Applied Clay Science 53 (2011) 553–560
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
ZnO/montmorillonite for photocatalytic and photochemical degradation of methylene blue Is Fatimah a,⁎, Shaobin Wang b, Dessy Wulandari a a b
Chemistry Department, Universitas Islam Indonesia, Yogyakarta, 55581, Indonesia Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 19 January 2011 Received in revised form 2 May 2011 Accepted 4 May 2011 Available online 31 May 2011 Keywords: Montmorillonite ZnO Photocatalysis Methylene blue dye degradation
a b s t r a c t Synthesis of a ZnO/montmorillonite photocatalyst based on an Indonesian natural montmorillonite was conducted using a sol–gel intercalation method. The physicochemical properties of the material were determined by XRD, N2 adsorption–desorption, SEM, TEM and UV–Vis diffuse reﬂectance. The activity was evaluated in photocatalytic and photochemical degradation of methylene blue (MB) with and without H2O2. Characterization showed that the ZnO particles were successfully distributed in montmorillonite support and ZnO/montmorillonite had lower band gap energy. The increased adsorption of MB on ZnO/montmorillonite resulted in faster photodegradation. The kinetics of the reaction obeyed the Langmuir–Hinshelwood model. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The presence of organic contaminants including dyes in wastewater has been an important problem in many countries since the textile industry is widely developed. The major issue related to these organic compounds in large quantities in wastewater is their chemical stability and low biodegradability in water systems, which is potentially harmful to the eco-environment. As a consequence, an effective and economic technique needs to be developed to reduce the concentrations of these contaminants before releasing the wastewater into the aquatic environment. Among the several techniques developed in wastewater treatment, advance oxidation processes (AOPs) are increasingly used as for the reduction of organic contaminants in a variety of wastewaters from different industrial plants. The AOPs are usually driven by hydroxyl radicals produced in the system (Bahnemann, 2004; Comninellis et al., 2008; Goswamee et al., 2001). The advantage of AOPs is the conversion of organic compounds to less toxic molecules. In perfect conditions, it is possible to oxidize the organic molecules completely to CO2 and H2O. Currently, H2O2 is reported as an effective oxidant for the production of hydroxyl radicals. Among several AOPs, heterogeneous photooxidation is presented as a low cost effective procedure for removing stable organic compounds including dye molecules. Heterogeneous photocatalysts trigger the formation of oxidant radicals through low energy photon absorption. The other beneﬁt of the heterogeneous
⁎ Corresponding author. E-mail address: [email protected]
(I. Fatimah). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.05.001
photocatalysis is the catalyst recycling that would minimize the use of the material and cost of the treatment (Kusic et al., 2006). Among metal oxide semiconductors, TiO2 is the most popular photocatalyst for removal of organic molecules. However, it has high band gap energy (3.2 eV) and no visible light sensitivity. Zinc oxide (ZnO) is also a material to be chosen as a low cost photocatalyst with, high photoactivity and comparable band gap energy (3.2 eV) to TiO2 (Aal et al., 2009; Schubnell et al., 1997). ZnO is suitable as an alternative photocatalyst for photobleaching of several dyes in bulk and suspension form, and sometimes exhibited higher activity than TiO2 (Behnajady et al., 2006; Daneshvar et al., 2004; Daneshvar et al., 2007; Lizama et al., 2002). However, ZnO is unstable in acid conditions and shows rapid deactivation in bulk use due to aggregation (Mihai et al., 2010). Previously, several researchers have attempted to prepare ZnO in alloy or composite forms by attaching or impregnating ZnO into a stable inorganic support such as activated carbon, MCM-41, SBA silica, and zeolites (Byrappa et al., 2006; Jeon et al., 2004; Silvestre-Albero et al., 2008; Zhai et al., 2010; Zhi et al., 2010). Attachment of ZnO in an inorganic support can extend life and reusability of the photocatalyst. An improvement in ZnO photocatalytic activity due to quantum size can also be obtained for effective absorption of photons (Baruah et al., 2009; Su et al., 2008; Viswanatha et al., 2004). Smectite clay is an important clay mineral having a unique structure related to its functional properties. Many works have reported the application and kinetic study of dye adsorption on various clays and their modiﬁed forms. Montmorillonite has been considered a potential adsorbent toward dyes. In pure form, montmorillonite demonstrates a high ability to adsorb dye molecules via a cationic exchange and molecular sieve mechanism (Liu and
