A novel layer-by-layer CNT/PbO2 anode for high-efficiency removal of PCP-Na through combining adsorption/electrosorption and electrocatalysis

Electrocatalytic oxidation of gaseous toluene in an all-solid cell using a foam Ti/Sb-SnO2/β-PbO2 anode

Mineralization of benzene, toluene, and xylene (BTX) with high efficiency at room temperature is still a challenge for the purification of indoor air. In this work, a foam Ti/Sb-SnO2/β-PbO2 anode catalyst was prepared for electrocatalytically oxidizing gaseous toluene in an all-solid cell at ambient temperature. The complex Ti/Sb-SnO2/β-PbO2 anode, which was prepared by sequentially deposing Sb-SnO2 and β-PbO2 on a foam Ti substrate, shows high electrocatalytic oxidation efficiency of toluene (80%) at 7 hr of reaction and high CO2 selectivity (94.9%) under an optimized condition, i.e., a cell voltage of 2.0 V, relative humidity of 60% and a flow rate of 100 mL/min. The better catalytic performance can be ascribed to the high production rate of ⋅OH radicals from discharging adsorbed water and the inhibition of oxygen evolution on the surface of foam Ti/Sb-SnO2/β-PbO2 anode when compared with the foam Ti/Sb-SnO2 anode. Our results demonstrate that prepared complex electrodes can be potentially used for electrocatalytic removal of gaseous toluene at room temperature with a good performance.

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.

High-efficiency removal of tetracycline from water by electrolysis-assisted NZVI: mechanism of electron transfer and redox of iron

A low-cost, stable and non-precious metal catalyst for efficient degradation of tetracycline (TC), one of the most widely used antibiotics, has been developed. We report the facile fabrication of an electrolysis-assisted nano zerovalent iron system (E-NZVI) that achieved TC removal efficiency of 97.3% with the initial concentration of 30 mg L^-1 at an applied voltage of 4 V, which was 6.3 times higher than the NZVI system without an applied voltage. The improvement caused by electrolysis was mainly attributed to the stimulation of corrosion of NZVI, which accelerated the release of Fe²⁺. And Fe³⁺ in the E-NZVI system could receive electrons to reduce to Fe²⁺, which facilitated the conversion of ineffective ions to effective ions with reducing ability. Moreover, electrolysis assisted to expand the pH range of the E-NZVI system for TC removal. The uniformly dispersed NZVI in the electrolyte facilitated the collection and secondary contamination could be prevented with the easy recycling and regeneration of the spent catalyst. In addition, scavenger experiments revealed that the reducing ability of NZVI was accelerated in the presence of electrolysis, rather than oxidation. TEM-EDS mapping, XRD and XPS analyses indicated that electrolytic effects could also delay the passivation of NZVI after a long run. This is mainly due to the increased electromigration, implying that the corrosion products of iron (iron hydroxides and oxides) are not formed mainly near or on the surface of NZVI. The electrolysis-assisted NZVI shows excellent removal efficiency of TC and is a potential water treatment method for the degradation of antibiotic contaminants.

Preparation and characterization of a novel blue-TiO2/PbO2-carbon nanotube electrode and its application for degradation of phenol

In this study, we fabricated a blue-TiO₂/PbO₂-carbon nanotube (CNT) electrode in which blue TiO₂ nanotube arrays (blue-TNA) served as the substrate for PbO₂-CNT eletrodeposition. Scanning electron microscope (SEM) showed compact surface structure of the electrode. The β-PbO₂ crystal structure was detected by X-ray diffraction (XRD). The distribution of Pb, O, C, and Na elements on the electrode surface have been confirmed by X-ray photoelectron spectroscopy (XPS). Blue-TiO₂/PbO₂-CNT electrode had higher response current (213.12 mA), larger active surface area and lower charge transfer resistance (2.22 Ω/cm²) than conventional TiO₂/PbO₂-CNT electrode. The influences of current density, initial phenol concentration, initial solution pH, and Na₂SO₄ concentration on the electrochemical oxidation of phenol have been analyzed. The results showed that the 100 mg/L phenol could be destroyed completely after 210 min, and chemical oxygen demand (COD) removal rate was 89.3% within 240 min. Additionally, the electrode showed long actual lifetime (5468.80 hr) and low energy consumption (0.08 kWh/gCOD). A phenol degradation mechanism was proposed by analyzing the intermediate products with high-performance liquid chromatography-mass spectrometry (HPLC-MS). Importantly, the blue-TiO₂/PbO₂-CNT electrode exhibited superior stability and high degradation efficiency after 15 times reuse, demonstrating its promising application potential on phenol-containing wastewater treatment.

