Polymerization regenerating Ca-zeolite by calcium inducing halloysite for enhancing toxic metal ions removal

Traditional techniques can be problematic due to secondary pollution while natural clay aroused great concern. For example, halloysite has high structural activity and excellent potential in toxic metals removal. The polymerization of halloysite to form Ca-zeolite with better metal-binding performance is of great significance for the immobilization of toxic metals. However, halloysite lacks exchangeable cations, which may restrict its polymerization conversion. Thus, it is efficacious to induce halloysite's structural recombination and speciation when calcium is introduced into the polymerization. The curly multilayer halloysite tubes in-situ unfolded and formed kelp-like Ca-zeolite by using calcium as a structure-directing agent. When the calcium content increased, the cubic zeolite was broken into pieces. The pore size distribution showed that micropores and mesoporous structure of halloysite disappeared and converted zeolite hierarchical shape. The characterization analysis indicated that new toxic metal compound groups were generated, confirming the lattice exchange of Ca and toxic metal ions in the Ca-zeolite. Otherwise, the lattice fringe showed that Pb was mainly immobilized on the 001 plane via ion exchange, while Cd formed inner complexes on the 110 plane. Above all, the halloysite-zeolite polymerization technology provides a new method for zeolite synthesis, which is significant for the control of toxic metal pollution in water.