Kinetics of Photocatalytic Degradation of Chlorophenol, Nitrophenol

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Ind. Eng. Chem. Res. 2006, 45, 482-486

APPLIED CHEMISTRY Kinetics of Photocatalytic Degradation of Chlorophenol, Nitrophenol, and Their Mixtures M. H. Priya and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

The kinetics of photocatalytic degradation of binary mixtures of chlorophenol and nitrophenol was investigated using solution combustion synthesized nano-TiO2. The degradation rates of both organics decreased in the binary mixture compared to the individual degradation rates of the organics. This was attributed to interaction between the mother compounds, interaction between the intermediates, and competition for the active reaction site on the catalyst. A modified Langmuir-Hinshelwood kinetic model was developed, and the kinetic parameters were determined from initial rate analysis for the degradation of chlorophenol and nitrophenol. The evolution of intermediates was also investigated, and a possible pathway for degradation of chlorophenol and nitrophenol was proposed. Introduction

Experimental Section

Phenol and its derivatives are the precursors or intermediates in many process industries, such as dyes, resin and plastics, pharmaceutics, and pulp and paper. They are used as disinfectants, explosives, herbicides, and pesticides. They are irritants at low levels and affect the respiratory system and the central nervous system and can induce cancer at higher doses. Eleven phenolic compounds have been listed by USEPA as priority pollutants.1 Among them, chlorophenols and nitrophenols appear commonly in industrial effluents. Voluminous literature is available on the photocatalytic degradation of chlorophenols2-7 and nitrophenols7-12 exploring the effect of number of substitutents2-4,9 and their positions,4,10,11 light intensity,5,11 temperature,6,11 pH,6,7,11 and influence of anions.7,12 Most of the studies are on degradation of an individual organic pollutant. However, industrial effluents contain a mixture of organic pollutants and interaction between two or more compounds during mineralization is likely. However, only a few studies have investigated the photocatalytic degradation of pollutant mixtures.7,13-15 The coexistence of pollutants retards the rate of degradation and is attributed to the effective competition for catalyst active sites by the compounds (original pollutants and intermediates).7,13-15 To the best of our knowledge, the photocatalytic degradation of a mixture of chlorophenol and nitrophenol has been studied for at only one binary composition,7 but no information was reported on the intermediates or on the kinetics of degradation. Combustion synthesized TiO2 was reported to be a more effective catalyst compared to Degussa P-25 for the degradation of chlorophenol2 and nitrophenol.8 The objective of this study is to investigate photodegradation of binary mixtures for a range of compositions using combustion synthesized TiO2 and to develop a model to determine the degradation kinetics. * To whom correspondence should be addressed. Tel.: 91-802932321. Fax: 91-80-23600683. E-mail: [email protected].

Materials. 4-Chlorophenol, 4- nitrophenol, acetic acid, and nitric acid (S. D. Fine Chemicals, India), acetonitrile and glycine (Merck, India), titanium isopropoxide (Lancaster chemicals, UK), and hydrogen peroxide (Sigma Aldrich, USA) were obtained. Water was double distilled and filtered through a Millipore membrane filter prior to use. Catalyst Preparation and Characterization. The combustion synthesis technique is a single-step process without any downstream processing. Stoichiometric amounts of precusor titanyl nitrate (obtained by nitration of titanium hydroxide, which is obtained by hydrolysis of titanium isopropoxide) and fuel (glycine) are mixed in water and combusted in a muffle furnace preset at 350 °C. The combustion is of the smoldering type without any flame wherein a spark appears at a corner and spreads over the mass, yielding a highly porous nanosized yellowish substance, pure anatase TiO2. Phase transition to other phases such as rutile or brookite is prevented in this process because the material is exposed to very high temperature for only a short duration. The catalyst has been characterized by various techniques such as XRD, TEM, BET, XPS, TG-DTA, and IR and UV spectroscopy. Further details on catalyst preparation and characterization are provided elsewhere.8,16 The solution combustion synthesized TiO2 is smaller in size (8-10 nm) and has a lower band gap energy, larger surface area, and higher bounded hydroxyl content compared to commercial Degussa P-25 catalyst.17 These attributes enhance the catalytic activity of the catalyst. Photochemical Reactor. The photochemical reactor employed in this study was comprised of a jacketed quartz tube of 3.4 cm i.d., 4 cm o.d., and 21 cm length and an outer Pyrex glass reactor of 5.7 cm i.d. and 16 cm length. A 125 W highpressure mercury vapor lamp (HPML) (Philips, India) was placed inside the jacketed quartz tube after removal of the outer shell. The ballast and capacitor were connected in series with the lamp to avoid fluctuations in the input supply. Water was circulated through the annulus of the quartz tube to avoid heating of the solution due to dissipative loss of UV energy. The solution