Stimulation of ethylene glycol electrooxidation on electrodeposited Ni–PbO2–GN nanocomposite in alkaline medium

In this work, a novel system composed of non-precious nickel-based metal oxide/reduced graphene oxide nanocomposite (Ni–PbO2–GN) is used for electrooxidation of ethylene glycol (EG) in 1.0 M NaOH solution and compares its activity with that of Ni, Ni–GN, and Ni–PbO2. The facile electrodeposition technique is used to prepare the catalysts on glassy carbon (GC) substrates. The outcomes of electrochemical measurements show a high performance towards EG oxidation is obtained for Ni-nanocomposite electrodes compared to that of Ni mainly due to their higher surface areas. The excellent electrocatalytic properties of the Ni-nanocomposite could be ascribed to the synergistic contributions of PbO2 and graphene (GN) nano-sheets that help the reduction of Ni grains. A smaller charge transfer resistance value of 34.5 Ω cm2 for EG oxidation reaction at + 360 mV is recorded for GC/Ni–PbO2–GN compared to the other prepared electrodes. Moreover, it exhibits higher kinetic parameters of EG such as diffusion coefficient (D = 3.9 × 10–10 cm2 s−1) and charge transfer rate constant (ks = 32.5 mol−1 cm3 s−1). The overall performance and stability of the prepared catalysts towards EG electrooxidation have been estimated to be in the order of GC/Ni–PbO2–GN > GC/Ni–GN > GC/Ni–PbO2 > GC/Ni.

Graphical abstract

Model Study of CNT-Based PEMFCs’ Electrocatalytic Layers

One of the most important problems in the development of proton-exchange membrane fuel cells (PEMFCs) is the selection of an efficient support material to serve as the electrocatalyst, which can ensure PEMFCs’ durability at low active metal loading, with minimal changes in the electrochemical surface and conductivity during long-term operations. Carbon nanostructures are now widely used in PEMFCs as such support materials, including carbon nanotubes (CNTs). In order to estimate the effect of the geometric parameters of a CNT-based support on the resulting size distribution of platinum nanoparticles for given synthesis conditions, in this work, we propose a semi-empirical model that assumes a random uniform distribution of platinum particles over the CNT surface. Based on the obtained distribution, the electrochemically active surface area (EASA) of the electrocatalyst is calculated and further used to evaluate the performance of the catalytic layer (CL) in the PEMFC. The applicability of the proposed model for calculating the parameters of CNT-based CLs and the output electrochemical characteristics of PEMFCs is shown.

A critical review on the electrosorption of organic compounds in aqueous effluent - Influencing factors and engineering considerations

Despite being an old process from the end of the 19^th century, electrosorption has attracted renewed attention in recent years because of its unique properties and advantages compared to other separation technologies and due to the concomitant development of new porous electrode materials. Electrosorption offer the advantage to separate the pollutants from wastewater with the possibility of selectively adsorbing and desorbing the targeted compounds. A comprehensive review of electrosorption is provided with particular attention given to the electrosorption of organic compounds, unlike existing capacitive deionization review papers that only focus on inorganic salts. The background and principle of electrosorption are first presented, while the influence of the main parameters (e.g., electrode materials, electrode potential, physico-chemistry of the electrolyte solutions, type of compounds, co-sorption effect, reactor design, etc.) is then detailed and the modeling and engineering aspects are discussed. Finally, the main output and future prospects about recovery studies and combination between electro-sorption/desorption and degradation processes are given. This review particularly highlights that carbon-based materials have been mostly employed (85% of studies) as porous electrode in organics electrosorption, while existing studies lack of electrode stability and durability tests in real conditions. These electrodes have been implemented in a fixed-bed reactor design most of the time (43% of studies) due to enhanced mass transport. Moreover, the electrode potential is a major criterion: it should be applied in the non-faradaic domain otherwise unwanted reactions can easily occur, especially the corrosion of carbon from 0.21 V/standard hydrogen electrode or the water oxidation/reduction. Furthermore, there is lack of studies performed with actual effluents and without addition of supporting electrolyte, which is crucial for testing the real efficiency of the process. The associated predictive model will be required by considering the matrix effect along with transport phenomena and physico-chemical characteristics of targeted organic compounds.

