Anodic oxidation for the degradation of organic pollutants: Anode materials, operating conditions and mechanisms. A mini review

Localized Alkaline Microenvironments Enhanced upon Tip Effects for Efficient Antibacterial Applications

The electrochemical disinfection mechanism primarily relies on a high-voltage pulsed electric field and electrocatalytic generation of highly oxidative radicals but suffers from shortcomings of either high voltages (dozens of kV) or significant gas supply. This study proposes an in situ electrocatalytic sterilization strategy that forms localized alkaline microenvironments in neutral electrolytes under a constant current of -12 mA at a relatively lower voltage (-0.5 V vs. RHE). The nanowire cathode surface can effectively adsorb hydronium ions (H3O+) from the solution during H2 evolution, leading to a rapid increase in hydroxide ions (OH-) concentration in the surrounding solution. This leads to a high diffusion zone of OH-, creating a localized alkaline microenvironment. Scanning electrochemical microscopy (SECM) and finite element analysis (FEA) reveal that the nanowire-derived tip-enhanced effect significantly enhances OH- accumulation around the cathode, effectively inactivating Escherichia coli (E. coli) at 107 CFU/mL within 3 minutes at -0.5 V. The resulting increase in OH- concentration drives rapid sterilization by inhibiting bacterial ATP synthesis and inducing oxidative stress. This method can efficiently inactivate microorganisms in a short time, while having minimal impact on the overall pH of the environment, making it a safe and environmentally friendly approach.

Development of a graphene based anodic membrane for efficient electrocatalytic oxidation of typical antibiotics from water

Traditional membrane separation processes were ineffective at degrading pollutant molecules, while catalytic reactors often faced inefficiencies due to their inability to simultaneously retain and concentrate reactant molecules. To address these challenges, we synthesized an anodic electrocatalyst (Ti4O7/SnO2-Sb) and combined it with reduced graphene oxide (rGO) sheets to develop a high-performance anodic electrocatalytic membrane (AOM). The AOM's tailored interlayer spacing facilitated simultaneous concentration and electrooxidation of antibiotic pollutants, enabling efficient degradation. At a permeate flux of 7.4 L m-2·h-1, the AOM achieved ∼95 % degradation of chloramphenicol (CAP, 20 mg L-1). The corresponding degradation rate constant (k) of 199.70 min-1 was two orders of magnitude higher than that of a conventional reactor without retention capability (1.06 min-1). Moreover, the AOM exhibited exceptional energy efficiency and long-term stability, highlighting its potential for advanced wastewater treatment. This innovative design, which leveraged pollutant concentration via the size-sieving effects of confined interlayer spaces, marked a significant advancement in high-performance water treatment technologies.

Treatment of Produced Water Using a Pilot-Scale Advanced Electrochemical Oxidation Unit

The main goal of this study is to optimize the treatment of produced water (PW) using a pilot-scale advanced electrochemical oxidation unit. The electro-cell is outfitted with a boron-doped diamond BDD anode and gas diffusion (GDE) cathode. Synthetic PW was prepared in the laboratory following a protocol designed to closely replicate the characteristics of real PW. The PW used in this study had a total dissolved solids (TDS) concentration of 16,000 mg/L and a total organic carbon (TOC) concentration of 250 mg/L. The effect of various electrooxidation parameters on the reduction in TOC was investigated including pH (2-12), electric current (I) (50-200 mA/cm2), and airflow rate (0-4 NL/min). Response surface method RSM with a Box-Behnken design at a confidence level of 95 percent was employed to analyze the impact of the above factors, with TOC removal used as a response variable. The results revealed that the TOC level decreased by 84% from 250 to 40 mg/L in 4 h, current density of 200 mA/cm2, pH of 12, and airflow rate 2 (NL/min). The investigation verified the influential role of diverse operational factors in the treatment process. RSM showed that reducing the airflow rate and increasing pH levels and electric current significantly enhanced the TOC removal. The obtained results demonstrated profound TOC removal, confirming the substantial potential of treating PW using the electrochemical method.

Photo-induced degradation of toxic recalcitrant compounds from surface water: Insights into advanced nanomaterials, hybrid photocatalytic systems, and real applications

The rapid increase in toxic recalcitrant organic compounds (ROCs) from various industrial, residential, and agricultural sources poses a significant public health concern and threatens environmental preservation. The presence of these toxic ROCs weakens the effectiveness of conventional water and wastewater treatment systems. As a result, numerous physicochemical and biological treatment processes have been explored, each demonstrating varying removal efficiencies depending on experimental conditions. Given the limitations of existing treatment methods, research has increasingly focused on advanced oxidation processes, particularly photocatalysis. Photocatalysis is a prominent treatment technique due to its low sludge production, non-toxic nature, reusable characteristics, and ability to harness visible light. This review comprehensively examines the ecotoxicological effects of ROCs, existing biological and physicochemical treatment methods, advancements in photocatalyst synthesis, the transition from conventional to advanced photocatalysts, and hybrid treatment systems. In the context of photocatalytic removal of ROCs, the review also addresses several influencing parameters, including initial pollutant concentration, solution pH, light intensity, catalyst dose, and catalyst type. Global case studies focusing on the mechanisms of photocatalytic degradation of ROCs are highlighted. The documented photocatalysts for removing ROCs from water and wastewater have shown promising results. Moreover, integrating photocatalysis with advanced physicochemical and biological processes has effectively removed various dissolved (e.g., ROCs) and suspended impurities, showcasing its practical applications. Thus, this study could serve as a valuable resource for researchers and engineers working on the treatment of various micropollutants, such as ROCs, in real wastewater.

