Ind. Eng. Chem. Res. 2006, 45, 913-921
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Kinetics of TiO2-Catalyzed Ultrasonic Degradation of Rhodamine Dyes M. H. Priya and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India
The ultrasonic degradation of two dyes, Rhodamine B (C28H31ClN2O3) and Rhodamine Blue (C28H32N2O3), were studied in the absence of catalyst and in the presence of two catalysts (combustion-synthesized anatase TiO2 and commercial Degussa P-25 TiO2. The rate of degradation of catalyzed reaction was higher than that obtained with in the absence of the catalysts. Among the catalysts, combustion-synthesized anatase TiO2 degraded the dyes faster when compared to the degradation with commercial Degussa P-25 catalyst. A Langmuir-Hinshelwood kinetic model was developed and the kinetic rate parameters were determined. The effect of other operating parameters, such as initial concentration, pH, temperature, and power intensity, was also investigated. The degradation rate increased with decreasing pH, increasing temperature, and higher intensity. 1. Introduction Recently, with increasing concerns for eco-conservation, advanced oxidation processes (AOPs) have received increasing attention for the destruction of organic pollutants commonly found in wastewaters. AOP is defined as a process that involves in situ generation of free radicals such as hydroxyl radicals. In addition to conventional AOP such as hydrogen peroxide oxidation, ozonation, photolysis, fenton process, photocatalytic oxidation, and wet-air oxidation, ultrasonic sonication is an emerging AOP. The application of ultrasonic irradiation to induce chemical transformations was first reported by Richards and Loomis1 in 1927. Subsequently, extensive studies have been performed to check its potential for application in wastewater treatment processes.2-9 Ultrasonic irradiation induces electrohydraulic cavitation in liquids,9 which is a process during which the radii of pre-existing gas cavities in the liquid oscillate in a periodically changing pressure field created by the ultrasonic waves. The gas bubbles implode violently when unstabilized by ultrasonic oscillations. In aqueous systems, rapid implosion of cavity bubbles containing entrapped gases and water vapor results in localized highpressure (313 atm) hot spots (T ) 4000 K), because of adiabatic heating.10 The apparent chemical effects observed in liquid reaction media are either direct or indirect consequences of these extreme conditions. Chemical reactions have been reported to occur in three reaction zones, viz. gas-phase cavity, interfacial shell, and bulk phase.11 Under extreme conditions (5000 K and 1000 atm),12 in first gas-phase zone, water molecules readily disassociate into OH• and H•, which can either decompose the volatile substrate in the cavity or produce hydrogen peroxide. Pyrolysis is the dominant process inside the cavity. Large temperature (∼2000 K) and pressure gradients exist in the interfacial zone; therefore, the substrate can be degraded by two reaction pathways, either by oxidation with OH• or by thermal reaction. Hydroxyl radicals that have diffused into the bulk solution degrade hydrophilic pollutants in bulk solution. Thus, oxidative reaction in the bulk liquid is controlled by the rate of diffusion of the radical into water. The efficacy of combinations of various AOPs has been reviewed.13 Several studies on the synergistic effect of ultrasonic/ * To whom correspondence should be addressed. Tel: +91-80309-2321. Fax: +91-80-309-2321. E-mail address: giridhar@ chemeng.iisc.ernet.in.
