Efficacy of Fresh and Used Supported Copper-Based Catalysts for

Nov 13, 2012 - Efficacy of Fresh and Used Supported Copper-Based Catalysts for Ferulic Acid Degradation by Wet Air Oxidation Process. Bholu R. Yadav a...
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Efficacy of Fresh and Used Supported Copper-Based Catalysts for Ferulic Acid Degradation by Wet Air Oxidation Process Bholu R. Yadav and Anurag Garg* Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Mumbai, India, 400076 ABSTRACT: Pulp mill effluents are not suitable for conventional biological treatment processes due to the presence of lignin which is found in cellulosic raw materials. The present study was undertaken to remove ferulic acid (a lignin model compound) from synthetic wastewater using a catalytic wet air oxidation (CWAO) process. The hydrothermal process was performed in the presence of three heterogeneous catalysts, namely, 60% CuO/40% CeO2, 60% CuO/40% Al2O3, and 60% CuO/40% 13X, in a temperature range of 90−160 °C, while the total pressure was maintained in the range of 0.55−0.8 MPa. CuO and CeO2 mixture (prepared by the sol gel peroxo method) exhibited the best performance and removed ca. 70% chemical oxygen demand (COD) from the wastewater at 120 °C temperature. The spent catalyst showed appreciable decline in the COD removal during reuse. The detailed results obtained from the catalyst characterization (fresh and used) and WAO studies are presented in the paper.

1. INTRODUCTION Pulp and paper mill effluent contains several organic and inorganic compounds like lignin, cellulosic compounds, phenols, mercaptans, and sulfides. Lignin is the principal compound in pulp mill effluent which is persistent in nature and not amenable to conventional biological processes. Therefore, the present study was undertaken to investigate the performance of a chemical oxidation process, known as wet air oxidation (WAO), for the degradation of a lignin model compound (ferulic acid). Ferulic acid is a phyto-chemical found naturally in the cell wall of plants. Its structure is based around a benzene ring, and it acts as an antioxidant.1 WAO is a hydrothermal destructive treatment process which causes the mineralization of suspended or dissolved organic or inorganic compounds on complete oxidation. The degradation of pollutants occurs in the presence of a source of oxygen (e.g., air and molecular oxygen). After the reaction, the organic matter containing carbon and hydrogen are converted into less harmful end products (carbon dioxide and water). Typically, WAO is performed in temperature and pressure ranges of 120− 325 °C and 0.5−20 MPa, respectively.2 The major constraint in its application for real effluents is the requirement of severe oxidation conditions which enhances the capital and operational costs. Hence, the use of a suitable catalyst in the reaction is suggested as one of the measures for reducing the operating conditions and overall waste treatment costs. The reaction occurring in the presence of a catalyst is termed as catalytic wet air oxidation (CWAO). Several supported noble and transition metal heterogeneous catalysts have been tested for the degradation of different model compounds as well as industrial effluents in the past.3,4 Despite the good performance of noble metals, their high costs restrict the use of such catalysts in the oxidative process commercially. On the other hand, the comparatively inexpensive transition metal-based catalysts are not found as effective as noble metals. A major problem encountered with the heterogeneous catalysts is leaching of the active metal species in hot and acidic conditions which causes the early catalyst deactivation. The catalyst preparation methods can have significant effect on the overall performance © 2012 American Chemical Society

of a catalyst (its efficiency and reusability). Hence, the studies focusing on the catalyst preparation methods and its deactivation during CWAO process should be performed so that a stable and efficient catalyst could be developed. To our knowledge, few studies have been performed on the degradation of ferulic acid using the CWAO process.5,6 In these studies, three heterogeneous catalysts (Cu−Ni−Ce-Al2O3, Cu− Mn−Al2O3, and Cu−Ni−Ce/Al2O3) were used. Some research studies have also been carried out on pulp and paper mill effluent degradation under moderate to high operating conditions.7−10 Most of these studies utilized a copper-based heterogeneous catalyst which showed better activities compared to the other catalysts. A group of researchers has investigated the degradation of alkali lignin (another model lignin compound) using the WAO/CWAO process.11 From previous experiences, it was found that CuO/CeO2 has good capability to degrade phenolic compounds and pretreated black liquor.9,12 It has been found that CuO/CeO2 prepared by the sol−gel method had better performance for phenol degradation than that obtained from the coprecipitation method.12 Besides, zeolite-supported copper-based catalysts have also been used recently for the treatment of persistent organics.13 The present work was undertaken to study the performance of copper supported heterogeneous catalysts for ferulic acid removal from synthetic wastewater and to examine the reusability of the best performing catalyst.

