Electrochemical degradation of per- and polyfluoroalkyl substances (PFAS) using low-cost graphene sponge electrodes

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.

Synergistic adsorption and photocatalytic degradation of perfluorooctanoic acid in aqueous solution by a regenerable biochar-titania nanotube composite

Perfluorooctanoic acid (PFOA), a recalcitrant perfluoroalkyl substance, presents escalating challenges for aquatic decontamination due to its extreme persistence and bioaccumulation. A biochar-titania nanotube (TNTs@biochar) combining the advantages of biochar and TNTs was synthesized for the first time via an alkaline hydrothermal approach and explored for the adsorption and photodegradation of PFOA in aqueous solution. Titania nanotubes interacted with biochar to form TNTs@biochar. The optimal composite was obtained at a biochar : TiO2 mass ratio of 1 : 1 and a calcination temperature of 550 °C. The composite efficiently adsorbed ∼99% of PFOA through hydrophobic and anion-π interactions and hydrogen bonding, concentrating PFOA on photoactive sites. The incorporation of biochar with TNTs enhanced light absorption in the 200-700 nm range, lowered the band gap energy to 3.10 eV, improved the formation rate and separation efficiency of e--h+ pairs, and enhanced interfacial charge transfer, resulting in promoted photocatalytic activity. The degradation of pre-concentrated PFOA on TNTs@biochar reached up to 99%. The photodegradation also regenerated the composite, allowing for four successive adsorption-photodegradation cycles. Hydroxyl radical and h+-driven oxidation played a paramount part, leading to decarboxylation and C-F bond cleavage. The byproducts of the photodegradation demonstrated lower acute and chronic toxicity compared with PFOA. The composite exhibits synergistic adsorption and photocatalytic activity as well as offers efficiently and economically scalable solutions for PFOA-laden water remediation.

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.

Thermochemical Recycling and Degradation Strategies of Halogenated Polymers (F-, Cl-, Br-): A Holistic Review Coupled with Mechanistic Insights

Handling the waste associated with halogenated polymers is a daunting task due to the well-documented emission of halogen-bearing toxicants during the disposal or recycling operation. According to the Stockholm Convention treaty, most of these products are classified as persistent organic pollutants due to their potential health hazards. This review aims to provide a holistic overview of the recent updates for treating halogenated polymeric waste through physical, chemical and biological approaches. In the line of inquiry, critical analysis of the obstacles and prospects associated with each degradation technique on the halogenated polymer has been performed, assessing based on the degradation efficiency, treatment upscaling, pollution control, and feasibility. Though many treatments show promising results, they also entail drawbacks. Thermal treatment exploiting various metal oxides, especially calcium additives, is considered the most executable technique for halogenated polymer valorization coupled with mineralization/metal extraction due to its intuitive operational feasibility and potential scalability. Strategies for combating the soaring halogenated polymeric wastes summarized herein tap into promoting a circular economy approach for their sustainable disposal and recycling.

Electrochemical destruction of PFAS at low oxidation potential enabled by CeO2 electrodes utilizing adsorption and activation strategies

The persistence and ecological impact of per- and poly-fluoroalkyl substances (PFAS) in water sources necessitate effective and energy-efficient treatment solutions. This study introduces a novel approach using cerium dioxide (CeO2) electrodes enhanced with oxygen vacancy (Ov) to catalyze the defluorination of PFAS. By leveraging the unique affinity between cerium and fluorine-containing species, our approach enables adsorptive preconcentration and catalytic degradation at low oxidation potentials (1.37 V vs. SHE). Demonstrating high removal and defluorination efficiencies of perfluorooctanoic acid (PFOA) at 94.0 % and 73.0 %, respectively, our approach also proves effective in the environmental matrix. It minimizes the impacts of co-existing natural organic matter and chloride ions, crucial benefits of operating at lower oxidation potentials. The role of Ov in CeO2 is validated by both experimental results and density functional theory modeling, demonstrating that these sites can activate the C-F bond and substantially reduce the energy barriers for defluorination. Consequently, our CeO2-based method not only achieves defluorination efficiencies comparable to more energy-intensive techniques but does so while requiring less than 0.62 kWh/m3 per order. This positions our approach as a promising, cost-effective alternative for the remediation of PFAS-contaminated waters, emphasizing its relevance and effectiveness in environmental remediation scenarios.