ozone,14 ultrasonic/H2O2,15-17 ultrasonic/Fenton process,16,17 ultrasonic/UV irradiation,14 ultrasonic/activated alumina,18 ultrasonic/biocatalyst,19 ultrasonic/metal or metal oxide,20,21 and ultrasonic/TiO2 processes22-24 have been reported. In such combinative processes, OH• radicals are generated in the bulk solution, either by thermal dissociation of ozone and hydrogen peroxide or by the Fenton process or by photolysis by ultraviolet (UV) irradiation or by the semiconductive property of oxide catalysts. Thus, reaction in the bulk zone is not diffusionlimited. Among various combinative processes, the application of semiconductive catalysts in ultrasonic degradation is promising. Semiconductors (e.g., TiO2, ZnO, CuO, Fe2O3) are characterized by a filled valence band and an empty conduction band. An electron ecb- can be promoted from the valence band (VB) into the conduction band (CB), leaving a hole hvb+ behind by furnishing energy matching or exceeding the band gap energy (Eg) of the semiconductor. UV light has been normally used as an energy source; however, the light is screened by catalyst particles itself, so that the region exhibiting the catalytic power is spatially limited in the reactor.23 The problem of spatial limitation can be eliminated when ultrasonic irradiation is used as an energy source. Moreover, the presence of solid particles in a liquid increases the nucleation sites for cavity formation, resulting in the generation of more free radicals. Symbiotically, ultrasonic vibration prevents blocking of catalyst active site, increases the surface area by breakage (heterocatalytic reactions are surface reactions), and reduces any mass-transfer limitations. Among all semiconductors, TiO2 finds wider application because of its availability, stability, low cost, and favorable Eg value. The mode of synthesis of TiO2 influences the activity of the catalyst (i.e., Eg, bounded hydroxyl species, crystallinity, and particle size). The pure anatase phase nano titania (TiO2) prepared via the solution combustion method25 has been reported to be a better photocatalyst in degradation of dyes, compared to the commercial Degussa P-25 catalyst. Its superiority was attributed to its lower Eg value, higher hydroxyl content, smaller particle size (8-10 nm), larger surface area, and higher crystallinity,25,26 compared to that of Degussa P-25. This study aims at exploring the utility of commercial TiO2 (Degussa P-25) and solution combustion-synthesized TiO2 (CST) as the catalyst in the sonocatalytic degradation of two Rhodamine dyes, Rhodamine B (RB) and Rhodamine Blue (RBL). Although ultrasonic degradation has been modeled using
10.1021/ie050966p CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
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an equation similar to that obtained by L-H kinetics in one study,27 no basis has been provided for its use. In this study, a detailed L-H kinetic model has been developed and the kinetic parameters were determined from the experimental data. The kinetics of ultrasonic degradation of these dyes using CST was compared against degradation in the absence of catalyst and in the presence of commercial Degussa P-25 catalyst. The influence of various parameters, such as initial dye concentration, pH, power intensity, and temperature, on the rate of ultrasonic degradation was also investigated to determine the optimum conditions for degradation. Thus, the objective of the study is to investigate the various parameters that influence the catalytic ultrasonic degradation of two dyes and propose a new model to determine the kinetics of degradation. 2. Experimental Section 2.1. Materials. The dyes, Rhodamine B (RB, C28H31N2O3Cl, CAS: 81-88-9), Rhodamine Blue (RBL, C28H32N2O3, CAS: 1326-03-0), and nitric acid was purchased from S. D. Fine Chemicals (India). Titanium isopropoxide (Lancaster Chemicals, UK), hydrogen peroxide (Sigma-Aldrich, USA), and glycine (Merck, India) also were used. Water was doubledistilled and filtered through a Millipore membrane filter prior to use. 2.2. Catalyst Preparation. Nanosized TiO2 was prepared25 via the solution combustion method, using precusor titanyl nitrate [TiO(NO3)2] and fuel glycine (H2N-CH2-COOH). The titanyl nitrate was synthesized by the reaction of titanyl hydroxide [TiO(OH)2] obtained by the hydrolysis of titanium isopropoxide [Ti(i-OPr)4] with nitric acid. In a typical combustion synthesis, a Pyrex dish (with a volume of 300 cm3) containing an aqueous redox mixture of stoichiometric amounts of titanyl nitrate (2 g) and glycine (0.8878 g) in 15 mL water was introduced into a muffle furnace that was preheated at 350 °C. The solution initially undergoes dehydration and a spark appears at one corner, which spreads throughout the mass, yielding anatase titania. In this system, the combustion reaction was of the smoldering type without the appearance of a flame. 