Comparison of a Non Electrostatic Surface Complexation Model and Surface Phase Theories for Prediction of Future Sorptio

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Comparison of a Non Electrostatic Surface Complexation Model and Surface Phase Theories for Prediction of Future Sorption Properties. Allan T. Emrén and Anna-Maria Jacobsson Materials and Surface Chemistry, Chalmers University of Technology, SE-41296 Goteborg, Sweden. E-mail [email protected] Abstract In performance assessments, sorption of radionuclides dissolved in groundwater is mostly handled by the use of fixed Kd values. It has been well known that this approach is unsatisfying. Only during the last few years, however, tools have become available that make it possible to predict the actual Kd value in an aqueous solution that differs from the one in which the sorption properties were measured. One such approach is surface complexation (SC) that gives a detailed knowledge of the sorption properties. In SC, one tries to find what kinds of sorbed species are available on the surface and the thermodynamics for their formation from species in the bulk aqueous solution. Recently, a different approach, surface phase method (SP), has been developed. In SP, a thin layer including the surface is treated as a separate phase. In the bulk aqueous solution, the surface phase is treated as a virtual component, and from the chemical potential of this component, the sorption properties can be found. In the paper, we compare advantages and disadvantages of the two kinds of models. We also investigate the differences in predicted sorption properties of a number of radionuclides (Co, Np, Th and U). Furthermore, we discuss under which circumstances, one approach or the other is preferable.

Introduction The safety of repositories for spent nuclear fuel is guaranteed by the stability of engineered barriers, but also by the ability of the rock to sorb radionuclides that may be released due to failed engineered barriers. In safety assessments, the Kd concept is commonly used in spite of weaknesses that have been pointed out during many years. The main drawback of the Kd model is that a Kd value is applicable only to a situation very similar to the one in which it was measured. Even a minor deviation in properties of the aqueous solution may cause the Kd value to change by orders of magnitude. In spite of that, Kd is widely used, as there is a lack of alternatives that can be used in practice. During the last years, SC (surface complexation) models have been developed [1] to give detailed insights in the sorption processes. Surface complexation models describe the sorption as formation of complexes between the surface groups and the sorbing species. The general surface complexation reaction considered here is: ≡ SOH rn + x z+ + yH 2 O ←→ ≡ SOH c x(OH) ry+z-y + (n − c + y)H +

(1)

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where ≡ SOH rn is some type of surface site of charge r, xz+ is a sorbing cation, y is the number of protons released in the reaction and ≡ SOH c x(OH) ry+z-y is the surface complex that is formed. The equilibrium constant, Kx, for the reaction is: Kx =

{≡ SOH x(OH) }{H {≡ SOH }{x c

r +z − y y r n

+ 1− c + y i − plane }

z+ i − pl