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

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.

Organic Electrochemical Transistors with Molecularly Imprinted Polymer Electrodes for Rapid Detection of Perfluorooctanoic Acid

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants linked to adverse health effects, and there is a need for sensors that can detect PFAS in challenging environments. Electrochemical sensors offer significant potential for achieving cost-effective, rapid, and real-time detection of PFAS, particularly in comparison to current detection techniques, which rely on costly chromatographic methods. Here, we report that organic electrochemical transistors (OECTs) containing a molecularly imprinted polymer (MIP) gate electrode can selectively detect perfluorooctanoic acid (PFOA) in seawater. We prepared a molecularly imprinted polyaniline (PANI) gate electrode by polymerizing aniline onto filter paper in the presence of PFOA, followed by rinsing to remove the PFOA. When used as a gate electrode in an organic electrochemical device (OECT), the presence of PFOA produced a measurable change in the OECT source-drain current due to adsorption of PFOA onto the gate electrode, which reduced capacitance and increased impedance. Other molecules produced a weak or no response. Specifically, we show that the device responds strongly to PFOA but only weakly to perfluoropropionic acid (PFPrA), perfluorohexanoic acid (PFHxA), and surfactant 4-dodecylbenzenesulfonic acid (DBSA). The device is also able to selectively detect PFOA in mixtures containing these other PFAS or surfactants. We achieved a detection limit of 1.6 parts per trillion (ppt) or 3.86 × 10-12 M, below the regulatory advisory level of 70 ppt set by the United States Environmental Protection Agency for PFOA. This work demonstrates low-cost sensors capable of rapid and molecularly specific detection of PFOA, which can potentially lead to low-cost sensors for monitoring the concentrations of PFOA and other PFAS in seawater and other challenging environments.

Photodegradation of steroid hormone micropollutants with palladium-porphyrin coated porous PTFE of varied morphological and optical properties

In flow-through reactors, the photodegradation rate can be improved by enhancing contact and increasing the photocatalyst loading. Both can be attained with a higher surface-to-volume ratio. While previous studies focused on thin membranes (30 - 130 µm) with small pore sizes of 20 - 650 nm, this work employed poly(tetrafluoroethylene) (PTFE) supports, of which pore sizes are in the order of 10 µm, while the porosities and thicknesses are variable (22.5 - 45.3 % and 0.2 - 3 mm, respectively). These porous materials were anticipated to allow a higher loading of porphyrin photosensitisers and better light penetration for subsequent photodegradation of steroid hormone micropollutants via singlet oxygen (1O2) generation. The reactor surface refers to the surface within the PTFE pores, while the reactor volume is the total void space inside these pores. The surface-to-volume ratios between 105 and 106 m2/m3 are higher than those of typical microreactors (103 to 104 m2/m3). The weighted average light transmittance varied from 38 % with the thinnest and most porous support to 4.8 % with the thickest support. Good light penetration combined with minimal absorption by PTFE enhanced the light utilisation of the porphyrins when coated in the porous supports. Changes in the support porosity of the coated supports minimally affected steroid hormone removal, because the collision frequency in the very large pores remained relatively constant. However, varying the support thickness, porphyrin loading (0.3 - 7.7 μmol/g), and water flux (150 - 3000 L/m2.h), hence the resulting hydraulic residence time, influenced the collision frequency and steroid hormone removal. Results showed that the supports did not outperform membranes most likely because the larger pore size in the former limited contact between the hormones and 1O2. From photostability testing of the pristine supports, perfluoroalkyl substances (PFAS) released from the supports were found at 10 - 300 ng/L concentrations during accelerated ageing. While PFAS formation was detectable, the quantities during water treatment operations would be extremely low. In summary, this study elucidates the capability and limitations of porous supports coated with photosensitisers to remove waterborne micropollutants.

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.

Promoting C-F Bond Activation for Perfluorinated Compounds Decomposition via Atomically Synergistic Lewis and Brønsted Acid Sites

Catalytic hydrolysis is a sustainable method for the degradation of perfluorinated compounds (PFCs) but is challenged by the high reaction temperatures required to cleave strong C-F bonds. Herein, we developed an innovative C-F activation strategy by constructing synergistic Lewis and Brønsted acid pairs over atomically dispersed Zn-O-Al sites to promote C-F bond activation for decomposition of typical PFCs, CF4. Density functional theory (DFT) calculations demonstrate tricoordinated Al (AlIII) sites and Zn-OH functional, respectively, as Lewis and Brønsted acid sites over Zn-O-Al, synergistically enhancing the adsorption and decomposition of CF4. X-ray absorption spectroscopy (XAS), pyridine infrared spectroscopy (Py-IR), and ammonia temperature-programmed desorption (NH3-TPD) verified the presence of both AlIII and Zn-OH on the atomically dispersed Zn-O-Al sites. CF4-TPD and in situ infrared spectroscopy confirmed that the Zn-O-Al sites facilitate CF4 adsorption and C-F bond activation. As a result, the Zn-O-Al sites with synergistic Lewis and Brønsted acid pairs achieved 100% CF4 decomposition at a low temperature of 560 °C and demonstrated outstanding stability for more than 250 h.

