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

Differentiation of adsorption and degradation in steroid hormone micropollutants removal using electrochemical carbon nanotube membrane

The growing concern over micropollutants in aquatic ecosystems motivates the development of electrochemical membrane reactors (EMRs) as a sustainable water treatment solution. Nevertheless, the intricate interplay among adsorption/desorption, electrochemical reactions, and byproduct formation within EMR complicates the understanding of their mechanisms. Herein, the degradation of micropollutants using an EMR equipped with carbon nanotube membrane are investigated, employing isotope-labeled steroid hormone micropollutant. The integration of high-performance liquid chromatography with a flow scintillator analyzer and liquid scintillation counting techniques allows to differentiate hormone removal by concurrent adsorption and degradation. Pre-adsorption of hormone is found not to limit its subsequent degradation, attributed to the rapid adsorption kinetics and effective mass transfer of EMR. This analytical approach facilitates determining the limiting factors affecting the hormone degradation under variable conditions. Increasing the voltage from 0.6 to 1.2 V causes the degradation dynamics to transition from being controlled by electron transfer rates to an adsorption-rate-limited regime. These findings unravels some underlying mechanisms of EMR, providing valuable insights for designing electrochemical strategies for micropollutant control.

Per- and polyfluoroalkyl substances contamination of drinking water sources in Africa: Pollution sources and possible treatment methods

Despite the detection of poly- and perfluorinated alkyl substances (PFAS) in the water system in Africa, the effort towards mitigating PFAS in water in Africa needs to be better understood. Therefore, this review evaluated the contamination status and mitigation methods for handling PFAS-contaminated water systems in Africa. The findings revealed the presence of PFAS in wastewater treatment plant (WWTP) effluents, surface water and commercially available bottled and tap water in African countries. The concentration of PFAS in drinking water sources reviewed ranged from < limits of quantification to 778 ng L-1. The sources of PFAS in water systems in Africa are linked to uncontrolled importation of PFAS-containing products, WWTP effluents and inappropriate disposal of PFAS-containing materials. The information on treatment methods for PFAS-contaminated water systems is scanty. Unfortunately, the treatment method is challenged by poor water research infrastructure and facilities, lack of awareness, poor research funding and weak legislation; however, adsorption and membrane technology seem favourable for removing PFAS from water systems in Africa. It is essential to focus on monitoring and assessing drinking water quality in Africa to reduce the disease burden that this may cause. Most African countries' currently implemented water treatment facilities cannot efficiently remove PFAS during treatment. Therefore, governments in Africa need to fund more research to develop an efficient water treatment technique that is sustainable in Africa.

Modulating crystal facets of photoanodes for photoelectrocatalytic scalable degradation of fluorinated pharmaceuticals in wastewater

Fluorinated pharmaceuticals pollution has become an ever-increasing environmental concern due to its negative impacts. Photoelectrocatalytic (PEC) degradation system is a desirable approach to tackle the pollution problems. However, photogenerated charge separation and interfacial mass transfer are the main bottlenecks for improving the PEC degradation performance. Herein, we report a TiO2 photoanode with tuned (101)/(110) facets in situ grown on a Ti mesh substrate for PEC degradation of fluorinated pharmaceuticals. The exposure of (101) facets facilitates efficient photogenerated charge separation and the desorption of generated •OH radical. Besides, the three-dimensional (3D) architecture of photoanode promotes macroscopic mass transfer. This system performed complete defluorination of 5-fluorouracil and more than 75 % total organic carbon (TOC) removal efficiency. The apparent reaction rate constant of high (101) facet-exposed TiO2 grown on Ti mesh is up to 6.96 h-1, 6‒fold faster than that of photoanode with low (101) facet-exposed TiO2 grown on Ti foil. It is demonstrated that a large-sized PEC system of 1200 cm2 can degrade 100 L of synthetic fluorinated pharmaceutical wastewater with more than 80 % elimination efficiency. This work showcases the facet and substrate modulated strategy of fabricating high-performed photoanode for PEC wastewater purification.

Energy-efficient treatment of refractory industrial effluent using flow-through electrochemical processes: Oxidation mechanisms and reduction of chlorinated byproducts

While flow-through anodic oxidation (FTAO) technique has demonstrated high efficiency to treat various refractory waste streams, there is an increasing concern on the secondary hazard generation thereby. In this study, we developed an integrated system that couples FTAO and cathodic reduction processes (termed FTAO-CR) for sustainable treatment of chlorine-laden industrial wastewater. Among four common electrode materials (i.e., Ti4O7, β-PbO2, RuO2, and SnO2-Sb), RuO2 flow-through anode exhibited the best pollutant removal performance and relatively low ClO3 and ClO4 yields. Because of the significant scavenging effect of Cl- in real wastewater treatment, the direct electron transfer process played a dominant role in contaminant degradation for both active and nonactive anodes though active species (i.e., active chlorine) were involved in the subsequent transformation of the organic matter. A continuous FTAO-CR system was then constructed for simultaneous COD removal and organic and inorganic chlorinated byproduct control. The quality of the treated effluent could meet the national discharge permit limit at low energy cost (∼4.52 kWh m3 or ∼0.035 kWh g1-COD). Results from our study pave the way for developing novel electrochemical platforms for the purification of refractory waste streams whilst minimizing the secondary pollution.

Comparative analysis of UV-initiated ARPs for degradation of the emerging substitute of perfluorinated compounds: Does defluorination mean the sole factor?

