Yin Wang, associate professor, civil & environmental engineering, and Xiaoli Ma, associate professor, materials science & engineering, are investigators on a two-year grant from the U.S. Department of Defense to improve the efficiency and precision of removing PFAS compounds from water.
In the project, funded at nearly $250,000, the researchers will focus on modifying a clay-like mineral called “layered double hydroxide” (LDH). Shangping Xu, UWM associate professor, geosciences, is also on the research team.
PFAS (per- and polyfluoroalkyl substances) are synthetic chemicals used in a wide variety of products, from stain-resistant carpets to firefighting foam. Called “forever chemicals” because of their persistence in the environment, PFAS compounds are linked to harmful health effects – even at very low concentrations.
“There are methods of removing some PFAS compounds, but we need more efficient and more cost-effective ways,” Wang said. “One challenge is to find a way to remove more from drinking water to make sure it meets the recent regulations.”
Because even a small amount is harmful, almost all the chemicals must be removed to meet the EPA’s recent limits set for six PFAS compounds.
Using electrostatic interactions to attract and repel
LDHs are promising adsorbents, materials that collect dissolved or minimal amounts of contaminants like a molecular magnet. The layers of LDH carry a highly positive charge on their surfaces, attracting the negatively charged PFAS ions, Wang said. And the layered structure of LDH may increase the area where adsorption occurs, boosting the materials effectiveness.
Relying on this electrostatic attraction alone, however, may not suffice, Wang said.
To take it a step further, the team will tweak the material so it becomes hydrophobic, or water-repellent.
The researchers have seen this interaction before. Once PFAS filters reach the end of their usefulness, the accumulated PFAS on them must be removed so that they can be recycled. Currently, an organic solvent is used to do this because PFAS is hydrophobic.
This work could lead to a powerful new way to clean contaminated filters that is safer for the environment.
“We are trying to see if we can change the interaction between LDH filter material and the PFAS so that a similar hydrophobic interaction happens,” Wang said, “and more importantly, if the LDH filter material can be regenerated without organic solvents.”
The biggest obstacle
The research builds on Wang and Xu’s earlier work developing powdered LDH adsorbents licensed by a national manufacturer through UWM’s Water Equipment and Policy Center.
The researchers hope the work will give insight into one of the biggest roadblocks in PFAS cleanup: military installations where fire-fighting foam was used for decades, and the contaminant has settled in surrounding soil. At these sites, just below the water table, the PFAS can be in concentrations thousands of times higher than in water reservoirs.
Researchers continue to look for solutions that more selectively remove greater amounts of PFAS in these locations.
