Selective Adsorption of Magnesium Ions and Structural Reconstruction in Acidified Palygorskite

Clay minerals play a crucial role in adsorption, crystallization, and elemental migration at interfaces, with processes such as the release of constituent elements and structural evolution in acidic aqueous solutions. In this study, acidification of palygorskite induces preferential Mg2+ dissolution from octahedral sites, creating distinct octahedral vacancy defects critical for selective cation adsorption. Acidified palygorskite exhibits exceptional Mg2+ selectivity in mixed ion solutions (Mg2+ ≫ Ca2+ > Na+ > K+ > Li+), surpassing affinities of competing ions by 1–2 orders of magnitude. The adsorption mechanism is predominantly governed by electrostatically induced chemisorption. Crucially, re-adsorbed Mg2+ drives structural reconstruction by occupying octahedral vacancies, as evidenced by SAED pattern restoration, Mg redistribution in TEM-EDS, FTIR recovery of Si-O-Mg vibrations (973 cm−1), NMR resurgence of Q3(0Al) edge silicon sites (−92.2 ppm), and increased bound and structural water content in TG-DTG. DFT calculations confirm the mechanism that octahedral vacancies exhibit lower formation energy (3.58 eV) than Si-leached defects (7.36 eV), and Mg2+ adsorption at vacancy sites (−1.95 eV) exceeds energies for other cations. This defect-mediated adsorption not only governs Mg2+ selectivity but also enables structural reconstruction via an adsorption-crystallization pathway, establishing a direct link between selective ion capture and mineral recrystallization processes. The findings advance predictive understanding of clay mineral reactivity in geochemical cycling and offer theoretical guidance for the development of novel adsorption materials.