1.Introduction

High-gravity technology is an innovative process intensification technique that was originally suggested in 1979[1].The technology is achieved through multiphase reactors which can create a high-gravity condition of several orders of magnitude greater than the Earth's gravitational field.A classic example of such reactors is a rotating packed bed(RPB)which has hitherto been employed in many applications[2,3],and is widely acknowledged to exhibit good mass transfer characteristics[4,5].However,RPBs still suffer some intrinsic limitations which hinder their application in the industry[6,7].

Owing to the challenges associated with RPBs,another high-gravity multiphase device – a rotor–stator reactor(RSR)– which is also regarded as an efficient gas–liquid contactor was developed on the basis of an RPB[8].In an RSR,a high-gravity environment is created by large centrifugal force generated by rotation of a rotor.As a result,fluid flowing through the reactor is split into fine liquid elements and films.Also,there is strong turbulence of gas and liquid streams as well as fast renewal of gas–liquid interface.All of these can significantly contribute to intense mixing and increased mass transfer efficiency in the reactor[9,10].Consequently,RSRs have been successfully employed in several applications in recent years[11–13].

第二，侦查工作是一项专业性很强的法律活动，其运行必须严格遵守法律法规，严格按照法律程序来办理，任何超越法律的行为都将可能导致侦查工作失败。如在讯问犯罪嫌疑人时，犯罪嫌疑人始终闭口不言，一时拿不下犯罪嫌疑人的供述，面对这种情形，虽然采取刑讯逼供或其它手段可能会很快突破犯罪嫌疑人的心理防线，但是一旦采取这种行为，将导致侦查工作走向非法，其所收集的证据材料将不能作为定案证据，更可能直接导致侦查人员触犯法律。

o-Phenylenediamine(o-PDA),also known as 1,2-benzenediamine,is an aromatic amine with wide spread applications in the chemical industry[14,15].However,it is also highly toxic and bio-recalcitrant compound that must be eliminated from industrial effluents[16].Some methods to remove o-PDA from waste waters have been reported in recentyears[17,18].However,these methods suffer the limitations of low removal efficiency,difficulty to operate,inability of continuous operation and low throughput capacity.Thus,there is a need to develop a simple and efficient alternative approach to remove o-PDA from effluents.

Ozonation is an advanced oxidation process that is widely recognized as an effective approach to degrade refractory organics in wastewater owing to the high oxidation potential of ozone(2.08 V)as well as its excellent disinfection ability[19,20].Ozonation can also increase the amount of dissolved oxygen necessary for subsequent bio-treatment[21].Nonetheless,the selectivity of ozone in reaction limits its application and ability to achieve complete mineralization of compounds[22].More efforts to enhance ozonation performance in order to improve removal efficiency of pollutants are therefore necessary.Although combined use of O3 with H2O2 has been applied to treat several bio-recalcitrant organics[23,24],there is hardly any report regarding itsapplication to treat o-PDA effluents.Moreover,the ozonation processes are limited by ozone-liquid mass transfer rate since ozone is rapidly consumed in aqueous solution[25].Consequently,a gas–liquid contacting unit with good mass transfer performance is desirable.

Table 1 shows the degradation intermediate products of o-PDAunder each of the oxidation approaches as identified by GC/MS analyses.It is evident that both processes can degrade o-PDA(1,2-benzenediamine)owing to the absence of its peak in both total ion chromatograms as shown in Fig.9(a)and(b).

The relative abundance of compounds D1,D3,D4,D5 and D7 in each of the oxidation processes is shown in Table 2.Although the products D1,D3,D4,D5 and D7 are presentin both oxidation processes,their relative abundance in the O3/H2O2 process is much less than that in the O3 process,suggesting that the degradation of o-PDA was more enhanced in the O3/H2O2 process than in the O3 process.This phenomenon further,perhaps explains the difference in CODreduction rates achieved by both processes.

纳米流体发电的本质是利用纳米孔与电解质溶液界面效应所产生的对阴/阳离子的选择透过性，将蕴含在溶液浓度梯度差中的吉布斯自由能转化为电能.基于纳米流体发电的基本原理可知，决定输出功率变化趋势的原因有两个：一是ΔC，随着它的增加意味着有更多的吉布斯自由能可以转化为电能；二是纳米孔阵列的离子选择透过性，它的强弱与纳米管道两侧双电荷层的厚度(λD)密切相关，λD的公式为：

2.Materials and Methods

2.1.Structure of an RSR

Proposed degradation pathways for o-PDA by O3/H2O2 and O3 processes,based on the identified intermediates,are respectively shown in Fig.10.

