Beyond Floc Growth: Unlocking Post-Precipitation CaF2 Removal through Magnetic Nanocoagulation

<title>Abstract</title> <p>Calcium precipitation converts dissolved fluoride into solid CaF2, yet the resulting fine particles can remain persistently suspended and difficult to recover. Here we develop magnetic nanocoagulation for actual semiconductor wastewater and resolve it into two sequential functions: Brownian capture, which incorporates non-magnetic CaF2 into a magnetic particle population, and magnetic addressability, which drives subsequent assembly and transport. Across four Fe3O4 size classes, M20 achieved the highest apparent capture (79.9%), whereas M200 clarified fastest (k50 = 0.541 min−1), revealing a functional inversion between capture and transport. Controls and pH-dependent experiments showed that interfacial capture establishes the addressable target fraction before field-driven separation. Dual-surface calculations further showed that strong confinement reorganizes interfacial water and favours a localized Ca–F-associated hydration state. These results establish a separation strategy that bypasses continuous floc growth by coordinating nanoscale capture, active magnetic transport and confinement-controlled hydration.</p>