Photocatalytic Degradation of 4-Chlorophenol in Aerated Aqueous

Improving removal of 4-chlorophenol using a TiO 2 photocatalytic system with microwave and ultraviolet ...... Bettina Bems , Friederike C. Jentoft , R...
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Langmuir 1996, 12, 6368-6376

Photocatalytic Degradation of 4-Chlorophenol in Aerated Aqueous Titanium Dioxide Suspensions: A Kinetic and Mechanistic Study J. Theurich, M. Lindner, and D. W. Bahnemann* Institut fu¨ r Solarenergieforschung GmbH, Sokelantstrasse 5, 30165 Hannover, Germany Received March 11, 1996. In Final Form: September 9, 1996X The photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous titanium dioxide suspensions has been studied and compared for two different catalysts: Sachtleben Hombikat UV 100 and Degussa P25. The influence of pH and initial 4-CP concentration on reaction rate and product distribution has been investigated in detail. Regarding the initial 4-CP concentration, the degradation kinetics formally can be discribed by a Langmuir-Hinshelwood expression. While the degradation of 4-CP itself is hardly influenced by the pH, the overall mineralization rate (measured in terms of total organic carbon reduction) changes considerably, decreasing strongly at pH > 7. With P25 used as the photocatalyst the mineralization of 4-CP is slightly faster than that with Hombikat UV 100, but smaller concentrations and numbers of intermediates are detected in the latter case. Most of the intermediates observed during this photocatalytic degradation process are identical with those identified during direct photolysis. In acidic solution (pH 3) hydroquinone and benzoquinone are the only intermediates when Hombikat UV 100 is the photocatalyst. Using P25 as a photocatalyst, seven additional intermediates, namely, hydroxyhydroquinone, hydroxybenzoquinone, phenol, 4-chlorocatechol, 4-hydroxyphenylbenzoquinone, 2,5,4′-trihydroxybiphenyl, and 5-chloro-2,4′-dihydroxybiphenyl, were found. Hydroquinone (HQ) and benzoquinone (BQ) have been found to be the main intermediates of the photocatalytic 4-CP degradation. It could be shown that the HQ/BQ photosystem acts as an electron relay which effectively short circuits the photocatalyst resulting in a reduced efficiency of the degradation of 4-CP and most probably other aromatic compounds. An electron shuttle mechanism is proposed to explain the highly efficient short circuiting of the photocatalyst in the presence of these compounds. A degradation mechanism for 4-chlorophenol is proposed and discussed to account in particular for the pH dependence of the 4-CP degradation.

Introduction The widespread presence of persistent organic chemicals as pollutants in waste water effluents from industries or even normal households is a serious environmental problem, since such compounds can be found in groundwater wells and surface waters where they have to be removed to achieve drinking water quality.1,2 Therefore many processes have been proposed over the years and are currently employed to destroy these toxins. In recent years attention has been drawn toward an alternative techniquesthe so-called photocatalytic detoxificationswhere the pollutants are degraded by irradiating suspensions of metal oxide semiconductor particles such as TiO2 or ZnO.3-7 Several reviews have been published recently describing the principal mechanism of photocatalysis and presenting various examples for its application.8-18 It has been pointed out that there is an * Author to whom correspondence should be addressed: tel, ++49 +511/3585037; fax, ++49 +511/3585010; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Ha¨fner, M. Natur 1989, 10, 20. (2) Dieter, H. H. Wechselwirkung 1989, 43, 4. (3) Matthews, R. W. Water Res. 1986, 20, (5), 569. (4) Al-Sayyed, G.; D’Oliveira, J.-C.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1991, 58, 99. (5) Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A.; Marchese, L.; Martra, G. J. Appl. Catal. 1991, 69, 323. (6) D’Oliveira, J.-C.; Minero, C.; Pelizzetti, E.; Pichat, P. J. Photochem. Photobiol. A: Chem. 1993, 72, 261. (7) Sehili, T.; Boule, P.; Lemaire J. Photochem. Photobiol. A: Chem. 1989, 50, 117. (8) Hoffmann, M.; Martin, S.; Choi, W.; Bahnemann, D. Chem. Rev. 1995, 95, 69. (9) Ollis, D. F.; Al-Ekabi, H. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (10) Blake, D. M. Bibliography of Work on the Photocatalytical Removal of Hazardous Compounds from Water and Air; National Renewal Energy Laboratory, 1994.

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apparent lack of knowledge concerning details of the photocatalytic degradation mechanism in particular for more complex molecules. This type of information, however, is important for a technical realization of the method avoiding undesired reactions (intermediates and toxic byproducts). In the following we present our results of a detailed mechanistic study employing 4-chlorophenol (4-CP) as the model pollutant. The photodegradation of chlorophenols in aqueous solution has received considerable attention because these compounds are important xenobiotic micropollutants of the aquatic environment orginating, for example, from industrial chemical synthesis. More specifically, 4-chlorophenol (4-CP) is used for the production of quinizarin (a dye), clofibrate (a drug), and chlorophenesin and dichlorophen (fungicides).19 Therefore, several investigations of the photocatalytic decomposition of chlorophenols using metal oxide semiconductors either in aqueous heterogenous suspensions4,6,7,20-22 or in an immobilized (11) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. Rev. 1993, 22, 417. (12) Kamat, P. V. Chem. Rev. 1993, 93, 267. (13) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (14) Bahnemann, D. W. Isr. J. Chem. 1993, 33, 115. (15) Pichat, P. Catal. Today 1994, 19, 313. (16) Aithal, U. S.; Aminabhavi, T. M.; Shukla, S. S. J. Hazard. Mater. 1993, 33, 369. (17) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (18) Bahnemann, D.; Cunningham, J.; Fox, M. A.; Pelizzetti, E.; Serpone, N. In Aquatic and Surface Photochemistry; Helz, G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; p 261. (19) Oudjehani, K.; Boule, P. J. Photochem. Photobiol. A: Chem. 1992, 68, 363. (20) Barbeni, M.; Pramauro, P.; Pelizzetti, E. Nouv. J. Chim. 1984, 8, 547. (21) Stafford, U., Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98, 6343. (22) Cunningham, J.; Sedla´k, P. J. Photochem. Photobiol. A: Chem. 1994, 77, 255.

