Recycling of Acidic and Alkaline Solutions by Electrodialysis in a

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Recycling of Acidic and Alkaline Solutions by Electrodialysis in a Treatment Process for Uranium Oxide Waste Using a Carbonate Solution with Hydrogen Peroxide Kwang-Wook Kim,* Jun-Taek Hyun, Keun-Young Lee, Eil-Hee Lee, Dong-Young Chung, and Jei-Kwon Moon Korea Atomic Energy Research Institute, Daedeok-daero 989-111, Yuseong-gu, Daejeon, 305-353, Republic of Korea ABSTRACT: In this work, an electrolytic system consisting of an electro-dialyzer and a gas-absorber was devised for recycling the carbonate salt solution produced in the process that uses a carbonate-hydrogen peroxide media for treating uranium-bearing wastes. The recovery characteristics of acidic and alkaline solutions in the electrodialysis system were investigated in a batch manner by changing several variables and then evaluated in continuous operation with an electro-dialyzers stacked system. When HNO3 and NaOH solutions from the effluent solution of NaNO3 after the treatment of uranium oxide waste were recovered by the electrodialysis system, the energy consumption required to treat the same amount of feed solution decreased with decreases in the applied current and in the applied voltage, even though it took a longer time. The electrodialysis yield to recover HNO3 and NaOH was proportional to the total electricity supplied in the system, regardless of whether the constant current or constant voltage operation mode was used.

1. INTRODUCTION A few carbonate-based processes to treat uranium mixture oxides, such as spent nuclear fuel, uranium scraps generated from nuclear fuel fabrication, and uranium-bearing waste, have recently been studied by many researchers because they are considered to have several advantages in terms of safety, economic competitiveness, and the generation of secondary wastes, compared with the conventional processes using acid media.1−8 We have introduced a process using a highly alkaline carbonate solution with H2O2, as shown in Figure 1, to treat impurity-contaminated uranium oxide, such as uranium fuel scraps.3,9,10 Uranium is selectively leached from the uranium mixture oxide in the form of a uranyl peroxo-carbonato complex ion of UO2(O2)x(CO3)y−z with a high solubility, and then the dissolved uranium ion is recovered as a precipitate of UO4 by acidifying the uranyl carbonato complex solution. The H2O2 supplies a ligand to form uranyl peroxo complex ions and acts as an oxidant for the dissolution of UO2.3,4,6−9 Most transient metals or their oxides are not dissolved in the carbonate solution at high pH because of their very low solubilities.3,4,6−9 If the carbonate salt used in such a process is not recycled, an enormous amount of carbonate salt will accumulate or be released to the environment, which weakens the advantages and the environmental friendliness of the carbonate-based processes. The most important characteristic of the process in Figure 1 is the complete recycling of all the reagents used in the process without any release of secondary waste solution to the environment. Accordingly, a way to recover the carbonate salt used in the process has to be devised without putting any chemicals into the process. When the uranyl peroxo-carbonato complex solution, which is generated during the uranium leaching step by the carbonate solution with hydrogen peroxide, is acidified by HNO3 to precipitate UO4 from the uranium solution, CO2 gas is released from the solution. The © 2012 American Chemical Society

released CO2 gas can be recovered again as a Na2CO3 solution using a CO2-absorber with a NaOH solution flowing down into it. Then, the recovered Na2CO3 solution can be recycled into the uranium leaching step. The HNO3 and NaOH solutions used in the steps for UO4 precipitation and CO2 absorption can be recovered by an electrodialysis cell from the supernatant solution consisting of Na+ ions and NO3− ions after the UO4 precipitation. For verification of the recycling of all of the inorganic solutions used in the process, it is necessary to experimentally confirm the UO4 precipitation and the recovery of Na2CO3 with an electrodialysis system. In this work, an electrolytic system consisting of an electrodialyzer and a gas-absorber was devised for recycling the carbonate salt solution in the process using a carbonatehydrogen peroxide medium for treating uranium-bearing wastes, and the recovery characteristics of the acidic and alkaline solutions in the electrodialysis system were investigated by varying several variables in a batch setup and were then evaluated in continuous operation with an electrodialyzersstacked system.

