US researchers have developed a sponge-like material to capture rare earth elements from industrial wastewater.
Rather than simply treating wastewater, a Rice University team viewed dirty water pools as resource pools to recover valuable rare earth materials. It could eventually open the possibility to recover critical minerals that are limited in supply in different regions.
”Instead of viewing industrial wastewater only as something that needs to be treated, we can begin thinking of it as a resource containing valuable materials that may be worth recovering,” said Qilin Li, the Karl F. Hasselmann Professor of Civil and Environmental Engineering at Rice and corresponding author of the study.
Metal-ligand chemistry
The idea may sound simple; however, it is not easy.
Besides the rare earth elements, the industrial wastewater is loaded with other metals in much higher concentrations, like iron, sodium, and aluminum. These high concentrations make it challenging to separate the minuscule amounts of the trace elements. Hence, it often interferes with the selective recovery of the valuable rare metals.
To circumvent this, the researchers borrowed from the principles of coordination chemistry and built an extremely porous membrane.
The approach uses metal-organic frameworks (MOFs), which are highly porous, sponge-like materials made by linking metal atoms with organic molecules. Scientists then chemically modified the walls of the tiny internal pores to improve performance.
In the experiments, the a water-stable MOF was modified with an organic ligand, Phosphono Methyl Imino Diacetic Acid (PMIDA).
Inside the MOF’s tiny pores, PMIDA acts like a chemical hook, strongly binding positively charged rare earth ions. The team tested the capture of three rare metals, namely neodymium, gadolinium, and dysprosium, using the ultra-porous material.
At the highest ligand concentration, PMIDA captured up to 160 milligrams of neodymium, 175 milligrams of gadolinium, and 195 milligrams of dysprosium per gram of material. These recovery capacities were at least twice as high as previously reported amounts using a similar approach.
Increasing the PMIDA ligand amount boosted the material’s overall performance by changing the available binding sites. Further, simulations revealed that adding extra ligand allowed the binding groups to cluster around individual rare earth ions within the pores. This arrangement formed stronger and more stable complexes to enhance capacity and structural stability.
”What was especially interesting was that we could see not just whether the material captured the rare earths but why,” Liu said. “By combining experiments with molecular simulations, we could look inside the pores and see how changing the number of ligands changes the way these ions are held.”
Selective capture, simple release
Another important finding was the ability of the material to capture the rare earth metals selectively and efficiently in the presence of unwanted materials, such as iron. The study reported minimal changes in the uptake of rare earth, even when iron was present at 100 times the concentration of the rare earth elements in the waste liquid.
In one test, rare earth uptake was only about 0.3% lower than it was without iron present.
Besides the capturing technology, the researchers also examined the release of the trace elements. A mild treatment with dilute hydrochloric acid easily extracted the trace elements, allowing the valuable material to be reused.
It is just a beginning. And further research is needed to improve the material’s selectivity, durability, and regeneration during extraction. However, the team remains optimistic about MOFs, which could provide a more sustainable replacement for the liquid-liquid solvent extraction methods used to recover rare earth elements.
The research was published in ACS Applied Materials & Interfaces.