Uranium retention by montmorillonite at high ionic strengths


Uranium retention by montmorillonite at high ionic strengths

Fritsch, K.; Schmeide, K.

Clay and clay minerals are viewed as potential host rock as well as backfill material for nuclear waste disposal in deep geological formations. While South German clay deposits have pore waters with ionic strengths below 0.5 mol l-1, North German clay deposits exhibit high ionic strengths up to 4 mol l-1 in the depth relevant for repositories.[1] Such high ionic strength conditions are rarely investigated and are thus in the focus of the present work. Montmorillonite is used as model clay for uranium sorption experiments in sodium and calcium chloride. Additionally, a mixed electrolyte was prepared that resembles the groundwater composition at the Konrad site, which serves as a reference site. Surface complexation modelling is employed to gain thermodynamic data from the sorption experiments.

Sorption experiments were conducted with Na montmorillonite SWy-2, which was purified according to Poinssot et al. [2] and Bradbury et al. [3]. Sorption edges were determined with an initial U(VI) concentration of 1.10-6 mol kg-1 and a solid-liquid ratio of 4 g kg-1 in a pH range of 4 to 10 with electrolyte concentrations from 0.1 to 3 mol kg-1. The U(VI) concentration for sorption isotherms ranged from 1.10-8 to 1.10-5 mol kg-1. The mixed electrolyte was composed of cNaCl = 2.52 mol kg-1, cCaCl2 = 0.12 mol kg-1 and cMgCl2 = 0.048 mol kg-1 (overall I = 3.02 mol kg-1).

The classic ionic strength effect describes the decrease of sorption with increasing ionic strength and is generally only observed in the acidic pH range, where cation exchange is the predominant sorption mechanism. Natural groundwaters at the Konrad site have pH values above 5.5, where cation exchange is not the dominant sorption mechanism anymore, which in turn causes a smaller influence of ionic strength on sorption. The sorption maxima in the different salt systems usually lie just below the neutral point and well within the pH range of groundwaters at the reference site. The sorption edge in NaCl is observed at pH 4.1. In the mixed electrolyte, sorption is increased in the alkaline pH range when compared to pure electrolytes. This is due to the magnesium content in this electrolyte, which forms hydroxide precipitates that influence uranium retention by co-precipitation.

Surface complexation modelling with the 2 site protolysis non-electrostatic surface complexation and cation exchange model (2SPNE SC/CE)[4] in conjunction with spectroscopic investigation of the sorption phenomena can be used to identify the surface species involved. The results show that ≡SOUO2 + is the dominant surface complex below pH 6 Uranium hydroxide complexes become the dominant surface complexes above pH 7 in absence of CO2. In presence of CO2, sorption above pH 7 is decreased due to aqueous uranium carbonate complexes.

In general, there is no influence of ionic strength on U(VI) retention by montmorillonite at environmentally relevant pH values. The classic ionic strength effect can only be observed in the acidic pH range, and only up to a sodium concentration of 2 mol kg-1. However, in the presence of calcium or magnesium, uranium retention is promoted by secondary phase formation which leads to partial irreversibility of immobilisation at high ionic strengths in the alkaline pH range.

[1] Brewitz, W. et al. Eignungsprüfung der Schachtanlage Konrad für die Endlagerung radioaktiver Abfälle. GSF-T 136; 1982.
[2] Poinssot, C. et al. Experimental studies of Cs, Sr, Ni, and Eu sorption on Na-illite and the modelling of Cs sorption. NTB 99-04; 1999.
[3] Bradbury, M.; Baeyens, B. Geochim Cosmochim Acta 2009, 73, 990–1003.
[4] Bradbury, M.; Baeyens, B. J Contam Hydrol 1997, 27, 223–248.

Keywords: uranium sorption; argillaceous rock; clay; uranium; montmorillonite; high ionic strength; surface complexation modelling

  • Contribution to proceedings
    Clays in natural and engineered barriers for radioactive waste confinement, 23.-26.03.2015, Brussels, Belgium
    Proceedings of Clays in natural and engineered barriers for radioactive waste confinement
  • Poster
    Clays in natural and engineered barriers for radioactive waste confinement, 23.-26.03.2015, Brussels, Belgium

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