Fe-ZSM-5 nanocomposite for the treatment of petroleum refinery wastewater: Optimization of process parameters by response surface methodology

Chemosphere 159 (2016) 552e564

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Preparation, characterization and photocatalytic application of TiO2/ Fe-ZSM-5 nanocomposite for the treatment of petroleum refinery wastewater: Optimization of process parameters by response surface methodology Zahra Ghasemi a, Habibollah Younesi a, *, Ali Akbar Zinatizadeh b a b

Department of Environmental Science, Faculty of Natural Resources, Tarbiat Modares University, Noor, P.O. Box 64414-356, Iran Department of Applied Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran

h i g h l i g h t s
Visible light active mesoporous TiO2/Fe-ZSM-5 photocatalyst was developed.
Photocatalytic degradation of PRWW was modeled and optimized using RSM.
The catalyst showed a mesoporous structure and anatase crystallites.
The catalyst showed good photocatalytic activity for PRWW degradation.
The catalyst exhibited good reusability for the degradation of PRWW.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 February 2016 Received in revised form 7 June 2016 Accepted 13 June 2016

Photocatalytic degradation of organic contaminants from petroleum refinery wastewater under UV and sunlight was investigated by immobilizing nanosized TiO2 photocatalyst into the structure of assynthesized Fe-ZSM-5 zeolite via sol-gel method. Pure phase of TiO2/Fe-ZSM-5 photocatalyst with specific surface area of 304.6 m2 g1 and loaded TiO2 of 29.28% was successfully synthesized. Effects of various operational parameters on treatment process were investigated by use of Response Surface Methodology (RSM). Maximum reduction of 80% COD was achieved at pH of 4, a photocatalyst concentration of 2.1 g l1, temperature of 45  C and UV exposure time of 240 min. Gas chromatography-mass demonstrated an apparent shift in molecular weight from a higher fraction to a lower fraction even under sunlight. It is expected that the prepared photocatalyst is able to use ultraviolet and visible light energy. Results indicated that removal of COD degradation did not decrease as the reuse cycle of photocatalyst increased. Moreover, the potential to use sunlight energy and the simplicity of operation make photocatalysis an attractive prospect in terms of petroleum refinery wastewater treatment. © 2016 Elsevier Ltd. All rights reserved.

Handling Editor: Jun Huang Keywords: Nano-TiO2/Fe-ZSM-5 RSM Petroleum refinery wastewater Sunlight Regeneration

1. Introduction Wastewater from petroleum refineries contains a high concentration of pollutants namely aliphatic and aromatic petroleum hydrocarbons. The toxic aromatic fraction poses a significant threat to environment because it is not destroyed easily by traditional treatments (Abdelwahab et al., 2009). Heterogeneous photocatalytic degradation is an attractive technology for advanced

* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Younesi). http://dx.doi.org/10.1016/j.chemosphere.2016.06.058 0045-6535/© 2016 Elsevier Ltd. All rights reserved.

treatment of petroleum refinery wastewater (PRWW). It can completely destroy a wide range of organic pollutants under moderate conditions (Diya’uddeen et al., 2011). This system is characterized by the generation of a hydroxyl radicals (
OH) with a high oxidation potential of þ2.8 V (Soon and Hameed, 2011). They are characterized as little selectivity of attack, which is a useful attribute for an oxidant when a multicontaminated wastewater such as the PRWW is considered (Saien and Nejati, 2007). Due to the point that TiO2 has various merits, such as optical and electronic properties, low cost, high photocatalytic activity, high photostability, high chemical stability and non-toxic nature, anatase form

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of TiO2 is the best semiconductor for practical wastewater treatment (Alwash et al., 2013). The heterogeneous photocatalytic degradation process using TiO2 as a popular photocatalyst has gradually attracted more interest and been widely used for the degradation of persistent organic pollutants and converting them to less toxic and relatively more biologically degradable substances (Chin et al., 2010). Photocatalytic degradation process using TiO2 offers some advantages, including no formation of sludge, complete conversion of contaminants to relatively harmless end products, reuse of catalyst TiO2, and removal of both organic and inorganic pollutants simultaneously. In recent years, development of new complex molecular assemblies as photocatalysts with improved efficiency is an active area of research. The particular focus of the most studies has been on nanosized anatase TiO2 particles, which have relative high photocatalytic activity compared to other form of
ska and Walendziewski, 2005). TiO2 (Galin It has been reported that ZSM-5 and transition metal modified zeolites exhibit high photocatalytic activity under UV irradiation for oxidation of organic compounds (Yan et al., 2008). The incorporation of heteroatoms such as iron can make the zeolite a photocatalyst (Yan et al., 2004). The photocatalytic activity of Fe-ZSM-5 is caused by the isolated Fe in zeolite framework. The charge transfer excited state of the isolated iron formed in Fe-ZSM-5 framework could have the same role as that of photogenerated electron-hole pairs on the semiconductors (Ohno et al., 2008). It ws found that impurity iron in the structure of HZSM-5 was responsible for the photocatalytic oxidation of several organic compounds (Yan et al., 2004, 2008) and more increase of iron content causes more increase of photocatalytic activity. Therefore, Fe-ZSM-5 as a support of TiO2 would affect the overlap of the conduction band due to the existence of Fe(d) orbital of iron in Fe-ZSM-5 framework and Ti(d) orbital of TiO2. The charge transfer of the 3d electrons from Fe3þ ions in Fe-ZSM-5 structure to TiO2 conduction band could decrease the band gap of TiO2 and increase visible light absorption ability of photocatalyst that increases with Fe3þ nominal concentration. The goal of the present work was to develop a novel complex assembly by a microporous matrix of Fe-ZSM-5 zeolite and TiO2 nanoparticles with high photocatalytic efficiency for the treatment of real PRWW without any pre-treatment. It is believed that the unique characteristics of Fe-ZSM-5 as support of TiO2 would effectively enhance the photocatalytic performance of this type of new complex zeolite based photocatalyst. The as-prepared nanoTiO2/Fe-ZSM-5 photocatalyst was characterized by various complementary techniques. Moreover, central composite design (CCD) under response surface methodology (RSM) was applied to evaluate the effects of four variables of pH, photocatalyst concentration, UV exposure time and temperature on the photocatalytic treatment of wastewater.

