Electrochemical oxidation of pyrrole, pyrazole and tetrazole using a TiO2 nanotubes based SnO2-Sb/3D highly ordered macro-porous PbO2 electrode

One-step synthesis of Pt-Nd co-doped Ti/SnO2-Sb nanosphere electrodes used to degrade nitrobenzene

Ti/SnO2-Sb electrodes possess high catalytic activity and efficiently degrade nitrobenzene (NB); however, their low service life limits their wide application. In this study, we used one-step hydrothermal synthesis to successfully prepare Pt-Nd co-doped Ti/SnO2-Sb nanosphere electrodes. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were applied to characterize the surface morphology, microstructure, and chemical composition of the electrodes, respectively. The electrochemical activity and stability of the electrodes were characterized via linear sweep and cyclic voltammetry, electrochemical impedance spectroscopy, and an accelerated service life test; their performance for NB degradation was also studied. An appropriate amount of Pt-Nd co-doping refined the average grain size of SnO2 and formed a uniform and compact coating on the electrode surface. The oxygen evolution potential, total voltammetric charge, and electron transfer resistance of the Ti/SnO2-Sb-Nd-Pt electrodes were 1.88 V, 3.77 mC/cm2, and 11.50 Ω, respectively. Hydroxy radical was the main active radical species during the electrolytic degradation of nitrobenzene with Ti/SnO2-Sb-Nd-Pt. After Pt-Nd co-doping, the accelerated service life of the electrodes was extended from 8.0 min to 78.2 h (500 mA/cm2); although the NB degradation rate decreased from 94.1 to 80.6%, the total amount of theoretical catalytic degradation of NB in the effective working time increased from 17.4 to 8754.1 mg/cm2. These findings reveal good application potential for the electrodes and provide a reference for developing efficient and stable electrode materials.

Three-dimensional structured electrode for electrocatalytic organic wastewater purification: Design, mechanism and role

Considering the growing need in decentralized water treatment, the application of electrocatalytic processes (EP) to achieve organic wastewater purification will be dominant in the near future due to high efficiency, small reactor assembly as well as the flexibility of operation and management. The catalytic performance of electrode materials determines the development of this technology. Among them, the unique three-dimensional (3D) structure electrode shows better performance than two-dimensional (2D) electrode in increasing mass transfer, enhancing adsorption and exposing more active sites. Hence, this review starts with the introduction of definition, classification, advantages and disadvantages of 3D electrode materials. Then a critical discussion on the design and construction of 3D electrode materials for organic wastewater purification application is provided. Next, the removal mechanism of organic pollutants on the surface of 3D electrode, the role of 3D structure, the design of reactor with 3D electrode, the conversion and toxicity of degradation products, electrode energy efficiency, stability and cost, are comprehensively reviewed. At last, current challenges and future perspectives for the development of 3D electrode materials are addressed. We deem that this review will provide a valuable insight into the design and application of 3D electrodes in environmental water purification.

Membrane Fouling and Electrochemical Regeneration at a PbO2-Reactive Electrochemical Membrane: Study on Experiment and Mechanism

Membrane fouling and regeneration are the key issues for the application of membrane separation (MS) technology. Reactive electrochemical membranes (REMs) exhibited high, stable permeate flux and the function of chemical-free electrochemical regeneration. This study fabricated a micro-filtration REM characterized by a PbO₂ layer (PbO₂-REM) to investigate the electro-triggered anti-fouling and regeneration progress within REMs. The PbO₂-REM exhibited a three-dimensional porous structure with a few branch-like micro-pores. The PbO₂-REM could alleviate Humic acid (HA) and Bisphenol A (BPA) fouling through electrochemical degradation combined with bubble migration, which achieved the best anti-fouling performance at current density of 4 mA cm^-2 with 99.2% BPA removal. Regeneration in the electro-backwash (e-BW) mode was found as eight times that in the forward wash and full flux recovery was achieved at a current density of 3 mA cm^-2. EIS and simulation study also confirmed complete regeneration by e-BW, which was ascribed to the air-water wash formed by bubble migration and flow. Repeated regeneration tests showed that PbO₂-REM was stable for more than five cycles, indicating its high durability for practical uses. Mechanism analysis assisted by finite element simulation illustrated that the high catalytic PbO₂ layer plays an important role in antifouling and regeneration.

