Beyond PFOA and PFOS: The Mid-Chain PFAS
EPA’s 2024 MCL rule regulates six PFAS, including PFNA (perfluorononanoic acid, C9) and PFHxS (perfluorohexane sulfonate, C6 sulfonate). These mid-chain compounds occupy an intermediate position in GAC adsorption efficiency—better captured than short-chain but with important performance differences from the benchmark long-chain compounds.
PFNA: The Longer Carboxylate
PFNA (C9 carboxylate) is actually longer-chain than PFOA (C8) and shows slightly stronger GAC adsorption. In systems designed for PFOA removal, PFNA is typically well-captured. However, PFNA’s lower environmental concentration compared to PFOA means it contributes less to driving early bed exhaustion, potentially causing problems in multi-PFAS contamination scenarios.
PFHxS: The Critical Mid-Range Sulfonate
PFHxS (C6 sulfonate) presents more challenging dynamics. Despite its regulatory priority, PFHxS adsorbs to GAC with significantly less affinity than PFOS. Studies show PFHxS breakthrough occurs substantially earlier than PFOS, meaning GAC systems changed based on PFOS breakthrough may be allowing PFHxS to pass above its hazard index threshold for extended periods.
Mixture Effects and Competitive Adsorption
In real water supplies containing multiple PFAS, competitive adsorption complicates performance predictions. Long-chain PFAS competitively displace previously adsorbed short-chain PFAS from carbon surfaces during operation, causing short-chain compounds to break through even earlier than predicted by single-compound studies. This mixture effect makes GAC less reliable in waters with diverse PFAS contamination.