Weizhou Jiao, Xingyue Wei, Shengjuan Shao, Youzhi Liu

1 Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering, National Demonstration Center for Experimental Comprehensive Chemical Engineering Education,North University of China, Shanxi 030051, China

2 Key Laboratory of Industrial Ecology and Environmental Engineering(MOE),School of Environmental Science and Technology,Dalian University of Technology,Dalian 116024,China

Keywords:Rotating packed bed Ozone Heterogeneous catalysis Overall decomposition rate constant Overall volumetric mass transfer coefficient

ABSTRACT This study investigated catalytic decomposition and mass transfer of aqueous ozone promoted by Fe-Mn-Cu/γ-Al2O3(Cat)in a rotating packed bed(RPB)for the first time.The results showed that the value of the overall decomposition rate constant of ozone (Kc) and overall volumetric mass transfer coefficient (KLa)are 4.28×10-3 s-1 and 11.60×10-3 s-1 respectively at an initial pH of 6,β of 40,CO3(g) of 60 mg·L-1 and QL of 85 L·h-1 in deionized water, respectively.Meanwhile, the Kc and KLa values of Fenhe water are 0.88 × 10-3 s-1 and 2.51 × 10-3 s-1 lower than deionized water, respectively.In addition, the Kc and KLa values in deionized water for the Cat/O3-RPB system are 44.86% and 47.41% higher than that for the Cat/O3-BR (bubbling reactor) system, respectively, indicating that the high gravity technology can facilitate the decomposition and mass transfer of ozone in heterogeneous catalytic ozonation and provide some insights into the industrial wastewater.

1.Introduction

In recent years, heterogeneous catalytic ozonation has been concerned as a green and sustainable development of advanced oxidation process (AOP) for industrial wastewater treatment and reclamation [1-3].It has many distinct advantages over O3/H2O2,O3/Fe2+and O3/Fenton such as higher the catalytic activity,reusability and less pollution to the environment [4,5].However,it is found that the degradation efficiency of organics by heterogeneous catalytic ozonation is lower than homogeneous catalyst,mainly due to the presence of phase interface [6-8].

Therefore, many researchers have tried to couple heterogeneous catalytic ozonation technology with external field enhancement (including electric fields [9], ultrasonic field [10], ultraviolet radiation[11],and high gravity field[12])to improve the degradation effect of organic pollution.But it is found that the electric field has low current efficiency and poor stability [13], and ultraviolet irradiation has low light energy utilization [14], and ultrasound also has a power consumption problem [15], which makes the above external field unsuitable for large-scale production.

The high gravity technology is a chemical process strengthening technology,the carrier of which is a rotating packed bed(RPB)[16].It has also been found that RPB has the advantages of reducing device volume, enhancing gas-liquid micro-mixing, improving ozone utilization rate, and facilitating industrial scale-up [17].What’s more, the mass transfer coefficient between gas and liquid phases was improved by 1-3 orders of magnitude compared with conventional packed bed [18].At present, it is widely used in the field of distillation [19,20], absorption [21,22], stripping [23,24],mixing [25]and photo catalytic degradation of wastewater [26].

At the same time, it is found that the high gravity technology coupled with single ozone and homogeneous catalytic ozone has a good effect on the degradation effect and the promotion of ozone gas-liquid mass transfer [27,28].Liuet al.[29]found that the ozone concentration in water reached 77% of saturation in RPB but only 49%in stirred tank reactor(STR),suggesting high absorption efficiency of RPB for ozone.In addition, high gravity technology can also accelerate the mass transfer between liquid and solid [30,31].Changet al.[32]studied the RPB coupling heterogeneous catalytic ozonation to mineralize phenol wastewater using Pt/Al2O3catalyst as packing.The mineralization rate of phenol reached 80%at 30 min,55%higher than that of RPB-O3system.Furthermore, the RPB can also fix the solid particle catalyst in the packing, which is beneficial to the separation of the catalyst and the wastewater.But so far, the decomposition and mass transfer behaviors of ozone by the coupling of high gravity technology and heterogeneous catalytic ozonation technology have not been reported.