Electrochemical oxidation of chloramphenicol with lead dioxide-surfactant composites

The PbO₂ -2 wt.% sodium dodecyl sulfate composite formed from methanesulfonate electrolyte consists of 93.1% of α-phase PbO₂ in contrast to the similar one synthesized from nitrate electrolyte, which contains 73.3% of β phase. The electrocatalytic activity of the obtained composites in the oxygen evolution reaction and oxidation of chloramphenicol was investigated. It was found that the Tafel slope significantly exceeds the theoretical value, which indicates a decrease in the degree of filling of the electrode surface with oxygen-containing particles. In the presence of organic compound and chloride ions in the solution, irreversible adsorption of the intermediate is observed, which leads to additional blocking of active centers on the oxide surface, which are involved in the oxidation of organic substance. It was established that the maximum rate of chloramphenicol conversion is 83.5% and 85% at 50 and 80 mA cm^-2 , respectively, under kinetic control. The heterogeneous oxidation rate constant of chloramphenicol is 0.0035 min^-1 . Oxidation of chloramphenicol occurs through the formation of 4-(-2-amino-1,3-dihydroxy-propanyl)-nitrobenzene with cleavage of dichloroacetic acid. Next, the amino group is oxidized to the nitro group to form 4-(2-nitro-1,3-dihydroxy-propanyl)-nitrobenzene. Subsequent electrolysis produces nitrobenzoic acid, which is oxidized to benzoic acid, later hydroquinone, then benzoquinone and a set of aliphatic compounds. PRACTITIONER POINTS: The PbO₂ -2 wt.% SDS composite consists of 93.1% of α phase of PbO₂ in contrast to those synthesized from nitrate electrolyte. The Tafel slope indicates a decrease of surface filling with oxygen-containing particles. Irreversible adsorption of the intermediate is observed in the presence of chloride ions.

Electrocatalytic degradation of diuron herbicide using three-dimensional carbon felt/β-PbO2 anode as a highly porous electrode: Influencing factors and degradation mechanisms

Traditional planar PbO₂ anodes have been used extensively for the electrocatalytic degradation process. However, by using porous PbO₂ anodes that have a three-dimensional architecture, the efficiency of the process can be significantly upgraded. In the current study, carbon felt (CF) with a highly porous structure and a conventional planar graphite sheet (G) were used as electrode substrate for PbO₂ anodes. Both CF/β-PbO₂ and G/β-PbO₂ anodes were prepared by the anodic deposition method. The main properties of the electrodes were characterized by XRD, EDX-mapping, FESEM, and BET-BJH techniques. The electrocatalytic degradation of diuron using three-dimensional porous CF/β-PbO₂ anode was modeled and optimized by a rotatable central composite design. After optimizing the process, the ability of porous CF/β-PbO₂ and planar G/β-PbO₂ anodes to degrade and mineralize diuron was compared. The electrocatalytic degradation of the diuron was well described by a quadratic model (R² > 0.99). Under optimal conditions, the kinetics of diuron removal using CF/β-PbO₂ anode was 3 times faster than the G/β-PbO₂ anode. The energy consumed for the complete mineralization of diuron using CF/β-PbO₂ anode was 2077 kWh kg^-1 TOC. However, the G/β-PbO₂ anode removed only 65% of the TOC by consuming 54% more energy. The CF/β-PbO₂ had more stability (115 vs. 91 h), larger surface area (1.6287 vs. 0.8565 m² g^-1), and higher oxygen evolution potential (1.89 vs. 1.84 V) compared to the G/β-PbO₂. In the proposed pathways for diuron degradation, the aromatic ring and groups of carbonyl, dimethyl urea, and amide were the main targets for HO^• radical attacks.

