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PVDF process in continuous submerged membrane photoreactor
Chemical Engineering and Processing 116 (2017) 68–75
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Central composite design optimization of Rhodamine B degradation using TiO2 nanoparticles/UV/PVDF process in continuous submerged membrane photoreactor Vahid Vatanpour* , Asma Karami, Mohsen Sheydaei Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran
A R T I C L E I N F O
Article history: Received 18 January 2017 Received in revised form 13 February 2017 Accepted 28 February 2017 Available online 2 March 2017 Keywords: Central composite design Submerged photocatalytic membrane reactor (SPMR) Advanced oxidation process Water treatment TiO2 nanoparticles
A B S T R A C T
In this paper, efficiency of a hybrid system combining UV/TiO2 nanoparticles and polyvinylidene fluoride (PVDF) membrane in a continuous pilot-scale submerged membrane photocatalysis reactor (SMPR) was investigated to degradation of Rhodamine B (RhB). The PVDF microfiltration membrane has potential to separate TiO2 nanoparticles from the treated wastewater. Effects of different operational parameters such as TiO2 dosages, UV light intensity, solution pH and polluted water flux in treatment reactor, which affect the performance of the photoreactor were evaluated. For the setup of the experimental design, a central composite design (CCD) matrix of the statistical response surface methodology (RSM) was used. The results indicated that the TiO2 photocatalyst at 0.1 g/L, 3 UV-C lamps, polluted water flux of 100 L/h m2 and pH of 8 were the optimum conditions for the removal of the RhB. Under the optimum conditions, 95.0% degradation was experimentally obtained. © 2017 Elsevier B.V. All rights reserved.
1. Introduction During the last years, widespread production and use of organic compounds for household, agriculture and industrial purposes have resulted in environment especially water resources pollutions. Simultaneously, increasing demand of clean water resources due to the rapid development of population growth has become an issue worldwide. Accordingly, polluted water treatment with the goal of returning it to the clean water network is of great importance in environmental processes [1]. Thus, there is considerable interest in developing and using effective treatment processes for polluted water. Recently, advanced oxidation processes (AOPs) have attracted more attention as effective methods for treatment of wastewater containing organic pollutants [2]. The AOPs is a promising alternative technique for common water treatment methods such as adsorption, sedimentation, coagulation and filtration processes, because of the formation of hydroxyl radicals (OH) which have a great potential for complete degradation of the organic pollutants [3].
* Corresponding author. E-mail addresses: [email protected], [email protected] (V. Vatanpour). http://dx.doi.org/10.1016/j.cep.2017.02.015 0255-2701/© 2017 Elsevier B.V. All rights reserved.
Heterogeneous photocatalysis is one of the AOPs which has been widely investigated and used for degradation of the organic pollutants in wastewater [4]. This method is composed of semiconductors acting as photocatalyst and a light as energy source. Among different photocatalytic processes, TiO2/UV is the most used process up to date, due to TiO2 has low-toxic properties, high photoactivity, low cost, high chemical stability and water insolubility under ambient conditions [5,6]. Heterogeneous photocatalysis usually are used in both suspended and immobilized form for formation of OH [7–10]. In the suspended system, available surface area of photocatalyst is considerably higher than the immobilized systems. By the way, separation of homogeneous catalyst from the treated wastewater restricts the utilization of this process [11]. Application of photocatalytic membrane reactors (PMRs) in one of the methods can be used to resolve this problem [12,13]. In PMRs, continuous photocatalytic degradation process and separation of treated wastewater from photocatalyst are performed, simultaneously. The PMRs have some advantages compared with conventional photoreactor (PRs) including: 1) simultaneously accomplishment of photocatalytic degradation process and separation of treated wastewater from photocatalyst without using additional operations and 2) energy and time costs with keeping of the photocatalyst in the reaction environment [13].
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In this study, polyvinylidene (PVDF) microfiltration membrane has been synthesized and used along with UV/TiO2 nanoparticles in continuous flow PMR for the removal of Rhodamine B (RhB) from aqueous media. Moreover, response surface methodology (RSM) based on central composite design (CCD) as statistical approach has been employed to investigate the effect of operational variables on degradation of RhB, identify the optimum level of them for the maximum degradation efficiency and develop model of the treatment process. Four variables including: TiO2 dose, pH of solution, dye solution flux, and UV light intensity were selected as independent variables and degradation efficiency was considered as dependent variable for the CCD. The UV light intensity was changed by varying the number of UV-C lamp(s) immersed into the photocatalytic zone of the photoreactor. 2. Experimental 2.1. Materials The nano-structured TiO2-P25 photocatalyst with mean primary pore size of about 20–30 nm and BET specific surface area of 50 m2/g was obtained from Degussa, Germany. Rhodamine B, a commercial dye, was chosen as a model pollutant. Commercially polyvinylidene fluoride (PVDF, Kynar1 K-761) powder was purchased from Elf Atochem, UK. N-methyl-2-pyrrolidone, NMP as a solvent was obtained from Merck, Germany. Other chemicals were provided in analytical grade and used without any further purification. Distilled water was used throughout the study. 2.2. Preparation of PVDF membranes Flat sheet microfiltration membranes were fabricated by nonsolvent induced phase separation (NIPS) technique [14] from dissolving 12 wt% of PVDF in NMP solvent. The casting solution was continuously stirred with 500 rpm for 8 h to form a homogenous solution. Then, the obtained solution was kept in an oven at temperature of 50 C to remove the air bubbles. To form a film layer at a nominal thickness of 170 mm, the casting solution was cast by a home-made applicator on a nonwoven fabric located on a flat glass
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plate. The role of nonwoven fabric was to induce of mechanical strengthening to the membranes. The resulted films were immediately immersed into a coagulation bath containing distilled water at room temperature for solidification. Next, the fabricated membranes were soaked in a bath of distilled water and maintained wet until using. 2.3. Experimental setup and procedure In this work, a pilot-scale submerged membrane photocatalysis reactor (SMPR) was used for degradation of a model pollutant in aqueous media using TiO2 nanoparticles as a photocatalyst and the PVDF as a membrane for separating of the nanoparticles from the treated water. The used microfiltration membrane had not any rejection for the RhB. The SMPR reactor (Fig. 1) was made of stainless steel vessel with volume of 5.6 L. Due to the UV radiation may affect the chemical bond of PVDF and destroy it, the reactor was separated to two parts by a baffle including: photocatalytic degradation part using the TiO2/UV and treated water filtration part using membrane modules (Fig. 2). The low pressure UV-C lamps (6 W) were suspended vertically in the middle of the photocatalytic degradation area. Air was bubbled into the solution using an air pump to supply dissolved oxygen required for photocatalytic reaction as well as to reduce the plugging of the PVDF membrane surface by TiO2 attachments. The photoreactor was equipped by a jacket to control temperature of suspension during the photodegradation process with the aid of water recirculation. The filtration was continuously obtained using a suction pump connected to the membrane module. The contents of the photoreactor were mixed at 600 rpm using a magnetic stirrer. In this study, the dye solution with adjusted pH was poured in SMPR and mixed with the predetermined dose of TiO2 for 15 min in the absence of any light. Then, the UV irradiation was initiated and the air was pumped into the reactor by a diaphragm pump. During the course of all UV/TiO2 photodegradation runs, the pH of the suspension was controlled. The pH was measured by pH-meter (Sana, SL 901, Iran) and controlled at specified value within a reasonable range (pH 0.2) using NaOH and HCl solutions. Meanwhile, a fresh RhB solution was fed into the reactor separately
Fig. 1. Schematic of pilot-scale submerged membrane photocatalysis reactor.
