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Perfluorinated Organic Compounds (PFCs) Degradation using Non-Thermal Plasma Enhanced by Boron Doped Graphene Oxide as Catalyst

Periodic Reporting for period 1 - PFCsByPlasCat (Perfluorinated Organic Compounds (PFCs) Degradation using Non-Thermal Plasma Enhanced by Boron Doped Graphene Oxide as Catalyst)

Reporting period: 2020-09-01 to 2022-08-31

Perfluorinated organic compounds (PFCs) are man-made chemicals which have spread in the environment, causing major concern nowadays due to their persistency, bioaccumulation and ascertained or suspected effects on human health. In 2019 US EPA extended the focus of regulatory measures to include both per- and polyfluoroalkyl substances, indicated by the acronym PFAS. PFAS are considered to be one of the major environmental pollution risks to humans and the biota of the 21st century. Perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) are the most common PFAS found in drinking water supplies and groundwater. In 2019 they were included in Annex A of the Stockholm Convention, which lists chemicals prohibited for use and production. The European Parliament and Council have established a maximum limit for total PFAS in water intended for human consumption of 100 ng/L in Directive 2020/2184/EU. In addition, PFOA and PFOS are included in the emerging compounds list by USEPA, which has recently lowered their health advisory levels in drinking water to 0.004 ppt and 0.02 ppt, respectively. Traditional water treatment stages are ineffective towards PFAS contaminants. Current methods for decontamination of PFAS polluted water involve their physical removal by adsorption on activated charcoal, a procedure which generates new PFAS-containing wastes, additional associated disposal costs and environmental issues. Therefore, searching for new treatment technologies capable of degrading PFOA and PFOS is essential, especially based on advanced oxidation/reduction processes (AORPs). Among the approaches being considered, cold plasma stands out for its unique capabilities of generating in situ short-lived, very reactive oxidizing and reducing species such as electrons and the hydroxyl radical. The overall research objective of the PFCsByPlasCat project consists in studying and developing a process based on the combination of cold plasma and nanocatalysts (plas+cat process) for treating PFAS-contaminated water.
The degradation of PFAS in water was studied at first by using only cold plasma, then the possible enhancement by adding to the system boron-doped graphene oxide, graphene oxide and reduced graphene oxide, selected as catalysts, was investigated. All catalysts were characterized before and after the plasma treatment by various techniques such as XRD, SEM-EDS, AFM and FT-IR. During the study, it was observed that plasma produced by a multipin self-pulsed discharge (MSPD) was the most effective for obtaining a synergistic effect with the catalysts. A new reactor was then designed in which the MSPD discharge is generated both at the bottom and on the surface of the treated water (Double MSPD), with the purpose of decomposing all the products formed during PFAS degradation, including short-chain compounds with no surfactant properties. The degradation kinetics studies were carried out with different water matrixes, gas atmospheres, PFAS concentrations, and catalyst concentrations. Overall, use of boron-doped graphene oxide was the most successful, leading to a complete degradation in the first minute of treatment. The results of the project were presented at five international conferences, in three seminars (two in universities, one in public organization) and in three dissemination activities. Moreover, I attended the European Researcher Night as a public engagement event.
The major progress points beyond the project's state-of-the-art are: (1) The development of a plas+cat process for PFAS degradation using graphene oxide-based catalysts. (2) The identification of the most suitable type of discharge producing a plasma able to activate the catalyst. During the project, it was noticed that shorter-chain perfluorinated acids, formed as products, especially perfluorobutanoic acid (PFBA), are harder to degrade than the precursor PFOA. PFBA can be present in the water to be treated from two main sources: it is formed as a product of the degradation of long-chain PFASs, and it is used in industry as an alternative to PFOA. It has been proven to be more durable in the environment and to have more toxic effects than long-chain analogues. PFBA pollution in the environment is becoming increasingly serious, making it urgent to develop effective degradation technologies. PFBA was treated with three different plasma reactors and the results were compared with those obtained by combining plasma with catalysts (graphene-based catalyst, nano zero valent iron, titanium dioxide-based catalysts). The study's results not only confirm the extreme inertia of PFBA, but also provide clues for improving and adapting the experimental set-up and conditions optimized so far for the treatment of PFOA and other long-chain PFAS, in such ways as to achieve optimal performance for the degradation of any type of PFAS.
Summary of PFCsByPlasCat