Chlorinated Byproduct Formation during the Electrochemical Advanced Oxidation Process at Magnéli Phase Ti4O7 Electrodes

High photothermal conversion efficiency of RF sputtered Ti4O7 Magneli phase thin films and its linear correlation with light absorption capacity

RF-sputtering is used to deposit Ti4O7-Magneli-phase films onto various substrates at deposition temperatures (Ts) ranging from 25 to 650 °C. Not only the structural, but also electrical conductivity, optical absorbance and photothermal properties of the Ti4O7 films are shown to change significantly with Ts. A Ts of 500 °C is pointed out as the optimal temperature that yields highly-crystalized pure-Ti4O7-Magneli phase with a densely-packed morphology and a conductivity as high as 740 S/cm. The Ti4O7 films deposited at Ts = 450-500 °C also exhibited the highest optical absorption over all the broad (200-1500) nm range. The absorbed sunlight (AM1.5) was efficiently converted into heat by raising the temperature of the Ti4O7 films up to ~ 54 °C. Thus, the external photothermal efficiency (ηext) of the Ti4O7 films, was found to be as high as ~ 74%. This is the highest ηext reported so far for sputtered-Ti4O7 coatings (just ~ 450 nm-thick), highlighting their significant potential for photothermal applications such as desalination, deicing and/or smart windows. Finally, the ηext of the Ti4O7 coatings is demonstrated, for the first time, to be linearly correlated to their integrated light absorption coefficient. This fundamental relationship paves the way towards the design and optimization of highly efficient solar-thermal conversion devices.

New insight of electrogenerated H2O2 into oxychlorides inhibition and decontamination promotion: From radical to nonradical pathway during anodic oxidation of high Cl--laden wastewater process

Anodic oxidation (AO) has been extensively hailed as a robust and promising technology for pollutant degradation, but the parasitic formation of oxychlorides (ClOx-) would induce a seriously over-evaluated electrochemical COD removal performance and dramatical biotoxicity increasement of the AO-treated Cl--laden effluents. Herein, we shed new light on the roles of H2O2 high-efficiently electrogenerated in three-dimensional (3D) reactor in inhibiting ClOx- production and promoting pollutant degradation, which has been overlooked in previous literature. Total yield of ClOx- in phenol simulated wastewater containing 30 mM Cl- was dropped from 25 mM and 24.3 mM to only 0.26 mM and 0.23 mM within 120 min after treating by 3D H2O2-involing systems with Ti/Ru-IrO2 and BDD anode, respectively. Meanwhile, the COD removal of 3D Ti/Ru-IrO2-based system was increased by 57 % (85 % removal at 0.011 kWh g-1 COD), comparable to that of 3D BDD-based system (90 % removal at 0.008 kWh g-1 COD), the energy consumption of which were far less than those of conventional 2D and 3D electro-Fenton systems (0.08-0.2 kWh g-1 COD). During degradation process of Cl--bearing phenol by 3D AO-H2O2 systems, the anodically produced species (Cl•, Cl2•-, ClO-) were rapidly quenched by the in-situ electrogenerated H2O2 and then successfully transformed into 1O2. The radical pathway of reaction between H2O2 and Cl•/Cl2•- had a more obviously thermodynamical advantage (∆G = 11.5 kJ mol-1) than nonradical pathway between H2O2 and ClO- (∆G = 171 kJ mol-1) based on DFT analysis. And the steady-state concentration of 1O2 was 8.8 × 10-9 M and 4.2 × 10-10 M in 3D Ti/Ru-IrO2 and BDD-based system, respectively, which collectively took responsibility for the termination of ClOx- production and promotion of organic pollutant degradation. This work provides a technical feasibility in the practical utilization of AO technology to wastewater treatment without toxic oxychloride by-products.

