Role of cations on the dissolution mechanism of kaolinite in high alkaline media

Abstract

Kaolinite and its dehydroxylated forms are widely used in industry, including the growing field of geopolymers. Until now, a deep understanding of the dissolution mechanism of kaolinite particles in alkaline media is still lacking. This work aims to investigate the dissolution mechanisms through aluminum and silicon leaching of two references kaolinites (KGa-1b and KGa-2) regarding various 2 M alkali treatment (KOH, NaOH and 50/50 NaOH + KOH) as a function of time and at 80 °C. For this, a wide range of methods have been used, from the determination of [Si] and [Al] in solution to the analysis of solids by combining X-ray diffraction (XRD), Fourier Transformed Infrared (FTIR), X-ray Absorption Near Edge Structure (XANES) and Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopies and Transmission Electronic Microscopy (TEM) observations. The results evidence a higher dissolution rate with poorly crystallized kaolinites, confirming the role of the crystallinity of the samples toward reactivity in an alkaline media. In addition, the nature of the cation significantly affect the degree of dissolution following the sequence KOH < NaOH + KOH < NaOH. This process is combined with a change in Al speciation from 6-coordinated to 4-coordinated sites, as evidenced by XAS and MAS-NMR experiments. Altogether, these results allow to propose different dissolution mechanisms at the particle scale depending on the cations. The dissolution in presence of KOH proceeds uniformly from the edges, starting from the edges of the octahedral sheet, whereas in the case of NaOH, the edges dissolution is oriented with respect to areas of less coherence starting from the tetrahedral sheet.