Electrochemical oxidation processes for PFAS removal from contaminated water and wastewater: fundamentals, gaps and opportunities towards practical implementation

Advances in PFAS electrochemical reduction: Mechanisms, materials, and future perspectives

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals that pose significant risks to both human and environmental health due to their widespread use and stability. Traditional remediation methods, such as adsorption and filtration, concentrate PFAS without breaking them down. Alternative methods, such as pyrolysis, chemical oxidation, and photodegradation, often require costly and complex conditions. Electrochemical technology is a promising alternative for PFAS removal. In particular, electrochemical reduction has been emerging in recent years as a promising alternative to promote C-F dissociation and H/F exchange reactions, thus generating less fluorinated compounds. This review summarizes the advances in technologies for PFAS electrochemical reduction, with proposed electrochemical reduction mechanisms, the factors that influence the removal of PFAS, and the challenges and future directions associated with these methods. Novel materials, such as nanocatalysts, molecularly inspired networks, or 2D/3D materials, are stable in aqueous environments and exhibit high electrochemical activity toward C-F bond dissociation. In addition, the above materials show potential for scalable applications in PFAS treatment, although further research is needed to optimize their performance. This review also aims to understand the opportunities and challenges in PFAS electrochemical reduction, offering insights for future research and development.

Robust titanium suboxide anodes doped by sintering enhance PFOS degradation in water

Per- and Polyfluoroalkyl Substances (PFAS) are a class of persistent organic pollutants that are ubiquitous in the environment, while PFOS is one most representative PFAS of extraordinary persistence. Electrochemical oxidation (EO) is promising for destructive treatment of PFAS in water, and Magnéli phase titanium suboxide (TSO) is regarded as one of only few suitable anode materials for this application. We herein conducted an in-silico survey with Density Functional Theory (DFT) simulations to identify possible beneficial dopant elements, and then prepared TSO anodes doped with Niobium (Nb-TSO) or Cerium (Ce-TSO) by sintering. The doped TSO thus prepared exhibited great robustness, having service lifetimes longer than the pristine Ti4O7 anode, making them useful for EO applications in PFAS treatment. PFOS degradation by EO using Nb-TSO anode was faster than that on the pristine Ti4O7 anode, with energy consumption approximately 1.8 times lower. Further characterizations and DFT simulations reveal that the enhanced efficiency of Nb-TSO anode is attributed to its reduced charge transfer resistance and increased effective electroactive surface area (EESA). The EESA of the Ce-TSO anode was reduced in comparison to the pristine Ti4O7, but PFOS degradation rates normalized by EESA were increased significantly for EO with Ce-TSO anode, due to its increased oxygen evolution potential (OEP) and hydroxyl radical production. The doped TSO anodes prepared in this study by sintering will be useful in EO treatment of PFAS-contaminated waters, with improved service life and performance, and the study provides understandings to guide further improvements of the TSO anodes via doping.

Per-and polyfluoroalkyl degradation in a hybrid dielectric barrier discharge plasma and electrooxidation system through involving more reactive species by air and water circulation

The presence of PFAS in water matrices has become a global environmental issue in the last half-century. Dielectric barrier discharge (DBD) and electrooxidation (EO) showed potential for PFAS degradation but have yet to find practical application due to relatively high energy consumption. In this study, a hybrid DBD-EO system for efficient degradation of PFAS was developed by involving more reactive oxygen, sulfate radicals (SO4•-) and nitrogen species (RONS). The results showed that using the hybrid DBD-EO system under optimal conditions (applied voltage = 6 kV and current density = 7.5 mA/cm2) could increase PFOA degradation efficiency from 65.0 % (DBD) and 62.5 % (EO) to 89.14 %. While the EE/O decreased from 67.0 kWh/m3 (DBD) and 47.82 kWh/m3 (EO) to 21.61 kWh/m3. In addition, the effect of operational parameters and water matrices revealed that the hybrid DBD-EO system had high potential for PFOA removal from water under various conditions. According to the EPR and DFT calculation results, integration of reactive species in EO (SO4•-, •OH, O2•-) and ONOOH) and DBD (•OH, O2•-, NO2•-, 1O2 and ONOOH) processes in the DBD-EO system led to efficient degradation of PFOA through a mechanism of decarboxylation/defluorination cycle. Our findings suggested the combination of DBD and EO is a promising approach for complete degradation of PFAS from water with low energy consumption and minimal environmental side effects.