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V. Vatanpour et al. / Chemical Engineering and Processing 116 (2017) 68–75 Table 1 Coded and real levels of independent test variables. Factor
Coded and real levels 2
1
0
+1
+2
X1, X2, X3, X4,
0 0 5 60
0.1 1 6 100
0.2 2 7 140
0.3 3 8 180
0.4 4 9 220
TiO2(g/L) UV Lamps count(s) pH flux (L/h.m2)
replication of the experiences using central point level of each factor was proposed to give an estimate of the experimental error. Experimental design matrix was tabulated in Table 2. The designed experiments were carried out using the method described in Section (2.3) and the degradation efficiency values of the CCD experiments provided the basis for the development of empirical second-order polynomial equation to predict and describe the relationships between the degradation efficiency and operational factors according to Eq. (3). Y ¼ b0 þ
Fig. 2. The photograph of used plate and frame module with effective membrane area of 140 cm2.
using a diaphragm pump and the treated water was continuously filtered by a peristaltic pump connected to the membrane module. The residual concentration of the RhB in the filtered solution at different times of sampling was determined by measuring the absorbance of the solution using a UV–vis spectroscopy (double beam spectrophotometer UV-210A) at l= 539 nm. The RhB concentration was determined using concentration-absorbance calibration plot. Degradation efficiency (%) was calculated through Eq. (1). Degradation ef f iciency ð%Þ ¼
C inlet C outlet 100 C inlet
ð1Þ
Where Cinlet and Coutlet are the input and output concentrations of the RhB in the wastewater, respectively. 2.4. Experimental design and statistical analysis Four operational factors (independent variables) of TiO2 dose (X1), number of UV-C lamp(s) (X2), solution pH (X3) and RhB solution flux in PMR (X4) were chosen in this study to design, model and optimize the treatment process by RSM with CCD. For statistical calculations, the real value of the independent variable (Xi) was converted to dimensionless (coded) value as xi using Eq. (2): xi = (Xi X0)/dX
(2)
where X0 is the real value of the independent variable at the central level and dX presents the step change [15]. Table 1 represents the original values and level codes of operational factors varied over five levels: low level (code: 1), high level (code: +1), central level (code 0) and two other levels corresponding to codes: 2 and +2. Degradation efficiency of the RhB solution (Y) was chosen as dependent variable (response) of experimental design. RSM study was performed at Design-Expert Software Version 7 environment. Thirty experiments were proposed by experimental design. The
k X
k X
k1 X k X
i¼1
i¼1
i¼1 j¼2
bi X i þ
bii X 2i þ
bij X i X j þ e
ð3Þ
where Y is the degradation efficiency (%) of RhB solution, b0, bi, bii, and bij are the constant and the regression coefficients of the model. k is number of studied factors and e is the error. Xi, X2i and XiXj are linear, quadratic, and interaction terms of model, respectively. i < j must be considered in interaction term (XiXj). When effect of one factor on model response, depends on the level of another factor, an interaction effect exists between these factors and it's important to recognize it. Statistical analysis of variance (ANOVA) was used to evaluate the fit quality of the experimental results to the developed polynomial model.
Table 2 Experimental design and the results of the CCD. Experiment order
Standard order
TiO2 (g/L)
UV Lamp
pH Flux (L/h. m 2)
Degradation efficiency (%, Experimental)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
6 25 1 22 17 19 9 2 14 8 10 5 18 12 7 11 29 15 28 27 24 4 30 26 16 23 21 3 13 20
0.3 0.2 0.1 0.2 0 0.2 0.1 0.3 0.3 0.3 0.3 0.1 0.4 0.3 0.1 0.1 0.2 0.1 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 0.2 0.1 0.1 0.2
1 2 1 2 2 4 1 1 1 3 1 1 2 3 3 3 2 3 2 2 2 3 2 2 3 2 2 3 1 0
8 7 6 9 7 7 6 6 8 8 6 8 7 6 8 6 7 8 7 7 7 6 7 7 8 7 5 6 8 7
46.41 71.65 49.90 61.02 61.39 82.20 32.08 33.68 45.98 88.33 21.22 60.04 60.95 58.13 95.00 63.73 68.89 76.81 66.69 61.5 38.19 86.68 70.44 74.26 73.59 78.30 56.41 85.70 26.53 4.12
100 140 100 140 140 140 180 100 180 100 180 100 140 180 100 180 140 180 140 140 220 100 140 140 180 60 140 100 180 140
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3. Results and discussion 3.1. Characterization of membrane For showing TiO2 rejection ability of the prepared MF membranes, the turbidity test was performed from permeated water after filtration of TiO2 suspension (without dye). The results show that there is no permeation of TiO2 and the rejection is 100%. Surface and cross-sectional SEM micrographs of the synthesized PVDF membrane and surface SEM micrograph of this membrane after use in SMPR are shown in Fig. 3. In the crosssectional SEM micrographs (Fig. 3a and b), two layers can be seen.
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The top layer is a thin and dense layer that forms an active and selective surface of the membrane. The bottom layer due to low concentration of PVDF in the casting solution have finger-like structure with large cavities that is supported the membrane surface layer [16]. Surface SEM image of the microfiltration membrane shows that the mean pore size is about 80–200 nm (analyzed by Image J software) [17]. In addition, Fig. 3d shows the membrane after testing, which the layer of TiO2 has deposited on the membrane surface. Comparison of SEM micrographs of fresh and used PVDF approved the ability of the membrane in effective removal of TiO2 nanoparticles from the treated water and maintaining of this
Fig. 3. SEM images of the used PVDF membrane (a and b) cross-section, (c) surface SEM image before and (d) after using in SMPR for RhB degradation.
