New Indole Derivative Heterogeneous System for the Synergistic Reduction and Oxidation of Various Per-/Polyfluoroalkyl Substances: Insights into the Degradation/Defluorination Mechanism

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

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

Concurrent redox reactions for perfluorocarboxylic acids decontamination via UV-activated tryptophan/carbon nanotubes

The contamination and persistence of Perfluorooctanoic Acid (PFOA) in aquatic environments have escalated environmental concerns, driving extensive research into effective decontamination strategies. To enhance the removal efficiency of PFOA via Advanced Reduction Processes (ARP) utilizing UV irradiation of tryptophan (Trp), carbon nanotubes (CNT) were incorporated, resulting in the development of a UV-Trp/CNT system. This novel process demonstrated a significant improvement in PFOA removal kinetics, as well as defluorination and Total Organic Carbon (TOC) reduction, and was effective across a broad spectrum of perfluoroalkyl carboxylic acids (PFCAs). In addition to the advanced reduction mechanism driven by hydrated electrons (eaq-), quenching experiments, material characterization, and chemical calculations indicated that CNTs facilitated the enrichment of Trp and PFOA, enabling electron transfer from PFOA to Trp via the CNT surface. This established a novel reaction pathway for PFOA oxidation coupled with ARP. The sequential defluorination of -CF₂- groups was facilitated by eaq-, while the electron transfer mechanism enabled oxidative decarboxylation, electron rearrangement, CC bond cleavage, and carbon chain shortening. These oxidative and reductive processes alternated systematically, advancing the development of a synergistic redox approach for the removal of PFCAs and inspiring further exploration into the use of carbon materials to construct confined domains and catalyze the degradation of PFASs.

Plasma-Generated Free Electrons Induced Perfluorooctanoic Acid Efficient Degradation at the Gas-Liquid Interface

Low-temperature plasma, generating both reductive electrons and diverse oxidative species, has demonstrated considerable potential for the degradation of perfluorooctanoic acid (PFOA). However, limited understanding of electron propagation mechanisms during discharge has led previous research to focus on hydrated electrons (eaq-) while neglecting free electrons (e-). In this study, a consistent and modeled dielectric barrier discharge (DBD) plasma was employed to degrade PFOA. Contribution analysis indicated that reactions driven by e- were dominant, with substantial contributions from hydroxyl radical (•OH)-mediated oxidation. By integrating a kinetic model with a streamer solver, a basic discharge unit model was developed. Simulation of e- streamer propagation identified a high-intensity response electric field formed by the e- memory effect, with a peak strength of 1.816 × 106 V/m. This electric field facilitated a secondary acceleration of e-, allowing e- to penetrate the surface water layer and directly attack PFOA via chain-shortening mechanisms. The delocalized state of e- restricted degradation primarily to the gas-liquid interface, minimizing interference from the surrounding medium. This study highlights the previously overlooked role of e- and provides essential theoretical insights for the plasma-based treatment of PFOA-contaminated water.

Quinones stimulate reactive oxygen species production from zero-valent iron over centimeter distances

Zero-valent iron (ZVI) has demonstrated high potential for in-situ remediation of contaminated groundwater and soils. Upon exposure to oxygen, ZVI generates reactive oxygen species (ROS). In parallel with the electron transfer mediated-reductive path, ROS plays a critical role in the oxidative degradation of organic pollutants during ZVI remediation of groundwater and soil. Yet, the efficiency is often constrained by the confined ROS production localized to the surface or immediate vicinity of ZVI particles. Here, we demonstrate that quinones significantly enhance ROS production from ZVI over centimeter-scale distances. H₂O₂ and •OH were detected over 1 cm from ZVI particles after 24 h of incubation, with production increasing alongside quinone concentration and incubation time, reaching 318.3 ± 50.0 µM and 1263.2 ± 143.5 nM at 2 mm, respectively. The broad applicability of quinone in promoting remote ROS generation was demonstrated for various ZVI materials. This remote ROS production is driven by sequential electron transfer from ZVI to quinone, long-distance electron transfer via quinone, and subsequent electron transfer from reduced quinone to oxygen. The resulting increase in ROS production amount and extended range improved ZVI remediation efficiency by 5- to 15-fold for organic pollutant degradation. These findings provide a promising strategy for enhancing ROS-mediated ZVI remediation in heterogeneous environments.

