“Zwitterion” may not be a word you encounter every day, but for Professor Patrick Doyle of MIT’s Department of Chemical Engineering, it’s the term his team is developing to remove micropollutants from water. Core. “Zwitterion” molecules, derived from the German word “zwitter,” meaning “mixture,” are molecules with equal numbers of positive and negative charges.
Devashish Gokhale, a doctoral student in Doyle’s lab, uses the example of magnets to describe zwitterionic materials. “On a magnet, you have a north pole and a south pole that stick to each other, whereas on a zwitterionic molecule you have a positive charge and a negative charge that stick to each other in a similar way.” Since many inorganic Micropollutants and some organic micropollutants are inherently charged, and Doyle and his team have been studying how to use zwitterionic molecules to capture micropollutants in water.
in a new paper Natural waterDoyle, Gokhale and undergraduate Andre Hamelberg explain how they use zwitterionic hydrogels to sustainably capture organic and inorganic micropollutants from water with minimal operational complexity. In the past, zwitterionic molecules have been used as coatings on water treatment membranes for their antifouling properties. But in Doyle’s group’s system, zwitterionic molecules are used to form the backbone within a scaffold material, or hydrogel—a porous three-dimensional network of polymer chains that contain large amounts of water. “Zwitterionic molecules have a very strong attraction for water compared to other materials used to make hydrogels or polymers,” Gokhale said. What’s more, the positive and negative charges on the zwitterionic molecules cause the hydrogel to be less compressible than what is typically observed in hydrogels. This makes the hydrogel more swollen, stronger, and more porous, which is important for scaling up hydrogel-based water treatment systems.
The early stages of this research were supported by seed funding from the Abdul Latif Jameel Water and Food Systems Laboratory (J-WAFS) at MIT. Doyle’s team is currently working to commercialize the platform for home use and industrial-scale applications with support from a J-WAFS Solutions Grant.
Looking for sustainable solutions
Micropollutants are chemically diverse substances that can be harmful to human health and the environment, although their concentrations are usually low (micrograms to milligrams per liter) relative to conventional pollutants. Micropollutants can be organic or inorganic, naturally occurring or synthetic. Organic micropollutants are primarily carbon-based molecules, including pesticides and per- and polyfluoroalkyl substances (PFAS), known as “forever chemicals.” Inorganic micropollutants, such as heavy metals such as lead and arsenic, tend to be smaller than organic micropollutants. Unfortunately, organic and inorganic micropollutants are ubiquitous in the environment.
Many micropollutants originate from industrial processes, but the effects of human-induced climate change also contribute to the environmental spread of micropollutants. Gokhale explained that in California, for example, fires can burn plastic cables and introduce leech micropollutants into natural ecosystems. Doyle added, “In addition to climate change, events such as epidemics can also cause spikes in the amount of organic micropollutants in the environment due to high concentrations of pharmaceuticals in wastewater.”
It is therefore not surprising that micropollutants have received increasing attention over the past few years. The chemicals attracted media attention and led to “significant changes in the environmental engineering and regulatory landscape,” Gokhale said. In March 2023, the U.S. Environmental Protection Agency (EPA) proposed a strict federal standard to regulate six different PFAS chemicals in drinking water. Just last October, the EPA proposed banning the micropollutant trichlorethylene, a carcinogenic chemical found in brake cleaners and other consumer products. Just last November, the U.S. Environmental Protection Agency (EPA) proposed requiring water companies nationwide to replace all lead pipes to protect the public from lead exposure. Internationally, Gokhale pointed to the Oslo Paris Convention, which has a mandate to protect the marine environment in the Northeast Atlantic, including phasing out offshore chemical releases from the oil and gas industry.
With each new necessary regulation to protect water security, the need for effective water treatment processes grows. Compounding this challenge is the need to make water treatment processes sustainable and energy efficient.
The benchmark method for treating micropollutants in water is activated carbon. However, making filters from activated carbon is energy-intensive and requires maintaining very high temperatures in large centralized facilities. Gokhale said approximately “four kilograms of coal are needed to produce one kilogram of activated carbon, so a significant amount of carbon dioxide is lost to the environment.” Global water and wastewater treatment accounts for 5% of annual emissions, according to the World Economic Forum. According to the EPA, drinking water and wastewater systems emit more than 45 million tons of greenhouse gases annually in the United States alone.
“We need to develop methods that have a smaller climate footprint than those used in industry today,” Gokhale said.
Support “high risk” projects
In September 2019, Doyle and his lab launched a preliminary project to develop a particle-based platform to remove a variety of micropollutants from water. Doyle’s team has been using hydrogels in drug processing to form drug molecules into pill form. When he learned about J-WAFS’s seed grant opportunities for early-stage research on water and food systems, Doyle realized his pharmaceutical work on hydrogels could be applied to environmental problems such as water treatment. “If I went to NSF, I would never have gotten funding for this project [National Science Foundation]because they would just say, ‘You’re not a water person.’ But the J-WAFS seed fund provides a way for high-risk, high-reward projects,” Doyle said.
