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

The Neogene-Paleogene glauconite sands of Belgiumcover 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 ismainly 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 wasmeasured in batch in a background solution of 0.1 M CaCl2 and 0.5 mM KCl. The log transformed 137Cs distribution coefficient Kd (L kg−1) after 30 days reaction ranged 3.4–4.3, surprisingly close to the Kd of the BoomClay (3.5). Isolated glauconite fractions exhibited similar 137Cs sorption potentials (log Kd 4.1–4.3) as the reference Illite du Puy (4.4). The small Kd 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 (b2 μm) confirmed slower reaction with intact pellets than with milled samples. Additionally, the Kd 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 Kd was similar for milled and intact pellets suggesting that cracks increase the accessibility of the inner sorption sites. After 8.5months the Kd 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.