Swelling mica-type clays: synthesis by NaCl melt method, NMR characterization and cation exchange selectivity

Abstract

Several swelling mica-type clays of nominal compositions, NaxSi8−xAlxMg6O20F4·yH2O where x = 4, 3 or 2 were prepared using kaolinite, silica gel or silicic acid, magnesium fluoride and sodium chloride in the temperature range of 800 to 1100 °C after treatment for 5 to 15 h. All the synthetic clays were characterized by powder X-ray diffraction (XRD). Some of the samples were characterized by scanning electron microscopy (SEM) and 27Al and 29Si solid-state magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Powder XRD showed that all syntheses yielded water-swollen micas with c-axis spacing of ∼1.2 nm along with various amounts of anhydrous mica phase with a c-axis spacing of about 0.96 nm. Solid-state 27Al MASNMR spectroscopy of some micas revealed that almost all the Al is present in the tetrahedral environment while solid-state 29Si MASNMR spectroscopy revealed different Si(Al) nearest neighbor environments depending upon the composition of the various mica-type clays. Selective cation exchange studies were performed on some of these micas and they showed that these novel micas are highly selective for cations such as Sr and Ba. These synthetic clays prepared by a cost-effective method are of relevance in remediation of soils and waters contaminated with hazardous metal ions.