Efficient electro-oxidation-based degradation of per- and polyfluoroalkyl (PFAS) persistent pollutants by using plasma torch synthesized pure-Magnéli phase-Ti4O7 anodes

Pure Magnéli-phase Ti4O7 were prepared by means of a Plasma Torch (PT) coating method and integrated into an advanced electro-catalytic oxidation (AEO) process in order to degrade perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) persistent pollutants present in waters. The X-ray diffraction analysis confirmed the polycrystalline nature of the pure Magnéli phase PT-Ti4O7 coatings (∼100 μm thick)). The Raman spectra of the PT-Ti4O7 coatings also exhibited the two characteristic peaks (at 138 and 183 cm-1) of the PT-Ti4O7 Magnéli phase. Scanning electron microscopy revealed the nanostructured hierarchical morphology of the PT-Ti4O7 thus conferring them high surface area. The PT-Ti4O7 anodes are shown to achieve higher degradation efficiencies towards PFOA and PFOS in comparison with the conventional boron-doped diamond anodes. By investigating several AEO parameters (including current density, treatment time, nature of the anode material), we were able to optimise the AEO process. Thus, for both PFOA and PFOS (at an initial concentration of 500 ppb in synthetic wastewaters), degradation efficiencies as high as 96.6% and 99.7% were achieved, respectively, with a current density of 20 mA/cm2, a treatment time of 120 min and PT-Ti4O7 mesh-type anodes. PFOA and PFOS can be degraded by both direct anodic electrochemical oxidation (•OH radicals) and indirect electrochemical oxidation via mediators, such as persulphate acid (H2S2O8) generated by sulphate anodic oxidation. The degradation of both compounds followed pseudo-first-order kinetics. The reaction rate constant (k) for PFOS removal was 4.63 × 10-2 min-1, whereas 2.76 × 10-2 min-1 was recorded for PFOA removal. Subsequently, we have used the above optimal AEO operating conditions to treat real wastewater effluents (containing 17 types of PFAS molecules with a total content of 8500 ppb) and achieved a degradation rate of 39.1%-87.4% for eight of the 17 PFAS compounds. The degradation rate was found to be dependent on the chemical structure and chain length of each PFOA/PFOS component.

Electrochemically producing high-valent iron-oxo species for phenolics-laden high chloride wastewater pretreatment

Electrochemical advanced oxidation processes (EAOPs) have shown great promise for treating industrial wastewater contaminated with phenolic compounds. However, the presence of chloride in the wastewater leads to the production of undesirable chlorinated organic and inorganic byproducts, limiting the application of EAOPs. To address this challenge, we investigated the potential of incorporating Fe(II) and Fe(III) into the EAOPs with a boron-doped diamond (BDD) anode under near-neutral conditions. Our findings revealed that both Fe(II) and Fe(III) facilitated the generation of high-valent iron-oxo species (Fe(IV) and Fe(V)) in the anodic compartment, thereby reducing the oxidation contribution of reactive chlorine species. Remarkably, the addition of 1000 μM Fe(II) under high chloride conditions resulted in over a 2.8-fold increase in the oxidation rate of 50 μM phenolic contaminants at pH 6.5. Furthermore, 1000 μM Fe(II) contributed to a reduction of more than 66% in the formation of chlorinated byproducts, consequently enhancing the biodegradability of the treated water. Additionally, transitioning from batch mode to continuous flow mode further amplified the positive effects of Fe(II) on the EAOPs. Overall, this study presents a modified electrochemical approach that simultaneously enhanced the degradation of phenolic contaminants and improved the biodegradability of wastewater with high chloride concentrations.

Exploring the viability of peracetic acid-mediated antibiotic degradation in wastewater through activation with electrogenerated HClO

Electrochemical advanced oxidation processes (EAOPs) face challenging conditions in chloride media, owing to the co-generation of undesirable Cl-disinfection byproducts (Cl-DBPs). Herein, the synergistic activation between in-situ electrogenerated HClO and peracetic acid (PAA)-based reactive species in actual wastewater is discussed. A metal-free graphene-modified graphite felt (graphene/GF) cathode is used for the first time to achieve the electrochemically-mediated activation of PAA. The PAA/Cl- system allowed a near-complete sulfamethoxazole (SMX) degradation (kobs =0.49 min-1) in only 5 min in a model solution, inducing 32.7- and 8.2-fold rise in kobs as compared to single PAA and Cl- systems, respectively. Such enhancement is attributed to the occurrence of 1O2 (25.5 μmol L-1 after 5 min of electrolysis) from the thermodynamically favored reaction between HClO and PAA-based reactive species. The antibiotic degradation in a complex water matrix was further considered. The SMX removal is slightly susceptible to the coexisting natural organic matter, with both the acute cytotoxicity (ACT) and the yield of 12 DBPs decreasing by 29.4 % and 37.3 %, respectively. According to calculations, HClO accumulation and organic Cl-addition reactions are thermodynamically unfavored. This study provides a scenario-oriented paradigm for PAA-based electrochemical treatment technology, being particularly appealing for treating wastewater rich in Cl- ion, which may derive in toxic Cl-DBPs.

