Silver-Mediated Electrochemical Oxidation: Production of Silver (II) in Nitric Acid Medium and in situ Destruction of Phenol in Semi-batch Process

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1 J. Ind. Eng. Chem., Vol. 13, No. 2, (2007) Silver-Mediated Electrochemical Oxidation: Production of Silver (II) in Nitric Acid Medium and in situ Destruction of Phenol in Semi-batch Process Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, and Il Shik Moon Department of Chemical Engineering, Sunchon National University, Chonnam , Korea Received August 14, 2006; Accepted November 30, 2006 Abstract: The mediated electrochemical oxidation was studied with Ag (II) as the mediator ion in nitric acid medium. The oxidation of Ag (I) was performed in an electrochemical cell under various conditions, including varying the temperature and the concentrations of nitric acid and mediator ions in a batch-type electrochemical reactor in recirculation mode. The formation of Ag (II) increased upon increasing the concentration of nitric acid, but it decreased at higher temperatures. The percentage of conversion decreased upon increaseing the concentration of Ag (I). The destruction of phenol was performed in batch and continuous organic feeding modes. A maximum destruction efficiency of 88.8 % (based on CO 2 ) was achieved in the batch process. The destruction efficiency during the continuous organic feeding increased upon increasing the temperature and initial concentration of Ag (I) up to 0.5 M. The destruction was also tested in the long run for 2 h; the steady state destruction efficiency was 75 %, based on CO 2 production. Keywords: mediated electrochemical oxidation, silver (I), destruction efficiency, phenol, organic destruction, carbon dioxide Introduction 1) The mediated electrochemical oxidation (MEO) process is an emerging new technology for the destruction of organic waste materials. In this process, the hazardous or mixed organic waste streams can be destructed at ambient temperatures (less than 100 o C) and atmospheric pressure. In the MEO process, the mediator oxidant is regenerated continuously by an electrochemical cell to minimize the production of secondary wastes [1-3]. Steel and coworkers [4] reported that mediated electrochemical oxidation might provide a new alternative for the safe disposal of chemical wastes. Mediated electrochemical oxidation is a cyclic process involving the electrochemical regeneration of mediator ions in the electrochemical cell and subsequent destruction of organic materials by the produced metal ion oxidant. The general reactions in the cell and in the reactor for organic destruction can be represented as follows: To whom all correspondence should be addressed. ( ismoon@sunchon.ac.kr) In the electrochemical cell, Red Ox + ne In the reactor during the organic destruction, Ox + S P + Red where Red represents the mediator in the reduced state, Ox represents the mediators in the oxidized state, S represents the reactants, and P is the product of the reaction [4]. Commonly used mediator redox pairs in MEO processes are Ag 2+ /Ag +, Mn 3+ /Mn 2+, Co 3+ /Co 2+, and Ce 4+ / Ce 3+. Among these systems, silver is the most powerful oxidizing agent in acid media owing to its high redox potential (1.98 V). It may react directly with the organic material to be destroyed or it may first react with water to form hydroxyl radicals that in turn oxidize the organic compounds. Sulfuric acid, nitric acid, or perchloric acid is generally employed as the electrolyte for the electrogeneration of silver [5-13]. Sulfuric acid possesses low solubility toward metal ions, perchloric acid is explosive, and hydro-

