Sorption of NpO2+ on montmorillonite: influence of ferrous iron

The interaction of radionuclides with clay minerals, major constituents of the multi-barrier system of a radioactive waste repository (i.e. host rocks and backfill materials), is a key process in retarding their migration. It is very important to develop a fundamental understanding of the uptake processes of (radio-)contaminants onto clay minerals and to implement sorption models to reliably predict their fate in the near- and far-fields of radioactive waste repositories. 
Redox phenomena play an important role for radionuclide retention in the safety case. In the near-field, the corrosion of steel canisters will release large amounts of ferrous iron (FeII) and produce H2 with a strong influence on the redox potential (Eh). In the undisturbed bentonite it is anticipated that FeII concentrations in the porewater will lie typically between 10-5 and 10-4 M, controlled by the solubility of siderite, and Eh is predicted to be ~-200 mV [1]. The presence of high concentrations of FeII and reducing conditions in the interstitial porewater of bentonite can potentially influence the sorption behaviour of redox-sensitive nuclides. The mobility of elements such as U, Np and Tc highly depends on their oxidation state. In their most oxidized form +V, +VI (as oxocations NpO2+, UO2+2+) and +VII (as oxoanion TcO4-) these radionuclides are highly mobile, whereas their transport is strongly decreased when reduced to the oxidation state +IV.
With respect to reduction reactions of inorganic and organic compounds, FeII bound to oxide/clay minerals (surface mediated reduction) has been observed to be much more reactive than dissolved FeII [2-6]. Not only the kinetics of reaction is faster but also the redox potential of the clay/oxide associated FeII/FeIII redox couple is lower than this of dissolved iron. 

The aim of this study is to investigate the sorption of the long-lived neptunium (237Np, t1/2 = 2.14·106 a) on montmorillonite under reducing conditions mimicking “in situ” geochemical repository conditions. Most of sorption, modelling and spectroscopic studies on clay minerals so far have focused on the uptake of the mobile and very soluble NpO2+ under oxic and anoxic conditions. Sorption data for Np under reducing conditions or in the presence of FeII on clay minerals are sparse. 

The sorption of NpV on montmorillonite (Na-STx) was measured under electrochemically established reducing conditions in a bulk electrolysis cell composed of a three electrode system at a fixed redox potential in the absence and presence of varying ferrous iron concentrations (up to 44 mmol·kg-1). Fig. 1a shows the temporal evolution of the sorption of NpV in the absence and presence of ~44 mmol·kg-1 FeII at pH 7.8 and Eh -291 mV (vs SHE). If no Fe is added the sorption of Np remained nearly constant and no noticeable increase of sorption compared to anoxic conditions could be observed. In the presence of Fe, the sorption of Np steadily increased up to log Rd ~ 3.8 L/kg, suggesting the reduction of NpV at the clay surface. 

Evidence for the oxidation state and type of surface complex formed was obtained from extended X-ray absorption fine structure (EXAFS) spectroscopy (Fig. 1b). The derived structural parameters confirm that under the above mentioned experimental conditions, Np sorbed on montmorillonite is fully reduced to NpIV and is strongly associated to surface bound Fe. 
The results of this study (i.e. influence of different FeII concentrations, nature of surface complex) will be presented at the conference.

Figs. 1: Sorption of NpO2+ on Na-STx under anoxic conditions and under reducing conditions (-291 mV) in the absence and presence of FeII. a) Temporal evolution and b) Fourier Transforms of the Np-LIII edge EXAFS spectra of Np adsorbed to Na-STx.

References:

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