROH,O3 concept:
Z ½OHdt
ROH;O3 ¼ Z
D½O3 t
¼
# Z "P kI;i MI;i ½O3 P kS;i MS;i
D½O3 t
dt
P P kI;i MI;i kI;i MI;i 1 ¼ ¼ P P kO3 D½O3 t kS;i MS;i kS;i MS;i P kI;i MI;i 1 P P ¼P kD;i MD;i þ kM ½Mi kS;i MS;i ½O3 dt
¼hP
1 kS;i MS;i
(4)
P where kI;i ½MI;i is the total formation rate of OH though the reP action of O3 with initiator (s1), kS;i ½MS;i is the total inhibition P 1 rate of OH (s ), and kD;i ½MD;i is the total direct consuming rate of O3 without OH production (s1). The radical formation yield (h) for each water sample was calculated by multiplying the ROH,O3 P value and the OH inhibition rate ( kS;i ½MS;i ) (Table 2). Interestingly, the NR has the highest radical yield (h ¼ 0:52) among the water samples, although the NR shows lowest oxidation efficiency as described above. This result could be explained as follows: both initiation rate and inhibition rate in NR might be higher than that in the other solutions, and the differences in inhibition rate between NR and the other waters might be larger than difference in initiation rate between them. In this case, the appearance radical yield in NR should be lower than the other waters even though the radical formation yield (h) of NR was higher than the other waters. In this respect, the ROH,O3, which can P be calculated by [h= kS;i ½MS;i ], is a more meaningful parameter for describing oxidation efficiency of ozonation.
Fig. 2. (a) Rct plot and (b) ROH,O3 plot for different water samples ([O3]0 ¼ 1.3 mg/L and Temp. ¼ 15 C).
decreasing O3 exposure or increasing OH exposure. For example, O3 exposure and OH exposure in SR were higher than those in NR
3.2.2. Effects of temperature on Rct and ROH,O3 values Shin et al. (2016) reported that temperature has a significant influence on oxidation kinetics in ozonation for drinking water treatment. The effects of temperature variations on the Rct and ROH,O3 values were evaluated for the four water samples at reaction temperatures varying from 5 to 25 C (Fig. 3). As temperature increases in each water sample, decomposition rates of both O3 and pCBA were increased (Fig. S2), which indicates higher O3 consumption and higher radical formation at higher temperature. As reported in previous studies (Elovitz et al., 2000; Shin et al., 2016), the Rct values of the four tested samples increased with increasing temperature, whereas the ROH,O3 values were constant. These results can be explained by a constant OH yield from O3 decomposition in conjunction with a constant OH inhibition rate under the temperature variations (5e25 C) (Equation (4)). The OH formation
Table 2 Determined oxidation kinetic parameters for four water samples ([O3]0 ¼ 1.3 mg/L and temperature ¼ 15 C). Samples HR-R HR-F NR SR a b
O3 exposurea (M$s) 3
4.91 10 10.00 103 2.76 103 7.45 103
Determined from 0 to 660 s. Determined for overall reaction.
OH exposurea (M$s)
2.76 2.42 2.14 6.97
10
10 1010 1010 1010
Rctb 4.16 2.02 5.21 5.18
ROH,O3b (s)
8
10 108 108 108
1.46 1.62 1.00 4.21
5
10 105 105 105
hb 0.40 0.30 0.52 0.45
M. Kwon et al. / Water Research 122 (2017) 172e182
rate could be increased with increasing temperature because of higher formation of O3 adducts during O3 initiation and radicalchain propagation reactions at higher temperature (Merenyi et al., 2010; Shin et al., 2016). The Rct value could therefore be increased by increasing the temperature, because it is based on the R R residual O3 exposure ½Rct ¼ ½OHdt= ½O3 dt; however, ROH,O3 is temperature independent because it is based on the consumed O3 R dose ½ROH;O3 ¼ ½OHdt=D½O3 . This temperature-independent characteristic of ROH,O3 concept is helpful for a kinetic modeling and operation of ozonation process. 3.2.3. Effects of initial O3 dose The O3 dose is a key operational parameter in ozonation, and variations in the O3 dose significantly affect kO3 and OH formation (Buffle et al., 2006). The effects of the O3 dose on Rct and ROH,O3 were investigated by adjusting the initial O3 dose from 0.7 to 2.0 mg/L. Because of the high IOD and kO3 in NR, the initial O3 dose tests were
177
conducted for HR-R, HR-F, and SR. Fig. 4 shows the Rct and ROH,O3 values of each water sample at various O3 doses. Overall, the Rct values decreased with increasing initial O3 dose; however, the effects of variation in the initial O3 dose on ROH,O3 were not significant. Buffle et al. (2006) also reported that a larger O3 dose decreases the Rct value because the R increase in the O3 exposure ( ½O3 dt) is much higher than that in R the OH exposure ( ½ OHdt) with increasing O3 dose. However, the ROH,O3 values were constant with variation of initial O3 dose R because the increased rate of OH exposure ( ½OHdt) was similar to that of O3 consumption (D½O3 ), indicating the constant OH yield from O3 decompositions with the variation. The data sources for the effects of initial O3 dose variations on Rct and ROH,O3 are provided in the supplementary material (Fig. S3). In practical terms, it is difficult to compare the performance of ozonation among the different water sources using a kinetic parameter which is dependent with initial O3 dose. For example, even