Optimizing electrochemical removal of perfluorooctanoic acid in landfill leachate using ceramic carbon foam electrodes by coupling CFD simulation and reactor design

Perfluorooctanoic acid (PFOA), a persistent and bioaccumulative pollutant classified as a 'forever chemical', poses a global environmental and health risk due to its widespread use and resistance to degradation. The development of effective and efficient removal technologies is crucial to mitigate its long-term impacts. In this study, we present a novel approach to address the growing concern of emerging contaminants, particularly PFOA, in landfill leachate. We investigate the use of ceramic carbon foam electrodes (CCFE) as a cost-effective and efficient alternative to traditional electrode materials for the electrochemical degradation of PFOA. Computed microtomography was used to reconstruct the actual three-dimensional geometries of the samples from which porosities were calculated. We also coupled computational fluid dynamics simulations of the actual geometries and optimized the working conditions to minimize pressure drop and improve treatment efficiency. Our design significantly reduces energy requirements by operating at lower potentials, a critical factor in sustainable waste management practices. The optimized CCFE system demonstrated superior performance in the degradation of PFOA in landfill leachate, offering a promising solution for the treatment of emerging contaminants. This study not only provides a viable method for mitigating the environmental impact of PFOA but also sets a precedent for the development of low-energy, high-efficiency treatment technologies for various persistent pollutants. In addition, the proposed solution, as part of closed-loop water systems, will enhance water reuse and recycling, thereby preserving and regenerating natural water bodies.

PFAS in landfill leachate: Practical considerations for treatment and characterization

Per- and polyfluoroalkyl substances (PFAS) are widely used in consumer products and are particularly high in landfill leachate. The practice of sending leachate to wastewater treatment plants (WWTPs) is an issue for utilities that have biosolids land application limits based on PFAS concentrations. Moreover, landfills may face their own effluent limit guidelines for PFAS. The purpose of this review is to understand the most appropriate treatment technology combinations for mitigating PFAS in landfill leachate. The first objective is to understand the unique chemical characteristics of landfill leachate. The second objective is to establish the role and importance of known and emerging analytical techniques for PFAS characterization in leachate, including quantification of precursor compounds. Next, an overview of technologies that concentrate PFAS and technologies that destroy PFAS is provided, including fundamental background content and key operating parameters. Finally, practical considerations for PFAS treatment technologies are reviewed, and recommendations for PFAS treatment trains are described. Both pros and cons of treatment trains are noted. In summary, the complex matrix of leachate requires a separation treatment step first, such as foam fractionation, for example, to concentrate the PFAS into a lower-volume stream. Then, a degradation treatment step can be applied to the concentrated PFAS stream.

Ab Initio Investigation of Per- and Poly-fluoroalkyl Substance (PFAS) Adsorption on Zerovalent Iron (Fe0)

In this study, dispersion-corrected density functional theory (DFT) calculations were employed to investigate the adsorption of per- and poly-fluoroalkyl substances (PFAS) onto zerovalent iron (Fe0). The main objective of this investigation was to shed light on the adsorption properties, including adsorption energies, geometries, and charge transfer mechanisms, for four PFAS molecules, namely, perfluorooctanesulfonic acid (PFOS), perfluorobutanesulfonic acid (PFBS), perfluorooctanoic acid (PFOA), and perfluorobutanoic acid (PFBA), on the most thermodynamically accessible Fe0 surface facets. Additionally, the DFT investigation examined the role of PFAS chain length, functional group, protonation/deprotonation state, and solvation in water in their adsorption to Fe0. Overall, the adsorption of the four PFAS molecules on various Fe0 surfaces exhibited thermodynamically favorable energetics. Nevertheless, solvation in water resulted in less exothermic adsorption energies, and the presence of preadsorbed oxygen blocked the Fe0 surface, preventing PFAS adsorption. Additionally, the inclusion of a monolayer of Ni on top of the Fe0 surface reduced the stability of PFAS adsorption compared to pristine Fe0. Results of the computational investigation were compared to experimental results from the literature for qualitative validation.

Emerging eco-friendly technologies for remediation of Per- and poly-fluoroalkyl substances (PFAS) in water and wastewater: A pathway to environmental sustainability

Per- and polyfluoroalkyl substances (PFAS) are rampant, toxic contaminants from anthropogenic sources, called forever chemicals for their recalcitrance. Although banned in several parts of the world for public health implications, including liver, kidney, and testicular diseases, PFAS are abundant in water sources due to easy dispersion. With chemical properties resulting from strong hydrophobic bonds, they defile many physicochemical removal methods. Though adsorption processes such as granular activated carbon (GAC) are widely used, they are marred by several limitations, including cost and secondary contamination. Thus, eco-friendly methods involving a synergy of the removal principles have been preferred for ease of use, cost-effectiveness, and near-zero effect on the environment. We present novel eco-friendly methods as the solution to PFAS remediation towards environmental sustainability. Current eco-friendly methods of PFAS removal from water sources, including electrocoagulation, membrane/filtration, adsorption, and phytoremediation methods, were highlighted, although with limitations. Novel eco-friendly methods such as microbial fuel cells, photoelectrical cells, and plasma treatment offer solutions to PFAS remediation and are quite efficient in terms of cost, result, and environmental sustainability. Overall, the successful integration of eco-friendly techniques in a seamless manner ensures the desired result. We also present a balanced position on the ecosystem impact of these ecofriendly methods, noting the successes towards environmental sustainability while exposing the gaps for further research.

