Analysis of Kinetic Models in Heterogeneous Catalysis of Methyl Orange Using TiO 2 /Silica Composite Photocatalyst

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1 Analysis of Kinetic Models in Heterogeneous Catalysis of Methyl Orange Using TiO 2 /Silica Composite Photocatalyst Kwena Y. Pete, Mika M Sillanpää, Maurice S. Onyango, and Ochieng Aoyi Abstract In this study, a composite photocatalyst of TiO 2/silica was prepared, analyzed and used in a batch reactor for the photocatalytic degradation of synthetic textile (methyl orange) wastewater. The successive photocatalyst was characterized by Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM- EDS) and zeta potential (ZP) analyses. Operating parameters such as loading, initial pollutant concentration and ph were optimized. The performance of the photocatalyst reactor was evaluated on the basis of color removal and degradation kinetics. The equilibrium data was analyzed using the Langmuir and Freundlich isotherm models. Experimental data was best represented by the Langmuir isotherm model. The maximum adsorption capacity of dye onto composite photocatalyst was found to be mg/g. The pseudo second order kinetic model adequately described the kinetic data. Keywords Composite photocatalyst, kinetics, methyl orange, photocatalysis T I. INTRODUCTION HE accelerated growth of textile industry and the increasing worldwide concern for environmental conservation have revealed the great problem of environmental pollution by azo dyes. Due to the stability and toxicity of azo dyes and the presence of residual surfactants, azo dye textile wastewaters are resistant to the conventional biological treatment [1]. With this growing demand, heterogeneous photocatalysis employing TiO 2 semiconductor catalyst is one of the few attractive alternatives to resolve this problem. A number of important features for the heterogeneous photocatalysis have extended their feasible applications in water treatment, such as operating at ambient temperature and pressure, low operative costs and complete mineralization of parents and Kwena Pete was with the Faculty of Technology, Lappeenranta University of Technology, Sammonkatu 12, FI Mikkeli, Finland. She is now with the Centre for Renewable and Water, Vaal University of Technology, Private Bag X021 Vandebijlpark 1900, South Africa (*Corresponding author. address: koendapete@webmail.co.za; Tel ; Fax: ). Mika M Sillanpää is with the Faculty of Technology, Lappeenranta University of Technology, Sammonkatu 12, FI Mikkeli, Finland Maurice S. Onyango is with the Department of Chemical and Metallurgical Engineering, Tshwane University of Technology, Pretorai,0001 Ochieng Aoyi is with the Centre for Renewable and Water, Vaal University of Technology, Private Bag X021 Vandebijlpark 1900, South Africa their intermediate compounds without secondary pollution [2]. The post-separation of the semiconductor TiO 2 catalyst after water treatment remains as the major problem towards the practicality at an industrial process. The fine particle size of the TiO 2, together with their large surface area-to-volume ratio and surface energy creates a strong tendency for catalyst agglomeration during the process. Such particles agglomeration is highly disadvantageous in views of surfacearea reduction and its reusable lifespan. To solve this problem, TiO 2 catalyst powder is loaded onto supporting silica material in such a way as to provide high surface area and accessibility of the catalyst. The main aim of this work was to investigate the heterogeneous photocatalytic degradation of methyl orange dye using TiO 2 /silica composite photocatalyst. The effect of operating parameters such as the composition of the composite particle size, catalyst loading, initial pollutant concentration and solution ph were determined. A. Materials II. MATERIALS AND METHODS Chemicals used were of analytical reagent grade, used as received and purchased from Sigma Aldrich. Methyl orange (MO, 99 %,), an azo dye pollutant in wastewater. The TiO 2 was used as photocatalyst with Ludox HS-30 colloidal silica, as the supporting material. B. Synthesis of Composite photocatalyst The TiO 2 supported-silica samples were prepared by adding Ludox HS-30 colloidal silica solution to TiO 2 with proper mixing to ensure a homogeneous TiO 2 supporting onto silica. After mixing, the mixture was dried at 110 C and screened to different particle sizes (particle sizes , µm). The coated particles were then dispersed in Mill Q-plus water, till the ph of the suspension was close to 6.5. By this procedure, a sample of 3-20 wt% of TiO 2 onto silica particles was prepared. C. Photocatalytic experiment Photocatalytic degradation experiments were carried out in 100 ml semi batch reactor at 25 ± 3 C. Milli Q-plus water (resistance = 18.2 M.Ω) was used for all experimental work. 169

