In water treatment, most contaminants follow familiar rules. Metals can be precipitated. Bacteria can be disinfected. Organic pollutants can often be broken down or filtered out.

PFAS do not behave like most contaminants. Their unusual chemistry makes them both useful in consumer products and stubbornly difficult to remove from water.

Over the last decade, the spread of PFAS contamination in groundwater and drinking water has forced utilities and regulators to confront a problem that is as much about physics and chemistry as it is about public health. Engineers now know how to capture many PFAS compounds. What they still struggle with is doing it efficiently, consistently, and affordably.

At the heart of the challenge is a molecular structure that resists the very processes water treatment systems normally rely on.

The bond that refuses to break

PFAS owe their persistence to a specific chemical feature: the carbon–fluorine bond.

In PFAS molecules, carbon atoms are surrounded by fluorine atoms, creating one of the strongest bonds in organic chemistry. Fluorine is extremely electronegative, meaning it pulls electrons tightly toward itself. The resulting bond is short, stable, and difficult to disrupt.

This strength gives PFAS their famous durability. It also explains why they were widely used in products that needed to repel water, oil, and heat – such as non-stick cookware, firefighting foams, waterproof fabrics, and industrial coatings.

But the same chemical resilience that makes PFAS useful in manufacturing makes them extraordinarily resistant to environmental breakdown.

Sunlight, biological activity, and many chemical reactions that degrade other pollutants have little effect on them. As a result, PFAS released into the environment can persist for decades, moving through soils and groundwater and eventually entering drinking water supplies.

For water engineers, that persistence means traditional treatment strategies often fail.

Why conventional treatment methods fall short

Most drinking water treatment plants were not designed with PFAS in mind. Their processes focus primarily on pathogens, sediment, nutrients, and natural organic matter.

Common techniques include:

• Coagulation and flocculation, which clump particles together so they can settle out
• Filtration, which removes suspended solids
• Disinfection, which kills microorganisms

PFAS slip through nearly all of these steps.

Unlike particles or microbes, PFAS molecules are extremely small and remain dissolved in water. They do not readily settle or form larger aggregates that can be filtered out. Nor are they easily destroyed by disinfectants such as chlorine or ozone.

Even advanced treatment technologies used for other organic contaminants – such as ultraviolet oxidation – often fail to break PFAS apart.

That leaves utilities relying on a different strategy: capturing PFAS rather than destroying them.

Adsorption: Trapping molecules on a surface

The most widely used method for removing PFAS from drinking water today is adsorption.

In adsorption processes, contaminants attach to the surface of another material. For PFAS removal, the most common adsorbent is granular activated carbon (GAC), a highly porous form of carbon with an enormous internal surface area.

A single gram of activated carbon can contain hundreds of square meters of microscopic pores. When contaminated water flows through a carbon filter, PFAS molecules stick to those surfaces.

The attraction occurs because PFAS molecules have two distinct regions:

• A hydrophobic fluorinated tail, which repels water
• A charged head group, which interacts with dissolved ions

These characteristics allow PFAS to bind to carbon surfaces under the right conditions.

But adsorption is not a perfect solution.

The short-chain problem

One of the biggest challenges in PFAS removal is distinguishing between long- and short-chain compounds.

Early PFAS compounds – such as PFOA and PFOS – contained relatively long carbon chains. These molecules bind relatively strongly to activated carbon, making them easier to remove.

In recent years, however, many industries have shifted toward short-chain PFAS, partly in response to regulatory pressure on older compounds.

Short-chain PFAS behave differently.

Because they have fewer carbon atoms, their fluorinated tails interact less strongly with carbon surfaces. As a result, they tend to pass through activated carbon filters more easily.

For water treatment operators, that means filters must be replaced more frequently or supplemented with additional treatment steps. Even then, removal efficiency can vary widely.

Ion exchange: Swapping contaminants out of water

Another major PFAS treatment approach uses ion exchange resins.

These synthetic materials contain charged sites that attract oppositely charged ions in water. When PFAS molecules pass through the resin, their negatively charged head groups bind to the resin surface, displacing other ions.