2. MATERIALS AND METHODS 2.1. Materials. 2.1.1. Chemicals. Ferulic acid was supplied by Sigma-Aldrich Mumbai, India. Cu(NO 3 ) 2 ·3H 2 O, CeCl3·7H2O and H2O2 (30% v/v) were purchased from Merck chemicals, Mumbai, India. The support materials (alumina and 13X zeolite) were supplied by Zeolite and Allied Received: Revised: Accepted: Published: 15778

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The synthesized catalysts were characterized using X-ray diffraction (XRD) and scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM−EDX) techniques. 2.2.2. Wet Air Oxidation (WAO) Experimentation. WAO batch studies were carried out in a 700 mL high pressure reactor (material of construction: SS-316) which was procured from Amar Equipments Pvt. Ltd., Mumbai, India. The instrument was well equipped with provisions for controlling the temperature and stirrer speed to ensure complete mixing of the reactants. The reactor cap had two ports: one for the introduction of air into the reactor and the other for the periodic wastewater sampling. The treated samples were passed through a stainless steel condenser with water flowing in the outer shell before collection. For a typical run, 300 mL of wastewater was charged into the reactor with a desired amount of catalyst (for catalytic runs), and the reactor was heated to a predetermined temperature. The time requirement to raise the reaction temperature from ambient to the set value was approximately 45 min. After achieving the desired temperature, a sample was withdrawn to monitor any change in COD of the wastewater during the heating period. Subsequently, the compressed air was introduced to start the oxidation reaction and to achieve the desired pressure level in the reactor. At this instant, the agitation of reactor contents was also started to ensure complete mixing of the reactants. The samples collected during the reaction were analyzed for pH and COD as a function of time. WAO and CWAO reactions were performed at moderate operation conditions (temperature 90−160 °C and total pressures 0.55−0.80 MPa) for 3 h duration. The use of moderate conditions may help in the enhancement of catalyst lifetime (due to less leaching of the catalyst species) and can bring improvement in the economics of the WAO process. The agitation speed of 1000 rpm was found to be enough to eliminate the mass transfer limitations18 and therefore, the same speed was maintained throughout the experimental study. 2.2.3. Analytical Methods. The pH of the wastewater was measured by a digital pH meter (Polmon, LP-1395, India). The COD of untreated and treated water samples was determined in a Hach COD reactor (DRB200, USA) using the standard closed reflux method described in American Public Health Association.19 The TOC of untreated and treated wastewater samples was determined using a TOC Analyzer (Shimadzu, TOC-VCSH, Japan). The XRD spectra of the catalysts were obtained on a Philips diffractometer (PANalytical “Xpert Pro”, Netherland). The X-ray source used was Cu Kα and the scan angle (2θ) was varied from 20−90°. SEM−EDX analysis was done using a HITACHI scanning electron microscope (S3700N, Japan). The concentration of the metal species (Cu and Ce) in wastewater samples was determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (HORIBA Jobin Yvon- Ultima 2000, France).