Due to the increasing attention for the residual of per- and polyfluorinated compounds in environmental water, Sodium p-Perfluorous Nonenoxybenzenesulfonate (OBS) have been considered as an alternative solution for perfluorooctane sulfonic acid (PFOS). However, recent detections of elevated OBS concentrations in oil fields and Frontal polymerization foams have raised environmental concerns leading to the decontamination exploration for this compound. In this study, three advanced reduction processes including UV-Sulfate (UV-SF), UV-Iodide (UV-KI) and UV-Nitrilotriacetic acid (UV-NTA) were selected to evaluate the removal for OBS. Results revealed that hydrated electrons (eaq-) dominated the degradation and defluorination of OBS. Remarkably, the UV-KI exhibited the highest removal rate (0.005 s-1) and defluorination efficiency (35 %) along with the highest concentration of eaq- (K = -4.651). Despite that nucleophilic attack from eaq- on sp2 carbon and H/F exchange were discovered as the general mechanism, high-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (HPLC/Q-TOF-MS) analysis with density functional theory (DFT) calculations revealed the diversified products and routes. Intermediates with lowest fluorine content for UV-KI were identified, the presence nitrogen-containing intermediates were revealed in the UV-NTA. Notably, the nitrogen-containing intermediates displayed the enhanced toxicity, and the iodine poly-fluorinated intermediates could be a potential-threat compared to the superior defluorination performance for UV-KI.

Per-and polyfluoroalkyl (PFAS) eternal pollutants: Sources, environmental impacts and treatment processes

Per- and polyfluoroalkyl substances (PFAS) have become a growing environmental concern due to their tangible impacts on human health. However, due to the large number of PFAS compounds and the analytical difficulty to identify all of them, there are still some knowledge gaps not only on their impact on human health, but also on how to manage them and achieve their effective degradation. PFAS compounds originate from man-made chemicals that are resistant to degradation because of the presence of the strong carbon-fluorine bonds in their chemical structure. This review consists of two parts. In the first part, the environmental effects of fluorinated compound contamination in water are covered with the objective to highlight how their presence in the environment adversely impacts the human health. In the second part, the focus is put on the different techniques available for the degradation and/or separation of PFAS compounds in different types of waters. Examples of removal/treatment of PFAS present in either surface or ground water are presented.

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.

Iron complex regulated synergistic effect between the current and peroxymonosulfate enhanced ultrafast oxidation of perfluorooctanoic acid via free radical dominant electrochemical reaction

Iron complex regulated electrochemical reaction was triggered for revealing the reaction mechanism, degradation pathway, and applied potential of perfluorooctanoic acid (PFOA). The increased PMS concentrations, electrode spacing, and current density significantly enhanced PFOA elimination, with current density exhibiting a relatively strong interdependency to PFOA complete mineralization. The synergy between PMS and electrochemical reactions greatly accelerated PFOA decomposition by promoting the generation of key reaction sites, such as those for PMS activation and electrochemical processes, under various conditions. Furthermore, density functional theory calculations confirmed that the reciprocal transformation of Fe2+ and Fe3+ complexes was feasible under the electrochemical effect, further promoting the generation of active sites. The developed electrochemical oxidation with PMS reaction (EO/PMS) system can rapidly decompose and mineralize PFOA while maintaining strong tolerance to changing water matrices and organic and inorganic ions. Overall, it holds promise for use in treating and purifying wastewater containing PFOA.

Electro-oxidation of Ibuprofen using carbon-supported SnOx-CeOx flow-anodes: The key role of high-valent metal

Exploiting electrochemically active materials as flow-anodes can effectively alleviate mass transfer restriction in an electro-oxidation system. However, the electrocatalytic activity and persistence of the conventional flow-anode materials are insufficient, resulting in limited improvement in the electro-oxidation rate and efficiency. Herein, we reported a rational strategy to substantially enhance the electrocatalytic performance of flow-anodes in electro-oxidation by introducing the redox cycle of high-valent metal in a suitable carbon substrate. The characterization suggested that the SnOx-CeOx/carbon black (CB) featured well-distributed morphology, rapid charge transfer, high oxygen evolution potential, and strong water adsorption, and stood out among three kinds of SnOx-CeOx loaded carbon materials. Mechanistic analysis indicated that the redox cycle of Ce species played a key role in accelerating the electron transfer from SnOx to CB directionally and could continuously create the electron-deficient state of the SnOx, thereby sustainably triggering the generation of ·OH. All these features enabled the resulting SnOx-CeOx/CB flow-anode to accomplish a calculated maximum kinetic constant of 0.02461 1/min, a higher current efficiency of 47.1%, and a lower energy consumption of 21.3 kWh/kg COD compared with other conventional flow-anodes reported to date. Additionally, SnOx-CeOx/CB exhibited excellent stability with extremely low leaching concentrations of Sn and Ce ions. This study provides a feasible manner for efficient water decontamination using the electro-oxidation system with SnOx-CeOx/CB.

Synergistic roles of oxidation and self-aggregation in efficient ultrafiltration membrane fouling alleviation using a flow-through Sb-SnO2 anode during wastewater reclamation

The application of ultrafiltration (UF) in wastewater reclamation alleviates the demand for limited water supplies. However, the membrane fouling caused by the effluent organic matter (EfOM) becomes a major obstacle for UF application. In this study, a pre-oxidation strategy for UF using a Sb-SnO2 (ATO) anode in flow-through mode was proposed with the hopes to improve the performance of UF during wastewater reclamation. The results indicated that this flow-through ATO (FA) anode significantly outperformed a boron-doped diamond (BDD) anode in terms of EfOM degradation and membrane fouling control. It is noteworthy that apart from oxidation, the self-aggregation behavior of foulants was also involved in the mechanisms of membrane fouling mitigation. On the one hand, FA pre-oxidation relieved the burden of membrane fouling by decomposing the macromolecular EfOM into small molecular organic matter, and even mineralizing it. The effective destruction of unsaturated EfOM by FA pre-oxidation made a remarkable contribution to fouling mitigation due to the strong correlation between the total fouling index and UV254. On the other hand, the surface morphology of membrane and interface properties of foulants revealed the self-aggregation behavior of foulants. FA pre-oxidation made the foulants aggregate spontaneously and reduced the potential of forming a dense cake layer on the membrane surface, which was conductive for water permeation. Overall, FA pre-oxidation proved to be a feasible and chemical-free option for UF pretreatment to simultaneously produce high-quality reused water and alleviate membrane fouling during wastewater reclamation.