2.2.Experimental set-up and procedures

This study investigated the effectiveness of O3/H2O2 process to degrade o-PDA in a rotor–stator reactor(RSR).The degradation efficiency of o-PDA(η)and the overall gas-phase volumetric mass transfer coefficient(K G a)were separately determined as a function of various operating parameters.Results reveal that higher initial o-PDA concentration favored the overall gas-phase volumetric mass transfer coefficient while it resulted in reduced degradation efficiency.Higher rotation speed of RSR as well as higher initial pH of o-PDA effluent favored both η and K G a.On the other hand,both η and K G a only increased up to peak values with increase in reaction temperature and H2O2 concentration.Comparison experiment results show that η,K G a and r COD achieved by O3/H2O2 process were 31.6,24.4 and 28.9%respectively higher than those of O3 process.Analyses of the intermediate products of degradation further reveal that O3/H2O2 process achieved faster and more complete degradation of o-PDA as compared to O3 process,a phenomenon which can be attributed to the generation of more hydroxyl radicals in O3/H2O2 process than in O3 process.These results indicate that H2O2 can greatly enhance ozone oxidation process,and confirm that simultaneous use ofozone with H2O2 is a promising alternative approach to pre-treat o-PDA effluents.This work further demonstrates that RSR can significantly boost ozone absorption and thus provides a feasible intensification means for the ozonation of o-PDA as well as other recalcitrant organic effluents.

Fig.1.Schematic structure of an RSR.(a)Structure of an RSR(b)An illustration of rotor rings and stator rings:(1)Gas inlet;(2)Cover cap;(3)Stator;(4,7,11)Bolts;(5)Nozzle;(6)Gas outlet;(8)Liquid outlet;(9)Seal;(10)Shaft;(12)rotor;(13)Rotor seat;(14)Casing.

2.3.Analytical methods

Ozone concentration at the inlet(C i)and outlet streams(C o)was monitored and measured by ozone analyzer(Double UV Light Ozone Meter,Limicen Ozone R&D Center,China).The degradation efficiency of o-PDA was calculated based on the concentrations of o-PDA before and after treatment as measured by a UV–Vis spectrophotometer(UV2600,Shimadzu,Japan)at 416 nm.The COD of the wastewater was determined using a COD analyzer(5B-3A,Lian-hua Tech.Co.,Ltd.,China)according to the Chinese standard(HJ/T 399-2007)while the intermediate products of degradation were identified by a gas chromatography-mass spectrometer(GC/MS,5977A,Agilent-USA).The degradation efficiency(η),which indicates the extent of removal of o-PDA,was calculated by Eq.(17)while r COD,which signifies the degree of removal of organic contaminants from the effluent,was obtained by Eq.(18).The overall gas-phase volumetric mass transfer coefficient(K G a),which indicates the degree of ozone absorption into the solution,was calculated by Eq.(19)which is a modification of the formula proposed by Li et al.[31].where C Ao and C Af are the concentrations of o-PDA(mg·L−1)before and after treatment respectively while CODo and CODf represent initial and final COD(mg·L−1)of the wastewater respectively.G is the gas volumetric flow rate(L·h−1),H is the axial length of RSR,R is the inner radius of the RSR casing while C i and C o represent the concentrations of ozone(mg·L−1)at the inlet and outlet gas streams respectively.

Fig.2.Experimental set-up for degradation of o-PDA by O3/H2O2 process in an RSR.

