Effect of indium doping on the PbO2 electrode for the enhanced electrochemical oxidation of aspirin: An electrode comparative study

Advancing nanoarchitectures of 2D WO3/MXene photoanode for enhanced photoelectrocatalytic oxidation of phenol and arsenic in synthetic wastewater

The photoelectrocatalytic advanced oxidation process (PEAOP) necessitates high-performing and stable photoanodes for the effective oxidation of complex pollutants in industrial wastewater. This study presents the construction of 2D WO3/MXene heteronanostructures for the development of efficient and stable photoanode. The WO3/MXene heterostructure features well-ordered WO3 photoactive sites anchored on micron-sized MXene sheets, providing an increased visible light active catalytic surface area and enhanced electrocatalytic activities for pollutant oxidation. Phenol, a highly toxic compound, was completely oxidized at an applied potential of 0.8 V vs. RHE under visible light irradiation. Systematic optimization of operational conditions for the photoelectrocatalytic oxidation of phenol was conducted. The phenol oxidation mechanism was elucidated via high-performance liquid chromatography (HPLC) analysis and the identification of intermediate compounds. Additionally, a mixed model of phenol and arsenic (III) in polluted water demonstrated the capability of WO3/MXene photoanode for the simultaneous oxidation of both organic and inorganic pollutants, achieving complete conversion of phenol and As(III) to non-toxic As(V). The WO3/MXene photoanode facilitated water oxidation, generating a substantial amount of O2•- and •OH oxidative species, which are crucial for the concurrent oxidation of phenol and arsenic. Recyclability tests demonstrated a 99% retention of performance, confirming the WO3/MXene photoanode's suitability for long-term operation in PEAOPs. The findings suggest that integrating WO3/MXene photoanodes into water purification systems can enhance economic feasibility, reduce energy consumption, and improve efficiency. This PEAOP offers a viable solution to the critical issue of heavy metal and organic chemical pollution in various water bodies, given its scalability and ability to preserve ecosystems while conserving clean water resources.

Photo-electrochemical activation of persulfate for the simultaneous degradation of microplastics and personal care products

The recent widespread use of microplastics (MPs), especially in pharmaceuticals and personal care products (PPCPs), has caused significant water pollution. This study presents a UV/electrically co-facilitated activated persulfate (PS) system to co-degrade a typical microplastic polyvinyl chloride (PVC) and an organic sunscreen p-aminobenzoic acid (PABA). We investigated the effect of various reaction conditions on the degradation. PVC and PABA degradation was 37% and 99.22%, respectively. Furthermore, we observed alterations in the surface topography and chemical characteristics of PVC throughout degradation. The possible degradation pathways of PVC and PABA were proposed by analyzing the intermediate products and the free radicals generated. This study reveals the co-promoting effect of multiple mechanisms in the activation by ultraviolet light and electricity.

Efficient electrochemical degradation of ceftazidime by Ti3+ self-doping TiO2 nanotube-based Sb-SnO2 nanoflowers as an intermediate layer on a modified PbO2 electrode

Ceftazidime (CAZ) is an emerging organic pollutant with a long-lasting presence in the environment. Although some PbO2 materials exhibit degradation capabilities, inefficient electron transport in the substrate layer and the problem of electrode stability still limit their use. Here, an interfacial design in which TiO2 nanotube arrays generate Ti3+ self-doping oxide substrate layers and highly active 3D Sb-SnO2 nanoflowers-like interlayers was used to prepare PbO2 anodes for efficient degradation of CAZ. Interestingly, after implementing Ti3+ self-doping in the PbO2 anode base layer and introducing 3D nanoflowers-like structures, the capacity for •OH generation increased significantly. The modified electrode exhibited 5-fold greater •OH generation capacity compared to the unmodified electrode, and a 2.7-fold longer accelerated electrode lifetime. The results indicate that interfacial engineering of the base and intermediate layers of the electrodes can improve the electron transfer efficiency, promote the formation of •OH, and extend the anode lifetime of the activated CAZ system.

Removal of ofloxacin using a porous carbon microfiltration membrane based on in-situ generated •OH

This study investigated the removal performance of ofloxacin (OFL) by a novel electro-Fenton enhanced microfiltration membrane. The membranes used in this study consisted of metal-organic framework derived porous carbon, carbon nanotubes and Fe2+, which were able to produce hydroxyl radicals (•OH) in-situ via reducing O2 to hydrogen peroxide. Herein, membrane filtration with bias not only concentrated the pollutants to the level that could be efficiently treated by electro-Fenton but also confined/retained the toxic intermediates within the membrane to ensure a prolonged contact time with the oxidants. After validated by experiments, the applied bias of -1.0 V, pH of 3 and electrolyte concentration of 0.1 M were the relatively optimum conditions for OFL degradation. Under these conditions, the average OFL removal rate could be reach 75% with merely 5% membrane flux loss after 4 cycles operation by filtrating 1 mg/L OFL. Via decarboxylation reaction, piperazinyl ring opening, dealkylation and ipso substitution reaction, etc., OFL could be gradually and efficiently degraded to intermediate products and even to CO2 by •OH. Moreover, the oxidation reaction was preferred to following first-order reaction kinetics. This research verified a possibility for antibiotic removal by electro-enhanced microfiltration membrane.