Voltammetric methodology for the quality control and monitoring of sulfamethoxazole removal from water

Sulfamethoxazole is an antibiotic that is among the drugs most frequently found in waters around the world because of its habitual consumption and its high chemical stability that prevents it from being eliminated from the environment. In this study, an electroanalytical methodology based on differential pulse voltammetry is developed for the analysis of sulfamethoxazole at trace levels in water. After the optimization of the instrumental parameters a linear range from 6.59 to 96.27 μM was found with limits of detection and quantification of 1.98 and 6.59 μM, respectively, with an RSD below 6 %. Moreover, several validation studies involving different pH values, water samples and instrumentation-techniques were performed in order to ensure the robustness of the method. For this purpose, the peak area was used as quantitative variable since it is not affected by the pH of the medium even if there is any modification of this parameter during the experiments. Furthermore, the effect of other drug such as trimethoprim on the analytical signal of sulfamethoxazole was also evaluated. Once the method was developed it was tested on the quality control of Soltrim®, obtaining recoveries between 98 and 102 %. Lastly, the voltammetric method was applied for the in situ monitoring of sulfamethoxazole's removal from water samples, specifically by anodic oxidation and electro-Fenton treatments. While the former was coupled to an adsorption process, the latter was carried out with different iron sources including commercial medicines that can be found in wastewater. The problem of significant variation in pH during the treatment was solved by working with the peak area, and so obtaining valid and reliable kinetic data. Although anodic oxidation proved to be faster considering the calculated kobs, electro-Fenton turned out to be more efficient in eliminating the drug, achieving the disappearance of its analytical signal in only 30 min of treatment.

A high-efficient electrochemical degradation of diclofenac in water on planar and microstructured 2D, and macroporous 3D boron-doped diamond electrodes: Identification of degradation and transformation products

The highly efficient degradation of persistent organic substances by electrochemical advanced oxidation processes (EAOPs), which don't result in the formation of potentially harmful by-products, is crucial for the future of water management. In this study, boron-doped diamond electrodes (BDDE) with three morphologies (planar 2D, microstructured 2D, and macroporous 3D) were employed for the anodic oxidation of diclofenac (DCF) in two working electrolytes (NaCl and Na2SO4). In total, 11 by-products formed during the electrochemical oxidation of DCF were identified via HPLC-HRMS. The identification of degradation products revealed the formation of various active chlorinated species. The utilization of a chlorine-free Na2SO4 electrolyte resulted in the formation of greater number of chlorinated species, while their elimination required a longer period compared to the use of NaCl electrolyte. The formation of by-products was also influenced by the specific type of BDD electrode, which was associated with variations in applied current density. This led to an uneven distribution of dichloro (2D BDDE) and trichloro (3D BDDE) patterns. However, none of the products showed signs of a high level of persistence. The results revealed that the type of electrolyte is the most significant factor affecting the removal efficiency of DCF, while the different electrode morphologies do not lead to differences in the removal rates. The electrode type exerted a notable influence on the removal rates, which was associated with varying applied current densities, exclusively in the case of the Na₂SO₄ electrolyte. Over 99 % removal efficiency for DCF in NaCl, with power consumption of 1.8 kWh m-3 was achieved.

Unique role of Mn(II) in enhancing electro-oxidation of organic pollutants on anodes with low oxygen evolution potential at low current density

This study systematically explored the role of Mn(II) in the removal of 4-chlorophenol (4-CP) by electro-oxidation (EO) employing anodes with low oxygen evolution potential (OEP), i.e., Ti/RuO2-IrO2, Ti/Pt, and Ti/Ti4O7, as well as anodes with high OEP, namely, Ti/PbO2, Ti/SnO2, and boron-doped diamond (Si/BDD). Mn(II) significantly promoted 4-CP removal on the anodes with low OEP at fairly low current density (0.04 to 1 mA/cm2), but had minimal to negative impact on those with high OEP. Cyclic voltammetry and X-ray photoelectron spectra revealed that Mn(II) was oxidized to Mn(III), then to Mn(IV) on the anodes with low OEP, whereas its was oxidized directly to Mn(IV) on those with high OEP. Deposition of manganese oxide on the anodes with low OEP suppressed oxygen evolution reaction (OER) in EO process, but enhanced OER on those with high OEP. Quenching and spectral results consistently indicated that Mn(III) and Mn(IV) were the primary species responsible for enhancing 4-CP removal on the anodes with low OEP. These findings provide mechanistic insights into the redox transformation of Mn(II) in EO and the theoretical basis for a novel strategy to boost pollutant degradation in EO systems using low OEP anodes through coupling with the redox chemistry of manganese.

Real-time in situ monitoring as a tool for comparison of electrochemical advanced oxidation processes for the decolourisation of azo and indigoid dyes

The widespread use of synthetic dyes has led to the release of substantial amounts of dye-contaminated wastewater, posing significant environmental and health concerns. This study focuses on the use of anodic and electrochemically activated persulfate oxidation for the degradation of organic contaminants. Specifically, the structural variations of nine dyes in the indigoid and azo families, and their impact on the efficiency of electrochemical oxidation were analysed. An in situ continuous monitoring apparatus with a UV-visible detector was employed to collect data in real-time. The electrochemically activated persulfate system demonstrated higher efficiency compared to the anodic oxidation approach. In both systems the efficiency of decolourisation was highly dependent on the structure of the pollutant. Electron-withdrawing substituents in direct conjugation with the chromophore, bulky auxochromes, and extended aromatic systems significantly decreased the decolourisation efficiency. Conversely, changing the location of electron-withdrawing groups and adding electron-donating substituents increased the decolourisation efficiency, even overcoming the detrimental effects of bulky groups and extended conjugation. This type of systematic structural comparison study is essential for highlighting the interconnected nature of pollutant structure and degradation speed so that efficient electrochemical oxidation systems can be designed for the treatment of genuine wastewater effluent containing more than one pollutant.

