Novel clays: Solid-state synthesis, characterization and cation exchange selectivity

Several novel swelling mica-type clays have been synthesized by solid-state processes. Synthetic clays of ideal compositions such as Na2Si6Al2Mg6O20F4 · xH2O (Na-2-mica), Na3Si5Al3Mg6O20F4 · xH2O (Na-3-mica) and Na4Si4Al4Mg6O20F4 · xH2O (Na-4-mica) have been prepared and characterized by powder X-ray diffraction (XRD), 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 except in the case of Na-2-mica which also showed a small peak of anhydrous mica phase with a c-axis spacing of 0.96 nm. Solid-state 27Al MASNMR spectroscopy revealed that almost all the Al is present in the tetrahedral environment of the different micas. Solid-state 29Si MASNMR spectroscopy revealed different Si (Al) nearest neighbor environments depending upon the composition of the various micas. Selective cation exchange studies were performed on the various micas using 0.5 N NaCl solution containing 12.9 ppm Sr2+ or 8.12 ppm of La3+. The results showed, for the first time, that Na-3-mica has a high selectivity for the trivalent cation tested. The previously reported high selectivity of these synthetic micas for the divalent cations has been confirmed. These selective cation exchange studies are of relevance in cation separations from drinking and waste waters.