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Figure 1. Photocatalytic degradation profiles of (9) 100 mg/L chlorophenol, (0) 100 mg/L chlorophenol in 50% (weight percentage) mixture with nitrophenol, (b) 100 mg/L nitrophenol, and (O) 100 mg/L nitrophenol in 50% (weight percentage) mixture with chlorophenol.

was taken in the outer reactor and continuously stirred to ensure that the suspension of the catalyst was uniform. The lamp radiated predominantly at 365 nm, corresponding to an energy of 3.4 eV, and the photon flux was 5.8 × 10-6 mol of photons/ s. Further details of the experimental setup can be found elsewhere.16 Degradation Experiments. A known mass of 4-chlorophenol and 4-nitrophenol was dissolved in Millipore-filtered doubledistilled water and subjected to UV irradiation in the photochemical reactor described above with a catalyst loading of 1 g/L. The reactions were carried out at 40 °C, which was maintained by circulating water in the annulus of the jacketed quartz reactor. Samples were collected at regular intervals, filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis, as described below. Sample Analysis. The extent of degradation of chlorophenol and nitrophenol and the evolution of intermediates were determined using an HPLC. The HPLC consisted of an isocratic pump (Waters 501), a Rheodyne injector, C-18 column, a UV detector (Waters 2487), and a data acquisition system. The eluent stream consisted of 80% water and 20% acetonitrile (volume percent) pumped at 0.8 mL/min. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the UV absorbance at 270 nm was continuously monitored using an UV detector and stored digitally. The chromatograph was converted to concentration units using calibration with pure compounds. The intermediates were confirmed by injecting standard solutions of the compounds. Results and Discussion The photocatalytic degradation of binary mixtures of chlorophenol and nitrophenol was investigated. No appreciable degradation of either chlorophenol or nitrophenol was observed either in absence of UV light or catalyst. TiO2 prepared by the solution combustion technique was employed for the study. Figure 1 shows the degradation profile of 100 mg/L chlorophenol and nitrophenol individually and in a mixture containing 100 mg/L concentration of each compound. The degradation of chlorophenol was faster than that of nitrophenol. The degradation of chlorophenol was almost complete in 30 min, while the degradation of nitrophenol took nearly 300 min to reach completion. In the binary mixture, the degradation of nitrophenol was significantly reduced due to the presence of

Figure 2. (a) Concentration profiles of chlorophenol and intermediates when chlorophenol (200 mg/L) was degraded individually. (9) Chlorophenol; (4) hydroxyhydroquinone; (b) hydroquinone. (b) Concentration profiles of nitrophenol and intermediates when chlorophenol (100 mg/L) was degraded individually. (0) Nitrophenol; (O) nitrocatechol; (b) hydroquinone; (4) hydroxyhydroquinone.

chlorophenol but the degradation of chlorophenol was only slightly reduced in the presence of nitrophenol. The reduction in the degradation rates of the organics in the mixture compared to the individual degradation rate is in agreement with earlier studies7,13-15 for other organics. To understand the mechanism and devise a kinetic model for the degradation of binary mixtures, degradation of 100 mg/L solution of nitrophenol was investigated by varying the initial chlorophenol concentration ([cp]0) from 0 to 200 mg/L. Similarly, the degradation of 200 mg/L chlorophenol was investigated by varying the initial nitrophenol concentration ([np]0) from 0 to 200 mg/L. The degradation of chlorophenol was carried out at a higher concentration of 200 mg/L to facilitate tracking of residual concentration of chlorophenol, which degrades faster and has a lower UV absorptivity compared to that of nitrophenol. The degradation profiles for chlorophenol and nitrophenol, when degraded individually, are shown in Figure 2, along with the evolution profile of intermediates. Two intermediates, hydroquinone and hydroxyhydroquinone, were detected during the degradation of chlorophenol. However, during the degradation of nitrophenol, in addition to the above two intermediates, a third intermediate, namely nitrocatechol, was detected. Pho-

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Figure 3. Photocatalytic degradation mechanism for the degradation of a binary mixture of chlorophenol and nitrophenol.

