Electrochemical degradation of per- and poly-fluoroalkyl substances using boron-doped diamond electrodes

Improved boron nitride nanomaterial morphologies for the enhanced photocatalytic remediation of perfluorooctanoic acid

Boron nitride nanotubes (BNNT), nanobarbs (BNNB), and nanoparticles (BNNP) were used as photocatalysts under 254 nm UV irradiation for the photodegradation of perfluorooctanoic acid (PFOA). Mass spectrometry was used to monitor PFOA degradation via a stepwise mechanism in which CF2 groups were excised from the parent chain. All BNNM were effective catalysts, with BNNB exhibiting the fastest photodegradation rate, and BNNT showing increased activity upon reuse. The enhanced activity of BNNB and BNNT, along with the ability to incorporate these materials into robust structures, makes them promising candidates for use in the fabrication of photocatalytic filters that can remediate aqueous per- and polyfluoroalkyl substances in flow reactors.

Realizing Direct Hot-Electron Transfer from Metal Nanoparticles to Per- and Polyfluoroalkyl Substances

Per- and polyfluoroalkyl substances (PFAS) are a group of forever synthetic chemicals. They are widely utilized in industries and household appliances because of their remarkable stability and distinctive oil- and water-repellent properties. Despite their broad applications, unfortunately, PFAS are hazardous to all forms of life, including humans. In recent years, the environmental persistence of PFAS has raised significant interest in degrading these substances. However, the strong C-F bonds in these chemicals pose several challenges to their degradation. Plasmons of noble metal nanoparticles (NPs) offer many exciting applications, including photocatalytic reactions. However, an atomistic understanding of plasmon-driven processes remains elusive. In this work, using the real-time time-dependent density functional theory, we have studied the real-time formation of plasmons, hot-carrier generation, and subsequent direct hot-carrier transfer from metal NPs to the PFAS. Our simulations show that there is an apparent direct hot-electron transfer from NPs to PFAS. Moreover, using Ehrenfest dynamics simulations, we demonstrated that the transferred hot electrons can efficiently degrade PFAS without requiring any external thermal bath. Thus, our work provides an atomistic picture of plasmon-induced direct hot-carrier transfer from NPs to PFAS and the efficient degradation of PFAS. We strongly believe that this work generates the impetus to utilize plasmonic NPs to mitigate PFAS.

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.

Role of LiOH in Aqueous Electrocatalytic Defluorination of Perfluorooctanoic Sulfonate: Efficient Li-F Ion Pairing Prevents Anode Fouling by Produced Fluoride

Per- and polyfluoroalkyl substances (PFAS) pose a significant environmental and health threat due to their high toxicity, widespread use, and persistence in the environment. Electrochemical methods have emerged as promising approaches for PFAS destruction, offering cost-effective and energy-efficient solutions. We established recently that electrocatalysis with nonprecious materials enabled the complete defluorination of perfluorooctanesulfonate (PFOS) in aqueous 8.0 M LiOH. Here, we reveal the mechanistic role of LiOH in the efficient aqueous electrocatalytic PFOS defluorination. Our results demonstrate that synergistic effects of high lithium and high hydroxide ion concentrations are essential for complete PFOS defluorination. Two-dimensional NMR data of electrolytes post pulsed electrolysis provide experimental evidence for Li-F ion pairing, which plays a crucial role in preventing anode fouling by produced fluoride, thus enabling sustained C-F bond cleavage. This Li-F ion pairing was increased at high pH, and elevated temperatures enhanced diffusion of Li-F ion pairs into the bulk electrolyte. High hydroxide ion concentrations additionally removed fluoride from the anode surface by competitive adsorption, corroborated by XPS data. Our findings provide quantitative mechanistic insights into the electrocatalytic defluorination process and offer a general route of enhancing the efficiency of anodic PFAS defluorination.

