Oxygen Evolution Potential Research Articles

Construction of the multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode for the highly efficient and selective oxidation of ammonia in aqueous solution: Characterization, performance and mechanism

Electrochemical oxidation is a promising technology to convert ammonia nitrogen to environmental-friendly gaseous nitrogen, but the high cost of the precious metal as an electrode limits its application. Herein, the multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode, a cheap and efficient dimensionally stable anode (DSA), was constructed for selective electrocatalytic oxidation of ammonia. The surface morphology, crystalline structure, and composition of the TiO2-NTs/Sb-SnO2/PbO2 electrode were investigated by the field emission scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The synergistic effect of the multi-layer structures of the electrode contributed to a stable structure and efficient electrocatalytic activity. Accelerated lifetime measurements and electrode recycling tests proved that the electrode possessed outstanding stability and long life with continuously used for 935 h. Compared with the commercial Ti/RuO2-IrO2, the TiO2-NTs/Sb-SnO2/PbO2 electrode displayed a more excellent performance of chlorine evolution and higher removal efficiency of ammonia. Under the optimal conditions, 30 mg/L of ammonia nitrogen could be completely removed with 92.3% selective conversion to gaseous nitrogen after 60 min. Moreover, the difference between oxygen evolution potential and chlorine evolution potential exceeded 0.45 V on the TiO2-NTs/Sb-SnO2/PbO2 electrode, which enhanced the anodic chlorine evolution reaction. Hypochlorite radical (ClO•) was supposed as the dominant oxidant to transform ammonia in water, which could promote ammonia removal and the selectivity of gaseous nitrogen with minimizing by-products formation such as nitrite and nitrate, etc. Therefore, this study suggests that the constructed multi-layer TiO2-NTs/Sb-SnO2/PbO2 electrode might be a promising anode for the electrochemical oxidation of ammonia nitrogen to N2 in an aqueous solution.

Efficient electrochemical oxidation of cephalosporin antibiotics by a highly active cerium doped PbO2 anode: Parameters optimization, kinetics and degradation pathways

Removing cephalosporin contaminants from aquatic environments is of great concern because of the hazards they pose to the environment and human health. In this study, a Ce-PbO2 anode with a SnO2-Sb intermediate layer was fabricated using an electrochemical deposition method and used for the electrocatalytic oxidation of cephalosporin antibiotics. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed the successful preparation of the Ce-PbO2 anode and revealed that Ce doping resulted in a highly stable structure and crystallinity of the PbO2 crystals. Moreover, the Ce-PbO2 anode exhibited excellent electrochemical performance with higher oxygen evolution potential (1.56 V), larger electroactive surface area (1.104 C cm−2), smaller corrosion current density (1.796 × 10−4 A cm−2), and higher hydroxyl radical (•OH) yield (8.64 mM min−1 m−2) compared with the intermediate layer and PbO2 anode. The effects of major operating parameters, including current density (20–80 mA cm−2), electrolyte concentration (0.050–0.125 M), initial concentration of target compounds (5–75 mg L−1), and pH (2−10) on the removal efficiencies of cefradine (RAD), and cephalothin (CEP) at the Ce-PbO2 electrode were investigated. The research results illustrated that when electrolysis was carried out for 90 min at the optimal current density of 40 mA cm−2, an initial concentration of 50 mg L−1, and an electrolyte concentration of 0.10 M, the removal efficiency of RAD and CEP were higher than 99%. Additionally, the degradation process followed pseudo-first-order kinetics. The total organic carbon (TOC) removal efficiency of RAD and CEP was 60.6% and 68.2% after 240 min, respectively, under the optimal conditions. The energy consumption for RAD and CEP degradation was 1.74 and 3.04 kWh g−1 TOC, respectively. In addition, the Ce-PbO2 electrode exhibited good stability over nine consecutive cycles. Using UPLC-MS, the intermediate products of RAD and CEP were confirmed, and the degradation intermediates were completely oxidized to CO2, H2O, and inorganic ions (SO42-, NO3-). During electrolysis, the intermediate toxicity was studied using Escherichia coli growth inhibition tests, and the growth inhibition rate was calculated. Considering these results, the Ce-PbO2 anode could be utilized to treat wastewater containing cephalosporin antibiotics with high efficiency.

