Electrochemical Molecular Switches for the Capture and Release of Uranium.

[NNadir]

The paper I'll discuss in this post is this one: Redox-switchable carboranes for uranium capture and release (Gabriel Ménard et al, Nature volume 577, pages 652–655(2020))

According to a DOE report, DOE/EM-0275 as of 1996, the US government had in stock about 585 million kg of depleted uranium, beyond the 25 million kg enriched to about 3% in U-235, about 610 million kg.

A kg of plutonium, the starting material for which is depleted uranium, contains about 80.3 trillion joules of energy, completely fissioned.

The world was, as of 2018, consuming about 600 exajoules (600 million trillion joules) of energy each year.

Recently there have been efforts by a number of companies, one of the most well known being Bill Gates' Terrapower, to commercialize "breed and burn" type reactors that transmute depleted uranium into plutonium in situ.

It follows that this US inventory is sufficient, at current levels of energy demand to power all the world's energy for about 80 years, no dangerous natural gas, no dangerous petroleum, no coal mining for energy purposes. Of course, there are other uranium inventories elsewhere in the world. In addition, a side product of the useless wind industry and the electric car industry, both of which depend on access to iron neodymium boride magnets, often doped with dysprosium - lanthanides - is the radioactive element thorium. Collected from lanthanide mile tailings, dumps, in which the thorium has been partially refined, this thorium is also a valuable nuclear fuel. It is reasonable to say that in a "breed and burn" powered world it would be unnecessary to mine any fuels for several centuries.

Fracked rock, which has been eternally pulverized for a few decades of "good times" by all of us self declared "green" people also represents a potential source of uranium: The radon dumped by the gas industry in Pennsylvania's Reading Pronge gas fields indicates that this pulverized rock, over which water may flow for millenia upon millenia, is also a potential uranium source.

Finally, since that establishment of oxygen in the planetary atmosphere, a continuous uranium cycle has been established in the planetary oceans; they contain about 4.5 - 5 billion tons of uranium.

There has much discussion of refining uranium from dilute sources, seawater, run-off from uranium mine tailings, and natural uranium formations both for the purposes of obtaining fuel as well as to remediate areas of anthropomorphic contamination or natural uranium flows. Uranium is a chemotoxic element, notably having effects on renal and other tissues. Many thousands of papers on this subject have been published in the scientific literature; I almost certainly have hundreds in my personal electronic files. Many of these papers concern organic resins, notably amidoxime functionalized resins. There are also inorganic species that have been advanced for this purpose. What is of interest about this laboratory scale material is that it can more or less breathe uranium, in effect "inhale" and "exhale" it by the application of electrical currents.

From the introduction:

Known for over 50 years, carboranes have been extensively studied in coordination chemistry (including with U), catalysis, luminescence, and energy storage applications10,11,12,13,14,15. Studies have shown that reduction of substituted closo-carboranes to the nido-carboranes results in rupture of the C–C bond and cage opening, with a simultaneous increase in ligand bite angle, θ (Fig. 1a; closo and nido refer to 2n + 2 and 2n + 4 framework bonding electrons, respectively, where n is the number of vertices)11,14,16,17,18. We rationalized that by incorporating donating groups to ortho-carborane, we could tune the chelating properties of the cluster switching from opened to closed conformations by redox control of the reduced and oxidized states, respectively, and enable the chemical or electrochemical capture and release of uranyl in solution (Fig. 1a).

Closo and nido refer to something known as the "Wade-Mingo" rules, and refer to the presence of a complete platonic solid structure, in this case icosahedral symmetry, having all vertices represented, closo or one vertex missing, nido. (The symmetry of in these cases is not truly icosahedral, since the symmetry is "disturbed" or "degraded" by the presence of the functionalized carbon. The carbon in this boron hydride structure is functionalized with diphenylphosphine oxide.