Against this background,the aims of this study were to investigate the decomposition and mass transfer of ozone catalyzed by Fe-Mn-Cu/γ-Al2O3in RPB for the first time, taking into consideration of initial pH,high gravity factor(β),gaseous ozone concentration (CO3(g)), liquid flow rate (QL) and different water quality.Besides, catalytic decomposition and mass transfer of ozone in the RPB were compared with those in a bubbling reactor (BR).

2.Experimental

2.1.Chemicals and analytical methods

In our previous study,Fe-Mn-Cu/γ-Al2O3(Cat)catalyst has been successfully prepared and characterized [33].Specific features are as follows: the size of the Fe-Mn-Cu/γ-Al2O3catalyst is 3-5 mm,the catalyst specific surface area is 212.122 m2·g-1, total pore volume is 0.551 cm3·g-1and average pore size is 5.613 nm, and the point of zero charge (pHpzc) of catalyst was 6.3.Potassium iodide(KI) was obtained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China).Sodium thiosulphate solution (Na2S2O3)and soluble starch were purchased from Tianjin Beichen Fangzheng Reagent Factory (Tianjin, China), and the pH was adjusted with sulfuric acid (Luoyang Chemical Reagent Factory, China) and sodium hydroxide (Tianjin Continental Chemical Reagent Factory,China).All chemicals were analytical grade and used with no further purification.Deionized water obtained from a GWA-UN5-F10 pure system was used in the experiment.Ozone was produced with an ozone generator(CF-G-3-20 g,Qingdao Guolin Technology Co.Ltd., China) supplied with pure oxygen.The concentration of ozone in the gaseous phase was determined using a wall mounted ozone concentration detector (UV-2200C, ZIBO ZHIPRER Automation Technology Co., LTD.,China);while that in the aqueous phase was measured by the iodometric method,the specific steps were as follows: firstly, 40 ml of ozone aqueous solution from the liquid outlet was taken, excessive KI solid was added and stirred evenly,and dropped dilute sulfuric acid (1 + 5) immediately to adjust the pH of the solution to 2, standing for 5 min.The calibrated 0.01 mol·L-1Na2S2O3standard solution was used for titration.When the titrated solution changes from yellow to light yellow,1 ml of 1% boiled starch supernatant was added, continuing the titration until the solution just becomes colorless to terminate the titration reaction,measuring the volume of sodium thiosulfate consumed, and further calculating the liquid phase ozone concentrationCO3l()according to Eq.(1).

wherec1is the concentration of the standard solution of Na2S2O3,0.01 mol·L-1,V1is the volume of Na2S2O3consumed (ml), andV2is the volume of the water sample taken (ml).

Moreover,the initial pH was detected by a pH meter(Shanghai Pioneer Technology PHS-3C, China).A magnetic stirrer (Shanghai Yuezhong instrument equipment Co.Ltd., China) was used in BR.

2.2.Ozonation processes

As seen in Fig.1,catalytic absorption and decomposition experiments were carried in RPB(Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology, China), with the motor power of 370 W and the liquid pump power of 15 W.The characteristic parameters of the packing bed were shown in Table 1.The Fe-Mn-Cu/γ-Al2O3catalyst was used as the packing with a voidage of 46% to react with ozone and recycle easily.

Table 1Characteristic parameters for rotating packed bed

2.3.Catalytic absorption experiment

Deionized water(2 L)with a pH of 2,4,6,8 and 10 was poured into the storage tank and pump it into the liquid inlet at a rate of 85 L·h-1and circulate continuously.By adjusting the gravitational force(20-60 g),the liquid would be thrown out of the inner diameter of the packing to the outer diameter and then flowed from the outlet to the tank for the next cycle.The ozone-containing gas was continuously introduced axially from the bottom of RPB at a constant rate ofQG=60 L·h-1until the ozone generator was stabilized.At this point, ozone absorption experiment was started.Different saturation concentrations of dissolved ozone could be obtained by varying the parameters.Unreactive ozone was absorbed by 5%KI solution.Aqueous samples were collected from the liquid outlet at regular time intervals to further detect the concentrations of dissolved ozone as a function of treatment time.The absorption process in RPB was repeated three times (the margin of errors was shown in Figs.6-9) and averaged for fitting analysis.Experiment was also carried out in BR under similar conditions.The diameter(Φ)of the bubbling reactor is about 85 mm,the height(H)is about 400 mm.