Differentiating the reaction mechanism of three-dimensionally electrocatalytic system packed with different particle electrodes: Electro-oxidation versus electro-fenton

Recently, there are still some controversial mechanisms of the 3D electrocatalytic oxidation system, which would probably confound its industrial application. From the conventional viewpoint, the Ti4O7 material may be the desired particle electrodes in the 3D system since its high oxygen evolution potential favors the production of •OH via H2O splitting reaction at the anode side of Ti4O7 particle electrodes. In fact, the incorporation of Ti4O7 particles showed phenol degradation of 88% and COD removal of 51% within 120 min, under the optimum conditions at energy consumption of 0.668 kWh g−1 COD, the performance of which was much lower than those in many previous literatures. In contrast, the prepared carbon black-polytetrafluoroethylene composite (CB-PTFE) particles with abundant oxygen-containing functional groups could yield considerable amounts of H2O2 (200 mg L−1) in the 3D reactor and achieved a complete degradation of phenol and COD removal of 80% in the presence of Fe2+, accompanying a low energy consumption of only 0.080 kWh g−1 COD. It was estimated that only 20% of Ti4O7 particles near the anode attained the potential over 2.73 V/SCE at 30 mA cm−2 based on the potential test and simulation, responsible for the low yield of •OH via the H2O splitting on Ti4O7 (1.74 × 10−14 M), and the main role of Ti4O7 particle electrodes in phenol degradation was through direct oxidation. For the CB-PTFE-based 3D system, current density of 10 mA cm−2 was sufficient for all the CB-PTFE particles to attain cathodic potential of −0.67 V/SCE, conducive to the high yield of H2O2 and •OH (9.11 × 10−14 M) in the presence of Fe2+, and the •OH-mediated indirect oxidation was mainly responsible for the phenol degradation. Generally, this study can provide a deep insight into the 3D electrocatalytic oxidation technology and help to develop the high-efficiency and cost-efficient 3D technologies for industrial application.

Electrochemical oxidation of phenol in chloride containing electrolyte using a carbon-coated Ti4O7 anode

Magnéli-phase Ti4O7 is an excellent anode material for electrochemical advanced oxidation process (EAOP). However, the development of Ti4O7 electrodes is always limited by the high cost and high energy consumption in their preparation process. In this study, the carbon-coated Ti4O7 electrode was successfully synthesized by an in-situ pyrolysis method, which exhibited high specific surface area (143.8 m2 g−1) and efficient electron transfer (47.3 Ω). The accelerated service life was measured to be 102.5 h, demonstrating its excellent stability. EAOP performance of the C-coated Ti4O7 electrode was evaluated under various operating conditions. The result showed that a complete degradation of phenol (100 mg L−1) was obtained in 60 min, under the condition of 0.1 M NaCl electrolyte and 20 mA cm−2 current density. Owing to the high oxygen evolution potential (OEP, 2.25 V vs. RHE) of the C-coated Ti4O7 electrode, high TOC removal ratio (68.9%) and low energy consumption (113.8 kWh kg−1TOC) were achieved under the optimal condition. Moreover, the mechanism of phenol degradation was investigated by linear sweep voltammetry (LSV) and free radical scavenging experiments. The results indicated that the degradation of phenol can be mainly attributed to the synergistic effects between adsorbed ·OH (M(·OH)) and Cl−, rather than the direct electron transfer process (DET) on the surface of the C-coated Ti4O7 electrode. This study provides an efficient strategy for the mineralization of phenolic wastewater by the C-coated Ti4O7 electrode.