2 232 Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, and Il Shik Moon Figure 1. Schematic diagram of the MEO system. chloric acid is unstable due to the oxidation of Cl - to Cl 2 [14]. Considering these working difficulties, nitric acid can be considered as the best choice in terms of its high solubilizing capacity, stability, and low viscosity. The potential of the Ag (II)/Ag (I) couple in a nitric acid medium lies between 1.91 and 1.98 V versus a standard hydrogen electrode. Some researchers have studied the kinetics of the oxidation of Ag (I) in nitric acid media. Fleishmann and coworkers [15] reported the oxidation kinetics of Ag (I) in 3 M nitric acid with a platinum electrode. However, a systematic study of the kinetics of Ag (I) oxidation at various nitric acid concentrations and its instantaneous usage towards phenol destruction has not been reported so far. The objectives of the present study were to investigate the generation of Ag (II) in a batch-type electrochemical reactor in recirculation mode with various operating parameters, including the temperature and the concentrations of the mediator and nitric acid, under the constant potential mode, and to destruct phenol in batch and continuous feeding modes under the optimum conditions of Ag (II) production. Experimental Section Figure 1 shows a schematic diagram of the mediated electrochemical oxidation process. The electrochemical reactor of flow type consisted of two compartments separated by a Nafion 324 membrane. The membrane is capable of allowing protons to move from the anodic to cathodic compartment to maintain the current flow. IrO 2 - Coated titanium and titanium mesh-type DSA electrodes, having an area of 4 cm 2 (2 cm 2 cm 0.5 cm), were used as the anode and cathode, respectively. A 50-mL solution of Ag (I) in nitric acid (0.5 M in 8 M) and nitric acid (12 M) were placed in the anolyte and catholyte reservoirs, respectively. The electrolyte solution was continuously circulated through the anolyte and catholyte compartments of the reactor at a constant flow rate. A constant potential was applied between the electrodes during the oxidation reaction. In both the anolyte and catholyte reservoirs, the temperature was maintained constant throughout the reaction using thermostated mantles. During the experiment, Ag (I) was oxidized to Ag (II) at the anode and nitric acid was reduced into nitrous acid at the cathode. In the catholyte reservoir, NO produced during the reduction of nitric acid was reoxidized with continuous passage of atmospheric oxygen into NO 2, and dissolved in water to reform nitric acid. During the electrochemical oxidation, the concentration of Ag (II) in solution was determined rapidly and conveniently using a method reported previously [16]. The procedure involved the quantitative oxidation of cerium (III) to cerium (IV) by Ag (II), followed by the photometric determination of the cerium (IV) concentration at a fixed nitric acid concentration. The anodic and cathodic reactions can be represented as follows [6]: At anode: Ag + Ag 2+ + e - Ag 2+ + NO 3 - AgNO 3 + At cathode: NO H + + 2e - HNO 2 + H 2 O After oxidation, Ag (II) is present as a dark-brown nitrate complex, AgNO 3 +, in the solution. After finding suitable and optimal operating conditions for silver oxidation, the destruction of phenol was investigated. The destruction was studied either in batch or in continuous feeding mode. In the case of continuous

3 Silver-Mediated Electrochemical Oxidation: Production of Silver (II) in Nitric Acid Medium and in situ Destruction of Phenol in Semi-batch Process 233 Figure 2. Effect of temperature on the formation of Ag (II). (Experimental conditions: [AgNO 3], 0.1 M; [HNO 3], 8 M; Temp., 25 o C; Cell Voltage, 2.5 V; q, 100 ml/min). organic feeding, phenol was added into the anolyte after the concentration of Ag (II) attained its steady state. The phenol solution (10,000 ppm) was constantly fed at a rate of 0.05 ml/min up to 30 or 120 min using a syringe pump. The phenol was oxidized into carbon dioxide and water and the mediator ion was reduced. C 6H 5OH + 28Ag(II) + 11H 2O 6CO Ag(I) + 28H + The reduced mediator ions were continuously oxidized in the cell. The CO 2 formed during the organic destruction reaction was removed by passing a