The effects of surface oxidation and fluorination of boron-doped diamond anodes on perchlorate formation and organic compound oxidation

Surface fluorination mediated electro-oxidative degradation of HFPO-DA on boron-doped diamond electrode

Heptafluoropropylene oxide dimer acid (HFPO-DA), as an alternative to perfluorooctanoic acid (PFOA), has been shown to pose similar environmental and health risks as other perfluorinated compounds. The electrochemical-based advanced oxidation processes are promising techniques for the treatment of perfluorinated compounds, and the boron-doped diamond (BDD) anode could degrade HFPO-DA under mild conditions. However, the roles of radicals in the degradation and how to overcome the steric hindrance of the -CF3 branch on the carboxyl group were not yet clear. In this study, we investigated the degradation mechanism of HFPO-DA on the BDD anode. Instead of other non-active anodes (PbO2 and SnO2 electrodes), HFPO-DA can be degradable on the BDD electrode with a rate constant logarithmic correlation to the applied current density. The hydroxyl radical (•OH) was one of the key factors in the degradation of HFPO-DA, accounting for almost 89% of the significant effect, and the direct electron transfer was the rate-limiting step in the degradation reaction. Physicochemical characterization including field emission scanning electron microscope (FE-SEM), X-ray photo-electron spectroscopy (XPS), water contact angle, and electrochemical property indicated that the BDD electrode was fluorinated after electrolysis, the electrode surface became more hydrophobic due to the bonding of -CxFy, leading to a decrease in the electrochemically active area. Moreover, degradation products (pentafluoropropionic acid, trifluoroacetic acid, and fluorine ion) were detected and the mass balance of carbon and fluorine was calculated during the degradation. Therefore, a degradation mechanism for HFPO-DA was proposed, which involved direct electron transfer, decarboxylation, radical reaction, decarboxylation, and decarboxylation. The de-CF3 step initiated the fluorination of the BDD electrode, which was initiated by the defluorination process. This study contributes to the understanding of the electro-oxidative degradation of perfluoroalkyl ether carboxylic acids and provides guidance for the application of electrochemical advanced oxidation processes.

Managing PFAS exhausted Ion-exchange resins through effective regeneration/electrochemical process

This study proposes an integrated approach that combines ion-exchange (IX) and electrochemical technologies to tackle problems associated with PFAS contamination. Our investigation centers on evaluating the recovery and efficiency of IX/electrochemical systems in the presence of five different salts, spanning dosages from 0.1 % to 8 %. The outcomes reveal a slight superiority for NaCl within the regeneration system, with sulfate and bicarbonate also showing comparable efficacy. Notably, the introduction of chloride ion (Cl-) into the electrochemical system results in substantial generation of undesirable chlorate (ClO3-) and perchlorate (ClO4-) by-products, accounting for ∼18 % and ∼81 % of the consumed Cl-, respectively. Several agents, including H2O2, KI, and Na2S2O3, exhibited effective mitigation of ClO3- and ClO4- formation. However, only H2O2 demonstrated a favorable influence on the degradation and defluorination of PFOA. The addition of 0.8 M H2O2 resulted in the near-complete removal of ClO3- and ClO4-, accompanied by 1.3 and 2.2-fold enhancements in the degradation and defluorination of PFOA, respectively. Furthermore, a comparative analysis of different salts in the electrochemical system reveals that Cl- and OH- ions exhibit slower performance, possibly due to competitive interactions with PFOA on the anode's reactive sites. In contrast, sulfate and bicarbonate salts consistently demonstrate robust decomposition efficiencies. Despite the notable enhancement in IX regeneration efficacy facilitated by the presence of methanol, particularly for PFAS-specific resins, this enhancement comes at the cost of reduced electrochemical decomposition of all PFAS. The average decay rate ratio of all PFAS in the presence of 50 % methanol, compared to its absence, falls within the range of 0.11-0.39. In conclusion, the use of 1 % Na2SO4 salt stands out as a favorable option for the integrated IX/electrochemical process. This choice not only eliminates the need to introduce an additional chemical (e.g., H2O2) into the wastewater stream, but also ensures both satisfactory regeneration recovery and efficiency in the decomposition process through electrochemical treatment.

