Aqueous ozone decomposition kinetics in a rotating packed bed

Aqueous ozone decomposition kinetics in a rotating packed bed

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Aqueous ozone decomposition kinetics in a rotating packed bed Peizhen Yang a, Shuai Luo b, Hongyan Liu a, Weizhou Jiao a,∗, Youzhi Liu a a b

Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, North University of China, Taiyuan, Shanxi 030051, China School of Environment, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 24 August 2018 Revised 19 October 2018 Accepted 30 October 2018 Available online xxx Keywords: Ozone Decomposition kinetics Rotating packed bed Inorganic ions

a b s t r a c t The decomposition kinetics of aqueous ozone in a rotating packed bed (RPB) at 18 ± 2 °C and in the pH range 3.0–11.0 was firstly investigated. The decomposition rate of ozone increased with increasing pH value and rotor speed. Moreover, a reaction kinetic model for ozone decomposition in RPB was developed, which can be applied for an extended range of pHs from acidic to alkaline operating conditions. It was deduced that ozone decomposition was based on pseudo-first-order kinetics with respect to the ozone concentration. The degree of the chain reaction between ozone molecule and hydroxyl ion in RPB was found higher than that in batch reactor. Furthermore, experiments showed that NO3 − , Cl− , HCO3 − and CO3 2 − ions could promote the self-decomposition of ozone to some extent and the promotion effect of CO3 2− ion was the largest. © 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Ozone (O3 , redox potential 2.08 V) is widely used as an alternative to chlorine disinfection in the field of water treatment. Additionally, ozone is still coupled with other technologies to degrade more complex wastewater [1,2]. It is known that the rate of microbial degradation by ozone is directly proportional to the concentration of ozone in solution. However, given the low solubility of ozone in water, the degradation of most types of pollutants with this technology is controlled by the mass transfer process. High gravity technology has been proposed as a process intensification, which was realized by using a rotating packed bed (RPB) [3]. The principle of RPB is to use a high-speed rotating packing to achieve a field equal to 100–1000 times that of general gravity, which can increase the gas-liquid mass transfer constant by 1–3 orders of magnitude. It has been widely applied in the chemical process industries, such as distillation [4,5], absorption [6,7], stripping [8,9], and mixing [10]. It is beneficial to improve the amount of ozone dissolved within a unit of time and it would significantly reduce the operating costs of wastewater treatment. The current research focused on coupling high-gravity technology and ozonation technology to improve wastewater treatment efficiency [11,12]. However, very few papers have been involved the process of ozone self-decomposition in RPB. It is worth noting that aqueous ozone decomposition plays a major role in the ozonation process. Because ozone is unstable, it can react via a direct reaction pathway involv-



Corresponding author. E-mail address: [email protected] (W. Jiao).

ing molecular ozone or by an indirect reaction pathway involving various highly reactive radicals (· OH, redox potential 2.80 V) that arise from its decomposition [13]. · OH can non-selectively and rapidly react with dissolved organics, generally with a rate constant in the range of 108 –1010 L/(mol·s), which is greater than the constant of direct molecule oxidation at 106 L/(mol s) [12]. · OH is mainly produced by the self-decomposition reaction of ozone in water under the action of an initiator. Therefore, in order to design a more efficient ozonation system, it is essential to determine the kinetics of decomposition of ozone and the variables affecting the rate of decomposition. The decomposition of ozone in water have been investigated in conventional reactors by various researchers for several decades [14–18]. The self-decomposition rate was found to be affected by ozone concentration, pH, ultraviolet light and dissolved anions [14]. Ozone decomposes rapidly in water at pH>8.0 because of the presence of hydroxide ions, which are the main chain initiators [15]. Some anions, such as HCO3 − , CO3 2− , SO4 2− , NO3 − , Cl− , etc., existing in water could scavenge hydroxyl radicals generated from O3 /OH− processes to inhibit the degradation rate of the dissolved organic matter [19]. Acero et al. investigated the influence of HCO3 − and CO3 2− ions on the O3 /H2 O2 process and found that the efficiency of degrading micro pollutant can be significantly decreased in presence of HCO3 − and CO3 2− [20]. Yalap et al. studied the effect of Cl− , SO4 2− , NO3 − and H2 PO4 2− on the degradation of tetracycline by these oxidation processes. The results showed that the nature and concentrations of these inorganic anions significantly affect the performance of photo catalytic and ozone oxidation processes [21]. The same experimental results were also observed by Chen et al., the degradation rate of 2,4-D was found to

