- Email: [email protected]
Studies on cerium oxidation in catalytic ozonation process: A novel approach for organic mineralization
Catalysis Communications 8 (2007) 1497–1501 www.elsevier.com/locate/catcom
Studies on cerium oxidation in catalytic ozonation process: A novel approach for organic mineralization Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, Il Shik Moon
*
Department of Chemical Engineering, Sunchon National University, 315 Maegok dong, Suncheon 540-742, Jeonnam, Republic of Korea Received 8 September 2006; received in revised form 19 December 2006; accepted 19 December 2006 Available online 23 December 2006
Abstract A novel regenerative catalytic system has been developed using cerium and ozone in nitric acid medium. It was found that cerium(III) was oxidized to cerium(IV) by ozone in nitric acid medium with good conversion yields. The conversion rate of Ce(III) was measured under various parameters viz. ozone–air flow rate, initial concentration of Ce(III), and concentration of nitric acid at 25 C. It was found that the conversion of Ce(III) increased with increasing ozone flow rate and concentration of nitric acid while decreased with increasing Ce(III) concentration. The pseudo first order kinetic constants were evaluated for Ce(III) oxidation. The efficiency of this hybrid system comprising of ozone and cerium redox pair towards organic mineralization was evaluated taking phenol as the model organic pollutant and compared with Ce(III) catalyzed and uncatalyzed ozonation processes. The presence of Ce(III) catalyst increased the destruction efficiency of phenol compared to uncatalyzed ozonation whereas a synergetic effect was observed between the cerium redox pair (Ce(III) and Ce(IV)) and ozone towards phenol mineralization and a maximum TOC removal was obtained in the latter case. Kinetic interpretations have been made with some simplifying assumptions owing to the much complex nature of ozone and metal ion interactions. This hybrid catalytic ozonation process may find its suitability for continuous organic destruction at room temperature. 2007 Elsevier B.V. All rights reserved. Keywords: Ozone; Cerium catalyst; Catalytic ozonation; Phenol mineralization
1. Introduction Ozone is capable of oxidizing organic and inorganic compounds due to its high reduction potential (E0 = 2.07 V vs SHE) [1]. The treatment of water and waste water by ozone for organic pollutant destruction is a well established research area. But ozone alone has been shown to achieve a very limited mineralization of organic compounds or removal of refractory pollutants particularly in acidic solutions. Recently, research focus has been laid on the catalytic ozonation processes with metal ions or metal oxides acting as catalysts in order to enhance the reactivity of ozone at lower pH conditions. Consequently, various ozonation processes have been investigated as efficient methods for degrading organics [2]. Ozonation pro*
Corresponding author. Tel./fax: +82 61 750 3581. E-mail address: [email protected] (I.S. Moon).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.12.017
cesses catalyzed by transition metals have been classified in to activation of ozone by metals in solution and heterogeneous catalytic ozonation in the presence of metal oxides or metals on supports [3]. The activation of ozone by Fe(II), Mn(II), Ni(II), or Co(II) as their sulfates was reported by Hewes et al. [4]. The presence of these metals increases the TOC removal efficiency as compared to ozonation alone and was reported by many authors as follows. Zinc or copper sulfate, silver nitrate, and chromium trioxide catalyzed ozonation was reported by Abdo et al. [5] for the treatment of dye effluents. Gracia et al. [6] showed that Mn, and Ag catalyzed the ozonation of humic acid substances in water. Andreozzi et al. [7] found that Mn(II) accelerates the oxidation of oxalic acid by ozone under acidic conditions. It was reported in the literature [8,9] that apart from transition metals some inner transition metals like cerium could also be used as an effective oxidation catalyst in an acidic
1498
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
medium for organic removal and decontamination applications by ozone. Delanoe et al. [10] and Fu et al. [11] have reported the use of ceria as the supporting catalyst for the oxidation of carboxylic acids and chlorinated carboxylic acids by the ozonation process. Freshour et al. [12] observed that the ozone decomposition rate strongly depends on pH and increases with increasing pH value. Generally the TOC removal efficiency of uncatalyzed ozonation was found to be less at lower pH conditions and Ni et al. [13] observed that for the organic removal by ozonation at an acidic pH, the TOC removal rate could be increased with Al2O3 catalyst. Although cobalt and iron could well be used for catalyzing the ozonation processes they possess some disadvantages. The stability of cobalt(III) is less due to water oxidation while the oxidizing ability of iron towards organic compounds is well below that of cerium. Considering these properties of other metal ions, cerium is chosen which possesses