The Effect of Boron Doping Concentration on the Electrochemical Oxidation of Chlorine Using BDD Electrode

Boron-doped diamond (BDD) electrode is an excellent candidate for anodic electrochemical oxidation of wastewater. However, higher concentrations of ClO3 − and ClO4 − of biotoxicity was found during chloride electrolysis using BDD electrodes compared to other electrode materials. In this study, BDD electrodes with five different boron doping concentrations were fabricated on silicon substrates using hot-filament chemical vapor deposition (HFCVD) method. The physical and electrochemical characterisation were conducted, which confirmed that with the decrease of boron concentration, the crystal size of the diamond decreased, oxygen evolution potential decreased, charge transfer resistance decreased while the electro-active surface area (EASA) increased. The electrochemical oxidation experiments of NaCl solution were carried out with these five BDD electrodes and time-dependent traces of product concentrations, current efficiencies and energy consumptions were compared and discussed. When the boron concentration increased, the minimum energy required to produce unit active chlorine (AC) decreased initially then increased, the same tendency was found in the yields of ClO3 − and ClO4 −. The lightly doped BDD (1.23 × 1020 cm−3) showed low energy consumption and high yield of AC, and low yields of ClO3 − and ClO4 −. Electrolysis of chlorine-containing organic wastewater was studied which further demonstrated the good performance of lightly doped BDD electrode.

Development of 3D network of Zn-oxide nanorods assisted with PbO2/Pb electrode for electrochemical oxidation of methylene blue in aqueous phase

BackgroundA three-dimensional (3D)-network of zinc oxide (ZnO) nanorods is deposited on lead-oxide/lead (PbO2/Pb), to improve anodic oxidation activity via in-situ generation of hydroxyl radical (•OH) used for Methylene blue (MB) removal in a batch process. The performance of modified electrode (ZnO-PbO2/Pb) as anode for decolorization of MB in the aqueous phase is compared with commercially available PbO2/Pb electrode. MethodsHydrothermal synthesis method is used to develop nano/micro 3D network of (metal-oxides of ZnO and PbO2, respectively), examined through scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDX) and X-Ray Diffraction (XRD) analysis were performed for compositional analysis. Synthesis conditions were optimized based on MB removal via electro-chemical oxidation (ECO) process. ZnO-PbO2/Pb electrode exhibited improved oxygen evolution potential (1.37 V) owing to 3D network of metal-oxide with enhanced electrochemical activity. FindingsUnder optimum conditions (100 mg/L, 60 mA/cm2, 0.1 M Na2SO4 and pH 4), a high MB removal efficiency 96.26% and 76% COD removal was achieved. Removal results indicates that 22.60% higher COD removal was achieved for ZnO-PbO2/Pb electrode than PbO2/Pb electrode. Trapping experiment confirmed •OH radical is possibly responsible for decolorization of MB. Service life time of modified electrode is stable up to 24 h, which is two folds higher than pristine PbO2/Pb electrodes (12 h). Based on results, surface modification of PbO2/Pb electrode followed pseudo-first-order rate kinetics is obtained for modified anode material.

Fabrication of a novel PbO2 electrode with rare earth elements doping for p-nitrophenol degradation

A novel PbO2 electrode modified with rare earth elements (La, Ce, Gd and Er) doping (named as Re-PbO2) was prepared by electrodeposition in the present study. The micro-morphology and crystal structure of Re-PbO2 were characterized by scanning electronic microscopy (SEM), energy dispersive spectroscope (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Their electrochemical properties were determined by linear sweep voltammetry (LSV), cyclic voltammetry (CV), accelerated life test and hydroxyl radicals (•OH) formation analysis. Electrochemical oxidation of p-nitrophenol (p-NP) by Re-PbO2 compared with un-doped PbO2 has been investigated and the degradation rate followed the order of Er-PbO2 > Gd-PbO2 > La-PbO2 > Ce-PbO2 > PbO2. Especially for Er-PbO2, the pseudo-first order kinetic for p-NP (kp-NP) degradation was 0.41, which was only 0.19 for un-doped PbO2. Rare earths elements doping improved the oxidation ability of Re-PbO2 mainly through reducing grain size, increasing oxygen evolution potential, enlarging electrochemical active surface area and enhancing •OH formation ability. In addition, existing formation of oxygen species on PbO2 electrode surface was investigated by XPS. For Re-PbO2, percentage of lattice oxygen species (Oads) were higher than that on the un-doped one. These results demonstrated that rare earth elements can enhance the oxidation ability of PbO2 electrode significantly.

