Magnetophoresis of Paramagnetic and Diamagnetic Metal Salts in Porous Media: Effects of Porous Media and Solutes Interactions*

We report experiments on magnetophoresis of a mixture of paramagnetic and diamagnetic metal solutes in porous media. Specifically, we focused on the magnetophoresis of individual paramagnetic (MnCl₂) and diamagnetic (ZnCl₂) metal salts, as well as their mixtures, at concentrations of 1, 10, and 100 mM. We find that compared to silica gel with a polydisperse size distribution, larger silica particles achieved moderately higher enrichment of paramagnetic ions. Additionally, diamagnetic metal salts (ZnCl₂) showed moderately stronger depletion at the surface of the magnet. When using smaller silica gel particles, no noticeable difference was observed in the concentration depletion of diamagnetic metal salts. However, for paramagnetic salts, the concentration enrichment was moderately lower in silica gel with smaller particle sizes compared to the polydisperse silica distribution. For binary mixtures of metal salts, we find enrichment in the region of highest magnetic field for both paramagnetic and diamagnetic metal salts. However, the overall enrichment for the mixture was lower compared to the enrichment levels observed for the individual metal salts. Our hypothesis is that in a mixture containing solvent, paramagnetic and diamagnetic ions, each individual ion experience magnetic forces in different directions. Presumably these opposing forces may induce competing convective flows that impact (lower) the overall magnetomigration of each ion in binary mixtures.