Challenges in PFAS Postdegradation Analysis: Insights from the PFAS-CTAB Model System

Per- and polyfluoroalkyl substances (PFAS) are synthetic chemicals widely used for their oil and water-repellent properties. Their environmental persistence and potential health risks have raised significant concerns. As PFAS degrades through remediation or natural processes, they form complex mixtures of the original chemicals, transformation byproducts, and degradation additives. Analyzing PFAS after degradation presents analytical challenges due to possible chemical and physical interactions, including ion pairing, micelle formation, and complexation. These factors can significantly impact the precision and accuracy of PFAS measurements, yet they are often overlooked in PFAS degradation studies. In this work, we demonstrate that with the addition of ppb-level cetyltrimethylammonium bromide (CTAB), a cationic surfactant used in PFAS plasma-based degradation, the PFAS calibration curve linearity, sensitivity, and reproducibility are severely compromised. Isotopically labeled internal standards cannot fully correct these issues. Furthermore, the standard EPA methods 537.1, 533, and 1633 could not accurately recover PFAS concentrations in the PFAS and CTAB mixtures, with severe matrix effects observed for longer-chain and nitrogen-containing PFAS. Among these methods, Method 1633 is currently the most suitable option for postdegradation analysis. Method 1633 showed the lowest CTAB interference because this method used another weak ion pair additive, formic acid or acetic acid (in commercial lab analysis), to acidify the sample before LC-MS/MS analysis and added an isotopically labeled internal standard. For future PFAS degradation studies, we recommend systematically evaluating the matrix effect on the PFAS quantification using a recovery matrix to validate the analytical methods before use.

Copper-Based Two-Dimensional Conductive Metal-Organic Framework Thin Films for Ultrasensitive Detection of Perfluoroalkyls in Drinking Water

Perfluoroalkyls (PFAS) continue to emerge as a global health threat making their effective detection and capture extremely important. Though metal-organic frameworks (MOFs) have stood out as a promising class of porous materials for sensing PFAS, detection limits remain insufficient and a fundamental understanding of detection mechanisms warrants further investigation. Here, we show the use of a 2D conductive MOF film based on copper hexahydroxy triphenylene (Cu-HHTP) to fabricate chemiresistive sensing devices for detecting PFAS in drinking water. We further show ultrasensitive detection using electrochemical impedance spectroscopy. Owing to excellent electrostatic attractions and electrochemical interactions between the copper-based MOF and PFAS, confirmed by high-resolution spectroscopy and theoretical simulations, the MOF-based sensor reported herein exhibits excellent affinity and sensitivity toward perfluorinated acids at concentrations as low as 0.002 ng/L.

Opportunity and Challenge of Advanced Porous Sorbents for PFAS Removal

Per- and polyfluoroalkyl substances (PFASs), comprising over 9,000 persistent synthetic organic contaminants, are extensively found in the environment and pose significant risks to both human and ecological health. Among the strategies for addressing PFAS contamination, adsorption processes have proven to be cost-effective. Traditional sorbents such as ion-exchange resins and activated carbon have been found to exhibit low adsorption capacities and slow equilibration times. Recent innovations in porous materials, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), however, offer significant improvements in the efficiency of PFAS adsorption. This review thoroughly examines the latest advancements in these materials, analyzing their mechanisms of adsorption, and concludes by suggesting directions for future research that could further enhance their effectiveness in PFAS management.

Unveiling Tetrafluoromethane Decomposition over Alumina Catalysts

Tetrafluoromethane (CF4), the simplest perfluorocompound known as a "forever chemical", presents substantial environmental challenges due to its health risks and contribution to the greenhouse effect. Designing efficient catalysts for CF4 decomposition remains difficult, compounded by limited understanding of the mechanisms. Here, we use constrained ab initio molecular dynamics (cAIMD) simulations and in situ experiments to elucidate the mechanism of alumina-catalyzed CF4 decomposition, highlighting the pivotal role of surface hydroxyl groups. The initial C-F bond breaking is the rate-determining step, with surface hydroxyl groups reducing the reaction free energy from 1.69 to 1.34 eV. These hydroxyl groups also facilitate the self-healing of oxygen vacancies generated during the decomposition. Contrary to the belief that CF4 decomposes directly into CO2, our cAIMD simulations, supported by synchrotron vacuum ultraviolet photoionization mass spectrometry data, reveal significant CF2O and CO byproducts. Experimental data in an anhydrous environment indicate that water primarily replenishes surface hydroxyl groups rather than directly participating in decomposition. We conclude that the relatively high efficiency of Al2O3 catalysts stems from three key properties: (1) the presence of active sites with a specific affinity for CF4 adsorption, ensuring efficient substrate interaction; (2) appropriate metal-oxygen bond strength, enabling the participation of lattice oxygen in the reaction; and (3) a high density of surface hydroxyl groups that facilitate the initial C-F bond cleavage and the self-healing of oxygen vacancies.