© 1996 American Chemical Society

Photocatalytic Degradation of 4-Chlorophenol

form23-25 have already been published. The kinetics of the photocatalytic 4-CP degradation have commonly been interpreted as being indicative of a Langmuir-Hinshelwood-type mechanism in which the limitation of the 4-CP decomposition rate at higher pollutant concentrations is related to the extent of adsorption of the pollutant molecule on the TiO2 surface.4,21 However, Cunningham et al. showed in a recent study22 that the observed adsorption constants of chlorophenols on TiO2 in the dark differ from equivalent data obtained from the kinetic analysis of their photocatalytic degradation employing the same TiO2 particles under UV irradiation. Although 4-CP is obviously an intensively studied model compound, the mechanism of its photocatalytic degradation is still not fully understood. In particular, the pH dependency of the photocatalytic 4-CP degradation has to the best of our knowledge so far been investigated only over a small range of initial pH values.4,20,26 Especially the influence of the pH on the concentration and number of intermediates has been studied only qualitatively. The present study has been concerned with the influence of the initial 4-CP concentration on its photocatalytic degradation in an attempt to gain further insights into the underlying reaction mechanism. The decomposition of 4-CP has furthermore been investigated in detail at different pH values using a pH-stat technique, which ensures constant pH throughout the irradiation even in the abscence of chemical buffer systems, thereby allowing the determination of photonic efficiencies that are completely independent from the pH. Moreover, the photocatalytic decomposition of the main intermediates observed during the photocatalytic 4-CP degradation, hydroquinone and benzoquinone, has been studied to identify mechanistic details which should also be of general importance for the photocatalytic degradation pathway of other aromatic molecules. Experimental Section Reagents. All chemicals were of reagent grade and used without further purification. The water employed was purified by a Milli-Q/RO system (Millipore) resulting in a resistivity >18 MΩ cm. P25 from Degussa and Hombikat UV 100 from Sachtleben Chemie were used as photocatalysts in this study. P25 consists of 75% anatase and 25% rutile with a specific BET surface area of 50 m2 g-1 and a primary particle size of 20 nm.27,28 Hombikat UV 100 is a newly developed TiO2 powder consisting of 100% pure anatase with a specific BET surface area of g250 m2 g-1 and a primary particle size of 5 nm.29 Procedures. Stock solutions containing the desired concentrations of 4-CP, hydroquinone (HQ) or benzoquinone (BQ) were prepared in water. The photochemical reactor was made of duran glass with a plain quartz window (on which the light beam is focused) equipped with a magnetic stirring bar, a watercirculating jacket, and five openings for electrodes and gas supplies. For the irradiation experiments 150 mL of the desired solution and the required amount of photocatalyst (5 g L-1) were added to the reactor. KNO3 (10 mmol L-1) was added to keep the ionic strength constant during irradiation. It was shown that the nitrate ion does not react with photogenerated electrons or holes under the chosen experimental conditions; i.e., no nitrite ions or nitroaromatics compounds were detected by HPLC. Thus, (23) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.; Draper, R. B. Langmuir 1989, 5, 250. (24) Hofstadtler K.; Bauer, R.; Novalic, S.; Heisler, G. Environ. Sci. Technol. 1994, 28, 670. (25) Vinodgopal, K.; Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98, 6797. (26) Matthews, R. W. J. Chem. Soc., Faraday. Trans. 1984, 180, 457. (27) Degussa Tech. Bull. 1984, 56, 8. (28) Bickley, R. I.; Gonzalez-Carreno, T.; Lees, J. S.; Palmisano, L.; Tilley, R. J.D. J. Solid State Chem. 1992, 92, 178. (29) Lindner, M.; Bahnemann, D. W.; Hirthe B.; Griebler, W.-D. In Solar Engineering; Stine, W. B., Tanaka, T., Claridge, D. E., Eds.; 1995, Book No. H0932A, p 399.