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

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

Catalytic electrodes' characterization study serving polluted water treatment: environmental healthcare and ecological risk assessment

Pesticide residues in the environment have irreparable effects on human health and other organisms. Hence, it is necessary to treat and degrade them from polluted water. In the current work, the electrochemical removal of the fenitrothion (FT), trifluralin (TF), and chlorothalonil (CT) pesticides were performed by catalytic electrode. The characteristics of SnO2-Sb2O3, PbO2, and Bi-PbO2 electrodes were described by FE-SEM and XRD. Dynamic electrochemical techniques including cyclic voltammetry, electrochemical impedance spectroscopy, accelerated life, and linear polarization were employed to investigate the electrochemical performance of fabricated electrodes. Moreover, evaluate the risk of toxic metals release from the catalytic electrode during treatment process was investigated. The maximum degradation efficiency of 99.8, 100, and 100% for FT, TF, and CT was found under the optimal condition of FT, TF, and CT concentration 15.0 mg L-1, pH 7.0, current density 7.0 mA cm-2, and electrolysis time of 120 min. The Bi-PbO2, PbO2, and SnO2-Sb2O3 electrodes revealed the oxygen evolution potential of 2.089, 1.983, 1.914 V, and the service lifetime of 82, 144, and 323 h, respectively. The results showed that after 5.0 h of electrolysis, none of the heavy metals such as Bi, Pb, Sb, Sn, and Ti were detected in the treated solution.

Electrochemical oxidation of chloramphenicol by modified Sm-PEG-PbO2 anodes: Performance and mechanism

Chloramphenicol (CAP) is used extensively in industry and daily life, but its abuse has seriously affected the environment and public health. In this paper, a new composite PbO₂ electrode was obtained through the modification Sm and polyethylene glycol (PEG), and an electrocatalytic oxidation technology of CAP degradation was investigated. The results showed that the catalytic degradation ability and industrial service life of the PEG-Sm-PbO₂ composite electrode were significantly enhanced. Co-doping inhibited the growth of grains, resulting in the formation of refined pyramidal grains on the surface of the electrode, which increased the number of active spots. The industrial service life of the modified electrode was improved by 87.0%. In addition, the degradation effect under different conditions and mechanism of CAP were also explored. The optimal conditions for CAP degradation were explored, at which time the CAP degradation rate reached 99.1%. The degradation process was in accordance with the primary reaction kinetics, and the apparent rate constant of CAP at the PEG-Sm-PbO₂ electrode was raised by 57.1% in comparison with the unmodified electrode, indicating that the modification facilitated the degradation of CAP in the electrode. Finally, two possible CAP degradation pathways were deduced. The results will provide technical support and a theoretical basis for the degradation of persistent organic pollutants.

Effect of (Sm, In) Doping on the Electrical and Thermal Properties of Sb2Te3 Microstructures

Doped Sb₂Te₃ narrow-band-gap semiconductors have been attracting considerable attention for different electronic and thermoelectric applications. Trivalent samarium (Sm)- and indium (In)-doped Sb₂Te₃ microstructures have been synthesized by the economical solvothermal method. Powder X-ray diffraction (PXRD) was used to verify the synthesis of single-phase doped and undoped Sb₂Te₃ and doping of Sm and In within the crystal lattice of Sb₂Te₃. Further, the morphology, structure elucidation, and stability have been investigated systematically by scanning electron microscopy (SEM), Raman analysis, and thermogravimetric analysis (TGA). These analyses verified the successful synthesis of hexagonal undoped Sb₂Te₃ (AT) and (Sm, In)-doped Sb₂Te₃ (SAT, IAT) microstructures. Moreover, the comparison of dielectric parameters, including dielectric constant, dielectric loss, and tan loss of AT, SAT, and IAT, was done in detail. An increment in the electrical conductivities, both AC and DC, from 1.92 × 10^-4 to 4.9 × 10^-3 Ω^-1 m^-1 and a decrease in thermal conductivity (0.68-0.60 W m^-1 K^-1) were observed due to the doping by trivalent (Sm, In) dopants. According to our best knowledge, the synthesis and dielectric properties of (Sm, In)-doped and undoped Sb₂Te₃ in comparison with their electrical properties and thermal conductivity have not been reported earlier. This implies that appropriate doping with (Sm, In) in Sb₂Te₃ is promising to enhance the electronic and thermoelectric behavior.

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.