good oxidizing ability and also not precipitated as chloride with chlorine containing organic compounds. Ce(IV) in nitric acid medium (E 0 = 1.72 V vs SHE) [1] is a powerful non-selective oxidant and was used extensively for organic mineralization reactions [14]. This combined cerium and ozone system is slightly different from catalytic ozonation process using transition metals. In catalytic ozonation processes a small quantity of a metal in the order of 106–103 M, is usually used to activate the ozone decomposition to produce more free radicals. But in the present study the amount of the metal ion catalyst employed is fairly high compared to the previous studies with other metal ions and also the oxidized form of cerium is a powerful oxidant in acid medium towards organic compounds. Therefore this combined effect of ozone, Ce(III) and Ce(IV) should have more pronounced effect for organic mineralization and forms an interesting system of applied research for organic removal and decontamination applications. Carie et al. [15] have reported the decontamination process of stainless steel by Ce(IV) in 4 M nitric acid in which the reduced Ce(IV) is reoxidized by ozone. The electrochemical production of Ce(IV) oxidant using an electrochemical cell could also be used for organic decontamination and destruction studies but restricted due to the low coulombic efficiency at room temperature for lower metal ion concentrations, formation of secondary pollutants such as nitrogen oxide gases in the cathodic compartment and also poisoning of the electrodes by undestructed organics during the regeneration of mediator metal ion [16]. Usually, in metal ion catalyzed ozonation systems the concentration of metal ions employed is fairly low but for metal ions with high redox potentials such as cerium a higher level of concentration produces a highly active redox couple which may increase the organic destruction efficiencies. Therefore, to have better understanding of the cerium catalyzed ozonation process it is necessary to optimize the system with respect to the metal ion concen-
tration, ozone flow rate, and acid concentration. As far as we know, no study on the oxidation of Ce(III) by ozone was published in the literature. The objectives of the present investigation were (i) to study the oxidation of Ce(III) by ozone in nitric acid medium under various conditions such as concentration of Ce(III), concentration of nitric acid, ozone–air flow rate at 25 C; (ii) to check the efficiency of this hybrid catalytic ozonation system for phenol destruction and to compare with uncatalyzed ozonation under the same conditions. 2. Experimental section 2.1. Materials Cerium(III) nitrate hexahydrate obtained from Aldrich (99%), nitric acid (60%) and phenol (extra pure) from Dae Jung Company, Korea, were used as received. The water used was doubly distilled and deionized. 2.2. Ozone generation The generation of ozone was accomplished by coronadischarge system (Ozone Tech Co. Ltd., Model OG-TY, South Korea). A dry gas of pure oxygen from a cylinder (2 l/min) was subjected to an electrical discharge in the ozonizer. The oxygen molecules in the gas were dissociated to form ozone. Power was supplied in the form of high alternating current voltage. 2.3. Ce(III) oxidation in nitric acid by ozone The oxidation of Ce(III) in nitric acid was performed in a batch reactor by bubbling ozone through a ceramic diffuser at 25 ± 1 C. In all experiments the flow rate of ozonized oxygen was 100 ml/min (unless otherwise specified) and the concentration of ozone at the reactor inlet was equal to 0.13 g l1. Before starting the experiment, 1000 ml of Ce(III) in nitric acid solution was placed in the reactor and ozone was continuously passed. The aliquots were periodically collected and titrated for Ce(IV) quantifications by potentiometry. In all cases ozonation was continued up to 180 min. The Ce(III) conversion was computed as the percentage ratio of the concentration of Ce(IV) formed to the initial Ce(III) in Molar units. The rate of Ce(III) conversion was obtained using the following simple pseudo first order equation assuming that the concentrations of ozone and nitric acid remain unchanged during the reaction [17,18]. Rate ¼ k obs ½CeðIIIÞ 2.4. Mineralization of phenol The phenol destruction experiments were conducted in a batch reactor by bubbling ozone through a ceramic diffuser (flow rate 100 ml/min) without and with cerium catalyst at
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
25 ± 1 C. The volume of the solution taken was 1000 ml in all the experiments. In the case of cerium catalyzed phenol destruction, the destruction studies were carried out with Ce(III) (50 mM) and phenol (2.7 mM) both added together at zero time along with ozonation and also to verify the role of Ce(IV) an experiment was conducted in which Ce(III) was first partially oxidized to Ce(IV) by passing ozone up to 60 min (7% Ce(IV) was produced in 60 min) and then phenol solution was added with ozonation continued throughout the reaction. The liquid phase organic removal was monitored by total organic carbon analyses (TOC analyzer, Shimadzu, 5000 A).
