Hemiacetal dynamic covalent shell-engineered magnetic nanoadsorbents with ultrahigh Pb²⁺ capture capacity and rapid magnetic recovery

Recyclable adsorbents for heavy metal ions have gained increasing attention. Here, a magnetic polymer core-shell nanoadsorbent (MP-CSN) was prepared via precipitation polymerization to overcome recycling challenges. The material features amino-functionalized magnetic nanoparticles as the core and a highly cross-linked hemiaminal polymer (PHM) shell. Characterization by DLS, FT-IR, and TGA confirmed high magnetic responsiveness. Adsorption tests at room temperature showed that MP-CSN reached equilibrium within 4 h for Pb²⁺ concentrations of 20–160 mg/L. Under optimal conditions (140 mg/L, 0.2 g/L), the maximum adsorption capacity was 620 mg/g based on the Langmuir model. Bare M-N particles showed <10% removal, highlighting the crucial role of the polymer shell. Competitive adsorption indicated preferential removal of Pb²⁺ (88.22%), Zn²⁺ (75.72%), Cu²⁺ (80.06%), Co²⁺ (67.73%), and Hg²⁺ (26.35%). This work provides a surface engineering strategy for magnetically responsive nanocomposites with tailored adsorption, especially for lead-contaminated water.