Electrocatalytic degradation of methyl orange and 4-nitrophenol on a Ti/TiO2-NTA/La-PbO2 electrode: electrode characterization and operating parameters

The anode material plays a crucial role in the process of electrochemical oxidation. Herein, a TiO₂ nanotube arrays (TiO₂-NTA) intermediate layer and La-PbO₂ catalytic layer were synthesized on a Ti surface by the electrochemical anodic oxidation and electrochemical deposition technology, respectively. The prepared Ti/TiO₂-NTA/La-PbO₂ electrode was used as an electrocatalytic oxidation anode for pollutant degradation. Scanning electron microscopy (SEM) analysis showed that the TiO₂-NTA layer possessed a highly ordered and well-aligned nanotube array morphology, and the La-PbO₂ layer with angular cone cluster was uniform and tightly bonded. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis indicated that the intermediate layer primarily consisted of the anatase crystal structure of TiO₂ and the catalyst layer was made of La-PbO₂. Electrochemical analysis revealed that Ti/TiO₂-NTA/La-PbO₂ electrode exhibited higher oxidation peak current, electrochemical active surface area, and oxygen evolution potential (OEP, 1.64 V). Using methyl orange and 4-nitrophenol as model pollutants, electrocatalytic properties of the prepared Ti/TiO₂-NTA/La-PbO₂ electrode were systematically investigated under different conditions, and the electrochemical degradation fitted well with the pseudo-first-order kinetics model. Efficient anodic oxidation of model pollutants was mainly attributed to the indirect oxidation mediated by hydroxyl radicals (•OH). The total organic carbon (TOC) removal efficiency of methyl orange and 4-nitrophenol was 70.2 and 72.8%, and low energy consumption (2.50 and 1.89 kWh g^-1) was achieved after 240 min of electrolysis under the conditions of initial concentration of model pollutant, electrode spacing, and electrolyte concentration were 50 mg L^-1, 2 cm, and 0.1 mol L^-1, respectively. This work provided a new strategy to develop the high-efficiency electrode for refractory pollutants degradation.

Wastewater treatment by anodic oxidation in electrochemical advanced oxidation process: Advance in mechanism, direct and indirect oxidation detection methods

Electrochemical Advanced Oxidation Process (EAOP) has been applied to the degradation of refractory pollutants in wastewater due to its strong oxidation capacity, high degradation efficiency, simple operation, and mild reaction. Among electrochemical processes, anodic oxidation (AO) is the most widely used and its mechanism is mainly divided into direct oxidation and indirect oxidation. Direct oxidation means that pollutants are oxidized at the anode by direct electron transfer. Indirect oxidation refers to the generation of active species during the electrolytic reaction, which acts on pollutants. The mechanism of AO process is controlled by many factors, including electrode type, electrocatalyst material, wastewater composition, pH, applied current and voltage levels. It is very important to explore the reaction mechanism of electrochemical treatment, which determines the efficiency of the reaction, the products of the reaction, and the extent of reaction. This paper firstly reviews the current research progress on the mechanism of AO process, and summarizes in detail the different mechanisms caused by influencing factors under common AO process. Then, strategies and methods to distinguish direct oxidation and indirect oxidation mechanisms are reviewed, such as intermediate product analysis, electrochemical test analysis, active species detection, theoretical calculation, and the limitations of these methods are analyzed. Finally some suggestions are put forward for the study of the mechanism of electrochemical advanced oxidation.

Energy-efficient electrochemical treatment of paracetamol using a PbO2 anode based on pulse electrodeposition strategy: Kinetics, energy consumption and mechanism

The purpose of this research is to study the pulse electrochemical oxidation of paracetamol (PCT) using a novel PbO₂ anode based on pulse electrodeposition strategy (PbO₂-PE). The pulse electrodeposition strategy used to prepare a PbO₂ anode resulted in rougher surface, higher directional specificity of β(101) and more redox couples of Pb⁴⁺/Pb²⁺. Additionally, the oxygen evolution potential (OEP) and charge transfer resistance were also improved. When compared to direct current electrochemical oxidation process, pulse electrolysis in had a slightly higher PCT removal efficiency and active species (·OH and active chlorine) production, while 72.04% of energy consumption was saved. The effects of operating parameters on PCT degradation efficiency and specific energy consumption were studied. The findings suggested that the pulse electrochemical oxidation of PCT followed a pseudo-first-order kinetic model, with PCT removal reaching 98.63% after 60 min of electrolysis under optimal conditions. Possible mechanisms describing reaction pathways for PCT were also proposed. Finally, combinating with the economic feasibility and safety evaluation, we could conclude that pulse electrolysis with a PbO₂-PE electrode was a promising option for improving the practicability of electrochemical treatment for refractory organic wastewater.

Elaboration of Highly Modified Stainless Steel/Lead Dioxide Anodes for Enhanced Electrochemical Degradation of Ampicillin in Water

Lead dioxide-based electrodes have shown a great performance in the electrochemical treatment of organic wastewater. In the present study, modified PbO2 anodes supported on stainless steel (SS) with a titanium oxide interlayer such as SS/TiO2/PbO2 and SS/TiO2/PbO2-10% Boron (B) were prepared by the sol–gel spin-coating technique. The morphological and structural properties of the prepared electrodes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that the SS/TiO2/PbO2-10% B anode led to a rougher active surface, larger specific surface area, and therefore stronger ability to generate powerful oxidizing agents. The electrochemical impedance spectroscopy (EIS) measurements showed that the modified PbO2 anodes displayed a lower charge transfer resistance Rct. The influence of the introduction of a TiO2 intermediate layer and the boron doping of a PbO2 active surface layer on the electrochemical degradation of ampicillin (AMP) antibiotic have been investigated by chemical oxygen demand measurements and HPLC analysis. Although HPLC analysis showed that the degradation process of AMP with SS/PbO2 was slightly faster than the modified PbO2 anodes, the results revealed that SS/TiO2/PbO2-10%B was the most efficient and economical anode toward the pollutant degradation due to its physico-chemical properties. At the end of the electrolysis, the chemical oxygen demand (COD), the average current efficiency (ACE) and the energy consumption (EC) reached, respectively, 69.23%, 60.30% and 0.056 kWh (g COD)−1, making SS/TiO2/PbO2-10%B a promising anode for the degradation of ampicillin antibiotic in aqueous solutions.