Enhanced perfluorooctanoic acid (PFOA) degradation by electrochemical activation of peroxydisulfate (PDS) during electrooxidation for water treatment

Improved treatment of per- and polyfluoroalkyl substances (PFAS) in water is critically important in light of the proposed United States Environmental Protection Agency (USEPA) drinking water regulations at ng L-1 levels. The addition of peroxymonosulfate (PMS) during electrooxidation (EO) can remove and destroy PFAS, but ng L-1 levels have not been tested, and PMS itself can be toxic. The objective of this research was to test peroxydisulfate (PDS, an alternative to PMS) activation by boron-doped diamond (BDD) electrodes for perfluorooctanoic acid (PFOA) degradation. The influence of PDS concentration, temperature, and environmental water matrix effects, and PFOA concentration on PDS-EO performance were systematically examined. Batch reactor experiments revealed that 99 % of PFOA was degraded and 69 % defluorination was achieved, confirming PFOA mineralization. Scavenging experiments implied that sulfate radicals (SO4-) and hydroxyl radicals (HO) played a more important role for PFOA degradation than 1O2 or electrons (e-). Further identification of PFOA degradation and transformation products by liquid chromatography-mass spectrometry (LC-MS) analysis established plausible PFOA degradation pathways. The analysis corroborates that direct electron transfers at the electrode initiate PFOA oxidation and SO4- improves overall treatment by cleaving the CC bond between the C7F15 and COOH moieties in PFOA, leading to possible products such as C7F15 and F-. The perfluoroalkyl radicals can be oxidized by SO4- and HO, resulting in the formation of shorter chain perfluorocarboxylic acids (e.g., perfluorobutanoic acid [PFBA]), with eventual mineralization to CO2 and F-. At an environmentally relevant low initial concentration of 100 ng L-1 PFOA, 99 % degradation was achieved. The degradation of PFOA was slightly affected by the water matrix as less removal was observed in an environmental river water sample (91 %) compared to tests conducted in Milli-Q water (99 %). Overall, EO with PDS provided a destructive approach for the elimination of PFOA.

Effect of electrolyte composition on electrocatalytic transformation of perfluorooctanoic acid (PFOA) in high pH medium

Recent regulatory actions aim to limit per- and polyfluoroalkyl substances (PFAS) concentrations in drinking water and wastewaters. Regenerable anion exchange resin (AER) is an effective separation process to remove PFAS from water but will require PFAS post-treatment of the regeneration wastestream. Electrocatalytic (EC) processes using chemically boron-doped diamond electrodes, stable in a wide range of chemical compositions show potential to defluorinate PFOA in drinking water and wastewater treatments. Chemical composition and concentration of mineral salts in supporting electrolytes affect AER regeneration efficiency, and play a crucial role in the EC processes. Their impact on PFAS degradation remains understudied. This study investigates the impact of 17 brine electrolytes with different compositions on perfluorooctanoic acid (PFOA) degradation in an alkaline medium and explores the correlation between the rate of PFOA degradation and the solution's conductivity. Results show that higher electrolyte concentrations and conductivity lead to faster PFOA degradation rates. The presence of chloride anions have negligible impact on the degradation rate. However, the presence of nitrate salts reduce PFOA degradation efficiency. Additionally, the use of mixed electrolytes may be a promising approach for reducing the cost of EC operations. PFOA degradation was not influenced by the pH of the bulk solution.

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.

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.

Combination of separation and degradation methods after PFAS soil washing

The current study evaluated a three-stage treatment to remediate PFAS-contaminated soil. The treatment consisted of soil washing, foam fractionation (FF), and electrochemical oxidation (EO). The possibility of replacing the third stage, i.e., EO, with an adsorption process was also assessed. The contamination in the studied soils was dominated by perfluorooctane sulfonate (PFOS), with a concentration of 760 and 19 μg kg-1 in soil I and in soil II, accounting for 97 % and 70 % of all detected per-and polyfluoroalkyl substances (PFAS). Before applying a pilot treatment of soil, soil washing was performed on a laboratory scale, to evaluate the effect of soil particle size, initial pH and a liquid-to-soil ratio (L/S) on the leachability of PFAS. A pilot washing system generated soil leachate that was subsequently treated using FF and EO (or adsorption) and then reused for soil washing. The results indicated that the leaching of PFAS occurred easier in 0.063-1 mm particles than in the soil particles having a size below 0.063 mm. Both alkaline conditions and a continual replacement of the leaching solution increased the leachability of PFAS. The analysis using one-way ANOVA showed no statistical difference in means of PFOS washed out in laboratory and pilot scales. This allowed estimating twenty washing cycles using 120 L water to reach 95 % PFOS removal in 60 kg soil. The aeration process removed 95-99 % PFOS in every washing cycle. The EO and adsorption processes achieved similar results removing up to 97 % PFOS in concentrated soil leachate. The current study demonstrated a multi-stage treatment as an effective and cost-efficient method to permanently clean up PFAS-contaminated soil.

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.

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.