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Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027

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be relatively sensitive to Cl− , PO4 3− , CO3 2− and NO3 − [22]. The decomposition of ozone in RPB are different to those conventional reactors because of its high turbulence and strong mixing effect on the liquid. Aqueous ozone decomposition is the chain reaction and hydroxide ions are the main chain initiators. The role of RPB in ozone self-decomposition is to accelerate the chain reaction rate between ozone molecules and hydroxide ions in the water, thereby triggering the next series of reactions, leading to an accelerated rate of ozone self-decomposition. The aim of this study was to develop an overall reaction kinetic model on the decomposition of ozone in RPB, taking into consideration of pH, rotor speed and inorganic species. The specific objectives of this paper were to reveal the self-decomposition behavior of ozone solution in RPB, and further provide some theoretical data for the further industrial application of the high gravity technology coupled with the ozonation process. 2. Experimental 2.1. Chemicals and analytical methods All working solutions were prepared in deionized water after boiling. All chemicals and solvents were of analytical grade and without any further purification. The solution pH values were kept constant at desired levels with NaOH and H2 SO4 solutions by a pH analyzer (PHS-3C, Shanghai Instrument Electric Scientific Instrument Co., Ltd, China). The pH values of this experiment were 3.0, 5.0, 7.0, 9.0 and 11.0. The concentration of ozone in the gaseous phases was determined by the wall mounted ozone concentration detector (UV-2200C, ZIBO ZHIPRER Automation Technology Co., LTD. China), and the concentration of ozone in the aqueous phases was determined by the water residual ozone online monitor (CL3630, B&C Electronics Srl. Italy). Before the start of the experiment, the instrument was calibrated by the KI titration method [23]. The flow rate of the oxygen inlet gas produced in the gas stream was controlled by changing the power input to the ozone generator (NPO10P-2, Shandong lvbang photoelectric equipment Co., Ltd, China). The device used in the comparison experiment was a constant temperature heating magnetic stirrer (DF-101S, Shanghai Anchun Instrument Co., Ltd.). These experiments were replicated three times. 2.2. Procedure The experiments were in a cross-flow rotating packed bed. The diameter of the RPB shell is 180 mm, the height of the RPB shell is 240 mm. The stainless wire mesh was used as the packing with a specific area of 935.07 m2 m−3 and a voidage of 74%. The axial height of the bed is 75 mm, and the inner and outer radii of the bed are 40 mm and 75 mm, respectively. The comparative experimental device was a magnetic stirrer with a tank size of 220 mm × 110 mm and an overall size of 290 mm × 280 mm × 230 mm, the reactor is a 2 L beaker. The experiments setup used in this study is shown in Fig. 1. The reaction volume is 2 L. The reaction was maintained at a constant temperature of 18 ± 2 °C. The production of ozone was controlled by changing the power input to ozone generator. At the beginning of the experiment, the ozone-containing gas was purged out of the system until the ozone generator was stabilized. Then the liquid entered the packing from a liquid distributor, sprayed radially from the inner bed, and moved outward as a result of the centrifugal force. The gas was introduced axially from the bottom of the RPB. Thus, gas and liquid contacted by cross-flow way in the RPB until the concentration of ozone in the aqueous phase reached steady state.

Fig. 1. Experimental setup (1) oxygen cylinder, (2) valve, (3) ozone generator, (4) wall mounted ozone concentration detector, (5) motor, (6) cross-flow rotating packed bed, (7) seal, (8) flowmeter, (9) pump, (10) liquid storage tank, (11) water residual ozone online monitor, (12) 10% KI solution.

For the ozone decomposition experiments, the ozonecontaining gas flowed to the bottom of the RPB until the concentration of ozone in the aqueous phase reached steady state. At this point, the gas flow was switched off. This was the starting point where the decomposition reaction occurred. Thus, the initial condition started from a steady-state condition. During the ozone decomposition, in tens of seconds, small aliquots of the solution were taken from outlet of the RPB and analyzed for the residual ozone concentration by the water residual ozone online monitor and KI method. 2.3. Theoretical analysis In this investigation, the kinetics of ozone decomposition was studied in RPB at constant temperature and under dynamic conditions whereby ozone solution was sheared continuously through high rotor speed to provide complete mixing. Hence, the rate of change of ozone with time can be written as:

O3 (aq ) + OH− (aq ) → Products

(1)

dCO3 = kCOm3 COn H− dt

(2)

r O3 = −

In the formula (2), m and n represented the reaction order of the entire reaction system relative to ozone and hydroxide concentration, k represented the intrinsic reaction rate constant of ozone decomposition. In the course of the experiment, we conducted the self-decomposition of ozone in different pH ranges, when the pH was a fixed value or the change of pH value was very small, and thus the concentration of OH− (COH− ) could be regarded as a constant, and the Eq. (5) could be simplified as:

r O3 = −

dCO3 = kdCOm3 dt

(3)

kd = kCOn H−

(4)

and then the following equations were obtained as:

CO3 ,t = exp(−kd t ) m = 1 CO3 ,0



CO3 ,t CO3 ,0

(5)

1−m

= 1 + (m − 1 )CO3 ,0 m−1 kd t

m = 1

(6)

The parameter kd represented the observed rate constant of ozone decomposition. After the reaction order m of the reaction system with respect to ozone was determined, kd could be obtained from the change of ozone concentration (CO3 ) with time. 3. Results and discussion The aqueous decomposition kinetic of ozone was studied on the grounds of the experiments under various solution pH values, rotor speed, and anion species in RPB.

Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027

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3

0.0 -0.4 -0.8

ln(CO3/CO3,0)

-1.2 -1.6

First-order fitting

-2.0 -2.4

-4 -1

2

pH=3.0±0.1, kd =7.28×10 s , R =0.96751 -4 -1

2

-3 -1

2

-3 -1

2

pH=5.0±0.1, kd =9.12×10 s , R =0.96793

-2.8

pH=7.0±0.1, kd =1.38×10 s , R =0.94513

-3.2

pH=9.0±0.1, kd =3.84×10 s , R =0.95207

-3.6

-3 -1

2

pH=11.0±0.1, kd =6.35×10 s , R =0.90582

-4.0 0

120

240

360

480

600

Fig. 2. Effect of rotor speed on mass-transfer of ozone from liquid phase to gas phase.

3.2. Effect of solution pH The pH is an important variable in ozone chemistry in aqueous ozone solution, which affects the change of hydroxide ion concentration in aqueous solution, and ultimately affects the selfdecomposition of ozone [13–15,24]. Typical decay curves of ozone at different solution pH values (3.0, 5.0, 7.0, 9.0, 11.0) are shown in Fig. 3 where the experimental data were regressed by a pseudofirst-order kinetics rate model with respect to ozone concentration. As expected, the decompose rate of ozone increased with the increasing pH in experiments due to the increasing concentration of OH− which was the prime species that could initiate the ozone decomposition. And with the increasing pH, the rate of self-decomposition of ozone is accelerated. The agreement among these data is fairly good, and the present data was correlated by:

kd = 1.03 × 10−2CO0.H125 −

(7)

0.0064

840

960

1080

1200

This work Chen et al. (2001)

0.0056 0.0048

-1

In order to eliminate the effect of ozone escaping from the liquid phase to the gas phase due to high-speed rotor in RPB, a controlled experiment was carried out in this experiment. By controlling the experimental operating parameters, the change of ozone concentration in water (CO3 ) can only be attributed to the emission of ozone into the gas phase caused by rotor speed (N). The results are shown as Fig. 2. As shown in Fig. 2, when the pH is 1.0, the liquid volume mass-transfer coefficient KL a increases from 6.23 × 10−5 s−1 to 11.06 × 10−5 s−1 with the increase of rotor speed from 0 rpm to 884 rpm, which proves that the concentration of ozone in water is not affected by rotor speed when the concentration of hydroxide ion in water is very low. The reason for the analysis is that the pH of the solution is 1.0 at this time, and the concentration of the main chain initiator of the hydroxide ion as a self-decomposition of ozone in water is very low. It can be considered that the change of ozone in water at this time is attributed to its transfer from the liquid phase to the gas phase due to the action of rotation. From the results of Fig. 2, it can be seen that the increase of the rotor speed does not cause the ozone to transfer from the liquid phase to the gas phase. This phenomenon also proves that the change of ozone concentration in water of this study is mainly attributed to the self-decomposition.

t/s

Fig. 3. Ozone decomposition rate regressed by the pseudo-first-order kinetic model with respective to ozone at various solution pH values Experimental conditions: temperature: 18 ± 2 °C; high gravity factor β = 40; liquid flow rate: 60 L/h; CO3,0 = 8.0 mg/L.

kd(s )

3.1. Effect of rotor speed on ozone transfer from liquid phase to gas phase

720

0.0040 0.0032 0.0024 0.0016 0.0008 0.0000 3

4

5

6

7

pH 8

9

10

11

12

Fig. 4. Effects of pH values on the observed rate constant of ozone decomposition.