Electrooxidation of chlorophene and dichlorophen by reactive electrochemical membrane: Key determining factors of removal efficiency

This study systematically investigated the variable main electrooxidation mechanism of chlorophene (CP) and dichlorophen (DCP) with the change of reaction conditions at Ti4O7 anode operated in batch and reactive electrochemical membrane (REM) modes. Significant degradation of CP and DCP was observed, that is, CP exhibited greater removal efficiency in batch mode at 0.5-3.5 mA cm-2 and REM operation (0.5 mA cm-2) with a permeate flow rate of 0.85 cm min-1 under the same reaction conditions, while DCP exhibited a faster degradation rate with the increase of current density in REM operation. Density functional theory (DFT) simulation and electrochemical performance tests indicated that the electrooxidation efficiency of CP and DCP in batch mode was primarily affected by the mass transfer rates. And the removal efficiency when anodic potentials were less than 1.7 V vs SHE in REM operation was determined by the activation energy for direct electron transfer (DET) reaction, however, the adsorption function of CP and DCP on the Ti4O7 anode became a dominant factor in determining the degradation efficiency with the further increase of anodic potential due to the disappeared activation barrier. In addition, the degradation pathways of CP and DCP were proposed according to intermediate products identification and frontier electron densities (FEDs) calculation, the acute toxicity of CP and DCP were also effectively decreased during both batch and REM operations.

Simultaneous modulation of CHO cell cytotoxicity, turbidity, and DOC by coagulation with or without pre-oxidation in water from the Pearl River Delta region, China

Coagulation with or without pre-oxidation are important drinking water treatment processes. However, the efficacy of these processes in mitigating water toxicity remains unknown. To further improve drinking water safety, we employed water from the Pearl River Delta region of southern China to investigate a treatment approach consisting of coagulation with or without pre-oxidation to simultaneously modulate health-relevant cytotoxicity to CHO cells, on top of the conventional foci of turbidity and dissolved organic carbon (DOC) during water treatment. Three coagulants (two aluminum-based and one iron-based salts) and three pre-oxidants (ozone, permanganate, and peroxymonosulfate) were studied. For coagulation without pre-oxidation, intermediate coagulant doses and pH reached optimum cytotoxicity to CHO cells, turbidity, and DOC control simultaneously. Introducing oxidants reduced cytotoxicity to CHO cells significantly, enhanced by increasing oxidant concentrations and pre-oxidation duration. The cytotoxicity to CHO cells mitigation capabilities of three pre-oxidants were: ozone > peroxymonosulfate > potassium permanganate. Modulation of water cytotoxicity to CHO cells was mostly attributable to controlling DOC (specifically humic-acid like substances, tyrosine, tryptophan). However, the addition of pre-oxidants led to significant shifts in water cytotoxicity to CHO cells forcing drivers, rendering humic-acid like substances the sole decisive cytotoxicity-inducing fluorophores. For the first time, 'sweet spots' were identified to simultaneously monitor cytotoxicity to CHO cells alongside turbidity and DOC. These methods better modulate water cytotoxicity to CHO cells without sacrificing conventional water treatment goals while shedding light onto the mechanisms behind.

Shifting Emphasis from Electro- to Catalytically Active Sites: Effects of Pore Size of Flow-Through Anodes on Water Purification

A flow-through anode has demonstrated high efficiency for micropollutant abatement in water purification. In addition to developing novel electrode materials, a rational design of its porous structure is crucial to achieve high electrooxidation kinetics while sustaining a low cost for flow-through operation. However, our knowledge of the relationship between the pore structure and its performance is still incomplete. Therefore, we systematically explore the effect of pore size (with a median from 4.7 to 49.4 μm) on the flow-through anode efficiency. Results showed that when the pore size was <26.7 μm, the electrooxidation kinetics was insignificantly improved, but the permeability declined dramatically. Traditional empirical evidence from hydrodynamic modeling and electrochemical tests indicated that a flow-through anode with a smaller pore size (e.g., 4.7 μm) had a high mass transfer capability and large electroactive area. However, this did not further accelerate the micropollutant removal. Combining an overpotential distribution model and an imprinting method has revealed that the reactivity of a flow-through anode is related to the catalytically active volume/sites. The rapid overpotential decay as a function of depth in the anode would offset the merits arising from a small pore size. Herein, we demonstrate an optimal pore size distribution (∼20 μm) of typical flow-through anodes to maximize the process performance at a low energy cost, providing insights into the design of advanced flow-through anodes in water purification applications.

Identifying priority PBT-like compounds from emerging PFAS by nontargeted analysis and machine learning models

As traditional per and polyfluoroalkyl substances (PFAS) are phased out, emerging PFAS are being developed and widely used. However, little is known about their properties, including persistence, bioaccumulation, and toxicity (PBT). Screening for emerging PFAS relies on available chemical inventory databases. Here, we compiled a database of emerging PFAS obtained from nontargeted analysis and assessed their PBT properties using machine learning models, including qualitative graph attention networks, Insubria PBT Index and quantitative EAS-E Suite, VEGA, and ProTox-II platforms. Totally 282 homologues (21.8% of emerging PFAS) were identified as PBT based on the combined qualitative and quantitative prediction, in which 140 homologues were detected in industrial and nonbiological/biological samples, belong to four categories, i.e. modifications of perfluoroalkyl carboxylic acids, perfluoroalkane sulfonamido substances, fluorotelomers and modifications of perfluoroalkyl sulfonic acids. Approximately 10.1% of prioritized emerging PFAS were matched to chemical vendors and 19.6% to patents. Aqueous film-forming foams and fluorochemical factories are the predominant sources for prioritized emerging PFAS. The database and screening results can update the assessment related to legislative bodies such as the US Toxic Substances Control Act and the Stockholm Convention. The combined qualitative and quantitative machine learning models can provide a methodological tool for prioritizing other emerging organic contaminants.

