Some per- and poly-fluoroalkyl substances (PFAS) are far more acidic than previously thought, a team led by researchers at the University of Buffalo in New York has determined. This matters because these values help predict the mobility of these persistent chemicals in the environment and their potential effects on human health.
Using fluorine and proton nuclear magnetic resonance (NMR) spectroscopy, which is more rigorous and accurate than current methods, the team determined the acidity of 10 types of PFAS and three of their common breakdown products. The study looked at the acid dissociation constants (pKa) of these PFAS and found that most were lower – and in some cases very significantly lower – than those reported in previous experimental studies and predicted by current computational chemistry models. pKa is a measure of the strength of an acid and the lower the number the stronger the acid; as pKa drops the solubility of the compound in water is likely to increase too.
PFAS – also known as ‘forever chemicals’ – are a family of an estimated 15,000 synthetic chemicals that have been widely used in consumer products globally since the 1950s. They all share a characteristic carbon chain with multiple fluorine atoms attached. They do not degrade easily in the environment because the carbon–fluorine bond is among the strongest in existence. The unique properties of these substances confer characteristics like repellence to oil, grease and water, as well as temperature resistance and friction reduction. This helps to create products that are non-stick and stain-resistant, for example.
However, PFAS are also highly mobile in the environment and they bioaccumulate, as well as biomagnify, up the food chain. PFOA and PFOS – the best studied of these substances – have been linked to serious health conditions like reproductive and developmental disorders, reduced immune function and certain types of cancer.
The Buffalo researchers augmented partial NMR datasets with computational predictions to arrive at more accurate pKa values, 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.
These findings indicate that previous measurements have underestimated the acidity of PFAS chemicals. Consequently, Hoepker and study co-author Diana Aga, a chemist and director of Renew, suggest that the ability of these substances to persist and spread in the environment has also been mischaracterised.
In one extreme example, the pKa of hexafluoropropylene oxide dimer acid, also known as Gen-X, turned out to be approximately 1000 times lower than the measurement listed in a previous study. The team also determined the pKa values for several prominent emerging PFAS that had never been measured before.
More precise pKa measurements will shed light on the behaviour of PFAS in the environment. The pKa of a PFAS can also affect how such a chemical bioaccumulates in a person’s body. ‘For example, if a chemical is more readily found in water, then it is more bioavailable and likely to reach our drinking water supplies, if not properly treated,’ Aga notes.
Hoepker says one major reason why previous scientific measurements have underestimated the acidity of PFAS is that the pKa of these substances traditionally has been determined by bulk chemical measurements that measure the liquid property as a whole as it responds to modifying the pH of the solution containing the PFAS. ‘The problem with that is that you need to have a full fluorine accounting, otherwise you’re going to get it wrong,’ he states.
Biased by glass no more
Accounting for fluorine, however, is very tricky because most PFAS have an affinity for glass, which is extensively used in research. ‘While NMR tubes are also made of glass, the direct NMR observation of fluorine or proton nuclear spins … in essence automatically corrects for such sorption events,’ he says. ‘You’re not seeing the fluorines that are on the glass, you’re only seeing the fluorines that are in solution, and that’s all we care about. That in itself, determines your pKa, and so you’re not biased by these other sorption processes.’
And another factor that has skewed pKa results for PFAS in the past is the use of organic solvents like methanol. The more methanol a solution contains, the higher the pKa reading, Hoepker says. ‘We have lowered the methanol content by 10 to 100-fold, compared to other studies, and have therefore virtually eliminated this effect.’
Graham Peaslee, a physicist at Notre Dame University in Indiana who was not involved in this work but has published numerous studies on detecting PFAS, calls the study ‘a very solid piece of experimental work’. He says the possibility that many PFAS are more acidic than thought would help explain why they are so soluble in water and readily get everywhere in the environment.
‘I don’t know if the average non-chemist cares too much about this work, but I think it will shape the models that people use to predict the solubility for thousands of other PFAS that haven’t been measured yet,’ Peaslee tells Chemistry World. ‘And to that end, it is exciting for chemists to have more (and better) data to which to compare their computer models.’
Robert Paton, a computational chemist at Colorado State University, agrees. He says the team’s experimental design avoids several of the challenges associated with traditional bulk pKa measurement methods, and that these revised values will be ‘valuable for our ongoing efforts to understand and model transport, dispersion and bioaccumulation’.