Effect of Ultrasonic Irradiation on the Catalytic Activity and Stability of

The objective of this research is to examine the effect of ultrasonic irradiation (35 kHz) on the catalytic activity and stability of goethite powder ...
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Ind. Eng. Chem. Res. 2007, 46, 691-698

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Effect of Ultrasonic Irradiation on the Catalytic Activity and Stability of Goethite Catalyst in the Presence of H2O2 at Acidic Medium Manickavachagam Muruganandham, Jing-Shen Yang, and Jerry J. Wu* Department of EnVironmental Engineering and Science, Feng Chia UniVersity, Taichung 407, Taiwan

The objective of this research is to examine the effect of ultrasonic irradiation (35 kHz) on the catalytic activity and stability of goethite powder (R-FeOOH) in the presence of H2O2 at pH 3. The catalytic activity of the catalyst was investigated on the decolorization of Direct Orange 39 (DO39) azo dye as a model pollutant, and the stability of the catalyst was examined by measuring contemporary dissolved total iron concentration. The decolorization of dye was strongly enhanced by ultrasonic irradiation on Fenton-like process (R-FeOOH/ H2O2). The kinetics of decolorization obeys first-order reaction and the rate constant was found to be 0.0189 min-1. The effect of R-FeOOH and H2O2 (2.5-15 g/L) dosage on the decolorization and iron dissolution was investigated and found to increase the decolorization rate with increasing dosage. However, both dosages did not influence the iron dissolution. The catalytic reusability was investigated in up to four successive cycles, and was found that the catalytic efficiency did not decrease appreciably. The effect of ultrasonic irradiation on the catalyst composition, surface area, pore volume, and pore size has been investigated and discussed in each cycle. The structure of the catalyst before and after four cycles was investigated using X-ray diffraction (XRD), cold-field emission scanning electron microscope (FESEM). The mechanism of degradation was discussed. It is concluded that ultrasonic irradiation in the presence of R-FeOOH and H2O2 is an efficient process and that the goethite is a promising catalyst for the abatement of dye pollutants in wastewater. 1. Introduction Environmental sonochemistry is a rapidly growing area and is an example of an advanced oxidation process (AOP) that deals with the destruction of organic species in aqueous solution. Iron is one of the most abundant elements on earth. Many classes of iron metal, compounds, and mixtures are extensively used in industrial production and daily life, especially in chemical engineering as catalysts. Fenton-like process (FeOOH/H2O2) is widely used as a catalytic treatment process to decompose various pollutants present in wastewater.1-4 It is well-known that solution pH is an important parameter for the iron oxide/ H2O2 process. Many authors observed that the most effective pH regime for the mineral-catalyzed treatment was at pH 3, which was attributed to increased soluble iron dissolution. Zinder et al. and Stone and Morgan studied the effect of ligand and reductive agent on metal oxides, indicating that the dissolution reaction was facilitated under reducing conditions and was influenced by complex forming organic ligand.5,6 Lu et al. observed that proton-promoted dissolution rate of goethite at pH 3 is smaller than the reductive dissolution rate.7 Hence, there is a need for developing a stable and efficient catalyst for industrial application of real wastewater treatment in an acidic medium. The typical ultrasound decomposition of toxic organics is 10 000 times faster than the natural aerobic oxidation. However, in a recent economic analysis of treatment of wastewater containing organics, the cost of sonochemical oxidation is comparable to incineration.8 Ultrasound can be used alone or in conjunction with other techniques such as photocatalytic, biological catalytic, and inorganic catalytic as well as other methods.9-15 The presence of solid particles in the aqueous system was found to enhance the production of microbubbles under ultrasound.16 Therefore, the combination of these two * To whom correspondence should be addressed. Tel: + 886-424517250 ext. 5206. Fax: +886-4-24517686. E-mail: [email protected].

