Using rust to clean water... but how?

Pouring flecks of rust into water usually makes it dirtier. But researchers have developed special iron oxide nanoparticles they call “smart rust” that actually makes it cleaner.

Smart rust can attract many substances, including oil, nano- and microplastics, and the herbicide glyphosate, depending on the particles’ coating. And because the nanoparticles are magnetic, they can easily be removed from water with a magnet and the pollutants. The team reports that they’ve tweaked the particles to trap estrogen hormones potentially harmful to aquatic life.

The researchers from Friedrich-Alexander-Universität Erlangen-Nürnberg will present their results today at the fall meeting of the American Chemical Society (ACS). ACS Fall 2023 is a hybrid meeting being held virtually and in-person Aug. 13–17 and features about 12,000 presentations on a wide range of science topics.

A video on the research is available at www.acs.org/SmartRust.

“Our ‘smart rust’ is cheap, nontoxic and recyclable,” says the project’s principal investigator, Marcus Halik, PhD. “And we have demonstrated its use for all kinds of contaminants, showing this technique’s potential to dramatically improve water treatment.”

How the smart rust in water project came about

For many years, Halik’s research team has been investigating environmentally friendly ways to remove pollutants from water. The base materials they use are iron oxide nanoparticles in a superparamagnetic form, which means they are drawn to magnets but not to each other, so the particles don’t clump.

To make them “smart,” the team developed a technique to attach phosphonic acid molecules onto the nanometer-sized spheres.

“After we add a layer of the molecules to the iron oxide cores, they look like hairs sticking out of these particles’ surfaces,” said Halik.

Then, by changing what is bound to the other side of the phosphonic acids, the researchers can tune the properties of the nanoparticles’ surfaces to strongly adsorb different pollutants.

Early versions of smart rust trapped crude oil from water collected from the Mediterranean Sea and glyphosate from pond water collected near the researchers’ university. Additionally, the team demonstrated that smart rust could remove nano- and microplastics added to lab and river water samples.

So far, the team has targeted pollutants present in mostly large amounts. Lukas Müller, a graduate student presenting new work at the meeting, wanted to know if he could modify the rust nanoparticles to attract trace contaminants, such as hormones.

When some of our body’s hormones are excreted, they are flushed into wastewater and eventually enter waterways. Natural and synthetic estrogens are one such group of hormones, and the primary sources of these contaminants include waste from humans and livestock. The amounts of estrogens are very low in the environment, says Müller, so they are difficult to remove. Yet even these levels have been shown to affect some plants’ and animals’ metabolism and reproduction. However, the effects of low levels of these compounds on humans over long periods aren’t fully known.

Trapping hormones in water with smart rust

“I started with the most common estrogen, estradiol, and then four other derivatives that share similar molecular structures,” said Müller.

Estrogen molecules have a bulky steroid body and parts with slight negative charges. To exploit both characteristics, he coated iron oxide nanoparticles with two sets of compounds: one that’s long and another that’s positively charged. The two molecules organized themselves on the nanoparticles’ surface, and the researchers hypothesize that they build many billions of “pockets” that draw in the estradiol and trap it in place.

Because these pockets are invisible to the naked eye, Müller has been using high-tech instruments to verify that these estrogen-trapping pockets exist. Preliminary results show efficient extraction of the hormones from lab samples. Still, the researchers need to look at additional experiments from solid-state nuclear magnetic resonance spectroscopy and small-angle neutron scattering to verify the pocket hypothesis.

“We are trying to use different puzzle pieces to understand how the molecules actually assemble on the nanoparticles’ surface,” explains Müller.

In the future, the team will test these particles on real-world water samples and determine the number of times that they can be reused. Because each nanoparticle has a high surface area with lots of pockets, the researchers say that they should be able to remove estrogens from multiple water samples, thereby reducing the cost per cleaning.

“By repeatedly recycling these particles, the material impact from this water treatment method could become very small,” concludes Halik.