Platinum Group Metal Extraction With Thermomorphic Ionic Liquids.

[NNadir]

Many elements in the periodic table are subject to depletion from ores in near term; others in the long term.

Those subject in the short term include the "platinum group metals" - often referred to in the scientific literature as "PGM."

These are the elements, ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt).

The first three elements are common fission products that can be isolated from used nuclear fuels. Two of them, ruthenium and rhodium can be obtained in a non-radioactive form with a few decades of cooling; pure non-radioactive (but monoisotopic) Pd can be obtained from the decay of ruthenium-106, which has a half life a few days longer than a year.

Palladium that is isolated as a fission product will remain slightly radioactive for millions of years, owing to the long lived isotope Pd-107. From my perspective this does not mean it is not useful; it can be used as a catalyst (one of the big uses for palladium) in closed systems, and off line I've been considering it as a component of superalloys that would prove superior (higher melting) to the nickel based superalloys which plays a key role in many technologies, notably power generation. The longer the half-life of an isotope, the lower its specific activity; which is why bananas, radioactive because of the potassium they contain, don't kill you. K-40 has a half-life of billions of years.

In the next few years, rhodium will become more available from used nuclear fuels than it is from domestic ores.

It is thus with interest that I came across a paper in the literature today that mentions the extraction of these valuable elements from used nuclear fuels, this one: Significant Acceleration of PGMs Extraction with UCST-Type Thermomorphic Ionic Liquid at Elevated Temperature (Arai et al, ACS Sustainable Chem. Eng., 2018, 6 (2), pp 1555–1559.

The authors describe an "ionic liquid" that is useful for the extraction of the light PGM from used nuclear fuel, where they are considered problematic because they interfere with the bad idea of throwing the stuff in used nuclear fuel away, that is dumping it. (This is a bad idea because all of the components of used nuclear fuel are potentially very useful materials to have. We need more of the stuff, not less, even if as a culture we're generally too stupid to figure that out.)

Here's what they say in their introduction which I've just echoed above:

Ionic liquids (ILs) are commonly defined as organic salts which melt below 100 °C. They have unique properties, e.g., nonflammability, nonvolatility, high conductivity, and diversity of combinations of cations and anions. Specifically, it is possible to synthesize an IL with potential to extract metal ions (Mn+) due to introducing functional groups on either its cationic or anionic components. Because of these properties, the use of ILs as an extraction solvent for Mn+ has been frequently investigated.(1-8) The above characteristics make these extraction systems more environmentally friendly compared with the ordinary organic/aqueous biphasic systems. One of expected applications is treatment of radioactive wastes.(9) For example, platinum group metals (PGMs) like Ru, Rh, and Pd in the high-level liquid wastes are sometimes problematic in the vitrification process.(10, 11) Hence, removal of these PGMs is significantly required. In this context, we are investigating the potential of ILs in the PGMs extraction. A few ILs undergo a temperature-responsive behavior, which shows transition of miscibility of IL with an aqueous solution at a critical temperature.(12-17) Specifically, an IL consisting of N,N,N-trimethylglycinium (or betainium, [Hbet]+) and bis(trifluoromethylsulfonyl)amide ([Tf2N]? ) is hydrophobic enough to form an organic phase immiscible with water at room temperature, whereas these layers are completely miscible with each other above 55 °C, namely, the upper-critical solution temperature (UCST).(18-24) Therefore, [Hbet][Tf2N] (Figure 1(a)) is highly promising for an energy-saving extraction process because ultimately homogeneous mixing of the aqueous/IL biphasic system is facilitated only by heating.

Whenever I look at a new chemical these days, I try to reflect on its environmental fate based on my general knowledge of biochemistry and toxicology. This is why I'm horrified at the latest trend in "green" solar technology, the perovskites, because these are compounds of the toxic element lead, which is even worse than the use of the toxic element cadmium used in commercial solar cells being distributed today with complete disregard for all future generations and too much regard for fads.

Things with a shorter half-life in the environment are obviously better than those with longer half-lives. The best case is compounds that occur naturally.