ways of treatment may produce a synergistic effect.17,18 These methods coupled (US/R-FeOOH/H2O2) increase the decomposition efficiency and reduce the time required for removing the pollutants. It was reported that coupling of the goethite (R-FeOOH/H2O2) process with ultrasonic irradiation increases the decomposition efficiency of the target organics.19 The increase in degradation rate by ultrasonic irradiation was attributed to the indirect chemical effects associated with continuous ultrasonic cleaning and activation of the goethite surface and the enhanced rate of transport resulting from the turbulent effects of cavitations. Stock et al. studied the degradation of azo dye, napthol blue using a high-frequency ultrasonic generator, and UV-photolysis20 and found that the sonolysis is effective for inducing faster degradation, while photolysis is effective for promoting mineralization. It is important to develop environment-friendly catalysts with high efficiency, less energy cost, and low price, which is one of the essential goals of green chemistry. In light of the above points, it is still a challenge to develop such a process for real wastewater treatment for industrial application. Hence, to develop highly efficient and stable catalysts, an attempt has been made to study the catalytic activity and stability of goethite catalyst in the presence of ultrasonic irradiation. DO39 was chosen as a model pollutant on the basis of the following considerations. (1) It is a good model compound. It is stable under ultrasonic irradiation (US/dye) and resistant to hydrogen peroxide oxidation (H2O2/dye). (2) Azo dyes are major environmental contaminants. The presence of a sulfonic group endows these molecules with high water solubility and produces deactivation of the aromatic ring against bacterial attack, so this compound is a non-biodegradable pollutant present in wastewater. The primary objectives of this study are to investigate DO39 decomposition and catalytic stability of commercially available industrial R-FeOOH catalysts. To the best of our knowledge, no reports are available in the literature for the detailed study of the influence of ultrasonic irradiation on the catalytic activity,

10.1021/ie060752n CCC: $37.00 © 2007 American Chemical Society Published on Web 12/24/2006

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Table 1. The Characteristics of Direct Orange 39 Dye

catalytic solubility, surface properties, and structure of the goethite catalyst under the above-mentioned experimental conditions using DO39 as pollutant. The main goal is to find an appropriate combination of goethite and H2O2 and to study catalytic stability in the presence of ultrasonic irradiation that could provide maximum efficiency concerning destruction of DO39. 2. Experiment 2.1. Reagents. Hydrogen peroxide (30% w/w) was obtained from Merck chemical company, Taiwan, was of analytical grade, and was commercially available. DO39 (85% purity) was received from local dye industries in Taiwan and was used without further purification. The initial concentration of DO39 in all experiments was 0.01 g/L. Uncharacterized R-FeOOH powder was purchased from local industries in Taiwan with bulk density of 1.16 (g/cm3) and absolute density 3.44 (g/cm3). The particle size distribution of the catalyst ranges from 100 to 500 nm measured by transmission electron microscopy (TEM). The catalyst was washed with deionized water (18.2 MΩ) and was dried in an oven at 100 °C before it was used in the reaction. For all experimental work, deionized water Milli Q-Plus, resistance ) 18.2 MΩ was used. The pH of aqueous solution was maintained at 3.0 ( 0.1 throughout the experiments and was adjusted by using 70% HClO4. The pH was determined using Consort C231 Electrochemical multimeter (Belgium) with a glass pH electrode. The structure and absorbance of the DO39 is shown in Table 1. 2.2. Methods. For the ultrasonic experiments, the Elma Transonic T460 ultrasonic sonicator with 35 kHz fixed frequency and 120 W maximum electric power input were obtained from Elma Germany. The schematic diagram of the reaction setup is shown in Figure 1. The decomposition reaction in the presence of the catalyst, H2O2, and dye solution at desired concentration was conducted in a 1-L semibatch reactor, which was made of borosilicate glass with the dimensions of 13-cm diameter and 20-cm tall, to facilitate the operation of all ultrasonic processes. A 500-mL solution containing appropriate concentration of DO39, catalyst, and H2O2 was placed in a glass reservoir. The solution was then ultrasonically irradiated and was operated at atmospheric pressure and room temperature. The initial temperature of the reaction solution was 25 °C. The temperature of the solution increases by ultrasound irradiation because of sonication. In our experiments, the temperatures were kept constant by circulating water connected to a thermobath. The temperature of working solutions was measured every 15 min with a mercury thermometer. The time of interruptions for temperature measurements was not included in the total time of samples sonication. The appropriate quantity of deionized water was used to keep the initial volume constant in all experiments in the sonicator. The necessary sonication time for the treatment of DO39 (90 min) was determined during preliminary research, and it was applied in all experiments presented in this paper. A required quantity of the sample was

Figure 1. Schematic diagram of ultrasonic reactor.

