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

Ultra-fast and ultra-efficient phenol removal from aqueous solution using a nano biocarbon adsorbent by RSM-CCD method: parameters, isotherm, kinetic, ANOVA

The purpose of this research is to investigate the ability of peanut shell activated carbon (PSAC) to adsorb phenol from aqueous solutions. Phenolic wastewater in various industries and their release to the environment are environmental problems. Among the various separation methods, adsorption is an accepted method because of its efficiency, simplicity, cost-effectiveness, and possibility to use different adsorbent materials to achieve maximum adsorption efficiency. Response surface methodology (RSM) was used to minimize the required experiments, modeling, finding the optimal point, and variance analysis. Among the studied variables, pH, adsorbent dosage, and initial concentration are important. The results show that it is possible to completely remove at 300 ppm of phenol concentration and 5 min. Characterization of PSAC was done using Fourier transform infrared spectroscopy spectrum (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmet-Teller (BET), and size analysis. By examining the isotherm models, it was found that the adsorption follows the Langmuir model. The maximum adsorption capacity was 250 mg g-1 based on the Langmuir model. The three combined features of complete removal, ultra-fast adsorption, and high adsorption capacity are the unique features of this nano biocarbon for phenol removal.

A novel continuous all-weather photo-electric synergistic treatment system for refractory organic compounds and its application in degrading enrofloxacin

A novel continuous all-weather photo-electric synergistic treatment system was proposed in this study for refractory organic compounds, which overcame the defects of conventional photo-catalytic treatments that rely on light irradiation and thus cannot achieve all-weather continuous treatment. The system used a new photocatalyst (MoS2/WO3/carbon felt) with the characteristics of easy recovery and fast charge transfer. The system was systematically tested in degrading enrofloxacin (EFA) under real environmental conditions in terms of treatment performance, pathways and mechanisms. The results showed that the EFA removal of photo-electric synergy substantially increased by 1.28 and 6.78 times, compared to photocatalysis and electrooxidation, respectively, with an average removal of 50.9% under the treatment load of 832.48 mg m-2 d-1. Possible treatment pathways of EFA and mechanism of the system were found to be mainly the loss of piperazine groups, the cleavage of the quinolone portion and the promotion of electron transfer by bias voltage.

CuFe2O4/diatomite actuates peroxymonosulfate activation process: Mechanism for active species transformation and pesticide degradation

Peroxymonosulfate (PMS) activation is a promising technology for water purification, but the removal performance of multiple pollutant matrices and the mechanism for reactive species transformation in the heterogeneous catalytic system remain ambiguous. Herein, a novel CuFe₂O₄/diatomite was fabricated for PMS activation to achieve efficient removal of typical pesticides. Uniform distribution of CuFe₂O₄ on diatomite efficiently alleviated the agglomeration of CuFe₂O₄ and increased specific surface area (57.20 m² g^-1, 3.8-fold larger than CuFe₂O₄). CuFe₂O₄/5% diatomite (5-CFD)/PMS system showed nearly 100% removal efficiency for mixed pesticide solution within 10 min (0.10 g L^-1 5-CFD and 0.40 g L^-1 PMS) and excellent anti-interference performance towards various coexisting substances (≥90% removal efficiency). The electrochemical measurements confirmed that the lower charge transfer resistance of 5-CFD significantly enhanced the electron-transfer capacity between 5-CFD and PMS, accelerating the reactions among Fe(III)/Fe(II), Cu(II)/Cu(I), and PMS, further generating •OH (261.3 μM), ¹O₂ (138.8 μM), SO₄^•- (11.8 μM), and O₂^•-. The O in reactive oxygen species didn't originate from dissolved oxygen (DO) but PMS, independent of the low solubility of DO and slow diffusion rate of O₂ in water. Furthermore, the production of ¹O₂ went through the process: PMS → O₂^•- → ¹O₂, and SO₄^•- could rapidly convert into •OH. The degradation pathways and the evolution of intermediates were proposed by HPLC-QTOF-MS/MS and DFT calculations. QSAR analysis illustrated that the toxicity became lower with the reaction process. This study provides novel insights into the mechanism for pesticide degradation and active species transformation and the anti-interference capability of systems.

