A self-consistent model describing the thermodynamics of Eu(III) adsorption onto hematite

The environmental fate of actinides is greatly influenced by interfacial reactions, including adsorption onto solid surfaces where the adsorption of trivalent and tetravalent actinides is generally a very strong and potentially irreversible reaction. Changes in the primary hydration sphere of the actinide during inner-sphere adsorption could greatly influence the thermodynamics of these reactions. However, few researchers have studied actinide adsorption thermodynamics. Therefore, using Eu(III) as an analog for trivalent actinides, we examined the thermodynamics of Eu(III) adsorption onto hematite, with particular emphasis on changes in the Eu(III) coordination number and the influence of temperature upon sorption. Our working hypothesis was that a decrease in hydration number upon adsorption, as indicated by a decrease in coordination number and an increase in adsorption with increasing temperature, results in energetically favorable sorption reactions, which are driven by a large, positive entropy term. To perform these studies, we applied the diffuse layer model to describe Eu(III) adsorption onto hematite at pH values ranging from ~3 to 7 and at 15, 25, 35, and 50°C. Additionally, we characterized the Eu(III)-hematite surface complex and changes in the Eu(III) primary hydration sphere using extended X-ray absorption fine structure spectroscopy (EXAFS) and computational modeling. High-resolution transmission electron microscopy (HRTEM) was used to identify possible europium surface precipitates or morphological changes in the hematite. The data indicate that the adsorption reaction (1) is endothermic, (2) proceeds with a decrease in the Eu(III) coordination number, and (3) results in the formation of a bidentate mononuclear surface complex, (FeO)₂Eu⁺. The enthalpy and entropy values for the formation of this surface complex, which were estimated using a van't Hoff plot, were 131±8 kJmol^-1 and 439±26JK^-1mol^-1, respectively, indicating that adsorption of Eu(III) onto hematite is entropically driven. Additionally, we suggest that the decrease in Eu(III) coordination number and the large entropy term are due to the loss of coordinating water molecules from the Eu(III) hydration sphere.