Research Updates on the Mechanism and Influencing Factors of the Photocatalytic Degradation of Perfluorooctanoic Acid (PFOA) in Water Environments

Perfluorooctanoic acid is ubiquitous in water bodies and is detrimental to the health of organisms. Effectively removing perfluorooctanoic acid (PFOA), a persistent organic pollutant, has been a hot topic around the world. With traditional physical, chemical, and biological methods, it is difficult to effectively and completely remove PFOA, the costs are high, and it is easy to cause secondary pollution. There are difficulties in applying some technologies. Therefore, more efficient and green degradation technologies have been sought. Photochemical degradation has been shown to be a low-cost, efficient, and sustainable technique for PFOA removal from water. Photocatalytic degradation technology offers great potential and prospects for the efficient degradation of PFOA. Most studies on PFOA have been conducted under ideal laboratory conditions at concentrations that are higher than those detected in real wastewater. This paper summarizes the research status of the photo-oxidative degradation of PFOA, and it summarizes the mechanism and kinetics of PFOA degradation in different systems, as well as the influence of key factors on the photo-oxidative degradation and defluoridation process, such as system pH, photocatalyst concentration, etc. PFOA photodegradation technology's existing problems and future work directions are also presented. This review provides a useful reference for future research on PFOA pollution control technology.

Application of Electrochemical Oxidation for Water and Wastewater Treatment: An Overview

In recent years, the discharge of various emerging pollutants, chemicals, and dyes in water and wastewater has represented one of the prominent human problems. Since water pollution is directly related to human health, highly resistant and emerging compounds in aquatic environments will pose many potential risks to the health of all living beings. Therefore, water pollution is a very acute problem that has constantly increased in recent years with the expansion of various industries. Consequently, choosing efficient and innovative wastewater treatment methods to remove contaminants is crucial. Among advanced oxidation processes, electrochemical oxidation (EO) is the most common and effective method for removing persistent pollutants from municipal and industrial wastewater. However, despite the great progress in using EO to treat real wastewater, there are still many gaps. This is due to the lack of comprehensive information on the operating parameters which affect the process and its operating costs. In this paper, among various scientific articles, the impact of operational parameters on the EO performances, a comparison between different electrochemical reactor configurations, and a report on general mechanisms of electrochemical oxidation of organic pollutants have been reported. Moreover, an evaluation of cost analysis and energy consumption requirements have also been discussed. Finally, the combination process between EO and photocatalysis (PC), called photoelectrocatalysis (PEC), has been discussed and reviewed briefly. This article shows that there is a direct relationship between important operating parameters with the amount of costs and the final removal efficiency of emerging pollutants. Optimal operating conditions can be achieved by paying special attention to reactor design, which can lead to higher efficiency and more efficient treatment. The rapid development of EO for removing emerging pollutants from impacted water and its combination with other green methods can result in more efficient approaches to face the pressing water pollution challenge. PEC proved to be a promising pollutants degradation technology, in which renewable energy sources can be adopted as a primer to perform an environmentally friendly water treatment.

Combination of adsorption/desorption and photocatalytic reduction processes for PFOA removal from water by using an aminated biosorbent and a UV/sulfite system

Per- and polyfluoroalkyl substances (PFAS) are stable organic chemicals, which have been used globally since the 1940s and have caused PFAS contamination around the world. This study explores perfluorooctanoic acid (PFOA) enrichment and destruction by a combined method of sorption/desorption and photocatalytic reduction. A novel biosorbent (PG-PB) was developed from raw pine bark by grafting amine groups and quaternary ammonium groups onto the surface of bark particles. The results of PFOA adsorption at low concentration suggest that PG-PB has excellent removal efficiency (94.8%-99.1%, PG-PB dosage: 0.4 g/L) to PFOA in the concentration range of 10 μg/L to 2 mg/L. The PG-PB exhibited high adsorption efficiency regarding PFOA, being 456.0 mg/g at pH 3.3 and 258.0 mg/g at pH 7 with an initial concentration of 200 mg/L. The groundwater treatment reduced the total concentration of 28 PFAS from 18 000 ng/L to 9900 ng/L with 0.8 g/L of PG-PB. Desorption experiments examined 18 types of desorption solutions, and the results showed that 0.05% NaOH and a mixture of 0.05% NaOH + 20% methanol were efficient for PFOA desorption from the spent PG-PB. More than 70% (>70 mg/L in 50 mL) and 85% (>85 mg/L in 50 mL) of PFOA were recovered from the first and second desorption processes, respectively. Since high pH promotes PFOA degradation, the desorption eluents with NaOH were directly treated with a UV/sulfite system without further adjustment. The final PFOA degradation and defluorination efficiency in the desorption eluents with 0.05% NaOH + 20% methanol reached 100% and 83.1% after 24 h reaction. This study proved that the combination of adsorption/desorption and a UV/sulfite system for PFAS removal is a feasible solution for environmental remediation.

