Thermodynamics of the α-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 $$\cdot \hbox {dm}^{-3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>·</mml:mo> <mml:msup> <mml:mtext>dm</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> HCl at T = 343.15 K. Enthalpies of mixing ( $$\Delta _{mix}H$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mrow> <mml:mi>mix</mml:mi> </mml:mrow> </mml:msub> <mml:mi>H</mml:mi> </mml:mrow> </mml:math> ), calculated from the experimental data, are small and positive. The available data allow for fitting the data as $$\Delta _{mix}H = W x (1-x)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mrow> <mml:mi>mix</mml:mi> </mml:mrow> </mml:msub> <mml:mi>H</mml:mi> <mml:mo>=</mml:mo> <mml:mi>W</mml:mi> <mml:mi>x</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>1</mml:mn> <mml:mo>-</mml:mo> <mml:mi>x</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , with the mixing parameter $$W = 15.2\pm$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>W</mml:mi> <mml:mo>=</mml:mo> <mml:mn>15.2</mml:mn> <mml:mo>±</mml:mo> </mml:mrow> </mml:math> 1.0 kJ $$\cdot \hbox {mol}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>·</mml:mo> <mml:msup> <mml:mtext>mol</mml:mtext> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . Using $$\Delta _fG^o$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Δ</mml:mi> <mml:mi>f</mml:mi> </mml:msub> <mml:msup> <mml:mi>G</mml:mi> <mml:mi>o</mml:mi> </mml:msup> </mml:mrow> </mml:math> 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 field observations from lateritic profiles show that Sc is primarily harbored by goethite via adsorption. It seems that under weathering conditions, thermodynamically driven partitioning of $$\hbox {Sc}^{3+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>Sc</mml:mtext> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> into the aqueous phases and its subsequent adsorption onto goethite surfaces controls the mobility of Sc in the weathering profiles.