carrier gas stream (500 ml/min of N 2 ) into the reservoir. The concentrations of CO 2 and CO produced were measured continuously (ppm) by using an Infrared CO 2 analyzer (Anagas CD 98, Environmental Instruments) and converted into volume at 25 o C. From the accumulated CO 2 and CO volumes, the destruction efficiency was calculated. Destruction Efficiency (% of CO 2 ) = (Expt. volume/theo. Volume) 100 Silver Oxidation Results and Discussion Effect of Temperature The effect of temperature on the electrochemical oxidation of Ag (I) was studied in the temperature range o C at fixed Ag (I) and nitric acid concentrations. Figure 2 shows the concentration of Ag (II) generated with respect to the reaction time at different tem- Figure 3. Effect of nitric acid concentration on Ag (II) generation. (Experimental conditions: [AgNO 3], 0.1 M; Temp., 25 o C; Cell Voltage, 2.5 V; q, 100 ml/min). peratures. As the temperature increased, the formation of Ag (II) decreased due to the increased reactivity of Ag (II) with water and corresponding decrease in the stability of Ag (II). Upon increasing the temperature, the nitrous acid production increased in the anolyte and, as a result, the acid concentration decreased in the anolyte. This reduced nitric acid concentration lowered the stability of the silver (II) complex and, hence, the silver (II) concentration also decreased. A similar trend was also observed by Sequeira and coworkers [1]. The reduction of Ag (II) by water is represented by 4AgNO H 2 O Ag + + O 2 + 4HNO 3 Effect of Nitric Acid The oxidation of Ag (I) was studied under different nitric acid concentrations at a fixed concentration of Ag (I) (0.1 M) at 25 o C. Figure 3 shows the concentration of Ag (II) produced with respect to the reaction time at different nitric acid concentrations. From the figure, we observe that the formation of Ag (II) increased upon increasing the nitric acid concentration, due to the higher stability of the [Ag(NO 3 )] + complex at higher acid concentrations [17]. The highest conversion, 45 % Ag (II), was obtained at 8 M nitric acid. From Figure 3 we also observe that the concentration of Ag (II) increased initially and attained a steady state near 30 min for a lower acid concentration (6 M); the same effect was observed at ca. 60 min at a high acid concentration (10 M nitric acid). Effect of Initial Ag (I) Concentration The oxidation of silver was studied under different initial concentrations of Ag (I) at a fixed concentration of nitric acid of 8 M at 25 o C. Figure 4 shows the increase

4 234 Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, and Il Shik Moon Figure 4. Effect of initial Ag (I) concentration on the formation of Ag (II). (Experimental conditions: [HNO 3], 8 M; Temp., 25 o C; Cell Voltage, 2.5 V; q, 100 ml/min). Figure 6. Effect of concentration of Ag (I) on the destruction efficiency of phenol in the continuous mode (Exper-imental conditions: Temp., 60 o C; [HNO 3], 8 M; Cell Voltage, 2.5 V; feed time, 30 min). Destruction of Phenol during Continuous Organic Feeding Figure 5. Effect of temperature on the destruction efficiency of phenol in the continuous feeding mode (Experimental conditions: [AgNO 3], 0.1 M; [HNO 3], 8 M; Cell Voltage, 2.5 V; feed time, 30 min). in the Ag (II) concentration with time at different initial concentrations of Ag (I). The percentage conversion of Ag (II) formation decreased upon increasing the concentration of Ag (I). At 0.1 M Ag (I), 45 % of Ag (II) was obtained at the steady state. In this case, the steady state concentration of Ag (II) was attained at ca. 30 min for almost all concentrations of silver (I), except for 1 M Ag (I). Short-term Feeding Effect of Temperature The destruction of phenol was undertaken at different temperatures with fixed concentrations of 0.1 M Ag (I) and 8 M nitric acid. The Ag (I) was oxidized into Ag (II). Initially, the Ag (II) concentration increased with increasing time and reached a steady state after 30 min. The