Electrochemical degradation of antibiotic enoxacin using a novel PbO2 electrode with a graphene nanoplatelets inter-layer: Characteristics, efficiency and mechanism

A novel PbO₂ electrode was fabricated by adding graphene nanoplatelets (GNP) inter-layer into β-PbO₂ active layer (called GNP-PbO₂) and utilized to degradation of antibiotic enoxacin (ENO). The GNP-PbO₂ electrode had a much rougher surface than the typical PbO₂ electrode, with smaller crystal size and lower charge-transfer resistance at the electrode/electrolyte interface. Notably, the GNP inter-layer increased the oxygen evolution potential of PbO₂ electrode (2.05 V vs. SCE), which was very beneficial to inhibit oxygen evolution and promote ·OH production. The relatively best operating parameters for ENO removal and energy efficiency were current density of 20 mA cm^-2, initial pH of 7, initial ENO concentration of 100 mg L^-1 and electrode distance of 4 cm. Furthermore, indirect radical oxidation was found to be the main way during electrolysis process. Based on the observed analysis of intermediate products, the main reaction pathways of ENO included hydroxylation, defluorination and piperazine ring-opening. Finally, combinating with the electro-oxidation capability, stability and safety evaluation, we can conclude that GNP-PbO₂ is a promising anode for treatment of various organic pollutants in wastewater.

Fabrication of PbO2 Electrodes with Different Doses of Er Doping for Sulfonamides Degradation

In the present study, PbO₂ electrodes, doped with different doses of Er (0%, 0.5%, 1%, 2%, and 4%), were fabricated and characterized. Surface morphology characterization by SEM-EDS and XRD showed that Er was successfully doped into the PbO₂ catalyst layer and the particle size of Er-PbO₂ was reduced significantly. Electrochemical oxidation of sulfamerazine (SMR) in the Er-PbO₂ anode system obeyed te pseudo first-order kinetic model with the order of 2% Er-PbO₂ > 4% Er-PbO₂ > 1% Er-PbO₂ > 0.5% Er-PbO₂ > 0% PbO₂. For 2% Er-PbO₂, k_SMR was 1.39 h^-1, which was only 0.93 h^-1 for 0% PbO₂. Effects of different operational parameters on SMR degradation in 2% Er-PbO₂ anode system were investigated, including the initial pH of the electrolyte and current density. Under the situation of an initial pH of 3, a current density of 30 mA·cm^-2, a concentration of SMR 30 mg L^-1, and 0.2 M Na₂SO₄ used as supporting electrolyte, SMR was totally removed in 3 h, and COD mineralization efficiency was achieved 71.3% after 6 h electrolysis. Furthermore, the degradation pathway of SMR was proposed as combining the active sites identification by density functional calculation (DFT) and intermediates detection by LC-MS. Results showed that Er-PbO₂ has great potential for antibiotic wastewater treatment in practical applications.

When MXene (Ti3C2Tx) meet Ti/PbO2: An improved electrocatalytic activity and stability

Stable electrode materials with high catalytic activity are urgently required for electrochemical degradation of refractory organic pollutants in wastewater treatment. Herein, high conductive MXene (Ti₃C₂T_x) was firstly fabricated by electrophoretic deposition (EPD) as an interlayer for preparing a novel PbO₂ electrode. The well-conducted Ti₃C₂T_x interlayer significantly improved the electrochemical performance of the EPD-2.0/PbO₂ (EPD time was 2.0 min) electrode with the charge transfer resistance decreased by 9.51 times, the inner active sites increased by 5.21 times and the ∙OH radicals generation ability enhanced by 4.07 times than the control EPD-0/PbO₂ anode. Consequently, the EPD-2.0/PbO₂ electrode achieved nearly 100% basic fuchsin (BF) and 86.78% COD removal efficiency after 3.0 h electrolysis. Therefore, this new PbO₂ electrode presented a promising potential for electrochemical degradation of BF and the new Ti₃C₂T_x middle layer could also be used to fabricate other efficient and stable anodes, such as SnO₂, MnO₂, TiO₂, etc.

Electrochemical removal of fluoxetine via three mixed metal oxide anodes and carbonaceous cathodes from contaminated water

In this work, the fluoxetine (FLX) removal has been studied via the anodic oxidation (AO) process. Anode electrodes were Ti/RuO₂, Ti/RuO₂-IrO₂, and Ti/RuO₂-IrO₂-SnO₂, and cathode electrodes were graphite and carbon nanotubes (CNTs). The performances of electrodes were compared in terms of FLX removal efficiency. As a result, Ti/RuO₂-IrO₂-SnO₂ and CNTs were the optimal anode and cathode, respectively. The properties of the optimal electrodes were investigated using scanning electron microscopy, atomic force microscopy and X-ray diffraction spectroscopy. Cyclic voltammetry analysis was performed to study the electrochemical behavior of electrodes. The effect of current intensity (mA), initial pH, initial FLX concentration (mg/L) and process time (min) on the FLX removal efficiency was investigated and the response surface methodology was applied for the optimization of the AO process. The results showed that at current intensity, pH, initial FLX concentration and process time of 500 mA, 6, 25 mg/L and 160 min, maximum FLX removal efficiency was observed, which was 96.25%. Gas Chromatography-Mass Spectrometry (GC-MS), and total organic carbon (TOC) analysis was determined to evaluate the intermediates, and mineralization efficiency. The TOC removal efficiency was reached 81.51% after 6 h under optimal experimental conditions, indicating the successful removal of the FLX.