Prospective Life Cycle Assessment for the Electrochemical Oxidation Wastewater Treatment Process: From Laboratory to Industrial Scale

Electrochemical oxidation (EO) is a promising technology for water purification, but indirect environmental burdens may arise in association with consumption of materials and energy during electrode preparation and process operation. This study evaluated the life cycle environmental impacts of emerging EO technology from laboratory scale to industrial scale using prospective life cycle assessment (LCA) on a quantitative basis. Environmental impacts of EO technology were assessed at laboratory scale by comparing three representative anode materials (SnO2, PbO2, and boron-doped diamond) and other two typical processes (adsorption and Fenton method), which verified the competitiveness of the EO process and identified the key factors to environmental hotspots. Thereafter, LCA of scale-up EO was performed to offer guidance for practical application, and the life cycle inventory was compiled upon thermodynamic and kinetic simulations, empirical calculation rules, and similar technical information. Results demonstrated EO to be effective for destructing recalcitrant organic pollutants, but visible direct benefits might be outweighed by increased indirect environmental burdens associated with the preparation of anode materials, use of electrolytes, and energy consumption during the operation stage at both laboratory scale and larger scale. This necessitated attention to overall life cycle profiles by taking into account reactor design, anode materials, electrolyte and flow pattern, and decentralized location with a large share of renewable power station and rigorous contamination control strategies for wastewater treatment plants.

Electrooxidation of per- and polyfluoroalkyl substances in chloride-containing water on surface-fluorinated Ti4O7 anodes: Mitigation and elimination of chlorate and perchlorate formation

Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFASs, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti₄O₇ anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti₄O₇ anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveal the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti₄O₇ anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti₄O₇ anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.

Formation of chlorate and perchlorate during electrochemical oxidation by Magnéli phase Ti4O7 anode: inhibitory effects of coexisting constituents

Formation of chlorate (ClO₃⁻) and perchlorate (ClO₄⁻) as by-products in electrooxidation process has raised concern. In the present study, the formation of ClO₃⁻ and ClO₄⁻ in the presence of 1.0 mM Cl⁻ on boron doped diamond (BDD) and Magneli phase titanium suboxide (Ti₄O₇) anodes were evaluated. The Cl⁻ was transformed to ClO₃⁻ (temporal maximum 276.2 μM) in the first 0.5 h on BDD anodes with a constant current density of 10 mA cm², while approximately 1000 μM ClO₄⁻ was formed after 4.0 h. The formation of ClO₃⁻ on the Ti₄O₇ anode was slower, reaching a temporary maximum of approximately 350.6 μM in 4.0 h, and the formation of ClO₄⁻ was also slower on the Ti₄O₇ anode, taking 8.0 h to reach 780.0 μM. Compared with the BDD anode, the rate of ClO₃⁻ and ClO₄⁻ formation on the Ti₄O₇ anode were always slower, regardless of the supporting electrolytes used in the experiments, including Na₂SO₄, NaNO₃, Na₂B₄O₇, and Na₂HPO₄. It is interesting that the formation of ClO₄⁻ during electrooxidation was largely mitigated or even eliminated, when methanol, KI, and H₂O₂ were included in the reaction solutions. The mechanism of the inhibition on Cl⁻ transformation by electrooxidation was explored.

Is HFPO-DA (GenX) a suitable substitute for PFOA? A comprehensive degradation comparison of PFOA and GenX via electrooxidation

Due to the potential hazard of perfluorooctanoic acid (PFOA), hexafluoropropylene oxide dimer acid (HFPO-DA, GenX) has become a typical alternative since 2009. However, GenX has recently been reported to have equal or even greater toxicity and bioaccumulation than PFOA. Considering the suitability of alternatives, it is quite essential to study and compare the degradation degree between PFOA and GenX in water. Therefore, in the present study, a comprehensive degradation comparison between them via electrooxidation with a titanium suboxide membrane anode was conducted. The degradation rate decreased throughout for PFOA, while it first increased and then decreased for GenX when the permeate flux increased from 17.3 L to 100.3 L m^-2·h^-1. The different responses of PFOA and GenX to flux might be attributed to their different solubilities. In addition, the higher k_obs of PFOA demonstrated that it had a better degradability than GenX by 2.4-fold in a mixed solution. The fluorinated byproduct perfluoropropanoic acid (PFPrA) was detected as a GenX intermediate, suggesting that ether bridge splitting was needed for GenX electrooxidation. This study provides a reference for assessing the degradability of GenX and PFOA and indicates that it is worth reconsidering whether GenX is a suitable alternative for PFOA from the point of view of environmental protection.

