Highly selective capture of Ni²⁺ from complex environments by a sandwich-like layered metal sulfide ion exchanger

Radionickel ion (⁶³Ni²⁺) remediation is critical for public health and the environment, but selectively capturing of Ni²⁺ from complex environments like seawater presents a challenge. Metal sulfide ion exchangers (MSIEs) are emerging as efficient adsorbents for radionuclides; however, the study of MSIEs for selectively removing Ni²⁺ is still in its infancy. Herein, the layered metal sulfide K₂Cu₂Sn₂S₆ (CTS-1) with a unique sandwich-like anionic framework was synthesized by the hydrothermal method for the first time, representing a novel approach in the selective capture of Ni²⁺ from complex environments. Single-crystal structural analysis confirmed the sandwich-like framework, in which a [Cu-S] sublayer is sandwiched by two [Sn-S] sublayers with parallel grooves. The charge-balancing K⁺ ions are located within these grooves. Due to its special structure, CTS-1 exhibits remarkable adsorption capacities for Ni²⁺ with rapid kinetics (a high rate constant k₂ of 7.26 ×10^-2 g/(mg·min)), broad pH durability (removal rates >97 % at pH 3-12), and high selectivity (separation factors for Ni²⁺ >700 against various cations). Impressively, it can efficiently remove Ni²⁺ from multiple complex environments, achieving a 90.28 % removal rate even in seawater (C₀^Ni ∼5 mg/L). CTS-1 is environmentally friendly and suitable for use in fixed-bed columns for the practical application. Moreover, Ni²⁺ ions are captured through ion exchange with K⁺, and the high selectivity stems from the strong affinity of S^2- for Ni²⁺ and the trapping effect of the grooves within the structure. In summary, this pioneering study demonstrates the highly selective capture of Ni²⁺ by a sandwich-like layered MSIE, potentially inspiring the development of efficient scavengers for radionuclides.