Oxidation of ammonia in dilute aqueous solutions over graphite-supported α- and β-lead dioxide electrodes (PbO2@G)

Simultaneous ammonia and organics degradation from municipal landfill leachate by electrochemical oxidation

The two primary issues for wide implementation of the electrochemical oxidation of wastewater are the significant cost of electrode and high energy consumption. On the other side, conventional biological processes and membrane technology have several drawbacks for recalcitrant landfill leachate (LL) treatment. To address these issues, graphite/PbO2 anode was used to treat medium to mature age (biodegradability index, 5-day biochemical oxygen demand/chemical oxygen demand: 0.25) LL. To reduce the cost of the oxidation process and maximize the efficiency, operating conditions were optimized. The optimum parameter values were obtained as 24.7 mA cm-2, 180 ± 3 rpm, and 1.9 cm of current density, stirring rate, and electrode gap, respectively. Dissolved organic carbon (DOC), chemical oxygen demand (COD), and ammonia-N removal efficiencies of 55 ± 1.4%, 81 ± 1.9%, and 56 ± 3% were obtained after 8 h of degradation at optimum conditions. The decrease in aromatic substances and ultraviolet (UV) quenching materials were evaluated by UV-Visible spectroscopy and Specific UV absorbance. The conversion of aromatic compounds into simpler molecule compounds was also verified by Fourier-transform infrared spectroscopy analysis. The lab-scale anode synthesis cost was evaluated as 0.42 USD.

Robust PbO2 modified by co-deposition of ZrO2 nanoparticles for efficient degradation of ceftriaxone sodium

In recent years, PbO2 electrodes have received widespread attention due to their high oxygen evolution reaction (OER) activity. However, due to the brittle nature of the plating layer, it is easy to cause the active layer to fall off. Pb2+ will leach out with the electrochemical process causing secondary pollution. The starting point of this study is established to improve the stability and adhesion of the electrode coating. Electrochemical oxidation technology has the characteristics of high treatment efficiency, wide range of applications, and non-polluting environment. In this study, conventional PbO2 electrodes were modified by using co-deposition of ZrO2 nanoparticles. In addition, α-PbO2 was added to increase the stability of the electrodes. At a high current density of 1 A/cm2, the accelerated life of the pure PbO2 electrode is 648 h, the accelerated life of the PbO2-ZrO2 electrode is 1.37 times that of the pure PbO2, and the electrode with an added α-PbO2 layer is 1.69 times that of the pure PbO2 electrode. The amount of dissolved Pb2+ was only 29% of that of pure PbO2. The electrochemical performance of the electrode is evaluated by studying the degradation effect of ceftriaxone sodium (CXM). The addition of ZrO2 nanoparticles alters the particle size and deposition content of PbO2, leading to a unique crystal structure distinct from pure PbO2. Compared to conventional PbO2 electrodes, the PbO2-ZrO2 can remove chemical oxygen demand (COD) and pollutants more efficiently, removing for 59% increased by 38.47%. Therefore, PbO2-ZrO2 is of great value in the field of electrochemical degradation of industrial pollutants.

B4C/Ce co-modified Ti/PbO2 dimensionally stable anode: Facile one-step electrodeposition preparation and highly efficient electrocatalytic degradation of tetracycline

The application of PbO2 for electrochemical oxidation technology is limited by its low electrocatalytic activity and short service life. Herein, based on the facile one-step electrodeposition, we prepared a boron carbide (B4C) and cerium (Ce) co-modified Ti/PbO2 (Ti/PbO2-B4C-Ce) electrode to overcome these shortcomings. Compared with Ti/PbO2 electrode, the denser surface is displayed by Ti/PbO2-B4C-Ce electrode. Meanwhile, electrochemical characterization indicates that the introduction of B4C and Ce significantly enhance the electrochemical performance of PbO2 electrode. In degradation experiments, under optimized conditions (current density 20 mA cm-2, pH 9, 0.15 M Na2SO4 and 30 °C), the fully degradation of tetracycline (TC) can be completed within 30 min. Furthermore, the trapping experiment demonstrates that ∙OH and SO4·- radicals have a synergistic effect in the degradation process of TC. Based on results of liquid chromatography-mass spectrometer, the generated ·OH preferentially attacks amides, phenols and conjugated double bond groups in TC. Importantly, Ti/PbO2-B4C-Ce electrode maintains a constant degradation efficiency even after 10 recycling tests, and its service life is 2.4 times of traditional Ti/PbO2 electrode. Hence, Ti/PbO2-B4C-Ce electrode is a promising electrode for degradation of organic wastewater containing amides, phenols, and conjugated double bond groups.

