Plutonium Polymer Mystery Solved

Earlier this week a paper by L. Soderholm et al. in Angewandte Chemie may have solved the great plutonium polymer mystery. Plutonium polymer is the ubiquitous noun often spoken by plutonium chemists in regards to the un-extractable ill-defined hydrous oxides of plutonium that will form in any solution of aqueous plutonium lying about the bench top. Often plutonium polymerization can be inhibited by storing aqueous plutonium solutions at high acid concentrations. It was thought to form from a series of olation reactions:Plutonium Olation Reaction: Pu—OH + Pu—OH₂ —> Pu—OH—Pu + H₂O

This old hypothesis is put to rest with the isolation of Li₁₄(H₂O)_n[Pu₃₈O₅₆Cl₅₄(H₂O)₈]. This occurred after repeated anion-exchange with an acidified alkaline peroxide solution of plutonium, that then crystallized in the presence of aqueous LiCl. This type of workup is common for samples containing plutonium polymer. The crystal is reported to have the same intracluster packing and structural topology as bulk PuO₂. The crystal structure of the [Pu₃₈O₅₄(H₂O)₈]⁴⁰⁺ is shown below-left and a picture of the [Pu₃₈O₅₆Cl₅₄(H₂O)₈]^14-is shown below right.

Reprinted pending permission from John Wiley & Sons, Inc.: Angewandte Chemie International Edition (Dec 2007).

The Plutonium is in green, oxygen from the oxide in red, and oxygen from water in blue. With this and other evidence of well defined Pu—O clusters and the lack of hard evidence for oxyhydroxides they expect plutonium to condensate through an oxolation reaction.

Plutonium Oxolation Reaction: 2Pu—OH —> Pu—O—Pu + H₂O

The one caveat with this work is that it was performed with the more stable plutonuium-242 (t_1/2=3.7 x 10⁵ y) and not the typical reactor plutonium-239 (t_1/2=2.4 x 10⁴ y). Perhaps in the presence of the >10x more radioactive Pu-239 the nanoclusters would become either too structurally damaged to resolve nice crystalline structures, or more chemically reactive towards hydrous oxide formation or oxyhydroxide formation. Regardless, this work may still lead to better methods of extracting plutonium out of the nuclear fuel cycle and represents a nice resolution to the nebulous plutonium polymer conundrum.

Link to Paper: http://dx.doi.org/10.1002/anie.200704420

Mitch

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