2.4.Catalytic decomposition experiment

The parameters in this experiment were the same as above absorption experiment.However, it should be noted that the feed valve would be closed once dissolved ozone became saturated.The liquid was circulated in RPB in the presence of Fe-Mn-Cu/γ-Al2O3catalyst, and the catalytic decomposition experiments were started.Samples were collected at regular time intervals and analyzed for the residual ozone concentration in water.The decomposition processes in RPB were repeated three times (the margin of errors is less than 0.354 mg·L-1) and averaged for fitting analysis.Again, experiment was also carried out in BR under similar conditions.

2.5.Theoretical analysis

In this study, catalytic decomposition and mass transfer of aqueous ozone promoted by Fe-Mn-Cu/γ-Al2O3in the RPB were studied.When the gaseous ozone continues mass transfer to the liquid phase, the overall decomposition rate constant of ozone refers to two parts, including ozone self-decomposition rate constant, and decomposition rate constant of catalytic ozone by catalyst[34].Furthermore,the rate of decomposition of ozone depends on the process of the mass transfer rate from gaseous ozone to liquid phase.The faster ozone decomposition is, the better degradation of organic pollutants will be.The absorption and mass transfer of ozone in water are related to the gaseous ozone concentration, the initial pH, the high gravity factor and the liquid flow rate.The overall volumetric mass transfer coefficient of ozone in Cat/O3-RPB system includes the ozone mass transfer coefficient from the gas phase to the liquid phase enhanced by high gravity and enhanced by the large surface area of heterogeneous catalyst.The ozone solution in water does not reach the stable ozone concentration, the reaction follows Eq.(2) [6,34,35]:

whereKLais the overall volumetric mass transfer coefficient of ozone,Kcis the overall decomposition rate constant of ozone, [O3]*is the aqueous ozone concentration in equilibrium with gaseous ozone concentration, [O3]is the concentration of ozone in water at timet, and d[O3]/dtis the accumulation rate of ozone in water.The concentration of dissolved ozone in water gradually increaseswith time until it reaches a stable value [O3]s.At this time, the change of ozone concentration in water is 0, and Eq.(3) becomes:

Fig.1. Schematic diagram of absorption and decomposition of heterogeneous catalytic ozonation in a rotating packed bed.1—oxygen cylinder;2—ozone generator;3,8—flow meter; 4,7—valve; 5—ozone gas detector; 6—rotating packed bed; 9—peristaltic pump; 10—storage tank; 11—exhaust gas absorption device.

And then, the Eq.(3) is converted to Eq.(4).

Eq.(5) is obtained by substituting Eq.(4) into Eq.(2), and integrating into Eq.(6):

The plot of ln([O3]s/([O3]s- [O3]))versusreaction timetis displayed in Eq.(6),and it satisfies the first order equation,where the slope isKLa+Kc.

It was found in literature [28]that the decomposition of ozone in water conforms to the pseudo-first-order reaction,and the addition of catalyst can accelerate the decomposition rate of ozone[7].Therefore,Eq.(7)was used to study the influence of corresponding parameters on the decomposition of ozone.

By exploring different experimental parameters, the corresponding liquid phase ozone concentration at the corresponding time was obtained, so as to obtainKc.Therefore,KLais obtained by subtractingKcfrom (KLa+Kc).

3.Results and Discussion

3.1.Effect of different factors on the catalytic decomposition of ozone in Cat/O3-RPB

3.1.1.Effect of initial pH

Initial pH is an important factor affecting the catalytic decomposition of ozone in deionized water.Fig.2 shows the effect of pH (2-10) on the decomposition rate of dissolved ozone in the Cat/O3-RPB system.TheKcvalues are calculated by plotting ln([O3]0/[O3])versustime, in line with a linear relationship, and the correlation coefficients (R2) are above 0.9834.With the increase of pH, theKcvalue first increases and then decreases, which may be related to the interaction between ozone and surface charge of the catalyst.The surface electrical properties of the catalyst at different pH values are shown in Eqs.(8) and (9).The metal oxide surface is electropositive under acidic conditions (pH ＜pHpzc), but electronegative under alkaline conditions (pH ＞pHpzc).The pHpzcof the Fe-Mn-Cu/γ-Al2O3catalyst is 6.3 by a Nano-Sizer.The density of hydroxyl groups on the catalyst surface is the highest,which exist in the form of M-OH [1,36].As seen in Fig.2, when solution pH value is 6,ozone is more easily adsorbed on the surface of catalyst under the action of high gravity.Thus,the decomposition rate constant of ozone is the largest (Kc= 4.28 × 10-3s-1), which is more effective for the degradation of organic pollutants[37].However, the density of hydroxyl groups on the surface of catalyst decreases at solution pH lower or higher than 6.3.As a result, the content of active free radicals in aqueous phase decreases,making it more difficult for the treatment of organic pollutions.This is also consistent with the conclusions of Valdéset al.[7].