Conceptualizing Controlling Factors for PFAS Salting Out in Groundwater Discharge Zones Along Sandy Beaches

Understanding fate and transport processes for per- and poly-fluoroalkyl substances (PFAS) is critical for managing impacted sites. "PFAS Salting Out" in groundwater, defined herein, is an understudied process where PFAS in fresh groundwater mixes with saline groundwater near marine shorelines, which increases sorption of PFAS to aquifer solids. While sorption reduces PFAS mass discharge to marine surface water, the fraction that sorbs to beach sediments may be mobilized under future salinity changes. The objective of this study was to conceptually explore the potential for PFAS Salting Out in sandy beach environments and to perform a preliminary broad-scale characterization of sandy shoreline areas in the continental U.S. While no site-specific PFAS data were collected, our conceptual approach involved developing a multivariate regression model that assessed how tidal amplitude and freshwater submarine groundwater discharge affect the mixing of fresh and saline groundwater in sandy coastal aquifers. We then applied this model to 143 U.S. shoreline areas with sandy beaches (21% of total beaches in the USA), indirectly mapping potential salinity increases in shallow freshwater PFAS plumes as low (<10 ppt), medium (10-20 ppt), or high (>20 ppt) along groundwater flow paths before reaching the ocean. Higher potential salinity increases were observed in West Coast bays and the North Atlantic coastline, due to the combination of moderate to large tides and large fresh groundwater discharge rates, while lower increases occurred along the Gulf of Mexico and the southern Florida Atlantic coast. The salinity increases were used to estimate potential perfluorooctane sulfonic acid (PFOS) sorption in groundwater due to salting out processes. Low-category shorelines may see a 1- to 2.5-fold increase in sorption of PFOS, medium-category a 2.0- to 6.4-fold increase, and high-category a 3.8- to 25-fold increase in PFOS sorption. The analysis presented provides a first critical step in developing a large-scale approach to classify the PFAS Salting Out potential along shorelines and the limitations of the approach adopted highlights important areas for further research.

Electrochemical Hydrogen Peroxide Generation and Activation Using a Dual-Cathode Flow-Through Treatment System: Enhanced Selectivity for Contaminant Removal by Electrostatic Repulsion

To oxidize trace concentrations of organic contaminants under conditions relevant to surface- and groundwater, air-diffusion cathodes were coupled to stainless-steel cathodes that convert atmospheric O2 into hydrogen peroxide (H2O2), which then was activated to produce hydroxyl radicals (·OH). By separating H2O2 generation from its activation and employing a flow-through electrode consisting of stainless-steel fibers, the two processes could be operated efficiently in a manner that overcame mass-transfer limitations for O2, H2O2, and trace organic contaminants. The flexibility resulting from separate control of the two processes made it possible to avoid both the accumulation of excess H2O2 and the energy losses that take place after H2O2 has been depleted. The decrease in treatment efficacy occurring in the presence of natural organic matter was substantially lower than that typically observed in homogeneous advanced oxidation processes. Experiments conducted with ionized and neutral compounds indicated that electrostatic repulsion prevented negatively charged ·OH scavengers from interfering with the oxidation of neutral contaminants. Energy consumption by the dual-cathode system was lower than values reported for other technologies intended for small-scale drinking water treatment systems. The coordinated operation of these two cathodes has the potential to provide a practical, inexpensive way for point-of-use drinking water treatment.

Unrefined and Milled Ilmenite as a Cost-Effective Photocatalyst for UV-Assisted Destruction and Mineralization of PFAS

Per- and polyfluoroalkyl substances (PFAS) are fluorinated and refractory pollutants that are ubiquitous in industrial wastewater. Photocatalytic destruction of such pollutants with catalysts such as TiO2 and ZnO is an attractive avenue for removal of PFAS, but refined forms of such photocatalysts are expensive. This study, for the first time, utilized milled unrefined raw mineral ilmenite, coupled to UV-C irradiation to achieve mineralization of the two model PFAS compounds perfluorooctanoic acid (PFOA) and perfluoro octane sulfonic acid (PFOS). Results obtained using a bench-scale photocatalytic reactor system demonstrated rapid removal kinetics of PFAS compounds (>90% removal in less than 10 h) in environmentally-relevant concentrations (200-1000 ppb). Raw ilmenite was reused over three consecutive degradation cycles of PFAS, retaining >80% removal efficiency. Analysis of degradation products indicated defluorination and the presence of shorter-chain PFAS intermediates in the initial samples. End samples indicated the disappearance of short-chain PFAS intermediates and further accumulation of fluoride ions, suggesting that original PFAS compounds underwent mineralization due to an oxygen-radical-based photocatalytic destruction mechanism induced by TiO2 present in ilmenite and UV irradiation. The outcome of this study implies that raw ilmenite coupled to UV-C is suitable for cost-effective reactor operation and efficient photocatalytic destruction of PFAS compounds.

