Article Type: Research Article Phenol Degradation of Industrial Wastewater by Photocatalysis Dheeaa Al Deen Atallah Aljoubory 1 and Ramaligham Senthilkumar 2 1 Caledonian College of Engineering, Muscat, Sultanate of Oman 2 College of Applied Sciences, Sohar, Sultanate of Oman * Corresponding author: Ramaligham Senthilkumar, College of Applied Sciences, Sohar, Sultanate of Oman, Email: kumar.soh@cas.edu.om Received March 29, 2014; Accepted April 15, 2014; Published April 18, 2014 Citation: Ramaligham Senthilkumar, Dheeaa Al Deen Atallah Aljoubory (2014) Phenol Degradation of Industrial Wastewater by Photocatalysis. Journal of Innovative Engineering 2(2): 5 Copyright: 2014 Ramaligham Senthilkumar et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract Photo-catalytic activity of Titanium dioxide (TiO2) under various irradiation conditions was explored. Experiments were conducted to investigate the effects of parameters such as ph, catalyst dosage, phenol concentration and ultra violet (UV) light intensity. This research addressed the photo degradation efficiency of phenol and the generation of hydroxyl radicals using the UV/TiO2 system. Experimental results revealed that the photo-catalytic activity of the TiO 2 was higher under UV light irradiation and indicated that the removal efficiency was highest for phenol when using 0.20 wt% TiO2 dose concentrations. The maximum phenol removal was achieved by using TiO 2 for industrial waste water having the initial concentrations of phenol 10 mg/l. Experimental data obtained under different conditions were fitted with kinetic models to describe the dependency of degradation rate as a function of the above mentioned parameters and pseudo-first order was found to describe the phenol degradation with high correlation coefficients compared to pseudo second order. Equilibrium data were analyzed using Langmuir-Hinshelwood kinetics. Keywords: Photo-catalysis; Phenol, Sunlight; Hydroxyl radical; TiO 2 ; UV; Photo-catalysis; Phenol degradation Abbreviations: Cp-Phenol concentration at time t, ppm; Cpo-Initial phenol concentration, ppm; dcp/dt- Differential of Cp polynomial with respect to t, ppm/min; e--photon generated electron on the photo catalyst; h+- Photon generated hole on the photo catalyst; k-reaction rate constant ppm/min; K-Adsorption equilibrium constant 1/ppm; K app -Apparent rate constant min-1; MWNT-Multi walled carbon nano tubes; OH -Hydroxide ion; -OH - Hydroxyl radical; -(-r)p-reaction rate, ppm/g.min; t-time, min; V-Volume of treated phenol solution, L; σ-photodegradation efficiency 1. Introduction Phenol and its derivatives are the commonly organic pollutants encountered in industrial effluents that will cause severe environmental problems without proper treatment Photo-catalysis by TiO 2 semiconductors is promising for elimination of hazard environmental pollutants especially for the degradation of bio-recalcitrant organic contaminants. In fact, many studies on photo-catalytic degradation of phenol and its derivatives by TiO 2 upon illumination with near UV light have been reported. This research addresses the photo degradation efficiency of
phenol radicals using the UV/TiO 2 system. The UV/TiO 2 process destroyed organic pollutants and yielded intermediates. Photo-catalysis is a promising technique for the treatment of contaminated water and groundwater, which has been widely studied in recent years because it is able to completely oxidize organic molecules at a low energy cost. Upon absorption of light energy that is equal to or greater than band gap energy, the electrons in the valence band of the semiconductors such as TiO 2 can be excited to those in the conduction band, leaving a positive hole in the valence band [1]. The positive hole is a strong oxidant, which can oxidize compound directly or react with the electron donors in the environment such as water or hydroxide ions to form hydroxyl radicals (OH) that are also potent oxidants. Phenol and its derivatives are the commonly organic pollutants encountered in industrial effluents in petroleum refining industries which cause severe environmental problems without proper treatment. The effects of various parameters such as the initial concentration of organic compound, ph, and catalyst dosage on the degradation efficiency have been examined. The effects of operating parameters including the initial concentrations of phenol, ph, and catalyst dosage on the degradation rates were discussed. The aim of this study is to evaluate the effectiveness of photo-degradation using TiO2. Wastewater derived from different chemical industries like oil - refineries has high concentration of phenol and their derivatives which are extremely toxic, carcinogenic and refractory in nature. Treating phenolic wastewater to harmless level is an arduous process for many biological and chemical processes due to its high solubility and stability in water. Recently, considerable attention has been focused on complete oxidation of organic compounds to harmless products of CO 2 and H 2 O by the advanced oxidation processes. The electron hole pairs will be formed in the TiO 2 semiconductor particles by irradiating TiO 2 with light or energy. The generated holes are a strong oxidizing agent. The reaction of the photo-generated holes with water molecules and hydroxyl ions adsorbed on the surface of TiO 2 yields formation of hydroxyl radicals (OH), capable of mineralizing organic compounds to carbon dioxide