As it happens, you contain ionic liquids and would die without them. This is choline, which is trimethylammonium ethanol amine chloride (or hydroxide), the cation being an peralkylated and reduced form of the amino acid glycine (albeit not biochemically synthesized from glycine, but rather from serine or methionine.)

Anyway...

Since used nuclear fuels have a very high energy to mass ratio, one should - with a little chemical sophistication - require trivial amounts of materials to process them, but this said, this has historically not been true, as we have learned from the interesting case of the Hanford tanks from the former weapons plutonium isolation plant in Washington State. (The interesting chemistry of these tanks is fascinating, by the way, but that's a topic for another day.)

Here is the structure of the ionic liquids that may prove useful for the extraction of PGM from used nuclear fuels:



The ion on the top left is betaine, a common constituent of plants that helps plant cells balance their osmotic pressure. The ion on the top right is dehydroxycholine; I'm not aware of its presence or lack of presence in living cells, but I image it's going to be metabolized much like either choline or betaine.

The ion on the bottom of both species is however, is bistrifluromethylsulfonyl imide. This is a derivative of triflate, a common reagent utilized as a protecting group in organic synthesis. Triflate is the salt of trifluorosulfonic acid, one of the more powerful acids in the world and regrettably, an acid that is extremely stable. It is therefore environmentally suspect, since it is likely to persist for a long time, rather like the problematic PFOS side product of the Teflon industry and the fabric protection industry, widely distributed, long lived and rather suspect as a potential carcinogen.

I would suspect that triflate might be subject to some radiological degradation, but a lot of radiation in the presence of lots of water would be required, which is why the stuff is good for processing nuclear fuels, but potentially problematic unless completely recovered and recycled.

Anyway, this ionic liquid is very good at the removal of PGMs not only from nuclear fuels, but from other materials from which they may need recovery, at least when they are heated in low concentrations of nitric acid. (PGM are very, very, very, very useful elements.)

A graphic from the paper:



The caption:

Figure 5. Dependence of the extraction efficiency (E%) of Mn+ on [HNO3] in HNO3(aq)–[Hbet][Tf2N] systems.

They may also be useful for partial separations from one another, given their differening distribution constants:



The caption:

Figure 6. Distribution ratio (D) of Ru(III), Rh(III), and Pd(II) as functions of (a) [[Hbet]+] in 0.3 M HNO3(aq)/[TMPA][Tf2N] and (b) [H+] in (H,Na)NO3(aq)/[Hbet][Tf2N] (total [NO3–]: 3.00 M). Initial condition: [Ru(III)] = 7 mM, [Rh(III)] = 3 mM, [Pd(II)] = 5 mM, T = 353 K.

The authors conclude thusly:

To answer what drives the extraction of inert PGMs from HNO3(aq) to the thermomorphic [Hbet][Tf2N] ionic liquid, we studied the distribution behavior of Ru(III) and Rh(III) at different temperatures as well as that of Pd(II), the labile PGM. As a result, the kinetics of the extraction reactions of the inert PGMs were successfully improved at elevated temperatures. Their interaction with [Hbet]+ to form extractable species is the rate-determining step, which has been successfully accelerated by convection heating. Thus, the extraction of these inert PGMs seems to be simply temperature controlled regardless of the heating methods like convection and microwave. The extraction mechanism of Ru(III), Rh(III), and Pd(II) in the current extraction system is concluded to follow the formation of the PGM:bet complexes to release H+ to the aqueous phase. Further detailed investigations are currently ongoing, for instance, separation from other Mn+, preparation and characterization of the extractable PGM:bet complexes, and stripping behavior of the extracted PGMs from [Hbet][Tf2N]. We wonder that the back-extraction kinetics would also be affected by the temperature.

The subtext of this is that despite public fear and ignorance, there are still some people intelligent enough to be figuring out what to do with used nuclear fuels. This can only be good for a future that may prove inhabited by wiser people than we have proved to be.

Have a nice day tomorrow.