In March 2022, Doyle, Gokhale and MIT undergraduate student Ian Chen published findings from seed-funded work describing their use of micelles within hydrogels for water processing. Micelles are spherical structures that form when surfactant molecules (found in substances such as soap) come into contact with water or other liquids. The team was able to synthesize hydrogel particles filled with micelles that can absorb micropollutants in water like a sponge. Unlike activated carbon, hydrogel particle systems are made from environmentally friendly materials. Additionally, the system’s materials are manufactured at room temperature, making it more sustainable than activated carbon.
Building on the success of the seed funding, Doyle and his team received the J-WAFS Solutions Fund in September 2022 to help take their technology from the lab to the market. With this support, the researchers have been able to build, test and refine a pilot-scale prototype of their hydrogel platform. Iterations of the system during solution funding included the use of zwitterionic molecules, a new development in seed funding efforts.
Rapid elimination of micropollutants is particularly important in commercial water treatment processes because water has a limited amount of time to remain within an operating filtration unit. This is called contact time, Gokhale explained. In municipal or industrial scale water treatment systems, contact time is typically less than 20 minutes and can be as short as 5 minutes.
“But as people have been trying to target these emerging micropollutants of concern, they have realized that they cannot reach low enough concentrations in the same time frame as traditional pollutants,” Gokhale said. “Most technologies only focus on specific molecules or specific classes of molecules. So you have entire technologies that focus just on PFAS, and then you have other technologies that target lead and metals. When you start thinking about removing all of these contaminants from the water, You end up with designs that have a lot of unit operations. That’s a problem because you have plants in the middle of large cities where they don’t necessarily have the expansion space to increase the contact time to effectively remove multiple micropollutants,” he added.
Because zwitterionic molecules have unique properties that confer high porosity, researchers have been able to design a system that absorbs micropollutants from water more quickly. Tests showed the hydrogel eliminated six chemically distinct micropollutants at least 10 times faster than commercial activated carbon. The system is also compatible with a variety of materials, giving it versatility. Micropollutants can bind to many different sites within the hydrogel platform: organic micropollutants bind to micelles or surfactants, while inorganic micropollutants bind to zwitterionic molecules. Micelles, surfactants, zwitterionic molecules, and other chelating agents can be exchanged to essentially tailor the system with different functions depending on the conditions of the water being treated. This “plug-and-play” addition of various functional agents does not require changes in the design or synthesis of the hydrogel platform, and adding more functionality does not take away existing functionality. In this way, zwitterion-based systems can rapidly remove multiple contaminants at lower concentrations in a single step without requiring large industrial installations or capital expenditures.
Perhaps most importantly, the particles in Doyle’s group’s system can be regenerated and used over and over again. Simply soaking the pellets in an ethanol bath washes away microcontaminants so they can be used indefinitely without losing efficacy. When activated carbon is used for water treatment, the activated carbon itself becomes contaminated with micropollutants and must be treated as toxic chemical waste and disposed of in specialized landfills. Over time, micropollutants in landfills will re-enter the ecosystem, perpetuating the problem.
Arjav Shah, a PhD-MBA candidate in the MIT Department of Chemical Engineering and the MIT Sloan School of Management, respectively, recently joined the team to lead commercialization efforts. The research team found that zwitterionic hydrogels can be used in a variety of real-world settings, from large industrial packed beds to small, portable, off-grid applications, such as tablets used to clean cafeteria water – which they have begun through MIT and Several commercialization projects in the greater Boston area are piloting the technology.
The combined strengths of each member of the team continue to move the project forward in impactful ways, including undergraduate students like Andre Hamelberg, the project’s third author Natural water Paper. Hamelberg is a participant in MIT’s Undergraduate Research Opportunities Program (UROP). Gokhale is also a J-WAFS researcher, providing training and mentoring to Hamelberg and other UROP students in the lab.
“We see this as an educational opportunity,” Gokhale said, noting that UROP students learn science and chemical engineering through research conducted in the lab. The J-WAFS project is also “a way to get undergraduate students interested in the more sustainable aspects of water treatment and chemical engineering,” Gokhale said. He added that this is “one of the few projects where you go from designing specific chemicals, to building small filters and devices, to scaling up and commercializing. It’s really a great learning opportunity for undergraduates, We’re always happy to have them work with us.”
In four years, the technology has grown from an initial idea to one with scalable practical applications, making it a model for the J-WAFS project. The fruitful collaboration between J-WAFS and the Doyle lab provides inspiration for any MIT faculty who wants to apply their research to water or food systems projects.
“The J-WAFS project is a way to demystify what chemical engineers do,” Doyle said. “I think there’s an old notion in chemical engineering that it only applies to oil and gas. But modern chemical engineering is about things that make life and the environment better.”