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.

Activated carbon fiber as an efficient co-catalyst toward accelerating Fe2+/Fe3+ cycling for improved removal of antibiotic cefaclor via electro-Fenton process using a gas diffusion electrode

The electro-Fenton (EF) based on gas-diffusion electrodes (GDEs) reveals promising application prospective towards recalcitrant organics degradation because such GDEs often yields superior H2O2 generation efficiency and selectivity. However, the low efficiency of Fe2+/Fe3+ cycle with GDEs is always considered to be the limiting step for the EF process. In this study, activated carbon fiber (ACF) was firstly employed as co-catalyst to facilitate the performance of antibiotic cefaclor (CEC) decomposition in EF process. It was found that the addition of ACF co-catalyst achieved a rapid Fe2+/Fe3+ cycling, which significantly enhanced Fenton's reaction and hydroxyl radicals (•OH) generation. X-ray photoelectron spectroscopy (XPS) results indicated that the functional groups on ACF surface are related to the conversion of Fe3+ into Fe2+. Moreover, DMSO probing experiment confirmed the enhanced •OH production in EF + ACF system compared to conventional EF system. When inactive BDD and Ti4O7/Ti anodes were paired to EF system, the addition of ACF could significantly improve mineralization degree. However, a large amount of toxic byproducts, including chlorate (ClO3-) and perchlorate (ClO4-), were generated in these EF processes, especially for BDD anode, due to their robust oxidation capacity. Higher mineralization efficiency and less toxic ClO4- generation were obtained in the EF + ACF process with Ti4O7/Ti anode. This presents a novel alternative for efficient chloride-containing organic removal during wastewater remediation.

Electrochemical oxidation of reverse osmosis concentrate using a pilot-scale reactive electrochemical membrane filtration system: Performance and mechanisms

Scale-up treatment of real wastewater holds the key to promoting the practical application of electrochemical filtration technology. This work used a pilot-scale Ti/Pd reactive electrochemical membrane (REM) system (12 REM modules with a total REM area of 0.144 m2) to treat high-salinity reverse osmosis concentrate (ROC) from a chemical industry park. The pilot-scale Ti/Pd REM system demonstrated effective electrochemical degradation of ROC wastewater, achieving removal efficiencies of 82.3 ± 1.9% for COD and 46.7 ± 5.6% for TN at a membrane flux of 90 L/(m2·h) and a cell voltage of 5 V, with an energy consumption of 0.045 kWh/g-COD. Singlet oxygen (1O2) and reactive chlorine species were identified as the two primary reactive oxygen species generated in the Ti/Pd REM system. Fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis indicated that the pilot-scale Ti/Pd REM treatment effectively oxidized humic acid-like substance and unsaturated aromatic compounds. Overall, the Ti/Pd REM technology shows a promising application potential for the treatment of high-salinity ROC from the chemical industry.

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.

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

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.

Formation of oxidation byproducts during electrochemical treatment of simulated produced water