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photocatalyst in SMPR for long time application in photocatalytic degradation process. 3.2. Central composite design model and analysis of variance The experiments designed by CCD method (Table 2) were performed and the resulted degradation efficiency (%) with time for each experiment was recorded (data not shown). Obtained results indicated that during each experiment, the degradation efficiency was increased with time up to reach a constant value and then was steady. Steady value of degradation efficiency of the designed experiments shown in Table 2 were introduced to Design-Expert Software Version 7 as dependent variable (response) of experimental design to develop corresponding second-order polynomial model to describe the RhB degradation process. The second-order polynomial response equations of Eqs. (4) and (5) were used to correlate the real and coded value of independent variables, respectively and degradation efficiency (%). Degradation efficiency (%) = 50.67 190.237X1(g/L) + 45.18X2 + 25.23X3 0.12X4 (L/h.m2) 134.06X1(g/L) X1(g/L) 5.84X2 X2 1.95X3 X3 1.29 103X4 (L/h.m2) X4 (L/h.m2) + 8.15X1(g/L) X2 + 16.95X1(g/L) X3 + 0.65X1(g/L) X4(L/h.m2) X4(L/h m2) 0.19X2 X3 0.02X2 X4 (L/h m2) + 0.02X3 X4(L/h.m2) X4(L/ 2 hm ) (4)
Degradation efficiency (%) = 69.24 1.79 x1 + 19.25 x2 + 3.80 x3 9.76 x4 + 0.81 x1 x2 + 1.70 x1 x3 + 2.60 x1 x4 0.19 x2 x3 0.81 x2 x4 + 0.84 x3 x4 1.34 x1 x1 5.84 x2 x2 1.95 x3 x3 2.07 x4 x4 (5)
100.0 80.0
R² = 0.9473
60.0
Source
DF
F-Value
P-Value
Model TiO2 dosage (g/L) UV lamp pH Flux (L/h.m2) TiO2 dosage (g/L)* TiO2 dosage (g/L) UV lamp*UV lamp pH*pH Flux (L/h.m2)*Flux (L/h m2) TiO2 dosage (g/L)*UV lamp TiO2 dosage (g/L)*pH TiO2 dosage (g/L)*Flux (L/h m2) UV lamp*pH UV lamp*Flux (L/h m2) pH*Flux (L/h m2) Error Lack-of-Fit Pure Error Total
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 16 10 6 30
20.98 1.76 204.10 7.97 52.45 1.13 21.5 2.41 2.70 0.24 1.06 2.48 0.01 0.24 0.26
<0.0001 0.2040 <0.0001 0.0128 <0.0001 0.3043 0.0003 0.1417 0.1210 0.6286 0.3205 0.1361 0.9083 0.6314 0.6185
4.03
0.0685
freedoms as well as absolute probability ‘P’ value is smaller than 0.05 at the confidence level of 95% [19]. According to the results shown in Table 3, the Fisher’s F-value of developed model was 20.98, which is higher than the critical F-value, 2.4, for degrees of freedoms equal to 14 for model and 16 for error at significant level of 0.05. The probability ‘P’ value of regression model was much lower than 0.05 approving high significance of the regression model. At the same time, the Fisher’s F-value and probability ‘P’ value of lack-of-fit were lower and higher than 4.06 and 0.05, respectively. This implied that the lack-of-fit as difference between experimental results and model predictions except random error was not significant relative to the pure error and the developed model is adequate [20]. Furthermore, the total determination coefficient (R2) of 0.95% obtained from the ANOVA, indicates that only 5% of the total experimental variation is not explained by the developed model. According to the result shown in Table 3, evaluation of absolute probability ‘P’ value related to terms of the model indicated that the coefficients related to UV lamp, pH, Flux (L/h.m2) and UV lamp UV lamp terms were statistically are significant model terms at 95% confidence level. Moreover, a Pareto analysis was used to investigation of the percentage effect of each factor on the degradation efficiency as
Percentage effect (%)
Model predicted degradation efficiency (%)
The curve fitting of predicted and experimental of RhB degradation rate is shown in Fig. 4. The experimental data were not statistically different from their predicted counterparts from a minimum of 4.12% to a maximum of 95.00% degradation efficiency, which indicates a good fit between the experimental and predicted responses. According to Fig. 4, R2 value of 0.9473 was obtained from curve fitting of predicted and experimentally obtained Rhodamine B degradation efficiency, which exhibited a close fit between the experimental results and model calculation. Similar results were reported in other papers [15,18]. Fig. 4. The ANOVA was performed to assess the validity and adequacy of the model and the effect of operational parameters and their interactions. 95% of confidence level was considered to be significant. The ANOVA data are shown in Table 3. It is well-known that a corresponding variable will be more significant if its Fisher’s F-value is higher than the critical F-value for related degrees of
Table 3 Analysis of variance (ANOVA) for response surface quadratic model.
40.0
100 90 80 70 60 50 40 30 20 10 0
20.0 0.0 0.0
20.0
40.0
60.0
80.0
100.0
Experimentally obtained degradation efficiency (%) Fig. 4. Comparison plot between the experimental and model predicted degradation efficiency (%).
Fig. 5. Pareto chart of standardized effects for the model of RhB degradation process.
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shown in Eq. (6). 2
n X
b2i Þ 100
Effect of Factorð%Þ ¼ ðbi =
ð6Þ
i¼1
In this equation, bi is the regression coefficient of the model and n is the number of studied factors [21]. According to the obtained result from the Pareto analysis shown in Fig. 5, among the variables, UV lamp and flux of polluted water in reactor created the main effects on the RhB degradation process. 3.3. Effect of pH and TiO2 dosage on RhB degradation process To understanding both the main and the interaction effects of the TiO2 dosage and pH on RhB degradation process, 3D surface plot and 2D contour plot of degradation efficiency (%) as a function of these factors is drawn using Eqs. (4) or (5) at a time, while polluted water flux and numbers of used UV lamps were kept at fixed middle (zero coded) level of about 140 L/h m2 and 2 lamp, respectively (Fig. 6). The pH is an important operating factor that can influence the heterogeneous photocatalytic degradation efficiency. The pH can affect in efficiency of reactive radicals development, oxidative property of developed radicals as well as tendency of catalyst particles for pollutants adsorption [22]. The obtained results, which are shown in Fig. 6, indicated that at all the degradation efficiency was increased with increasing pH up to about 7–8. The pH of zero point charge (pHzpc) of TiO2 is 6.25 [23]. This means that the TiO2 nanoparticles are nearly not surface charged at pH of 6.25. The nanoparticles surfaces become positive or negative when pH is below or above this value, respectively. At pH of 5, the surface of TiO2 in solution is positively charged. Accordingly, tendency of RhB cations to migrate and adsorb on the surface of TiO2 particles is
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low where reactive radicals are developed, which cause decrease in photocatalytic degradation efficiency. At the pH between 7 and 8, the TiO2 surface carries a negative charge which led to increase in tendency of TiO2 particles for adsorption of RhB cations. However, more increase in pH of suspension up to 9 led to decrease in degradation efficiency which can be attributed to decrease in oxidation potential of developed reactive radicals [22]. As can be seen in this figure, presence of TiO2 led to increase in the degradation efficiency of RhB solution. This increase in the photodegradation efficiency is due to heterogeneous photocatalytic degradation process. Though, more increase in the TiO2 dosage leads to decrease in the color removal. When the TiO2 dosage exceeds, the suspension becomes too thick and may cause light barrier to admit enough UV light for all photocatalysis nanoparticles [24]. Furthermore, at high TiO2 dosage, nanoparticle aggregation phenomenon may decrease effective surface area of photocatalyst [24]. In addition, parallel lines of contour plot and surface plot shown in Fig. 6 mean that the interaction effect between applied pH and TiO2 dosage is not significant. This was also confirmed by the probability ‘P’ value of 0.3205 through ANOVA. 3.4. Effect of light intensity and polluted water flux on RhB degradation process Effects of RhB solution flux and number of applied UV lamp(s) in the PMR on the photocatalytic degradation efficiency are illustrated using 3D surface plot and 2D contour plot in Fig. 7, while the TiO2 dosage and pH were 100 mg/L and 7, respectively. As shown in Fig. 7, in the slower permeate flux corresponding to higher residence times (volume of the storage tank divided by the permeate flow rate), the degradation efficiency was enhanced due
Fig. 6. The response surface plot and contour plot of the degradation efficiency (%) as the function of pH and TiO2 dosage.
