8-Rahmani

JRHS 2009; 9(2): 48-51

Copyright © Journal of Research in Health Sciences

Investigation of Photocatalytic Degradation of Phenol by UV/TiO2 Process in Aquatic Solutions

Rahmani AR (PhD)a, Samadi  MT (PhD)a, Enayati Moafagh Ab

a Department  of Environmental Health Engineering, Hamadan University of Medical Sciences, Iran

b Department of Environmental Science, Science and Research branch, Azad Eslamic University, Iran

*Corresponding author: Dr Alireza Rahmani ,   E-mail: rahmani@umsha.ac.ir

Received: 19 April 2008; Accepted: 29 Aug ust2008

Abstract

Background: The main objective of this research was to study photocatalytic oxidation of phenol in aquatic solutions by using of UV, TiO2 and the combination of them. Phenol compounds are widely used in industries and other daily life. Highly toxicity and carcinogenicity of these compounds can causal considerable adverse effects on the aquatic ecosystems and human health.

Methods: The study was an experimental research and carried out by phenol 100 mg L-1 solution and contact time, pH and amount of TiO2 were considered in photocatalytic system as the basic variables affecting removal efficiencies. Phenol concentration in inlet and outlet samples was detected by spec­trophotometery at 500nm.

Results: Combination of UV and TiO2 can cause higher efficiencies of phenol removal. The removal efficiencies of UV, TiO2 and UV/TiO2 photocatalytic oxidation system with various operation condi­tions had a range of 1.8%-19.64%, 2.38%-17.8% and 34.65%-82.91%, respectively. It was found that increasing of TiO2 concentration and contact time and pH , increasing the amount of phenol that oxi­dized in combined system and maximum removal coefficients obtained in pH=11 , 9 h contact time and 0.2 gr of TiO2.

Conclusion: Combinated UV/TiO2 process may be applied as an effective process for the removal of phenol from aquatic solutions such as industrial wastewaters and polluted water resources. 

Keywords: Phenol, UV ray, TiO2, Photocatalysis

Introduction

In industries and daily life, phenol com­pounds are widely used and have become common pollutants in water bodies (1). Due to their stability and bioaccumulation, they remain in the environment for longer peri­ods. Highly toxicity and carcinogenicity of these compounds can cause considerable ad­verse effects on the aquatic ecosystems and human health. Therefore use of an effective and economic method for the elimination of phenol in water solutions has been in urgent demand (2). Traditional wastewater treat­ment techniques for phenol removal are ac­tivated car­-

bon adsorption, chemical oxida­tion and bio­logical digestion (1). However each tech­nique has some limitations and dis­advantages (2).

The application of photocatalysis in the treatment of phenol in wastewater is an in­teresting alternative and is the object of a great interest over the last years by many re­searchers (1, 3-5). In these researches atten­tion has been drawn toward an alterna­tive technique where the pollutants are de­graded by irradiation suspension of metal oxide semi­conductor particles such as TiO2 or ZnO (1, 4). TiO2 is known to be an excel­lent photo­catalyst for complete mineraliza­tion of phe­nol in water and wastewater(6, 7). It is non-toxic, insoluble in water and com­paratively cheap (8, 9). Many organic com­pounds are de­composed in aqueous solution in the pres­ence of titanium dioxide powder illuminated with UV ray (6).

TiO2 (anatase) has an energy band gap of 3.2 eV and is capable to be activated by near UV-light with wavelengths up to 388 nm. Light energy from ultraviolet radiation in the form of photons, excites the electrons on the surface of titanium atoms suspended in the contaminated water, moving them from the valence band to the conductance band. This absorption corresponds to between 3 to 4 per­cent of the solar spectrum (10). The result of this energy change is the formation of holes in the surface of titanium atom, and free elec­trons, which are now available to form hy­dro­xide (OH) or other radicals, which can oxi­dize organic chemicals, or re­duce metal spe­cies (1, 2, 11). In all cases, the degradation process mainly involved the formation of hy­droxyl radicals (12). Al­though this is a small fraction of the spec­trum, many studies have been carried out to develop an efficient method for using natural solar radiation to destroy toxic organic com­pounds (9, 10) and to re­duce toxic metals (13).

The main objective of this research was to study of photocatalytic oxidation of phenol in the presence of UV ray, TiO2 and investi­gation of the role of main factors that af­fecting the process.

Methods

Phenol solution obtained from Merck Chemi­cal Co., was prepared in 100 mg L-1 concen­trations. The concentration of phenol was meas­ured at 500 nm by a Shimatsu spectro­photometer. TiO2 was used mostly anatase and had a BET surface area of 50 m2 g-1 and an average particle diameter of 30 nm. pH adjustments in samples were done by using 0.1 N HCl acid (Merck, 37%) and 0.1N NH4OH (Merck 25%). A low pressure UV lamp, 0.8 m in length was installed 10 cm above the samples surface and the light in­tensity were 0.9 J s-1m-2 measured by a Ra­di­ometer at 300 to 400 nm (Hagner EC1-UV-A). All analysis were done according to the standard methods (14).

