Remarkably enhanced direct dissolution of plutonium oxide in task-specific ionic liquid

[NNadir]

Plutonium oxide is a challenging substance. The purification of plutonium involves (in the commonly used PUREX process) precipitation of the oxalate and thermal treatment giving the oxide. The oxide can sometimes take a polymeric which is challenging to dissolve.

Therefore I read with interest this paper, Remarkably enhanced direct dissolution of plutonium oxide in task-specific ionic liquid: insights from electrochemical and theoretical investigations Jayachandran, Kavitha , Gupta, Ruma , Chandrakumar, K. R. S., Goswami, Dibakar, Noronha, D. M., Paul, Sumana , Kannan, S. 2019, 1474-1477 Chemical Communications 55 10 2019, 1474-1477.

I don't necessarily agree that the description of electrochemical pyroprocessing in the introductory text but here it is:

Nuclear energy provides about 11% of the world's electricity from about 450 power reactors.1 It is one of the world's largest sources of low-carbon power which is free from greenhouse gases or other environmental contaminants and hence this has been profoundly considered as a solution for mitigating climate change.2 However, the central bottleneck is the disposal of radioactive waste which is one of the major drawbacks inhibiting the widespread acceptance of nuclear energy.3,4 Existing methods for the purification and recovery of actinides are based on a very well-known PUREX (Plutonium Uranium EXtraction) process.5 Another method is the pyrochemical reprocessing method in which the actinides are separated from the high-temperature molten salt media electrochemically.6,7 There are, however, many disadvantages associated with the above traditional methods viz. a lot of aqueous radioactive waste generation, the requirement of high temperature and the strongly corroding nature of the molten salt media.8–10 Hence, the suitability of these methods has thus been critically questioned for the purification and recovery of nuclear fuel materials. Thus, it warrants the development of alternative approaches that could lend a hand to solve several concerns with the current methodology including non-proliferation issues, criticality, safety, apparatus corrosion, and stiffness of the process.

In recent years, room temperature ionic liquids (RTILs) have been considered for several applications including the separation of lanthanides and actinides.11 This has been primarily attributed to the unique physical properties of the ionic liquids and more importantly, their less corrosive nature and lower operating temperatures. The solubility, coordination and speciation of f-block elements in ionic liquids have been recently explored.12–15 In spite of the usage of ionic liquids for the separation of f-block elements, the primary issue of poor solubility of lanthanides and actinides in ILs is still unsolved...

The authors explored the use of betaine bis trifluoromethylsulfonylimide (Tf2N-). Betaine is a quaternary permethylated derivative of the simplest amino acid, glycine. (It is a constituent of sugar beets.)

The authors showed the reversible redox capability between Pu(III) and Pu(IV) after showing that his ionic liquid dissolves the hitherto difficult to dissolve Pu(IV)O2. More interestingly the were able to reduce the plutonium to a metallic form on an iron electrode:

Let us now discuss our attempts towards electrochemical deposition of the dissolved plutonium from [Hbet][NTf2] medium. The plutonium was deposited at a stainless steel planchette by applying a cathodic potential of ?1.5 V, employing controlled potential coulometry. The alpha spectrum of the stainless steel planchette recorded after the deposition of plutonium is given in Fig. 3.

Fig 3:



The caption:

Fig. 3 Alpha spectra of the stainless steel planchette after the electrochemical deposition of plutonium.

The authors continue:

The alpha spectrum shows that there are two characteristic peaks at energies 5.157 MeV and 5.499 MeV, respectively for 239,240Pu and 238Pu isotopes. No other peaks corresponding to other actinides viz. U, Th or Np were seen in the spectrum. There is a possibility of spectral interference of 241Am (E? = 5.486 MeV) at 5.499 MeV alpha energy, however, the observed gamma spectrum of the sample confirms the absence of 241Am (the 59.54 keV peak is absent). The tailing of the alpha energy in Fig. 3 is attributed to the deposition of a higher concentration of plutonium on the planchette, which leads to the loss of kinetic energy of alpha particles within the source itself. The outcome of the above results clearly reveals that the dissolved plutonium can be quantitatively recovered from the ionic liquid medium via electrochemical reduction...

