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Ind. Eng. Chem. Res. 1999, 38, 3238-3245
Photocatalytic Oxidation of Chlorophenols in Single-Component and Multicomponent Systems Gianluca Li Puma and Po Lock Yue* Department of Chemical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
The photocatalytic oxidation of single-component and multicomponent systems consisting of chlorophenols in aqueous TiO2 suspension was investigated using long, medium, and short wavelength UV radiation. In the single-component experiments, 2-chlorophenol (2-CP), 2,4dichlorophenol, 2,4,6-trichlorophenol, pentachlorophenol, and 4-chloro-3-methylphenol (4-Cl-3MP) were found to degrade at the same rate. The rate of mineralization of 4-Cl-3-MP was higher than that of the others. Power law kinetic models for the temporal prediction of the degradation and mineralization profiles fitted the experimental results well for the single-component systems. The application of these models to represent the simultaneous oxidation of a binary mixture of 2-CP and 4-Cl-3-MP was examined for several equimolar and nonequimolar mixtures. Competitive inhibition kinetics were observed. Under equimolar feed conditions the rates of degradation of the total mixture were comparable to those for single-component systems and could be represented well by the model developed for single-component systems. When one of the two substrates was in excess, the overall oxidation kinetics were controlled by that substrate and the system could therefore be treated as a single-component system. Introduction Chlorophenols belong to a notable group of pollutants because of their high toxicity and carcinogenic properties. They are thus found on both the U.S. Environmental Protection Agency list and the European Community red list of priority pollutants. One of the most direct sources of human exposure to chlorophenols is through drinking water that has been disinfected with chlorine.1,2 Although this method of disinfection is widely used in North America,2,3 it is not practiced in many European countries because chlorine can easily react with organic substrates, present in the raw water supply, to yield a wide range of chlorinated substrates, many of which are of environmental concern.2 These chlorinated substrates are normally formed at trace levels (e10 ppm), making their effective elimination by conventional treatment methods difficult. Adsorption on activated carbon can be an effective removal process, but the spent adsorbent needs further treatment. The reduction in concentration of chlorophenols to parts per billion levels in drinking water can be successfully carried out by advanced oxidation processes (AOP’s) and particularly by heterogeneous photocatalysis. The application of heterogeneous photocatalysis for the oxidation of chlorophenols has been extensively studied for single-component systems.4,5 In most of these studies, the modeling of reaction kinetics has been limited only to the prediction of the initial reaction rates. The prediction of the reaction for the entire time course is of primary importance in order to meet the permitted levels of discharge set by various administrative authorities. Chlorophenols are often encountered as a multicomponent mixture. Studies on the photocatalytic oxidation of multicomponent systems of chlorophenols are scarce. * To whom correspondence should be addressed. Fax: (852) 2335 9030. E-mail:
[email protected].
One of the very few multicomponent studies was by AlEkabi et al.,6 in which the photocatalytic degradation of an equimolar mixture of 4-chlorophenol, 2,4-dichlorophenol, and 2,4,5-trichlorophenol in a reactor with supported titanium dioxide (TiO2) was examined. Competitive kinetics of these three chlorophenols for the active sites at the catalyst surface were observed. However, the authors did not take into account the effect of mass transfer of reactants at the catalyst surface of supported TiO2. Mass-transfer limitations could have a significant influence on the kinetics observed.7,8 A study conducted using a slurry suspension of fine particles of TiO2 should eliminate the effect of mass transfer, thus simplifying the analysis of the observed kinetics. D’Oliveira et al.9 reported the photocatalytic degradation of 2-, 3-, and 4-chlorophenols in a single experiment performed at equal initial concentrations using TiO2 slurry solutions. The results showed marked differences in kinetics between these pollutants in comparison with the results observed for the single-component systems. The authors attributed this to the effect of the intermediate products that would have competed with the three chlorophenols for the same sites of TiO2, and suggested that the modeling of the kinetics for multicomponent systems would be difficult. Koster et al.10 investigated the photocatalytic oxidation of a mixture of organochlorine compounds, using a multilamp slurry photoreactor. For the simultaneous oxidation of a mixture of 1,1,1-trichloroethane, tetrachloroethene, and chlorobenzene, they found that chlorobenzene was oxidized at the highest rate, followed by tetrachloroethene and saturated 1,1,1-trichloroethane, but single-component experiments showed the highest oxidation rates for tetrachloroethene. The authors explained these results by suggesting that a higher adsorption of chlorobenzene and its oxidation products was occurring on the sites at the TiO2 surface, thus
10.1021/ie9807598 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/27/1999
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3239
Figure 1. Schematic representation of the photocatalytic reactor rig.