Tackling Challenges of Long-Term Electrode Stability in Electrochemical Treatment of 1,4-Dioxane in Groundwater

Electrochemical advanced oxidation is an appealing point-of-use groundwater treatment option for removing pollutants such as 1,4-dioxane, which is difficult to remove by using conventional separation-based techniques. This study addresses a critical challenge in employing electrochemical cells in practical groundwater treatment─electrode stability over long-term operation. This study aims to simulate realistic environmental scenarios by significantly extending the experimental time scale, testing a flow-through cell in addition to a batch reactor, and employing an electrolyte with a conductivity equivalent to that of groundwater. We first constructed a robust titanium suboxide nanotube mesh electrode that is utilized as both anode and cathode. We then implemented a pulsed electrolysis strategy in which reactive oxygen species are generated during the anodic cycle, and the electrode is regenerated during the cathodic cycle. Under optimized conditions, single-pass treatment through the cell (effective area: 2 cm2) achieved a remarkable 65-70% removal efficiency for 1,4-dioxane in the synthetic groundwater for over 100 h continuous operation at a low current density of 5 mA cm-2 and a water flux of 6 L m-2 h-1. The electrochemical cell and pulse treatment scheme developed in this study presents a critical advancement toward practical groundwater treatment technology.

Low-Temperature Mineralization of Fluorotelomers with Diverse Polar Head Groups

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental pollutants linked to harmful health effects. Currently employed PFAS destruction methods are energy-intensive and often produce shorter-chain and recalcitrant partially fluorinated byproducts. We report the mineralization of five fluorotelomer compounds via a base-mediated degradation using NaOH and mild temperatures (120 °C) in a mixture of DMSO:H2O (8:1 v/v). The studied fluorotelomers have varying polar head groups-carboxylic acids, sulfonic acids, alcohols, and phosphonic acids, which are the most common polar head groups used in commercial and industrial applications. The degradation intermediates and byproducts were characterized using 1H, 13C, and 19F NMR spectroscopy. Density functional theory computations at the M06-2X/6-311 + G(2d,p)-SMD-(DMSO) level were consistent with the observed intermediates and guided an overall mechanistic hypothesis. Degradation of each fluorotelomer occurs through a similar process, in which the nonfluorinated carbons and the first fluorinated carbon are cleaved from the remaining perfluoroalkyl fragment, which degrades through previously identified pathways. These findings provide important insight into PFAS degradation processes and suggest that PFAS containing at least one C-H bond within or adjacent to its fluoroalkyl chain can be degraded under these mild conditions. Many PFAS in current use as well as recalcitrant fluorinated byproducts generated from other PFAS degradation methods are candidates for this approach.

A review of electrooxidation systems treatment of poly-fluoroalkyl substances (PFAS): electrooxidation degradation mechanisms and electrode materials

The prevalence of polyfluoroalkyls and perfluoroalkyls (PFAS) represents a significant challenge, and various treatment techniques have been employed with considerable success to eliminate PFAS from water, with the ultimate goal of ensuring safe disposal of wastewater. This paper first describes the most promising electrochemical oxidation (EO) technology and then analyses its basic principles. In addition, this paper reviews and discusses the current state of research and development in the field of electrode materials and electrochemical reactors. Furthermore, the influence of electrode materials and electrolyte types on the deterioration process is also investigated. The importance of electrode materials in ethylene oxide has been widely recognised, and therefore, the focus of current research is mainly on the development of innovative electrode materials, the design of superior electrode structures, and the improvement of efficient electrode preparation methods. In order to improve the degradation efficiency of PFOS in electrochemical systems, it is essential to study the oxidation mechanism of PFOS in the presence of ethylene oxide. Furthermore, the factors influencing the efficacy of PFAS treatment, including current density, energy consumption, initial concentration, and other parameters, are clearly delineated. In conclusion, this study offers a comprehensive overview of the potential for integrating EO technology with other water treatment technologies. The continuous development of electrode materials and the integration of other water treatment processes present a promising future for the widespread application of ethylene oxide technology.