Comparison of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and perfluorobutane sulfonate (PFBS) removal in a combined adsorption and electrochemical oxidation process

This study focused on three of the most studied PFAS molecules, namely perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and perfluorobutane sulfonate (PFBS). They were compared in terms of their adsorption capacity onto graphite intercalated compound (GIC), a low surface area, highly conductive and cheap adsorbent. The adsorption on GIC followed a pseudo second order kinetics and the maximum adsorption capacity using Langmuir was 53.9 μg/g for PFOS, 22.3 μg/g for PFOA and 0.985 μg/g for PFBS due to electrostatic attraction and hydrophobic interactions. GIC was added into an electrochemical oxidation reactor and >100 μg/L PFOS was found to be fully degraded (<10 ng/L) leaving degradation by-products such as PFHpS, PFHxS, PFPeS, PFBS, PFOA, PFHxA and PFBA below 100 ng/L after 5 cycles of adsorption onto GIC for 20 min followed by regeneration at 28 mA/cm2 for 10 min. PFBS was completely removed due to degradation by aqueous electrons on GIC flakes. Up to 98 % PFOA was removed by the process after 3 cycles of adsorption onto GIC for 20 min followed by regeneration at 25 mA/cm2 for 10 min. When PFBS was spiked individually, only 17 % was removed due to poor adsorption on GIC. There was a drop of 3-40 % by treating PFOS, PFOA and smaller sulfonates in a real water matrix under the same electrochemical conditions (20 mA/cm2), but PFOS and PFOA removal percentage were 95 and 68 % after 20 min at 20 mA/cm2.

Iron complex regulated synergistic effect between the current and peroxymonosulfate enhanced ultrafast oxidation of perfluorooctanoic acid via free radical dominant electrochemical reaction

Iron complex regulated electrochemical reaction was triggered for revealing the reaction mechanism, degradation pathway, and applied potential of perfluorooctanoic acid (PFOA). The increased PMS concentrations, electrode spacing, and current density significantly enhanced PFOA elimination, with current density exhibiting a relatively strong interdependency to PFOA complete mineralization. The synergy between PMS and electrochemical reactions greatly accelerated PFOA decomposition by promoting the generation of key reaction sites, such as those for PMS activation and electrochemical processes, under various conditions. Furthermore, density functional theory calculations confirmed that the reciprocal transformation of Fe2+ and Fe3+ complexes was feasible under the electrochemical effect, further promoting the generation of active sites. The developed electrochemical oxidation with PMS reaction (EO/PMS) system can rapidly decompose and mineralize PFOA while maintaining strong tolerance to changing water matrices and organic and inorganic ions. Overall, it holds promise for use in treating and purifying wastewater containing PFOA.

Controllable chemical redox reactions to couple microbial degradation for organic contaminated sites remediation: A review

Global environmental concern over organic contaminated sites has been progressively conspicuous during the process of urbanization and industrial restructuring. While traditional physical or chemical remediation technologies may significantly destroy the soil structure and function, coupling moderate chemical degradation with microbial remediation becomes a potential way for the green, economic, and efficient remediation of contaminated sites. Hence, this work systematically elucidates why and how to couple chemical technology with microbial remediation, mainly focused on the controllable redox reactions of organic contaminants. The rational design of materials structure, selective generation of reactive oxygen species, and estimation of degradation pathway are described for chemical oxidation. Meanwhile, current progress on efficient and selective reductions of organic contaminants (i.e., dechlorination, defluorination, -NO2 reduction) is introduced. Combined with the microbial remediation of contaminated sites, several consideration factors of how to couple chemical and microbial remediation are proposed based on both fundamental and practical points of view. This review will advance the understanding and development of chemical-microbial coupled remediation for organic contaminated sites.

Structural dependence of PFAS oxidation in a boron doped diamond-electrochemical system

