Preparation of Zn–Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid by coprecipitation: uptake of rare-earth metal ions from aqueous solutions

Abstract

A Zn–Al layered double hydroxide intercalated with triethylenetetramine-hexaacetic acid (TTHA·Zn–Al LDH) was prepared by the dropwise addition of a Zn–Al nitrate solution to a TTHA solution at a constant pH value of 10.0. The TTHA·Zn–Al LDH could uptake rare-earth metal ions such as Nd³⁺ ions from aqueous solutions. This can be attributed to the chelating ability of the TTHA ions in the interlayer, i.e., Nd–TTHA complexes could be formed in the interlayer. However, the TTHA·Zn–Al LDH could hardly uptake Sr²⁺ ions from aqueous solutions, indicating that the TTHA ions in the interlayer did not function as effective chelating agents for Sr²⁺ ions. Therefore, TTHA·Zn–Al LDH could uptake Nd³⁺ ions selectively from an aqueous mixture of Nd³⁺ and Sr²⁺ ions. This was confirmed by the fact that the uptake amount after 120 min was 0.078 and 0.007 mmol g⁻¹ for Nd³⁺ and Sr²⁺, respectively, when TTHA·Zn–Al LDH was added to a 1 : 1 mixed nitrate solution of Nd³⁺ and Sr²⁺ ions. In this case, the degree of selectivity, i.e., the Nd/Sr molar ratio, was 10.7. Furthermore, TTHA·Zn–Al LDH was superior to ethylenediaminetetraacetate (EDTA)·Zn–Al LDH for the uptake of Nd³⁺ ions, which can be attributed to the order of the stability of the metal–chelate complexes: Nd–TTHA complex > Nd–EDTA complex.