Technical Report NTB 94-11

Sorption by Cation Exchange Incorporation of a Cation Exchange Model into Geochemical Computer Codes

A procedure for introducing cation exchange into geochemical/surface complexation codes is described. Beginning with selectivity coefficients, Kc, defined in terms of equivalent fractional ion occupancies, a general expression for the molar based exchange code input parameter, Kex, is derived. In natural systems the uptake of nuclides onto complex sorbents often occurs by more than one mechanism. The incorporation of cation exchange and surface complexation into a geochemical code therefore enables sorption by both mechanisms to be calculated simultaneously. The code and model concepts are tested against sets of experimental data from widely different sorption studies. A proposal is made to set up a data base of selectivity coefficients. Such a data base would form part of a more general one consisting of sorption mechanism specific parameters to be used in conjunction with geochemical/sorption codes to model and predict sorption.