Driven by long-term persistence and adverse health impacts of legacy perfluorooctanoic acid (PFOA), production has shifted towards shorter chain analogs (C4, perfluorobutanoic acid (PFBA)) or fluorinated alternatives such as hexafluoropropylene oxide dimer acid (HFPO-DA, known as GenX) and 6:2 fluorotelomer carboxylic acid (6:2 FTCA). Yet, a thorough understanding of treatment processes for these alternatives is limited. Herein, we conducted a comprehensive study using an electrochemical approach with a boron doped diamond anode in Na2SO4 electrolyte for the remediation of PFOA common alternatives, i.e., PFBA, GenX, and 6:2 FTCA. The degradability, fluorine recovery, transformation pathway, and contributions from electro-synthesized radicals were investigated. The results indicated the significance of chain length and structure, with shorter chains being harder to break down (PFBA (65.6 ± 5.0%) < GenX (84.9 ± 3.3%) < PFOA (97.9 ± 0.1%) < 6:2 FTCA (99.4 ± 0.0%) within 120 min of electrolysis). The same by-products were observed during the oxidation of both low and high concentrations of parent PFAS (2 and 20 mg L-1), indicating that the fundamental mechanism of PFAS degradation remained consistent. Nevertheless, the ratio of these by-products to the parent PFAS concentration varied which primarily arises from the more rapid PFAS decomposition at lower dosages. For all experiments, the main mechanism of PFAS oxidation was initiated by direct electron transfer at the anode surface. Sulfate radical (SO4•-) also contributed to the oxidation of all PFAS, while hydroxyl radical (•OH) only played a role in the decomposition of 6:2 FTCA. Total fluorine recovery of PFBA, GenX, and 6:2 FTCA were 96.5%, 94.0%, and 76.4% within 240 min. The more complex transformation pathway of 6:2 FTCA could explain its lower fluorine recovery. Detailed decomposition pathways for each PFAS were also proposed through identifying the generated intermediates and fluorine recovery. The proposed pathways were also assessed using 19F Nuclear Magnetic Resonance (NMR) spectroscopy.

Combined adsorption and electrochemical oxidation of perfluorooctanoic acid (PFOA) using graphite intercalated compound

Perfluorooctanoic acid (PFOA) is a bioaccumulative synthetic chemical containing strong C-F bonds and is one of the most common per- and polyfluoroalkyl substances (PFAS) detected in the environment. Graphite intercalated compound (GIC) flakes were used to adsorb and degrade PFOA through electrochemical oxidation. The adsorption followed the Langmuir model with a loading capacity of 2.6 µg PFOA g-1 GIC and a second-order kinetics (3.354 g µg-1 min-1). 99.4% of PFOA was removed by the process with a half-life of 15 min. When PFOA molecules broke down, they released various by-products, such as short-chain perfluoro carboxylic acids like PFHpA, PFHxA, and PFBA. This breakdown indicates the cleavage of the perfluorocarbon chain and the release of CF2 units, suggesting a transformation or degradation of the original compound into these smaller acids. Shorter-chain perfluorinated compounds had slower degradation rates compared to longer-chain ones. Combining these two methods (adsorption and in situ electrochemical oxidation) was found to be advantageous because adsorption can initially concentrate the PFOA molecules, making it easier for the electrochemical process to target and degrade them. The electrochemical process can potentially break down or transform the PFAS compounds into less harmful substances through oxidation or other reactions.

Scientometric analysis of electrocatalysis in wastewater treatment: today and tomorrow

Electrocatalytic methods are valuable tools for addressing water pollution and scarcity, offering effective pollutant removal and resource recovery. To investigate the current status and future trends of electrocatalysis in wastewater treatment, a detailed analysis of 9417 papers and 4061 patents was conducted using scientometric methods. China emerged as the leading contributor to publications, and collaborations between China and the USA have emerged as the most frequent partnerships. Primary article co-citation clusters focused on oxygen evolution reaction and electrochemical oxidation, transitioning towards advanced oxidation processes ("persulfate activation"), and electrocatalytic reduction processes ("nitrate reduction"). Bifunctional catalysts, theoretical calculations, electrocatalytic combination technologies, and emerging contaminants were identified as current research hotspots. Patent analysis revealed seven types of electrochemical technologies, which were compared using SWOT analysis, highlighting electrochemical oxidation as prominent. The technological evolution presented the pathway of electro-Fenton to combined electrocatalytic technologies with biochemical processes, and finally to coupling with electrocoagulation. Standardized evaluation systems, waste resource utilization, and energy conservation were important directions of innovation in electrocatalytic technologies. Overall, this study provided a reference for researchers to understand the framework of electrocatalysis in wastewater treatment and also shed light on potential avenues for further innovation in the field.

