Environ. Sci. Technol. 2008, 42, 913–919
Kinetics of Simultaneous Photocatalytic Degradation of Phenolic Compounds and Reduction of Metal Ions with Nano-TiO2 R. VINU AND GIRIDHAR MADRAS* Department of Chemical Engineering, Indian Institute of Science, Bangalore-560 012, India
Received August 16, 2007. Revised manuscript received October 13, 2007. Accepted October 16, 2007.
The simultaneous photocatalytic degradation of phenol and 4-nitrophenol and reduction of metal ions like copper (Cu2+) and chromium (Cr6+) was studied with solution combustion synthesized nanoanatase TiO2 (CS TiO2) and commercial titania, Degussa P-25. The presence of metal ion reduces the rate of degradation of phenol and 4-nitrophenol. It was found that Cu2+ reduction to Cu+ is accelerated in the presence of phenol. In the case of Cr6+, CS TiO2 enhances the initial adsorption of Cr6+ and complete reduction is achieved within the first 10 min of UV irradiation. The presence of phenol or 4-nitrophenol also enhances the initial adsorption of Cr6+ and its reduction. The metal ion reduction in the presence of CS TiO2 is compared with that of Degussa P-25. The rate of reduction of metal ions in presence of Degussa P-25 is twice as slow as that of CS TiO2 in presence of both phenol and 4-nitrophenol. The presence of Cu2+ and Cr6+ also induces the formation of the intermediates which were not observed for the phenol-CS TiO2 system. The formation and consumption of the intermediates are modeled with a simple series reaction mechanism. A detailed dual-cycle, multistep reaction mechanism of TiO2 photocatalysis for the simultaneous degradation and reduction is proposed and the model is developed following the network reduction technique. The kinetic rate constants in the model are evaluated for the systems studied.
Introduction
Experimental Section
Heterogeneous photocatalysis involves the semiconductor mediated redox reactions for the effective detoxification of harmful pollutants in water. Energetics dictates that for a semiconductor to be photochemically active, the redox potential of the photogenerated valence band hole must be sufficiently positive to generate hydroxyl (OH · ) radicals and that of the conduction band electron must be sufficiently negative to generate superoxide (O2– .) radicals for the oxidation of the pollutants (1). Many reviews in the recent past have evaluated the efficiency of TiO2, which has an optimum band gap of 3.2 eV, in the oxidative degradation of numerous organic compounds and reduction of inorganic metal ions (2–4). We have recently developed anatase phase nano TiO2 by solution combustion synthesis that has a higher specific surface area, higher surface hydroxyl content, and lower band gap compared to Degussa P-25 (5). It is well proven that CS * Corresponding author tel.: +91-80-2293-2321; fax: +91-80-23600683; e-mail:
[email protected]. 10.1021/es0720457 CCC: $40.75
Published on Web 12/19/2007
TiO2 can degrade organic pollutants such as phenolic compounds and dyes at a faster rate than Degussa P-25 (6, 7). The TiO2 mediated photocatalytic degradation of phenolic compounds has been a subject of intense research and thorough studies have been conducted on the degradation of phenol (8, 9), chlorophenols (10, 11), nitrophenols (12, 13), substituted phenols (14), their mixtures (15) and multisubstituted phenols (16) and the intermediates that are formed during the degradation. Metal ions pose an acute threat to the environment in their toxic state, which must essentially be reduced to their stable nontoxic states before discharging them into the environment. Metal ions and aromatic compounds are discharged together in industrial processes such as wood preserving, metal finishing, petroleum refining, leather tanning and finishing, paint and ink formulation, and manufacturing of automobile parts (17). Many studies report the reduction of metal ions such as Ag+, Cu2+, Cr6+, Hg2+, Fe2+, and Fe3+, etc. using commercial TiO2 (18–21). Only a few studies have dealt with the simultaneous photocatalytic synergistic degradation of dyes and organic compounds with the reduction of metal ions (22–24). Although the mechanism proposed in a recent work (24) was solved by applying the pseudo-steady-state assumption, the model does not account for the reduction rate of metal ions in the presence of dye, and a nonlinear equation was fit between the initial dye degradation rate and the initial metal ion concentration. In another study, cyclic network mechanism was applied for the photocatalytic decomposition of methylene blue in two batch slurry reactors (25). Overall, a thorough mechanistic description of the photocatalytic simultaneous degradation-reduction of organics and metal ions is lacking. Hence our current investigation of the kinetics of simultaneous photocatalytic degradation and reduction assumes importance. In the present study, the simultaneous photocatalytic degradation of phenol and 4-nitrophenol and reduction of Cu2+ and Cr6+ was investigated in presence of CS TiO2. The metal ions were chosen based on a recent work (26) which reports that among a wide variety of metal ions, only Cu2+ and Cr6+ show significant reduction with CS TiO2. The effect of varying the concentration of phenol and 4-nitrophenol on the reduction rate of Cu2+ and Cr6+ and vice-versa were studied. The metal ion reduction profiles in the presence of phenolic compound with CS TiO2 were compared with that of Degussa P-25. A detailed kinetic model based on the dualcycle, multistep network mechanism was formulated and the rate constants were determined.