inhibiting the oxidation of tetrachloroethene and the saturated compounds. For multicomponent systems, only one kinetic model has been presented in the literature. The model describes the time course of the photocatalytic oxidation of the binary system perchloroethylene-benzene.11 The model was found to fit the experimental data well only for the results obtained at low initial concentrations of perchloroethylene (e100 µM). The data showed that the effect of perchloroethylene concentration on the rate of degradation of benzene was negligible, while benzene and its intermediates significantly inhibited the degradation of perchloroethylene. In the present study, the photocatalytic oxidation of aqueous suspensions of chlorophenols is investigated. The main objective is to develop kinetic models for the temporal prediction of both degradation and mineralization of single-component and multicomponent systems of chlorophenols. The suspensions were irradiated by the simultaneous use of short, medium, and long wavelength ultraviolet light to exploit the integration of simultaneous photocatalysis and photolysis.12 Experimental Section Reagents and Materials. All the chemicals used in the experiments and analyses were reagent grade or higher and were used as received without further purification. 2-Chlorophenol (2-CP) (99.5%), 2,4,6trichlorophenol (98%), ethyl acetate, methanol, phosphoric acid, hydrochloric acid, ferric trichloride anhydrous, sodium acetate, and 1,10-phenanthroline hydrate were supplied by BDH Merck. 2,4-Dichlorophenol (99%), 4-chloro-3-methylphenol (4-Cl-3-MP) (99%), pentachlorophenol (99%), acetic anhydride (99%), and potassium oxalate monohydrate (ACS reagent) were from Aldrich. Potassium persulfate was supplied by Sigma, while sulfuric acid (1.84 specific gravity) was supplied by Fisons. Titanium dioxide P25 powder was obtained from Degussa (70:30% anatase to rutile; average particle
Table 1. Spectral Distribution for a 250 W Medium Pressure Mercury Arc Lamp As Supplied by the Lamp Manufacturer wavelength (nm)
power (W)
less than 254 254 257 265 270-275 280-289 296-302 313 334 366 404 435 546 578
negligible 1.077 0.344 0.323 0.129 0.215 0.425 1.078 0.196 1.634 1.274 1.830 2.06 0.818
energy photon flux (kcal einstein-1) (× 106 einstein s-1) negligible 112.56 111.25 107.89 105.89 102.11 96.59 91.35 85.6 78.12 70.77 65.73 52.36 49.47
negligible 2.29 0.74 0.72 0.29 0.51 1.06 2.82 0.55 5.00 4.30 6.65 9.4 3.95
diameter 30 nm in 100 nm aggregates; surface area 50 ( 15 m2 g-1). Ultrapure water with a maximum TOC concentration of 0.1 ppmC was used. Photoreactor. A schematic representation of the 3.6 L stirred multilamp batch photoreactor is shown in Figure 1. The reactor vessel consisted of a QVF borosilicate glass tube (300 mm high, 152 mm diameter) sealed at both ends with stainless steel 316 plates. The photoreactor was fitted with three ultraviolet lamps arranged 40 mm from the reactor center at a pitch of 120°. The lamps used were 250 W medium-pressure mercury arc lamps of 152 mm in length and 19 mm in diameter, housed inside quartz tubes with an external diameter of 38 mm. The spectral power distribution of the lamp (Table 1) extended into the short, medium, and long wavelength of ultraviolet light. Oxygen was supplied to the suspensions through a sintered Teflon plate at the bottom of the reactor. The reactor was fitted with a mixer unit with a turbine impeller, a glass cooling coil placed close to the internal walls of the photoreactor, and probes for temperature and pH measurements. The temperature of the suspension was controlled by circu-
