International audienceIn this work, we consider magnetic separation of iron oxide nanoparticles when a nanoparticle suspension (diluted ferrofluid) passes through a closed-loop filter composed of a packed bed of micro-beads magnetized by an externally applied magnetic field. We show that the capture of nanoparticles of a size as small as 60 nm is easily achieved at low-to-moderate magnetic fields (15 kA/m) thanks to relatively strong magnetic interactions between them. The key parameter governing the capture process is the Mason number-the ratio of hydrodynamic-to-magnetic forces exerted to nanoparticles. The filter efficiency, Λ, defined through the ratio of the inlet-to-outlet concentration shows a power-law dependency on Mason number, 0.83 Ma − Λ ∝ , in the range 2 4 10 10 Ma < <. The proposed theoretical model allows a correct prediction of the Mason number dependency of the filter efficiency. The obtained results could be of potential interest for water purification systems based on chemical adsorption of micro-pollutants on magnetic nanoparticles, followed by magnetic separation of the nanoparticles