if same O3 dose is applied to different water samples, the concentration of O3 after IOD phase will be different because of the different IOD depending on the water characteristics (see Section 3.1), and the Rct value will be measured at different initial O3 condition. The ROH,O3 concept could be used for kinetic modeling and comparison of ozonation efficiency among various water samples regardless initial O3 dose. These independent characteristics of ROH,O3 from variations in temperature and initial O3 dose allows monitoring the change in the OH yield from O3 decomposition without the influence of the two crucial parameters in drinking water treatment plant. This study focused on the effect of temperature and initial O3 dose, but it should be noted that other parameters such as pH, alkalinity, and DOC will change the OH yield from O3 decomposition and the ROH,O3 value. Further investigations will be followed to evaluate the effect the variation in pH, alkalinity, and DOC concentrations. 3.2.4. Relationship between Rct and ROH,O3 In Equation (3), the ROH,O3 concept was derived from the correlation between kO3 and Rct. Fig. 5 shows the linear correlations between the kO3 and Rct values at various temperatures and O3 doses. All the water sources gave good linear correlations, but the line slopes significantly differed depending on the water sample indicates different oxidation characteristics depending on the water source. Table 3 lists the Rct/kO3 and ROH,O3 values. The ROH,O3 values for each water source matched the Rct/kO3 values well. These results indicate that performing a large number of experimental conditions to establish the correlation between kO3 and Rct that is unnecessary by using the ROH,O3 concept. The determined parameters, i.e., IOD, kO3, Rct, and ROH,O3, in all tested conditions are listed in Table 4. 3.3. Kinetic modeling using ROH,O3 concept Removal prediction tests were conducted to evaluate the ROH,O3 concept by comparison with experimental results. In ozonation, the overall degradation of a target compound includes both direct O3 reactions and OH reactions (Elovitz and von Gunten, 1999):
ln
Fig. 3. Effect of temperature on (a) Rct concept and (b) ROH,O3 concept ([O3]0 ¼ 1.3 mg/L).
½M ½M0
Z ¼ kO3 ;M
Z ½O3 dt þ kOH;M
½OHdt
(5)
where [M] is the molar concentration of a micropollutant M (M), kO3,P is the second-order rate constant of the reaction between the target pollutant and O3 (M1 s1), and kOH,M is the second-order rate constant of the reaction between the target pollutant and R OH (M1 s1). The O3 exposure (M s) ( ½O3 dt) can be determined
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M. Kwon et al. / Water Research 122 (2017) 172e182
Fig. 5. Correlations between Rct and kO3 values for different water samples.
Table 3 Comparison of Rct/kO3 and ROH,O3 values.
Fig. 4. Effect of initial O3 dose on (a) Rct concept and (b) ROH,O3 concept (Temperature ¼ 15 C).
from kO3 using the following equation:
Z
½O3 0 IOD ½O3 dt ¼ 1 exp kO3 t kO3
(6)
½OHdt is the OH exposure, which is difficult to measure directly. Similarly to the Rct concept, the unknown OH exposure can be established using the ROH,O3 value:
Rct/kO3 (s) 1.49 1.95 1.20 4.36
Z Z ½M ¼ kO3 ;M ½O3 dt þ kOH;M ½OHdt ½M0
Z Z ¼ kO3 ;M ½O3 dt þ kOH;M ROH;O3 ½O3 dt kO3 (7)
105 105 105 105
ROH,O3 (s) 1.49 1.67 1.06 4.14
105 105 105 105
In the kinetic model for ozonation of natural water, the prediction of OH formation from IOD is important because considerable amount of contaminants are oxidized by the OH during IOD phase (Shin et al., 2016; Westerhoff et al., 1997). In this study, we supposed that during the IOD phase O3 was consumed via two pathways: i) decomposition with OH formation [available IOD (AIOD)] and ii) consumption without OH formation [non-available IOD (NAIOD)]. The NAIOD can be calculated from the ROH,O3 concept. In the linear plot of ROH,O3 (lnð½pCBA=½pCBA0 Þ=D½O3 ) (Fig. 6a), the x-intercept represents O3 consumption without OH formation, i.e. NAIOD. The determined IOD and NAIOD can be used to calculate the AIOD:
IOD NAIOD ¼ AIOD
R
ln
Samples HR-R HR-F NR SR
(8)
Table 4 lists the determined IOD, NAIOD, and AIOD values for all tested conditions. To investigate the relationship between IOD and AIOD, the ratios of [AIOD/IOD] were calculated in each condition, and the average values for each water sample were determined as 0.53 ± 0.03 (NR), 0.56 ± 0.06 (HR-R), 0.59 ± 0.12 (HR-F), and 0.64 ± 0.08 (SR). Interestingly, similar [AIOD/IOD] ratios (0.53e0.64) were observed for the four different water samples despite the difference in the ratio of [IOD/[O3]0] (0.16e0.47). However, the [AIOD/IOD] ratio can be varied depends on the water characteristic and will be decreased by increasing the substances which instantaneously consume O3 without OH production; therefore it should be measured for each water sample. From the [AIOD/IOD] ratio for each water sample, the AIOD can be calculated
M. Kwon et al. / Water Research 122 (2017) 172e182
179
Table 4 Determined parameters for all tested conditions used to characterize and model ozonation kinetics.