Products, Mumbai, India. All the chemicals were of analytical grade. 2.1.2. Wastewater. The synthetic wastewater was prepared by dissolving a predetermined amount of ferulic acid (1 to 10 g/L) in tap water. The pH of the synthetic wastewater was in the acidic range from 3.78 to 4.13 with the highest value for the lowest ferulic acid concentration and vice versa. Chemical oxygen demand (COD) and total organic carbon (TOC) of the wastewater ranged from 1550 to 15500 mg/L and from 530 to 5300 mg/L, respectively. 2.2. Experimental Methods. 2.2.1. Synthesis of Catalysts. In the present study, different heterogeneous catalysts, CuO/CeO2, CuO/Al2O3, and CuO/13X were used for CWO runs. For preparation of all the catalysts, CuO and a support ratio of 60:40 (w/w) were selected. CuO/CeO2 catalyst was prepared by two chemical methods: coprecipitation and sol−gel peroxo route methods. The other two catalysts, CuO/Al2O3 and CuO/13X were prepared by the wet impregnation method as per the procedure outlined by Liu et al.14 To prepare a mixture of copper and cerium oxides by the coprecipitation method, stoichiometric amounts of Cu(NO3)2·3H2O and CeCl3·7H2O were dissolved in distilled water, and the mixture was agitated by means of a magnetic stirrer. To adjust the pH of the solution near to 8.0, ammonium hydroxide solution (2 M) was added dropwise so that the precipitation of the metal hydroxides could be maximized. The suspension was then left undisturbed for 2 h to allow the settling of coarser solids. The precipitation process facilitates the good deposition of metal oxide on the support.15 The precipitate was washed five times with distilled water to remove excess ammonia. The resulting wet solid mass was dried at a temperature of 103−105 °C for 24 h in an oven. The dried mass was ground with a pestle and mortar. Subsequently, the dried mixture was calcined in a muffle furnace at 500 °C temperature for 2 h. To prepare CuO/CeO2 mixture by the sol−gel peroxo route method,12 the salts of the respective metal species were allowed to react with excess H2O2 (30 vol % solution) in two separate glass beakers. Later both solutions were mixed properly in a glass container. The pH of the mixed solution was adjusted to around 8.0 by adding the ammonium hydroxide solution. The basic medium (pH = 8−11) and H2O2 help in achieving a higher surface area of the synthesized material.16 The excess peroxide was evaporated by heating the solution at 80 °C temperature for 30 min. After cooling the remaining solution, 20 mL of ethanol was added to increase the uniformity.17 The mixture was then partially dried at an 80 °C temperature for 24 h to allow the occurrence of hydrolysis and polymerization reactions for the formation of gel-like mass. This resulting mass was oven-dried at a temperature of 105−110 °C for 24 h and then calcined at 500 °C temperature for 2 h. The wet impregnation method used for the synthesis of the other two copper catalysts (supported on alumina and 13X) was completed in three steps. In the first step, the dry support (13X or alumina as the case may be) was added to an aqueous solution containing a predetermined amount of copper nitrate to allow penetration of the metal precursor into the support mainly due to capillary forces. The second step involved the oven drying of the solution to evaporate excess water (temperature 105−110 °C for 24 h). Finally, the oven-dried powder was thermally treated at 500 °C temperature for 2 h in the third step.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. 3.1.1. XRD Analysis. The presence of catalytic species was confirmed from the XRD spectra obtained for different catalysts (Figure 1). The XRD patterns for CuO/CeO2 catalyst prepared by coprecipitation and sol−gel peroxo routes showed the formation of CuO and CeO2 species at similar positions. However, two additional peaks representing CuO could be seen at 48.3° and 75.6° angles for the catalyst prepared by sol gel peroxo method. The 15779

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3.1.2. SEM−EDX Analysis. The results obtained from SEM− EDX are illustrated in Figure 2. In all three images, the nonuniform dispersion of copper can be seen upon the support material. The lighter and darker colors represent Cu and support material, respectively. From the image shown in Figure 2a, it can be observed that majority of the particles were generally circular in shape. The SEM image of CuO/13X catalyst demonstrates the presence of needle-shaped particles, and copper appeared to be impregnated over the support only in patches. The CuO/Al2O3 catalyst was found to contain particles with irregular shapes, and copper was present in clusters over Al2O3 support. Compared to Figure 2b,c, the catalyst prepared by the sol−gel peroxo method (Figure 2a) showed the better dispersion of copper over CeO2 support. 3.2. Noncatalytic WAO Process. Noncatalytic WAO of synthetic wastewater (ferulic acid concentration = 1 g/L, COD = 1550 mg/L) was studied in the temperature range of 90−150 °C. The total pressure of the reactor was maintained at 0.8 MPa in all the runs. The oxygen partial pressures at the reaction temperatures of 90, 100, 120, and 150 °C were varied from 0.168−0.071 MPa. The reaction was conducted at the original pH (= 4.13) of the wastewater. The COD reductions obtained at different temperatures are shown in Figure 3. As expected, an