Langmuir, Vol. 12, No. 26, 1996 6369 KNO3 can be regarded as an inert compound. Before the irradiation was started, the reactor containing the suspension was placed into an ultrasonic bath for 10 min to dash larger agglomerates, then the suspension was stirred for at least 1 h in the dark to allow equilibration of the system. To ensure a constant pH value throughout the experiment making the addition of any buffer solution unnecessary, a pH-stat technique was employed. Details about this technique have been reported elsewhere.30 To ensure a constant O2 concentration the suspensions were continuously purged with molecular oxygen throughout the experiment. Irradiations were carried out with a highpressure mercury lamp (Osram HBO 500W). IR radiation and short-wavelength UV radiation were eliminated by a 10 cm water filter and a 320 nm cut off filter. Samples (4.5 mL) were taken before and at regular intervals during the irradiation. They were centrifuged with a Heraeus Sepatech Labufuge GL before analyses. An actinometry was performed using Aberchrome 54031 in order to determine the total incident light intensity in the wavelength region between 310 and 370 nm which can be absorbed by TiO2. This technique allows the determination of the incident photon flux entering a photoreactor of a specific geometry at the inner front window. This avoids the necessity of corrections for any influences of light reflections taking into account the reactor geometry. The light intensity throughout all experiments in this study was between 1.01 and 1.06 mmol of photons L-1 min-1. The photonic efficiency ξ was calculated for each experiment as the ratio of the photocatalytic degradation rate and the incident light intensity.32 Its validity for a better comparison of photocatalytic systems has been experimentally verified in several papers recently published.8,33,34 For a wide range of organic pollutants (including also 4-CP) a square root dependency of the photonic efficiency ξ on the light intensity has been observed for moderate photon fluxes.4,18,29,43 It was not the aim of the present work to study the influence of the light intensity on ξ, however, as this paper concentrates on the underlying reaction mechanism. For each experiment the degradation rate was determined from the initial slope of the individual concentration versus time profiles. In addition to photocatalysis, blank photolysis experiments were carried out at pH 3 using band cutoff filters of 295 and 320 nm, respectively, under the same conditions as indicated above. Analyses. The concentrations of 4-CP and most of its reaction intermediates were measured by HPLC using a Dionex 4500i chromatograph equipped with an ET 250/8/4 Nucleosil 10 C18 reversed phase column (Macherey & Nagel). All substances were detected by a UV detector at 280 nm. The eluent consisted of a ternary mixture of water (containing 1 vol % acetic acid), methanol, and acetonitrile (60:30:10 by volume); the flow rate was 1 mL min-1. Concentrations of compounds, which were not commercially available, were calculated using the equation which was derived from calibration measurements for hydroquinone, except for hydroxybenzoquinone which was treated according to the BQ calibration. These calculated concentrations should, of course, not be taken as exact data, but they represent a reasonable estimate of the concentrations of intermediates formed. The real concentrations will generally be smaller than the calculated values because all of these intermediates are higher substituted phenols, biphenyls, or quinone derivatives of biphenyls (see Results for details). Therefore, these compounds represent larger chromophoric systems than the two reference substances HQ and BQ. Thus, the molar extinction coefficients of these compounds will be higher than those for HQ and BQ and, consequently, the calculated should be higher than the real concentrations. Further intermediates were determined by GC/MS analysis. The samples were prepared by extracting the aqueous solution at pH 3 three times with 30 mL dichloromethane and then again three times with 30 mL of diethyl ether. The dichloromethane (30) Bahnemann, D.; Bockelmann, D.; Goslich, R. Sol. Energy Mater. 1991, 24, 564. (31) Heller, H. G.; Langan, J. R. EPA Newslett. 1981, October, 71. (32) Serpone, N.; Terzian, R.; Lawless, D.; Kennepohl, P.; Sauve´ G. J. Photochem. Photobiol. A: Chem. 1993, 73, 11. (33) Tahiri, H.; Serpone, N.; Le van Mao, R. J. Photochem. Photobiol. A: Chem. 1996, 93, 199. (34) Serpone, N.; Sauve´, G.; Koch, R.; Tahiri, H.; Pichat, P.; Piccinini, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol. A: Chem. 1996, 94, 191.

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Figure 1. Influence of the initial 4-CP concentration on the photonic efficiency of the 4-CP and the TOC degradation. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL extracts contained more than 80% of the remaining 4-CP, about 50% of the benzoquinone (BQ), and almost quantitatively traces of less polar substances, e.g., chlorinated biphenyls. While most of the hydrophilic compounds, especially hydroxyhydroquinone (HHQ) and hydroquinone (HQ), remained in the aqueous phase, moderately polar intermediates, e.g., trihydroxybiphenyls or 4-chlorocatechol (4-CC), could be found in the etheric phase. The etheric extracts were evaporated to a final volume of 20 mL, dried over sodium sulfate, and taken directly for GC/MS analysis. Chloride measurements were carried out using a Metrohm system, consisting of an ion selective electrode (6.0502.120), a reference electrode (6.0726.100) and a voltmeter (691). The voltage was recorded continuously on a personal computer. Total organic carbon (TOC) was monitored with a Shimadzu TOC 500 analyzer measuring directly the aqueous solutions after centrifugation. TOC concentrations were calculated under the assumption that all organic molecules consist of six carbon atoms. The pH-stat technique not only guarantees constant pH conditions but also allows an extremely sensitive in-situ measurement of H+aq formation or depletion during the photolysis experiments. It has been shown that this method is suitable to monitor the kinetics of the photocatalytic degradation of rather simple molecules such as chloroform,35 tetrachloromethane,36 or dichloroacetic acid29 where practically no intermediates are formed, and therefore the formation of H+aq corresponds to the generation of chloride ion. However, it has been observed during this study that for the destruction of more complex molecules such as 4-CP the formation of H+aq only agrees with the formation of chloride ions when the concentration of intermediates can be neglected. Thus, quantitative results from the pH-stat measurements have not been used for kinetic analyses here.

Results Concentration Dependence. The photocatalytic degradation of 4-CP can be described by the following overall reaction stoichiometry h*ν

h*ν

ClC6H4OH + 13/2 O2 9 8 [intermediates] 9 8f [TiO ] [TiO ] 2

2

6CO2 + HCl + 2H2O (1) As shown in other publications4,22,23 the rate, and therefore the photonic efficiency, of the degradation process is a function of the initial 4-chlorophenol concentration, at least in a certain concentration range, when other parameters, such as the concentration of molecular oxygen, the ionic strength, and the light intensity, were kept constant. Figure 1 shows the dependence of the (35) Lindner, M. Dissertation im Fachbereich Chemie der Universita¨t Hannover, in progress. (36) Hilgendorff M.; Hilgendorff M.; Bahnemann, D. J. Adv. Oxid. Technol. 1996, 1, 35.