2. EXPERIMENTAL SECTION It is necessary to design a system to embody the functions described in Figure 1 to treat uranium oxide waste in an environmentally friendly manner without releasing a secondary waste solution. The system shown in Figure 2 was devised to precipitate UO4 and to simultaneously recover the carbonate salt solution to be recycled to the uranium oxide leaching step with a gas absorber and an electrodialysis cell. An acidic Received: Revised: Accepted: Published: 6275

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Figure 1. A process to recover pure uranium from contaminated uranium scrap generated nuclear fuel fabrication by using oxidative leaching of uranium oxide and UO4 precipitation in a carbonate solution with hydrogen peroxide.

mL). All the parts of cell were made of polyethylene. Silicon gaskets of 0.5 mm were placed between the frames and ion exchange membranes. The ion exchange membranes were used after storing in demineralized water as received without any pretreatment. For the anodes of the cell, IrO2-coated Ti electrodes with the dimensions of 2 × 4 × 0.1 cm3 and with a mesh form of 6 mm × 3 mm were used. For the cathodes of the cell, Ti electrodes of the same dimensions and form as the anodes were used. The IrO2 electrode was prepared by coating multi-iridium oxide layers onto a Ti mesh in the same way as that used in our previous work.11 The supernatant solution of NaNO3 remained after the precipitation of UO4 was fed in between the anion and cation exchange membranes, as shown in Figure 2. For batch experiments with the electrodialysis cell, a cathodic solution with an initial concentration of 0.1 M NaOH, an anodic solution with an initial concentration of 0.1 M HNO3, and a feed solution of 0.5 M NaNO3 were circulated through the 250 mL volumetric flask reservoirs by peristaltic pumps (Ismatec, Reglo). The volume of each circulating solution was 100 mL. For the continuous electrodialysis experiments, an electrodialysis cells-stacked electrolyzer with three cells was used, where the feed, cathodic, and anodic solutions independently flowed in series through their compartments of the electrolyzer by the pumps. The volume of each cell and the electrodes of the stack were the same as those used in the batch experiment. To control the experimental temperature in the batch and continuous experiments, the electrodialysis cell was placed in a tray filled with water that was circulated into a chiller set at 25 ± 1 °C. Constant voltage or constant current were supplied to the electrodialysis cell by a potentiostat/galvanostat (Wonatec, WMPG1000). In all of the experiments with the electrodialysis cells, the pH value of each anolyte, catholyte, and feed solution was recorded online by a pH-meter and a data-logger (Almemo 5990-2) controlled by a computer. During the electrodialysis experiments, each electrolyte solution was sampled at regular intervals to analyze the concentrations of NO3− and OH− or Na+ ions with an autotitrator or ion chromatography (Dionex ICS 90). The nitrate concentration was analyzed by a TOC-TN analyzer (Shimadzu TOC-V CSH/TNM-1). The concentration of uranium was analyzed by a colorimetric method using Arsenazo III.12 All of the chemicals used in this work were reagent grade, and they were dissolved, as received, in demineralized water of 18.2 MΩ prepared by double distillation and an ion-exchanger (Milli-Q plus).

Figure 2. Schematic diagram of an apparatus used for recoveries of uranium and carbonate salt solution by using an electro-dialysis cell and a gas-absorber.

