(Nanowerk News) Northwestern University engineers have developed a new sponge that can remove metals — including toxic heavy metals like lead and critical metals like cobalt — from contaminated water, leaving water safe and drinkable.
In a proof-of-concept experiment, the researchers tested their new sponge on a sample of tap water that was highly contaminated, containing more than 1 part per million of lead. With single use, filtered sponges lead to levels below detectability.
After using the sponge, the researchers also managed to recover the metal and reused the sponge for several cycles. The new sponge holds promise for future use as an inexpensive and easy-to-use tool in home water filters or large-scale environmental improvement efforts.
The study was published May 10 in the journal Water ACS ES&T (“Nano-SCHeMe: Nanomaterial Sponge Coatings for Heavy Metals, Environmental Remediation Platform”). This paper outlines the new research and establishes design rules for optimizing such platforms to remove – and recover – other toxic heavy metals, including cadmium, arsenic, cobalt and chromium.
“The presence of heavy metals in water supplies is a huge public health challenge for the whole world,” said Northwestern’s Vinayak Dravid, senior author of the study. “This is a gigatonne problem that requires a solution that can be implemented easily, effectively and inexpensively. That’s where our sponge comes into play. This sponge can remove pollution and can be used repeatedly.”
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.
This project builds on previous Dravidian work to develop highly porous sponges for various aspects of environmental improvement. In May 2020, his team launched a new sponge designed to clean up oil spills. Nanoparticle-coated sponges, now being commercialized by Northwestern spin-off MFNS Tech, offer a more efficient, economical, environmentally friendly and reusable alternative to current approaches to oil spills.
But Dravid knew that wasn’t enough.
“When there is an oil spill, you can get the oil out,” he said. “But there were also toxic heavy metals – such as mercury, cadmium, sulfur and lead – in the spill. So even if you remove the oil, some other toxins may remain.
Rinse and repeat
To address this aspect of the problem, the Dravid team, once again, turned to sponges coated with a very thin layer of nanoparticles. After testing different types of nanoparticles, the team found that a layer of manganese-doped goethite performed best. Not only are manganese-doped goethite nanoparticles not only inexpensive to manufacture, readily available and non-toxic to humans, they also possess the properties necessary to selectively recover heavy metals.
“You want a material with a high surface area, so there is more room for lead ions to attach to it,” says Benjamin Shindel, Ph.D. student in Dravid’s lab and first author of the paper. “These nanoparticles have a high surface area and abundant reactive surface sites for adsorption and are stable, so they can be reused many times.”
The team synthesized manganese-doped goethite nanoparticle slurries, as well as several other nanoparticle compositions, and coated commercially available cellulose sponges with these slurries. Then, they rinsed the coated sponge with water to remove any loose particles. The final layer is measured to be only tens of nanometers thick.
When immersed in contaminated water, the nanoparticle-coated sponge effectively absorbs lead ions. The US Food and Drug Administration requires that bottled water contain below 5 parts per billion of lead. In a filtration experiment, the sponge lowered the amount of lead to about 2 parts per billion, making it safe to drink.
“We’re very excited about that,” Shindel said. “Obviously this performance can vary based on several 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 large lake.
Recovery goes through mining
From there, the team rinsed the sponge with slightly acidified water, which Shindel likened to “has the same lemonade acidity.” The acid solution causes the sponge to release lead ions and is ready to be used again. Although the performance of the sponge decreases after the first use, it still recovers more than 90% of the ions during the next use cycle.
The ability to collect and recover these heavy metals is invaluable for removing rare and critical metals, such as cobalt, from water sources. A common ingredient in lithium-ion batteries, cobalt is extremely expensive to mine and comes with a laundry list of environmental and human costs.
If researchers can develop sponges that selectively remove rare metals, including cobalt, from water, then those metals could be recycled into products such as batteries.
“For renewable energy technologies, such as batteries and fuel cells, metal recovery is required,” said Dravid. “Otherwise, the world’s cobalt will not be enough to increase the number of batteries. We have to find a way to recover metals from very dilute solutions. Otherwise, it becomes poisonous and toxic, just sitting in the water. We might as well make something valuable with it.”
As part of the research, Dravid and his team established new design rules to help others develop tools to target specific metals, including cobalt. Specifically, they pinpointed which nanoparticles were inexpensive and non-toxic as well as having high surface areas and affinity for attaching to metal ions. They studied the performance of manganese, iron, aluminum and zinc oxide layers on lead adsorption. Then, they established the relationship between the structure of these nanoparticles and their adsorptive properties.
On the phone Nanomaterial Sponge Coating for Heavy Metals (or “Nano-SCHeMe”), an environmental remediation platform can help other researchers distinguish which nanomaterials are best suited for a particular application.
“I’ve read a lot of literature comparing different coatings and adsorbents,” said Caroline Harms, an undergraduate student in the Dravid lab and co-author of the paper. “Indeed there is a lack of standardization in the field. By analyzing different types of nanoparticles, we develop a comparative scale that actually works for all of them. That could have a lot of implications in moving this field forward.
Dravid and his team envision their sponge could be used in commercial water filters, for environmental cleaning or as an additional step in water reclamation and treatment facilities.
“This work may be related to water quality issues both locally and globally,” said Shindel. “We want to see this out in the world, where it can make a real impact.”