Energy-efficient electrochemical degradation of ciprofloxacin by a Ti-foam/PbO2-GN composite electrode: Electrode characteristics, parameter optimization, and reaction mechanism

Reducing energy comsuption is crucial to commercialize electrochemical oxidation technologies. In this study, a novel PbO₂ composite electrode (Ti-foam/PbO₂-GN) was successfully fabricated based on a porous titanium (Ti) foam substrate and a β-PbO₂ active layer embedded with multiple graphene (GN) interlayers, and applied as an anode for energy-efficient pulse electrochemical oxidation of ciprofloxacin (CIP). In contrast to PbO₂ and Ti-foam/PbO₂ electrodes, the Ti-foam/PbO₂-GN electrode surface exhibited a more compact structure, smaller crystal grain size, and greater electrochemical active surface area. CIP removal of 89.7% was obtained with a low energy consumption (EE/O) of 6.17 kWh m^-3 under pulse electrolysis conditions with a current density of 25.00 mA cm^-2, pulse frequency of 5000 Hz, and pulse duty cycle of 50.0%. Up to 70.7% of the energy was saved in the pulse current mode compared to the direct current mode. Narrowing the electrode spacing to 2 cm facilitated the mass transfer process and enhanced oxidation efficiency. According to the intermediates identified, the pulse electrolysis of CIP primarily involved hydroxylation of the quinolone ring, breaking of the piperazine ring, defluorination, and decarboxylation processes, and a possible degradation mechanism of CIP was proposed. The continuous oxidation performance of CIP and the relatively low leaching of Pb²⁺ suggested that the Ti-foam/PbO₂-GN electrode exhibited excellent stability, repeatability, and safety. The degradation results of CIP in real water also exhibits the great potential of environmental application. As a result, pulse electrochemical oxidation using a Ti-foam/PbO₂-GN electrode has proven to be an energy-efficient and promising alternative for antibiotic wastewater treatment.

PbO2 materials for electrochemical environmental engineering: A review on synthesis and applications

Lead dioxide (PbO₂) materials have been widely employed in various fields such as batteries, electrochemical engineering, and more recently environmental engineering as anode materials, due to their unique physicochemical properties. Key performances of PbO₂ electrodes, such as energy efficiency and space-time yield, are influenced by morphological as well as compositional factors. Micro-nano structure regulation and decoration of metal/non-metal on PbO₂ is an outstanding technique to revamp its electrocatalytic activities and enhance environmental engineering efficiency. The aim of this review is to comprehensively summarize the recent research progress in the morphology control, the structure constructions, and the element doping of PbO₂ materials, further with many environmental application cases evaluated. Concerning electrochemical environmental engineering, the lead dioxide employed in chemical oxygen demand detection, ozone generators, and wastewater treatment has been comprehensively reviewed. In addition, the future research perspectives, challenges and the opportunities on PbO₂ materials for environmental applications are proposed.

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.

Electrochemical oxidation of palm oil mill effluent using platinum as anode: Optimization using response surface methodology

This work investigates the electrochemical oxidation of palm oil mill effluent (POME) treatment using platinum (Pt) as anode and graphite as a cathode. The response surface methodology was used to investigate the relationships between different factors conditions (voltage, electrolysis time and chemical support) and responses of the treatment (chemical oxygen demand reduction, colour removal, and total oil removal). A quadratic mathematical model was chosen for all responses using Box-Behnken Design (BBD) with R² 0.9853 for COD reduction, R² 0.9478 for colour removal and R² 0.9185 for total oil removal. According to Derringer's function desirability, under the optimum condition (Voltage 15, electrolysis time 2 h, and 19.95 mg/L NaCl) of POME treatment, 84% of COD reduction, 98% of colour removal and 99% total oil of removal could be achieved. These results indicate that platinum as an anode material is effective for the electrochemical oxidation treatment of POME.