New insight to superoxide radical-mediated degradation of pentachlorophenate: Kinetic determination and theoretical calculations

This study reported the reactivity and mechanisms of superoxide radical (O₂˙^-)-mediated transformation of pentachlorophenate. Our results indicated that O₂˙^- alone exhibits limited effects on its degradation, and bimolecular nucleophilic substitution is the dominant reaction pathway.

Electrochemical study of a novel high-efficiency PbO2 anode based on a cerium-graphene oxide co-doping strategy: Electrodeposition mechanism, parameter optimization, and degradation pathways

A novel and efficient Ti/SnO₂-Sb/PbO₂-GO-Ce electrode was successfully fabricated based on the co-deposition of Ce ions and graphene oxide (GO) into β-PbO₂ crystals and used as an anode for electrocatalytic oxidation of phenol. The electrodeposition mechanism, parameter optimization, mechanism analysis, and potential degradation pathways were discussed in depth. The co-doping of GO and Ce resulted in the high directional specificity of β(301), orderly and dense grain arrangement of PbO₂ crystals. At the same time, the oxygen evolution potential, •OH generation capacity and lifetime were also improved. The effects of experimental parameters on phenol removal efficiency were evaluated, including the applied current density, electrode gap, supporting electrolyte, initial NaCl concentration, initial pH, and initial phenol concentration. Under the optimal conditions, the removal efficiency of phenol can reach 375.6 g m^-2 h^-1 for 20 min electrolysis, which is about 1.2 times that of the pure PbO₂ electrode. The active oxygen species (•OH, ClO^- and HClO) were important attributes to the degradation of phenol. Additionally, a potential degradation pathway for phenol was proposed. After 10 successive recycles, there was no significant difference of the electro-generated •OH, cell voltage and phenol removal rate, which confirms the stability and admirable reusability of Ti/SnO₂-Sb/PbO₂-GO-Ce electrode.

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.

Development of a novel graphitic carbon nitride and multiwall carbon nanotube co-doped Ti/PbO2 anode for electrocatalytic degradation of acetaminophen

In this work, we have constructed a novel graphitic carbon nitride/multiwall carbon nanotube (GCN/CNT) doped Ti/PbO₂ as anode for highly effective degradation of acetaminophen (ACE) wastewater. The ACE removal efficiency of 83.2% and chemical oxygen demand removal efficiency of 76.3% are achieved under the optimal condition of temperature 25 °C, initial pH 7, current density 15 mA cm^-2 and Na₂SO₄ concentration 6.0 g L^-1. The excellent electrocatalytic activity of Ti/PbO₂-GCN-CNT anode for ACE oxidation is ascribed to the effective suppression of oxygen evolution and the enhanced electron transfer after introducing GCN and CNT. Furthermore, Ti/PbO₂-GCN-CNT electrode displays excellent stability and reusability. ACE degradation is accomplished by direct oxidation and indirect oxidation, and ∙OH radical plays primary role in the indirect oxidation of ACE wastewater. The intermediates of ACE degradation are detailly investigated using LC-MS analysis and a possible degradation mechanism is proposed.

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.

Performance of photocatalytic ozonation process for pentachlorophenol (PCP) removal in aqueous solution using graphene-TiO2 nanocomposite (UV/G-TiO2/O3)