1499
mum yield was reached around 3 h. At the ozone–air flow rate of 100 ml/min the maximum conversion of 33% Ce(IV) was obtained where as at 1000 ml/min flow rate, 95% yield was obtained. Under the experimental conditions of large excess of ozone and hydrogen ion concentrations the pseudo first order equation can be applied for Ce(III) oxidation. When the pseudo first order rate constant values (kobs) were plotted against various ozone–air flow rates a good linearity was observed (Fig. 2), suggesting that the reaction is of first order with respect to ozone. 3.2. Effect of initial Ce(III) concentration on its oxidation
3. Results and discussion 3.1. Effect of ozone–air mixture flow rate on Ce(III) oxidation The mechanism of ozone with reduced metal ions though not well understood so far [3], the overall reaction of Ce(III) with molecular ozone can be represented as follows [1]; 2Ce(III) + O3 + 2Hþ ! 2Ce(IV) + O2 + H2 O The percentage conversion with time for 50 mM Ce(III) in 1 M nitric acid at various flow rates of ozone–air mixture (100, 250, 500 and 1000 ml/min) is shown in Fig. 1. As expected, the conversion of Ce(III) increased with increasing ozone flow rates because of the increased dissolved ozone concentration in the liquid phase. It can be observed that the rate of Ce(III) conversion increased linearly with respect to time at low ozone–air flow rates compared to high flow rates. In all the cases the conversion rate of Ce(III) deviated from linearity after 50 min and the maxi-
The effect of initial metal ion concentration on the oxidation rate (Fig. 3) was studied for the Ce(III) concentrations viz. 10, 50 and 100 mM in 1 M nitric acid with 100 ml/min ozone–air flow rate. It is seen from the Fig. 3 that at the concentration of 10 mM Ce(III) a maximum of 70% conversion was obtained at 180 min. But for concentrations of 50 and 100 mM Ce(III), the final conversion yields were decreased to 24% and 15%, respectively. This decrease in the oxidation rate may be due to the increased ozone decomposition and decreased liquid phase ozone concentration at higher concentrations of metal ions. So the concentration of metal ion catalyst employed plays an important role in determining the reactivity of ozone in solution. The pseudo first order rate constants calculated for Ce(III) oxidation were found to be 1.0 · 103, 1.5 · 103 and 6.8 · 103 min1, respectively, for 100, 50 and 10 mM of Ce(III), respectively. Sanchez et al. [19] have reported that in Fe(II) catalyzed ozonation of phenol, the mineralization efficiency decreased at higher concentrations of metal ion with a maximum reaction rate obtained
100
0.02
y = 2E-05x R2 = 0.9918
0.016
60
0.012
k obs
Conversion of Ce(III) (%)
80
40
0.008
20
0.004 100 ml/min 500 ml/min
250 ml/min 1000 ml/min
0 0
50
100 Time (min)
150
200
Fig. 1. Effect of various ozone–air mixture flow rates on the percentage of Ce(IV) formation ([Ce(III)], 50 mM; [HNO3], 1 M; T, 25 ± 1C).
0 0
300
600
900
1200
Ozone - air mixture flow rate (ml/min) Fig. 2. Effect of various ozone–air mixture flow rates on the pseudo first order rate constants ([Ce(III)], 50 mM; [HNO3], 1 M; T, 25 ± 1C).
1500
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
observation of increasing oxidation rate with increasing nitric acid concentration was noted by Wei et al. [20,21] for the electrochemical oxidation of Ce(III) in nitric acid. They explained that high proton concentration promotes the electron transfer of the Ce(III)/Ce(IV) couple in nitric acid media. Also, Asano et al. [9] have found that the dissolution of UO2 using ozone and 0.1 M Ce(IV) in nitric acid solutions increased with the nitric acid concentration from 0.5 to 3 M, but further increase from 3 to 5 M resulted only is small increase in dissolution. This increased dissolution of UO2 found by Asano et al. could be explained as due to the faster regeneration rate of reduced Ce(IV) oxidant at 3 M compared to 0.5 M, based on our results.
Fig. 3. Percentage formation of Ce(IV) with respect to time at different initial concentrations of Ce(III) (flow rate of ozone–air mixture, 100 ml/ min; [HNO3], 1 M; T, 25 ± 1C).
for 4 mM of Fe(II). This observation is in line with the results obtained in this study that increasing the cerium ion concentration decreases the conversion yield since the metal ion concentration used is in the higher range. 3.3. Effect of nitric acid concentration on Ce(III) oxidation The effect of nitric acid concentration on the rate of Ce(III) oxidation (Fig. 4) was studied under the conditions of 50 mM Ce(III) in 0.3, 1, 3 and 4 M nitric acid solutions with 100 ml/min of ozone–air flow rate. It is seen from Fig. 4 that the conversion of Ce(III) increased with nitric acid concentration in well defined jumps. This may be due to the facile oxidation of metal ions at high proton concentrations. Also, the higher acid concentrations (lower pH) decrease the decomposition rate [2] and increase the life time of ozone and hence higher conversions. A similar
3.4. Comparison between uncatalyzed and catalyzed ozonation processes for phenol mineralization To check the efficiency of cerium catalyzed ozonation process over the uncatalyzed ozonation, the destruction of 250 mg/l (2.7 mM) phenol was carried out at 25 ± 1C. The uncatalyzed ozonation was carried out taking 1000 ml of 2.7 mM phenol solution and passing ozone continuously. In the case of Ce(III) catalyzed ozonation process the solution containing 2.7 mM phenol and 50 mM Ce(III) in 3 M nitric acid was fed in to the reactor and ozone was bubbled continuously. In order to confirm the role of Ce(IV) species the mineralization of phenol was carried out with the partially oxidized Ce(III) by bubbling ozone during the first 60 min before adding phenol. The latter experiment was designed to check the destruction efficiency of the catalyzed ozonation process by the cerium redox pair which contains the active oxidant 100
TOC removal (%)
80
60
40
20
0
0
50
100
150
200
Time (min)
Fig. 4. Effect of various concentrations of nitric acid on percentage Ce(IV) formation ([Ce(III)], 50 mM; flow rate of ozone–air mixture, 100 ml/min; T, 25 ± 1C).
Fig. 5. Effect of added cerium(III) catalyst: (i) at zero time (m) (ii) after 60 min ozonation of Ce(III) (j) on the percentage TOC removal of phenol compared to (iii) uncatalyzed ozonation (d) ([Ce(III)], 50 mM; flow rate of ozone–air mixture, 100 ml/min; [HNO3], 1 M; T, 25 ± 1C; [phenol], 250 mg/lit.).
