Photoelectrochemical removal of chlorfenvinphos by using WO3 nanorods: Influence of annealing temperature and operation pH

Synthesis of CuOx nanostructures in novel electrolytes under hydrodynamic conditions for photoelectrochemical applications

In this work, CuOx (x = 1 and 2) nanostructures have been synthesized by electrochemical anodization in ethylene glycol based electrolytes using oxalic acid or NaF (with or without NaOH) as complexing agents. The influence of hydrodynamic conditions and time during anodization of copper have also been evaluated. A comprehensive morphological, structural, electrochemical and photoelectrochemical characterization of the nanostructures has been performed. The results revealed the convenient use of oxalic acid and 250 rpm for 5 minutes during electrochemical anodization to obtain homogeneous CuOx nanostructures formed by spheres with Cu2O as a predominant phase. Using this nanostructure as a photocathode for N2O photoelectron-reduction, almost 100% of N2O removal was achieved after 1 h, showing the improvement of the photoelectrochemical approach vs. the photo or the electro performance.

Degradation of Methylparaben Using Optimal WO3 Nanostructures: Influence of the Annealing Conditions and Complexing Agent

In this work, WO₃ nanostructures were synthesized with different complexing agents (0.05 M H₂O₂ and 0.1 M citric acid) and annealing conditions (400 °C, 500 °C and 600 °C) to obtain optimal WO₃ nanostructures to use them as a photoanode in the photoelectrochemical (PEC) degradation of an endocrine disruptor chemical. These nanostructures were studied morphologically by a field emission scanning electron microscope. X-ray photoelectron spectroscopy was performed to provide information of the electronic states of the nanostructures. The crystallinity of the samples was observed by a confocal Raman laser microscope and X-ray diffraction. Furthermore, photoelectrochemical measurements (photostability, photoelectrochemical impedance spectroscopy, Mott-Schottky and water-splitting test) were also performed using a solar simulator with AM 1.5 conditions at 100 mW·cm^-2. Once the optimal nanostructure was obtained (citric acid 0.01 M at an annealing temperature of 600 °C), the PEC degradation of methylparaben (C_O 10 ppm) was carried out. It was followed by ultra-high-performance liquid chromatography and mass spectrometry, which allowed to obtain the concentration of the contaminant during degradation and the identification of degradation intermediates. The optimized nanostructure was proved to be an efficient photocatalyst since the degradation of methylparaben was performed in less than 4 h and the kinetic coefficient of degradation was 0.02 min^-1.

Visible Light-Driven Advanced Oxidation Processes to Remove Emerging Contaminants from Water and Wastewater: a Review

The scientific data review shows that advanced oxidation processes based on the hydroxyl or sulfate radicals are of great interest among the currently conventional water and wastewater treatment methods. Different advanced treatment processes such as photocatalysis, Fenton's reagent, ozonation, and persulfate-based processes were investigated to degrade contaminants of emerging concern (CECs) such as pesticides, personal care products, pharmaceuticals, disinfectants, dyes, and estrogenic substances. This article presents a general overview of visible light-driven advanced oxidation processes for the removal of chlorfenvinphos (organophosphorus insecticide), methylene blue (azo dye), and diclofenac (non-steroidal anti-inflammatory drug). The following visible light-driven treatment methods were reviewed: photocatalysis, sulfate radical oxidation, and photoelectrocatalysis. Visible light, among other sources of energy, is a renewable energy source and an excellent substitute for ultraviolet radiation used in advanced oxidation processes. It creates a high application potential for solar-assisted advanced oxidation processes in water and wastewater technology. Despite numerous publications of advanced oxidation processes (AOPs), more extensive research is needed to investigate the mechanisms of contaminant degradation in the presence of visible light. Therefore, this paper provides an important source of information on the degradation mechanism of emerging contaminants. An important aspect in the work is the analysis of process parameters affecting the degradation process. The initial concentration of CECs, pH, reaction time, and catalyst dosage are discussed and analyzed. Based on a comprehensive survey of previous studies, opportunities for applications of AOPs are presented, highlighting the need for further efforts to address dominant barriers to knowledge acquisition.

Enhanced photoelectrocatalytic degradation of diclofenac sodium using a system of Ag-BiVO4/BiOI anode and Ag-BiOI cathode

We report the photoelectrocatalysis of diclofenac sodium using a reactor consisting of Ag-BiVO₄/BiOI anode and Ag-BiOI cathode. The electrodes were prepared through electrodeposition on FTO glass and modified with Ag nanoparticles through photodeposition. The structural and morphological studies were carried out using XRD, SEM, and EDS which confirmed the successful preparation of the materials. The optical properties as observed with UV-DRS revealed that the electrodes were visible light active and incorporation of metallic Ag particles on the surface increased the absorption in the visible light region. Presence of p-n heterojunction in the anode led to decrease in the spontaneous recombination of photoexcited electron–hole pairs as seen in the photocurrent response. The results from photoelectrocatalytic degradation experiments revealed that replacing platinum sheet with Ag-BiOI as counter electrode resulted in higher (92%) and faster removal of diclofenac sodium as evident in the values of apparent rate constants. The reaction mechanism further revealed that efficiently separated photogenerated holes played a major role in the degradation of the pharmaceutical. The prepared electrodes showed good stability and impressive reusability. The reports from this study revealed that the dual photoelectrodes system has a great potential in treating pharmaceutical polluted wastewater using visible light irradiation.