Research trends in the development of anodes for electrochemical oxidation of wastewater

The review focuses on the recent development in anode materials and their synthesis approach, focusing on their compatibility for treating actual industrial wastewater, improving selectivity, electrocatalytic activity, stability at higher concentration, and thereby reducing the mineralization cost for organic pollutant degradation. The advancement in sol–gel technique, including the Pechini method, is discussed in the first section. A separate discussion related to the selection of the electrodeposition method and its deciding parameters is also included. Furthermore, the effect of using advanced heating approaches, including microwave and laser deposition synthesis, is also discussed. Next, a separate discussion is provided on using different types of anode materials and their effect on active •OH radical generation, activity, and electrode stability in direct and indirect oxidation and future aspects. The effect of using different synthesis approaches, additives, and doping is discussed separately for each anode. Graphene, carbon nanotubes (CNTs), and metal doping enhance the number of active sites, electrochemical activity, and mineralization current efficiency (MCE) of the anode. While, microwave or laser heating approaches were proved to be an effective, cheaper, and fast alternative to conventional heating. The electrodeposition and nonaqueous solvent synthesis were convenient and environment-friendly techniques for conductive metallic and polymeric film deposition.

Electrochemical degradation performance and mechanism of dibutyl phthalate with hydrophobic PbO2 electrode

A polytetrafluoroethylene (PTFE) doped PbO₂ anode with a highly hydrophobicity was fabricated by electrodeposition method. In this process, vertically aligned TiO₂ nanotubes (TiO₂NTs) are formed by the anodic oxidation of Ti plates as an intermediate layer for PbO₂ electrodeposition. The characterization of the electrodes indicated that PTFE was successfully introduced to the electrode surface, the TiO₂NTs were completely covered with β-PbO₂ particles and gave it a large surface area, which also limited the growth of its crystal particles. Compared with the conventional Ti/PbO₂ and Ti/TiO₂NTs/PbO₂ electrode, the Ti/TiO₂NTs/PbO₂-PTFE electrode has enhanced surface hydrophobicity, higher oxygen evolution potential, lower electrochemical impedance, with more active sites, and generate more hydroxyl radicals (·OH), which were enhanced by the addition of PTFE nanoparticles. The electrocatalytic performance of the three electrodes were investigated using dibutyl phthalate (DBP) as the model pollutant. The efficiency of the DBP removal of the three electrodes was in the order: Ti/TiO₂NTs/PbO₂-PTFE > Ti/TiO₂NTs/PbO₂ > Ti/PbO₂. The degradation process followed the pseudo-first-order kinetic model well, with rate constants of 0.1326, 0.1266, and 0.1041 h^-1 for the three electrodes, respectively. The lowest energy consumption (6.1 kWh g^-1) was obtained after 8 h of DBP treatment using Ti/TiO₂NTs/PbO₂-PTFE compared to Ti/TiO₂NTs/PbO₂ (6.7 kWh g^-1) and Ti/PbO₂ (7.4 kWh g^-1) electrodes. Moreover, the effects of current density, initial pH and electrolyte concentration were investigated. Finally, the products of the DBP degradation process were verified based on gas chromatography-mass spectrometry analysis, and possible degradation pathways were described.

Application of lead oxide electrodes in wastewater treatment: A review

Electrochemical oxidation (EO) based on hydroxyl radicals (·OH) generated on lead dioxide has become a typical advanced oxidation process (AOP). Titanium-based lead dioxide electrodes (PbO₂/Ti) play an increasingly important role in EO. To further improve the efficiency, the structure and properties of the lead dioxide active surface layer can be modified by doping transition metals, rare earth metals, nonmetals, etc. Here, we compare the common preparation methods of lead dioxide. The EO performance of lead dioxide in wastewater containing dyes, pesticides, drugs, landfill leachate, coal, petrochemicals, etc., is discussed along with their suitable operating conditions. Finally, the factors influencing the contaminant removal kinetics on lead dioxide are systematically analysed.