solution and an alkaline solution are necessary for the acidification of uranyl peroxo-carbonato complex to precipitate UO4 and the recovery of carbonate salt solution, respectively. The acidic and alkaline solutions are prepared from the supernatant solution after the UO4 precipitation by an electrodialysis cell. The functions of the uranium precipitation and the recoveries of the carbonate, acidic, and alkaline solutions shown in Figure 2 are interconnected. The uranyl peroxo carbonato complex ion solution of UO2(O2)x(CO3)y2−2x−2y used in this work was prepared by dissolving UO2 powder in a carbonate solution of 0.25 M Na2CO3 with 1 M H2O2. The acidification of the uranium complex solution to precipitate UO4 from the solution was carried out by continuously mixing the uranium solution and 0.5 M HNO3 solution at a flow rate of 2 mL/min in a volumetric flask. The recovery of carbonate salt solution was performed by a gas absorber 40 cm in length and 2.5 cm in diameter packed with silica beads 1 mm in diameter. The CO2 gas released during the acidification of the uranium carbonate complex solution went into the bottom, and a NaOH solution of 0.5 M flowed down from the top. The electrodialysis cell used to recover the HNO3 and NaOH used in the precipitation and the gas absorption steps consisted of three compartments, which were divided by a cation exchange membrane (Nafion 424) and an anion exchange membrane (Electrolytica A-7001). Each anodic, cathodic and feed compartment of the electrodialysis cell divided by the membranes was formed by frames with a vacant space of 3.5 × 8 × 0.7 cm3 (approximately 30

3. RESULTS AND DISCUSSION To first confirm complete uranium precipitation and recovery of the carbonate salt solution before studying the recovery characteristics of acid and alkali solutions with the electro6276

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if the HNO3 and NaOH solutions are recovered from the NaNO3 solution after the precipitation with an electrodialysis system, the process suggested in Figure 1 can be said to treat uranium oxide waste, such as uranium fuel scraps, in a very environmentally friendly way without any generation of secondary waste solutions. To investigate the recovery characteristics of HNO3 and NaOH from an electrodialysis cell, several variables such as the concentrations of NaNO3, NaOH, and HNO3 (A), the cell voltages or cell currents (B), and the pH values (C) in the feed, cathodic, and anodic reservoirs were measured as a function of time in batch operation according to the electrodialysis methods of supplying a constant voltage and constant current (Figures 4 and 5). In those experiments, a feed solution of 0.5 M NaNO3, a cathodic solution with initial concentration of 0.1 M NaOH, and an anodic solution with initial concentration of 0.1 M HNO3 were circulated within their compartments through their respective reservoirs. Figure 4 shows the results of the constant applied current operation mode. As NO3− and Na+ ions migrated into the anodic and cathodic compartments through the anion and cation exchange membranes, respectively, water split reactions occurred at each electrode to generate hydroxyl ions and hydrogen gas in the cathodic compartment and proton ions and oxygen gas in the anodic compartment, as shown in eq 2 and eq 3, which resulted in the production of NaOH and HNO3 solutions from the electrodialysis cell.15 The transport rates of Na+ and NO3− ions increased with the applied constant current. As the NaNO3 in the feed decreased linearly with time at each applied current, the concentrations of HNO3 and NaOH on the anodic and cathodic sides almost linearly increased. This means that the amount of ions migrating across the membranes are linear with the total electricity passing through the cell. However, as the ions in the feed solution were exhausted by being transported through the membranes, the conductivity of the feed solution rapidly decreased, resulting in an abrupt increase in the cell voltage, which caused electrodialysis breakoff because the voltage compliance limit of the power supply was reached, as shown in Figure 4B. The rapid rise of cell voltage began when the concentration of NaNO3 in the feed solution reached approximately 0.1 M, in other word, when about 80% of the NaNO3 in the feed solution was electrodialyzed. The pH values in the anodic and cathodic compartments decreased further, below pH 1, or increased to over pH 13 with time and with an increase in the applied constant current, respectively. The pH of the feed solution dropped rapidly, as soon as the electrodialysis began, then decreased gradually. However, it rose slightly again from the point at which the cell voltage rapidly increased, as shown in Figure 4C. If the Na+ ion and NO3− ion in the feed solution migrate equally across the cation and anion exchange membranes, respectively, the pH in the feed solution should remain constant during the electrodialysis. The decrease in the pH of the feed solution in the beginning is thought to be caused by the difference in the number of the ions transported across the anion and cation exchange membranes. The later increase in the pH of the feed solution during the rapid rise of the cell voltage is thought to be due to some of the OH− ions in the cathodic compartment being strongly attracted toward the anode side under the high potential gradient that existed between the electrodes, causing the back-transfer of OH− ions into the feed compartment, even across the cation exchange membrane. This result implies that ions can move across an ion exchange membrane designed to

