WANG Ai－jie，ZHOU Ai－juan，WANG Li－yan，LIU Min，CHEN Zhao－bo

王愛杰， 周愛娟， 王麗燕， 劉 敏， 陳兆波

(1.State Key Laboratory of Urban Water Resource and Environment，Harbin Institute of Technology，Harbin 150090，China，waj0578@hit.edu.cn; 2.School of Municipal＆Environmental Engineering，Harbin Institute of Technology，Harbin 150090，China; 3.School of Chemistry＆Chemical Engineering，Harbin Normal University，Harbin 150025，China)

The problems of environment have become crucial.One of the main preoccupations is the treatment of wastewaters.Particularly，textile industries are the important pollution source for the aquatic system.Dyes used in the textiles industries are azo dyes about 60%－70%，which contained one or more azo bonds(—N =N—).It has been reported that some of them are toxic，mutagenic［1］and carcinogenic［2－3］compounds. For the majority of them，their half－lives under sunlight are greater than 2000 h and their resistance to biological and even chemical degradation［4］makes them hazardous for the environment even at low concentration.

Photocatalysis processbased on UV－irradiated TiO2represents one of AOP’s that provide an interesting route to reach a total mineralization［5］.For example，this photocatalytic process can be successfully used to photodegrade the azo dyes using either artificial light［5－7］or solar irradiation［8－9］.Besides，more and more publications are focused on the efficiency of immobilized titania catalysts［10－11］.Nowadays，because of its low cost，high photoactivity，nontoxicity，photocorrosion resistance and other physical and chemical properties，TiO2is the most promising photocatalyst.So in this paper TiO2dispersed on Fe3O4－SiO2is used as the catalyst for photooxidative degradation of Procion Red MX－5B(PR).The oxide supporting material(Fe3O4－SiO2)can enhance the reaction and produce additional OH radicals under UV－vis light.Additionally，as a type of superparamagnetic nanoparticles with unusual magnetic response and ultrasmall particle size Fe3O4－SiO2has no magnetic response when the external magnetic field is zero or weak enough，while the particle can present great magnetic response once the external magnetic field is powerful and move with external magnetic field［12－13］.In respect that Fe3O4has magnetically，we could reclaim the Fe3O4－SiO2－supported TiO2catalyst use ferromagnetic.Hence，producing a new catalyst combining this kind of superparamagnetic nanoparticle and TiO2nanoparticles，that is，a magnetic separable photocatalyst，is of emerging interest.

In order to enhance the PR decolorization performance，an optimization approach，such as response surface methodology(RSM)，should be employed.Several works have proved that RSM is a powerful statistical tool for the optimization of photocatalytic oxidation processes［14－18］.The main advantage of RSM is the reduced number of experimental trials needed to evaluate multiple factors and their interactions.The study of the individual and interactive effects of these factors will be helpful in efforts to find the target value［19］.Hence，in order to determine a suitable polynomial equation for describing the response surface，RSM can be adopted to optimize the process［20］.

This paper provides data concerning a simulated wastewater polluted by PR，containing a commercial available TiO2/Fe3O4－SiO2powder as catalyst.The optimization of the reaction parameters of Procion Red MX－5B photodegradation was performed by response surface methodology and experimental design.The decoloration rate was selected as the response for optimization and the functional relationship between the response and the most significant independent variables(factors) was established by means of experimental design.The most important factors employed into experimental design are the TiO2/Fe3O4－SiO2catalyst loading，pH value and the irradiation time.

1 Materials and Methods

1.1 Materials and Reagents

The dye PR(purity 50%;Mw=595.4g/mol; λmax=538 nm)，tetraethoxysilane，3－aminopropyltrimethoxysilane，Ti(OC4H9)4，F(xiàn)eCl2·4H2O and FeCl3·6H2O were purchased from Sigma－Aldrich and used without further purification.Solutions were prepared in ultra－pure water obtained from a Millipore waters Milli－Q equipment.

The magnetic TiO2/Fe3O4－SiO2photocatalyst was synthesized as follows:stable Fe3O4nanoparticles were prepared by the coprecipitation method through adding NaOH to the mixture of FeCl2and FeCl3(molar ratio 1∶2)under an inert environment which was made by argon gas flow through the system.The Fe3O4－SiO2support was prepared as follows:freshly prepared 3－aminopropyltrimethoxysilane solution was added in the mixture of Fe3O4magnetic nanoparticles(500 mg)and H2O(500 mL)standing 40 min，subsequently，adding isopropanol(100 mL)and adjusting pH value higher than 11，and then tetraethoxysilane(2 mL)was added and mechanically stirred for 12 h.After ultrasonic washing by ethanol for three times and magnetic separation，the Fe3O4－SiO2support was obtained.TiO2/Fe3O4－SiO2photocatalyst was synthesized by the sol－gel process which was as follows:the formed Fe3O4－SiO2support (800 mg)was dispersed in the absolute alcohol(140 mL)and then further refluxed at 90℃ for 120 min. Subsequently，the mixture of Ti(OC4H9)4(2.7 mL) and absolute alcohol(60 mL)was added dropwise.The pH value was set at 5 by adding dilute nitric acid.The solution was continuously stirred and refluxed at 90℃for 120 min after reacting 15 min.After ultrasonic washing by ethanol for three times and magnetic separation，the sample was dried at 60℃ and grinded homogeneously，and then calcined at 450℃ for 120 min.

