Size effects in magnetic separation for rapid and efficient bacteria removal

Magnetic separation offers a promising strategy for bacterial removal from aqueous systems due to its tunable specificity, biocompatibility, and facile recovery. However, the influence of nanoparticle size on removal efficiency and adsorption capacity remains unclear. Here, we systematically investigate size-dependent effects using porous magnetic particles covalently modified with polyethyleneimine (PEI). Porous magnets with diameters ranging from ∼50 to ∼420 nm were synthesized and functionalized with PEI to enhance electrostatic interactions with negatively charged bacteria. The resulting PEI-modified porous magnets induced extensive bacterial aggregation and exhibited high adsorption capacities for both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus. Adsorption capacity increased with decreasing particle size under equal iron mass, reaching up to 18.8 × 10⁹ CFU mg^-1 for E. coli and 28.9 × 10⁹ CFU mg^-1 for S. aureus. Adsorption kinetics were also size-dependent, reaching over 95% of maximum capacity within 10 minutes for all particle sizes. The smallest particles (50 nm) were difficult to recover without sufficient bacterial contact. In low-concentration bacterial separations, larger particles resulted in minimal residual bacteria and achieved nearly 100% removal. These results highlight the critical role of particle size in magnetic bacterial separation and provide guidance for designing efficient magnetic adsorbents.