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Zhang, 2007). This adsorptive property is an advantage to enforce heterogeneous photoactivity when a semiconductor is immobilized. Previous investigations of Meng et al. (2008), Zhang et al. (2010), and Kun et al. (2005) observed an increase of photoactivity by dispersing TiO2 in montmorillonite supports. Results of these investigations suggested that the porous structure and high speciﬁc surface area of montmorillonite were beneﬁcial to photoactivity via enhancing adsorption, which is the determining step in the heterogeneous photocatalytic reaction. Therefore, a combination of adsorption and heterogeneous photocatalysis makes photooxidation more effective for the removal of dye compounds from wastewater. In the present work, synthesis of a ZnO/montmorillonite and its activity were reported. Photooxidation of methylene blue was chosen as a reaction model. Similar to other previous works of metal oxide immobilization into clay structure, the preparation of ZnO/montmorillonite adopted a sol–gel method. The sol–gel solution was believed to reduce the acidity, which could damage clay structure as reported in the preparation of TiO2/montmorillonite. The sol–gel method of ZnO dispersion was also reported by Mihai et al. (2010). In general, before ZnO dispersion a pre-intercalation process of montmorillonite was taken as reported in TiO2/montmorillonite system (Ding et al., 2008; Manova et al., 2010). The preintercalation of a montmorillonite was achieved using cetyl trimethyl ammonium chloride (CTMACl) to create hydrophobic properties of the montmorillonite surface and also to expand the interlayer of the montmorillonite for metal oxide dispersion. The prepared material was characterized by X-ray diffraction (XRD), BET surface area analyzer, scanning electron microscopy with Energy Dispersive Analysis (SEM–EDS), transmission electron microscopy (TEM) and UV–Visible diffuse reﬂectance spectrophotometry (DRUV–Vis).
2. Materials and methods The montmorillonite sample supplied by PT. Tunas Inti Makmur, Indonesia was ﬁrstly activated with 0.5 M H2SO4 solution under reﬂux for 6 h. The solid was ﬁltered and washed with double distilled water until the ﬁltrate pH was 7. The chemicals used, zinc chloride (ZnCl2·2H2O), NaOH, H2SO4, cetyl trimethyl ammonium chloride (CTMACl), methylene blue and H2O2 were supplied by Merck. Before metal dispersion, a clay suspension was prepared by diluting 5 wt.% montmorillonite powder in aquadest with the addition of CTMACl drop by drop at 2.5 mmol CTMACl/g-montmorillonite stirred at room temperature overnight. A pillarization agent of zinc chloride was prepared by mixing zinc chloride and NaOH at 1:1 M concentration in H2O:isopropanol (50:50) solution with vigorous stirring for 4 h. The Zn 2+ precursor solution was then dropped slowly into the montmorillonite suspension to achieve a 5.0 wt.% Zn loading. The mixture was stirred overnight and then was ﬁltered and washed using deionized water until the ﬁltrate had pH 7 and was free from Cl − ions (AgNO3 test). The solids were dried in an oven for overnight and calcined at 500 °C for 4 h under N2 ﬂow in order to remove carbonaceous material which might have been deposited in pores and to further remove impurities from material. The obtained sample was ground to 200-mesh and then was encoded as ZnO/montmorillonite. X-ray diffraction (XRD) patterns of powder samples were obtained with a Shimadzu X6000 diffractometer using Ni-ﬁltered Cu Kα radiation (λ = 1.54 Å) and scanning speed 5°/min DRUV–Vis spectra of the samples were recorded on a JASCO V-670 spectrophotometer equipped with a diffuse reﬂectance accessory; the % R values were transformed via the Kulbelka–Munk formula. A gas sorption analyzer (NOVA 1200e) was used to determine the speciﬁc surface area and adsorption–desorption proﬁle using N2 sorbate gas at 196 °C. Elemental analysis was performed with a Seico EDX. Photo-oxidation was investigated, with a UV–Visible spectrophotometer (HITACHI U-2080) and Shimadzu HPLC.
