Some years back, in 2007 - which in NNadir time is about 150 billion tons of dumped dangerous fossil fuel ago (or in more traditional units of time, about 5 years ago) - I wrote a diary in this space about a fellow who was shot in the head in 1915, Henry Moseley, who made a very important discovery about the nature of matter, which explained some things that confused scientists around that time, in particular with regard to the fact that cobalt's atomic weight was higher than that of nickel, and that the atomic weight of tellurium exceeded that of iodine.
That diary is here:
A Very Fine Exploded Head: Henry Moseley and "Dangerous Nuclear Waste."
That diary was on the subject of a metal that is only found in almost undectable amounts in uranium ores, technetium.
I haven't commented, as I recall, in quite some time, on this remarkable element which despite its rarity in nature, is now available in metric ton quantities from used nuclear fuel. It's time for an update, since a lot is going on in technetium chemistry these days.
I was going to write a diary on lanthanide mining in Greenland - addressing the question of what our oil and gas drilling friends in Denmark intend to do with the side product of said mining - the naturally occurring element uranium - but I'm in a good mood tonight, despite everything, and would rather discuss something pleasant, technetium.
There has been a huge burst of work recently on the chemistry of technetium, and I thought I'd touch upon them.
Technetium, element 43, is the lightest element in the periodic table table that has only radioactive isotopes. No stable isotopes exist. Of the first 83 elements in the periodic table, only 79 have stable isotopes, promethium, technetium and bismuth are all radioactive, the latter so weakly so that its radioactivity was only recently discovered.
Promethium, like technetium, is a fission product, however the half-life of promethium isotopes - the most commonly available is Pm-147 which has a half-life of 2.6234 years - is so short that it is possible (and then at fairly high expense) to isolate a few kilograms at most. (Promethium has one longer lived isotope, Pm-145 but it is not commonly available from used nuclear fuel because it is not neutron rich. Thus Pm-145 is commonly made using accelerators as opposed to fission sources.)
By contrast, the half-life of technetium-99 - which represents a product of about 6% of nuclear fissions in the core of a nuclear reactor - has a half-life of 211,100 years and thus, as mentioned above, is available in multi-ton quantities.
Recently there has been quite a bit published on this metal and I thought I'd touch on some aspects of technetium metallurgy and chemistry.
The main source of technetium compounds in the United States seems to be from Oak Ridge National Laboratory. The element is supplied in the form of ammonium pertechnate, which has the formula NH4TcO4. The pertechnate ion, and the ammonium salt are very water soluble compounds, which leads to some level concern when discussed in the context of the somewhat insipid concept of so called "nuclear waste."
Nothing, absolutely nothing, is "waste" if you have a use for it.
In its environmental chemistry - and in many other aspects of its chemistry - technetium is present as the TcO4- ion, which is analogous to the permanganate ion formed by technetium's cogener in the period table, manganese. (The other cogener is the very rare and very expensive element rhenium.) However, permanganate is a fairly powerful oxidizing agent and is readily reduced to manganese dioxide, for instance by organic compounds. Pertechnate, which has a standard electrode potential that is less than half that of permanganate, is a much less powerful oxidant, and is thus fairly stable. The chemistry of pertechnate is more analagous to perrhenate than permanganate.
Even so there is some very interesting approaches to the preparation of nanoscale technetium metal that involves (more or less) the oxidation of organic matter. An expert in the chemistry of technetium is Dr. Frederic Poineau - predictably he received his Ph.D. in France - who now works at the Harry Reid Center for Environmental Studies at the University of Nevada at Las Vegas. Because technetium's chemistry is anionic, it may be separated from various matrices (water for instance) by the use of anionic exchange resins.
Such an exchange resin is generally made from a plastic such as polystyrene - my most recent diary The Use of Carbon Dioxide To Oxidize Ethylbenzene to Styrene was about this polymer. Chemists use this to make exchange resins since it contains easily modifiable aromatic (benzene) rings. In cation exchange resins a typical functionality might be a sulfonyl group; in anion exchange resins things like quartenary amino functions are used. (More exotic functionalities are used in resins for things like peptide synthesis.)
Dr. Poineau and his co-workers describe in a recent paper Journal of Radioanalytical and Nuclear Chemistry, Vol. 279, No.1 (2009) 43–48 the preparation of technetium metal microspheres by collecting pertechnate on such resins (Dowex Marathon and Reillex HPQ) and then strongly heating them in a tube furnace.
Actually the resin is not directly reduced by pertechnate. Ammonium pertechnate is reduced by the thermal decomposition of the compound by the following chemical reaction: NH4TcO4 → TcO2 + 1/2 N2 + 2H2O.