dialysis cell shown in Figure 2, a continuous experiment investigating the acidification of a uranyl peroxo-carbonato complex ion solution of UO2(O2)x(CO3)y2−2x−2y and the recovery of NaNO3 from the released CO2 gas was carried out in a volumetric flask. A peroxo-carbonato complex solution with a uranium concentration of 1.0 g/L was fed into the volumetric flask at a flow rate of 2 mL/min, and the solution pH was controlled between 2 and 3 by feeding a HNO3 solution of 0.5 M into the flask. The CO2 gas released from the flask during the acidification went into the bottom of a gas absorber into which a NaOH solution of 0.5 M flowed down from the top at a flow rate of 2 mL/min. The changes in the carbonate concentrations and the pH values with time at the flask, where the acidification occurred, and at the outlet of the gas absorber, where the recovered Na2CO3 solution came out, are shown in Figure 3. Complete decarbonation occurred from

Figure 3. Changes of carbonate concentrations and pHs with time in the acidification flask and at the outlet of the gas absorber.

the beginning according to the reaction eq 1,3,9 and the carbonate concentration in the acidification flask solution was almost zero the entire time. The carbonate concentration of the recovered Na2CO3 solution coming out of the gas absorber increased with time and took some time to reach a steady-state value because it took some time for the released CO2 gas to fill the volume of the acidification flask at the beginning of the experiment. The recovery yield of carbonate salt, which was calculated from the total carbonate moles of the feed uranyl peroxo-carbonato complex solution and the Na2CO3 solution coming out of the gas absorber, was more than 99%. The uranium precipitation yield as UO4 in the acidification flask measured after the precipitate settled was approximately 99.9% because the solubility of the UO4 is very low at Ksp = 1.34 × 10−3.13,14 UO2 (O2 )x (CO3)2y − 2x − 2y + mH+ + 2y H 2O → UO2 (O2 ) ·4H 2O + y H 2CO3 (CO2 ↑) 1

(1) 2

3

where m = 4, 6, 8 at y = 0, 1, 2 and x/y = /2, /1, /0. From the results of Figure 3, it was verified that it is possible to completely recover the Na2CO3 solution to be recycled into the dissolution step of the contaminated-uranium oxide materials, while simultaneously recovering pure uranium as UO4, if the HNO3 and NaOH solutions are properly supplied to the system. After the precipitation in the acidification flask, the supernatant contains only Na+ and NO3− ions. Accordingly, 6277

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Figure 5. Changes of the concentrations of NaNO3, NaOH, and HNO3 (A), voltages (B), and pHs (C) in feed, cathodic, and anodic reservoirs with batch electrodialysis time at several applied constant voltages.

Figure 4. Changes of the concentrations of NaNO3, NaOH, HNO3 (A), voltages (B), and pH values (C) in feed, cathodic, and anodic reservoirs with batch electrodialysis time at several applied constant currents.

at the cathode: 2H 2O + 2e− → H 2 + 2OH−

transfer opposite-polarity ions and then move toward the

Figure 5 shows the results of the constant applied voltage operation mode. Because the NaNO3 in the feed decreased linearly as a function of time in the beginning, the concentrations of HNO3 and NaOH in the anodic and cathodic compartments almost linearly increased, but they

opposite-polarity electrode under a high potential gradient. at the anode: 2H 2O → O2 + 4H+ + 4e−

(3)