Figure 1. XRD spectra of different catalysts.

location of peaks was also confirmed from the information available in literature.12,20−22 From the catalyst spectra, it can be suggested that copper mainly exists in an oxidation state of +2 (as CuO) and cerium in +4 oxidation state (as CeO2). The presence of more Ce4+ enhances the oxygen storage capacity of the catalyst. The XRD spectra for both CuO/13X and CuO/alumina catalysts showed the prominent CuO peaks at 2θ values of 36°, 39°, and 49°. The peaks for 13X could be seen at 30°, 33°, and 42°, whereas for alumina, the peaks were observed at 29°, 58°, and 69°. The results are in agreement with the previously reported studies characterizing 13X and alumina.13,23−26 One major difference between the spectra obtained from wet impregnation methods and coprecipitation/sol gel preoxo methods is the sharpness and intensity of CuO peaks which were greater in the former case.

Figure 3. COD reduction with time at different temperatures during noncatalytic WAO (initial COD = 1550 mg/L, total pressure (TP) = 0.8 MPa, initial pH = 4.13).

Figure 2. SEM images for different catalysts (a) CuO/CeO2 prepared by sol−gel; (b) CuO/13X; (c) CuO/alumina. 15780

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increase in the reaction temperature showed higher COD reductions. The maximum COD reduction of 67% (final COD = 510 mg/L) could be achieved at 150 °C temperature after 180 min of reaction. The COD reductions at 90, 100, and 120 °C temperatures were found to be 26, 41, and 51%, respectively, after the same reaction time corresponding to the final COD values of 1150, 910, and 760 mg/L in the same order. It can be observed that ferulic acid degradation was started right from the beginning and no induction period was observed. All the curves can be split in two segments: First a fast step (60−150 min at different temperatures) followed by slow degradation for rest of the period. At the highest temperature (i.e., 150 °C), the reaction occurred at the faster rate and almost 50% COD reduction could be achieved within 60 min. Thereafter, the reaction was slowed down and only ca. 17% reduction could be achieved in next 120 min. At 90 and 100 °C temperatures, the fast reaction occurred till 90 min afterward; almost no significant decrease in COD was observed. Although at 120 °C temperature, the COD reduction up to 90 min was almost similar to that observed at 100 °C temperature, it further continued till 150 min showing ca. 10% more COD reduction. It is a well established phenomenon that the WAO reaction is governed by active free radicals species. To generate free radicals (like HO·) which have the capability of oxidizing complex organic molecules, sufficient external energy should be added. At the lower temperature conditions, the formation of active free radical species is not easy and therefore the inferior performance of WAO process could be found. With further enhancement in input energy (or with an increase in the reaction temperature), more free radicals are formed, and the collisions among different reactant species increase which improved WAO efficiency. The extent of degradation also depends upon the nature and structure of the pollutant. The availability of oxygen is another factor that may affect the overall performance of the WAO process. No increase in COD reduction beyond 60 min reaction at the highest temperature may be due to the following two reasons: nonavailability of oxygen and the formation of low molecular weight carboxylic acids (particularly acetic acid). The reduction in wastewater pH from 4.13 to 4.0 also confirms the formation of a weak acidic species. Acetic acid is highly resistant to chemical oxidation at the reaction temperature and pressure used in the present study. During the reaction, formic and oxalic acids can also be formed which are comparatively easier to oxidize into the final products (i.e., CO2 and H2O). Hence, higher COD reduction and low pH decrease could be seen at the highest temperature. In the present study, the reaction intermediates were not determined but previous research studies on the same model compound reveal that 2-methoxy-4-vinylphenol and vanillin are the two key intermediates formed during the degradation of ferulic acid.6 It has been reported that the WAO process is very complicated even for a single compound and several parallel degradation pathways are possible.4 3.3. Catalytic WAO Process. 3.3.1. Screening of the Catalysts. The performance of different catalysts (CuO/CeO2, CuO/13X, and CuO/alumina) were compared under identical operating conditions (i.e., temperature, pressure, and catalyst concentration). The temperature and total pressures were maintained at 120 °C and 0.8 MPa, respectively. The pH of the wastewater was raised to 4.78, 7.10, and 7.22 from an initial pH of 4.13 when 3 g/L of each of the catalysts, CuO/CeO2