2.3. Catalyst Characterization. The catalyst was characterized by various methods, as discussed below. The X-ray diffraction (XRD) patterns of catalysts were recorded on a Siemens model D-5005 diffractometer using Cu KR radiation with a scan rate of 2°/min. The XRD pattern of combustionsynthesized TiO2 (CST) was recorded in the 2θ range of 5°100°. The pattern can be indexed to the pure anatase phase of TiO2 with the space group I1/amd. The data were then refined using the Fullprof-98 program. The lattice parameter for TiO2 was a ) 3.7865(5) Å and c ) 9.5091(1) Å. The crystallite size was determined from the XRD pattern, using the Scherrer formula, and, based on the full width half-maxima (fwhm) of the XRD pattern, the mean crystallite size was estimated to be 10 ( 2 nm. Transmission electron microscopy (TEM) of powders was performed using a JEOL model JEM-200CX TEM system that was operated at 200 kV. TEM studies also showed the crystallites of TiO2 are homogeneous, with a mean size of 8 ( 2 nm, which agrees well with the XRD measurements. The surface area of the catalyst was determined with a standard Brunauer-Emmett-Teller (BET) apparatus (model NOVA1000, Quantachrome) and was determined to be 240 m2/g; this value is greater than the surface area of commercial catalysts such as Degussa P-25 (50 m2/g). Fourier transform infrared
(FTIR) studies were performed in the frequency range of 4004000 cm-1 in the transmission mode (Perkin-Elmer, FTIRSpectrum-1000) and showed higher surface hydroxyl content for the CST. The as-synthesized TiO2 was subjected to thermogravimetry-differential thermal analysis (TG-DTA) (Perkin-Elmer, Pyris Diamond), which showed an 11% weight loss, indicating more surface hydroxyl groups. X-ray photoelectron spectra (XPS) of these materials were recorded with a model ESCA-3 Mark II spectrometer (VG Scientific, Ltd., England), using Al KR radiation (1486.6 eV). The ultraviolet-visible (UV-vis) absorption spectra of TiO2 powders were obtained for the dry-pressed disk samples, using a UV-vis spectrophotometer (GBC Cintra 40, Australia) within a wavelength range of 270-800 nm. The CST exhibits two optical absorption thresholds, at 570 and 467 nm, which correspond to band gap energies of Eg ) 2.18 eV and Eg ) 2.65 eV, respectively. Further details on catalyst preparation and characterization are provided elsewhere 25,26 2.4. Degradation Experiments. A horn-type ultrasonic processor (Vibronics, India) was used. Most of the experiments were operated at a fixed supply voltage (180 V), power intensity (36 W/cm2), and frequency (25 kHz). The temperature of the reaction mixture was maintained at 27 °C ((2 °C), using a thermostated ice water bath. Double-distilled water after Millipore filtration was used as the solvent. Ultrasonic irradiation was performed by dipping the horn in dye solution of known concentration and treating it in a 100-mL beaker for 120 min. A constant reaction volume of 80 mL was used for all sonication experiments. Samples were collected at regular intervals for subsequent analysis by spectrophotometry. The effect of operational parameters such as initial dye concentration, initial pH, power intensity, and temperature was also investigated. Several experiments were repeated in triplicate, and the variation was Degussa P-25 > noncatalyzed reactions. The degradation rates of the two dyes indicate that the presence of chlorine in RB is responsible for its lower degradation rate, compared to the degradation rate of RBL.
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Figure 4. Variation of the inverse of the initial rate versus the inverse of the initial concentration of (a) RB and (b) RBL to determine the parameters. Legend: (9) no catalyst, (b) in the presence of CST, and (2) in the presence of Degussa P-25.
Figure 3. Normalized ultrasonic degradation profile of RBL for various initial concentrations (a) without catalyst, (b) with CST, and (c) with Degussa P-25. (Legend for panel a: (9) 6.5 mg/L, (b) 20 mg/L, (2) 30 mg/L, and (1) 50 mg/L; legend for panel b: (9) 6.5 mg/L, (b) 17 mg/L, (2) 24 mg/L, and (1) 43 mg/L; legend for panel c: (9) 6.5 mg/L, (b) 16 mg/L, (2) 30 mg/L, and (1) 49 mg/L.)
