Biodegradation of fluorinated octanoic acids in an O₂-based membrane biofilm reactor

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants due to their widespread use and recalcitrance to degradation. Aerobic bioreactors provide a possible path for further defluorination and mineralization of reductively defluorinated PFAS, which are generated by hydrogenative defluorination. This study colonized an O₂-based membrane biofilm reactor (O₂-based MBfR) with octanoic acid (OA) as the primary electron-donor substrate and evaluated the ability of its biomass to co-biodegrade PFOA and analogs with different degrees of hydrogen substitution – 2-FOA for 2-fluorooctanoic acid, 2H-PFOA for 2H,2H-perfluorooctanoic acid, and 4H-PFOA for 2H,2H,3H,3H-perfluorooctanoic acid. The O₂-based MBfR achieved complete removal (>95%) and defluorination (>95%) of 2-FOA and partial removal (∼48%) and defluorination (∼14%) of 2H-PFOA in continuous-flow operation. However, PFOA was not biodegraded at all. Batch experiments indicated that biofilms require the oxidation of a primary donor substrate for the biodegradation of 2H-PFOA, and perfluorohexanoic acid was a biodegradation product. Microbial-community analysis revealed that genera Cupriavidus and Pseudomonas were important during OA-enabled biodegradation of 2-FOA and 2H-PFOA, which involved b-oxidation reactions. In summary, sufficiently hydrodefluorinated PFOA could be completely mineralized through aerobic biological treatment, while minimally hydrodeflourinated PFOA could be biologically defluorinated, but at a much slower rate due to its insufficient electron and energy flows to sustain defluorination. These results underscore that perfluoroalkyl substances must be sufficiently hydrodefluorinated to enable rapid aerobic biodegradation.