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Penn State detector able to locate rare earth mineral in acid mine drainage

The date of: 2021-09-02
viewed: 7
source:Bradford Era
Penn State researchers have developed a sensor to find a valuable and rare earth element, terbium, in samples drawn from the environment such as acid mine waste.
Terbium is one of the rarest of the rare earth elements. It is used to produce the green color in cell phone displays as well as in high-efficiency lighting and solid-state devices.
While there are a range of chemical, environmental and political challenges to obtaining terbium and other rare earth elements through existing methods, the elements are abundant in non-traditional sources in the United States, including coal byproducts, acid mine drainage and electronic waste.
The luminescence-based sensor developed at Penn State takes advantage of a protein that binds to rare earth elements and could be harnessed to help develop a domestic supply of these metals.
The most reliable current method of detecting rare earth elements in a sample is a type of mass spectrometry that is expensive and not portable. Meanwhile, portable methods are not as sensitive and do not perform well in complex environmental samples, where acidic conditions and other metals can interfere with detection.
“There is not currently a domestic supply chain of rare earth elements like terbium but they are actually quite abundant in non-traditional sources in the U.S., including coal byproducts, acid mine drainage, and electronic waste,” said Joseph Cotruvo Jr., a chemistry professor at Penn State and senior author of the study. “In this study, we developed a luminescence-based sensor that can be used to detect and even quantify low concentrations of terbium in complex acidic samples.”
The new sensor relies on lanmodulin, a protein that the researchers previously discovered that is almost a billion times better at binding to rare earth elements than to other metals.
The protein’s selectivity to bind rare earth elements is ideal for a sensor, as it is most likely to bind to rare earth instead of other metals that are common in environmental samples.
To optimize lanmodulin as a sensor for terbium, the researchers altered the protein by adding the amino acid tryptophan to the protein.
“Tryptophan is what is called a ‘sensitizer’ for terbium, which means that light absorbed by tryptophan can be passed to the terbium, which the terbium then emits at a different wavelength,” Cotruvo said.
The green color of this emission is actually one of the main reasons terbium is used in technologies like Smartphone displays. For our purposes, when the tryptophan-lanmodulin compound binds to terbium, we can observe the emitted light, or luminescence, to measure the concentration of terbium in the sample.”
The researchers developed many variants of the tryptophan-lanmodulin sensor and tested the most promising variant to determine the lowest concentration of terbium the sensor could detect in idealized conditions.
Even under highly acidic conditions, like that found in acid mine drainage, the sensor could detect environmentally relevant levels of terbium.
“One challenge with extracting rare earth elements is that you have to get them out of the rock,” Cotruvo said. “With acid mine drainage, nature has already done that for us, but looking for the rare earths is like finding a needle in a haystack.
“We have existing infrastructure to treat acid mine drainage sites at both active and inactive mines to mitigate their environmental impact. If we can identify the sites with the most valuable rare earth elements using sensors, we can better focus extraction efforts to turn waste streams into revenue sources.”
The researchers next tested the sensor in actual samples from an acid mine drainage treatment facility in Pennsylvania with many other metals present and just 3 parts per billion of terbium.
The sensor determined a concentration of terbium in the sample that was comparable with the determinations made by the top existing method, suggesting that the new sensor is a viable way to detect low concentrations of terbium in complex environmental samples.
We plan to further optimize the sensor so that it is even more sensitive and can be used more easily,” Cotruvo said. “We also hope to target other specific rare earth elements with this approach.”

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