Enhanced electrochemical degradation of perfluorooctanoic acid by ligand-bridged PtII at Pt anodes

A new mechanism for the electro-oxidation (EO) degradation of perfluorooctanoic acid (PFOA) by Pt anode was reported. Using bridge-based ligand anions (SCN-, Cl- and N3-) as electrolytes, the degradation effect of PFOA by Pt-EO system was significant. Characterization of the Pt anode, the detection and addition of dissolved platinum ions, and the comparison of Pt with DSA anodes determined that the Pt- ligand complexes resulting from the specific binding of anodically dissolved PtII with ligand ions and C7F15COO- ((C7F15-COO)PtII-L3, L = SCN-, Cl- and N3-) on the electrode surface played a decisive role in the degradation of PFOA. Density functional theory (DFT) calculations showed that inside (C7F15-COO)PtII-L3 complexes, the electron density of the perfluorocarbon chain (including the F atom) compensated toward the carboxyl group and electrons in the PFOA ion transferred to the PtII-Cl3. Moreover, the (C7F15-COO)PtII-Cl3, as a whole, was calculated to migrate electrons toward the Pt anode, leading to the formation of PFOA radical (C7F15-COO•). Finally, with the detection of a series of short chain homologues, the CF2-unzipping degradation pathway of PFOA was proposed. The newly developed Pt-EO system is not affected by water quality conditions and can directly degrade alcohol eluent of PFOA, which has great potential for treating industrial wastewater contaminated with PFOA.

Unveiling the contemporary progress of graphene-based nanomaterials with a particular focus on the removal of contaminants from water: a comprehensive review

Water scarcity and pollution pose significant challenges to global environmental sustainability and public health. As these concerns intensify, the quest for innovative and efficient water treatment technologies becomes paramount. In recent years, graphene-based nanomaterials have emerged as frontrunners in this pursuit, showcasing exceptional properties that hold immense promise for addressing water contamination issues. Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, exhibits extraordinary mechanical, electrical, and chemical properties. These inherent characteristics have led to a surge of interest in leveraging graphene derivatives, such as graphene oxide (GO), reduced graphene oxide and functionalized graphene, for water treatment applications. The ability of graphene-based nanomaterials to adsorb, catalyze, and photocatalyze contaminants makes them highly versatile in addressing diverse pollutants present in water sources. This review will delve into the synthesis methods employed for graphene-based nanomaterials and explore the structural modifications and functionalization strategies implemented to increase their pollutant removal performance in water treatment. By offering a critical analysis of existing literature and highlighting recent innovations, it will guide future research toward the rational design and optimization of graphene-based nanomaterials for water decontamination. The exploration of interdisciplinary approaches and cutting-edge technologies underscores the evolving landscape of graphene-based water treatment, fostering a path toward sustainable and scalable solutions. Overall, the authors believe that this review will serve as a valuable resource for researchers, engineers, and policymakers working toward sustainable and effective solutions for water purification.

Efficient Decomposition of Perfluoroalkyl Substances by Low Concentration Indole: New Insights into the Molecular Mechanisms

Perfluoroalkyl substances (PFAS) are a class of persistent organic pollutants known as "forever chemicals". Currently, the hydrated electron-based advanced reduction process (ARP) holds promise for the elimination of PFAS. However, the efficiency of ARP is often challenged by an oxygen-rich environment, resulting in the consumption of hydrated electron source materials in exchange for the high PFAS decomposition efficiency. Herein, we developed a ternary system constructed by indole and isopropyl alcohol (IPA), and the addition of IPA significantly enhanced the PFOA degradation and defluorination efficiency in the presence of low-concentration indole (<0.4 mM). Meanwhile, opposite results were obtained with a higher amount of indole (>0.4 mM). Further exploring the molecular mechanism of the reaction system, the addition of IPA played two roles. On one hand, IPA built an anaerobic reaction atmosphere and improved the yield and utilization efficiency of hydrated electrons with a low concentration of indole. On the other hand, IPA suppressed the attraction between indole and PFOA, thus reducing the hydrated electron transfer efficiency, especially with more indole. In general, the indole/PFAS/IPA system significantly improved the PFAS destruction efficiency with a small amount of hydrated electron donors, which provided new insights for development of simple and efficient techniques for the treatment of PFAS-contaminated wastewater.

Degradation of emerging per- and polyfluoroalkyl substances (PFAS) using an electrochemical plug flow reactor

