.Applied Clay Science 13 1998 21–34 /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 Cr O2y through ion exchange2 7 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 Al3q 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 Cr O2y solution2 7 has no affect on adsorption by calcined LDH. Release of Cr O2y from the adsorbed product2 7 depends upon the type of anion in the aqueous phase and CO2y being the most effective ion.3 Calcination of the Cr O2y adsorbed product up to 12508C has not resulted in the formation of2