Balancing interaction energy and collision frequency for engineering-efficient algal removal by covalently functionalized micrometer-scale magnetic particles.

Magnetic separation enables rapid algal removal without the addition of chemical coagulants, but current studies rely predominantly on functionalized magnetic nanoparticles, whose high cost limits practical application. Here, we show that micrometer-scale magnetic particles (MPs), although intrinsically less surface-reactive, can enable engineering-efficient algal removal through stable covalent functionalization. Polyethyleneimine (PEI) was grafted onto MPs via a silane-coupling strategy to enhance algal removal, and the effect of particle size was systematically investigated over the range of 90 nm to 32 μm. PEI grafting reversed the surface charge of MPs from negative to positive values, thereby promoting strong electrostatic attraction toward algal cells. Algal removal performance improved markedly as particle size decreased from 32 μm to 5 μm, whereas MPs between 90 nm and 5 μm exhibited comparable removal efficiencies. Among the tested particle sizes, 5 μm MPs showed the best overall performance, achieving 90% algal removal within 1 min at a dosage of 7.26 g·g-1, while smaller MPs (90 nm-1 μm) required longer separation times (≥ 3 min) to reach similar efficiencies. Mechanistic analysis combining extended Derjaguin-Landau-Verwey-Overbeek theory, Smoluchowski collision theory, and separation kinetics suggested that the final algal removal performance was governed by the coupled effects of attachment-related factors and the subsequent separation of the formed aggregates, resulting in a favorable particle-size window rather than a simple smaller-is-better trend. The selected 5 μm MPs maintained 87.2-91.8% removal over four reuse cycles and achieved 85% algal removal in bloom-affected pond water. Together with their faster separation kinetics and lower material cost relative to nanometer-scale MPs, these results identify covalently functionalized micrometer-scale MPs as a practical and cost-effective strategy for algal removal.