Fabrication of PbO2/PVDF/CC Composite and Employment for the Removal of Methyl Orange

The in situ electrochemical oxidation process has received considerable attention for the removal of dye molecules and ammonium from textile dyeing and finishing wastewater. Nevertheless, the cost and durability of the catalytic anode have seriously limited industrial applications of this technique. In this work, the lab-based waste polyvinylidene fluoride membrane was employed to fabricate a novel lead dioxide/polyvinylidene fluoride/carbon cloth composite (PbO2/PVDF/CC) via integrated surface coating and electrodeposition processes. The influences of operating parameters (pH, Cl− concentration, current density, and initial concentration of pollutant) on the oxidation efficiency of PbO2/PVDF/CC were evaluated. Under optimal conditions, this composite achieves a 100% decolorization of methyl orange (MO), 99.48% removal of ammonium, and 94.46% conversion for ammonium-based nitrogen to N2, as well as an 82.55% removal of chemical oxygen demand (COD). At the coexistent condition of ammonium and MO, MO decolorization, ammonium, and COD removals still remain around 100%, 99.43%, and 77.33%, respectively. It can be assigned to the synergistic oxidation effect of hydroxyl radical and chloride species for MO and the chlorine oxidation action for ammonium. Based on the determination of various intermediates, MO is finally mineralized to CO2 and H2O, and ammonium is mainly converted to N2. The PbO2/PVDF/CC composite exhibits excellent stability and safety.

Clean and effective removal of Cl(-I) from strongly acidic wastewater by PbO2

Recycling strongly acidic wastewater as diluted H₂SO₄ after contaminants contained being removed was previously proposed, however, Cl(-I), a kind of contaminant contained in strongly acidic wastewater, is difficult to remove, which severely degrades the quality of recycled H₂SO₄. In this study, the removal of Cl(-I) using PbO₂ was investigated and the involved mechanisms were explored. The removal efficiency of Cl(-I) reached 93.38% at 50℃ when PbO₂/Cl(-I) mole ratio reached 2:1. The identification of reaction products shows that Cl(-I) was oxidized to Cl₂, and PbO₂ was reduced to PbSO₄. Cl₂ was absorbed by NaOH to form NaClO, which was used for the regeneration of PbO₂ from the generated PbSO₄. Cl(-I) was removed through two pathways, i.e., surface oxidation and •OH radical oxidation. •OH generated by the reaction of PbO₂ and OH^- plays an important role in Cl(-I) removal. The regenerated PbO₂ had excellent performance to remove Cl(-I) after six-time regeneration. This study provided an in-depth understanding on the effective removal of Cl(-I) by the oxidation method.

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.

Improved degradation of tetracycline antibiotic in electrochemical advanced oxidation processes (EAOPs): bioassay using bacteria and identification of intermediate compounds

Among the pharmaceutical compounds, tetracycline is the second most common group of antibiotics in terms of production and consumption worldwide, which their entrance in to hospital, domestic and industrial wastewaters pollute water sources and environment and finally leads to antibiotic resistance. The aim of this study was to determine the efficiency of electrochemical processes, Fenton, electro-Fenton (EF) and sono-electro-Fenton (SEF) separately and using Graphite (G)/β-PbO2 anode to remove tetracycline from aqueous solutions. First, experiments for the electrochemical process by the response-surface methodology (RSM) using variables including pH (3–9), initial tetracycline concentration (20–100 mg/L), electrolysis time (4–45 min) and current density (0.5–4.5 mA/cm2) was designed and the optimal conditions of these variables were 3.5, 25.6 mg/L, 42.6 min, and 1.98 mA/cm2, respectively. Under the optimal conditions of the electrochemical process, the effect of FeSO4 with values of 0.02-0.08 g/250 mL in the Fenton process and the effect of H2O2 of 0.05–0.5 mg/L in the EF process were investigated, and the optimal values of 0.06 g/250 mL and 0.2 mg/L was obtained for FeSO4 and H2O2, respectively. Under optimal conditions, the removal efficiencies of SEF, EF, sono-electrochemical (SEC), electrochemical, Fenton and ultrasonic processes were 98.8%, 93.6%, 87.9%, 81.3%, 71.6%, and 11.5%, respectively. G/β-PbO2 anode had only 37.5% higher removal efficiency than graphite anode. Under the optimal conditions of SEF process, changes in toxicity reduction by bioassay with E. coli and Staphylococcus aureus bacteria were 86% and 58.4%, respectively, and the kinetic study showed that the removal of tetracycline by SEF process with R2=0.9975 followed the pseudo-first-order kinetics. Finally, intermediate compounds obtained from tetracycline analysis were identified using LC-MS analysis.

