X-ray Study Explores Atomic Structure of Tiny Traps for Heavy metals

Researchers have developed a specialized type of glowing metal organic framework, or LMOF (molecular structure at center), that is designed to detect and remove heavy-metal toxins from water. At upper left, mercury (Hg) is taken in by the LMOF. The graph at lower left shows how the LMOF’s fluorescence is turned off as it binds up the mercury. Its properties make this LMOF useful for both detecting and trapping heavy-metal toxins. (Credit: Rutgers University)

Tiny, glowing crystals that are designed to identify and capture heavy-metal toxins such as mercury or lead could be a powerful new tool ideal for both locating and cleaning up water sources that are contaminated.

Motivated by publicized cases where high levels of heavy metals were detected in drinking water in Flint, Mich., and Newark, N.J., a science team headed by researchers at Rutgers University used intense X-rays at Lawrence Berkeley National Laboratory (Berkeley Lab) to explore the structure of the crystals they developed and study how they bind to heavy metals.

The crystals function like reusable, miniature traps and sensors, and are known as luminescent metal-organic frameworks (LMOFs).

Top Performer for Detecting and Trapping Heavy Metals

Recent results featured in Applied Materials and Interfaces highlight that one particular type of LMOF tested by the team was found to selectively take up more than 99% of mercury from a test mixture of light and heavy metals within 30 minutes. The team reported that no other MOFs have performed as well in this dual role of identifying and capturing, or “adsorbing,” toxic heavy metals.

Simon Teat, a Berkeley Lab staff scientist, analyzed individual LMOF crystals with X-rays at the lab’s Advanced Light Source (ALS). Each crystal measured about 100 microns (millionths of a meter). Teat used diffraction patterns developed as the X-ray light struck the LMOF samples and used software tools to map their three-dimensional structure with atomic resolution.

The ALS is just one of the few synchrotron X-ray light sources in the world that have committed experimental stations for chemical crystallography studies on crystallized chemical compounds such as MOFs.

It All Starts with Structure

He discovered a patterned, grid-like 3-D structure comprising of zinc, nitrogen, oxygen, hydrogen, and carbon atoms that framed large, open channels. These atomic-scale structural details are vital for understanding how the LMOFs bind heavy metals, and can also help in designing extremely highly specialized structures.

With MOFs, you’re typically interested in using the holes for something.

Simon Teat, Staff Scientist, Berkeley Lab

In this situation, the structure allows heavy metals to enter these open channels and then chemically bind to the MOFs.

Their open framework provides the MOFs with an abundant surface area corresponding to their size, which allows them to take in huge volumes of contaminants.

The LMOF structure was engineered to glow by integrating a fluorescent chemical component, or ligand. “When the metal binds to the fluorescent ligand, the resulting framework fluoresces,” Teat said. When interacting with the heavy metals, the fluorescence of the LMOFs switches off.

Jing Li, a chemistry professor at Rutgers University who headed the research, stated that the technology could be a money-saving solution.

Others had developed MOFs for either the detection of heavy metals or for their removal, but nobody before had really investigated one that does both.

Jing Li, Chemistry Professor, Rutgers University

Li further added that intense X-rays produced at synchrotrons are considered to be the most ideal way to map the 3-D structure of the MOFs. He stated, “Knowing the crystal structures is one of the most important aspects of our research. You need those in order to perform subsequent characterizations and to understand the properties of these materials.”

Tests Show MOFs are Chemically Selective, Reusable

Researchers conducted tests where they discovered that LMOFs bind strongly to lead and mercury, but bind weakly to lighter metals such as calcium and magnesium that are detected in water supplies but do not pose the same risks.

Li highlighted that this selective trait, based on the molecular makeup of the LMOFs, is significant. “We need to have a MOF that is selective and will only take the harmful species,” he stated.

It is also possible to recycle the LMOFs. Researchers discovered the possibility of collecting, cleaning, and reusing the LMOFs for three cycles of toxic cleansing even before their performance could begin to degrade.

What’s Next?

The research suggests that heavily industrialized areas, cities with antiquated water regulations, and also agricultural communities can be specifically susceptible to groundwater contamination, which can result in soil contamination if not addressed. This could further result in the contaminants being used by animals and plants to develop a solid film. “These filters could be used for capture on a larger scale,” she said.

Li additionally stated that research and development could further study inexpensive and more durable LMOFs that could survive for more cycles, and researchers could also pursue the production of water filters by blending the LMOFs with polymers to create a solid film. “These filters could be used for capture on a larger scale,” she said.

“We would like to continue with this research,” Li said, emphasizing on the fact that her team would be interested in testing the system’s performance on actual contaminated water sources if funding becomes available. “These are promising results, but we have a long way to go.”

The team led by Li also used Berkeley Lab’s ALS for determining the crystal structures of MOFs for a variety of other applications, including toxin detection in foods; and new varieties of light-emitting components for LEDs, called phosphors, that incorporate more abundant, inexpensive materials.

The Advanced Light Source is a DOE Office of Science User Facility.

Researchers from the University of Texas at Dallas and Rider University were also part this research. The DOE Office of Science supported this work.

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