In recent years, shorter-chain fluorinated compounds have been manufactured as alternatives to legacy per- and polyfluoroalkyl substances (PFAS) after a global ban on some long-chain PFAS. This study is the first to investigate the degradability of emerging PFAS by an electrochemical plug flow reactor (EPFR). Ten different emerging PFAS, representing classes of fluorotelomer alcohol, perfluoroalkyl ether carboxylate, polyfluoroalkyl ethersulfonic acids, perfluoroalkyl ether/polyether carboxylates, perfluoroether sulfonate, N-alkyl perfluoroalkylsulfonamido carboxylate, fluoroalkyl phosphonic acid, and perfluoro alkane sulfonamide were investigated. The process kinetics was performed. The degradation of parent compounds increased with increasing retention time (RT). At 45.2 min of RT, the degradation of parent compounds ranged between 68%-100% with a current density of 17.2 mA/cm2. A linear increase in pseudo-first order rate constants was observed for all PFAS with increasing current density from 5.7 to 28.7 mA/cm2 (R2 > 0.91). The effect of pH, natural organic matter, and bicarbonate on the degradation, defluorination, and fluorine mass balance are reported. Alkaline pH (11) caused a decrease in degradation for all PFAS. While the presence of natural organic matter (NOM) significantly decreased the degradation and defluorination processes, the presence of bicarbonate at all studied concentrations (25, 50, and 100 mg/L) did not affect the process efficiency. The defluorination reduced to 34% from 81% with 15 mg/L NOM. The unknown/undetected fluorine fraction also increased in the presence of 15 mg/L NOM indicating the formation of NOM-PFAS complexes. Additionally, C2-C8 perfluoro carboxylic acids (PFCAs), one perfluoro sulfonic acid (PFSA), two H-PFCAs, and 4:2 fluorotelomer sulfonate (FTS) were identified as degradation byproducts in suspect screening. The electrical energy per order for PFAS ranged between 1.8 and 19.4 kWh/m3. This study demonstrates that emerging types of PFAS can potentially be degraded using an EPFR with relatively low electrical energy requirements.

Advanced electrocatalytic redox processes for environmental remediation of halogenated organic water pollutants

Halogenated organic compounds are widespread, and decades of heavy use have resulted in global bioaccumulation and contamination of the environment, including water sources. Here, we introduce the most common halogenated organic water pollutants, their classification by type of halogen (fluorine, chlorine, or bromine), important policies and regulations, main applications, and environmental and human health risks. Remediation techniques are outlined with particular emphasis on carbon-halogen bond strengths. Aqueous advanced redox processes are discussed, highlighting mechanistic details, including electrochemical oxidations and reductions of the water-oxygen system, and thermodynamic potentials, protonation states, and lifetimes of radicals and reactive oxygen species in aqueous electrolytes at different pH conditions. The state of the art of aqueous advanced redox processes for brominated, chlorinated, and fluorinated organic compounds is presented, along with reported mechanisms for aqueous destruction of select PFAS (per- and polyfluoroalkyl substances). Future research directions for aqueous electrocatalytic destruction of organohalogens are identified, emphasizing the crucial need for developing a quantitative mechanistic understanding of degradation pathways, the improvement of analytical detection methods for organohalogens and transient species during advanced redox processes, and the development of new catalysts and processes that are globally scalable.

Optimisation of GraPhage13 macro-dispersibility via understanding the pH-dependent ionisation during self-assembly: towards the manufacture of graphene-based nanodevices

GraPhage13 aerogels (GPAs) are micro-porous structures generated through the self-assembly of graphene oxide (GO) and M13 bacteriophage. As GPA fabrication involves the aggregation of GO and M13 in aqueous solution, we aim to understand its dispersibility across a wide pH range. Herein, a novel technique has been developed to relate the ionisation of functional groups to the surface charge, offering insights into the conditions required for GPA fabrication and the mechanism behind its self-assembly. The aggregation of GO and M13 was observed between pH 2-6 and exhibited dependence on the surface charge of the resulting aggregate with the M13 bacteriophage identified as the primary factor contributing to this, whilst originating from the ionisation of its functional groups. In contrast, GO exhibited a lesser impact on the surface charge due to the deprotonation of its carboxylic, enolic and phenolic functional groups at pH 6 and above, which falls outside the required pH range for aggregation. These results enhance our understanding of the GPA self-assembly mechanism, the conditions required for their fabrication and the optimal processability, laying the foundation towards its broad range of applications and the subsequent manufacture of graphene-based nanodevices.

