|Fig. 1: Ion exchange resin beads. (Source: Wikimedia Commons)|
Nitrates, or chemical species containing NO3, are naturally formed via vegetable and animal decomposition and are present in soil, food, and water.  These species exist primarily in highly water-soluble forms such as potassium nitrate (KNO3) and sodium nitrate (NaNO3), which dissociate completely in aqueous environments.  This facilitates their ability to contaminate water sources, which has become an environmental and health concern in recent years due to increased nitrate waste from industries such as fertilizer production and nuclear power plants.  In general, nitrates in the environment are stable; however, they can be reduced to nitrites via biological processes, meaning they can potentially lead to adverse health effects if ingested by humans.  Although the CDC does not have a carcinogenicity evaluation for nitrates and nitrites, research has linked nitrites to increased risks of gastrointestinal cancer, as well as to blue-baby syndrome.  Consequently, the United States Environmental Protection Agency has placed a maximum contaminant level in drinking water of 10 ppm and 1 ppm for nitrates and nitrites, respectively. 
As was mentioned above, one perhaps surprising source of nitrates being released to the environment is the nuclear power industry, in which nitrates arise primarily from processes associated with uranium fuel production and purification. Before nitrates are introduced to the process, uranium ore is mined.  In order to begin to separate the valuable uranium from the remainder of the ore, the ore is crushed into small particles and the uranium is leached using a strong acid (sulfuric acid is generally used, but please refer to reference  for a complete discussion of this process). During leaching, UO2 in the ore undergoes a reaction with sulfuric acid to form a uranyl sulfate species.  This solution is called the leach liquor. The competition between the interactions of both this species and nitrates with a porous resin can then be exploited in an ion exchange process to fully separate the uranium from the remainder of the ore sludge.
Ion exchange involves separating different species via the exchange of ions between a liquid and a solid resin (Fig. 1) based on the ions affinities for the resin.  In this process, the solution meant to be separated (the leach liquor) is flowed across a resin made of porous beads containing a single anionic species. As was mentioned previously, the uranium is associated with a sulfate ion following the uranium leaching process, meaning the ion of interest is the anion SO42-.  Because the ion of interest is an anion, a strong base anion (SBA) exchange resin (a resin containing a single anionic species) is used.  The anion within the resin must have an affinity to the beads that is less than that of the SO42- anions in the leaching liquor.  This allows the sulfate anions in the leaching liquor to be more strongly attracted to the resin than the resin anions, promoting an ion exchange in which the sulfate anions sorb onto the resin, and the resin anions are released into the solution as waste. Nitrate or chloride anions are generally the resin anion species of choice because both anions have a lower affinity to resin beads than sulfate anions.  However, nitrate has been shown to exhibit a higher elution efficiency than chloride, and it is consequently the most common choice of resin anion.  Therefore, as the leaching liquor flows over the beads, the nitrate anion in the resin is exchanged with the leaching liquor uranyl sulfate anion such that nitrates are expelled in the wastewater while the uranyl sulfate anions remain sorbed to the beads. 
The beads are then regenerated by flowing a highly concentrated nitrate solution across the beads, which, according to Le Chatelier's principle and the reaction quotient Q, defined by 
|AR + Bn±||⇆||BR + An±||[BR] [A]
where the resin contains ion A, and ion B is dissolved in the solution passing over the resine. This shifts the reaction equilibrium back toward the original nitrates sorbed to the resin.  In this way, the resin is restored back to its original state, and the uranium in the form of uranyl sulfate has now been isolated from the ore and can be concentrated and dried into the final yellowcake product. 
The primary concern with this ion exchange process is the high concentration of nitrates in the wastewater. Any nitrate left on the resin after elution is lost in the waste and therefore is present in the waste tailings, leading to environmental concerns of nitrate contamination of groundwater.  It has been reported that nuclear waste streams can consist of up to 20% of nitric acid, and it should be noted that nitrates also are present in processes such as pH control of nuclear power plant cooling towers and removal of isotopes from radioactive waste.  Consequently, because nitrates are so prevalent in the nuclear industry, it is desirable to develop efficient means of denitrification of nuclear waste streams.
Current methods of denitrification include biological denitrification using enzymes, as well as reverse osmosis and heterogeneous catalysis. However, biological denitrification requires constant monitoring of the process pH and other parameters, while the latter two methods produce further pollution that would have to be dealt with.  A promising alternative to these methods is electrochemical reduction of nitrate to less harmful and more useful products such as nitrogen and ammonia, because this method is highly tunable by changing electrode materials and overpotential, as well as requires a relatively low electrode surface area. 
|Table 1: Formal potentials of nitrate reduction to various products versus the saturated calomel electrode. |
Possible products of nitrate reduction can be seen in Table 1. The difficulties of this reaction lie in the facts that: (i) hydrogen evolution reaction competes at the overpotentials required for nitrate reduction, (ii) the overpotentials required to produce different products are very similar such that it is difficult to control what products are produced, and finally (iii) the mechanism of the reaction is not well understood.  This reaction has been studied quite extensively on platinum, but other cathode metals such as Ti, Cu, Fe, Zn, Pt/Ir, Pd, and Au are still being investigated. Ideally, the cathode should be as inactive as possible toward the hydrogen evolution reaction in order to maximize the conversion of nitrate to desired products.  If this becomes possible and selectivity toward specific products could be maximized, many possibilities concerning wastewater purification could be created. For instance, if nuclear wastewater nitrates could be selectively reduced to ammonia only, this ammonia could then be fed back into the process to precipitate the uranium from the ion exchange eluent, both allowing nitrates to be removed from wastewater and potentially lowering cost by regenerating chemicals used in the uranium production process.
© Sarah Blair. 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.
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