The deposition of plutonium on a steel electrode is something I find very interesting. Plutonium forms a low melting eutectic with iron, and briefly, in the 1960's a reactor operated at Los Alamos using this liquid metal as a fuel, the LAMPRE experiment. While the technology has basically been more or less forgotten, I personally believe it warrants another look for various reasons.

The authors add an interesting note to their paper suggesting a wider utility for this system, noting the work of Chinese workers:

Note added in proof: After the present manuscript was accepted for publication, we noticed the most recently published work of Qin and co-workers27 wherein the authors have reported the separation of lanthanide oxides from actinide oxides through the carboxyl-functionalized ionic liquid, [Hbet][Tf2N]. They claim that lanthanide oxide is dissolved in [Hbet][Tf2N] selectively whereas the dissolution of uranium is found to be negligible at the temp of 40 °C. We however claim in the present study that plutonium oxide can be remarkably dissolved in the same ionic liquid [Hbet][Tf2N] at elevated temperature. The differences in the dissolution process can be attributed to the different operating temperature.

The utilization of plutonium in my opinion is a key to saving what can be saved as climate change engulfs the world as it is doing now (and not in some far off future), and perhaps restore what can be restored of that already lost.

It's a cool, if esoteric little paper.

[eppur_se_muova]

1. Bookmarking. Noting ref. 27 for future interest. nt

[NNadir]

2. Ref. 27 is a very interesting paper.

It's this one: Highly Efficient and Selective Dissolution Separation of Fission Products by an Ionic Liquid [Hbet][Tf2N]: A New Approach to Spent Nuclear Fuel Recycling Fang-Li Fan, Zhi Qin, Shi-Wei Cao, Cun-Min Tan, Qing-Gang Huang, De-Sheng Chen, Jie-Ru Wang, Xiao-Jie Yin, Chao Xu, and Xiao-Gui Feng Inorganic Chemistry 2019 58 (1), 603-609.

They give a nice prep of the ionic liquid which is a simple metathesis reaction, they mix the lithium salt of the imide and the chloride salt of betaine, and the ionic liquid oils out, can be washed with water and made chloride free. I am always interested ionic liquids that are insoluble in water, so I'm making a mental note of this one.

One difference between the extraction used in the paper cited in the OP and the procedure utilized in this paper, besides the temperature (400K, 127°C) as noted by the authors of the Chem. Commun. paper, is that in this case, the ionic liquid is saturated with water.

The authors of the Inorg. Chem. paper just cited have a nice table of solubility for the Ln and An species:



Under these conditions, the water saturated ionic liquid at 40°C, uranium and neptunium oxides are only sparingly soluble, americium not at all, but all of the lanthanides are very soluble. It would be interesting to see how the solubility changes with desiccation by heating and with heat itself. It suggests a path toward continuous crystallization if both papers are accurate, which conceivably as the authors of the Chem. Commun. paper suggest.

I should check out some of the other references in the Chem. Commun. paper, particularly references 17-21, the majority of which are Binnemanns' papers. Binnemanns is a "go to" guy for ionic liquids and actinides.

Betaine, of course, is very cheap and readily available. I don't know much about it's radiation stability, but it's a simple enough molecule that it should be easy to remove any radiolysis products. I know I have a bunch of papers in my files from James Wishart at BNL on the radiation stability of ionic liquids, but I haven't looked at them in a number of years. I'm sure he must have commented on the radiation stability of Tf2N anions. Probably the degradation products are simply triflamides.

These are both nice papers. I like them a lot.

[eppur_se_muova]

3. Thanks! I see ceria, thoria, and zirconia are still rocks. :)

The crucible makers chose well.

Looks like there's more than one option to tweak for separation processes there -- variable temp and water content. These guys sound like they're well ahead of the pack and have a good chance of finding a practical way to separate and burn-up actinides.

Some of those other data are unexpectedly interesting to me, as well. Somebody else will probably get to exploit them before I can ever make use of them, but still interesting.