Fluorine-free foams exhibit incomplete aerobic and anaerobic biodegradation, create redox-specific byproducts and shift microbial communities.

The phaseout of per- and polyfluoroalkyl substances (PFAS) in firefighting foams has motivated the adoption of fluorine-free foams (F3), yet their environmental fate remains poorly understood. This study provides the first comprehensive assessment of F3 biodegradation under both aerobic and anaerobic conditions, combining modified OECD protocols, high-resolution mass spectrometry (HRMS), and microbial community profiling to elucidate surfactant degradation pathways, byproduct formation, and ecological impacts. Aerobic systems achieved greater than 80% bulk dissolved organic carbon (DOC) removal for two commercial F3 formulations within 28 days, yet targeted analyses revealed persistent, hydrophobic surfactants (e.g., ethylene glycol dodecyl ether, EGDE) in sludge phases (2-5 μg L-1) and transient byproducts like short-chain glycol ethers. Anaerobic degradation diverged sharply with minimal DOC reduction (<5%), limiting surfactant transformation, including sulfate reduction-driven hydrogen sulfide generation and accumulation of alkylamine intermediates (e.g., N-methyldodecylamine). Non-target HRMS screening identified 21 byproducts. Nine were exclusive to late-stage samples at day 60, with unknown peaks constituting 15-20% of residual DOC as uncharacterized compounds absent from formal assessments. Microbial diversity was reduced by 79% in anaerobic systems, selecting for specialist taxa (e.g., Azospira, Nitrospira) with the potential for surfactant hydrolysis. In contrast, aerobic communities retained metabolic flexibility but showed concentration-dependent inhibition. These findings challenge the adequacy of standardized biodegradability tests, which overlook sludge-phase residuals, redox-specific byproducts, and non-target compounds. This work shows that assumed "readily biodegradable" F3 surfactants may also leave persistent residues, necessitating advanced frameworks with phase-specific analytics and pathway prediction tools to ensure replacements reduce and do not redistribute risks to water quality and ecosystems.