A drop of water injected on an octahedral crystal. (Credit: Wendy Lee Queen/EPFL)
Toxic heavy metals in water are a serious problem all over the world. The World Health Organization (WHO) lists 10 chemicals of major Public Health concern, and four of them are metal or metalloid: mercury, lead, cadmium, and arsenic. WHO also indicates that 2.1 billion people cannot access clean drinking water in their homes, while nearly 1 billion people lack access to clean drinking water at all—and of course those figures will grow more dire as climate change alters the environment and reduces access.
However, heavy metals find their way into water in multiple ways. Industrial concerns cause metals to leach into the water supply, either accidentally or by actively dumping waste. Crumbling infrastructure across the US often puts municipal officials into a permanently reactive mode. And for every breakthrough in metal removal, it seems we see another potential threat arise as our need for energy and industrial products prompts regulations that at times may be too lax.
One of the more promising strategies for heavy metal removal today surround the use of metal organic frameworks or MOFs. Commercial techniques currently in use tend to consume tremendous amounts of energy and be very costly, not to mention insufficiently effective. MOFs, however, may be the answer to effective removal of heavy metals without burning through excessive energy and money.
Professor Wendy Lee Queen at École polytechnique fédérale de Lausanne (EPFL), her lab team, and colleagues from the Lawrence Berkeley National Laboratory and the University of California Berkeley have come up with a new MOF-based solution to the heavy metal problem. Professor Queen corresponded with EM about the research, which is published in ACS Central Science.
“This work was inspired by the many news articles in recent years highlighting a large amount of water contamination throughout developed countries like the US,” explains Professor Queen. “If these countries have deep rooted water contamination issues, I cannot imagine what it must be like in developing and underdeveloped countries.”
MOFs are created by interlinking metal nodes with struts of organic chemicals. This allows them to have internal surface area that is far greater than other materials.
“Think of a MOF as a sponge, with holes that are 50,000 times smaller than the diameter of the human hair,” Professor Queen offers. “It is the high density of these very tiny holes that give MOFs record-breaking surface areas. This just means you access the outside and most all of the inside as well.”
It also means they are easily tunable, and can be “customized” in a sense to grab various compounds from air or water—including heavy metals.
“A few years ago, I noticed some work that came out of Cornell; a research group there designed a new porous polymer that could extract several common hazardous organics from water in higher quantities and at faster rates than some state of the art materials,” Professor Queen describes. “I realized that the reason these materials were special was because of the functionality inside the polymer, and also the polymer’s high accessible surface area. Considering we are working with some of the world’s most porous materials, which we call MOFs, we thought to try to use these to introduce porosity to polymers that are not inherently porous. And it worked beautifully. This high surface area and high density of metal scavenging functionality is what makes our new composite special.”
The team started with a water-stable composite of MOF and polymer, Fe-BTC, designed using sustainable materials by an EPFL-Valais PhD student, Daniel T. Sun. The team then treated the MOF with dopamine, in order to trap polydopamine (PDA) inside the Fe-BTC. The final composite, Fe-BTC/PDA, can remove heavy metals such as mercury and lead selectively and quickly from water samples—more than 0.4 times and 1.6 times its own weight in lead and mercury, respectively. Fe-BTC/PDA was even successful at removing lead from the worst Flint, Michigan water samples to a safe level for drinking water under EPA and WHO standards—in seconds.
“After evaluation of our composite, we do believe it could meet or surpass the requirements for actual implementation,” remarks Professor Queen. “We have high capacities, we can remove small concentrations of lead and mercury in the presence of high concentrations of other common interferents, and we can do so in a matter of seconds. We have further demonstrated that the process can be reversed. It should be noted that it is often difficult to gain an accurate comparison between materials, because many are for instance only tested in distilled water and there are other important factors regarding toxicity, stability, and selectivity that are often overlooked.”
This MOF is unique in several ways, because it is extremely selective and cost-effective.
“Based on our calculation for the cost of the starting materials, if bought on a ton scale, the material would cost around 2.50 USD per kg,” details Professor Queen. “This is not including the actual cost of making the material. This would have to undergo a more serious cost analysis after developing a large, scaled up process.”
Obviously, this MOF might be applied and put to use in multiple ways. The team has been contacted by several companies that are interested in implementing the technology in several different applications.
“While it is still early for us, as we would like to do more work to further test their longevity and also work on scaling the materials up and structuring the powders into larger particles, I would of course love to see these materials make it to market and help people around the world live healthier lives,” states Professor Queen. “I could see them being used in water filters for a more in-home purification process, used in waste-treatment or contamination cleanup, for example.”
Other MOFs have been created that are very specific, and select for other substances. Deploying various MOFs together to create filters that collectively select for multiple contaminants is an interesting prospect for the future.
“We use our knowledge to put special functionality inside our sponges in order to make them specific to certain substances,” Professor Queen explains. “Currently, about 15% of global energy is expended on separations in chemical industry. As such, there is much interest in using MOFs to separate small molecules from various gases or liquids. And as you have seen from this work it can be extended to heavy metals and also many other analytes. So yes, we can put the functionality inside the MOF that is desired for one or several contaminants. You could also use a combination of different MOFs (as in a mixture) for the selection of a variety of contaminants.”