photonic efficiency of the 4-CP and TOC degradation on the initial 4-CP concentration. The photonic efficiency increases as the initial concentration of 4-CP is raised up to 5 mmol L-1 for both, TOC and 4-CP degradation, followed by a plateau region with a photonic efficiency around 0.7%. It should be noted that the photonic efficiencies of the 4-CP and TOC degradation do not differ greatly from each other; i.e., 4-CP is obviously completely mineralized under the chosen experimental conditions yielding CO2 and H2O without any indication of higher concentrations of intermediates. Identification of Intermediates. Nine different intermediates were detected by HPLC while studying the influence of the pH on the photocatalytic degradation of 4-CP. These intermediates were identical with those formed when 4-CP was photolyzed directly (which could be excluded under the selected experimental conditions), but the concentrations were about 10 times higher in the latter case.37 The individual compounds were identified either by comparision with the relative retention times of external standards or, in the case where the assumed byproducts were not commercially available, by comparison with the relative retention times reported by Oudjehani et al.,19 who employed similar HPLC conditions. Table 1 shows the observed retention times in comparison to the retention times of the external standards or those reported by Oudjehani et al., respectively. The structural formulas of all identified intermediates are shown in Figure 2. The main intermediates are in good agreement with the results of other research groups: 4,21,38 benzoquinone (BQ), hydroxybenzoquinone (HBQ), hydroquinone (HQ) and (especially for alkaline pH values) hydroxyhydroquinone (HHQ) (compare also Figures 4 and 5). Four minor intermediates were identified as phenol, 2,5,4′-trihydroxybiphenyl (THB), 4-hydroxyphenylbenzoquinone (HPBQ), and 4-chlorocatechol (4-CC), respectively. THB and 4-CC were also clearly identified by GC/ MS analysis. For alkaline pH values another byproduct was detected, which in analogy to the study of Oudjehani et al. was identified as 5-chloro-2,4′-dihydroxybiphenyl (CDHB).19 pH Effect. Although the pH can be one of the most important parameters for the photocatalytic process, its influence on the photocatalytic degradation has been investigated in detail only by a few research groups. Especially for the 4-CP degradation, to the best of our knowledge, no detailed investigation concerning the pH dependency has been published yet. The pH generally influences a semiconductor in an electrochemical system, e.g.,TiO2 particles suspended in water, by shifting the valence and the conduction band following the Nernst law.39-41 Additionally, the charge of the semiconductor surface, and therefore the adsorption properties, depends upon the pH.42 Since in photocatalysis the adsorption of a pollutant is a prerequisite for its degradation, a change in pH can lead to a change of the degradation rate and of the amounts and concentrations of intermediates. Therefore, in the present study the influence of the pH has been examined in detail maintaining constant pH throughout the irradiation with the pH-stat technique. Figure 3 shows the photonic efficiencies of the decomposi(37) Theurich, J. Diplomarbeit im Fachbereich Chemie der Universita¨t Hannover 1994. (38) Mills, A.; Morris, S.; Davies, R. J. Photochem. Photobiol. A: Chem. 1993, 70, 183. (39) Fujihira, M.; Satoh, Y.; Osa, T. Bull. Chem. Soc. Jpn. 1982, 55, 666. (40) Fujishima, A.; Inoue, T.; Honda, K. J. Am. Chem. Soc. 1979, 101, 5582. (41) Gra¨tzel, M. Ann. Chim. (Rome) 1987, 77, 411. (42) Augustinsky, J. Struct. Bonding 1988, 69, 1.

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Table 1. Relative Retention Times of Detected Compounds retention time (min) compound

experiment

ext standard

Oudjehani

hydroxyhydroquinone (I) hydroquinone (II) hydroxybenzoquinone (III) benzoquinone (IV) 2,5,4′-trihydroxybiphenyl phenol (VI) 4′-hydroxyphenylbenzoquinone (VII) 4-chlorocatechol (VIII) 4-chlorophenol 5-chlor-2,4′-dihydroxybiphenyl (IX)

2.9 3.2 3.7 4.1 5.0 6.1 7.5 8.9 15.1 25.4

2.9 3.2 n.a. 4.2 n.a. 6.1 n.a. n.a. 15.1 n.a.

n.d./n.d. 3.3/2.6 3.7/3.3 4.2/3.7 n.d./3.2 7.8/n.d. 11.3/5.4 10.2/n.d. 17.0/7.9 n.d./15.7

Figure 4. Maximum concentration of intermediates detected during the photocatalytic degradation of 4-CP with Hombikat UV 100 at different pH values. Experimental conditions: 5 g L-1 Hombikat UV 100, 1 mmol L-1 4-CP, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL

Figure 2. Intermediates detected by HPLC during photocatalytic degradation of 4-CP.

Figure 5. Maximum concentration of intermediates detected during the photocatalytic degradation of 4-CP with Degussa P25 at different pH values. Experimental conditions: 5 g L-1 Degussa P25, 1 mmol L-1 4-CP, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL.

Figure 3. Influence of the pH on the photonic efficiency of the 4-CP and the TOC degradation. Experimental conditions: 5 g L-1 TiO2, 1 mmol L-1 4-CP, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL.

tion of 4-CP and the TOC depletion as a function of pH when P25 or Hombikat UV 100 was used as the photocatalyst, respectively. It is obvious that the photonic efficiency of the 4-CP degradation is hardly influenced by the pH. The observed reaction rates vary by a factor of 2 for both photocatalysts. A local maximum at pH 7 and a rise above pH 9 are observed with P25, while with Hombikat UV 100 a decrease in the photonic efficiency from pH 2 to pH 5 followed by an increase for higher pH values (slightly until pH 11, strongly above) is obtained.

