Anodic oxidation of wastewater containing the Reactive Orange 16 Dye using heavily boron-doped diamond electrodes

Potential of low-cost TiO2-PVC composite in photoelectrocatalytic degradation of reactive orange 16 under visible light

In recent years, previously reported studies revealed a high efficiency of pollutant degradation by coupling photocatalysis and electrochemical processes (PECs) using titanium dioxide (TiO₂) photoelectrode rather than using photocatalysis or electrocatalysis alone. However, some of the TiO₂ photoelectrodes that have been reported were not cost-effective. This is due to the use of expensive chemicals and certain expensive equipment in the fabrication process, other than involving complicated preparation steps. Therefore, this study is aimed at investigating the PEC performance and stability of low-cost TiO₂-polyvinyl chloride (TiO₂-PVC) composite photoelectrode for Reactive Orange 16 (RO16) degradation. The materials characterisation using the ATR-FTIR, XRD and UV-Vis DRS proved that TiO₂ and TiO₂-PVC were successfully synthesised. The micrograph obtained for the surface characterisation using the FESEM showed that the smooth surface of freshly prepared photoelectrodes turned slightly rough with tiny pits formation after five continuous PEC processes. Nevertheless, the photoelectrode retained its original shape in good condition for further PEC processes. By PEC process, the fabricated photoelectrode showed 99.4% and 51.1% of colour and total organic carbon (TOC) removal, respectively, at optimised PEC parameters (1.0 mol L^-1 NaCl concentration, 10 V applied voltage, 120 min degradation time and initial pH 2). Moreover, the fabricated photoelectrode demonstrated sufficient reusability potential (~ 96.3%) after five cycles of PEC processes. In summary, a low-cost and stable composite photoelectrode with high efficiency in RO16 degradation was successfully fabricated and could be potentially applied for other emerging pollutants degradation via the PEC degradation technique.

Advances on removal of organophosphorus pesticides with electrochemical technology

Widespread use of organophosphorus pesticides (OPs), especially superfluous and unreasonable use, had brought huge harm to the environment and food chain. It is because only a small part of the pesticides sprayed reached the target, and the rest slid across the soil, causing pollution of groundwater and surface water resources. These pesticides accumulate in the environment, causing environmental pollution. Therefore, in recent years, the control and degradation of OPs have become a public spotlight and research hotspot. Due to its unique advantages such as versatility, environmental compatibility, controllability, and cost-effectiveness compatibility, electrochemical technology has become one of the most promising methods for degradation of OPs. The fundamental knowledge about electrochemical degradation on OPs was introduced in this review. Then, a comprehensive overview of four main types of practical electrochemical technologies to degrade pesticides were presented and evaluated. The knowledge contained herein should conduce to better understand the degradation of pesticides by electrochemical technology, and better exploit the degradation of pesticides in the environment and food. Overall, the objective of this review is to provide comprehensive guidance for rational design and application of electrochemical technology in the degradation of OPs for the safety of the environment and food chain in the future.

Occurrence, statutory guideline values and removal of contaminants of emerging concern by Electrochemical Advanced Oxidation Processes: A review

A wide variety of chemical compounds are used in human activities; however, part of these compounds reach surface water, groundwater and even water considered for potable uses. Due to the limited efficiency of water treatment by the Water and Wastewater Treatment Plants, the presence of these compounds in natural and human consumption waters can be very harmful due to their high persistence and adverse effects; these characteristics define the contaminants of emerging concern (CECs). Water treatment by Electrochemical Advanced Oxidation Processes (EAOPs) has been evaluated as a promising process for the removal of persistent and recalcitrant organic contaminants. With this background, the present review aims to gather studies and information published between 2015 and 2020 regarding the occurrence of CECs in surface, potable and groundwater, its treatment by EAOPs, the main operating conditions and by-product generation of EAOPs, contaminant toxicity assessments and international statutory guideline values concerning CEC standards and allowable concentrations in the environment and treated drinking water. Therefore, in this review it was found that the compounds bisphenol A (BPA), diethyltoluamide (DEET), 17α-ethinyl estradiol (EE2), perfluorobutanoic acid (PFBA), carbamazepine, caffeine and atrazine were the most frequently detected in water sources, with concentrations ranging from 35.54-4800, 1.21-98, 0.005-38.5, 5-742.904, 0.0071-586, 0.89-1040, and 100-323 (ng L-1), respectively. Among the operational conditions of EAOPs, current density, pH and oxidant concentration are the main operational parameters that have an influence on these treatment technologies, besides the by-products generated, which might be removed by the integration of EAOPs with biological digestion treatments. Regarding the values of water quality standards, many CECs do not have established standard allowable concentration values, which represents a concern toward the possible toxic effects of these compounds on non-target organisms.