steady state Ag (II) concentration was established when the rates of Ag (II) generation and loss were equivalent. At this point, the only loss of Ag (II) was due to the reduction by water. After the steady state was reached, the organic solution was continuously fed into the anolyte for 30 min. The organic fed to the anolyte was completely oxidized into carbon dioxide and water. Also, a minor quantity of carbon monoxide was detected during the destruction reaction. Figure 5 shows the destruction efficiency for various temperatures in the continuous feeding mode. The destruction efficiency was calculated based on the volume of CO 2 produced up to the end point of organic feeding (30 min). The destruction efficiency increased upon increasing the temperature (Figure 5). Effect of Silver (I) Concentration The effect of the silver (I) concentration on phenol destruction was studied at a fixed temperature of 60 o C with a fixed nitric acid concentration of 8 M. After reaching the steady state concentration of Ag (II), the destruction of phenol was performed in continuous mode. Figure 6

5 Silver-Mediated Electrochemical Oxidation: Production of Silver (II) in Nitric Acid Medium and in situ Destruction of Phenol in Semi-batch Process 235 Figure 7. (a) Phenol conversion to CO 2 and moles of CO 2. (b) Phenol conversion to CO and moles of CO produced during phenol destruction in continuous feeding mode (Experimental conditions: [AgNO 3], 0.1 M; Temp., 60 o C; [HNO 3], 8 M, Cell Voltage, 2.5 V; [Phenol], ppm). shows the destruction efficiency plotted against the initial concentration of Ag (I). The destruction efficiency increased upon increasing the concentration of Ag (I). However, the increase in the destruction efficiency was not significant over 0.5 M Ag (I). An increase in the Ag (II) concentration beyond 0.5 M did not result in increased destruction efficiencies. This finding coincides with the results reported by many authors for silver MEO systems [1,6,8,18], in which 0.5 M Ag (I) has been used for organic destruction to obtain maximum destruction efficiency. Long-Term Feeding Figure 7a shows the percentage conversion of CO 2 and moles of CO 2 produced during continuous feeding of phenol for 120 min at a constant flow rate of 0.05 ml/min at 60 o C. The steady state of CO 2 evolution was reached after 20 min. The steady state destruction efficiency of the process was 80 % based on CO 2 formation. The accumulated concentration of CO 2 increased linearly with respect to the reaction time. During the destruction of phenol, a small amount of CO was also observed. Figure 7b shows the percentage of CO produced and the moles of CO produced over time during phenol destruction. Destruction of Phenol in Batch-Type Organic Feeding In this case, both the phenol and silver solutions [with fixed concentrations of silver (I) and nitric acid] were mixed to obtain the desired concentrations of phenol (10,000 ppm) and silver (I) (0.1 M); this solution was then fed into the anolyte tank. A constant potential of 2.5 Figure 8. Phenol conversion to CO 2 plotted against time in terms of (a) percentage and concentration and (b) concentration of CO in batch mode of organic addition (Experimental conditions: [AgNO 3], 0.1 M; Temp., 60 o C; [HNO 3], 8 M; Cell Voltage, 2.5 V; [Phenol], ppm). V was applied between the electrodes throughout the experiment at 60 o C. Figure 8a shows the percentage conversion of CO 2 and the amount of CO 2 formed during the reaction time. The destruction efficiency of the process was observed to be 88 % based on CO 2 produced. Figure 8b shows the concentration of CO formed during the reaction. A maximum destruction of 80 % was obtained at ca. 30 min and then the extent of further destruction slowed down. At 60 min, the CO 2 production had almost ceased, and the increase in the destruction efficiency after 30 min was observed to be only 8 %. This typical pattern of an initial fast destruction and then slow kinetics is the inherent