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Fig. 7. The response surface plot and contour plot of the degradation efficiency (%) as the function of number of applied UV lamp(s) and polluted water flux.
3.5. Determination of optimal conditions for degradation of RhB The optimization was done to determine the optimum values for degradation of RhB solution, using the membrane photocatalytic process. Optimum values of the factors were obtained by numerical analyses of the polynominal models of process (Eqs. (4) or (5)). The desired goal in degradation efficiency (%) as dependent factor was defined as “maximize” to achieve best treatment execution. The optimum degradation efficiency (%) was achieved at TiO2 dosage, number of UV-C lamp, solution pH and RhB solution flux in PMR of 0.1 g/L, 3, 8 and 100 L/h m2, respectively. The predicted value for degradation efficiency (%) at optimum conditions was 92.5%, whereas the experimental corresponding value was 95.0%. As mentioned, reducing of membrane flux could cause to high degradation efficiency. However, as can be seen in Fig. 7 of manuscript, by reducing in solution flux from 100 L/h m2 to 60 L/h m2, very low increase in the RhB degradation efficiency was resulted. Therefore, in this work, 100 L/h m2 was selected as optimum solution flux due to economic consideration and production of higher treated water.
3.6. Comparison of optimized TiO2/UV/PVDF process with UV/PVDF process In order to compare the performance of TiO2/UV photocatalytic process with only UV irradiation on degradation of RhB, supplementary experiments in the absence and presence of 0.1 g/L TiO2 were conducted, while the number of UV-C lamp, solution pH and RhB solution flux in the PMR were in the optimized condition of 3, 8 and 100 L/h m2, respectively. According to the obtained results illustrated in Fig. 8, a constant degradation efficiency of 84.9% was yielded in the presence of 3 UV lamps without TiO2 after 200 min. The photocatalytic decomposition of RhB under UV irradiation resulted in 95% degradation efficiency. Comparison of the obtained result indicated that the oxidative ability of UV light is notable at optimum condition. By the way, TiO2 addition led to increase in the formation of highly reactive radicals especially OH and accordingly enhance in degradation efficiency.
100 90 Degradation effiiciency (%)
to the increasing contact time among the pollutants and the produced radicals. Accordingly, reaction opportunity between RhB molecules and the developed reactive radicals on the surface of TiO2 and/or bulk of solution was improved [25]. It is found that another very important factor that controls photocatalytic reaction rate is irradiated light intensity. As can be seen in Fig. 7, increment in the number of UV lamp(s) corresponding to UV light intensity enhanced degradation efficiency, which can be attributed to increasing probability of photocatalytic production of reactive radicals as well as efficiency of direct photolytic degradation of RhB molecules [26]. The similar result was also reported for photodegradation of iopromide using UV/S2O82/H2O2 process [27].
80 70 60 50 40 30 20
TiO2/UV/PVDF process
10
UV/PVDF process
0 0
50
100 150 Time (min)
200
250
Fig. 8. Ability of TiO2/UV/PVDF and UV/PVDF process on degradation of RhB (3 UV-C lamps, solution pH = 8, RhB solution flux = 100 L/h m2).
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The porous structure and considerable surface area of TiO2 nanoparticles is beneficial for the improvement of photodegradation. In the beginning of TiO2 nanoparticles/UV/PVDF process, adsorption of RhB on the unoccupied surface of photocatalyst led to considerable enhancement of RhB removal efficiency in photocatalytic process rather than the photolysis. However, the number of occupied sites on the TiO2 nanoparticles increased with time; thus, role of adsorption was decreased. In this work, due to use of high portion of UV light eradiated from applied lamps in Rhodamine degradation, these lamps were immersed in dye solution. Accordingly, efficiency of direct photolysis of Rhodamine was considerable. By the way, as can be seen in Fig. 8, the TiO2 nanoparticles led to more than 10% enhancement in RhB degradation efficiency. 4. Conclusion The present study investigated and approved the ability of TiO2/ UV/PVDF process in a continuous pilot-scale submerged membrane photocatalysis reactor on RhB degradation. Response surface methodology has allowed the design of a range of experiments to investigate the effect of TiO2 dosage, number of UV-C lamp, solution pH and RhB solution flux on degradation of RhB and develop a mathematical model having the best fit to the data obtained from the experimental design. The optimum conditions and result for degradation of RhB using the continuous pilot-scale SMPR were successfully obtained and showed ability of this system for wastewater treatment. Acknowledgment The authors thank the Kharazmi University, Iran, for financial and other supports. References [1] M.N. Chong, B. Jin, C.W. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997–3027. [2] M. Sheydaei, A. Khataee, Sonocatalytic degradation of textile wastewater using synthesized g-FeOOH nanoparticles, Ultrason. Sonochem. 27 (2015) 616–622. [3] N. Daneshvar, M.H. Rasoulifard, A.R. Khataee, F. Hosseinzadeh, Removal of C.I. Acid Orange 7 from aqueous solution by UV irradiation in the presence of ZnO nanopowder, J. Hazard. Mater. 143 (2007) 95–101. [4] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B 49 (2004) 1–14. [5] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater. 170 (2009) 560–569. [6] B. Ayoubi-Feiz, S. Aber, M. Sheydaei, Effect of oxidants on photoelectrocatalytic decolourization using a-Fe2O3/TiO2/activated charcoal plate nanocomposite under visible light, RSC Adv. 5 (2015) 19368–19378. [7] J. Byrne, B. Eggins, N. Brown, B. McKinney, M. Rouse, Immobilisation of TiO2 powder for the treatment of polluted water, Appl. Catal. B 17 (1998) 25–36.
