Evaluation of gliding arc plasma for the treatment of PFAS-contaminated water

Per- and polyfluoroalkyl substances (PFAS) are a large class of synthetic organofluorine compounds that have been designated contaminants of emerging concern due to their historical widespread use in tandem with their potential for bioaccumulation and toxicity. Ubiquitous in the environment, PFAS present a significant challenge to regulatory bodies and researchers aiming to minimize human exposure and associated health risks. One major route of exposure is through ingestion of contaminated water. The U.S. EPA recently set enforceable Maximum Contaminant Levels (MCLs) for six different PFAS in drinking water; a variety of treatment methods are being explored to meet these levels. Gliding arc plasma (GAP), a type of non-equilibrium plasma, is one novel destructive treatment method that has shown promise for this application. However, the aspects of the reactive environment produced by GAP (e.g. heat, reactive species, etc.) conducive to PFAS degradation in water and the mechanisms at play were not previously well understood. This work aimed to fill this research gap, as this knowledge could help with optimization and assessment of the feasibility of GAP for treatment of PFAS-contaminated water. Following mechanistic studies, this work also investigated the feasibility of GAP for this application based on a variety of factors including scalability as well as the effects of plasma gas, water quality, co-contaminants, and plasma enthalpy on treatment. To close, recommendations were made for future studies, specifically regarding optimization of air GAP for treatment of PFAS-contaminated water. To begin, this work investigated the energy required for PFAS mineralization in water by assessing this reaction through a thermodynamic lens. Fundamental thermodynamic equations were used to estimate the enthalpy of formation (-4,640 kJ/mol) of perfluorooctanoic acid (PFOA), which was then used to estimate the enthalpy of reaction (480 kJ/mol) of PFOA mineralization in water. The positive enthalpy of reaction indicated that PFOA mineralization in water is endothermic; thus, energy input (e.g. heat) is required, and plasma reactive species alone cannot drive the reaction. Comparison of a thermodynamic model, experimental results in air and argon GAP, and literature on thermal treatment processes revealed that while energy input (e.g. heat) was required for GAP degradation of PFOA in water, a lower energy barrier closer to that predicted by thermodynamics was present with GAP treatment than with conventional thermal treatments, and therefore mineralization was improved. It was hypothesized that plasma reactive species such as hydrated electrons (e⁻_[aq]) and reactive oxygen and nitrogen species (RONS), though unable to accelerate an endothermic reaction alone, likely served as catalysts for PFOA mineralization, helping to lower the energy barrier. To explore the hypothesis that plasma reactive species play an important role in PFAS mineralization via air GAP and to develop a better understanding of the degradation mechanism, experiments were conducted scavenging aqueous reactive species (e⁻_[aq] and RONS) and gas-phase charged particles during treatment of PFOA, perfluorooctane sulfonic acid (PFOS), or 6:2-fluorotelomersulfonic acid (6:2 FTS)-contaminated water (individually) via air GAP. Based on results from these scavenging experiments, degradation products, and previous literature on PFAS degradation pathways and GAP treatment of PFAS-contaminated water, several dominant pathways were hypothesized: (1) thermal mineralization initiated by gas-phase charged particles, (2) H/F exchange driven by free electrons (e⁻) at the plasma-water interface or e⁻_[aq] at the plasma-water interface/in the bulk liquid, and (3) fragmentation via charge transfer from gas-phase charged particles. While the significance of gas-phase charged particles to PFAS degradation in water during non-equilibrium plasma treatment has been hypothesized and modeled, this work provided the first experimental evidence of their role. To close, several factors were used to evaluate the feasibility of GAP as a destructive treatment method for PFAS-contaminated water. Treatment of PFOA and PFOS-contaminated water (individually) with air and argon GAP revealed that, while argon achieved more energy efficient degradation, both gases were capable of PFOA and PFOS degradation and generated a similar distribution of products. Generally, E_[EO] for achieving 90% PFOA and PFOS degradation, respectively, was 505 and 461 kWh/m³ in argon and 2,270 and 1,190 kWh/m³ in air. Based on fluorine mass balance results, similar energy requirements are expected for 90% defluorination (i.e. mineralization). Based on the volume of gas required for treatment and the cost of argon, it was concluded that air GAP is more feasible for large-scale treatment and that future optimization studies should focus on lowering the energy required to generate GAP in air. Elevated alkalinity, elevated salt content, and presence of toluene had little effect on PFAS degradation and fluoride production when treating mixtures of 6:2 FTS, PFOA, and PFOS. Further, efficiency of PFOA degradation was generally retained during a scale-up study. Finally, it was hypothesized that the relationship between plasma enthalpy and PFAS degradation/fluoride production rates might be a useful metric for future optimization efforts. While additional work is required to optimize air GAP for treatment of PFAS-contaminated water, results from this study demonstrate that it is a feasible, robust, and scalable method for this application. The findings of this work warrant a variety of future studies. Foremost, optimization of air GAP, including enhancing the efficiency of generating GAP discharge in air and exploring alterations to the reactor, is needed. Further, a closed version of the GAP liquid treatment system, which would make use of argon as the plasma gas more feasible, should be constructed and tested. Optimal waste stream (i.e., concentrated vs. environmentally relevant PFAS levels) for air GAP treatment should also be evaluated. Finally, water quality and safety post-treatment with air GAP should be assessed.