Concerning the TOC measurementssto our mind generally one of the most important measurements for detoxification processessthe photonic efficiencies decrease with rising pH, only with P25 a slight increase is observed above pH 9. The rate of 4-CP decomposition is always higher than that of the TOC degradation; for pH values e5 both values correspond with each other when Hombikat UV 100 is used as the photocatalyst. With rising pH the discrepancy between the TOC and the 4-CP measurements increases for both photocatalysts employed in this study. Apparently, a larger amount and higher concentrations of intermediates are formed at higher pH in excellent agreement with Figures 4 and 5. The degradation rates of 4-CP are higher with P25 than with Hombikat UV 100 throughout the whole pH-range studied. However, the resulting photonic efficiencies are still very low in comparison to photonic efficiencies obtained when the photocatalytic degradation of other nonaromatic organic

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Figure 6. Plot of the concentrations of the two main intermediates HQ and BQ as a function of the irradiation time during the photocatalytic degradation of 4-CP. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL

model compounds such as dichloroacetic acid29 or chloroform43 was studied. Figures 4 and 5 show the number and the maximum concentrations of the detected intermediates at different pH values for both catalysts employed in this study. Usually the maximum concentrations were detected when approximately half of the initial 4-CP was degraded (180 min). For both TiO2 powders the number and concentrations of intermediates increase with increasing pH and reaches a maximum at pH 9, the pH value were the lowest depletion rate of the TOC is observed (compare Figure 3). When P25 is the photocatalyst, the number of detected intermediates and their total concentration are higher than with Hombikat UV 100 throughout the whole pHrange examined. The formation and destruction of the two main intermediates (HQ and BQ) are shown in Figure 6 for the photocatalytic degradation of 1 mmol L-1 4-CP with Hombikat UV 100 as the photocatalyst at pH 3. While the concentration of BQ increases almost linearly during the whole irradiation time, the concentration of HQ reaches a maximum after 180 min, i.e., the first half-life time for the photocatalytic degradation of 4-CP, before its concentration is decreasing again. The shape of the HQ curve is typical for a compound which is formed as a primary intermediate in a consecutive reaction of the following kind:

AfBfC

(2)

Degradation of HQ and BQ. For a better understanding of the degradation pathway of 4-CP under photocatalytic conditions, analogous experiments were carried out for the degradation of the main intermediates under the same experimental conditions as used for the irradiation of the 4-CP suspensions. Figure 7 illustrates the decrease in the concentrations of HQ, TOC, and of the sum HQ and BQ at pH 3 as a function of the irradiation time observed during the photocatalytic degradation of 1 mmol L-1 HQ. Furthermore, the concentration of BQ, which is the only detected intermediate during the photocatalytic treatment of HQ, is shown in this figure. In another study HBQ has also been detected as an intermediate during the degradation of BQ over illuminated TiO2 suspensions.44 HQ is rapidly oxidized to BQ in the beginning of the irradiation, but with increasing (43) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci. Technol. 1991, 25 (3), 494. (44) Richard, C. New. J. Chem. 1994, 18, 443.

Figure 7. Photocatalytic degradation of HQ (c° ) 1 mmol L-1). Comparison of the HQ and TOC degradation, the formation and degradation of BQ, and the sum of the concentrations of HQ and BQ. C6,TOC is the TOC concentration calculated under the assumption that all formed intermediates consist of six carbon atoms. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL

Figure 8. Photocatalytic degradation of BQ (c° ) 1 mmol L-1). Comparison of the HQ and TOC degradation, the formation and degradation of BQ and the sum of the concentrations of HQ and BQ. C6,TOC is the TOC concentration calculated under the assumption that all formed intermediates consist of six carbon atoms. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL.

concentration of BQ this rate is decreasing. The rate of mineralization (i.e., TOC degradation) is constant over the whole irradiation time and corresponds very well with the decrease of the sum of the concentrations of HQ and BQ. This observation indicates that the concentrations of other intermediates should indeed be neglectable. The data of an identical experiment, but performed with BQ instead of HQ, are presented in Figure 8. Although this experiment was also carried out under a continuous oxygen stream, i.e., under oxidizing conditions, BQ is rapidly reduced to HQ within the first minutes of irradiation. But in parallel BQ can be efficiently mineralized under photocatalytic conditions as the marked decrease in the TOC concentration within the first 30 min demonstrates. With increasing HQ concentration the overall mineralization rate is decreasing and becomes identical to the rate observed for the mineralization of HQ. In both cases the ratio between HQ and BQ is 2:1. This distribution is reached within 30 min of irradiation starting with pure BQ and after 2 h beginning with pure HQ. Table 2 summarizes the photonic efficiencies calculated for the decrease of the TOC, the decrease of the sum of [HQ] and [BQ], and the decrease (increase) of the single

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Table 2. Photonic Efficiencies ξ (%) for the Hydroquinone and Benzoquinone Degradation ξ(start) HQ ξ(balance) HQ ξ(start) BQ ξ(balance) BQ ξ HQ + BQ ξ TOC

1 mmol of HQ

1 mmol of BQ

0.52 0.10 0.47 0.06 0.15 0.15

1.82 0.11 2.93 0.06 0.17 0.16

Figure 10. Plot of the inverse of the initial rate of 4-CP disappearance as a function of the reciprocol initial 4-CP concentration. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL.

Figure 9. Plot of the normalized 4-CP concentrations as a function of the irradiation time. Experimental conditions: 5 g L-1 Hombikat UV 100, pH 3, continuous O2-stream, I ≈ 1.0 mmol min-1 L-1, 293 K, 10 mmol L-1 KNO3, V ) 150 mL.

compounds calculated from Figures 7 and 8. For the single compounds two different photonic efficiencies were calculated: one for the first 30 min of the irradiation (ξ(start)) and one for the balanced system (ξ(balance)).

as a function of the irradiation time. At high initial concentrations the 4-CP decay is obeying zero-order kinetics (eq 4) while the degradation kinetics at low concentrations can be interpreted as an example for firstorder kinetics (eq 5). Assuming that solvent and reaction intermediates compete with the reacting substrate for the active surface sites on the semiconductor particle, eq 3 may be written in a more general form

) dt

Degradation Kinetics. As shown in Figure 1, the rate of the photocatalytic degradation of 4-CP over TiO2 is a function of the initial pollutant concentration increasing up to a 4-CP concentration of 5 mmol L-1 and remaining constant for higher 4-CP concentrations. One possible explanation for this behavior is that the adsorption of the substrate molecule is rate limiting. Assuming that this adsorption can be described by the LangmuirHinshelwood (L-H) equation (eq 3) one obtains

kaK[4-CP] d[4-CP] ) dt 1 + K[4-CP]

