Reusable sponge can capture and recover critical metals and heavy metal contaminants

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In proof-of-concept experiments, the researchers tested their new sponge on a sample of highly contaminated tap water, which contained more than 1 part per million of lead. With a single use, the filtered sponge leads to levels below detectable.

After using the sponge, the researchers were also able to successfully recover metals and reuse the sponge for multiple cycles. The new sponge shows promise for future use as an inexpensive, easy-to-use tool in home water filters or large-scale environmental remediation efforts.

The study was published yesterday (May 10) in the journal DHW ES&T Water. The document describes the new research and establishes design rules to optimize similar platforms to remove and recover other heavy metal toxins, such as cadmium, arsenic, cobalt, and chromium.

“The presence of heavy metals in the water supply is a huge public health challenge worldwide,” said Northwestern’s Vinayak Dravid, lead author of the study. “It’s a gigaton problem that requires solutions that can be implemented easily, effectively and cheaply. That’s where our sponge comes in. It can remove contamination and then be used over and over again.”

Dravid is the Abraham Harris Professor of Materials Science and Engineering at Northwestern’s McCormick School of Engineering and director of global initiatives at the International Institute for Nanotechnology.

absorbing spills

The project builds on Dravid’s previous work to develop highly porous sponges for various aspects of environmental remediation. In May 2020, his team introduced a new sponge designed to clean up oil spills. The nanoparticle-coated sponge, which is now being commercialized by Northwestern spin-off MFNS Tech, offers a more efficient, economical, environmentally friendly and reusable alternative to current approaches to oil spills.

But Dravid knew that it was not enough.

“When there is an oil spill, the oil can be removed,” he said. “But there are also toxic heavy metals, such as mercury, cadmium, sulfur and lead, in those spills. So even when the oil is removed, some of the other toxins can remain.”

rinse and repeat

To address this aspect of the problem, Dravid’s team again turned to sponges coated with an ultra-thin layer of nanoparticles. After testing many different types of nanoparticles, the team found that a manganese-doped goethite coating worked best. Manganese-doped goethite nanoparticles are not only inexpensive to manufacture, readily available, and non-toxic to humans, but also have the necessary properties to selectively remediate heavy metals.

“You want a material with a high surface area, so there’s more room for lead ions to stick to it,” said Benjamin Shindel, Ph.D. student in Dravid’s lab and first author of the article. “These nanoparticles have high surface areas and abundant reactive surface sites for adsorption and are stable so they can be reused many times.”

The team synthesized sludges of manganese-doped goethite nanoparticles, as well as various other nanoparticle compositions, and coated commercially available cellulose sponges with these sludges. They then rinsed the coated sponges with water to remove any loose particles. The final coatings were only tens of nanometers thick.

When submerged in contaminated water, the nanoparticle-coated sponge effectively sequestered lead ions. The US Food and Drug Administration requires bottled drinking water to have less than 5 parts per billion of lead. In filtration tests, the sponge reduced the amount of lead to about 2 parts per billion, making it safe to drink.

“We’re very happy with that,” Shindel said. “Of course, this performance can vary based on a number of factors. For example, if you have a large sponge in a small volume of water, it will perform better than a small sponge in a huge lake.”

Recovery prevents mining

From there, the team rinsed the sponge with slightly acidified water, which Shindel likened to “having the same acidity as lemonade.” The acid solution caused the sponge to release the lead ions and make it ready for another use. Although the performance of the sponge decreased after the first use, it still recovered more than 90% of the ions during subsequent use cycles.

This ability to collect and then recover heavy metals is particularly valuable for removing rare and critical metals, such as cobalt, from water sources. A common ingredient in lithium-ion batteries, cobalt is energetically expensive to mine and comes with a long list of environmental and human costs.

If the researchers could develop a sponge that selectively removes rare metals, including cobalt, from water, then those metals could be recycled into products like batteries.

“For renewable energy technologies, such as batteries and fuel cells, metal recovery is necessary,” Dravid said. “Otherwise, there isn’t enough cobalt in the world for the increasing number of batteries. We must find ways to recover metals from very dilute solutions. Otherwise, it becomes poisonous and toxic, just sitting in water. We might as well do something valuable with him.”

standardized scale

As part of the study, Dravid and his team established new design rules to help others develop tools to target particular metals, including cobalt. Specifically, they identified which low-cost, non-toxic nanoparticles also have high surface areas and affinities for binding to metal ions. They studied the performance of manganese, iron, aluminum and zinc oxide coatings on lead adsorption. Then, they established relationships between the structures of these nanoparticles and their adsorption properties.

Called Nanomaterial Sponge Coatings for Heavy Metals (or “Nano-SCHeMe”), the environmental remediation platform can help other researchers differentiate which nanomaterials are best suited for particular applications.

“I’ve read a lot of literature comparing different coatings and adsorbents,” said Caroline Harms, an undergraduate student in Dravid’s lab and a co-author on the paper. “There really is a lack of standardization in the field. By looking at different types of nanoparticles, we developed a comparative scale that really works for all of them. It could have many implications for advancing the field.”

Dravid and his team envision their sponge could be used in commercial water filters, for environmental cleanup, or as an extra step in water treatment and reclamation facilities.

“This work may be relevant to water quality issues both locally and globally,” Shindel said. “We want to see this out in the world, where it can have a real impact.”

Dravid and Northwestern have financial interests (shares, royalties) in MFNS Tech.