2008 American Chemical Society
Catalyst Preparation and Characterization. Nanosize anatase titania was prepared by solution combustion synthesis. This method involves the combustion of aqueous solutions containing stoichiometric amounts of the precursor compound, titanyl nitrate (TiO(NO3)2) and fuel, glycine (H2N-CH2-COOH). In combustion synthesis, a pyrex dish (300 cm3) containing an aqueous redox mixture of stoichiometric amounts of titanyl nitrate (2 g) and glycine (0.8878 g) in 30 mL of water was introduced into the muffle furnace, which was preheated to 350 °C. The solution initially undergoes dehydration and a spark appearing at one corner spreads throughout the mass and yields anatase phase TiO2. Further details are reported elsewhere (7). The catalyst has been characterized by various techniques such as XRD, TEM, BET, porosimetry, TG-DTA, XPS, FT-IR, and UV spectroscopic techniques. The XRD pattern of CS TiO2 was indexed to the pure anatase phase of TiO2. TEM studies showed that the crystallites of TiO2 are homogeneous with a mean size of 8 ( 2 nm, which agrees well with the XRD VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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measurements. The BET surface area of the catalyst was 240 m2/g, which is higher than that of Degussa P-25 (50 m2/g). The pore size distribution of the catalysts was determined by the BJH method and the average pore diameter was 25 nm in the case of CS TiO2 compared to that of 4 nm for Degussa P-25 (27). The TG-DTA of CS TiO2 showed a 15.6% total weight loss, indicating more surface hydroxyl groups, thus confirming the FT-IR study. UV–vis absorption spectra of CS TiO2 in the 270-800 nm range showed two optical absorption thresholds at 570 and 467 nm that correspond to band gap energies of 2.18 and 2.65 eV, respectively. This is significantly different from the bandgap of Degussa P-25 (3.1 eV), because of the carbide ion substitution for oxide ion of the form TiO2–2xCx0x (5). Photochemical Reactor. The photochemical reactor used in the present study consisted of a jacketed quartz tube of 3.4 cm i.d., 4 cm o.d., and 21 cm length. A high-pressure mercury vapor lamp of 125 W (Samson, India) was placed inside the reactor after carefully removing the outer shell. The ballast and capacitor were connected in series with the lamp to avoid the uneven fluctuation in the input supply voltage. Cold water was circulated in the annulus of the reactor to maintain the solution temperature below 35 °C as excess temperature may deplete the dissolved oxygen in the solution. The source assembly was placed concentrically inside the pyrex glass container of 5.7 cm i.d. and 16 cm height filled with 100 mL of the solution to be degraded. The distance between the source and the bottom of the vessel was 2 cm to aid better stirring using a magnetic stirrer. The lamp radiated predominantly at 365 nm (3.4 eV) and the photon flux determined by o-nitrobenzaldehyde actinometry (28) was 15.56 Wm-2. The degradation was performed in an open system wherein the top surface of the photoreactor was open to the atmosphere. This facilitates atmospheric air to provide enough oxygen for oxidative degradation of the pollutants. Further details are reported elsewhere (7). Analysis. Prior to analysis, the samples withdrawn at each time interval were filtered through a Millipore membrane filter and then centrifuged to remove any nanosized catalyst particles which might interfere in the analysis of the phenolic compound and metal ions. Because every aliquot must be analyzed for its phenolic compound and metal ion concentration, the analysis procedures were chosen such that there was no interference caused by the metal ion or phenolic