Speaker
Prof.
Tobias Reich
(Johannes Gutenberg-Universität Mainz, Institute of Nuclear Chemistry)
Description
The sorption of Np(V) on Na-montmorillonite (STx-1) has been studied by batch experiments, spectroscopic measurements, and surface complexation modelling with the aim to contribute toward a better understanding of the sorption of 237Np (t1/2 = 2.1 × 106 a) in the near field (bentonite backfill material) and far field (argillaceous rocks) of high-level nuclear waste repositories. Batch experiments were performed in the absence of inorganic carbon and under air-equilibrated conditions with 0.1 and 0.01 M NaClO4 as background electrolyte, 8 × 10-12 and 9 × 10-6 M Np(V), and 3 ≤ pH ≤ 10. At pH > 8 the presence of inorganic carbon has a strong influence on the sorption behavior of Np(V) due to the formation of aqueous Np(V) complexes with carbonate.
Neptunium LIII-edge extended X-ray absorption fine structure (EXAFS) measurements on Np(V)/montmorillonite samples with Np(V) loadings in the range of 0.3-3.5 µmol/g have been performed to determine the speciation of Np at the solid-liquid interface. The EXAFS spectra of samples prepared under ambient air conditions (pCO2 = 10-3.5 atm) revealed the formation of Np(V)-carbonate complexes at the montmorillonite surface.
The results of the batch experiments obtained under CO2-free conditions could be modeled using the two site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model described in [1]. For modeling the sorption behavior of Np(V) on montmorillonite in the air-equilibrated system, the aqueous complexation of Np(V) with carbonate [2] was included and the following additional surface complexation reaction was required: ≡SOH + NpO2+ + CO32- ⇔ ≡SONpO2CO32- + H+.
[1] M.H. Bradbury and B. Baeyens, Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides, Geochim. Cosmochim. Acta 69, 875-892, 2005.
[2] Chemical Thermodynamics of Neptunium and Plutonium, (Eds. J. Fuger et al.) Elsevier, Amsterdam 2001.
Primary author
Prof.
Tobias Reich
(Johannes Gutenberg-Universität Mainz, Institute of Nuclear Chemistry)
Co-authors
Dr
Andreas C. Scheinost
(Forschungszentrum Dresden-Rossendorf, Institute of Radiochemistry)
Dr
Bart Baeyens
(Paul Scherrer Institut, Laboratory for Waste Management)
Dr
Michael H. Bradbury
(Paul Scherrer Institut, Laboratory for Waste Management)
Dr
Rainer Dähn
(Paul Scherrer Institut, Laboratory for Waste Management)
Dr
Samer Amayri
(Johannes Gutenberg-Universität Mainz, Institute of Nuclear Chemistry)
Dr
Sonja Wendt
(Johannes Gutenberg-Universität Mainz, Institute of Nuclear Chemistry)