2 The UV lamp (low-pressure mercury lamp, wave length, 254 nm (Pen-Ray), with an intensity of 5.5 mw/m 2 ), protected by a quartz sleeve was used. For all experiments, the solution was left to equilibrate for 30 min in the dark before the lamp was switched on. This was sufficient to reach an equilibrated adsorption as deduced from the steady-state concentrations. Samples were taken every 30 minutes and immediately filtered through a polypropylene syringe filter (0.45 µm) in order to remove the photocatalyst. D. Characterization The surface morphology of the the successive composite photocatalyst was characterised by SEM equipped with EDS (Hitachi S-4800 Ultra-High Resolution Field Emission Scanning Electron Microscope). Zeta sizer Nano Series model ZEN 3600 (Malvern, the UK) was employed to measure the isoelectric points before the degradation tests. E. Chemical analysis The changes in the intensity of dye colour were observed from its characteristics absorption band, at 466 nm using UVvis spectrophotometer (Perkin-Elmer Lambda-45 spectrophotometer). A. Characterization III. RESULTS AND DISCUSSION Prior to photodegradation experiments, material characterization was done using SEM-EDS and ZP analyses techniques. These techniques helped to interpret the photocatalysis process under TiO 2 /silica composite photocatalyst composition. 1. SEM-EDS analysis The morphology of the successive photocatalysts with µm particle size was characterized by SEM technique, its images and EDS spectra are shown on Fig. 1 and Fig. 2. The prepared TiO 2 /silica photocatalyst exhibited a regular morphology which is related to the dispersion of TiO 2 on silica surface. The EDS spectras revealed that the Ti was detected, which clearly shows that crystallites of TiO 2 is well dispersed on the surface of the silica supporting material. Fig. 2 EDS spectra of 15% TiO 2/silica composite photocatalyst 2. Isoelectric point measurements (IEP) The point of zero charge (PZC) of TiO 2 /silica composite was determined by zeta potential measurements (Fig. 3). The IEP for the particles was at ph 2.0. This results are consistent with the findings revealed by Papirer [3] and Persello [4] where the silica s point of zero charge ranges between 2 and 3. In addition, the surface charges were changed from positive to negative as a function of ph due to the unprotonization of the surface groups. At a ph less than the IEP, the surface has a positive charge due to the formation of Si-OH 2 +. At a ph above the IEP, the surface of the silica has a negative charge due to the deprotonation of the silanol group resulting in Si-O -. Fig. 3 Zeta potential of TiO 2/silica as a function of ph Fig. 1 SEM images of 15 % TiO 2/silica composite photocatalyst B. Photocatalytic degradation of methyl orange Control experiment was conducted on two different conditions, dark adsorption over composite photocatalysts and photolysis under UV light only (Error! Reference source not found.4). No significant change was observed during adsorption test after 180 minutes. This elucidates the attainment of adsorption equilibrium and 30 min was used in all the experiments throughout. In direct photolysis, 58% colour removal was achieved during 210 min by the assistance of light. To recognize the role of support material 170

3 during the photocatalytic degradation of methyl orange, the 15 wt% TiO 2 /silica composite was compared with unsupported TiO 2 amount equivalent to the one available on both photocatalyst for the degradation efficiency. It can be clearly seen from Fig. 4 that the complete degradation of methyl orange was observed over supported system, whereas the degradation over TiO 2 itself did not reach complete degradation even at 210 min. This can be explained in terms of the interaction between TiO 2 and supporting material (silica), as well as the different structure from bulk TiO 2. Taking into account the surface analysis result, the modified photocatalyst TiO 2 samples having a larger surface area could allow a larger amount of surface adsorbed species [5]. Fig. 4 Adsorption of methyl orange over silica loading ( ); photocatalytic degradation of methyl orange over UV ( ); UV- TiO 2/silica ( ); UV-TiO 2 ( ) C. Effect of composite photocatalyst loading To find the optimum photocatalyst loading, the effect of photocatalyst was investigated by varying the loading of the photocatalyst in 25 mg/l methyl orange solution. It is apparent from Fig. 5 that above 0. 