compound on one another. Both the metal ion concentrations were determined spectrometrically (Shimadzu UV 2100) in the wavelength range 400–600 nm. Cu2+ was estimated at 510 nm by analyzing the complex formed with the anthraquinone dye, alizarin red S (29). Cr6 + ion was estimated at 540 nm by analyzing the complex formed with the reagent 1,5-diphenylcarbazide (30). A calibration based on Beer– Lambert law was used to quantify the concentration. The concentration of phenol, 4-nitrophenol, and the intermediates formed during the degradation were quantitatively analyzed using HPLC, which consisted of an isocratic pump (Waters 501), Rheodyne injector (sample loop, 50 µL), C-18 column, UV detector (Waters 2487), and a data acquisition system. The mobile phase consisted of 90 vol. % water and 10 vol. % methanol pumped at a flow rate of 0.5 mL/min. The UV absorbance detector was set at 270 and 320 nm for phenol and 4-nitrophenol, respectively. The retention time for every intermediate was found either by injecting the pure species or by comparison with the existing standards. The chromatographic areas were converted to concentration values using the calibration curves. Network Mechanism and Rate Equations. In the discussion to follow, it is assumed that adsorption of phenolic compound and metal ion is the first step before irradiation of the TiO2 nanoparticle suspension. This is consistent with the experiments wherein before degradation, the phenolic 914
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FIGURE 1. Cyclic network mechanism for the simultaneous photocatalytic degradation of phenolic compounds and reduction of metals. compound and/or the metal ion TiO2 suspension is equilibrated by stirring in the dark for 30 min. The adsorption is competitive between the phenolic compound and the metal ion, which constitute the two independent cyclic pathways C1 and C2 for the degradation of phenolic compound and the reduction of metal ion, respectively (Figure 1). When an organic compound or metal ion adsorbed TiO2 is irradiated with UV light, conduction band electrons and valance band holes are generated in a time scale of femtoseconds (2). These photogenerated electron–hole pairs can recombine within a time scale of 100 nanoseconds to radiate heat, leaving back the TiO2 vacant site (2). Herein we assume that the above two reactions are reversible. The next step in the cycle C1 is the detoxification of the phenolic compound by oxidation. This occurs by the formation of hydroxyl and superoxide radicals. Hydroxyl radicals are generated when surface adsorbed water or hydroxyl anions react with the valence band hole of the UV excited TiO2. Superoxide radicals are generated by the interaction of atmospheric oxygen and dissolved oxygen in the solution with the conduction band electron of the UV excited TiO2. Herein we also include the formation of hydroxyl radical by an alternate route, where hydrogen peroxide (generated by the reaction of superoxide radical with H+ ions), reacts with the conduction band electrons. Thus the hydroxyl and superoxide radicals attack the adsorbed phenolic compound to give hydroxylated intermediates and finally yield ring opened fragments, primarily linear chain carboxylic acids thereby freeing the occupied TiO2 site. In a parallel metal reduction cycle C2, the surface adsorbed metal ions are reduced by the photogenerated conduction band electrons to their thermodynamically stable, less toxic states. The cycle is complete when the surface adsorbed metal ion in its reduced state is freed from the occupied TiO2 site. The equations in Table 1 are proposed for the above dual site simultaneous phenolic compound degradation and metal ion reduction mechanism (2, 7, 25).
