In-situ synthesis of layered double hydroxides with tunable basal spacing for efficient iodide over iodate adsorption selectivity

Separating I₃⁻ and IO₃⁻ from aqueous solutions is technically challenging due to their very similar charge density. Herein, we describe the in-situ synthesis of cobalt-based layered double hydroxides (Co-LDHs) by decomposition of zeolite imidazole framework (ZIF) nanocrystals, whilst modulating the basal spacing in the Co-LDHs through anion intercalation (such as Br⁻, NO₃⁻, and ClO₄⁻). Co-LDH-NO₃⁻ possesses an optimal basal spacing of ∼4.69 Å, which is slightly smaller than the van der Waals diameter of IO₃⁻ (∼4.79 Å), thus enabling selective adsorption of linear I₃⁻ (diameter ∼4.22 Å) via ion exchange in the LDH interlayer. Co-LDH-NO₃⁻ showed ultrafast I₃⁻ uptake kinetics involving strong “host-guest” interaction forces, achieving high dynamic uptake capacities of 119.94 and 127.18 mg/g in I₃⁻/IO₃⁻ contaminated groundwater and tap water, respectively, and ∼100% separation efficiencies towards I₃⁻ over IO₃⁻. Co-LDH-Br⁻ with a smaller basal spacing (4.06 Å) exhibited much slower separation kinetics, and Co-LDH-ClO₄⁻ with a ∼5.36 Å basal spacing demonstrated poor separation efficiency under similar conditions. Ab initio molecular dynamics (AIMD) simulations were used to visually simulate the ion exchange process, with the results being consistent with the experimental observations. Furthermore, Co-LDH-NO₃⁻ demonstrated excellent reusability during dynamic breakthrough tests and I₃⁻/IO₃⁻ separation cycles due to a “special memory effect” of the 2D interlayer. This work guides adsorbent design for the efficient removal and separation of iodine species from contaminated water.