Closed-circuit magnetic separation of nanoparticles on a packed bed of spheres

International audienceIn this work, we consider magnetic separation of iron oxide nanoparticles when a nanoparticlesuspension (diluted ferrofluid) passes through a closed-circuit filter composed of a packed bed of microspheresmagnetized by an externally applied magnetic field. Recent studies on isolated magnetic collectors have shown thatthe capture of nanoparticles of a size as small as 50 nm is still very efficient if the magnetic interactions between themis sufficiently strong to induce a phase separation in the nanoparticle suspension [Magnet et al. (2012, 2014)]. In thepresent case of the packed bed of magnetizable spheres, such phase separation (manifesting by appearance of droplikeaggregates of nanoparticles) is easily achieved at magnetic fields as low as 15 kA/m. We show that the keyparameter governing the capture of magnetic nanopartilces in the present system is the Mason number - the ratio ofhydrodynamic-to-magnetic forces exerted to nanoparticles. The capture efficiency, λ, defined through a ratio of theinlet-to-outlet concentration shows a power-law dependency on Mason number, λ∝Ma-0.8, in the range Ma<104. Wedeveloped a theoretical model based on the trajectory analysis of particle motion in a concentric-sphere representativecell of the porous filter. This model allows a correct prediction of the Mason number dependency of the captureefficiency. The obtained results could be of potential interest for water purification systems based on chemicaladsorption of micro-pollutants on magnetic nanoparticles, followed by a magnetic separation on the considered type ofmagnetic filter. Preliminary results on nickel ion adsorption on oleate-covered iron oxide nanoparticles have alreadybeen obtained and are also reported in this work