Electrochemical Oxidation for Treatment of PFAS in Contaminated Water and Fractionated Foam-A Pilot-Scale Study

Per- and polyfluoroalkyl substances (PFAS) are persistent synthetic contaminants that are present globally in water and are exceptionally difficult to remove during conventional water treatment processes. Here, we demonstrate a practical treatment train that combines foam fractionation to concentrate PFAS from groundwater and landfill leachate, followed by an electrochemical oxidation (EO) step to degrade the PFAS. The study combined an up-scaled experimental approach with thorough characterization strategies, including target analysis, PFAS sum parameters, and toxicity testing. Additionally, the EO kinetics were successfully reproduced by a newly developed coupled numerical model. The mean total PFAS degradation over the designed treatment train reached 50%, with long- and short-chain PFAS degrading up to 86 and 31%, respectively. The treatment resulted in a decrease in the toxic potency of the water, as assessed by transthyretin binding and bacterial bioluminescence bioassays. Moreover, the extractable organofluorine concentration of the water decreased by up to 44%. Together, these findings provide an improved understanding of a promising and practical approach for on-site remediation of PFAS-contaminated water.

Opportunities for in situ electro-regeneration of organic contaminant-laden carbonaceous adsorbents

Adsorptive separation technologies have proven to be effective on organic contaminant removal in aqueous water. However, the breakthrough of contaminants is inevitable and can be at relatively low bed volumes, which makes the regeneration of spent adsorbents an urgent need. Electrochemically induced regeneration processes are given special attention and may provide ease of operation through in situ regeneration avoiding (i) removal and transport adsorbents, and (ii) avoiding use of hazardous chemicals (i.e., organic solvents, acids, or bases). Therefore, this review article critically evaluates the fundamental aspects of in situ electro-regeneration for spent carbons, and later discusses specific examples related to the treatment of emerging contaminants (such as per- and polyfluoroalkyl substances or PFAS). The fundamental concepts of electrochemically driven processes are comprehensively defined and addressed in terms of (i) adsorbent characteristics, (ii) contaminant properties, (iii) adsorption/regeneration driving operational parameters and conditions, and (iv) the competitive effects of water matrices. Additionally, future research needs and challenges to enhance understanding of in situ electro-regeneration applications for organic contaminants (specifically PFAS)-laden adsorbents are identified and outlined as a future key perspective.

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 %.

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.

The Need for Testing Isomer Profiles of Perfluoroalkyl Substances to Evaluate Treatment Processes

Many environmentally relevant poly-/perfluoroalkyl substances (PFASs) including perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) exist in different isomeric (branched and linear) forms in the natural environment. The isomeric distribution of PFASs in the environment and source waters is largely controlled by the source of contamination and varying physicochemical properties imparted by their structural differences. For example, branched isomers of PFOS are relatively more reactive and less sorptive compared to the linear analogue. As a result, the removal of branched and linear PFASs during water treatment can vary, and thus the isomeric distribution in source waters can influence the overall efficiency of the treatment process. In this paper, we highlight the need to consider the isomeric distribution of PFASs in contaminated matrices while designing appropriate remediation strategies. We additionally summarize the known occurrence and variation in the physicochemical properties of PFAS isomers influencing their detection, fate, toxicokinetics, and treatment efficiency.

Electrochemical Advanced Oxidation of Perfluorooctanoic Acid: Mechanisms and Process Optimization with Kinetic Modeling