Adsorption of per- and poly-fluoroalkyl substances (PFAS) on Ni: A DFT investigation

Electrocatalytic destruction of per- and polyfluoroalkyl substances (PFAS) is an emerging approach for treatment of PFAS-contaminated water. In this study, a systematic ab initio investigation of PFAS adsorption on Ni, a widely used electrocatalyst, was conducted by means of dispersion-corrected Density Functional Theory (DFT) calculations. The objective of this investigation was to elucidate the adsorption characteristics and charge transfer mechanisms of different PFAS molecules on Ni surfaces. PFAS adsorption on three of the most thermodynamically favorable Ni surface facets, namely (001), (110), and (111), was investigated. Additionally, the role of PFAS chain length and functional group was studied by comparing the adsorption characteristics of different PFAS compounds, namely perfluorooctanesulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorobutanesulfonic acid (PFBS), and perfluorobutanoic acid (PFBA). For each PFAS molecule-Ni surface facet pair, different adsorption configurations were considered. Further calculations were carried out to reveal the effect of solvation, pre-adsorbed atomic hydrogen (H), and surface defects on the adsorption energy. Overall, the results revealed that the adsorption of PFAS on Ni surfaces is energetically favorable, and that the adsorption is primarily driven by the functional groups. The presence of preadsorbed H and the inclusion of solvation produced less exothermic adsorption energies, while surface vacancy defects showed mixed effects on PFAS adsorption. Taken together, the results of this study suggest that Ni is a promising electrocatalyst for PFAS adsorption and destruction, and that proper control for the exposed facets and surface defects could enhance the adsorption stability.

Fe2+-NTA synergized UV254 photolytic defluorination of perfluorooctane sulfonate (PFOS): Enhancing through intramolecular electron density perturbation via electron acquisition

Perfluorooctane sulfonate (PFOS) is a persistent organic pollutant posing a risk in environmental persistence, bioaccumulation and biotoxicity. This study was to reach a comprehensive and deeper understanding of PFOS elimination in a UV254 photolytic treatment with the co-presence of Fe2+ and nitrilotriacetic acid trisodium salt (NTA). PFOS defluorination was noticeably enhanced in the UV/Fe2+-NTA treatment compared with UV/NTA, UV/Fe2+ and our previously studied UV/Fe3+ treatments. UV-vis, FTIR, and UPLC/MS-MS results indicated the formation of PFOS-Fe2+-NTA complex in PFOS, Fe2+ and NTA mixture. The transition energy gap of PFOS-Fe2+-NTA decreased below the excitation energy supplied by UV254 irradiation, corresponding with red shift appearing in UV-vis scanning spectrum. This favored intramolecular electron transfer from Fe2+-NTA to PFOS under UV254 irradiation to form electron-accepting PFOS. Molecular electrostatic potential and atom charge distribution analyses suggested electron density rearrangement and perturbation in the perfluorinated carbon chain of electron-accepting PFOS, leading to the decrease in bond dissociation energies. Intermediate products detection suggested the parallel defluorination pathways of PFOS desulfonation, middle carbon chain scission and direct C-F cleavage. NTA exhibited crucial functions in the UV/Fe2+-NTA treatment by holding Fe2+/Fe3+ in soluble form as a chelant and favoring water activation to generate hydrated electrons (eaq-) under UV irradiation as a photosensitizer. Fe2+ acting as the conduit for electron transfer and the bridge of PFOS anion and NTA was thought functioning best at 200 µM in this study. The degree of UV/Fe2+-NTA -synergized PFOS defluorination also depended on eaq- yield and UV254 photon flux. The structure dependence on the electron transfer process of PFOS and PFOA was explored incorporating molecular structure descriptors. Because of possessing greater potential to acquire electrons or less likeliness to donate its electrons than PFOA, PFOS exhibited faster defluorination kinetics in the published "reduction treatments" than "oxidation" ones. Whereas, PFOA defluorination kinetics were at similar level in both "reduction" and "oxidation" treatments.

Application of electron beam technology to decompose per- and polyfluoroalkyl substances in water

The widespread detection of per- and polyfluoroalkyl substances (PFAS) in environmental compartments across the globe has raised several health concerns. Destructive technologies that aim to transform these recalcitrant PFAS into less toxic, more manageable products, are gaining impetus to address this problem. In this study, a 9 MeV electron beam accelerator was utilized to treat a suite of PFAS (perfluoroalkyl carboxylates: PFCAs, perfluoroalkyl sulfonates, and 6:2 fluorotelomer sulfonate: FTS) at environmentally relevant levels in water under different operating and water quality conditions. Although perfluorooctanoic acid and perfluorooctane sulfonic acid showed >90% degradation at <500 kGy dose at optimized conditions, a fluoride mass balance revealed that complete defluorination occurred only at/or near 1000 kGy. Non-target and suspect screening revealed additional degradation pathways differing from previously reported mechanisms. Treatment of PFAS mixtures in deionized water and groundwater matrices showed that FTS was preferentially degraded (∼90%), followed by partial degradation of long-chain PFAS (∼15-60%) and a simultaneous increase of short-chain PFAS (up to 20%) with increasing doses. The increase was much higher (up to 3.5X) in groundwaters compared to deionized water due to the presence of PFAS precursors as confirmed by total oxidizable precursor (TOP) assay. TOP assay of e-beam treated samples did not show any increase in PFCAs, confirming that e-beam was effective in also degrading precursors. This study provides an improved understanding of the mechanism of PFAS degradation and revealed that short-chain PFAS are more resistant to defluorination and their levels and regulation in the environment will determine the operating conditions of e-beam and other PFAS treatment technologies.

