NETL researchers have developed a low-cost compact fiber-optic sensor that exploits a process called “photoluminescence sensitization” to detect rare earth elements (REEs) — critical materials needed by U.S. manufacturers to make high-tech systems and devices — in waste byproducts generated from mining and fossil energy production.

“Other nations produce the vast majority of the REEs needed to manufacture everything from smartphones, computer hard drives and batteries for electric vehicles to solar panels, windmill turbines and defense systems,” said NETL’s Scott Crawford, a research chemist.

“This new NETL technology can help generate a strong domestic supply chain of REEs to manufacture these and other essential products and reduce U.S. reliance on offshore producers.”      

NETL researchers previously demonstrated that a metal-organic framework (MOF) material could detect trace concentrations of REEs using a commercial photoluminescence spectrometer.

In a recently published study, Crawford, the study’s principal investigator, and NETL colleagues Ward Burgess, Ki-Joong Kim, John Baltrus and Nathan Diemler explain they have advanced the technology by integrating the material onto a piece of optical fiber and connecting that fiber to NETL’s patented fiber-optic fluorescence sensor.

The researchers noted their technique significantly lowers the cost of REE characterization as compared to state-of-the-art methods such as inductively coupled plasma mass spectrometry by a factor of 5 to 10. 

The enhanced technology targets REEs in acid mine drainage, an environmental hazard that flows from working and abandoned mines in regions across the United States, and coal fly ash leachate, which also can cause environmental issues.

Crawford explained that MOFs work as “porous microscopic sponges” and are placed in the tip of the optical fiber to collect REEs and other materials from the mine drainage and leachate.

Ultraviolet (UV) light is then directed at the MOFs. The MOFs undergo photoexcitation. As the MOFs absorb the UV light, some of that energy is transferred to the REEs, which produce a distinct luminescent signal. For instance, the REE terbium, which is used to manufacture low-energy lightbulbs, turns green, while europium becomes red or pink, and dysprosium becomes various shades and tints of cyan.

Researchers found the luminescent signal can be significantly improved by drying the fiber tip between exposure to REEs, which is particularly important when analyzing complicated samples of acid mine drainage or fly ash leachate that contain high concentrations of interfering ions and low pH levels.

The immobilization of the material on the fiber tip also makes it easier to recover and re-use it across multiple sensing cycles. As a proof-of-concept, the system was applied to track europium and terbium extraction in a simulated extraction process and was successfully used to detect part-per-billion or low part-per-million spikes of terbium, europium and dysprosium added to an acid mine drainage matrix.

“This work represents a simple, effective method for REE detection using immobilized sensing materials, which will significantly reduce the time and financial costs associated with characterizing rare earth elements during prospecting and process monitoring,” Crawford said.

This NETL technology is outlined in the article “Zinc adeninate metal-organic framework-coated optical fibers for enhanced luminescence-based detection of rare earth elements,” which was published in March in RSC Applied Interfaces, an open-access journal of the Royal Society of Chemistry.

NETL is a U.S. Department of Energy national laboratory that drives innovation and delivers technological solutions for an environmentally sustainable and prosperous energy future. By leveraging its world-class talent and research facilities, NETL is ensuring affordable, abundant and reliable energy that drives a robust economy and national security, while developing technologies to manage carbon across the full life cycle, enabling environmental sustainability for all Americans.