Fabrication of a Magnesium–Aluminum Layered Double-Hydroxide-Based Honeycomb-like Gel Microsphere and Its Synergistic Adsorption–Reduction Removal of Cr(VI) from Wastewater

A composite hydrogel microsphere (LDH/SA/PEI) with a three-dimensional honeycomb-like porous structure was fabricated using Mg–Al layered double hydroxide (LDH) as the matrix material, sodium alginate (SA) and polyethylenimine (PEI) as gel precursors, and a CaCl2/FeCl3 mixed solution as the hybrid cross-linking agent. The LDH/SA/PEI has realized the synergistic adsorption–reduction removal of Cr(VI) from wastewater. The optimal preparation process consists of 1.5 wt % SA, 4 vol % PEI, 4.5 wt % CaCl2, and 0.1 mol/L FeCl3. The maximum Cr(VI) adsorption capacities of 79.6–118 mg/g between 25 and 55 °C are accessed at 12 g/L LDH and pH 2 within 300 min. Kinetic and thermodynamic analyses reveal that the removal of Cr(VI) by the LDH/SA/PEI involves three distinct stages, surface diffusion, intramesopore diffusion, and intramicropore diffusion, which exhibits strong agreement with the intraparticle diffusion model (R2 > 0.95). The ΔG values of the reactions are all less than 0, with a ΔH of 35.3 kJ/mol and a ΔS of 126.8 J mol–1 K–1, indicating a chemisorption-dominated spontaneous endothermic process. Scale-up experiments confirm that 3 g/L LDH/SA/PEI can reduce the Cr(VI) concentration from 20 to <0.5 mg/L within 180 min. This study provides a novel material fabrication strategy with high adsorption capacity, mechanical stability, and sustainability for Cr(VI)-containing wastewater treatment, which has significant implications for anion-polluted industrial wastewater and the functional development of LDH-based composites.