The role of suspended biomass in PFAS enrichment in wastewater treatment foams

Foaming in aerated bioreactors at wastewater treatment plants (WWTPs) has been identified as an operational issue for decades. However, the affinity of per- and polyfluoroalkyl substances (PFAS) for air-liquid interfaces suggests that foam harvesting has the potential to become a sustainable method for PFAS removal from sewage. Aerated bioreactors' foams are considered three-phase systems, comprising air, aqueous and solid components, the latter consisting of activated sludge biomass. To achieve a comprehensive understanding of the capability of aerated bioreactors' foams to enrich PFAS, we analysed PFAS concentrations from WWTPs in both the solid and aqueous phases of the collapsed foams (foamate) and underlying bulk mixed liquors. Our findings show that PFAS enrichment occurs not only in the aqueous phase but also in the solid phase of the foamate. This suggests that previous field studies that only analysed the aqueous phase may have underestimated the capability of the aerated bioreactors' foams to enrich PFAS. Fractions of PFOA and PFOS sorbed to the solid phase of the foamate can be as high as 60 % and 95 %, respectively. Our findings highlight the importance of implementing effective foamate management strategies that consider both the aqueous and solid phases.

Electrochemical Reduction of Perfluorooctanoic Acid (PFOA): An Experimental and Theoretical Approach

Perfluorooctanoic acid (PFOA) is an artificial chemical of global concern due to its high environmental persistence and potential human health risk. Electrochemical methods are promising technologies for water treatment because they are efficient, cheap, and scalable. The electrochemical reduction of PFOA is one of the current methodologies. This process leads to defluorination of the carbon chain to hydrogenated products. Here, we describe a mechanistic study of the electrochemical reduction of PFOA in gold electrodes. By using linear sweep voltammetry (LSV), an E0' of -1.80 V vs Ag/AgCl was estimated. Using a scan rate diagnosis, we determined an electron-transfer coefficient (αexp) of 0.37, corresponding to a concerted mechanism. The strong adsorption of PFOA into the gold surface is confirmed by the Langmuir-like isotherm in the absence (KA = 1.89 × 1012 cm3 mol-1) and presence of a negative potential (KA = 3.94 × 107 cm3 mol-1, at -1.40 V vs Ag/AgCl). Based on Marcus-Hush's theory, calculations show a solvent reorganization energy (λ0) of 0.9 eV, suggesting a large electrostatic repulsion between the perfluorinated chain and water. The estimated free energy of the transition state of the electron transfer (ΔG‡ = 2.42 eV) suggests that it is thermodynamically the reaction-limiting step. 19F - 1H NMR, UV-vis, and mass spectrometry studies confirm the displacement of fluorine atoms by hydrogen. Density functional theory (DFT) calculations also support the concerted mechanism for the reductive defluorination of PFOA, in agreement with the experimental values.

A review of foam fractionation for the removal of per- and polyfluoroalkyl substances (PFAS) from aqueous matrices

The detection of per- and polyfluoroalkyl substances (PFAS) in aqueous matrices is an emerging environmental concern due to their persistent, bioaccumulative and toxic properties. Foam fractionation has emerged as a viable method for removing and concentrating PFAS from aqueous matrices. The method exploits the surface-active nature of the PFAS to adsorb at the air-liquid interfaces of rising air bubbles, resulting in foam formation at the top of a foam fractionator. The removal of PFAS is then achieved through foam harvesting. Foam fractionation has gained increasing attention owing to its inherent advantages, including simplicity and low operational costs. The coupling of foam fractionation with destructive technologies could potentially serve as a comprehensive treatment train for future PFAS management in aqueous matrices. The PFAS-enriched foam, which has a smaller volume, can be directed to subsequent destructive treatment technologies. In this review, we delve into previous experiences with foam fractionation for PFAS removal from various aqueous matrices and critically analyse their key findings. Then, the recent industry advancements and commercial projects that utilise this technology are identified. Finally, future research needs are suggested based on the current challenges.

A review of innovative approaches for onsite management of PFAS-impacted investigation derived waste

The historic use of aqueous film-forming foam (AFFF) has led to widespread detection of per- and polyfluoroalkyl substance (PFAS) in groundwater, soils, sediments, drinking water, wastewater, and receiving aquatic systems throughout the United States (U.S.). Prior to any remediation activities, in order to identify the PFAS-impacted source zones and select the optimum management approach, extensive site investigations need to be conducted. These site investigations have resulted in the generation of considerable amount of investigation-derived waste (IDW) which predominantly consists of well purging water and drill fluid, equipment washing residue, soil, drill cuttings, and residues from the destruction of asphalt and concrete surfaces. IDW is often impacted by varying levels of PFAS which poses a substantial challenge concerning disposal to prevent potential mobilization of PFAS, logistical complexities, and increasing requirement for storage as a result of accumulation of the associated wastes. The distinct features of IDW involve the intermittent generation of waste, substantial volume of waste produced, and the critical demand for onsite management. This article critically focuses on innovative technologies and approaches employed for onsite treatment and management of PFAS-impacted IDW. The overall objective of this study centers on developing and deploying end-of-life treatment technology systems capable of facilitating unrestricted disposal, discharge, and/or IDW reuse on-site, thereby reducing spatial footprints and mobilization time.