Binary electrochemical mineralization of heterocyclic nitrogenous compounds: parametric optimization using Taguchi method and mineralization mechanism

The main objective of the present work was to understand the interactive behaviour of various operating parameters including concentration of pollutants during binary electrochemical mineralization of the two nitrogenous heterocyclic pollutants in the aqueous solution. Indole and pyrrole were selected as pollutants, whereas Pt/Ti was selected as anode and cathode. The effects of different operating parameters like current density, solution conductivity, initial concentration of the pollutants and time were studied. Taguchi method was used to optimize these operating parameters for obtaining the ultimate rate of degradation for the nitrogenous compounds. There were basically two responses, i.e. chemical oxygen demand (COD) degradation and specific energy consumption. These responses were maximized and minimized, respectively. At the optimum condition, removal efficiencies of pyrrole, indole and COD were found to be 46.1%, 62.4% and 61.4%, respectively. The optimum value of specific energy consumption was found to be 159.5 kWh per kg COD removed. Possible mineralization pathways are also proposed on the basis of the identified intermediates by gas chromatography coupled with mass spectroscopy. The operating cost was also calculated for the binary lab-scale treatment of the indole and pyrrole and compared with reported cost analysis for the electrochemical treatment.

Preparation and characterization of ZnO/PEG-Co(II)-PbO2 nanocomposite electrode and an investigation of the electrocatalytic degradation of phenol

A ZnO/PEG (polyethylene glycol) -Co(II)-PbO₂ nanocomposite electrode was constructed by using the anodic electrodeposition method and used for the electrocatalytic degradation phenol. The results showed that the electrode surface formed numerous PbO₂ nanosphere structures, and the average size of a single nanosphere is approximately 0.4 μm. XRD and EDS results showed the active layer consisted of β-PbO₂, and contained small amounts of cobalt and carbon. The electrochemical measurements showed that the electrode possessed a lower activation energy (E_a = 17.517 kJ∙mol^-1) and charge transfer resistance (R_ct = 7.564 Ω cm²) and a larger exchange current density (i°=1.476 × 10^-4 mA cm^-2). The phenol degradation process was controlled by the adsorption process and kinetic parameters were obtained with an initial concentration of 100 mg L^-1. The electrode possessed a shorter half-life, larger reaction rate constant, and degradation efficiency (RE = 91.1 %) after 180 min. Reaction order was also calculated, and the degradation followed the pseudo-first-order reaction kinetics. HPLC results showed that the degradation pathway is as follows: firstly, phenol is gradually decomposed into o-diphenol, p-diphenol and benzoquinone under hydroxyl radicals attack. Then, benzoquinone is broken into maleic acid and fumaric acid. Finally, these acidic compounds are broken into oxalic acid, which is eventually mineralized.

Removal of aniline from wastewater by electro-polymerization with superior energy efficiency

Removal of toxic aniline from wastewater is of great importance in industrial manufacture. Traditional electrochemical methods encounter obstacles such as high energy consumption in mineralization and severe electrode passivation in electro-polymerization. In this paper, we report a practical electro-polymerization method by using Ti/Sb-SnO₂/PbO₂ anode to treat high concentration aniline wastewater. The cyclic voltammetric experiment was conducted and the problem of electrode passivation was solved by increasing the electrode potential. In the experiments of treating aniline wastewater, the produced solid polymer can separate from water rather than sticking to electrode surface. Elemental analysis shows that oxygen is incorporated in the polymer. Experiments were conducted under different conditions, including current density, pH and initial concentration of aniline and Na₂SO₄. The electro-polymerization route accounts for nearly 50% contribution in the removal of chemical oxygen demand (COD). Our electro-polymerization method gives an apparent current efficiency (ACE) of 232.15% and an energy consumption (E_p) of 0.008658 kWh g^-1_COD^-1 when half of COD is removed at a current density of 15 mA cm^-2, pH of 7.0, initial aniline concentration of 1.2 g L^-1 and Na₂SO₄ concentration of 4 g L.^-1.