TABLE 1. Proposed Equations for Dual Site Simultaneous Phenolic Compound Degradation and Metal Ion Reduction Mechanism Cycle C1 TiO2 + Ph T TiO2 - Ph TiO2 - Ph + hυ T TiO2 * (e-, h+) - Ph TiO2 (h+) - Ph + H2O(ads) f TiO2 (OH · )(ads) - Ph + H+ TiO2 (h+) - Ph + OH-(ads) f TiO2 (OH · )(ads) - Ph TiO2 (e-) - Ph + O2(ads) f TiO2 (O2–.)(ads) - Ph 2TiO2 (O2–.)(ads) - Ph + 2H+ f TiO2 - H2O2 - Ph + O2 TiO2 - H2O2 - Ph + TiO2 (e-) - Ph f TiO2 (OH · )(ads) - Ph + OHTiO2 (OH · )(ads) - Ph + Ph/ TiO2 - Ph f Intermediates f ROF + TiO2 TiO2 (O2–.)(ads) - Ph + Ph/ TiO2 - Ph f Intermediates f ROF + TiO2
adsorption: photoexcitation: hole pathway: electron pathway: hydroxy and superoxide radicals attack:
Cycle C2 TiO2 + Mn+ T TiO2 - Mn+ TiO2 - Mn+ + hυ f TiO2* (e-, h+) - Mn+ f TiO2 - Mm+ (m < n) TiO2 - Mm+ f Mm+ + TiO2
adsorption: metal ion reduction: desorption from the active site:
Though the reaction equations in Table 1 can be solved for the rate of degradation of the phenolic compound and reduction of metal ions by the quasi equilibrium approximation or pseudo-steady-state assumption, the resulting equations are cumbersome to solve for the rate expressions. The present method of multicycle network reduction technique (25, 31) allows one to directly obtain the rate of degradation of phenolic compound and the reduction of metal ion in terms of the steady state rate through each of the cycles C1 and C2, respectively. One more advantage the method possesses is that there are more than one variable (such as light intensity, dissolved oxygen concentration, and TiO2 suspension concentration) that can be tweaked at a time to get a more realistic picture of the photocatalytic process. The basic rate equation, its simplification, and the expressions for the rate constants are provided in the SI 1 Appendix A in the Supporting Information. Though the model can account for the different operating parameters like UV intensity, TiO2 concentration, and dissolved oxygen concentration, we have initially used this approach to model a two component system with only substrate concentration as a variable parameter. Our future work will examine the influence of various other parameters. The reduced rate equations for simultaneous degradation-reduction are as follows:
( (
-
-
) )
(
n+
1 1 [M ] 1 - K2 ) + rph,o [Ph]o K1 K3
(
)
1 1 1 [Ph] - K5 ) n+ + rMn+,o K6 [M ]o K4
)
(1) (2)
Thus a plot of (-1/rPh,o - K2) and (-1/rMn+,o - K5) against 1/[Ph]o and 1/[Mn+]o, respectively, will be a straight line passing through the origin. The slope gives the value of the right-hand term in the parentheses. The rate constants K1, K3, K4, and K6 are found by performing various experiments at different phenol and 4-nitrophenol with metal ion concentrations. The rate constants K2 and K5 are found by degrading the phenolic compound and reducing the metal ion individually with CS TiO2. The reduced rate equations are as follows (see SI 1 Appendix A in the Supporting Information):
-
( ) ( )
1 1 1 ) + K2 rph,o [Ph]o K ′ 1
(3)
1 1 1 ) + K5 rMn+,o [Mn+] K ′ o 4
(4)
-
Results and Discussion The TiO2 suspension concentration was maintained at 1 gL-1 for all the photo degradation-reduction experiments. Cu2+ and Cr6+ were introduced into the aqueous system as their salts, cupric nitrate and potassium dichromate, respectively. No detectable degradation of phenol and 4-nitrophenol or metal ions was observed without the catalyst or irradiation with UV light alone. Some of the photocatalysis experiments were performed with two different stirring rates and the results were the same confirming that the rate measurements were done within the kinetic regime. All the photodegradation-reduction reactions were carried out at the natural pH of the suspension, which was near-acidic to acidic. This is validated by the fact that adsorption and photocatalytic reduction of Cr6+ is facilitated in acidic conditions (20). At pH < 2, Cr6+ exists as neutral chromic acid molecule (H2CrO4), which exhibits low affinity to the positive TiO2 surface. At 2 < pH < 6, Cr6+ exists as negatively charged HCrO4-, CrO42-, and Cr2O72- species, which are adsorbed due to the balance with the positive surface charge of TiO2. Near the pHzpc () 6–7) of anatase TiO2, maximum level of adsorption is observed. The reduction in acidic and neutral solution pH is reported to follow the mechanism (18): 2Cr2O7 + 16H+ f 4Cr3+ + 8H2O + 3O2 (acidic pH) 4CrO4 + 20H+ f 4Cr3+ + 10H2O + 3O2 (neutral pH) Moreover, in the basic pH, Cr3+ precipitates as the hydroxide and is no more in the TiO2 suspension. Another study (32) proves that only single-electron processes are involved in the photocatalytic reduction of Cr6+ to Cr3+ by TiO2. The proposed mechanism was as follows: e-
