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

Graphene oxide with 1-nm-thick adlayer for efficient and near-instant removal of per- and polyfluoroalkyl substances

The environmental occurrence of anthropogenic chemicals-especially persistent micropollutants of per- and polyfluoroalkyl substances (PFAS)-raises pressing concerns for global drinking-water safety. Adsorption is an effective technology for removing PFAS but is limited by unsatisfactory adsorption capacity and efficiency. We report a strategy to attach polyamine adlayers to graphene oxide (GO) nanosheets that produces highly charged and monodispersed 2D adsorbents of a GO nanosheet sandwiched between two 1-nm-thick polyamine adlayers. This adsorbent has a high adsorption capacity for PFAS of ∼3070 mg/g-tens of times greater than that of GO and commercial activated carbon. It also provides almost instant adsorption of a variety of PFAS and reaches 57%-95% of its equilibrium capacity in a minute and removes ∼100% of PFAS from contaminated water sources within a few minutes, transforming real-life PFAS-contaminated water into safe drinking water. Experiment and theory show that the planar nature of the 2D adsorbent combined with its abundant surface adsorption sites that electrostatically attract the polar groups of the PFAS, and hydrogen bonding and hydrophobic-hydrophobic interactions with their non-polar groups, account for its ultra-high adsorption capacity and rapid removal efficiency. We also show that regeneration of the adsorbents removes the adsorbed PFAS and allows subsequent destruction, demonstrating a closed-loop treatment solution for micropollutant contamination.

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.

Durable Ti4O7 Heterojunction Composite Membrane Encapsulating N-Doped Graphene Nanosheets for Efficient Electro-Oxidation of GenX and Other PFAS in Fluorochemical Wastewater

Rational interfacial engineering design of an electrocatalyst, such as a heterojunction structure, can effectively enhance its catalytic activity. This study aims to address a critical challenge associated with the use of carbon material@Ti4O7 heterojunction composite electrodes for wastewater treatment─electrode stability over long-term operation. Herein, we report a highly stabilized interfacial engineering strategy, i.e., the use of conductive inorganic CeO2 as a "cement" to firmly encapsulate N-doped graphene oxide nanosheets (N-GS) on the Ti4O7 surface. The defect-rich N-GS encapsulated on the Ti4O7 surface significantly enhances interfacial charge transfer. This enhancement results in the N-GS/CeO2@Ti4O7 heterojunction composite electrode exhibiting excellent efficiency in the electro-oxidation of hexafluoropropylene oxide dimer acid (HFPO-DA or GenX). Furthermore, a flow-through N-GS/CeO2@Ti4O7 reactive electrochemical membrane system effectively mineralizes other 35 PFASs in a real fluorochemical wastewater sample, achieving a high defluorination rate of 70-90% and exhibiting better performance in PFAS destruction and energy efficiency compared to the UV/KI-SO32- process. Results of this study enhance our understanding of the electrochemical oxidation of PFAS and offer valuable insight into the design of stabilized Ti4O7 heterojunction composites. These findings are instrumental in advancing the development of effective treatments for PFAS-contaminated environments.

Integrating kinetic modeling and experimental insights: PFAS electrochemical degradation in concentrated streams with a focus on organic and inorganic effects

This study investigated the impact of organic and inorganic constituents on electrochemical degradation of per- and poly-fluoroalkyl substances (PFAS) in a sulfate-based brine from regeneration of spent ion exchange (IX) resin. The system's performance was assessed in the presence of natural organic matter (NOM) and common inorganic constituents: chloride, nitrate, and bicarbonate. Results revealed distinct outcomes based on constituent type, concentration, and specific PFAS variant. NOM hindered PFAS decomposition, especially for more hydrophobic compounds. Chloride reduced degradation and defluorination efficiencies through competitive interactions with PFAS for the anode's active sites and scavenging effects on SO4•- and •OH. Nitrate and bicarbonate minimally impacted degradation but significantly reduced defluorination. Investigating the electrochemical process in real brine solutions showed higher efficiency and lower electrical energy consumption when methanol was distilled, as methanol scavenges reactive radicals and competes for active anode sites. A kinetic model was also developed to determine the direct electron transfer (DET) and mass transfer coefficients for the species present, considering both surface and bulk solution interactions. The model predicted mass transfer (mol m-2 s-1) and DET (m2 mol-1 s-1) coefficients of 6:2 FTCA, PFOA, GenX, and PFBA to be (5.0 ×10-10, 3.7 ×1011), (1.0 ×10-9, 8.0 ×108), (6.0 ×10-8, 7.5 ×108), and (6.2 ×10-8, 4.2 ×108), respectively.