Fig.2. The effects of initial pH on the heterogeneous catalytic ozone decomposition in the deionized water (β = 40, QL = 85 L·h-1, CO3(l) = 20, 14.8, 14.2, 12.24 and 9.3 mg·L-1, at pH = 2, 4, 6, 8, and 10, respectively).

3.1.2.Effect ofβ

β is a dimensionless indicator of the strength of the high gravity field, which is the ratio of the centrifugal accelerationversusthe gravitational acceleration, β is defined as follows Eq.(10) [38,39]:

where ω is the angular rotation velocity of the rotor, s-1;ris the rotor radius, m;gis the gravitational acceleration, m·s-2; andNis the rotor speed, r·min-1.These parameters could be controlled by adjusting the frequency of the converter.

Fig.3 shows the effect of β(20-60)on the catalytic decomposition of ozone in deionized water in the Cat/O3-RPB system.TheKcvalues are calculated by plotting ln([O3]0/[O3])versustime,in line with a good linear relationship,and theR2values are above 0.9937.It is seen that theKcvalue increases significantly from 3.1 × 10-3s-1to 4.28 × 10-3s-1with the increase of β value from 20 to 40.This is because the increase of β value makes the thickness of the liquid film thinner and favors the adsorption of more aqueous ozone on the surface of the catalyst, leading to an increase in the decomposition rate of ozone and a large increase in ·OH, which can accelerate the degradation of organic pollutants [32].As the β value exceeds 40, the decomposition rate constant decreases due to the short residence of the liquid on the surface of the catalyst in unit time[33].Thus,when β is 40,Kcis the largest.In addition, Yanget al.[28]studied the decomposition of ozone under high gravity field and found that theKcincreases from 7.32 × 10-4s-1to 1.58 × 10-3s-1as β increases from 10 to 50.It can be seen that the addition of catalyst can significantly improve the decomposition rate of ozone.

Fig.3. Effects of β on the heterogeneous catalytic ozone decomposition in the deionized water(pH=6,QL=85 L·h-1,CO3(l) =11.55,13,14.2,14.89 and 14 mg·L-1,at β = 20, 30, 40, 50 and 60, respectively).

3.1.3.Effect of CO3(g)

The effect ofCO3(g)on the catalytic decomposition of ozone in water in the Cat/O3-RPB system is shown in Fig.4.TheKcvalues are calculated by plotting ln([O3]0/[O3])versustime, in line with a linear relationship, and theR2values are above 0.9853.TheKcvalue increases from 2.37 × 10-3s-1to 4.32 × 10-3s-1with the increase ofCO3(g), which can be attributed to the increase in the partial pressure of gas and the saturation concentration of ozone in water.Thus, more ozone in the liquid phase will contact with the active sites on the catalyst surface under high gravity force,which can accelerate the adsorption and decomposition processes on the catalyst surface.Consequently, the increase of ozone concentration favors the generation of more free radicals in the liquid phase, leading to the efficient mineralization of organic compounds [32].At the same time, Gaoet al.[40]studied the decomposition rate of ozonation alone in Karman contactor, and found that when theCO3(l)was low (4.8 mg·L-1), theKcwas also low(4.167 × 10-5s-1).However, when the concentration of gaseous ozone concentration increases from 60 mg·L-1to 80 mg·L-1, theKcdoes not increase significantly.But at this time, the ozone production input to the system increased, resulting in the lower utilization rate of ozone, and increasing the power consumption[33].Consequently, theCO3(g)of 60 mg·L-1contributes to improve decomposition rate of catalytic ozone.

Fig.4. Effects of CO3(g) on the heterogeneous catalytic ozone decomposition in the deionized water (pH = 6, QL = 85 L·h-1, CO3(l) = 6, 9.5, 14.2, and 18.6 mg·L-1, at CO3(g) = 30, 40, 60 and 80 mg·L-1, respectively).