Indirect oxidation mechanism governing in P-rGO/Ti anode with C2-PO2/rGO configuration for efficient 2-Methyl-4-Isothiazolin-3-one electrooxidation

A stable phosphorus-doped reduced graphene (P-rGO/Ti) anode applied in electrochemical oxidation was prepared for the degradation of 2-Methyl-4-Isothiazolin-3-one (MIT), a broad-spectrum biocide with high concentration and toxicity widely exists in reverse osmosis concentrates. The P-rGO/Ti anode had high oxygen evolution potential (2.434 V), which inhibited side reactions and accelerated the mineralization of MIT. The •OH quenching tests verified that the indirect oxidation on MIT degradation was up to 84.17 %, and the mechanism of indirect oxidation was proposed about the C2-PO2/rGO configuration of P-rGO was more conducive to adsorb H2O and supply electron, resulting in the elongated OH bond and then easily cleaved to produce more •OH via density functional theory calculations. Furthermore, an empirical kinetic model was established to predict MIT removal trend. The P-rGO/Ti electrode maintained favorable MIT removal efficiency and structural stability with a long-term operation experiment in 1200 min. Moreover, the degradation pathways of MIT were proposed based on UPLC-Q-TOF-MS analysis. This study profoundly reveals the indirect oxidation mechanism of different P-rGO configurations on inducing the generation of •OH, and provides a new perspective for the design of stable and efficient carbon materials in electrochemical oxidation for wastewater purification.

Carbon nanofibre microfiltration membranes tailored by oxygen plasma for electrocatalytic wastewater treatment in cross-flow reactors

The engineering of electrocatalytic membrane reactors provides potential perspectives to integrate membrane separation with electrocatalytic technology for efficient removal of emerging organic pollutants from wastewater. Here, electro-responsive microfiltration carbon nanofibre (CNF) membranes were synthesized by electrospinning of poly(acrylonitrile) PAN and subsequent carbonization, followed by oxygen plasma treatment to induce their surface wettability and reactivity for electrocatalytic water treatment. The electrocatalytic performance of CNF membranes was fine-tuned via oxygen plasma treatment to yield reaction kinetic constants up to 29.6 × 10−3 and 15.6 × 10−3 min−1 against methylene blue (MB) and acetaminophen (ACP), respectively, which were 1.4–1.8 times higher than that exhibited by pristine CNF membranes. The water permeance across CNF membrane was gradually enhanced with increasing the plasma exposure time up to 5 min to exhibit 4.65 × 103 L m−2 h−1.bar−1, while the removal efficiency of MB and ACP was significantly improved to reach 99 and 91%, respectively during combined microfiltration and electrocatalytic reaction, which was 2.4–10.3 times higher than that achieved during microfiltration alone. The achieved performance of oxygen plasma treated CNF membranes was attributed to their enhanced wettability (water contact angle ∼24°) and raised electro-oxidation capacity (oxygen evolution potential ∼1.6 V) with introducing oxygen-containing groups on the membrane surface. This work offers an effective scalable fabrication methodology to engineer flexible and functional CNF membranes with excellent electrocatalytic performance towards cost-effective water treatment.

Constructing dimensionally stable TiO2 nanotube arrays/SnO2/RuO2 anode via successive electrodeposition for efficient electrocatalytic oxidation of As(III)

