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PostPosted: Nov 26, 2009 12:09 am 
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I am always interested in the chemistry of the transuranium actinides. I hope I'm not too dissident here, as someone who does not wish to discount the utility of plutonium - deep respect and appreciation for the use of thorium aside - I would like to see more work done on the chemistry of the elements americium, curium, berkelium and californium, since I believe these elements can and should be of increasing technological importance should humanity survive.

Of the elements I listed, only americium is of true technological and economic importance, mostly because of its use in smoke detectors, where it has probably worked to save tens of thousands - if not hundreds of thousands - of lives.

The amount of americium required for this application is exceeding small, however, and it would surprising to learn that the world demand for the element is greater than a few kg, if that. I would expect - without direct knowledge - that almost all of this americium has been produced using raffinates from solvent extraction for plutonium isolation, followed by some kind of ion exchange protocol.

If you must know, I don't really like it when americium is formed. Every atom of americium-241 on earth these days represents an atom of an isotope I like very much, Pu-241, that has been allowed to decay before being put to good use. I'd rather that didn't happen, but it does happen, routinely and in fairly large quantities. (It is, in any case, inevitable that when one has Pu-241, one also has some Am-241.)

That said, I can think of good uses to which americium may be put if one has it, besides some elaborate scheme to dump it. It is potentially a very convenient source of highly isotopically pure Pu-238, for instance, via Cm-242. Probably one can obtain Pu-238 that is isotopically more pure than what one can obtain from Np-237. Also the element has interesting metallurgy.

Anyway, I read with interest a recent paper that I thought I’d share with this group on the subject of the purification of Am. Although Am can be oxidized to the V oxidation state, it is very lanthanide like in its chemistry, with a well defined and stable III state.

This property makes it somewhat problematic in redox systems to separate from the lanthanides which are, of course, fission products.

The paper in question is Hydrometallurgy 99 (2009) 18–24 and it out of the Bhabha Atomic Research Centre, in Mumbai, India, where so much excellent work on actinide chemistry is now being performed. The title of the paper is Ethyl-bis-triazinylpyridine (Et-BTP) for the separation of americium(III) from trivalent lanthanides using solvent extraction and supported liquid membrane methods.

Indulging any organic chemists here, let me state briefly that Et-BTP is obtained from 2,6 cyanopyridine - which I happen to know is commercially available - via condensation with hydrazine, followed by condensation with 3,4 hexane dione, which doesn’t seem all that synthetically challenging in itself, not that I am aware of any industrial preparation of the compound. The latter step is classic heterocyclic chemistry.

The process of the actual separation is a solvent extraction method, and although I sometimes deprecate solvent extraction – I’m not a PUREX or TRUEX kind of guy – I am also not a “one size fits all kind” of guy either. Solvent extraction has its place, I suppose.

More problematic is the solvent – rather than the heterocyclic complexation agent/extractant represented by the triazinylpyridine – used in some of the experiments, 2-nitrophenyloxyoctane, or aka 2-nitrophenyl octyl ether, NPOE. This solvent is obtained from 2-nitrophenyl phenol, which industrially must be separated from the 4-nitrophenol isomer, nasty chemistry, and then reacted with what I would expect to be a fairly expensive primary halogenated octane.

Still, as the quantities of americium available are probably always going to be at a scale of a few tons, the price is probably acceptable, particularly with recycling of solvents.

In any case, distribution constants in this system, using Eu(III) as a model lanthanide, probably because strictly put, Eu is a “cogener” of americium although their chemistry differs considerably, are on the order of 500 or greater.

Pretty good results were also obtained with n-alkane type solvents as well.

This is a pretty good separation. Actually I would expect the separation of europium and americium to be relatively simple compared to other lanthanides in the absence of such a process, since europium is readily reduced to the II oxidation state, where its chemistry is remarkably like that of barium. It may be of more interest for situations where the lanthanide is more problematic like say, gadolinium, samarium or even neodymium.

I found this paper fun and interesting, if esoteric.


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