tocatalysis is primarily a hydroxyl radical oxidation18 reaction involving sequential hydroxylation of the organic pollutant. Hence, chlorophenol (CP) primarily degrades to hydroquinone (HQ) while both hydroquinone and nitrocatechol (NC) are the primary hydroxylated intermediates of nitrophenol (NP), which subsequently converts to hydroxyhydroquinone (HHQ) by further hydroxylation followed by ring fragmentation. Figure S1a-g (see Supporting Information) shows the concentration profiles for chlorophenol, nitrophenol, and intermediates for the photodegradation of various binary mixtures. However, when a mixture of chlorophenol and nitrophenol was degraded, an additional intermediate was observed and was detected to be succinic acid (SUC). Succinic acid (C4 compound) is a ringopening fragment reported to have been observed during the degradation of chlorophenol.19 A closer inspection of the degradation profiles reveals that the rate of evolution of succinic acid is higher than that of other intermediates: hydroquinone, nitrocatechol, and hydroxyhydroquinone. This suggests that succinic acid could be formed by direct degradation of chlorophenol and nitrophenol in a mixture in addition to ring breakage of hydroxyhydroquinone. The proposed mechanism of degradation of binary mixtures is given in Figure 3. The degradation profiles of chlorophenol and nitrophenol for various concentrations of chlorophenol and nitrophenol and the same initial concentration of another organic in the mixture are shown in the insets of Figure 4. The initial rate of degradation of the compound was determined from the initial slope of the concentration profile. The initial rate of degradation of an organic compound in a mixture increased with increase in its concentration, irrespective of the presence of another organic compound at a fixed initial concentration, as observed during

Figure 4. Variation of inverse of initial rate with inverse of initial concentration of (a) chlorophenol when [np]0 ) 100 mg/L and (b) nitrophenol when [cp]0 ) 200 mg/L. The points represent the experimental data, and the line denotes the model fit. (insets) Photocatalytic degradation profiles of various concentrations of (a) chlorophenol when [np]0 ) 100 mg/L and (b) nitrophenol when [cp]0 ) 200 mg/L. (9) 50 mg/L; (b) 100 mg/L; (2) 150 mg/L; (1) 200 mg/L.

degradation of these compounds individually.2 The insets of parts a and b of Figure 5 show the degradation profiles of 200 and 100 mg/L chlorophenol and nitrophenol, respectively, when the concentration of other compound varies from 0 to 200 mg/ L. The initial rate of degradation of chlorophenol/nitrophenol decreases with an increase in the initial concentration of nitrophenol/chlorophenol in the mixture (Figure 5). When 100 mg/L nitrophenol was degraded in the presence of various concentrations of chlorophenol, the degradation rate of nitrophenol was reduced and the corresponding rate of formation of nitrocatechol also decreased. However, when the degradation was carried out at a constant concentration of 200 mg/L chlorophenol at various concentrations of nitrophenol, the rate of formation of nitrocatechol was reduced with increasing nitrophenol concentration. This implies that nitrophenol preferentially degrades to hydroquinone or succinic acid in the presence of chlorophenol. Further, the detection of succinic acid as an intermediate during the degradation of the binary mixture suggests that the coexistence of chlorophenol and nitrophenol influences the degradation pathways of individual compounds. The reduction in the degradation of the compounds in the mixture compared to individual compounds is not just due to competition for the catalyst active site but is also due to interaction between these compounds. A previous study that investigated the degradation of benzene and perchloroethylene in a binary mixture14 suggested a mechanism based on the interaction between two compounds.

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interaction between and with intermediates is unknown, the apparent degradation rate coefficients can be obtained from an initial rate analysis, i.e., when the concentration of intermediates is negligible. The competition between chlorophenol and nitrophenol for the active site of catalyst can be accounted in the kinetic expression by the term (1 + KcpCcp + KnpCnp), in the denominator of an L-H expression, where Kcp and Knp are the equivalent adsorption equilibrium constants and Ccp and Cnp are the initial concentrations of chlorophenol and nitrophenol, respectively. From Figure 5, it is observed that the initial rate of degradation of chlorophenol decreases almost linearly with the increase in the concentration of nitrophenol and vice versa. Therefore, the expression (kii - kijCj) can account for the interaction between two species of the binary mixture. Here, kii is the kinetic constant influenced by the concentration of individual species concerned whose rate is considered while kij is the interaction parameter influenced by both species of the binary mixture. Thus, the parameter kij represents the influence of the jth species over the degradation of the ith species. Therefore, the initial rates of degradation of chlorophenol (rcp) and nitrophenol (rnp) can be expressed mathematically as

rcp )

(kcc - kcnCnp)Ccp ; 1 + KcpCcp + KnpCnp

rnp )

