Uptake and separation of rare earth elements and late transition metal cations by nanoadsorbent grafted with diamino ligands

Recycling of magnetic materials based on Rare Earth Elements (REE) is of major interest in the view of growing clean energy production and transportation. One of the major challenges in its realization is the need to separate smaller amounts of Late Transition Metals (LTM) from REE. Hybrid adsorbents are very attractive in finding such a solution. Here, novel silica-based nanoadsorbents were synthesized by grafting the surface of dense silica nanoparticles with a diamino functional ligand grafted via an arene linker to improve selectivity towards LTM. The produced adsorbent materials were characterized using SEM, TEM, AFM, XPS, FTIR, and TGA in its pure form and by DLS in suspension, and tested for the adsorption and separation of LTM (Co2+ and Ni2+) and REE (Sm3+ and Nd3+) in single and mixed solutions. Prepared organo-silica material showed rapid uptake of all tested cations with higher affinity towards LTM. Adsorption capacities reached values of 1.18-1.45 mmol/g for Co2+ and Ni2+, respectively, with a 1:1 metal-to-ligand stoichiometry for Ni cations. Investigation of reusability demonstrated the potential of the prepared materials as an environmentally friendly alternative in specific separation of LTM to conventional separation techniques. Investigations of the molecular structures of the Ni2+ complex with the selected molecular function and of Co3+ with a closely related tris-aminoethyl amine ligand in combination with XPS data for corresponding surface complexes helped explaining the molecular mechanisms for adsorption and desorption of the LTM cations