The aim of this study was to evaluate the efficiency of photocatalytic ozonation process using graphene-dioxide titanium nanocomposite in removing Pentachlorophenol (PCP) from aqueous solutions. In this study, nanocomposites with graphene to TiO2(G/T) ratios of 1:10 and 1:20 were synthesized by hydrothermal method, and its characteristics were assessed using various analyses, SEM, XRD, FTIR, TEM, BET and TGA. In this process, the effects of parameters including O3 concentration (0.25-1.25 mg/L), nanocomposite concentration (50-500 mg/L), initial PCP concentration (10-100 mg/L), and time (10-60 min), were studied. The results showed that PCP removal efficiency was increased by decreasing solute concentration. Increasing nanocomposite dose to 100 mg/L was led to an increase in efficiency (99.1%), but then a decreasing trend was observed. Increasing the concentration of ozone, up to specific value, also enhanced the efficiency but then had a negative effect on process efficiency. Furthermore, the optimum ratio of the catalyst was determined to be 1:20. The highest efficiency of the process for initial pentachlorophenol concentration of 100 mg/L was obtained 98.82% in optimum conditions (catalyst dose of 100 mg/L and 60 min). It is concluded that the photocatalytic ozonation process using graphene-dioxide titanium nanocomposite had the highest efficiency in removal and mineralization of PCP.

Fabrication of Co/Pr co-doped Ti/PbO2 anode for efficiently electrocatalytic degradation of β-naphthoxyacetic acid

The existence of β-naphthoxyacetic acid (BNOA) pesticide in water system has aroused serious environmental problem because of its potential toxicity for humans and organisms. Therefore, exploiting an efficient method without secondary pollution is extremely urgent. Herein, a promising Ti/PbO₂-Co-Pr composite electrode has been successfully fabricated through simple one-step electrodeposition for efficiently electrocatalytic degradation of BNOA. Compared with Ti/PbO₂, Ti/PbO₂-Co and Ti/PbO₂-Pr electrodes, Ti/PbO₂-Co-Pr electrode with smaller pyramidal particles possesses higher oxygen evolution potential, excellent electrochemical stability and outstanding electrocatalytic activity. The optimal degradation condition is assessed by major parameters including temperature, initial pH, current density and Na₂SO₄ concentration. The degradation efficiency and chemical oxygen demand removal efficiency of BNOA reach up to 94.6% and 84.6%, respectively, under optimal condition (temperature 35 °C, initial pH 5, current density 12 mA cm^-2, Na₂SO₄ concentration 8.0 g L^-1 and electrolysis time 3 h). Furthermore, Ti/PbO₂-Co-Pr electrode presents economic energy consumption and superior repeatability. Finally, the possible degradation mechanism of BNOA is put forward according to the main intermediate products identified by liquid chromatography-mass spectrometer. The present research paves a new path to degrade BNOA pesticide wastewater with Ti/PbO₂-Co-Pr electrode.

Degradation of aqueous cefotaxime in electro-oxidation - electro-Fenton -persulfate system with Ti/CNT/SnO2-Sb-Er anode and Ni@NCNT cathode

Due to the potential threatening of antibiotics in aqueous environment, a novel electro-oxidation (EO) - electro-Fenton (EF) -persulfate (PS) system with the addition of peroxydisulfate and Fe²⁺ was installed for the degradation of cefotaxime. Ti/CNT/SnO₂-Sb-Er with an ultra-high oxygen evolution potential (2.15 V) and enhanced electrocatalytic surface area was adopted as anode. The OH production and electrode stability test demonstrated great improvement in the electrochemical performances. Ni@NCNT cathode was tested with higher H₂O₂ generation by the presence of nitrogen functionalities due to the acceleration of electron transfer of O₂ reduction. Experiment results indicated CNT and ErO₂ modification increased the molecular and TOC removal of cefotaxime. Coupling processes of EO-EF and EO-PS both resulted in shorter electrolysis time for complete cefotaxime removal, however, the mineralization ability of EO-PS process was lower than EO-EF, which might result from the immediate vanishing of PS. Thus, a further improved treatment EO-EF-PS system achieved an 81.6% TOC removal towards 50 mg L^-1 cefotaxime after 4 h electrolysis, under the optimal working condition Fe²⁺ = PS = 1 mM. The influence of current density and initial concentration on the performance of all processes was assessed. Methanol and tert-butanol were added in the system as OH and SO₄^- scavengers, which illustrating the mechanism of EO-EF-PS oxidizing process was the result of the two free radicals. Major intermediates were deduced and the degradation pathway of cefotaxime was analyzed. This research provides a potential coupling process with high antibiotic removal efficiency and effective materials for practical uses.

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

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