Thermodynamics of the $\alpha$-FeOOH (goethite)-ScOOH solid solution

Abstract

Scandium (Sc) is a rare element that finds uses in modern technologies. Thermodynamic properties of Sc phases could help in the development of innovative technologies to extract Sc from mining waste. In this work, we investigated the FeOOH-ScOOH solid solution with the goethite structure. The end members and five intermediate compositions were synthesized and characterized. The lattice parameters show that the solid solution is non-ideal, with complex behavior induced by the Fe-Sc substitution. The excess unit-cell volume deviates negatively for the Sc-rich region, and positively for the Fe-rich region from the ideal behavior (Vegard's law). Enthalpies of dissolution were determined by acid-solution calorimetry in 5 mol \(\cdot\) dm \(^{-3}\) HCl at T = 343.15 K. Enthalpies of mixing ($\Delta_{mix}H$), calculated from the experimental data, are small and positive. The available data allow for fitting the data as $\Delta_{mix}H = W x (1-x)$, with the mixing parameter $W = 15.2\pm$1.0 kJ$\cdot$mol$^{-1}$. Using $\Delta_fG^o$ of ScOOH from earlier literature, we calculated a Lippmann diagram that shows that Sc should strongly partition into the aqueous phase upon goethite precipitation. The thermodynamic conclusions disagree with field observations that Sc is primarily harbored by goethite in lateritic profiles. It seems that under weathering conditions, adsorption of Sc$^{3+}$ and kinetic hindrance to exsolution prevail over thermodynamic driving forces.