(2) 6278

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then approached their steady-state values without the electrodialysis breakoff. These results are different from the results for the constant applied current operation mode shown in Figure 4. The rates of migration of NO3− and Na+ increased with an increase in the applied cell voltage. A small difference in the steady-state concentrations of HNO3 in the anolyte and of NaOH in the catholyte were observed. This difference was due to the transfer of water from the feed solution to the anodic and cathodic sides following the migration of NO3− and Na+ due to electrolytic osmosis. Approximately 8% and 2% of the feed solution were experimentally observed to transfer into the cathodic side and the anodic side, respectively. On the basis of the corrected concentrations of HNO3 in the anolyte and of NaOH in the catholyte, taking into consideration the water transfer, the decreased amount of NaNO3 in the feed solution is almost the same as the increased amount of NO3− and Na+ on the anodic and cathodic sides. When NO3− and Na+ ions in the feed solution migrate to the anodic and cathodic sides after some time has elapsed, the concentrations of HNO3 and NaOH in each compartment increase to the point at which the total cell conductivity is the highest; that is, at the lowest cell resistance, the current shows a peak, as shown in Figure 5B. Similar to the pH behavior of the feed solution for the constant current operation mode, the pH decreased rapidly and then gradually rose again, but the final pH values at the end of the electrolysis were much higher than those in the constant current operation mode. The rerising rate of the pH increase in the feed solution was faster with a higher applied cell voltage. In the case of 16 V, even the anodic pH was observed to rise again. This behavior implies that some OH− ions in the cathodic compartment were more attracted to the feed compartment across the cation exchange membrane than those in the constant current operation mode, and they even moved to the anodic compartment. These phenomena eventually caused a decrease in energy efficiency in the constant voltage operation mode. During the electrodialysis operation in Figure 4 and Figure 5, instant overall cell resistance can be evaluated with the relationship between the applied total current and the measured cell voltage. The overall cell resistance under constant applied current operation has the same pattern as the change of measured cell voltage in Figure.4B. However, the overall cell resistance under constant applied voltage operation has just a reversed pattern of the measured current in Figure 5B. In Figure 4 and Figure 5, the lowest cell resistances during the electrodialysis are observed to have a trend to appear near minimum pH in the feed solution. That occurred when approximate 50% of ions in the feed solution was migrated in Figures 4 and 5, because the migrated Na+ and NO3− ions generated NaOH and HNO3 solutions with high conductivities in cathodic and anodic compartments, respectively, which resulted in the decrease in overall cell resistance. However, as the ions in the feed solution were further exhausted, the conductivity of the feed compartment decreased less and less, which resulted in the rapid increase of overall cell resistance even though the anodic and cathodic compartments had higher conductivity solutions due to more concentrated HNO3 and NaOH. Figure 6 shows the energy consumption for the time taken to recover 80% of the NaNO3 in the feed solution as HNO3 and NaOH in the electrodialysis cell under the constant current or constant voltage operation mode, which is calculated from the results of Figures 4 and 5. The value of 80% removal of NaNO3 in the feed solution was chosen to use all the measured data,

Figure 6. Calculated energy consumption for the time taken to electrodialyze 80% of NaNO3 at constant current and constant voltage operations.

including the final point in the constant current operation mode at 70 mA/cm2, at which the experiment was stopped early because of the voltage compliance limit of the power supply. The energy consumption required to electrodialyze the same amount of feed solution decreases with decreases in the applied current or in the applied voltage, even though it takes longer to treat the same volume of feed solution. In addition, the constant voltage operation mode consumed more energy than the constant current operation mode to treat the same amount of NaNO3 in the feed solution. This result can be explained by the more back attractions of OH− ions in the cathodic compartment to feed the solution side under the constant applied voltage operation mode than under the constant applied current operation mode, as described in Figure 5. Figure 7 shows the electrodialyzed yield of NaNO3 in the feed solution according to the total electricity supplied in the batch operation for both the constant current and constant voltage operation modes using all of the data points in Figures 4 and 5. The plot displays good linearity with a regression coefficient of 0.995. This result means that the electrodialyzed

Figure 7. Electrodialyzed yield of NaNO3 in the feed solution according to the total supplied electricity in the batch operation. 6279