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
Ce(IV). The course of destruction was followed by TOC analysis. Fig. 5 shows the comparison of TOC removal efficiencies of phenol by non-catalytic and catalytic ozonation processes. From the figure it can be observed that at 120 min the uncatalyzed ozonation gave 54% TOC removal whereas Ce(III) catalyzed ozonation yielded 71%. But the efficiency of cerium redox pair catalyzed ozonation was found to be 79%. A near complete mineralization was seen for the redox pair catalyzed destruction process at 180 min which contained 7% Ce(IV) and 93% Ce(III). Therefore, it may be concluded that the cerium redox pair catalyzed ozonation process was found to be effective for organic mineralization and the synergetic effect of ozone and cerium redox pair on the TOC removal was revealed. As the reduced cerium was reoxidized continuously, ozone was effectively utilized for both cerium oxidation as well as organic destruction. Although extensive experimental investigations have been carried out and some hypothesis were proposed, the interaction between ozone molecules and metal ions needs to be explored from a mechanistic point of view and also the kinetic treatment needs a thorough investigation. Further experiments are being carried out to optimize the cerium redox couple catalyzed ozonation process with continuous organic addition. 4. Conclusions The oxidation of cerium(III) to cerium(IV) by ozone was achieved in good conversion yields in nitric acid solutions. The conversion of Ce(III) was found to be increased with increasing ozone–air flow rate, nitric acid concentration but decreased with increasing initial Ce(III) concentration. The efficiency of this hybrid system containing cerium redox couple and ozone was tested for the mineralization of phenol. The mineralization efficiency of this hybrid system was to be good compared to the Ce(III) catalyzed and uncatalyzed ozonation processes. The main advantage of this system is the regeneration of Ce(IV) oxidant by ozone at room temperature in nitric acid medium. This method could be used as a simple one pot oxidation or mineralization of organics at room temperature. Acknowledgements This work performed in the framework of the project by the Ministry of Commerce, Industry and Energy (MOCIE)
1501
through Regional Innovation Centre (RIC). Research performed as part of the Core Environmental Technology Development Project for Next Generation (Eco-Technopia-21) of Korea Institute of Environmental Science and Technology (KIEST). Research partially supported by the Korea Research Foundation Grant funded by MOEHRD (Ref. No.: KRF-2005-210-D00028), Republic of Korea. The financial help from the above funding agencies is gratefully acknowledged here. One of the authors (Dr.S.B.) thanks the management of Sri Chandrasekharendra Saraswathi Viswa Maha Vidyalaya (Deemed University), Kanchipuram, India for granting research leave. References [1] J.P. Caire, F. Laurent, S. Cullie, F. Dalard, J.M. Fulconis, H. Delagrange, J. Appl. Electrochem. 33 (2003) 703. [2] K.H. Barbara, Z. Maria, N. Jacek, Appl. Catal. B 46 (2003) 639. [3] B. Legube, N. Karpel Vel Leitner, Catal. Today 53 (1999) 61. [4] C.G. Hewes, R.R. Davinson, Water AIChE Symp. Ser. 69 (1972) 71. [5] M.S.E. Abdo, H. Shaban, M.S.H. Bader, J. Environ. Sci. Health A23 (1988) 697. [6] R. Gracia, J.L. Aragues, J.L. Ovelleiro, Ozone Sci. Eng. 18 (1996) 195. [7] R. Andreozzi, A. Insola, V. Caprio, M.G. D’Amore, Water Res. 26 (1992) 917. [8] T.T. Suwa, N. Kuribayashi, E. Tachikawa, Corros. Eng. 37 (1988) 73. [9] Y. Asano, M. Kataoka, Y. Ikeda, S. Hasegawa, Y. Takashima, H. Tomiyasu, Prog. Nucl. Energ. 29 (1995) 243. [10] F. Delanoe, B. Acedo, N. Karpel Vel Leitner, B. Legube, Appl. Catal. B-Environ. 29 (2001) 315. [11] H. Fu, N. Karpel Vel Leitner, B. Legube, New J. Chem. 26 (2002) 1662. [12] A.R. Freshour, S. Mawhinney, D. Bhattacharyya, Water Res. 30 (1996) 1949. [13] C.H. Ni, J.N. Chen, Water Sci. Technol. 43 (2001) 213. [14] S. Balaji, S.J. Chung, T. Ramesh, I.S. Moon, Chem. Eng. J. 126 (2007) 51–57. [15] J.P. Caire, F. Laurent, S. Cullie, F. Dalard, J.M. Fulconis, H. Delagrange, J. Appl. Electrochem. 33 (2003) 709. [16] T. Tzedakis, A. Savall, J. Appl. Electrochem. 27 (1997) 589. [17] A.V. Levanov, I.V. Kuskov, K.B. Koiaidarova, A.V. Zosimov, E.E. Antipenko, V.V. Lunin, Kinet. Catal. 46 (2005) 138. [18] E. Barron, M. Deborde, S. Rabouan, P. Mazellier, Water Res. 40 (2006) 2181–2189. [19] L. Sanchez, X. Domenech, J. Casado, J. Peral, Chemosphere 50 (2003) 1085–1093. [20] Y. Wei, B. Fang, T. Arai, M. Kumagai, J. Appl. Electrochem. 35 (2005) 561–566. [21] M. Matheswaran, S. Balaji, S.J. Chung, I.S. Moon, J. Ind. Eng. Chem. (2007), in press.
