Highly charged swelling micas of different charge densities: Synthesis, characterization, and selectivity for Sr and Ba

Highly charged swelling sodium micas (Na-2-, Na-3-, and Na-4-micas) with different layer charge densities were synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and solid-state ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Their ion exchange properties were examined for Sr²⁺ and Ba²⁺ ions using isotherms and Kielland plots. XRD patterns after 2Na⁺ → Sr²⁺ or Ba²⁺ exchange were investigated to check for change in 'd' values of 0 0 1 reflections of Na-micas. The 'd' values of 0 0 1 reflections were shifted from two-layer hydrate to one-layer hydrate depending on layer charge density and the extent of ion exchange by divalent cations. A comparison of shifting in 'd' values of 0 0 1 reflections indicates that there are two different hydration states of Sr²⁺ and Ba²⁺ in the interlayer space of Na-micas. Na-2-mica showed the highest selectivity for Sr²⁺ and Ba²⁺ among Na-micas because of easier expansibility of its interlayers and thus accessibility to exchanging sites. Therefore, Na-2-mica is a good candidate for decontamination of Sr²⁺ and Ba²⁺ from drinking water and contaminated soils or nuclear waste solutions.