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extent for recovery of HNO3 and NaOH from NaNO3 solution is simply proportional to the total amount of electricity supplied to the system, regardless of the use of the constant current or constant voltage operation mode. On the basis of the results shown in Figures 4 and 5, the constant voltage operation mode can electrodialyze more than 98% of the feed solution, even though it takes longer to treat the feed solution for the same amount of energy supplied, compared with the constant current operation mode. With the constant current operation mode, it is difficult to treat more than 90% of the feed solution because of the abrupt rise in the cell voltage, which causes the power supply to shut off. Figure 8 shows the results of electrodialysis with a change in the feed concentration of NaNO3 at a constant applied voltage of 12 V. The total amounts of electricity used to recover 98% of the initial feed solutions of 0.1, 0.3, and 0.5 M NaNO3 were 1.98 × 103, 5.45 × 103, and 9.68 × 103 coulombs, respectively, which are almost linear to the initial amount of NaNO3 to be electrodialyzed in the feed solution. This linearity is also true for the time to treat the solution and the amplitude of the peak current. Figure 9 shows the concentrations of NaNO3, NaOH, and HNO3 (A), the voltages (B), and the pH values (C) as a function of time in continuous flow-through electrodialysis at constant applied currents of 30, 40, and 50 mA/cm2 using an electrodialysis stacked-cell electrolyzer with three cells with a flow rate of the feed and anodic and cathodic solutions of 2 mL/min. In the case of 50 mA/cm2 because the cell voltage at the third cell of the stacked-cell electrolyzer exceeded the compliance voltage of the power supply used in this work (20 V) at approximately 20 min, as shown in Figure 9B, the experiment stopped. This behavior implies that the NaNO3 in the feed solution was almost completely exhausted in the third unit cell such that the solution resistance abruptly increased. In contrast, in the cases of 30 and 40 mA/cm2, the electrodialysis reached a steady state within approximately 60 min. The residence time of each solution in the electrolyzer was approximately 44.8 min. In the case of 30 mA/cm2, the total electricity supplied to the electrolyzer during the residence time was 6196 coulombs. The time required to supply the same amount of electricity to the batch operation at 30 mA/cm2 (Figure 4) is 135 min, at which the concentration of NaNO3 in the feed solution in the batch system is approximately 0.146 M, which is similar to the value of 0.14 M measured at the feed outlet of this continuous electrolyzer. In the case of 40 mA/ cm2, the total supplied electricity during the residence time was 8225 coulombs, and more than 98% of the NaNO3 in the feed solution was electrodialyzed. The values of the supplied electricity and the electrodialyzed yield similarly correspond to the plot line obtained in the batch operation in Figure 7. This implies that the continuous electrodialysis yield for recovering the acid and alkali is proportional to the total electricity supplied to the system, as well. Figure 10 shows the results at constant applied voltages of 9, 12, and 15 V using the same electrolyzer as that used in Figure 9. At all the applied voltages, the electrodialysis reached a steady state within approximately 60 min and approximately 98% of the NaNO3 in the feed solution was recovered as HNO3 and NaOH. The current peaks, which were observed in the batch operation in Figure 5B, were also observed before steady state was reached. As the feed solution passed consecutively through the second and third cells, the current peak amplitudes changed depending on the solution resistance formed in each

Figure 8. Changes of the concentrations of NaNO3, NaOH, and HNO3 (A), voltages (B), and pH values (C) in feed, cathodic, and anodic reservoirs with time at several feed concentrations at a constant applied voltage of 12 V.

electrodialysis cell and the steady state current decreased. The steady state pH in the feed stream at the outlet of the electrodialysis system increased with the applied constant voltage because the OH− ions in the cathodic stream are more attracted to the feed compartment across the cation exchange membrane with higher applied voltages. From all of the results on the electrodialysis of NaNO3 to recover HNO3 and NaOH, the process suggested in Figure 1 6280

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Figure 10. Changes of the concentrations of NaNO3, NaOH, and HNO3 (A), voltages (B), and pH values (C) in feed and cathodic and anodic reservoirs with time at several applied constant voltages in a three electro-dialysis cells-stacked electrolyzer.

