Ind. Eng. Chem. Res. 2008, 47, 2919-2925
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Catalytic Ozonation of Oxalic Acid Using Carbon-Free Rice Husk Ash Catalysts J. J. Wu,* S. H. Chen, and M. Muruganandham Department of EnVironmental Engineering and Science, Feng Chia UniVersity, Taichung 407, Taiwan
A highly efficient, carbon-free rice husk ash (RHA) has been successfully used as a catalyst in oxalic acid (OA) decomposition by combining with ozonation at pH 3. The catalyst was characterized using various analytical techniques. X-ray fluorescence (XRF) analysis showed that RHA catalyst contains SiO2 as a major constituent and MgO, P2O5, SO3, K2O, CaO, MnO2, Fe2O3, CuO, ZnO as minor constituents. X-ray diffraction (XRD) and EDX analysis consistently confirmed the presence of silica as a major constituent. Cold field scanning electron microscopic (FESEM) analysis revealed nonuniform particle size and the presence of external pores on the catalyst surface. Elemental analysis (EA) indicated that RHA contains negligible percentage of carbon content. Oxalic acid degradation as well as its TOC removal was studied at various physicochemical conditions. Adsorption and ozonation processes are not effective for oxalic acid and its TOC removal. The addition of RHA catalyst in the ozonation process caused 76.2% OA decomposition and 75% TOC removal in 60 min. Also, 0.25 g/L RHA dosage was found to be optimum for effective removal of oxalic acid. The efficiencies of RHA was compared with other commercially available catalysts, and the order was found to be RHA ≈ TiO2-P25 > FeOOH > SiO2 > Al2O3 > ZnO. Oxalic acid decomposition was enhanced by increasing inlet gaseous ozone concentration from 40 to 120 mg/L. The catalytic ozonation process is more pronounced than the ozonation process alone at pH 3, 7, and 10. It is concluded that RHA is an efficient green catalyst for oxalic acid removal. 1. Introduction Although new developments in various fields have led to the presence of new compounds in effluent streams, growing concerns about the environment have resulted in the development of new environmental technologies, new materials, and new approaches to reduce and minimize wastes.1,2 It is important to develop environmentally friendly catalysts that are effective, economical, and energy-efficient, the latter which is an important goal of green chemistry. Rice husk (RH) is a waste material which is generated in the milling process when the grain is separated from paddy, and it is very cheap and available in large quantities. Rice husk ash (RHA) has been used as a silica source to prepare zeolite beta,3 ZSM-5 zeolite/porous carbon,4 MCM41 molecular sieve,5 and support material for many catalysts,6-8 and as an adsorbent.9,10 Existing physicochemical technologies are expensive and commercially unattractive for the treatment of pollutants in wastewater. Chemical oxidation is one of the recommended technologies for the removal of refractory compounds in water treatment. Thus, catalytic ozonation is an important environmental remediation processes and it is a relatively novel subject with tremendous potential in the near future. Ozone is an environmentally friendly oxidant since it decomposes to oxygen without producing self-derived byproducts in the oxidation reaction. Unfortunately, some oxidation byproducts are refractory to further oxidative conversion by means of ozone, thus preventing a complete elimination of total organic carbon (TOC). However, the high energy cost for ozone generation limits many practical applications of direct ozonation. Therefore, it is highly recommended to conduct very effective methods for pollutant removal from water by degrading them, either to less harmful intermediates or to complete mineralization. In this way, more efficient pollutant removal has been reported by combining ozonation with UV, H2O2, homogeneous catalysts, and hetero* To whom correspondence should be addressed. Tel.: + 886-424517250, ext 5206. Fax: +886-4-24517686. E-mail:
[email protected].
