Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles

Abstract : Sorption/desorption kinetics for selected radionuclides (Tc-99(VII), Th-232(IV), U-233(VI), Np-237(V), Pu-242 and Am-243(III)) under Grimsel (Switzerland) ground water conditions (pH 9.7 and ionic strength of similar to 1mM) in the presence of synthetic Zn or Ni containing montmorillonite nanoparticles and granodiorite fracture filling material (FFM) from Grimsel were examined in batch studies. The structurally bound Zn or Ni in the octahedral sheet of the synthetic colloids rendered them suitable as colloid markers. Only a weak interaction of the montmorillonite colloids with the fracture filling material occurs over the experimental duration of 10,000 h (similar to 13 months). The tri- and tetravalent radionuclides are initially strongly associated with nanoparticles in contrast to Tc-99(VII), U-233(VI) and Np-237(V) which showed no sorption to the montmorillonite colloids. Radionuclide desorption of the nanoparticles followed by sorption to the fracture filling material is observed for Th-232(IV), Pu-242 and Am-243(III). Based on the conceptual model that the driving force for the kinetically controlled radionuclide desorption from nanoparticles and subsequent association to the FFM is the excess in surface area offered by the FFM, the observed desorption kinetics are related to the colloid/FFM surface area ratio. The observed decrease in concentration of the redox sensitive elements Tc-99(VII), U-233(VI) and Np-237(V) may be explained by reduction to lower oxidation states in line with Eh-pH conditions prevailing in the experiments and thermodynamic considerations leading to (i) precipitation of a sparingly soluble phase, (ii) sorption to the fracture filling material, (iii) possible formation of eigencolloids and/or (iv) sorption to the montmorillonite colloids. Subsequent to the sorption/ desorption kinetics study, an additional experiment was conducted investigating the potential remobilization of radionuclides/ colloids attached to the FFM used in the sorption/desorption kinetic experiments by contacting this FFM with pure Grimsel groundwater for 7 days. A positive correlation of Pu-242, Th-232(IV) and Np-237 was observed with the Zn and Ni concentrations in the desorption experiments indicating a remobilization of sorbed montmorillonite colloids. The results of the study in hand highlight (i) the novel use of structural labeled colloids to decrease the uncertainties in the determination of nanoparticle attachment providing more confidence in the derived radionuclide desorption rates. Moreover, the data illustrate (ii) the importance of radionuclide colloid desorption to be considered in the analysis and application of colloid facilitated transport both in laboratory or in-situ experiments and numerical model simulations and (iii) a possible remobilization of sorbed colloids and associated radionuclides by desorption from the matrix material (FFM) under non-equilibrium conditions.
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Geochimica et Cosmochimica Acta, Elsevier, 2015, 148, pp.426-441. 〈10.1016/j.gca.2014.10.010〉
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https://hal.univ-lorraine.fr/hal-01281804
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Soumis le : mercredi 2 mars 2016 - 18:47:38
Dernière modification le : mercredi 30 mai 2018 - 01:20:04

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F. M. Huber, S. Heck, Laurent Truche, M. Bouby, J. Brendle, et al.. Radionuclide desorption kinetics on synthetic Zn/Ni-labeled montmorillonite nanoparticles. Geochimica et Cosmochimica Acta, Elsevier, 2015, 148, pp.426-441. 〈10.1016/j.gca.2014.10.010〉. 〈hal-01281804〉

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