Scientists Unveil New Insights into the Acidity and Environmental Impact of PFAS
The persistent nature of per- and polyfluoroalkyl substances (PFAS), often referred to as “forever chemicals,” is partly due to their acidic properties. This characteristic allows these substances to dissolve easily and spread in water, posing environmental and health challenges.
Recent research led by the University at Buffalo has revealed that some PFAS are even more acidic than previously believed. This discovery is crucial for understanding their environmental mobility and potential health implications.
A team introduced an innovative experimental method to more accurately determine the acidity of 10 PFAS types and three of their common breakdown products. Their findings, published in Environmental Science & Technology Letters, showed that the acid dissociation constant (pKa) for these chemicals was often significantly lower than previously reported. For instance, the pKa of GenX, a PFOA substitute used in Teflon production, was found to be about a thousand times lower than previous estimates.
A lower pKa indicates a greater propensity for a chemical to release a proton and become negatively charged, influencing its environmental behavior.
“These findings suggest that previous measurements have underestimated PFAS’ acidity. This means their ability to persist and spread in the environment has been mischaracterized, too,” says Alexander Hoepker, PhD, the study’s corresponding author and a senior research scientist at the UB RENEW Institute.
Accurate pKa measurements are essential for understanding how PFAS interact with their surroundings. A chemical’s pKa can determine whether it remains dissolved in water, adheres to soil or biological membranes, or volatilizes into the air.
“If we’re going to understand how these concerning chemicals spread, it’s very important we have a reliable method for the accurate determination of their pKa values,” says Diana Aga, PhD, director of RENEW and SUNY Distinguished Professor and Henry M. Woodburn Chair in the UB Department of Chemistry.
This research was supported by the National Science Foundation and conducted in collaboration with Scott Simpson, PhD, from the St. Bonaventure Department of Chemistry, and scientists from Spain’s Institute of Environmental Assessment and Water Research.
Experimental and Computational Techniques
PFAS molecules consist of a highly fluorinated tail that repels water and an acidic headgroup. The pKa value indicates the pH level at which a PFAS switches between neutral and charged states, depending on its environment. However, discrepancies in pKa values have been noted among different research groups, partly due to PFAS’ tendency to adhere to glassware used in experiments.
“PFAS likes to stick to glass. When that happens, it throws off traditional, so-called bulk measurements that quantify how much PFAS is in a solution,” Hoepker explains. “In other cases, too much organic solvent is used to get PFAS into solution, which similarly biases the pKa measurement.”
The UB team employed fluorine and proton nuclear magnetic resonance (NMR) spectroscopy to address these challenges. NMR allows researchers to detect atom-level signatures, providing insights into whether a PFAS molecule is charged or neutral. This method bypasses issues like glass adsorption, which have affected past measurements.
Some PFAS are so acidic that their neutral form requires extremely low pH conditions, impractical for standard labs. In these cases, the team combined NMR data with electronic-structure calculations to predict pKa values.
“We augmented partial NMR datasets with computational predictions to arrive at more accurate pKa values,” Hoepker says. “This NMR-centered hybrid approach — integrating experimental measurements with computational analyses — enhanced our confidence in the results and, to our knowledge, has not previously been applied to PFAS acidity.”
Challenges and Implications for Regulation
The study’s most challenging measurement was for PFOA, historically used in nonstick cookware and labeled hazardous by the EPA. The team found its pKa to be -0.27, indicating it remains negatively charged across most pH levels. Prior studies reported its pKa ranging from 1 to 3.8, with computational models estimating it between 0.24 and 0.34.
Trifluoroacetic acid (TFA), a PFAS increasingly detected in global waters and likely spread through atmospheric deposition, showed a pKa of around 0.03, much lower than previous estimates of 0.30 to 1.1.
Importantly, the team measured pKa values for several emerging PFAS, such as 5:3 fluorotelomer carboxylic acid (5:3 FTCA) and ethers like NFDHA and PFMPA, which pose regulatory challenges due to their health risks.
“This new experimental approach of determining pKa values for PFAS will have wide-ranging applications, from being able to validate computationally derived values, to facilitating the development of machine learning models that can better predict pKa values of newly discovered PFAS contaminants when reference standards are not available,” Aga states. “In turn, knowledge of the pKa values of emerging PFAS will allow researchers to develop appropriate analytical methods, remediation technologies, and risk assessment strategies more efficiently.”
Besides Scott Simpson, co-authors include Silvia Lacorte from the Spanish Institute of Environmental Assessment and Water Research, Aina Queral Beltran from the University of Barcelona, and UB Chemistry graduate students Damalka Balasuriya and Tristan Vick.
Original Story at www.sciencedaily.com