Sorption and desorption behavior of PFOS and PFOA onto a Gram-positive and a Gram-negative bacterial species measured using particle-induced gamma-ray emission (PIGE) spectroscopy

In this study we examined whether a Gram-positive and a Gram-negative bacterial species have the capability to remove perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) from solution through sorption reactions under two different pH conditions. We developed a novel approach for quantifying PFAS in solution using particle-induced gamma-ray emission (PIGE) spectroscopy, and we used the results to constrain the important PFAS-bacterial sorption mechanisms. A mass balance experiment measuring PFOS in the aqueous phase and solid phase confirmed that PIGE spectroscopy measurements can account for all PFOS in the system, demonstrating for the first time how PIGE spectroscopy can be used to quantify the extent of PFAS sorption in experimental systems. Significant sorption occurred under most conditions studied, with up to 40 +/- 5% and 67 +/- 11% sorption of PFOA and PFOS, respectively. PFOA and PFOS sorption were both rapid, with steady-state being attained within minutes. PFOS sorption was pH dependent while PFOA sorption was pH independent, and greater sorption occurred onto Bacillus subtilis biomass compared to Pseudomonas putida biomass for both PFAS types. This sorption behavior suggests that pH, the PFAS head molecule, and cell wall structure all have a significant effect on the extent and mechanisms of bacterial sorption of PFAS. The observed sorption behaviors suggest that sorption of both molecules involves electrostatic and hydrophobic components. PFOS desorption was rapid and exhibited complete reversibility for both bacterial species, indicating that sorption is likely a cell surface phenomenon with no internalization occurring on the timescale of these experiments, and that the process can be modeled as an equilibrium reaction. Our work demonstrates that different PFAS molecules react to bacteria differently, and thus likely have markedly different mobilities in the environment. PFAS sorption in geologic systems is strongly influenced by organic matter, and our results suggest that biosorption may represent an inexpensive and effective PFAS remediation technique.