XAFS and LIBD Investigation of the Formation and Structure of Colloidal Pu(IV) Hydrolysis Products.

Abstract

Colloids ColloidsG 2000 XAFS and LIBD Investigation of the Formation and Structure of Colloidal Pu(IV) Hydrolysis Products. -The structures of Pu(IV) oxyhydroxide eigen-colloids formed upon increasing the pH of an aqueous Pu(IV) solution are characterized by XAFS and laser induced breakdown detection (LIBD). A model of colloid formation is proposed, which leads to a face-centered cubic Pu sublattice having cation defects. The solubility for Pu(IV) measured with LIBD is close to the lower limit of the solubility curve from previously reported data. -(ROTHE, J.; WALTHER, C.; DENECKE*, M. A.; FANGHAENEL, T.; Inorg. Chem. 43 (2004) 15, 4708-4718; Inst. Nukl. Entsorgungstech., Forschungszent.

Relevance of formation conditions to the size, morphology and local structure of intrinsic plutonium colloids

“…Hence, higher amount of surface defects (e.g., edges, corners, strains, steps…), surface groups (-OH functions, µ 3 -oxo bonds, adsorbed water molecules…) and under-coordinated surface atoms can be expected for such nanoparticles. 15,22,36,46,51,53,63,68 These differences may also explain the slight WL XANES differences noted in Fig. 5a between our very similar samples.…”

“…This observation also agrees with XANES spectra of Pu(IV) intrinsic colloid from the literature. 15,22,50,51 The energy position of the white line (WL) evidences a similar electronic structure (< 1 eV difference) for all of the samples: 18068.4 eV for SC-0.8-H 2 O, 18068.0 eV for PuO 2 reference and 18067.4 eV for all of the hydrolytic colloids (HC-10-HNO ; HC-5-HNO 3 and HC-5-H 2 O). In comparison to molecular Pu(IV) 47,49 plutonium intrinsic colloids show a similar position with a lower amplitude combined to a wider and asymmetrical WL as recently evidenced by Vitova et al 50 In this study, the authors combined XANES and computations to demonstrate that intrinsic Pu(IV) colloids exhibit band-like 6d states similar to bulk PuO 2 contrasting with the narrower 6d states expected in molecular Pu(IV) species.…”

“…It was previously suggested that the EXAFS amplitude reduction results from a strong local disorder in the bulk Pu atoms (decrease of crystallinity and increase of the DWF) and/or by a nanoparticle surface effect attributed to the increasing proportion of plutonium atoms standing at the particle surface in comparison to the bulk when decreasing in size (S/V atomic ratio). 15,22,36,51,53 The more specific distortion of the Pu-O coordination sphere has been suggested to result from a splitting of the Pu-O distances in the Pu oxide local structure. Several hypotheses have been discussed to explain the nature of these bonds including the presence of Pu-OH, Pu-O and Pu-H 2 O distances, O defects or also the presence of short Pu=O oxo bonds found in Pu(V) or Pu(VI) moieties.…”

“…Several hypotheses have been discussed to explain the nature of these bonds including the presence of Pu-OH, Pu-O and Pu-H 2 O distances, O defects or also the presence of short Pu=O oxo bonds found in Pu(V) or Pu(VI) moieties. 15,50,53,58,59,62 The two above hypotheses (bulk disorder and enhanced surface atom contribution) result in similar effects on the EXAFS spectra and are hence difficult to distinguish. .…”

“…6,[12][13][14] The formation of very stable Pu(IV) colloidal suspensions (referred to as intrinsic colloids but also polymers or eigen colloids in the literature) may result through olation or oxolation mechanisms even at relatively high nitric acid concentration (0.5 M). 12,[15][16][17] The preparation of stable suspensions of intrinsic Pu(IV) colloids can be managed through several methods including the precipitation from Pu(IV) solutions with alkali and peptization, 6,13 radiolytic processes with Pu ions, 6,18 or the "simple" pouring of nitric Pu(IV) solutions at room temperature or boiling pure water. 6,[19][20][21] Recently, an alternative synthesis method using sonochemistry was proposed for the preparation of stable intrinsic Pu(IV) colloids in pure water free of electrolytes or stabilizing agents.…”