Studies on cerium oxidation in catalytic ozonation process: A novel approach for organic mineralization Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, Il Shik Moon
*
Department of Chemical Engineering, Sunchon National University, 315 Maegok dong, Suncheon 540-742, Jeonnam, Republic of Korea Received 8 September 2006; received in revised form 19 December 2006; accepted 19 December 2006 Available online 23 December 2006
Abstract A novel regenerative catalytic system has been developed using cerium and ozone in nitric acid medium. It was found that cerium(III) was oxidized to cerium(IV) by ozone in nitric acid medium with good conversion yields. The conversion rate of Ce(III) was measured under various parameters viz. ozone–air flow rate, initial concentration of Ce(III), and concentration of nitric acid at 25 C. It was found that the conversion of Ce(III) increased with increasing ozone flow rate and concentration of nitric acid while decreased with increasing Ce(III) concentration. The pseudo first order kinetic constants were evaluated for Ce(III) oxidation. The efficiency of this hybrid system comprising of ozone and cerium redox pair towards organic mineralization was evaluated taking phenol as the model organic pollutant and compared with Ce(III) catalyzed and uncatalyzed ozonation processes. The presence of Ce(III) catalyst increased the destruction efficiency of phenol compared to uncatalyzed ozonation whereas a synergetic effect was observed between the cerium redox pair (Ce(III) and Ce(IV)) and ozone towards phenol mineralization and a maximum TOC removal was obtained in the latter case. Kinetic interpretations have been made with some simplifying assumptions owing to the much complex nature of ozone and metal ion interactions. This hybrid catalytic ozonation process may find its suitability for continuous organic destruction at room temperature. 2007 Elsevier B.V. All rights reserved. Keywords: Ozone; Cerium catalyst; Catalytic ozonation; Phenol mineralization
1. Introduction Ozone is capable of oxidizing organic and inorganic compounds due to its high reduction potential (E0 = 2.07 V vs SHE) [1]. The treatment of water and waste water by ozone for organic pollutant destruction is a well established research area. But ozone alone has been shown to achieve a very limited mineralization of organic compounds or removal of refractory pollutants particularly in acidic solutions. Recently, research focus has been laid on the catalytic ozonation processes with metal ions or metal oxides acting as catalysts in order to enhance the reactivity of ozone at lower pH conditions. Consequently, various ozonation processes have been investigated as efficient methods for degrading organics [2]. Ozonation pro*
Corresponding author. Tel./fax: +82 61 750 3581. E-mail address: [email protected] (I.S. Moon).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.12.017
cesses catalyzed by transition metals have been classified in to activation of ozone by metals in solution and heterogeneous catalytic ozonation in the presence of metal oxides or metals on supports [3]. The activation of ozone by Fe(II), Mn(II), Ni(II), or Co(II) as their sulfates was reported by Hewes et al. [4]. The presence of these metals increases the TOC removal efficiency as compared to ozonation alone and was reported by many authors as follows. Zinc or copper sulfate, silver nitrate, and chromium trioxide catalyzed ozonation was reported by Abdo et al. [5] for the treatment of dye effluents. Gracia et al. [6] showed that Mn, and Ag catalyzed the ozonation of humic acid substances in water. Andreozzi et al. [7] found that Mn(II) accelerates the oxidation of oxalic acid by ozone under acidic conditions. It was reported in the literature [8,9] that apart from transition metals some inner transition metals like cerium could also be used as an effective oxidation catalyst in an acidic
1498
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
medium for organic removal and decontamination applications by ozone. Delanoe et al. [10] and Fu et al. [11] have reported the use of ceria as the supporting catalyst for the oxidation of carboxylic acids and chlorinated carboxylic acids by the ozonation process. Freshour et al. [12] observed that the ozone decomposition rate strongly depends on pH and increases with increasing pH value. Generally the TOC removal efficiency of uncatalyzed ozonation was found to be less at lower pH conditions and Ni et al. [13] observed that for the organic removal by ozonation at an acidic pH, the TOC removal rate could be increased with Al2O3 catalyst. Although cobalt and iron could well be used for catalyzing the ozonation processes they possess some disadvantages. The stability of cobalt(III) is less due to water oxidation while the oxidizing ability of iron towards organic compounds is well below that of cerium. Considering these properties of other metal ions, cerium is chosen which possesses good oxidizing ability and also not precipitated as chloride with chlorine containing organic compounds. Ce(IV) in nitric acid medium (E 0 = 1.72 V vs SHE) [1] is a powerful non-selective oxidant and was used extensively for organic mineralization reactions [14]. This combined cerium and ozone system is slightly different from catalytic ozonation process using transition metals. In catalytic ozonation processes a small quantity of a metal in the order of 106–103 M, is usually used to activate the ozone decomposition to produce more free radicals. But in the present study the amount of the metal ion catalyst employed is fairly high compared to the previous studies with other metal ions and also the oxidized form of cerium is a powerful oxidant in acid medium towards organic compounds. Therefore this combined effect of ozone, Ce(III) and Ce(IV) should have more pronounced effect for organic mineralization and forms an interesting system of applied research for organic removal and decontamination applications. Carie et al. [15] have reported the decontamination process of stainless steel by Ce(IV) in 4 M nitric acid in which the reduced Ce(IV) is reoxidized by ozone. The electrochemical production of Ce(IV) oxidant using an electrochemical cell could also be used for organic decontamination and destruction studies but restricted due to the low coulombic efficiency at room temperature for lower metal ion concentrations, formation of secondary pollutants such as nitrogen oxide gases in the cathodic compartment and also poisoning of the electrodes by undestructed organics during the regeneration of mediator metal ion [16]. Usually, in metal ion catalyzed ozonation systems the concentration of metal ions employed is fairly low but for metal ions with high redox potentials such as cerium a higher level of concentration produces a highly active redox couple which may increase the organic destruction efficiencies. Therefore, to have better understanding of the cerium catalyzed ozonation process it is necessary to optimize the system with respect to the metal ion concen-
