Letter pubs.acs.org/ac
Separation of Americium from Curium by Oxidation and Ion Exchange Jonathan D. Burns,*,† Thomas C. Shehee,‡ Abraham Clearfield,† and David T. Hobbs‡ †
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States Savannah River National Laboratory, Savannah River Nuclear Solutions LLC, Aiken, South Carolina 29808, United States
‡
S Supporting Information *
ABSTRACT: Nuclear energy has the potential to be a clean alternative to fossil fuels, but in order for it to play a major role in the US, many questions about the back end of the fuel cycle must be addressed. One of these questions is the difficult separation of americium from curium. Here, we report the oxidation of Am in two systems, perchloric acid and nitric acid and the affect of changing the acid has on the oxidation. Kd values were observed and a direct separation factor was calculated and was seen to be as high as 20 for four metal(IV) pillared phosphate phosphonate inorganic organic hybrid ion exchange materials. These ion exchangers are characterized by very low selectivity for cations with low charge but extremely high uptake of ions of high charge.
T
factors (SF) for these systems are presented in Table 1. Unlike traditional ion exchange resins, which can be converted into
he worldwide demand for energy is at an all time high and growing at a rapid rate. Over the past decade, the US Department of Energy has renewed its interest in closing the nuclear fuel cycle1 as a means to create a major clean energy source without increasing greenhouse gas emissions. However, the problem of handling and disposing the waste generated in such operations still remains. As uranium dioxide fuel is consumed in a nuclear reactor, many radioactive species are generated including fission products, like cesium, strontium, and lanthanides as well as plutonium and minor actinides. The minor actinides lead to the majority of the long-term radiotoxicity of the waste stream.2 It is therefore desired to purify Am for transmutation and reduce the nuclear waste legacy for future generations. The zirconium and tin(IV) based phosphate phosphonate unconventional metal organic frameworks (UMOFs) have been shown to have high affinity for lanthanides and other high charge actinide ions (≥3+), while discriminating against actinide ions of low charge (≤2+).3 These ion exchange materials provide a robust platform to implement a separation in the extreme environments found in the nuclear fuel cycle without generating large volumes of hazardous, radioactive mixed waste, as do solvent extraction techniques. Here, we report the high efficiency separation of oxidized americium from curium and a system for oxidation and stabilizing americium in a pentavalent state. It is also shown that these hybrid ion exchange materials exhibit unique charge discrimination toward all cations of low charge (≤2+) completely ignoring cations such as Na+ and Ca2+, with only a slight affinity for other mono- and divalent cations. These ion exchange materials provide a simple medium for separating Nd3+ from Cs+, Sr2+, or Ni2+ in nitric acid at a pH of 2; the separation © 2012 American Chemical Society
Table 1. Separation Factors of Nd3+ from Cs+, Sr2+, or Ni2+ at pH 2 sample
Nd3+/Cs+
Nd3+/Sr2+
Nd3+/Ni2+
H−Zr−hybrid Na−Zr−hybrid H−Sn−hybrid Na−Sn−hybrid
15 78 480 800
>1000 >100 000 1900 2400
19 520 910 1000
specific countercation phases by treatment with a concentrated salt solution, e.g., 1 M NaCl, these UMOFs show no response even to titration with NaOH. In the trivalent state, americium has a similar ionic radius and almost identical effective nuclear charges to Cm3+, Nd3+, and Sm3+ resulting in very similar chemical properties.3d,4 These similarities make the separation using conventional techniques very difficult if not impossible. We have therefore undertaken the task of developing a method to oxidize Am3+ to AmO2+ and significantly changing the chemical properties of the polyatomic ion.3d As has been reported previously by the authors3d,5 and elsewhere,6 Am3+ can be oxidized to AmO22+ using Na2S2O8 at 80 °C in slightly acidic solutions. The dioxoamericium(VI) ion is present as long as there is enough energy in the system to allow the persulfate to radicalize and form the sulfate radical SO4•−. Once the reaction is cooled, Am(VI) reduces rapidly to Received: July 2, 2012 Accepted: July 25, 2012 Published: July 25, 2012 6930
dx.doi.org/10.1021/ac3018394 | Anal. Chem. 2012, 84, 6930−6932
Analytical Chemistry
Letter
Figure 1. Spectrum of pure AmO2+ in 0.01 M HNO3 after the solution was allowed to age resulting in precipitation of CaSO4 and coprecipitation of any Am3+ not oxidized.
OCl−, and small amounts of ClOx− from the decomposition of the hypochlorite. Traditionally, the separation of Am from Cm has been a very difficult problem to tackle because of their very similar chemical properties which require complex solvent extraction systems7 with exotic ligands or chromatography8 that employs organic resins, which are highly susceptible to radiolydic degradation. We have previously reported on an increased affinity for highly charged (≥3+) cations,3c,d which resulted in a separation of americium from neodymium and europium. More recently, it has been shown that, using the same procedure of oxidation, stabilization, and ion exchange, americium can be separated from curium (see Table 2). The Kd values for AmO2+ were
Am(V) and Am(III). However, if hypochlorite is added while the reaction is still warm, Am is pushed to the pentavalent state, with only a slight amount of Am(III) present (
