Researchers from Northwestern University’s McCormick School of Engineering (Evanston, Illinois, USA) have developed a new sponge that can remove metals—including toxic heavy metals like lead and critical metals like cobalt—from contaminated water, leaving safe, drinkable water behind.
In proof-of-concept experiments, the researchers tested their new sponge on a highly contaminated sample of tap water, containing more than 1 part per million of lead. With one use, the sponge filtered lead to below detectable levels.
After using the sponge, researchers were 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.
In a study published in the journal ACS ES&T Water, the researchers outline the new project and set design rules of optimizing similar platforms for removing—and recovering—other heavy-metal toxins, including cadmium, arsenic, cobalt, and chromium.
“The presence of heavy metals in the water supply is an enormous public health challenge for the entire globe,” says the McCormick School’s Vinayak Dravid, who also served as senior author of the study. “It is a gigaton problem that requires solutions that can be deployed easily, effectively, and inexpensively. That’s where our sponge comes from. It can remove the pollution and be used again and again.”
The project builds on Dravid’s previous work to develop 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, which is now being commercialized by Northwestern spinoff MFNS Tech, offers a more efficient, economic, ecofriendly, and reusable alternative to current approaches to oil spills.
But Dravid knew it wasn’t enough. “When there is an oil spill, you can remove the oil,” he says. “But there are also toxic heavy metals—like mercury, cadmium, sulfur, and lead—in those spills. So even when you remove the oil, some of the other toxins might remain.”
To tackle this aspect of the issue, Dravid’s team turned once again to sponges coated with an ultrathin layer of nanoparticles. After testing many different types of nanoparticles, the team found that a manganese-doped goethite coating worked best. Not only are these nanoparticles inexpensive to make, easily available, and nontoxic to humans, they also have the properties necessary to selectively remediate heavy metals.
“You want a material with a high surface area, so there’s more room for the lead ions to stick to it,” says Benjamin Shindel, a PhD student in Dravid’s lab and the paper’s first author. “These nanoparticles have high-surface areas and abundant reactive surface sites for adsorption and are stale, so they can be reused many times.”
The team synthesized slurries of manganese-doped goethite nanoparticles, as well as several other compositions of nanoparticles, and coated commercially available cellulose sponges with these slurries. They then rinsed the coated sponges with water in order to wash away any loose particles. The final coatings measured just tens of nanometers in thickness.
From there, the team rinsed the sponge with mildly acidified water, which Shindel likened to “having the same acidity of lemonade.” The acidic solution caused the sponge to release the lead ions and be ready for another use. Although the sponge’s performance declined after the first use, it still recovered more than 90% of the ions during 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. 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.
As a part of the study, Dravid and his team set new design rules to help others develop tools to target particular metals, including cobalt. Specifically, they pinpointed which low-cost and nontoxic nanoparticles also have high-surface areas and affinities for sticking to metal ions. They studied the performance of coatings of manganese, iron, aluminum, and zinc oxides on lead adsorption. Then, they established relationships between the structures of these nanoparticles and their adsorptive 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. Dravid and his team imagine that their sponge could be used in commercial water filters, for environmental clean-up, or as an added step in water reclamation and treatment facilities.
“This work may be pertinent to water quality issues both locally and globally,” Shindel says. “We want to see this out in the world, where it can make a real impact.”
Source: Northwestern University – McCormick School of Engineering, www.mccormick.northwestern.edu.