Arsenic-containing water needs to be purified to reduce the global hazards of arsenic to human health and ecosystems. Herein, successive electrodeposition of SnO2 and RuO2 nanoparticles on the TiO2 nanotube arrays (TNTAs) was performed to synthesize dimensionally stable TNTAs/SnO2/RuO2 anodes for highly efficient electrocatalytic oxidation of As(III) in aqueous solution. Such a TNTAs/SnO2/RuO2 anode possessed fast charge transfer, higher oxygen evolution potential (OEP), larger electrocatalytic active area, and excellent stability, and delivered effectively electrocatalytic oxidation As(III), of which 10 mg/L of As(III) was completely oxidized into As(Ⅴ) with lower toxicity within 20 min owing to the synergistic effect of RuO2 and SnO2. Impressively, the optimized TNTAs/SnO2/RuO2-10 anode presented an excellent electrochemical stability and longer service lifetime, and its oxidation conversion efficiency of As(III) still could be maintained to be 98% as well as its kinetic reaction constant decreased by only 0.03 even after 11 recycled usage, which was ascribed to the intensified interaction between RuO2 and TNTAs/SnO2. The excellence of the innovative anode was also evidenced through its strong anti-interference ability to coexisting ions and the elucidation of the associated oxidation reaction mechanism of As(III) on the surface of the anode by electron spin resonance and radical scavenger tests. This novel electrode integrates high electrocatalytic activity, high electron transport and high stability, ensures the long-term stable use of the electrode, while realizes efficient and rapid electrocatalytic oxidation of As(III) in aquatic environments. This work provides new ideas for further improving the catalytic performance and stability of dimensionally-stable anode materials.

Oxygen vacancies modified MnS/MnO2 heterostructure anode catalyst for efficiently electrocatalytic removal of dye wastewater

Electrochemical advanced oxidation processes (EAOPs) are favorable technologies to remove organic wastewater, but the development of high-efficiency and low-energy consumption anode catalysts is still a priority. Herein, a carbon cloth-supported MnS/MnO2 heterostructure rich in oxygen vacancies is successfully synthesized as an anode for the electrocatalytic removal of methyl orange (MO) wastewater. The anode catalyst possesses lower charge transfer resistance (77.4 Ω), higher oxygen evolution potential (2.13 V vs RHE), and removes nearly 100% methyl orange (20 mg/L) in 30 min with a high mineralization current efficiency of 73.2% and low energy consumption of 23.4 kWh/kg TOC. Through density functional theory (DFT) studies, oxygen vacancies increase the positive charge of Mn atoms in the vicinity of the heterostructure interface, thus improving its catalyst activity. The calculated adsorption energy of H2O (ΔEH2O) is reduced from − 0.672 eV for MnO2 to − 0.887 eV for oxygen-vacancy heterostructure, which is conducive to the water splitting to form •OH. Overall, this study provides a promising strategy for enhancing the electrocatalytic removal performance of manganese dioxide by using heterostructure design and oxygen vacancy engineering.

Thermodynamic and Kinetic Investigation on Electrogeneration of Hydroxyl Radicals for Water Purification

The water oxidation reaction (WOR) is of fundamental importance for the development of promising technologies in the field of energy conversion and environmental remediation. Although effort has been devoted to uncovering the mechanism of the oxygen evolution reaction (OER) by four-electron WOR, little attention has been paid to an in-depth understanding of one-electron WOR for •OH formation, which is the key to the electrochemical advanced oxidation process (EAOP) for environmental applications. Currently, oxygen evolution potential (OEP) is generally regarded as an important thermodynamic descriptor responsible for •OH production, but the kinetic descriptor has been overlooked; thus, the trade-off between thermodynamic and kinetic factors remains unclear. In this study, the thermodynamic feasibility and kinetic activity of electrogeneration of •OH were regulated by doping three kinds of low-electronegativity elements (Ce, Al, and In) into the prototype PbO2 nonactive anode. We investigated these electrodes in terms of •OH production and electrochemical decontamination performance in the context of sulfamethoxazole (SMX) removal. The results showed that 0.8%Ce-PbO2 anode (OEP = 3.67 V vs Ag/AgCl) achieved the highest SMX removal performance (∼100%; kobs = 0.027 min–1) and the highest •OH production with a relatively high number of •OH = 2.7 × 107. In comparison, the highest OEP (5.98 V vs Ag/AgCl) for the 1.6%Ce-PbO2 anode exhibited the lowest SMX removal (∼59%; kobs = 0.008 min–1) and 1 order of magnitude lower •OH production (relative number of •OH = 1.5 × 106). This suggested that the electrogeneration of •OH was unlikely to be always positively correlated to the OEP of the anode. Both experimental results and theoretical calculations verified the importance of suitably high OEP and relative fast electron transfer rate toward WOR to generate H2O+ (oxidation intermediate of H2O) for the ideal anode of EAOP. This study not only demonstrates the trade-off existing between thermodynamic (OEP) and kinetic (charge/electron transfer) factors responsible for electrogeneration of •OH via WOR fundamentally but also suggests a possible strategy for developing a high-efficiency anode by doping low-electronegativity elements to optimize one-electron WOR for environmental applications.