1.2 Pilot-scale Setup and Operation

A schematic representation of the photoreactor is shown in Fig.1.The reactor mainly consisted of electric stirrer，125 W mercury lamp，quartzose cold trap and beaker.The irradiation experiments were carried out in four parallel 100 mL quartz beaker.The light source was a 125 W mercury lamp setting in a quartzose cold trap，emitting in the near－UV(mainly around 365 nm)，with a Corning 0－52 filter to avoid direct photolysis of naphthalene by UV－irradiation below 340 nm.The warp of photoreactor was made of polymethyl methacrylate(PMMA)，inner surface of which cling silver paper to return UV light.For all experiments，the suspensions were electric stirred without any permanent air bubbling.In these conditions，the oxygen dissolved in water following Henry’s Law was sufficient to ensure a constant oxygen pressure and a constant resulting coverage of the surface of TiO2/ Fe3O4－SiO2.

Fig.1 Schematic diagram of the photoreactor

A volume of 50 mL of aqueous PR solution was introduced in the parallel reactors and vigorously stirred at 1000 r/min(higher rotate speed may result in naphthalene volatilization without degradation).Once thermal and volatilization equilibria reached，the appropriate amount of the semiconductor(TiO2/Fe3O4－SiO2) powder was introduced in the reactor.Before irradiation，the reaction mixture was then premixed in the dark during 20 min to reach adsorption equilibrium. And then the mercury lamp was switched on to initiate the photocatalytic reaction and the stirring was maintained during irradiation.Specific quantities of samples were withdrawn at periodic intervals and separated by centrifugal machine in order to remove the catalyst particles.In this study the interval of time was 20 min.In the course of time，samples immediately were analyzed by UV－vis spectrophotometer.

1.3 Multivariate Experimental Design

In this study，the degradation of PR in the presence of UV radiation using a magnetic TiO2/Fe3O4－SiO2photocatalyst was optimized with response surface methodology(RSM)by Design Expert 7.1.6(Stat－Ease，2008)without any blocking.The runs were designed in accordance with the central composite experimental design(CCD)which is well suited for fitting a quadratic surface and usually work well for the process optimization.The central composite criterion can be used to select points for a mixture design in a constrained region.This criterion selects design points from a list of candidate points so that the variances of the model regression coefficients are minimized.

In the present study，the independent variables of irradiation time，pH value and catalyst loading were coded with low and high levels in the central composite design，while the decoloration rate of PR was the response(dependent variable).Process variable ranges were determined by means of preliminary experiments，hat is:irradiation time，40—60 min;pH value，1—2 and catalyst loading in the range of 0.60—0.80 g/L.

A 23－factorial central composite experimental design，with eight factorial points，6 axial points(α= 1.68179)and 6 replications at the center point leading to a total number of 20 experiments(as shown in Tab. 1)was employed for response surface modeling.

1.4 Analytical Methods

The decoloration of PR solution was monitored by measuring absorbance with an UV－vis spectrophotometer Perkin Elmer at a wavelength of 538 nm.

2 Results and Discussions

2.1 The Analysis of Variance of Regress Equation

Results of different runs of experiments are shown in Tab.1.

Tab.1 Matrix for hybrid design with the experiment

In optimizing a response surface，an adequate fit of the model should be obtained to avoid poor or ambiguous results［21］.It is important to ensure the adequacy of the employed model.Tab.2 shows the analysis of variance(ANOVA)of regression parameters of the predicted response surface quadratic model for the decoloration rate of PR.As it can be seen from Tab.2，the model F－value of 13.4955 and a low probability value(Prob＞F=0.0001)indicate that the model is significant for the decoloration rate of PR.There is only a 0.01% chance that a Model F－Value this large could occur due to noise.The Lack of Fit F－value of 69.2069 implying the Lack of Fit is significant.There is a 0.06%chance that a Lack of Fit F－value can occur due to noise.The Adeq Precision is 11.004 which indicates an adequate signal，measuring the signal to noise ratio greater than 4 is desirable［22］.Tab.1 also shows a satisfactory correlation between the values of experimental data and predictive values.The largest standard error is 6.88%which shows the quadratic regression equation model is a good fit to experimental data.The value of correlation coefficient(R2=0.9310)indicates that only 6.90%of the total variation can not be explained by the empirical model and expresses good enough quadratic fits to navigate the design space.Joglekar and May［23］suggested that R2should be at least 0.80 for a good fit of a model. The R2value(0.9310)obtained in the present study for this response variable was higher than 0.80，indicating that the regression models explained the reaction well. Hence，the response surface model developed in this study for predicting PR decolorization efficiency was considered to be satisfactory.

