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.

​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.

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