Some per- and poly-fluoroalkyl substances (PFAS) exhibit significantly greater acidity than previously assumed, as determined by a research team from the University of Buffalo in New York. This is important because these values are essential for predicting how these enduring chemicals move in the environment and their possible impacts on human health.
Employing fluorine and proton nuclear magnetic resonance (NMR) spectroscopy, a more precise method than existing techniques, the team assessed the acidity of 10 different PFAS types and three of their common breakdown products. The research analyzed the acid dissociation constants (pKa) for these PFAS and found that most were lower – in some instances considerably lower – than values previously indicated in earlier experimental studies and those forecasted by current computational chemistry models. pKa serves as an indicator of an acid’s strength; a lower pKa signifies a stronger acid, and as pKa decreases, the solubility of the compound in water typically increases.
PFAS – referred to as ‘forever chemicals’ – comprise an estimated 15,000 synthetic compounds that have been extensively utilized in global consumer products since the 1950s. They all possess a distinctive carbon chain with multiple fluorine atoms attached. These substances resist environmental degradation due to the strength of the carbon–fluorine bond. Their unique properties impart qualities such as oil, grease, and water repellence, along with temperature resistance and reduced friction, enabling the production of non-stick and stain-resistant products.
Nonetheless, PFAS are also extremely mobile in the environment and can bioaccumulate and biomagnify throughout the food chain. PFOA and PFOS – the most extensively studied of these substances – have been associated with severe health issues including reproductive and developmental disorders, weakened immune responses, and certain cancer types.
According to the study’s corresponding author, Alexander Hoepker, a cross-disciplinary R&D biochemist and senior research scientist at Buffalo’s Research and Education in Energy, Environment and Water (Renew) Institute, the researchers enhanced partial NMR datasets with computational predictions to achieve more precise pKa values.
These results suggest that prior measurements have undervalued the acidity of PFAS compounds. As a result, Hoepker and study co-author Diana Aga, a chemist and director of Renew, propose that the persistence and spread of these substances in the environment have also been inaccurately characterized.
In one notable instance, the pKa for hexafluoropropylene oxide dimer acid, or Gen-X, was found to be roughly 1000 times lower than the figure cited in an earlier study. The team also established the pKa values for several significant emerging PFAS that had not been previously measured.
More accurate pKa measurements will illuminate the behavior of PFAS in the environment. The pKa of a PFAS also influences how readily such a chemical bioaccumulates in human bodies. ‘For instance, if a chemical is more frequently found in water, it is more bioavailable and likely to contaminate our drinking water supplies if not adequately treated,’ notes Aga.
Hoepker indicates that a primary reason prior scientific measurements have underestimated the acidity of PFAS is that their pKa has typically been determined through bulk chemical measurements that assess the liquid property as a whole in response to changes in the pH of the PFAS-containing solution. ‘The issue with this approach is that a full fluorine accounting is necessary; otherwise, inaccuracies arise,’ he explains.
No longer influenced by glass
Considering fluorine is complex since most PFAS has a tendency to adhere to glass, which is commonly used in research. ‘Although NMR tubes are also fashioned from glass, the direct NMR observation of fluorine or proton nuclear spins … effectively compensates for these sorption events,’ he states. ‘You’re not observing the fluorines on the glass; you’re only measuring the fluorines that are in the solution, which is what matters. This alone establishes your pKa, thereby removing bias from these other sorption processes.’
Another element that has distorted pKa outcomes for PFAS in prior studies is the reliance on organic solvents such as methanol. According to Hoepker, higher methanol content leads to elevated pKa readings. ‘We reduced the methanol concentration by 10 to 100 times compared to previous research, virtually eliminating this influence.’
Graham Peaslee, a physicist at Notre Dame University in Indiana who was not involved in this study but has conducted numerous research projects on detecting PFAS, describes the study as ‘a very solid piece of experimental work’. He suggests that the likelihood of many PFAS being more acidic than previously believed could help clarify their high solubility in water.