Enhanced perfluorooctane acid mineralization by electrochemical oxidation using Ti3+ self-doping TiO2 nanotube arrays anode

Perfluorooctanoic acid (PFOA) is of increasing concern due to its worldwide application and extremely environmental persistence. Herein, we demonstrated the electrochemical degradation of PFOA with high efficiency using the Ti³⁺ self-doping TiO₂ nanotube arrays (Ti³⁺/TiO₂-NTA) anode. The fabricated Ti³⁺/TiO₂-NTA anode exhibited vertically aligned uniform nanotubes structure, and was demonstrated good performance on the electrochemical degradation of PFOA in water. The degradation rate, total organic carbon (TOC) removal rate and defluorination rate of PFOA reached 98.1 %, 93.3 % and 74.8 %, respectively, after electrolysis for 90 min at low current density of 2 mA cm^-2. The energy consumption (7.6 Wh L^-1) of this electrochemical oxidation system using Ti³⁺/TiO₂-NTA anode for PFOA degradation was about 1 order of magnitude lower than using traditional PbO₂ anodes. Cathodic polarization could effectively prolong the electrocatalytic activity of the anode by regenerating Ti³⁺ sites. PFOA molecular was underwent a rapidly mineralization to CO₂ and F^-, with only low concentration of short-chain perflfluorocarboxylic acids (PFCAs) intermediates identified. A possible electrochemical degradation mechanism of PFOA was proposed, in which the initial direct electron transfer (DET) on the anode to yield PFOA free radicals (C₇F₁₅COO•) and hydroxyl radicals (•OH) oxidation were greatly enhanced. This presented study provides a novel approach for the purification of the recalcitrant PFOA from wastewaters.

Electrochemical removal of tetrabromobisphenol A by fluorine-doped titanium suboxide electrochemically reactive membrane

This study reports fluorine-doped titanium suboxide anode for electrochemical mineralization of hydrophobic micro-contaminant, tetrabromobisphenol A. Fluorinated TiSO anode promoted electro-generated hydroxyl radicals (•OH) with higher selectivity and activity, due to increased O₂ evolution potential and more loosely interaction with hydrophobic electrode surface. For electro-oxidation process, fluorine doping had an insignificant impact on outer-sphere reaction and exerted inhibition on inner-sphere reaction, as indicated by cyclic voltammogram performed on Ru(NH₃)₆^3+/2+, Fe(CN)₆^3-/4- and Fe^3+/2+ redox couple. This facilitated electrochemical conversion of TBBPA and intermediates via more efficient outer-sphere reaction and hydroxylation route. Additionally, generated O₂ micro-bubbles could be stabilized on hydrophobic F-doped TiSO anode, which extended the three-phase boundary available for interfacial enrichment of TBBPA and subsequent mineralization. Under action of these comprehensive factors, 0.5% F-doped TiSO electrochemically reactive membrane could achieve 99.7% mineralization of TBBPA upon energy consumption of 0.52 kWh m^-3 at current density of 7.8 ± 0.24 mA cm^-2 (3.75 V vs SHE) and flow rate of 1628 LHM based on flow-through electrolysis. The modified anode exhibited superior performances compared with un-modified one with more efficient TBBPA removal, less toxic intermediate accumulation and lower energy consumption. The results may have important implications for electrochemical removal and detoxification of hydrophobic micro-pollutants.

Comparative removal of Suwannee River natural organic matter and perfluoroalkyl acids by anion exchange: Impact of polymer composition and mobile counterion

Anion exchange resin (AER) adsorption is an established technology for water treatment and groundwater remediation. Two contaminants amenable to AER treatment are natural organic matter (NOM) and per- and polyfluoroalkyl substances (PFAS), specifically anionic perfluoroalkyl acids (PFAAs) such as perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS). NOM is ubiquitous in natural waters and is often targeted for removal. PFAS occurrence in water resources is a human health concern. Accordingly, the goal of this research was to provide new insights on the use of AER for water treatment considering separate and combined removal of NOM and PFAAs. Batch experiments were conducted comparing polystyrene and polyacrylic AER in both chloride- and sulfate-forms using natural groundwater spiked with Suwannee River natural organic matter (SRNOM) and/or six PFAAs. The polymer composition of the AER had a significant impact on contaminant removal with polystyrene resin more effective for PFAA removal and polyacrylic resin more effective for SRNOM removal. Both resins had type I quaternary ammonium functional groups; however, the polyacrylic resin had trimethyl ammonium whereas the polystyrene resin had triethyl ammonium. Therefore, the influence of polymer composition could not be isolated conclusively from functional group chemistry. Polystyrene AER showed greater removal of PFAAs with sulfonate than carboxylate head group and 8-carbon than 4-carbon chain length. Removal of SRNOM and PFAAs by both resin polymer compositions were greater when sulfate was the mobile counterion ion than chloride. The results of this research have important implications for using AER for water treatment and remediation. Foremost, polymer composition and mobile counterion form of the resin can be selected to target specific contaminants and maximize contaminant removal. When contaminants have unique interactions with AER such as SRNOM and polyacrylic resin and PFAAs and polystyrene resin, the presence of one contaminant does not impact removal of the other contaminant.

