Rice study tests MOF recovery of rare earths from wastewater

Rice University researchers reported that a ligand-modified metal-organic framework could selectively adsorb neodymium, gadolinium and dysprosium from industrial wastewater, with measured capacities as high as 160 milligrams of neodymium, 175 milligrams of gadolinium and 195 milligrams of dysprosium per gram of material. The peer-reviewed experimental study, supported by molecular simulations, was published in ACS Applied Materials & Interfaces and focused on recovering scarce rare earth elements from complex waste streams.

The researchers began with MIL-101, a water-stable metal-organic framework composed of metals and organic molecules arranged into an exceptionally porous structure. They modified its internal surfaces with phosphonomethyliminodiacetic acid, abbreviated PMIDA, whose binding groups form strong bonds with positively charged rare earth ions. By varying the amount of PMIDA added to the framework, the team tested how ligand density affected adsorption and used simulations to examine how the rare earth ions and ligand molecules arranged themselves inside the material’s nanoscale pores.

Higher PMIDA loading did not produce a proportional increase in capacity. Instead, the experiments and simulations indicated that multiple binding groups could coordinate around individual rare earth ions as ligand loading increased, creating stronger and more stable complexes within the pores. At the highest loading tested, the material’s neodymium and gadolinium adsorption capacities were at least twice the values previously reported for the same water-stable metal-organic framework under similar conditions. The results identify ligand loading and coordination strength as related design factors for this adsorption process, rather than treating added ligand solely as a means of increasing the number of binding sites.

The researchers also tested selectivity against iron, a common contaminant that can greatly outnumber rare earth elements in industrial waste. When iron was present at 100 times the concentration of the target rare earth elements, rare earth uptake was only about 0.3% lower in one test than it was without iron. Most of the captured rare earth elements could subsequently be released from the framework with dilute hydrochloric acid, a step that may allow the valuable metals to be collected and the material to be reused. Study first author Yiming Liu, who conducted the work as a Rice postdoctoral researcher and is now an assistant professor of environmental engineering at South Dakota State University, said the combination of experiments and simulations showed not only whether the framework captured the elements but also how changes in ligand loading affected the way the ions were held.

Rare earth elements have distinctive magnetic and electronic properties that make them important in permanent magnets, electric vehicles, wind turbines, drones, aerospace applications, medical imaging and health care. Their supply chains remain geographically concentrated, while separating small amounts from wastewater is difficult because other metals may be present at much higher concentrations. The Rice team’s findings suggest that engineered metal-organic frameworks could offer an alternative to liquid-liquid solvent extraction for capturing rare earth elements. Liu said further work is needed to refine the technique, particularly improving selectivity while maintaining durability, regeneration and ligand attachment, and preserving the material’s ability to release captured elements.

Subject of Research: Selective recovery of rare earth elements from wastewater using ligand-modified metal-organic frameworks

Article Title: Rice researchers modify Nobel-prize winning material to recover valuable rare earth elements from industrial wastewater

Article References: Rice researchers modify Nobel-prize winning material to recover valuable rare earth elements from industrial wastewater. Original publication (Bibliographic details unavailable.)

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DOI: Not provided

Keywords: rice university, rare earth elements, industrial wastewater, metal-organic frameworks, neodymium, gadolinium, dysprosium, selective adsorption, iron interference, molecular simulation, ACS Applied Materials & Interfaces, Rice