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

Abstract

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 μm sized pellets in the sand. Recent studies have shown a surprisingly strong sorption of radiocaesium (137Cs) on these sands, despite the coarse sized pellet structure. This study was set up to characterize the selective 137Cs+ 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.