The decomposition reaction orders compared with other studies under the specific experimental conditions are shown in Table 1. In a wide range of pH values, the overall reaction order with respect to OH− concentration was obtained in earlier study (0.12) [25,26], which was in agreement with that obtained in this study (0.125). In addition, the rate of self-decomposition of ozone increased more slowly at a pH range of 3.0–7.0 than that at the range of 7.0– 11.0. The slope of the fitted line was 1.61 × 10−4 when the pH range was from 3.0 to 7.0. In contrast, when the pH range was from 7.0 to 11.0, the slope of the fitted line was 1.24 × 10−3 . The experimental results indicated that ozone was predominantly in molecular form in solution under acidic conditions, reducing the changing sensitivity of the reaction rate between ozone and hydroxide ions. Moreover, the reaction rate of ozone molecules with hydroxide ions in solution was relatively slow (70 L/(mol · s)) under acidic conditions [13]. When the pH increased from 7.0 to 11.0, the rate of ozone self-decomposition had been significantly improved. The main reason was attributed to the dominant hydroxyl ions in water, because the ozone molecules could react with hydroxyl ions to generate another important ozone self-decomposition initiator HO2 − , which reacted quickly with ozone molecules (2.2 × 106 L/(mol s)) [13]. Similar results were also obtained by Chen et al. (Fig. 4), at the temperature of 25 °C, the pH range was at 3.5–13.0. Their

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P. Yang, S. Luo and H. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx Table 1 Decomposition kinetics of ozone with respect to hydroxide ion concentration in a wide range of pH. Reaction order with respect to pH

Temperature, °C

Ozone

Hydroxide ion

Investigators

Year

0.5–10 3.2–13.0 3.0–11.0

3.5–60 20–25 18 ± 2

1.0 1.0 1.0

0.12 0.12 0.125

Sullivan et al. [25] Chen et al. [26] This work

1980 2001 –

0.0 -4 -1

-0.2

ln(CO3/CO3,0)

2

β=0, kd= 7.32×10 s , R =0.91628 -4 -1

2

-3 -1

2

-3 -1

2

-3 -1

2

β=10, kd= 9.51×10 s , R =0.98531

-0.4

β=20, kd= 1.42×10 s , R =0.97275

-0.6

β=40, kd= 1.43×10 s , R =0.96011

-0.8

β=50, kd= 1.58×10 s , R =0.99699

First-order fitting

-1.0 -1.2

observed rate constant was rising to 1.58 × 10−3 s−1 , only with a small effect on the rate increase of ozone decomposition. The reason was that liquid was cut into tiny droplets with higher rotor speed, but there was a certain limit to the effect of liquid mixture, even if the high gravity factor β continued to increase, indicating that the rotor speed should not be excessive in practical applications. The rate equation of ozone decomposition was summarized and shown as the following (N is rotor speed, and the unit is r/min):

kd = 5.05 × 10−4 β 0.282

-1.4

(9)

or kd = 3.32 × 10−5 N 0.564

-1.6 -1.8 -2.0 0

300

600

900

1200

t/s

1500

1800

2100

2400

Fig. 5. Ozone decomposition rate regressed by the first-order kinetic model with respective to ozone at various high gravity factors. Experimental conditions: temperature: 18 ± 2 °C; pH = 7.0 ± 0.1; liquid flow rate: 60 L/h; CO3,0 = 8.0 mg/L.

experimental results indicated that ozone decomposition is faster in alkaline conditions than in acid conditions [26]. Furthermore, it showed that the decomposition rate constant of the ozone in RPB was 4.03–41.83 times than that of Chen et al. under similar experimental conditions.

(10)

Combining Eqs. (7) and (10), the overall decomposition rate equation of ozone under high gravity fields in the range of solution pH 3.0–11.0 was given as:

r O3 = −

dCO3 −5 1.0 0.564 = 1.03 × 10−2CO1.30CO0.H125 CO3 N (11) − + 3.32 × 10 dt

The orders of ozone and OH− in Eq. (11) were consistent with Sullivan et al. [25] and Chen et al. [26] It can also be seen from Eq. (11) that in the case of changing the pH and the rotor speed separately, the apparent kinetics of ozone self-decomposition in the water has a reaction order of 0.125 and 0.564 for the hydroxide ion and the rotor speed, respectively. It is proved that the change of the rotor speed has a greater influence on the kinetics of ozone self-decomposition in the high-gravity environment than the change of the hydroxide ion.

