Selective exchange and fixation of strontium ions with ultrafine Na-4-mica

A simple preparation process of Na-4-mica (ideal brittle mica composition of Na₄Mg₆Al₄Si₄O₂₀F₄ ·xH₂O) from kaolinite yielded a nearly pure and very fine Na-4-mica with a crystallite size of about 0.2 μm. Strontium-sodium ion exchange in the hydrated interlayers of the Na-4-mica was demonstrated with this fine mica. The forward ion exchange of Sr²⁺ → 2Na⁺ (Sr-ion adsorption) was determined after equilibration for 4 weeks, and the Sr exchange capacity reached a value of about 200 mequiv (100 g)^-1. The parabolic diffusion or modified Freundlich model described the forward exchange process well. Only a small reverse exchange 2Na⁺ → Sr²⁺ (Sr-ion desorption) occurred from the hydrated Sr²⁺-saturated sample in 4 weeks. The irreversible sodium-strontium ion-exchange isotherm was obtained at room temperature after 4 weeks of equilibration. This mica showed much higher selectivities for Sr and Ba than for Mg and Ca, as expected. This selective Sr-ion exchanger with a large ion-exchange capacity and extremely low Sr leachability is expected to be useful for ⁹⁰Sr removal followed by its immobilization at room temperature.