Open access publications in the SLU publication database

Abstract

Rare Earth Elements (REE) are a group of 17 chemically similar metals that have gained an increasing importance over the past decades, due to their unique properties that make them crucial for many applications, especially in high-tech products. The term “rare” is rather historical than descriptive, since they are quite abundant in the Earth’s crust. However, their extraction and separation can be challenging. This thesis aims to develop a competitive technology, based on functional nanoadsorbents, for extraction and separation of REE in solution that can be industrially applied.

Silica (SiO2) nanoparticles (NPs) with an iron oxide core were selected as the base for the nanoadsorbents. Nanoscale SiO2 exhibits a large surface area that is very advantageous for adsorption purposes. Furthermore, they can be surface functionalized with different organic molecules (ligands). The iron oxide core allows the solid nanoadsorbents to be easily removed magnetically from solution.

First, the synthesis of the nanoadsorbents was optimized and three different organosilanes were synthesized, grafted onto SiO2 NPs and tested for REE uptake. One of the three organosilanes showed to be reasonably efficient and was further tested with magnetic NP for selective uptake of REE. Structural studies of molecular model compounds gave molecular insights into the observed selectivity.

Next, adsorption conditions were carefully modified to, presumably, double the obtained REE uptake. Unexpectedly, the uptake increased up to 30 times, which suggested that a different uptake mechanism was involved. The mechanism was investigated and revealed the induced seeding of a crystalline phase of REE(OH)3 on the surface. Preliminary tests on REE carbonate from REE ores gave very encouraging results.

Finally, a deeper understanding into the selective REE uptake was achieved via structural studies of different ligands and their interaction with different REE, creating the basis for molecular recognition approach. Four ligands specific to different REE were identified and their interaction with REE explained from a molecular point of view. One of these ligands was chosen for upscaling of the technology, by evaluating its potential as packing material for chromatographic separation of REE. The studies provided very attractive results, with good separation of 6 different REE from a solution.