Klar2O Reorder
Podcast

Why Some PFAS Are Harder to Remove Than Others

PFAS are often discussed as if they were one single contaminant.

They are not.

PFAS is a large group of chemicals with different structures and properties. Because of these differences, a treatment method that works well for one PFAS may perform much less effectively for another. [U.S. EPA, Reducing PFAS in Drinking Water with Treatment Technologies, https://www.epa.gov/sciencematters/reducing-pfas-drinking-water-treatment-technologies] US EPA

This is particularly clear when comparing long-chain and short-chain PFAS.

Long-chain PFAS are generally easier to adsorb onto activated carbon. Short-chain PFAS tend to remain more mobile in water and can be harder to capture with conventional carbon-based filters. ScienceDirect

So why does chain length matter?

The answer lies in the chemistry of the molecule.

PFAS are a chemical family, not one substance

The term PFAS covers a very large group of substances.

They can differ in:

These differences affect how a PFAS behaves in water and how strongly it interacts with a filter medium.

That is why saying “this filter removes PFAS” is often too broad.

A better question is:

Which PFAS, at what concentration, under which conditions?

Long-chain vs. short-chain PFAS

One of the most important differences is the length of the fluorinated carbon chain.

In simplified terms:

Longer chain → generally stronger interaction with activated carbon

Shorter chain → generally weaker interaction with activated carbon

This does not apply identically to every PFAS or every filter material. However, the general trend is well established for conventional activated carbon treatment. [U.S. EPA, Predicting How Effective Water Filters Are at Removing a Variety of PFAS, https://www.epa.gov/sciencematters/predicting-how-effective-water-filters-are-removing-variety-pfas] US EPA

Why does chain length make a difference?

Activated carbon removes contaminants mainly through adsorption.

That means molecules interact with surfaces and pores within the carbon rather than simply being caught by a sieve.

Longer-chain PFAS tend to have stronger hydrophobic interactions with activated carbon.

Short-chain PFAS are more water-compatible and generally have weaker interactions with conventional carbon surfaces.

As a result, short-chain compounds can remain in the water more easily.

Recent research confirms that short-chain PFAS are less strongly adsorbed by activated carbon than longer-chain compounds. ScienceDirect

A smaller molecule is not automatically easier to remove

This sounds counterintuitive.

You might expect a smaller molecule to move through a filter more easily, but that does not explain the entire process.

With adsorption, the question is not simply:

“Can the molecule fit into the pores?”

It is also:

“Does the molecule interact strongly enough with the filter surface to stay there?”

Short-chain PFAS can enter water more readily and may have weaker interactions with conventional activated carbon.

Therefore, their smaller size does not automatically make them easier to capture.

Short-chain PFAS are more mobile

The same properties that make some short-chain PFAS difficult to adsorb can also make them more mobile in the environment.

They tend to be more water-soluble and less strongly retained by many soils and carbon-based materials.

This can allow them to move more readily through water systems.

It creates an important connection:

The PFAS that can travel easily through water can also be harder to capture with some conventional treatment methods.

That combination makes short-chain PFAS an important challenge for water treatment.

PFBA and PFBS are useful examples

Two commonly studied short-chain PFAS are:

PFBA

and

PFBS

The EPA reports that granular activated carbon generally performs better for longer-chain PFAS such as PFOA and PFOS, while PFBA and PFBS are less strongly adsorbed. US EPA

This does not mean that activated carbon has no effect on short-chain PFAS.

It means that the same carbon system can have very different performance depending on which PFAS is present.

That distinction is essential when evaluating treatment claims.

PFAS also have different functional groups

Chain length is only part of the story.

PFAS can also contain different functional groups.

Two important groups are:

carboxylic acids (PFCAs)

and

sulfonic acids (PFSAs)

These groups influence how the molecule interacts with water and filter surfaces.