Multilayered TNAs/SnO2/PPy/β-PbO2 anode achieving boosted electrocatalytic oxidation of As(III)

Dealing with arsenic pollution has been of great concern owing to inherent toxicity of As(III) to environments and human health. Herein, a novel multilayered SnO₂/PPy/β-PbO₂ structure on TiO₂ nanotube arrays (TNAs/SnO₂/PPy/β-PbO₂) was synthesized by a multi-step electrodeposition process as an efficient electrocatalyst for As(III) oxidation in aqueous solution. Such TNAs/SnO₂/PPy/β-PbO₂ electrode exhibited a higher charge transfer, tolerable stability, and high oxygen evolution potential (OEP). The intriguing structure with a SnO₂, PPy, and β-PbO₂ active layers provided a larger electrochemical active area for electrocatalytic As(III) oxidation. The as-synthesized TNAs/SnO₂/PPy/β-PbO₂ anode achieved drastically enhanced As(Ⅲ) conversion efficiency of 90.72% compared to that of TNAs/β-PbO₂ at circa 45.4%. The active species involved in the electrocatalytic oxidation process included superoxide radical (•O₂^-), sulfuric acid root radicals (•SO₄^-), and hydroxyl radicals (•OH). This work offers a new strategy to construct a high-efficiency electrode to meet the requirements of favorable electrocatalytic oxidation properties, good stability, and high electrocatalytic activity for As(III) transformation to As(V).

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.

Electrochemical degradation of vanillin using lead dioxide electrode: influencing factors and reaction pathways

In this study, a novel PbO₂-CeO₂ composite electrode was applied it to the electrocatalytic degradation of vanillin. The operating parameters such as applied current density, initial vanillin concentration, supporting electrolyte concentration and pH value were investigated and optimised. After 120 min, in a 0.10 mol L^-1 Na₂SO₄ solution with a current density of 50 mA cm^-2 and a pH value of 5.0 containing 30 mg L^-1 vanillin, the vanillin removal efficiency can reach 98.03%, the COD removal efficiency is up to 73.28%. The results indicate that electrochemical degradation has a high ability to remove vanillin in aqueous solution. The reaction follows a pseudo-first-order reaction kinetics model with rate constants of 0.03036 min^-1. In the process of electrochemical degradation, up to eight hydroxylated or polyhydroxylated oxidation by-products were identified through hydroxylation, dealkylation and substitution reactions. Furthermore, the degradation pathways were proposed, which eventually mineralised into inorganic water and carbon dioxide.

Further reuse of phosphorus-laden biochar for lead sorption from aqueous solution: Isotherm, kinetics, and mechanism

Biochar and engineered biochar have been used for phosphorous recovery from wastewater, but the resulted phosphorous-laden (P-laden) biochar needs further disposal. In this study, the feasibility of reusing P-laden biochar for Pb immobilization as well as the underlying mechanism was explored. Three types of engineered biochar, i.e., Ca modified biochar, Mg modified biochar, and Fe modified biochar, were selected to sorb P and then the exhausted biochar was further used for Pb sorption. Results showed that Mg and Ca modified biochar exhibited considerable Pb sorption capacity after P sorption with the maximum value of 3.36-4.03 mmol/g and 5.49-6.58 mmol/g, respectively, while P-laden Fe modified biochar failed to sorb Pb due to its acidic pH. The removal of Pb by P-laden Mg modified biochar involved more precipitation including PbHPO₄, Pb₅(PO₄)₃(OH), and Pb₃(CO₃)₂(OH)₂ because of its higher P sorption capacity and more -OH group on the surface. Cation exchange with CaCO₃ to form PbCO₃ was the main mechanism for Pb removal by P-laden Ca modified biochar despite the formation of Pb₅(PO₄)₃(OH) precipitate. Our results demonstrate that waste P-laden biochar can be further used for the effective removal of Pb, which provides a potential approach for waste adsorbent disposal.