Ion exchange systems can remove PFAS more effectively than activated carbon in some cases, particularly for shorter-chain compounds.

However, the process still faces limitations:

• Competing ions in water can reduce efficiency
• Resins eventually become saturated and must be replaced
• Regeneration processes create concentrated PFAS waste streams

Like adsorption, ion exchange does not destroy PFAS – it simply concentrates them.

Membranes and the brute-force approach

Some utilities turn to reverse osmosis (RO) or similar membrane technologies for PFAS removal.

Reverse osmosis forces water through an extremely fine membrane under high pressure. The membrane allows water molecules to pass while rejecting many dissolved contaminants, including PFAS.

RO systems can achieve very high removal efficiencies, often above 90%.

But they come with significant trade-offs.

Membrane systems require substantial energy to maintain the necessary pressure. They also produce a concentrated waste stream, known as brine, that contains the rejected contaminants.

That brine must be disposed of or treated further, effectively shifting the PFAS problem into a smaller but more concentrated volume of waste.

The chemistry of stubborn molecules

The difficulty of removing PFAS is not only about size or charge. It also reflects how these molecules interact with water itself.

PFAS have unusual amphiphilic properties – meaning they contain both water-repelling and water-interacting components. This dual nature allows them to move easily between surfaces and water.

In many ways, PFAS behave similarly to surfactants, the same class of chemicals used in soaps and detergents. They tend to accumulate at interfaces between water and other materials.

This behaviour complicates treatment in several ways:

• PFAS may cling to surfaces within treatment systems.
• They can travel long distances in groundwater without breaking down.
• Their interactions with other organic compounds in water can alter removal efficiency.

For engineers designing reliable treatment systems, these behaviours introduce significant uncertainty.

The destruction challenge

Capturing PFAS is one problem. Destroying them is another.

Because of the strength of the carbon-fluorine bond, breaking PFAS molecules apart requires substantial energy or specialised chemical conditions.

Researchers are exploring several approaches:

• Electrochemical oxidation, which uses electric current to generate reactive species
• Plasma reactors, which expose PFAS to highly energetic ionised gases
• Supercritical water oxidation, where water at extremely high temperature and pressure can break down resistant molecules
• Advanced photochemical reactions that combine ultraviolet light with catalysts

Some of these technologies have shown promise in laboratory experiments and pilot systems.

Yet scaling them up to treat millions of litres of drinking water each day remains a formidable engineering challenge. Energy consumption, reactor design, and by-product formation all remain open questions.

A moving target

Complicating matters further is the sheer diversity of PFAS compounds.

Scientists estimate that thousands of PFAS variants exist, with new ones occasionally entering the market as older chemicals face restrictions.

Each compound has slightly different physical and chemical properties – different chain lengths, functional groups, and environmental behaviours.

A treatment system optimised for one PFAS compound may not work equally well for another.

This variability forces engineers to design systems capable of handling a wide range of contaminants simultaneously, often under changing water conditions.

Engineering around chemistry

Despite these challenges, utilities around the world are deploying PFAS treatment systems at an increasing scale.

Activated carbon filters, ion exchange systems, and membrane plants are already operating in communities affected by contamination. In many cases, these systems can reduce PFAS concentrations to levels that meet emerging regulatory standards.

But the solutions come with costs.

Treatment systems require ongoing monitoring, frequent media replacement, and careful disposal of contaminated materials. Large facilities can cost tens or even hundreds of millions of dollars to build and operate.

For smaller communities, those expenses can be difficult to absorb.

The deeper lesson of PFAS

PFAS contamination highlights a broader reality in environmental engineering: the chemistry of modern industrial compounds can outpace the infrastructure designed to manage them.

Water treatment systems evolved over decades to address pathogens and conventional pollutants. PFAS, with their extraordinary chemical stability, challenge those systems in ways few other contaminants have.

Removing them from water is possible. Doing so efficiently and sustainably remains an active area of research.

Ultimately, the problem of PFAS removal is not just about technology. It is about the fundamental physics and chemistry of molecules engineered to resist the forces that normally break pollutants down.

These molecular properties – once prized for their durability – are precisely what make them so difficult to remove from the world’s water supplies.