withdrawn from the reservoir at specific time intervals. Before the analysis, all sonicated samples were filtrated through Whatman glass microfiber filter paper (47 mm) to remove goethite particles. The absorbance at 411 nm (n f π* transition of -NdN- group) is due to the color of the dye solution, and it is used to monitor the decolorization of dye. The respective calibration curve was obtained using solutions of known concentrations of the studied DO39; it was linear with the high correlation coefficient (R2 ) 0.999). From the calibration curve, the initial and final concentrations of DO39 were calculated in case necessary. 2.3 Analytical Methods. Total iron was measured using Hitachi AA-464 flame atomic absorption spectrometry (AAS) with Hollow cathode lamp. The surface area, pore size, and pore volume of the unused and used goethite powders were measured by nitrogen adsorption at 77.350 K using an accelerated surface area and porosity apparatus (ASAP 2010, Micromeritics). Prior to analysis, 1-2 g of powder was degassed at 473 K and 200 mmHg for 2 h (cold free space ) 53.0324 cm3, warm free space ) 17.4242 cm3, equilibration interval ) 10 s). X-ray diffraction analysis was used to analyze the structure of the catalysts. The X-ray diffraction (XRD) patterns were recorded using a MAX SCIENCE MXP3 diffractometer and Cu KR radiation and 2θ scanned angle from 20° to 70° at the scanning rate of 2° per min. The morphology of the catalyst was examined using a Hitachi S-4800 cold-field emission scanning electron microscope (FESEM) equipped with HORIBA EMAX 400 energydispersive X-ray microanalysis (EDX) system. Prior to FESEM measurements, the samples were mounted on gold platform using silicon wafer and were coated with a layer of catalyst. The plate containing the sample was placed in the electron microscope for the analysis with magnification of 30 000 and 100 000. The atomic composition of the iron oxide surface was elucidated by energy-dispersive spectra for the analysis with 20-µm surface area. Elemental analysis was carried out using Vario EL III CHNOS elemental analyzer (Germany). UV spectral analysis was done using spectronics Genesys5 spectrophotometer.

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Figure 2. Effect of ultrasonic irradiation on goethite/H2O2 process on (a) adsorption and degradation of DO39 and (b) total dissolved iron. a: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, stirring speed ) 150 rpm, pH ) 3.0. b: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, stirring speed ) 150 rpm, pH ) 3.0. c: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, US frequency ) 35 kHz, pH ) 3.0. d: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, US frequency ) 35 kHz, pH ) 3.0.

3. Results and Discussion 3.1. Adsorption and Degradability. Figure 2a depicts the absorption value during DO39 adsorption and decolorization in the absence and presence of ultrasonic irradiation as a function of reaction time under similar experimental conditions. Initial controlled experiments show that the dye is resistant to ultrasonic irradiation, H2O2 oxidation, presence of both, and presence of the catalyst with dye (data not shown in the Figure 2a). In the absence of ultrasonic irradiation (R-FeOOH/H2O2), about 30% of adsorption and 47% of decolorization is observed at 90 min, whereas in the same time the presence of ultrasonic irradiation causes 45% and 83%, respectively. The decomposition occurs more rapidly under ultrasonic irradiation than without irradiation. The degradation of DO39 by R-FeOOH/H2O2 process is due to the formation of hydroxyl radicals by thermal decomposition of hydrogen peroxide on the surface of goethite. These results clearly show that ultrasonic irradiation enhances both adsorption and decolorization appreciably. The fast decolorization of dye is due to the initial electrophilic cleavage of its chromophoric azo (-NdN-) bond. Azo bonds are more active in these dyes, and they are oxidized by hydroxyl generated during the reaction. DO39 contains one azo bond, and decolorization shows that the chromophoric, azo bond of dye molecule is destroyed. Adsorption of pollutants on the goethite surface is an important parameter in heterogeneous catalysis process. The surface equilibrium constants of goethite can be written as eqs (1 and 2). In acidic solution, the pH is lower than Pzc (Pzc ) 6.4) and hence

tFeOH + H+ a tFeOH2+ tFeOH a tFeO- + H +

pKa1 ) 6.4 pKa2 ) 9.2

dye-SO3 Na f (dye-SO3-) + Na+

(1) (2) (3)

