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New Nanoparticle-Coated Sponge Aids in the Recovery of Heavy-Metal Toxins

Engineers from Northwestern University have created a new type of sponge that can extract metals such as lead and cobalt from polluted water, resulting in clean and safe drinking water.

The researchers conducted a proof-of-concept experiment to test the effectiveness of their new sponge on tap water contaminated with more than 1 part per million of lead. After using the sponge once, the lead levels in the water were reduced to below detectable levels.

The new sponge developed by Northwestern University engineers can remove metals from contaminated water, including toxic heavy metals like lead and critical metals like cobalt, leaving safe and drinkable water behind. In experiments, the sponge was tested on highly contaminated tap water, and after one use, it filtered lead to below detectable levels. Additionally, the researchers were able to recover metals and reuse the sponge for multiple cycles. The new sponge has potential for use in home water filters or large-scale environmental remediation efforts.

Yesterday (May 10,2023), a study was published in the journal ACS ES&T Water. The study discusses a new method for removing heavy-metal toxins like lead, and also suggests ways to improve the design of this method for removing other toxins such as cadmium, arsenic, cobalt, and chromium.

Vinayak Dravid, the senior author of the study, said that the existence of heavy metals in water is a significant public health problem for the entire world. He said that it is a major issue that requires solutions that are easy to use, effective, and cost-effective. Dravid also stated that their sponge can remove pollution from water 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.

Sopping up Spills

Dravid's previous work involved developing highly porous sponges for environmental remediation purposes. In May 2020, he and his team created a sponge that could clean up oil spills. The sponge, coated with nanoparticles, is more efficient, economical, ecofriendly, and reusable than current oil spill clean-up methods. MFNS Tech, a Northwestern spinoff, is now commercializing the nanoparticle-coated sponge.

But Dravid knew it wasn't enough.

Dravid explains that during an oil spill, not only oil but also toxic heavy metals such as mercury, cadmium, sulfur, and lead may be present in the water. While the oil can be removed, the toxic heavy metals might still be left behind.

Rinse and Repeat

Dravid's team used sponges coated with an ultra-thin layer of nanoparticles to tackle the toxic heavy metals, such as mercury, cadmium, sulfur, and lead, found in oil spills. They experimented with various types of nanoparticles and found that a coating of manganese-doped goethite worked best. Manganese-doped goethite nanoparticles are easy to produce, affordable, and safe for humans. They also have the necessary properties to selectively eliminate heavy metals.

According to Benjamin Shindel, a Ph.D. student in Dravid's lab and the first author of the paper, the material used should have a large surface area to allow more space for lead ions to attach to it. The manganese-doped goethite nanoparticles fulfill this requirement as they have a high surface area and plenty of reactive surface sites for adsorption. Moreover, they are stable and can be reused several times.

To create the sponge, the researchers produced a mixture of manganese-doped goethite nanoparticles and other types of nanoparticles, which they then used to coat cellulose sponges. The coated sponges were then rinsed with water to remove any excess particles, leaving a very thin coating measuring just tens of nanometers in thickness.

The researchers coated commercially available cellulose sponges with manganese-doped goethite nanoparticles, which were then rinsed with water to remove any loose particles. These nanoparticle-coated sponges were then submerged in contaminated water and were able to effectively trap lead ions. In filtration trials, the sponge was able to lower the amount of lead to approximately 2 parts per billion, which is below the FDA's requirement for safe drinking water (below 5 parts per billion).

Benjamin Shindel, the paper's first author, expressed his satisfaction with the results of the nanoparticle-coated sponge. However, he noted that the performance of the sponge could vary depending on the situation. For example, a large sponge in a small amount of water would work better than a small sponge in a large lake.

Recovery Bypasses Mining

After using the sponge to filter out lead from contaminated water, the researchers rinsed it with slightly acidic water. The acidic solution made the sponge release the trapped lead ions and become reusable. Although the sponge's performance decreased slightly after the first use, it was still able to recover over 90% of the lead ions in subsequent use cycles.

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

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

Dravid emphasizes the importance of metal recovery in renewable energy technologies such as batteries and fuel cells. He says that without metal recovery, there would not be enough cobalt in the world for the increasing number of batteries. Metals sitting in water become toxic and poisonous, so finding ways to recover metals from diluted solutions is necessary. Dravid suggests that creating something valuable from the recovered metals could be a beneficial solution.

Standardized Scale

In the study, Dravid and his team established new guidelines to assist others in creating tools to target specific metals like cobalt. They identified low-cost and nontoxic nanoparticles that have high-surface areas and can adhere to metal ions. They evaluated how coatings of manganese, iron, aluminum, and zinc oxides performed in lead adsorption and established a connection between the structure of these nanoparticles and their adsorptive characteristics.

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.

Caroline Harms, a co-author of the paper and an undergraduate student in Dravid's lab, mentioned that there is a lack of standardization in the field of comparing different coatings and adsorbents. The team analyzed various types of nanoparticles and established a comparative scale that can be used for all of them, which can have significant implications in advancing the field.

The team led by Dravid envisions that their sponge could serve various applications such as in commercial water filters, environmental remediation efforts, or as an additional step in water treatment and reclamation facilities.

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

The study, "Nano-SCHeME: Nanomaterial Sponge Coatings for Heavy Metals, an environmental remediation platform," was supported by the National Science Foundation and U.S. Department of Energy.


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