Influence of the doping level of boron-doped diamond anodes on the removal of penicillin G from urine matrixes

The objective of this study is to understand the influence of the characteristics of boron-doped diamond anodes on the degradation of Penicillin G contained in urine. Therefore, five commercial BDD anodes with different boron doping levels (100 ppm - 8000 ppm) were studied. These electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, and electrolysis. The boron doping was found to correlate well with the electrochemical properties of the electrodes, and results indicate a different behavior in drug degradation. The improvement in the toxicity and the reduction of the antibiotic effect of urine were the most innovative inputs monitored. For this, the concentration of Penicillin G, the toxicity toward Vibrio fisheri, and the antibiotic effect in Enterococcus faecalis were monitored. The best results were found for the BDD with a boron content of 200 ppm, capable of removing 100% of the antibiotic, reducing toxicity by 90%, and eradicating the antibiotic effect. These results indicate that low doping levels are more efficient for urine removal by anodic oxidation.

Electrochemical oxidation of indanthrene blue dye in a filter-press flow reactor and toxicity analyses with Raphidocelis subcapitata and Lactuca sativa

Alternative routes to degrade dyes are of crucial importance for the environment. Hence, we report the electrochemical removal of indanthrene blue by using a boron-doped diamond anode, focusing on the toxicity of the treated solutions. Different operational conditions were studied, such as current density (5, 10, and 20 mA cm-2) and electrolyte composition (Na2SO4, Na2CO3, and NaNO3). Besides, the pH was monitored throughout the experiment to consider its direct influence on the ecotoxicity effects. The highest electrochemical oxidation efficiency, measured as color removal, was seen in the 180 min condition of electrolysis in 0.033 M Na2SO4, applying 20 mA cm-2, resulting in a color removal of nearly 91% and 40.51 kWh m-3 of energy consumption. The toxicity towards Lactuca sativa depends solely on pH variations being indifferent to color removal. While the inhibition concentration (IC50) for Raphidocelis subcapitata increases 20% after treatment (in optimized conditions), suggesting that the byproducts are more toxic for this specific organism. Our data highlight the importance of analyzing the toxicity towards various organisms to understand the toxic effect of the treatment applied.

Advanced Oxidation Processes for the Removal of Antibiotics from Water. An Overview

In this work, the application of advanced oxidation processes (AOPs) for the removal of antibiotics from water has been reviewed. The present concern about water has been exposed, and the main problems derived from the presence of emerging pollutants have been analyzed. Photolysis processes, ozone-based AOPs including ozonation, O3/UV, O3/H2O2, and O3/H2O2/UV, hydrogen peroxide-based methods (i.e., H2O2/UV, Fenton, Fenton-like, hetero-Fenton, and photo-Fenton), heterogeneous photocatalysis (TiO2/UV and TiO2/H2O2/UV systems), and sonochemical and electrooxidative AOPs have been reviewed. The main challenges and prospects of AOPs, as well as some recommendations for the improvement of AOPs aimed at the removal of antibiotics from wastewaters, are pointed out.

Conductive diamond: synthesis, properties, and electrochemical applications

This review summarizes systematically the growth, properties, and electrochemical applications of conductive diamond.

An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes

Wastewater containing dyes are one of the major threats to our environment. Conventional methods are insufficient for the removal of these persistent organic pollutants. Recently much attention has been received for the oxidative removal of various organic pollutants by electrochemically generated hydroxyl radical. This review article aims to provide the recent trends in the field of various Electrochemical Advanced Oxidation Processes (EAOPs) used for removing dyes from water medium. The characteristics, fundamentals and recent advances in each processes namely anodic oxidation, electro-Fenton, peroxicoagulation, fered Fenton, anodic Fenton, photoelectro-Fenton, sonoelectro-Fenton, bioelectro-Fenton etc. have been examined in detail. These processes have great potential to destroy persistent organic pollutants in aqueous medium and most of the studies reported complete removal of dyes from water. The great capacity of these processes indicates that EAOPs constitute a promising technology for the treatment of the dye contaminated effluents.