e-
e-
Cr(VI) 98 Cr(V) 98 Cr(IV) 98 Cr(III) where Cr(V) and Cr(IV) are highly unstable species. Hence our mechanism of Cr6+ reduction to Cr3+ is inline with the above observation. Similarly, Cu2+ reduction to Cu+ is also thermodynamically favorable at acidic pH. This is because of the fact that standard reduction potentials of Cu(II,I), Cu(I,0), and Cu(II,0) couples are independent of pH and Cu2+ precipitates as Cu(OH)2 in the basic pH. Moreover, Cu2+ reduction to metallic Cu0 is infeasible in the absence of hole scanvenger like formate ion and is well reported (21). Phenol and 4-Nitrophenol Degradation. Figure 2 shows the degradation profiles of phenol and 4-nitrophenol and VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Concentration profiles of (a) phenol, (b) 4-nitrophenol, (c) Cu2+, and (d) Cr6+ when oxidized/reduced individually with CS TiO2.
FIGURE 3. Variation of inverse of initial rate with inverse of initial concentration for phenol, 4-nitrophenol, Cu2+, and Cr6+. reduction profiles of Cu2+ and Cr6+ for different initial concentrations when each were individually photocatalytically oxidized/reduced with CS TiO2. From the linear fit of 1/Co versus 1/ro (Figure 3), the rate constants K2, Ph, K2, NP, K5, Cu2+, and K5, Cr6+ were evaluated to be 0.0734, 0.048, 1.084, and 2 × 10-4 L-min mg-1, respectively. 916
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Figure SI 2 in the Supporting Informaiton shows the concentration profiles and linear fits for phenol when degraded with Cu2+ and Cr6+. Figure SI 3 shows the concentration profiles and linear fits for 4-nitrophenol when reduced with Cr6+. Comparing the initial rates of degradation of phenol and 4-nitrophenol in the absence and presence of any metal ion, it is obvious that the presence of Cu2+ or Cr6+ reduces the rate of degradation of phenol in the order Phenol only > Phenol-Cr6+ > Phenol-Cu2+, though the overall concentration profiles are the same. Similar is the case for 4-nitrophenol with Cr6+ and the rate of degradation follows the order NP only > NP-Cr6+. To confirm whether any increase or decrease in the concentration of metal ions would better degrade phenol and 4-nitrophenol, experiments were performed with four different concentrations of Cu2+ and Cr6+. From Figure 4 it is clear that there is no net effect of varying the concentration of metal ions on the oxidation of phenol and 4-nitrophenol. Thus, the observed reduction in the rate of degradation of phenol and 4-nitrophenol can be attributed to the fact that Cu+ and Cr3+ ions create acceptor and donor surface centers that behave as recombination sites for photogenerated electrons and holes (17). The time evolution of the straight chain aliphatic acids, mostly consisting of maleic acid and oxalic acid, which are the ring opened fragments and CO2 during the oxidation of the phenol and 4-nitrophenol and HNO2 (13) in the case of 4-nitrophenol is calculated by mass balance and is also plotted in Figure 4. Cu2+ and Cr6+ Adsorption-Reduction. Both phenol and 4-nitrophenol did not show any appreciable adsorption onto CS TiO2, but Cu2+ and Cr6+ showed appreciable adsorption for certain systems. In the case of Cu2+, the initial adsorption onto CS TiO2 in the initial equilibration period was from 5
