Magnetophoresis of Trivalent Lanthanides in Viscosity Standards to Uncover Paramagnetism Relationships

The separation of rare earth elements (REEs) remains a critical challenge as global supply chains face increasing vulnerability. State-of-the-art methods such as solvent extraction are impractical for domestic use due to their reliance on large volumes of harsh chemicals and the generation of hazardous waste. Furthermore, solvent extraction suffers from limited selectivity, requiring hundreds of mixing/settling stages to achieve adequate separation. These limitations highlight the urgent need for innovative technologies that are both environmentally benign and highly efficient. Field-assisted separation techniques represent a promising alternative, leveraging differences in ionic transport properties to achieve high-resolution separations. Established approaches such as electrophoretic separations enable fully aqueous separations, yielding high separation factors between REEs. Magnetophoretic separations fall within this same category; however, their application to REEs remains largely unexplored. Given that lanthanides exhibit distinct paramagnetic properties, this characteristic offers a potential basis for selective separation. Nevertheless, the conditions necessary for practical magnetophoretic separations are not well understood. In this study, we investigated the magnetophoretic migration of aqueous REE droplets under varying experimental conditions. Specifically, we examined the influence of anionic counterparts (i.e., chloride, nitrate, and trifluoromethanesulfonate) on the magnetophoretic velocity of REE droplets under an external magnetic field gradient. Additionally, we assessed the effects of REE concentration and the viscosity of the organic separation medium on migration behavior. Our results reveal systematic trends correlating net magnetic force with average magnetophoretic velocity for six REEs (Pr, Nd, Sm, Dy, Ho, and Er), each possessing distinct magnetic moments. These findings demonstrate the potential viability of magnetophoretic separation as a complementary technique to electrophoresis, particularly for difficult adjacent pairs such as Eu-Gd.