Uptake of Ni²⁺ and Cu²⁺ by Zn–Al layered double hydroxide intercalated with carboxymethyl-modified cyclodextrin: Equilibrium and kinetic studies

Zn–Al layered double hydroxides (Zn–Al LDHs) intercalated with carboxymethyl-modified cyclodextrin (CMCD) ions (CMCD•Zn–Al LDHs) were prepared by the co-precipitation method. The adsorption of Ni²⁺ by CMCD•Zn–Al LDHs followed the Langmuir-type mechanism, as evident from the adsorption isotherm. ESI-TOF-MS results implied that the uptake of Ni²⁺ by CMCD•Zn–Al LDHs could be attributed to the formation of chelate complexes between Ni²⁺ and CMCD ions in the interlayers of the CMCD•Zn–Al LDH. Kinetic studies revealed that the reaction involved in the uptake of Ni²⁺ by CMCD•Zn–Al LDH proceeded under mass transfer control, suggesting that the CMCD ion in the interlayer of the CMCD•Zn–Al LDH quickly formed a chelate complex with Ni²⁺. The transfer rate of Ni²⁺ through the product layer was rate-limiting. Similar results were obtained for the uptake of Cu²⁺ by CM-β-CD•Zn–Al LDH. It was also found that the CM-β-CD•Zn–Al LDH was more selective toward Cu²⁺ uptake than Ni²⁺ in the mixed solution. This was attributed to the Irving-Williams order of stability of bivalent transition metal complexes; Cu²⁺ formed a more stable chelate complex with the CM-β-CD ion in the interlayer of CM-β-CD•Zn–Al LDH as compared to Ni²⁺.