Technical Report NTB 17-14

The development of a thermodynamic sorption data base for illite and the application to argillaceous rocks

A thermodynamic sorption database has been developed for application to argillaceous rocks based on radionuclide sorption experiments on illite. Sorption edges/isotherms have been measured on illite for Co(II), Ni(II), Zn(II), Eu(III), Am(III), Cm(III), Sn(IV), Th(IV), Np(V), Pa(V) and U(VI) and modelled using the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model. A generalised sorption model for the uptake of caesium on illite is also given. All of these data are presented here together with the modelled curves, the corresponding surface complexation constants on the strong and weak sites, the selectivity coefficients and the hydrolysis constants used.

Linear free energy relationships (LFERs) between surface complexation constants and the appro¬priate corresponding aqueous hydrolysis constants have been derived separately for divalent transition metals, trivalent lanthanides/actinides and tetravalent actinides. These relationships allow surface site binding constants to be estimated for metals where the data are either very poor or absent, thus providing a means to calculate sorption values. In order to give an idea of how well such a procedure could work, test cases are presented in which sorption edges/isotherms were calculated using surface complexation constants taken from LFERs and compared with measured sorption values. A general methodology for using LFERs to calculate the sorption of metals for which no sorption data exist is suggested and applied to predict sorption edges/isotherms for Mn(II), Cu(II), Pu(III), U(IV), Np(IV) and Pu(IV) on illite.

The 2SPNE SC/CE sorption model, and the associated model parameters, were then used in blind predictions of sorption isotherms for Co(II), Ni(II), Eu(III), Th(IV) and U(VI) on Opalinus Clay for various groundwater compositions and were then compared with the measured values. The main assumption was that the sorption on Opalinus Clay is controlled by the illite content (where illite/smectite mixed layers were present they were assumed to have the same sorption characteristics as illite).

The main conclusion drawn from this work is that the tables of surface complexation constants and cation exchange selectivity coefficients compiled for illite and used in conjunction with the LFERs and the 2SPNE SC/CE sorption model, provide a powerful means of calculating the sorption of many radionuclides in complex porewater/argillaceous rock systems. Together they constitute a Thermodynamic Sorption Data Base (TSDB) for illite/argillaceous rock systems.