Ultra-high oxygen evolution potential of CuO5-Zn1 active sites on SnO2(1 1 0) surface and its origin: DFT theoretical study

SnO2 is an anode material with the potential to form a high oxygen evolution potential for organic wastewater treatment. Previous experiments found that CuZn, co-doped with SnO2 surfaces were suspected to form Cu-O-Zn sites with high catalytic activity. By utilising DFT calculations combined with previous experimental results, it was found that CuO5-Zn1 active sites were formed on the (110) surface of CuZn co-doped SnO2 with an oxygen evolution potential of 2.80 V (vs CHE). The electronic density of states, charge density distribution, Bader charge and charge density difference analysis indicated that the formation of ultra-high oxygen evolution potential was caused by the enhanced covalency of Cu-O-Zn. The indirect covalent enhancement between transition metal atoms found on the SnO2 surface means that atomically regulated and controlled catalytic activity can be achieved on the SnO2 surface.

Fabrication and characterization of PEG-In2O3 modified PbO2 anode for electrochemical degradation of metronidazole

An In2O3-PbO2 anode with high electrocatalytic activity was prepared in our previous study. However, the service life of In2O3-PbO2 anode needed to be further improved. Thus, a typical stabilizer of polyethylene glycol (PEG) was used to modify the In2O3-PbO2 anode in this study. Theoretical and experimental results show that a small amount of PEG promoted the doping of In2O3 into β-PbO2 film, while excessive PEG reduced the doping of In2O3. Doping a moderate amount of PEG not only reduced the PbO2 crystal size, improved the density of electrode film, reduced the charge transfer resistance, increased the oxygen evolution potential, number of active sites and capacity of •OH generation of In2O3-PbO2 anode, but also prolonged the service life of In2O3-PbO2 anode from 96 to 137 h. In the process of electrochemical degradation of metronidazole, the PEG/In2O3-PbO2 anode exhibited superior electrocatalytic activity. The degradation rate constant at the PEG/In2O3-PbO2 anode was 1.682 times higher than that at In2O3-PbO2 anode. After 60 min of electrolysis, COD and TOC removals reached up to 80.92% and 78.52%, respectively, much higher than those of In2O3-PbO2 anode (69.24% and 55.83%, respectively). Additionally, a total of 13 intermediates generated in the degradation process were identified by LC-MS, and two possible degradation pathways were proposed accordingly. Overall, this study offers a promising approach for enhancing the electrocatalytic activity and stability of the PbO2 anode.

Novel Sb-SnO2 Electrode with Ti3+ Self-Doped Urchin-Like Rutile TiO2 Nanoclusters as the Interlayer for the Effective Degradation of Dye Pollutants.

Stable and efficient SnO2 electrodes are very promising for effectively degrading refractory organic pollutants in wastewater treatment. In this regard, we firstly prepared Ti3+ self-doped urchin-like rutile TiO2 nanoclusters (TiO2-x NCs) on a Ti mesh substrate by hydrothermal and electroreduction to serve as an interlayer for the deposition of Sb-SnO2 . The TiO2-x NCs/Sb-SnO2 anode exhibited a high oxygen evolution potential (2.63 V vs. SCE) and strong ⋅OH generation ability for the enhanced amount of absorbed oxygen species. Thus, the degradation results demonstrated its good rhodamine B (RhB), methylene blue (MB), alizarin yellow R (AYR), and methyl orange (MO) removal performance, with the rate constant increased 5.0, 1.9, 1.9, and 4.7 times, respectively, compared to the control Sb-SnO2 electrode. RhB and AYR degradation mechanisms are also proposed based on the results of high-performance liquid chromatography coupled with mass spectrometry and quenching experiments. More importantly, this unique rutile interlayer prolonged the anode lifetime sixfold, given its good lattice match with SnO2 and the three-dimensional concave-convex structure. Consequently, this work paves a new way for designing the crystal form and structure of the interlayers to obtain efficient and stable SnO2 electrodes for addressing dye wastewater problems.

