Effects of Ligand Chemistry and Geometry on Rare Earth Element Partitioning from Saline Solutions to Functionalized Adsorbents

Rare earth elements
(REE) are elements that drive the development
of new technologies in many sectors, including green energy. However,
the supply chain of REE is subject to a complex set of technical,
environmental, and geopolitical constraints. Some of these challenges
may be circumvented if REE are recovered from naturally abundant alternative
sources, such as saline waters and brines. Here, we synthesized and
tested aminated silica gels, functionalized with REE-reactive ligands:
diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic
dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono­(methyl)­glycine
(BPG). A suite of characterization techniques and batch adsorption
experiments were used to evaluate the properties of the functionalized
silica adsorbents and test the REE-uptake chemistry of the adsorbents
under environmentally relevant conditions. Results showed that BPG
and DTPADA yielded the most REE-reactive adsorbents of those tested.
Moreover, the DTPADA adsorbents demonstrated chemical and physical
robustness as well as ease of regeneration. However, as in previous
studies, amino-poly­(carboxylic acid) adsorbents showed limited uptake
at midrange pH and low-sorbate concentrations. This work highlighted
the complexity of intermolecular interactions between even moderately
sized reactive sites when developing high-capacity, high-selectivity
adsorbents. Additional development is required to implement an REE
recovery scheme using these materials; however, it is clear that BPG-
and DTPADA-based adsorbents offer a highly reactive adsorbent warranting
further study.