Uptake of Nd³⁺ and Sr²⁺ by Li-Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid: Kinetic and equilibrium studies

A Li-Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid (TTHA·Li-Al LDH) was prepared by the drop-wise addition of an Al solution to a Li-TTHA solution at a constant pH of 8.0. TTHA·Li-Al LDH was found to take up Nd³⁺ and Sr²⁺ ions from aqueous solutions. This can be attributed to the metal chelating functions of the TTHA species in the interlayer of TTHA·Li-Al LDH, i.e., Nd-TTHA and Sr-TTHA complexes were formed in the interlayers. TTHA·Li-Al LDH was found to take up Nd³⁺ ions preferentially over Sr²⁺ ions from aqueous solutions, which can be attributed to the higher stability of the Nd-TTHA complex than Sr-TTHA. The mass-transfer-controlled shrinking core model described the uptake behavior better than the surface reaction-controlled model. The TTHA ions in the TTHA·Li-Al LDH interlayer quickly formed chelate complexes with Nd³⁺ or Sr²⁺, as a result of which the transfer rate of Nd³⁺ or Sr²⁺ through the product layer was rate limiting. Furthermore, this reaction was suitably represented by the Langmuir-type adsorption mechanism, indicating that it involves chemical adsorption, which is consistent with the formation of chelate complexes between Nd³⁺ or Sr²⁺ and TTHA ions in the interlayers of TTHA·Li-Al LDH.