2.1. Photocatalytic activity Photooxidation reaction of methylene blue (MB) was carried out in a 500 mL glass beaker in a thermo-controlled water bath. The beaker was charged with 250 mL aqueous solution of MB at varying concentrations and 0.2 g/L catalyst. The reactor was equipped with an ultraviolet lamp at and was held in a closed box. The radiation source was a Philips 20 W UV-B lamp of placed on top at a distance of 40 cm from the glass beaker. Water from the thermostatic bath was circulated through a reactor jacket to ensure a constant temperature of 25± 0.5 °C inside the reactor. In order to carry out the experiments under aerobic conditions, the solution was saturated by bubbling air at atmospheric pressure. H2O2 solution was added into the reactor at a molar ratio of 1:5 (H2O2:MB) before stirring and photon exposure. The concentration of MB was determined using a spectrophotometer (UV–Visible HITACHI U-2010) at 663 nm. HPLC analysis was also employed using a Shimadzu HPLC with acetonitrile:H2O (50:50) as a mobile ﬂow. 3. Results and discussion 3.1. Material characterization XRD patterns of raw montmorillonite, acid activated montmorillonite, CTMA-intercalated and ZnO/montmorillonite are presented in Fig. 1. The (001) diffraction peak of montmorillonite occurs at 6.3° 2θ (d001 =14.9 Å). Other reﬂections are at 2θ=19.89° and 35.6° corresponding to (hk) reﬂections of (11,02) and (13,20) respectively. These peaks are analogs to the XRD pattern reported by Tabak et al.(2007) and Shahwan et al. (2006). The acid activated montmorillonite displayed a higher intensity of the (9001) reﬂection due to removal of impurities during activation. The (001) reﬂection of CTMA-intercalated montmorillonite was shifted to a lower angle (2θ=5.85°). The shift was in line with the hypothesis that CTMA caused the opening of SiO2 interlayer space. The characteristic peaks were maintained in the reﬂection of ZnO/montmorillonite. The (001) reﬂection of ZnO/montmorillonite was positioned at 5.8° 2θ (d=16.2 Å). The reﬂection showed a shift compared to CTMA-intercalated montmorillonite, however the shift is not signiﬁcant to show the ZnO pillar formation in interlayer space. In addition, there was a new reﬂection for ZnO at 2θ=31.6° that could be (100) reﬂection of ZnO within the wurtzite phase (Lu et al., 2009; Mihai et al., 2010; Zhi et al., 2010). The low intensity and very broad peak (FWHM=0.541°θ) reﬂects the formation of low crystalline ZnO at a low content. The presence of ZnO in the prepared ZnO/montmorillonite material was checked by EDS analysis. The composition of the raw
Fig. 1. XRD pattern of (a) raw montmorillonite, (b) acid activated montmorillonite, (c) CTMA-montmorillonite and (d) ZnO/montmorillonite, *=(100) reﬂection of ZnO wurtzite phase.