A comparative review on the mitigation of metronidazole residues in aqueous media using various physico-chemical technologies

In the last few decades, pharmaceuticals have emerged as a new class of serious environmental pollutants. The presence of these emerging contaminants even in minimal amounts (micro- to nanograms) has side effects, and they can cause chronic toxicity to health and the environment. Furthermore, the presence of pharmaceutical contaminants in water resources leads to significant antibiotic resistance in bacteria. Hence, the removal of antibiotics from water resources is essential. Thus far, a wide range of methods, including adsorption, photodegradation, oxidation, photolysis, micro-/nanofiltration, and reverse osmosis, has been used to remove pharmaceutical contaminants from water systems. In this article, research related to the processes for the removal of metronidazole antibiotics from water and wastewater, including adsorption (carbon nanotubes (CNTs), magnetic nanocomposites, magnetic molecularly imprinted polymer (MMIP), and metal-organic frameworks), filtration, advanced oxidation processes (photocatalytic process, electrochemical advanced oxidation processes, sonolysis and sonocatalysis) and aqueous two-phase systems (ATPSs), was reviewed. Results reveal that advanced oxidation processes, especially photocatalytic and sonolysis processes, have high potential in removing MNZ (more than 90%).

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Comparative study of MMO and BDD anodes for electrochemical degradation of diuron in methanol medium

Treating emerging pollutants at low concentrations presents significant challenges in terms of degradation efficiency. Anodic oxidation using active and non-active electrodes shows great potential for wastewater treatment. Thus, this study compared the efficiency of a commercial mixed metal oxide anode (MMO: Ti/Ti0.7Ru0.3O2) and a boron-doped diamond anode (BDD) for the electrochemical oxidation of diuron in methanol, in chloride and sulfate media. The MMO anode achieved diuron removal rates of 94.9% and 92.8% in chloride and sulfate media, respectively, with pseudo-first-order kinetic constants of 0.0177 and 0.0143 min-1. The BDD anode demonstrated slightly higher removal rates, achieving 96.2% in sulfate medium and 96.9% in chloride medium, with respective kinetic constants of 0.0193 min⁻1 and 0.0177 min⁻1. Increasing the current density enhanced diuron removal by up to 15% for both electrodes; however, excessively high current densities led to increased energy consumption due to side reactions. The present of water had antagonistic effects, resulting in removal rates of 91.1% for chloride media using the BDD anode; and 87.4% and 90.4% in sulfate media with MMO and BDD anodes, respectively. The MMO anode in chloride medium did not show significant difference in the degradation percentage, reaching 96% of diuron removals. The degradation mechanism was proposed based on the detection of various by-products. The primary reactions observed during the oxidation of diuron in methanol involved chlorine substitution in the aromatic ring and dealkylation. These processes generated several intermediates and by-products at low concentrations, ultimately leading to high diuron removal.

New MMO coatings for electro-refinery applications: Promoting the production of carboxylates

Within the new circular economy paradigm, this work evaluates the performance of tailored mixed metal oxides (MMO) anodes, based on ruthenium and antimony, for their application into an electrochemically-assisted organic refinery process. This process is designed to transform pollutants into value-added products with minimal mineralization. Oxidation of synthetic wastes consisting of phenol solutions was used to validate the electrochemical conversion of phenolic wastes into carboxylates, which are then considered as bricks to be used for electrosynthesis or to produce fuels. The MMO anodes were manufactured using two synthesis routes (Pechini method and ionic liquid method), each followed by one of three different heating treatments: furnace, microwave, and CO2 laser. The selection of the optimal electrode for the organic electrorefinery was based on a combination of physical and electrochemical properties, degradation performance of phenol to carboxylates, and long-term stability, looking for a truly sustainable solution. Results indicate that anodes synthesized by the ionic liquid (IL) method, regardless of the heating treatment, demonstrated superior performance, with larger active areas (with furnace 82 mC cm-2, microwave 97 mC cm-2, and laser 127 mC cm-2) and higher phenol degradation rates, resulting in a greater generation of carboxylates during electrolysis, yielding primarily oxalate and achieving up to 40% conversion with furnace heating. However, laser-treated anodes exhibited greater stability than furnace-made ones, attributed to the formation of an insulating TiO2 layer. Although the electrode with the longest service life did not show the best catalytic properties for minimizing mineralization, the observed variations in coatings with identical chemical compositions highlight the importance of this research. This study positions itself at the forefront of developing more efficient and sustainable electrochemical technologies for organic waste treatment.

Assessment of Ti, Ir, Ta and Ru influence on mixed metal oxide electrodes for photoelectrochemical generation of persulfate: Impact on sulfamethoxazole degradation

The presence of persulfate (S2O82-) in decontamination processes favors the oxidation of organic pollutants due to its strong oxidation power. In this research we study the photoelectrochemical generation of persulfate using five mixed metal oxides electrodes (MMO) with different compositions and its effect on the degradation of sulfamethoxazole antibiotic (SMX) by photoelectrocatalysis (PEC) and electro-oxidation (EO). By PEC, all anodes generated a higher concentration of S2O82- than those not exposed to light. The high S2O82-concentration obtained by PEC was 0.150 mM using MMO[Ti/Ir/Ta] in a solution with Na2SO4 100 mM applying a current density of 2 mA/cm2. On the other hand, the maximum concentration obtained was 0.250 mM at 30 min of electrolysis for MMO[Ti/Ir/Ta] using Na2SO4 50 mM and applying current density of 5 mA/cm2. S2O82-production by EO was between 0.005 and 0.089 mM. It is observed that MMO based in Ta2O5 showed the best S2O82- production. The effect of S2O82- electro-generation (using the anode with the highest and the anode with the lowest S2O82- production) on the degradation of sulfamethoxazole by PEC and EO was studied using the experimental conditions with the best production of this oxidant. MMO[Ti/Ir/Ta] and MMO[Ti/Ru] were used as anodes, and it was observed that by PEC, 100% of SMX was degraded after 30 min of electrolysis using MMO[Ti/Ir/Ta] and 60 min using MMO[Ti/Ru]. By EO, the degradation of SMX was partial, demonstrating that the electrophotocatalytic effect favors the generation of S2O82-, enhancing the degradation of SMX at short electrolysis times.

