Adsorption-desorption of chromium(VI) on Mg-Al hydrotalcite-like compounds part I. Adsorption

Adsorption properties of Cr(VI) on Mg-Al hydrotalcite-like compounds (HTlc) were studied. The influences of initial solution pH, additives of inorganic electrolytes, NaCl, NaNO3, Na2SO4 and Na3PO4, and organic ligands, EDTA and citric acid on the adsorption of Cr(VI) were investigated, and the adsorption mechanism was discussed in combination with the results of IR and XRD experiments. It was found that Mg-Al HTlc showed good adsorption ability for Cr(VI) from solution, indicating that the use of HTlc as a promising inorganic sorbent for the removal of Cr(VI) from wastewater is possible. The adsorption kinetics and the adsorption isotherms of Cr(VI) on the HTlc can be described by the pseudo-second order kinetic and Langmuir isotherm, respectively. With increasing pH, the adsorption amount decreased. The additives of inorganic electrolytes, NaCl, NaNO3, Na2SO4 and Na3PO4, and organic ligands, EDTA and citric acid, could obviously restrain the adsorption of Cr(VI) on the HTlc, and the orders of the restraining ability on the adsorption are Na3PO4 >= Na2SO4 >= NaCl >> NaNO3 and citric acid > EDTA, respectively. The adsorption of Cr(VI) on the HTlc arose from the anion exchange in the interlayer space and the adsorption on the external surface, respectively. Furthermore, the adsorption layer of Cr(VI) on the external surface may be divided into two layers: the inner-sphere surface complexes arising from the chemical binding and the outer-sphere surface complexes arising from the electrostatic interaction.