Attachment of Tridentate Ligand onto the Polyhedral Oligomeric Silsesquioxanes as an Efficient Strategy to Capture La(III) Ions: A Comparative Study of Homogeneous and Heterogeneous Systems Using Potentiometric Titration

Developing sustainable methods for lanthanide ion recovery is a relevant area of research that seeks to promote the efficient reuse of these valuable chemical elements present in several electronic devices. Adsorption techniques using materials functionalized with chelating groups, such as modified Polyhedral Oligomeric Silsesquioxanes (POSS) matrices, may play a crucial role in capturing these metals. This work investigates the coordination of the lanthanum-(III) ion with two tridentate organic ligands: bis-(pyridin-2-ylmethyl)-amine (L1) and (pyridin-2-ylmethyl)-(thiophen-2-ylmethyl)-amine (L2). The results revealed stronger interactions for L1, with higher formation constants (9.0 for [LaL1]³⁺ and 16.9 for [La-(L1)₂]³⁺) compared to L2 (4.37 for [LaL2]³⁺). Further studies on ligand L1 attached to silsesquioxane (POSS-L1) showed significant metal ion interactions, showing the formation of [LaPOSS-L1]³⁺ (81.0%) at pH 5.42 and La-(OH)₃POSS-L1 above pH 8.0, with a formation constant of 10.33, which is higher than that for free L1. These results demonstrate the efficient binding of La-(III) to the POSS-L1 in a heterogeneous system. The modified silsesquioxane matrix presents a promising alternative for lanthanide ion adsorption, contributing to sustainable metal recovery strategies.