Efficient multi-stage electrochemical flow-through system for refractory organic pollutant treatment: Kinetics, mass transfer, and thermodynamic analysis

Improving the kinetics rate and mass transfer is essential for expanding the potential of electrochemical technologies in wastewater treatment. The electrochemical flow-through configuration promises a high oxidation efficiency and low energy consumption. We aimed to provide a thorough understanding of the enhanced kinetics, mass transfer, and thermodynamic parameters during the degradation of amoxicillin (AMX) in a multi-stage flow-through (MSFT) system using porous Ti-ENTA/SnO2-Sb anodes. All operating conditions strongly influenced the kinetics of AMX degradation and followed pseudo-first-order rate kinetic model (R2 > 0.85), with the highest kobs of 0.228 min-1 at high temperature (318 K). In comparison to the flow-by mode, the AMX removal rate in the three-stage flow-through mode was greatly enhanced by 70%, exhibiting the superior capacity of a porous anode. This system exhibited outstanding performance regarding the high kinetics rate and mass transfer rate (km), which increased by factors of 3.46 and 10.74, respectively, obtained in the flow-by mode. It also revealed that •OH generation was 5.64 times higher, and the EE/O was 19.89-fold lower than those in flow-by mode. Temperature plays a vital role in the reaction process, and thermodynamic features found the positive enthalpy (ΔHo) of +27.06 kJ mol-1, signifying the process was endothermic. A Hatta number (Ha) of >0.02 at all temperatures proved this finding, confirming an undeniable role in mass transfer. Finally, these findings reveal the system's performance and offer the possibility of establishing a multi-stage flow-through for wastewater treatment.

Flow-through electrochemical organophosphorus degradation and phosphorus recovery: The essential role of chlorine radical

Phosphorus scarcity and the deleterious ecological impact of the release of organophosphorus pesticides have emerged as critical global issues. Previous research has shown the ability of electrochemistry to induce the precipitation of calcium phosphate from phosphorus-laden wastewater to recover the phosphorus. The current study presents a flow-through electrochemical system consisting of a column-shaped electrochemical reactor, a tubular stainless-steel (SS) cathode, and a titanium suboxides (TiSO) anode. This system simultaneously oxidizes tetrakis (hydroxymethyl) phosphonium sulfate (THPS) and recycles phosphates. The influence of current density, flow rate, and initial calcium ions concentration were examined under continuous flow operation. To enhance the electrochemical reactor's performance, we elevated the current density from 5 to 30 mA cm-2, which caused the phosphorus recovery efficiency to increase from 37% to 72% within 120 min, accompanied by an enhancement of the THPS mineralization efficiency from 57% to 90%. These improvements were likely due to the higher yield of reactive species chloride species (Cl•) formed at the TiSO anode and the higher local pH at the cathode. By investigating the formation of Cl• at the TiSO anode, we found that THPS mineralization exceeded 75% in the presence of NaCl at a current density of 20 mA cm-2. The demonstrated performance of the flow-through electrochemical system should enable the utilization of anodic oxidation-cathodic precipitation for the recovery of phosphorus from organophosphorus-contaminated wastewater.

Sequential anodic oxidation and cathodic electro-Fenton in the Janus electrified membrane for reagent-free degradation of pollutants

Electrified membrane technologies have recently demonstrated high potential in tackling water pollution, yet their practical applications are challenged by relying on large precursor doses. Here, we developed a Janus porous membrane (JPEM) with synergic direct oxidation by Magnéli phase Ti4O7 anode and electro-Fenton reactions by CuFe2O4 cathode. Organic pollutants were first directly oxidized on the Ti4O7 anode, where the extracted electrons from pollutants were transported to the cathode for electro-Fenton production of hydroxyl radical (·OH). The cathodic ·OH further enhanced the mineralization of organic pollutant degradation intermediates. With the sequential anodic and cathodic oxidation processes, the reagent-free JPEM showed competitive performance in rapid degradation (removal rate of 0.417 mg L-1 s-1) and mineralization (68.7 % decrease in TOC) of sulfamethoxazole. The JPEM system displayed general performance to remove phenol, carbamazepine, and perfluorooctanoic acid. The JPEM runs solely on electricity and oxygen that is comparable to that of PEM relies on large precursor doses and, therefore, operation friendly and environmental sustainability. The high pollutant removal and mineralization achieved by rational design of the reaction processes sheds light on a new approach for constructing an efficient electrified membrane.

Y-mediated optimization of 3DG-PbO2 anode for electrochemical degradation of PFOS

In our previous study, the three-dimensional graphene-modified PbO2 (3DG-PbO2) anode was prepared for the effective degradation of perfluorooctanesulfonat (PFOS) by the electrochemical oxidation process. However, the mineralization efficiency of PFOS at the 3DG-PbO2 anode still needs to be further improved due to the recalcitrance of PFOS. Thus, in this study, the yttrium (Y) was doped into the 3DG-PbO2 film to further improve the electrochemical activity of the PbO2 anode. To optimize the doping amount of Y, three Y and 3DG codoped PbO2 anodes were fabricated with different Y3+ concentrations of 5, 15, and 30 mM in the electroplating solution, which were named Y/3DG-PbO2-5, Y/3DG-PbO2-15 and Y/3DG-PbO2-30, respectively. The results of morphological, structural, and electrochemical characterization revealed that doping Y into the 3DG-PbO2 anode further refined the β-PbO2 crystals, increased the oxygen evolution overpotential and active sites, and reduced the electron transfer resistance, resulting in a superior electrocatalytic activity. Among all the prepared anodes, the Y/3DG-PbO2-15 anode exhibited the best activity for electrochemical oxidation of PFOS. After 120 min of electrolysis, the TOC removal efficiency was 80.89% with Y/3DG-PbO2-15 anode, greatly higher than 69.13% with 3DG-PbO2 anode. In addition, the effect of operating parameters on PFOS removal was analyzed by response surface, and the obtained optimum values of current density, initial PFOS concentration, pH, and Na2SO4 concentration were 50 mA/cm2, 12.21 mg/L, 5.39, and 0.01 M, respectively. Under the optimal conditions, the PFOS removal efficiency reached up to 97.16% after 40 min of electrolysis. The results of the present study confirmed that the Y/3DG-PbO2 was a promising anode for electrocatalytic oxidation of persistent organic pollutants.

