Toward Molecular Recognition of REEs: Comparative Analysis of Hybrid Nanoadsorbents with the Different Complexonate Ligands EDTA, DTPA, and TTHA.

Highly efficient tailored SiO2-based nanoadsorbents were synthesized for the selective extraction of rare-earth elements (REEs). Three different complexonates (EDTA, DTPA, and TTHA) were investigated in terms of uptake capacity and selectivity, showing capacities of up to 300 mg of RE3+/g and distinct preferential trends depending on the complexonate. EDTA-functionalized nanoadsorbents showed higher uptake for Dy3+, DTPA-functionalized ones for Nd3+, and TTHA-functionalized ones for La3+. The selectivity was even more pronounced in desorption at pH 3, with separation factors of up to 76 in ternary mixtures. A broad comparative study of single-crystal structures of the complexes between REE and the nongrafted complexonates at different pHs led to a molecular understanding of their individual modes of action. EDTA-derived nanoadsorbents combine concerted action and chelation, whereas the latter is the preferential coordination mechanism for DTPA- and TTHA-derived nanoadsorbents. These different mechanisms result in quite specific REE affinities, which opens great possibilities toward molecular recognition of REEs and for tailoring nanoadsorbents for a particular REE or group of REEs in their production from minerals and in recycling. It also brings new insights into how REEs are adsorbed on nanomaterials applied in a broad variety of fields, including bioimaging and MRI.