Evaluation of diclofenac degradation effect in "active" and "non-active" anodes: A new consideration about mineralization inclination

This work investigates the electrochemical oxidation (EO) of diclofenac (DCF) in water with Ti/Ti₄O₇, Ti/Ru-Ir, Ti/Sb-SnO₂ and Ti/PbO₂ electrodes. Scanning electron microscope and X-ray diffraction results suggest that Ti/Ti₄O₇ has porous stacked surface morphology and Ti/Sb-SnO₂ possesses the smallest grain size. Linear sweep voltammetry test results indicate that PbO₂ has the highest oxygen evolution potential, while Ti/Ti₄O₇ and Ti/Ru-Ir show better oxygen evolution activity. DCF degradation results reveal that PbO₂ possessed the highest DCF removal (R_DCF = 99.2%) and chemical oxygen demand (COD) removal (R_COD = 97.0%), the fastest COD degradation rate (k = 0.0275 min^-1, R² = 0.964), the lowest specific energy consumption (EC_DCF = 1.81 kWh^.g DCF^-1, EC_TOC = 6.90 kWh^.g TOC^-1). The toxicity variation of DCF during EO process on PbO₂ is rise first and then to fall. Considering the differences of the four electrodes in residual, conversion and mineralization aspects, mineralization selectivity (MS) was proposed to estimate the mineralization inclination of electrodes during EO process, and PbO₂ displays the strongest mineralization inclination (MS = 0.594). In addition, the possible degradation pathway of DCF on PbO₂ electrode indicates a composite behavior of conversion and mineralization. All of them above indicate the promising application potential of PbO₂ in lower concentration pharmaceuticals and personal care products wastewater treatment. Moreover, MS could be employed as a supplementary index to assess the different inclinations of this composite behavior on various electrodes used for electrochemical treatment of organics in later studies.

Current progress in electrochemical anodic-oxidation of pharmaceuticals: Mechanisms, influencing factors, and new technique

Various pharmaceuticals have been detected in natural water and wastewater bodies, causing threats to water ecosystem and human health. Although electrochemical anodic-oxidation (EAO) has been shown to be efficient for pharmaceuticals degradation from aqueous solution, it still has a distinct need to apply EAO technology for pharmaceuticals removal rationally. This review provides the most recent progress on the mechanisms, influencing factors, and new technique of EAO for pharmaceuticals degradation. The mechanism and superiority of EAO were analyzed. Major influencing factors (e.g., electrode materials, electrochemical reactor, applied current density, anode-cathode distance, electrolyte type and concentration, initial solution pH value, and initial pharmaceuticals concentration) were discussed on the removal of pharmaceuticals. The latest development of reactive electrochemical membranes (REM) was regarded as an emerging EAO technique, and it was also highlighted. This work revealed that the EAO of pharmaceuticals has extraordinary application prospects in the field of water and wastewater treatment.

Treatment of Ni-EDTA containing wastewater by electrochemical degradation using Ti3+ self-doped TiO2 nanotube arrays anode

Ethylene diamine tetraacetic acid (EDTA) could form stable complexes with nickel due to its strong chelation. Ni-EDTA has significant impacts on human health because of its acute toxicity and low biodegradability, thus some appropriate approaches are required for its removal. In this research, a Ti³⁺ self-doped TiO₂ nanotube arrays electrode (ECR-TiO₂ NTA) was prepared and employed in electrochemical degradation of Ni-EDTA. The oxygen evolution potential of ECR-TiO₂ NTA was 2.6 V vs. SCE. More than 96% Ni-EDTA and 88% TOC was removed after reaction for 120 min at current density 2 mA cm^-2 at pH 4.34. The degradation of Ni-EDTA was mainly through the cleavage of amine group within Ni-EDTA and furthermore decomposed it into small molecular acids and inorganic ions including NH₄⁺and NO₃^-. The electro-deposition of nickel ions at cathode was confirmed by XPS and was greatly affected by the pH of solution. The effects of current density, initial Ni-EDTA concentration, initial pH of solution and HCO₃^- concentration on Ni-EDTA degradation were investigated. The results exhibited that the ECR-TiO₂ NTA had excellent efficiencies in electrochemical degradation of Ni-EDTA. The LSV analysis suggested that Ni-EDTA oxidation on ECR-TiO₂ NTA anode and the production of hydroxyl radical (·OH) on the anode played an important role in the removal of Ni-EDTA.

Micro-area investigation on electrochemical performance improvement with Co and Mn doping in PbO2 electrode materials

PbO₂-Co₃O₄-MnO₂ electrodes, used in the electrowinning industry and in the degradation of organic pollutants, have demonstrated an elevated performance through macroscopic electrochemical measurements. However, few reports have investigated localized electrochemical performance, which plays an indispensable role in determining the essential reasons for the improvement of the modified material. In this study, the causes of the increase in electrochemical reactivity are unveiled from a micro perspective through scanning electrochemical microscopy (SECM), X-ray diffraction (XRD), Raman microscopy (Raman), and X-ray photoelectronic energy spectroscopy (XPS). The results show that the increase of electrochemical reactivity of the modified electrodes results from two factors: transformation of the microstructure and change in the intrinsic physicochemical properties. Constant-height scanning maps indicate that the electrochemical reactivity of the modified electrodes is higher than that of the PbO₂ electrode on the whole and high-reactivity areas are orderly distributed, coinciding with the observations from SEM and XRD. Thus, one of the reasons for the improvement of the modified electrode performance is the refinement of the microscopic morphology. The other reason is the surge of the oxygen vacancy concentration on the surface of the coating, which is supported by XRD, Raman and XPS. This finding is detected by the probe approach curve (PAC), which can quantitatively characterize the electrochemical reactivity of a substrate. Heterogeneous charge transfer rate constants of the modified electrode are 4-5 times higher than that of the traditional PbO₂ electrode. This research offers some insight into the electrochemical reactivity of modified PbO₂ electrodes from a micro perspective.