3240 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
lating water from a thermostatic water bath. The reactor was accommodated inside a black wooden box for protection from UV radiation. Experimental Procedures. Suspensions were prepared in ultrapure water by mixing TiO2 with appropriate solutions of chlorophenols to give the selected concentrations to a final volume of 3.6 L. Irradiation commenced after the suspensions had been equilibrated in the dark for 40 min with constant mixing and an oxygen sparge at 1 L min-1. The oxygen sparging was maintained at the same rate throughout the irradiation experiment. Samples collected at appropriate time intervals were analyzed for concentrations of substrates and total organic carbon. Analyses. The degradation of chlorophenol substrates and the formation of intermediate products were followed by a Hewlett-Packard Model 5890 Series II gas chromatograph equipped with a Ni-63 electron capture detector and an on-column injection system (GC-ECD). The instrument was fitted with a Hewlett-Packard Model 7673 autosampler and an SGE capillary column BP-X5, 25 m, 0.22 mm i.d., and 0.25 µm film. For these analyses, the samples were acetylated with acetic anhydride according to the method described in Mathew and Elzerman13 and concentrated by solid-phase extraction. Ethyl acetate was used as the eluting solvent, and Bond Elut CH sorbent, as the stationary phase. The GCECD analytical method used was the same as that described in Li Puma and Yue.14 The course of mineralization of a chlorophenol to carbon dioxide was traced with a Dohrman DC-180 carbon analyzer. Total organic carbon (TOC) was calculated as the difference between the total carbon (TC) and the inorganic carbon (IC). The sensitivity of the analyzer was in the range 10 ppb to 30000 ppm carbon. Chemical Actinometry. The photon flux from the ultraviolet lamps was measured by using potassium ferrioxalate actinometry.15 Actinometric experiments were performed individually for each of the three lamps. The combined radiant power for the three lamps was estimated to be 4.3 × 10-5 einstein s-1, using an average quantum yield for Fe2+ production of 1.185. The radiant power per unit reactor volume was calculated to be 1.19 × 10-5 einstein s-1 L-1. In the presence of a TiO2 water suspension the absorption of photons would be less than that in these estimations because of (i) the dispersive effect of light scattering, (ii) the absorption of photons from the walls of the photoreactor and from the lamp bulbs, and (iii) the fact that a ferrioxalate actinometer solution absorbed photons up to 436 nm, while TiO2 photocatalytic reactions are driven by photons of wavelength less than 380 nm. Results and Discussion Oxidation of Chlorophenols in Single-Component Systems. The procedure of continuous sparging of oxygen in the reactor could have resulted in losses of volatile organic carbon (VOC) from the liquid phase. To assess this effect, a number of control experiments were conducted with the outflow gas from the reactor being bubbled through a cold trap system. The cold trap system consisted of a cylinder containing 200 mL of ethyl acetate maintained at a temperature of 0 °C. After the trapping experiments, the total volume of ethyl acetate was reduced to 10 mL by evaporation in a vacuum and the sample was analyzed by GC-ECD and GC-MS. In all the experiments no VOC peaks were found.
Figure 2. Effect of the catalyst loading on the rate of mineralization of 2-CP. Initial concentration ) 400 µM. Temperature ) 20 °C.