Distinct microbial communities enriched in water-saturated and unsaturated reactors influence performance of integrated hydroponics-microbial electrochemical technology

This study aimed to understand the wastewater treatment and electricity generation performance besides the microbial communities of the integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) systems operated with water-saturated and water-unsaturated reactors. The organics removal was slightly higher in the water-unsaturated system (93 ± 4 %) than in the water-saturated system (87 ± 2 %). The total nitrogen removal and electric voltage were considerably higher in the water-saturated system (42 ± 5 %; 111 ± 8 V per reactor) than in the water-unsaturated system (18 ± 3 %; 95 ± 9 V per reactor). The enhanced organics and nitrogen removal and high voltage output in respective conditions were due to the dominance of polysaccharide-degrading aerobes (e.g., Pirellula), anammox bacteria (e.g., Anammoximicrobium), denitrifiers (e.g., Thauera and Rheinheimera), and electroactive microorganisms (e.g., Geobacter). The differential performance governed by distinct microbial communities under the tested conditions indicates that an appropriate balancing of water saturation and unsaturation in reactors is crucial to achieving optimum iHydroMET performance.

A Co-Doping Materials Design Strategy for Selective Ozone Electrocatalysts

Catalysts for electrochemical ozone production (EOP) face inherent selectivity challenges stemming from thermodynamic constraints. This work establishes a design strategy for minimizing these limitations and inducing EOP activity in tin oxide, which is an intrinsically EOP-inactive material. We propose that selective ozone production using tin oxide catalysts can be broadly achieved by co-doping with two elements: first, n-type dopants to enhance electrical conductivity, and second, transition metal dopants that leach and homogeneously generate essential hydroperoxyl radical intermediates. Synthesizing tantalum, antimony, and tungsten n-type dopants with nickel, cobalt, and iron as transition metal dopants confirms that properly co-doping tin oxide yields EOP-active catalysts. This study offers a robust framework for advancing EOP catalyst design and serves as a case study for the application of fundamental co-catalysis and solid-state physics principles to induce catalytic activity in inert materials.

A review of electrooxidation systems treatment of poly-fluoroalkyl substances (PFAS): electrooxidation degradation mechanisms and electrode materials

The prevalence of polyfluoroalkyls and perfluoroalkyls (PFAS) represents a significant challenge, and various treatment techniques have been employed with considerable success to eliminate PFAS from water, with the ultimate goal of ensuring safe disposal of wastewater. This paper first describes the most promising electrochemical oxidation (EO) technology and then analyses its basic principles. In addition, this paper reviews and discusses the current state of research and development in the field of electrode materials and electrochemical reactors. Furthermore, the influence of electrode materials and electrolyte types on the deterioration process is also investigated. The importance of electrode materials in ethylene oxide has been widely recognised, and therefore, the focus of current research is mainly on the development of innovative electrode materials, the design of superior electrode structures, and the improvement of efficient electrode preparation methods. In order to improve the degradation efficiency of PFOS in electrochemical systems, it is essential to study the oxidation mechanism of PFOS in the presence of ethylene oxide. Furthermore, the factors influencing the efficacy of PFAS treatment, including current density, energy consumption, initial concentration, and other parameters, are clearly delineated. In conclusion, this study offers a comprehensive overview of the potential for integrating EO technology with other water treatment technologies. The continuous development of electrode materials and the integration of other water treatment processes present a promising future for the widespread application of ethylene oxide technology.

Interplay between Catalyst Corrosion and Homogeneous Reactive Oxygen Species in Electrochemical Ozone Production

Electrochemical ozone production (EOP), a six-electron water oxidation reaction, offers promising avenues for creating value-added oxidants and disinfectants. However, progress in this field is slowed by a dearth of understanding of fundamental reaction mechanisms. In this work, we combine experimental electrochemistry, spectroscopic detection of reactive oxygen species (ROS), oxygen-anion chemical ionization mass spectrometry, and computational quantum chemistry calculations to determine a plausible reaction mechanism on nickel- and antimony-doped tin oxide (Ni/Sb-SnO2, NATO), one of the most selective EOP catalysts. Antimony doping is shown to increase the conductivity of the catalyst, leading to improved electrochemical performance. Spectroscopic analysis and electrochemical experiments combined with quantum chemistry predictions reveal that hydrogen peroxide (H2O2) is a critical reaction intermediate. We propose that leached Ni4+ cations catalyze hydrogen peroxide into solution phase hydroperoxyl radicals (•OOH); these radicals are subsequently oxidized to ozone. Isotopic product analysis shows that ozone is generated catalytically from water and corrosively from the catalyst oxide lattice without regeneration of lattice oxygens. Further quantum chemistry calculations and thermodynamic analysis suggest that the electrochemical corrosion of tin oxide itself might generate hydrogen peroxide, which is then catalyzed to ozone. The proposed pathways explain both the roles of dopants in NATO and its lack of stability. Our study interrogates the possibility that instability and electrochemical activity are intrinsically linked through the formation of ROS. In doing so, we provide the first mechanism for EOP that is consistent with computational and experimental results and highlight the central challenge of instability as a target for future research efforts.