FIGURE 4. Concentration profiles of (a) 50 mg L-1 of phenol for different initial Cu2+ concentrations, (b) 30 mg L-1 of phenol for different initial Cr6+ concentrations, (c) 30 mg L-1, and (d) 50 mg L-1of 4-nitrophenol for different initial Cr6+ concentrations and time evolution of ROF+CO2 and HNO2. to 10%. The percentage of Cu2+ adsorption showed no significant trend with varying the initial phenol concentration in the system, but showed a steady increase with increase in the initial Cu2+ concentration. Hence the initial adsorption of Cu2+ has only a negligible role compared to the photoreduction of Cu2+. In the case of Cr6+, very high levels of adsorption were observed with CS TiO2 for all the systems comprising (i) individual Cr6+, (ii) phenol-Cr6+, and (iii) 4-nitrophenol-Cr6+ which prove the environmental efficacy of this catalyst in the removal of trace amounts of Cr6+. It is observed that the percentage of Cr6+ adsorption with CS TiO2 decreases with increase in the initial Cr6+ content in the system (Figures SI 4 and SI 5) and increases with increase in the initial concentration of phenol and 4-nitrophenol (Figure 5). Because up to 80% of Cr6+ is adsorbed initially, photoreduction is complete within the first 10 min of UV irradiation. This initial adsorption observed for CS TiO2 can be compared with that of Degussa P-25 (Figures SI 6 and SI 7), where only 17 and 6% initial adsorption were observed for 60 and 120 µM Cr6+, respectively, with 50 mg L-1 of phenol and almost zero initial adsorption with 40 mg L-1 of 4-nitrophenol. Such high adsorption levels of Cr6+ observed with CS TiO2 can be attributed to its high specific surface area (240 m2/g) compared to that of Degussa P-25 (50 m2/g). Cu2+ reduction was very sluggish in the absence of phenol in the CS TiO2 system. The overall percentage reduction of Cu2+ was 28.5, 21, 7, and 39% for initial concentrations of 250, 300, 400, and 515 µM, respectively. But the presence of phenol accelerates the reduction of Cu2+ to Cu+. The concentration profiles for the reduction of Cu2+ in the presence of phenol show that 70–90% reduction is achieved
FIGURE 5. Concentration profiles of 90 µM and 120 µM Cr6+ for different initial concentrations of (a) phenol and (b) 4-nitrophenol. within the total time period of UV exposure (Figure SI 8). The reduction profiles suggest that in the initial time period of 20 min, the rate of reduction is very slow, which is less than 7% of the initial concentration. After this initial lag period, the reduction rate increases drastically so that even complete reduction is observed in some cases. The reason for the observed Cu2+ reduction profile may be due to phenol degradation caused by OH · and O2–. radicals in the initial period dominating the metal reduction pathway, which is only due to the conduction band electrons. This is validated VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Concentration profiles of 400 µM Cu2+ for different initial concentrations of phenol.
TABLE 2. Kinetic Rate Constants for the Degradation of Phenol with Simultaneous Reduction of Cu2+ and Cr6+ and That of 4-Nitrophenol with Cr6+ with CS TiO2 phenolic compound phenol 4-nitrophenol
metal ion K1 min-1 Cu2+ Cr6+ Cr6+
0.047 1.193 0.114
K3 mg L-1 min-1
K4 min-1
K6 mg L-1 min-1
3.02 × 103 4.28 51.52
0.011 0.213 0.212
5.58 24.63 42.23
by the fact that almost 70% of phenol is degraded in this initial lag period compared to the Cu2+ reduced. The initial concentration of phenol is also detrimental in the reduction of Cu2+. From Figure 6 it is evident that higher initial concentrations of phenol increase the rate and hence completely reduce Cu2+. The reduction profiles of Cu2+ in presence of phenol for CS TiO2 are also compared with those of Degussa P-25 (Figure SI 9), which shows a sluggish reduction rate compared to CS TiO2. This shows that CS TiO2 is effective for Cu2+ reduction. The values of rate constants (Table 2) also validate the above observation. From the relative magnitudes of K5 for Cu2+ and Cr6+, K5, Cr6+ is 3 orders of magnitude less than that of K5, Cu2+. This shows that the value of rate constants, k56 and k60 are very high for Cr6+ than that for Cu2+ (from eq SI 1 (A.13)), indicating that Cr6+ is reduced very rapidly due to irradiation and subsequent desorption from the TiO2 vacant site. Comparing the rate constants that signify the simultaneous degradation-reduction, K3, Cu2+ for phenol is 3 orders of magnitude higher than that of K3, Cr6+ for phenol and one order higher for 4-nitrophenol. This is validated by the reasoning that K3 is inversely related to k56 and k60 and these rate constants are much smaller for Cu2+ than that for Cr6+. Hence, from eq SI 1 (A.11), K3, Cu2+ > K3, Cr6+. For Cr6+, K4 is the same for both phenol and 4-nitrophenol, which indicates that phenol and 4-nitrophenol affect the adsorption and photoreduction of Cr6+ to the same extent. The rate constant K6, signifying the relative role of phenol degradation pathway on the reduction of metal ions is 1 order of magnitude higher for phenol-Cr6+ system than that for phenol-Cu2+ system. The argument is consistent with the observation that D’00 and k56k60 are of the same order of magnitude for Cr6+ and Cu2+. Hence, K6 is dependent only on D00, which contains the rate constants signifying the phenol degradation pathway. 918