Electrochemical oxidation (EO) can effectively remove recalcitrant organic contaminants from produced water (PW) but the formation of toxic oxidation byproducts (OBPs) is an unintended consequence. This study has rigorously investigated the OBPs formation during the EO treatment of a simulated PW containing phenol - a common organic contaminant existing in PW, as a model contaminant. In the absence of ammonia, free chlorine was generated from Cl- oxidation to serve as the main oxidant for phenol oxidation. During the EO process, 2,4,6-trichlorophenol and 2,6-dichlorobenzoquinone were identified as the critical intermediates that led to the formation of carbonaceous OBPs (C-OBPs). Some C-OBPs like chloroform (TCM), chloral hydrate (CH), and trichloroacetic acid (TCAA) reached their peak concentrations of 15 - 180 μM that were then reduced to 1 - 115 μM via volatilization and/or electrochemical reduction. When ammonia was present, nitrogenous OBPs (N-OBPs) were formed with the peak levels of 1 - 10 μM at the chlorination breakpoint (when ammonia was completely removed) that were subsequently reduced below 1 uM via volatilization and/or hydrolysis. It was observed that ammonia significantly decreased the formation of both C-OBPs and chlorate due to the consumption of free chlorine. A higher current density accelerated OBPs formation rates with different effects on volatile and non-volatile OBPs. The results of this study will enhance our understanding of OBPs formation precursors and mechanisms during electrochemical process and help develop strategies for proper control of OBPs to achieve safer electrochemical wastewater treatment.

Electrochemical Removal of Metal-Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

Cu and Ni complexes with ethylenediaminetetraacetic acid (Cu/Ni-EDTA), which are commonly present in metal plating industry wastewaters, pose a serious threat to both the environment and human health due to their high toxicity and low biodegradability. In this study, the treatment of solutions containing either or both Cu-EDTA and Ni-EDTA using an electrochemical process is investigated under both oxidizing and reducing electrolysis conditions. Our results indicate that Cu-EDTA is decomplexed as a result of the cathodic reduction of Cu(II) with subsequent electrodeposition of Cu(0) at the cathode when the cathode potential is more negative than the reduction potential of Cu-EDTA to Cu(0). In contrast, the very negative reduction potential of Ni-EDTA to Ni(0) renders the direct reduction of EDTA-complexed Ni(II) at the cathode unimportant. The removal of Ni during the electrolysis process mainly occurs via anodic oxidation of EDTA in Ni-EDTA, with the resulting formation of low-molecular-weight organic acids and the release of Ni2+, which is subsequently deposited as Ni0 on the cathode. A kinetic model incorporating the key reactions occurring in the electrolysis process has been developed, which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall, this study improves our understanding of the mechanism of removal of heavy metals from solution during the electrochemical advanced oxidation of metal plating wastewaters.

Ultrasound-enhanced Magnéli phase Ti4O7 anodic oxidation of per- and polyfluoroalkyl substances (PFAS) towards remediation of aqueous film forming foams (AFFF)

Per-and polyfluoroalkyl substances (PFAS) remediation is still a challenge. In this study, we propose a hybrid system that combines electrochemical treatment with ultrasound irradiation, aiming for an enhanced degradation of PFAS. Equipped with a titanium suboxide (Ti₄O₇) anode, the electrochemical cell is able to remove perfluorooctanoic acid (PFOA) effectively. Under the optimal conditions (50 mA/cm² current density, 0.15 M Na₂SO₄ supporting electrolyte, and stainless steel/Ti₄O₇/stainless steel electrode configuration with a gap of ∼10 mm), the electrochemical process achieves ∼100 % PFOA removal and 43 % defluorination after 6 h. Applying ultrasound irradiation (130 kHz) alone offers a limited PFOA removal, with 33 % PFOA removal and 5.5 % defluorination. When the electrochemical process is combined with ultrasound irradiation, we observe a significant improvement in the remediation performance, with ∼100 % PFOA removal and 63.5 % defluorination, higher than the sum of 48.5 % (43 % achieved by the electrochemical process, plus 5.5 % by the ultrasound irradiation), implying synergistic removal/oxidation effects. The hybrid system also consistently shows the synergistic defluorination during degradation of other PFAS and the PFAS constituents in aqueous film forming foam (AFFF). We attribute the synergistic effect to an activated/cleaned electrode surface, improved mass transfer, and enhanced production of radicals.

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.

