Highly efficient recovery of Zn²⁺/Cu²⁺ from water by using hydrotalcite as crystal seeds

The efficient and waste-free recovery of heavy metals is critical for heavy metal wastewater treatment. In this work, we explored how heavy metals can be recovered as valuable chemicals in the presence of crystal seeds. Hydrotalcite (one kind of layered double hydroxides (LDHs)) was used as crystal seeds to recover Zn²⁺ in the presence of Al³⁺ from water (i.e., seed-Zn²⁺-Al³⁺ system), which was compared with the monometallic heterogeneous system (seed-Zn²⁺) and direct coprecipitation (Zn²⁺-Al³⁺) system. Our results demonstrated that the seed-Zn²⁺-Al³⁺ system possessed a recovery rate of 2.6-2.8 times and a recovery kinetic rate of 2.7-5.9 times higher than those of the other two systems. Differing from the latter two systems, hydrotalcite seeds could induce Zn²⁺ and Al³⁺ to form ZnAl-LDH in seed-Zn²⁺-Al³⁺. Interestingly, the ZnAl-LDH presents a compositional divalent/trivalent cation molar ratio of ca. 3, which is comparable with the value in the hydrotalcite. It was demonstrated that the hydrotalcite seeds could act as a template to significantly induce the formation of ZnAl-LDH complying with the seed's structure and compositional ratio. Similar induction effect of seeds as the Zn²⁺ system was further verified in Cu²⁺ systems. This work provides a novel strategy for efficient recovery of heavy metals with product selectivity.