Physicals aspects of magnetically assisted water purification
Résumé
Capture of micron-sized magnetic particles on high gradient magnetic separators has been studied for decades in view of broad applications to ore benefication and to magnetic separation of biological cells. Filtration of magnetic nanoparticles, as small as 10-20 nm in diameter, is often considered as practically irrelevant because of the weakness of magnetic interactions as compared to Brownian effects. We propose to use nanoparticles combined with microparticles to remove pollutants from water. The nanoaparticles which present a large surface area adsorb the pollutants and the microparticles attract the nanoparticles under an external magnetic field. So we can separate the purified water from the waste water containing the two sizes magnetic particles with pollutants. After washed these particles can be re-use for another purification cycle. We show in this work that, despite a strong Brownian motion, the nanoparticles can be effectively captured by a magnetic filter element if the interparticle interactions are large enough to provoke a condensation phase transition in the vicinity of the filter element. In experiments, under an external magnetic field we observe formation of thick anisotropic "clouds" of nanoparticles around a micron-sized spherical filter element, and try to estimate the shape and the concentration profile of these clouds from the fundamental thermodynamic relations.