Investigation of lepidolite surface properties: Insights from solid-gas and solid-liquid interfaces analyses

This study proposes an in-depth exploration of the surface properties of lepidolite, a lithium-bearing phyllosilicate mineral, using nitrogen and argon adsorption isotherms, electrophoretic mobility measurements, and surface titration experiments. The solid-gas and solid-liquid interfaces analyses allow characterizing the surface heterogeneity of lepidolite, highlighting the dominance (similar to 80%) of the basal surfaces over the lateral surfaces (similar to 20%) in the total specific surface area. The N-2 adsorption isotherm exhibited various adsorption sites, including the exposed interfoliar cations and the empty hexagonal cavities related to the cleavage process, on the basal planes, or the exposed oxygen and fluorine atoms on the lateral surfaces. Besides, the solid-liquid interfaces analyses highlighted a significant difference between the isoelectric point and point of zero charge of lepidolite, with pH values of 2-3 and 8.5-9, respectively, which demonstrated the importance of monitoring surface observables such as the surface potential rather than the zeta potential. Noteworthily, surface titration experiments allowed to identify distinct surface sites with specific pKa values that were discussed and demonstrated that the potassium interfoliar cations are exchanged with protons when the pH is decreased. These findings allow to gain understanding in the lepidolite's surface properties and will provide valuable implications for lithium extraction by the froth flotation method, a mineral beneficiation process based on the surface properties of minerals. Also, this research will pave the road to the in-depth investigation of physicochemical properties of surfaces in order to finely tune the metal extraction in the current energy transition context.