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

Abstract

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.