Electrochemical degradation of methylene blue by Pb modified porous SnO2 anode

A significant number of pollutants in wastewater can be electrocatalytically oxidized by SnO₂-Sb, a relatively inactive electrode. However, the arduous process of environmental remediation due to poor electrochemical performance and short service life of the traditional Ti/SnO₂-Sb electrode. In this work the SnO₂ electrode with a micron-sized sphere structure was prepared by in-situ hydrothermal. The results of the study that the electrode (Pb-10%) synthesized from the precursor solution in which the Pb:Sn molar ratio is 10% exhibits excellent electrooxidation properties. Impressiveing, the Pb-10% electrode displayed the small charge transfer resistance (10.71 Ω) and the high oxygen evolution potential (2.26 V vs. SCE). Thus, the electrochemical degradation experiment demonstrates that 100 mg L^-1 MB was degraded by Pb-10% electrode under the condition of initial pH = 5, and the decolorization rate reached 94.6%. Moreover, the influence of different parameters such as Pb doping amount, initial pH value of solution, initial concentration of MB and inorganic ions on degradation efficiency were also explored, in turn the practical application of electrodes in the field of purifying water resources is optimized. It is worth noting that the service life of the optimized electrode (100 mA cm^-2, 0.5 M H₂SO₄, 90 h) is about 12 times longer than that of the bare electrode (Sn-Sb). Therefore, the high-performance Ti/SnO₂-Sb electrode prepared in this work possesses vast application prospects in the electrocatalytic oxidation.

Electrocatalytic performance of oxygen-activated carbon fibre felt anodes mediating degradation mechanism of acetaminophen in aqueous environments

Carbon felts are flexible and scalable, have high specific areas, and are highly conductive materials that fit the requirements for both anodes and cathodes in advanced electrocatalytic processes. Advanced oxidative modification processes (thermal, chemical, and plasma-chemical) were applied to carbon felt anodes to enhance their efficiency towards electro-oxidation. The modification of the porous anodes results in increased kinetics of acetaminophen degradation in aqueous environments. The utilised oxidation techniques deliver single-step, straightforward, eco-friendly, and stable physiochemical reformation of carbon felt surfaces. The modifications caused minor changes in both the specific surface area and total pore volume corresponding with the surface morphology. A pristine carbon felt electrode was capable of decomposing up to 70% of the acetaminophen in a 240 min electrolysis process, while the oxygen-plasma treated electrode achieved a removal yield of 99.9% estimated utilising HPLC-UV-Vis. Here, the electro-induced incineration kinetics of acetaminophen resulted in a rate constant of 1.54 h^-1, with the second-best result of 0.59 h^-1 after oxidation in 30% H₂O₂. The kinetics of acetaminophen removal was synergistically studied by spectroscopic and electrochemical techniques, revealing various reaction pathways attributed to the formation of intermediate compounds such as p-aminophenol and others. The enhancement of the electrochemical oxidation rates towards acetaminophen was attributed to the appearance of surface carbonyl species. Our results indicate that the best-performing plasma-chemical treated CFE follows a heterogeneous mechanism with only approx. 40% removal due to direct electro-oxidation. The degradation mechanism of acetaminophen at the treated carbon felt anodes was proposed based on the detected intermediate products. Estimation of the cost-effectiveness of removal processes, in terms of energy consumption, was also elaborated. Although the study was focussed on acetaminophen, the achieved results could be adapted to also process emerging, hazardous pollutant groups such as anti-inflammatory pharmaceuticals.

