Radiocaesium sorption on natural glauconite sands is unexpectedly as strong as on Boom Clay

The Neogene-Paleogene glauconite sands of Belgium cover the Boom Clay deposits that are candidate host for radioactive waste disposal. It is unclear if the highly permeable sand formations may act as an additional barrier for radiocesium (¹³⁷Cs) or could be added as a complementary sorption sink in a surface disposal concept. Glauconite is an Fe-rich phyllosilicate that is mainly present as 250-125 μm sized pellets in sand, it is unknown to what extent and how fast these pellets may bind ¹³⁷Cs. Pelletized clays embedded in sand may have poorly accessible high affinity sites for ¹³⁷Cs. The ¹³⁷Cs sorption on 11 different glauconite sands was measured in batch in a background solution of 0.1 M CaCl₂ and 0.5 mM KCl. The log transformed ¹³⁷Cs distribution coefficient K_d (L kg^-1) after 30 days reaction ranged 3.4-4.3, surprisingly close to the K_d of the Boom Clay (3.5). Isolated glauconite fractions exhibited similar ¹³⁷Cs sorption potentials (log K_d 4.1-4.3) as the reference Illite du Puy (4.4). The small K_d variation among the Neogene-Paleogene sands was explained by its glauconite content (r = 0.82). The ¹³⁷Cs sorption kinetics (1-57 days) of milled pellets (<2 μm) confirmed slower reaction with intact pellets than with milled samples. Additionally, the K_d values of milled samples (57 days) sorption are 1.1-1.5 fold larger than the corresponding intact pellets, suggesting that not all Cs binding sites are accessible in intact pellets. Strongly weathered pellets exhibited cracks (visible with SEM). In these pellets the K_d was similar for milled and intact pellets suggesting that cracks increase the accessibility of the inner sorption sites. After 8.5 months the K_d values were 1.6-1.8-fold above corresponding 1 month data and these long-term reactions were more pronounced as total sand K content was larger. An adsorption-desorption experiment illustrated that ¹³⁷Cs sorption is not fully reversible.