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.

Foam fractionation and electrochemical oxidation for the treatment of per- and polyfluoroalkyl substances (PFAS) in environmental water samples

Treatment of waters contaminated by per- and polyfluoroalkyl substances (PFAS) in large volumes remains a challenge to date. Treatment trains comprising separation and destruction technologies are promising to manage PFAS contamination. Foam fractionation (FF) and electrochemical oxidation (EO) are two cost-effective technologies for PFAS separation and destruction, respectively. This work systematically explored the performance of a treatment train of FF followed by EO (FF-EO) for treating PFAS in environmental water samples. For each treatment step, the dependence of the treatment performance on operational factors and other variables were analyzed statistically. The statistical analysis revealed PFAS enrichment and removal depend significantly on PFAS carbon chain length, solution conductivity, and PFAS concentration. Whether FF-EO treatment costs less energy than direct EO without FF mainly relies upon PFAS carbon chain length and TOC content in the sample. Both correlations were found to be linear. For all environmental water samples in this study, FF-EO is more energy-efficient than EO alone.

Titanium Oxide Electrocatalytic Membrane Filtration: "Two Faces" of Oxygen Vacancies in Generation and Transformation of Reactive Oxygen Species

Oxygen vacancies are crucial to the production of reactive oxygen species (ROS) in the metal oxide electrocatalytic membrane (MOx EM) process. Here, using cathodic TiOx EM as a model, we thoroughly reveal the roles of oxygen vacancies in ROS generation and transformation. Oxygen vacancies significantly promote H2O2 and •OH production at low concentrations (increment <35%) but inhibit their production at high concentrations (increment >35%). Electrochemical analysis discloses that the enhancement of ROS production profits from the acceleration of charge transfer kinetics by both bulk and surface oxygen vacancies, whereas we attribute the decline in ROS production to the strong adsorption of ROS by surface oxygen vacancies. It is strongly supported by theoretical calculations that reveal the promoted adsorption of *OOH and *OH by oxygen vacancies, which intensifies the capture and scavenging of H2O2 and •OH. Moreover, the gradual increase of interaction time between ROS and oxygen vacancies (from ∼1 to ∼5 s) notably reduces the generation and transformation efficiency of ROS, further highlighting the detrimental impact of oxygen vacancies. In summary, oxygen vacancies show "two faces" toward ROS generation and transformation, acting as ROS promoters at low concentrations but inhibitors at high concentrations. A medium oxygen vacancy concentration is preferred for ROS production, thus causing impressive pollutant removal (>95% removal of bisphenol A within 1.2-1.5 s at 360-440 LMH). This study provides guidance on regulating ROS generation and transformation by manipulating the oxygen vacancy concentration to enhance the decontamination efficiency of MOx EMs.

Oxygen vacancies induced band gap narrowing for efficient visible-light response in carbon-doped TiO2

The band gap of rutile TiO2 has been narrowed, via the formation of oxygen vacancies (OVs) during heat treatment in carbon powder (cHT) with embedding TiO2 coatings. The narrowed band gap efficiently improves the visible light response of TiO2 coatings, to further enhance the visible-light-driven photocatalytic activity. The change in OVs during cHT has been studied by manipulation of cHT temperature and time. The effect of OVs on the band structure of nonstoichiometric TiO2-x has been further calculated by first-principles calculations. With raising the temperature, SEM images show that the nano-size fiber-like structure forms on the surface of TiO2 coatings, and the amount of the fiber-like structure significantly increases and their size changes from nano to micro under 800 °C, contributing to cause an increase in accessible surface area. The UV-Vis results reveal that the band gap of TiO2 has been narrowed during cHT, due to the formed oxygen vacancies. The XPS results further confirm that the formation of surface defects including OVs, and the XPS depth profile further shows the decreased relative amount of O whereas increased relative amount of carbon. Notably, after cHT for TiO2 coatings, the photocatalytic activity first increases then decreases with raising the temperature, achieving approximately 3 times at 850 °C. The first-principles calculation suggest that the OVs in TiO2 coatings with localized electrons could facilitate the band gap narrowing, further favoring to enhance the photocatalytic activity under visible light.

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.

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.