3.3. Effect of high gravity factor The high gravity factor β is the dimensionless indicator of the strength of the high gravity field, which is a radio of the centrifugal acceleration versus the gravitational acceleration as shown below [27]:

β=

w2 r N2 r = g 900

(8)

where w is angular velocity of the rotation of rotor, s−1 ; r is rotor radius, m; g is gravitational acceleration, m2 /s; N is rotor speed, r/min. These parameters could be controlled by adjusting the frequency of the converter. The high gravity factor was proportional to the square of the rotor speed, and the intensity of the high gravity field could be achieved by adjusting the rotor speed. Fig. 5 showed the dependence of the observed rate constant of ozone decomposition on the high gravity factor at different time periods. It was seen that the observed rate constant kd significantly increased from 7.2 × 10−4 s−1 to 1.43 × 10−3 s−1 with the increase of the high gravity factor in the range from 0 to 40. This would be due to the fact that a higher rotor speed would produce a massive shearing force to cut the liquid molecule to tiny liquid drops or thin liquid films, which greatly increased the degree of liquid turbulence [28]. The fluid constantly collided and merged, whose turbulence level would accelerate the chemical reaction between dissolved ozone molecules and hydroxide ions in water. However, when the high gravity factor β was further increased to 50, the

3.4. Effect of inorganic anions In order to avoid the influence of pH value on different inorganic ions, the pH value was 7.0 in this experiment. The decomposition rates of ozone in the presence of various anions (HCO3 − , CO3 2− , SO4 2− , NO3 − , Cl− , 10 mmol/L) for the system at pH = 7.0 were shown in Fig. 6. The decomposition rates of ozone were almost constant despite the presence of various anions. SO4 2− ions had almost no effect for hydroxyl radicals and had the minimum influence on the decomposition of ozone, the similar experimental results were reported by Zhang et al. [19]. Cl− , NO3 − ions were found to be very weak scavengers for hydroxyl radicals, similar to the results of the study reported by Andreozzi et al. [29]. HCO3 − ions promoted ozone decomposition to some extent, which was consistent with Chen et al. [22]. CO3 2− ions are common inorganic ions detected in natural water and is generally used as a radical scavenger in the advanced ozone oxidation process in order to directly inspect the oxidation process, the reaction equation can be described as [20,30]: − HCO− 3 + · OH → ·HCO3 +OH

k1 = 8.5 × 106 L/(mol · s )

(12)

− CO23− + · OH → ·CO− 3 +OH

k2 = 3.9 × 108 L/(mol · s )

(13)

− 2− ·CO− 3 + · CO3 → CO2 +CO4

k3 = 2 × 107 L/(mol · s )

(14)

Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027

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0.0

-0.4

-0.4

5

-0.8

ln(CO3/CO3,0)

ln(CO3/CO3,0)

-0.8

-1.2

-1.6

-2.0

-1

-3

-1

NaSO4, kd=1.48×10 s -3

NaNO3, kd=1.54×10 s

-2.4

-3

NaCl, kd=1.70×10 s

Na2CO3, kd=2.68×10 s

120

-4 -1 2 N=559 rpm, kd= 9.50×10 s , R =0.98818

-1

-3

60

2 -4 -1 N=395 rpm, kd= 7.64×10 s , R =0.99566

-2.0

NaHCO3, kd=2.18×10 s

0

-5 -1 2 N=0 rpm, kd= 8.42×10 s , R =0.99264

-1

-3

-2.8

First-order fitting -1.6

-3

Blank, kd=1.42×10 s

-1.2

180

-3 -1 2 N=791 rpm, kd= 1.38×10 s , R =0.98031

-1 3

-2.4

-1

240

300

t/s

360

420

480

540

600

Fig. 6. Effects of inorganic ions on the observed rate constant of ozone decomposition. Experimental conditions: temperature: 18 ± 2 °C; pH = 7.0 ± 0.1; high gravity factor β = 40; liquid flow rate: 60 L/h; CO3,0 = 8.0 mg/L. 0.0 -4 -1