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montmorillonite is similar to that reported in previous report (Fatimah et al., 2010). Raw montmorillonite contained silica (SiO2) and alumina(Al2O3) at 59.80 ± 0.67 and 22.19 ± 0.65 wt.%, respectively. The ratio of Si/Al was 2.45 (~ 2.0), a typical value for silicaalumina sheets in the ratio of 2:1 structure. The domination of sodium content as an exchangeable cation indicating a sodium type of smectite, which makes the montmorillonite easily to be swollen and to be intercalated by CTMA. After was modiﬁed by ZnO, the Zn content in the ZnO/montmorillonite was found as 4.31 at.%. The surface morphology of ZnO dispersion was detected by SEM. A more open surface of ZnO/montmorillonite relative to surface image of montmorillonite was observed. However, the presence of ZnO particles was not clearly identiﬁed on the surface. However, TEM photos showed the presence of ZnO in montmorillonite. Compared to the TEM image of montmorillonite, there were some darker dots appearing in ZnO/montmorillonite samples conﬁrmed as dispersed ZnO particles. The ZnO particles were distributed in irregular size at about 2–10 nm verifying the non-uniformed distribution of ZnO. The nitrogen adsorption–desorption isotherms and pore size distribution of montmorillonite and ZnO/montmorillonite are displayed in Fig. 2. Both adsorption–desorption proﬁles and pore size distributions suggested the increase of adsorption capability of ZnO/montmorillonite
(a) 140 ZnO-Mont-ads ZnO-Mont-des raw Mont-ads raw Mont-des
Adsorbed Volume (cc/g)
over the original montmorillonite. The proﬁles indicated an increase in pore volume by a hysteresis loop and this was also in accordance with the shift of the pore distribution to a higher pore radius. There was no dominant pore created by ZnO insertion/pillaring since there was no bimodal pore size distribution suggesting that micropores were still dominant in both raw and prepared materials. The particle size of ZnO was also conﬁrmed by particle size measurement based on the Scherer formula (Eq. (1)). D = 0:9λ = β cos θ
where λ is wavelength of Cu-Kα radiation (1.54 Å), β is full width at half maximum (FWHM) of wurzite reﬂection and is correspond to 2θ in XRD reﬂection (Chen et al., 2008). Calculation based on the FWHM data of 0.541°θ the reﬂection at 2θ = 31.6° resulted a particle size of 10.07 nm. Fig. 2 shows the adsorption–desorption proﬁle of raw montmorillonite and ZnO/montmorillonite. The speciﬁc surface area, pore radius and pore volume compared to acid activated montmorillonite and CTMA-intercalated montmorillonite are listed in Table 1. As expected, in agreement with XRD data, the speciﬁc surface area of the material increased after ZnO insertion in montmorillonite structure. The enhanced speciﬁc surface area suggests an increase in adsorption capacity for dye molecules. The DRUV–Vis spectra of ZnO/montmorillonite and bulk-ZnO (Fig. 3) display the lower Kulbelka–Munk function, F[R] of ZnO/ montmorillonite compared to ZnO. The spectrum of ZnO/montmorillonite lied between the spectra of ZnO and montmorillonite indicating the formation of small size ZnO particles in the matrix. Based on Kulbelka–Munk equation, Kulbelka–Munk function (F[R∞]) can be determined from reﬂectance spectrum data (Eq. (2)):
ð1−R∞ Þ2 2R∞
½FðR∞ Þhv = C2 ðhv−Eg Þ
where R is reﬂection intensity, h is Max Planck constant, v is wave number and Eg band gap energy. Using the Eq. (3), band gap energy (Eg) can be derived by plotting [F(R∞)hv] 2 versus hv (Fig. 3b,c). The band gap energy of ZnO/ montmorillonite was calculated as 3.22 eV, slightly higher than that of ZnO (3.20 eV). This blue shift means increasing band gap energy due to particle conﬁnement effect in that immobilization of ZnO produced in nanosize of ZnO particle. Similar pattern of shift was also reported by Zhi et al. (2010) and Fernandez et al. (2008).