(prepared by sol−gel method), CuO/13X, and CuO/alumina, respectively, were added to the reactor. The results from CWAO reactions are illustrated in Figure 4.

Figure 4. Comparison of noncatalytic and catalytic WO (with different catalysts) (initial COD = 1550 mg/L, temperature (T) = 120 °C, TP = 0.8 MPa, initial pH = 4.13).

Among all the catalysts, CuO/CeO2 (prepared by the sol− gel method) exhibited the best activity and removed ca. 70% of the total COD from the synthetic wastewater. The same catalyst prepared by the coprecipitation method showed slightly lower activity (ca. 66% COD reduction) which was similar to that obtained with the CuO/13X catalyst. The CuO/alumina catalyst was found to be the least effective catalyst and showed only 60% COD reduction. During a noncatalytic reaction under the same conditions, the overall COD reduction was only 51%. One advantage with CeO2 support is the presence of storage capacity which helps in the oxidation reaction. CeO2 peaks could be seen in the XRD patterns for CuO/CeO2 catalyst. The final pH of the treated wastewater was found to decline to 4.43, 6.61, and 6.68 for the catalytic runs with CuO/CeO2 (prepared by sol−gel method), CuO/13X, and CuO/alumina catalysts, respectively. The catalytic reaction generally occurs at the external surface and/or inside the pores of a heterogeneous catalyst due to the adsorption of reactants.6 After the reaction, the products are oxidized into other simple compounds or gaseous end products which are desorbed from the active sites and create space for other molecules. The pore size of a catalyst plays an important role in the availability of active sites for reactant molecules. The pore size should be adequate enough so that the reactant molecules could enter the internal pore surface of the catalyst particle. Besides, the sufficient adhesive force should be present to keep the molecules on the surface for the reaction to occur. Due to the superior catalytic activity, CuO/CeO2 (prepared by sol gel peroxo method) was chosen for subsequent CWAO runs. 3.3.2. Effect of Operating Parameters on the Activity of CuO/CeO2 Catalyst (Prepared by Sol Gel Peroxo Method). The effectiveness of CuO/CeO2 catalyst was determined under varying operating conditions. The effect of wastewater pH, catalyst concentration, total reaction pressure, temperature, and 15781

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pressure was in the range of 0.082−0.134 MPa). A decrease in total pressure showed negative impact on the overall COD reduction (Figure 6). At 0.55 MPa pressure, COD reduction of

initial ferulic acid concentration was studied on the CWAO process. 3.3.2.1. Effect of pH. To investigate the effect of pH on the CWAO reaction, a set of experimental runs was carried out in a pH range of 4.13 (original) to 9.0 while maintaining the other reaction conditions the same (T = 120 °C, TP = 0.8 MPa, and catalyst concentration = 3 g/L). The pH of the wastewater was increased to the desired level by adding an appropriate amount of ammonium hydroxide solution. The increase in pH from 4.13 to 9.0 showed a gradual reduction in the performance of the catalyst, and the COD removal was decreased from 70% to 63%. The depletion in CWAO performance may be attributed to the scavenging action of free radicals at a pH in highly alkaline regions.4 Apart from this, the chemistry of free radical formation also changes with alteration in the pH of a wastewater. The intermediates species and their degradation patterns may be different in the changed conditions. In the view of above result, further studies were performed at the original pH of the wastewater. 3.3.2.2. Effect of Catalyst Concentration. The catalyst dose was changed from 0.5 to 5.0 g/L to examine the performance of the catalytic reaction at mild temperature and pressure conditions (i.e., 120 °C temperature and 0.8 MPa total pressures). It can be seen from Figure 5 that the COD

Figure 6. Effect of pressure on COD reduction (initial COD = 1550 mg/L, T = 120 °C, catalyst concentration = 3 g/L).