Simultaneous decolorization and desalination of dye wastewater through electrochemical process

Salt-containing dye wastewater discharged from textile industries causes serious environmental problems. Simultaneous decolorization and desalination of dye wastewater in a laboratory scale electrochemical cell are realized for the first time with boron-doped diamond anode. With initial methyl orange (MO) and NaCl of 50 and 3000 mg L^-1, decolorization and desalination efficiencies of 70.2 and 88.7% were achieved after 6-h treatment with applied voltage of 6 V. Increasing applied voltages resulted in the improvements of both color and salt removal, while higher MO concentrations suppressed decolorization and higher NaCl concentration accelerated desalination rate. MO dissociated into anions transferred through the anion exchange membrane into the anode compartment and reacted with the active species as ·OH, H₂O₂, and ClO^- generated in anode compartment, leading to color removal. Component analysis confirmed the destruction of MO, with generation of low molecular weight compounds such as phenol and indole. Ions balance analysis indicated that Cl^- and Na⁺ moved to the anode and the cathode compartments respectively through the employed membranes driven by external voltage, realizing salt removal. This study has collectively demonstrated an efficient alternative for satisfactory treatment of salt-containing dye wastewater based on electrochemical technology.

Nickel-foam-supported β-Ni(OH)2 as a green anodic catalyst for energy efficient electrooxidative degradation of azo-dye wastewater

Electrochemical oxidative degradation (EOD) is a particularly promising technique for removing organic pollutants from wastewater. However, due to the high overpotential of EOD in conventional anode materials, the energy cost of EOD is usually very high, which greatly promotes the search for highly active, stable, and energy-efficient anodic catalysts. Herein, we demonstrated that nickel-foam-supported (NF-supported) β-Ni(OH)₂ (NF/β-Ni(OH)₂) prepared via a facile hydrothermal method could be used as an energy efficient anode for EOD. The as-prepared 3D porous NF/β-Ni(OH)₂ exhibited high activity toward the electrochemical oxidation of methyl orange (MO) in the low potential region (<1.07 V vs. SCE). This property differs greatly from those of the conventional anode materials that require a high positive potential to keep them active for EOD, making NF/β-Ni(OH)₂ an energy-efficient and active anode material for EOD. With an oxidation current density of 0.25 mA cm⁻², the decolorization of MO was completed within 30 min, and the COD removal after 3h of reaction was 63.0%. The normalized energy consumption for the 3 h degradation of MO was 22.2 kW h (kg COD)⁻¹, which is only a fraction of (or even one tenth of) the values reported in the literature. Moreover, NF/β-Ni(OH)₂ had a good stability and recyclability for EOD. No activity decay was observed during 10 h of EOD and the COD removal remained almost unchanged after four consecutive reaction cycles. We demonstrated experimentally that the NF/β-Ni(OH)₂ anode could generate large amounts of hydroxyl radicals and that the oxidation of MO by hydroxyl radicals was the main mechanism during EOD. We believe that this work opens a new avenue for developing highly active and energy-efficient anode materials that can work in the low potential region for EOD.

A novel NF/β-Ni(OH)₂ catalyst for energy efficient electrochemical degradation of methyl orange was fabricated via a facile hydrothermal method.

Electrochemical oxidation of COD from real textile wastewaters: Kinetic study and energy consumption

In the present study, the electrochemical oxidation of real wastewaters discharged by textile industry was carried out using a boron-doped diamond (BDD) anode. The effect of operational variables, such as applied current density (20-100 mA·cm^-2), NaCl concentration added to the real wastewaters (0-3 g·L^-1), and pH value (2.0-10.0), on the kinetics of COD oxidation and on the energy consumption was carefully investigated. The obtained experimental results could be well matched with a proposed kinetic model, in which the indirect oxidation mediated by electrogenerated strong oxidants would be described through a pseudo-first-order kinetic constant k. Values of k exhibited a linear increase with increasing applied current density and decreasing pH value, and an exponential increase with NaCl concentration. Furthermore, high oxidation kinetics resulted in low specific energy consumption, but this conclusion was not suitable to the results obtained under different applied current density. Under the optimum operational conditions, it only took 3 h to complete remove the COD in the real textile wastewaters and the specific energy consumption could be as low as 11.12 kWh·kg^-1 COD. The obtained results, low energy consumption and short electrolysis time, allowed to conclude that the electrochemical oxidation based on BDD anodes would have practical industrial application for the treatment of real textile wastewater.