Tab.2 Notability of the coefficient and of variance analysis regress equation

By means of Multi Linear Regression method［24］，a quadratic regression equation was developed based on statistical experimental design.The regression model in terms of coded factors is presented as follows:

In this design，A，C，BC，A2，B2，C2are significant model terms.So in terms of actual factors，an empirical relationship between PR decolorization efficiency and the variables has been expressed by the following second－order polynomial equation:

2.2 Analysis of Interrelation between the Variables

The surface response and contour plots of the quadratic model with one variable kept at central level and the other two varying within the experimental ranges are shown in Figs.2－4.

In Fig.2，the response surface and contour plot were developed as a function of pH value and irradiation time while the catalyst loading was kept constant at 0.7 g/L being the central level.99%PR decolorization efficiency was realized at 0.7 g/L catalyst loading while pH value was above 1.05 and irradiation time were higher than 51 min.The decoloration rate gradually increased with increasing irradiation time and went flatter after 48 min.As can be seen，an increase of pH value from 1 up to the value of about 1.35 led to the increase of decoloration rate and again gradually decreased above pH value with 1.35.The interrelation between the variables is important in terms of optimization;however，the irradiation time has a more significant impact on PR decolorization efficiency than pH value.

Fig.2 The effect of pH value and irradiation time on decoloration rate(catalyst loading:0.7g/L)

To study the effect of irradiation time and catalyst loading on PR decolorization experiments were carried out with irradiation time varying from 40 min to 60 min and under different catalyst loading at constant applied pH value with 1.5.The results are displayed in Fig. 3.This figure clearly shows PR decolorization efficiency increases with the increase of irradiation time at all catalyst loading studied.As seen from the contour plot，almost complete PR decolorization efficiency can be achieved at pH value with 1.5 while the irradiation time is higher than 50min and catalyst loading is lower than 0.72 g/L.In order to achieve high level decolorization efficiency(＞90%)，catalyst loading values become relatively important for irradiation time greater than 47 min.The interrelation between the variables，therefore，is important in terms of optimization，however，the irradiation time has a more significant impact on PR decolorization efficiency than catalyst loading.

Fig.3 The effect of irradiation time and catalyst loading on decoloration rate(pH value:1.5)

The effect of the variables，i.e.，pH value and catalyst loading on PR decolorization efficiency is illustrated in Fig.4.When the irradiation time is kept constant at 50 min，PR decolorization efficiency increases with the decrease of catalyst loading.Similar to interrelation between pH value and the catalyst loading，it can be concluded from the correlation between pH value and the catalyst loading that both has almost the same significant impact on PR decolorization efficiency.

2.3 Confirmation of Optimal Experiment Factors and Model Assess

Eq.(2)can be switched to the form hereinafter:

where X=［A，B，C］T.

Fig.4 The effect of pH value and catalyst loading on decoloration rate(irradiation time:50 min)

That is:

We can take a result that A=49.9488 min，B= 1.3236，C=0.5738 g/L.The decoloration rate of PR is maximum when the irradiation time，pH value and the catalyst loading are 49.9488 min，1.3236 and 0.5738 g/L，respectively.

In order to confirm the validity of Response Surface Methodology Regression Model，we changed the operation parameters and the experimental result was represented in Tab.3.As a consequence of which we have received，the maximum standard error between the observed value and the predicted value was less than 4%which indicated that the quadratic model can predict experimental results well.Furthermore，the standard error was only 3.85%in code 3 when the catalyst loading was 0.90 g/L from which we can know that the model had extrapolation ability.As the result in the optimal experimental condition，we had done experiments for three times repetitively and the degradation rate of PR was 98.68%，97.47%and 96.69% respectively prominently with optimal effect which can prove the regression model was validity，credible and the model had preferable instructional ability for experiments.

Tab.3 Confirm analyses for the model on procion red MX-5B decoloration

3 Conclusions

The photocatalytic degradation of PR in aqueous solution was studied using TiO2/Fe3O4－SiO2as a semiconductor catalyst，focusing on the influence of some parameters such as，the irradiation time，pH value and TiO2/Fe3O4－SiO2catalyst loading.The Response Surface Methodology is a suitable approach to determine easily the factor effects with considerably less experi－mental effort，as well as to facilitate process modeling and to find the optimum.The central composite experimental design allowed to develope a quadratic model as a functional relationship between the decoloration rate of PR and the independent variables.In the present study the RSM together with CCD were used to find the significance of factors at different levels.A satisfactory goodness－of－fit was observed between the predicted and experimental results，which reflected the applicability of RSM to optimize the process for the simulated PR decoloration efficiency.

The optimal conditions of photocatalytic decoloration rate are as follows:the irradiation time，pH value，and the catalyst loading are 49.9488 min，1.3236 and 0.5738 g/L，respectively.Under these conditions the color removal efficiency was of 99.47%being the best experimental point determined into the region of experimentation.By the ANOVA analysis and model confirmation the optimal solution obtained using RSM was experimentally validated and credible with preferable instructional ability for experiments.

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