Electrochemical advanced oxidation processes (EAOPs) are promising technologies for perfluorooctanoic acid (PFOA) degradation, but the mechanisms and preferred pathways for PFOA mineralization remain unknown. Herein, we proposed a plausible primary pathway for electrochemical PFOA mineralization using density functional theory (DFT) simulations and experiments. We neglected the unique effects of the anode surface and treated anodes as electron sinks only to acquire a general pathway. This was the essential first step toward fully revealing the primary pathway applicable to all anodes. Systematically exploring the roles of valence band holes (h⁺), hydroxyl radicals (HO^•), and H₂O, we found that h⁺, whose contribution was previously underestimated, dominated PFOA mineralization. Notably, the primary pathway did not generate short-chain perfluoroalkyl carboxylic acids (PFCAs), which were previously thought to be the main degradation intermediates, but generated other polyfluorinated alkyl substances (PFASs) that were rapidly degraded upon formation. Also, we developed a simplified kinetic model, which considered all of the main processes (mass transfer with electromigration included, surface adsorption/desorption, and oxidation on the anode surface), to simulate PFOA degradation in EAOPs. Our model can predict PFOA concentration profiles under various current densities, initial PFOA concentrations, and flow velocities.

Efficient removal of trifluoroacetic acid from water using surface-modified activated carbon and electro-assisted desorption

Trifluoroacetic acid (TFA) is a very persistent, very mobile substance (vPvM) with potential toxicity, and causes increasing environmental concerns worldwide. Conventional wastewater treatment strategies are inefficient for selective TFA removal in the presence of inorganic anions. Here we show that surface defunctionalized activated carbon felt (DeACF) carrying anion exchange sites exhibits an outstanding adsorption efficiency towards TFA thanks to introduced electrostatic attraction and enhanced interactions between hydrophobic carbon surface and CF₃ moieties (q_max = 30 mg/g, K_d = (840 ± 80) L/kg at c_TFA = 3.4 mg/L in tap water). Flow-cell experiments demonstrated a strongly favored TFA uptake by DeACF from tap water over Cl^- and SO₄^2- but a remarkable co-adsorption of the inorganic water contaminant NO₃^-. Electro-assisted TFA desorption using 10 mM Na₂SO₄ as electrolyte and oxidized ACF as anode showed high recoveries of ≥ 87% at low cell voltages (< 1.1 V). Despite an initial decrease in TFA adsorption capacity (by 33%) caused by partial surface oxidation of DeACF after the 1st ad-/desorption cycle, the system stability was fully maintained over the next 4 cycles. Such electro-assisted 'trap&release' approach for TFA removal can be exploited for on-site regenerable adsorption units and as a pre-concentration step combined with degradation technologies.

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

Electrochemical oxidation (EO) is emerging as one of the most promising methods for the degradation of recalcitrant per- and poly-fluoroalkyl substances (PFASs) in water and wastewater, as these compounds cannot be effectively treated with conventional bio- or chemical approaches. This review examines the state of the art of EO for PFASs destruction, and comprehensively compares operating parameters and treatment performance indicators for both synthetic and real contaminated water and wastewater media. The evaluation shows the need to use environmentally-relevant media to properly quantify the effectiveness/efficiency of EO for PFASs treatment. Additionally, there is currently a lack of quantification of sorption losses, resulting in a likely over-estimation of process' efficiencies. Furthermore, the majority of experimental results to date indicate that short-chain PFASs are the most challenging and need to be prioritized as environmental regulations become more stringent. Finally, and with a perspective towards practical implementation, several operational strategies are proposed, including processes combining up-concentration followed by EO destruction.

Ultrasensitive Diamond Microelectrode Application in the Detection of Ca2+ Transport by AnnexinA5-Containing Nanostructured Liposomes

This report describes the innovative application of high sensitivity Boron-doped nanocrystalline diamond microelectrodes for tracking small changes in Ca²⁺ concentration due to binding to Annexin-A5 inserted into the lipid bilayer of liposomes (proteoliposomes), which could not be assessed using common Ca²⁺ selective electrodes. Dispensing proteoliposomes to an electrolyte containing 1 mM Ca²⁺ resulted in a potential jump that decreased with time, reaching the baseline level after ~300 s, suggesting that Ca²⁺ ions were incorporated into the vesicle compartment and were no longer detected by the microelectrode. This behavior was not observed when liposomes (vesicles without AnxA5) were dispensed in the presence of Ca²⁺. The ion transport appears Ca²⁺-selective, since dispensing proteoliposomes in the presence of Mg²⁺ did not result in potential drop. The experimental conditions were adjusted to ensure an excess of Ca²⁺, thus confirming that the potential reduction was not only due to the binding of Ca²⁺ to AnxA5 but to the transfer of ions to the lumen of the proteoliposomes. Ca²⁺ uptake stopped immediately after the addition of EDTA. Therefore, our data provide evidence of selective Ca²⁺ transport into the proteoliposomes and support the possible function of AnxA5 as a hydrophilic pore once incorporated into lipid membrane, mediating the mineralization initiation process occurring in matrix vesicles.