(knn - kncCcp)Cnp 1 + KcpCcp + KnpCnp (1)

where, kcc and knn are the kinetic constants of individual organic compounds and knc and kcn are the interaction parameters. The linearized forms of the rate expression are Figure 5. (a) Effect of initial concentration of nitrophenol on initial rate of degradation of 200 mg/L chlorophenol. The points represent the experimental data, and the line denotes the model fit. The inset shows the photocatalytic degradation of 200 mg/L concentration of chlorophenol in binary mixture when the concentration (mg/L) of nitrophenol is 0 (9), 50 (0), 100 (b), 150 (O), and 200 (2). (b) Effect of initial concentration of chlorophenol on initial rate of degradation of 100 mg/L nitrophenol. The points represent the experimental data, and the line denotes the model fit. The inset shows the photocatalytic degradation of 100 mg/L concentration of nitrophenol in binary mixture when the concentration (mg/L) of chlorophenol is 0 (9), 50 (0), 100 (b), 150 (O), and 200 (2).

A simple Langmuir-Hinshelwood (L-H) kinetic model incorporating the competition among benzene, perchloroethylene, and intermediates for catalyst active sites was proposed. The model, which used the L-H parameters obtained from pure perchloroethylene decomposition data, overestimated the rate of degradation of perchloroethylene in the binary mixture. Because of the rapid degradation of benzene, its degradation was not significantly influenced by the presence of perchloroethylene. Therefore, the model predicted the degradation rate for benzene in the mixture satisfactorily. Similarly, in this case, a model assuming only competitive inhibition by the intermediates with no interaction among various compounds of the mixture overestimated the rates of degradation of nitrophenol and evolution of intermediates but could predict the degradation profile of chlorophenol satisfactorily. This indicates that the kinetic rate expression for degradation of binary mixtures should account for competitive inhibition by other compounds including intermediates and also the interaction between these intermediates. The determination of the individual kinetic parameters for all chemical species is not possible because the interaction between two intermediates varies in the presence of a third intermediate. It is, therefore, important to introduce a term in the rate expression to explain the interaction between reacting species. However, because the

1 + KnpCnp 1 Kcp 1 ) + ; rcp kcc - kcnCnp Ccp kcc - kcnCnp 1 + KcpCcp 1 Knp 1 ) + (2) rnp knn - kncCcp Cnp knn - kncCcp The variations of the inverse of initial rate with the inverse of initial concentration for chlorophenol at a constant concentration of nitrophenol and for nitrophenol at a constant concentration of chlorophenol are shown in parts a and b, respectively, of Figure 4. The plots are linearly regressed, and the slopes and the intercepts were determined. The ratios of slope and intercept (1 + KnpCnp)/Kcp and (1 + KcpCcp)/Knp from Figure 4 result in two linear equations in Kcp and Knp (the values of Cnp and Ccp are known). By simultaneously solving the linear equations, the values of adsorption equilibrium constants for chlorophenol and nitrophenol were evaluated to be 1.3 × 10-4 and 9.6 × 10-3 (mg/L)-1. The values of kinetic and interaction coefficients were estimated by fitting all the experimental data (Figures 4 and 5) with eq 2. Thus, the rate coefficients, kcc and knn, for chlorophenol and nitrophenol are 0.15 and 0.045 min-1, respectively. This indicates that chlorophenol degrades nearly 3 times faster than nitrophenol. This could be attributed to the high susceptibility of chlorophenol to hydroxyl radical attack. The rate coefficients for interaction, kcn and knc, are 9 × 10-6 and 1.4 × 10-4 ((min)(mg/L))-1, respectively. Thus, kcnCnp < kcc and kncCcp ∼ knn, indicating that the degradation of chlorophenol is not significantly influenced by the presence of nitrophenol. However, the presence of chlorophenol significantly influences the degradation of nitrophenol. Conclusions The photocatalytic degradation of a mixture of phenolic compounds (chlorophenol and nitrophenol) by combustion

