Extraction of Rare Earth Elements (REEs) using Functionalized Mesoporous Carbons

D. Saha
Widener University,
United States

Keywords: rare earth elements, extraction technology, Adsorption


Rare Earth Elements (REEs) are 17 elements of the periodic table including 15 lanthanides, scandium, and yttrium. These REEs play a critical role in almost all fields of modern society, including renewable sectors, electronics, batteries, and military applications. The separation of Rare Earth Elements (REEs) from various non-conventional sources is critically important to maintain the supply-demand balance of REEs in the Western world. It is very challenging to separate REEs from other elements or each other owing to very similar properties. In this work, we have fabricated mesoporous carbon grafted with various heteroatom-containing chelating agents, including single-stranded oligo (DNA) and carboxylate groups. The grafting of oligo was performed using 1-[ethyl-3-(3-dimethylaminopropyl) carbodiimide] and N-hydroxyl succinimide on C4-amino linker of the oligo sequence (with varying thymine units) whereas the carboxylate group was grafted using the specific acid mixtures on the pristine mesoporous carbons. The efficiency of these functionalized mesoporous carbons was examined from the synthetic and real-world sources of the REEs. It was demonstrated these adsorbents successfully adsorbed and extracted REEs from those different sources. REE adsorption in both types of carbons was greater than that of pristine carbon demonstrating the key role of functionalities in capturing the REEs. The uptake of 16 lanthanides of REEs was examined from the synthetic mixture and it was revealed that extracted amounts of REEs are linearly increased with the increase in metallic radii of the REE atoms. For the carboxylated mesoporous carbons, the extracted amounts increased with the decrease in atonic radii (or increase in density of REEs) suggesting that carboxylated mesoporous carbon is more selective to the heavier REEs. Carboxylated mesoporous carbon was employed to extract REEs from coal fly ash (CFA). The leachate produced from CFA confirmed the presence of sixteen REEs, except promethium, and the carboxylated mesoporous carbon was successfully extracted about 80-90% of all the REEs present in the leachate in three consecutive cycles. The cyclability study also confirmed that the REEs were successfully recovered from the carboxylated carbons. X-ray absorption spectroscopy (XAS) including X-ray absorption near edge structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) were performed to understand the interactions between REE atom and the surface functionalities. XANES analysis confirmed that the valence states of the REEs remained unaltered. EXAFS analysis confirmed the shortening of bond distance between REE and oxygen and confirmed the chelation or complex formation of REEs on the adsorbent surface. The overall results suggested that these adsorbents can potentially be used to enrich REEs from various sources Acknowledgements: This work was supported National Science Foundation (NSF) with grant number 1837202. Publications: (i) D. Saha et al. Separation and purification Technology 2023, 314, 123583 (ii) D. Saha et al. Langmuir 2022, 38, 203-210. (iii) D. Saha et al. ACS Applied Materials and Interfaces, 2020, 12, 43180-43190.