3.1.4.Effect of liquid flow rate

Fig.5 shows the effect of differentQLon the decomposition of catalytic ozone in the Cat/O3-RPB system.TheKcvalues are calculated by plotting ln([O3]0/[O3])versustime, in line with a linear relationship, and theR2values are above 0.9826.It is seen that theKcvalue increases significantly from 3.89 × 10-3s-1to 4.28 × 10-3s-1with the increase ofQLfrom 65 L·h-1to 85 L·h-1.This phenomenon is caused by the fact that gas/liquid turbulence is strengthened, the wettability of liquid microelements to the catalyst is increasing [29], and the probability of aqueous ozone in water contacting the Lewis acid sites on the surface of the catalyst gradually increases, resulting in the continuous increase ofKc.While theQLexceeds 85 L·h-1, theQLin the packing is too fast,which shortens the reaction time of aqueous ozone and the catalyst, resulting in theKcfalling.The literature has examined the change ofQL(from 65 to 105 L·h-1) and found that the phenol mineralization rate is the largest (96.42%) when theQLis 85 L·h-1[33].Consequently, theQLof 85 L·h-1helps to improve decomposition rate of catalytic ozone.

Fig.5. The effects of different QL on the heterogeneous catalytic ozone decomposition in the deionized water(pH=6,β=40,CO3(l) =14.04,14.07,14.2,14.16 and 14,12 mg·L-1, at QL = 65, 75, 85, 95 and 105 L·h-1, respectively).

Fig.6. The effects of different pH on ozone concentration in water in Cat/O3-RPB system (CO3(g)=60 mg·L-1, β = 40, QL = 85 L·h-1).

3.2.Effect of different factors on the catalytic absorption of ozone in Cat/O3-RPB

3.2.1.Effect of initial pH

Fig.6 shows the effect of pH on the absorption process of ozone in deionized water in the Cat/O3-RPB system and the margin of error was less than 0.369 mg·L-1.When the concentration of liquid phase ozone (CO3(l)) is stable,CO3(l)decreases gradually with the increase of pH, because ozone is difficult to decompose under acidic conditions, while OH-as an initiator can accelerate ozone decomposition under alkaline conditions [41].In the process of unsteady-state ozone absorption, the linear relationship between ln([O3]s/([O3]s- [O3])) and timetis fitted, and the results are shown in Table 2.The slopeKc+ KLavalues are obtained, and theR2values are greater than 0.9786.It is found by subtractingKcfromKc+ KLathat theKLavalue changes between 10.96 × 10-3and 12.60 × 10-3s-1with the change of pH, which has little effect on theKLacompared to that at pH = 6.This weak change may be due to the different charge of the catalyst at different pH, which affects the rate of ozone adsorption on the catalyst surface [42],and thus affects the overall mass transfer rate process.Gaoet al.[40]found that the change in pH (2-8) did not affect the ozone mass transfer during the single ozonation process,and theKLavalues were only around 4.6×10-3s-1,which also illustrates that the RPB and catalyst play an important role in the mass transfer of ozone.

3.2.2.Effect ofβ

Fig.7 shows the effect of β on the absorption process of ozone in the Cat/O3-RPB system and the margin of error was less than 0.54 mg·L-1.With β increasing from 20 to 40, theCO3(l)increases all the time at the steady stage.This is mainly because the highspeed rotation of the high gravity technology and the large specific surface area of the catalyst are beneficial to the mass transfer of ozone.As a result,the mass transfer rate is greater than the corresponding decomposition rate [15,42].However, When the β increases from 40 to 60, theCO3(l)slightly decreases at the steady stage.This is because the mass transfer rate of ozone is equal to the decomposition rate of ozone.The linear relationship between ln([O3]s/([O3]s- [O3])) and time t is fitted in the unsteady-state ozone absorption process, and the results are shown in Table 3.The slopeKLa+Kcvalues are obtained,and theR2values are greater than 0.9825.Using subtraction,theKLavalue changes significantly between 7.53×10-3and 11.6×10-3s-1,and it also increases first and then decreases with the increase of β.When β is increased from 20 to 40,the centrifugal force on the droplets,liquid filaments and liquid film in the packing is constantly increasing, which increases the surface velocity of liquid film, accelerates droplet movement, resulting in the enhanced gas-liquid mass transfer effect.In addition, the high gravity technology can make the catalyst turn over at high speed, which is beneficial for the catalyst with large specific surface area to adsorb gaseous ozone to the liquid phase faster.For the above two reasons,theKLaincreases constantly.However, when β exceeds 40, theKLavalue decreases gradually.This may be due to the shorter residence time of the gas into the liquid phase per unit time, resulting in the reduction of the overall volumetric mass transfer coefficient [17,29].Therefore, the β is 40, which is conducive to the mass transfer of ozone.

