Adsorption of Cr(VI) in layered double hydroxides

Adsorption of Cr(VI) by various uncalcined and calcined layered double hydroxides (LDH) of Mg–Al, Ni–Al and Zn–Cr types was investigated. Adsorption of Cr2O72− through ion exchange in uncalcined LDH is slower in rate and smaller in amount than in calcined one where the adsorption occurs via rehydration. The nature and content of bi- and trivalent metal ions in LDH influence the adsorption. Calcined Mg–Al LDH with higher Al3+ content in the precursor structure shows higher adsorption capacity. Adsorption is higher in the Mg–Al LDH than in Ni–Al and Zn–Cr LDH. A two-step first-order rate kinetics was observed for the adsorption process which follows the Freundlich-type adsorption isotherm. The explanation of adsorption phenomenon has been supported by X-ray diffraction patterns. The pH of initial Cr2O72− solution has no affect on adsorption by calcined LDH. Release of Cr2O72− from the adsorbed product depends upon the type of anion in the aqueous phase and CO32− being the most effective ion. Calcination of the Cr2O72− adsorbed product up to 1250°C has not resulted in the formation of Cr-immobilized phases.