At pH 3.0, goethite surfaces are protonated and the density of

this functional group (FeOH2+) behaves as an electrophilic center. The DO39 dye in solution is negatively charged. The sodium sulfonate group is hydrolyzed to form dye anion and sodium ion (eq 3). Since DO39 contains one sulfonate group, the hydrolyzed molecule behaves as a monoanionic dye. At pH 3.0, the electrostatic attraction between the positively charged goethite surface (FeOH2+) and the negatively charged sulfonic group (dye-SO3-) leads to strong adsorption. Bandara et al. observed the adsorption of Orange II dye on goethite surface through sulfonic group present in dye and suggested three possible ways of bond formation between the sulfonic group and goethite surface, that is, (1) chelating bidentate, (2) bridged bidentate, and (3) unidentate complex.21 Moreover, the detail of the mode of adsorption is out of the focus of this article. The dye adsorption is almost saturated in 30 min. However, after 45 min a very slight increase in the absorption value is noted, which indicates desorption of the dye from the catalyst surface under the function of ultrasound. Similar adsorptiondesorption behavior is also reported in sonocatalytic degradation of methyl orange on TiO2 catalyst.22 The noted desorption is less than 0.2% from 45 to 90 min, so the influence of ultrasonic waves on the desorption of pollutant from the surface of the catalyst is almost negligible. Hydrogen peroxide did not degrade the dye solution in the presence of ultrasonic irradiation, which implies that the direct homolytic cleavage of H2O2 is not possible. Therefore, the decolorization in bulk phase because of H2O2 is completely ruled out. The degradation is mainly due to goethite catalyzing hydrogen peroxide decomposition, which generates a powerful oxidizing agent, namely, hydroxyl radicals. The enhancement of removal rate by ultrasonic is due to (1) ultrasonic irradiation having the ability to substantially increase the dye adsorption on the catalyst surface, which favors catalyzed degradation, (2) indirect chemical effects associated with the continuous ultrasonic cleaning and activation of the goethite surface, and (3) the enhanced rate of mass transport resulting from the turbulent effects of the cavitations. Figure 2b shows the total dissolved iron with ultrasonic irradiation (US/R-FeOOH/H2O2) and without (R-FeOOH/H2O2) irradiation as a function of reaction course. The results clearly show that ultrasonic irradiation induces more iron dissolution than without irradiation. During the ultrasonic irradiation process, the total dissolved iron concentration decreases when the reaction time increases; this may be due to adsorption of dissolved iron on the surface of goethite to form the complex [FeOOH-Fe3+]. The role of dissolved iron on the decomposition process is also important. Since H2O2 is present in the solution, the dissolved ferric iron also reacts with H2O2. On the basis of the mechanism proposed in the literature for homogeneous systems,9,23,24 the reactions of ultrasound coupled with the Fenton-like reagent can be described by the eqs 4-7, where the symbol t represents the iron species bound to the surface of the catalyst.

tFe3+ + H2O2 f Fe (OOH) 2+ + H+ T Fe2++HO2• (4) tFe(OOH)2+ + US f tFe2++HO2•

(5)

tFe2+ + H2O2 f tFe3++HO- + HO•

(6)

tFe3++HO2• f tFe2++H+ + O2

(7)

3.2. Effect of FeOOH Dosage. Since the amount of sonocatalysts is an important parameter in the heterogeneous sonocatalytic process, it is necessary to investigate the role of

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Figure 3. Effect of goethite dosage on (a) decomposition of DO39 (b) total dissolved iron. [DO39] ) 0.01 g/L, [H2O2] ) 10 g/L, pH ) 3.0 ( 0.1, ultrasonic frequency ) 35 kHz.

goethite in the decomposition process. Figure 3 depicts the effect of catalyst loading (2.5-15 g/L) on the (a) decolorization of DO39 and the (b) total dissolved iron at pH 3 in the presence of 10 g/L of H2O2 by ultrasonic irradiation. After 15 min of reaction time, 28.0, 34.0, 51.2, 57.2, and 63% of decolorization is noted in 2.5, 5.0, 7.5, 10.0, and 15 g/L of goethite after 15 min, respectively. These results demonstrate the effectiveness of goethite as a catalyst in the heterogeneous degradation process. The decomposition rate of DO39 at various goethite concentrations was linear. The adsorption of DO39 increased by increasing the amount of catalyst, which is due to the increase of total surface area at higher concentration. In sonicated solution, with an increase in the amount of catalyst, the number of cavities and radicals increased and led to a higher removal rate. To support the above statements, the adsorption experiment was also carried out under similar experimental conditions without adding hydrogen peroxide with ultrasonic irradiation. About 8.9, 25, 31, 36.6, and 47.2% of dye adsorption was observed at the same catalyst dosage, respectively. We did not observe the optimum dosage under the experimental conditions, which implies that an optimum amount of R-FeOOH is not necessary for efficient decomposition. This is controversial to results reported in the literature. Wang et al. found that 1 g/L of nanometer size, ordinary anatase TiO2 is optimum dosage in methyl parathion degradation in sonocatalytic process.25 However, different results were observed in sonocatalytic methyl orange degradation in anatase and rutile TiO2. About 1 g/L of rutile was the optimum dosage, whereas the degradation increases with an increase in the catalyst dosage from 0.25 to 1.25 g/L in anatase catalyst.22 In general, it is believed that the optimal adding amount depends on the nature of the organic pollutants and on many other experimental factors. Dissolved iron results show that an increase in the catalyst loading did not influence the iron dissolution appreciably. After 15 min, 1.75, 1.90, 2.1, 2.4, and 2.1 mg/L of iron were observed in AAS measurements at the same catalyst dosage, respectively. However, when the reaction time increases, iron concentration decreases with increasing catalyst loading. Because of higher concentration of goethite particle on reaction solution, there is