Electrochemical degradation of per- and poly-fluoroalkyl substances in the presence of natural organic matter

Per- and poly-fluoroalkyl substances (PFAS), a contentious group of highly fluorinated, persistent, and potentially toxic chemicals, have been associated with human health risks. Currently, treatment processes that destroy PFAS are challenged by transforming these contaminants into additional toxic substances that may have unknown impacts on human health and the environment. Electrochemical oxidation (EO) is a promising method for scissoring long-chain PFAS, especially in the presence of natural organic matter (NOM), which interferes with most other treatment approaches used to degrade PFAS. The EO method can break the long-chain PFAS compound into short-chain analogs. The underlying mechanisms that govern the degradation of PFAS by electrochemical processes are presented in this review. The state-of-the-art anode and cathode materials used in electrochemical cells for PFAS degradation are overviewed. Furthermore, the reactor design to achieve high PFAS destruction is discussed. The challenge of treating PFAS in water containing NOM is elucidated, followed by EO implementation to minimize the influence of NOM on PFAS degradation. Finally, perspectives related to maximizing the readiness of EO technology and optimizing process parameters for the degradation of PFAS are briefly discussed.

Management of per- and polyfluoroalkyl substances (PFAS)-laden wastewater sludge in Maine: Perspectives on a wicked problem

This article discusses the challenges and potential solutions for managing wastewater sludge that contains per- and polyfluoroalkyl substances (PFAS), using the experience in Maine as a guide toward addressing the issue nationally. Traditional wastewater treatment, designed to remove excess organic waste and nutrients, does not eliminate persistent toxic pollutants like PFAS, instead partitioning the chemicals between discharged effluent and the remaining solids in sludge. PFAS chemistry, the molecular size, the alkyl chain length, fluorine saturation, the charge of the head group, and the composition of the surrounding matrix influence PFAS partitioning between soil and water. Land application of sludge, incineration, and storage in a landfill are the traditional management options. Land application of Class B sludge on agricultural fields in Maine peaked in the 1990s, totaling over 2 × 106 cu yd over a 40-year period and has contaminated certain food crops and animal forage, posing a threat to the food supply and the environment. Additional Class A EQ (Exceptional Quality) composted sludge was also applied to Maine farmland. The State of Maine banned the land application of wastewater sludge in August 2022. Most sludge was sent to the state-owned Juniper Ridge Landfill, which accepted 94 270 tons of dewatered sludge in 2022, a 14% increase over 2019. Between 2019 and 2022, the sum of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) concentrations in sludge sent to the landfill ranged from 1.2 to 104.9 ng/g dw. In 2022, the landfill generated 71.6 × 106 l of leachate. The concentration of sum of six PFAS in the leachate increased sixfold between 2021 and 2022, reaching 2 441 ng/l. The retention of PFAS within solid-waste landfills and the potential for long-term release of PFAS through liners into groundwater require ongoing monitoring. Thermal treatment, incineration, or pyrolysis can theoretically mineralize PFAS at high temperatures, yet the strong C-F bond and reactivity of fluorine require extreme temperatures for complete mineralization. Future alternatives may include interim options such as preconditioning PFAS with nonpolar solvents prior to immobilization in landfills, removing PFAS from leachate, and interrupting the cycle of PFAS moving from landfill, via leachate, to wastewater treatment, and then back to the landfill via sludge. Long-term solutions may involve destructive technologies such as electron beam irradiation, electrochemical advanced oxidation, or hydrothermal liquefaction. The article highlights the need for innovative and sustainable solutions for managing PFAS-contaminated wastewater sludge.

Influence of PFDA on the nutrient removal from wastewater by hydrogels containing microalgae-bacteria