3.2.3.Effect of inlet ozone concentration

Fig.8 shows the effect ofCO3(g)on the absorption process of ozone in the Cat/O3-RPB system and the margin of errors is less than 0.432 mg·L-1.TheCO3(l)increases gradually with the increase ofCO3(g)at the steady stage.Because the higher the gaseous ozone concentration, the higher the gas phase pressure.According to Henry’s law, the saturation concentration of ozone in the liquid phase increases.Moreover,the plot of ln([O3]s/([O3]s-[O3]))versustime in the unsteady-state ozone absorption process is fitted, and the results are shown in Table 4.The slopeKc+KLavalues are obtained, in line with a linear relationship, and theR2value is greater than 0.9825.Using subtraction, theKLavalue changes between 7.12 × 10-3and 13.7 × 10-3s-1with the increase ofCO3(g).This is mainly because ozone absorption is controlled by the liquid film [39], and the increased external driving force can lead to an increase in the mass transfer rate, which favors to the degradation of organic matter,lining with the results of Aghaeinejad [43,44].

3.2.4.Effect of the liquid flow rate

As shown in Fig.9,the effects of differentQLon absorption process of ozone in Cat/O3-RPB system were studied and the margin of error was less than 0.426 mg·L-1.It is noted that the concentration of dissolved ozone in the deionized water is almost constant with the increase ofQLat the steady stage.The reason could be attributed to the constant amount of ozone entering the system and a cer-tain value of β.In addition,the plot of ln([O3]s/([O3]s-[O3]))versustime in the unsteady-state ozone absorption process is fitted, and the results are shown in Table 5.TheKc+KLavalues are obtained,in line with a linear relationship,and theR2value is above 0.9825.Using subtraction,the calculatedKLavalue first increases and then decreases as theQLincreases.When theQLranges from 65 L·h-1to 85 L·h-1, under a certain shear force, the liquid entering the catalyst packing gradually increases per unit time, and the increased wetting of the catalyst packing by the liquid leads to more and more sufficient contact with the ozone, which will reduce the resistance for mass transfer [45], resulting in enhancing theKLavalue.When theQLis more than 85 L·h-1,the liquid film in the catalyst packing is relatively thickened per unit time for a givenQGand β, and the contact area between phases is reduced, which is detrimental to the increase ofKLavalue.

Table 2Fitted data with ln([O3]s/([O3]s - [O3])) - t at different pH values

Table 3Fitted data with ln([O3]s/([O3]s - [O3])) versus t at different high gravity factors

Fig.7. The effects of different β on ozone concentration in water in Cat/O3-RPB(pH = 6, CO3(g) = 60 mg·L-1, T = 20 °C, QG = 60 L·h-1, QL = 85 L·h-1).

3.3.Catalytic ozone decomposition and mass transfer correlation

3.3.1.Catalytic ozone decomposition correlation

whereA,a,b,c,and dare required coefficients.The correlative expression of catalytic ozone decomposition in Cat/O3-RPB system is obtained with the Matlab program as following Eq.(12).

The average error of this correlation is 8.91%.The expression can be used when the pH is 2-10, β is 0-60,CO3(g)is 0-80 mg·L-1,andQLis 0-105 L·h-1.From the correlation, it can be seen thatQLhas a negative effect on ozone decomposition,while it can be seen thatCO3g(), pH and β have a positive correlation effect on the decomposition of ozone, and the influence relationship isCO3(g)＞pH ＞β ＞QL.

3.3.2.Catalytic ozone mass transfer correlation

whereA′,a′,b′,C′andd′are required coefficients.The correlative expression of catalytic ozone mass transfer in Cat/O3-RPB system is obtained with the Matlab program as following Eq.(14).