The effect of catalyst loading on the oxidation of 2-CP to carbon dioxide was first established to determine the optimal loading of the photocatalyst for the geometry of the reactor. Figure 2 illustrates the rates of mineralization as a function of the catalyst loading for experiments performed at an initial concentration of 2-CP of approximately 400 µM. The adsorption of 2-CP on TiO2 (Degussa P25) has been reported to be poor, with monolayer coverage being reached at an equilibrium concentration in the aqueous phase of approximately 150 µM.16 This suggests that maximum coverage of the active sites of the photocatalyst was likely for experiments performed at an initial concentration of 2-CP of approximately 400 µM. The mineralization of 2-CP was found to follow zero-order kinetics. In the absence of the photocatalyst, the oxidation of 2-CP was caused by photolysis by the UVC radiation from the lamps. In the presence of the catalyst, the mineralization rates showed a 3-fold enhancement, increasing to a maximum at a catalyst concentration of 0.5 mg L-1. The increase in mineralization rates was the result of a greater availability of photoactivated reaction sites, while the plateau of high TOC reduction rate that followed was the consequence of complete light absorption and complete lateral mixing bringing fresh reactant into the illuminated portion of the reactor. Figure 3 shows a comparison of degradation (a) and the mineralization (b) kinetics of single-component experiments with 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, pentachlorophenol, and 4-chloro3-methylphenol conducted under the same experimental conditions, at initial chlorophenol concentrations in the range from 40 to 45 µM. This value of initial concentrations, which corresponded to one-third of the monolayer coverage of TiO2, was selected in order to minimize the influence of the intermediate products on the kinetics of initial oxidation of chlorophenols. The degradation profiles of all five chlorophenol substrates overlapped, indicating a similar mechanism of initial oxidation. In contrast, the influence of the intermediate products became significant in the TOC reduction profiles. As the reaction proceeded, monolayer coverage of the photoactive sites on the surface of the photocatalyst by intermediate products with different degrees of resistance to oxidation was obtained progressively. Of the five chlorophenols, 2-CP yielded the lowest rate of mineralization while 4-Cl-3-MP showed the fastest rate of
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3241
-
dC ) kCn dt
(1)
where C refers to the concentration of either chlorophenol or total organic carbon, t is time, k is the specific reaction rate, and n is the reaction order. The integration of eq 1 yields the dependence of the concentration of C on time (eq 2).
C ) (Ct)01-n + (1 - n)kt)1/(1-n)
(2)
For kinetics other than first-order, it is worth noting that this kinetic model predicts that the concentration will fall to zero at a finite time, which can be evaluated by
t)
Figure 3. Degradation (a) and mineralization (b) profiles of selected chlorophenols. Initial concentration ) 40-45 µM. [TiO2] ) 0.5 g L-1. Temperature ) 20 °C.
TOC reduction. The other three chlorophenols gave similar rates of mineralization. The differences observed in the mineralization rates are probably due to the different intermediate products formed from the oxidation of the chlorophenols. Although much of the literature presents a Langmuirian treatment of the kinetics of photocatalytic reactions, Cunningham et al.16,17 have shown very clearly the limitations inherent with the treatment of adsorption kinetics in photocatalytic reactions. In particular, they argued against the validity of the Langmuir-Hinshelwood mechanism in photocatalysis. This is especially true in the present case with chlorophenols weakly absorbed on TiO2. The development of a Langmuir-Hinshelwood model with an additional term for photolysis would increase the mathematical complexity of the photocatalytic reactor model without the guarantee of true representation of the mechanistic processes. Recently, it has been shown that the use of a power law kinetic equation allows simpler models of photocatalytic reactors to be formulated with a good degree of success.18,19 Because of its relative simplicity, this approach is extended here to account for the additional effect of photolysis. In this study, the kinetics of oxidation of chlorophenols in single-component systems were modeled using an nth-order power law kinetics equation. A material balance for an ideal batch reactor is given by
Ct)01-n (1 - n)k
(3)