2

-3 -1

2

-3 -1

2

-3 -1

2

-3 -1 2 N=884 rpm, kd= 1.40×10 s , R =0.97192

0

180

360

540

720

900

t/s

1080

1260

1440

1620

1800

Fig. 8. Ozone decomposition rate regressed by the first-order kinetic model with respective to ozone at various magnetic stirrer speed. Experimental conditions: temperature: 18 ± 2 °C; pH = 7.0 ± 0.1; CO3,0 = 8.0 mg/L.

pH=3.0±0.1, kd= 7.36×10 s , R =0.97826

-0.2

pH=5.0±0.1, kd= 1.06×10 s , R =0.95701

-0.4

pH=7.0±0.1, kd= 1.24×10 s , R =0.97882 pH=9.0±0.1, kd= 1.54×10 s , R =0.93948

ln(CO3/CO3,0)

-0.6

-3 -1

2

pH=11.0±0.1, kd= 2.18×10 s , R =0.90902

-0.8

First-order fitting

-1.0 -1.2 -1.4 -1.6 -1.8 -2.0 0

300

600

900

1200

t/s

1500

1800

2100

2400

Fig. 7. Ozone decomposition rate regressed by the first-order kinetic model with respective to ozone at various solution pH values. Experimental conditions: temperature: 18 ± 2 °C; magnetic stirrer speed: 791 rpm; CO3,0 = 8.0 mg/L.

As expected, CO3 2− ions still showed a promoting effect on the self-decomposition of ozone in this experiment, it could be shown as the reaction (13) and (14) [31]. 3.5. Comparison of different processes 3.5.1. Effect of pH and rotor speed on the observed rate constant of ozone decomposition in the magnetic stirrer In order to compare the ozone self-decomposition performance in different reactors, a comparative experiment with same experimental condition (pH, rotor speed) in a magnetic stirrer was carried out. Fig. 7 showed the ozone decomposition rate properly fitted by pseudo-first-order kinetics in the pH range of 3.0–11.0 in a magnetic stirrer. This phenomenon showed that the dependence of ozone decomposition rate on pH was consistent with RPB. Fig. 8 showed the effect of the magnetic stirrer speed on the observed rate constant of ozone decomposition. The observed rate constant of ozone decomposition fitted by pseudo-first order kinetics in the magnetic stirrer speed range of 0–884 rpm. The decomposition kinetics of the ozone was improved as the magnetic stirrer speed, which was consistent with the results in RPB.

Fig. 9. Comparison of the observed rate constant of ozone decomposition varies with various pH values between RPB and magnetic stirrer.

3.5.2. Comparison of the observed rate constant of ozone decomposition between RPB and magnetic stirrer Figs. 9 and 10 presented the comparison of the observed rate constant of ozone decomposition in RPB and magnetic stirrer. As is apparent from the Fig. 9, when pH increased from 3.0 to 7.0, the self-decomposition reaction of ozone is slower, and there was no big change in the self-decomposition rate between RPB and magnetic stirrer. When the pH exceeded 7.0, the decomposition reaction of ozone molecule was accelerated due to the predominant hydroxide ions. The observed rate constant of ozone decomposition in RPB was 6.35 × 10−3 s−1 at a pH of 11.0, but the observed rate constant of ozone decomposition of magnetic stirrer was only 2.18 × 10−3 s−1 . This anomaly might result from the turbulence level in RPB, which was significantly greater than that of the magnetic stirrer at the same speed (791 rpm). This degree of turbulence was beneficial for ozone self-decomposition reaction. Fig. 10 showed the effect of same rotor speed on the decomposition rate of ozone between RPB and magnetic stirrer. As shown in Fig. 10, the estimated kinetic rate constant of magnetic stirrer speed at 0 rpm was 8.42 × 10−5 s−1 and increased to 1.40 × 10−3 s−1 as the magnetic stirrer speed increased to 884 rpm. However, the observed rate constant of ozone decomposition in

Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027

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4. Conclusions

Fig. 10. Comparison of the observed rate constant of ozone decomposition varies with various rotor speed between RPB and magnetic stirrer.

