The sorption of caesium to glauconite sands obeys local equilibrium at environmentally relevant water flow rates

Glauconite has a radiocaesium interception potential that is comparable to illite, suggesting that glauconite containing sands may be an effective geological barrier for radiocaesium. However, glauconite is present as coarse pellets (~0.5 mm diameter) and considerable sorption non-equilibrium may occur during reactive transport in these permeable sands. Here, we analysed trace caesium (Cs) sorption kinetics in agitated batch suspensions to predict reactive transport at variable flow rates in glauconite containing sands. Batch sorption experiments showed that Cs+ sorption KD values in agitated suspension of glauconite sand of the Diest Fm (Dessel Member) increased by a factor 6 to 8 between 48 h and 94 days at trace Cs+ concentration (~10-9 M). A breakthrough (BT) experiment was set up with 13 packed columns of the same Fm that were leached with 10-6 M Cs+ at variable flow rates equivalent to water residence times 0.07–3.4 days for a duration of 154 days. The BT was observed in treatments with the higher flow rates after 890–1170 pore volumes. The breakthrough expressed in pore volumes revealed earlier breakthrough at only the highest flow rate, indicating chemical non-equilibrium in that treatment. The BT curves were modelled with HP1 (Hydrus-PhreeqC), thereby relying on batch data obtained at 90 days and assuming local reaction equilibrium. The BT curves were well predicted with this model, corroborating local equilibrium except at the highest flow rates. It is concluded that Cs+ sorption on highly permeable glauconite sands is sufficiently fast to delay breakthrough with flow rates below 2.4 m d-1. Only in high flow rate, unrealistic for the Neogene and Paleogene formations, early breakthrough could occur.