1 g photocatalyst loading the reaction rate aggravate and becomes independent of the photocatalyst loading. This can be ascribed in terms of availability of active sites on TiO 2 /silica surface and the light penetration of photoactivating light into the suspension. The availability of active sites increases with the suspension of photocatalyst loading. Additionally, the decline in the percentage of degradation at higher catalyst loading may be due to deactivation of activated molecules by collision with ground state molecules [6]. D. Effect of dye initial concentration The characteristic of organic dye concentrations in wastewater from textile industry is usually in the range of mgl -1 [7]. Therefore, methyl orange solutions were varied in the range 5-50 mgl -1 during the photocatalytic degradation at solution ph 6.3. The degradation rate was found to increase up to an initial concentration of 35 mgl -1 and then decreased (Error! Reference source not found.6). The inadequate number of surface sites on silica/ TiO 2 particles may affect the photodegradation efficiency. As the concentration of methyl orange pollutant increase, more molecules of the compound get adsorbed on the surface of the photocatalyst. As a result, the reactive species ( OH and O 2- ) needed for the degradation of dye also increases. However, the formation of OH and O 2- on the composite photocatalyst surface remains constant for a 5.5 mw/m 2 light intensity and 0.1 g catalyst loading for 210 minutes duration of irradiation. Hence, the available OH radicals are inadequate to attack the methyl orange molecules at higher concentrations due to constant reaction conditions. Fig. 5 Effect of TiO 2/silica photocatalyst loading 0.1 g ( ); 0.5 g ( ); 1 g ( ) Fig. 6 Effect of initial concentration on TiO 2/silica photocatalyst 5 mgl -1 ( ); 15 mgl -1 ( ); 25 mgl -1 ( ); 35 mgl -1 ( ); 50 mgl -1 ( ) E. Effect of solution ph To study the effects of H + concentrations on dye degradation, comparative experiments were performed at three ph values: 3, 7 and 9 (Fig. 11). In the acidic ph, minimization of electron hole recombination is a key factor for the enhanced degradation of methyl orange. At ph 3, the surface of silica/tio 2 is positively charged and thus attracting the largest amount of negatively charged anions of methyl orange, hence there is a substantial degradation. Zeta potential (ZP) measurements indicate that the surface charge 171

4 of the TiO 2 /silica composite photocatalyst decreased as the ph increased signifying repulsive forces between photocatalyst surface and dye in alkaline media. Furthermore, the limiting behavior in alkaline medium is due to the negative charge of a composite photocatalyst and therefore methyl orange transformation products can be repelled away from the silica/tio 2 surface which opposes the adsorption of substrate molecules on the surface of the composite photocatalyst. As a result, methyl orange degradation declines in alkaline medium F. Photodegradation isotherms and kinetics of methyl orange The isotherm modelling basically reflects the interaction between substrate and photocatalyst until the state of equilibrium is reached. In order to optimize the effectiveness of the TiO 2 /silica on the photodegradation of methyl orange, linear form of Langmuir and Freundlich models were applied for the TiO 2 /silica composite photocatalyst. For brevity, the linear form of the Langmuir and Freundlich isotherm model are described by (1) and (2), [8, 9]. The correlation coefficients (R 2 ) along with other parameters for two different models were calculated and listed in Table I. Clear deviations were observed in the Langmuir and Freundlich isotherm models. Fig. 7 indicates a straight line with a correlation coefficient (R 2 ) of signifying that the degradation of methyl orange onto the composite photocatalyst fits the Langmuir isotherm reasonably well. The Freundlich constants, K f and n, calculated from this investigation, are 2.3*10-3 and 0.399, respectively. This implies poor fitting for the Freundlich isotherm model. Comparison of the correlation coefficients of both models suggests that the Langmuir model is suitable. The fact that the Langmuir isotherm fits the experimental data well can be due to homogeneous distribution of active sites on the TiO 2 /silica surface. photodegradation of methyl orange on the composite photocatalyst was