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filtered and washed with distilled water until the pH of washing water dropped to 7.0. The products were dried at 110  C in an oven for 24 h and then calcined at 450  C for 4 h at a heating rate of 3  C min1. The schematic figure of different stages of hydrothermal synthesis of Fe-ZSM-5 zeolite is shown in Fig. S1. 2.2. Synthesis of TiO2/Fe-ZSM-5 photocatalyst To prepare the TiO2/Fe-ZSM-5, the Na-form of synthesized FeZSM-5 was first converted into an H-form using ammonium chloride (ACS reagent, Merck) solution (S1). Then, nanometer size TiO2 was loaded on an H-form Fe-ZSM-5 via a sol-gel method using acid as a catalyst, which catalyzed hydrolysis of titanium (IV) isopropoxide in dry 2-propanol as solvent with hydrochloric acid (Ko et al., 2009). Typically, 10 ml of 0.1 M hydrochloric acid (37%, Merck) was added to 440 ml of anhydrous 2-propanol (ACS reagent, Merck) under stirring. 50 ml of 0.5 M titanium (IV) isopropoxide (Merck) dissolved in 2-propanol was added dropwise to the above solution under vigorous stirring at 0  C. The solution was stirred for 6 h to allow the acid-catalyzed hydrolysis of titanium (IV) isopropoxide in 2-propanol. Finally, 5 g of prepared H-form Fe-ZSM-5 was added to the as-prepared titania sol under stirring for 4 h. The solvent was then removed under vacuum by rotary evaporation. The photocatalyst was dried at 110  C and then calcined at 550  C for 4 h at a heating rate of 5  C min1 under air to induce the crystallinity of TiO2 on zeolite. The schematic representation of different stages of the synthesis of TiO2/Fe-ZSM-5 is shown in Fig. S2. 2.3. Photocatalyst characterization

2. Material and methods

The powder X-ray diffraction (XRD) analysis of Fe-ZSM-5 zeolite and TiO2/Fe-ZSM-5 photocatalyst were performed by a PW1800 Philips X’Pert diffractometer using CuKa as radiation. The X-ray wavelength derived from a copper anode (CuKa) is l ¼ 1.54 Å and the data was collected in 2q range 5e70 with a step size of 0.02 /s. The synthesized materials were characterized by X-ray fluorescence (XRF, Philips, Spectrometer PW2404), scanning electron microscopy (SEM, Philips, XL30, operated at 30 kV) and Fourier transform infrared (FT-IR) spectrophotometer (Shimadzo, FTIR1650, Japan) with a wave number range of 400e4000 cm1. The nitrogen adsorption/desorption isotherms were measured at 77 K using a conventional volumetric apparatus (BET, Bel Japan, Inc.). The Brunauer-Emmett-Teller (BET) method was used for the determination of surface area. The loss of ignition (LOI) test was carried out following the SIRIM procedure (ISO 3262-1975). UVeVis absorption spectra of products were recorded by a V/650 spectrophotometer (Jasco Inc., Japan) with a wavelength range from 200 to 1000 nm. However, the outdoor light measurement showed that a sunlight exposure of between 2000 and 3500 lux was attained between 9 a.m. and 3 p.m.

2.1. Synthesis of Fe-ZSM-5 zeolite

2.4. Wastewater characteristics

A mixture of 10SiO2:0.0493 Fe2O3:1TPABr:3Na2O:500H2O was prepared for the hydrothermal synthesis solution. Sodium silicate (Carlo Erba) was dissolved in deionized water. On the other hand, tetrapropylammonium bromide (Merck) was dissolved in deionized water and heated at 50  C for 20 min. The solution of TPABr was added to that of sodium silicate solution and stirred for 15 min. The iron (III) nitrate (nonahydrate 98%, Sigma-Aldrich) was dissolved in 24.9 g of deionized water. After the solution got clear, the combined solution of sodium silicate and TPABr was slowly poured into the latter solution followed by vigorous stirring. The resultant mixture was transferred into a stainless steel autoclave placed vertically in an oven at 170  C for 72 h. The solid product was

To evaluate the performance of TiO2/Fe-ZSM-5 photocatalyst, an oily PRWW influent to a dissolved air flotation (DAF) unit at the Bandar Abbas refinery plant, southern Iran, was a field test sample. The wastewater was stored at 4  C. Table 1 summarizes the average and standard deviation (SD) parameters of PRWW measured according to the standard methods for the examination of water and wastewater (APHA, 1998). 2.5. Photoreactor configuration The experimental setup of the photoreactor designed for the treatment of PRWW is presented in Fig. 1. The capacity of the

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Table 1 Physicochemical characteristic parameters of petroleum refinery wastewater. Characteristics

Range

Average ± SD

pHa Total CODb Soluble CODb Total BOD (TBOD)b Total solids (TS)b Total dissolved Solids (TDS)b Total suspended solids (TSS)b Fixed suspended solids (FSS)b Volatile suspended solids (VSS)b SO2b 4 b PO3 4 eP b Cl b NO 3 eN TKNb ECc DOb Turbidityd

7.63 602 247 392 1725 1624 101 79 22 114 0.275 12.5 0.496 40.32 1983 4.40 96.36

7.63 ± 0.5 602 ± 30 247 ± 12.5 392 ± 20 1725 ± 86.3 1624 ± 81 101 ± 5 79 ± 4 22 ± 1.1 114 ± 5.5 0.275 ± 0.02 12.5 ± 63 0.496 ± 0.025 40.32 ± 2.02 1983 ± 100 4.4 ± 0.22 96.36 ± 4.82

a b c d

dimensionless. mg/l. ms. NTU at 25  C.