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As Cr6+ degrades phenol faster than Cu2+, the value of K6 is higher for phenol-Cr6+ system. Intermediates Analysis. The kinetics of reaction, the rates of formation, and consumption of the primary hydroxylated intermediates during the degradation of phenol and 4-nitrophenol were studied. Phenol does not yield any intermediates when degraded individually with CS TiO2, but results in the formation of two intermediates, namely hydroquinone and catechol, with Degussa P-25. The same intermediates were also observed in presence of Cu2+ and Cr6+ with CS TiO2. Similarly, 4-nitrophenol yields hydroquinone and 4-nitrocatechol when degraded individually as well as in the presence of Cr6+ with CS TiO2. The primary hydroxylated intermediates undergo secondary hydroxylation, and then ring opening to form straight chain aliphatic acids denoted here as ring opened fragments (ROF). On further exposure to UV light these finally mineralize to give CO2 and water vapor. Figures SI 10, 11, and 12 show the concentration profiles of the primary hydroxylated intermediates for phenol degraded with different initial concentrations of Cu2+ and Cr6+ and 4-nitrophenol degraded with different initial concentrations of Cr6+. From the concentration profiles, it is observed that the concentrations of the intermediates increase initially and after attaining a maximum value, they decrease and in some cases get completely consumed. This behavior is due to the competition between the primary and the secondary hydroxylated species. The formation and consumption pathway can be represented as
where A denotes the initial pollutant, B and C denote the primary hydroxylated intermediates, and D and E denote the secondary hydroxylated intermediates. Assuming all the reactions to be first order, for a series reaction, the concentration of the intermediate is given by Cint/CA0 ) kf/(kc kf)(exp(-kft) - exp(-kct)), where Cint is the concentration of the intermediate at any time t, CA0 is the initial concentration of the pollutant A, and kf and kc denote the formation and consumption rate constants of the intermediate, respectively. Table SI 14 lists the kf and kc values of the intermediates formed during the simultaneous phenolic compound degradation and metal ion reduction. In all the cases kc > kf, confirming that secondary hydroxylation is faster than primary hydroxylation step. Thus, for the phenol-Cu2+ system, the ratio kc/kf for catechol and hydroquinone is the highest for 300 µM Cu2+. This can be correlated with the high pH attained during the reaction, which accelerates the photooxidation of the intermediates due to the OH.- radicals. For the phenol-Cr6+ system, 90 µM and 150 µM Cr6+ concentrations show high values of kc/kf for hydroquinone and catechol, respectively. This may be due to the reaction mixture attaining basic nature during the intermediate reaction period (Figure SI 13). For the 4-nitrophenol-Cr6+ system, the concentration profiles of hydroquinone are similar for all the concentrations of Cr6+. Hence, it can be concluded that phenol has a significant effect on the photoreduction of Cu2+ in presence of CS TiO2, and the presence of phenol and 4-nitrophenol increase the adsorption of Cr6+ onto CS TiO2 and hence its photoreduction. In an alternate pathway, Cu2+ and Cr6+ reduce the rate of degradation of phenol.
Acknowledgments G.M. thanks the Department of Science and Technology for financial support and Swarnajayanthi fellowship.
Supporting Information Available SI 1 Appendix A, Figures SI 2-14. This material is available free of charge via the Internet at http://pubs.acs.org.
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