(3)

where d[4-CP]/dt is the rate of 4-CP degradation, ka the apparent reaction rate constant, K the adsorption coefficient of 4-CP, and [4-CP] the concentration of 4-CP. Two extreme cases have to be considered: For high concentrations of the pollutant, where saturation coverage of the TiO2 surface is achieved (i.e., K[4-CP] . 1) eq 3 simplifies to a zero-order rate equation (eq 4):

-

d[4-CP] ) ka dt

(4)

For very low concentrations of 4-CP (i.e., K[4-CP] , 1) the L-H equation changes into a pseudo-first-order kinetic law (eq 5)

-

d[4-CP] ) ka[4-CP] dt

n

1 + K[4-CP] +

Discussion

-

kaK[4-CP]

d[4-CP] -

(5)

with ka′ ) kaK being the pseudo-first-order rate constant. These two extreme cases are illustrated (in a normalized presentation) in Figure 9 for two different initial concentrations of 4-CP (10 and 0.2 mmol L-1, respectively)

(6)

Kici ∑ i)1

where i is the number of substances (solvent, molecular oxygen, or byproducts) competing with 4-CP for free adsorption places on the surface of the photocatalyst, Ki is the respective adsorption coefficient, and ci is the concentration of the component i. Again, for high 4-CP concentrations eq 6 reduces to the form of eq 4 and the photocatalytic degradation reaction will follow zero-order kinetics. The transformation of the Langmuir-Hinshelwood equation (eq 3) into its inverse function results in a linear relationship (eq 7) with an intercept of ka-1 and a slope of (ka-1K-1)

-

dt 1 1 ) + d[4-CP] ka kaK[4-CP]

(7)

A plot of the inverse of the initial rate of 4-CP disapperance as a function of the reciprocal initial 4-CP concentration is shown in Figure 10. The following values can be derived from the intercept and the slope of this straight line: ka ) 0.2 mmol L-1 h-1 and K ) 2.4 × 104 L mol-1. The value of ka depends strongly on the experimental conditions and, consequently, cannot be compared with data found by other research groups. However, the intercept can be interpreted as the maximum rate which can be achieved for infinite high initial 4-CP concentrations under the chosen experimental conditions. It should be pointed out that the above kinetic treatment employing a Langmuir-Hinshelwood adsorption behavior of the substrate just presents one possible model to explain the experimental results. Independent adsorption measurements are certainly warrented to decide whether the derived K value represents a realistic equilibrium con-

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Theurich et al.

stant.45 However, as shown in Table 3 the value for the equilibrium constant K agrees very well with those obtained in recently published works from Al-Sayyed et al.4 and Al-Ekabi et al.23 where P25 was used as a photocatalyst (Table 3). pH Dependency. While the influence of the initial pH on the degradation rate of 4-CP has been studied before, no efforts were made in these studies to maintain a constant pH throughout the irradiation. On the contrary, in most of these experiments the inital pH of the model waste water was varied, and the decrease in pH observed during irradiation was used besides other analytical techniques such as HPLC to calculate the reaction rate. Matthews26 and Al-Sayyed et al.4 observed no significant change in the decomposition rate of 4-CP (and therefore an unchanged photonic efficiency) within a pH range of 2.9-6.0; Barbeni et al.20 reported an increase in the rate of the photocatalytic decomposition of 4-CP at pH 12 in comparison to neutral pH. The latter results indicate that the degradation rate of 4-CP may depend upon the chosen pH. These rates are of course apparent values because they do not account for a change in the reaction rate with changing pH. The pH-stat technique employed in this study should exclude any influence of the pH on the degradation kinetics of the model compound and, consequently, the degradation rates or photonic efficiencies obtained at a given pH value do not present apparent data any longer. While we also observed only a weak influence of the pH on the degradation rate of 4-CP itself, it is important to note the significant influence of the pH on the TOC degradation rate (see Figure 1). Even though TOC measurements are not regularly performed when the degradation kinetics of a model compound are studied, we strongly believe that they present one of the most important criteria to realistically assess the efficiency of a photocatalytic process. Al-Ekabi et al.23 observed that the decrease in pH during the irradiation is more pronounced than that predicted by eq 1. They interpreted this observation by the formation of acidic intermediates (probably of the type RCOOH), which are subsequently converted into further intermediates and finally to the mineralization products. In agreement with the results in that study, our simultaneous measurements of the proton formation and the chloride ion evolution (results are not explicitly shown here), respectively, also confirm an overestimation of H+ evolution in comparision to the Cl- formation. Matthews26 observed an increased formation of hydroxylated products with increasing pH. AlSayyed et al.4 found no change in the number of intermediates detected by HPLC but observed some changes of their concentrations, a result which they attributed to varying stabilities of the intermediates at different pH values. The intermediates identified in that study were hydroquinone (HQ), benzoquinone (BQ), 4-chlorocatechol (4-CC), and probably hydroxybenzoquinone (HBQ). Three other intermediates were detected, but could not be identified due to their low concentrations. LC/MS analysis showed the abscence of catechol, resorcinol and biphenyl derivatives under the experimental conditions employed in that paper. Mills et al.38 also detected traces of 4-chlororesorcinol (4-CR) at pH 2 besides the intermediates mentioned above in their investigation of the photocatalytic decomposition of 4-CP.

