NiAl layered double hydroxides modified with citrate, malate, and tartrate: Preparation by coprecipitation and uptake of Cu² from aqueous solution

We report on aqueous Cu² uptake by NiAl layered double hydroxides (NiAl LDHs) modified with citrate (C₆H₅O ₇^3-), malate (C₄H₄O₅ ^2-), and tartrate (C₄H₄O₆ ^2-) anions via coprecipitation. Dropwise addition of a mixed aqueous solution of Ni(NO₃)₂ and Al(NO₃)₃ to the respective organic acid solutions at a constant pH of 7.09.0 afforded LDHs with intercalated C₆H₅O₇^3- and Ni(C₆H₅O₇^)-, C₄H ₄O₅^2-, and C₄H₄O ₆^2- in their interlayers. The anions were also likely adsorbed on the LDH surface. Citrate·NiAl LDH could rapidly take up Cu² at a constant pH of 5.0, mainly via chelation by the intercalated and adsorbed anions, rather than coprecipitation with dissolved Al³ to form CuAl LDH. By contrast, malate and tartrate were not active as chelating agents, probably because they formed bridges between brucite-like layers by direct coordination of the two -COO^- groups with Al³ in those layers.