photoreactor was about 1000 ml and placed in a water bath which provided an adjustable temperature. The radiation source was a mercury UV lamp (Philips, 8W BLB T5, Netherlands) which was vertically located at the center of the reactor (S2). The reactor vessel was placed on a magnetic stirrer for well mixing of the water surrounding the lamp inside the reactor and making the photocatalyst particles suspended along the quartz tube to achieve an average equal exposure. An air pump located below the reactor equipped with a fine bubble diffuser provided dissolved oxygen supply for photocatalysis in the reaction system with a constant air flow rate of 2.0 l min1. For photocatalytic degradation, the pH of wastewater was adjusted with 1.0 M NaOH (Merck) and 1.0 M HCl (Merck). After adjustment of temperature, the reactor was covered with a

protective wood shield and then the UV irradiation was started. 2.6. Analytical methods Samples (5 mL) were taken before and after UV exposure at the specific time. Thereafter, the samples were allowed to settle for a period of 60 min for separation of the photocatalyst particles. The concentrations of chemical oxygen demand (COD) of the samples were measured according to standard methods (APHA, 1998). The digestion of samples accomplished by an ECO16 Thermoreactor (Velp Scientifica, UK) at 150  C for 120 min and a Plaintest system, Photometer 8000 (UK) at 600 nm was used to measure CODs. To identify the present organic compounds in the wastewater and determine the efficiency of degradation under optimum condition, 250 mL samples of wastewater were taken before and after treatment under optimum condition and analyzed using a 7890A, Agilent Technologies (HP, USA) gas chromatograph (GC) coupled to 5975C, Agilent Technologies (HP, USA) mass detector (MS). The extraction procedure for wastewater samples was manual separatory funnel liquid-liquid extraction as described in US EPA SW-846 Method 3510C (EPA, 1996). The column used for detecting the organic compounds by GC-MS was HP-5 MS column of 30 m length, 0.25 mm inner diameter and 0.25 mm film thickness. An injection volume of 1 mL with split less capillary injection system was used for analysis of samples. The operating conditions were as described in S3. 2.7. Experimental design and mathematical modeling The experimental data were statistically and graphically analyzed using the Design Expert Software 7.0 (DOE, Stat-Ease Inc., Minneapolis, MN, USA). The CCD under RSM was used to characterize the interaction between the process variables and the response. Preliminary experiments were performed for determination of the independent variables and their experimental ranges to design the experimental runs. Based on the literature and the

Fig. 1. The experimental setup of the photoreactor designed for the treatment of PRWW.

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preliminary results, the four most important operating variables, which affect the efficiency of a photocatalytic process are pH (A), photocatalyst concentration (B), temperature (C) and UV exposure time (D). These parameters were considered as the system independent variables and COD removal efficiency was calculated as the process response (Amini et al., 2008, 2009). A design of 30 experiments was planned for 16 factorial points (24), 8 axial points (2  4) and 6 replicates at the center point. Independent variables and their experimental ranges and levels are given in Table S1. The experimental conditions designed by the DOE software are shown in Table 2 by order of runs. The following empirical second-order polynomial model (Eq. (1)) explains the behavior of the system:

Y ¼ b0 þ

k X

bi xi þ

i¼1

k X

bii x2i þ

i¼1

k1 X k X

bij xi xj þ ε

(1)

i¼1 j¼2

where, Y is the predicted response, xi, xj,. .,xk are the input variables which affect the response Y, x2i; x2j;. .; x2k are the square effects, xixj, xixk and xjxk are the interaction effects, b0 is the intercept term, bi (i ¼ 1,2. ., k) is the linear effect, bii (i ¼ 1,2 …, k) is the squared effect, bij (i ¼ 1, 2 …, k; j ¼ 1, 2 …, k) is the interaction effect and ε is a random error. Regression analysis was employed in order to fit the response function (the second-order polynomial equation) to the experimental data and to recognize the relevant model terms. The finest model and the significance of each coefficient were determined by analysis of variance (ANOVA), including sequential F-test, lack-of-fit test and other adequacy measures. The larger the magnitude of the F-value and therefore the smaller the P-value, the more significant of the corresponding coefficient (Daneshi et al., 2010).

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3. Results and discussion 3.1. Characterization of the photocatalyst The XRD patterns of Fe-ZSM-5 and TiO2/Fe-ZSM-5 are shown in Fig. 2a and b. The existence of peaks at 2q ¼ 7.94, 8.01 and 8.90 and 2q ¼ 23e25 , which correspond to the Fe-ZSM-5 and MFI framework, respectively, confirmed the presence of Fe-ZSM-5 crystal phase (Treacy and Higgins, 2007). Fig. 2a confirms that the pure form of Fe-ZSM-5 with no impurity phase was prepared and no diffraction peak to any iron oxides species was observed. The high crystallinity of synthesized zeolite was confirmed by high intensities of all peaks and low background lines. The appearance of a strong diffraction peak at 2q ¼ 25.3 in Fig. 2c in compare to the XRD pattern of pure TiO2 (Degussa P25, Sigma-Aldrich) in Fig. 2b is characteristic of anatase form of TiO2 (Liu et al., 2012). The average crystal size of anatase TiO2 loaded on Fe-ZSM-5 was ~14 nm, calculated by applying the Scherer’s equation on the anatase diffraction peak at 2q ¼ 25.3 (S4). The synthesis method employed in this study has successfully loaded nanosized TiO2 crystals on Fe-ZSM-5. The photogenerated electrons have to travel shorter in the nanosized TiO2 crystals until they reach the surface reaction sites of the photocatalyst (Kudo and Miseki, 2009). The XRF results, shown in Table 3, indicated that the SiO2/Fe2O3 and Na2O/SiO2 ratios of the Fe-ZSM-5 were 9.34 and 0.025, respectively. After loading TiO2 on Fe-ZSM-5, the Na2O/SiO2 ratio and the TiO2 percentage of TiO2/Fe-ZSM-5 were 0.006 and 29.28, respectively. Considering the percentages of Fe2O3 and TiO2 in the synthesized photocatalyst, the amounts of the active components of Fe and Ti in the catalyst were calculated as 3.95 and 17.55%, respectively. To obtain additional information of hybrid

Table 2 Experimental design based on central composite design (CCD) used in this study. Run

Independent values

Responses

Real (coded) values

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

Observed values

Predicted value

Residual

A

B

C

D

YObs.

Ypred.

YObs.  Ypred.

