The chemistry of PFAS.

The carbon fluorine bond

Every PFAS is built on the carbon fluorine (C-F) bond, the strongest single bond in organic chemistry, with a bond dissociation energy on the order of 480 kJ/mol. Fluorine is the most electronegative element, so it holds its electrons tightly and forms a short, highly polar, very stable bond. A perfluorinated carbon chain wraps the carbon backbone in a sheath of fluorine that shields it from chemical, thermal, and biological attack. That single fact drives everything else: the persistence, the difficulty of destruction, and the need for high energy methods to mineralize it. ITRC ↗

Structure and classes

A PFAS molecule has a fluorinated tail and a functional head group. Perfluoroalkylsubstances are fully fluorinated on the carbon chain; polyfluoroalkylsubstances are only partly fluorinated and can transform into perfluoroalkyl acids over time.

• PFCAs (perfluoroalkyl carboxylic acids, for example PFOA) carry a carboxyl head.
• PFSAs (perfluoroalkyl sulfonic acids, for example PFOS, PFHxS) carry a sulfonate head and are generally more stable and more strongly sorbed.
• Fluoropolymers (PTFE, PVDF) are high molecular weight solids, distinct from the small mobile non polymer acids that dominate water contamination.

The combination of a hydrophobic, lipophobic fluorinated tail and a charged head makes most PFAS surfactants, which is exactly why they were valuable in firefighting foam and coatings. EPA ↗

Persistence and transport

Under environmental conditions PFAS do not meaningfully hydrolyze, photolyze, or biodegrade. Short chain compounds are less bioaccumulative but more water soluble and mobile, so they travel further in groundwater and are harder to capture. Polyfluorinated precursorsact as a slow reservoir, transforming into terminal perfluoroalkyl acids such as PFOA and PFOS, which is why measured PFAS can rise over time even without new releases. ITRC fate and transport ↗

What it takes to destroy them

Destruction means breaking the C-F bond and mineralizing the molecule to fluoride ion, carbon dioxide, and salts. Defluorination (release of fluoride) is the true endpoint, not just a drop in target analyte concentration. The leading routes, with the conditions that define them:

• Supercritical water oxidation (SCWO). Above water's critical point (about 374 C and 22.1 MPa), water becomes a single dense phase that dissolves organics and oxygen together. Added oxidant drives rapid oxidation to near complete mineralization, with reported destruction efficiencies above 99 percent. Energy intensive, and a small residual stream can remain. Peer-reviewed review ↗
• Hydrothermal alkaline treatment (HALT). Hot pressurized water below the critical point plus a strong base (sodium hydroxide) defluorinates PFAS without an oxidant, allowing lower complexity subcritical operation.
• Electrochemical oxidation. At the anode (often boron doped diamond), direct electron transfer and hydroxyl radicals cleave the chain. Effective on concentrated streams; electrode material and energy use govern cost.
• Plasma. Electrical discharge at a gas to liquid interface generates reactive species and electrons that attack PFAS concentrated at the interface.
• High temperature incineration. Thermal destruction at roughly 1,000 C and above. Effective in testing, but products of incomplete combustion and stack emissions remain an open regulatory question.

Note the order of operations: granular activated carbon, ion exchange, and reverse osmosis separate and concentrate PFAS but do not destroy it. The destruction routes above are what finally close the loop on the concentrate they produce. See the destruction technologies directory for providers and stage, and water treatmentfor the separation side. EPA destruction guidance ↗

Analytical methods

PFAS are regulated at parts per trillion, so the method and its accreditation matter as much as the result.

• EPA Method 1633 / 1633A. The reference method for 40 PFAS in water, soil, biosolids, and tissue by isotope dilution LC-MS/MS, finalized by EPA. The standard for compliance grade data. EPA Method 1633 ↗
• Total Oxidizable Precursor (TOP) assay. Oxidizes precursors to measurable terminal acids, estimating the hidden precursor load that targeted methods miss.
• Total organic fluorine (TOF) and adsorbable organic fluorine (AOF). Mass balance approaches that capture fluorine beyond the named analytes, useful when unknown PFAS are suspected.
• Lab accreditation. Defensible data comes from ISO/IEC 17025 accredited laboratories, and for defense work the DoD ELAP under the DoD Quality Systems Manual. See standards and accreditation.