Enhanced Feammox activity and perfluorooctanoic acid (PFOA) degradation by Acidimicrobium sp. Strain A6 using PAA-coated ferrihydrite as an electron acceptor

Acidimicrobium sp. Strain A6 (A6) can degrade perfluoroalkyl acids (PFAAs) by oxidizing NH₄⁺ while reducing Fe(Ⅲ). However, supplying and distributing Fe(III) phases in sediments is challenging since surface charges of Fe(III)-phases are typically positive while those of sediments are negative. Therefore, ferrihydrite particles were coated with polyacrylic acid (PAA) with four different molecular weights, resulting in a negative zeta potential on their surface. Zeta potential was determined as a function of pH and PAA loading, with the lowest value observed when the PAA/ferrihydrite ratio was > 1/5 (w/w) at a pH of 5.5. Several 50-day incubations with an A6-enrichment culture were conducted to determine the effect of PAA-coated ferrihydrite as the electron acceptor of A6 on the Feammox activity and PFOA degradation. NH₄⁺ oxidation, PFOA degradation, production of shorter-chain PFAS, and F^- were observed in all PAA-coated samples. The 6 K and 450 K treatments exhibited significant reductions in PFOA concentration and substantial F^- production compared to incubations with bare ferrihydrite. Electrochemical impedance spectroscopy showed lowered charge transfer resistance in the presence of PAA-coated ferrihydrite, indicating that PAAs facilitated electron transfer to ferrihydrite. This study highlights the potential of PAA-coated ferrihydrite in accelerating PFAS defluorination, providing novel insights for A6-based bioremediation strategies.