Multifunctional gel beads derived from coal gangue–phosphorus tailings layered double hydroxides for Cd(II) immobilization

Remediating heavy metal-polluted soils requires not only effective contaminant immobilization but also water conservation and fertility restoration. In this study, polyacrylamide-layered double hydroxide gel beads (hereafter referred to as RFW-LDH beads, where RFW denotes Cd(II) remediation, fertilizer release, and water retention) were synthesized using solid wastes derived from coal gangue and phosphorus tailings. Their multifunctional performance, including Cd(II) immobilization, water retention, controlled fertilizer release, and magnetic recovery, was systematically evaluated. RFW-LDH beads exhibited excellent Cd(II) immobilization, primarily attributed to the synergistic contributions of polymer matrix complexation/coordination, LDH surface adsorption, and isomorphic substitution. Their stability was confirmed through the toxicity characteristic leaching procedure (TCLP), dry–wet cycling, and freeze–thaw tests. Sequential extraction further demonstrated the transformation of Cd into more stable fractions following remediation. Urea release from the beads was independent of environmental conditions and followed a two-stage pattern: an initial rapid release driven by surface swelling, followed by a slower diffusion-controlled phase. Swelling–deswelling and soil water-holding experiments showed that the beads effectively delayed evaporation and improved soil moisture retention. Plant toxicity assays revealed reduced Cd accumulation and mitigated phytotoxicity, while tissue carbon and nitrogen analyses confirmed enhanced soil fertility resulting from the encapsulated urea. Microbial community shifts indicated improved carbon and nitrogen cycling and overall enhancement of soil properties. Additionally, the incorporation of magnetic Fe 3 O 4 imparted strong magnetic responsiveness, enabling rapid and nondestructive recovery under an external magnetic field. Collectively, the RFW-LDH beads integrate heavy metal immobilization, water retention, nutrient supply, and magnetic recovery into a single platform, offering a green, efficient, and sustainable strategy for remediating polluted soils.