Study of the chlorfenvinphos pesticide removal under different anodic materials and different reactor configuration

The present manuscript focuses on the study of the electrochemical oxidation of the insecticide Chlorfenvinphos (CVP). The assays were carried out under galvanostatic conditions using boron-doped diamond (BDD) and low-cost tin dioxide doped with antimony (Sb-doped SnO₂) as anodes. The influence of the operating variables, such as applied current density, presence or absence of a cation-exchange membrane and concentration of supporting electrolyte, was discussed. The results revealed that the higher applied current density the higher degradation and mineralization of the insecticide for both anodes. The presence of the membrane and the highest concentration of Na₂SO₄ studied (0.1 M) as a supporting electrolyte benefited the oxidation process of CVP using the BDD electrode, while with the ceramic anode the elimination of CVP was lower under these experimental conditions. Although the BDD electrode showed the best performance, ceramic anodes appear as an interesting alternative as they were able to degrade CVP completely for the highest applied current density values. Toxicity tests revealed that the initial solution of CVP was more toxic than the samples treated with the ceramic electrode, while using the BDD electrode the toxicity of the sample increased.

A CeO2 Semiconductor as a Photocatalytic and Photoelectrocatalytic Material for the Remediation of Pollutants in Industrial Wastewater: A Review

The direct discharge of industrial wastewater into the environment results in serious contamination. Photocatalytic treatment with the application of sunlight and its enhancement by coupling with electrocatalytic degradation offers an inexpensive and green technology enabling the total removal of refractory pollutants such as surfactants, pharmaceuticals, pesticides, textile dyes, and heavy metals, from industrial wastewater. Among metal oxide—semiconductors, cerium dioxide (CeO2) is one of the photocatalysts most commonly applied in pollutant degradation. CeO2 exhibits promising photocatalytic activity. Nonetheless, the position of conduction bands (CB) and valence bands (VB) in CeO2 limits its application as an efficient photocatalyst utilizing solar energy. Its photocatalytic activity in wastewater treatment can be improved by various modification techniques, including changes in morphology, doping with metal cation dopants and non-metal dopants, coupling with other semiconductors, and combining it with carbon supporting materials. This paper presents a general overview of CeO2 application as a single or composite photocatalyst in the treatment of various pollutants. The photocatalytic characteristics of CeO2 and its composites are described. The main photocatalytic reactions with the participation of CeO2 under UV and VIS irradiation are presented. This review summarizes the existing knowledge, with a particular focus on the main experimental conditions employed in the photocatalytic and photoelectrocatalytic degradation of various pollutants with the application of CeO2 as a single and composite photocatalyst.

Photoelectrocatalyzed degradation of organophosphorus pesticide fenamiphos using WO3 nanorods as photoanode

In this study, WO₃ nanostructures were synthesized by the electrochemical anodization technique to use them on the degradation of persistent organic compounds such as the pesticide fenamiphos. The acids electrolyte used during the anodization were two different: 1.5 M H₂SO₄ - 0.05 M H₂O₂ and 1.5 M CH₄O₃S - 0.05 M H₂O₂. Once the samples have been manufactured, they have been subjected to different tests to analyze the properties of the nanostructures. With Field Emission Scanning Electron Microscopy (FE-SEM) the samples have been examined morphologically, their composition and crystallinity has been studied through Raman Spectroscopy and their photoelectrochemical behaviour by Photoelectrochemical Impedance Spectroscopy (PEIS). Finally, degradation tests have been carried out using the technique known as photoelectrocatalysis (PEC). The conditions that were applied in this technique were a potential of 1 V_Ag/AgCl and simulated solar illumination. The degradation process was monitored by UV-Visible and High-Performance liquid Chromatography (HPLC) to control the course of the experiment. The nanostructures obtained with 1.5 M CH₄O₃S - 0.05 M H₂O₂ electrolyte showed a better photoelectrochemical behaviour than nanostructures synthesized with 1.5 M H₂SO₄ - 0.05 M H₂O₂. The fenamiphos degradation was achieved at 2 h of experiment and the intermediate formation was noticed at 1 h of PEC experiment.