PFAS have demonstrated to affect some aerobic microorganisms applied for wastewater treatment. This study evaluated the nutrient removal of three types of hydrogels containing a consortium of microalgae-bacteria (HB), activated carbon (HC), or both (HBC) in presence of perfluorodecanoic acid (PFDA). The nutrients evaluated were ammonium nitrogen (NH₄-N), nitrate nitrogen (NO₃-N), phosphate (PO₄), and chemical oxygen demand (COD). Fluorine (F^-) concentration and the integrity of HB exposed to PFDA were also determined at the end of experiments to understand the potential sorption and effects of PFDA on hydrogel. The results indicated that the presence of PFDA did affect the nitrification process, 13% and 36% to HB and HBC, respectively. Mass balance confirmed negative impact of PFDA on nitrogen consumption in HB (-31.37%). However, NH₄-N was removed by all types of hydrogels in a range of 61-79%, while PO₄ was mainly removed by hydrogels containing activated carbon (AC), 37.5% and 29.2% for HC and HBC, respectively. The removal of both NH₄ and PO₄, was mainly attributed to sorption processes in hydrogels, which was enhanced by the presence of AC. PFDA was also adsorbed in hydrogels, decreasing its concentration between 18% and 28% from wastewater, and up to 39% using HC. Regarding COD concentration, this increased overtime but was not related to hydrogel structure, since Transmission Electron Microscopy imaging revealed that their structure was preserved in presence of PFDA. COD increasement could be attributed to soluble algal products as well as to PVA leaching from hydrogels. In general, the presence of AC in hydrogels can contribute to mitigate the toxic effect of PFDA over microorganisms involved in biological nutrient removal, and hydrogels can be a technique to partially remove this contaminant from aqueous matrices.

Electrogenerated oxychlorides induced overlooked negative effects on electro-oxidation wastewater treatment in terms of over-evaluated COD removal efficiency and biotoxicity

The high-efficiency and environmentally-friendly electro-oxidation (EO) would lose its competitive edge because of the production of oxychloride by-products (ClO_x^-), which has not yet drawn significant attention in academic and engineering communities. In this study, the negative effects of the electrogenerated ClO_x^- were compared among four commonly used anode materials (BDD, Ti₄O₇, PbO₂ and Ru-IrO₂) in terms of ClO_x^- interference on the evaluation of electrochemical COD removal performance and biotoxicity. Apparently, the COD removal performance of various EO systems were highly enhanced with increasing current density in the presence of Cl^-, e.g., the amounts of COD removed by various EO systems from the phenol solution with an initial COD content of 280 mg L^-1 at 40 mA cm^-2 within 120 min decreased in the order: Ti₄O₇ of 265 mg L^-1 > BDD of 257 mg L^-1 > PbO₂ of 202 mg L^-1 > Ru-IrO₂ of 118 mg L^-1, which was different from the case with the absence of Cl^- (BDD of 200 mg L^-1 > Ti₄O₇ of 112 mg L^-1 > PbO₂ of ¹08 mg L^-1 > Ru-IrO₂ of 80 mg L^-1) and the results after removing ClO_x^- by anoxic sulfite-based method (BDD of 205 mg L^-1 > Ti₄O₇ of 160 mg L^-1 > PbO₂ of 153 mg L^-1 > Ru-IrO₂ of 99 mg L^-1). These results can be ascribed to the ClO_x^- interference on COD evaluation, the extent of which decreased in the order: ClO₃^- > ClO^- (where ClO₄^- cannot impact COD test). The highest overrated electrochemical COD removal performance of Ti₄O₇ may be associated with its relatively high production of ClO₃^- and the low mineralization extent. The chlorella inhibition ratio of ClO_x^- decreased in the order: ClO^- > ClO₃^- >> ClO₄^-, which accounted for the biotoxicity increasement of the treated water (PbO₂ 68%, Ti₄O₇ 56%^, BDD 53%, Ru-IrO₂ 25%). Generally, the inevitable problems of overrated electrochemical COD removal performance and biotoxicity increasement induced by ClO_x^- should deserve significant attention and effective countermeasures should be also developed when employing EO process for wastewater treatment.

Effective PFAS degradation by electrochemical oxidation methods-recent progress and requirement

The presence of per- and poly-fluoroalkyl substances (PFASs) in water is of global concern due to their high stability and toxicity even at very low concentrations. There are several technologies for the remediation of PFASs, but most of them are inadequate either due to limited effectiveness, high cost, or production of a large amount of sludge. Electrochemical oxidation (EO) technology shows great potential for large-scale application in the degradation of PFASs due to its simple procedure, low loading of chemicals, and least amount of waste. Here, we have reviewed the recent progress in EO methods for PFAS degradation, focusing on the last 10 years, to explore an efficient, cost-effective, and environmentally benign remediation technology. The effects of important parameters (e.g., anode material, current density, solution pH, electrolyte, plate distance, and electrical connector type) are summarized and evaluated. Also, the energy consumption, the consequence of different PFASs functional groups, and water matrices are discussed to provide an insight that is pivotal for developing new EO materials and technologies. The proposed degradation pathways of shorter-chain PFAS by-products during EO of PFAS are also discussed.

