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.