tration, ozone flow rate, and acid concentration. As far as we know, no study on the oxidation of Ce(III) by ozone was published in the literature. The objectives of the present investigation were (i) to study the oxidation of Ce(III) by ozone in nitric acid medium under various conditions such as concentration of Ce(III), concentration of nitric acid, ozone–air flow rate at 25 C; (ii) to check the efficiency of this hybrid catalytic ozonation system for phenol destruction and to compare with uncatalyzed ozonation under the same conditions. 2. Experimental section 2.1. Materials Cerium(III) nitrate hexahydrate obtained from Aldrich (99%), nitric acid (60%) and phenol (extra pure) from Dae Jung Company, Korea, were used as received. The water used was doubly distilled and deionized. 2.2. Ozone generation The generation of ozone was accomplished by coronadischarge system (Ozone Tech Co. Ltd., Model OG-TY, South Korea). A dry gas of pure oxygen from a cylinder (2 l/min) was subjected to an electrical discharge in the ozonizer. The oxygen molecules in the gas were dissociated to form ozone. Power was supplied in the form of high alternating current voltage. 2.3. Ce(III) oxidation in nitric acid by ozone The oxidation of Ce(III) in nitric acid was performed in a batch reactor by bubbling ozone through a ceramic diffuser at 25 ± 1 C. In all experiments the flow rate of ozonized oxygen was 100 ml/min (unless otherwise specified) and the concentration of ozone at the reactor inlet was equal to 0.13 g l1. Before starting the experiment, 1000 ml of Ce(III) in nitric acid solution was placed in the reactor and ozone was continuously passed. The aliquots were periodically collected and titrated for Ce(IV) quantifications by potentiometry. In all cases ozonation was continued up to 180 min. The Ce(III) conversion was computed as the percentage ratio of the concentration of Ce(IV) formed to the initial Ce(III) in Molar units. The rate of Ce(III) conversion was obtained using the following simple pseudo first order equation assuming that the concentrations of ozone and nitric acid remain unchanged during the reaction [17,18]. Rate ¼ k obs ½CeðIIIÞ 2.4. Mineralization of phenol The phenol destruction experiments were conducted in a batch reactor by bubbling ozone through a ceramic diffuser (flow rate 100 ml/min) without and with cerium catalyst at
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
25 ± 1 C. The volume of the solution taken was 1000 ml in all the experiments. In the case of cerium catalyzed phenol destruction, the destruction studies were carried out with Ce(III) (50 mM) and phenol (2.7 mM) both added together at zero time along with ozonation and also to verify the role of Ce(IV) an experiment was conducted in which Ce(III) was first partially oxidized to Ce(IV) by passing ozone up to 60 min (7% Ce(IV) was produced in 60 min) and then phenol solution was added with ozonation continued throughout the reaction. The liquid phase organic removal was monitored by total organic carbon analyses (TOC analyzer, Shimadzu, 5000 A).
1499
mum yield was reached around 3 h. At the ozone–air flow rate of 100 ml/min the maximum conversion of 33% Ce(IV) was obtained where as at 1000 ml/min flow rate, 95% yield was obtained. Under the experimental conditions of large excess of ozone and hydrogen ion concentrations the pseudo first order equation can be applied for Ce(III) oxidation. When the pseudo first order rate constant values (kobs) were plotted against various ozone–air flow rates a good linearity was observed (Fig. 2), suggesting that the reaction is of first order with respect to ozone. 3.2. Effect of initial Ce(III) concentration on its oxidation
3. Results and discussion 3.1. Effect of ozone–air mixture flow rate on Ce(III) oxidation The mechanism of ozone with reduced metal ions though not well understood so far [3], the overall reaction of Ce(III) with molecular ozone can be represented as follows [1]; 2Ce(III) + O3 + 2Hþ ! 2Ce(IV) + O2 + H2 O The percentage conversion with time for 50 mM Ce(III) in 1 M nitric acid at various flow rates of ozone–air mixture (100, 250, 500 and 1000 ml/min) is shown in Fig. 1. As expected, the conversion of Ce(III) increased with increasing ozone flow rates because of the increased dissolved ozone concentration in the liquid phase. It can be observed that the rate of Ce(III) conversion increased linearly with respect to time at low ozone–air flow rates compared to high flow rates. In all the cases the conversion rate of Ce(III) deviated from linearity after 50 min and the maxi-
The effect of initial metal ion concentration on the oxidation rate (Fig. 3) was studied for the Ce(III) concentrations viz. 10, 50 and 100 mM in 1 M nitric acid with 100 ml/min ozone–air flow rate. It is seen from the Fig. 3 that at the concentration of 10 mM Ce(III) a maximum of 70% conversion was obtained at 180 min. But for concentrations of 50 and 100 mM Ce(III), the final conversion yields were decreased to 24% and 15%, respectively. This decrease in the oxidation rate may be due to the increased ozone decomposition and decreased liquid phase ozone concentration at higher concentrations of metal ions. So the concentration of metal ion catalyst employed plays an important role in determining the reactivity of ozone in solution. The pseudo first order rate constants calculated for Ce(III) oxidation were found to be 1.0 · 103, 1.5 · 103 and 6.8 · 103 min1, respectively, for 100, 50 and 10 mM of Ce(III), respectively. Sanchez et al. [19] have reported that in Fe(II) catalyzed ozonation of phenol, the mineralization efficiency decreased at higher concentrations of metal ion with a maximum reaction rate obtained
100
0.02
y = 2E-05x R2 = 0.9918
0.016
60
0.012
k obs
Conversion of Ce(III) (%)
80
40
0.008
20
0.004 100 ml/min 500 ml/min
250 ml/min 1000 ml/min
0 0
50
100 Time (min)
150
200
Fig. 1. Effect of various ozone–air mixture flow rates on the percentage of Ce(IV) formation ([Ce(III)], 50 mM; [HNO3], 1 M; T, 25 ± 1C).