Efficient Electrochemical Oxidation of Chloramphenicol by Novel Reduced TiO2 Nanotube Array Anodes: Kinetics, Reaction Parameters, Degradation Pathway and Biotoxicity Forecast

The key component of electrochemical advanced oxidation technology are high-efficiency anodes, and highly efficient and simple-to-prepare materials have generated a lot of interest. In this study, novel self-supported Ti³⁺-doped titanium dioxide nanotube arrays (R-TNTs) anodes were successfully prepared by a two-step anodic oxidation and straightforward electrochemical reduction technique. The electrochemical reduction self-doping treatment produced more Ti³⁺ sites with stronger absorption in the UV-vis region, a band gap reduction from 2.86 to 2.48 ev, and a significant increase in electron transport rate. The electrochemical degradation effect of R-TNTs electrode on chloramphenicol (CAP) simulated wastewater was investigated. At pH = 5, current density of 8 mA cm^-2, electrolyte concentration of 0.1 M sodium sulfate (Na₂SO₄), initial CAP concentration of 10 mg L^-1, CAP degradation efficiency exceeded 95% after 40 min. In addition, molecular probe experiments and electron paramagnetic resonance (EPR) tests revealed that the active species were mainly •OH and SO₄^-, among which •OH played a major role. The CAP degradation intermediates were discovered using high-performance liquid chromatography-mass spectrometry (HPLC-MS), and three possible degradation mechanisms were postulated. In cycling experiments, the R-TNTs anode demonstrated good stability. The R-TNTs prepared in this paper were an anode electrocatalytic material with high catalytic activity and stability, which could provide a new approach for the preparation of electrochemical anode materials for difficult-to-degrade organic compounds.

Environment-friendly and efficient electrochemical degradation of sulfamethoxazole using reduced TiO2 nanotube arrays-based Ti membrane coated with Sb-SnO2

This study focused on the preparation, characterization, and sulfamethoxazole (SMX) removal performance of the SnO₂-coated reactive electrochemical membrane (REM). This REM was fabricated by loading SnO₂ on the reduced TiO₂ nanotube arrays (RTNA)-based Ti membrane (TM). Regarding the dopant for SnO₂, Sb was more effective in boosting the electrocatalytic activity than Bi, and the energy consumption for Sb-SnO₂-coated REM (TM/RTNA/ATO) was lower than Bi-SnO₂-coated REM (TM/RTNA/BTO). As for the internal layer, RTNA provided TM/RTNA/ATO with more electroactive surface areas and prolonged the service lifetime. Compared with batch mode, the SMX removal efficiency in flow-through mode was increased up to 8.4-fold. The SMX degradation performances were also affected by fluid velocity, current density, initial SMX concentration, and electrolyte concentration. The synergistic effects of ^•OH oxidation and direct electron transfer were responsible for the effective removal of SMX. TM/RTNA/ATO was proved to be stable and durable by multi-cycle and accelerated lifetime tests. Its extensive applicability was verified with high removal efficiencies of SMX in the surface water and wastewater effluent. These results demonstrate the promise of TM/RTNA/ATO for water treatment applications.

Carbon electrochemical membrane functionalized with flower cluster-like FeOOH catalyst for organic pollutants decontamination

Decorating active catalysts on the reactive electrochemical membrane (REM) is an effective way to further improve its decontamination performance. In this work, a novel carbon electrochemical membrane (FCM-30) was prepared through coating FeOOH nano catalyst on a low-cost coal-based carbon membrane (CM) through facile and green electrochemical deposition. Structural characterizations demonstrated that the FeOOH catalyst was successfully coated on CM, and it grew into a flower cluster-like morphology with abundant active sites when the deposition time was 30 min. The nano FeOOH flower clusters can obviously boost the hydrophilicity and electrochemical performance of FCM-30, which enhance its permeability and bisphenol A (BPA) removal efficiency during the electrochemical treatment. Effects of applied voltages, flow rates, electrolyte concentrations and water matrixes on BPA removal efficiency were investigated systematically. Under the operation condition of 2.0 V applied voltage and 2.0 mL·min^-1 flow rate, FCM-30 can achieve the high removal efficiency of 93.24% and 82.71% for BPA and chemical oxygen demand (COD) (71.01% and 54.89% for CM), respectively, with only a low energy consumption (EC) of 0.41 kWh·kgCOD^-1, which can be ascribed to the enhancement on OH yield and direct oxidation ability by the FeOOH catalyst. Moreover, this treatment system also exhibits good reusability and can be adopted on different water background as well as different pollutants.

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 %.

A Review of PFAS Destruction Technologies

Per- and polyfluoroalkyl substances (PFASs) are a family of highly toxic emerging contaminants that have caught the attention of both the public and private sectors due to their adverse health impacts on society. The scientific community has been laboriously working on two fronts: (1) adapting already existing and effective technologies in destroying organic contaminants for PFAS remediation and (2) developing new technologies to remediate PFAS. A common characteristic in both areas is the separation/removal of PFASs from other contaminants or media, followed by destruction. The widely adopted separation technologies can remove PFASs from being in contact with humans; however, they remain in the environment and continue to pose health risks. On the other hand, the destructive technologies discussed here can effectively destroy PFAS compounds and fully address society's urgent need to remediate this harmful family of chemical compounds. This review reports and compare widely accepted as well as emerging PFAS destruction technologies. Some of the technologies presented in this review are still under development at the lab scale, while others have already been tested in the field.

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.

Magnéli phase titanium sub-oxides synthesis, fabrication and its application for environmental remediation: Current status and prospect

Sub-stoichiometric titanium oxide, also called titanium suboxides (TSO), had been a focus of research for many decades with a chemical composition of Ti_nO_2n-1 (n ≥ 1). It has a unique oxygen-deficient crystal structure which provides it an outstanding electrical conductivity and high corrosion resistance similar to ceramic materials. High electrical conductivity and ability to sustain in adverse media make these phases a point of attention for researchers in energy storage and environmental remediation applications. The Magnéli phase-based reactive electroconductive membranes (REM) and electrodes have demonstrated the electrochemical oxidation of pollutants in the water in flow-through and flow by configuration. Additionally, it has also shown its potential for visible light photochemical degradation as well. This review attempts to summarize state of the art in various Magnéli phases materials synthesis routes and their electrochemical and photochemical ability for environmental application. The manuscript introduces the Magnéli phase, its crystal structure, and catalytic properties, followed by the recent development in synthesis methods from diverse titanium sources, notably TiO₂ through thermal reduction. The various fabrication methods for Magnéli phase-base REMs and electrodes have also been summarized. Furthermore, the article discussed the environmental remediations via electrochemical and photochemical advanced oxidation processes. Additionally, the hybrid technology with REMs and electrodes is used to counter membrane biofouling and develop electrochemical sensing devices for the pollutants. The Magnéli phase materials have a bright future for both electrochemical and photochemical advanced oxidation of emerging contaminants in water and wastewater treatment.

