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| Fig. 1: The nitrates of the nuclear industry are found in the wastewaters from uranium processing plants, such as the Hanford processing plant in the U.S. state of Washington, as pictured. (Source: Wikimedia Commons) |
In recent decades, many studies have been made into the handling, treatment, and management of radioactive waste in a safe method. Two main categories exist in radioactive waste treatment: traditional and non-traditional techniques. Whereas ion exchange, evaporation, and chemical precipitation are considered traditional treatment methods, electrochemical treatment is considered a non-conventional one. [1] Traditional treatment methods are energy intensive and require numerous, hazardous chemicals while electrochemical treatment processes consume less energy and have milder operating conditions. Furthermore, electrochemical treatment processes can be controlled remotely and automatically, are tunable via system parameters such as operating voltage, and avoid direct exposure to radioactive wastes. Thus it is no surprise that electrochemical treatment is rapidly becoming implemented throughout the world to treat streams containing aqueous wastes containing Cs-137 Co-60, nitrate wastes resulting from nuclear fuel fabrication plants, and organic wastes such as scintillation cocktails which contain Tritium, C-14, I-125, P-32, and S-35. [2]
The concept of electrochemically controlled ion-exchange (EIX) was first developed in the United States from the 1950s to 70s to investigate desalinating brackish water. The principle was simple: leverage local pH changes induced at an electrode surface by passage of small electrolytic currents. When the electrode contains acidic cation exchange groups, they activate to cation absorption at cathodic potentials. Often, carbon electrodes modified with carboxyl groups (-COOH) were used. The research into this method was dropped when more cost-effective methods of desalination were developed. It wasn't until 1981 when the Atomic Energy Research Establishments Harwell Laboratory began looking into the application of electrochemical ion exchange to treat radioactive waste. The lab developed an electrochemical cell where a pair of outer counter electrodes was in contact with in EIX membrane of powered ion-exchanger bonded together with an elastomeric binder. The cell removed Cs with high decontamination factors and consumed only 0.25% of the energy needed for the equivalent removal by the traditional evaporation technique. This breakthrough led to use of EIX in the use of treating nuclear wastewaters on an industrial scale. [3]
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| Table 1: Electrochemical reduction reactions of nitrate and nitrite. |
Nitrate (NO3-) and nitrite (NO2- ) are two of the major hazardous species present in nuclear waste waters. They come from processes associated with uranium fuel production and purification (Fig 1), and are linked to health effects including increased risks of gastrointestinal cancer and blue-baby syndrome.. Electrochemical reduction of nitrate leads to less harmful and more useful products such as nitrogen and ammonia, and the method is highly tunable by changing electrode materials and overpotential. [4] Electrode materials, cell design, and operating conditions are the main parameters affecting nitrate/nitrite reduction using an electrochemical cell. Lead (Pb) has been found to the best cathode with respect to current efficiency, and operation at high current densities (300-600 mA/cm2) and moderate temperatures (80°C) have been shown to remove over 99% of nitrate/nitrite from nuclear waste. [5] The cathodic (reduction) reactions are shown in Table 1.
The use of polyvalent metal ions for removal of organic pollutants from radioactive effluent streams is well known. These pollutants include nitrobenzene, chlorobenzenes, chlorinated naphthalenes, and polychlorinated dibenzofurans, which are known to be toxic, bioaccumulative, and even carcinogenic. Rather than use coagulant as an external source of polyvalent metal ions, electrochemistry can be leveraged to generate Al(III) and hydroxyl ions at the anode to produce several monomeric and polymeric hydroxylated species which ultimately precipitate as Al(OH)3. These aggregates have a large surface area, allowing them to rapidly adsorb organic pollutants onto them. These flocks are then floated to the top from evolved hydrogen gas. [6] Furthermore, some of these hydroxyl radicals oxidize organic pollutants directly, leading to mineralization and release of carbon dioxide.
© Matthew Liu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Handling and Treatment of Radioactive Aqueous Wastes," International Atomic Energy Agency, IAEA-TECDOC-654, July 1992.
[2] M. M. A. El-Aziz and M. A. Khalifa, "Electrochemistry and Radioactive Wastes: A Scientific Overview," J. Turkish Chem. Soc. A 3, 47 (2016).
[3] R. O. A. Rahman et al., "Overview on Recent Trends and Developments in Radioactive Liquid Waste Treatment Part 1: Sorption/Ion Exchange Technique." Int. J. Environ. Eng. Sci. 2, 1 (2011).
[4] S. Blair, "Electrochemical Denitrification of Nuclear Wastewater," Physics 241, Winter 2018, Stanford University.
[5] J. D. Genders, D. Hartsough, and D. T. Hobbs, "Electrochemical Reduction of Nitrates and Nitrites in Alkaline Nuclear Waste Wolutions," J. Appl. Electrochem. 26, 1 (1996).
[6] H. R. Ghatak, "Electrochemical Treatment of Hazardous Organic Pollutants." India J. Energy Technol. Policy 3, No. 11, 84 (2013).
