Synthesis of Na-2-mica from talc and kaolinite: characterization and Sr2+ uptake

Abstract

Na-2-mica, a highly charged sodium fluorophlogopite mica of ideal composition, Na₂Mg₆Si₆Al₂O₂₀F₄·xH₂O was synthesized from a mixture of talc, kaolinite, ultrafine magnesium oxide or magnesium nitrate, and sodium fluoride at a temperature of 825 °C for 12 h. The resulting powders were characterized by X-ray diffraction (XRD), ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR), and scanning electron microscopy (SEM). Both magnesium sources resulted in the formation of highly charged expandable mica; however, the sample prepared with magnesium nitrate also contained forsterite as an impurity. The presence of forsterite in the sample was confirmed by a strong resonance at −62 ppm relative to the tetramethylsilane reference. Each mica had four distinctly non-equivalent Si environments. The ²⁷Al MAS-NMR spectra indicated that all Al in the samples was tetrahedrally coordinated. Use of magnesium nitrate as a magnesium source rather than magnesium oxide resulted in slightly smaller particles; however, both sources resulted in mica flakes less than 2 µm. The cation exchange capacity (CEC) of the mica prepared from MgO was close to the theoretical value for Na-2-mica (247 meq 100 g⁻¹); however, the CEC of the sample prepared from magnesium nitrate was much larger than expected. Part of the layer charge for these samples was the result of cation vacancies in the octahedral sheet of the mica as indicated by a Li-migration technique. Ion exchange studies indicated that both micas were highly selective for strontium, whose radioisotope Sr-90 is commonly present in nuclear waste.