PNNL

Abstract

Exerting control over materials’ composition and structure to selectively adsorb rare earth elements (REE) is important for critical ions separation. Understanding how local electric potentials influence REE adsorption and how they can be modified via chemical substitution is essential to the realization of control. The study presents the calculated Mean Inner Potentials for muscovite and phlogopite micas, both yielding Vo = +10.6 V, in excellent agreement with measurement Vo = +10.6 V. We show that several natural substituents for aluminum (Al) significantly affect the electric potentials at the basal surface, thereby altering the adsorption energies (Ead ) in agreement with their differences in electronegativity (?). These changes in interfacial electric potential and their spatial extent correlate with the variations in Ead for ligated rare earth element (REE) cations: Nd·(NO3)3 and Yb·(NO3)3. We find that Nd3+ adsorption is generally preferred on both muscovite and phlogopite compared to Yb3+. Substituents with lower ? than Al increase the Lewis basicity of adjacent oxygen atoms and enhance the Ead by 20 to 30 kcal/mol. This occurs because the Lewis basicity of the oxygen atom’s lone pairs is increased by the adjacent substituent atom lower ?. Such atomic substitutions modify the magnitude and spatial extent of the electric potential by ~ 1 nm or more, whether on the interface or within the mica layers. The findings highlight the potential of high-resolution electron holography and the importance of understanding atomic scale variations in quantum mechanical electric potentials to improve REE cation adsorption and selectivity on micas.