Rare-earth elements (REEs) are critical materials in modern industry, but their production has a significant environmental footprint. Environmentally friendly separation methods would enable efficient, sustainable recycling of REEs. This work introduces a class of cyclen-based macrocyclic chelators that induce significant differences in solubility for REE chelates, enabling their selective precipitation from pH-neutral aqueous solution. The process was refined using simple coordinating additives (e.g., acetate) to form ternary coordination compounds to fine-tune these chelate solubilities. Conditions were optimized for the REEs found in NdFeB magnets, allowing separations of even adjacent lanthanides by repeated precipitations. Separation factors comparable to those of industrial solvent extraction methods were achieved without organic solvents. Analysis of NdFeB magnets from current electric car motors revealed an unexpected presence of holmium as a supplement and/or replacement for terbium and dysprosium, suggesting shifting industrial trends with implications for future recycling efforts. In a case study, one such automotive magnet was processed to obtain a 99.7% pure neodymium product. Scalable, tunable, and entirely aqueous, this approach advances the sustainable use of REEs toward a circular economy.

Department of Applied Electronics Faculty of Electrical Engineering and Computer Science VSB─Technical University of Ostrava 17 listopadu 2172 15 708 00 Ostrava Czech Republic

Department of Inorganic Chemistry Faculty of Science Charles University Hlavova 2030 128 40 Prague 2 Czech Republic

Institute of Inorganic Chemistry Czech Academy of Sciences Husinec Řež 1001 250 68 Husinec Řež Czech Republic

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo náměstí 542 2 160 00 Prague 6 Czech Republic

Institute of Physics Czech Academy of Sciences Na Slovance 1999 2 182 00 Prague 8 Czech Republic

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