Degradation of Perfluorooctanesulfonate by Reactive Electrochemical Membrane Composed of Magnéli Phase Titanium Suboxide

This study investigated the degradation of perfluorooctanesulfonate (PFOS) in a reactive electrochemical membrane (REM) system in which a porous Magnéli phase titanium suboxide ceramic membrane served simultaneously as the anode and the membrane. Near complete removal (98.30 ± 0.51%) of PFOS was achieved under a cross-flow filtration mode at the anodic potential of 3.15 V vs standard hydrogen electrode (SHE). PFOS removal efficiency during the REM operation is much greater than that of the batch operation mode under the same anodic potential. A systematic reaction rate analysis in combination with electrochemical characterizations quantitatively elucidated the enhancement of PFOS removal in REM operation in relation to the increased electroactive surface area and improved interphase mass transfer. PFOS appeared to undergo rapid mineralization to CO₂ and F^-, with only trace levels of short-chain perfluorocarboxylic acids (PFCAs, C₄-C₈) identified as intermediate products. Density functional theory (DFT) simulations and experiments involving free radical scavengers indicated that PFOS degradation was initiated by direct electron transfer (DET) on anode to yield PFOS free radicals (PFOS^•), which further react with hydroxyl radicals that were generated by water oxidation and adsorbed on the anode surface (^•OH_ads). The attack of ^•OH_ads is essential to PFOS degradation, because, otherwise, PFOS^• may react with water and revert to PFOS.

Enhancing Electrochemical Efficiency of Hydroxyl Radical Formation on Diamond Electrodes by Functionalization with Hydrophobic Monolayers

Electrochemical formation of high-energy species such as hydroxyl radicals in aqueous media is inefficient because oxidation of H₂O to form O₂ is a more thermodynamically favorable reaction. Boron-doped diamond (BDD) is widely used as an electrode material for generating ^•OH radicals because it has a very large kinetic overpotential for O₂ production, thus increasing electrochemical efficiency for ^•OH production. Yet, the underlying mechanisms of O₂ and ^•OH production at diamond electrodes are not well understood. We demonstrate that boron-doped diamond surfaces functionalized with hydrophobic, polyfluorinated molecular ligands (PF-BDD) have significantly higher electrochemical efficiency for ^•OH production compared with hydrogen-terminated (H-BDD), oxidized (O-BDD), or poly(ethylene ether)-functionalized (E-BDD) boron-doped diamond samples. Our measurements show that ^•OH production is nearly independent of surface functionalization and pH (pH = 7.4 vs 9.2), indicating that ^•OH is produced by oxidation of H₂O in an outer-sphere electron-transfer process. In contrast, the total electrochemical current, which primarily produces O₂, differs strongly between samples with different surface functionalizations, indicating an inner-sphere electron-transfer process. X-ray photoelectron spectroscopy measurements show that although both H-BDD and PF-BDD electrodes are oxidized over time, PF-BDD showed longer stability (≈24 h of use) than H-BDD. This work demonstrates that increasing surface hydrophobicity using perfluorinated ligands selectively inhibits inner-sphere oxidation to O₂ and therefore provides a pathway to increased efficiency for formation of ^•OH via an outer-sphere process. The use of hydrophobic electrodes may be a general approach to increasing selectivity toward outer-sphere electron-transfer processes in aqueous media.

Electrochemical properties of fluorinated boron-doped diamond electrodes via fluorine-containing plasma treatment

It was systematically demonstrated that the electrochemical properties of fluorinated boron-doped diamond electrodes could be attributed to interfacial band bending.

