Magnetically assisted separation of weakly magnetic metal ions in porous media. Part 1: experiments

We report experiments on the magnetophoresis of paramagnetic (MnCl₂) and diamagnetic (ZnCl₂) metal ion species in porous media under the influence of a non-uniform magnetic field generated by a permanent magnet. Experiments were carried out in a range of initial ion concentrations (1-100 mM), porous media particle sizes (polydisperse, 63 µm and 500 µm), and varying mixture ratios of metal ion concentrations. For solutions containing single metal ion species, paramagnetic MnCl₂ migrated toward the magnet surface, with an enrichment of approximately 2-4% near regions of high magnetic field. Conversely, single species diamagnetic ZnCl₂ moved away from regions of highest magnetic field gradients, with depletion levels of 0.5-1.8% relative to the initial concentration. Our results demonstrate that magnetophoresis is directly proportional to porous media particle size, increasing with larger particle sizes, a trend attributed to the reduced drag forces experienced by the ions in media with larger particles. Interestingly, in solutions containing species, both MnCl₂ and ZnCl₂ migrated toward regions of highest magnetic field, contrary to their individual behaviors. The magnetophoretic effect of MnCl₂ was diminished with increasing concentrations of ZnCl₂, indicating interactions between the two ion species. These findings suggest that both metal ion species undergo field-induced cluster formation, with cluster sizes in the micrometer range, in both single and binary species systems. In binary mixtures, the two ion species appear to interact, potentially forming mixed clusters containing both MnCl₂ and ZnCl₂.