From waste to function: Hierarchical brush-like gypsum micro-assemblies enable distinct immobilization mechanisms for hazardous Cd2+, Pb2+, and Cu2+

Hierarchical brush-like calcium sulfate dihydrate micro-assemblies (H-B-CSDM) were synthesized as a high-efficiency adsorbent via a synergistic additive-mediated aqueous precipitation method. Their adsorption performance toward Cd 2+ , Pb 2+ , and Cu 2+ was systematically evaluated, and the distinct mechanisms were elucidated through multi-scale characterization and density functional theory (DFT) calculations. Under optimized conditions (pH 6.0, an adsorbent dosage of 2 g·L −1 ), H-B-CSDM achieves adsorption equilibrium for all three heavy metal ions within 60 min. Pb 2+ exhibits the highest adsorption capacity and pronounced selectivity, with an uptake of 49.0 mg·g −1 in the ternary system, which is 1.16 and 1.26 times those of Cd 2+ and Cu 2+ , respectively. The adsorption of Cd 2+ and Pb 2+ followed pseudo-second-order kinetics and the Freundlich isotherm model, indicating chemisorption-dominated heterogeneous multilayer adsorption, whereas Cu 2+ displayed multi-mechanism coupling behavior. XRD, FTIR, XPS, and SEM analyses collectively demonstrate three distinct immobilization pathways: ion-exchange-induced surface precipitation (PbSO 4 formation) for Pb 2+ , electrostatic attraction with reversible outer-sphere complexation for Cd 2+ , and surface complexation with partial lattice substitution for Cu 2+ . Density functional theory (DFT) calculations corroborate these findings: Pb 2+ exhibits the most negative adsorption energy (−0.988 eV) at surface sites, making it thermodynamically most prone to surface deposition; whereas Cu 2+ and Cd 2+ exhibit adsorption energies of −0.385 eV and − 0.211 eV, respectively, indicating a decreasing order of chemical adsorption strength, with Cd 2+ primarily undergoing weak electrostatic outer-sphere complexation. H-B-CSDM maintained high adsorption efficiency over five regeneration cycles, demonstrating the best regeneration capacity for Pb 2+ . This study provides a theoretical basis and design paradigm for upgrading gypsum-based minerals from low-cost carriers to highly efficient active adsorption platforms for treating complex wastewater.