Thomas Rabung

Macroscopic and spectroscopic investigations on Eu(III) and Cm(III) sorption onto bayerite (β-Al(OH)3) and corundum (α-Al2O3).

The interaction of trivalent Cm and Eu with the aluminum hydroxide bayerite (β-Al(OH)3) and the aluminum oxide corundum (α-Al2O3) was investigated by batch sorption experiments and time resolved laser fluorescence spectroscopy (TRLFS). The experimental methods for both polymorphs show similar pH dependent sorption behavior at trace metal ion concentrations (∼10(-7) M), i.e. similar Eu sorption edges and nearly identical Cm speciation between pH=3 and 13. In this pH range the Cm aquo ion as well as the Cm(III) surface species surface⋯Cm(OH)x(H2O)(5-x) (x=0, 1, 2) can be distinguished by TRLFS. The similar sorption data point to a (surface) transformation of the thermodynamically unstable Al2O3 surface into bayerite, in agreement with the similar isoelectric points obtained for both minerals (pH(IEP)=8.6-8.8). The pH dependent surface charge is most likely due to the protonation/deprotonation of singly coordinated Al-OH surface groups, prevailing on the edge planes of the rod-like bayerite crystals and the surface of the colloidal Al2O3 particles. These surface groups are also believed to act as ligands for lanthanide/actinide(III) surface complexation. In contrast to the similar sorption behavior at trace metal ion concentrations, discrepancies are observed at higher Eu levels. While similar sorption edges occur up to 7×10(-7) M Eu for corundum, the pH edge on bayerite is gradually shifted to higher pH values in this Eu concentration range. The latter behavior may be related either to the existence of multiple sorption sites with different sorption affinities, or to the influence of an additional amorphous Al-phase, forming in the course of the bayerite synthesis.

Interaction of Nd(III) and Cm(III) with borate in dilute to concentrated alkaline NaCl, MgCl2 and CaCl2 solutions: solubility and TRLFS studies

The interaction of lanthanides and trivalent actinides with borate in dilute to concentrated alkaline NaCl, MgCl2 and CaCl2 solutions was investigated at 22 ± 2 °C by a comprehensive series of solubility experiments with Nd(OH)3(am), and complemented with Cm(III)–TRLFS studies (TRLFS: time resolved laser fluorescence spectroscopy) under analogous pH and ionic strength conditions. Although there was clear evidence of borate complexation in the pH range of 8.5 to 10, overall no significant increase in Nd(III) solubility occurred in any of the investigated salt systems in the presence of [B]tot ≤ 0.4 M, compared with analogous borate-free solutions. On the contrary, a significant decrease in Nd(III) concentration was observed at pHc ≤ 9 in NaCl and MgCl2 systems with [B]tot ≥ 0.16 M (diluted salt systems) or [B]tot ≥ 0.04 M (concentrated salt systems). This observation, together with a clear change in the slope of the solubility curve and the further confirmation by XPS analyses, indicates the transformation of Nd(OH)3(am) into a so far unknown Nd(III)–borate solid phase with significantly lower solubility. Similar Nd(III) concentrations in the aqueous phase are obtained in undersaturation solubility experiments conducted with a synthesized crystalline phase Nd[B9O13(OH)4](cr). TRLFS confirmed the formation of aqueous Cm(III)–borate complexes in dilute to concentrated NaCl and MgCl2 systems at pHc = 8 and [B]tot ≥ 0.04 M. Two different Cm(III)–borate species are proposed based on the peak shift of the spectra, although the resulting fluorescence emission bands do not allow the definition of an unequivocal chemical model for this system. TRLFS also shows that no Cm(III)–borate complexes form under hyperalkaline conditions (pHc = 12), due to the stronger competition posed by hydrolysis and the predominance of weakly coordinating B(OH)4− in the aqueous phase. These results show the impact of An(III)–borate interactions on An(III) speciation and highlight the hitherto unknown role of borate in the immobilization of trivalent actinides under repository-relevant conditions due to the formation of borate-bearing solid phases with significantly lower solubility than the corresponding hydroxides.

Actinide-Nanoparticle Interaction: Generation, Stability and Mobility

Actinide ions are widely considered immobile under environmental conditions relevant to their disposal notably under reducing conditions prevalent in subsoil systems. Colloid formation is, however, known to increase their mobility. Observations of this in environmental systems are regularly reported. Different processes may lead to actinide colloid formation, by either generation of polymeric oxide/hydroxide nanoparticles or sorption to already existing or neo-formed ground water colloids. In order to assess actinide mobilization by colloidal species, the cascade of individual colloidal processes under given geochemical conditions must be considered and quantified: colloid formation, actinide–colloid interaction, colloid stabilization and migration. This chapter describes and discusses actinide colloid reactions with a focus on the underlying mechanisms.

The specific sorption of Np(V) on the corundum (α-Al2O3) surface in the presence of trivalent lanthanides Eu(III) and Gd(III): A batch sorption and XAS study.

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as a competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl(V) ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10(-5)M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was applied to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopy (XAS) measurements were carried out on the samples containing only Np(V) and Np(V)+Gd(III). The results reveal the presence of a bidentate Np(V) edge-sharing complex on the corundum surface in the absence of Gd(III), while the coordination environment of Np(V) on the corundum surface could be changed when Gd(III) is added to the sample before the sorption of Np(V).