Enhanced Performance in Uranium Extraction by Quaternary NH4-Functionalized Amidoxime-Based Fibers.
The paper I'll discuss in this post is this one: Enhanced Performance in Uranium Extraction by Quaternary Ammonium-Functionalized Amidoxime-Based Fibers (Lu Xu,* and Hongjuan Ma*, Ind. Eng. Chem. Res. 2020, 59, 5828−5837)
I'm less alone than I used to be in my long held contention that nuclear energy is the only form of energy that is environmentally sustainable, particularly if one embraces the ethical concepts of human development goals and environmental justice. The widely held theory that so called "renewable energy" is somehow superior to nuclear energy or even that it is remotely sustainable is being experimentally tested at vast expense - on a scale of trillions of dollars of "investment" - and the results of this vast experiment are increasingly clear: So called "renewable energy" has done nothing to address climate change; is doing nothing to address climate change; and won't do anything to address climate change. In fact the rate of the accumulation of the dangerous fossil fuel waste carbon dioxide concentrations has reached the highest rate ever observed going back to the 1950's, averaging between 2.4-2.5 ppm/year in the most recently passed decade, compared to a rate of between 1.5-1.6 ppm/year averages as recently as the period between 1990 and 2000. In the 42 years between 1958 and 2000, there were 5 years in which the increases in carbon dioxide were higher than 2.0 ppm. Since 2001, there have been 17 such years in which carbon dioxide increases were greater than 2.0 ppm.
Mauna Loa CO2 annual mean growth rates
The "investment" in so called "renewable energy" is detailed here:
Frankfurt School/UNEP Global Renewable Energy Investment, 2018, Figure 3, page 14
All of the above consists of facts. Facts matter.
Although the scientific literature remains littered with new approaches to so called "renewable energy" and oodles of papers on how to store intermittent energy - not all of them as useless as so called "renewable energy" itself - one seldom sees anymore announcements accompanying them that nuclear energy in unacceptable because of that big historical (and ignorant) bugaboo, public acceptance.
One also sees a plethora of papers that state some realities about nuclear energy, that it is, in fact, essentially infinitely sustainable.
There are three potential nuclear fuel cycles that might save the world, the thorium/uranium (233) cycle, the uranium/plutonium cycle and, somewhat more speculatively, the lithium/deuterium (fusion) cycle.
It is possible that the last cycle might prove the most sustainable, but as a practical matter, it will not be available on anything like a meaningful commercial scale until at least the concentration of carbon dioxide has increased by more than 50 ppm, as it will so long as we continue to embrace ideology that hasn't worked, isn't working and won't work. After more than a decade of attending lectures at the Princeton Plasma Physics lab, many of which are on the subject of advances being made in fusion energy systems - and there have been significant advances - there are still, even if the ITER (which I strongly support) is able to show a net energy gain from fusion, huge hurdles to overcome, particularly in the areas of heat transfer and materials science.
For the two remaining cycles, both of which have been utilized on an industrial scale, I favor the one with which humanity has the overwhelmingly largest experience, the uranium/plutonium cycle, which is more or less infinitely sustainable because of the vast amounts of uranium found in seawater, recoverable because of uranium's very high energy to mass ratio, only exceeded by the extraordinary energy to mass ratio of the lithium/tritium/deuterium system which again, is not currently available. I have nothing against the thorium cycle, and can certainly think of many ways where it can provide useful synergies, but the low water solubility of thorium means that it is not infinitely sustainable. The paper under discussion is about recovering uranium from seawater.