63% could be found which was increased to 70% at 0.8 MPa pressure. At lower pressure, oxygen deficiency will be more and thus the formation of active free radicals will be affected. 3.3.2.4. Effect of Ferulic Acid Concentration. The effect of initial ferulic acid concentration on the CWAO process was observed under similar temperature and pressures of 120 °C and 0.8 MPa, respectively. The initial ferulic acid concentration was varied from 1 to 10 g/L (COD = 1550−15500 mg/L). In all the runs, a catalyst dose of 3 g/L was added to the synthetic wastewater. It can be observed from Figure 7 that COD removal from the wastewater having ferulic acid concentration of 10 g/L was 80% (final COD = 3094 mg/L). The percent COD removal was found to decrease with reduction in the initial substrate concentration (varying from 67−70%). The results suggest the rapid conversion of ferulic acid into other lighter molecular weight intermediates which gradually further

Figure 5. Effect of catalyst concentration on COD reduction (initial COD = 1550 mg/L, initial pH = 4.13, T = 120 °C, TP = 0.8 MPa).

reduction was enhanced with an increase in the catalyst concentration. A catalyst concentration of 0.5 g/L increased the COD removal to around 58% from 51% obtained during noncatalytic oxidation. The COD reduction was further increased to ca. 64% with 1.0 g/L catalyst concentration. The maximum COD reduction of 76% (final COD = 371 mg/L) was observed when the reaction was performed with a catalyst concentration of 5 g/L. It can be noted that the requirement of the catalyst was very high (∼4 g/L) to achieve an increment from 64% to 76%. This finding shows that the addition of catalyst reduced the energy barrier and probably assisted in the generation rate of free radicals though the enhancement in COD removal was low. 3.3.2.3. Effect of Total Pressure. The effect of total pressure on COD reduction was investigated by varying the reaction pressure from 0.55 to 0.8 MPa (corresponding oxygen partial

Figure 7. Effect of initial substrate concentration on COD reduction (T = 120 °C, TP = 0.8 MPa, catalyst concentration = 3.0 g/L). 15782

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oxidation reaction at 120 °C temperature and 0.8 MPa total pressures. The spent catalyst was recovered from the treated water after a 3 h reaction under the same operating conditions. No wastewater sample was withdrawn from the reactor during this run. The treated wastewater was filtered and the deposited solid mass at the filter paper was oven dried at 100−105 °C temperature before it was subjected to the physical characterization. Subsequently, the recovered catalyst was reused for oxidative degradation of ferulic acid. 3.4.1. Characterization of the Spent Catalyst. The used catalyst was characterized using the same techniques applied to the fresh catalyst (i.e., XRD and SEM−EDX). The XRD spectrum was compared with that obtained for fresh CuO/ CeO2 catalyst (Figure 9). It can clearly be seen that the

converted into persistent carboxylic acids. The pH of wastewater having the highest ferulic acid concentration (i.e., 10 g/L) was decreased to 3.55 (from an initial pH of 3.78) after 3 h of reaction. It was observed that the degradation of wastewater with the highest ferulic acid was still progressing even after 3 h of reaction which indicates the presence of short chain easily degradable compounds (like alcohols and aldehydes) other than carboxylic acids (mainly acetic acid). 3.3.2.5. Effect of Temperature. The effect of temperature on CWAO reaction was studied by performing the experiments in a temperature range of 90−160 °C. The total pressures were maintained at 0.8 MPa while the catalyst concentration was kept at 3 g/L. The results revealed that an increase in temperature enhanced the overall COD reduction up to a certain temperature (Figure 8). It was observed that the COD

Figure 9. XRD spectra of the fresh and recovered heterogeneous catalyst (60% CuO/40% CeO2) prepared by sol−gel.

intensity of peaks was reduced after using the catalyst once at elevated temperature and pressures. The slightly broader peaks at 27°, 33°, 48°, and 55° scanning angles representing CuO and CeO2 species could still be seen. The broadening may probably due to the merging of peaks. The SEM−EDX image obtained for the used catalyst was almost similar to that for the fresh one, indicating the absence or little carbon deposition on the catalyst surface (Figure 10). However, no information could be obtained from the image about the deposition of carbonaceous material inside the pores of catalyst which may also adversely affect the catalytic activity.