(b) 4.5 raw montmorillonite
3.2. Photocatalytic activity
The photocatalytic activities of raw montmorillonite, standard ZnO and ZnO/montmorillonite are presented in Fig. 5. The kinetics of MB degradation was obtained by determining the reduction in MB concentration as a function of reaction time. The data were collected from the experiments with MB concentration of 10 ppm, catalyst loading
2 1.5 1 0.5
Table 1 Textural properties of ZnO/montmorillonite and raw montmorillonite.
Pore Radius (A) Fig. 2. (a) Adsorption–desorption proﬁle of materials and (b) pore distribution of materials.
Speciﬁc surface area (m2/g)
Pore volume (cc/g)
Pore radius (Å)
Raw montmorillonite ZnO/montmorillonite
I. Fatimah et al. / Applied Clay Science 53 (2011) 553–560 Table 2 Initial rate of MB degradation in various systems. Reaction condition
Initial rate (ppm/min)
Montmorillonite Montmorillonite–UV Montmorillonite–H2O2–UV ZnO/montmorillonite ZnO/montmorillonite–UV ZnO/montmorillonite–H2O2–UV ZnO–UV ZnO–H2O2–UV
0.578 0.626 1.165 0.700 1.744 2.652 2.772 2.827
of 2 g/L with and without 20 ppm H2O2 (½MB 2=2 2:0). In each run, prior to UVB illumination MB solution and the catalysts were mixed for 30 min in the dark, which showed MB adsorption process on each catalyst. The adsorption of MB dye on ZnO/montmorillonite was higher than that on natural montmorillonite. Montmorillonite and ZnO/ montmorillonite could achieve 20 and 40% reduction of MB after 150 min. The signiﬁcant improvement of MB adsorption by ZnO/ montmorillonite was related to the enhanced speciﬁc surface area from 48.6 m 2/g to 231.6 m 2/g. The advantage of the pre-intercalation process is the formation of uniformed porous structure in the composite material (Manova et al., 2010). Similar observations were made by Ding et al. (2008) photodegradation of MB by TiO2 dispersed in clay via alkylamine intercalation. The advantage of pre-intercalation is the formation of uniformed porous structure in the composite material (Manova et al., 2010). Table 2 presents the initial rate of MB degradation in all the tests. Photocatalytic oxidation of MB was inﬂuenced by H2O2. For either montmorillonite or ZnO/montmorillonite, addition of H2O2 increased MB degradation. H2O2 accelerates the reaction rate via increasing hydroxyl radical formation by its initiation with UV light in the photooxidation (Banat et al., 2005). For ZnO/montmorillonite, photodegradation of MB reached 80% after 30 min and 97% after 150 min, reaction time. For ZnO, photocatalytic degradation of MB was generally higher and MB degradation reached 100% after 150 min. However, in terms of net ZnO loading in solution, it was seen that ZnO/ montmorillonite exhibited the highest photodegradation of MB (Fig. 4). Photocatalytic MB degradation at varying MB initial concentrations on ZnO/montmorillonite with H2O2 was further investigated (Fig. 5). The initial reaction rate increased with increase of initial
Fig. 3. (a) DRUV–Visible spectra of montmorillonite, standard ZnO powder and ZnO/ montmorillonite, (b) and (c) plot of [F(R∞)hv]2 to obtain band gap energy of standard ZnO and ZnO/montmorillonite. Fig. 4. Comparison of kinetics curves of MB degradation using montmorillonite, ZnO/ montmorillonite and bulk-ZnO at varied conditions of adsorption, photo-oxidation without H2O2 addition, and photo-oxidation with H2O2 addition.
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Table 4 Parameters of isotherm modeling.