Energy-efficient removal of trace antibiotics from low-conductivity water using a Ti4O7 reactive electrochemical ceramic membrane: Matrix effects and implications for byproduct formation

The inevitably high energy consumption of traditional electrochemical processes to treat low-conductivity water has limited their wider application. Herein, we present an energy-efficient alternative, i.e., a Ti₄O₇ reactive electrochemical ceramic membrane (Ti₄O₇-REM) system with a superior mass transfer ability. For the removal of 10-200 μM norfloxacin (NOR) from low-conductivity (178-832 μS cm^-1) water, the Ti₄O₇-REM system increased the kinetics rate constant by 4.3-34.0 times, thus decreasing the energy cost by 80.5-97.3% compared with a flow-by system. The rapid NOR removal was related to the enhanced direct electron transfer process in the Ti₄O₇-REM system, which allowed for higher resistance to HCO₃^- scavenging and a favorable reaction between NOR and the active sites. Meanwhile, this mechanism likely contributed to the less formation of inorganic chlorinated product, ClO₃^-, in the presence of Cl^-. Although organic chlorinated byproducts were not detected during NOR degradation in the Ti₄O₇-REM system, Cl^- influenced the speciation of the intermediates. A single-pass Ti₄O₇-REM system demonstrated 94-97% removal of trace antibiotics from real water samples in 30 s. The additional energy consumption (<0.02 kWh m^-3) using a Ti₄O₇-REM system only contributed to 5.0-6.4% of the total in a typical tertiary wastewater treatment plant. Based on the above results, we can conclude that the convection-enhanced REM technique is viable for the purification of low-conductivity natural waters.

Kinetics and mechanism of the reaction of hydrogen peroxide with hypochlorous acid: Implication on electrochemical water treatment

Reduction of HOCl to Cl^- by in-situ electrochemical synthesis or ex-situ addition of H₂O₂ is a feasible method to minimize Cl-DBPs and ClO_x^- (x = 2, 3, and 4) formation in electrochemical oxidative water treatment systems. This work has investigated the kinetics and mechanism of the reaction between H₂O₂ and HOCl. The kinetics study showed the species-specific second order rate constants for HOCl with H₂O₂ (k₁), HOCl with HO₂^- (k₂) and OCl^- with H₂O₂ (k₃) are 195.5 ± 3.3 M^-1s^-1, 4.0 × 10⁷ M^-1s^-1 and 3.5 × 10³ M^-1s^-1, respectively. The density functional theory calculation showed k₂ is the most advantageous thermodynamically pathway because it does not need to overcome a high energy barrier. The yields of ¹O₂ generation from the reaction of H₂O₂ with HOCl were reinvestigated by using furfuryl alcohol (FFA) as a probe, and an average of 92.3% of ¹O₂ yields was obtained at pH 7-12. The second order rate constants of the reaction of ¹O₂ with 13 phenolates were determined by using the H₂O₂/HOCl system as a quantitative ¹O₂ production source. To establish a quantitative structure activity relationship, quantum chemical descriptors were more satisfactory than empirical Hammett constants. The potential implications in electrochemical oxidative water treatment were discussed at the end.

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.

Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism

Deliberate addition of mildly oxidative chlorate (ClO₃^-), so-called "chemical oxygen demand (COD) remover", into wastewater in China or electrochemical production of ClO₃^- from Cl^- induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO₃^- from the water samples before COD determination to eliminate this interference of ClO₃^-. In this reaction system, keeping the reaction temperature of sulfite reducing ClO₃^- at 60 ^°C was crucial for fast ClO₃^- removal rate, fixed molar [sulfite]_ini/[chlorate]_ini ratio value and the synchronous exhaustion of sulfite and ClO₃^-, which were of great significance for the real application of this improved COD determination method. The ClO₃^- interference on COD determination could be successfully eliminated after 20 min reduction of ClO₃^- by sulfite at pH_ini 4.0∼6.0 with the molar [sulfite]_ini/[chlorate]_ini ratio value in the range of 5∼6 when concentration of ClO₃^- was below 5 mM. Despite of the involvement of SO₄^·- in the sulfite reducing ClO₃^- system, the degradation of organic matters by SO₄^·- could be greatly impeded due to the lessened dissolved oxygen for SO₄^·- production at high reaction temperature and the scavenging of SO₄^·- by sulfite. In this reaction system, ClO₂, ClO₂^- and ClO^- were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl^- as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO₃^--laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.

