Adsorption of chromate from aqueous solutions by HDTMA-modified clinoptilolite, glauconite and montmorillonite

Adsorption of chromate on natural clinoptilolite (Cp), glauconite (Gl), and montmorillonite (Mt) treated with hexadecyl trimethylammonium (HDTMA) bromide at amounts equivalent to 1.0 and 2.0 of their cation-exchange capacities (CEC) was investigated by batch study. In the case of the Cp and Gl, adsorption of HDTMA took place on the zeolite and clay mineral surfaces only and thus relates to the external CEC (ECEC). The amount of chromate removed from the solution by the organo-silicates continuously decreased with increasing pH in the range 1.3-10. The highest values were obtained at pH between 1.3 and 6, and decreased rapidly above pH 6. At an initial chromate concentration of 6.25 mmol/L, its amounts bound to the Cp, Gl and Mt modified using 1.0 CEC loadings of the surfactant were 47, 102, and 168 mmol/kg, respectively. When amount of HDTMA used was equivalent to 2.0 CEC of the silicate, these values increased to 182, 240, and 285 mmol/kg, respectively. The results obtained suggest that the surfactant molecules, bound beyond the CEC to the external surfaces of the Cp and Gl, show a greater chromate adsorption ability in comparison with such molecules located in the interlayer spaces of the Mt. It is also evident that an excess of the surfactant, not adsorbed on the silicate surfaces, participated in the removal of chromate from the solution by formation of a precipitate of alkylammonium chromate.