geneous catalysts.11-14 Some studies on the heterogeneous catalytic ozonation of in particular manganese oxide, iron oxide, titanium dioxide, and activated carbon substances have been previously reported.14-17 The majority of the processes employed industrially involve catalysis by various metal complexes, and an increasing variety of catalytic processes has been developed over the past several years. In light of the points mentioned above, it is still a challenge to develop an efficient and inexpensive catalyst for industrial application. Therefore, in order to develop highly efficient and stable catalysts, an attempt has been made to study the catalytic activity of RHA in the presence of ozone at acidic media. Oxalic acid was chosen as a model pollutant based on the following considerations. (1) Oxalic acid is persistent to ozonation at acidic medium. (2) Oxalic acid is a typical refractory compound to conventional chemical oxidation. Catalytic ozonation of oxalic acid has been reported using various catalysts.13-23 The primary objective of this study was to investigate oxalic acid degradation and to test the catalytic activity of RHA catalysts. To the best of our knowledge, no reports are available in the literature for the detailed study of the oxalic acid degradation using RHA as a catalyst in catalytic ozonation processes. 2. Experiments 2.1. Chemicals. Anhydrous oxalic acid, obtained from Showa Chemical Co. (Japan), was of analytical grade and used without further purification. HPLC-grade acetonitrile and tert-butyl alcohol (99.0%) were purchased from Merck Chemical Co. in Taiwan. Indigo dye from Acros Organics was of analytical grade and used as received. Al2O3 (purity 99.9%) and SiO2 (purity 99.0%) from Showa Chemical Co., TiO2-P25 from Degussa Germany, ZnO (purity 99.8%) from Tedia Chemical USA, and R-FeOOH (98.0%) from ESCO Tech. Corp. Taiwan, were used as received, and the particle sizes of these catalysts measured by transmission electron microscopy (TEM) were 50-100, 100-500, 30, 50-100, and 50-100 nm, respectively. The surface area measurement results of RHA are shown in Table
10.1021/ie071093x CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
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then purged by nitrogen gas to remove inorganic carbon. The percentage of oxalic acid removal and its TOC removal were calculated from the following expressions:
Table 1. Surface Analysis and Elemental Analysis of the RHA Catalyst surface areaa (m2/g)
average pore diameterb (Å)
micropore areac (m2/g)
C
H
N
S
33.5550
116.7181
1.9429
0.053
0.122
0.318
0.448
a
content (%)
OA removal(%) )
By BET. b By BJH adsorption. c By t-plot.
1. For all experimental work, deionized water MilliQ-Plus, resistance ) 18.2 MΩ, was used. The pH of aqueous solution was adjusted using stock sulfuric acid (6 N) at pH 3, phosphate buffer at pH 7 (KH2PO4 + NaOH), and boric buffer (Na2B4O7 + NaOH) at pH 10 in all catalytic ozonation processes. Oxalic acid was prepared in deionized water before the experiments. 2.2. Catalytic Ozonation Experiments. Ozone was generated from dried pure oxygen by corona discharge using an ozone generator (RXO-5, Ozonair, USA), which can produce 6% ozone concentration (w/w) in the oxygen-enriched gas stream. To better maintain the performance of the ozonation system, oxygen was dried using a molecular sieve (Model 23988, Supelco, USA) before entering the ozone generator. A 0.5-L semibatch reactor, which was made of glass with dimensions of 9 cm diameter and 15 cm height, was used to facilitate the operation of all catalytic oxidation processes (Figure 1). Ozone was introduced through a porous fritted diffuser that can produce fairly fine bubbles with diameter less than 1 mm, which had been determined using a camera with a close-up lens and image analysis software Matrox Inspector 2.0.24 The gas flow rate was regulated by a mass flow controller (5850E, Brooks, USA). The gaseous ozone concentrations in the inlet and outlet stream were determined and monitored spectrophotometrically by the absorbance of ozone measured in a 2 mm flow-through quartz cuvette at the wavelength 254 nm. An extinction coefficient of 3000 M-1 cm-1 was used to convert absorbance into concentration units.25 Applied ozone dosage is typically defined as the product of gas flow rate, ozone concentration, and ozonation period, divided by the reactor volume. Thus, absorbed ozone dosage can be defined as the difference between applied ozone dosage and the sum of dissolved ozone in the reactor and outlet gaseous ozone quantities leaving from the reactor. A 500 mL solution containing appropriate