Additivity rule and its application to the sorption of radionuclides on soils

SummaryPrevious authors reported that for some sorption systems the overall distribution coefficient (Kd) for a radionuclide or trace element on a mixture of minerals could be predicted from the additivity rule, the distribution coefficients on individual minerals and their mass fractions and that for some sorption systems, deviations from the additivity rule were found. In this paper, the additivity rule and its applicability to soils were analyzed from the theoretical considerations and the nature of soils. It was found that under the simplifying assumptions: 1) reversible and independent sorption reactions, 2) at equilibria of sorption reactions, 3) homogeneous surfaces, 4) insolubility of ion exchangers or sorbents, 5) no new phase formation, and 6) no interactions between individual minerals of the mixture, the additivity rule is formally valid. In practice, because of these crucial assumptions, the nature of soils and the difference in the experimental conditions for the sorption on mixtures and for the sorption on individual minerals, in general, the additivity rule is not applicable to the sorption systems of soils.