PFAS in landfill leachate: Practical considerations for treatment and characterization

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

Unveiling the Mechanism and Kinetics of Pollutant Attenuation by Free Radicals Triggered from Goethite in Water Distribution Systems

Investigating the fate of persistent organic pollutants in water distribution systems (WDSs) is of great significance for preventing human health risks. The role of iron corrosion scales in the migration and transformation of organics in such systems remains unclear. Herein, we determined that hydroxyl (•OH), chlorine, and chlorine oxide radicals are generated by Fenton-like reactions due to the coexistence of oxygen vacancy-related Fe(II) on goethite (a major constituent of iron corrosion scales) and hypochlorous acid (HClO, the main reactive chlorine species of residual chlorine at pH ∼ 7.0). •OH contributed mostly to the decomposition of atrazine (ATZ, model compound) more than other radicals, producing a series of relatively low-toxicity small molecular intermediates. A simplified kinetic model consisting of mass transfer of ATZ and HClO, •OH generation, and ATZ oxidation by •OH on the goethite surface was developed to simulate iron corrosion scale-triggered residual chlorine oxidation of organic compounds in a WDS. The model was validated by comparing the fitting results to the experimental data. Moreover, the model was comprehensively applicable to cases in which various inorganic ions (Ca2+, Na+, HCO3-, and SO42-) and natural organic matter were present. With further optimization, the model may be employed to predict the migration and accumulation of persistent organic pollutants under real environmental conditions in the WDSs.

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.

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.

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

The susceptibility of 19 representative per- and polyfluoroalkyl substances (PFAS) to direct photolysis and defluorination under far-UVC 222 nm irradiation was investigated. Enhanced photolysis occurred for perfluorocarboxylic acids (PFCAs), fluorotelomer unsaturated carboxylic acids (FTUCAs), and GenX, compared to that at conventional 254 nm irradiation on a similar fluence basis, while other PFAS showed minimal decay. For degradable PFAS, up to 81% of parent compound decay (photolysis rate constant (k222 nm) = 8.19-34.76 L·Einstein-1; quantum yield (Φ222 nm) = 0.031-0.158) and up to 31% of defluorination were achieved within 4 h, and the major transformation products were shorter-chain PFCAs. Solution pH, dissolved oxygen, carbonate, phosphate, chloride, and humic acids had mild impacts, while nitrate significantly affected PFAS photolysis/defluorination at 222 nm. Decarboxylation is a crucial step of photolytic decay. The slower degradation of short-chain PFCAs than long-chain ones is related to molar absorptivity and may also be influenced by chain-length dependent structural factors, such as differences in pKa, conformation, and perfluoroalkyl radical stability. Meanwhile, theoretical calculations indicated that the widely proposed HF elimination from the alcohol intermediate (CnF2n+1OH) of PFCA is an unlikely degradation pathway due to high activation barriers. These new findings are useful for further development of far-UVC technology for PFAS in water treatment.

Electrosorption, Desorption, and Oxidation of Perfluoroalkyl Carboxylic Acids (PFCAs) via MXene-Based Electrocatalytic Membranes

MXenes exhibit excellent conductivity, tunable surface chemistry, and high surface area. Particularly, the surface reactivity of MXenes strongly depends on surface exposed atoms or terminated groups. This study examines three types of MXenes with oxygen, fluorine, and chlorine as respective terminal atoms and evaluates their electrosorption, desorption, and oxidative properties. Two perfluorocarboxylic acids (PFCAs), perfluorobutanoic acid (PFBA) and perfluorooctanoic acid (PFOA) are used as model persistent micropollutants for the tests. The experimental results reveal that O-terminated MXene achieves a significantly higher adsorption capacity of 215.9 mg·g^-1 and an oxidation rate constant of 3.9 × 10^-2 min^-1 for PFOA compared to those with F and Cl terminations. Electrochemical oxidation of the two PFCAs (1 ppm) with an applied potential of +6 V in a 0.1 M Na₂SO₄ solution yields >99% removal in 3 h. Moreover, PFOA degrades about 20% faster than PFBA on O-terminated MXene. The density functional theory (DFT) calculations reveal that the O-terminated MXene surface yielded the highest PFOA and PFBA adsorption energy and the most favorable degradation pathway, suggesting the high potential of MXenes as highly reactive and adsorptive electrocatalysts for environmental remediation.