PHOTOCATALYTIC DEGRADATION OF PHENOL FERYAL AKBAL1 and A. NUR ONAR2 1 Ondokuz Mayıs University, Faculty of Engineering, Environmental Engineering Department, Kurupelit-Samsun, Turkey; 2 Ondokuz Mayıs University, Faculty of Arts and Science, Chemistry Department, Kurupelit-Samsun, Turkey (∗ author for correspondence, e-mail:
[email protected])
(Received 11 December 2001; accepted 7 June 2002)
Abstract. In this study photocatalytic degradation of phenol in the presence of UV irradiated TiO2 catalyst and H2 O2 was investigated. Effects of TiO2 and H2 O2 concentrations and pH on photocatalytic degradation were examined. The rate constants for photocatalytic degradation were evaluated as a function of TiO2 and H2 O2 concentrations and pH of the solution. It was found that photodegradation is an effective method for the removal of phenol and disappearance of phenol obeyed first order kinetics. The amount of CO2 produced during photocatalytic degradation was corresponding to the complete mineralization. Photodegradation can be an alternative method for the treatment of phenol containing wastewaters. Keywords: phenol, photocatalytic degradation, titanium dioxide, ultraviolet light
1. Introduction Contamination of water supplies is an increasing problem. Most conventional treatment processes are effective in water treatment but they only transfer the contaminants from one medium to another or generate waste that requires further treatment and disposal (Crittenden et al., 1997; Topodurti et al., 1993). There is a need to develop effective methods for degradation of resistant pollutants to less harmful compounds or for their complete mineralization. The transfer of contaminants from water to another phase is not an ideal remedy. Destructive oxidation treatments provide more permanent solutions (Matthews, 1991). Advanced oxidation processes have lately shown promise for water and wastewater treatment. Photodegradation is a promising method for the treatment of toxic and bioresistant pollutants (D’Oliveria et al., 1990; Kochany, 1993). The primary oxidant responsible for the oxidation of organic compounds is the highly reactive hydroxyl radical. Many different techniques can be employed to generate hydroxyl radicals with all methods involving the use of an oxidant together with an activating system (Winterbottom et al., 1997). Photocatalytic reactions on irradiated semiconductor powders have good potential for the removal of organic and inorganic waste materials from water. Titanium dioxide is a semiconductor that is frequently used in organic degradation experiments; it is non toxic, insoluble in water and comparatively cheap (Lakshmi et Environmental Monitoring and Assessment 83: 295–302, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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al., 1995). Many organic compounds are decomposed in aqueous solution in the presence of titanium dioxide powder illuminated with UV light (Tennakone et al., 1995). Phenols and phenolic compounds are common pollutants of aquatic system. The chlorination of natural waters for disinfection produces chlorinated phenols. Activated carbon adsorption is generally accepted for phenolic pollutant removal from aqueous phase. However this process produces spent carbon as a waste byproduct. Therefore it must be desirable to examine mineralization technologies for detoxification that chemically treat the pollutant (Davis and Huang, 1990). The aim of this study is to examine the decomposition of phenol by photodegradation. Photocatalytic degradation rates of phenol have been examined in the presence of TiO2 and H2 O2 under UV illumination. Effects of TiO2 and H2 O2 concentrations and pH on photocatalytic degradation efficiency were also investigated.
2. Experimental Phenol and photocatalyst were obtained from Merck Chemical Company (Darmstadt, Germany). All the chemicals used were analytical grade. Phenol solutions used in the experiments were prepared at 100 ppm concentration. Photodegradation experiments performed in the batch system in the presence of H2 O2 and/or TiO2 under UV illumination. The illumination was provided by Osram 300 W Ultravitalux sunlamp. Effects of the parameters such as H2 O2 and TiO2 concentrations and pH on the photodegradation efficiency were investigated. Aqueous suspensions containing phenol and TiO2 and/or H2 O2 were stirred during irradiation. For the all experiments the pH of reaction mixture was adjusted at the start of the reaction. 0.1 M H2 SO4 and 0.1 M NaOH solutions were used to change the pH of the solution. If necessary the samples were centrifuged at 4000 rpm for 15 min. The progress of reaction was followed by monitoring the disappearance of the phenol at 500 nm (APHA, 1985). The amount of phenol decomposed with respect to time was estimated. The rate constants were evaluated from the plots of log concentration versus irradiation time. Carbon dioxide evolved during photocatalytic mineralization of phenol was determined as BaCO3 gravimetrically after absorption in Ba(OH)2 solution.