Optimization of Operating Conditions for Electrochemical Decolorization of Methylene Blue with Ti/α-PbO2/β-PbO2 Composite Electrode

α-PbO2 was introduced into the intermediate layer of an electrode to prevent the separation of the electrodeposited layer and maintain oxidizing power. The resulting Ti/α-PbO2/β-PbO2 composite electrode was applied to the electrochemical decolorization of methylene blue (MB) and the operating conditions for MB decolorization with the Ti/α-PbO2/β-PbO2 electrode were optimized. The morphology, structure, composition, and electrochemical performance of Ti/α-PbO2 and Ti/α-PbO2/β-PbO2 anode were evaluated using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The optimum operating parameters for the electrochemical decolorization of MB at Ti/α-PbO2/β-PbO2 composites were as follows: Na2SO4 electrolyte 0.05 g L−1, initial concentration of MB 9 mg L−1, cell voltage 20 V, current density 0.05–0.10 A cm−2, and pH 6.0. MB dye could be completely decolorized with Ti/α-PbO2/β-PbO2 for the treatment time of less than one hour, and the dye decolorization efficiency with Ti/α-PbO2/β-PbO2 was about 5 times better, compared with those obtained with Ti/α-PbO2.

A highly activated iron phosphate over-layer for enhancing photoelectrochemical ammonia decomposition

Environmentally friendly ammonia (NH₃) decomposition has attracted a lot of interests in recent years to resolve the issue of water eutrophication from a wastewater and achieve a clean H₂ storage. Here, we report a novel strategy for solar-driven ammonia decomposition by introducing a highly-activated iron phosphate (FePi) over-layer on the surface of α-Fe₂O₃ nanorods photoanode (FePi/Fe₂O₃), and innovatively propose a photoelectrochemical (PEC) ammonia degradation system with enhanced performance. After a facile electrochemical (EC) activation, the FePi over-layer is converted into FeOOH. The EC-activated over-layer provides the efficient active sites for the ammonia adsorption process, which promotes the high catalytic kinetics for ammonia oxidation reaction (AOR). Due to the synergistic effect of the electrocatalytic and the photocatalytic process, the FePi/Fe₂O₃ exhibits the enhanced PEC AOR performance, which competes with water oxidation reaction (WOR). Comparing to the initial concentration of ammonia, the FePi/Fe₂O₃ achieves a 54.4% ammonia degradation rate within 3 h at 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, which demonstrates the reliable ammonia decomposition performance. This study confirms that it is feasible to achieve PEC ammonia decomposition in an aqueous solution without chloride mediators and provides a promising strategy for the harmless treatment of ammonia wastewater.

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.

Enhancement of the Activity of Electrochemical Oxidation of BPS by Nd-Doped PbO2 Electrodes: Performance and Mechanism

The electrochemical oxidation processes have attracted tremendous attention on the destruction of toxic and non-biodegradable organics. A series of neodymium (Nd)-doped PbO2 electrodes (Ti/PbO2-Nd) were synthesized through a pulse electrodeposition method, and its activity of bisphenol S (BPS) removal was further examined. The morphologies and structures were characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and an energy dispersive spectrometer (EDS). The performance, energy consumption and mechanism of electrochemical oxidation of BPS by Ti/PbO2-Nd electrode were also discussed. Compared to the traditional Ti/PbO2 electrode, the Ti/PbO2-Nd enables finer crystal particles, facilitating the oxygen evolution overpotential (OEP) from 1.41V to 1.55V and the generation of hydroxyl radicals (•OH). Moreover, lower duty cycles during the preparation of the electrode also contribute to the tapering size of crystals. The results show that the Ti/PbO2-Nd electrode exhibits relatively high activity in the anodic oxidation of BPS. Over 95% of BPS could be removed with the current density of 15 mA cm−2. Moreover, the energy consumption of BPS degradation on Ti/PbO2-Nd electrode is 60.26 kWh m−3, much lower than that on Ti/PbO2 electrode (95.45 kWh m−3). To conclude, the Ti/PbO2-Nd electrode has been proven to be a promising material for BPS removal.