Figure 8. Effect of temperature on COD reduction (initial COD = 1550 mg/L, TP = 0.8 MPa, catalyst concentration = 3 g/L).

reduction even at 90 °C reaction temperature was around 50% during the CWAO process which was almost double to that obtained during the noncatalytic reaction (as shown in Figure 3). At 150 °C temperature, the overall COD reduction was 80% which was only 10% more than the percentage observed at 120 °C temperature. A subsequent increase in temperature to 160 °C did not cause any additional COD reduction. The most likely reason for a small or no increase in COD reduction at higher temperatures (i.e., 150 °C or more) should be the deficiency of oxygen in the reaction mixture. The curves obtained at different reaction temperatures indicate the completion of CWAO reactions in 2−3 steps. The first step can be characterized as the fastest reaction zone that was continued only for 30 min duration except for the reaction conducted at 160 °C temperature at which the fast reaction was continued till 90 min. The second slower reaction step was continued from 30 to 150 min for the reactions performed at 90, 120, and 150 °C temperatures. The third and final step can be characterized as the slowest one in which little or no COD reduction could be found. The treated wastewater pH was changed to 4.39, 4.43, 4.44, and 4.97 at 90, 120, 150, and 160 °C temperatures, respectively from an wastewater initial pH of 4.13. 3.4. Catalyst Deactivation Study. The reusability of CuO/CeO2 catalyst was investigated by performing the

Figure 10. SEM image for recovered CuO/CeO2 catalyst. 15783

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1 1 − = kt C C0

3.4.2. CWAO of Ferulic Acid with Recovered Catalyst. The performance of spent catalyst was studied for the degradation of ferulic acid at 120 °C temperature and 0.8 MPa total pressures. An appreciable decrease in overall COD reduction (ca. 60%) was observed with the reuse of the spent catalyst compared to the CWAO run with the fresh catalyst (for which COD reduction = 70%) (Figure 11). It can be seen that COD

(2)

where C represents COD of the wastewater at time t, C0 is initial COD, and k is the rate of reaction. From the temperature profiles for both catalytic and noncatalytic reactions (Figures 3 and 8), it has been found that the reaction was almost completed within 150 min. Therefore, the COD data for this duration were used to determine the kinetic constants. Table 1 Table 1. Rate Constants Obtained for Catalytic and Noncatalytic WAO Reaction of Ferulic Acid noncatalytic WAO temp (°C)

k (min−1)

regression coeff (r2)

k (min−1)

regression coeff (r2)

1 2 3

90 120 150

1 × 10−6 4 × 10−6 8 × 10−6

0.954 0.993 0.989

4 × 10−6 8 × 10−6 1 × 10−5

0.971 0.982 0.984

k = koe−Ea / RT

and TOC reductions were almost the same until 30 min of reaction beyond which the degradation of substrate and intermediates decreased with time. The deposition of carbonaceous material on the internal pore surface of the catalyst may be one of the possible reasons. As a result, the accessibility of active sites for the new reactant molecules will hinder. Another reason attributing to the decrease in catalytic activity may be leaching of the metal species from the catalyst. To determine the leaching of active metal species, the wastewater treated by fresh and used catalyst was analyzed for Cu and Ce concentrations and it was found that 2.5% and 12% of the total Cu and Ce were leached out from the fresh catalyst (initial Cu = 1438 mg/L and Ce = 977 mg/L). After the second run, the concentrations of Cu and Ce in the treated water were increased to 106 mg/L and 265 mg/L, respectively (7.4% and 27% of the original values). These results demonstrate the susceptibility of the catalyst under thermal conditions. Loss of significant amount of ceria also has an adverse impact on the oxygen storage capacity of the catalyst.