A review on superior advanced oxidation and photocatalytic degradation techniques for perfluorooctanoic acid (PFOA) elimination from wastewater

Perfluorooctanoic acid (PFOA) has been identified as the most toxic specie of the family of perfluorinated carboxylic acids (PFCAs). It has been widely distributed and frequently detected in environmental wastewater. The compound's unique features such as inherent stability, rigidity, and resistance to harsh chemical and thermal conditions, due to its multiple and strong C-F bonds have resulted in its resistance to conventional wastewater remediations. Photolysis and bioremediation methods have been proven to be inefficient in their elimination, hence this article presents intensive literature studies and summarized findings reported on the application of advanced oxidation processes (AOPs) and photocatalytic degradation techniques as the best alternatives for the PFOA elimination from wastewater. Techniques of persulfate, photo-Fenton, electrochemical, photoelectrochemical and photocatalytic degradation have been explored and their mechanisms for the degradation and defluorination of the PFOA have been demonstrated. The major advantage of AOPs techniques has been centralized on the generation of active radicals such as sulfate (SO₄^•-) hydroxyl (•OH). While for the photocatalytic process, photogenerated species (electron (e) and holes (h ⁺ _vb)) initiated the process. These active radicals and photogenerated species possessed potentiality to attack the PFOA molecule and caused the cleavage of the C-C and C-F bonds, resulting in its efficient degradation. Shorter-chain PFCAs have been identified as the major intermediates detected and the final stage entails its complete mineralization to carbon dioxide (CO₂) and fluoride ion (F^-). The prospects and challenges associated with the outlined techniques have been highlighted for better understanding of the subject matter for the PFOA elimination from real wastewaters.

Electrochemical-based approaches for the treatment of forever chemicals: Removal of perfluoroalkyl and polyfluoroalkyl substances (PFAS) from wastewater

Electrochemical based approaches for the treatment of recalcitrant water borne pollutants are known to exhibit superior function in terms of efficiency and rate of treatment. Considering the stability of Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are designated as forever chemicals, which generating from various industrial activities. PFAS are contaminating the environment in small concentrations, yet exhibit severe environmental and health impacts. Electro-oxidation (EO) is a recent development that treats PFAS, in which different reactive species generates at anode due to oxidative reaction and reductive reactions at the cathode. Compared to water and wastewater treatment methods those being implemented, electrochemical approaches demonstrate superior function against PFAS. EO completely mineralizes (almost 100 %) non-biodegradable organic matter and eliminate some of the inorganic species, which proven as a robust and versatile technology. Electrode materials, electrolyte concentration pH and the current density applying for electrochemical processes determine the treatment efficiency. EO along with electrocoagulation (EC) treats PFAS along with other pollutants from variety of industries showed highest degradation of 7.69 mmol/g of PFAS. Integrated approach with other processes was found to exhibit improved efficiency in treating PFAS using several electrodes boron-doped diamond (BDD), zinc, titanium and lead based with efficiency the range of 64 to 97 %.

Wastewater treatment by anodic oxidation in electrochemical advanced oxidation process: Advance in mechanism, direct and indirect oxidation detection methods

Electrochemical Advanced Oxidation Process (EAOP) has been applied to the degradation of refractory pollutants in wastewater due to its strong oxidation capacity, high degradation efficiency, simple operation, and mild reaction. Among electrochemical processes, anodic oxidation (AO) is the most widely used and its mechanism is mainly divided into direct oxidation and indirect oxidation. Direct oxidation means that pollutants are oxidized at the anode by direct electron transfer. Indirect oxidation refers to the generation of active species during the electrolytic reaction, which acts on pollutants. The mechanism of AO process is controlled by many factors, including electrode type, electrocatalyst material, wastewater composition, pH, applied current and voltage levels. It is very important to explore the reaction mechanism of electrochemical treatment, which determines the efficiency of the reaction, the products of the reaction, and the extent of reaction. This paper firstly reviews the current research progress on the mechanism of AO process, and summarizes in detail the different mechanisms caused by influencing factors under common AO process. Then, strategies and methods to distinguish direct oxidation and indirect oxidation mechanisms are reviewed, such as intermediate product analysis, electrochemical test analysis, active species detection, theoretical calculation, and the limitations of these methods are analyzed. Finally some suggestions are put forward for the study of the mechanism of electrochemical advanced oxidation.

A Review of PFAS Destruction Technologies

Per- and polyfluoroalkyl substances (PFASs) are a family of highly toxic emerging contaminants that have caught the attention of both the public and private sectors due to their adverse health impacts on society. The scientific community has been laboriously working on two fronts: (1) adapting already existing and effective technologies in destroying organic contaminants for PFAS remediation and (2) developing new technologies to remediate PFAS. A common characteristic in both areas is the separation/removal of PFASs from other contaminants or media, followed by destruction. The widely adopted separation technologies can remove PFASs from being in contact with humans; however, they remain in the environment and continue to pose health risks. On the other hand, the destructive technologies discussed here can effectively destroy PFAS compounds and fully address society's urgent need to remediate this harmful family of chemical compounds. This review reports and compare widely accepted as well as emerging PFAS destruction technologies. Some of the technologies presented in this review are still under development at the lab scale, while others have already been tested in the field.