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Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Central composite design optimization of Rhodamine B degradation using TiO2 nanoparticles/UV/PVDF process in continuous submerged membrane photoreactor Vahid Vatanpour* , Asma Karami, Mohsen Sheydaei Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran
A R T I C L E I N F O
Article history: Received 18 January 2017 Received in revised form 13 February 2017 Accepted 28 February 2017 Available online 2 March 2017 Keywords: Central composite design Submerged photocatalytic membrane reactor (SPMR) Advanced oxidation process Water treatment TiO2 nanoparticles
A B S T R A C T
In this paper, efficiency of a hybrid system combining UV/TiO2 nanoparticles and polyvinylidene fluoride (PVDF) membrane in a continuous pilot-scale submerged membrane photocatalysis reactor (SMPR) was investigated to degradation of Rhodamine B (RhB). The PVDF microfiltration membrane has potential to separate TiO2 nanoparticles from the treated wastewater. Effects of different operational parameters such as TiO2 dosages, UV light intensity, solution pH and polluted water flux in treatment reactor, which affect the performance of the photoreactor were evaluated. For the setup of the experimental design, a central composite design (CCD) matrix of the statistical response surface methodology (RSM) was used. The results indicated that the TiO2 photocatalyst at 0.1 g/L, 3 UV-C lamps, polluted water flux of 100 L/h m2 and pH of 8 were the optimum conditions for the removal of the RhB. Under the optimum conditions, 95.0% degradation was experimentally obtained. © 2017 Elsevier B.V. All rights reserved.
1. Introduction During the last years, widespread production and use of organic compounds for household, agriculture and industrial purposes have resulted in environment especially water resources pollutions. Simultaneously, increasing demand of clean water resources due to the rapid development of population growth has become an issue worldwide. Accordingly, polluted water treatment with the goal of returning it to the clean water network is of great importance in environmental processes [1]. Thus, there is considerable interest in developing and using effective treatment processes for polluted water. Recently, advanced oxidation processes (AOPs) have attracted more attention as effective methods for treatment of wastewater containing organic pollutants [2]. The AOPs is a promising alternative technique for common water treatment methods such as adsorption, sedimentation, coagulation and filtration processes, because of the formation of hydroxyl radicals (OH) which have a great potential for complete degradation of the organic pollutants [3].
* Corresponding author. E-mail addresses: [email protected], [email protected] (V. Vatanpour). http://dx.doi.org/10.1016/j.cep.2017.02.015 0255-2701/© 2017 Elsevier B.V. All rights reserved.
Heterogeneous photocatalysis is one of the AOPs which has been widely investigated and used for degradation of the organic pollutants in wastewater [4]. This method is composed of semiconductors acting as photocatalyst and a light as energy source. Among different photocatalytic processes, TiO2/UV is the most used process up to date, due to TiO2 has low-toxic properties, high photoactivity, low cost, high chemical stability and water insolubility under ambient conditions [5,6]. Heterogeneous photocatalysis usually are used in both suspended and immobilized form for formation of OH [7–10]. In the suspended system, available surface area of photocatalyst is considerably higher than the immobilized systems. By the way, separation of homogeneous catalyst from the treated wastewater restricts the utilization of this process [11]. Application of photocatalytic membrane reactors (PMRs) in one of the methods can be used to resolve this problem [12,13]. In PMRs, continuous photocatalytic degradation process and separation of treated wastewater from photocatalyst are performed, simultaneously. The PMRs have some advantages compared with conventional photoreactor (PRs) including: 1) simultaneously accomplishment of photocatalytic degradation process and separation of treated wastewater from photocatalyst without using additional operations and 2) energy and time costs with keeping of the photocatalyst in the reaction environment [13].
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In this study, polyvinylidene (PVDF) microfiltration membrane has been synthesized and used along with UV/TiO2 nanoparticles in continuous flow PMR for the removal of Rhodamine B (RhB) from aqueous media. Moreover, response surface methodology (RSM) based on central composite design (CCD) as statistical approach has been employed to investigate the effect of operational variables on degradation of RhB, identify the optimum level of them for the maximum degradation efficiency and develop model of the treatment process. Four variables including: TiO2 dose, pH of solution, dye solution flux, and UV light intensity were selected as independent variables and degradation efficiency was considered as dependent variable for the CCD. The UV light intensity was changed by varying the number of UV-C lamp(s) immersed into the photocatalytic zone of the photoreactor. 2. Experimental 2.1. Materials The nano-structured TiO2-P25 photocatalyst with mean primary pore size of about 20–30 nm and BET specific surface area of 50 m2/g was obtained from Degussa, Germany. Rhodamine B, a commercial dye, was chosen as a model pollutant. Commercially polyvinylidene fluoride (PVDF, Kynar1 K-761) powder was purchased from Elf Atochem, UK. N-methyl-2-pyrrolidone, NMP as a solvent was obtained from Merck, Germany. Other chemicals were provided in analytical grade and used without any further purification. Distilled water was used throughout the study. 2.2. Preparation of PVDF membranes Flat sheet microfiltration membranes were fabricated by nonsolvent induced phase separation (NIPS) technique [14] from dissolving 12 wt% of PVDF in NMP solvent. The casting solution was continuously stirred with 500 rpm for 8 h to form a homogenous solution. Then, the obtained solution was kept in an oven at temperature of 50 C to remove the air bubbles. To form a film layer at a nominal thickness of 170 mm, the casting solution was cast by a home-made applicator on a nonwoven fabric located on a flat glass
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plate. The role of nonwoven fabric was to induce of mechanical strengthening to the membranes. The resulted films were immediately immersed into a coagulation bath containing distilled water at room temperature for solidification. Next, the fabricated membranes were soaked in a bath of distilled water and maintained wet until using. 2.3. Experimental setup and procedure In this work, a pilot-scale submerged membrane photocatalysis reactor (SMPR) was used for degradation of a model pollutant in aqueous media using TiO2 nanoparticles as a photocatalyst and the PVDF as a membrane for separating of the nanoparticles from the treated water. The used microfiltration membrane had not any rejection for the RhB. The SMPR reactor (Fig. 1) was made of stainless steel vessel with volume of 5.6 L. Due to the UV radiation may affect the chemical bond of PVDF and destroy it, the reactor was separated to two parts by a baffle including: photocatalytic degradation part using the TiO2/UV and treated water filtration part using membrane modules (Fig. 2). The low pressure UV-C lamps (6 W) were suspended vertically in the middle of the photocatalytic degradation area. Air was bubbled into the solution using an air pump to supply dissolved oxygen required for photocatalytic reaction as well as to reduce the plugging of the PVDF membrane surface by TiO2 attachments. The photoreactor was equipped by a jacket to control temperature of suspension during the photodegradation process with the aid of water recirculation. The filtration was continuously obtained using a suction pump connected to the membrane module. The contents of the photoreactor were mixed at 600 rpm using a magnetic stirrer. In this study, the dye solution with adjusted pH was poured in SMPR and mixed with the predetermined dose of TiO2 for 15 min in the absence of any light. Then, the UV irradiation was initiated and the air was pumped into the reactor by a diaphragm pump. During the course of all UV/TiO2 photodegradation runs, the pH of the suspension was controlled. The pH was measured by pH-meter (Sana, SL 901, Iran) and controlled at specified value within a reasonable range (pH 0.2) using NaOH and HCl solutions. Meanwhile, a fresh RhB solution was fed into the reactor separately
Fig. 1. Schematic of pilot-scale submerged membrane photocatalysis reactor.
