PFBS: Short-Chain, Widespread, and GAC-Resistant
Perfluorobutane sulfonate (PFBS) is a four-carbon PFAS compound that replaced PFOS following the PFOS phaseout. It is now one of the most widely detected PFAS in drinking water sources globally, particularly in areas where shorter-chain PFAS have been used in industrial and consumer applications. EPA has set an MCL of 2,000 ng/L for PFBS—500 times higher than the PFOA MCL of 4 ng/L—partly because treatment technology cannot achieve the same low-level removal for this short-chain compound.
Why GAC Fails for PFBS
PFBS has a log Kow (octanol-water partition coefficient) far below long-chain PFAS, reflecting minimal hydrophobicity. Activated carbon relies primarily on hydrophobic interactions to retain PFAS—the more hydrophobic, the better the adsorption. PFBS’s low hydrophobicity means it has minimal attraction to carbon surfaces, resulting in rapid breakthrough even in well-designed systems.
Scale of the Performance Gap
Studies at PFBS-impacted utilities show that GAC systems achieving 99% PFOA removal simultaneously allow 80–90% of PFBS to pass through to finished water. At source water concentrations of 500 ng/L PFBS (below the MCL of 2,000), finished water may contain 400–450 ng/L—technically compliant but representing a substantial exposure burden unaddressed by the carbon system.
The Regulatory and Treatment Mismatch
The high PFBS MCL effectively acknowledges the treatment technology limitation: EPA cannot set a 4 ng/L limit for a compound that current treatment cannot achieve. This pragmatic approach protects utilities from unachievable compliance requirements but leaves consumers with higher permissible exposure to a compound that has replaced one of the most toxic PFAS in history.