concentration of oxalic acid and RHA catalyst was placed in a reactor. In order to make homogeneous slurries, the solution was magnetically stirred at the speed of 900 rpm. After a few minutes of stirring, ozone was introduced into the reactor. All experiments were operated at 25 °C using a water jacket around the reactor in which water was circulated through a temperature-controlled water bath. 2.3. Oxalic Acid Analysis. Samples were withdrawn from the reactor at desired time intervals in the course of the experiments. The residual dissolved ozone in the samples was removed immediately by purging with nitrogen gas (99.99%). The catalyst was removed from the experimental solutions by centrifugation, and the supernatant was filtered through Whatman glass microfiber filter paper (diameter ) 47 mm) with a pore size of 0.22 µm. The dissolved ozone concentration was measured using the indigo method.26 Oxalic acid concentration was analyzed by HPLC (Spectra System-1000, Chrom Quest) equipped with an ultra aqua C18 column (5 µm, 250 × 4.6 mm) employing UV detection at wavelength of 240 nm. The mobile phase consisted of water containing phosphoric acid, and the flow rate was controlled at 1.0 mL/min. The retention time of oxalic acid is 3 min, and the standard deviation of retention times was lower than 0.1 min. TOC concentrations were measured according to Standard Methods 5310 B27 using a TOC analyzer (5000A, Shimadzu), and the sample was acidified and
TOC (%) )
C(OA)0 - C(OA)t C(OA)0
C(TOC)0 - C(TOC)t C(TOC)0
× 100
× 100
where C(OA)0 and C(TOC)0 are initial concentrations of oxalic acid and TOC and C(OA)t and C(TOC)t are concentrations after time t, respectively. 2.4. Rice Husk Ash Preparation from Rice Husk. The rice husk was received from a local rice mill at Taichung city in Taiwan. The rice husk as received was washed with water and 1 M HNO3 several times and dried at room temperature. The impurity-free RH was then made into a fine powder using a grinding machine (D96S-90000, AGENTEC, Taiwan). The fine powder RH was again washed with a copious amount of water and then dried at room temperature for 2 days. A small quantity of RH powder was taken in a porcelain dish and placed inside a muffle furnace. The furnace was heated in open atmospheric conditions at the rate of 25 °C/min. The RH was combusted at 800 °C for 2 h, and then the furnace was allowed to cool at room temperature. A white ash was collected, stored at room temperature in a glass bottle, and used for further analysis. 2.5. Analytical Methods. The surface area, pore size, and pore volume of the RHA were measured by nitrogen adsorption at 77.35 K using an accelerated surface area and porosity apparatus (ASAP 2010, Micromeritics). X-ray fluorescence spectrometry elemental composition was determined using an X-ray fluorescence spectrometer, type PHILIPS TW 2400 with an Rh 4 kW tube directly, in solid mode. The X-ray diffraction (XRD) patterns were recorded using a MAX SCIENCE MXP3 diffractometer and Cu KR radiation and 2θ scanned angle from 10° to 80° at a scanning rate of 2°/min. The morphology of the catalyst was examined using a Hitachi S-4800 cold field emission scanning electron microscope (FESEM) equipped with a HORIBA EMAX 400 energy dispersive X-ray microanalysis (EDX) system. Prior to FESEM measurements, pinches of samples were mounted on a carbon platform and made a coating on the carbon platform. The plate containing the sample was placed in the scanning electron microscope for the analysis with desired magnifications. Elemental analysis was carried out using a Vario EL III CHNOS elemental analyzer (Germany). UV spectral analysis was done using a Spectronics Genesys 5 spectrophotometer (USA). 3. Results and Discussion 3.1. Characterization of As-Prepared RHA. X-ray fluorescence analysis showed that RHA contains SiO2 (89.4%), CaO (3.6%), Fe2O3 (0.47%), MnO2 (0.37%), ZnO (0.03%), CuO (1.2%), K2O (1.38%), P2O5 (0.70%), MgO (0.39%), and SO3 (1.74%). The XRD pattern of RHA is shown in Figure 2. A broad peak appearing around 2θ ) 22° clearly indicates that RHA mainly consists of silica; the ash is in the amorphous form and due to the presence of a low percentage of minor constituents did not yield any XRD peaks. The XRD pattern of as-prepared ash is very similar to pure silica extracted from RHA reported in the literature.28,29 The FESEM analysis shows no definite shape of silica with various particle sizes (Figure 3a), and high-resolution images indicate large pores on the surface
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Figure 1. System setup for ozonation and catalytic ozonation experiments.