Sonolysis of per- and poly fluoroalkyl substances (PFAS): A meta-analysis

Human ingestion of per- and polyfluoroalkyl substances (PFAS) from contaminated food and water is linked to the development of several cancers, birth defects and other illnesses. The complete mineralisation of aqueous PFAS by ultrasound (sonolysis) into harmless inorganics has been demonstrated in many studies. However, the range and interconnected nature of reaction parameters (frequency, power, temperature etc.), and variety of reaction metrics used, limits understanding of degradation mechanisms and parametric trends. This work summarises the state-of-the-art for PFAS sonolysis, considering reaction mechanisms, kinetics, intermediates, products, rate limiting steps, reactant and product measurement techniques, and effects of co-contaminants. The meta-analysis showed that mid-high frequency (100 - 1,000 kHz) sonolysis mechanisms are similar, regardless of reaction conditions, while the low frequency (20 - 100 kHz) mechanisms are specific to oxidative species added, less well understood, and generally slower than mid-high frequency mechanisms. Arguments suggest that PFAS degradation occurs via adsorption (not absorption) at the bubble interface, followed by headgroup cleavage. Further mechanistic steps toward mineralisation remain to be proven. For the first time, complete stoichiometric reaction equations are derived for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) sonolysis, which add H2 as a reaction product and consider CO an intermediate. Fluorinated intermediate products are derived for common, and more novel PFAS, and a naming system proposed for novel perfluoroether carboxylates. The meta-analysis also revealed the transition between pseudo first and zero order PFOA/S kinetics commonly occurs at 15 - 40 µM. Optimum values of; ultrasonic frequency (300 - 500 kHz), concentration (>15 - 40 μM), temperature (≈20 °C), and pH range (3.2 - 4) for rapid PFOX degradation are derived by evaluation of prior works, while optimum values for the dilution factor applied to PFAS containing firefighting foams and applied power require further work. Rate limiting steps are debated and F- is shown to be rate enhancing, while SO42- and CO2 by products are theorised to be rate limiting. Sonolysis was compared to other PFAS destructive technologies and shown to be the only treatment which fully mineralises PFAS, degrades different PFAS in order of decreasing hydrophobicity, is parametrically well studied, and has low-moderate energy requirements (several kWh g-1 PFAS). It is concluded that sonolysis of PFAS in environmental samples would be well incorporated within a treatment train for improved efficiency.

Study of MOF incorporated dual layer membrane with enhanced removal of ammonia and per-/poly-fluoroalkyl substances (PFAS) in landfill leachate treatment

Landfill leachate is a highly polluted and complex wastewater as it contains large amounts of organic matters, ammonia‑nitrogen, heavy metals, and per-/poly-fluoroalkyl substances (PFAS), which makes its treatment very challenging. In this paper, hydrophilic/hydrophobic dual layer membranes combining advantages of pervaporation and membrane distillation was employed to treat leachate in a direct contact membrane distillation (DCMD) configuration. An aluminum fumarate (AlFu) metal organic framework (MOF) incorporated poly(vinyl alcohol) (PVA) hydrophilic layer was coated on hydrophobic PTFE membrane to overcome the low separation efficiency of PFAS and ammonia and wetting issues encountered by the conventional hydrophobic PTFE membrane used for DCMD. The rejections of dual layer membranes with different MOF loading to PFAS, ammonia, TOC and TDS were assessed based on the amount of AlFu MOF incorporated into the PVA layer. Based on the conducted adsorption tests, it was found that AlFu MOF increases the rejection of PVA layer to PFAS and ammonia. The coating of the hydrophilic layer could enhance the wetting resistance with/without MOF addition. In comparison with the pristine PTFE membrane using synthetic feed containing 3 wt% NaCl, 1 wt% addition of AlFu MOF into the PVA layer showed slightly increased flux. All the tested membranes showed more than 99% rejection to TOC. The rejection to ammonia was increased as more MOF was incorporated into the PVA layer. The maximum rejection of ammonia was 99.8% when the PVA layer containing 10% MOF. All the membranes showed more than 99% rejection to PFOS and PFHxS. However, PTFE membrane did not show any rejection to PFOA. As more MOF was added into the hydrophilic layer, the rejection to PFOA increased, but plateaued at 65.6% with 5% MOF incorporation into the hydrophilic layer.