The initial concentration of the pollutant is an important parameter in wastewater treatment processes. The degradation of four different initial concentrations of RB and RBL was investigated for all three cases (i.e., noncatalyzed, Degussa P-25catalyzed, and CST-catalyzed systems). Figures 2a-c and 3a-c
show the degradation profiles of RB and RBL for various initial concentrations (a) in the absence of catalyst, (b) in the presence of CST, and (c) in the presence of Degussa P-25. The initial rates of the reaction were determined by extrapolating the tangent (based of the linear fit of the first four points) of the concentration profile (Figures 2a-2c and 3a-3c) back to initial conditions. The initial rates increased with increase in initial concentration. The initial degradation rate of RBL was higher than that of RB. The initial rates followed L-H kinetics, as indicated by the linear fit of the inverse of the initial rate r0 versus the inverse of the initial concentration C0 of the dyes (see Figure 4a and b). Thus, the experimental observations confirmed the validity of the kinetic model previously discussed. The L-H parameters, which are determined from the slope and intercept of Figure 4a and 4b, are given in Table 1. Thus, ultrasonic degradation follows L-H rate kinetics, similar to that of degradation by photocatalysis.25,28,30-34 As discussed previously, the rate of degradation of dyes using CST was the highest. Therefore, the effect of various parameters such as pH, power intensity, and temperature was studied for the ultrasonic degradation of 10 mg/L of RB and 17 mg/L of RBL in the presence of CST. The effect of pH on degradation of dyes using the CST catalyst was investigated at three pH values, covering the acidic,
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Table 1. Langmuir-Hinshelwood (L-H) Parameters of the Ultrasonic Degradation of Rhodamine Dyes catalyst
k0 (× 103/min)
K0 (× 102 L/mg)
no catalyst CST Degussa P-25
Rhodamine B 2.4 16.5 11.4
13.3 66.0 66.0
no catalyst CST Degussa P-25
Rhodamine Blue 11.4 15.0 13.8
3.5 4.3 4.6
neutral/near neutral, the basic regime (pH 3-11). After the dye solution with the catalyst was stirred for 30 min, the pH of the mixture was altered using dilute NaOH and dilute HNO3 solutions and sonicated immediately. The rate of degradation of two Rhodamine dyes decreased almost exponentially as the initial pH increased (see Figure 5a), which is a trend that has been observed for the ultrasonic degradation of many organic pollutants, such as methyl orange, azo dyes, and nitrophenol.20,22,35,36 The ultrasonic degradation of azo dyes and nitrophenol increased as the pH decreassed, because of hydrophobic enrichment of the molecules to enhance their reactivity under ultrasonic cavitation.35,36 At low pH, the surface of TiO2 is positively charged and capable of adsorbing the dye ion, which has a negative charge. At higher pH, there is negligible adsorption. Although the formation of OH radicals at higher pH values could be facilitated at the interface of the solution by the transfer of holes to the more-abundant OH, the increase in the number of hydroxyl radicals on the surface of TiO2 by trapping photons increases the degradation rate of the organic compound in acidic pH.22 The enhancement of degradation rates under acidic conditions can also be attributed to the formation/ accumulation of HOO radicals in the liquid bulk, which enhances the degradation of the dye.35 Rhodamine dyes were degraded at three different power intensities (24, 30, and 36 W/cm2). The rate of degradation increased as the power intensity increased (see Figure 5b). This effect is due to the increased cavitational activity that occurs at higher levels of power and is in accordance with ultrasonic degradation studies of chlorophenol, polycyclic compounds, and methyl orange.4,19,22 The increase in power intensity results in an increase in the total quantum of pressure energy liberation, via an enormous generation of cavities compensating the decreased collapse energy of a single cavity.3 The effect of bulk temperature on the ultrasonic degradation is not clear. Some studies have shown that the sonication rate constants decreased as the temperature increased.7,37 This was attributed to the presence of vapor in gas bubbles at higher temperature, which cushions the implosion growth and the use of enthalpy for condensation, despite accelerated bubble formation at that temperature. However, the rate constants of thymine increased with temperature.38 It has been reported that the ultrasonic degradation of methyl orange22 increased with temperature for reactions in the absence of any catalyst and in the presence of rutile TiO2. The degradation of methyl orange increased by 10% when the temperature was increased from 20 °C to 40 °C, whereas an increase of 20% was observed when the temperature was increased from 40 °C to 60 °C. However, these studies have not quantified the increase of degradation with temperature, in terms of fundamental parameters such as the activation energies. The experimental data indicate that the degradation rates of both dyes increased as the temperature increased (see Figure 5c). To model this, eq 13 is rewritten in terms of the initial concentration and initial rates as rD0(1 + K0[D]0)/[D]0 ) k0. Substituting an Arrhenius rate expression
Figure 5. Variation of the initial rate with (a) initial pH, (b) power intensity, and (c) bulk temperature. Legend: (b) RB and (O) RBL.