Insight into the Photocatalytic Degradation Mechanism for "Forever Chemicals" PFNA by Reduced Graphene Oxide/WO3 Nanoflower Heterostructures

Water contamination with "forever chemicals" like per- and polyfluoroalkyl substances (PFAS) poses significant toxicity to the environment. Since they are the most persistent synthetic chemicals that hardly degrade in the natural environment and are carcinogenic to humans, there is an urgent need to discover novel processes for destroying PFAS. Herein, we report on the design of a reduced graphene oxide (r-GO)/WO3 nanoflower (WO3-NF)-based heterostructure for harnessing 365 nm light-driven photocatalytic oxidation and reduction process toward the photocatalytic degradation of perfluorononanoic acid (PFNA). Moreover, reported data reveal that using an r-GO/WO3-NF heterostructure photocatalyst, 100% PFNA degradation and 14% defluorination can be achieved in the presence of isopropyl alcohol as the hydroxy radical (•OH) quencher or glucose as a hot hole (h+) quencher after exposure to 365 nm light for 22 h. A reported mechanistic study shows synergistic oxidation and reduction processes are vital for the complete degradation of PFNA, where the hydrated electron (eaq -) plays a key role as a reducing agent and h+ and •OH act as oxidation agents. Furthermore, the photocatalytic destruction mechanism study indicates that chain shortening via C-C bond breaking and defluorination via C-F bond breaking are major pathways for PFNA degradation. A wavelength-dependent study shows that only 22% degradation can be achieved after exposure to 532 nm light for 22 h, which is due to the lack of the formation of hydrated electrons (eaq -). The current study sheds light on the construction of the r-GO/WO3 NF heterojunction for the highly efficient degradation of PFAS.

Unveiling the Contribution of Hydrogen Radicals to Per- and Polyfluoroalkyl Substances (PFASs) Defluorination: Applicability and Degradation Mechanisms

At present, the defluorination of per- and polyfluoroalkyl substances (PFASs), including perfluoroether compounds as substitutes of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate, is limited by the effective active species produced during the oxidation-reduction process. The contribution of the hydrogen radical (•H) as a companion active substance in the photoreduction and electrocatalytic degradation of PFASs has been neglected. Herein, we demonstrate that perfluorocarboxylic acids and perfluoroether compounds such as PFOA and hexafluoropropylene oxide dimer acid (GenX) underwent near-complete photodegradation and effective defluorination by continuously generating •H through perfluoroalkyl radical activation of water under UV irradiation without any reagents and catalysts. Importantly, the initial dissolved oxygen, H+, and impurities in surface water scarcely inhibited the defluorination of the PFASs. The difference in the defluorination mechanism between PFOA and GenX under the action of •H was elucidated by combining theoretical calculations with targeted and nontargeted analysis methods. The investigation of the photodegradation of different PFASs indicates that perfluoroether compounds were not easily photodegraded via reduction of •H compared with other compounds, whereas polyfluorinated compounds in which some F atoms were replaced with Cl were more prone to elimination. However, the UV/•H system was ineffective against perfluorosulfonic acids. This study provides an unprecedented perspective for further development of the removal technology of PFASs and the design of alternative PFASs that are easy to eliminate.