Integrated flow anodic oxidation and ultrafiltration system for continuous defluorination of perfluorooctanoic acid (PFOA)

While flow anodic oxidation systems can efficiently generate hydroxyl radicals (·OH) and significantly enhance direct electron transfer (DET) processes that result in the oxidation of target contaminants via the charge percolating network of flow anode particles, challenges remain in constructing a flow anodic oxidation system that can be operated continuously with stable performance. Here we incorporate an ultrafiltration (UF) membrane module into the flow anodic oxidation system and achieve the continuous defluorination of perfluorooctanoic acid (PFOA) for 12 days with high efficiency (94.1%) and reasonable energy consumption (38.1 Wh mg^-1) compared to other advanced oxidation processes by using a mixture of conducting Ti_xO_2x-1 and Pd/CNT particles as the flow anode. The results indicate that DET, ·OH mediated oxidation and adsorption processes play critical roles in the degradation of PFOA during the flow anodic oxidation processes. The synergistic effect of the Ti_xO_2x-1 and Pd/CNT particles enhances the defluorination efficiency by 3.2 times at 4.5 V vs Ag/AgCl compared to the control experiment (no flow anode particles present) and promotes the release of F^- into solution while other intermediate products remain adsorbed to the surface of the Pd/CNT particles. Although the Pd/CNT particles were oxidized after the long-term operation, no obvious Pd ion leakage into solution was observed. Results of this study support the feasibility of continuous operation of a flow anode/UF system with stable performance and pave the way for the translation of this advanced oxidation technology to practical application.

On-Demand Atomic Hydrogen Provision by Exposing Electron-Rich Cobalt Sites in an Open-Framework Structure toward Superior Electrocatalytic Nitrate Conversion to Dinitrogen

Electrocatalytic nitrate (NO₃^-) reduction to N₂ via atomic hydrogen (H*) is a promising approach for advanced water treatment. However, the reduction rate and N₂ selectivity are hindered by slow mass transfer and H* provision-utilization mismatch, respectively. Herein, we report an open-framework cathode bearing electron-rich Co sites with extraordinary H* provision performance, which was validated by electron spin resonance (ESR) and cyclic voltammetry (CV) tests. Benefiting from its abundant channels, NO₃^- has a greater opportunity to be efficiently transferred to the vicinity of the Co active sites. Owing to the enhanced mass transfer and on-demand H* provision, the nitrate removal efficiency and N₂ selectivity of the proposed cathode were 100 and 97.89%, respectively, superior to those of noble metal-based electrodes. In addition, in situ differential electrochemical mass spectrometry (DEMS) indicated that ultrafast *NO₂^- to *NO reduction and highly selective *NO to *N₂O or *N transformation played crucial roles during the NO₃^- reduction process. Moreover, the proposed electrochemical system can achieve remarkable N₂ selectivity without the additional Cl^- supply, thus avoiding the formation of chlorinated byproducts, which are usually observed in conventional electrochemical nitrate reduction processes. Environmentally, energy conservation and negligible byproduct release ensure its practicability for use in nitrate remediation.

Nitrate Removal in an Electrically Charged Granular-Activated Carbon Column

Nitrate removal from groundwater remains a challenge. Here, we report on the development of a flow-through, electrically charged, granular-activated carbon (GAC)-filled column, which effectively removes nitrate. In this system, the GAC functioned as an anode, while a titanium sheet acted as a cathode. The high removal rate of nitrate was achieved through a combination of electrosorption and electrochemical transformation to N₂. The column could be readily regenerated in situ by reversing the polarity of the applied potential. We demonstrate that in the presence of chloride, the mechanism responsible for the observed nitrate removal involves a combination of electroadsorption of nitrate to the anodically charged GAC, electroreduction of nitrate to ammonium, and the oxidation of ammonium to N₂ gas by reactive chlorine and other oxidative radicals (with nearly 100% N₂ selectivity). Given the ubiquitous presence of chloride in groundwater, this method represents a ready, green, and sustainable treatment process with significant potential for the remediation of contaminated groundwater.