for the kinetic rate constant, k0 ) k00 exp[-E/(RT)] and assuming that the parameter K0 is independent of temperature, indicates that a plot of ln{rD0(1 + K0[D]0)/[D]0} versus the inverse of temperature (1/T) would be linear, as shown in Figure 6. The activation energies for the RB and RBL dyes, obtained from the slope of the linear plots, are 3.3 and 2.4 kcal/mol, respectively. These low activation energies indicate that the effect of temperature on the degradation rate is not significant.
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[OH•] )
2kp
(A.5)
ks3[H2O2] + kox[D]
Hence, eq A.1 becomes
rD )
2kpkox[D] ks3[H2O2] + kox[D]
)
k0[D] 1 + K0[D]
(A.6)
where
k0 )
2kpkox ks3[H2O2]
(
K0 )
kox ks3[H2O2]
)
(A.6a)
II. In the presence of a catalyst: Figure 6. Arrhenius plot for the degradation of dyes to determine the activation energy. (See Figure 5 for legend.)
4. Conclusion The efficacy of combustion-synthesized TiO2 (CST)-catalyzed ultrasonic degradation of two Rhodamine dyes (Rhodamine B and Rhodamine Blue) was compared against degradation without catalyst and in the presence of the commercial Degussa P-25 catalyst. The presence of a catalyst enhanced the rate of degradation of Rhodamine dyes. Rhodamine Blue degraded faster than Rhodamine B. The initial rates of degradation of the dyes, which were followed a Langmuir-Hinshelwood kinetics and the kinetic parameters were obtained from the model. The effect of pH, power intensity, and bulk temperature on the degradation rate of the dyes was also investigated. The rate was observed to increase in acidic pH and with an increase in power intensity and temperature. Appendix Based on the mechanism presented in the kinetic modeling section, the kinetic expression for degradation of dye can be derived as follows: I. In the absence of a catalyst: The rate of degradation of dye is given as
rD ) -
dD ) kox[D][OH•] dt
(A.1)
Mass-balance equations for hydrogen peroxide and all the radicals are given as
d[H2O2] ) khf[OH•]2 - khb[H2O2] + ks2[HO2•]2 dt ks3[H2O2][OH•] (A.2) d([H•] + [HO2•]) ) kp - ks2[HO2•]2 + ks3[H2O2][OH•] dt (A.3) d[OH•] ) kp - khf[OH•]2 + khb[H2O2] - ks3[H2O2][OH•] dt kox[D][OH•] (A.4) Under the assumption of quasi-steady-state conditions for the hydrogen peroxide and all the radicals, the concentration of the hydroxyl radical is defined as
dD ) khc1[S-D][hVB+] + khc2[D][hVB+] + dt kox1[D][OH•] + kox2[S-D][OH•] + kox3[D][S-OH•] +
rD ) -
kox4[S-D][S-OH•] (A.7) This involves degradation by both direct hole capture and oxidation by hydroxyl species. The surface hydroxyl species and surface adsorbed dye molecules can be expressed in terms of the concentration of bulk species, using equilibrium constants:
[S-OH•] ) KOH[S][OH•]
(A.8)
[S-D] ) KD[S][D]
(A.9)
Applying the assumption of quasi-steady-state conditions for the various intermediate species yields the following relations: (a) For holes,
