Highly selective and efficient Pb²⁺ capture using PO₄-loaded 3D-NiFe layer double hydroxides derived from MIL-88A

Lead (Pb) contamination in water requires improved decontamination technologies. The addition of phosphate to precipitate Pb²⁺ is a widely used method for remediating Pb in soil and water, though it has certain limitations. This study focuses on novel 3D mesoporous layered double hydroxide (LDH) sorbents functionalized with phosphate anions for Pb²⁺ removal from contaminated waters. Our innovative strategy involves converting a sacrificial template metal-organic frameworks (MOFs) structure (MIL-88A(Fe)) into Ni_xFe LDH, followed by an anion exchange reaction with phosphate anions. This process preserves the 3D microrod architecture of MIL-88A and prevents deleterious LDH particle aggregation. The synthesis results in stable microrod crystals, 1-2 μm long, composed of 3D assemblies of NixFe-PO₄ LDH nanoplatelets with a specific surface area exceeding 110 m²/g. The novel LDH materials display fast adsorption kinetics (pseudo-second order model) and remarkably high Pb²⁺ removal performances (Langmuir isotherm model) with a capacity of 538 mg/g, surpassing other reported adsorbents. LDH-PO₄ exhibits high selectivity for Pb²⁺ over competing ions like Ni²⁺ and Cd²⁺ (selectivity order is: Pb²⁺ > Ni²⁺ > Cd²⁺). Removal of Pb²⁺ from Ni_xFeLDH/88A-PO₄ involves various mechanisms, including surface complexation and surface precipitation of lead phosphate or lead hydroxide phases as revealed by structural characterization techniques.