Determination of sorption isotherms for Eu, Th, U and Am on the gel layer of corroded HLW glass

The retention of Eu(III), Th(IV), U(VI) and Am(III) in the alteration layer (gel layer) of precorroded simulated HLW glass is investigated by sorption experiments in water, NaCl and MgCl2 brine as a function of the pH. In water, the sorption behaviour of the four elements shows a strong dependence on the pH and the sorption coefficients (Rs) determined reflect the different charges of the three ions and the corresponding sorption properties. The Rs values of Th are the highest values, followed by the Rs values of Eu and Am, which are lower by about a factor of 10 at the respective pH, and finally by the Rs values of U(VI) which are the lowest. The sorption behaviour of the four elements can be described by a Langmuir isotherm for pH values <5. As shown by modelling calculations, the strong increase of the Rs values of Eu and Am, as well as of Th, with increasing pH can be described by the formation of a bidentate surface complex. Above pH 4–6, colloidal particles could be detected. From the concentrations of Nd in the solutions leached from the glass, `desorption' coefficients (Rd) of Nd can be calculated. The Rd values of Nd correspond quite well with the Rs data of the chemically homologue Eu and Am. In NaCl brine the sorption behaviour of Th and U is rather similar to that in water, whereas in MgCl2 brine the sorption of Eu, Am, and U in the gel layer is much lower than in water or NaCl brine. The results of the sorption tests can explain the very low release and solution concentrations of REE and actinides found in corrosion experiments with the radioactive CEA glass R7T7 in NaCl-rich solution under neutral to alkaline conditions.