The average error of this correlation is 6.75%.The expression can be used when the pH is 2-10, β is 0-60,CO3(g)is 0-80 mg·L-1,andQLis 0-105 L·h-1.From the correlation,it can be seen that pH has a negative effect on ozone mass transfer,while β,CO3(g)andQLhave a positive correlation effect.But the relationship affecting the mass transfer of ozone isQL＞CO3(g)＞β ＞pH.

3.4.Effect of different water quality on ozone decomposition and mass transfer

In addition to studying the effects of deionized water on ozone decomposition and mass transfer in the Cat/O3-RPB system, the effects of Fenhe water on catalytic ozone decomposition and mass transfer were also studied under pH=6,CO3(g)=60 mg·L-1,β=40,andQL= 85 L·h-1.As shown in Fig.10(a), The water quality of Fenhe River has a linear relationship with the overall decomposition rate of ozone,and its correlation coefficient is 0.9907.It is generally believed that there are accelerating and inhibiting ions in the real water [46].As known from the experimental results, theKcvalue of Fenhe water is 0.88 × 10-3s-1lower than deionized water.This is mainly due to the presence of inhibiting ions (common)in the Fenhe water.Some anions in Fenhe water adsorb onto catalystviaa complexation reaction expressed in Eq.(15), competing for interaction with strong Lewis acid sites of the catalyst where ozone aqueous decomposition takes place [7,47], leading to a reduction hydroxyl content in the surface of the catalyst, thereby reducing theKcvalue.Qiet al.[1]has reported sulfate ion (50 mmol·L-1) and nitrate ions(50 mmol·L-1) could be adsorb onto the surface of γ-Al2O3via a complexation reaction, theKcwas inhibited 3.67 × 10-3s-1and 3.3 × 10-3s-1, respectively.At the same time, Luet al.[48]explored the effect of anions on the degradation of metronidazole,and the degradation rate of metronidazole by addingdecreased by 11.3%, 41.4% and 13%, respectively.

Fig.8. The effects of different gaseous ozone concentration on ozone concentration in water in Cat/O3-RPB system (pH = 6, β = 40, QL = 85 L·h-1).

Fig.9. The effects of different QL on ozone concentration in water in Cat/O3-RPB system (pH = 6, CO3(g) = 60 mg·L-1, β = 40).

Futhermore,the effect of Fenhe water quality on ozone absorption process was studied, and the result is shown in Fig.10(b).It can be seen that the ozone concentration in Fenhe water quality is 2.2 mg·L-1lower than that of deionized water at stable state.This is due to the fact that there are many ions in Fenhe water that can quickly deplete ozone.In addition, the plot of ln([O3]s/([O3]s- [O3]))versustime in the unsteady-state ozone absorption process is fitted,and the results are shown in Table 6.TheKc+KLavalues are obtained,in line with a good linear relationship,and theR2value is above 0.9946.Using subtraction,it can be seen that the calculatedKLavalue is 2.51×10-3s-1lower than that of deionized water.This is mainly because the anions occupy the active sites on the catalyst surface, so that the adsorption rate of aqueous ozone on the catalyst surface decreases, resulting in a slower rate of ozone decomposition in the liquid phase, which in turn reduces the rate of mass transfer of ozone gas into the liquid phase.

Table 4Fitted data with ln([O3]s/([O3]s - [O3])) versus t at different gaseous ozone concentration

Table 5Fitted data with ln([O3]s/([O3]s - [O3])) versus t at different liquid flow rate

Table 6Fitted data with ln([O3]s/([O3]s - [O3])) versus t at different water quality

