Facing the Challenge of Poly- and Perfluoroalkyl Substances in Water: Is Electrochemical Oxidation the Answer?

Abstract

Electrochemical treatment systems have the unique ability to completely mineralize poly-and perfluoroalkyl substances (PFASs) through potential-driven electron transfer reactions. In this review, we discuss the state-of-the-art on electrooxidation of PFASs in water, aiming at elucidating the impact of different operational and design parameters, as well as reported mechanisms of PFAS degradation at the anode surface. We have identified several shortcomings of the existing studies that are largely limited to smallscale laboratory batch systems and unrealistic synthetic solutions, which makes extrapolation of the obtained data to real-world applications difficult. PFASs are surfactant molecules, which display significant concentrationdependence on adsorption, electrosorption, and dissociation. Electrooxidation experiments conducted with high initial PFAS concentration and/or in high conductivity supporting electrolytes likely overestimate process performance. In addition, the formation of organohalogen byproducts, chlorate and perchlorate, was seldom considered. Nevertheless, the first step toward advancing from laboratory-scale to industrial-scale applications is recognizing both the strengths and limitations of electrochemical water treatment systems. More comprehensive and rigorous evaluation of novel electrode materials, application of scalable proof-of-concept studies, and acknowledgment of all treatment outputs (not just the positive ones) are imperative. The presence of PFASs in drinking water and in the environment is an urgent global public health issue. Developments made in material science and application of novel threedimensional, porous electrode materials and nanostructured coatings are forging a path toward more sustainable water treatment technologies and potential chemical-free treatment of PFAS-contaminated water.

Electrochemical technologies for per‐ and polyfluoroalkyl substances mitigation in drinking water and water treatment residuals

“…Electrochemical treatment is capable of destroying per-and polyfluoroalkyl substances, but future research should reflect more realistic drinking water sources. recently started to be evaluated in the context of drinking water treatment (Chaplin, 2019;McBeath, Mohseni, & Wilkinson, 2020;Radjenovic et al, 2020;Ryan et al, 2021), improved understanding of the performance of electrochemical processes for water treatment is needed to assess their feasibility for use in the water sector.…”

“…Of note, prior electrode materials such as SnO 2 and PbO 2 may not have feasible application for drinking water treatment due to the potential risk of secondary metal contamination (such as lead) which is recommended to be excluded from water infrastructure. DSA electrodes (also described as mixed metal oxide in the literature) are generally not feasible for PFAS electrooxidation compared to boron‐doped diamond due to poor anode stability when operating at high current densities / potentials required for PFAS mitigation (Radjenovic et al, 2020).…”

“…Nondestructive treatment technologies such as granular activated carbon, ion exchange, nanofiltration, and reverse osmosis can remove PFAS. However, nondestructive technology can be limited by the presence of PFAS in concentrated waste streams resulting from treatment (e.g., ion exchange regenerant, membrane concentrate/backwash, and reverse osmosis concentrate), which may require additional treatment before the waste streams are discharged (Radjenovic et al, 2020; Stoiber et al, 2020). For example, Glover et al (2018) measured 400 ng/L total PFAS in ultrafiltration and nanofiltration membrane backwash water and 600–1800 ng/L in reverse osmosis concentrate.…”

“…Electrochemical treatment may also be applied at various points in the drinking water treatment train, such as after particle separation or for treating the water treatment residuals. Electrocoagulation and electrooxidation have primarily been studied in the context of wastewater, contaminated groundwater, and fundamental electrolyte matrices (i.e., buffered matrices free of scavengers and interfering parameters) While they have recently started to be evaluated in the context of drinking water treatment (Chaplin, 2019; Garcia‐Segura, Nienhauser, et al, 2020; McBeath, Mohseni, & Wilkinson, 2020; Radjenovic et al, 2020; Ryan et al, 2021), improved understanding of the performance of electrochemical processes for water treatment is needed to assess their feasibility for use in the water sector.…”

Novel Niobium-doped Titanium Oxide Towards Electrochemical Destruction of Forever Chemicals

“…10 EAOP have garnered intense interest due to the utilization of an electromotive force to induce ROS generation, which provides an attractive pathway for destroying recalcitrant PFAS contaminants. 10,11,12,13,14 The key advantage of EAOP in destroying PFAS lies in its environmental compatibility, being based on electricity rather than additional chemicals that are themselves potentially toxic or require subsequent removal. 10,11,15 The underlying mechanism for the destruction of PFAS is governed by the electrochemical production of hydroxyl free radicals (  OH) as a byproduct of water oxidation at the anode surface, which then react unselectively with a myriad of recalcitrant organic contaminants in water (Equation 1): 16 H2O →  OH + H + + e …”