Synthetic Smectite Colloids: Characterization of Nanoparticles after Co-Precipitation in the Presence of Lanthanides and Tetravalent Elements (Zr, Th)

Abstract

The magnesian smectite hectorite is a corrosion product frequently detected in nuclear waste glass alteration experiments. The structural incorporation of a single trivalent lanthanide was previously demonstrated. Hectorite was presently synthesized, for the first time, in the presence of several lanthanides (La, Eu, Yb) following a multi-step synthesis protocol. The smallest-sized particles (nanoparticles, NPs) were isolated by centrifugation and analyzed by asymmetrical flow field-flow fractionation (AsFlFFF) coupled to ICP-MS, in order to obtain information on the elemental composition and distribution as a function of the size. Nanoparticles can be separated from the bulk smectite phase. The particles are able to accommodate even the larger-sized lanthanides such as La, however, with lower efficiency. We, therefore, assume that the incorporation proceeds by substitution for octahedral Mg accompanied by a concomitant lattice strain that increases with the size of the lanthanides. The presence of a mixture does not seem to affect the incorporation extent of any specific element. Furthermore, syntheses were performed where in addition the tetravalent zirconium or thorium elements were admixed, as this oxidation state may prevail for many actinide ions in a nuclear waste repository. The results show that they can be incorporated as well.

Yttrium co-precipitation with smectite: A polarized XAS and AsFlFFF study

“…The hectorite octahedral sheet mostly contains Mg which can be used as an indicator of the pres ence of the smectite. Information on the Y(III) binding mode with the smectite can be derived from the variation of the Mg:Y ratio as a func tion of the size (Bouby et al, 2011(Bouby et al, , 2012(Bouby et al, , 2015Finck et al, 2012). For Y structurally incorporated within the hectorite structure, the Mg:Y ratio is expected to be constant for all particle sizes.…”

“…For Y structurally incorporated within the hectorite structure, the Mg:Y ratio is expected to be constant for all particle sizes. For Y(III) adsorbed at the surface of hectorite, the NP associated amount of Y(III) is expected to follow the surface area to volume ratio, and thus the Mg:Y ratio is ex pected to change with the NP size (Bouby et al, 2015).…”

“…The theoretical background of AsFlFFF and the coupling to sensitive detection tech niques is well documented (e.g., Hassellöv et al, 1999;Schimpf et al, 2000;Bouby et al, 2008;Dubascoux et al, 2008). Recently, AsFlFFF coupled to ICP MS has been applied to study the interaction of Lu(III), Eu(III) or a mixture of lanthanides (La(III), Eu(III) and Yb(III)) (Bouby et al, 2015) with hectorite. Analysis of NP mobilized from the bulk hectorite synthesized in the presence of Lu(III) (Bouby et al, 2012) in dicated a homogeneous Lu(III) association (incorporation) to hectorite over the whole size range (17 200 nm).…”

Fate of Lu(III) sorbed on 2-line ferrihydrite at pH 5.7 and aged for 12 years at room temperature. I: insights from ICP-OES, XRD, ESEM, AsFlFFF/ICP-MS, and EXAFS spectroscopy

“…At this stage, it thus can only be concluded that a minor but significant fraction of lutetium present is intimately associated with the detected iron colloids. Due to the irregular shape of the iron colloids (elongated needles on rhombohedra), results might better suggest a lutetium immobilization by incorporation within iron colloids rather than by surface adsorption which might have resulted in a less constant Fe/Lu ratio according to previous studies (Bouby et al 2015;Finck et al 2012). Information on the nature of the Lu species and on the phase(s) immobilizing Lu(III) will be obtained by probing the Lu L 3 -edge by X-ray absorption spectroscopy (see below).…”

“…The morphology and mineralogical composition of the transformation products, and the Lu(III) repartition between the solid and the liquid phase were determined. Information on the Lu(III) association mode with the particles was obtained by application of the asymmetrical flow field-flow fractionation (AsFlFFF) technique coupled to sensitive detection techniques using similar methodology as in earlier works with various NPs (e.g., (Bouby et al 2011;Bouby et al 2012;Bouby et al 2015;Huber et al 2012)) whereby Fe was used as fingerprint of the presence of Fe phases (hematite and/or goethite). Information on the retention mode and on the nature of the host phase was obtained by X-ray absorption spectroscopy at the Lu L 3 -edge.…”