nature of MEO systems featuring the formation of slowly degrading intermediates [19,20]. It was reported by Balazs and coworkers [19,20] that in the silver MEO process, the coulombic efficiency [21] was ca. 80 % to obtain 95 % organic destruction, whereas to obtain 99 % destruction, the coulombic efficiency was observed to be only 40 %, indicating the slow destruction of remaining organics and the greater amount of energy required to achieve complete destruction. Conclusions The electrochemical oxidation of Ag (I) was studied with respect to various parameters, including the temperature and the concentrations of nitric acid and mediator in an electrochemical cell. The concentration of Ag (II) increased upon increasing the concentration of nitric acid and decreased upon increasing the temperature. The conversion ratio also decreased upon increasing the initial

6 236 Manickam Matheswaran, Subramanian Balaji, Sang Joon Chung, and Il Shik Moon concentration of Ag (I). The destruction of phenol was studied in batch and continuous feeding modes under the optimum oxidation conditons for Ag (II) production by the cell. The destruction efficiency in the batch mode was ca. 88 % (based on CO 2 evolution) and in the continuous mode it was 80 %. Long-term organic feeding experiments showed that the efficiency of the process, with large amounts of organic destruction and consistent Ag (II) production by the cell. Acknowledgments This study was supported by the Ministry of Commerce, Industry, and Energy (MOCIE), Republic of Korea, through the project of the Regional Invocation Center (RIC). References 1. C. A. C. Sequeira, D. M. F. Santos, and P. S. D. Brito, Appl. Surf. Sci., 252, 6093 (2006). 2. (a) I. S. Kim, S. W. Choi, C. D. Heo, and S. C. Park, J. Korean Ind. Eng. Chem., 13, 33 (2002). (b) C. W. Lee, J. Ind. Eng. Chem., 12, 967 (2006). 3. S. C. Park and I. S. Kim, J. Korean Ind. Eng. Chem., 16, 206 (2002). 4. D. F. Steele, D. Richardson, J. D. Campbell, D. R. Craig, and J. D. Quinn, Trans. IChemE., 68, 115 (1990). 5. R. M. Spotnitz, R. P. Kreh, J. T. Lundquist, and P. J. Press, J. Appl. Electrochem., 20, 209 (1990). 6. J. Bringmann, K. Ebert, U. Galla, and H. J. Schimider, J. Appl. Electochem., 25, 846 (1995). 7. V. Devadoss, M. Noel, K. Jayaraman, and C. Ahmed Basha, J. Appl. Electrochem., 33, 319 (2003). 8. J. C. Farmer, F. T. Wang, R. A. Hawley Fedder, P. R. Lewis, L. J. Summers, and L. Follies, J. Electrochem. Soc., 139, 654 (1992). 9. J. J. Jow and T. C. Chou, J. Appl. Electrochem., 18, 298 (1988). 10. A. T. Kuhn and T. H. Randle, J. Chem. Soc. Faraday Trans., 1, 403 (1985). 11. T. H. Randle and A. T. Kuhn, Electrochim. Acta., 31, 739 (1986). 12. M. P. Sah, J. Indian Chem. Soc., 72, 173 (1995). 13. T. Vijayabarathi, D. Velayutham, and M. Noel, J. Appl. Electrochem., 31, 976 (2001). 14. Y. Liu, X. Xia, and H. Liu, J. Power Sources, 130, 299 (2004). 15. M. Fleischmann, D. Pletcher, and A. Rafinski, J. Appl. Electrochem., 1, 1 (1971). 16. J. B. Kirwin, F. B. Peat, P. J. Proll, and L. H. Sutcliffe, J. Phys. Chem., 67, 1617 (1963). 17. A. A. Noyes, D. De Vault, C. D. Coryell, and T. J. Deahl, J. Am. Chem. Soc., 59, 1326 (1937). 18. Z. Chiba, B. Schumacher, P. Lewis, and L. Murguia, WM95 Symposia, Tucson, AZ, March 1 (1995). 19. S. Balaji, S. J. Chung, T. Ramesh, and I. S. Moon, Chem. Eng. J., 126, 51 (2007). 20. S. J. Chung, S. Balaji, M. Matheswaran, T. Ramesh, and I. S. Moon, Water Sci. Technol., 55, 261 (2007). 21. B. Balazs, Z. Chiba, P. Lewis, L. Murguia, and M. Adamson, Sixth International Confernece on Radioactive Waste Management & Environmental Remediation, Singapore, October (1997).

Electrochemical recovery of silver from waste aqueous Ag(I)/Ag(II) redox mediator solution used in mediated electro oxidation process

Korean J. Chem. Eng., 26(4), 1053-1057 (2009) DOI: 10.2478/s11814-009-0175-x RAPID COMMUNICATION Electrochemical recovery of silver from waste aqueous Ag(I)/Ag(II) redox mediator solution used in mediated