Bismuth-doped 3D carbon felt/PbO2 electrocatalyst for degradation of diuron herbicide and improvement of pesticide wastewater biodegradability

A bismuth-doped three-dimensional networked carbon felt/PbO2 (CF/Bi-PbO2) anode was developed for the electrocatalytic degradation of diuron and treatment of real pesticide wastewater. The prepared PbO2 anodes were characterized by FESEM, XRD, EDX, BET-BJH, accelerated lifetime test, LSV, CV and EIS. Bi doping in the PbO2 matrix significantly increased the electrochemical active area (3.17 vs. 1.57 m2 g−1), oxygen evolution potential (2.3 vs. 1.95 V) and service time (143 vs. 76 h) of CF/Bi-PbO2 anode compared to pristine CF/PbO2. The results showed that the optimized degradation efficiency of diuron by CF/Bi-PbO2 anode at pH 5 and a current density of 15 mA cm−2 after 25 min of reaction is 97.6 %. However, it took 50 min to remove COD with 98.7 % efficiency. The possible pathways of electrocatalytic degradation of diuron were discussed using HPLC-mass data. The electrocatalytic performance of PbO2 anodes was compared under optimal conditions for real pesticide wastewater treatment. Accordingly, the kinetic constant of COD removal by Bi-doped anode (0.0101 min−1) was about 2.4 times higher than pristine PbO2 (0.0043 min−1). The biodegradability index (BOD/COD ratio) of pesticide wastewater after 180 min of treatment by CF/Bi-PbO2 and CF/PbO2 anodes was 0.62 and 0.29, respectively, which shows that the wastewater treated with Bi-doped anode is completely biodegradable.

Influence of MnOx deposition on TiO2 nanotube arrays for electrooxidation

TiO2 has demonstrated outstanding performance in electrochemical advanced oxidation processes (EAOPs) due to its structural stability and high oxygen overpotential. However, there is still much room for improving its electrochemical activity. Herein, narrow bandgap manganese oxide (MnOx) was composited with TiO2 nanotube arrays (TiO2 NTAs) that in-situ oxidized on porous Ti sponge, forming the MnOx-TiO2 NTAs anode. XANES and XPS analysis further proved that the composition of MnOx is Mn2O3. Electrochemical characterizations revealed that increasing the composited concentration of MnOx can improve the conductivity and reduce oxygen evolution potential so as to improve the electrochemical activity of the composited MnOx-TiO2 NTAs anode. Meanwhile, the optimal degradation rate of benzoic acid (BA) was achieved using MnOx-TiO2 NTAs with a MnOx concentration of 0.1 mmol L−1, and the role of MnOx was proposed based on DFT calculation. Additionally, the required electrical energy (EE/O) to destroy BA was optimized by varying the composited concentration of MnOx and the degradation voltage. These quantitative results are of great significance for the design and application of high-performance materials for EAOPs.

Peroxydisulfate production in situ by blue titania nanotube array electrode for tetracycline degradation in groundwater: Performance and mechanism

• PDS generation in situ by Blue-TNA was used to TC degradation in groundwater. • Coexisting anions inhibit the generation of PDS. • The hydroxyl radical is an important precursor of PDS generation. • TC is degraded through the combined action of PDS, ·OH and SO 4 ·- . In this study, a blue titania nanotube array (Blue-TNA) electrode was prepared and applied in oxidizing sulfate ions (SO 4 2- ) in groundwater to peroxydisulfate (PDS) for antibiotics degradation. Influencing factors, the mechanism of PDS formation and the efficiency of antibiotics degradation were studied. There was no PDS generation in both the PbO 2 and Pt electrolysis systems over the experimental time, but 2.8 mmol/L PDS was generated in the Blue-TNA electrolysis system in 90 min, which can be attributed to a higher oxygen evolution potential of Blue-TNA than that of PbO 2 and Pt. An increase in current density and Na 2 SO 4 concentration favored PDS generation and the current efficiency improvement. The coexisting anions (CO 3 2- 、HCO 3 - and PO 4 3- ) inhibited the PDS generation with the strength of inhibition following the sequence: CO 3 2- ＞ HCO 3 - > PO 4 3- . The effect of dissolved oxygen on PDS generation was negligible, but ·OH was an important precursor for PDS generation based on radical quenching experiments. The degradation efficiency of tetracycline (TC, 20 mg/L) by Blue-TNA was more than 90% in 30 min due to the synergy of ·OH and SO 4 ·- in the sulfate electrolysis system. This study proposed a novel and economic method for PDS generation by the oxidation of SO 4 2- in situ using Blue-TNA, which is an eco-friendly and efficient method in TC degradation.