Research has found that PFSAs can show stronger sorption to activated carbon than comparable PFCAs, although the exact behaviour depends on the compound and treatment conditions. ScienceDirect

So even two PFAS with similar chain lengths may not behave identically.

Water chemistry can change the result

A filter never treats PFAS in isolation.

Real water contains many other substances.

These can include:

Some of these compounds can compete for adsorption sites.

Natural organic matter is especially important for activated carbon.

Recent research shows that different fractions of natural organic matter can inhibit adsorption of short- and long-chain PFAS in different ways. ScienceDirect

Therefore, the same filter can behave differently in different water sources.

Flow rate matters too

Even the right filter medium can perform differently depending on how quickly water passes through it.

A higher flow rate generally means less time for the water to interact with the filter medium.

For activated carbon, the EPA identifies factors such as:

as important influences on PFAS removal. US EPA

That is why a laboratory result cannot automatically be transferred to every real-world installation.

Contact time affects adsorption

Imagine two systems using the same activated carbon.

In one system, water moves slowly through a deep carbon bed.

In the other, water moves much faster through a smaller bed.

The carbon is the same.

However, the treatment conditions are different.

The water has a different opportunity to interact with the carbon in each system.

This can influence how much PFAS is captured before the water leaves the filter.

What is breakthrough?

No adsorption filter has unlimited capacity.

As PFAS accumulate on the filter medium, available adsorption sites gradually become occupied.

Eventually, PFAS can begin appearing in the treated water.

This is called breakthrough.

Short-chain PFAS can break through activated-carbon columns earlier than many long-chain PFAS. Recent column studies have observed rapid breakthrough of short-chain PFAS compared with longer-chain compounds. ScienceDirect

That means a filter can still be physically functioning while its performance for a particular PFAS has already declined.

One filter can capture several PFAS differently

Real water may contain a mixture of PFAS.

For example:

They do not necessarily compete equally for the available adsorption sites.

Longer-chain compounds can interact more strongly with activated carbon.

Research has also shown that long-chain PFAS can interfere with the adsorption of shorter-chain compounds by occupying accessible sites and affecting pore access. ScienceDirect

This makes PFAS mixtures more complicated than single-compound laboratory tests.

The filter material matters

Not all activated carbon is the same.

Different carbons have different:

Research has found substantial variation between activated carbons in their ability to adsorb PFAS. ScienceDirect

Therefore:

“Activated carbon removes PFAS”

is not a complete technical statement.

The relevant question is:

“Which activated carbon, for which PFAS, under which water conditions?”

Other technologies can behave differently

The fact that short-chain PFAS can be difficult for activated carbon does not mean they are impossible to remove.

Different treatment technologies rely on different mechanisms.

Options used for PFAS treatment include:

The EPA identifies all three as established treatment approaches for PFAS in drinking water. US EPA

Ion exchange, for example, can be particularly useful for certain short-chain PFAS because its removal mechanism differs from conventional hydrophobic adsorption. Reviews have found strong anion-exchange resins can perform well across both short- and long-chain PFAS. ScienceDirect

Reverse osmosis works differently again

Reverse osmosis does not depend on the same adsorption mechanism as activated carbon.

Instead, a semipermeable membrane creates a separation between treated water and a concentrated reject stream.

Because the mechanism is different, RO can provide high rejection for a broad range of dissolved substances, including many PFAS.

However, it also creates a concentrate stream that needs to be managed.

So the question is not simply:

“Which filter removes the most?”

It is:

“Which treatment mechanism fits the target PFAS and the application?”

What about ultra-short-chain PFAS?

The challenge becomes even more interesting with ultra-short-chain PFAS.

Compounds such as TFA are much smaller and highly water-soluble.

Their behaviour can be very different from compounds such as PFOS or PFOA.

Conventional activated carbon is generally much less effective for these highly mobile compounds.

Recent research into short-chain and ultra-short-chain PFAS is therefore focusing on multiple interaction mechanisms rather than relying only on conventional hydrophobic adsorption. ScienceDirect

This is an active area of water-treatment research.