3(þ1) 2(0) 2(0) 3(þ1) 2(0) 1(1) 4 (þa) 3(þ1) 1(1) 2(0) 1(1) 2(0) 2(0) 1(1) 3(þ1) 3(þ1) 3(þ1) 2(0) 2(0) 2(0) 2(0) 3(þ1) 3(þ1) 1(1) 2(0) 2(0) 1(1) 0(a) 1 1

8(þ1) 6(0) 6(0) 4(1) 10(þa) 8(þ1) 6(0) 4(1) 8(þ1) 6(0) 4(1) 2(a) 6(0) 4(1) 4(1) 8(þ1) 8(þ1) 6(0) 6(0) 6(0) 6(0) 8(þ1) 4(1) 8(þ1) 6(0) 6(0) 4(1) 6(0) 4 8

45(þ1) 55(þ a) 35(0) 25(1) 35(0) 45(þ1) 35(0) 45(þ1) 25(1) 35(0) 25(1) 35(0) 15(a) 25(1) 25(1) 45(þ1) 25( 1) 35(0) 35(0) 35(0) 35(0) 25(1) 45(þ1) 25(1) 35(0) 35(0) 45(þ1) 35(0) 45 45

240(þ1) 180(0) 180(0) 240(þ1) 180(0) 240(þ1) 180(0) 240(þ1) 240(þ1) 300(þa) 120(1) 180(0) 180(0) 240(þ1) 120(1) 120(1) 120(1) 180(0) 180(0) 60(a) 180(0) 240(þ1) 120(1) 120(1) 180(0) 180(0) 240(þ1) 180(0) 120(1) 120(1)

40.14 68.1 66.60 48.05 75.12 65.93 66.15 82.72 67.68 62.15 57.52 67.51 66.31 55.54 75.00 37.54 58.36 61.25 65.62 65.11 62.15 41.09 81.9 67.68 58.97 61.10 77.44 19.72 60.05 42.46

40.77 69.29 62.62 50.30 75.81 64.42 Ignore 80.48 68.51 63.33 58.04 Ignore 65.82 53.60 74.66 37.63 58.76 62.62 62.62 64.63 62.62 39.28 82.22 68.07 62.62 62.62 78.20 20.41 60.02 41.36

0.63 1.19 þ3.98 2.25 0.69 þ1.51 Ignore þ2.24 0.83 1.18 0.52 Ignore þ0.49 þ1.94 þ0.34 0.09 0.4 1.37 þ3 þ0.48 0.47 þ1.81 0.32 0.39 3.65 1.52 0.76 0.69 þ0.03 þ1.1

A, Photocatalyst Dose, mg/L; B, pH; C, Temperature, Deg.  C; D, UV expose, min.

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consists of a sharp knee at P/P0 lower than 0.1 due to the volume filling of micropores (MP). According to IUPAC classification, it is type I isotherm, characteristic of microporous solids. According to the MP pore size distribution plot (Fig. 3a), the mean pore size of zeolite was 0.6 nm, confirming that the zeolite had very regular pore channels in the microporous region. The structural and textural parameters of products are summarized in Table 4. Fig. 3b shows the typical N2 adsorption/desorption isotherm and the corresponding MP pore size distribution plot of photocatalyst, which exhibits different characteristics than the zeolite. It is a type IV isotherm curve with a hysteresis loop, characteristic of mesoporous solids (Yao et al., 2012). The hysteresis loop starting at P/P0 z 0.45 is an H2 type mainly because adsorption and desorption branches are parallel. H2 type hysteresis is the characteristic of mesoporous materials (Yao et al., 2012). After TiO2 loading on Fe-ZSM-5, the surface area of the photocatalyst decreases compared to the zeolite. Considering the IUPAC classification of adsorption isotherms, the photocatalyst had a larger average pore diameter than that of FeZSM-5 and can be classified as mesoporous materials (IUPAC definition: pore size 2e50 nm). This could be due to growth and deposition of TiO2 nanoparticles on the surface of photocatalyst without blocking the pore system of zeolite.

90 Adsorption Desorption

80 70

Table 3 The chemical compositions of as-prepared ZSM-5 zeolite and TiO2/Fe-ZSM-5 photocatalyst.

800

dVp/dd p

3

Fig. 2. The XRD patterns of synthesized and calcined Fe-ZSM-5 zeolite (a), pure TiO2 (Degussa P25, Sigma-Aldrich) (b) and TiO2/Fe-ZSM-5 photocatalyst (c).

Va, c m (ST P) g

-1

1000 60 50 40

600 400 200

30

0 0.0

20

Fe-ZSM-5 (%)

TiO2/Fe-ZSM-5 (%)

Na2O SiO2 Fe2O3 Al2O3 MgO CaO Pb TiO2 L.O.I.

2.13 83.87 8.98 0.275 0.060 0.090 0.034 nd 4.56

0.367 59.79 5.66 0.316 0.085 0.099 nd 29.283 4.4

0.8

10 0.0

1.2

1.6

2.0

2.4

0.2

0.4

0.6

0.8

1.0

1.2

Relative pressure (p/p o)

(a) 220

0.05

200

nd ¼ not detected.

0.04

-1

180

3

Va, c m (ST P) g

nanocomposite and loading of TiO2, the TiO2/Fe-ZSM-5 was characterized by elemental mapping at the microstructural level by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) and the elemental mapping images were supplied to prove a good dispersion of TiO2 nanoparticles throughout Fe-ZSM-5 (S5 and Fig. S3). The Si/Fe ratio evaluated by EDS, which computed according to the SiO2 and Fe2O3 percentages, decreased from 7.06 to 6.23 after TiO2 loading. This slight decrease is due to the replacement of the tetrahedral Si sites with Ti during the photocatalyst preparation. During the photocatalyst synthesis, the Ti species affect the zeolite framework and a further polymerization of the Ti species in the zeolite cavities takes place. The TiOþ 2 species and titanium in the form of TieOeTi are incorporated into the zeolite framework (Anandan and Yoon, 2003; Kim and Yoon, 2001; Liu et al., 1992). N2 adsorption/desorption isotherm of Fe-ZSM-5 zeolite (Fig. 3a),

0.4

d p, nm

dVp/dd p

Composition

160

0.03 0.02 0.01

140

0.00 0.8

120

1.2

1.6

2.0

2.4

2.8

3.2

d p, nm

100 80

Adsorption Desorption

60 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p o)

(b) Fig. 3. The N2 adsorption/desorption isotherm and the pore size distribution curve of prepared zeolite (a), and the N2 adsorption/desorption isotherm and the pore size distribution curve of prepared photocatalyst (b).

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Table 4 Surface texturing data of as-prepared ZSM-5 zeolite and TiO2/Fe-ZSM-5 photocatalyst. Sample

SBETa, m2/g

Dav.b, nm

Vmicc, cm3g1

Vmesd, cm3g1

Vtotale, cm3/g

Vmicf, %

Fe-ZSM-5 TiO2/Fe-ZSM-5

399.7 304.6

0.6 1.42

0.11 0.052

0.01 0.198

0.12 0.25

91.67 20.8

a b c d e f

BET surface area. Average pore diameter. Micropore volume. Mesopore volume. Total pore volume. Microporosity percentages ¼ (Micropore volume/Total pore volume) * 100.

