Synthesis of Li-Al layered double hydroxide intercalated with amino tris(methylene phosphonic acid) and kinetic and equilibrium studies of the uptake of Nd3+ and Sr2+ ions

A Li-Al layered double hydroxide intercalated with amino tris(methylene phosphonic acid) (AMP Li-Al LDH) was synthesized by the drop-wise addition of an Al-containing solution to a Li-AMP solution at a constant pH of 8.0. The AMP Li-Al LDH was found to take up Nd3+ and Sr2+ ions from aqueous solutions; this phenomenon was attributable to the metal-chelating functionality of the AMP ions in the interlayers of the AMP Li-Al LDH. Further, the AMP Li-Al LDH was found to take up Nd3+ ions preferentially than Sr2+ ions. This was attributable to the stability of the Nd-AMP complex being higher than that of the Sr-AMP complex. The mass-transfer-controlled shrinking-core model could describe the uptake behavior better than the surface-reaction-control model. The AMP ions in the AMP Li-Al LDH interlayers rapidly formed chelate complexes with the Nd3+ or Sr2+ ions. As a result, the transfer of Nd3+ and Sr2+ ions through the product layer was the rate-limiting step. Furthermore, this reaction could be explained by a Langmuir-type adsorption mechanism, indicating that it involved chemical adsorption; this was consistent with the formation of chelate complexes between Nd3+ and Sr2+ ions and the AMP ions in the interlayers of the AMP Li-Al LDH. (C) 2016 Elsevier B.V. All rights reserved.