FOCUS4PFAS · Functionalized low-cost graphene sponge electrodes for sustainable water treatment and complete defluorination of per- and polyfluoroalkyl substances (PFAS) FOCUS4PFAS

Functionalized low-cost graphene sponge electrodes for sustainable water treatment and complete defluorination of per- and polyfluoroalkyl substances (PFAS) FOCUS4PFAS

PFAS have been used since the 1940s and are known as “forever chemicals” due to their extreme persistency to advanced water and wastewater treatment strategies. Due to the strength of the C-F bond, each released molecule of PFAS remains in the environment. There are more than 4,700 PFAS-related CAS numbers identified in the Organization for Economic Cooperation and Development (OECD) global database, nearly all of them being extremely resistant to environmental and metabolic degradation, and with high bioaccumulation potentials and toxicities. The common feature of most PFAS is the presence of one or more perfluoroalkyl groups, (CnF2n+1), with the rest of the molecule either partially (poly-) or fully (per-) fluorinated. Long-chain PFAS such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) have been gradually phased out under REACH regulation and replaced with the shorter-chain homologues and more polar PFAS with fewer fluorinated carbons. Although the initial, industry-funded studies suggested the short-chain PFAS as a “harmless alternative”, recent studies demonstrate that the human and environmental health risk from shorter-chain PFAS was significantly underestimated, and that they may have a significant bioaccumulation potential with multiple potential toxic effects, including oxidative stress, immunosuppression and increased risk of cancer. Short-chain PFAS are significantly more mobile in the environment and more difficult to remove from the contaminated water than long-chain PFAS. Due to their high mobility, short-chain PFAS reach water bodies that are of special concern for human exposure, such as drinking water resources. In addition, short-chain PFAS are formed as unintended byproducts during the manufacturing of long-chain PFAS. There is no established (waste)water treatment technology capable of dealing with PFASs and even more so, their short-chain, C2-C6 homologues. Wastewater treatment plant (WWTP) effluents are important sources of PFAS in the environment, especially in the water-scarce Mediterranean areas. Significant research efforts have been directed towards optimization of the existing technologies for the removal of both long- and short-chain PFAS, yet with limited success. PFAS are recalcitrant to ozonation and •OH-based advanced oxidation processes (AOPs) (e.g., UV/H2O2, Fenton) due to the strength of the C–F bonds and the high electronegativity of fluorine. Separation of PFAS from water using ion-exchange resins (IXR), granular activated carbon (GAC) and reverse osmosis (RO) shows satisfactory efficiencies in removing long-chain PFAS but have very limited performance in removing the short-chain PFAS. Furthermore, these non-destructive techniques result in a concentrated PFAS residual that needs further treatment. RO, being our last line of defense against PFAS and other trace contaminants, is cost effective only at large scale due to excessive capital, maintenance, and operating costs. The development of technically simple, low-cost, and sustainable water treatment technologies, capable of energy-efficient PFAS degradation, is a critical challenge to be addressed by water researchers. Although commercial anodes can defluorinate PFAS, they suffer from major limitations – high price, high energy consumption, and formation of toxic chlorinated byproducts. These limitations have been overcome with the recent development of graphene sponge electrodes. In this project, tailoring of the graphene sponge electrodes with the introduction of atomic dopants, functional groups and/or two dimensional (2D) materials were conducted to improve their surface interaction with PFAS through electrosorption and adsorption, and thus enhance the electrocatalytic C-F bond cleavage. To gain insight into the electrooxidation mechanisms of PFAS at the newly developed, functionalized graphene sponge anodes, this project will employ high resolution mass spectrometry (HRMS) to identify any partially defluorinated byproducts formed during the treatment. Focus will be placed on low molecular weight (MW), polar PFAS, including shorter-chain homologues (e.g., C2-C6), which pose a major challenge for any type of advanced water treatment, including electrochemical technology. This project aims at developing graphene sponge anodes tailored for electrochemical degradation of poly- and perfluoroalkyl substances (PFAS), including long- (>C6) and highly polar shorter-chain (C2-C6) homologues, and their complete defluorination and removal from the contaminated water of different origin. Electrooxidation is capable of complete defluorination of PFAS, initiated by the direct electron transfer (DET) from the PFAS molecule to