Synthesis of Na-2-mica from metakaolin and its cation exchange properties

Na-2-mica of the ideal composition of Na₂Mg₆Al₂Si₆O₂₀F ₄·xH₂O was prepared from a mixture of NaF, magnesium nitrate, SiO₂ and calcined kaolinite (the latter serving as a cost-effective aluminosilicate source) at 800°C and was found to excel in Sr removal from solutions at room temperature. This mica was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and ²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Fundamental studies of nNa⁺→Mⁿ⁺ (Mⁿ⁺ = Sr²⁺, Ca²⁺, Li⁺, K⁺ and Cs⁺) equilibria revealed a high selectivity and large capacity for Sr²⁺ ion uptake. A relatively low-temperature heat treatment (200°C) of the Sr-exchanged mica resulted in collapse of the hydrated interlayer spacings of the mica, which will be useful to immobilize strontium in the stable unhydrated phase. This selective Sr-ion exchanger is expected to be useful for ⁹⁰Sr removal and its immobilization.