(3)

where k is the reaction rate at temperature T, ko is the specific rate constant, Ea is activation energy of the reaction, and R is the universal gas constant. The activation energy for noncatalytic WAO and CWAO were found to be 41.89 and 19.76 kJ mol−1, respectively, whereas the specific rate constants for the two reactions were 1.136 and 0.0029 min−1. The values for activation energies confirm the little dependency of WAO or CWAO processes on temperature (within the range used in present study).

5. CONCLUSIONS AND SCOPE FOR FUTURE WORK Several conclusions can be drawn from the presented study. From the results, it can be suggested that the catalyst preparation method may have a pronounce effect on its performance. For instance, CuO/CeO2 catalyst prepared by the sol−gel method showed better COD removal than the same catalyst synthesized by the coprecipitation method. The catalyst exhibited the highest capability among all the tested catalysts at mild operating conditions (i.e., at 120 °C temperature and 0.8 MPa total pressures) which is highly desirable in this field of research. However, the catalyst showed much inferior performance after the first run (10% lower COD removal than that obtained with the fresh catalyst) which suggests the need for more work aiming at the enhancement of the thermal stability of a catalyst by altering the operating conditions used for its synthesis. The probable poisoning of active sites (inside the pores) by carbonaceous deposition and leaching of active species are suggested to be the two major reasons for deteriorating catalytic activity. With increase in ferulic acid concentration in the synthetic wastewater percent COD degradation was enhanced since the higher amount of parent compound was converted into low molecular weight

4. KINETIC STUDY FOR NONCATALYTIC AND CATALYTIC WAO The noncatalytic and catalytic WAO processes were assumed to follow second order kinetic equation. The kinetic constants were determined using COD obtained periodically for the wastewater treated at different temperatures (90, 120, and 150 °C). The rate expression for a second order reaction can be written as dC = kC 2 dt

sample no.

shows the rate constants and regression coefficient values at different temperatures for both reactions. The rate constants for the catalytic runs were 1.25−4 times that for noncatalytic runs at the reaction temperatures. The increase in temperature from 90 to 150 °C enhanced the reaction rate constant by a factor of 8 and 2.5 for noncatalytic WAO and CWAO processes, respectively. The activation energy of WAO and CWAO reactions was calculated using the Arrhenius expression:

Figure 11. Deactivation study for CuO/CeO2 catalyst (initial COD = 1550 mg/L, T = 120 °C, TP = 0.8 MPa, catalyst concentration = 3.0 g/L).

−

catalytic WAO

(1)

The solution for eq 1 is given as 15784

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compounds. The findings from the present study indicate that the partial destruction of refractory pollutants is possible by a mild WAO process which may cause significant improvement in the process economics. It has been reported that the capital cost (including the installation costs) of the process can be around 82% of the total expenditure depending upon the reaction conditions and volume of wastewater to be treated.27 Meanwhile the operating costs can also be reduced significantly with the decrease in reaction conditions. The future studies can focus on the prediction of the ferulic acid degradation mechanism by detecting the intermediates and end-products formed during the reaction. The conditions for catalyst preparation (such as calcination temperature) should be optimized to achieve higher degree of stability and prevent leaching of the metal species in acidic and hot environment. To evaluate the feasibility of the WAO process for real pulp and paper mill wastewater, a detailed economic analysis (considering capital and operating costs) should be carried out.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-22-25767861. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Department of Science and Technology (DST), New Delhi, for providing the financial support to carry out the research work through “Fast Track Scheme for Young Scientists”. We acknowledge the support of the Sophisticated Analytical Instrumentation Facility (SAIF), IIT Bombay, and the Metallurgical Engineering & Materials Science Department, IIT Bombay, for helping us in the catalyst characterization.

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REFERENCES

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dx.doi.org/10.1021/ie301787g | Ind. Eng. Chem. Res. 2012, 51, 15778−15785