In real systems, fractional reaction orders were observed and the value of n will shift upward to unity as the reactant is depleted. Consequently the model should not be used when the concentration of the reactants falls 95% from the initial value. The results of the oxidation of single-component experiments performed at initial substrate concentrations of 2-CP and 4-Cl-3-MP in the range 40 to 700 µM are shown in Figure 4. The kinetic parameters for a given set of experimental conditions were determined using a differential method of data analysis.20 Taking the logarithm of eq 1, the kinetics coefficients k and n were respectively calculated from the slope and the intercept of a plot of -ln(dC/dt) versus ln C. Figure 4c illustrates the initial rates of degradation of 2-CP and 4-Cl-3-MP as a function of the initial concentrations of substrate. A single straight line was found to fit the degradation rates of both 2-CP and 4-Cl-3-MP. The experimental results in Figure 4a and b were found to be represented well by eq 2 with n ) 0.203 and k ) 1.34 min-1 µM(1-0.203). In the same experiments, the apparent quantum efficiency, defined as the ratio of the initial rates of degradation divided by the radiant power per unit reactor volume, was calculated to be in the range 0.4-0.8%, which is in agreement with the data published in the literature.4 A common mechanism of initial oxidation of chlorophenols may be postulated in view of the similarity observed in the degradation kinetics of 2-CP and 4-Cl3-MP for a relatively wide region of initial concentrations, and in light of the results in Figure 3. As the suspensions were irradiated by the simultaneous use of short, medium, and long wavelength ultraviolet light, oxidation of the organic substrates could be assumed to have occurred by simultaneous photolysis and photocatalysis. The photolysis of chlorophenols has been reported to proceed through the cleavage of a carbonchlorine bond followed by hydroxylation or protonation. Tetrachlorophenol, tetrachlorocatechol, tetrachlororesorcinol, and tetrachlorohydroquinone have been reported to be intermediate products of the photolysis of pentachlorophenol.14,21 The photolysis of monochlorophenols has been shown to progress through the displacement of chloride by hydroxyl ions with the formation of 1,2-dihydroxybenzene from 2-chlorophenol, 1,3-dihydroxybenzene (resorcinol) from 3-chlorophenol, and 1,4-dihydroxybenzene (catechol) from 4-chlorophenol.6,22,23 In contrast, under photocatalytic conditions,
3242 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
Figure 5. Mineralization profiles of experiments in Figure 4a and estimation of kinetic parameters in eq 4 for 2-chlorophenol. Solid lines are model predictions (eqs 2 and 4).
Figure 4. Degradation profiles of 2-CP (a) and 4-Cl-3-MP (b) at different initial concentrations. Solid lines are model predictions (eq 2). Plot c illustrates the estimation of the kinetic parameters in eq 2. [TiO2] ) 0.5 g L-1. Temperature ) 20 °C.
the oxidation of chlorophenols has been shown to progress through successive hydroxylation of the aromatic ring preferentially on the para and ortho positions14,22-24 with the formation of chloro- and hydroxyquinones. For TiO2-sensitized oxidation of chlorophenolic compounds, the build-up of hydroxyl radicals and/or peroxy species on the solid-liquid interface has been shown by several authors. Cunningham16 suggested that, under irradiation with relatively high radiation intensities, the build-up of peroxy species would greatly exceed the extent of 2-CP adsorption, thus making it feasible for such species to control the kinetics of the process. Given the complexity of the kinetics under study, it was not possible to distinguish whether the photolytic pathway would be preferential to the photocatalytic pathway or vice versa. If the kinetics were controlled by the accumulation of photogenerated hydroxyl radicals at the TiO2 surface or by the cleavage of one carbon-chlorine bond, it could lead to similar degradation rates of different chlorophenols. The kinetics of TOC reduction observed in the singlecomponent experiments on 2-CP and 4-Cl-3-MP were
Figure 6. Mineralization profiles of experiments in Figure 4b and estimation of kinetic parameters in eq 4 for 4-chloro-3-methylphenol. Solid lines are model predictions (eqs 2 and 4).
found to be zero-order (Figures 5 and 6). For these experiments, the specific reaction rate k in eq 1 was found to be a linear function of the initial TOC concentration of organics, as expressed in eq 4.
kTOC ) k1CTOC,t)0 + k2
(4)
A comparison of the coefficients derived from the linear fitting of the results shown in Figures 5b and 6b for 2-CP and 4-Cl-3-MP resulted in a similar value of the parameter k2 but a different k1 value. Equation 4 shows that k has two parts; one is dependent on the
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3243
Figure 7. Effect of temperature on the oxidation of 2-CP. Solid lines are model predictions (eq 6). [TiO2] ) 0.5 g L-1.