This work presents a decomposition kinetic study of ozone at a wide range of pH value (3.0–11.0). It was demonstrated that the decomposition rate of ozone increased with increasing pH and rotor speed, and further to be affected by inorganic ions species for the RPB. The scavenging effect of CO3 2− was identified as an effective scavenger of · OH among the anion species studied. The overall decomposition rate equation of ozone under high-gravity fields in the range of solution pH 3.0–11.0 was also obtained. In the process of high-gravity technology and ozonation process for treating wastewater, high-gravity technology is considered to strengthen the gas-liquid mass transfer process, but this research show that the strong turbulence characteristics of the RPB will not only enhance the gas-liquid mass transfer but also accelerate the self-decomposition reaction of ozone in water to produce more oxidative hydroxyl radicals. The results obtained from this study provide a promising insight into the application of the high gravity technology coupled with ozonation process. Acknowledgments This work was supported by the Natural Science Foundations of China (U1610106) and Shanxi excellent talent science and technology innovation project (201705D211011), Specialized Research Fund for Sanjin Scholars Program of Shanxi Province (201707) and North University of China Fund for Distinguished Young Scholars (201701). References

Fig. 11. Half-lives of ozone at various pH values in different water quality between RPB and magnetic stirrer.

RPB increased from 7.32 × 10−4 to 1.58 × 10−3 s−1 as rotor speed of RPB increased from 0 to 884 rpm. These results indicated that the turbulence level significantly influenced the decomposition of ozone. Thus, RPB was a competent reactor to assist the ozone decomposition with stronger mass transferring efficiency and higher production of · OH. In conclusion, the overall decomposition rate equation of ozone in magnetic stirrer in the range of solution pH 3.0–11.0 is given as (N is rotor speed of magnetic stirrer, and the unit is r/min):

r O3 = −

dCO3 −6 1.0 0.807 = 3.00 × 10−3CO1.30CO0.H055 CO3 N (15) − + 6.32 × 10 dt

3.5.3. Half-lives of ozone at various pH values in RPB and magnetic stirrer The determination of the half-life of ozone in water provides very important information because it refers directly to ozone decomposition [32]. Fig. 11 shows the half-lives of ozone vs. pH when decomposition of ozone is considered as a first-order reaction rate. The figure shows that half-life reduced when the water quality changed from deionized water after boiling to tap water, because some anions in tap water may promote ozone decomposition in water. It also be seen from the figure that the decomposition rate of ozone in RPB is faster than of magnetic stirrer under the same water quality.

[1] Aghaeinejad-Meybodi A, Ebadi A, Shafiei S, Khataee A, Rostampour M. Modeling and optimization of antidepressant drug Fluoxetine removal in aqueous media by ozone/H2 O2 , process: comparison of central composite design and artificial neural network approaches. J Taiwan Inst Chem E 2015;48:40–8. [2] Charles J, Crini G, Morin-Crini N, Badot P M, Trunfio G, Sancey B, Carvalho B, Avramescu S, Winterton P, Gavoille S, Torri G. Advanced oxidation (UV-ozone) and cyclodextrin sorption: effects of individual and combined action on the chemical abatement of organic pollutants in industrial effluents. J Taiwan Inst Chem E 2014;45:603–8. [3] Burns JR, Ramshaw C. Process intensification: visual study of liquid maldistribution in rotating packed beds. Chem Eng Sci 1996;51:1347–52. [4] Luo Y, Chu GW, Zou HK, Xiang Y, Shao L, Chen JF. Characteristics of a two-stage counter-current rotating packed bed for continuous distillation. Chem Eng Process 2012;52:55–62. [5] Mondal A, Pramanik A, Bhowal A, Datta S. Distillation studies in rotating packed bed with split packing. Chem Eng Res Des 2012;90:453–7. [6] Lin CC, Liu WT, Tan CS. Removal of carbon dioxide by absorption in a rotating packed bed. Ind Eng Chem Res 2003;42:2381–6. [7] Yi F, Zou HK, Chu GW, Shao L, Chen JF. Modeling and experimental studies on absorption of CO2 by benfield solution in rotating packed bed. Chem Eng J 2009;145:377–84. [8] Li YM, Ji JB, Yu YL, Xu ZC, Li XH. Hydrodynamic behavior in a rotating zigzag bed. Chin J Chem Eng 2010;18:34–8. [9] Yuan MH, Chen YH, Tsai JY, Chang CY. Ammonia removal from ammonia-rich wastewater by air stripping using a rotating packed bed. Process Saf Environ 2016;102:777–85. [10] Wenzel D, Górak A. Review and analysis of micromixing in rotating packed beds. Chem Eng J 2018;345:492–506. [11] Zeng ZQ, Zou HK, Li X, Arowo M, Sun BC, Chen JF, Chu GW, Shao L. Degradation of phenol by ozone in the presence of Fenton reagent in a rotating packed bed. Chem Eng J 2013;229:404–11. [12] Jiao WZ, Luo S, He Z, Liu YZ. Applications of high gravity technologies for wastewater treatment: a review. Chem Eng J 2017;313:912–27. [13] Lovato ME, Martín CA, Cassano AE. A reaction kinetic model for ozone decomposition in aqueous media valid for neutral and acidic pH. Chem Eng J 2009;146:486–97. [14] Sotelo JL, Beltrán FJ, Benítez FJ, Beltrán-Heredia J. Ozone decomposition in water: kinetic study. Ind Eng Chem Res 1987;26:39–43. [15] Jung Y, Hong E, Kwon M, Kang JW. A kinetic study of ozone decay and bromine formation in saltwater ozonation: effect of O3 dose, salinity, pH, and temperature. Chem Eng J 2017;312:30–8. [16] Tomiyaso H, Fukutomi H, Gordon G. Kinetics and mechanisms of ozone decomposition in basic aqueous solution. Inorg Chem 1985;24:2962–6. [17] Hewes CG, Davison RR. Kinetics of ozone decomposition and reaction with organics in water. AIChE J 1971;17:141–7.

Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027

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ARTICLE IN PRESS

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P. Yang, S. Luo and H. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (xxxx) xxx [18] Gurol MD, Singer PC. Kinetics of ozone decomposition: a dynamic approach. Environ Sci Technol 1982;16:377–83. [19] Zhang SG, Qin YJ, Zhang DM, Jiao WZ, Guo L, Liu YZ. Effects of coexisting substances on nitrobenzene degradation with O3 /H2 O2 process in high-gravity fields. China Pet Process Petrochem 2016;18:32–40. [20] Acero JL, Gunten UV. Influence of carbonate on the ozone/hydrogen peroxide based advanced oxidation process for drinking water treatment. Ozone-Sci Eng 20 0 0;22:305–28. [21] Yalap KS, Balcioglu IA. Effects of inorganic anions and humic acid on the photocatalytic and ozone oxidation of oxytetracycline in aqueous solution. J Adv Oxid Technol 2008;12:134–43. [22] Chen L, Shi HX, Wang DH. Kinetics of 2,4-dichlorophenoxyacetic acid oxidation by ozone. J Chem Ind Eng 2005;56:2204–6. [23] Snell FD, Hilton CL, Ettre LS. Encyclopedia of industrial chemical analysis. New York: John Wiley & Sons, Inc.; 1987. [24] Alvárez PM, García-Araya JF, Beltrán FJ, Giráldez I, Jaramillo J, Gómez-Serrano V. The influence of various factors on aqueous ozone decomposition by granular activated carbons and the development of a mechanistic approach. Carbon 2006;44:3102–12.

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[25] Sullivan D, Roth J. Kinetics of ozone self-decomposition in aqueous solution. AIChE Symp Ser 1980;76:142–9. [26] Chen Y, Li YL, Zhang H, Zhong L, Chen HQ. Studies on kinetics of ozone decomposition in water. J Chem Eng Chin Univ 20 01;15:50 0–4. [27] Jiao WZ, Liu YZ, Qi GS. Gas pressure drop and mass transfer characteristics in a cross-flow rotating packed bed with porous plate packing. Ind Eng Chem Res 2010;49:3732–40. [28] Chen JF, Gao H, Zou HK, Chu GW, Zhang L, Shao L, Xiang Y. Cationic polymerization in rotating packed bed reactor: experimental and modeling. AIChE J 2010;56:1053–62. [29] Andreozzi R, Caprio V, Insola A, D’Amore MG. Quinoxaline ozonation in aqueous solution. Ozone-Sci Eng 1990;12:329–40. [30] Drzewicz P, Trojanowicz M, Zona R, Solar S, Gehringer P. Decomposition of 2,4-dichlorophenoxyacetic acid by ozonation, ionizing radiation as well as ozonation combined with ionizing radiation. Radiat Phys Chem 2004;69:281–7. [31] Ku Y, Su AW, Shen YS. Decomposition kinetics of ozone in aqueous solution. Ind Eng Chem Res 1996;35:3369–74. [32] Gardoni D, Vailati A, Canziani R. Decay of ozone in water: a review. Ozone-Sci Eng 2012;34:233–42.

Please cite this article as: P. Yang, S. Luo and H. Liu et al., Aqueous ozone decomposition kinetics in a rotating packed bed, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2018.10.027