A Novel Porous Ni, Ce-Doped PbO2 Electrode for Efficient Treatment of Chloride Ion in Wastewater

The porous Ti/Sb-SnO2/Ni-Ce-PbO2 electrode was prepared by using a porous Ti plate as a substrate, an Sb-doped SnO2 as an intermediate, and a PbO2 doped with Ni and Ce as an active layer. The surface morphology and crystal structure of the electrode were characterized by scanning electron microscope(SEM), energy dispersive spectrometer(EDS), and X-Ray diffraction(XRD). The electrochemical performance of the electrodes was tested by linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and electrode life test. The results show that the novel porous Ni-Ce-PbO2 electrodes with larger active surface area have better electrochemical activity and longer electrode life than porous undoped PbO2 electrodes and flat Ni-Ce-PbO2 electrodes. In this work, the removal of Cl− in simulated wastewater on three electrodes was also studied. The results show that the removal effect of the porous Ni-Ce-PbO2 electrode is obviously better than the other two electrodes, and the removal rate is 87.4%, while the removal rates of the other two electrodes were 72.90% and 80.20%, respectively. In addition, the mechanism of electrochemical dechlorinating was also studied. With the progress of electrolysis, we find that the increase of OH- inhibits the degradation of Cl−, however, the porous Ni-Ce-PbO2 electrode can effectively improve the removal of Cl−.

Development of a 3D ordered macroporous RuO2 electrode for efficient pyrazole removal from water

Inability to remove biologically toxic and persistent contaminants is a critical issue in traditional water treatment processes. In this study, a novel 3D macroporous RuO₂ (3D-RuO₂) electrode with uniform and interconnected cavities has been fabricated via templated electrodeposition approach for treatment of persistent pyrazole. The physicochemical properties of the electrodes are characterized by means of SEM, BET, XRD, LSV and CV measurements. The results show that structural features of the 3D-RuO₂ play important roles in the electrocatalysis performance. Thanks to the abundant crystal defect sites, 3D-RuO₂ electrode possesses more mesopores within the skeleton, resulting in 17.9 and 2.2 times larger specific surface area compared to traditional flat thermal-deposited (TF-RuO₂) and electrodeposited RuO₂ (EF-RuO₂) respectively. At a current density of 5 mA cm^-2, the pyrazole removal rate on 3D-RuO₂ is 1.7 times and 1.3 times that of TF-RuO₂ and EF-RuO₂. The energy consumption for 50% of pyrazole removal on 3D-RuO₂ is 0.05 kWh g^-1pyrazole, much lower than that of TF-RuO₂ (0.11 kWh g^-1pyrazole) and EF-RuO₂ (0.075 kWh g^-1pyrazole). The improved removal performance of 3D-RuO₂ electrode is attributed to its strong electro-adsorption capacity (270.3 μg cm^-2), leading to enhanced mass transfer of pollutants to the electrode surface. The mass transfer coefficient (κ_m) is estimated as 2.4 × 10^-6 m s^-1 for 3D-RuO₂, which is 3.9 and 2.3 times as much as that of TF-RuO₂ and EF-RuO₂. Finally, contribution of different electron transfer approaches to pyrazole degradation under anodic polarization was investigated by ROS scavenging experiments.

A 2.5D Electrode System Constructed of Magnetic Sb–SnO2 Particles and a PbO2 Electrode and Its Electrocatalysis Application on Acid Red G Degradation

A novel electrode consisting of a Ti/PbO2 shell and Fe3O4/Sb–SnO2 particles was developed for electrochemical oxidation treatment of wastewater. Scanning electron microscope (SEM), X-ray diffraction (XRD), the current limiting method, toxicity experiments, and high-performance liquid chromatography were adopted to characterize its morphology, crystal structure, electrochemical properties, the toxicity of the wastewater, and hydroxyl radicals. Acid Red G (ARG), a typical azo dye, was additionally used to test the oxidation ability of the electrode. Results indicated that the 2.5D electrode could significantly improve the mass transfer coefficient and •OH content of the 2D electrode, thereby enhancing the decolorization, degradation, and mineralization effect of ARG, and reducing the toxicity of the wastewater. The experiments revealed that, at higher current density, lower dye concentration and higher temperature, the electrochemical oxidation of ARG favored. Under the condition of 50 mA/cm2, 25 °C, and 100 ppm, the ARG, Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) removal efficiency reached 100%, 65.89%, and 52.52%, respectively, and the energy consumption and the current efficiency were 1.06 kWh/g COD, 8.29%, and energy consumption for TOC and mineralization current efficiency were 3.81 kWh/g COD, 9.01%. Besides, the Fe3O4/Sb–SnO2 particles after electrolysis for 50 h still had remarkable stability. These results indicated that the ARG solution could be adequately removed on the 2.5D electrode, providing an effective method for wastewater treatment.