Recent Trends in Pharmaceuticals Removal from Water Using Electrochemical Oxidation Processes

Nowadays, the research on the environmental applications of electrochemistry to remove recalcitrant and priority pollutants and, in particular, drugs from the aqueous phase has increased dramatically. This literature review summarizes the applications of electrochemical oxidation in recent years to decompose pharmaceuticals that are often detected in environmental samples such as carbamazapine, sulfamethoxazole, tetracycline, diclofenac, ibuprofen, ceftazidime, ciprofloxacin, etc. Similar to most physicochemical processes, efficiency depends on many operating parameters, while the combination with either biological or other physicochemical methods seems particularly attractive. In addition, various strategies such as using three-dimensional electrodes or the electrosynthesis of hydrogen peroxide have been proposed to overcome the disadvantages of electrochemical oxidation. Finally, some guidelines are proposed for future research into the applications of environmental electrochemistry for the degradation of xenobiotic compounds and micropollutants from environmental matrices. The main goal of the present review paper is to facilitate future researchers to design their experiments concerning the electrochemical oxidation processes for the degradation of micropollutants/emerging contaminants, especially, some specific drugs considering, also, the existing limitations of each process.

Co/Sm-modified Ti/PbO2 anode for atrazine degradation: Effective electrocatalytic performance and degradation mechanism

In this work, Ti/PbO₂-Co-Sm electrode has been successfully prepared using electrodeposition and further applied for the electrocatalysis of atrazine (ATZ) herbicide wastewater. As expected, Ti/PbO₂-Co-Sm electrode displays highest oxygen evolution potential, lowest charge transfer resistance, longest service lifetime and most effective electrocatalytic activity compared with Ti/PbO₂, Ti/PbO₂-Sm and Ti/PbO₂-Co electrodes. Orthogonal and single factor experiments are designed to optimize the condition of ATZ degradation. The maximum degradation efficiency of 92.6% and COD removal efficiency of 84.5% are achieved in electrolysis time 3 h under the optimum condition (current density 20 mA cm^-2, Na₂SO₄ concentration 8.0 g L^-1, pH 5 and temperature 35 °C). In addition, Ti/PbO₂-Co-Sm electrode exhibits admirable recyclability in degradation progress. The degradation of ATZ is accomplished by indirect electrochemical oxidation and ∙OH is tested as the main active substance in ATZ oxidation. The possible degradation mechanism of ATZ has been proposed according to the degradation intermediates detected by LC-MS. This research suggests that Ti/PbO₂-Co-Sm is a promising electrode for ATZ degradation.

Fabrication and characterization of titanium-based lead dioxide electrode by electrochemical deposition with Ti4 O7 particles

A novelly modified Ti/PbO₂ electrode was synthesized with Ti₄ O₇ particles through electrochemical deposition method (marked as PbO₂ -Ti₄ O₇ ). The properties of the as-prepared electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), hydroxyl radical concentration, accelerated life test, etc. Azophloxine was chosen as the model pollutant for electro-catalytic oxidation to evaluate electrochemical activity of the electrode. The experimental results indicated that Ti₄ O₇ modification could prominently improve the properties of the electrodes, especially, improve the surface morphology, enhance the current response, and reduce the impedance. However, the predominant phases of PbO₂ electrodes were unchanged, which were completely pure β-PbO₂ . During the electrochemical oxidation process, the PbO₂ -Ti₄ O₇ (1.0) electrode showed the best performance on degradation of AR1 (i.e., the highest removal efficiency and the lowest energy consumption), which could be attributed to its high oxygen evolution potential (OEP) and strong capability of HO· generation. Moreover, the accelerated service lifetime of PbO₂ -Ti₄ O₇ (1.0) electrode was 175 hr, 1.65 times longer than that of PbO₂ electrode (105.5 hr). PRACTITIONER POINTS: PbO₂ /Ti₄ O₇ composite anode was fabricated through electrochemical co-deposition. Four concentration gradients of Ti₄ O₇ particle were tested. PbO₂ -Ti₄ O₇ (1.0) showed optimal electrocatalytic ability due to its high OEP and HO· productivity.

Fabrication and characterization of porous titanium-based PbO2 electrode through the pulse electrodeposition method: Deposition condition optimization by orthogonal experiment

Porous titanium-based PbO₂ electrodes were successfully fabricated by pulse electrodeposition method. The primary pulse electrodeposition parameters, including pulse frequency (f), duty ratio (γ), average current density (J_a) and electrodeposition time (t) were considered in this study. An orthogonal experiment was designed based on those four factors and in three levels. SEM images and XRD results suggest that the surface morphology and structure of PbO₂ electrodes could be easily changed by varying pulse electrodeposition parameters. Orthogonal analysis reveals that the increase of f and J_a could decrease the average grain size of PbO₂ electrodes, which is conducive to create more active sites and promote the generation of hydroxide radicals. The electrochemical degradation of Azophloxine was carried out to evaluate the electrochemical oxidation performance of pulse electrodeposited electrodes. The results indicate that the influences of four factors can be ranked as follow: J_a >γ≈ t > f. The higher f, larger J_a and longer t could facilitate the optimization of the integrated electrochemical degradation performance of prepared PbO₂ electrode. The accelerated life time is dominated by J_a and t, coincident with the average weight increase of β-PbO₂ layer. The optimal parameters of pulse electrodeposition turn out to be: f = 50 Hz, γ = 30%, J_a = 25 mA cm^-2, t = 60 min. Together, the consequences of the experiments give assistance to uncover and roughly conclude the mechanism of pulse electrodeposition.