Figure 1:



The caption:

a, General chemical or electrochemical mono- or bi-phasic capture of uranyl from UO2X2L2 (X = Cl−, OAc−; L = THF, Ph3PO) using the reduced ‘open’-cage nido-carboranes (2a/2b) generated by reduction (for example, CoCp∗2CoCp2∗ or negative bias) of the ‘closed’-cage closo-carborane (1). The corresponding relative bite angles (θ are also shown. Oxidation (for example, [FeCp2][PF6] or positive bias) of the captured products 3/4 or 3N/4N leads to UO22+ release. Compounds labelled in green have been chemically isolated, whereas compounds in orange are proposed electrochemical products (see Methods). Blue and red pathways represent UO22+ capture and release, respectively. b, c, Solid-state molecular structures of 4 (b) and 3 (c) obtained from XRD studies. H atoms, [CoCp∗2]+[CoCp2∗]+ counter cations, phenyl C–H linkages and all co-crystallized solvent molecules are omitted for clarity. See Extended Data Fig. 1for the structures of 1 and 2a.

Many of the experiments described in the full paper take place in organic solvents, which of course, is not seawater, but nevertheless the system is definitely quite interesting, and one can imagine modifications.

Anyway, the system operates electrochemically.

Figure 2:



The caption:

a, Illustration of the H-cell used, incorporating excess Fc/Fc+ (left) and 1, TPO and [UO2Cl2(THF)2]2 (right) in a 3:1 PC:benzene solvent mixture. Charging the cell (blue) leads to the capture of UO22+, converting 1 to 4N (major product) and 3N (minor product, not shown). b, Quantification of products and reactants by 31P{1H} NMR spectroscopy against an inert internal standard, [Ph3PNPPh3][PF6] (not shown). The initial spectrum is shown in grey, whereas spectra acquired during charge and discharge cycles (1–6) are shown in blue and red, respectively. c, Bottom, applied galvanostatic potentials for charge (blue) and discharge (red) cycles. Dashed lines represent wait periods, which were necessary for 31P{1H} NMR data acquisition. Each cycle is 24 h. Top, instrumental measure of delivered charge (teal) versus charge used for the reduction of 1, measured by quantifying the total reduced products, 3N and 4N, by 31P NMR spectroscopy. See Methods and Extended Data Figs. 6, 8 for additional experimental details and data.

The issue of organic solvents is addressed as shown in figure 3, which essentially is an extraction procedure.

Figure 3:



The caption:

Fig. 3: Simplified depiction of half H-cell and spectroscopic measurements for the biphasic electrochemical capture/release of dissolved UO22+ (yellow sphere) from/to buffered aqueous solutions. See Methods and Extended Data Fig. 7 for an expanded stepwise figure and all experimental details. a, Biphasic mixture of UO2X2 dissolved in a NaOAc-buffered aqueous solution (pH 5.4) and of electrochemically generated 2bfrom 1 (X = OAc− or NO3− . Inset, aqueous UV-Vis and organic 31P{1H} NMR spectra after reduction of 1 to 2b, but before phase mixing. Residual 1 is observed in the latter owing to the set SOC. b, Simplified depiction of the captured UO2X2 in the form of 3N and/or 4N. Inset, aqueous UV-Vis spectrum showing the capture of UO2X2 by the 2b/DCE layer (top); the corresponding 31P{1H} NMR spectrum of the DCE layer showing the captured major product (3N/4N) and minor residual 1 (bottom). c, Biphasic release of UO2X2 from the DCE layer to a fresh NaOAc-buffered solution (pH 5.4), following electrochemical oxidation of 3N/4N. Inset, aqueous UV-Vis and organic 31P{1H} NMR spectra of free UO2X2 and 1, respectively—both consistent with the release of captured UO2X2 from the DCE to the aqueous phase. A small amount (~20%) of unknown byproducts (marked by asterisks) is also observed in the 31P{1H} NMR spectrum.

Note that exposure to organic solvents would not be acceptable unless the organics were destroyed by subsequent processing. One such available approach to processing would involve subjecting the resultant aqueous solution to supercritical conditions, whereupon the solvent residues would be oxidized to carbon dioxide and the water reduced to hydrogen.

This is a lab scale procedure, and it seems to me that a number of issues need to be addressed before anything like this could be run on an industrial scale. Then again, as stated at the outset, the "breed and burn" concept means that there is really no need to obtain more uranium than has already been mined, at least for several centuries, so there's plenty of time to do that, to make nuclear energy essentially inexhaustible. (At the end of my life, it does seem that ultimately fusion energy may be viable, but current isolated uranium might make the world survivable in the interim.

It's a nice little interesting paper, I think.

Have a nice day tomorrow.