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synthesized nano-TiO2 was investigated at various compositions. Chlorophenol degraded faster than nitrophenol, indicating its higher susceptibility to hydroxyl radical attack. The rate of degradation of the phenols was retarded by the presence of another phenol in the mixture. The extent of inhibition was more prominent for the degradation of nitrophenol than for chlorophenol. The evolution of intermediates for the degradation of a single component and mixture was also studied. The difference in observed kinetics of intermediates of a mixture and a single component is due to interaction between compounds in the mixture. A modified L-H kinetic expression incorporating the effect of both competitive inhibition and interaction among the components of the mixture was proposed. The kinetic parameters were determined based on the initial rate analysis for chlorophenol and nitrophenol. Supporting Information Available: Figure S1a-g shows the concentration profiles of chlorophenol, nitrophenol, and intermediates for the degradation of a mixture of chlorophenol and nitrophenol of various initial concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Kieth, L. H. EPA’s priority pollutants: where they come from, where they’re going? AIChE Symp. Ser. 1980, 77, 249. (2) Sivalingam, G.; Priya, M. H.; Madras, G. Kinetics of the photodegradation of substituted phenols by solution combustion synthesized TiO2. Appl. Catal. B: EnViron. 2004, 51, 67-76. (3) Pandiyan, T.; Rivas, O. M.; Martinez, J. O.; Amezcua, G. B.; Marinez-Carrillo, M. A. Comparison of methods for the photochemical degradation of chlorophenols. J. Photochem. Photobiol. A: Chem. 2002, 146, 149-155. (4) D’liveira, J. C.; Minero, C.; Pelizzetti, E.; Pichat, P. Photodegradation of dichlorophenols and trichlorophenols in TiO2 aqueous suspensions: kinetic effects of the positions of the Cl atoms and identification of the intermediates. J. Photochem. Photobiol. A: Chem. 1993, 72, 261-267. (5) Chen, D.; Ray, A. K. Photocatalytic kinetics of phenol and its derivatives over UV irradiated TiO2. Appl. Catal. B: EnViron. 1999, 23, 143-157. (6) Pera-Titus, M.; Garcia-Molina, V.; Banos, M. A.; Gimenez. J.; Esplugas, S. Degradation of chlorophenols by means of advanced oxidation processes: a general review. Appl. Catal. B: EnViron. 2004, 47, 219256.

(7) Wuang, K. H.; Hsieh, Y. H.; Chou, M. Y.; Chang, C. Y. Photocatalytic degradation of 2-chloro and 2-nitrophenol by titanium dioxide suspensions in aqueous solution. Appl. Catal. B: EnViron. 1998, 21, 1-8. (8) Nagaveni, K.; Sivalingam, G.; Hedge, M. S.; Madras, G. Photocatalytic degradation of organic compounds over combustion synthesized Nano-TiO2. EnViron. Sci. Technol. 2004, 38, 1600-1604. (9) Ksibi, M.; Zemzemi, A.; Boukchina, R. Photocatalytic degradability of substituted phenols over UV irradiated TiO2. J. Photochem. Photobiol. A: Chem. 2003, 159, 61-70. (10) Maurino, V.; Minero, C.; Pelizzetti, E.; Piccinini, P.; Serpone, N.; Hidaka, H. The fate of organic nitrogen under photocatalytic degradtion of nitrophenols and aminophenols on irradiated TiO2. J. Photochem. Photobiol. A: Chem. 1997, 109, 171-176. (11) Lea, J.; Adesina, A. A. Oxidative degradation of 4-nitrophenol in UV-illuminated titania suspension. J. Chem. Technol. Biotechnol. 2001, 76, 803-810. (12) Augugliaro, V.; Palmisano, L.; Schiavello, M.; Sclafani, A. Photocatalytic degradation of nitrophenols in aqueous titanium dioxide dispersion. Appl. Catal. 1991, 69, 323-340. (13) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.; Draper, R. B. Kinetic studies in heterogeneous photocatalysis. 2. TiO2mediated degradation of 4-chlorophenol alone and in a three-component mixture of 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5-trichlorophenol in air-equilibrated aqueous media. Langmuir 1989, 5, 250-255. (14) Turchi, C. S.; Ollis, D. F. Mixed reactant photocatalysis: intermediates and mutual rate inhibition. J. Catal. 1989, 119, 483-496. (15) Marci, G.; Sclafani, A.; Augugliaro, V.; Palmisano, L.; Schiavello, M. Influence of some aromatic and aliphatic compounds on the rate of photodegradation of phenol in aqueous suspensions of TiO2. J. Photochem. Photobiol. A: Chem. 1995, 89, 69-74. (16) Nagaveni, K.; Sivalingam, G.; Hedge, M. S.; Madras, G.; Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO2. Appl. Catal. B: EnViron. 2004, 48, 83-93. (17) Sivalingam, G.; Nagaveni, K.; Hedge, M. S.; Madras, G. Photocatalytic degradation of various dyes by combustion synthesized nano anatase TiO2. Appl. Catal. B: EnViron. 2003, 45, 23-38. (18) Turchi, C. S.; Ollis, D. F. Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack. J. Catal. 1990, 122, 178-192. (19) Bandara, J.; Mielczarski, J. A.; Lopex, A.; Kiwi, J. Sensitized degradation of chlorophenols on iron oxides induced by visible light comparison with titanium oxide. Appl. Catal. B: EnViron. 2001, 34, 321333.

ReceiVed for reView July 19, 2005 ReVised manuscript receiVed October 28, 2005 Accepted November 3, 2005 IE050846H