Influence of ionic strength on U(VI) sorption on montmorillonite at high salinities

Clay and clay minerals are potentially suitable as host rock for nuclear waste disposals due to their high sorption capacities and low permeability. In Germany, there are two types of clay that in principle are eligible to be used for the construction of deep geological disposals: South German Opalinus clay with low ionic strength pore waters and north German clay deposits with high ionic strength pore waters, e. g. with sodium chloride concentrations of up to 4 mol/l in the depths relevant for nuclear waste disposal. [1] The present work focuses on U(VI) sorption onto the clay mineral montmorillonite under high ionic strength conditions, with the Konrad mine serving as reference site for experimental conditions. The experiments are conducted in sodium and calcium chloride as well as in a mixed electrolyte that resembles the groundwater at the Konrad site.
The classic ionic strength effect, where sorption decreases with increasing ionic strength, can only be observed in the acidic pH range where cation exchange is the predominant sorption mechanism for U(VI). However, natural groundwaters at the Konrad site have pH values from 5.75 to 6.85, where the ionic strength has an already diminished influence on sorption. The sorption maxima for U(VI) in the different salt systems lie slightly below the neutral point and well within the pH range of groundwaters of the reference site. For high ionic strengths like those in north German groundwaters, U(VI) retention becomes partly irreversible. [2] Furthermore, in the calcium chloride system, U(VI) retention increases strongly with ionic strength in the alkaline pH range. Both these effects are attributed to secondary phase formation, which is promoted by increasing ionic strength.

[1] Brewitz, W. et al. (1982) Eignungsprüfung der Schachtanlage Konrad für die Endlagerung radioaktiver Abfälle. GSF-T 136.
[2] Zehlike, L. (2013) Durchführung von Sorptions- und Desorptionsversuchen von Uran(VI) an Montmorillonit, Bachelor thesis, TU Dresden.