A mechanical filter sponge removes lead from the water, a reusable sponge can capture and recover precious metals and heavy metal contaminants
Northwestern University engineers have developed a new sponge that can remove metals — including toxic heavy metals like lead and dangerous metals like cobalt — from polluted water, leaving clean, drinkable water.
In a proof-of-concept study, the researchers tested their new sponge on samples of tap water that was highly contaminated, with more than one part per million of lead. With just one use, this woven sponge leads to levels below detection.
After using the stage, the researchers were able to recover the metals and use the sponge for several cycles. The new stage holds promise for future use as a cheap, easy-to-use tool in household water filtration or large-scale environmental improvement efforts.
The study was published May 10 in the journal ACS ES&T Water. The paper outlines new research and establishes design principles for developing similar platforms to remove – and recover – other heavy metal contaminants, including cadmium, arsenic, cobalt and chromium.
“The presence of heavy metals in water is a major public health challenge for the entire world,” said Northwest’s Vinayak Dravid. “This is a gigatonne problem that requires a solution that can be deployed quickly, efficiently and cheaply. This is where our platform comes in. It can remove pollution and use it 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.
Clean up spills
This work builds on Dravid’s earlier work to develop absorbent sponges for various aspects of environmental remediation. In May 2020, his team released a new sponge designed to clean up oil spills. The nanoparticle coating platform, now commercialized by Northwest spinoff MFNS Tech, offers a safe, economical, environmentally friendly and reusable alternative to current methods for oil spills. But Dravid knew that was not enough.
“When there’s oil, you can pull the oil out,” he said. “But there are also toxic heavy metals – such as mercury, cadmium, sulfur and lead – in these spills. So, even when you remove the oil, some of the other toxins may remain.
Rinse and repeat
To solve this part of the problem, Dravid’s team turned again to a sponge coated with a layer of ultrathin nanoparticles. After testing several types of nanoparticles, the team found that the goethite coating of manganese worked best. Not only are manganese-doped goethite nanoparticles inexpensive to manufacture, readily available, and non-toxic to humans, but they also have the necessary properties to extract heavy metals.
“You want something that has a large surface area, so there’s room for the main ions to adhere to it,” said Benjamin Shindel, a Ph.D. student. in Dravid’s lab and the first author of the paper said. “These nanoparticles have a high surface area and a highly reactive surface site for adsorption and stability, so they can be used many times.”
The group synthesized manganese-doped goethite nanoparticle slurries, along with many other nanoparticle components, and commercially available cellulose sponge these slurries. They also wash the covered stages with water to remove any loose material. The final coating is only a few tens of nanometers.
When placed in polluted water, the well-coated nanoparticle surface emits ions. The United States Food and Drug Administration requires drinking water to contain less than five parts per billion of lemon. In a filter test, the stage reduced the amount of lead to about two parts per billion, making it drinkable.
“We’re very happy with it,” Shindel said. “Of course, this performance can vary depending on many factors. For example, if you have a large sponge in a small pool of water, it will work better than a small sponge in a large pool.
Recovery passes the mine
From there, the team washed the stage with slightly acidified water, which Shindel compared to “having the same acidity as lemonade.” The acidic solution causes the stage to release lead ions and is ready for further use. Although the efficiency of the sponge is reduced after the first use, it still receives more than 90% of the ions during the subsequent use.
This ability to collect and recover heavy metals is particularly useful for removing rare and highly precious metals, such as cobalt, from water sources. A common ingredient in lithium-ion batteries, cobalt is expensive to produce and comes with a long list of environmental and human costs.
If researchers can create a stage that selectively removes rare metals, including cobalt, from water, those metals can be recycled into products like batteries.
“For renewable energy technologies, such as batteries and fossil fuels, there is a need for iron recovery,” Dravid said. “Otherwise, there is not enough cobalt in the world for the ever-increasing number of batteries, and we need to find a way to recover those metals from a mild solution. Otherwise, it becomes poison and poison, just sitting there in the water. And we can turn it into something useful.
Standard flag
As part of the study, Dravid and his team developed new design principles to help others develop tools to target certain metals, including cobalt. Specifically, they discovered non-toxic nanoparticles that also have a high surface area and affinity for metal ions. They learned to work with manganese, iron, aluminum and zinc oxide coatings and lead media. Then, they established a relationship between the structure of these nanoparticles and their properties.
Called the Nanomaterial Sponge for Heavy Metals (or “Nano-SCHeMe”), the environmental modification platform could help other researchers identify the best nanomaterials for applications.
“I’ve read a lot of papers comparing different coatings and adsorbents,” said Caroline Harms, an undergraduate student in Dravid’s lab and co-author of the paper. . By comparing different types of nanoparticles, we have created a comparison scale that works for each of them. This can have many implications for moving the field forward.
Dravid and his team think their sponge could be used in commercial water filtration, for environmental cleaning, or as the next step in water preparation and treatment facilities. “This work could be important for water quality issues both locally and globally,” Shindel said. “We want to see this in the world, where it can have a real impact.”
Source: Northwestern University