Figure 4. Effect of H2O2 dosage on (a) the decolorization of DO39 and (b) total dissolved iron. [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, pH ) 3.0 ( 0.1, ultrasonic frequency ) 35 kHz.

a possibility of readsorption of the dissolved iron, which then forms a complex with the catalysts. This might be the reason for the decrease in dissolved iron concentration when reaction time increases at higher catalyst dosage. In conclusion, the amount of catalyst has positive influence on decolorization rate and does not promote iron dissolution appreciably. 3.3. Effect of H2O2 Dosage. Although hydrogen peroxide can be produced by application of ultrasound alone to a dilute aqueous solution, the amount may be too small to be significant. The degradation was not induced by the H2O2, although its concentration was much higher than that of DO39. The H2O2 may be mainly present not inside the cavitation bubbles but in the bulk solution because of the high water solubility and the low volatility. The sonolytic decomposition of H2O2, therefore, would be too slow, although the dissociation energy of the O-O bond (213 kJ mol-1) in H2O2 is less than that of the O-H bond (418 kJ mol-1) in water. Since there is no direct chemical interaction between ultrasonic irradiation and hydrogen peroxide, it is expected that the amount of H2O2 does not influence the direct sonocatalytic process. However, many studies have revealed that H2O2 concentration can dramatically influence the Fenton-like degradation of organic compounds. Since hydrogen peroxide is directly related to the amount of hydroxyl radicals produced in the goethite catalytic reaction, it is important to study its concentration effect on the treatment process. Figure 4 shows the effect of H2O2 dosage on the decolorization (3a) and iron dissolution 3b as a function of reaction time. It seems the removal efficiency decreases when hydrogen peroxide dosage increases from 5 to 15 g/L. However, it does not influence the overall removal rate. About 74, 83, 83, and 86% of decolorization is observed after 90 min at 2.5, 5, 10, and 15 g/L of H2O2 dosage, respectively. Hence, 5 g/L of H2O2 dosage can be taken as optimum dosage for efficient degradation. Similar results were reported in Fenton-like studies.26,27 Mixed results on the correlations among iron oxides, H2O2 decomposition, and target pollutants oxidation during Fenton-like oxidation were reported. The oxidation efficiency of octachlorodibenzo-

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Figure 5. First-order DO39 removal by US/R-FeOOH/H2O2 process. [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, US frequency ) 35 kHz, pH ) 3.0.

p-dioxin by 3.5% of H2O2 was the same as that of 35% H2O2 in a natural soil.28 On the contrary, Gates and Siegrist29 found that the removal of trichloroethylene by Fenton-like oxidation in natural soil increased with the H2O2 dosage. The best oxidation stoichiometry for tetrachloroethylene in a natural soil occurred at low H2O2 doses,29 which was in contrast to the report that a higher dose of H2O2 was required for the effective oxidation in silica sand.30 Increasing the hydrogen peroxide concentration would accelerate the reaction between H2O2 and catalyst, and thus more hydroxyl radicals are produced when the concentration of H2O2 increases from 2.5 to 15 g/L. Consequently, degradation is also increased. Nevertheless, further increase in the H2O2 dosage from 10 to 15 g/L would decrease the decomposition rates because of hydroxyl radical scavenging effect of hydrogen peroxide, which produces hydroperoxy radicals (eqs 8 and 9).31 These hydroperoxy radicals are much less reactive and do not contribute to oxidation. The dissolved iron results show that hydrogen peroxide does not influence the iron dissolution and the dissolved iron concentration does not exceed 3 mg/L in all dosages.

