Phosphoric Triamide-Based Silica Magnetic Nanocomposites as Efficient Adsorbents for Removal of Lead and Copper from Aqueous Solutions

The development of magnetically separable adsorbents with precisely engineered surface functionalities remains a challenge in heavy-metal remediation. In this work, we report design and application of phosphoric triamide-functionalized magnetic nanocomposites (Fe3O4/SiO2/PTAs) as efficient adsorbents for heavy metal ions, establishing phosphoric triamides as a ligand platform for magnetic adsorption systems. This innovative ligand architecture provides multiple donor sites capable of cooperative coordination with metal ions, thus enhancing both selectivity and binding affinity. Three tailored ligands bearing 2-, 3-, and 4-aminopyridinyl substituents were synthesized and covalently anchored onto silica-coated magnetite nanoparticles. Comprehensive physicochemical characterization confirmed the successful grafting of the ligands and the formation of nanocomposites with favorable surface properties. The phosphoric triamide-functionalized Fe3O4/SiO2 nanoparticles (∼30 nm) exhibited increased surface area (∼50–71 m2/g), larger pore volume (∼0.39 cm3/g), and strong magnetic properties (saturation magnetization up to 55.41 emu/g), providing abundant active sites while enabling efficient magnetic separation after adsorption. Among them, Fe3O4/SiO2/PTA1 showed the highest adsorption capacity toward Pb2+ (113 mg/g) and Cu2+ (16 mg/g), together with superior removal efficiency for both metal ions. Adsorption equilibrium data were best described by the Sips isotherm model, whereas kinetic analysis indicated that the integrated kinetic Langmuir (IKL) and fractal-like pseudo-first-order (FL-PFO) models adequately represent the adsorption process. Furthermore, the adsorbent demonstrated excellent magnetic recoverability and reusability with no loss of selectivity. Overall, this study introduces phosphoric triamide-based surface chemistry for magnetic nanomaterials, bridging molecular ligand design with efficient heavy-metal removal and offering opportunities for the development of advanced and selective hybrid adsorbents.