From the paper's introduction:
The total amount of uranium in seawater is about 4.5 billion tons, which is 1000 times larger than that on land.(6,7) Uranium extraction from seawater was studied in the United Kingdom after the Second World War.(8) Researchers at that time prepared inorganic adsorbents and investigated promising extraction functional groups. In the early 1980s, several polymeric adsorbents were prepared and used in uranium extraction from seawater by Japan. Since the beginning of the 21st century, United States, Japan, and other countries have devoted their research on novel adsorbents and large-scale marine experiments.(9−11) At present, many methods have been used for the extraction of uranium from seawater.(12−15) Among these methods, adsorption is one of the most promising methods because of its low commercial cost, high efficiency, and ease of operation.(16−22) A large number of studies have been carried out in the field of adsorbent materials, among which amidoxime (AO) is considered to be one of the functional groups with the best coordination performance for uranium.(23−25)
Reference 4, (Review of cost estimates for uranium recovery from seawater Harry Lindner ⁎, Erich Schneider, Energy Economics 49 (2015) 9–22) which I happened to have in my files is rather glib in this statement: "...land-based uranium sources can only sustain nuclear power plants for the next 80-120 years..." which is obviously mistaken unless it assumes without any real justification that all nuclear reactors built over the next century will have the same operating procedures as was utilized in the 450 or so nuclear reactors successfully built and operated in the 20th century. Many of the small modular reactor designs now in development are designed to not be refueled for periods extending through several decades, the reason being that they are "breed and burn" reactors which generate and consume plutonium in situ. In this was depleted uranium long considered by people who can't think very well to be so called "nuclear waste" is transformed into useful fuel. Although many of these designs rely on the use of enriched uranium as a "starter," there is no intrinsic reason that they should. Plutonium is completely acceptable for this purpose, and, in many ways, in fact superior.
The world inventory of plutonium, excluding that released in above ground and underground nuclear weapons testing but including that currently available from used nuclear fuel, is on the order of 2,000 tons. Completely fissioned, using a recoverable energy value of 190 MeV/fission, this much plutonium contains about 160 exajoules of energy. The most recent IEA report indicates that as of 2018, humanity was consuming, for all sources of energy, oil, gas, coal, nuclear, hydroelectric and the (trivial) "renewable energy" industry, about 600 exajoules of energy per year. Thus the energy content of the plutonium already in existence is equivalent to about a three month supply of all of humanity's energy demand. However, "breed and burn" reactors operate (unlike the vast majority of nuclear reactors now in operation) on the fast neutron spectrum and are designed to contain an arrangement of depleted uranium in such a geometry as to assure that every single plutonium atom that undergoes nuclear fission converts more than exactly one atom of "depleted uranium" (U-238) into a new fissionable plutonium atom.
Some time ago, in this space, I reported on some literature concerning the critical mass of plutonium: Bare Metal Critical Masses of Commonly Available Plutonium Isotopes. Commercial plutonium - that found in power reactors - is in generally a mixture of isotopes, usually dominated by Pu-239, but also including significant quantities of Pu-240 and, depending on the amount of time it has been stored without use, Pu-241. Use MOX fuel will also contain appreciable Pu-242, and perhaps Pu-241, again dependent on the storage time, and finally in percentage terms amounting to the low single digits, Pu-238. Referring to the reported critical masses, we can crudely estimate, depending on the geometry and other components in the fuel matrix and control materials, that a reasonable critical mass (in a fast neutron spectrum) is on the order of 20 kg for commercial grade plutonium. Obtaining energy from plutonium of course, requires that a critical mass be present. Theoretically therefore, it is possible using plutonium present right now, to utilize it to start, loosely, 100,000 nuclear reactors, albeit small reactors, all “fired up” using plutonium.
The current inventory of depleted uranium is on the order of 1.2 million tons, already mined, and already isolated. This means, therefore, in a breed and burn setting if we shut every coal plant, every oil well, every gas mine, abandoned all fracking, all offshore oil wells, restored every wilderness area at sea and on land from the destruction associated with the construction of wind farms, set all the rivers free by dismantling dams, and used only already mined and isolated uranium, thus shutting all existing uranium mines as well - leaving the contents for future generations to use - at current levels of world energy demand, this uranium would last over 150 years. This does not count the quantities of already mined thorium, most of which is found in the tailings of lanthanide mines used to provide materials for, among other things, the so called "renewable energy" industry. The high energy to mass ratio associated with the already mined uranium and thorium makes this possible.
So much for reference 4.
As noted in the excerpt from the introduction, discussions of the recovery of seawater have been going on for over half a century, accelerating appreciably from my somewhat informal and desultory purview beginning in the 1980's. Certainly discussions of amidoxime functionalized resins has been much discussed in the literature - a rough count in my own files shows well over 100 papers and it's not like I spend a lot of time focusing on this subject. So it is reasonable to ask what's new here.