A molecular metal-organic cage as a recyclable sponge for PFOS removal from water

A metal-organic cage (MOC) is shown to be an efficient molecular sponge for PFOS. A large association constant is observed for the 2 : 1 PFOS : MOC host-guest complex. Up to 12 equivalents of PFOS per MOC are removed from water. The recycling procedure developed allows for the recovery and reuse of the MOC.

Soluble metal porphyrins - Zero-valent zinc system for effective reductive defluorination of branched per and polyfluoroalkyl substances (PFASs)

Nano zero-valent metals (nZVMs) have been extensively utilized for decades in the reductive remediation of groundwater contaminated with chlorinated organic compounds, owing to their robust reducing capabilities, simple application, and cost-effectiveness. Nevertheless, there remains a dearth of information regarding the efficient reductive defluorination of linear or branched per- and polyfluoroalkyl substances (PFASs) using nZVMs as reductants, largely due to the absence of appropriate catalysts. In this work, various soluble porphyrin ligands [[meso‑tetra(4-carboxyphenyl)porphyrinato]cobalt(III)]Cl·7H2O (CoTCPP), [[meso‑tetra(4-sulfonatophenyl) porphyrinato]cobalt(III)]·9H2O (CoTPPS), and [[meso‑tetra(4-N-methylpyridyl) porphyrinato]cobalt(II)](I)4·4H2O (CoTMpyP) have been explored for defluorination of PFASs in the presence of the nZn0 as reductant. Among these, the cationic CoTMpyP showed best defluorination efficiencies for br-perfluorooctane sulfonate (PFOS) (94%), br-perfluorooctanoic acid (PFOA) (89%), and 3,7-Perfluorodecanoic acid (PFDA) (60%) after 1 day at 70 °C. The defluorination rate constant of this system (CoTMpyP-nZn0) is 88-164 times higher than the VB12-nZn0 system for the investigated br-PFASs. The CoTMpyP-nZn0 also performed effectively at room temperature (55% for br-PFOS, 55% for br-PFOA and 25% for 3,7-PFDA after 1day), demonstrating the great potential of in-situ application. The effect of various solubilizing substituents, electron transfer flow and corresponding PFASs defluorination pathways in the CoTMpyP-nZn0 system were investigated by both experiments and density functional theory (DFT) calculations. SYNOPSIS: Due to the unavailability of active catalysts, available information on reductive remediation of PFAS by zero-valent metals (ZVMs) is still inadequate. This study explores the effective defluorination of various branched PFASs using soluble porphyrin-ZVM systems and offers a systematic approach for designing the next generation of catalysts for PFAS remediation.

An Atomic and Molecular Insight into How PFOA Reduces α-Helicity, Compromises Substrate Binding, and Creates Binding Pockets in a Model Globular Protein

Per- and polyfluoroalkyl substances (PFAS) pose significant health risks due to their widespread presence in various environmental and biological matrices. However, the molecular-level mechanisms underlying the interactions between PFAS and biological constituents, including proteins, carbohydrates, lipids, and DNA, remain poorly understood. Here, we investigate the interactions between a legacy PFAS, viz. perfluorooctanoic acid (PFOA), and the milk protein β-lactoglobulin (BLG) obtained using a combination of experimental and computational techniques. Circular dichroism studies reveal that PFOA perturbs the secondary structure of BLG, by driving a dose-dependent loss of α-helicity and alterations in its β-sheet content. Furthermore, exposure of the protein to PFOA attenuates the on-rate constant for the binding of the hydrophobic probe 8-anilino-1-naphthalene sulfonic acid (ANS), suggesting potential functional impairment of BLG by PFOA. Steered molecular dynamics and umbrella sampling calculations reveal that PFOA binding leads to the formation of an energetically favorable novel binding pocket within the protein, when residues 129-142 are steered to unfold from their initial α-helical structure, wherein a host of intermolecular interactions between PFOA and BLG's residues serve to insert the PFOA into the region between the unfolded helix and beta-sheets. Together, the data provide a novel understanding of the atomic and molecular mechanism(s) by which PFAS modulates structure and function in a globular protein, leading to a beginning of our understanding of altered biological outcomes.