This larger average pore diameter causes the exhibition of different characteristics of the typical N2 adsorption/desorption isotherm and the corresponding MP pore size distribution plot of photocatalyst than those of Fe-ZSM-5. The specific surface area of photocatalyst was 304.6 m2 g1. The large specific surface area of photocatalyst is a key role in photocatalytic activity and would increase the active sites and O2 adsorption. All these may contribute to the higher photocatalytic degradation of organic pollutants in refinery wastewater. Fig. 4a and b shows Fe-ZSM-5 SEM images, indicating the particle size of synthesized zeolite is within a narrow range of 1 mm. The morphology of the zeolite presents cube-like crystals with a clean and smooth surface. Fig. 4c and d shows SEM images of TiO2/ Fe-ZSM-5 photocatalyst. It is evident that the originally clean and smooth surface of zeolite particles (Fig. 4b) is covered with TiO2 nanoparticles (Fig. 4d). The small particles located on cubic FeZSM-5 particles attributed to nano-TiO2 anatase. As it is clear from the SEM image, the TiO2 particles are as nanoparticles attaching to the surface of zeolite and typical morphology of the

zeolite has not changed after titania loading. Moreover, Fig. 4c and d reveal that nano-TiO2 particles are well distributed on the surface of zeolite, slightly increased the particle size of zeolite and consequently unsmooth surface. To obtain additional confirmation of hybrid nanocomposite and loading of TiO2, the Fe-ZSM-5 and TiO2/ Fe-ZSM-5 were characterized by TEM (S6 and Fig. S4). The FT-IR spectrum of Fe-ZSM-5 (Fig. 5a) is typical of pentasil zeolites. The iron-oxo species are isolated tetrahedrally in zeolite framework as FeO4 units instead of AlO4 (Yan et al., 2008). The band at 462 cm1is attributed to the TeOeT (T ¼ Si, Fe) a symmetric stretch of the SiO4 and FeO4 internal tetrahedral (S7). The TiO2/FeZSM-5 spectra are shown in Fig. 5b. The peak at 3610 cm1, which corresponds to the stretching vibration of the surface eOH, shows a little weaker after loading of TiO2, revealing that a certain amount of eOH was demolished by TiO2 loading. Furthermore, a new absorption band covering a range from 859 to 1087 cm1, which corresponds to the stretching vibration of TieOeT (T ¼ Si, Fe, Ti), was appeared in the FT-IR spectra of photocatalyst as compared to zeolite. This is in line with the results from Liu et al. (Liu et al., 1992).

Fig. 4. Scanning electron microscopy images of Fe-ZSM-5 zeolite (a and b) and TiO2/Fe-ZSM-5 photocatalyst (c and d).

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1095

558

Transmmittance, %

a

1231 1654

a

200

917 1068

b

680 462

897

Absorbance, a.u.

b

300

400

500

600

700

800

Wavelength, nm

1008 Fig. 6. The UVeVis absorption spectra of prepared (a) zeolite and (b) photocatalyst.

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavelenght, cm-1 Fig. 5. The FT-IR spectra in the 400e4000 cm and TiO2/Fe-ZSM-5 photocatalyst (b).

1

range of prepared Fe-ZSM-5 zeolite (a)

However, the bending vibration of TieO bonds were observed in the range of 450e700 cm1 (Bineesh et al., 2010). The vibration peaks within 900e1068 cm1 are clearly resolved in the spectrum of photocatalyst. Xie et al (Xie et al., 2004) observed that the intensity of IR vibration bands in the region of 500e1000 cm1, corresponded to TieOeTi linkages in the TiO2 nanoparticles would be decreased and disappeared with the addition of silica, incorporating Ti atoms into the tetrahedral sites in the silica network. The disappearance of the peaks in the region of 900e1068 cm1 confirmed the presence of Ti atoms in photocatalyst and their incorporation into the zeolite framework. In addition, the intensity of symmetric vibration of SieOeSi at 792 cm1 decreased with TiO2 loading confirming the mentioned incorporation. This is ascribed to intense TieO stretching overlapped with SieOeSi vibration (Mahalakshmi et al., 2009). According to the results of the infrared spectra analysis, TiO2 reacts with the surface eOH of raw zeolite during the process of synthesis, and produced the new bonds such as TieOeSi and TieOeFe. The bond stress is strong enough to keep the TiO2 caught on the zeolite during the removal process by the TiO2-Fe-ZSM-5. Therefore, the photocatalyst is not a simple mechanical mixture of TiO2 anatase and zeolite but it is a nanocomposite regarding the formation of mentioned new bonds. The light absorption property of products is presented in Fig. 6. The strong bands recorded in the range of 200e300 nm (Fig. 6a) were attributed to Fe3þ in tetrahedral or octahedral coordination (Kowalska-Kus et al., 2013) (S8). The UVeVis absorption spectrum for photocatalyst is shown in Fig. 6b. The band at about 328 was characteristic for Ti4þspecies in large anatase titania particles (Vayssilov, 1997). The light absorption by TiO2/Fe-ZSM-5 was extended to 750 nm as compared to the zeolite (650 nm) and pure TiO2 (less than 400 nm). It shows the formation of an impurity energy level within the zeolite and TiO2 band gap, improving light absorption of photocatalyst in the visible light region. This, in turn, increased the number of photo generated electrons and holes and improved the photocatalyic properties of the TiO2/Fe-ZSM-5