Iodinated contrast media electro-degradation: process performance and degradation pathways

The electrochemical degradation of six of the most widely used iodinated contrast media was investigated. Batch experiments were performed under constant current conditions using two DSA® electrodes (titanium coated with a proprietary and patented mixed metal oxide solution of precious metals such as iridium, ruthenium, platinum, rhodium and tantalum). The degradation removal never fell below 85% (at a current density of 64 mA/cm(2) with a reaction time of 150 min) when perchlorate was used as the supporting electrolyte; however, when sulphate was used, the degradation performance was above 80% (at a current density of 64 mA/cm(2) with a reaction time of 150 min) for all of the compounds studied. Three main degradation pathways were identified, namely, the reductive de-iodination of the aromatic ring, the reduction of alkyl aromatic amides to simple amides and the de-acylation of N-aromatic amides to produce aromatic amines. However, as amidotrizoate is an aromatic carboxylate, this is added via the decarboxylation reaction. The investigation did not reveal toxicity except for the lower current density used, which has shown a modest toxicity, most likely for some reaction intermediates that are not further degraded. In order to obtain total removal of the contrast media, it was necessary to employ a current intensity between 118 and 182 mA/cm(2) with energy consumption higher than 370 kWh/m(3). Overall, the electrochemical degradation was revealed to be a reliable process for the treatment of iodinated contrast media that can be found in contaminated waters such as hospital wastewater or pharmaceutical waste-contaminated streams.

Electrochemical oxidation of imazapyr with BDD electrode in titanium substrate

In this work we have studied the treatment of imazapyr by electrochemical oxidation with boron-doped diamond anode. Electrochemical degradation experiments were performed in a one-compartment cell containing 0.45 L of commercial formulations of herbicide in the pH range 3.0-10.0 by applying a density current between 10 and 150 mA cm(-2) and in the temperature range 25-45 °C. The maximum current efficiencies were obtained at lower current densities since the electrochemical system is under mass transfer control. The mineralization rate increased in acid medium and at higher temperatures. The treatment was able to completely degrade imazapyr in the range 4.6-100.0 mg L(-1), although the current charge required rises along with the increasing initial concentration of the herbicide. Toxicity analysis with the bioluminescent bacterium Vibrio fischeri showed that at higher pollutant concentrations the toxicity was reduced after the electrochemical treatment. To clarify the reaction pathway for imazapyr mineralization by OH radicals, LC-MS/MS analyses we performed together with a theoretical study. Ions analysis showed the formation of high levels of ammonium in the cathode. The main final products of the electrochemical oxidation of imazapyr with diamond thin film electrodes are formic, acetic and butyric acids.

Recent updates on electrochemical degradation of bio-refractory organic pollutants using BDD anode: a mini review

Boron-doped diamond (BDD) is playing an important role in environmental electrochemistry and has been successfully applied to the degradation of various bio-refractory organic pollutants. However, the review concerning recent progress in this research area is still very limited. This mini-review updated recent advances on the removal of three kinds of bio-refractory wastewaters including pharmaceuticals, pesticides, and dyes using BDD electrode. It summarized the important parameters in three electrochemical oxidation processes, i.e., anodic oxidation (AO), electro-Fenton (EF), and photoelectro-Fenton (PEF) and compared their different degradation mechanisms and behaviors. As an attractive improvement of PEF, solar photoelectro-Fenton using sunlight as UV/vis source presented cost-effectiveness, in which the energy consumption for enrofloxacin removal was 0.246 kWh/(g TOC), which was much lower than that of 0.743 and 0.467 kWh/(g TOC) by AO and EF under similar conditions. Finally the existing problems and future prospects in research were suggested.

A comparative study of the electrochemical oxidation of the herbicide tebuthiuron using boron-doped diamond electrodes

The thiadiazolylurea derivative tebuthiuron (TBH) is commonly used as an herbicide even though it is highly toxic to humans. While various processes have been proposed for the removal of organic contaminants of this type from wastewater, electrochemical degradation has shown particular promise. The aim of the present study was to investigate the electrochemical degradation of TBH using anodes comprising boron-doped (5000 and 30,000 ppm) diamond (BDD) films deposited onto Ti substrates operated at current densities in the range 10-200 mA cm(-2). Both anodes removed TBH following a similar pseudo first-order reaction kinetics with k(app) close to 3.2 × 10(-2) min(-1). The maximum mineralization efficiency obtained was 80%. High-pressure liquid chromatography with UV-VIS detection established that both anodes degraded TBH via similar intermediates. Ion chromatography revealed that increasing concentrations of nitrate ions (up to 0.9 ppm) were formed with increasing current density, while the formation of nitrite ions was observed with both anodes at current densities ≥ 150 mA cm(-2). The BDD film prepared at the lower doping level (5000 ppm) was more efficient in degrading TBH than its more highly doped counterpart. This unexpected finding may be explained in terms of the quantity of impurities incorporated into the diamond lattice during chemical vapor deposition.