Electrochemical oxidation of surfactants as an essential step to enable greywater reuse

The practical application of electrochemical oxidation technology for the removal of surfactants from greywater was evaluated using sodium dodecyl sulfate (SDS) as a model surfactant. Careful selection of electrocatalysts and optimization of operational parameters demonstrated effective SDS removal in treating a complex greywater matrix with energy consumption below 1 kWh g-1 COD (Chemical Oxygen Demand), paving the way for a more sustainable approach to achieving surfactant removal in greywater treatment when aiming for decentralized water reuse. Chromatographic techniques identified carboxylic acids as key byproducts prior to complete mineralization. These innovative approaches represent a novel pathway for harnessing electrochemical technologies within decentralized compact devices, offering a promising avenue for further advancements in this field.

Ultrasonic cavitation: Tackling organic pollutants in wastewater

Environmental pollution and energy shortages are global issues that significantly impact human progress. Multiple methods have been proposed for treating industrial and dyes containing wastewater. Ultrasonic degradation has emerged as a promising and innovative technology for organic pollutant degradation. This study provides a comprehensive overview of the factors affecting ultrasonic degradation and thoroughly examines the technique of acoustic cavitation. Furthermore, this study summarizes the fundamental theories and mechanisms underlying cavitation, emphasizing its efficacy in the remediation of various water pollutants. Furthermore, potential synergies between ultrasonic cavitation and other commonly used technologies are also explored. Potential challenges are identified and future directions for the development of ultrasonic degradation and ultrasonic cavitation technologies are outlined.

Impact of Anodic Oxidation Reactions in the Performance Evaluation of High-Rate CO2 /CO Electrolysis

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO2 /CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, several impacts of anodic oxidation of liquid products in terms of performance evaluation are investigated. First, the crossover behavior of several typical liquid products through an anion-exchange membrane is analyzed. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the Faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, a theoretical guideline with hierarchical indicators is further developed to predict and regulate the possible AORs in an electrolyzer. The perspective concludes by giving some suggestions on rigorous performance evaluations for high-rate CO2 /CO electrolysis in an MEA-based setup.

Real-time in situ monitoring using visible spectrophotometry as a tool for probing electrochemical advanced oxidation processes for dye decolorisation

An apparatus for real-time in situ monitoring of electrochemical processes using UV-visible spectrophotometry has been used to optimise the electrochemically-activated persulfate decolorisation of Acid Orange 7. The impacts of varying electrode composition, current density, persulfate loading, and stirring speed on the rate of decolorisation have been probed. Decolorisation through this activated persulfate approach was compared to that using anodic oxidation for nine dyes; three from each of the azo, triarylmethane, and xanthene families. The core structure and presence of functional groups have a significant impact on the rate of decolorisation. Azo and xanthene dyes decolorise faster than triarylmethane dyes, while electron-withdrawing groups and halogens are especially detrimental to the rate of decolorisation. Electrochemically-activated persulfate resulted in faster decolorization than anodic oxidation for almost every dye, an effect that was enhanced with the electron-deficient substrates. This type of systematic structural comparison study is essential for designing electrochemical degradation procedures for the remediation of real wastewater.

(Electro)catalytic oxidation of sulfide and recovery of elemental sulfur from sulfide-laden streams

MnxOy coated over TiO2 nanotube array substrate was doped with Mo and polyaniline (PANI) and applied for electrochemical desulfurization of concentrated sulfide (HS-) solutions at basic pH, typical of biogas scrubbing solutions and industrial wastewater. Mo and PANI co-dopants significantly enhanced the anode activity towards sulfide oxidation and ensured its complete stability even in highly corrosive sulfide solutions (e.g., 200 mM HS-). This was due to the increased electrochemically active surface area, improved coating conductivity and reduced charge transfer resistance. The (electro)catalytic oxidation of HS- demonstrated robust performance with very limited impact of different operational parameters (e.g., dissolved oxygen, anode potential, HS- concentration). Due to the formation of elemental sulfur (S0) layer at the anode surface at basic pH, longer term anode usage requires its periodic removal. Chemical dissolution of S0 with toluene allows its rapid removal without affecting the anode activity, and easy recrystallization and recovery of pure sulfur.

Electrochemical Oxidation of Organic Pollutants in Aqueous Solution Using a Ti4O7 Particle Anode

Anodes based on substoichiometric titanium oxide (Ti₄O₇) are among the most effective for the anodic oxidation of organic pollutants in aqueous solutions. Such electrodes can be made in the form of semipermeable porous structures called reactive electrochemical membranes (REMs). Recent work has shown that REMs with large pore sizes (0.5-2 mm) are highly efficient (comparable or superior to boron-doped diamond (BDD) anodes) and can be used to oxidize a wide range of contaminants. In this work, for the first time, a Ti₄O₇ particle anode (with a granule size of 1-3 mm and forming pores of 0.2-1 mm) was used for the oxidation of benzoic, maleic and oxalic acids and hydroquinone in aqueous solutions with an initial COD of 600 mg/L. The results demonstrated that a high instantaneous current efficiency (ICE) of about 40% and a high removal degree of more than 99% can be achieved. The Ti₄O₇ anode showed good stability after 108 operating hours at 36 mA/cm².

Treatment of wastewater from the washing process of a municipal solid waste collection container by electrochemical treatment using different anode materials: a statistical optimization

An underground municipal solid waste (MSW) container should be washed periodically to prevent/reduce odor and leachate production. In this study, the treatment process of wastewater derived from the washing process of an MSW container was investigated using the electrochemical (EC) treatment process with different anode materials (Fe, TiO₂, and graphite). Response surface methodology (RSM) was used to evaluate the effect of process parameters such as initial pH, applied current, and reaction time on chemical oxygen demand (COD), Tannin/Lignin, and color removals. According to the results obtained from the RSM models, all process parameters were significant. The optimum process parameters in terms of COD removal were derived from the models for each anode material. Under the optimized conditions, the COD removals were determined to be 93.25%, 75.95%, and 98.46% for Fe-Fe, TiO₂-Fe, and graphite-Fe electrode pairs, respectively. The color and Tannin/Lignin removals were determined as 98.12% and 77.78% for the Fe-Fe, 92.76% and 98.45% for the TiO₂-Fe and 94.50% and 79.56% for the graphite-Fe electrode pair, respectively. The specific energy consumption (SEC) values were found as 46.95, 300.02, and 32.95 kWh/m³ for each electrode combination given above, respectively. In terms of both removal efficiencies and SEC, the most effective anode material was determined as graphite, followed by iron.