H2O2 + •OH f HO2° + H2O

(8)

HO2° + •OH f H2O +O2

(9)

The kinetics of DO39 decolorization has been analyzed. Literature reports showed that degradation of organic pollutants in ultrasonic-induced Fenton-like process obeys first-order kinetics.19 Since ultrasonic irradiation does not degrade DO39 dye at all, the degradation of DO39 by ultrasonic-irradiated Fenton-like process can be described by a linear combination of first-order term (eq 10).

-dC(DO39)/dt ) kFe/US‚C(DO39)

(10)

kFe/US ) rate constant for ultrasonic-irradiated Fenton-like process (US/R-FeOOH/H2O2). Integrating eq 10 gives

ln[C(DO39)0/C(DO39)] ) kFe/US‚t

(11)

A representative kinetic run was studied at the following conditions: [FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, US frequency ) 35 kHz, pH ) 3.0. Thus, in Figure 5 it can be seen when ln[C(DO39)/C(DO39)0] is plotted against time, the data substantially fit a straight line with a regression coefficient of 0.99. The first-order rate constant, kFe/US, for DO39 degradation by ultrasound-irradiated Fenton-like process is 0.0189 min-1. 3.4. Catalytic Stability. Any catalysts after finishing their regular tasks in chemical investigation and chemistry industry

Figure 6. Effect of catalytic reusage on (a) the decomposition DO39 and (b) total dissolved iron. [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, pH ) 3.0 ( 0.1, ultrasonic frequency ) 35 kHz.

are expected to be reclaimed and recycled by proper and simple treatment. In fact, deactivation of the catalyst is a frequently observed phenomenon. For example, Liu et al. have found that the maximum mass loss after 100 h of runtime is 3.6% by treating on a commercial titanium silicalite-1 with NH3 or H2O2 to simulate the hydrothermal environment of cyclohexanone ammoximation.32 The combined action of water and coke proved to be responsible for the AlPO4 deactivation in 1-butanol dehydration,33 whereas it was the formation of residual tar products as well as coke on the active surface that produced a reversible deactivation of the V2O5/TiO2 catalyst in the oxidation of o-xylene.34-36 Since the catalytic stability and reusability are important factors in catalyzed reactions, it is necessary to study the stability of the goethite catalyst. If the stability is poor or deactivation of the catalyst is severe, the catalyst will be useless in a practical industrial application. The entire catalytic reusability test has been investigated under identical reaction conditions. At the end of the oxidation process, the catalyst can be easily removed from the reactor and then be washed gently with deionized water (18.2 MΩ). The washed catalysts were dried in atmospheric condition and in an oven at 100 °C for 30 min to remove molecular water present on the surface of the catalysts. Moreover, this heating does not influence the catalytic activity of the catalysts. Figure 6 shows DO39 decomposition results (4a) and iron dissolution (5b) of the recovered catalysts. After 15 min, about 50% decolorization was observed in all cycles. This sharp decrease in concentration is mainly due to adsorption effect. This result implies that successive use of the catalysts does not influence the dye adsorption on the catalysts. At the end of the reaction, 83, 81.3, 81, and 80% of decomposition was noted in four successive cycles. However, a considerable difference in iron dissolution was noted between the first and second cycle. The iron dissolution is almost the same in the first two successive uses. Furthermore, the iron dissolution in the second, third, and fourth cycles is almost the same. In conclusion, there is no obvious decrease in the DO39 decomposition efficiency in successive usages. These results indicate that the catalyst has an excellent long-term stability. The

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Figure 7. XRD pattern of goethite catalysts: (A) Unused catalyst, (B) after first cycle, (C) after second cycle, (D) after third cycle, and (E) after fourth cycle.

excellent stability of the catalytic activity could be attributed to the stable structure of the catalyst, which is also confirmed by the XRD and SEM measurements. The catalyst stability of the catalyst was tested in each cycle and was compared with unused catalysts. Figure 7 shows the XRD patterns of the used and unused catalysts in successive cycles. The XRD pattern of the catalyst is similar to pure goethite pattern reported in the XRD standard data base library and is similar to pure goethite pattern reported in the literature.37-38 There is no difference between all five samples, indicating that no obvious change in goethite crystallites occurred during the catalytic reaction. The morphology of the catalyst was examined by using FESEM as shown in Figure 8. Initially, the FESEM was recorded at two different magnitudes (×100k, ×30k). The FESEM results

Figure 8. FESEM of iron oxide catalysts: (a) before reaction and (b) after reaction. (a) Unused catalyst (a1) × 100k, (a2) × 30k, (b) after first cycle (30k), (c) after second cycle (30k), (d) after third cycle (30k), and (e) after fourth cycle (30k).