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.

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.

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.

Electrochemical oxidation combined with UV irradiation for synergistic removal of perfluorooctane sulfonate (PFOS) in water

The effect of electrochemical degradation on Magnéli phase Ti₄O₇ anode combined with UV irradiation on the removal of PFOS was systematically evaluated in the present study. A synergistic effect of electrolysis and UV irradiation rather than a simple additive effect for PFOS degradation was demonstrated experimentally and theoretically. The short wavelength irradiation within 400 nm is the main contribution to enhance the electrochemical degradation of PFOS, while the initial pH of the solution has little effect on the PFOS degradation. The increase of current density accelerates the removal of PFOS either by electrolysis treatment or the joint process. The time-dependent density functional theory (TD-DFT) calculation indicates that the synergistic effect of the electrolysis and UV irradiation is most likely due to the involvement of the excited PFOS induced under UV irradiation in the electrochemical reaction. This study provides the first mechanistic explanation for the electrochemical degradation of PFOS enhanced by UV irradiation.

A critical review on classifications, characteristics, and applications of electrically conductive membranes for toxic pollutant removal from water: Comparison between composite and inorganic electrically conductive membranes

Research efforts have recently been directed at developing electrically conductive membranes (EMs) for pressure-driven membrane separation processes to remove effectively the highly toxic pollutants from water. EMs serve as both the filter and the electrode during filtration. With the assistance of a power supply, EMs can considerably improve the toxic pollutant removal efficiency and even realize chemical degradation to reduce their toxicity. Organic-inorganic composite EMs and inorganic EMs show remarkable differences in characteristics, removal mechanisms, and application situations. Understanding their differences is highly important to guide the future design of EMs for specific pollutant removal from water. However, reviews concerning the differences between composite and inorganic EMs are still lacking. In this review, we summarize the classifications, fabrication techniques, and characteristics of composite and inorganic EMs. We also elaborate on the removal mechanisms and performances of EMs toward recalcitrant organic pollutants and toxic inorganic ions in water. The comparison between composite and inorganic EMs is emphasized particularly in terms of the membrane characteristics (pore size, permeability, and electrical conductivity), application situations, and underlying removal mechanisms. Finally, the energy consumption and durability of EMs are evaluated, and future perspectives are presented.

Efficient decomposition of perfluorooctane sulfonate by electrochemical activation of peroxymonosulfate in aqueous solution: Efficacy, reaction mechanism, active sites, and application potential

The electrochemical oxidation method is a promising technology for the degradation of perfluorooctane sulfonate (PFOS). However, the elimination processes of PFOS are still unknown, including the electron transfer pathway, key reactive sites, and degradation mechanism. Here, we fabricated diatomite and cerium (Ce) co-modified Sb₂O₃ (D-Ce/Sb₂O₃) anode to realize efficient degradation of PFOS via peroxymonosulfate (PMS) activation. The transferred electron and the generated hydroxyl radical (•OH) can high-effectively decompose PFOS. The electron can be rapidly transferred from the highest occupied molecular orbital of the PFOS to the lowest unoccupied molecular orbital of the PMS via the D-Ce/Sb₂O₃ driven by a potential energy difference under electrochemical process. The active site of Ce-O in the D-Ce/Sb₂O₃ can greatly reduce the migration distance of the electron and the •OH, and thus improving the catalytic activity for degrading various organic micropollutants with high stability. In addition, the electrochemical process shows strong resistance and tolerance to the changing pH, inorganic ions, and organic matter. This study offers insights into the electron transfer pathway and PMS activation mechanism in PFOS removal via electrochemical oxidation, paving the way for its potential application in water purification.

Flow-through electrochemical removal of benzotriazole by electroactive ceramic membrane

Benzotriazole (BTA) is a widely used anticorrosive additive that is of endurance, bioaccumulation and toxicity, and BTA industrial wastewater treatment remains a challenge. This study reports efficient electrochemical removal of BTA by titanium oxide (TiSO) electroactive ceramic membrane (ECM), indicated by 98.1% removal at current density of 20 mA∙cm-2 and permeate flux of 692 LHM under cathode-to-anode flow pattern (1 h). Electrochemical analysis demonstrated the pH-dependent formation of anti-corrosive BTA film on the TiSO anode, which was responsible for improved BTA removal for cathode-to-anode (CA) flow pattern compared with that for anode-to-cathode (AC). The modelling results showed the CA flow pattern to be more favourable for BTA oxidation mediated by electro-generated •OH by preventing the formation of deactivation film via creating an alkaline boundary layer at the anode/electrolyte interface. Intermediates and essential active sites were identified by using experimental analysis and theoretical density functional theory (DFT) calculations, thereby the most likely degradation pathways were underlined. Toxicity analysis revealed remarkable decrease in oral rat LD50 values and bioaccumulation factor during electrochemical degradation of BTA. This study provides a proof-in-concept demonstration of effective removal for anti-corrosive emerging pollutants by TiSO-ECM under flow-through pattern.

