Adsorption mechanism of rare earth elements by adsorbents with diglycolamic acid ligands

Previously, we investigated silica gel adsorbents bearing immobilized diglycolamic acid ligands (designated EDASiDGA) for the selective recovery of rare earth elements (REEs) from acidic solution, but the details of the adsorption mechanism remained to be elucidated. In this study, to clarify the adsorption mechanism, we prepared some derivatives of EDASiDGA and investigated the differences in their adsorption behaviors with respect to REE ions. On the basis of our results, we concluded that adsorbents with diglycolamic acid ligands adsorb REE ions via three oxygen atoms (that is, the ether, amide, and carboxylic acid oxygen atoms of the diglycolamic acid ligands) and that this tridentate chelation confers high selectivity for REE ions.