Scientists finally know why 'forever chemicals' spread so easily in water

Scientists have just rechecked how acidic many PFAS really are – and the results were surprising. The new numbers show they’re even stronger acids than once thought, helping explain why these “forever chemicals” move so easily through water.

Acidity controls whether a molecule holds onto a proton or lets it go, which determines if it carries a charge. Charged molecules dissolve more readily in water, giving them the ability to travel farther and faster through the environment.

Acidity matters for forever chemicals

Alexander Hoepker, Ph.D., a senior research scientist at the University at Buffalo (UB) RENEW Institute, and his team focused on measuring acidity with a method that avoids the usual traps that can skew results.

In the new study, the researchers determined pKa values for 10 PFAS and three small fluorinated acids. A pKa value indicates the pH at which a molecule is half charged and half neutral.

Lower pKa numbers mean stronger acids and greater ionization at typical environmental pH. That tendency increases solubility and makes long-distance movement in groundwater more likely.

Acidity data tell a new story

For perfluorooctanoic acid (PFOA), the team measured a pKa near −0.27, which implies it will exist in the charged form across almost any natural water. Earlier experiments and models often placed it between 0.24 and 3.8 – a very different picture.

The replacement chemical GenX showed a pKa roughly one thousand times lower than that reported in one earlier study. That shift places GenX in the same very strong acid category as PFOA.

Trifluoroacetic acid (TFA) came in at about 0.03, while past estimates ranged from 0.30 to 1.1. That makes it a stronger acid than many had assumed.

“These findings suggest that previous measurements have underestimated PFAS acidity. This means their ability to persist and spread in the environment has also been mischaracterized,” said Hoepker.

Where past methods fell short

PFAS molecules tend to adsorb to glass, a detail that confuses bulk measurements because some of the sample sticks to the container. In other tests, adding too much organic solvent drives the numbers upward.

The team avoided those pitfalls by analyzing the molecules directly with nuclear magnetic resonance (NMR), a spectroscopic method that tracks changes in nuclear signals within a strong magnetic field.

When the head group carries a negative charge, nearby fluorine atoms shift to a different radio frequency.

This approach captures the charge state without being distorted by losses to surfaces or by micelles that form at higher concentrations.

For the most acidic compounds, where neutral forms are hard to prepare, the team combined experiments with density functional theory (DFT) calculations to predict the reference signals.

“We augmented partial NMR datasets with computational predictions to arrive at more accurate pKa values,” said Hoepker. That strategy narrowed the estimates where conventional titrations would struggle.

Cleaning up forever chemicals from water

Accurate acidity values anchor the models that forecast where these chemicals go in real settings. Acidity influences whether the forever chemicals remain dissolved in water, bind to soil, or move into the air.

The stakes are high because PFOA and PFOS are now listed as hazardous substances under CERCLA in a federal rule that took effect in July 2024. That designation triggers reporting and cleanup authority for releases.

The EPA also set national drinking water limits for several PFAS in 2024, including GenX. Utility monitoring, public notification, and treatment choices flow from those enforceable standards.

“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,” said Diana Aga, Ph.D., director of the UB RENEW Institute. Those tasks become easier when the underlying numbers are trustworthy.

The overlooked PFAS problem

TFA appears in rain, rivers, and even drinking water in many locations, and multiple lines of evidence point to atmospheric transport.

A 2024 review noted widespread detections and emphasized the difficulty of removing this small, mobile acid. The newly measured pKa of about 0.03 helps explain that pattern, because the charged form remains in water and moves with it.

That makes tracking sources more important – from the breakdown of fluorinated refrigerants to the degradation of longer PFAS.

Lessons from the findings

The take-home message is simple: when acidity is measured correctly, the story of how PFAS move becomes clearer and easier to predict.

These values feed into better laboratory methods, stronger models, and more practical cleanup plans. They also help regulators and communities focus resources where they can do the most good.

The study is published in Environmental Science & Technology Letters.

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