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V. Vatanpour et al. / Chemical Engineering and Processing 116 (2017) 68–75 Table 1 Coded and real levels of independent test variables. Factor
Coded and real levels 2
1
0
+1
+2
X1, X2, X3, X4,
0 0 5 60
0.1 1 6 100
0.2 2 7 140
0.3 3 8 180
0.4 4 9 220
TiO2(g/L) UV Lamps count(s) pH flux (L/h.m2)
replication of the experiences using central point level of each factor was proposed to give an estimate of the experimental error. Experimental design matrix was tabulated in Table 2. The designed experiments were carried out using the method described in Section (2.3) and the degradation efficiency values of the CCD experiments provided the basis for the development of empirical second-order polynomial equation to predict and describe the relationships between the degradation efficiency and operational factors according to Eq. (3). Y ¼ b0 þ
Fig. 2. The photograph of used plate and frame module with effective membrane area of 140 cm2.
using a diaphragm pump and the treated water was continuously filtered by a peristaltic pump connected to the membrane module. The residual concentration of the RhB in the filtered solution at different times of sampling was determined by measuring the absorbance of the solution using a UV–vis spectroscopy (double beam spectrophotometer UV-210A) at l= 539 nm. The RhB concentration was determined using concentration-absorbance calibration plot. Degradation efficiency (%) was calculated through Eq. (1). Degradation ef f iciency ð%Þ ¼
C inlet C outlet 100 C inlet
ð1Þ
Where Cinlet and Coutlet are the input and output concentrations of the RhB in the wastewater, respectively. 2.4. Experimental design and statistical analysis Four operational factors (independent variables) of TiO2 dose (X1), number of UV-C lamp(s) (X2), solution pH (X3) and RhB solution flux in PMR (X4) were chosen in this study to design, model and optimize the treatment process by RSM with CCD. For statistical calculations, the real value of the independent variable (Xi) was converted to dimensionless (coded) value as xi using Eq. (2): xi = (Xi X0)/dX
(2)
where X0 is the real value of the independent variable at the central level and dX presents the step change [15]. Table 1 represents the original values and level codes of operational factors varied over five levels: low level (code: 1), high level (code: +1), central level (code 0) and two other levels corresponding to codes: 2 and +2. Degradation efficiency of the RhB solution (Y) was chosen as dependent variable (response) of experimental design. RSM study was performed at Design-Expert Software Version 7 environment. Thirty experiments were proposed by experimental design. The
k X
k X
k1 X k X
i¼1
i¼1
i¼1 j¼2
bi X i þ
bii X 2i þ
bij X i X j þ e
ð3Þ
where Y is the degradation efficiency (%) of RhB solution, b0, bi, bii, and bij are the constant and the regression coefficients of the model. k is number of studied factors and e is the error. Xi, X2i and XiXj are linear, quadratic, and interaction terms of model, respectively. i < j must be considered in interaction term (XiXj). When effect of one factor on model response, depends on the level of another factor, an interaction effect exists between these factors and it's important to recognize it. Statistical analysis of variance (ANOVA) was used to evaluate the fit quality of the experimental results to the developed polynomial model.
Table 2 Experimental design and the results of the CCD. Experiment order
Standard order
TiO2 (g/L)
UV Lamp
pH Flux (L/h. m 2)
Degradation efficiency (%, Experimental)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
6 25 1 22 17 19 9 2 14 8 10 5 18 12 7 11 29 15 28 27 24 4 30 26 16 23 21 3 13 20
0.3 0.2 0.1 0.2 0 0.2 0.1 0.3 0.3 0.3 0.3 0.1 0.4 0.3 0.1 0.1 0.2 0.1 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 0.2 0.1 0.1 0.2
1 2 1 2 2 4 1 1 1 3 1 1 2 3 3 3 2 3 2 2 2 3 2 2 3 2 2 3 1 0
8 7 6 9 7 7 6 6 8 8 6 8 7 6 8 6 7 8 7 7 7 6 7 7 8 7 5 6 8 7
46.41 71.65 49.90 61.02 61.39 82.20 32.08 33.68 45.98 88.33 21.22 60.04 60.95 58.13 95.00 63.73 68.89 76.81 66.69 61.5 38.19 86.68 70.44 74.26 73.59 78.30 56.41 85.70 26.53 4.12
100 140 100 140 140 140 180 100 180 100 180 100 140 180 100 180 140 180 140 140 220 100 140 140 180 60 140 100 180 140
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3. Results and discussion 3.1. Characterization of membrane For showing TiO2 rejection ability of the prepared MF membranes, the turbidity test was performed from permeated water after filtration of TiO2 suspension (without dye). The results show that there is no permeation of TiO2 and the rejection is 100%. Surface and cross-sectional SEM micrographs of the synthesized PVDF membrane and surface SEM micrograph of this membrane after use in SMPR are shown in Fig. 3. In the crosssectional SEM micrographs (Fig. 3a and b), two layers can be seen.
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The top layer is a thin and dense layer that forms an active and selective surface of the membrane. The bottom layer due to low concentration of PVDF in the casting solution have finger-like structure with large cavities that is supported the membrane surface layer [16]. Surface SEM image of the microfiltration membrane shows that the mean pore size is about 80–200 nm (analyzed by Image J software) [17]. In addition, Fig. 3d shows the membrane after testing, which the layer of TiO2 has deposited on the membrane surface. Comparison of SEM micrographs of fresh and used PVDF approved the ability of the membrane in effective removal of TiO2 nanoparticles from the treated water and maintaining of this
Fig. 3. SEM images of the used PVDF membrane (a and b) cross-section, (c) surface SEM image before and (d) after using in SMPR for RhB degradation.