the anode. In addition, electrochemical processes have other major advantages: they do not use chemical reagents - only current, they do not form a residual waste stream, they operate at ambient temperature and pressure, and are easily automated. Electrochemical systems are very well-suited for decentralized and distributed water treatment, for example as point-of-entry (POE) and point-of-use (POU) devices, and can be designed to treat varying types of contaminated water, from tap water, groundwater, to municipal and industrial wastewater. Nevertheless, electrochemical water treatment systems are struggling to be applied at a wider-scale due to major limitations of the existing electrode materials (i.e., boron-doped diamond (BDD), mixed metal oxide (MMO), and Ti4O7): (i) high energy consumption of the treatment, due to the low electrode surface area of the pricey commercial electrodes, and (ii) rapid oxidation of Cl− ions to free chlorine (HOCl/OCl−), which reacts with the organic matter to form chlorinated organic byproducts. In the case of high-oxidizing power anodes typically capable of PFAS degradation (i.e., BDD, Ti4O7), Cl− is further oxidized to toxic and persistent chlorate (ClO3−) and perchlorate (ClO4−). All commercial anodes perform poorly in removing the short-chain PFAS (>C6) due to their limited interaction with the anode surface. These limitations can be addressed by the low-cost graphene sponge electrodes, which have been recently developed and patented at the ICRA institute. Graphene-based sponges are produced using a scalable, bottom-up approach that allows easy introduction of dopants into the reduced graphene oxide (RGO) coating. Graphene sponge anode is electrochemically inert to chloride, as there is no chlorine, ClO3− and ClO4− formation even at high anodic currents applied; in brackish water (20 mM NaCl), the current efficiency for Cl2 production at 173 A m-2 of anodic current density was only 0.04%. At the same time, graphene sponge electrodes form strong oxidant species (•OH, O3, H2O2) and are electrocatalytically active for the degradation of persistent organic and microbial contaminants, while maintaining excellent stability in both anodic and cathodic polarization due to the covalent C-Si and C-O bonding between the RGO coating and SiO2, a major component of the mineral wool template used in their production. Although the observed efficiencies of C-F bond cleavage were relatively low (i.e., 13-23%), this was achieved in a low conductivity supporting electrolyte (1 mS cm-1) and in one-pass, flow-through mode. Electrochemical cleavage of the C-F bond without producing toxic chlorinated byproducts opens a plethora of possibilities for the treatment of not only PFAS-contaminated water with the direct impact on humans (e.g., drinking water, groundwater, surface water), but also complex, PFAS-rich brackish streams (e.g., landfill leachate, RO brine) for which electrochemical treatment using commercial anodes cannot be employed due to up to >200-fold increase in toxicity of the treated effluent. The cost of the developed graphene sponge electrodes is estimated at €46 per m2 of the projected surface area, which makes them extremely cost-competitive compared with the state-of-the art BDD anodes with an approximate cost of €6,000 per m2. Driven by the need for overcoming the limitations of the existing advanced water treatment strategies (RO, AOPs) and enabling a sustainable PFAS remediation technology, this project aims at developing graphene sponge anodes tailored to achieve a more efficient and complete defluorination of not only long-chain PFAS, but more importantly of their more polar and short-chain homologues. Enhanced electrosorption and adsorption of PFAS at the newly developed graphene sponge anode will improve their subsequent electrochemical degradation, as the initial DET step from the PFAS headgroup (e.g., carboxylic, sulfonate) to the anode is the rate-limiting step for the process. The results of this research will contribute towards wider-scale implementation of electrochemical processes for the degradation of emerging and highly persistent contaminants in water. The main research objectives are: • Identification of the key properties of modification/doping of graphene sponge electrodes that enhance electrosorption and electrocatalysis of PFAS, with the focus on shorter-chain (<C6) homologues and highly polar PFAS recently introduced in the industry. • Study of the impact of common water constituents on the electrooxidation of PFAS and optimization of the operational parameters (e.g., flowrate, current, potential) for different treatment scenarios. • Structural elucidation of the transformation products (TPs) of PFAS and definition of the electrooxidation pathways. • Proof-of-concept studies with optimized reactor design and operation, treating PFAS-contaminated water of different origin (e.g., tap water, groundwater, municipal wastewater, reverse osmosis (RO) brine)

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