Objective

Due to the complex composition of aqueous film-forming foams (AFFFs), none of the existing treatment techniques can effectively mineralize all per- and polyfluoroalkyl substances (PFAS) present at sites impacted by AFFFs. Thus far, heat-activated persulfate oxidation (HAPO) is the only in situ technology that can mineralize perfluoroalkyl carboxylic acids (PFCAs) and many polyfluoroalkyl substances. Recently, SERDP-funded research showed that perfluorooctanesulfonic acid (PFOS) can be transformed by a laccase-mediator system into shorter chain polyfluoroalkyl substances. To expand the technology to treat the full suite of PFAS, a method is needed to convert perfluoroalkyl sulfonates (PFSA) to PFCA or other species that can be mineralized by HAPO. An integrated treatment that can mineralize PFCA, PFSA, and polyfluorinated compounds would help the Department of Defense (DoD) meet its need for a holistic treatment tackling the diverse PFAS at AFFF-impacted sites.

While coupling this laccase-mediator treatment with HAPO seems like a logical, innovative step, additional research and development is needed to determine the feasibility of this strategy. The goal of this proof-of-concept project is to evaluate the potential for using enzymatic laccase treatment in conjunction with HAPO to mineralize PFOS under conditions relevant to in situ treatment. The approach consists of two crucial steps: 1) assess rates of PFSA transformation that can be achieved by a mediator radical system activated by laccase enzymes; and, 2) assess the potential for using HAPO to mineralize the products of the mediator-laccase system.

Technical Approach

Although the reported laccase-mediator system can transform PFOS to partially fluorinated compounds, the reported reaction was slow and required the addition of relatively high concentrations of metals and mediators. While diverse laccases and mediators have been identified, the reactivity of different laccase-mediator combinations vary widely. The effectiveness of different laccase-mediator systems in PFOS transformation still needs to be fully evaluated. In this research project, the laccase-mediator combinations that yield the fastest PFOS biotransformation will be identified. Then, the most effective PFOS transformation will be integrated with the HAPO treatment. A mass balance will be established by measuring PFAS for which analytical standards are available, along with measurements of fluoride and the total oxidizable precursor assay. Compounds not amenable to these analyses will be identified using high-resolution mass spectrometry.

Benefits

The project approach is a high-risk, high-gain venture that capitalizes on SERDP‚Äôs previous research investments, and that is backed by preliminary data that has been collected. Successful results from this research will pave the road to develop a non-toxic, novel treatment train that mineralizes the most common types of PFAS in AFFF, thus enabling the DoD to achieve closure at AFFF-impacted sites with less time and cost.