Figure 2. XRD analysis of as-prepared RHA.
of the RHA catalyst as shown in Figure 3b,c. FESEM-EDX analysis also confirmed the presence of silica as a major constituent (Figure 3d). The elemental analysis showed (Table 1) that rice husk ash contains 0.053% carbon; hence carbon catalyzed decomposition is negligible. 3.2. Aqueous Ozone Decomposition in the Presence of RHA. The aqueous ozone decomposition experiments were performed at various RHA concentrations to determine the role of RHA in the aqueous ozone decomposition at pH 3. The effect of catalyst dosage on ozone decomposition was investigated
from 0 to 0.25 g/L, and the results are shown in Figure 4a. Increasing the catalyst loading from 0.025, 0.05, 0.1, and 0.25 g/L enhances the decomposition efficiency up to 61.8%, 71.3%, 77.1%, and 85.2%, respectively, at the time of 15 min. If a firstorder decomposition of dissolved ozone was assumed, the firstorder decomposition rate constant (k) could be fitted by the slope determined by a linear regression as shown in Figure 4b. It is noted that the increase in the RHA dosage would significantly enhance the decomposition rate of aqueous ozone from k ) 0.0087 to 0.1197 min-1, demonstrating the effectiveness of RHA as a catalyst in reaction toward ozone molecules. The decomposition rate of ozone at the various RHA dosages was found to be proportionally related to the ozone decomposition rate, because the increased amount of RHA would provide more active sites for ozone adsorption and decomposition. Our results are in good agreement with earlier reports on ozone decomposition.30 It is concluded that RHA catalyst is an efficient catalyst for aqueous ozone decomposition. 3.3. Adsorption and Degradability. Figure 5 depicts oxalic acid decomposition and its TOC removal in various processes under identical experimental conditions. Adsorption experiments showed that about 2.7% of oxalic acid is removed after 60 min of contact time, which implies that the adsorption process is not effective. Similar oxalic acid adsorption results were also reported in the literature in various metal oxide catalysts. Beltran et al. noted that only 8% oxalic acid removal was observed in
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Figure 4. Aqueous ozone decomposition in the presence of RHA catalysts. [O3]0 ) 2.3 mg/L, pH 3.0, and temperature ) 25 °C.
Figure 3. FESEM pictures of RHA: (a) [×1000]; (b, c) [×20000]. (d) EDX figure of RHA.
adsorption process after 4 h contact time on activated carbon catalyst in acidic media.17 Similarly, only 15% and 7% adsorption removal was noted on TiO2/Al2O3 and Fe2O3/Al2O3 catalysts at pH 2.5, respectively.20,23 Oxalic acid removal in the ozonation process is not effective. Only 14.3% oxalic acid decomposition and 14% TOC removal were noted at the end of 60 min of reaction time, which implies that the ozonation process alone is not effective for oxalic acid decomposition and mineralization. The low removal rate in the ozonation process
Figure 5. Oxalic acid degradation (a) and its TOC removal (b) by various processes. [OA] ) 100 mg/L, [RHA] ) 0.25 g/L, pH 3.0, tert-butyl alcohol ) 50 mg/L, applied ozone dosage ) 18.4 mg/L‚min, and temperature ) 25 °C.
is due to the slow reaction of ozone with oxalic acid, and the reported direct rate constant values (kD) were ,0.04 M-1 s-1.31
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Figure 6. Effect of catalyst dosage on degradation of oxalic acid. [OA] ) 100 mg/L, pH 3.0, applied ozone dosage ) 18.4 mg/L‚min, and temperature ) 25 °C.
