Electrochemical Mineralization of Per- And Polyfluoroalkyl Substances
Per- and polyfluoroalkyl substances (PFAS) are fluorinated organic compounds with broad applications in aqueous film-forming foams (AFFF) for firefighting, lubricants, waterproof and stain-resistant products. Perfluorooctanoic acid (PFOA), a toxic and carcinogenic PFAS, is replaced by GenX. Nevertheless, PFOA is expected to be present in the environment for an extended period after its phasing out due to its recalcitrant nature. In addition, GenX is predicted to have similar toxicity as PFOA. Various PFAS, including PFOA and GenX, have been widely detected in surface water and groundwater in the United States and worldwide. The current treatment practice for PFAS fails to provide a permanent solution and is likely to increase the risk of recontamination of surface water and groundwater. Among various destructive methods, electrochemical mineralization, which uses electric power to transform PFAS into bicarbonate and fluoride, is a promising option. However, past studies on electrochemical mineralization of PFAS have limitations such as low PFAS mineralization and incomplete fluorine mass balance. This study focused on addressing these issues and examined the treatment performance using PFOA, GenX, and AFFF waste streams as examples. The first task of the study was to identify the suitable anode material and achieve complete fluorine mass balance for electrochemical mineralization, using PFOA as the example compound. Boron-doped diamond (BDD) was selected as the best out of the three anode materials tested (Ebonex Plus, Ti/RuO2, and BDD) based on its PFOA degradation efficiency and life span. In a batch study conducted at 20 mA/cm2, more than 80% PFOA degradation and complete fluorine mass balance were achieved. In addition to screening suitable anode materials, the reactors used for electrochemical mineralization were also continuously redesigned during the study, and multiple issues were identified and fixed to achieve the best performance. The second task of this study was to assess the electrochemical mineralization of GenX using boron-doped diamond electrodes in two different reactor types: continuous reactors with recirculation and batch reactors. Experiments using the continuous reactors with recirculation were carried out at 10, 20, 25, and 30 mA/cm2. Based on GenX degradation and defluorination ratio, 20 mA/cm2 was selected as the optimum current density. At this current density, GenX degradation, defluorination ratio, and mass balance of 65%, 14%, and 70% were achieved, respectively. In batch reactor studies conducted at 20 mA/cm2, 1M NaOH was found best for fluoride capture and methanol for organic fluorine capture. The third task of this study was to explore the electrochemical mineralization of PFAS in simulated AFFF waste streams from firefighting practice. Batch studies on electrochemical mineralization of two AFFF solutions were conducted at 20 mA/cm2. Utilizing the total oxidable precursor assay and targeted analysis, the degradation of perfluoroalkyl acids and their precursor was evaluated. The two tested AFFF solutions achieved 25-42% fluorine mass balance based on TOP assay and fluoride analysis. This research serves as a guide for future electrochemical research by providing optimal reactor design and suitable analyses. Particularly, results from this study demonstrated the significance of PFAS loss in aerosol forms generated during the electrochemical process and possible solutions to avoid such loss by capturing and recirculating aerosols to achieve high PFAS degradation and complete fluorine mass balance.