(Izadyar and Fatemi, 2013). After loading TiO2, visible light absorption was increased. The absorption edge at 650 nm was redshifted relative to that of the zeolite at 524 nm and pure TiO2 at 390 nm (not shown for clearness). This shift is attributed to the charge transfer of the 3d electrons from Fe3þ ions to TiO2 conduction band (Carrettin et al., 2007). As the absorption edge is 650 nm for the photocatalyst, the band gap energy was calculated equal to1.91 eV (S9). This value indicates the photocatalyst’s potential for the light absorption in the visible region. 3.2. Photocatalytic treatment of PRWW In preliminary experiments, to examine the effect of UV irradiation on photodegradation of COD and adsorption of pollutants by photocatalyst, experiments were conducted both in the absence (dark condition) and presence of UV light. The effects of adsorption and photodegradation on COD removal at a photocatalyst concentration of 2.0 g l1, pH of 6.0, temperature of 35  C and UV exposure time of 180 min (Fig. 7) were investigated. Experimental data showed 61.35% COD removal in the presence of UV light under photocatalyst concentration of 2.0 g l1, while low COD removal of 4% was obtained in the absence of light. However, when the temperature increased from 25 to 45  C under the same aforementioned conditions, there was no significant change in the COD removal efficiency. The reason of low removal efficiency in the absence of light may be due to either very low adsorption of organic pollutants by the photocatalyst particles or the volatility of a part of light hydrocarbons due to the airflow. In subsequent experiments, the COD removal of about 19.7% was achieved in the presence of UV light and absence of photocatalyst under the above condition. The results indicate that photodegradation plays a more important role than adsorption process. All these results suggested that both UV light and photocatalyst are needed for effective treatment of wastewater. 3.3. RSM approach for maximal removal efficiency of COD Experimental matrix (coded and actual values) and results of central composite design for photocatalytic COD removal from PRWW using TiO2/Fe-ZSM-5 are given in Table 2. From Table 2, the

Z. Ghasemi et al. / Chemosphere 159 (2016) 552e564

parameters on the COD removal efficiency was discussed in S10, Table S2 and S3 and Fig. S5.

100 Under UV light Absence of light

COD removal, %

80

3.4. Effect of process variables on COD removal efficiency

60

40

20

0 0

50

100

150

200

250

300

Time, min Fig. 7. The COD removal of wastewater at pH of 6, temperature of 35  C and photocatalyst concentration of 2 g l1 under (a) absence of light and (b) UV light.

experimental runs 8, 23 and 27 resulted in the maximum COD removal of 82.72, 81.90 and 77.44%, respectively. The application of RSM gave percentage COD removal as function of photocatalyst concentration (A), pH (B), temperature (C) and UV exposure time (D) as expressed by Eq. (2): YCOD% ¼ þ 62.62e1.76A  7.42B  6.48AB þ 1.40AC  4.98AD  7.17BC þ 5.65CD  11.43A2 þ 7.01B2 þ 1.23C2 (2) Fig. 8 presents the predicted versus actual plot of COD photocatalytic degradation. Actual values were determined performing experiments for a particular run, and the predicted values were measured using Eq. (2) for the model. It can be seen from Fig. 8 that the predicted values from quadratic model equation fitted well within an acceptable variance range when compared to the responses from experimental results. The reliability of the RSM method in describing the effect of each variable on the photocatalytic degradation and the significance of the studied

83.00

Predicted value

67.00

51.00

35.00

19.00 19.72

35.47

559

51.22

66.97

Actual value Fig. 8. Predicted versus actual values plot for COD removal.

82.72

Fig. 9a shows the effect of photocatalyst concentration and UV exposure time on COD removal, while the other variable was set at the middle value. From this figure, the maximum removal efficiency was obtained at a photocatalyst concentration of 2.1 g l1 and also increased with an increase in the UV exposure time. Fig. 9b shows the interaction effect of photocatalyst concentration and temperature on COD removal efficiency, while the other variable was set at the middle value. From this figure, the maximum removal efficiency was obtained at a photocatalyst concentration of 2.1 g l1 and also increased with an increase in the temperature. These results are consistent with those reported in the literature for different organic materials (Chiou and Juang, 2007; Saien and Nejati, 2007). Many researchers have reported that organic removal efficiency decreases when the photocatalyst concentration increases beyond a certain limit. A possible explanation can be that the number of available active sites in solution increases with photocatalyst loading, while beyond the optimum concentration, the light transmission through the solution will compromise due to the increase of turbidity. It causes non-uniform light intensity distribution, so that with the increase of photocatalyst concentration, the photocatalytic degradation rate would be lower. On the other hand, the tendency toward agglomeration increases in high photocatalyst concentrations. This behavior would reduce the available surface area and active sites for light absorption and consequently would reduce the photocatalytic degradation efficiency (Saien et al., 2009). However, the photocatalyst surface and the absorption of light by photocatalyst particles are limited below the optimum level of the photocatalyst concentration. Another reason may be an almost total light absorption which is occurred by photocatalyst particles at an optimum concentration (Saien et al., 2009) and so, the maximum limit of photons absorption is reached within the photoreactor. The pH value plays an important role on photocatalyst surface charge, the mechanism and the rate of hydroxyl radical generation (Rubio-Clemente et al., 2014) and consequently on the rate of photocatalytic degradation of organic pollutants (Khodadoust et al., 2012). At a specific pH value, an organic compound can attain a positive or negative charge, as well as the neutral form in PRWW. Electrostatic interaction between photocatalyst surface, organic molecules and formation of charged radicals during the photocatalytic oxidation is strongly dependent on the pH of wastewater (Ahmed et al., 2010). According to the literature, different organic materials have different activity in the photocatalytic degradation process. Some organic materials are posed on fast degradation under acidic conditions, while others are degraded faster at higher pH, so it is important to determine the optimum pH for photocatalytic degradation (Ahmed et al., 2010; Diya’uddeen et al., 2011). Fig. 9c shows the interaction effects of pH and UV exposure time on removal of COD, while the other variable was set at the middle value. From this figure, the pH had a strong reverse effect on the response and the COD removal efficiency was increased with a decrease in pH value and increase of UV exposure time. The effect of pH on the photocatalytic reaction can be argued with the help of the point of zero charge of TiO2 (Chou and Liao, 2005) and the adsorption of the pollutants on the photocatalyst in different pH value (Evgenidou et al., 2005). The point of zero charge (pzc) of TiO2 is at pH~6.25 (Bahnemann et al., 2007). The surface of TiO2 is protonated under acidic condition (pH < 6.2), whereas it is deprotonated under alkaline conditions (pH > 6.2) as shown in the following reactions:

560

Z. Ghasemi et al. / Chemosphere 159 (2016) 552e564

240

45

Prediction

83.632

76.85

Prediction

180

72.3013

83.632

76.85

C: Temperature, oC

D: UV ex pos e, mi n

82.0496

210

81.3987 81.3987

67.7525

150

40

76.5448

35

65.5352 71.04

30

65.5352

60.0304

63.2038

60.0304

120

25 1.0

1.5

2.0

2.5

3.0

1.0

A: Photocatalyst concentration, g/l

1.5

2.0

2.5

3.0

A: Photocatalyst concentration, g/l

(a)

