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

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.