Fig. 5. Kinetics curve of MB degradation at varied initial concentrations of MB.
concentration of MB (Table 3). However, MB degradation efﬁciency decreased with increasing MB concentration. The kinetics of photooxidation of MB on ZnO/montmorillonite was tested using the Langmuir, Freundlich and Langmuir–Hinshelwood (L–H) models. From the data presented in Fig. 5 and Table 3, simulation for the Langmuir and Freundlich models were constructed by applying data of degraded MB concentration by photocatalyst (qe) as a function of MB concentration in equilibrium state (MBe) by Eq. (4) and (5) respectively. For Langmuir–Hinshelwood model, Eq. (6) was used. 1 1 1 1 : = + qe qm qm :b Ce
lnqe = lnKF + =n lnCe
1 1 1 = + r0 k kKC0
where is qm is monolayer capacity and b is adsorption constant in Langmuir model, and KF in Eq. (5) is the Freundlich constant. In Eq. (6) ro is the initial rate, Co is the initial concentration of MB, k is the apparent kinetic constant and K is the adsorption constant. The chosen model was referred to previous investigation on kinetic study of adsorption and photodegradation (Saepurahman and Chong, 2010; Valente et al., 2006). The models were evaluated based on regression coefﬁcient (R 2) (Table 4) and it was concluded that the Langmuir– Hinshelwood model ﬁt the experimental data quite well (R 2 = 0.999).
Coefﬁcient of determination (R2)
Langmuir Freundlich Langmuir–Hinshelwood
0.6355 0.5027 0.9981
b = 11.90, qm = 6.56 KF = 17.21 K = 8.315 × 10− 5, k = 498.94
The value of adsorption–desorption coefﬁcient (K) was obtained as 8.315 × 10 − 5. Fig. 6 shows the correlation between the initial reaction rate with the initial concentration of MB. The conformity with the L–H model is in accordance with earlier studies stating suitability of photodegradation reaction over semiconductor photocatalyst (TiO2 and ZrO2) with L–H model (Houas et al., 2001; Lizama et al., 2002; Rajeshwar et al., 2008). The L–H model indicated that photodegradation was strongly affected by an adsorption process (Belohlav and Zamostny, 2000). Fig. 7 shows MB spectra at 0, 30, 60, and 120 min during photocatalytic oxidation. The characteristic absorption of MB was at 245, 291, and 663.5 nm. At 30 min, all the peaks related with MB absorption were observed, indicating the presence of MB molecules in the solution. An additional peak at 360 nm also appeared. This peak was referred to the intermediate product degraded from MB in solution. At longer time, this peak and the maximum absorption peak in the visible range (663.5 nm) disappeared along with the increased absorbance at the wavelength lower than 260 nm suggesting formation of aromatic groups. Furthermore, at 180 min, absorbance at all wavelength range was minimized, indicating complete degradation of MB. This UV–Vis spectra change with time of photooxidation is in accordance with was reported by Galagan and Su (2008). Fig. 8 presents the chromatograms of initial MB solution and the treated solutions. Initially, MB showed a single peak at 3.093 min. After 30 min, the area of the peak decreased and two new peaks were identiﬁed at 1.596 and 2.456 min, respectively. These new peaks indicated the intermediate compounds produced from the degradation of MB. At 120 min, only a single strong peak occurred. These results are in accordance with the MB degradation reported by Rajeshwar et al. (2008). Based on a previous work (Houas et al., 2001) the possible product in methylene blue oxidation is phenol. A spiking
Table 3 Kinetics parameter of initial rate, correlation coefﬁcient of reaction order simulations from varied initial concentration of MB. Initial concentration [MBo]/ppm
Initial reaction rate (M/min)
Concentration in equilibrium [MBe]/(× 10− 5 M)
Equilibrium amount of degraded MB(qe) (ppm/g)
10 15 20 25 30 40 60
0.615 0.895 1.219 1.445 1.850 2.544 3.737
0.132 0.886 1.220 2.887 3.660 7.190 31.200
12.335 17.225 20.425 16.013 32.925 41.013 23.500
1/ro Fig. 6. Simulation curve of Langmuir–Hinshelwood model.
