Technical Report NTB 94-11

A procedure for introducing cation exchange into geochemical/surface complexation codes is described. Beginning with selectivity coefficients, K_c, defined in terms of equivalent fractional ion occupancies, a general expression for the molar based exchange code input parameter, K_ex, 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.