Impact of supporting electrolyte on electrochemical performance of borophene-functionalized graphene sponge anode and degradation of per- and polyfluoroalkyl substances (PFAS)

Graphene sponge anode functionalized with two-dimensional (2D) boron, i.e., borophene, was applied for electrochemical oxidation of C4-C8 per- and polyfluoroalkyl substances (PFASs). Borophene-doped graphene sponge outperformed boron-doped graphene sponge anode in terms of PFASs removal efficiencies and their electrochemical degradation; whereas at the boron-doped graphene sponge anode up to 35% of the removed PFASs was recovered after the current was switched off, the switch to a 2D boron enabled further degradation of the electrosorbed PFASs. Borophene-doped graphene sponge anode achieved 32-77% removal of C4-C8 PFASs in one-pass flow-through mode from a 10 mM phosphate buffer at 230 A m-2 of anodic current density. Higher molarity phosphate buffer (100 mM) resulted in lower PFASs removal efficiencies (11-60%) due to the higher resistance of the graphene sponge electrode in the presence of phosphate ions, as demonstrated by the electrochemical impedance spectroscopy (EIS) analyses. Electro-oxidation of PFASs was more efficient in landfill leachate despite its high organic loading, with up to 95% and 75% removal obtained for perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), versus 77% and 57% removal in the 10 mM phosphate buffer, respectively. Defluorination efficiencies as determined relative to the electrooxidized fraction of PFASs indicated up to 69% and 82% of defluorination of PFOS and PFOA in 10 mM phosphate buffer, which was decreased to 16 and 29% defluorination, respectively, for higher buffer molarity (100 mM) due to the worsened electrochemical performance of the sponge. In landfill leachate, relative defluorination efficiencies of PFOS and PFOA were 33% and 45%, respectively, indicating the inhibiting effect of complex organic and inorganic matrix of landfill leachate on the C-F bond breakage. This study demonstrates that electrochemical degradation of PFASs is possible to achieve in complex and brackish streams using a low-cost graphene sponge anode, without forming toxic chlorinated byproducts even in the presence of >7 g L-1 of chloride.

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.

Occurrence of per- and polyfluoroalkyl substances in aquatic environments and their removal by advanced oxidation processes

Per- and polyfluoroalkyl substances (PFAS), one of the main categories of emerging contaminants, are a family of fluorinated organic compounds of anthropogenic origin. PFAS can endanger the environment and human health because of their wide application in industries, long-term persistence, unique properties, and bioaccumulation potential. This study sought to explain the accumulation of different PFAS in water bodies. In aquatic environments, PFAS concentrations range extensively from <0.03 (groundwater; Melbourne, Australia) to 51,000 ng/L (Groundwater, Sweden). Additionally, bioaccumulation of PFAS in fish and water biota has been stated to range from 0.2 (Burbot, Lake Vättern, Sweden) to 13,900 ng/g (Bluegill samples, U.S.). Recently, studies have focused on PFAS removal from aqueous solutions; one promising technique is advanced oxidation processes (AOPs), including microwaves, ultrasound, ozonation, photocatalysis, UV, electrochemical oxidation, the Fenton process, and hydrogen peroxide-based and sulfate radical-based systems. The removal efficiency of PFAS ranges from 3% (for MW) to 100% for UV/sulfate radical as a hybrid reactor. Therefore, a hybrid reactor can be used to efficiently degrade and remove PFAS. Developing novel, efficient, cost-effective, and sustainable AOPs for PFAS degradation in water treatment systems is a critical area of research.

Nanomaterial-Based Advanced Oxidation/Reduction Processes for the Degradation of PFAS

This review focuses on a critical analysis of nanocatalysts for advanced reductive processes (ARPs) and oxidation processes (AOPs) designed for the degradation of poly/perfluoroalkyl substances (PFAS) in water. Ozone, ultraviolet and photocatalyzed ARPs and/or AOPs are the basic treatment technologies. Besides the review of the nanomaterials with greater potential as catalysts for advanced processes of PFAS in water, the perspectives for their future development, considering sustainability, are discussed. Moreover, a brief analysis of the current state of the art of ARPs and AOPs for the treatment of PFAS in water is presented.

Is sorption technology fit for the removal of persistent and mobile organic contaminants from water?

Persistent, Mobile, and Toxic (PMT) and very persistent and very mobile (vPvM) substances are a growing threat to water security and safety. Many of these substances are distinctively different from other more traditional contaminants in terms of their charge, polarity, and aromaticity. This results in distinctively different sorption affinities towards traditional sorbents such as activated carbon. Additionally, an increasing awareness on the environmental impact and carbon footprint of sorption technologies puts some of the more energy-intensive practices in water treatment into question. Commonly used approaches may thus need to be readjusted to become fit for purpose to remove some of the more challenging PMT and vPvM substances, including for example short chained per- and polyfluoroalkyl substances (PFAS). We here critically review the interactions that drive sorption of organic compounds to activated carbon and related sorbent materials and identify opportunities and limitations of tailoring activated carbon for PMT and vPvM removal. Other less traditional sorbent materials, including ion exchange resins, modified cyclodextrins, zeolites and metal-organic frameworks are then discussed for potential alternative or complementary use in water treatment scenarios. Sorbent regeneration approaches are evaluated in terms of their potential, considering reusability, potential for on-site regeneration, and potential for local production. In this context, we also discuss the benefits of coupling sorption to destructive technologies or to other separation technologies. Finally, we sketch out possible future trends in the evolution of sorption technologies for PMT and vPvM removal from water.