Electrocatalytic oxidation of PCP-Na by a novel nano-PbO2 anode: degradation mechanism and toxicity assessment

This study aims at investigating the electrocatalytic oxidation of sodium pentachlorophenate (PCP-Na) using a novel nano-PbO₂ powder anode. The nano-PbO₂ powder (marked as HL-PbO₂) was prepared by a simple hydrolysis process, and hydrothermal treatment was followed to improve the activity of HL-PbO₂. The HL-PbO₂ treated for 24 h by hydrothermal process (HL/HT-PbO₂-24) was confirmed to possess higher crystallinity, higher oxygen evolution potential, and more active sites, resulting in stronger OH radical generation capacity and higher electrochemical activity. Compared with conventional electrodeposited PbO₂ (ED-PbO₂) anode, the HL/HT-PbO₂-24 anode showed higher PCP-Na degradation rate. Under the same operating conditions, the mineralization current efficiency at HL/HT-PbO₂-24 was 2.7 times than that at ED-PbO₂. Five intermediates were detected in PCP-Na degradation solution and possible degradation mechanism of PCP-Na was discussed. In addition, the acute toxicity of PCP-Na degradation solution to zebrafish embryos and the oxidative stress induced in zebrafish embryos/larvae were studied to evaluate the ecological security of electrocatalytic oxidation of PCP-Na.

Electrocatalytic degradation of perfluoroocatane sulfonate (PFOS) on a 3D graphene-lead dioxide (3DG-PbO2) composite anode: Electrode characterization, degradation mechanism and toxicity

In this work, a three-dimension grapnene-PbO₂ (3DG-PbO₂) composite anode was prepared using coelectrodeposition technology for electrocatalytic oxidation of perfluorooctane sulfonate (PFOS). The effect of 3DG on the surface morphology, structure and electrocatalytic activity of PbO₂ electrode was investigated. The results indicated that the 3DG-PbO₂-0.08 anode (3DG concentration in electrodeposition solution was 0.08 g L^-1) possessed the best electrocatalytic activity due to its stronger ·OH radicals generation capacity, more active sites and smaller charge-transfer resistance. The degradation rate constant of PFOS on 3DG-PbO₂-0.08 anode was 2.33 times than that of pure PbO₂ anode. Additionally, the by-products formed in electrocatalytic degradation of PFOS were identified and a PFOS degradation pathway was proposed accordingly, which was dominated by the dissociation of -CF₂- groups via the attack of ·OH radicals. Finally, the toxicity evolution of degradation solution was examined to evaluate the ecological risk of electrocatalytic oxidation of PFOS by acute toxicity assays to zebrafish embryos.

Low-Temperature Removal of Refractory Organic Pollutants by Electrochemical Oxidation: Role of Interfacial Joule Heating Effect

Low temperature presents a challenge to wastewater treatment in the winters of cold regions. In the electrochemical oxidation (EO) process, the interfacial Joule heating (IJH) effect results in interfacial temperature higher than that of bulk electrolytes, which would alleviate the negative impact of low water temperature on organic oxidation occurring within the boundary layer of the anode. This study investigated the electrochemical oxidation of the representative recalcitrant organic pollutant, i.e., phenol, p-chlorophenol (p-CP), and 2,4-dichlorophenoxyacetic acid (2,4-D) on titanium suboxide (TiSO) anode at a low water temperature (8.5 ± 1 °C). At a low current density of 2 mA cm^-2, the IJH effect was insignificant and thus had a slight impact on interfacial temperature, leading to a low-efficiency and incomplete organic removal via direct electron transfer (DET) oxidation. Increasing the current density to 20 mA cm^-2 promoted the working up of the IJH effect and thus resulted in a dramatic increase in the interfacial temperature from 8.1 to 38.7 °C. This almost eliminated the negative impact of low temperature on the abatement of organic pollutants as though the low temperature of the bulk solution did not interact with interfacial reactions at all. This was indicated by the oxidation rates of 0.158 min^-1 (phenol), 0.084 min^-1 (p-CP), and 0.070 min^-1 (2.4-D) at a temperature of 8.5 ± 1 °C, the values being almost comparable to that obtained at room temperature (23.5 ± 1 °C). Both theoretical and experimental results demonstrated that the extent to which the low- and room-temperature cases deviated from each other was positively correlated with the activation energy of organic pollutants when reacting with ^•OH. The improvement of organic oxidation at low temperature should result from the compensation of the IJH effect, giving rise to higher ^•OH reactivity, more activated organic molecules, and enhanced mass transfer. This study may prompt new possibilities to develop an IJH effect-based electrochemical manner for decentralized water decontamination in cold regions.