Semiconductor Electrode Materials Applied in Photoelectrocatalytic Wastewater Treatment—an Overview

Industrial sources of environmental pollution generate huge amounts of industrial wastewater containing various recalcitrant organic and inorganic pollutants that are hazardous to the environment. On the other hand, industrial wastewater can be regarded as a prospective source of fresh water, energy, and valuable raw materials. Conventional sewage treatment systems are often not efficient enough for the complete degradation of pollutants and they are characterized by high energy consumption. Moreover, the chemical energy that is stored in the wastewater is wasted. A solution to these problems is an application of photoelectrocatalytic treatment methods, especially when they are coupled with energy generation. The paper presents a general overview of the semiconductor materials applied as photoelectrodes in the treatment of various pollutants. The fundamentals of photoelectrocatalytic reactions and the mechanism of pollutants treatment as well as parameters affecting the treatment process are presented. Examples of different semiconductor photoelectrodes that are applied in treatment processes are described in order to present the strengths and weaknesses of the photoelectrocatalytic treatment of industrial wastewater. This overview is an addition to the existing knowledge with a particular focus on the main experimental conditions employed in the photoelectrocatalytic degradation of various pollutants with the application of semiconductor photoelectrodes.

TiO2 Modified with Organic Acids for the Decomposition of Chlorfenvinphos under the Influence of Visible Light: Activity, Performance, Adsorption, and Kinetics

Photocatalytic decomposition of chlorfenvinphos (CFVP) in the presence of titanium dioxide (TiO₂) modified with organic acids: pyruvic (PA) and succinic (SA) under the visible light radiation has been studied. The following tests were examined: dose of photocatalysts, adsorption time, pH of the model solution, deactivation of catalysts, the role of oxygen, identification of free radicals for the CFVP decomposition, Langmuir-Hinshelwood kinetics. The synthesized materials were characterized by Scanning Electron Microscopy (SEM) and UV-Vis. At 10 wt.% of acid (90:10) decomposition of chlorfenvinphos was the most effective in the following conditions: dose of catalyst 50.0 mg/L, time of adsorption = 20 min, pH of model solution = 3.0. Under these conditions the order of photocatalyst efficiency has been proposed: TiO₂/PA/90:10 > TiO₂/SA/90:10 > TiO₂ with the removal degree of 85, 72 and 48%. The mathematically calculated half-life at this conditions was 27.0 min and 39.0 min for TiO₂/PA/90:10 and TiO₂/SA/90:10 respectively, compared to 98 min for pure TiO₂. It has been determined that the O₂^- radicals and holes (h⁺) are the main reactive species involved in the photodegradation of chlorfenvinphos. The results of this study showed that method may be an interesting alternative for the treatment of chlorfenvinphos contaminated wastewater.

Photoelectrocatalyzed degradation of a pesticides mixture solution (chlorfenvinphos and bromacil) by WO3 nanosheets

A photoelectrocatalyst consisting of WO₃ nanosheets or nanorods has been synthesized by electrochemical anodization under hydrodynamic conditions, and has been used for the degradation of two toxic pesticides: chlorfenvinphos and bromacil. Nanostructures have been characterized by FESEM and Raman spectroscopy. Photoelectrochemical degradation tests have been carried out both for individual pesticide solutions and for a mixture solution, and the concentration evolution with time has been followed by UV-Vis spectrophotometry. For individual pesticides, pseudo-first order kinetic coefficients of 0.402h^-1 and 0.324h^-1 have been obtained for chlorfenvinphos and bromacil, respectively, while for the mixture solution, these kinetic coefficients have been 0.162h^-1 and 0.408h^-1. The change in behavior towards pesticide degradation depending on whether individual or mixture solutions were used might be indicative of a competitive process between the two pesticide molecules when interacting with the WO₃ nanostructures surface or when approaching the semiconductor/electrolyte interface.

Interrogating solar photoelectrocatalysis on an exfoliated graphite–BiVO4/ZnO composite electrode towards water treatment

A novel photoanode consisting of an exfoliated graphite–BiVO₄/ZnO heterostructured nanocomposite was fabricated. The material was characterised with scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). Photoelectrochemical studies were carried out with cyclic/linear sweep voltammetry and chronoamperometry. The solar photoelectrochemical properties of the heterojunction photoanode were investigated through the degradation of rhodamine B in water. The results revealed that the nanoparticles of BiVO₄ and ZnO were well entrapped within the interlayers of the exfoliated graphite (EG) sheets. Improved charge separation was achieved in the EG–BiVO₄/ZnO composite electrode which resulted in superior photoelectrochemical performance than individual BiVO₄ and ZnO electrodes. A higher degradation efficiency of 91% of rhodamine B was recorded using the composite electrode with the application of 10 mA cm⁻² current density and a solution pH of 7. The highest total organic carbon removal of 74% was also recorded with the EG–BiVO₄/ZnO. Data from scavenger studies were used to support the proposed mechanism of degradation. The electrode has high stability and reusability and hence lends itself to applications in photoelectrocatalysis, especially in water treatment.

Band alignment between ZnO and BiVO₄ on exfoliated graphite (EG) support.