A Robust Model for Prediction of U(VI) Adsorption onto Ferrihydrite Consistent with Spectroscopic Observations

A robust model that can predict the adsorption behavior of U(VI) on ferrihydrite under a wide range of environmental conditions was developed with the aid of an extended triple-layer model. X-ray absorption spectroscopic observations from previous studies showed that the predominant U(VI) surface species on ferrihydrite was commonly a bidentate inner-sphere species under ambient CO₂ conditions. Previous surface complexation models, however, could not predict U(VI) surface speciation because of the lack of sufficient macroscopic adsorption datasets with which to estimate the surface complexation reaction. In this study, we obtained U(VI) adsorption data at U(VI) concentrations of 10nM under a wide range of pH, ionic strength, and solid concentrations in NaNO₃ solutions with/without atmospheric CO₂. We determined the stoichiometries of the U(VI) adsorption reactions and the equilibrium constants with the adsorption data and the U(VI) hydroxyl constants recently estimated from direct luminescence measurements. A single set of equilibrium constants for the reactions could reproduce reasonably well the reported adsorption datasets obtained under a wide range of pH values (2-12), U(VI) concentrations (10^-8 to 10^-4 M), ionic strengths (0.004-0.5), and CO₂ partial pressures (<10^-6 to 10^-1.7 atm). The model could also predict all U(VI) surface speciation consistent with previous spectroscopic observations under a wide range of solution conditions.