It is an important result of our study that not only the number but also the concentration of the detected intermediates is strongly influenced by the reaction pH. In agreement with Matthews26 an increase of hydroxylated products (especially HHQ) was observed for increasing pH values. We attribute this to higher stabilities of polyhydroxylated benzenes (e.g., HHQ is unstable at pH < 337) and to a change in the adsorption properties of these intermediates at alkaline pH values. HHQ was found to be strongly adsorbed on TiO2 in acidic media while it was poorly adsorbed at pH 11.37 Besides a fast TOC degradation the suppression of the formation of byproducts (especially when they are highly toxic) is an important criterion for a good photocatalyst. Thus, the two materials employed in this study should be considered as equally active, although slightly higher photonic efficiencies were obtained with P25 but, on the other hand, markedly lower concentrations of byproducts were obtained when Hombikat UV 100 was the photocatalyst. This conclusion is specifically valid for the degradation of 4-CP, while earlier studies have clearly shown that Hombikat UV 100 was a much better photocatalyst for the degradation of aliphatic compounds such as dichloroacetic acid,29 chloroform,35 and tetrachloromethane,36 resulting in photonic efficiencies which were by a factor of three higher than with P25. A similar result was published recently by Mills et al.46 where Degussa P25 beside several other commercial TiO2 powders was compared with laboratory made TiO2. Significantly smaller amounts of intermediates were detected when the latter materials were used as the photocatalyst. Furthermore the results presented in this study have clearly shown that it can be very important to select the optimum pH for the photocatalytic degradation process to combine a high photonic efficiency with the surpression of the formation of (toxic) intermediates. Thus, our results are in agreement with those published before,4,20,23,26,38 yielding, however, additional knowledge especially about the influence of the chosen TiO2 material on the photocatalytic efficiency and the influence of the pH not only on the degradation kinetics of the model compound itself but also on its influence on the rate of mineralization and on the formation of byproducts in a more quantitave way. Electron Shuttle Mechanism. The experiments carried out with the two main intermediates of the 4-CP degradation (HQ and BQ) showed nearly identical photonic efficiencies for the overall mineralization (TOC) and for the decrease of the sum of HQ and BQ (see Table 2), irrespective whether the experiment was performed with HQ or BQ. Moreover, these values were constant over the whole irradiation time, while the photonic efficiencies of the decrease or increase of the individual components themselves were considerably higher in the beginning of the irradiation. An increasing concentration of the intermediate (BQ in the case of HQ as the model pollutant, HQ in the case of BQ) is obviously inhibiting the degradation rate of the model pollutant itself until a ratio from HQ to BQ of 2:1 is reached. These observation indicate the existence of a photocatalytic balance between HQ and BQ, which we attribute to a fast electron shuttle mechanism between HQ and BQ. A recent study has shown that BQ can act as a very effective electron scavenger over illuminated TiO2 or ZnO44 and is able to compete successfully with molecular oxygen for the photogenerated electrons in the conduction band of the semiconductor particle (eq 8), but in parallel the formation of hydroxybenzoquinone (HBQ)

(45) Cunningham, J.; Al-Sayyed G. J. Chem. Soc., Faraday Trans. 1990, 86, 3935.

(46) Mills, A.; Sawunyama, P. J. Photochem. Photobiol. A: Chem. 1994, 84, 305.

Table 3. Adsorption Constants K experiment (104 L mol-1)

Al-Sayyed4

Al-Ekabi23

(104 L mol-1)

(104 L mol-1)

2.4

1.66

1.9

Photocatalytic Degradation of 4-Chlorophenol

Langmuir, Vol. 12, No. 26, 1996 6375

as an intermediate for the photocatalytic oxidation of BQ was observed as a side reaction (eq 9).

BQ + OH• f [BQOH•] f f HBQ

(9)

Assuming that ring cleavage takes place only from the quinone form (or some oxidized derivatives), this “equilibrium” between HQ and BQ is acting as a photocatalytic short circuit for the mineralization process and is a possible explanation for the low photonic efficiency for the photocatalytic degradation of 4-CP and other aromatic compounds such as phenol.47,48 A similar behavior has been observed for the inorganic system Fe2+/Fe3+ (eqs 10 and 11),49 which is well known as the photo-Fenton reaction.50

e-CB + Fe3+surface f Fe2+surface

(10)

h+VB + Fe2+surface f Fe3+surface

(11)

Scheme of the 4-CP Degradation. The high number of intermediates which has been identified during the photocatalytic 4-CP degradation indicates a complex reaction mechanism. Figure 11 illustrates the main envisaged pathways of the photocatalytic/photolytic decomposition of 4-CP. Because of the manifold possibilities of the intermediary radicals to undergo further reactions (disproportionation, radical abstraction, recombination, etc.), this scheme is probably still oversimplyfied. The primary oxidation products of 4-CP are 4-chlorocatechol (4-CC), hydroquinone (HQ), and benzoquinone (BQ). 4-CC is formed by the addition of a hydroxyl radical to the ortho position of the hydroxyl group of 4-CP (reaction a), followed by an elimination of a hydrogen atom to recover the aromatic ring (reaction e). The degradation pathway via 4-CC seems to be a minor pathway under the chosen experimental conditions, though other research groups found in their studies that it is a major pathway for the degradation of 4-CP.35 4-CC is further degraded to hydroxyhydroquinone (HHQ) by oxidation with another hydroxyl radical (or a valence band hole) (reaction m) or by a reductive reaction with an electron (reaction k) followed by a reaction with molecular oxygen (reaction p) or by Cl• abstraction (reaction l), both yielding HBQ (reaction w). HQ is formed when the attack of the hydroxyl radical takes places in the para position (reaction c) and a chlorine atom (reaction f) or, if the generated intermediate reacts first with an electron, a chloride ion is released. HQ can either be oxidized to BQ (reaction j), resulting in the photocatalytic short circuit described above, or be transformed to HHQ (reaction n and r) by a second attack of an hydroxyl radical in analogy to the transformation of 4-CP to 4-CC. HHQ can be further oxidized to hydroxybenzoquinone (HBQ) (reaction u), which probably leads to a similar photocatalytic short circuit as observed for the system HQ/BQ, with the difference that HHQ and HBQ are less stable, especially against direct photolysis, (47) Augugliaro, V.; Palmisano, L.; Sclafani, A.; Minero, C.; Pelizzetti, E. Toxicol. Environ. Chem. 1988, 16, 89. (48) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (49) Bahnemann, D. W.; Fischer, C.-H.; Janata, E.; Henglein, A. J. Chem. Soc., Faraday Trans. 1987, 83, 2559. (50) Zepp, R. G.; Faust, B. C.; Hoigne´, J. Environ. Sci. Technol. 1992, 26, 313.

