Researchers have developed a device that filters heavy metals — particularly uranium — from drinking water using graphene foam.
A reusable 3D functionalized reduced graphene oxide foam is used to extract uranium from contaminated water. Image Credit: MIT
Billions of people worldwide do not have access to clean drinking water, with UNICEF estimating that as of 2019, this life-threatening issue affecting as many as 1 in 3 people¹.
While some forms of water contamination, such as plastics and algal blooms, are fairly easy to identify visibly, others are much more difficult to trace, making them more dangerous. Amongst these difficult-to-spot substances are heavy metals, and in particular, uranium.
Uranium can reach drinking water due to mining operations, nuclear waste sites, and even from natural deposits located beneath the ground, bringing with it a number of health risks if consumed². These can range from kidney damage to cancer risk depending on how soluble the consumed uranium is and its concentration.
Drinking water supplies in many regions of the U.S. are affected by uranium contamination. A concerning number of reservoirs and aquifers now showing levels of uranium contamination above the recommended safe level of 30 parts per billion (ppb). As an idea of the scale of this problem, just two contaminated aquifers in High Plains and Central Valley alone supply drinking water to over 6 million U.S. citizens.
The problem of uranium contamination extends beyond the borders of the U.S., with many other countries demonstrating worryingly high levels of this heavy metal in drinking water.
Now researchers from the Massachusetts Institute of Technology (MIT) led by Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and materials science and engineering professor, may have found a pioneering new solution to this problem. The team has developed a graphene foam that is highly efficient in removing uranium from drinking water.
“Within hours, our process can purify a large quantity of drinking water below the EPA limit for uranium,” says Li, adding that the technique could be modified for other metals.
The technique, which originated in a project started three years ago with the aim of cleaning up heavy metals from mining sites, hinges on a graphene oxide foam that, when exposed to an electric charge, can capture uranium from a solution. One benefit of the technique is the fact that it is highly reusable, with the foam retaining its electrostatic properties after as many as seven uses, the team says.
The breakthrough is documented in a paper published in the latest edition of the journal Advanced Materials³ by lead authors and postdocs Ahmed Sami Helal, the Department of Nuclear Science and Engineering, MIT, and Chao Wang, the School of Materials Science and Engineering, Tongji University, Shanghai.
How Does Graphene Foam Improve on Current Methods of Uranium Removal?
Of course, the MIT-devised graphene foam is not the first platform developed for the removal of heavy metals like uranium, arsenic, lead, mercury, and radium, from drinking water. These prior platforms have suffered from several drawbacks that limit their use, however.
“These techniques are highly sensitive to organics in water and are poor at separating out the heavy metal contaminants,” says Helal. “So they involve long operation times, high capital costs, and at the end of extraction, generate more toxic sludge.”
Of these heavy metals, the MIT team chose to focus on uranium. This is due to results from the U.S. Geological Service and the Environmental Protection Agency (EPA), which have revealed natural rock sources, mining activities, and even dumped nuclear weapons, leaching unhealthy metal levels into reservoirs and aquifers in the northeastern region of the U.S.
The removal of uranium, without leaving a toxic residue, is the major problem that the team aimed to tackle. Previous studies had shown that using an electrically charged carbon fiber could do this, but the results of practical testing were somewhat imprecise.
Wang found the solution to this problem while exploring the use of graphene foam in lithium-sulfur batteries.
The physical performance of this foam was unique because of its ability to attract certain chemical species to its surface. I thought the ligands in graphene foam would work well with uranium.
Chao Wang, School of Materials Science and Engineering, Tongji University, Shanghai
Making an Effective ‘Uranium Magnet’
The MIT team found that when they passed an electric charge through graphene foam it split water and released hydrogen with the knock-on effect of increasing local pH levels. This drives a chemical that pulls uranium ions out of a solution and causes it to graft to the surface of the foam.
The net result of this process is the creation of crystalline uranium hydroxide — a mineral that has never been seen before — hanging off the foam in an arrangement reminiscent of fish scales. The foam sheds the uranium when the electrical charge is reversed.
Perfecting the graphene foam was a painstaking process for the MIT crew, with intense chemical engineering and hundreds of attempts needed to develop a material that wasn’t too fragile.
The result was a strong and durable graphene foam that effectively captures uranium and is reusable. The end product can function equally well in drinking water and seawater — making it highly adaptable for a multitude of roles.
Each time it’s used, our foam can capture four times its own weight of uranium, and we can achieve an extraction capacity of 4,000 mg per gram, which is a major improvement over other methods. We’ve also made a major breakthrough in reusability because the foam can go through seven cycles without losing its extraction efficiency.
Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering, Massachusetts Institute of Technology (MIT)
The team even believes there is the possibility that the graphene foam could be adapted for home use, fitting it on taps and faucets for effective in-situ filtration and improving upon current activated carbon alternatives.
As mentioned above, the next step for the team is to adapt the graphene foam to capture other heavy metals. “There is a science to this, so we can modify our filters to be selective for other heavy metals such as lead, mercury, and cadmium,” Li concludes. “In the future, instead of a passive water filter, we could be using a smart filter powered by clean electricity that turns on electrolytic action, which could extract multiple toxic metals, tell you when to regenerate the filter, and give you quality assurance about the water you’re drinking.”
Sources
1. 1 in 3 people globally do not have access to safe drinking water, Unicef, [https://www.unicef.org.uk/press-releases/1-in-3-people-globally-do-not-have-access-to-safe-drinking-water-unicef-who/#:~:text=1%20in%203%20people%20globally,%E2%80%93%20UNICEF%2C%20WHO%20%2D%20Unicef%20UK]
2. Uranium Health Effects, DUF6 Guide, [https://web.evs.anl.gov/uranium/guide/ucompound/health/index.cfm]
3. Wang. C., Li. J., Helal. A.S., et al, [2021], ‘Uranium In Situ Electrolytic Deposition with a Reusable Functional Graphene-Foam Electrode,’ Advanced Materials, [https://doi.org/10.1002/adma.202102633]
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