and water [1]. On the other hand, generated electrons are scavenged by adsorbed oxygen with the formation of superoxide radicals, which can be reduced further to react with hydrogen peroxide to form hydroxyl radicals again. Furthermore, the process usually utilizes TiO 2 as photo-catalyst, which is non-toxic, largely available, has long-term stability, strong oxidizing power and high photo-activity. From this viewpoint, TiO 2 photo-catalysts that can operate efficiently under visible light irradiation would be ideal for practical and widespread use. Here, we explored the photo-catalytic activity of TiO 2 under UV light and solar irradiation. This work may provide new insights and understanding on the mechanisms of photo-activity enhancement by TiO 2. It has been extensively employed as photo-catalytic material for solving environmental problems, especially for eliminating toxic chemicals from wastewater. The photo-catalytic activities of TiO2 powder greatly depend upon its microstructure and physical properties. Many studies have investigated the relationship between the conditions and the properties of the resulting TiO2, such as surface area, particle size, pore volume and pore size distribution, crystal structure, phase composition, thermal stability, band gap energy, among others. Photo-catalytic technology has been widely studied and it has been proposed as an effective method for treatment of toxic and polluted water. The influence of UV intensity on the efficiency of the process was investigated. 2. Experimental Procedure 2.1. Materials and Methods Samples of industrial waste water were collected from wastewater treatment plant in petroleum refining industry and the concentration of phenol in the sample was analyzed. The amount of phenol in the samples were found to be 10,
30 and 50 mg/l. CHEMetrics' phenokits and mass spectro-photometer was used for the analysis and to determine concentration of phenol in waste water. 2.3 Batch Process In a batch reactor, the reactants were charged into a container, well mixed, and are left to react for a certain period (Figure 1). The effects of the ph, initial phenol concentration, amount of catalyst and light intensity on photodegradation of phenol were investigated and optimal conditions were found out. Experiments on photo-degradation of phenol using TiO2 under UV irradiation, sunlight and dark were carried out. 2.4 Continuous Process Continuous experiments were carried out by taking the optimum parameters such as ph, initial concentration, and dosage and UV irradiation obtained from batch experiments. The flow rate of 1.2 L/min and 1.6 L/min were tried and initial concentration of 10 mg/l was used for period of 2 hr. The same conditions were used for both UV irradiation and sunlight and the efficiency of phenol degradation was analyzed. 3. Results and Discussion 3.1 Batch Reactor 3.1.1 Effect of light intensity The incident light intensity was expected to be one of the rate-controlling parameters. The effect of UV intensity on the photo-catalytic efficiency of catalysts was investigated by changing the power of UV source lamps from 5 to 15 W. Experimental result showed that the degradation of phenol increases with the UV intensity irradiated, and the photo-catalytic efficiency with 15 W lamps in catalyst was more than the 5W lamp. Results indicated (Table 1) that the efficiency of phenol degradation increased with increasing the light intensity [2]. It showed that the efficiency of phenol removal with increasing the light intensity was that the photons were present in excessive amount leading to more reactive species generation and more destruction of phenol and proved that higher catalyst activity occurred under high light intensity [1].
Table 1: Efficiency of phenol removal with reference to light intensity Light intensity Effect of light intensity on degradation of phenol 5 W 1 15 W 8.2 3.1.2 Effect of ph At the optimized intensity of UV light ph of the sample were studied at 3, 5, 7 and 9. Better removal efficiency of phenol was found to be at ph 7, and it was considered as the optimal ph for further study (Figure 2). The process of the photo-catalytic degradation of phenol involves with radical oxidation, direct electron transfer and surface sorption reaction between ph 6.7 to 7.2 [1]. Besides, TiO 2 coagulates at ph 9 and the samples of the reaction slurry could not be filtrated and analyzed. 3.1.3 Effect of initial concentration Experiments were carried out for 2 hr with 15 W irradiation for different initial concentrations of phenol (2 mg/l, 10 mg/l, 30 mg/l, 50 mg/l) using 0.20 wt% of catalyst. It was found that the degradation of phenol increased with increasing the initial concentration of phenol. The maximum level for efficiency is 80 % at the initial concentration of phenol (10 mg/l). Degradation of phenol increased with increasing the initial concentration of phenol, but the maximum efficiency for degradation of phenol is at the initial concentration of phenol (25 mg/l) [3]. The current study reveals that the reason for the smaller degradation rate at the latter stage of the process (after 25 mg/l) was likely due to the coverage of active sites by reaction intermediates. Another study by Jesus Morera del Rio, [4] reveals that the phenol conversion increases with increase in concentration of phenol and light intensity. The maximum efficiency for degradation of phenol is at the initial concentration of phenol (40 mg/l).