A Review on Removal and Destruction of Per- and Polyfluoroalkyl Substances (PFAS) by Novel Membranes

Per- and Polyfluoroalkyl Substances (PFAS) are anthropogenic chemicals consisting of thousands of individual species. PFAS consists of a fully or partly fluorinated carbon-fluorine bond, which is hard to break and requires a high amount of energy (536 kJ/mole). Resulting from their unique hydrophobic/oleophobic nature and their chemical and mechanical stability, they are highly resistant to thermal, chemical, and biological degradation. PFAS have been used extensively worldwide since the 1940s in various products such as non-stick household items, food-packaging, cosmetics, electronics, and firefighting foams. Exposure to PFAS may lead to health issues such as hormonal imbalances, a compromised immune system, cancer, fertility disorders, and adverse effects on fetal growth and learning ability in children. To date, very few novel membrane approaches have been reported effective in removing and destroying PFAS. Therefore, this article provides a critical review of PFAS treatment and removal approaches by membrane separation systems. We discuss recently reported novel and effective membrane techniques for PFAS separation and include a detailed discussion of parameters affecting PFAS membrane separation and destruction. Moreover, an estimation of cost analysis is also included for each treatment technology. Additionally, since the PFAS treatment technology is still growing, we have incorporated several future directions for efficient PFAS treatment.

Hydroxyl radicals in anodic oxidation systems: generation, identification and quantification

Anodic oxidation has emerged as a promising treatment technology for the removal of a broad range of organic pollutants from wastewaters. Hydroxyl radicals are the primary species generated in anodic oxidation systems to oxidize organics. In this review, the methods of identifying hydroxyl radicals and the existing debates and misunderstandings regarding the validity of experimental results are discussed. Consideration is given to the methods of quantification of hydroxyl radicals in anodic oxidation systems with particular attention to approaches used to compare the electrochemical performance of different anodes. In addition, we describe recent progress in understanding the mechanisms of hydroxyl radical generation at the surface of most commonly used anodes and the utilization of hydroxyl radical in typical electrochemical reactors. This review shows that the key challenges facing anodic oxidation technology are related to i) the elimination of mistakes in identifying hydroxyl radicals, ii) the establishment of an effective hydroxyl radical quantification method, iii) the development of cost effective anode materials with high corrosion resistance and high electrochemical activity and iv) the optimization of electrochemical reactor design to maximise the utilization efficiency of hydroxyl radicals.

Interface engineering strategy of a Ti4O7 ceramic membrane via graphene oxide nanoparticles toward efficient electrooxidation of 1,4-dioxane

Although Ti₄O₇ ceramic membrane has been recognized as one of the most promising anode materials for electrochemical advanced oxidation process (EAOP), it suffers from relatively low hydroxyl radical (^•OH) production rate and high charge-transfer resistance that restricted its oxidation performance of organic pollutants. Herein, we reported an effective interface engineering strategy to develop a Ti₄O₇ reactive electrochemical membrane (REM) doped by graphene oxide nanoparticles (GONs), GONs@Ti₄O₇ REM, via strong GONs-O-Ti bonds. Results showed that 1% (wt%) GON doping on Ti₄O₇ REM significantly reduced the charge-transfer resistance from 73.87 to 8.42 Ω compared with the pristine Ti₄O₇ REM, and yielded ^•OH at 2.5-2.8 times higher rate. The 1,4-dioxane (1,4-D) oxidation rate in batch experiments by 1%GONs@Ti₄O₇ REM was 1.49×10^-2 min^-1, 2 times higher than that of the pristine Ti₄O₇ REM (7.51×10^-3 min^-1) and similar to that of BDD (1.79×10^-2 min^-1). The 1%GONs@Ti₄O₇ REM exhibited high stability after a polarization test of 90 h at 80 mA/cm², and within 15 consecutive cycles, its oxidation performance was stable (95.1-99.2%) with about 1% of GONs lost on the REM. In addition, REM process can efficiently degrade refractory organic matters in the groundwater and landfill leachate, the total organic carbon was removed by 54.5% with a single-pass REM. A normalized electric energy consumption per log removal of 1,4-D (EE/O) was observed at only 0.2-0.6 kWh/m³. Our results suggested that chemical-bonded interface engineering strategy using GONs can facilitate the EAOP performance of Ti₄O₇ ceramic membrane with outstanding reactivity and stability.

Recent advances in electrocatalytic membrane for the removal of micropollutants from water and wastewater

The increasing occurrence of micropollutants in water and wastewater threatens human health and ecological security. Electrocatalytic membrane (EM), a new hybrid water treatment platform that integrates membrane separation with electrochemical technologies, has attracted extensive attention in the removal of micropollutants from water and wastewater in the past decade. Here, we systematically review the recent advances of EM for micropollutant removal from water and wastewater. The mechanisms of the EM for micropollutant removal are first introduced. Afterwards, the related membrane materials and operating conditions of the EM are summarized and analyzed. Lastly, the challenges and future prospects of the EM in research and applications are also discussed, aiming at a more efficient removal of micropollutants from water and wastewater.

Exploration of perfluorooctane sulfonate degradation properties and mechanism via electron-transfer dominated radical process

Polyfluoroalkyl and perfluoroalkyl chemicals (PFCs) widely used in lubricants, surfactant, textiles, paper coatings, cosmetics, and fire-fighting foams can release a large deal of organics contaminants into wastewater and pose great risks to the health of humans and eco-environments. Although advanced oxidation processes can effectively deconstruct various organic contaminants via reactive radicals, the stable structure of PFCs makes it difficult to be degraded. Here, we confirm that electrochemical oxidation process coupled with peroxymonosulfate (PMS) reaction can efficiently destroy stable structure of PFCs via electron transfer and meanwhile completely degrade PFCs via generated active radicals. We further studies via capturing and scavenging radicals, and DFT calculations find that electron hydroxyl radials play a dominant role in degrading PFCs. Based on the calculations of adsorption energy and molecular orbital energy we further demonstrate that many active sites on the surface of Ti₄O₇ (1 0 4) plane can rapidly take part in electrochemical reaction for generating radials and removing organic contaminants. These results give a promising insight towards high-effective and deep degradation of PFCs via electrochemical reaction coupled with advanced oxidation processes, as well as providing guidance and technical support for the remove of multiple organic contaminants.