Northwestern University engineers have developed a new sponge that can remove minerals — including toxic heavy metals like lead and important metals like cobalt — from polluted water, leaving potable water behind.
In proof-of-concept experiments, the researchers tested their new sponge on a highly contaminated sample of tap water, which contained more than one part per million of lead. With one use, the filter sponge leads to below detectable levels.
After using the sponge, the researchers were also able to successfully recover the minerals and reuse the sponge for multiple cycles. The new sponge shows promise for future use as an inexpensive and easy-to-use tool in household water filters or large-scale environmental remediation efforts.
The study was published late yesterday (May 10) in the journal ACS ES&T Water. The paper outlines the new research and lays out design rules for optimizing similar platforms to remove — and recover — other heavy metal toxins, including cadmium, arsenic, cobalt, and chromium.
“The presence of heavy metals in water supplies presents an enormous public health challenge for the entire world,” said Vinayak Dravid of Northwestern, senior author of the study. “It’s a gigaton problem that requires solutions that can be deployed easily, effectively and inexpensively. That’s where a sponge comes in. It can decontaminate and then be used again and again.”
Dravid is the Abraham Harris Professor of Materials Science and Engineering in Northwestern University’s McCormick School of Engineering and Director of Global Initiatives at the International Institute for Nanotechnology.
Clean up spills
The project builds on Dravid’s previous work developing highly porous sponges for various aspects of environmental remediation. In May 2020, his team unveiled a new sponge designed to clean up oil spills. The nanoparticle coated sponge, now commercialized by Northwestern spinoff MFNS Tech, provides a more efficient, economical, environmentally friendly, and reusable alternative to current oil spill approaches.
But Dravid knew that was not enough.
“When there is an oil leak, you remove the oil,” 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 other toxins may remain.
Rinse and repeat
To address this aspect of the problem, Dravid’s team turned, once again, to sponges coated with an extremely thin layer of nanoparticles. After testing many different types of nanoparticles, the team found that a manganese-doped goethite coating worked best. Not only are manganese-doped goethite nanoparticles inexpensive, readily available and non-toxic to humans, they also have the properties to selectively treat heavy metals.
“You want a material with a high surface area, so there is more surface area for the lead ions to stick to,” said Benjamin Schindel, Ph.D. student in Dravid’s lab and first author of the paper. “These nanoparticles have high surface areas, abundant reactive surface sites for adsorption and are stable, so they can be reused many times.”
The team made slurries of manganese-doped goethite nanoparticles, as well as several other formulations of nanoparticles, and commercially available cellulose sponges with this slurry. Then they rinse the coated sponge with water to wash off any loose particles. The final coatings are only tens of nanometers thick.
When immersed in contaminated water, sponges coated with nanoparticles effectively trap lead ions. The US Food and Drug Administration requires that bottled drinking water be less than 5 parts per billion of lead. In filtration experiments, the sponge reduced the amount of lead to approximately 2 parts per billion, making it safe to drink.
“We’re really happy about that,” Schindel said. “Of course, this performance can vary based on several factors. For example, if you have a large sponge in a small volume of water, it will do better than a small sponge in a huge lake.”
Recovery goes beyond mining
From there, the team rinsed the sponge with slightly acidified water, which Schindel likened to “the same acidity as lemon juice.” The acidic solution caused the sponge to release lead ions and prepare for another use. Although the performance of the sponge decreases after the first use, it still recovers more than 90% of its ions during subsequent cycles of use.
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 energy expensive to mine and comes with a laundry list of environmental and human costs.
If researchers can develop a sponge that selectively removes trace metals, including cobalt, from water, those metals could be recycled into products such as batteries.
“For renewable energy technologies, such as batteries and fuel cells, there is a need for mineral recovery,” said Dravid. “Otherwise, there just isn’t enough cobalt in the world for the growing number of batteries. We must find ways to recover the metals from highly dilute solutions. Otherwise, they become toxic and toxic, just by sitting there in the water. We could also make something of value.”
As part of the study, Dravid and his team created new design rules to help others develop tools to target specific minerals, including cobalt. Specifically, they identified low-cost, non-toxic nanoparticles that also have high surface areas and affinity for adhesion to metal ions. They studied the performance of manganese, iron, aluminum and zinc oxide coatings on lead adsorption. They then established relationships between the structures of these nanoparticles and their adsorption properties.
The environmental remediation platform, called Sponge Coatings for Heavy Metal Nanomaterials (or “Nano-SCHeMe”), could help other researchers distinguish which nanomaterials are best suited for specific applications.
“I’ve read a lot of literature comparing different coatings and sorbents,” 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 this field. By analyzing different types of nanoparticles, we developed a comparative metric that actually works for all of them. It could have a lot of implications in moving the field forward.”
Dravid and his team imagine their sponges could be used in commercial water filters, to clean the environment or as an extra step in water reclamation and treatment facilities.
“This work may be relevant to water quality issues both locally and globally,” Schindel said. “We want to see that out in the world, where it can have a real impact.”
Dravid and Northwestern have financial interests (stock, equity) in MFNS Tech.