Removal of chromate from water by a new CTAB-silica gelatin composite

A novel composite able to remove hexavalent chromium Cr(VI) from aqueous solutions was obtained by adding the silica precursor tetraethoxysilane (TEOS) to the hexadecyltrimethylammonium bromide (CTAB) microemulsion-based gel. A physical characterization of the new matrix revealed high stability of the silica gelatin composite in water at high temperatures and at neutral pH. Good efficiency in removing chromate from neutral solutions was also demonstrated by the adsorption kinetics. In particular, the adsorption data of chromate obtained with the CTAB–silica gelatin composite at 25 ◦C and pH 7.5 are described by the Freundlich isotherm model. The specific role of CTAB in the silica gelatin composite was also evaluated by comparing the kinetics of the anionic AOT–silica gelatin composite to the CTAB one. The data collected clearly showed that the positively charged surfactant was necessary to efficiently adsorb Cr(VI) from aqueous solutions. SEM and pulsed gradient spin–echo NMR analysis of the composite demonstrated that the silicon is well assembled in the gelatin network, in which water molecules maintain a high mobility. The diffusion coefficient of water in this system was shown to remain close to the value of pure water. Finally, X-ray analyses of the elemental content in the CTAB–silica gelatin composite indicated no difference in terms of percentage of silica distributions in different areas of the matrix and suggested that chromium adsorption could take place in internal areas.