Figure 11. Degradation scheme of the photocatalytic/photolytic degradation of 4-CP.

than HQ and BQ. BQ can either be formed by the oxidation of HQ (reaction j) or directly by the reaction of molecular oxygen (reaction g) with the hydroxy phenyl radical (HPR) generated by the abstraction of a Cl• atom (reaction d), leading to a peroxyl radical which can disproportionate to HQ and BQ (reactions h and i).23 The formation of HPR as a short-lived radical intermediate has been suggested in a recent study by electron paramagnetic resonance spin trapping detection during the direct photolysis of 4-CP.51 Flash photolysis experiments have shown that BQ is directly produced from 4-CP being one of its first oxidation products.52 BQ itself can either be reduced to HQ (reaction j) or oxidized (by the attack of a hydroxyl radical) to HBQ (reaction o and s). Therefore, HHQ and HBQ are formed as secondary intermediates from each of the three primary oxidation products (4-CC, HQ, and BQ) (see Figure 11). Assuming that the ring cleavage is possible only for aromatic compounds which are hydroxylated in positions 1 and 2, HHQ and its quinone derivative HBQ are the last aromatic intermediates in this degradation scheme. An attack of another hydroxyl radical leads to the ring cleavage and therefore to noncyclic intermediates. In the case of HBQ these intermediates should be 3-hydroxy derivatives of muconic aldehyde or muconic acid, compounds which should undergo rapid oxidation to the final mineralization products carbon dioxide and water. Okamoto et al.48 detected during their investigation of the photocatalytic oxidation of phenol many unidentified peaks by HPLC. Because of the very short retention times of these compounds under the chosen experimental conditions, they assumed that these com(51) Lipcynska-Kochany, E.; Kochany, J.; Bolton, J. R. J. Photochem. Photobiol. A: Chem. 1991, 62, 229. (52) Lipczynska-Kochany, E.; Bolton, J. R. J. Photochem. Photobiol. A: Chem. 1991, 58, 315.

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degraded under photocatalytic conditions probably into monocyclic compounds and can be mineralized completely within the irradiation time required for the total oxidation of 4-CP. 2,4′-Dihydroxy- and 4,4′-dihydroxybiphenyl (DHBP), which are formed during the direct photolysis of 4-CP,19 could not be detected in this study. Especially the lack of DHBP is a surprising result as it should be formed easily by a reaction of either HPR with a 4-CP molecule and the following release of a chlorine atom or by a combination of two HPR species (reaction B). Conclusions

Figure 12. Further reactions of the hydroxyphenyl radical (HPR) at higher pH-values, forming bicyclic byproducts and phenol.

pounds are very polar products, i.e., aldehydes and/or carboxylic acids. The possible formation of phenol and bicylic compounds, which were observed in our study at pH > 7, is explained in Figure 12. As shown in Figure 11 HPR can be formed directly from 4-CP by a homolytic cleavage (reaction d) of the ring-chlorine bond or by a reductive attack of an electron on 4-CP releasing a chloride ion (reaction b). The formation of HPR by the reaction with an electron becomes more likely with increasing pH because of the Nernstian behavior of the semiconductor. Thus, the observed increase in the concentrations of bicyclic compounds is the expected result for alkaline pH values. HPR can abstract a hydrogen atom to form phenol (reaction C), while the formation of 5-chloro-2,4′-dihydroxybiphenyl (CDHB) can be explained by the reaction of HPR with 4-CP (reaction A). CDHB is further oxidized by the attack of a hydroxyl radical to 2,5,4′-trihydroxybiphenyl (THB) (reaction D), which can also be formed directly by a reaction between HPR and HQ (reaction E). Further oxidation of THB leads to 4-hydroxyphenylbenzoquinone (HPBQ) (reaction G); a reaction of HPR and BQ should also form HPBQ (reaction F). All bicyclic compounds will be

The photocatalytic degradation of 4-chlorophenol (4CP) has been examined using two different TiO2 photocatalysts. In particular, the influences of the initial 4-CP concentration and of the pH have been studied. At low 4-CP concentrations (1 mmol L-1) zero-order kinetics were observed. This behavior has been formally analyzed by a Langmuir-Hinshelwood adsorption treatment of the kinetic data. In contrast to the degradation rate of 4-CP itself, the overall mineralization rate (as studied by the TOC decrease) is significantly influenced by the chosen pH. For pH values >7 this rate decreases strongly, which is in excellent agreement with the fact that larger amounts and higher concentrations of intermediates are formed at alkaline pH. Besides the formation of 4-CC, the major degradation pathway is the dechlorination of 4-CP resulting in HQ and BQ as primary intermediates. These compounds can be further oxidized to HHQ and HBQ, the secondary intermediates. HHQ with its quinone derivative HBQ is most likely the final aromatic intermediate before ring cleavage to the final mineralization products. The formation of small amounts of bicyclic compounds which was observed for pH values above 5 could be explained through intermediate free radical pathways. The results obtained in the present study demonstrate the necessity to choose the optimum photocatalytic conditions for the degradation of hazardous compounds. Besides the pH the chosen photocatalyst can influence the distribution and the concentrations of detected intermediates. A better knowledge about the semiconductors surface, i.e., surface reactions and surface complexes, would be helpful to obtain a better understanding of heterogeneous photocatalytic reactions. The HQ/BQ photosystem appears to be a short circuit for photocatalytic processes and therefore seems to have a key function for the photocatalytic oxidation of aromatic pollutants. LA960228T