Novel AgNWs-PAN/TPU membrane for point-of-use drinking water electrochemical disinfection

The safety of drinking water remains a major challenge in developing countries and point-of-use (POU) drinking water treatment device plays an important role in decentralised drinking water safety. In this study, a novel material, i.e. a silver nanowires-polyacrylonitrile/thermoplastic polyurethane (AgNWs-PAN/TPU) composite membrane, was fabricated via electrospinning and vacuum filtration deposition. Morphological and structural characterisation showed that the PAN/TPU fibres had uniform diameters and enhanced mechanical properties. When added to these fibres, the AgNWs formed a highly conductive network with good physical stability and low silver ion leaching (<100 ppb). A POU device equipped with a AgNWs-PAN/TPU membrane displayed complete removal of 10⁵ CFU/mL bacteria, which were inactivated by silver ions released from the AgNWs within 6 h. Furthermore, under a voltage of 1.5 V, the bacteria were completely inactivated within 20-25 min. Inactivation efficiency in 5 mM NaCl solution was higher than those in Na₂SO₄ and NaNO₃ solutions. We concluded that a strong electric field was formed at the AgNW tips. Additionally, silver ions and chlorine compounds worked synergistically in the disinfection process. This study provides a scientific basis for research and development of silver nanocomposite membranes, with high mechanical strength and high conductivity, for POU drinking water disinfection.

Fluorination of Boron-Doped Diamond Film Electrodes for Minimization of Perchlorate Formation

This research investigated the effects of surface fluorination on both rates of organic compound oxidation (phenol and terephthalic acid (TA)) and ClO₄^- formation at boron-doped diamond (BDD) film anodes at 22 °C. Different fluorination methods (i.e., electrochemical oxidation with perfluorooctanoic acid (PFOA), radio frequency plasma, and silanization) were used to incorporate fluorinated moieties on the BDD surface, which was confirmed by X-ray photoelectron spectroscopy (XPS). The silanization method was found to be the most effective fluorination method using a 1H,1H,2H,2H-perfluorodecyltrichlorosilane precursor to form a self-assembled monolayer (SAM) on the oxygenated BDD surface. The ClO₄^- formation decreased from rates of 0.45 ± 0.03 mmol m^-2 min^-1 during 1 mM NaClO₃ oxidation and 0.28 ± 0.01 mmol m^-2 min^-1 during 10 mM NaCl oxidation on the BDD electrode to below detectable levels (<0.12 μmoles m^-2 min^-1) for the BDD electrode functionalized by a 1H,1H,2H,2H-perfluorodecyltrichlorosilane SAM. These decreases in rates corresponded to 99.94 and 99.85% decreases in selectivity for ClO₄^- formation during the electrolysis of 10 mM NaCl and 1 mM NaClO₃ electrolytes, respectively. By contrast, the oxidation rates of phenol were reduced by only 16.3% in the NaCl electrolyte and 61% in a nonreactive 0.1 M KH₂PO₄ electrolyte. Cyclic voltammetry with Fe(CN)₆^3-/4- and Fe^3+/2+ redox couples indicated that the long fluorinated chains created a blocking layer on the BDD surface that inhibited charge transfer via steric hindrance and hydrophobic effects. The surface coverages and thicknesses of the fluorinated films controlled the charge transfer rates, which was confirmed by estimates of film thicknesses using XPS and density functional theory simulations. The aliphatic silanized electrode also showed very high stability during OH^• production. Perchlorate formation rates were below the detection limit (<0.12 μmoles m^-2 min^-1) for up to 10 consecutive NaClO₃ oxidation experiments.

Perchlorate formation during the electro-peroxone treatment of chloride-containing water: Effects of operational parameters and control strategies

This study investigated the degradation of clofibric acid and formation of perchlorate during the electro-peroxone (E-peroxone) treatment of chloride-containing (26.1-100 mg L(-1)) water (Na2SO4 electrolytes and secondary effluents). The E-peroxone process involves sparging O2 and O3 gas mixture into an electrolysis reactor where a carbon-based cathode is used to electrochemically convert the sparged O2 to H2O2. The electro-generated H2O2 then reacts with sparged O3 to produce OH, which can rapidly oxidize pollutants in the bulk solution. When boron-doped diamond (BDD) electrodes were used as the anode, perchlorate concentrations increased significantly from undetectable levels to ∼15-174 mg L(-1) in the different water samples as the applied current density was increased from 4 to 32 mA cm(-2). In contrast, no ClO4(-) was detected when Pt/Ti anodes were used in the E-peroxone process operated under similar reaction conditions. In addition, when sufficient O3 was sparged to maximize OH production from its peroxone reaction with electro-generated H2O2, the E-peroxone process with Pt/Ti anodes achieved comparable clofibric acid degradation and total organic carbon (TOC) removal yields as that with BDD anodes, but did not generate detectable ClO4(-). These results indicate that by optimizing operational parameters and using Pt/Ti anodes, the E-peroxone process can achieve the goal of both fast pollutant degradation and ClO4(-) prevention during the treatment of chloride-containing wastewater.