3.Results and Discussion

3.1.Effect of H2O2 concentration

Fig.3 illustrates the effect of H2O2 concentration(C H2O2)on K G a andη respectively under a constant supply of ozone.It was noted that η increased with increase in H2O2 concentration,reaching a peak value at C H2O2=30 mg·L−1,and thereafter slightly declined with further rise in H2O2 concentration.On the other hand,K G a increased throughout with increase in H2O2 concentration.Higher H2O2 concentration led to more HO2–in the solution[Eq.(6)],which then reacted rapidly with the absorbed ozone in the solution to generate more•OH radicals(Eq.(7))besides those generated as a result of ozone decomposition alone[Eq.(3)].Consequently,there was reduction in the concentration of ozone in the solution,leading to increased mass trans ferdriving force and therefore larger K G a.The availability of extra•OH radicals in the solution resulted in increased chance of reaction with o-PDA leading to higher η.Nonetheless,H2O2 can also acts as a scavenger for •OHradicals according to Eqs.(12)and(13),leading to reduction in the amount of•OH radicals in the solution and hence degradation rate.This phenomenon seemingly predominated at C H2O2 beyond 30 mg·L−1,and hence the slight reduction in η observed.

Fig.3.EffectofH2O2 concentration on η and K G a:(T=25°C;pH=6.5;C i=51.0 mg·L−1;C Ao=60 mg·L−1;N=1000 r·min−1).

3.2.Effect of initial o-PDA concentration

Fig.4 shows the variation of η and K G a with initial o-PDA concentration(C Ao).It is evident that higher initial o-PDA concentration boosted K G a while it led to a reduction in η.The results can be attributed to the following reasons.Firstly,since H2O2 concentration and ozone supply was fixed during the experiment,the amount of generated hydroxyl radicals in the solution remained almost constant with a varying initial o-PDA concentration.Thus,a lower initial o-PDA concentration favored η.Also,the degradation by-products of o-PDA in the solution increased accordingly with increase in initial o-PDA concentration,leading to increased competition for ozone and the generated hydroxyl radicals between the by-products and o-PDA,and hence the decline in η.However,the increase in the concentration of o-PDA and its degradation byproducts also resulted in increased consumption rate of absorbed ozone in the solution.Consequently,there was reduction in the concentration of ozone in the solution,leading to increased mass transfer driving force and hence larger K G a.

The variation of η and K G a with reaction temperature(T)is presented in Fig.7.The temperature was varied from 15 to 65°C by circulating water at a particular temperature around the reactor casing as well as adjusting the temperature of the solution accordingly prior to pumping into the reactor.It is evident that both η and K G a increased with increase in temperature,reaching peak values separately at 35°C and afterwards gradually declined with further rise in temperature.Higher temperature increased the rate of ozone decomposition to generate more•OH radicals.As a result,the concentration of the absorbed ozone in the solution decreased,leading to greater ozone-liquid mass transfer driving force and hence increased absorption of ozone.Besides,increasing the amount of•OH radicals through ozone decomposition,higher temperature increased the reaction rate between o-PDA and•OH radicals,resulting in higher degradation efficiency.Nonetheless,higher temperature can also lead to a reduction in solubility of ozone in the solution,and thereby limit ozone-liquid mass transfer rate.The effects of the latter factor superseded the aforementioned benefits of higher temperature at temperatures beyond 35°C,which probably explains the drop in the overall gas-phase volumetric mass transfer coefficient as well as degradation efficiency of o-PDA.

Fig.4.Effect of initial o-PDA concentration on η and K G a:(T=25 °C;pH=6.5;C i=51.0 mg·L−1;C H2O2=30 mg·L−1;N=1000 r·min−1).

3.3.Effect of initial pH

Fig.5 illustrates the effect of initial pH of the solution on ηand K G a.It was noted that higher initial pH favored both η and K G a.Higher pH means more hydroxyl ions in the solution and thus favored ozone decomposition in the solution to generate more•OH radicals.Higher pH also favors decomposition rate of H2O2 to yield hydroxyl radicals[32].Consequently,there was increased rate of reaction between o-PDA and •OH radicals,leading to higher η.The enhanced decomposition rates of both ozone and H2O2 also resulted in a reduction in the concentration of absorbed ozone in the solution,leading to increased mass transfer driving force and consequently increased absorption of ozone.The optimal initial pH was determined as 6.5 in view of extra costs associated with pH adjustments,since it is also the natural pH of o-PDA effluent.

Fig.5.Effect of initial pH on η and K G a:(T=25 °C;C Ao=60 mg·L−1;C i=51.0 mg·L−1;C H2O2=30 mg·L−1;N=1000 r·min−1).

