Adsorption of Plutonium Oxide Nanoparticles

Abstract

Adsorption of monodisperse cubic plutonium oxide nanoparticles ("Pu-NP", [Pu(38)O(56)Cl(x)(H(2)O)(y)]((40-x)+), with a fluorite-related lattice, approximately 1 nm in edge size) to the muscovite (001) basal plane from aqueous solutions was observed in situ (in 100 mM NaCl background electrolyte at pH 2.6). Uptake capacity of the surface quantified by α-spectrometry was 0.92 μg Pu/cm(2), corresponding to 10.8 Pu per unit cell area (A(UC)). This amount is significantly larger than that of Pu(4+) needed for satisfying the negative surface charge (0.25 Pu(4+) for 1 e(-)/A(UC)). The adsorbed Pu-NPs cover 17% of the surface area, determined by X-ray reflectivity (XR). This correlates to one Pu-NP for every 14 unit cells of muscovite, suggesting that each particle compensates the charge of the unit cells onto which it adsorbs as well as those in its direct proximity. Structural investigation by resonant anomalous X-ray reflectivity distinguished two different sorption states of Pu-NPs on the surface at two different regimes of distance from the surface. A fraction of Pu is distributed within 11 Å from the surface. The distribution width matches the Pu-NP size, indicating that this species represents Pu-NPs adsorbed directly on the surface. Beyond the first layer, an additional fraction of sorbed Pu was observed to extend more broadly up to more than 100 Å from the surface. This distribution is interpreted as resulting from "stacking" or aggregation of the nanoparticles driven by sorption and accumulation of Pu-NPs at the interface although these Pu-NPs do not aggregate in the solution. These results are the first in situ observation of the interaction of nanoparticles with a charged mineral-water interface yielding information important to understanding the environmental transport of Pu and other nanophase inorganic species.

Effects of the background electrolyte on Th(IV) sorption to muscovite mica

“…The second peak is broader and reaches up to 50 Å height from the surface. This distribution is similar to what has been observed upon adsorption (Schmidt et al, 2012b) and formation (Schmidt et al, 2013) of Pu nanoparticles on the muscovite-water interface, but the coverage of Th is smaller than those of Pu in the adsorbed nanoparticles (4.9 Th/A UC compared to 9.9 and 10.8 Pu/A UC , respectively).…”

“…Uptake quantification performed ex situ after two flushing steps finds a significantly smaller coverage of h a = 1.6 ± 0.3 Th/A UC than the coverage h RAXR = 4.9 Th/A UC found by RAXR. The plutonium nanoparticles reported previously were not desorbed by similar flushing steps, and RAXR and alpha spectrometry found matching coverages in these experiments (Schmidt et al, 2012b(Schmidt et al, , 2013. For Th extended outer sphere complexes, however, it has been demonstrated that the sorption reaction is partly reversible (Schmidt et al, 2012a).…”

“…The liquid film was then enclosed with a 7.5 lm-thick Kapton Ò film and the cell was closed with two additional Kapton Ò domes for containment. This setup has been described in more detail previously (Schmidt et al, 2012b). For the exchange experiment with sample ThClClO 4 the sorption reaction was performed in NaCl background media under the same conditions as described above.…”

Sorption of tetravalent thorium on muscovite

“…The measured Pu coverage was also larger than the value expected for the surface charge compensation by trivalent Pu, and was interpreted to be indicative of an in situ oxidation of plutonium followed by formation of Pu clusters on the surface. In a second study, the sorption of Pu-oxo-nanoparticles at the muscovite (0 0 1) basal plane has been elucidated (Schmidt et al, 2012) revealing specifics of the nanoparticle-mineralsurface interaction. The nanoparticles were shown to adsorb on the (0 0 1) basal plane in one well-defined layer as well as in randomly distributed aggregates from an aqueous solution of monodisperse plutonium-oxide nanoparticles ([Pu 38 O 56 Cl x (H 2 O) y ] (40Àx)+ , [Pu] tot = 10 À3 mol/L, pH = 2.6).…”

“…An understanding of Th chemistry for its own sake is useful to help delineate the similarities and differences in the behavior of these two elements. Toward this end, and to complement our efforts relevant to the understanding Pu migration in the environment (Soderholm et al, 2008;Fenter et al, 2010;Wilson et al, 2010;Schmidt et al, 2012), we report on experiments probing Th adsorption onto the (0 0 1) surface of muscovite mica.…”