d[hVB+] ) ke[S] - krc[hVB+][eCB-] - krg[S-OH-][hVB+] + dt k′rg[S-OH•] - krg[S-H2O][hVB+] + k′rg[S-OH•][H+] ) 0 (A.10) Because the rate of recombination of the hole-electron pair is much faster than any other trapping steps,28 the hole concentration can be obtained as
[hVB+] )
x
ke[S] krc
(A.11)
(b) For hydrogen peroxide,
d[H2O2] ) khf[OH•]2 - khb[H2O2] + ks2[HO2•]2 dt ks3[H2O2][OH•] (A.12) (c) For the H• and HO2• radical species combined,
d([H•] + [HO2•]) ) kp - ks2[HO2•]2 + ks3[H2O2][OH•] ) 0 dt (A.13)
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(d) For total hydroxyl content,
d([OH•] + [S-OH•]) ) kp - khf[OH•]2 + khb[H2O2] dt ks3[H2O2][OH•] + krg[S-OH-][hVB+] - k′rg[S-OH•] + krg[S-H2O][hVB+] - k′rg[S-OH•][H+] - kox1[D][OH•] kox2[S-D][OH•] - kox3[D][S-OH•] kox4[S-D][S-OH•] ) 0 (A.14) Upon simplification of eq A.14, using eqs A.8-A.13, the concentration of hydroxyl radical in the bulk is given as
[OH•] ) {2kp + krg[hVB+]([S-OH -] + [S-H2O])}/ ({k′rgKOH[S](1 + [H+]) + ks3[H2O2] + kox1[D] + kox2KD[S][D] + kox3KOH[S][D] + kox4KDKOH[S]2[D]}) (A.15) Thus, the rate of dye degradation reduces to
rD ) khc[D] +
koxK0[D] 1 + K0[D]
)
khc(1 + K0[D] + kox)[D] 1 + K0[D]
(A.16)
where
khc ) (khc1KD[S] + khc2)
x
ke[S] krc
(A.17)
x
(A.18)
kox ) 2kp + krg([S-OH-] + [S-H2O]) K0 )
ke[S] krc
kox1 + kox2KD[S] + kox3KOH[S] + kox4KOHKD[S]2 k′rgKOH[S](1 + [H+]) + ks3[H2O2]
(A.19)
The concentration of dye ([D]) is low; therefore, the secondorder term can be neglected:
rD )
(khc + kox)[D] 1 + K0[D]
)
k0[D] 1 + K0[D]
(A.20)
The kinetic term (k0) is comprised of both hole-capturing and oxidation kinetic terms. The final simplified rate expressions in eqs A.6 and A.20 are similar to the Langmuir-Hinshelwood expression that is frequently used to describe photocatalytic reactions. Literature Cited (1) Richards, W. T.; Loomis, A. L. The Chemical Effects Of HighFrequency Sound Waves I. A Preliminary Survey. J. Am. Chem. Soc. 1927, 49, 3086. (2) Gogate, P. R.; Sivakumar, M.; Pandit, A. B. Destruction of Rhodamine B Using Novel Sonochemical Reactor with Capacity of 7.5 L. Sep. Purif. Technol. 2004, 34, 13. (3) Sivakumar, M.; Pandit, A. B. Ultrasound Enhanced Degradation Of Rhodamine B: Optimization With Power Intensity. Ultrason. Sonochem. 2001, 8, 233. (4) Psillakis, E.; Goula, G.; Kalogerakis, N.; Mantzavinos, D. Degradation of Polycyclic Aromatic Hydrocarbons in Aqueous Solutions by Ultrasonic Irradiation. J. Hazard. Mater. 2004, B108, 95. (5) Tezcanli-Guyer, G.; Ince, N. H. Degradation and Toxicity Reduction of Textile Dyestuff by Ultrasound. Ultrason. Sonochem. 2003, 10, 235.
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ReceiVed for reView August 23, 2005 ReVised manuscript receiVed November 29, 2005 Accepted November 30, 2005 IE050966P