Experimental set-up and measurements

In this research, the photochemical cell con­sisted of five 250 ml beaker and 5 magnetic stirrers that used for stirring the samples. The temperature of reactors was controlled at 20o C. In the first phase, the beakers were filled with 200 ml of phenol solution (100 mg L-1) and in separate stages were contacted with UV, TiO2 and combination of UV/TiO2. Also the effects of various parameters include con­tact time (1, 3, 5, 7 and 9 h), pH (3, 7 and 11) and amount of TiO2 (0.5 and 1.0 gr L-1) in the removal efficiency of phenol were inves­ti­gated. The polished aluminium was used as the reflective material to protect the samples in the absence of radiation.

The samples were withdrawn from the beak­ers with the syringe (10 ml) before and after the photochemical reaction and the residual phe­nol was measured. Before phenol meas­ure­ment, the samples containing TiO2 were cen­trifuged at 5000 rpm for 10 min, and then the upper liquid layer were sucked with a glass syringe and filtered. Then concentra­tion of phenol in the prepared samples was measured. All experiments were conducted at lab tem­perature (nominally 20° C). The experimental apparatus as shown in Fig. 1 was set up in Ha­madan University of Medi­cal Sciences in 2006.

Figure 1:  Schematic of phenol removal photocatalytic pilot

Results

The results from examinations in pH= 3, 7 and 11 as the function of exposed time in the stud­ied processes are shown in Figures 2-4, respec­tively. It is found that phenol degra­daion ef­ficiency is affected by TiO2 con­cen­-

tration, UV irradiation and pH of the so­lution.

Figure 2: Removal efficiency of phenol at various conditions (pH=3)

Figure 3: Removal efficiency of phenol at various conditions (pH=7)

Figure 4: Removal efficiency of phenol at various conditions (pH=11)

Also the effect of time variation on removal phenol at different pH values is shown in Fig. 5.  The results show that degradation yield in a 9 h period with pH= 3, 7 and 11 has been 59%, 52% and 83 %, respectively.

Figure 5: Effect of time variation on phenol removal (TiO2=1 gr L-1, initial phenol concentration=100 mg L-1)

Discussion

The photocatalytic degradation of various toxic organic compounds has been proposed as a viable process to detoxify aquatic solu­tions. Irradiating pulverulent semi-conduc­tors like TiO2 in suspension or fixed to vari­ous supports in aqueous solutions containing organic pollutants, creates a redox environ­ment able to destroy these pollutants. Solar photocatalytic mineralization of organic water pollutants has a strong potential in the indus­trial destruction of toxic organics in water as this has been widely demonstrated in recent years, and the applications and tar­get com­pounds are numerous.

Nowadays, phenol compounds are one of the most common pollutants of water resources. Photocatalytic oxidation of phenol by using TiO2 as a catalyst and UV ray is one of the developing methods. In this study, photo­cata­lytic removal of phenol was investigated by stressing on contact time, pH and initial phe­nol concentrations. The results from ex­ami­nations in pH= 3, 7 and 11 as the function of exposed time in the studied processes are shown that phenol degradation efficiency is affected by TiO2 concentration, UV irradia­tion and pH of the solution. Also Mansouri and yang reported removal of Cr6+ and hu­mic acid in aquatic solutions by application of UV/TiO2 (15, 16).

By increasing the contact time, efficiency of phenol removal is raised. The most observed yield is related to pH=11 and the least to pH=7. Use of UV ray to degrade phenol has less efficiency. Presence of TiO2 in these proc­esses can promote removal efficiency, remarkably. The results show that the phenol degradation increases with increasing TiO2 concentration but the difference neglect able, Sun and et al reported that TiO2 concentra­tion affected the phenol oxidation by visible light (17). Phenol was more effectively elimi­nated by using combined UV/TiO2 condition than either than UV or TiO2 sepa­rately. Lee et al. reported a similar results for the re­moval of organic compounds  (18). 

The results show that in all mentioned pH scales, UV and TiO2 in separate stage can not degrade phenol remarkably. In use of UV and TiO2 synchronize, pH has an im­portant effect. Yang and Iliev reported that the in­crease of pH, increased the removal effi­ciency of humic acid and phenol in aquatic solu­tions (15, 19). 

As a result, increasing pH leads to reducing the process time. The process efficiency in pH=11, during 3 h was 74% and by in­creas­ing contact time to 9 h, the yield is just 8.8 % developed (3). The reason is that the reaction rate is related to phenol concentra­tion and by going on time, decrease in con­centration of phenol results in decreasing the efficiency (1, 2, 4).

Thus, when phenol as a hydroxide possess­ing material was solute in water bodies with alkali pH, it was converted to phenoxide ion that more degradable than phenol. Con­versely in acidic pH, phenol has a little de­gradability. Since TiO2 that was used in this study, is a recoverable photocatalyst, UV/TiO2 process for phenol degradation is compatible with the environmental.  

In conclusion, the reaction rates of photo­catalytic degradation of phenol were influ­enced by pH value. Photodegradation can be an alternative treatment method for those contaminants resistant to conventional meth­ods. Photodegradation can be used for com­plete mineralization of phenol. Since TiO2 that was used in this study, is a recov­erable photo­catalyst, UV/TiO2 process for phenol degra­dation is compatible with the environmental.

Acknowledgments

We gratefully acknowledge financial support for this project from the Environmental Health Engineering Dept., Faculty of Health, Hama­dan University of Medical sciences. We also appreciate Mrs. Parvin Ahmadi for their kindly helps and efforts. The authors declare that they have no conflict of inter­ests.

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