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.

Three-dimensional structured electrode for electrocatalytic organic wastewater purification: Design, mechanism and role

Considering the growing need in decentralized water treatment, the application of electrocatalytic processes (EP) to achieve organic wastewater purification will be dominant in the near future due to high efficiency, small reactor assembly as well as the flexibility of operation and management. The catalytic performance of electrode materials determines the development of this technology. Among them, the unique three-dimensional (3D) structure electrode shows better performance than two-dimensional (2D) electrode in increasing mass transfer, enhancing adsorption and exposing more active sites. Hence, this review starts with the introduction of definition, classification, advantages and disadvantages of 3D electrode materials. Then a critical discussion on the design and construction of 3D electrode materials for organic wastewater purification application is provided. Next, the removal mechanism of organic pollutants on the surface of 3D electrode, the role of 3D structure, the design of reactor with 3D electrode, the conversion and toxicity of degradation products, electrode energy efficiency, stability and cost, are comprehensively reviewed. At last, current challenges and future perspectives for the development of 3D electrode materials are addressed. We deem that this review will provide a valuable insight into the design and application of 3D electrodes in environmental water purification.

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 %.

Electrochemical degradation of PFOA and its common alternatives: Assessment of key parameters, roles of active species, and transformation pathway

This study investigates an electrochemical approach for the treatment of water polluted with per- and poly-fluoroalkyl substances (PFAS), looking at the impact of different variables, contributions from generated radicals, and degradability of different structures of PFAS. Results obtained from a central composite design (CCD) showed the importance of mass transfer, related to the stirring speed, and the amount of charge passed through the electrodes, related to the current density on decomposition rate of PFOA. The CCD informed optimized operating conditions which we then used to study the impact of solution conditions. Acidic condition, high temperature, and low initial concentration of PFOA accelerated the degradation kinetic, while DO had a negligible effect. The impact of electrolyte concentration depended on the initial concentration of PFOA. At low initial PFOA dosage (0.2 mg L^-1), the rate constant increased considerably from 0.079 ± 0.001 to 0.259 ± 0.019 min^-1 when sulfate increased from 0.1% to 10%, likely due to the production of SO₄^•-. However, at higher initial PFOA dosage (20 mg L^-1), the rate constant decreased slightly from 0.019 ± 0.001 to 0.015 ± 0.000 min^-1, possibly due to the occupation of active anode sites by excess amount of sulfate. SO₄^•- and ^•OH played important roles in decomposition and defluorination of PFOA, respectively. PFOA oxidation was initiated by one electron transfer to the anode or SO₄^•-, undergoing Kolbe decarboxylation where yielded perfluoroalkyl radical followed three reaction pathways with ^•OH, O₂ and/or H₂O. PFAS electrooxidation depended on the chemical structures where the decomposition rate constants (min^-1) were in the order of 6:2 FTCA (0.031) > PFOA (0.019) > GenX (0.013) > PFBA (0.008). PFBA with a shorter chain length and GenX with -CF₃ branching had slower decomposition than PFOA. While presence of C-H bonds makes 6:2 FTCA susceptible to the attack of ^•OH accelerating its decomposition kinetic. Conducting experiments in mixed solution of all studied PFAS and in natural water showed that the co-presence of PFAS and other water constituents (organic and inorganic matters) had adverse effects on PFAS decomposition efficiency.

Design of nanomaterials for the removal of per- and poly-fluoroalkyl substances (PFAS) in water: Strategies, mechanisms, challenges, and opportunities

Due to their persistent and pervasive distribution and their adverse effects on human health, the removal of per- and polyfluoroalkyl substances (PFAS) from the environment has been the focus of current research. Recent studies have shown that engineered nanomaterials provide great opportunities for their removal by chemical, physical and electrochemical adsorption methods, or as photo- or electrocatalysts that promote their degradation. This review summarizes and discusses the performance of recently reported nanomaterials towards PFAS removal in water treatment applications. We discuss the performance, mechanisms, and PFAS removal conditions of a variety of nanomaterials, including carbon-based, non-metal, single-metal, and multi-metal nanomaterials. We show that nanotechnology provides significant opportunities for PFAS remediation and further nanomaterial development can provide solutions for the removal of PFAS from the environment. We also provide an overview of the current challenges.