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Fig. 7. UV–Visible spectra of MB in initial concentration compared with treated solution for 30, 60, 90, and 120 min.
analysis of the photooxidation solution at 120 min was performed. The spiked solution was derived by addition of 1 μL of 20% phenol standard solution in 1 μL of the photooxidation sample. The chromatogram is presented in Fig. 8(d). The chromatogram conﬁrmed the presence of phenol.
3.3. Effect of catalyst dosage The effect of catalyst dosage on photo-oxidation of MB on ZnO/ montmorillonite was further investigated and the results are illustrated in Fig. 9. The initial rate of MB photooxidation increased
Fig. 8. Chromatogram of HPLC analysis result for (a) MB in initial concentration compared with photo-oxidated solution for 30 min and 120 min (b–c), and spiked 120 min photooxidated solution.
I. Fatimah et al. / Applied Clay Science 53 (2011) 553–560
Initial rate (ppm/minute)
3 2.5 2 1.5 1 0.5 0 0
Catalyst dosage (g/L) Fig. 9. Plot of initial rate as function of MB initial concentration.
as ZnO/montmorillonite dosage increased in the solution until the dosage exceeded 5 g/L. Further increase in catalyst loading would not enhance the rate. The higher loading of the photocatalyst provided higher surface area for adsorption and photoactive sites to adsorb UV light. But at much higher dosage, the catalyst would strongly affect the solution turbidity and reduce light absorption. The decolorization tended to occur during adsorption. 4. Conclusions The present study shows the improvement of physicochemical properties and photocatalytic activity of ZnO/montmorillonite for degradation of MB. The speciﬁc surface area, crystallinity and band gap energy were improved by ZnO insertion in montmorillonite matrix. Increased adsorption capability of ZnO/montmorillonite helped to enhance the rate of photooxidation reaction. From the kinetics data, it was observed that ZnO/montmorillonite catalyzed the MB photooxidation according to the Langmuir–Hinshelwood mechanism. Acknowledgement The authors thank DPPM Universitas Islam Indonesia for their partial ﬁnancial support to this research. References Aal, A., Mahmoud, S., Aboul-Gheit, A., 2009. Sol–gel and thermally evaporated nanostructured thin ZnO ﬁlms for photocatalytic degradation of trichlorophenol. Nanoscale Res. Lett. 4, 627–634. doi:10.1007/s11671-009-9290-1. Bahnemann, D., 2004. Photocatalytic water treatment: solar energy applications. Sol. Energy. 77 (5), 445–459. doi:10.1016/j.solener.2004.03.031. Banat, F., Al-Asheh, S., A1-Rawashdeh, M., Nusair, M., 2005. Photodegradation of methylene blue dye by the UV/H202 and UV/acetone oxidation processes. Desalination 181, 225–232. doi:10.1016/j.desal.2005.04.005. Baruah, S., Sinha, S.S., Ghosh, B., Pal, S.K., Raychaudhuri, A.K., Dutta, J., 2009. Photoreactivity of ZnO nanoparticles in visible light: effect of surface states on electron transfer reaction. J. Appl. Phys. 105, 074308. doi:10.1063/1.3100221. Behnajady, M.A., Modirshahla, N., Hamzavi, R., 2006. Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst. J. Hazard. Mater. 133, 226–232. doi:10.1016/j.jhazmat.2005.10.022. Belohlav, Z., Zamostny, P., 2000. A rate-controlling step in Langmuir– Hinshelwood kinetic models. Can. J. Chem. Eng. 78 (3), 513–521. doi:10.1002/ cjce.5450780310. Byrappa, K., Subramani, A.K., Ananda, S., Rai, K.M., Sunitha, M.H., Basavalingu, B., Soga, K., 2006. Impregnation of ZnO onto activated carbon under hydrothermal conditions and its photocatalytic properties. J. Mater. Sci. 41, 1355–1362. doi:10.1007/s10853-006-7341-x.
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