3.4.Effect of rotation speed

Fig.6 shows the effectof rotation speed(N)of RSR onη and K G a.The rotation speed was varied from 400 to 14000 r·min−1.It is evident from the figure that both η and K G a increased throughout with increase in rotation speed.This phenomenon can be attributed to the fact that higher rotation speed enhanced the relative movement of rotors and stators of the RSR and as a result created a high-gravity environment through generation of larger shear force.Consequently,liquid flowing through the reactor was splitin to finerelements and thinner films,leading to increased turbulence,larger effective gas–liquid contact area as well as faster gas–liquid surface renewal rate.All of these led to significant intensification of mass transfer rate between the gas and liquid phase,which is the limiting step in the ozonation process of o-PDA,leading to increased ozone absorption and subsequently higher degradation rate of o-PDA.

Fig.6.Effect of rotation speed on η and K G a:(T=25 °C;C Ao=60 mg·L−1;C i=51.0 mg·L−1;C H2O2=30 mg·L−1;pH=6.5).

3.5.Effect of reaction temperature

Fig.7.Effect of reaction temperature on η and K G a:(C Ao=60 mg·L−1;C i=51.0 mg·L−1;C H2O2=30 mg·L−1;pH=6.5;N=1000 r·min−1).

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3.6.Comparison experiment

Based on the above results,another experiment to determine the achievable η,K G a and r COD at a particular set of operating conditions of C Ao=60 mg·L−1;(at C Ao=60 mg·L−1,CODo=116.3 mg·L−1),C i=51.0 mg·L−1,pH=6.5,T=25 °C,N=1000 r·min−1 and C H2O2=30 mg·L−1 was performed,and results compared with those of a different experiment using ozone alone(O3 process)carried out under the same set of conditions but without introduction ofH2O2 in the solution.The comparison results,as illustrated in Fig.8,show that η,K G a and r COD achieved by O3/H2O2 process were 24.4%,31.6%and 28.9%respectively higher than those of O3 process,indicating the synergistic role played by O3 and H2O2 in ozonation.H2O2 dissociates to yield HO2−ions which then react with ozone to generate more•OH radicals.Consequently,there was increased rate of reaction between o-PDA and•OH radicals,leading to higher degradation efficiency.The increased rate of reaction between HO2−and ozone also resulted in reduced concentration of absorbed ozone in the solution,leading to increased ozone-liquid mass transfer driving force and therefore increased ozone absorption.

Fig.8.Comparison of η,r COD and K G a achieved by O3/H2O2 and O3 processes in RSR.

3.7.Intermediate products

This study therefore supposes that simultaneous use of ozone and hydrogen peroxide(O3/H2O2 process)can enhance ozonation performance and consequently improve degradation rate of ophenylenediamine(o-PDA).A high-gravity environment to intensify ozone-liquid mass transfer rate was achieved through large centrifugal force generated by rapid rotation of a rotor in an RSR.The degradation efficiency of o-PDA(η)as well as the overall gas-phase volumetric mass trans ferco efficient(K G a)were determined under different operating conditions including H2O2 concentration(C H2O2),initial o-PDA concentration(C AO),initial pH,temperature of reaction(T)and rotation speed(N)of RSR,and the results were validated by comparison with those of sole use of ozone(O3 process).Intermediate products of degradation of o-PDA under each of the ozonation processes were also identified using a gas chromatography–mass spectrometer(GC/MS)to further evaluate the extent of o-PDA degradation as well as establish its possible degradation pathway.

Table 1 Identified intermediate products in O3 and O3/H2O2 processes

No. Retention time/min O3/H2O2 process D1 1.95 Acetic acid,methyl ester C3H6O2 Exists Exists D2 2.36 3-Methyl but anal C5H10O– Exists D3 5.13 5-O-Methyl-d-gluconic acid dimethyl amide Compound identified by MS Molecular formula O3 process C9H19NO6 Exists Exists D4 6.75 Methyl 2-hydroxyacatetae C3H6O3 Exists Exists D5 7.29 Acetic acid C2H4O2 Exists Exists D6 9.01 Butane 1,2-diol C4H10O2 Exists Exists D7 9.39 Sec-butyl 4-ethylbenzoate C8H9NO2 Exists Exists D8 10.76 2-Pyridinecarbonitrile C6H4N2 Exists Exists

Fig.9.GC/MS total ion chromatograms for o-PDA effluent:(a)treated by O3 process and(b)treated by O3/H2O2 process.