Flow electrochemical inactivation of waterborne bacterial endospores

Waterborne pathogens have the risk of spreading waterborne diseases and even pandemics. Some Gram-positive bacteria can form endospores, the hardiest known life form that can withstand heat, radiation, and chemicals. Electrochemical inactivation may offer a promising solution, but is hindered by low inactivation efficiencies resulting from limitation of electrode/endospores interaction in terms of electrochemical reaction selectivity and mass transfer. Herein, these issues were addressed through modifying selectivity of active species formation using electroactive ceramic membrane with high oxygen evolution potential, improving mass transfer property by flow-through operation. In this way, inactivation (6.0-log) of Bacillus atrophaeus endospores was achieved. Theoretical and experimental results demonstrated synergistic inactivation to occur through fragmentation of coat via interfacial electron transfer and electro-produced transient radicals (•OH primarily, •Cl and Cl2•– secondarily), thereby increasing cell permeability to facilitate penetration of electro-produced persistent active chlorine for subsequent rupture of intracellular structures. Numbering-up electrode module strategy was proposed to scale up the system, achieving average 5.3-log inactivation of pathogenic Bacillus anthracis endospores for 30 days. This study demonstrates a proof-of-concept manner for effective inactivation of waterborne bacterial endospores, which may provide an appealing strategy for wide-range applications like water disinfection, bio-safety control and defense against biological warfare.

Mechanism of highly efficient electrochemical degradation of antibiotic sulfadiazine using a layer-by-layer GNPs/PbO2 electrode

A PbO2 electrode integrating electrocatalytic and adsorptive functions was successfully fabricated by embedding layer-by-layer graphene nanoplatelets (GNPs) into β-PbO2 active layer (GNPs/PbO2) and employed as anode for high-efficient removal of sulfadiazine (SDZ). In electrochemical degradation experiments, SDZ was quickly enriched on the surface of GNPs/PbO2 film via adsorption and then oxidized by ⋅OH in-site. In terms of the electrocatalytic performance and adsorption of electrode, the optimal electrodeposition time for each β-PbO2 outer layer was 4 min (GNPs/PbO2-4). Compared with conventional PbO2 electrode, the layer-by-layer GNPs resulted in the smaller crystal size and denser surface of PbO2 electrode, thus facilitating the generation of active oxygen species. At the same time, the specific surface area, oxygen evolution potential (OEP) of the anode were enhanced and the charge-transfer resistance was reduced. For GNPs/PbO2-4 anode, the optimal conditions of electrochemical oxidation of SDZ were identified as initial pH 9, 50 mg/L of SDZ and 20 mA/cm2 of current density using response surface methodology (RSM), 98.15% of SDZ could be removed in this case. The contribution of radical oxidation and non-radical oxidation to SDZ removal was about 79% and 21%, respectively. Moreover, the reaction pathways of SDZ on the GNPs/PbO2-4 electrode involving hydroxylation, radical reaction and ring cleavage were speculated. Finally, the continuous SDZ degradation and accelerated service lifetime test suggested that the GNPs/PbO2-4 electrode was shown to be stable and repeatable, and the Pb2+ concentration was measured to ensure the safety of the treated solution. Consequently, the above findings provide an innovative way to design and prepare an effective and stable PbO2 electrode for electrochemical degradation of antibiotic wastewater.