Tetracycline removal from aqueous solution by electrooxidation using ruthenium-coated graphite anode

This paper reports the electrochemical oxidation treatment of 80 mL of acidic aqueous solutions with 0.2 mM of the drug tetracycline in 25 mM Na₂SO₄ using a lab-scale electrochemical cell. The performance of tetracycline removal with Ru-coated graphite by the chemical bath deposition (CBD) and raw graphite anode has been demonstrated. The effects of operating parameters were tested such as pH, applied current, supporting electrolyte concentration, and initial tetracycline concentration. The best tetracycline degradation was obtained with Ru-coated graphite anode due to its higher oxidation power, which allowed the complete degradation of refractory compounds. The modified surface structure of the Ru-coated graphite anode was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray (EDX). The EO process with Ru-coated graphite anode allowed 93.8% tetracycline abatement after 100 min of electrolysis at an applied current of 100 mA. In all cases, tetracycline decay obeyed pseudo-first-order kinetics. The tetracycline removal performance of graphite electrodes with nano coating on graphite has offered a performing alternative. A Comparative study revealed that electrolysis with Ru-coated graphite acted as a better electrode material than raw graphite for the catalytic reaction.

Differently Prepared PbO2/Graphitic Carbon Nitride Composites for Efficient Electrochemical Removal of Reactive Black 5 Dye

In this paper, electrochemical degradation of Reactive Black 5 (RB5) textile azo dye was examined in regard to different synthesis procedures for making PbO2–graphitic carbon nitride (g-C3N4) electrode. The reaction of PbOH3- with ClO− in the presence of different surfactants, i.e., cetyltrimethylammonium bromide (CTAB) and tetrabutylammonium phosphate (TBAP), under conventional conditions, resulted in the formation of PbO2 with varying morphology. The obtained materials were combined with g-C3N4 for the preparation of the final composite materials, which were then characterized morphologically and electrochemically. After optimizing the degradation method, it was shown that an anode comprising a steel electrode coated with the composite of PbO2 synthesized using CTAB as template and g-C3N4, and using 0.15 M Na2SO4 as the supporting electrolyte, gave the best performance for RB5 dye removal from a 35 mg/L solution. The treatment duration was 60 min, applying a current of 0.17 A (electrode surface 4 cm2, current density of 42.5 mA/cm2), while the initial pH of the testing solution was 2. The reusability and longevity of the electrode surface (which showed no significant change in activity throughout the study) may suggest that this approach is a promising candidate for wastewater treatment and pollutant removal.

Multivalent Effect of Defect Engineered Ag2S/g-C3N4 3D Porous Floating Catalyst with Enhanced Contaminant Removal Efficiency

Chlorophenols, as a major environmental pollutant, enter water systems through industrial wastewater, agricultural runoff and chemical spills, and they are stable, persistent under natural conditions, and highly hazardous to water resources. The objective of this article is to prepare Ag2S-modified C3N4 three-dimensional network photocatalyst by calcination method to use photocatalysis as an efficient, safe, and environmentally friendly method to degrade chlorophenols. Ag2S/C3N4 has an excellent visible light absorption range, low band gap, effective separation of photogenerated charges, and active free radicals production, all of which make for the enhancement of photocatalytic degradation performance of the Ag2S/C3N4 system. Under the light irradiation (λ ≥ 420 nm), the photocatalytic degradation efficiency of 2,4,6-Trichlorophenol reach 95% within 150 min, and the stable photocatalytic degradation activity can still be maintained under different pH water environment and four degradation cycles. When Ag2S is loaded on ACNs, more photogenerated electrons are generated and subsequent reactions produce highly reactive groups such as •O2- and •OH that will originally be able to continuously attack TCP molecules to degrade pollutants. Therefore, this study shows that the photocatalyst provides a novel research approach for realizing the application in the field of pollutant degradation.

Elaboration of Highly Modified Stainless Steel/Lead Dioxide Anodes for Enhanced Electrochemical Degradation of Ampicillin in Water

Lead dioxide-based electrodes have shown a great performance in the electrochemical treatment of organic wastewater. In the present study, modified PbO2 anodes supported on stainless steel (SS) with a titanium oxide interlayer such as SS/TiO2/PbO2 and SS/TiO2/PbO2-10% Boron (B) were prepared by the sol–gel spin-coating technique. The morphological and structural properties of the prepared electrodes were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that the SS/TiO2/PbO2-10% B anode led to a rougher active surface, larger specific surface area, and therefore stronger ability to generate powerful oxidizing agents. The electrochemical impedance spectroscopy (EIS) measurements showed that the modified PbO2 anodes displayed a lower charge transfer resistance Rct. The influence of the introduction of a TiO2 intermediate layer and the boron doping of a PbO2 active surface layer on the electrochemical degradation of ampicillin (AMP) antibiotic have been investigated by chemical oxygen demand measurements and HPLC analysis. Although HPLC analysis showed that the degradation process of AMP with SS/PbO2 was slightly faster than the modified PbO2 anodes, the results revealed that SS/TiO2/PbO2-10%B was the most efficient and economical anode toward the pollutant degradation due to its physico-chemical properties. At the end of the electrolysis, the chemical oxygen demand (COD), the average current efficiency (ACE) and the energy consumption (EC) reached, respectively, 69.23%, 60.30% and 0.056 kWh (g COD)−1, making SS/TiO2/PbO2-10%B a promising anode for the degradation of ampicillin antibiotic in aqueous solutions.