At high temperatures, TcO2 disproportionates (auto oxidizes and autoreduces) to technetium metal and the volatile compound Tc2O7, the latter being distilled away. However to avoid this disproportionation, Poineau kept the temperature below 1000C. In this case the polystyrene resin (in the presence of water present as steam) was oxidized in a reformation reaction giving hydrogen and carbon monoxide gas, both of which can reduce technetium dioxide to the metal.
The disproportionation of technetium (along with its variable oxidation states) suggests its use in so called "chemical looping" processes as an oxygen carrier, although in truth there are many other transition (and non transition) metals that can do the job equally well and are available more cheaply and more on scale.
"Chemical looping" is a scheme wherein carbon compounds can be burned under conditions such that there is no need for use of a smokestack. It's esoteric but real.
The paper (if you can access it) contains some nice micrographs of broken technetium metal nanospheres.
The volatility of Tc2O7 has been used for a novel separation of techentium from used nuclear fuel rods. A process of this type is described in a paper that is readily accessible on the internet: Fission Product Removal From Spent Oxide Fuel By Head On Processing.
This is a very elegant - elegant because it is simple - approach to the facile separation of several valuable elements (only one of which is technetium) from nuclear fuel via the use of pure oxygen or oxygen/ozone mixtures. The other elements separated are ruthenium (the tetraoxide is however a very powerful oxidizing agent and can be explosive in high concentrations), rhodium oxide, tellurium oxide, and cesium oxide.
The rhodium is a very, very, very, very, very valuable element, and as I have pointed out elsewhere, within a few decades the primary reserves of this element will all be in used nuclear fuel.
Supply of Rhodium in Used Nuclear Fuel To Exceed World Supply From Ores by 2030.
Although there are many plans in various facilities to recover rhodium during nuclear fuel reprocessing, the chemistry is somewhat awkward, especially when compared to this approach. I like it. It's cool.
Because of the popularity on the internet of liquid phase reactors - many of these involve fluoride based molten salts, we tend to think of volatile fluorides as an excellent mode of volatilizing various products including uranium but also things like technetium. I have argued in various settings that just as Jesus is said to have said that "man does not live by bread alone" one can say that fluid phase reactors need not live by fluoride alone. But that's just my opinion. I like to see oxygen spoken for. (There are many other approaches that need not involve simple anions like fluoride or oxide.)
One of the very interesting aspects of technetium chemistry involves the remarkable antiferromagnetic properties of group II salts of technate, compounds like CaTcO3 and the corresponding strontium and barium salts. J. Am. Chem. Soc., 2011, 133 (6), pp 1654–1657 These types of materials are very useful in the development of things like disk drives, although you cannot collect much anti-nuke stupidity on a radioactive disk drive I would imagine.
Perish the thought.
Technetium, as is widely known among people who think of such things, as the pertechnate ion discussed above, has remarkable anti-corrosion properties for steel.
The metal itself is also a powerful refractory metal, with a melting point of over 2000C. This suggests the use of the metal or its alloys in specialized superalloys. Many such alloys are known that contain rhenium, again the cogener of techentium. Superalloys are often expensive, but are widely used. The most important current use is in jet turbine engines, but many nuclear applications, and other energy applications are possible. The accumulation of this element allows one to dream of large scale applications for a variety of such alloys.
It is worth noting that with the use of nuclear energy, it is relatively easy to imagine the supply of technetium easily exceeding the world supply of rhenium.
This fact has already received much notice in the case of evaluating various types of catalysts for organic synthesis, and technetium has been evaluated in certain kinds of redox ketone/alcohol systems.
Recently there has been an explosion of interest in technetium alloys, but not enough to do anything but whet my appetite for thinking about such things.
Some recent publications along these lines include Inorg. Chem., 2010, 49 (4), pp 1433–1438, more of Poineau's work. This paper is about zirconium/techentium alloys, and the rationale for the paper concerns long term storage of technetium, rather than metallurgy itself, which is hopefully driven not by a real sense of the need to "dispose" of this element, but rather as a cover scheme for getting grants.
Other reports in the literature consist of discussions of the properties of iron technetium alloys Journal of Nuclear Materials 408 (2011) 183–187 and alloys of zirconium and the various constituents of stainless steel. Journal of Nuclear Materials 277 (2000) 333±338
These papers contain - regrettably - "waste" talk as well, and I would have enjoyed reading more about their metallurgy.
Right now, the primary use of technetium today involves its use in medicine. A nuclear isomer of the common fission product 99Tc, 99mTc, is considered the "workhorse" of nuclear medicine, both in imaging and treatment. The latter is made in cyclotrons from molybdenum and decays (with the release of gamma radiation) to the former, whereupon it is pissed away by the patient.
Technetium literature is dominated by literature on medicinal chemistry.
I could say more, but I'm as tired as hell, and actually hope to get some sleep this evening.
Have a wonderful day tomorrow.
