Per and polyfluoroalkyl substances (PFAS) are a diverse set of organofluorine surfactants that are of great concern as emerging chemicals of concern due to their potential human and ecosystem health risks. PFAS are persistent in the environment and are recalcitrant to degradation by many traditional remediation strategies. The overarching goal of this project is to determine the feasibility of applying reverse vortex gliding arc plasmatron (GAP) and dielectric barrier discharge (DBD) non-thermal (“cold”) plasma technologies for the treatment of PFAS-impacted liquids and solids, respectively, in investigation-derived wastes (IDW).
This project is being conducted in two phases. The objective of the Phase I, proof-of-concept project was to demonstrate the application of non-thermal plasma technologies for the treatment of PFAS from IDW. Specifically, this project focused on the degradation of perfluoroalkyl sulfonates (PFSAs), perfluoroalkyl carboxylic acid (PFCAs), and fluorotelomer sulfonates (FtSs) by non-thermal plasma. The recent findings from Phase I demonstrated that these technologies are capable of rapid degradation of PFAS in water and solids. Results from Phase I can be found in the Phase I Final Report and are summarized below in the Interim Results section.
In an effort to further advance knowledge of these non-thermal plasma treatment technologies and evaluate their potential for full-scale application, the research for Phase II aims to achieve the following objectives:
- Objective 1: Develop a thorough understanding of the mechanisms involved and identify the transformation products formed during non-thermal plasma degradation of PFAS in liquid and solid matrices, respectively;
- Objective 2: Examine the effects of co-occurring chemicals, solution chemistry of impacted liquids, and the characteristics of impacts solids on non-thermal air plasma treatment of PFAS;
- Objective 3: Perform testing of GAP and DBD systems for continuous flow-through non-thermal air plasma treatment of impacted liquids and solids to generate data that will assist in the designing, engineering, and planning of these technologies for full-scale treatment;
- Objective 4: Estimate the capital and operational expenses (CAPEX and OPEX) for full-scale flow-through demonstration of the GAP and DBD systems.
Under Phase I, PFAS were tested for degradation by non-thermal plasma. A laboratory-scale GAP reactor was adapted for treatment of PFAS from liquid solutions to reach the greatest destruction of PFAS while minimizing energy consumed. For treatment of solid materials laden with PFAS, a laboratory-scale DBD reactor was constructed. To measure success of treatment, an “in-house” reverse phase liquid chromatography with suppressed ion conductivity detection (RPLC-IC) method was developed and validated for measuring the concentration of targeted perfluoroalkyl acids (PFAAs) and FtSs.
The degradation of PFAS was characterized by assessing the removal of PFAS and the generation of fluoride ions (F-) under different plasma regimes by adjusting the electronics (i.e., applied voltage and current) and changing the plasma gas (i.e., air, O2, N2). Approximate energy costs associated with treatment of perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) by GAP and DBD reactors were calculated. Byproducts of the degradation were quantified and detected by using an ELAP certified laboratory to analyze samples from the treatment processes. IDW was obtained as both soil and liquid and treated in either the GAP or DBD reactor to understand how treatment efficiency would be affected.
The Phase II project will continue work from the Phase I effort to further elucidate the mechanisms and pathways involved in the degradation of PFAS in liquids and sorbed onto solids in GAP and DBD systems, respectively. For GAP treatment of PFAS in liquids, these mechanistic studies will either be completed in the reactor built in the previous project, or in a newly designed, smaller reactor that is more suitable for mechanistic studies. In the existing reactor, the contribution of reactive oxygen and nitrogen species (RONS) in the bulk liquid versus reaction mechanisms where the plasmatron interacts with the liquid surface will be studied by manipulating the recirculation rate; in the new reactor, the role of hydrated electrons and RONS will be evaluated using fluorescent probes to determine their concentrations and by targeted quenching. For DBD treatment of PFAS-impacted solids, mechanistic studies will be completed using the reactor built in the previous project. The degradation of PFAS in this reactor using different gases (e.g., N2, O2, air, and Ar) will be studied to deduce the major mechanisms. This project also will identify degradation products formed during treatment in these non-thermal plasma systems using quantitative assessment of loss of the parent compound (relative to an appropriate control), quantitative or qualitative assessment of transformation products, and evaluation of mass balances for fluorine and total PFAS. Data for this assessment will be provided using total oxidizable precursor (TOP) assay for the evaluation of the cumulative PFAS concentration, suspect screening and nontarget analysis for semi-quantitative and qualitative analysis, measurement of the amount of fluoride generated to close the fluorine mass balance, and measurement of the total organofluorine (TOF) content to close the fluorine mass balance. The effects of co-occurring chemicals, solution chemistry, and solid characteristics on PFAS degradation in these non-thermal plasma systems will also be studied. This analysis will include the assessment of the inhibitory effects of toluene, benzo-a-pyrene, and common chlorinated solvent, as well as the evaluation of the effects of bulk water quality parameters (e.g., pH) on PFAS transformation kinetics by GAP and varying soil characteristics on PFAS transformation kinetics by DBD. Lastly, this project will evaluate scaled up semi-continuous and/or continuous GAP and DBD systems for the destruction of PFAS in liquids and solids, respectively, and develop design and cost estimates of these scaled systems.
Under Phase I, this project demonstrated the promising potential of GAP and DBD non-thermal plasma technologies for the respective treatment of PFAS-impacted water and solids. Transformation of PFAS by air plasma was observed in both the GAP and DBD laboratory-scale reactor. Greater than 90% removal was achieved within 60 minutes of treatment in the GAP system for a significant number of PFAS tested. The percent degradation and defluorination of the tested PFAS in the GAP system tended to decrease with shorter chain length PFAAs and FtSs, but minimal differences in the extent of degradation was observed among PFCAs, PFSAs, and FtSs with identical perfluoroalkyl chain lengths. During the treatment of PFOS, PFOA, and mixtures of PFAS in representative samples in the GAP and DBD systems, shorter chain PFAAs were generated. Preliminary estimates on the amount of energy required to achieve a desired amount of degradation was determined for PFOS and PFOA in the GAP system and was found to be on par with other non-thermal plasma methods (150 kJ/L to 1500 kJ/L) and approximately five times less than the amount of energy required to evaporate water (~3000 kJ/L).
The Phase I effort demonstrated that GAP and DBD non-thermal plasma technologies for the treatment of PFAS-impacted water and solids are promising in both effectiveness and energy efficiency. The work described in this project will advance and optimize these treatment technologies for removal of PFAS from matrices of greatest liability to the DoD. It will also determine whether this treatment technique will be efficient and economically viable for fullscale application. (Anticipated Phase II Completion - 2023)
Lewis, A., T. Joyce, M. Hadaya, F. Ebrahimi, I. Dragiev, N. Giardetti, J. Yang, G. Fridman, A. Rabinovich, A.A. Fridman, E.R. McKenzie, and C.M. Sales. 2020. Rapid Degradation of PFAS in Aqueous Solutions by Reverse Vortex Flow Gliding Arc Plasma. Environmental Science: Water Research & Technology. DOI: 10.1039/C9EW01050E.