Two Compounds That Replaced Regulated PFAS — and Evaded Filter Certification
Perfluorobutane sulfonic acid (PFBS) and perfluorohexanoic acid (PFHxA) are among the most widely detected short-chain PFAS in environmental samples globally. PFBS is a four-carbon sulfonate that was introduced as a replacement for PFOS following that compound’s restriction under the Stockholm Convention. PFHxA is a six-carbon carboxylate that replaced PFOA in many industrial applications after PFOA was phased out under EPA’s 2006 Stewardship Program. Both compounds are now detectable in drinking water sources, surface waters, and human serum worldwide — yet neither is currently required in NSF/ANSI 53 PFAS certification testing.
Understanding why these compounds escape standard filtration requires examining both their molecular properties and the limitations of certification frameworks that were designed around their longer-chain predecessors.
PFBS: Short Enough to Pass Through Most Carbon Beds
PFBS has a four-carbon perfluorinated backbone attached to a sulfonate head group. Its shorter chain produces a significantly lower log Kow than PFOS, meaning it adsorbs far more weakly to activated carbon surfaces. Laboratory and field studies consistently report GAC removal efficiencies of 10–30% for PFBS under conditions that achieve >95% removal for PFOS. In practical terms, a filter bed that would be effective for PFOS for several years may be essentially ineffective for PFBS from the point of installation.
PFBS has been detected in tap water samples in the United States, Europe, and China at concentrations ranging from below the detection limit to several hundred parts per trillion. In areas where PFBS-containing aqueous film-forming foam (AFFF) was used or where fluorochemical manufacturing occurred, PFBS can be the dominant PFAS detected — yet consumers using GAC-based point-of-use filters certified for PFOA/PFOS removal would receive no protection against it.
PFHxA: The C6 Carboxylate That Eludes Standard Adsorption
PFHxA sits at six carbons, which places it just below the PFCA long-chain threshold (C7+). Its log Kow is higher than PFBS but substantially lower than PFOA, producing intermediate but still inadequate GAC performance. Reported removal rates for PFHxA on GAC range from 20–40% under standard conditions, compared to >95% for PFOA. Like PFBS, PFHxA was introduced as a ‘shorter-chain, less bioaccumulative’ substitute for its regulated predecessor.
The bioaccumulation argument has some support: PFHxA has a shorter serum half-life in humans than PFOA. However, shorter half-life does not mean harmless. Animal studies have identified liver effects, and the compound’s ubiquity in global water samples means ongoing exposure is continuous rather than episodic. The relevant question for water treatment is not whether PFHxA bioaccumulates as persistently as PFOA, but whether the filter being used actually removes it — and in most certified systems, the answer is: poorly.
What Adequate Short-Chain PFAS Removal Requires
Addressing PFBS and PFHxA in treated water requires moving beyond GAC as the sole treatment barrier. Strong base anion exchange resins show improved but not complete removal for these compounds. High-pressure membrane systems (nanofiltration, RO) generally achieve better short-chain removal than adsorption-based systems. Advanced oxidation processes are under investigation for short-chain PFAS destruction but are not yet broadly deployed.
The immediate practical requirement is accurate characterization of what is in the water. A treatment system designed and certified around PFOA/PFOS performance cannot be assumed to address PFBS or PFHxA. Site-specific PFAS profiling, including short-chain compound analysis, is a prerequisite for selecting treatment technologies that will actually perform in the presence of the compound mixture at hand.