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photocatalyst in SMPR for long time application in photocatalytic degradation process. 3.2. Central composite design model and analysis of variance The experiments designed by CCD method (Table 2) were performed and the resulted degradation efficiency (%) with time for each experiment was recorded (data not shown). Obtained results indicated that during each experiment, the degradation efficiency was increased with time up to reach a constant value and then was steady. Steady value of degradation efficiency of the designed experiments shown in Table 2 were introduced to Design-Expert Software Version 7 as dependent variable (response) of experimental design to develop corresponding second-order polynomial model to describe the RhB degradation process. The second-order polynomial response equations of Eqs. (4) and (5) were used to correlate the real and coded value of independent variables, respectively and degradation efficiency (%). Degradation efficiency (%) = 50.67 190.237X1(g/L) + 45.18X2 + 25.23X3 0.12X4 (L/h.m2) 134.06X1(g/L) X1(g/L) 5.84X2 X2 1.95X3 X3 1.29 103X4 (L/h.m2) X4 (L/h.m2) + 8.15X1(g/L) X2 + 16.95X1(g/L) X3 + 0.65X1(g/L) X4(L/h.m2) X4(L/h m2) 0.19X2 X3 0.02X2 X4 (L/h m2) + 0.02X3 X4(L/h.m2) X4(L/ 2 hm ) (4)
Degradation efficiency (%) = 69.24 1.79 x1 + 19.25 x2 + 3.80 x3 9.76 x4 + 0.81 x1 x2 + 1.70 x1 x3 + 2.60 x1 x4 0.19 x2 x3 0.81 x2 x4 + 0.84 x3 x4 1.34 x1 x1 5.84 x2 x2 1.95 x3 x3 2.07 x4 x4 (5)
100.0 80.0
R² = 0.9473
60.0
Source
DF
F-Value
P-Value
Model TiO2 dosage (g/L) UV lamp pH Flux (L/h.m2) TiO2 dosage (g/L)* TiO2 dosage (g/L) UV lamp*UV lamp pH*pH Flux (L/h.m2)*Flux (L/h m2) TiO2 dosage (g/L)*UV lamp TiO2 dosage (g/L)*pH TiO2 dosage (g/L)*Flux (L/h m2) UV lamp*pH UV lamp*Flux (L/h m2) pH*Flux (L/h m2) Error Lack-of-Fit Pure Error Total
14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 16 10 6 30
20.98 1.76 204.10 7.97 52.45 1.13 21.5 2.41 2.70 0.24 1.06 2.48 0.01 0.24 0.26
<0.0001 0.2040 <0.0001 0.0128 <0.0001 0.3043 0.0003 0.1417 0.1210 0.6286 0.3205 0.1361 0.9083 0.6314 0.6185
4.03
0.0685
freedoms as well as absolute probability ‘P’ value is smaller than 0.05 at the confidence level of 95% [19]. According to the results shown in Table 3, the Fisher’s F-value of developed model was 20.98, which is higher than the critical F-value, 2.4, for degrees of freedoms equal to 14 for model and 16 for error at significant level of 0.05. The probability ‘P’ value of regression model was much lower than 0.05 approving high significance of the regression model. At the same time, the Fisher’s F-value and probability ‘P’ value of lack-of-fit were lower and higher than 4.06 and 0.05, respectively. This implied that the lack-of-fit as difference between experimental results and model predictions except random error was not significant relative to the pure error and the developed model is adequate [20]. Furthermore, the total determination coefficient (R2) of 0.95% obtained from the ANOVA, indicates that only 5% of the total experimental variation is not explained by the developed model. According to the result shown in Table 3, evaluation of absolute probability ‘P’ value related to terms of the model indicated that the coefficients related to UV lamp, pH, Flux (L/h.m2) and UV lamp UV lamp terms were statistically are significant model terms at 95% confidence level. Moreover, a Pareto analysis was used to investigation of the percentage effect of each factor on the degradation efficiency as
Percentage effect (%)
Model predicted degradation efficiency (%)
The curve fitting of predicted and experimental of RhB degradation rate is shown in Fig. 4. The experimental data were not statistically different from their predicted counterparts from a minimum of 4.12% to a maximum of 95.00% degradation efficiency, which indicates a good fit between the experimental and predicted responses. According to Fig. 4, R2 value of 0.9473 was obtained from curve fitting of predicted and experimentally obtained Rhodamine B degradation efficiency, which exhibited a close fit between the experimental results and model calculation. Similar results were reported in other papers [15,18]. Fig. 4. The ANOVA was performed to assess the validity and adequacy of the model and the effect of operational parameters and their interactions. 95% of confidence level was considered to be significant. The ANOVA data are shown in Table 3. It is well-known that a corresponding variable will be more significant if its Fisher’s F-value is higher than the critical F-value for related degrees of
Table 3 Analysis of variance (ANOVA) for response surface quadratic model.
40.0
100 90 80 70 60 50 40 30 20 10 0
20.0 0.0 0.0
20.0
40.0
60.0
80.0
100.0
Experimentally obtained degradation efficiency (%) Fig. 4. Comparison plot between the experimental and model predicted degradation efficiency (%).
Fig. 5. Pareto chart of standardized effects for the model of RhB degradation process.
V. Vatanpour et al. / Chemical Engineering and Processing 116 (2017) 68–75
shown in Eq. (6). 2
n X
b2i Þ 100
Effect of Factorð%Þ ¼ ðbi =
ð6Þ
i¼1
In this equation, bi is the regression coefficient of the model and n is the number of studied factors [21]. According to the obtained result from the Pareto analysis shown in Fig. 5, among the variables, UV lamp and flux of polluted water in reactor created the main effects on the RhB degradation process. 3.3. Effect of pH and TiO2 dosage on RhB degradation process To understanding both the main and the interaction effects of the TiO2 dosage and pH on RhB degradation process, 3D surface plot and 2D contour plot of degradation efficiency (%) as a function of these factors is drawn using Eqs. (4) or (5) at a time, while polluted water flux and numbers of used UV lamps were kept at fixed middle (zero coded) level of about 140 L/h m2 and 2 lamp, respectively (Fig. 6). The pH is an important operating factor that can influence the heterogeneous photocatalytic degradation efficiency. The pH can affect in efficiency of reactive radicals development, oxidative property of developed radicals as well as tendency of catalyst particles for pollutants adsorption [22]. The obtained results, which are shown in Fig. 6, indicated that at all the degradation efficiency was increased with increasing pH up to about 7–8. The pH of zero point charge (pHzpc) of TiO2 is 6.25 [23]. This means that the TiO2 nanoparticles are nearly not surface charged at pH of 6.25. The nanoparticles surfaces become positive or negative when pH is below or above this value, respectively. At pH of 5, the surface of TiO2 in solution is positively charged. Accordingly, tendency of RhB cations to migrate and adsorb on the surface of TiO2 particles is
73
low where reactive radicals are developed, which cause decrease in photocatalytic degradation efficiency. At the pH between 7 and 8, the TiO2 surface carries a negative charge which led to increase in tendency of TiO2 particles for adsorption of RhB cations. However, more increase in pH of suspension up to 9 led to decrease in degradation efficiency which can be attributed to decrease in oxidation potential of developed reactive radicals [22]. As can be seen in this figure, presence of TiO2 led to increase in the degradation efficiency of RhB solution. This increase in the photodegradation efficiency is due to heterogeneous photocatalytic degradation process. Though, more increase in the TiO2 dosage leads to decrease in the color removal. When the TiO2 dosage exceeds, the suspension becomes too thick and may cause light barrier to admit enough UV light for all photocatalysis nanoparticles [24]. Furthermore, at high TiO2 dosage, nanoparticle aggregation phenomenon may decrease effective surface area of photocatalyst [24]. In addition, parallel lines of contour plot and surface plot shown in Fig. 6 mean that the interaction effect between applied pH and TiO2 dosage is not significant. This was also confirmed by the probability ‘P’ value of 0.3205 through ANOVA. 3.4. Effect of light intensity and polluted water flux on RhB degradation process Effects of RhB solution flux and number of applied UV lamp(s) in the PMR on the photocatalytic degradation efficiency are illustrated using 3D surface plot and 2D contour plot in Fig. 7, while the TiO2 dosage and pH were 100 mg/L and 7, respectively. As shown in Fig. 7, in the slower permeate flux corresponding to higher residence times (volume of the storage tank divided by the permeate flow rate), the degradation efficiency was enhanced due
Fig. 6. The response surface plot and contour plot of the degradation efficiency (%) as the function of pH and TiO2 dosage.