3.1.4 Effect of the dose of TiO2 Four different TiO 2 dose concentrations (0.10, 0.20, 0.30 and 0.35 wt %) were used for 2 hr under 15 W UV irradiation. It was noted that increase in dosage of TiO2concentration from 0.10 wt % to 0.20 wt %, the efficiency degradation of phenol increases from 7.6 % to 39% (Figure 4). Further increase in TiO2 % at 0.30 wt % had showed the degradation of phenol was only 43 % which was a very small difference when compared to 0.20 wt % [3]. Hence, 0.20 wt% was considered to be the best dosage for further investigations 3.1.5 Removal of phenol by TiO2 under sunlight Under the optimized conditions of ph and UV light intensity, different initial concentration of phenol (10 mg/l, 30 mg/l, 50 mg/l) using 0.20 wt % of TiO 2 were studied under sunlight for 2 hr (Figure 5). It was noted that the efficiency of degradation of phenol under sunlight was poor. The efficiency of degradation of phenol under sunlight was 13% while, in the same conditions, the efficiency of degradation of phenol under UV irradiation was 82% [5].
3.1.5 Phenol removal under UV irradiation: Phenol removal from its solution by TiO 2 and their composite catalyst are plotted against UV irradiation time (Figure 6). Neat TiO 2 can only reach 44.20 % of phenol conversion within the same reaction time. The vertical line at time zero isolates the dark adsorption period from that under UV illumination [6]. Experiments were conducted for three samples under the same conditions which have different initial concentration of phenol (10 mg/l, 30 mg/l, 50 mg/l) using 0.2 wt % of TiO2. 15W UV irradiations were applied for 2 hr. It was observed that the efficiency of degradation of phenol under UV i radiation is the highest under sunlight, and the efficiency of phenol removal increased with increase in light intensity (Figure 7). These results agree with study by McManamon et al., [3] that show that when the UV light was turn on, the phenol removal exhibited the highest phenol removal efficiency from dark. Another study by Wei Li and Liu, [7] showed that that the activity of pure TiO 2 was after 180 min and 70.3% efficiency of phenol removal was achieved.
3.1.6 Kinetics of the reactions: The results of kinetic study shows that the degradation phenol followed pseudo first order kinetics: r = dc/ dt= krkc= k app C (1) where: k app is the apparent rate constant of a pseudo first order reaction. Thus, equation 1 can be simplified to a first order reaction: ln(c0 /C) = k app t (2) From Figure 8, the apparent rate constant kapp, was found to be 0.49 (slope of the curve), ie plotting a graph between ln (C0/C) versus t [3]. It was found that k app decreases with increasing initial concentration, indicating that the present TiO 2 /UV system was dominated by the concentration of photons. At higher initial phenol concentrations, light penetration will decrease via the shielding effect of the suspended particles [8].