Optimized Mo-doped IrOx anode for efficient degradation of refractory sulfadiazine

Electrochemical advanced oxidation processes (EAOPs) is considered to be an efficacious method to degrade antibiotics. However, the performance of the anode has become the main limiting factor of this technology. In this study, due to the electron-deficient characteristics and the improvement of OER performance of Mo, we chose to use thermal decomposition to incorporate Mo into IrO2 to prepare anodes with industrial applicability. Under the optimal ratio of Ir to Mo is 7:3, (Ir0.7Mo0.3)Ox electrode's particular pore structure can expose more active sites and create a channel for the transportation of electrons, thereby promoting the formation of free radicals and degrading pollutants more efficiently. (Ir0.7Mo0.3)Ox electrode also has a higher mass activity (6.332 A g-1, three times that of the IrO2 electrode) and a larger electrochemical active area (ECSA, 375.43 cm2, seven times that of the IrO2 electrode). In addition, the optimal conditions of (Ir0.7Mo0.3)Ox electrode for degrading sulfadiazine(SDZ) were explored, which achieved a higher removal than traditional electrodes (90% removal within 4 h) when the Ti plate was the substrate. Through the intermediate products of SDZ degradation and related literatures, two possible degradation pathways of SDZ were speculated. This research provides a new type of anode catalyst for the degradation of sulfonamide antibiotics, which is possible for industrial application.

Effect of Persulfate Activation by Electrogenerated H2O2 and Anodic Oxidation on the Color Removal of Dye Solutions at Pt and BDD Anodes

In this study, tartrazine solutions were oxidized using innovative electrochemical advanced oxidation processes (EAOPs) that combined persulfate (PS) activation with electrogenerated H₂O₂, cathodic reduction and anodic oxidation at Pt and BDD anodes, and graphite cathode in an undivided stirred reactor. For the Pt anode, SO₄·^- was generated from a reduction reaction at the cathode and a reaction between the PS and electrogenerated H₂O₂. For the BDD anode, SO₄·^- was generated from a reduction reaction at the cathode, an oxidation reaction at the anode, and a reaction between PS and electrogenerated H₂O₂. Among these activation methods, the activation efficiency of PS by electrogenerated H₂O₂ is much better than other methods. The effects of PS concentration up to 36 mM, applied current density between 6 to 15 mA cm^-2, and temperatures between 25 to 45 °C were investigated. For the electro-Fenton process with Pt anode (Pt-H₂O₂/PS process), the best result for oxidizing 250 mg L^-1 tartrazine solution was obtained with 37.5 mM Na₂SO₄ + 9.0 mM Na₂S₂O₈, applied current density at 12 mA cm^-2 and 45 °C, acquiring total color removal after 30 min reaction. For the electro-Fenton process with BDD anode (BDD-H₂O₂/PS process), the best result for oxidizing 250 mg L^-1 tartrazine solution was obtained with 25 mM Na₂SO₄ + 18 mM Na₂S₂O₈, applied current density at 12 mA cm^-2 and 45 °C, yielding 100% color removal after 30 min reaction. The main oxidizing agents are SO₄·^- and OH· in the anodic oxidation process with PS and the electro-Fenton process with PS. It is concluded that the additions of PS tremendously improve the oxidation power of electro-Fenton processes with PS, especially the Pt-H₂O₂/PS process.

Hydrothermal Synthesis of MnO2/Reduced Graphene Oxide Composite for 4-Nitrophenol Sensing Applications

Recently, the electrochemical sensing approach has attracted materials/electrochemical scientists to design and develop electrode materials for the construction of electrochemical sensors for the detection of para-nitrophenol (4-NP). In the present study, we have prepared a hybrid composite of MnO2 and rGO (MnO2/rGO) using a hydrothermal approach. The morphological features of the prepared MnO2/rGO composite were studied by scanning electron microscopy, whereas the phase purity and formation of the MnO2/rGO composite were authenticated via the powder X-ray diffraction method. Energy-dispersive X-ray spectroscopy was also employed to analyze the elemental composition of the prepared MnO2/rGO composite. In further studies, a glassy carbon electrode (GCE) was modified with MnO2/rGO composite (MnO2/rGO/GCE) and explored as 4-nitrophenol (4-NP) sensor. The fabricated MnO2/rGO/GCE exhibited a reasonably good limit of detection of 0.09 µM with a sensitivity of 0.657 µA/µMcm2. The MnO2/rGO/GCE also demonstrates good selectivity, stability and repeatability in 50 cycles.

Decontamination of wastewater containing contaminants of emerging concern by electrooxidation and Fenton-based processes - A review on the relevance of materials and methods

In recent years, there has been an increasingly growing interest regarding the use of electrochemical advanced oxidation processes (EAOPs) which are considered highly promising alternative treatment techniques for addressing environmental issues related to pollutants of emerging concern. In EAOPs, electrogenerated oxidizing agents, such as hydroxyl radical (HO^•), can react non-selectively with a wide range of organic compounds, degrading and mineralizing their structures to unharmful molecules like CO₂, H₂O, and inorganic ions. To this date, a broad spectrum of advanced electrocatalysts have been developed and applied for the treatment of compounds of interest in different matrices, specifically aiming at enhancing the degradation performance. New combined methods have also been employed as alternative treatment techniques targeted at circumventing the major obstacles encountered in Fenton-based processes, such as high costs and energy consumption, which still contribute significantly toward inhibiting the large-scale application of these processes. First, some fundamental aspects of EAOPs will be presented. Further, we will provide an overview of electrode materials which have been recently developed and reported in the literature, highlighting different anode and cathode structures employed in EAOPs, their main advantages and disadvantages, as well as their contribution to the performance of the treatment processes. The influence of operating parameters, such as initial concentrations, pH effect, temperature, supporting electrolyte, and radiation source, on the treatment processes were also studied. Finally, hybrid techniques which have been reported in the literature and critically assess the most recent techniques used for evaluating the degradation efficiency of the treatment processes.

