Enthalpy of Uranium Adsorption onto Hematite

The influence of temperature on the adsorption of metal ions at the solid-water interface is often overlooked, despite the important role that adsorption plays in metal-ion fate and transport in the natural environment where temperatures vary widely. Herein, we examine the temperature-dependent adsorption of uranium, a widespread radioactive contaminant, onto the ubiquitous iron oxide, hematite. The multitemperature batch adsorption data and surface complexation models indicate that the adsorption of uranium, as the hexavalent uranyl (UO₂²⁺) ion, increases significantly with increasing temperature, with an adsorption enthalpy (ΔH_ads) of +71 kJ mol^-1. We suggest that this endothermic, entropically driven adsorption behavior is linked to reorganization of the uranyl-ion hydration and interfacial water structures upon U^VI adsorption at the hematite surface. Overall, this work provides fundamental insight into the thermodynamics driving metal-ion adsorption reactions and provides the specific enthalpy value necessary for improved predictive geochemical modeling of U^VI adsorption in the environment.