Comparison of Approaches to Calibrate a Surface Complexation Model for U(VI) Sorption to Weathered Saprolite

A surface complexation model describing the sorption of uranyl ions and uranyl carbonate on weak and strong sites was used to analyze experiments conducted on pH-dependent U(VI) sorption to weathered shale/limestone saprolite. Sorption data were collected at two different solid to solution ratios. Various methods of estimating equilibrium reaction coefficients and site densities were investigated. As a first approximation, extractable iron oxides were assumed to behave as ferrihydrite with reaction coefficients as reported by Waite (Geochim Cosmochim Acta 58:5465–5478, 1994). A generalized composite (GC) approach was then employed with coefficients estimated by an inverse modeling method applied both in a stepwise fashion and simultaneously to whole data set. Uncertainty in model parameters and predictions was lowest using the simultaneous inverse method, but results from the stepwise method were very similar. The generalized reaction network accurately described pH-dependent U(VI) sorption on weathered saprolite between pH 4 and 9.