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Crystals to the Rescue

How microporous minerals trap chemicals

One of the many drawers of zeolites in the Mineral Sciences collection at NHMLA.

 

Zeolites (zee-oh-lights) are crystals with a superpower: they can trap chemicals, even radioactive ones. NHMLA Mineral Sciences Associate Curator Aaron Celestian wants to know how exactly they do this, and how we can design zeolites to trap the chemicals we want.

a photo of a mineral that looks like a white crystal koosh ball
This drawer of zeolites in the NHMLA Mineral Sciences collection includes Celestian’s favorite in the foreground, a mineral called mesolite that looks like a crystalline koosh ball.

These zeolites are a diverse bunch, but they all have one thing in common — a channel right through their crystalline structure. It’s this tube that allows them to trap chemicals, but not any ol’ random chemical. The size and shape of each channel makes it very specific about what will get stuck in the trap. Sitinakite, a zeolitic crystal, is naturally fond of cesium. While other materials either pass through the channel without incident or can’t begin to fit inside at all, cesium is just the right size to fit within the crystalline structure where it trips a mechanism like a spring-loaded trap, causing sitinakite to change shape and lock the cesium in place.

This is great news, as cesium is one of the most common and troublesome components of nuclear waste. The radioactive form, called cesium-137, can linger in the environment for decades and accumulate in living things because it behaves similarly to potassium, which we use in our cells. But if cesium is trapped in sitinakite, it’s not “bioavailable” any more — it can’t be incorporated into living things and cause long-term damage.

a photo taken with two kinds of powerful microscopes shows what this crystal looks like up close. it appears like tiny stacks of white boxes on a black background
These are extreme closeups of sitinakite using different kinds of microscopes, but even blown up this big, you still can’t see the tiny channels inside the crystals’ structure. The image on the left was taken with a Transmission Electron Microscope (TEM), and the image on the right is from a Scanning Electron Microscope (SEM).

Celestian is studying sitinakite and the other zeolitic crystals in our collections to see which substances they naturally capture in their traps, taking detailed measurements when they do it.

“We’ve known for decades that zeolites trap certain atoms, but no one knows the fundamental molecular processes that make them work. I’m watching the chemical process happen in real time,” explains Celestian.

When we understand how these minerals trap certain substances, and what makes them eventually let go of their chemical captives, we can use them for a variety of applications and even design new zeolites with different powers. They could gobble up toxic chemicals in the environment or deliver cancer drugs to tumors in the human body.

“We can give these crystals the ability to go into water and scavenge the elements of interest and leave everything else behind,” says Celestian.

a photo of the setup celestian uses. there are microscope lenses pointed at a coiled piece of metal around a tube. and a green laser is lighting it up
This is how Celestian studies these fascinating minerals. The machine is a Raman Spectrometer, and it measures how atoms vibrate in a sample. The green laser is aimed at a tube inside the coil which holds small amounts of the zeolite and the chemical it can trap. Celestian takes ongoing measurements with the Raman Spectrometer so he can better understand what is happening on the atomic scale when zeolitic crystals trap something.

 

 

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