(b)

240

240

Prediction

83.632

D: UV ex pos e, mi n

D: UV ex pos e, mi n

67.6001

210

64.5602

80.98 75.5067 70.0335

180

59.087

150

210

Prediction

85.4735

72.0685

180

81.0051

76.5368

150

120

120 4

5

6

7

8

25

30

B: pH

35

40

45

C: Temperature, oC

(c)

(d)

8

45 82.947 Prediction

C: Temperature, oC

62.3213

54.7708

6

69.8719

5

83.632

78.4006

47.2202

7

B: pH

83.632

40 73.8542

69.3078 64.7614

35

30 64.7614

77.4225

Prediction

83.632

4

25 1.0

1.5

2.0

2.5

3.0

A: Photocatalyst concentration, g/l

4

5

6

7

8

B: pH

(e)

(f)

Fig. 9. The contour plots of COD removal efficiency as a function of photocatalyst concentration and UV exposure time (a), photocatalyst concentration and temperature (b), pH and UV exposure time (c), temperature and UV exposure time (d), photocatalyst concentration and pH (e), pH and temperature (f).

TiOH þ Hþ 4 TiOHþ 2 at pH < pzc

(3)

TiOH þ OH 4 TiO þ H2O at pH > pzc

(4)

Depending on the anionic or cationic form of the organic compound, the photo degradation efficiency was enhanced or inhibited

by the electrostatic attraction or repulsion, respectively between the photocatalyst’s surface and the organic molecule (Khodadoust et al., 2012). Moreover, protonation and deprotonation of the organic contaminants can happen depending on the solution pH. In addition, under UV-light TiO2 of the composite produces OH radicals, which in turn may increase the reaction rate in acidic

Z. Ghasemi et al. / Chemosphere 159 (2016) 552e564

3.5. Numerical optimization using the desirability function and model verification In the numerical optimization, the desirability function was applied to evaluate response in experiments to find optimum points where the desired condition could be obtained for the photocatalytic treatment of wastewater. The numerical optimization finds a point that maximizes the desirability value. In the present study, there were three series of optimum conditions. Table 5 shows the optimized condition and the obtained results for COD removal efficiency at their respective optimum points. In sets 1 and 2, the goal of all variables was set in experimental ranges and in the third one, the goal of photocatalyst concentration was set at “minimize” in order to reduce the photocatalyst consumption. Photocatalytic degradation of PRWW consequently yielded more than 80% removal of organic pollutants under the optimum conditions. In order to verify the optimum points generated by the model and to confirm the validity of the proposed model, three confirmatory experiments were performed under recommended optimum operating conditions by the software. As shown in Table 5, experimental COD removal efficiencies were in good agreement with the values predicted by the model. An error less than 5.00% is acceptable. Furthermore, the error for the optimum conditions was 3.9, 4.1 and 3.4%, respectively.

P25 (Sigma-Aldrich) as a reference. As can be seen in Fig. 10, TiO2/ Fe-ZSM-5 (including 0.6 g l1 TiO2) shows the highest photocatalytic activities compared to simple mix of zeolite and the Degussa P25 (including 1.49 g l1 Fe-ZSM-5 and 0.6 g l1 TiO2). This higher photocatalytic activity may be caused by the further synergistically enhancement of the photocatalytic performance of zeolite supported TiO2 photocatalyst. It is believed that loading of TiO2 nanoparticles on Fe-ZSM-5 as a support and the existence of iron ions in the framework of zeolite play a significant role in the photocatalytic removal of COD in the present study. This results in the enhancement of photo-degradation activity of TiO2/Fe-ZSM-5. Under UV light irradiation, the FeeO of isolated [FeO4] units may be excited to an excitation state of [Fe2þO]* (Yan et al., 2004) which in turn may activate both O2 and organic components, leading to oxidation of organic components by O2 (Yan et al., 2008). On the other hand, iron can act as both electron and hole traps leading to higher photoactivity of TiO2/Fe-ZSM-5. The Fe2þ 4 Fe3þreversible redox reactions may easily proceed in zeolite without being
za
r, 2013). The beneficial effect of removed from the framework (La Fe3þ may be explained by considering that the redox potential of Fe3þ/Fe2þ is positive (0.771 V) (Ohno et al., 2008), so Fe3þ ions can be easily reduced to Fe2þ by transfer of photogenerated electrons from TiO2 to Fe3þ. Thus, by photoactivity property of the Fe-ZSM-5 framework and by loading of the TiO2 nanoparticles as semiconductor oxides, the efficiency of photocatalyst was enhanced effectively. Photocatalyst recovery was also easier compared to the literature (Zhang et al., 2011) considering that no centrifuge or filtration process was needed and just 60 min sedimentation time was enough for photocatalyst separation. This merit is economically and environmentally attractive.

100

(a)

80

COD removal, %

condition (Chen et al., 2001) while in a strong alkaline environment such radical species are rapidly scavenged and the reaction rate decreases. In the present study, the process of COD removal was favored by the protonated surface of TiO2. The maximum removal efficiency was achieved at pH values around 4.0 and UV exposure time of 240 min due to strong electrostatic attraction between the cationic TiO2 and anionic organic pollutants. The interaction effect of pH and temperature on removal efficiency (Fig. 9d) shows that the maximum removal efficiency was achieved at pH value of 4.0 at 45  C. However, a removal efficiency of about 65% was occurred at pH 8.0 and temperature of 25  C. At pH 7 and above, the surface of TiO2 becomes negatively charged TiO2(OH) (Akpan and Hameed, 2009). It shows that a part of organic compound degradation was favored by the negative charge of TiO2. Fig. 9e shows interaction effect of temperature and UV exposure time, while the other variable was set at the middle value. This figure shows that an increase of the removal efficiency of the COD with a simultaneous increase in temperature and UV exposure time. The maximum removal efficiency was achieved at a temperature of 45  C and UV exposure time of 240 min. This is due to the transfers of TiO2 electron in valance bond to higher energy levels and hence facilitating the production of electron-hole pair.

