An Approach to Sorption on Minerals Making Use of Surface Phase Thermodynamics

Abstract

In performance assessments for nuclear wastes repositories, K_d values are often used to describe sorption of radionuclides on fracture surfaces. The weakness of the K_d concept is that values determined under certain conditions can only be used when the chemistry is similar to those. Alternatives include surface complexation models. The problem with such models is that the parameters required are not easily determined, and thus are missing for several systems.

Another alternative is to use surface phase thermodynamics. Such a theory has been developed, in which the surface of a solid phase and the species adsorbed upon it are treated as a separate phase of variable composition. The composition is described in terms of pure end members. For each end member, the free enthalpy is described by a series expansion. To calculate coefficients, the theory makes use of the fact that at equilibrium, the chemical potential of a component has the same value in all phases. Thus, speciation of the bulk aqueous phase can be used. Making use of only the linear term in the expansion, one measured K_d value is enough to determine the corresponding coefficient.

The theory has been applied to sorption of U(VI) on alumina with the aid of data from the literature. The coefficient was determined from the measured K_d values in the interval pH = 4 – 12. The resulting model curve was found to agree reasonably well with observed values, although the K_d varies by a factor of 200.