initial TOC concentration while the other is not. This result may also be related to the formation of intermediate products. The appearance of an unidentified peak, which occurred only in the chromatogram of acetylated and extracted samples of irradiated 4-Cl-3-MP, and the similarity in the degradation rates of 2-CP and 4-Cl-3MP support the hypothesis that the differences in the kinetics of mineralization observed with these two substrates were the result of the formation and buildup of intermediate breakdown products with different resistances to oxidation. The parameter k1 in eq 4 may be associated with the effect of the intermediate products, as their influence on the reaction would be greater at higher initial TOC concentrations. In contrast, at low initial TOC concentrations, the influence of the intermediate products is expected to be small, and the process would be more controlled by the parameter k2 in eq 4. The effects of the intensity of the incident radiation and the temperature may be considered to be included in k2. As the intrinsic mechanism of initial oxidation is believed to be the same for all chlorophenols, the similarity in the value of k2 observed for 2-CP and 4-Cl-3-MP should be expected. The application of the kinetic model to fit the mineralization profiles of 2-CP and 4-Cl-3-MP is shown by the solid lines in Figures 5 and 6. A good fit was obtained between the experimental data and the model. Effect of Temperature. Although only negligible temperature effects were observed on the oxidation rates of 2-CP when the TiO2 suspensions were irradiated with UVA radiation alone, a significant influence has been reported when short, medium, and long wavelength ultraviolet light was used simultaneously.25 To include the effect of temperature in the kinetic model, the temperature-dependent term in eq 1 was expressed in terms of Arrhenius’ law: -E/RT
k ) k0e
(5)
where k0 is the frequency factor, E is the activation energy of the reaction, R is the gas constant, and T is the temperature. Figure 7 shows the degradation profiles of 2-CP in the range of temperature from 10 to 40 °C. The energy of activation E, obtained from the Arrhenius-type plot shown in Figure 7, was found to be 18.4 ( 3.0 kJ mol-1. This value is more than three times higher than the
Figure 8. Mineralization profiles of experiments in Figure 7. Solid lines are model predictions (eq 8).
value of 5.5 kJ mol-1 measured by Al-Sayyed et al.22 for the destruction of 4-chlorophenol in aqueous solution containing UVA-illuminated TiO2 powder and greater than the values found by Matthews26 for salicylic acid (11 kJ mol-1) and by Okamoto et al.27 for phenol (10 kJ mol-1). k0 calculated from eq 5 was 2403 min-1 µM(1-0.211). The concentration profiles of 2-CP calculated by integrating eq 6 were found to describe satisfactorily the experimental results shown in Figure 7. As chlorophenols degraded at similar rates, eq 6, with the Arrhenius parameters determined from Figure 7 (inset), is also expected to represent the degradation of the other chlorophenols considered in this study.
-
dCchlorophenol n ) k0e-E/RTCchlorophenol dt
(6)
The effect of temperature on the kinetics of mineralization was studied only for 2-CP oxidation. In establishing the effect of temperature on the reaction rate equation, it was assumed that, as explained earlier, the changes in temperature would influence the parameter k2 only and not k1. k2 should follow the Arrhenius form:
k2 ) k2,0e-E2,0/RT
(7)
The apparent energy of activation E2,0 obtained from the linear fitting of the data in Figure 8 (inset) was found to be 36.0 ( 13.8 kJ mol-1, and k2,0 was 27.07 × 104 ppmC min-1. The equation derived from the integration of the general rate equation,
-
dCTOC ) k1CTOC,t)0 + k2,0e-E2,0/RT dt
(8)
fitted the TOC reduction profiles in Figure 8 well, supporting the explanation given for the parameters k1 and k2 of eq 4. Oxidation of Chlorophenols in Multicomponent Systems. The application of the kinetic models developed earlier to represent the simultaneous oxidation of a binary mixture of 2-CP and 4-Cl-3-MP was examined for several equimolar and non-equimolar mixtures. Figure 9 illustrates respectively the results from the simultaneous photooxidation of 2-CP and 4-Cl-3-MP with one of the two reactants approximately 10 times in excess in the initial substrate concentration. In both
3244 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
Figure 10. Simultaneous oxidation of an equimolar mixture of 2-CP and 4-Cl-3-MP. [TiO2] ) 0.5 g L-1. Temperature ) 20 °C.
