Conventional water treatment relies on sequential flocculation and disinfection, which inflates infrastructure costs and heightens the risk of disinfection byproduct formation. Here, we report a magnetic lignin-based flocculant (LS-DMC-AM@Fe
3O
4) that integrates flocculation, sterilization, and rapid magnetic separation within a
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Conventional water treatment relies on sequential flocculation and disinfection, which inflates infrastructure costs and heightens the risk of disinfection byproduct formation. Here, we report a magnetic lignin-based flocculant (LS-DMC-AM@Fe
3O
4) that integrates flocculation, sterilization, and rapid magnetic separation within a single material. The composite was synthesized by thermally initiated graft copolymerization of methacryloyloxyethyl trimethylammonium chloride (DMC) and acrylamide (AM) onto sodium lignosulfonate (LS), followed by incorporation of Fe
3O
4 nanoparticles (NPs) at 15 wt% loading; the product exhibited a saturation magnetization of 12.8 emu g
−1. LS-DMC-AM@Fe
3O
4 achieved 98.2% kaolin turbidity removal at 1 mg L
−1 and 98.6%
E. coli removal at 8 mg L
−1, and displayed a markedly broader effective dosage window than its non-magnetic analog. We attribute this broadened window to Fe
3O
4-enhanced membrane disruption, which liberates anionic intracellular contents that buffer excess cationic charge and thereby suppress restabilization. The bactericidal efficiency reached 90% at 18 mg L
−1, 1.6-fold higher than LS-DMC-AM, governed by a synergistic dual mechanism: quaternary ammonium contact-killing coupled with Fe
3O
4 NP-induced intracellular reactive oxygen species (ROS) accumulation. Under an external magnetic field, flocs underwent rapid phase separation and displayed enhanced shear-regrowth capacity (
E. coli floc recovery factor: 53% vs. 26%); Fe
3O
4 NPs were recovered at >95% efficiency over two cycles. Despite higher unit production costs, LS-DMC-AM@Fe
3O
4 delivers competitive per-unit-volume treatment economics through its ultralow effective dosage and magnetic seed recyclability. These results establish a viable strategy for engineering multifunctional, recyclable flocculants from industrial lignin waste.
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