Ozonation of compounds in aqueous solution involves mass transfer with chemical reactions,i.e.,ozone mass transfer from gas phase into liquid phase,decomposition of ozone and finally mineralization of the contaminants[26].Ozone can react directly with the target compound via electrophilic attack or indirectly by first decomposing to yield hydroxyl radical(•OH),which then reacts with contaminants through abstraction of hydrogen,addition of OH or its substitution,or by means of electron transfer[27].The decomposition mechanism of O3 in the presence of H2O2 in water and its subsequent oxidation of o-PDA can be described by the following equations[28,29].

4.4.2 强化服务监督，完善管理制度。进一步完善和优化项目立项制、项目公示制、项目审计制、项目检查验收制、后续管护等制度，加强项目事中、事后监督。

Table 2 Relative abundance of compounds D1,D3,D4,D5 and D7 in the O3 and O3/H2O2 processes

Compound Retention time/min Abundance O3 process O3/H2O2 process D1 1.95 1.6×106 1.3×106 D3 5.13 8.4×105 6.3×105 D4 6.75 1.5×106 4.9×105 D5 7.29 1.1×106 8.4×105 D7 9.39 2.3×106 1.7×106

Fig.1(a)and(b)shows a schematic structure of a rotor–stator reactor.It comprises a series of concentric rotor rings and stator rings arranged consecutively in a radial direction with a spacing of about 1 mm.The rotor rings are fixed on a motor-driven seat whereas the stator rings are fixed on a cover cap.As shown in Fig.1(b),the open slots on the stator rings and holes on the rotor rings are passages for fluid flow.Rotation of the rotor generates large centrifugal force which causes liquid to flow radially outwards through the open slots and holes.The employed RSR measures 120 mm both in radius and axial length.More details on its specifications are available in our previous work[30].

Fig.10.Proposed degradation pathways of o-PDA by O3/H2O2 and O3 processes.

4.Conclusions

Fig.2 shows a diagram of the experimental setup.It mainly comprises an RSR,oxygen gas cylinder,ozone generator,three liquid tanks,two peristaltic pumps and flow meters.Ozone was produced from pure oxygen using an ozone generator(Tonglin Sci.and Tech.Ltd.,China).Concentrations of ozone at the inlet and outlet gas streams were monitored and measured using ozone analyzer(Double UV Light Ozone Meter,Limicen Ozone R&D Center,China).Simulated o-PDA wastewater was prepared by dissolving o-PDA(purity 99.8%)into de-ionized water and adjusting its pH to a desired value using either sodium hydroxide(0.1 mol·L−1,NaOH)or sulfuric acid(0.1 mol·L−1,H2SO4),both supplied by Beijing Chemical Works Co.Ltd.,China.Hydrogen peroxide solution was also obtained from Beijing Chemical Works Co.Ltd.,China.The solutions of o-PDA and H2O2 were introduced into the RSR at volumetric flow rates of 18 and 2 L·h−1 respectively(total liquid volumetric flow rate=20 L·h−1)through different liquid inlets.The two liquid streams sprayed uniformly via the respective nozzles of the liquid inlets to the innermost layer of the rotor where they merged into a single stream and then flowed radially outwards through the rotor and stator under the influence of centrifugal force generated by the rotation of the rotor.The concentrations of o-PDA and H2O2 solution were adjusted accordingly to ensure that the mixed liquid stream achieved the desired concentrations of o-PDA and H2O2 solution.At the same time,the ozone-containing gas stream was fed into the RSR at a volumetric flow rate of 240 L·h−1 through a gas inlet and flowed inwardly through the rotor and stator.The gas and liquid streams contacted counter-currently and mixed rapidly inside the RSR,leading to absorption of ozone into the liquid stream and subsequent degradation of o-PDA.Finally,the gas and liquid streams left RSR via the gas outlet and liquid outlet respectively.When the concentration of ozone in the gas outlet stream reached a steady state,the outlet liquid samples were collected and immediately analyzed.The system was operated in a continuous mode without recycle.The gas stream was introduced into the RSR when the desired ozone concentration had been achieved.A comparison experiment(O3 process)was performed in the same way though without introduction of hydrogen peroxide.

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