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 (ΔHads) 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 UVI 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 UVI adsorption in the environment.