The aim of this project was to develop a novel proof-of-concept in situ treatment train to mineralize perfluorooctane sulfonate (PFOS), by first transforming PFOS with a laccase-mediator system (LMS), an enzyme-based treatment method, and then subjecting transformation products to heat-activated persulfate oxidation (HAPO), a chemical treatment method. The project team considered that a dual-pronged treatment approach could address the challenges of treating complex mixtures of PFAS at sites impacted with aqueous-film forming foam (AFFF); namely, the initial biotransformation step of PFOS, which is notoriously difficult to treat in situ.

The objectives of this project were to 1) Identify laccase-mediator systems that could quickly transform PFOS; 2) quantify and characterize the transformation products formed during laccase-mediator reactions; and 3) evaluate the mineralization of transformation products by HAPO. In addressing these objectives, the project team sought to validate and optimize a treatment train approach for complex AFFF-impacted matrices.

The breadth of PFAS and non-fluorinated organics in AFFF as well as the co-occurring chemicals present at sites is further compounded by accumulation of perfluoroalkyl acids from precursor transformation. A single, isolated treatment unit is not a viable approach for remediation of these sites; however, a treatment train that takes advantage of the field-readiness and non-specificity of HAPO in combination with the novel enzyme-based LMS treatment approach, could make PFOS amenable to mineralization. The project team viewed the LMS strategy as a tunable and optimizable treatment method due to the multiple components of the treatment system; namely, the choice of enzyme, the choice of mediator, and the choice of buffer. In the initial phase of the research, the project team addressed the ‘optimization’ of the LMS for PFOS transformation, screening a variety of enzymes, mediator compounds, and buffers. The project team planned to evaluate the transformation rates and transformation products of PFOS subjected to these various conditions, and then to evaluate the subsequent treatment step, HAPO. The project team was unable to successfully or reliably show PFOS transformation in the screening studies. This led to a revision of the approach and for the project team to undertake a mechanistic investigation of the LMS, evaluating each of the oxidative reactions in the system. The mechanistic investigation led to important insights about the nature of PFOS ‘loss’ in enzyme-based treatment systems. 

 

Screening studies of enzymes, mediators, and buffers for PFOS transformation did not indicate reactivity of either the laccase enzyme or the radical mediator compound toward PFOS. Upon validating the reactivity of the LMS toward a proxy compound, as well as validating enzyme activity and generation of the radical mediator, the project team concluded that the LMS was not able to reliably transform PFOS. It was found, however, that PFOS could be removed from solution when subjected to the enzyme-mediator treatment through a sorption-based mechanism.

While unable to accomplish the initial objective of PFOS mineralization in a dual-step in situ treatment train approach, the findings suggested alternative removal mechanisms for enzyme-based treatment of PFOS. While not necessarily a feasible strategy at a large scale, the affinity of PFOS for proteins may be utilized for bio-inspired, selective materials for PFOS removal in complex impacted matrices. (Project Completion - 2023)

Steffens, S.D., E.H. Antell, E.K. Cook, G. Rao, R.D. Britt, D.L. Sedlak, and L. Alvarez-Cohen. 2023. An Artifact of Perfluoroalkyl Acid (PFAA) Removal Attributed to Sorption Processes in a Laccase Mediator System. Environmental Science and Technology Letters, 10(4):337-342. doi.org/10.1021/acs.estlett.3c00173.