HR-F
HR-R
SR
NR
Temp. ( C)
O3 dose (mg/L)
IOD (mg/L)
NAIOD (mg/L)
AIOD (mg/L)
kO3 (s1)
Rct
ROH,O3 (s)
5 10 15 20 25 15 15 15 15 15 5 10 15 20 25 15 15 15 15 15 5 10 15 20 25 15 15 15 15 15 5 10 15 20 25
1.30 1.30 1.30 1.30 1.30 0.70 1.00 1.30 1.60 2.00 1.30 1.30 1.30 1.30 1.30 0.70 1.00 1.30 1.60 2.00 1.30 1.30 1.30 1.30 1.30 0.70 1.00 1.30 1.60 2.00 1.30 1.30 1.30 1.30 1.30
0.17 0.21 0.25 0.23 0.14 0.13 0.13 0.25 0.29 0.30 0.45 0.56 0.54 0.50 0.54 0.36 0.44 0.54 0.56 0.57 0.46 0.56 0.51 0.59 0.58 0.36 0.46 0.51 0.61 0.67 0.61 0.61 0.60 0.62 0.64
0.03 0.10 0.11 0.09 0.03 0.06 0.06 0.11 0.15 0.15 0.19 0.27 0.29 0.19 0.21 0.13 0.18 0.29 0.29 0.27 0.18 0.23 0.16 0.25 0.17 0.07 0.18 0.16 0.21 0.29 0.28 0.31 0.26 0.29 0.31
0.13 0.11 0.14 0.14 0.11 0.07 0.07 0.14 0.14 0.15 0.27 0.30 0.25 0.31 0.32 0.23 0.26 0.25 0.27 0.29 0.29 0.33 0.35 0.34 0.42 0.29 0.28 0.35 0.40 0.38 0.33 0.30 0.33 0.33 0.33
0.55 103 0.99 103 1.19 103 2.04 103 3.42 103 2.19 103 1.66 103 1.19 103 1.08 103 1.01 103 1.08 103 1.83 103 2.67 103 5.04 103 7.57 103 5.31 103 3.73 103 2.67 103 2.17 103 1.96 103 0.50 103 0.77 103 1.23 103 2.14 103 3.79 103 2.58 103 1.63 103 1.23 103 1.09 103 0.94 103 1.45 103 2.89 103 5.13 103 8.00 103 11.20 103
0.83 108 1.48 108 2.02 108 3.98 108 6.76 108 3.02 108 2.66 108 2.02 108 2.14 108 1.81 108 1.62 108 2.65 108 4.16 108 8.13 108 11.80 108 6.95 108 5.05 108 4.16 108 3.43 108 3.41 108 2.04 108 3.44 108 5.18 108 9.86 108 1.62 108 11.60 108 7.54 108 5.18 108 5.17 108 4.02 108 1.69 108 3.18 108 5.21 108 8.76 108 13.50 108
1.48 1.50 1.62 1.91 1.88 1.41 1.57 1.62 1.90 1.74 1.43 1.37 1.46 1.54 1.58 1.35 1.37 1.46 1.56 1.73 3.91 4.26 4.21 4.41 4.01 4.06 4.57 4.21 3.82 4.00 1.04 1.06 1.00 1.07 1.13
by measuring the IOD, and the OH exposure during IOD can be calculated as:
ZIOD ½OHdt ¼ AIOD ROH;O3
(9)
0
It should be noted that although the determined ratio of AIOD from values were similar for different water sources, the OH exposures during the IOD phase will vary because of the different ROH,O3 values depending on the water sources. Taking the AIOD into consideration, Equation (7) can be rewritten as:
ln
½M ½M0
Z ½O3 dt þ kOH;M
¼ kO3 ;M
kO3
Z ROH;O3 ½O3 dt
þ AIOD ROH;O3 (10)
Fig. 6b shows the importance of AIOD in the prediction of iopromide removal (kO3,iopromide ¼ 0.8 M1 s1, kOH,iopromide ¼ 3.3 109 M1 s1). The experimental results of iopromide removal in SR were compared with the model predictions, with and without consideration of the AIOD. The degradation of iopromide showed a two-stage profile, with rapid degradation rate then a slower degradation period. The removal predictions and experimental data matched well when the AIOD was considered, while the initially rapid degradation of iopromide was unpredictable without consideration of the AIOD. Similar results were observed using the Rct concept (Fig. 6c). To accurately
105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105 105
predict the initially rapid degradation of a target compound, Rct concept is also needed to consider the AIOD which determined by ROH,O3 concept plot. To verify the model equation (Equation (10)), prediction tests were conducted in various conditions; different water sources, temperature, initial O3 dose, and target compounds, i.e. ibuprofen and iopromide. Fig. 7a shows degradation of iopromide at 20 C in the four different water sources. Among the water sources, SR had the highest removal efficiency of iopromide. This result could be explained by the highest ROH,O3 value of SR among the water sources, which could lead the higher formation yield of OH in SR than that in others. In the case of NR, the removal efficiency before 300 s was higher than that for HR-F, but it was lower than that for HR-F after 300 s. These are because of a higher kO3 and lower ROH,O3 