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 (–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.