Ind. Eng. Chem. Res., Vol. 46, No. 3, 2007 697 Table 2. Surface Chemical Composition (atomic %) and Elemental Analysis of the Goethite Catalyst before and after Each Cycles elements

Fe (atomic weight %)a

O (atomic weight %)a

C contentb (%)

H contentb (%)

N contentb (%)

unused catalysts after first cycle after second cycle after third cycle after fourth cycle

32.66 34.18 32.64 32.14 33.99

63.35 65.78 64.17 64.51 62.12

0.007 0.010 0.010 0.010 0.007

0.455 0.489 0.523 0.539 0.428

0.007 0.005 0.008 0.007 0.005

a

Determined by EDX. b Determined by elemental analysis.

Table 3. Effect of Ultrasonic Irradiation on Surface Area, Pore Size, and Pore Volume of Goethite Catalysts before and after Each Cycle parameters

unused catalysts

after first cycle

after second cycle

after third cycle

after fourth cycle

BET surface area (m2/g) Langmuir surface area (m2/g) single-point surface area (p/p0) (m2/g) t-plot external surface area (m2/g) t-plot micropore area (m2/g) BJH adsorption cumulative surface area of pores (m2/g)a BJH desorption cumulative surface area of pores (m2/g)a single-point adsorption total pore volume of pores (cm3/g) t-plot micropore volume (cm3/g) BJH adsorption cumulative volume of pores (cm3/g)b BJH desorption cumulative volume of pores (cm3/g)b adsorption average pore width (4V/A by BET) A° BJH adsorption average pore width (4V/A) A° BJH desorption average pore width (4V/A) A°

19.6786 26.8023 19.5734 12.2175 7.4611 13.1416 12.6677 0.008229 0.003355 0.037254 0.036969 16.7261 113.393 116.735

23.1241 31.4279 23.1322 13.1624 9.9617 13.9763 15.6619 0.009775 0.004524 0.041394 0.042201 16.9092 118.468 107.780

21.0618 28.6263 21.0367 12.4051 8.6566 13.4785 15.1310 0.008871 0.003920 0.040034 0.040619 16.8479 118.810 107.378

19.157 26.6341 20.0962 11.0975 8.0595 12.7547 13.8182 0.008056 0.003728 0.036224 0.037647 16.7731 115.737 105.923

18.3324 24.7391 18.2618 10.7781 7.5543 11.6274 12.9421 0.007711 0.003405 0.033072 0.033673 16.8245 113.772 104.073

a

Surface area of pores between 17.000 A° and 3000.000 A° width. b Cumulative volume of pores between 17.000 A° and 3000.000 A° width.

revealed a large number of external pores, and the silica was dispersed nonuniformly on the iron oxide surface. Unused and used catalyst morphology was analyzed and compared at ×30K magnitude. It seems all five photographs are identical, which shows that ultrasonic irradiation did not affect the morphology of the catalysts either. To support the above statements, we also studied the elemental composition of the catalyst. As can be seen from Table 2, no significant changes in the Fe content as well as other elements were observed in EDX and elemental analysis. SEM, XRD, and EDX results confirm that the catalyst itself is very stable after the fourth run. This implies that the catalysts can have a good catalytic activity for a long period. 3.5. Effect of Ultrasonic Irradiation on the Surface Properities of Used and Unused Goethite. The detailed effect of ultrasonic irradiation on the surface properties of goethite has not been reported in the literature. Moreover, catalytic properties are major influence factors in heterogeneous catalytic process. It shows details of surface area pore size and pore volume of used and unused catalysts in each cycle in Table 3. Ultrasonic irradiation increases not only surface area but also pore size and pore volume. However, all these parameters start to decrease after the first cycle and then decrease slowly until the end of the fourth cycle. As evidenced from the table, all three methods of surface area measurements show that the surface area of the catalyst after the third cycle falls below the value of unused catalyst. However, micropore area and micropore volume did not fall below the original value and the adsorption of average pore width was not been influenced even after the fourth cycle, which shows the stable structure of the catalysts. However, a slow decrease of surface area after the first cycle was noted, and it did not affect either dye adsorption or decolorization. This slight decrease in surface area is due to ultrasonically induced iron dissolution from the catalyst surface area. 3.6. Mechanism. Though Fenton-like process (R-FeOOH/ H2O2) mechanism is well documented in the literature,1,2,4,39,40 it is necessary to discuss the mechanism of R-FeOOH/H2O2 in the presence of ultrasound. Although physics and chemistry of the reactions associated with cavitations are fairly well understood, many questions on reaction mechanisms remain un-