3. Results and Discussion The effect of amount of catalyst on photocatalytic decomposition was investigated. Experiments were performed with various amount of catalyst at constant phenol concentration. Catalyst concentrations used in this study are 0.1, 0.2, 0.3 (w/w) %. The oxidation process is very sensitive to the pH of aqueous system and effect of pH was studied for two different reaction systems employing UV irradiation with
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TABLE I Decomposition of phenol solutions using different processes Process
Initial pH
% Phenol decomposition Concentration (ppm)
15 min
30 min
60 min
UV TiO2 (TiO2 = 0.1 w/w %)
3.0 5.0 8.0
100 100 100
11.0 11.0 6.0
25.0 38.8 20.5
53.5 70.7 58.8
UV/TiO2 (TiO2 = 0.2 w/w %)
3.0 5.0 8.0
100 100 100
12.5 20.0 8.5
23.8 45.5 25.0
62.0 77.5 68.8
UV/H2 O2 (H2 O2 = 0.1 M)
3.0 5.0 8.0
100 100 100
40.0 55.0 42.4
60.0 72.7 77.0
86.8 92.0 96.5
UV/H2 O2 (H2 O2 = 0.3 M)
3.0 5.0 8.0
100 100 100
64.0 72.4 50.4
82.0 86.7 87.5
98.0 99.1 99.9
and without TiO2 catalyst. Effect of H2 O2 addition on phenol photodecomposition in the presence of TiO2 was also investigated. Enhanced oxidation rates were observed for phenol oxidation when H2 O2 was added UV TiO2 system. Decomposition of phenol solutions using different processes were given in Table I. Complete mineralization of 100 ppm phenol solutions was achieved in very short irradiation times. Phenol photodegradation efficiencies with different processes ranged between 53.5–99.9%. The amount of CO2 evolved was determined by trapping in Ba(OH)2 solution. These measurements demonstrate that phenol can be completely mineralized. The dependence of phenol concentration on the irradiation time was fitted to an exponential function. First order kinetics with respect to phenol concentrations were found to fit all the experimental data and first order rate constants were estimated. The rate of disappearance of phenol may be described by following equation. r = −dC/dt = kC ,
(1)
where, k is the first order rate constant. First order rate constants for phenol oxidation were given in Table II. At the beginning of irradiation degradation rate is expressed as initial degradation rate. rin = kC0 ,
(2)
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TABLE II First order rate constants for phenol photodegradation Process
Initial pH
Rate constant (k, min−1 )
Correlation coefficient (R2 )
UV-TiO2 (TiO2 = 0.1 w/w %)
3.0 5.0 8.0
0.0129 0.0212 0.0147
0.980 0.987 0.922
UV-TiO2 (TiO2 = 0.2 w/w %)
3.0 5.0 8.0
0.0162 0.0258 0.0192
0.950 0.992 0.917
UV-H2 O2 (H2 O2 = 0.1 M)
3.0 5.0 8.0
0.0323 0.0400 0.0562
0.987 0.994 0.989
UV-H2 O2 (H2 O2 = 0.3 M)
3.0 5.0 8.0
0.0624 0.0746 0.1205
0.987 0.991 0.955
UV-TiO2 -H2 O2 (TiO2 = 0.1 w/w%, H2 O2 = 0.1 M)
3.0 5.0 8.0
0.0772 0.0786 0.0732
0.997 0.996 0.984
where, C0 is initial phenol concentration in the system. The calculated values of the initial rate were used for a comparison of the efficiency of the photocatalytic process under different reaction conditions. Initial oxidation rates were accelerated as compared with reactions without any H2 O2 added to the reaction mixture. Initial photocatalytic degradation rates for phenol oxidation were given in Table III. It was found that photodegradation efficiency affected by TiO2 and H2 O2 concentrations and pH of the solution. The photodegradation curves for phenol decomposition at various H2 O2 and TiO2 concentrations are given Figure 1. The effect of pH on the degradation of phenol was investigated for the pH range 3.0– 8.0. High pH values favour the formation of carbonate ions which are effective scavengers of hydroxyl ions and can reduce the efficiency of degradation process. The reaction at high pH is slower consistent with the decrease in the adsorption, as the semiconductor surface becomes negatively charged and phenolate becomes dominate organic form. Figure 2 shows the effect of pH on phenol decomposition in aqueous TiO2 suspension. It can be seen that a more rapid rate of phenol degradation occurred for lower pH solutions, in particular at pH 5.0 best results were observed. At pH
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TABLE III Initial degradation rates for phenol solutions Process
Initial pH
Initial oxidation rate (mg L−1 min)
UV/TiO2 (TiO2 = 0.1 w/w %)
3.0 5.0 8.0
1.29 2.12 1.50
UV/H2 O2 (H2 O2 = 0.1 M)
3.0 5.0 8.0
3.23 4.00 5.60
UV/TiO2 /H2 O2 (TiO2 = 0.1 w/w%, H2 O2 = 0.1 M)
3.0 5.0 8.0
7.72 7.86 7.32
Figure 1. Effects of TiO2 and H2 O2 concentrations on photocatalytic decomposition of phenol.
5.0 70.3% phenol degradation in 60 min of irradiation was achieved when UV with TiO2 catalyst was used. The addition of H2 O2 has been observed to enhance phenol oxidation in the presence of TiO2 . The removal efficiency increased to 99.2% when H2 O2 was added to UV/TiO2 system. Figure 3 shows photocatalytic decomposition of phenol with different processes. It is well known that hydrogen peroxide coupled with ultraviolet radiation is used
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Figure 2. Effect of pH on phenol decomposition.
Figure 3. Photocatalytic decomposition of phenol with different process.
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for the photodegradation of pollutants in homogenous media and also in heterogeneous systems. Experiments have demonstrated the beneficial effect of the addition of hydrogen peroxide on the efficiency of phenol photodegradation in the presence of titanium dioxide. The effect of hydrogen peroxide on phenol photodecomposition can be explained by the oxidative action of radical species produced from oxygen photoreduction and hydrogen peroxide photodecomposition on titanium dioxide surface.
4. Conclusion This study shows the potentialities of photocatalytic degradation in water purification. Photodegradation can be an alternative treatment method for those contaminants resistant to conventional methods. Photodegradation can be used for complete mineralization of phenol. First order rate expression can be used to describe photodegradation reaction of phenol. Titanium dioxide is inexpensive catalyst and can be reused. It was found that photodegradation reactions were dependent on the value of pH and concentrations of H2 O2 and TiO2 . It was observed that the amount of CO2 produced during the photodegradation was corresponding to the complete mineralization. The data point to the possible use of photocatalytic degradation as an effective method for completely removal of phenol. In conclusion, photodegradation can be a recommended approach for the treatment of phenolic wastewaters.
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