Adsorption of Cr(VI) on STAC-modified rectorite

Adsorption of Cr(VI) on stearyl trimethylammonium chloride modified rectorite (STAC-rectorite) was investigated by batch studies, X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) analyses. The Cr(VI) adsorption on STAC-rectorite is well described by the Freundlich isotherm. The adsorption capacity reached as high as 21 g/kg for Cr(VI), or 400 mmol/kg of chromate. The enthalpy change of adsorption (ΔH) is − 9.4 kJ/mol, indicating retention of Cr(VI) on STAC-rectorite is via physical adsorption. Higher Cr(VI) adsorption on STAC-rectorite occurred in acidic solutions (pH 4) and the amount of adsorbed Cr(VI) decreased rapidly with increasing pH as a higher (OH)− concentration will compete against Cr(VI) for adsorption sites, thus, inhibiting formation of Cr(VI)-STAC complex. FTIR analysis shows that the adsorbed STAC forms an admicelle surfactant surface coverage, which is responsible for Cr(VI) adsorption. Band position shifting of Si–O vibration and H–O vibration of water molecules before and after reaction with Cr(VI) indicates that the adsorbed Cr(VI) influences the strength of the Si–O chemical bonds in the siloxane layer and H–O bonds of intercalated water molecules.