The answer to that question involves a rather clever approach to polymer design that takes into consideration the chemical speciation of uranium in seawater. In the planetary uranium cycle, water extracts uranium from crustal rocks uplifted from the mantle, both terrestrial and on the seafloor in the form of its doubly charged oxo ion, (UO2^(2+)), the uranyl ion in which uranium is in the +6 oxidation state. (The presence of oxygen in the atmosphere is necessary for this species to be observed.) In turn, however, in seawater, this oxo cation is mostly complexed with two or three carbonate ions, each of which has a charge of -2, with the result that most of the uranium is present in the form of negative ion complexes having either a charge of -2 or -4.
For this reason, the authors have chosen to incorporate positively charged quaternary ammonium complexes, this to minimize uptake by the resin of positively charged metal ions of other metals. (For example, amidoxime resins also have an affinity for seaborne vanadium.) Their experiments show a higher affinity for uranium in seawater than do other amidoxime type resins, of which many have been explored and tested.
For synthesis of their resins, they utilized ultrahigh molecular weight polyethylene to which they radiation grafted acrylonitrile, which is functionalized as an amidoxime with hydroxylamine, as well as 2-(dimethylamino)ethyl methacrylate which they abbreviate as "DMAEMA."
The following schematic from the paper shows the synthetic strategy:
The caption:
To get a feel for the different types of absorbents represented in this scheme, it is useful to look at some excerpts of how they were synthesized:
...The AN fibers were then irradiated with an electron beam at an absorbed dose of 20 kGy in air at room temperature. Then, the irradiated AN fibers were immersed in a solution consisting of 10 vol % DMAEMA, 22.5 vol % MeOH, and 67.5 vol % water after purging with nitrogen for 30 min for deoxygenation. The grafting reaction was carried out at 60 °C for 5 h. The obtained fibers were washed with deionized water four times and dried to a constant weight in a vacuum oven at 60 °C. The resultant fibers were referred to as AN-DMAEMA fibers...
...To modify the fibers with AO groups and quaternary ammonium groups, two synthesis approaches were investigated according to the sequence of amidoximation and quaternization (Scheme 1): Method A: amidoximation was carried out first before quaternization of the tertiary amino group, and the samples obtained were referred to as AO-Q fibers. Method B: amidoximation was carried out after quaternization of the tertiary amino group, and the synthesized samples were referred to as Q-AO fibers. AO density (D(AO)) and quaternary ammonium density (D(Q)) on the modified fibers were evaluated...
The quaternization was performed using n-bromobutane.
The IR spectra of the fibers giving a feel for their differences:
The caption:
The relative ability of the different resins to absorb uranium on a weight basis:
The resins were tested in simulated and real seawater.
The kinetics in simulated seawater:
The caption:
The selectivity in real seawater:
To grasp the meaning of the data in the table that follows, it is useful to take a look at two equations for the variables described therein and the associated text (as a graphics object):
Q sub M here is a weight ratio essentially obtained by digesting the resin in a microwave in the presence of acid (which oxidizes it) and determining via inductively coupled plasma mass spectrometry (ICP/MS) the weight of the uranium in the resin.
Table 2 showing the values of K(M), K(U) and the selectivity β.
The authors' conclusions:
As I indicated above, these resins are not likely to be necessary to maintain access to uranium to power nuclear reactors for sustainable energy for many centuries, assuming we use our existing isolated uranium more wisely in "breed and burn" scenarios. (Wise use of nuclear resources would involve also utilizing the other actinides, specifically neptunium, americium, and curium as well as uranium and plutonium. I also note that valuable radioactive and nonradioactive fission products should also be recovered and put to use.)
We thus have many centuries to develop superior technologies not only for the recovery of elements (and fresh water) from seawater, but which may include getting past the goal line with respect to fusion power. A more immediate use would be to remove chemotoxic uranium from drinking or agricultural water where it may exist as a result of natural geologic or anthropogenic activities.
I hope, in spite of the immediate threat of Covid-19, that you are enjoying the great privilege of being alive as well as is possible in these circumstances. Here in New Jersey, it's a beautiful spring day.
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