561

(b)

60

40

20

0 0

50

3.6. Photocatalytic activity of nano-TiO2/Fe-ZSM-5 compared to commercial Degussa P25 Some experiments (under optimum condition) were conducted to evaluate the activity of the synthesized photocatalyst compared with a simple mix of zeolite and commercial Degussa

100

150

200

250

300

Time, min Fig. 10. Photocatalytic activity of (a) synthesized nano-TiO2/Fe-ZSM-5 (including 0.6 g l1 TiO2) and (b) simple mix of zeolite and commercial Degussa P25 (including 1.49 g l1 Fe-ZSM-5 and 0.6 g l1 TiO2). Experimental conditions: pH of 4, photocatalyst concentration of 2.1 g l1 and temperature of 45  C after 240 min.

Table 5 Optimum conditions and experimental results of photocatalytic treatment of petroleum refinery wastewater. No.

1 2 3

Variables

COD removal, %

Desirability value

pH

Photocatalyst concentration, mg/l

Temperature,  C

Time, min

Predicted value

Experimental value

4 4 4

2.1 2.1 1.3

45 45 45

210 240 240

83.57 84.55 83.08

80.44 81.24 80.39

1.000 1.000 1.000

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Z. Ghasemi et al. / Chemosphere 159 (2016) 552e564

3.7. Identification analysis

80

60

COD removal, %

Fig. 11 shows the obtained GC-MS chromatograms of untreated and treated samples under optimum conditions. Fig. 11a shows that the heavy fraction of petroleum hydrocarbons are predominant. The majority of the identified peaks consisted of aliphatic hydrocarbons, including n-branched and cycloalkanes; and polyaromatic hydrocarbons. In the present study, experiments were also performed on COD PRWW under optimum condition for solar photocatalytic oxidation in a batch reactor (Fig. 12). The outdoor experiments were carried out under natural sunlight on a clear sunny day in May. Fig. 11 shows that a high efficiency degradation of all organic pollutants were obtained under UV (Fig. 11b) and visible light (Fig. 11c). GC-MS analyses demonstrated that an apparent shift in molecular weight from a higher fraction to a lower fraction was observed under sunlight. Thus, the TiO2/Fe-ZSM-5 was able to degrade organic pollutants under UV and sunlight. The photocatalyst can effectively utilize visible light or solar energy. The effective overlap of the conduction band due to Ti(d) of TiO2 and the Fe (d) orbital of iron in the zeolite framework could decrease the band gap of TiO2 and increase visible light absorption ability of photocatalyst.

40

20

0 0

50

100

150

200

250

300

Time, min Fig. 12. COD removal under optimum condition and natural sunlight.

removal percentage was probably due to the accumulation of organic intermediates in the cavities and on the surface of the photocatalyst affecting the adsorption of organic pollutants, and reducing the activity of the photocatalyst (Gomez et al., 2013). The photocatalyst recovered after the third cycle was calcined at 600  C for 4 h and reused up to fifth cycle. At the end of the fourth cycle, the photocatalyst was reused for the fifth cycle via the same procedure employed after the first and second cycles of usage. As it can be observed in Fig. 13 (for the fourth and fifth degradation cycles), the original activity of the photocatalyst was restored after calcination at the end of the third cycle. Therefore, calcination is necessary to regenerate the activity of the reused photocatalyst after three cycles. These results clearly showed that the photocatalyst is stable and reusable for several treatment cycles without losing its original activity.

3.8. Photocatalyst recycling studies In order to examine the stability of nanoTiO2/Fe-ZSM-5 photocatalyst, recycling experiments were conducted under optimum condition. At the end of the first and second photocatalytic cycles, the photocatalyst was allowed to settle for a period of 60 min for separation of the photocatalyst particles and then the treated wastewater was discharged. The settled photocatalyst was then reused without any treatment. The results are presented in Fig. 13. A slight decrease in the COD removal percentage was observed at the end of the first, second and third cycles. The decrease in

3,5-di-tert-Butyl-4-hydroxyacetophenone C18 Benzaldehyde C17 C16 Phenol, 2,6-bis(1,1-dimethylethyl)-4-methyl

C19

C20

1,2-Benzenedicarboxylic acid

C22

C21

C25 C23 C24 C26

C15 2,6-di(t-butyl)-4-hydroxy-4-methyl-2,5-cyclohexadien-1-one

C27

2,6-di-butyl-2,5-cyclohexadiene-1,4-dione C28

C14

(a)

(b)

(c) 0

20

40

60

80

100

Retention time, min Fig. 11. The GC/MS chromatograms of the wastewater (a) before and (b) after the photocatalytic treatment under optimum conditions and UV light; and (c) sunlight.

Z. Ghasemi et al. / Chemosphere 159 (2016) 552e564 100

COD removal, %

80

60

40

20

0 1

2

3

4

5

Recycling activity, cycles Fig. 13. COD removal as a function of the number cycle for the TiO2/Fe-ZSM-5 photocatalyst. Experimental conditions: pH of 4, photocatalyst concentration of 2.1 g l1 and temperature of 45  C after 240 min.

4. Conclusion A novel photocatalyst nano-TiO2/Fe-ZSM-5 was successfully synthesized by immobilization of TiO2 into the structure of assynthesized Fe-ZSM-5 via sol-gel method. According to the results, the loaded TiO2 in the structure of Fe-ZSM-5 was in the form of anatase with a particle size of less than 20 nm. The prepared photocatalyst was a nanocomposite regarding the formation of TieOeSi new bonds. Photocatalyst was then applied for treatment of real PRWW using a designed photoreactor. The CCD under RSM was used for the design of the experiments and determination of optimum condition for efficient photocatalytic removal of organic pollutants from PRWW. The maximum COD removal of 80% was observed under photocatalyst concentration of 2.1 g l1, pH of 4, temperature of 45  C and UV exposure time of 240 min. The GC-MS analysis indicated that different petroleum compounds were degraded with high efficiencies under UV and visible light. This method has an industrially interest when regarded as a synergistic process for biological degradation or as the wastewater treatment of petroleum refinery after the oil and grease removal. Acknowledgement The study was funded by a grant Iran National Science Foundation (INSF grant No. 92011962), Iran Nanotechnology Initiative Council and the Tarbiat Modares University (TMU). The authors wish to thank Mrs Haghdoust for her assistant (Technical Assistant of Environmental Laboratory) and Tarbiat Modares University, Ministry of Science and National Science Foundation for their financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.chemosphere.2016.06.058. References Abdelwahab, O., Amin, N.K., El-Ashtoukhy, E.S.Z., 2009. Electrochemical removal of

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