Loofah-derived activated carbon supported on nickel foam (AC/Ni) electrodes for the electro-sorption of ammonium ion from aqueous solutions

Activated carbon (AC), prepared from dried loofah sponge, was supported on nickel foam to fabricate AC/Ni electrodes. The characteristics of ammonium electrosorption on AC/Ni electrodes was studied. Results showed that AC prepared in one-step activation (without pre-pyrolysis), i.e., OAC, had relatively low crystallinity, high mesoporosity, and high specific capacitance compared to those made in two-step carbonation followed by activation. Adsorption and desorption density of NH₄⁺ were measured at constant potential of -1.0 V (vs. Hg/HgO) and +0.1 V (vs. Hg/HgO), respectively. Non-faradaic charging contributed to the electrochemical storage and adsorption of ammonium ions on the AC surface with a maximal charge efficiency of 80%, at an applied potential of -1.0 V (vs. Hg/HgO). Multiple-layer adsorption isotherm better described the electrosorption of ammonium ion on OAC/Ni electrodes yielding a maximum adsorption capacity of 6 mg-N g^-1, which was comparable with other similar systems. Overall, results clearly demonstrated the effect of synthesis strategy on the capacitive charging behaviors of AC/Ni electrodes and its relationship to NH₄⁺ electrosorption.

Electrochemical degradation of oxalic acid over highly reactive nano-textured γ- and α-MnO2/carbon electrode fabricated by KMnO4 reduction on loofah sponge-derived active carbon

Manganese dioxide incorporated activated carbon (MnO₂/AC) was synthesized and used to electrochemically degrade oxalic acid in aqueous solutions. The highly porous carbon provided reactive sites for the electro-sorption of oxalic acid and MnO₂, with a specific polymorphism efficiently mediating the electron transfer between the electrode and organic pollutants. The activated carbon, made from the pyrolysis of dry loofah sponge using ZnCl₂ as activating agent, exhibited a high double-layer capacitance dependent upon the heating temperature (100 F/g at 800 °C). The γ-MnO₂ was in-situ deposited over the microporous structure of activated carbon through the redox reaction between KMnO₄ and carbon. Simple further calcination converted γ-MnO₂ to α-MnO₂ nano-whisker at temperatures above 500 °C. Cyclic voltammetry showed that oxalic acid significantly improved the anodic current of the Mn(III)/Mn(IV) redox couple on the MnO₂/AC electrode at an electrode potential around + 0.6 V (vs. Ag/AgCl). About 95% of oxalic acid degradation was achieved at pH < 4; meanwhile, 80% of the mineralization (total organic carbon removal) was attained independent of pH. Calcination converted γ-MnO₂ to α-MnO₂ which had higher electrochemical stability and inhibited the dissolution of Mn(II) from the electrode.

Differential control of anode/cathode potentials of paired electrolysis for simultaneous removal of chemical oxygen demand and total nitrogen

Paired electrolysis can take advantage of both anodic oxidation and cathodic reduction, and thus improve current efficiency for electrochemical wastewater treatment. In this work, differential control of anode/cathode potentials of paired electrolysis for simultaneous removal of chemical oxygen demand (COD) and total nitrogen (TN, including ammonia, nitrate, and nitrite) was studied. We first determined the optimal potentials for anodic oxidation of COD/NH₄⁺ or cathodic reduction of NO₃^-/NO₂^- (minimization of over-oxidation or over-reduction) by preliminary cyclic voltammetry and constant-potential electrolysis experiments, i.e., 1.6 V for anodic oxidation and -1.26 V for cathodic reduction in this case. The optimal working potential of the cathode was achieved at appropriate current density in the paired electrolysis system, the working potential of the anode was independently controlled by adjusting the ratio of its surface area to that of the cathode. In this way, both the cathode and anode could work under optimal potentials. At an optimized cathodic current density of 5.0 mA cm^-2 and cathode/anode surface area ratio of 2:1, the removal efficiencies of COD and TN from simulated wastewater reached 91.9% and 86.2%, respectively. Additionally, the developed paired electrolysis system was validated by treating an actual pharmaceutical wastewater, results for which showed that a total current efficiency of 84.8% was achieved, which was at least twice as high as that of traditional electrochemical processes.