Ozone reacts with organic compounds in water directly through molecular or selective reactions with specific functional groups (double bond). Since oxalic acid does not possess any specific functional groups in its molecule, direct reaction with ozone is very low. As we can see from the figure, oxalic acid removal is effective in RHA-catalyzed ozonation by the observation of 76.2% oxalic acid decomposition and 75% TOC removal. This enhancement of both removal rates is believed to be due to the generation of hydroxyl radicals by the RHA-catalyzed ozonation process which substantially increases the removal rate. The rate constant of hydroxyl radical with oxalic acid (k•OH) was reported as 7.7 × 106 M-1 s-1,32 which is extremely higher than kD values. Therefore, greater oxalic acid removal would be expected in the presence of hydroxyl radicals. The formation of hydroxyl radicals in catalytic ozonation is also confirmed by using tertbutyl alcohol as a radical scavenger. A separate experiment has been carried out in the presence of 50 mg/L tert-butyl alcohol, and the results are also presented in Figure 5a. The oxalic acid decomposition rate decreases in the presence of tert-butyl alcohol due to scavenging of hydroxyl radicals generated in the reaction. In conclusion, RHA is an effective catalyst in the catalytic ozonation process for the removal of oxalic acid. 3.4. Effect of RHA Loading. Catalyst loading is an important parameter in the catalytic ozonation process, and greater radical generation would be expected at higher catalyst dosage. In addition, in order to avoid the use of excess catalyst, it is necessary to find out the optimum catalyst loading for efficient application. Several authors have investigated the reaction rate as a function of catalyst loading in the catalytic ozonation process using various catalysts.33-36 The influence of catalyst loading from 0 to 0.5 g/L RHA on the decomposition of oxalic acid has been analyzed. Figure 6 shows the effect of various catalyst doses on the decomposition of oxalic acid at different reaction periods. It seems that the catalyst dose exerted a positive influence on both removal rates in catalytic ozonation process. However, the increase of catalyst dose from 0.25 to 0.5 g/L did not increase the removal rate appreciably. About 27.1%, 61.2%, 68.1%, 76.3%, and 78.6% oxalic acid decomposition in 60 min were observed at 0.025, 0.05, 0.1, 0.25, and 0.5 g/L dose of RHA catalyst. In addition, TOC removal data followed almost the same trend as the decomposition of oxalic acid, indicating that fast mineralization of oxalic acid into carbon dioxide and water was carried out by catalytic ozonation using RHA catalyst. These results demonstrate the effectiveness of RHA as a catalyst in the heterogeneous degradation process. The removal of oxalic acid increases by increasing RHA dose, which is due to the generation of more free radicals on the catalyst surface when the catalyst dosage increases. This result is quite consistent with some results published in the literature
Figure 7. Effect of various catalysts on the degradation of oxalic acid. [OA] ) 100 mg/L, [catalysts] ) 0.25 g/L, pH 3.0, applied ozone dosage ) 18.4 mg/L‚min, and temperature ) 25 °C.