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Fig. 7. The response surface plot and contour plot of the degradation efficiency (%) as the function of number of applied UV lamp(s) and polluted water flux.
3.5. Determination of optimal conditions for degradation of RhB The optimization was done to determine the optimum values for degradation of RhB solution, using the membrane photocatalytic process. Optimum values of the factors were obtained by numerical analyses of the polynominal models of process (Eqs. (4) or (5)). The desired goal in degradation efficiency (%) as dependent factor was defined as “maximize” to achieve best treatment execution. The optimum degradation efficiency (%) was achieved at TiO2 dosage, number of UV-C lamp, solution pH and RhB solution flux in PMR of 0.1 g/L, 3, 8 and 100 L/h m2, respectively. The predicted value for degradation efficiency (%) at optimum conditions was 92.5%, whereas the experimental corresponding value was 95.0%. As mentioned, reducing of membrane flux could cause to high degradation efficiency. However, as can be seen in Fig. 7 of manuscript, by reducing in solution flux from 100 L/h m2 to 60 L/h m2, very low increase in the RhB degradation efficiency was resulted. Therefore, in this work, 100 L/h m2 was selected as optimum solution flux due to economic consideration and production of higher treated water.
3.6. Comparison of optimized TiO2/UV/PVDF process with UV/PVDF process In order to compare the performance of TiO2/UV photocatalytic process with only UV irradiation on degradation of RhB, supplementary experiments in the absence and presence of 0.1 g/L TiO2 were conducted, while the number of UV-C lamp, solution pH and RhB solution flux in the PMR were in the optimized condition of 3, 8 and 100 L/h m2, respectively. According to the obtained results illustrated in Fig. 8, a constant degradation efficiency of 84.9% was yielded in the presence of 3 UV lamps without TiO2 after 200 min. The photocatalytic decomposition of RhB under UV irradiation resulted in 95% degradation efficiency. Comparison of the obtained result indicated that the oxidative ability of UV light is notable at optimum condition. By the way, TiO2 addition led to increase in the formation of highly reactive radicals especially OH and accordingly enhance in degradation efficiency.
100 90 Degradation effiiciency (%)
to the increasing contact time among the pollutants and the produced radicals. Accordingly, reaction opportunity between RhB molecules and the developed reactive radicals on the surface of TiO2 and/or bulk of solution was improved [25]. It is found that another very important factor that controls photocatalytic reaction rate is irradiated light intensity. As can be seen in Fig. 7, increment in the number of UV lamp(s) corresponding to UV light intensity enhanced degradation efficiency, which can be attributed to increasing probability of photocatalytic production of reactive radicals as well as efficiency of direct photolytic degradation of RhB molecules [26]. The similar result was also reported for photodegradation of iopromide using UV/S2O82/H2O2 process [27].
80 70 60 50 40 30 20
TiO2/UV/PVDF process
10
UV/PVDF process
0 0
50
100 150 Time (min)
200
250
Fig. 8. Ability of TiO2/UV/PVDF and UV/PVDF process on degradation of RhB (3 UV-C lamps, solution pH = 8, RhB solution flux = 100 L/h m2).
V. Vatanpour et al. / Chemical Engineering and Processing 116 (2017) 68–75
The porous structure and considerable surface area of TiO2 nanoparticles is beneficial for the improvement of photodegradation. In the beginning of TiO2 nanoparticles/UV/PVDF process, adsorption of RhB on the unoccupied surface of photocatalyst led to considerable enhancement of RhB removal efficiency in photocatalytic process rather than the photolysis. However, the number of occupied sites on the TiO2 nanoparticles increased with time; thus, role of adsorption was decreased. In this work, due to use of high portion of UV light eradiated from applied lamps in Rhodamine degradation, these lamps were immersed in dye solution. Accordingly, efficiency of direct photolysis of Rhodamine was considerable. By the way, as can be seen in Fig. 8, the TiO2 nanoparticles led to more than 10% enhancement in RhB degradation efficiency. 4. Conclusion The present study investigated and approved the ability of TiO2/ UV/PVDF process in a continuous pilot-scale submerged membrane photocatalysis reactor on RhB degradation. Response surface methodology has allowed the design of a range of experiments to investigate the effect of TiO2 dosage, number of UV-C lamp, solution pH and RhB solution flux on degradation of RhB and develop a mathematical model having the best fit to the data obtained from the experimental design. The optimum conditions and result for degradation of RhB using the continuous pilot-scale SMPR were successfully obtained and showed ability of this system for wastewater treatment. Acknowledgment The authors thank the Kharazmi University, Iran, for financial and other supports. References [1] M.N. Chong, B. Jin, C.W. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997–3027. [2] M. Sheydaei, A. Khataee, Sonocatalytic degradation of textile wastewater using synthesized g-FeOOH nanoparticles, Ultrason. Sonochem. 27 (2015) 616–622. [3] N. Daneshvar, M.H. Rasoulifard, A.R. Khataee, F. Hosseinzadeh, Removal of C.I. Acid Orange 7 from aqueous solution by UV irradiation in the presence of ZnO nanopowder, J. Hazard. Mater. 143 (2007) 95–101. [4] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B 49 (2004) 1–14. [5] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater. 170 (2009) 560–569. [6] B. Ayoubi-Feiz, S. Aber, M. Sheydaei, Effect of oxidants on photoelectrocatalytic decolourization using a-Fe2O3/TiO2/activated charcoal plate nanocomposite under visible light, RSC Adv. 5 (2015) 19368–19378. [7] J. Byrne, B. Eggins, N. Brown, B. McKinney, M. Rouse, Immobilisation of TiO2 powder for the treatment of polluted water, Appl. Catal. B 17 (1998) 25–36.
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