Electro-sorption of ammonium ion onto nickel foam supported highly microporous activated carbon prepared from agricultural residues (dried Luffa cylindrica)

An electrode made of loofah sponge derived activated carbon supported on nickel foam (AC/Ni) was successfully fabricated and used to remove ammonium ion (NH₄⁺) from aqueous solution. A multilayer adsorption isotherm was used to describe ammonium electro-sorption on AC/Ni electrodes at different temperature, initial NH₄⁺ concentration, and electrical field. The cyclic voltammetry (CV) results suggested that the electrical capacitance of AC/Ni electrodes, with the AC being prepared without preheating (OAC) or with low temperature heating (i.e., 300 AC), were higher than those prepared at high preheating temperature (i.e., 400 AC and 500 AC). Increasing the electro-sorption temperature from 10 to 50 °C decreased the monolayer NH₄⁺ adsorption capacity from 5 to ca. 2-3 mg-N g^-1, respectively. Background electrolyte, namely, sodium sulfate, exhibited significant competitive effect on the adsorption of ammonium ion at sodium ion concentration > 10^-2 M. The activation energy and heat of adsorption were 9-23.2 kJ mol^-1 and -3.7--10.7 kJ mol^-1, respectively, indicating a physisorption and exothermic adsorption characteristics. Based on the kinetics and thermodynamics analysis, there was slight increase in the activation energy with elevating preheating temperature, which increased the quantity of micro-pores and surface heterogeneity of the AC materials. Overall, results clearly demonstrated that carbon pyrolysis played a role on the capacitive charging behaviors of electrodes and the efficiency of NH₄⁺ electro-sorption on the AC/Ni electrodes.

Parameter optimization and degradation mechanism for electrocatalytic degradation of 2,4-diclorophenoxyacetic acid (2,4-D) herbicide by lead dioxide electrodes

2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most commonly used herbicides in the world. In this work, the electro-catalytic degradation of 2,4-D herbicide from aqueous solutions was evaluated using three anode electrodes, i.e., lead dioxide coated on stainless steel 316 (SS316/β-PbO₂), lead dioxide coated on a lead bed (Pb/β-PbO₂), and lead dioxide coated on graphite (G/β-PbO₂). The structure and morphology of the prepared electrodes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The process of herbicide degradation was monitored during constant current electrolysis using cyclic voltammetry (CV). In this study, the experiments were designed based on the central composite design (CCD) and were analyzed and modeled by response surface methodology (RSM) to demonstrate the operational variables and the interactive effect of three independent variables on 3 responses. The effects of parameters including pH (3–11), current density (j = 1–5 mA cm⁻²) and electrolysis time (20–80 min) were studied. The results showed that, at j = 5 mA cm⁻², by increasing the reaction time from 20 to 80 min and decreasing the pH from 11 to 3, the 2,4-D herbicide degradation efficiency using SS316/β-PbO₂, Pb/β-PbO₂ and G/β-PbO₂ anode electrodes was observed to be 60.4, 75.9 and 89.8%, respectively. Moreover, the results showed that the highest COD and TOC removal efficiencies using the G/β-PbO₂ electrode were 83.7 and 78.5%, under the conditions pH = 3, electrolysis time = 80 min and j = 5 mA cm⁻², respectively. It was also found that G/β-PbO₂ has lower energy consumption (EC) (5.67 kW h m⁻³) compared to the two other studied electrodes (SS316/β-PbO₂ and Pb/β-PbO₂). The results showed a good correlation between the experimental values and the predicted values of the quadratic model (P < 0.05). Results revealed that the electrochemical process using the G/β-PbO₂ anode electrode has an acceptable efficiency in the degradation of 2,4-D herbicide and can be used as a proper pretreatment technique to treat wastewater containing resistant pollutants, e.g., phenoxy group herbicides (2,4-D).

Optimization of process parameters by the CCD method and electrocatalytic degradation and the electrochemical degradation mechanism of 2,4-D using modified electrode anodes were investigated.

Electrodegradation of 2,4-dichlorophenoxyacetic acid herbicide from aqueous solution using three-dimensional electrode reactor with G/β-PbO2 anode: Taguchi optimization and degradation mechanism determination

Optimization of process parameters using the Taguchi method, electrochemical degradation and electrochemical degradation mechanism of 2,4-D herbicide using 2D and 3D reactors with G/β-PbO₂ anode were investigated.