Electrostatic Field in Contact-Electro-Catalysis Driven C-F Bond Cleavage of Perfluoroalkyl Substances

Perfluoroalkyl substances (PFASs) are persistent and toxic to human health. It is demanding for high-efficient and green technologies to remove PFASs from water. In this study, a novel PFAS treatment technology was developed, utilizing polytetrafluoroethylene (PTFE) particles (1-5 μm) as the catalyst and a low frequency ultrasound (US, 40 kHz, 0.3 W/cm2) for activation. Remarkably, this system can induce near-complete defluorination for different structured PFASs. The underlying mechanism relies on contact electrification between PTFE and water, which induces cumulative electrons on PTFE surface, and creates a high surface voltage (tens of volts). Such high surface voltage can generate abundant reactive oxygen species (ROS, i.e., O2⋅-, HO⋅, etc.) and a strong interfacial electrostatic field (IEF of 109~1010 V/m). Consequently, the strong IEF significantly activates PFAS molecules and reduces the energy barrier of O2⋅- nucleophilic reaction. Simultaneously, the co-existence of surface electrons (PTFE*(e-)) and HO⋅ enables synergetic reduction and oxidation of PFAS and its intermediates, leading to enhanced and thorough defluorination. The US/PTFE method shows compelling advantages of low energy consumption, zero chemical input, and few harmful intermediates. It offers a new and promising solution for effectively treating the PFAS-contaminated drinking water.

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

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

Novel Defluorination Pathways of Perfluoroether Compounds (GenX): α-Fe2O3 Nanoparticle Layer Retains Higher Concentrations of Effective Hydrated Electrons

The development of efficient defluorination technology is an important issue because the kind of emerging pollutant of hexafluoropropylene oxide dimer acid (GenX) as an alternative to perfluorooctanoic acid (PFOA) has the higher environmental risks. In the UV/bisulfite system, we first developed a hydrophobic confined α-Fe2O3 nanoparticle layer rich in oxygen vacancies, which accelerated the enrichment of HSO3- and GenX on the surface and pores through electrostatic attraction and hydrophobic interaction, retaining more hydrated electrons (eaq-) and rapidly destroying GenX under UV excitation. Especially, under anaerobic and aerobic conditions, the degradation percentage of GenX obtain nearly 100%, defluorination of GenX to 88 and 57% respectively. It was amazed to find that the three parallel H/F exchange pathways triggered by the rapid reactions of eaq- and GenX, which were unique to anaerobic conditions, improved the efficiency of fluoride removal and weaken the interference of dissolved oxygen and H+. Therefore, this study provided an available material and mechanism for sustainable fluoride removal from wastewater in aerobic and anaerobic conditions.

Integrating molecular descriptors for enhanced prediction: Shedding light on the potential of pH to model hydrated electron reaction rates for organic compounds

Hydrated electron reaction rate constant (ke-aq) is an important parameter to determine reductive degradation efficiency and to mitigate the ecological risk of organic compounds (OCs). However, OC species morphology and the concentration of hydrated electrons (e-aq) in water vary with pH, complicating OC fate assessment. This study introduced the environmental variable of pH, to develop models for ke-aq for 701 data points using 3 descriptor types: (i) molecular descriptors (MD), (ii) quantum chemical descriptors (QCD), and (iii) the combination of both (MD + QCD). Models were screened using 2 descriptor screening methods (MLR and RF) and 14 machine learning (ML) algorithms. The introduction of QCDs that characterized the electronic structure of OCs greatly improved the performance of models while ensuring the need for fewer descriptors. The optimal model MLR-XGBoost(MD + QCD), which included pH, achieved the most satisfactory prediction: R2tra = 0.988, Q2boot = 0.861, R2test = 0.875 and Q2test = 0.873. The mechanistic interpretation using the SHAP method further revealed that QCDs, polarizability, volume, and pH had a great influence on the reductive degradation of OCs by e-aq. Overall, the electrochemical parameters (QCDs, pH) related to the solvent and solute are of significance and should be considered in any future ML modeling that assesses the fate of OCs in aquatic environment.