This is why “PFAS removal” is not one number

Imagine two filters.

Filter A

Removes:

99% PFOA

under a defined laboratory condition.

Filter B

Removes:

90% PFBA

under a different test condition.

Which filter is better?

There is no meaningful answer without knowing the application.

The target contaminant is different.

The test conditions are different.

The water chemistry may be different.

The result is therefore not a simple ranking.

What should a good PFAS filter claim tell you?

A useful performance claim should provide context.

Look for information about:

Target compounds

Which PFAS were actually tested?

Concentration

At what starting concentration?

Water matrix

Was the test performed in clean laboratory water or representative source water?

Flow conditions

What flow rate and contact time were used?

Filter medium

What material was used?

Capacity

How long did the removal performance remain effective?

Breakthrough

When did the target PFAS begin appearing in the treated water?

These details tell you much more than a single removal percentage.

Why one test cannot cover every PFAS

There are too many PFAS compounds to test every possible combination of:

PFAS + filter + water chemistry + concentration + flow rate + temperature.

The EPA is therefore developing models that can help predict treatment performance for different PFAS and treatment conditions. US EPA

At the same time, direct testing remains important.

Models can help identify likely behaviour.

Application-specific testing can show what actually happens in the intended system.

The future is moving toward more selective treatment

The challenge of short-chain PFAS highlights a broader trend in water filtration.

The goal is not simply to make the filter physically tighter.

Instead, researchers are looking at how molecules interact with surfaces.

That includes:

Recent research into advanced adsorbents suggests that combining several interaction mechanisms can improve the capture of short-chain PFAS. ScienceDirect

This is an important shift.

Filtration is increasingly about chemistry, not just size.

What this means for modern water filters

A good water filter should be matched to the actual problem.

If the target is a long-chain PFAS, conventional activated carbon may be highly effective under suitable conditions.

If the target is a short-chain PFAS, the same approach may provide much lower removal.

For a mixture, the challenge becomes even greater.

That is why modern water treatment increasingly focuses on selectivity.

The objective is not to remove everything.

It is to create the right interaction between:

the contaminant

the water

and

the filter surface.

The Klar2O perspective

This is also where advanced surface technology becomes interesting.

Klar2O’s Smart Surface Technology uses biochemically coated silica beads designed to interact with target contaminants through adsorption rather than relying only on physical pore size. [Klar2O, About Us, https://klar2o.com/about-us]

The underlying principle is important:

a dissolved molecule does not need to be physically trapped by a tiny pore to be removed.

It can instead interact with a surface and be retained through chemical or biochemical interactions.

That opens a different way of thinking about selective filtration.

Instead of asking:

“How small can we make the pores?”

we can ask:

“How can we make the surface interact more effectively with the target molecule?”

So, why are some PFAS harder to remove?

There is no single reason.

Their removal depends on a combination of:

chain length

functional group

molecular structure

water chemistry

filter material

flow rate

contact time

and treatment technology.

Short-chain PFAS are generally harder to remove with conventional activated carbon because they are more water-soluble and interact less strongly with carbon surfaces. ScienceDirect

Long-chain PFAS often adsorb more readily.

However, even strong adsorption does not guarantee permanent removal.

Eventually, the filter can reach capacity and require replacement, regeneration or another form of treatment.

The bottom line

There is no universal “PFAS filter.”

There are different PFAS.

There are different filter materials.

And there are different water conditions.

A treatment system that performs very well for PFOA or PFOS may not deliver the same result for PFBA, PFBS or ultra-short-chain PFAS.

That is why effective PFAS treatment starts with knowing which compounds are actually present.

Only then can the right treatment mechanism be selected.

The future of PFAS filtration is not about making one filter remove everything.

It is about designing the right interaction for the right contaminant.

Verschlagwortet , , , , , , , , ,

Klar2O
Safe water
for safe life