Unraveling how minerals fight toxic waste using Raman spectroscopy

Energy production and consumption have shattered the earth’s ecological balance.

The burning of fossil fuels, in transportation and industry has led to growing climate change. And clean energy technologies like nuclear fission still leaves its own toxic footprint.

Yet scientists have identified valuable, naturally occurring substances to mediate the harmful effects of the first two industrial revolutions. Spectroscopy plays a key role in understanding these substances as the newest industrial age unfolds.

Minerals are simply naturally occurring inorganic solids that have a crystalline structure. That gives it a larger surface area. Thousands of named minerals exist, and researchers discover about 30 to 50 new ones each year.

The body needs a group of 16 minerals, called essential minerals. It includes calcium, phosphorus, potassium, sulfur, sodium, chloride, magnesium, iron, zinc, copper, manganese, iodine, selenium, molybdenum, chromium and fluoride.

Minerals serve various roles. Some help the body grow, develop, and perform different functions — from building strong bones to transmitting nerve impulses. Scientists even use minerals to make hormones or maintain a normal heartbeat.

Understanding of how minerals react with other compounds is leading to new ways to use these substances. Aaron Celestian, Ph.D., the associate curator of the Natural History Museum of Los Angeles County applies Raman spectroscopy and elemental analysis to decipher the reactions of minerals with various substances.

Raman Spectroscopy is a non-destructive, “dry” chemical analysis technique based on the interaction of light with the chemical bonds within a material. It delivers detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions.

Elemental analysis yields the elemental composition of minerals, chemical compounds, soil and waste. It can be qualitative and quantitative, and applied to bulk and surface analysis.

Celestian is a mineralogist and geochemist. But he also considers himself an educator and conservationist.

His research aims at deciphering chemical mechanisms at the atomic and molecular level, and determining how minerals grow and behave in varied environmental and industrial conditions. He hopes to understand, predict, and even manipulate mineral behaviors and properties at the macroscopic scale.

Celestian works extensively with zeolites, a group of minerals with common characteristics.

Zeolites are solids with a relatively open, three-dimensional crystal structures built from aluminum, oxygen, and silicon. Zeolites form with crystalline structures, with large open pores, or cavities, in a regular arrangement. The cavities are roughly the same size as small molecules. It’s open, cage-like, framework structure can trap other molecules inside it.

Zeolites are robust – it’s stable under temperature and pressure extremes, and it doesn’t dissolve in water or oxidize in the air.

Zeolites are good absorbers. It also adsorbs, or captures on its surface, a variety of heavy metals and ammonia. This allows it to remove a wide range of pollutants, and commercial applications include neutralizing industrial waste. In fact, zeolites are ingredients in cat litter, absorbing the ammonium out of urine. Other uses include water filters and softeners, laundry detergent, animal food and industrial catalysts.

The mineral group has other invaluable applications. A little background first.

Nuclear fission involves splitting of an atom, yielding heat energy. Its primary use is energy generation in nuclear power plants.

But fission leaves behind radioactive waste. That waste must be stored safely until it eventually decays or disintegrates into to harmless materials. That can take thousands of years.

In the meantime, its storage and protection has become a growing burden on the nuclear industry and the environment.

Minerals provide a possible solution to that problem.

Zeolites are effective at separating heavy metals in gases and streams. Engineers use zeolites in water treatment because of its unique adsorption properties, which allows it to remove a wide range of pollutants.

One particular zeolite is sitinakite. When you put it in water, sitinakite absorbs cesium, a byproduct of spent nuclear fuel. Engineers can use sitinakite to clean up nuclear waste sites.

Celestian’s research looks at how these minerals adsorb radioactive waste. Raman spectroscopy’s sensitivity helps researchers understand the chemical reactions involved in the process and maximize its effectiveness.

“We use Raman spectroscopy to see how that mineral changes on the molecular level,” Celestian said. “Then we can try to decipher what mechanisms are involved in capturing those metals. We then go back and see what other minerals share those properties.”

By understanding how these minerals adsorb radioactive waste, industrial entities can apply the information to scale up the application for large environments and disasters.

“I’m trying to use minerals to remediate things that humans are doing to cause environmental problems,” Celestian said.

For Raman analysis, Celestian uses the HORIBA XploRA™ Plus for real-time monitoring of mineral chemical reactions. He also uses it for characterizing the fluid inclusions - the microscopic bubbles of liquid and gas trapped within a crystal - inside of minerals on earth and potentially other planets.

That’s right – he studies other planets too. NASA plans a sample mission to Mars with a return, to launch in 2028. Celestian and his team at the Jet Propulsion Lab in Pasadena, California are preparing for that mission. The team is doing some of the groundwork to look for bacteria and beta-carotenes trapped inside minerals. That would show life once existed on Mars.

Celestian also uses a HORIBA XGT 7200 X-Ray microscope to characterize mineral reactions. The instrument mergers optical observation and elemental analysis functions for non-destructive elemental analysis. As an X-ray microscope, the XGT uses electromagnetic radiation in the soft X-ray band to produce magnified images of objects. X-rays do not reflect or refract easily, unlike visible light, and it is invisible to the human eye.