applied to pseudo first and second order kinetic model, and the results are presented in Table II. Fig. 9 and Fig. 10 shows the plot of pseudo first order and pseudo second order kinetic model, respectively. The correlation coefficient for the second order kinetic model (0.999) is greater than that of the first order kinetic model (0.936). Therefore the dye photocatalytic system by TiO 2 /silica is a second order reaction. The Pseudo-first-order and second order model equation are given in (4) and (5), [10]: (4) where q t and q e are the adsorption capacity (mmol/g) at time t and at equilibrium respectively, while k 1 represents the pseudo first order rate constant (min -1 ). The pseudo first order model was generalized to two-site-occupancy adsorption to form a pseudo-second-order equation: where k 2 is the pseudo second order rate constant (g/mmol min). It has been distinguished that this model is able to estimate experimental qe values quite well and is not very sensitive for the influence of the random errors [10] (5) (1) where, qm (mgl -1 ) and K L (Lmg -1 ) are the Langmuir constants related to adsorption capacity and rate of adsorption, respectively, qe is dye concentration at equilibrium onto adsorbent ((mgl -1 ), C e is dye concentration at equilibrium in solution (mgl -1 ). Fig. 6 Effect of solution ph on TiO 2/silica photocatalyst ph 3 ( ); ph 7 ( ); ph 9 ( ) (2) where K f is the Freundlich constant related to adsorption capacity, n f is measure of the surface heterogeneity, ranging between 0 and 1. For linearization of the data, the Freundlich equation is written in logarithmic form: (3) Photocatalysis is time dependent process and it is very imperative to determine the rate of photocatalysis for designing and evaluating the photocatalyst in removing the pollutants from waste water. The data for the 172

5 Fig. 7 Linear transform of Langmuir isotherm TABLE I ISOTHERMS CONSTANTS AND CORRELATION COEFFICIENTS FOR THE DEGRADATION OF METHYL ORANGE ONTO COMPOSITE PHOTOCATALYST Langmuir isotherm Freundlich isotherm qm (mg/g) k (min -1 ) R 2 n K f R * Fig. 9 Pseudo second order plots for the photocatalytic degradation TABLE II KINETIC PARAMETERS FOR DEGRADATION OF METHYL ORANGE Pseudo first order model k (min -1 ) Pseudo second order model k (min -1 ) IV. CONCLUSION qe (mg/g) 0.95 qe (mg/g) R R The characterization of TiO 2 supported silica revealed the well dispersion of TiO 2 on the surface of silica. The optimum conditions for the degradation of methyl orange at ph 3 include an initial concentration of 25 mg/l and 0.1 g of TiO 2 /silica composite photocatalyst loading. The equilibrium studies showed that the Langmuir model was most accurate and the methyl orange dye photocatalytic system by TiO 2 /silica composite photocatalyst is a second order reaction. ACKNOWLEDGMENT The This research was funded by the European Union and the city of Mikkeli, Finland and the Water Research Commission, South Africa. Fig. 8 Linear transform of Freundlich isotherm Fig. 8 Pseudo first order plots for the photocatalytic degradation of methyl orange dye onto TiO 2/silica REFERENCES [1] J. Virkutyte and M. Sillanpää, The hindering effect of experimental strategies on advancement of alkaline front and electro osmotic flow during electro kinetic lake sediment treatment, J. Hazard. Mater. vol. 143, pp , 2007 [2] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard and J.M. Hermann, Photcatalytic degradation path-away of methylelne blue in water, Appl. Catal. B: Environmental, vol 31, pp. 145, 2001 [3] E. Papirer, Adsorption on Silica Surfaces, John Wiley and Sons Ltd., New York, [4] J. Persello, Surface and interface structure of silicas, in: Adsorption on Silica Surfaces, Papirer, E. (Ed), John Wiley and Sons Ltd., New York, pp , [5] J.M. Hermann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catal. Today. 53, pp , [6] I.K. Konstantinou, and T.A. Albanis, TiO 2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations : a review. Appl. Catal. 49, 1 14, [7] S.K. Kansal, M. Singh and D. Sud, Studies on photodegradation of two commercial dyes in aqueous phase using different photocatalysts, J. of Hazard. Mater. 141, pp , [8] I. Langmuir, The Constitution and Fundamental Properties of Solids and Liquids, J. Am. Chem. Soc. vol. 40 (9), pp , [9] H. M. F. Freundlich, Over the Adsorption in Solution, J. Phys. Chem 57, pp , [10] E. Repo, M. Mäkinen, S. Rengarajb, G. Natarajanc, A. Bhatnagar, M. Sillanpää, Lepidocrocite and its heat-treated forms as effective arsenic adsorbents in aqueous medium, Chem. Eng. J. 180, pp ,