Figure 9. Changes of the concentrations of NaNO3, NaOH, HNO3 (A), voltages (B), and pHs (C) in feed, cathodic, and anodic reservoirs with time at several applied constant currents in a three electro-dialysis cells-stacked electrolyzer.

released during the acidification, can be sufficiently recovered with a continuous electrodialysis system. using a carbonate salt solution with hydrogen peroxide can be said to treat uranium oxide waste in a very environmentally friendly manner without any generation of secondary waste solutions because the HNO3 and NaOH solutions, which are used to acidify the uranyl peroxo-carbonato complex solution and to recover the carbonate salt solution from CO2 gas

4. CONCLUSIONS Several characteristics of an electrodialysis system using a cation exchange membrane and an anion exchange membrane to recover HNO3 and NaOH solutions from the effluent solution of NaNO3 after the treatment of uranium oxide waste were 6281

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(9) Kim, K.-W.; Hyun, J.-T.; Lee, E.-H.; Park, G.-I.; Lee, K.-W.; Yoo, M.-J.; Song, K.-C.; Moon, J.-K. Recovery of uranium from (U,Gd)O2 nuclear fuel scrap using dissolution and precipitation in carbonate media. J. Nucl. Mater. 2011, 418, 93. (10) Kim, K.-W.; Chung, D.-Y.; Lee, E.-H.; Lee, K.-Y.; Lee, K.-W.; Moon, J.-K. Environmentally-friendly recovery of uranium from nuclear fuel scraps generated at nuclear fuel fabrication process. Proc. ICEM 2011, the ASME 2011 14th International Conference on Environmental Remediation and Radioactive Waste Management; Reims, France, September 25−29, 2011. (11) Kim, K. W.; Lee, E. H.; Kim, J. S.; Shin, K. H.; Chung, B. I. Material and Organic Destruction Characteristics of High Temperature-Sintered RuO2 and IrO2 Electrodes. J. Electrochem. Soc. 2002, 47, 2525. (12) Strelow, F. W.; KoKot, M. L.; Walt, T. N. V. D.; Bhaga, B. Rationalized determination of uranium in rocks for geochemical prospecting using separation by ion exchange chromatography and spectrophotometry with arsenazo III. J. S. Afr. Chem. Inst. 1976, XXIX, 97. (13) Kim, K.-W.; Kim, Y.-H.; Lee, S.-Y.; Lee, J.-W.; Joe, K.-S.; Lee, E.H.; Kim, J.-S.; Song, K.; Song, K.-C. Precipitation characteristics of uranyl ions at different pH values depending on the presence of carbonate ions and hydrogen peroxide. Environ. Sci. Technol. 2009, 43 (7), 2355. (14) Debets, P. C. X-ray diffraction data on hydrated uranium peroxide. J. Inorg. Nucl. Chem. 1963, 53, 727. (15) Kim, K.-W.; Kim, Y.-H.; Lee, S.-Y.; Lee, E.-H.; Song, K.-C.; Song, K. Study on electrolytic recoveries of carbonate salt and uranium from a uranyl peroxo carbonato complex solution generated from a carbonate-leaching process. Ind. Eng. Chem. Res. 2009, 48, 2085.

evaluated in batch and continuous operation modes. The energy consumption required to electrodialyze the same amount of feed solution decreases with decreases in the applied current and in the applied voltage, even though the latter mode takes longer. The constant voltage operation mode consumes more energy than the constant current operation mode. The electrodialysis yield for HNO3 and NaOH from NaNO3 solution is proportional to the total electricity supplied to the system, regardless of the constant current or constant voltage operation mode. The constant voltage operation mode can electrodialyze more than 98% of the feed solution, even though it takes longer to treat the feed for the same amount of energy supplied compared with the constant current operation mode. However, with the constant current operation mode, it is difficult to treat more than 90% of the feed solution because of the abrupt rise in the cell voltage, which causes the power supply to shut off. It was confirmed that the carbonate-based uranium leaching process can be used to treat uranium oxide waste, such as uranium fuel scraps, in a very environmentally friendly manner without any generation of secondary waste solutions.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 42 868 2044. Fax: +82 42 868 2351. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Science and Technology (MEST) of the Republic of Korea under the nuclear R&D Project.

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REFERENCES

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