Tailoring wood waste biochar as a reusable microwave absorbent for pollutant removal: Structure-property-performance relationship and iron-carbon interaction

This study innovated the concept in designing an efficient and reusable microwave (MW) absorbent through concurrent exploitation of carbon graphitization, oxygen functionalization, and carbothermal iron reduction underpinned by an endothermic co-pyrolysis of wood waste and low-dosage iron. A powerful MW assimilation was accomplished from nanoscale amorphous magnetic particles as well as graphitized microporous carbon-iron skeleton in the biochar composites. Relative to a weak magnetic loss derived from the iron phase, the graphitic carbon architecture with abundant surface functionalities (i.e., CO and CO) exhibited a strong dielectric loss, which was thus prioritized as major active sites during MW reuse. The MW-absorbing biochar demonstrated a fast, robust, and durable removal of a refractory herbicide (2,4-dichlorophenoxy acetic acid) under mild MW irradiation with zero chemical input, low electricity consumption, and negligible Fe dissolution. Overall, this study will foster carbon-neutral industrial wastewater treatment and wood waste valorization.

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.

Using Fe-Cu/HGF composite cathodes for the degradation of Diuron by electro-activated peroxydisulfate

An iron-copper graphite felt (Fe-Cu/HGF) electrode was successfully prepared by heat treatment and impregnation of graphite felt as the support followed by calcination, and an electro-activated peroxydisulfate (E-PDS) system with Fe-Cu/HGF as the cathode was constructed to degrade Diuron. This system synergistically activated PDS through electrochemical processes and transition metal catalysis. High-valence metal ions could be converted into low-valence metal ions by reduction at the cathode, and low-valence metal ions continuously activated PDS to generate more sulfate radicals (SO₄^-) and hydroxyl radicals (OH) to accelerate Diuron degradation. The Fe-Cu/HGF composite cathode exhibited a performance superior to graphite felt (RGF) obtained using pretreatment only, including increased hydrophilicity, significantly increased number of defect sites and larger electroactive surface area. Under optimized experimental degradation conditions, Diuron could be completely removed in 35 min, at which time copper ion leaching was not detected in the solution, while the total iron ion concentration was 0.27 mg L^-1. Extending the reaction time to 6 h, the amount of total organic carbon was reduced to 32.2%. In addition, the free radicals that degraded Diuron were identified as mainly SO₄^- and OH with a slightly higher contribution of SO₄^-. The mechanism and pathways of Diuron degradation in the E-PDS system were determined. The E-PDS system was successfully applied to the degradation of other pollutants and the degradation of Diuron in different simulated water environments. In summary, the E-PDS system using Fe-Cu/HGF as the cathode is a promising treatment method for Diuron-containing wastewater.

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.

Electrochemical oxidation of sulfamethoxazole by nitrogen-doped carbon nanosheets composite PbO2 electrode: Kinetics and mechanism

In this study, nitrogen-doped carbon nanosheets (NCNSs) were prepared and successfully combined into the PbO₂ electrode by the composite electrodeposition technology, thereby NCNS-PbO₂ electrode was obtained. The electrochemical degradation of sulfamethoxazole (SMX) in aqueous solution by NCNS-PbO₂ electrode was studied. The main influence factors on the degradation of SMX, such as the initial concentration of SMX, current density, electrolyte concentration and initial pH value, were analyzed in detail. Under the optimal process conditions, after 120 min of treatment, the removal ratio of SMX and chemical oxygen demand (COD) reached 99.8 % and 60.7 %, respectively. The results showed that the electrochemical degradation of SMX fitted pseudo-first-order reaction kinetics. The electrochemical performance of NCNS-PbO₂ electrode was better than that of PbO₂ electrode by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, as well as the use of cyclic voltammetry and electrochemical impedance spectroscopy for electrochemical performance testing. This was because the doping of nitrogen atoms improved the properties of carbon nanosheets. After the composite, the active sites on the surface of PbO₂ were improved, the particle size of PbO₂ was reduced, and the electrical conductivity and electrocatalytic activity of the electrode were improved. In addition, the intermediate products were determined by GC-MS method, and the possible degradation pathways of SMX were proposed.

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.