in the catalytic ozonation process. Although Carbajo et al. found that the degradation efficiency of phenolic wastewaters at the end of the reaction period was independent of catalyst dosage,34 other references had results similar to those of this work. Park et al. reported that the pCBA degradation rate linearly depends on the goethite catalyst concentration from 2 to 15 g/L.30 Jung and Choi also noted similar results in pCBA degradation using nanosize ZnO as a catalyst ranging from 26.9 to 108.2 mg/L.33 From these results we could understand that the catalyst dosage is an important parameter and need to be optimized in the catalytic ozonation process. In summary, the optimum amount of catalyst for efficient removal was found to be 0.25 g/L. In all other experiments, 0.25 g/L RHA catalyst dosage was used. 3.5. Effect of Various Catalysts. The efficiency of the catalyst is another important parameter for the industrial and commercial use of any catalytic operation. If the catalyst is not efficient, the catalyst would become useless in practical applications. It is important to compare the catalytic efficiency of RHA with other commercially available metal oxide catalysts. Generally, in the heterogeneous catalytic ozonation process, the catalytic efficiency is fundamentally influenced by its physical and chemical properties, such as size, morphology, acidity, surface functional groups, and hydrophobic/hydrophilic properties. Thus, it would be somehow difficult to compare catalysts with similar physical and chemical properties. Generally, metal oxides such as aluminum oxide (Al2O3), silica (SiO2), titanium dioxide (TiO2), zinc oxide (ZnO), and goethite (FeOOH) are used as catalysts as well as supported catalysts in the catalytic ozone decomposition of pollutants.37-39 The RHA catalyst efficiency is compared with the catalysts mentioned above using the same dose (0.25 g) under identical experimental conditions. As shown in Figure 7, it is noted that oxalic acid decomposition percentages by Al2O3, SiO2, TiO2, ZnO, FeOOH, and RHA are 15.1%, 19%, 72.7%, 13.3%, 27%, and 76.2% at 60 min as well as TOC removal. Thus, RHA catalyst was found to have the best efficiency among the catalysts studied in this research for oxalic acid decomposition and TOC removal. Moreover, the oxalic acid decomposition efficiency of TiO2 is close to the efficiency of RHA and all other catalysts are not so effective in both oxalic acid decomposition and TOC removal. RHA catalyst has 5.0-, 4.1-, 5.7-, and 2.8-fold higher efficiency than Al2O3, SiO2, ZnO, and FeOOH, respectively. This might be due to the larger specific surface area (33.6 m2/g) on RHA catalyst compared to the other metal oxides, of which as used in the study the specific areas are among 2-5 m2/g. The order of efficiencies of these catalysts is RHA ≈ TiO2 > FeOOH > SiO2 > Al2O3 > ZnO. These results clearly imply that RHA is a potential catalyst for oxalic acid decomposition and its TOC removal when compared to other catalysts used in this study.
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Figure 8. Effect of inlet ozone gas concentration on degradation of oxalic acid. [OA] ) 100 mg/L, [catalysts] ) 0.25 g/L, pH 3.0, applied ozone dosage ) 18.4 mg/L‚min, and temperature ) 25 °C.
ozonation process is more pronounced at pH 3 than at pH 7 and 10. About 62.9%, 18.5%, and 14.8% oxalic acid decomposition were observed at pH 3, 7, and 10 in the catalytic ozonation process over the ozonation process. Since the point of zero charge (PZC) of SiO2 is less than 3,40 the lower pH as used would significantly attract more ozone and oxalic acid molecules toward the surface of RHA catalyst and catalytic decomposition of oxalic acid would proceed by hydroxyl radicals produced over the interface of solid-liquid phases. When the solution pH is increased to be neutral or basic, the generation of hydroxyl radicals might be mainly due to the initiation mechanisms by hydroxide ions. Thus, the difference in using the RHA catalytic ozonation process and the single ozonation process to decompose oxalic acid molecules became insignificant at higher pHs. 4. Conclusions
Figure 9. Effect of various initial pHs on degradation of oxalic acid. [OA] ) 100 mg/L, [catalysts] ) 0.25 g/L, applied ozone dosage ) 18.4 mg/L‚ min, and temperature ) 25 °C.
