Surfactant-modified clay as adsorbent for chromate

The adsorption of oxyanion of chromate was not given much attention, perhaps due to the fact that clays are negatively charged and unmodified clay mineral surface shows no affinity for chromate. After modification by cationic surfactant, clay was found to adsorb considerable amounts of chromate. Hexadecyl trimethylammonium (HDTMA) bromide was used to modify the surface of clay minerals such as kaolinite, montmorillonite, and pillared montmorillonite. Montmorillonite adsorbed a quantity of HDTMA equivalent to the cation exchange capacity. Kaolinite and pillared clay adsorbed relatively small amounts of the surfactant. ount of chromate adsorbed was maximum at and below pH 1 and proportional to the amount of HDTMA fixed on motmorillonite. The amount of chromate adsorbed between pH 2 and 6 was one half of that at pH 1, and above pH≈8 the adsorption was negligible. The pH dependence of chromate adsorption was attributed to the pH dependent-equilibria HCr2O7−⇋Cr2O72−⇋CrO42−. The adsorption data obtained was well described by Langmuir adsorption isotherm. Kinetics of chromate adsorption (diffusion constant) was calculated and a mechanism for the adsorption is proposed.