value of NR than that of HR-F (Table 4). The high kO3 value of NR led to both rapid decomposition of O3 and rapid formation of OH, but the OH formation from the consumed O3, i.e. ROH,O3, of NR was lower than that of HR-F. Fig. 7b shows the degradation of ibuprofen and iopromide in HR-F at 5 and 20 C. The removal efficiency of ibuprofen was higher than that of ioprominde at same temperature (5 C) because ibuprofen has higher rate constants with O3 and OH than iopromide (kO3,ibuprofen ¼ 9.6 M1 s1, kOH,ibuprofen ¼ 5.6 109 M1 s1, kO3,iopromide ¼ 0.8 M1 s1, kOH,iopromide ¼ 3.3 109 M1 s1). By increasing the temperature from 5 C to 20 C, the removal efficiency of ibuprofen was increased. These results could be explained by the increased kO3 and OH formation as temperature increases, which could increase the removal efficiency of ibuprofen. The complex oxidation properties in each water source and target
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Fig. 6. Importance of instantaneous O3 demand (IOD) and available IOD (AIOD) for removal prediction in ozonation; (a) determination of non-available IOD (NAIOD); removal prediction of iopromide with (w/) and without (w/o) consideration for AIOD by the (b) ROH,O3 and (c) Rct concepts.
compounds were well predicted using the suggested method using Equation (10) and the measured ROH,O3 value. All experimental results for iopromide and ibuprofen in the four water samples were compared with the model prediction (Fig. 8). The overall prediction test results using the ROH,O3 values showed good agreement (R2 ¼ 0.98) between the model predictions and experimental results over a wide range of test conditions (n ¼ 124): temperature range 5e20 C, initial O3 dose range 1.0e1.3 mg/L, and OH inhibition rate range 10,000e50,000 s1. 4. Conclusion In this study, the ROH,O3 concept was proposed to simplify the characterization and kinetic modeling of ozonation. The ROH,O3 value is derived from the Rct/kO3 value, which is used to cover the
entire range of seasonal and operational variations in the Rct values. The ROH,O3 concept allows simple determination of the Rct/kO3 value. Furthermore, following advantages of the ROH,O3 concept were found. - The ROH,O3 value is independent from significant seasonal and operational variations, i.e., temperature and initial O3 dose. - The ROH,O3 value increases with increasing OH exposure or decreasing O3 consumption. The oxidation efficiencies in water samples can therefore be indirectly compared using the ROH,O3 values because a higher ROH,O3 value indicates a higher oxidation efficiency. - Even if ROH,O3 values in different water sources are measured in different temperature and initial O3 dose, the oxidation
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Fig. 8. Comparison between the experimental results and the model predictions for the removal of iopromide and ibuprofen under different testing conditions (ROH,O3 range of 1.1e4.2 s, temperature range of 5e20 C, initial O3 dose range of 1.0e1.3 mg/L, and OH scavenging factor range of 10,000e50,000 s1).
References
Fig. 7. Removal prediction of (a) iopromide in different water source and (b) different target compound and temperature using the ROH,O3 concept.
efficiency can be compared because ROH,O3 concept is independent from the parameters. - The dynamic O3 and OH kinetics, and removal of target compounds were well predicted using the ROH,O3 concept. The AIOD value, which is determined using the ROH,O3 concept, enables prediction of initial rapid degradation of the target compound.
Acknowledgements This work is supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (NRF2016R1A2B4015598).
Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.watres.2017.05.062.
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