Figure 9. Simultaneous oxidation of a mixture of 2-CP and 4-Cl3-MP with (a) an excess of 2-CP or (b) an excess of 4-Cl-3-MP. [TiO2] ) 0.5 g L-1. Temperature ) 20 °C.
experiments, it was observed that the overall oxidation kinetics were controlled by the reactant in excess, as the substrate present in smaller concentrations was found to degrade at a much slower rate than that in the single-component experiments. The kinetic model developed earlier for the degradation of chlorophenols as a single-component system fitted the degradation profiles of the reactant present in excess well (solid lines in Figures 9). In addition, the reduction of TOC was also successfully represented by the model developed earlier with the kinetic parameters derived from the experiments performed under single-component conditions for the reactant in excess (cf., Figures 5 and 6). However, deviations in fitting the model to the results in Figures 9 may be expected as the concentrations of the two substrates become closer to each other during the reaction. A number of experiments were conducted using equimolar mixtures of 2-CP and 4-Cl-3-MP at total molar concentration ([2-CP] + [4-Cl-3-MP]) in the range from 80 to 800 µM. Figure 10 shows the results of one of these experiments. In these experiments, the rates of degradation of each of the two substrates were found to be slower than those observed in the single-component systems. However, the rates of degradation in terms of total molar concentration ([2-CP] + [4-Cl-3MP]) were comparable to those of single-component systems and could be well represented by the model developed earlier for the oxidation of chlorophenols in the single-component system (solid line in Figure 10). In addition, there were no substantial differences in the
Figure 11. Comparison between the model developed for singlecomponent systems and the TOC rates measured during simultaneous oxidation of 2-CP and 4-Cl-3-MP. [TiO2] ) 0.5 g L-1. Temperature ) 20 °C.
degradation profiles of 2-CP and 4-Cl-3-MP. The results of the experiments conducted with equimolar and nonequimolar mixtures suggested a mechanism that includes a competitive inhibition of the two reactants. This mechanism would support the hypothesis that chlorophenols would have a similar TiO2-mediated photooxidative mechanism for the initial destruction of the parent organic substrates. From an environmental point of view, the reduction in TOC may be more important than just the degradation of the initial organic substrates, as intermediate products may be refractory to further oxidation. In addition, the elimination of the initial organic substrates does not necessarily lead to a reduction in the overall hazard of the wastewater. A comparison between the model developed for singlecomponent systems and the TOC rates measured during the simultaneous oxidation of 2-CP and 4-Cl-3-MP is shown in Figure 11. The rates of mineralization with mixtures of these two substrates were found to be in closer agreement with the model representing the mineralization of 4-Cl-3-MP, with the exception of the experiment in which 2-CP was in excess. This suggests that a greater fraction of 4-Cl-3-MP was mineralized compared with that of 2-CP, possibly because less refractory intermediate products are generated during the oxidation of the former.
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In summary, it has been shown that the entire course of photocatalytic oxidation of single-component and multicomponent systems of chlorophenols can be predicted satisfactorily using simple kinetic models that could be useful in the design and modeling of large scale photocatalytic reactors. Nomenclature C ) concentration of substrate, mol L-1, or concentration of total organic carbon, ppmC CTOC ) concentration of total organic carbon, ppmC Cchlorophenol ) concentration of chlorophenol, mol L-1 E ) energy of activation of the reaction, J mol-1 k ) specific reaction rate, s-1 (mol L-1)1-n or s-1 (ppmC)1-n k0 ) frequency factor, s-1 (mol L-1)1-n k1 ) coefficient in eq 4, s-1 k2 ) k2.0eE2.0/RT, coefficient in eq 4, ppmC s-1 kTOC ) k1CTOC,t)0 + k2, specific reaction rate for destruction of TOC, ppmC s-1 n ) order of the reaction, dimensionless R ) gas constant ()8.314 J mol-1 K-1) t ) time, s T ) temperature, K TOC ) total organic carbon
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Received for review December 1, 1998 Revised manuscript received April 26, 1999 Accepted May 22, 1999 IE9807598
