Characterisation of the highly selective caesium sorption on glauconite rich sands of contrasting geological formations

摘要

The Neogene-Paleogene glauconite sands are investigated in the framework of the Belgian radioactive waste disposal as material for an additional sorption sink. Glauconite is an Fe-rich phyllosilicate that is mainly present as 125-250 mu m sized pellets in the sand. Recent studies have shown a surprisingly strong sorption of radiocaesium (Cs-137) on these sands, despite the coarse sized pellet structure. This study was set up to characterize the selective Cs-137(+) sorption sites on glauconite that are likely Frayed Edge Sites (FES) as in illite. First, the NH4/K selectivity on the FES was determined as a fingerprint for the highly selective FES. The NH4/K selectivity coefficient on the FES for three different sand formation ranged between 3.8 and 5.0, well in line with the range (4-7) that has been reported for illite clay and clay formations. Second, the Cs+ sorption isotherms were determined on three sand formations in a background solution of 0.5 mM K+, 100 mM Ca+ and 10 8-10 4 M Cs+; isotherms were fitted with an optimised three-site model adapted after the illite model of Bradbury and Baeyens (2000). The optimised model fitted the data adequately. The FES capacities of the sands ranged from 0.04 to 0.06% of the CEC which is significantly below that of illite (0.25%). Similarly, a strong decrease in the FES-type II site capacities was observed (1.7-2.2% of the CEC vs 20% for illite). The lower fractions of FES-type I and FES-type II sites detected in glauconite sand are related to higher fractions of smectites (XRD confirmed) that do not have specific Cs+ sites. Taken together, this study suggests that the glauconite sands have highly selective sites that bind 137Cs(+) with similar properties as in illite, however the fractions of these sites in the total CEC pool are about a factor five lower than in pure illite.