Disproportionation and Polymerization of Plutonium(IV) in Dilute Aqueous Solutions

Abstract

The rates of polymerization and disproportionation of Pu(IV) have been studied using low concentrations: (1.7 − 10) × 10−6M Pu, (0.8 − 12) × 10−4M HCI and 0.01M ionic strength. Osmium(II) complexes such as the tris−4,41−2,21−bipyridine complex were found to react rapidly with Pu(IV) but very slowly, if at all, with Pu(IV) polymer, Pu(lll), or Pu(V). Thus, it is possible to determine unreacted Pu(IV) in the presence of reaction products by using Os(II) complexes. Disproportionation reaction products, Pu(IlI) and Pu(V), were determined using their reactions with Ce(IV) sulfate. We find −d[Pu(IV)]/dt = k'[Pu(IV)]2 at constant pH. Log k1 varies from about 4.25 at pH 3 to about 7.0 at pH 4.1 (units for k1 are M−1min−1). The [H+] dependence varies from about −2 to −3 over the pH range studied. The measured rate is the sum of those for polymerization and disproportionation; the latter reaction amounts to about 75% of the total at pH 3 and 20% at pH 4. The second-order rate constants for disproportionation are very much larger than expected on the basis of extrapolation from 0.2 to 1.OM HClO, solutions. The products of the reaction do not affect the rate, but U(VI), aged Pu(IV) polymer, and CO2 increase the rate.

Surface chemistry of Pu oxides

“…Pu 4f binding energies for the colloidal material are very similar to those observed for the hydroxide compound. This is not surprising, since colloidal Pu(IV) is thought to be composed of extended polymer-like hydroxylated Pu oxides, in which different O ligands will form different types of bonds with Pu [18][19][20]. Water may also be a component of this material and is consistent with the greater intensity observed at higher binding energies in the O 1s region.…”

Plutonium uptake by brucite and hydroxylated periclase

“…Pu ions exposed to this elevated pH will tend to hydrolyze immediately and then adsorb on the brucite surface. The polymerization reaction for Pu at 10 −8 M is slow, so colloid formation under the batch sorption conditions is expected to be negligible [14].…”

New insights in the formation processes of Pu(IV) colloids

“…The role of Pu(IV) polynuclear complexes, however, is still not clear. Their existence is generally accepted [13][14][15][16][17] and it is assumed that formation processes involve hydroxide or oxygen bridging of Pu nuclei [17], but two recent reviews [18,19] consider Pu(IV) polynuclear species not thermodynamically stable and favor a model which describes the formation of colloids from mononuclear hydroxide complexes (Pu(OH) n 4−n ). Alternatively, a two step model was suggested [15]: In analogy to the behavior of Th(IV) [17,20], small polymeric species such as the Pu trimer should form and after an induction period depending on the degree of oversaturation of the solution [21] the colloids should grow rapidly.…”