Elucidation of the sorbent role in sorption thermodynamics of uranium(VI) on goethite.

The sorption process of radionuclides, often conducted at ambient temperature, shows significant sensitivity to the surrounding temperature. Prediction of fate and transport in the environment, therefore, requires accurate thermodynamic data of their species defining sorption-desorption onto solid surfaces. Herein, we examined the thermodynamics of uranium(VI), U(VI), sorption onto goethite with particular emphasis on directly calculating the enthalpy of U(VI) surface species formed under slightly acidic pH conditions. To achieve this aim, a sorption study of U(VI) was carried out on goethite in the pH range 3-10 and modelled using a 2-pK single-site diffuse layer surface complexation model. A binuclear bidentate species of U(VI), (FeO)UO, reproduces the sorption profile at pH 3-5 while the sorption was under-estimated in the pH >5 region. Precipitation of schoeptite at pH 5-8 was attributed to the underestimation of the predicted sorption behaviour. The species complexation constant was employed in the analysis of heat consumed, measured using an isothermal titration calorimeter, in the titration of the goethite suspension with U(VI) at pH 4.5 ± 0.1. Enthalpy for the U(VI) species was found to be 41 ± 7 kJ mol, suggesting that sorption is an entropically driven process. Comparing thermodynamic data with that of similar U(VI)-iron oxide systems, binding energy of U(VI) surface species, surface hydration and hydrogen binding are suggested as main factors in the sorbent role towards the thermodynamics of the sorption process.