0 0
300
600
900
1200
Ozone - air mixture flow rate (ml/min) Fig. 2. Effect of various ozone–air mixture flow rates on the pseudo first order rate constants ([Ce(III)], 50 mM; [HNO3], 1 M; T, 25 ± 1C).
1500
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
observation of increasing oxidation rate with increasing nitric acid concentration was noted by Wei et al. [20,21] for the electrochemical oxidation of Ce(III) in nitric acid. They explained that high proton concentration promotes the electron transfer of the Ce(III)/Ce(IV) couple in nitric acid media. Also, Asano et al. [9] have found that the dissolution of UO2 using ozone and 0.1 M Ce(IV) in nitric acid solutions increased with the nitric acid concentration from 0.5 to 3 M, but further increase from 3 to 5 M resulted only is small increase in dissolution. This increased dissolution of UO2 found by Asano et al. could be explained as due to the faster regeneration rate of reduced Ce(IV) oxidant at 3 M compared to 0.5 M, based on our results.
Fig. 3. Percentage formation of Ce(IV) with respect to time at different initial concentrations of Ce(III) (flow rate of ozone–air mixture, 100 ml/ min; [HNO3], 1 M; T, 25 ± 1C).
for 4 mM of Fe(II). This observation is in line with the results obtained in this study that increasing the cerium ion concentration decreases the conversion yield since the metal ion concentration used is in the higher range. 3.3. Effect of nitric acid concentration on Ce(III) oxidation The effect of nitric acid concentration on the rate of Ce(III) oxidation (Fig. 4) was studied under the conditions of 50 mM Ce(III) in 0.3, 1, 3 and 4 M nitric acid solutions with 100 ml/min of ozone–air flow rate. It is seen from Fig. 4 that the conversion of Ce(III) increased with nitric acid concentration in well defined jumps. This may be due to the facile oxidation of metal ions at high proton concentrations. Also, the higher acid concentrations (lower pH) decrease the decomposition rate [2] and increase the life time of ozone and hence higher conversions. A similar
3.4. Comparison between uncatalyzed and catalyzed ozonation processes for phenol mineralization To check the efficiency of cerium catalyzed ozonation process over the uncatalyzed ozonation, the destruction of 250 mg/l (2.7 mM) phenol was carried out at 25 ± 1C. The uncatalyzed ozonation was carried out taking 1000 ml of 2.7 mM phenol solution and passing ozone continuously. In the case of Ce(III) catalyzed ozonation process the solution containing 2.7 mM phenol and 50 mM Ce(III) in 3 M nitric acid was fed in to the reactor and ozone was bubbled continuously. In order to confirm the role of Ce(IV) species the mineralization of phenol was carried out with the partially oxidized Ce(III) by bubbling ozone during the first 60 min before adding phenol. The latter experiment was designed to check the destruction efficiency of the catalyzed ozonation process by the cerium redox pair which contains the active oxidant 100
TOC removal (%)
80
60
40
20
0
0
50
100
150
200
Time (min)
Fig. 4. Effect of various concentrations of nitric acid on percentage Ce(IV) formation ([Ce(III)], 50 mM; flow rate of ozone–air mixture, 100 ml/min; T, 25 ± 1C).
Fig. 5. Effect of added cerium(III) catalyst: (i) at zero time (m) (ii) after 60 min ozonation of Ce(III) (j) on the percentage TOC removal of phenol compared to (iii) uncatalyzed ozonation (d) ([Ce(III)], 50 mM; flow rate of ozone–air mixture, 100 ml/min; [HNO3], 1 M; T, 25 ± 1C; [phenol], 250 mg/lit.).