Graphene-based sponges for electrochemical degradation of persistent organic contaminants

Graphene-based sponges doped with atomic nitrogen and boron were applied for the electrochemical degradation of persistent organic contaminants in one-pass, flow-through mode, and in a low-conductivity supporting electrolyte. The B-doped anode and N-doped cathode was capable of >90% contaminant removal at the geometric anodic current density of 173 A m^-2. The electrochemical degradation of contaminants was achieved via the direct electron transfer, the anodically formed O₃, and by the OH^• radicals formed by the decomposition of H₂O₂ produced at the cathode. The identified transformation products of iopromide show that the anodic cleavage of all three C-I bonds at the aromatic ring was preferential over scissions at the alkyl side chains, suggesting a determining role of the π- π interactions with the graphene surface. In the presence of 20 mM sodium chloride (NaCl), the current efficiency for chlorine production was <0.04%, and there was no chlorate and perchlorate formation, demonstrating a very low electrocatalytic activity of the graphene-based sponge anode towards chloride. Graphene-based sponges were produced using a low-cost, bottom-up method that allows easy introduction of dopants and functionalization of the reduced graphene oxide coating, and thus tailoring of the material for the removal of specific contaminants.

Novel niobium-doped titanium oxide towards electrochemical destruction of forever chemicals

Electrochemical advanced oxidative processes (EAOP) are a promising route to destroy recalcitrant organic contaminants such as per- and polyfluoroalkyl substances (PFAS) in drinking water. Central to EAOP are catalysis-induced reactive free radicals for breaking the carbon fluorine bonds in PFAS. Generating these reactive species electrochemically at electrodes provides an advantage over other oxidation processes that rely on chemicals or other harsh conditions. Herein, we report on the performance of niobium (Nb) doped rutile titanium oxide (TiO2) as a novel EAOP catalytic material, combining theoretical modeling with experimental synthesis and characterization. Calculations based on density functional theory are used to predict the overpotential for oxygen evolution at these candidate electrodes, which must be high in order to oxidize PFAS. The results indicate a non-monotonic trend in which Nb doping below 6.25 at.% is expected to reduce performance relative to TiO2, while higher concentrations up to 12.5 at.% lead to increased performance, approaching that of state-of-the-art Magnéli Ti4O7. TiO2 samples were synthesized with Nb doping concentration at 10 at.%, heat treated at temperatures from 800 to 1100 °C, and found to exhibit high oxidative stability and high generation of reactive oxygen free radical species. The capability of Nb-doped TiO2 to destroy two common species of PFAS in challenge water was tested, and moderate reduction by ~ 30% was observed, comparable to that of Ti4O7 using a simple three-electrode configuration. We conclude that Nb-doped TiO2 is a promising alternative EAOP catalytic material with increased activity towards generating reactive oxygen species and warrants further development for electrochemically destroying PFAS contaminants.

Energy-efficient for advanced oxidation of bio-treated landfill leachate effluent by reactive electrochemical membranes (REMs): Laboratory and pilot scale studies

This study for the first time investigated the advanced treatment of bio-treated landfill leachate effluent using a novel reactive electrochemical membrane (REM) technology at the laboratory and pilot scales. At the laboratory scale, RuO₂-Ir-REM, Ti₄O₇-REM, and β-PbO₂-REM featured similar properties in pore size and water flux. Although RuO₂-Ir-REM holds more reactive sites than the other two REMs, β-PbO₂-REM and Ti₄O₇-REM featured higher oxidation ability than RuO₂-Ir-REM, causing their high yield of hydroxyl radical. Consequently, β-PbO₂-REM and Ti₄O₇-REM performed better than RuO₂-Ir-REM, which removed total organic carbon and ammonia nitrogen by 70%-76% and 100%, respectively, after 45 minutes of treatment. Fluorescence spectroscopy analysis showed that humic acid-like substances were oxidized by the REM treatment. Using the β-PbO₂-REM in the lab-scale setup with the solutions circulated, we observed a greater removal of chemical oxygen demand (COD) at a higher applied current or a faster water flux. The pilot system with four large size of β-PbO₂-REMs modules in series was developed based on the lab-scale setup, which steadily treated landfill leachate in compliance with the disposal regulations of China, at an energy consumption of 3.6 kWh/m³. Also, a single-pass REM can effectively prevent the transformation of chloride to chlorate and perchlorate. Our study showed REM technology is a powerful and promising process for the advanced treatment of landfill leachate.