Functionalization of synthetic saponite: Identification of grafting sites and application for anions sequestration

Highlights: • The monodentate and bidentate alkoxysilane was grafted on pristine and acid pre-treated saponite surfaces. • Amount of grafted alkoxysilane only slightly increased upon the acid treatment. • Intercalation of the alkoxysilane was favoured by the acid pre-treatment of the clay. • Adsorption of anionic compounds onto clay mineral improved following alkoxysilane grafting. The present work reports the study of the mechanism of grafting of an alkoxysilane on the surface of a synthetic saponite. XRD and SEM/EDX characterizations confirmed the effective synthesis of the clay mineral with structural formula ^INTNa_0.50^TET[Si_3.50Al_0.50]^OCT[Mg₃]O₁₀(OH)₂. The functionalization of this material in its pristine or acid pre-treated form with a cationic silane (tetradecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride) yielded a nanohybrid organoclay with interesting structural and chemical properties highlighted by ²⁹Si solid state NMR, XRD, FTIR, TGA and electrochemical characterizations. The monodentate and bidentate grafting of the alkoxysilane was observed regardless the starting material was pristine or acid pre-treated saponite. The more significant layer-to-layer distance increase observed after the functionalization of acid pre-treated saponite indicated some interlayer intercalation/grafting of the alkoxysilane. Electrochemical characterization (cyclic voltammetry and electrochemical impedance spectroscopy) showed that the modified clay mineral exhibited anionic exchange property due to the presence of grafted cationic alkoxysilane. This anionic exchange capacity was confirmed when successfully applied for the adsorption of the anionic dye Congo Red. In all cases, the acid pre-treatment tended to improve the anion exchange property of the organoclay.