Figure 9. Effect of t-butanol on the decolorization of DO39 in the presence of ultrasonic irradiation. [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, pH ) 3.0 ( 0.1, [t-BuOH] ) 0.01mol/L, ultrasonic frequency ) 35 kHz.

answered. Up until now, there has been no ready-made mechanism and satisfying explanation yet on the sonocatalytic degradation of organic pollutants using goethite as a catalyst. In the sono-oxidation process, ultrasound plays a dual role of reactant and catalyst. As reactant, ultrasound could be responsible for sonolytic degradation of organic molecule. As a catalyst, ultrasound causes sonolysis of the oxidant like H2O2 to create potent oxidizing free radicals, such as hydroxyl and hydroperoxy radicals, which initiate a complex chain of oxidative degradation of organic molecule. However, both mechanisms are completely ruled out as per our results. Nevertheless, the indirect chemical effects associated with the continuous cleaning and activation of the goethite surface is mainly responsible for the enhancement of removal rate. Neppolian et al. also suggest a similar mechanism on p-chlorobenzoic acid in the presence of ultrasound and goethite catalysts.19 It is expected that the sonocatalytic degradation of DO39 would mainly occur by goethite-catalyzed hydrogen peroxide decomposition resulting in the generation of hydroxyl radical attacks on the azo bond of DO39. To investigate the dependence of the degradation of DO39 on hydroxyl radicals, the sonocatalytic degradation has been carried out in the presence of t-butanol, a widely known hydroxyl radical scavenger (kOH ) 5 × 108 M-1 S-1.41 Figure 9 depicts DO39 decomposition in the absence and presence of 0.1mol/L of t-butanol. The results clearly show that t-butanol addition decreases the removal rate appreciably.

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tion, which is confirmed by the addition of hydroxyl scavengers. It is concluded that the goethite catalyst is a stable and efficient catalyst for industrial wastewater treatment. Literature Cited

Figure 10. Effect of NO3- on the adsorption and decolorization of DO39 in the presence of ultrasonic. (1) Decolorization: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, stirring speed ) 150 rpm, pH ) 3.0, reaction time ) 15 min. (2) Adsorption: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, stirring speed ) 150 rpm, pH ) 3.0, reaction time ) 15 min. (3) Decolorization: [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [H2O2] ) 5 g/L, [NO3-] ) 100 mg/L, US frequency ) 35 kHz, pH ) 3.0, reaction time ) 15 min. (4) Adsorption [R-FeOOH] ) 5 g/L, [DO39] ) 0.01 g/L, [NO3-] ) 100 mg/L, US frequency ) 35 kHz, pH ) 3.0, reaction time ) 15 min.

Beyond a doubt, the main reactive species in this process is hydroxyl radicals. Generally, it is believed that first both target pollutant and hydrogen peroxide is adsorbed on the catalytic surface, and then the degradation is initiated on the surface of the catalyst. To evaluate the importance of the adsorption of DO39 on goethite surface, the reaction has been carried out in the presence of NO3- (100 mg/L), and the results are shown in Figure 10. Obviously, both the adsorption and the decolorization of the dye on goethite decreased dramatically, when NO3- is added in solution along with DO39, about 35% and 38% (without nitrate ion) and 15, 6% (with nitrate ion) of decomposition and adsorption is observed respectively. The results indicate that there is a competitive nature between the anions and the dye during the adsorption process. This suggests that the sulfonic group of DO39 may be the binding site for this dye on goethite and suggests the importance of dye adsorption on the mechanism of degradation. Ge and Qu also reported a similar inhibition effect on acid red dye degradation in the presence of ultrasound and MnO2 catalysts.42 A detailed mechanism of sonocatalytic degradation is expected to be further studied, and adopting multifold methods are necessary to understand the mechanism clearly. Conclusions Ultrasonic waves not only enhance the decomposition rate but also increase the adsorption process by indirect chemical effect, which is more favorable to the degradation of DO39 dye. Increase in catalyst dosage enhances the removal rate, however, it does not facilitate more iron dissolution. Although 10 g/L of H2O2 was found to be optimum dosage for efficient removal, it did not influence the iron dissolution. In all experiments,