3.2 Continuous Reactor Journal of Innovative Engineering The samples were analyzed at different irradiation time viz., 2, 4, 10 hrs using 0.20 wt % of TiO 2 with UV irradiation of 15 W as well as under sunlight and the efficiency of degradation of phenol under UV irradiation was found to be the highest than that of under sunlight, and the efficiency of phenol removal increased with increasing the irradiation time. By comparing photo-degradation efficiency with TiO 2 / UV method in batch reactor and continuous reactor under same condition (Concentration =10 mg/l, t = 2 hr, T = 25oC, P = 15 W), it was noted that the photodegradation efficiency (%) using batch reactor was better than using continuous mode of study because the mechanism of degradation could be achievable under prolonged exposures to UV irradiation. However the variation in terms of degradation was very small and therefore continuous process was better since handling of feed liquid was more compared to batch method. Similarly though the time of exposure increases the degradation efficiency, the difference was very small between 2 hr and 10 hr contact time. Therefore it is concluded that 2 hr operation is suggested as it reduces the cost of pumping. 3.2.1 Phenol removal under UV irradiation in continuous reactor The same conditions (UV irradiation, 15W) were used under sunlight for three samples which have different irradiation time (2, 4 and 10 hr) using 0.20 wt % of TiO 2 (Figure 9) and found that the efficiency of degradation of phenol under UV irradiation was higher than that of under sunlight, and the efficiency of phenol removal increased with increasing the irradiation time. 3.2.2 Effect of flow rate recirculation: Different flow rate (1.2 l/min and 1.6 l/min) have been tested in continuous mode of study using 0.2 wt% of TiO 2 under UV in the irradiation time (2 hr), and it was found that the efficiency of degradation of phenol in a flow rate of 1.2 l/min was better than that with a flow rate of 1.6 l/min and the apparent rate constants (k app ) also more for flow rate 1.2 l/min compared to flow rate of 1.6 l/min. Better removal efficiency of phenol was found to be in a lower flow rate compared to higher flow rate [1]. The same conditions (UV irradiation, 15W) were used under sunlight for three samples which have different irradiation time (2, 4 and 10 hr) using 0.20 wt % of TiO2 (Figure 9) and found that the efficiency of degradation of
phenol under UV irradiation was higher than that of under sunlight, and the efficiency of phenol removal increased with increasing the irradiation time. 3.2.3 Effect of flow rate recirculation: Different flow rate (1.2 l/min and 1.6 l/min) have been tested in continuous mode of study using 0.2 wt% of TiO 2 under UV in the irradiation time (2 hr), and it was found that the efficiency of degradation of phenol in a flow rate of 1.2 l/min was better than that with a flow rate of 1.6 l/min and the apparent rate constants (kapp) also more for flow rate 1.2 L/min compared to flow rate of 1.6 l/min. Better removal efficiency of phenol was found to be in a lower flow rate compared to higher flow rate [1]. 4. Conclusions: The performance of the photo-degradation of phenol in presence of TiO 2 was found to be the best method for its degradation. Several parameters have been studied, it appeared that irradiation time, light intensity, ph, dose of TiO 2 and concentration of phenol mainly controls the rate of degradation. It is concluded that Photo-degradation by TiO 2 is an effective method for the removal of phenol from wastewaters. The efficiency of the process depends strongly on the experimental conditions. An increase in UV light intensity resulted in a higher degradation rate of phenol. ph 7 is the optimum value for the rapid generation of OH radicals. This gives the TiO 2 a positive charge which attracts the negatively charged phenol onto the surface of the catalyst resulting in efficient degradation. At low loading from 0 to 0.20 wt% the degradation of phenol increased with increased TiO 2 concentration and at loading over 0.20 wt % degradation of phenol is a bit or constant with increased TiO 2 concentration. Pseudo-first order was best to describe the phenol degradation as per kinetic studies. The efficiency of degradation of phenol under UV Irradiation is the higher than that under sunlight. The photo-degradation by titanium dioxide (TiO 2 ) has low operation cost and a very high probability of phenol degradation. The Langmuir Hinshelwood kinetic model fits the degradation data Phenol removal under UV irradiation in continuous reactor has been modeled with References: 1. Laoufi NA, Tassalit D, Bentahar F (2008) The degradation of phenol in water solution by TiO2 photocatalysis in a helical reactor. Global NEST Journal. 10: 404-418. 2. Hosseini SN, Borghei SM, Vossoughi M, Taghavinia N (2007) Immobilization of TiO2 on perlite granules for photocatalytic degradation of phenol. Applied Catalysis B : Environmental. 74: 53-62. 3. McManamon C, Holmes JD, Morris MA (2011) Improved photocatalytic degradation rates of phenol achieved using novel porous ZrO2-doped TiO2 nanoparticulate powders. Journal of Hazardous Materials. 193: 120 127. 4. http://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=1365&context=etd 5. Sonawane RS, Dongare MK (2006) Sol gel synthesis of Au/TiO2 thin films for photocatalytic degradation of phenol in sunlight. Journal of Molecular Catalysis A: Chemical. 243: 68-76. 6. Wang Wendong, Serp Philippe, Kalck Philippe, Luis Faria Joaquim (2005) Photo-catalytic degradation of phenol on MWNT and titanium composite catalysts prepared by a modified sol gel method. Applied Catalysis B : Environmental. 56: 305-312. 7. Wei Li, Liu S (2012) Bifunctional activated carbon with dual photocatalysis and adsorption capabilities for efficient phenol removal. Adsorption 18: 67-74.
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