3.6. Effect of Inlet Ozone Gas Concentration. Another important variable that influences the catalytic ozonation process is the inlet ozone concentration. The experiment has been studied at 200 mL/min ozone gas flow rate and with varying the ozone concentration from 40 to 120 mg/L. Figure 8 depicts the effect of inlet ozone concentration on oxalic acid decomposition as a function of reaction time. Obviously, the inlet ozone concentration influenced both removal rates positively. When the inlet concentration increases from 40 to 120 mg/L, oxalic acid decomposition increases from 33.3% to 76.3% in 60 min as well as TOC removal. This enhancement of removal rate is believed to be due to the generation of more hydroxyl radicals as well as the increase in the mass transfer rate of ozone into aqueous solution when the ozone gas concentration is increased. Similar results were also reported by Beltran et al. in TiO2catalyzed oxalic acid decomposition; they found that oxalic acid degradation increases when the inlet ozone gas concentration increases from 7 to 55 mg/L.15 3.7. Effect of pH. The pH of solution may influence the surface properties of catalyst, charge of the pollutants, and generation of free radicals. Therefore, the study of pH influence on the catalytic ozonation process would be helpful to understand the mechanism of the process as well as for a higher degree of removal. The degradation experiments of oxalic acid were performed at pH 3, 7, and 10 in the ozonation and catalytic ozonation processes, and the decomposition results are shown in Figure 9. The catalytic ozonation process is more efficient than the ozonation process for all these three pHs, implying that catalytic ozonation would enhance the degradation of oxalic acid in solution. In addition, it is apparent that the catalytic
The research has drawn several conclusions: 1. According to surface characterization of as-prepared RHA catalyst using several analytical instruments, silica has been demonstrated to be the major constituent in the catalyst composition and surface texture, and the other metal oxides play minor roles. 2. Oxalic acid is effectively decomposed in the RHAcatalyzed ozonation process. Increase in the catalyst loading from 0 to 0.25 g/L is able to promote the degradation rate of oxalic acid as well as its TOC removal. However, further increase in the catalyst dosage would not increase the removal rate appreciably. 3. RHA was found to be an efficient catalyst when compared to Al2O3, SiO2, TiO2, ZnO, and FeOOH catalysts using the same weight. This might be due to the larger surface area of RHA than those of the other metal oxides used in this study. 4. Higher concentrations of gaseous ozone (40-120 mg/L) would result in better mass transfer and absorption into solution to enhance the degradation of oxalic acid by more hydroxyl radicals produced from the catalytic ozonation process. 5. Due to the low point of zero charge on the surface of the RHA catalyst, catalytic ozonation using RHA becomes more efficient at acidic media in oxalic acid decomposition and TOC removal than at higher pHs, indicating the efficacy of this oxidation technique as applied in the removal of organic components especially in wastewater with lower pH. Acknowledgment The authors wish to acknowledge financial support by the National Science Council (NSC) in Taiwan under Contract No. NSC-96-2221-E-035-020. Literature Cited (1) Cecil, L. H.; Peter, M.; Juraj, N.; Nagaharu, O.; Ludovico, S. Sludge Management in Highly Urbanized Areas. Water Sci. Technol. 1996, 34, 517. (2) Jeyaseelan, S.; Lu, G. Q. Development of Adsorbent/Catalyst from Municipal Wastewater Sludge. Water Sci. Technol. 1996, 34, 499. (3) Prasetyoko, D.; Ramli, Z.; Endud, S.; Hamdan, H.; Sulikowski, B. Conversion of Rice Husk Ash to Zeolite Beta. Waste Manage. 2006, 26, 1173. (4) Katsuki, H.; Furuta, S.; Watari, T.; Komarneni, S. ZSM-5 Zeolite/ Porous Carbon Composite: Conventional- and Microwave-Hydrothermal Synthesis from Carbonized Rice Husk. Microporous Mesoporous Mater. 2005, 86, 145. (5) Chiarakorn, S.; Areerob, T.; Grisdanurak, N. Influence of Functional Silanes on Hydrophobicity of MCM-41 Synthesized from Rice Husk. Sci. Technol. AdV. Mater. 2007, 8, 110.
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ReceiVed for reView August 10, 2007 ReVised manuscript receiVed February 4, 2008 Accepted February 6, 2008 IE071093X