3.5.Comparison of decomposition and mass transfer of ozone in different devices

Experiments were performed in the Cat/O3-RPB system,O3-RPB system and the Cat/O3-BR system to evaluate the decomposition and mass transfer of ozone.As shown in Fig.11(a), theR2values of the overall decomposition rate equation of ozone in both devices are greater than 0.991, in line with a good linear relationship.TheKcvalue for the Cat/O3-RPB system (4.28 × 10-3s-1) is 44.86%higher than that for the Cat/O3-BR system(2.36×10-3s-1).Under the centrifugal force in RPB, the liquid is dispersed uniformly by the liquid distributor at a high speed and the catalyst can be rotated rapidly, so that the absorption and desorption ability of aqueous ozone on the catalyst surface is accelerated, generating more ·OH, and thus the decomposition rate constant of ozone can be greatly improved.Meanwhile theKcvalue for the Cat/O3-RPB system is 56.07%higher than that for the O3-RPB system,indicating that the addition of catalyst greatly promotes to increase theKc.Xuet al.[49]study the decomposition of ozone catalyzed by Al2O3under US field.The results showed that theKcwas 1.33 × 10-3s-1under the conditions of power density of 51.7 W/L andQLof 30 L·h-1.TheKcvalue in the Cat/O3-RPB system is 68.93% higher than that in the US-Al2O3/O3system.

Fig.10. (a)The effects of different water qualities on catalytic ozone decomposition in Cat/O3-RPB system;(b)The effects of different water qualities on catalytic ozone mass transfer in Cat/O3-RPB system.(pH = 6, CO3(g) = 60 mg·L-1, β = 40, QL = 85 L·h-1).

Fig.11. (a)Catalytic ozone decomposition rate constant under different systems.(b)The effects of different systems on ozone concentration in water in Cat/O3-RPB system(pH = 6, CO3(g) = 60 mg·L-1, β = 40, QL = 85 L·h-1).

Fig.11(b) shows that the concentration of dissolved ozone reaches 14.2 mg·L-1in the Cat/O3-RPB system and 9.3 mg·L-1in the Cat/O3-BR system at stable state, respectively, which shows that the high gravity technology helps improve ozone mass transfer, thereby increasing the accumulation of aqueous ozone in unit time.TheR2values of the overall ozone mass transfer rate equation in both devices are greater than 0.9847,and conform to a linear relationship.As shown in Table 7, theKLavalue in the Cat/O3-RPB system is 47.41% higher than that in the Cat/O3-BR system.This is due to the fact that under the action of high-speed centrifugal force, the liquid is sheared into tiny liquid microelements accelerates the transfer of ozone from the gas phase to the liquid phase, compared with traditional BR.TheKLavalue in the Cat/O3-RPB system is 8.19% higher than that in the O3-RPB system.This is likely because the catalytic particles may concentrate at the interface and enhance the mass transfer due to the chemical reaction.Xuet al.[49]study the absorption of ozone catalyzed by Al2O3under US field.The results showed that theKLawas 9.8 × 10-3s-1under the conditions of power density of 51.7 W/L andQLof 30 L·h-1.TheKLavalue of ozone in the Cat/O3-RPB system is 15.2% higher than that in the Al2O3/O3-US system.It can be inferred that high gravity enhanced heterogeneous catalytic ozone system is beneficial to improve the mass transfer effect of ozone.

Table 7Fitted data with ln([O3]s/([O3]s - [O3])) versus t at different systems

4.Conclusions

In this study,the heterogeneous catalyst was packed in the RPB to enhance the decomposition and mass transfer of ozone.The results found that theKcandKLavalues are relatively large, which are 4.28 × 10-3s-1and 11.60 × 10-3s-1at an initial pH of 6, β of 40,CO3(g)of 60 mg·L-1andQLof 85 L·h-1in deionized water, respectively.Meanwhile, the relationship affecting the catalytic ozone decomposition and the mass transfer of ozone is in the descending order ofCO3(g)＞pH ＞β ＞QLandQL＞CO3(g)＞β ＞pH in Cat/O3-RPB system,respectively.In addition,some ions in Fenhe water can also hinder to some extent the decomposition and mass transfer rate of catalytic ozone.And more importantly, theKcandKLavalues for the Cat/O3-RPB system are 44.86%and 47.41%higher than that for the Cat/O3-BR system, respectively, illustrating that high gravity device can facilitate the decomposition and mass transfer of ozone in heterogeneous catalytic ozonation technology and provides some theoretical insights into the industrial applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Specialized Research Fund for Sanjin Scholars Program of Shanxi Province(201707),Key Research& Development Plan of Shanxi Province(201903D321059), Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province(20200004), Transformation and Cultivation Projects of Scientific and Technological Achievements in Universities of Shanxi Province Institutions(2020CG040), and the China National Key Project of Science and Technology ‘‘Major Science and Technology Program for Water Pollution Control and Treatment” (2018ZX07601001).