An investigation into mechanism of cation adsorption by reconstruction of calcined layered double hydroxide

Abstract This work investigated the mechanism and limit of cation adsorption by reconstruction of calcined layered double hydroxide (CHT) for the first time. Mg x Al-CHT was used to adsorb Zn 2+ and Ni 2+ from aqueous solution. As a result, adsorption amount of cation was approximately equal to dissolved amount of Mg 2+ from CHT. In other words, the molar ratio was close to 1:1 between exchanged cation (Zn 2+ or Ni 2+ ) and released Mg 2+ . Besides, Zn 2+ adsorption and Mg 2+ dissolution took place synchronously until equilibrium. XRD and SEM indicated that CHT rebuilt layered double hydroxide (LDH) after adsorbing Zn 2+ and Ni 2+ . Furthermore, molar ratios of Mg 2+ /Zn 2+ and Mg 2+ /Ni 2+ were 1:1 in reconstructed CHT regardless of the original Mg 2+ /Al 3+ ratio. Based on these results, it was proposed that half of octahedral Al 3+ in LDH transformed into tetrahedral ones after calcination, resulting in half of Mg 2+ bond being distorted in CHT and ready for a potential exchange. When CHT was used to adsorb bivalent cation (Me 2+ ), the maximum amount of adsorption was the half amount of Mg 2+ , and MgMeAl-LDH was formed by dissociation-deposition.