M. Matheswaran et al. / Catalysis Communications 8 (2007) 1497–1501
Ce(IV). The course of destruction was followed by TOC analysis. Fig. 5 shows the comparison of TOC removal efficiencies of phenol by non-catalytic and catalytic ozonation processes. From the figure it can be observed that at 120 min the uncatalyzed ozonation gave 54% TOC removal whereas Ce(III) catalyzed ozonation yielded 71%. But the efficiency of cerium redox pair catalyzed ozonation was found to be 79%. A near complete mineralization was seen for the redox pair catalyzed destruction process at 180 min which contained 7% Ce(IV) and 93% Ce(III). Therefore, it may be concluded that the cerium redox pair catalyzed ozonation process was found to be effective for organic mineralization and the synergetic effect of ozone and cerium redox pair on the TOC removal was revealed. As the reduced cerium was reoxidized continuously, ozone was effectively utilized for both cerium oxidation as well as organic destruction. Although extensive experimental investigations have been carried out and some hypothesis were proposed, the interaction between ozone molecules and metal ions needs to be explored from a mechanistic point of view and also the kinetic treatment needs a thorough investigation. Further experiments are being carried out to optimize the cerium redox couple catalyzed ozonation process with continuous organic addition. 4. Conclusions The oxidation of cerium(III) to cerium(IV) by ozone was achieved in good conversion yields in nitric acid solutions. The conversion of Ce(III) was found to be increased with increasing ozone–air flow rate, nitric acid concentration but decreased with increasing initial Ce(III) concentration. The efficiency of this hybrid system containing cerium redox couple and ozone was tested for the mineralization of phenol. The mineralization efficiency of this hybrid system was to be good compared to the Ce(III) catalyzed and uncatalyzed ozonation processes. The main advantage of this system is the regeneration of Ce(IV) oxidant by ozone at room temperature in nitric acid medium. This method could be used as a simple one pot oxidation or mineralization of organics at room temperature. Acknowledgements This work performed in the framework of the project by the Ministry of Commerce, Industry and Energy (MOCIE)
1501
through Regional Innovation Centre (RIC). Research performed as part of the Core Environmental Technology Development Project for Next Generation (Eco-Technopia-21) of Korea Institute of Environmental Science and Technology (KIEST). Research partially supported by the Korea Research Foundation Grant funded by MOEHRD (Ref. No.: KRF-2005-210-D00028), Republic of Korea. The financial help from the above funding agencies is gratefully acknowledged here. One of the authors (Dr.S.B.) thanks the management of Sri Chandrasekharendra Saraswathi Viswa Maha Vidyalaya (Deemed University), Kanchipuram, India for granting research leave. References [1] J.P. Caire, F. Laurent, S. Cullie, F. Dalard, J.M. Fulconis, H. Delagrange, J. Appl. Electrochem. 33 (2003) 703. [2] K.H. Barbara, Z. Maria, N. Jacek, Appl. Catal. B 46 (2003) 639. [3] B. Legube, N. Karpel Vel Leitner, Catal. Today 53 (1999) 61. [4] C.G. Hewes, R.R. Davinson, Water AIChE Symp. Ser. 69 (1972) 71. [5] M.S.E. Abdo, H. Shaban, M.S.H. Bader, J. Environ. Sci. Health A23 (1988) 697. [6] R. Gracia, J.L. Aragues, J.L. Ovelleiro, Ozone Sci. Eng. 18 (1996) 195. [7] R. Andreozzi, A. Insola, V. Caprio, M.G. D’Amore, Water Res. 26 (1992) 917. [8] T.T. Suwa, N. Kuribayashi, E. Tachikawa, Corros. Eng. 37 (1988) 73. [9] Y. Asano, M. Kataoka, Y. Ikeda, S. Hasegawa, Y. Takashima, H. Tomiyasu, Prog. Nucl. Energ. 29 (1995) 243. [10] F. Delanoe, B. Acedo, N. Karpel Vel Leitner, B. Legube, Appl. Catal. B-Environ. 29 (2001) 315. [11] H. Fu, N. Karpel Vel Leitner, B. Legube, New J. Chem. 26 (2002) 1662. [12] A.R. Freshour, S. Mawhinney, D. Bhattacharyya, Water Res. 30 (1996) 1949. [13] C.H. Ni, J.N. Chen, Water Sci. Technol. 43 (2001) 213. [14] S. Balaji, S.J. Chung, T. Ramesh, I.S. Moon, Chem. Eng. J. 126 (2007) 51–57. [15] J.P. Caire, F. Laurent, S. Cullie, F. Dalard, J.M. Fulconis, H. Delagrange, J. Appl. Electrochem. 33 (2003) 709. [16] T. Tzedakis, A. Savall, J. Appl. Electrochem. 27 (1997) 589. [17] A.V. Levanov, I.V. Kuskov, K.B. Koiaidarova, A.V. Zosimov, E.E. Antipenko, V.V. Lunin, Kinet. Catal. 46 (2005) 138. [18] E. Barron, M. Deborde, S. Rabouan, P. Mazellier, Water Res. 40 (2006) 2181–2189. [19] L. Sanchez, X. Domenech, J. Casado, J. Peral, Chemosphere 50 (2003) 1085–1093. [20] Y. Wei, B. Fang, T. Arai, M. Kumagai, J. Appl. Electrochem. 35 (2005) 561–566. [21] M. Matheswaran, S. Balaji, S.J. Chung, I.S. Moon, J. Ind. Eng. Chem. (2007), in press.