Evaluation of anode materials in sonoelectrochemistry processes: Kinetic, mechanism, and cost estimation

Eco-friendly pollutant treatment technology has a developing tendency in future. The combination of ultrasound (US) and electrochemical (EC) is a promising technology, because they are efficient, clean and environmentally friendly. In this study, the impacts of anode material have been investigated in US (300 kHz) and EC (10V) system. The results of all systems revealed that the kinetic constant decreased with increasing pH. The results are also shown that ΔG^# > 0 and ΔH^# > 0 during PCP degradation in EC or US-EC systems are non-spontaneous and endothermic reactions. Meanwhile, in the US-EC system, TiO₂, Ti₄O₇, PbO₂, SnSb, RuIr, and BDD, except for TiO₂, all the anode materials showed a synergistic index (SI) of 106-197%, and the activation energies were 19.32, 33.4, 33.74, 32.84, 10.41, 36.44 kJ mol^-1, respectively. In EC and US-EC systems, PCP can be completely mineralized by BDD anode within 30 min. TBA scavenger experiments verified that hydroxyl radicals were the main oxidant in each system using BDD and PbO₂ anode. As a result of estimating the cost according to the anode material when removing PCP using the EC or US-EC system, BDD was the smallest in the two systems, 1.58 and 1.12 $ m^-3, respectively. Finally, this study may serve as a reference for implementation of US-EC system in wastewater treatment.

Electrochemical oxidation process on palm oil mill effluent waste activated sludge: optimization by response surface methodology

Biological-based treatment as the conventional treatment for palm oil mill effluent (POME) in open-ponding system face a well known rate-limiting step which is hydrolysis. In this study, electrochemical oxidation (EO) by a ruthenium oxide-coated titanium (Ti/RuO₂) electrode was introduced as a pre-treatment for POME waste activated sludge (WAS). Surface morphology and elemental analysis were investigated using field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Response surface methodology type central composite design was used in this study to understand the relationship between the independent and dependent variables. Analysis of variance (ANOVA) was used to validate the model of the studied variables. The correlation coefficients (R²) indicated a close agreement between the experimental results and the predicted values, with high R² values of 0.9044-0.9773. Multiple response optimization suggested that the range of current density (17-27 mA/cm²) and electrolysis time (55-75 min) at a fixed concentration of sodium chloride (10 g/L), resulted in mixed liquor volatile suspended solids (MLVSS) removal >20%, capillary suction timer (CST) reduction >43%, extracellular polymeric substances (EPS) increment <19% and soluble chemical oxygen demand (sCOD) increment >25%. EO appears to be an efficient pre-treatment as well as practical way to improve the POME WAS disintegration and dewaterability.

Electrochemical oxidation of landfill leachate using boron-doped diamond anodes: pollution degradation rate, energy efficiency and toxicity assessment

Electrochemical oxidation (EO), due to high efficiency and small carbon footprint, is regarded as an attractive option for on-site treatment of highly contaminated wastewater. This work shows the effectiveness of EO using three boron-doped diamond electrodes (BDDs) in sustainable management of landfill leachate (LL). The effect of the applied current density (25-100 mA cm^-2) and boron doping concentration (B/C ratio: 500 ppm, 10,000 ppm and 15,000 ppm) on the performance of EO was investigated. It was found that, of the electrodes used, the one most effective at COD, BOD₂₀ and ammonia removal (97.1%, 98.8% and 62%, respectively) was the electrode with the lowest boron doping. Then, to better elucidate the ecological role of LLs, before and after EO, cultivation of faecal bacteria and microscopic analysis of total (prokaryotic) cell number, together with ecotoxicity assay (Daphnia magna, Thamnocephalus platyurus and Artemia salina) were combined for the two better-performing electrodes. The EO process was very effective at bacterial cell inactivation using each of the two anodes, even within 2 h of contact time. In a complex matrix of LLs, this is probably a combined effect of electrogenerated oxidants (hydroxyl radicals, active chlorine and sulphate radicals), which may penetrate into the bacterial cells and/or react with cellular components. The toxicity of EO-treated LLs proved to be lower than that of raw ones. Since toxicity drops with increased boron doping, it is believed that appropriate electrolysis parameters can diminish the toxicity effect without compromising the nutrient-removal and disinfection capability, although salinity of LLs and related multistep-oxidation pathways needs to be further elucidated.

Rust triggers rapid reduction of Cr6+ by red phosphorus: The importance of electronic transfer medium of Fe3

Red phosphorus (P) is one of the metalloid materials, with five external electrons, it should be an excellent electron donor. However, the activity of red P to reduce Cr⁶⁺ is limited. Due to electrostatic repulsion, it is difficult for the electrons on the red P to transfer to the chromate anion (Cr⁶⁺). Interestingly, we found that Fe³⁺ derived from rust, waste iron or Fe³⁺ reagents can be used as the electron transport medium to solve the electron transport obstacles between red P and Cr⁶⁺. As a result, the reduction of Cr⁶⁺ by red P/rust system takes only 20 min, which is far lower than the 140 min of red P. The reduction rate of Cr⁶⁺ in the red P/rust system is about 12.3 times that of red P. The reaction mechanism is that red P is not easy to access chromate anions but can easily adsorb Fe³⁺. The adsorbed Fe³⁺ will be reduced to Fe²⁺ by red P, and the regenerated Fe²⁺ will diffuse into the solution to rapidly reduce Cr⁶⁺. Therefore, this work provides an alternative waste iron reuse pathway and also sheds light on the important role of electron medium in reduction reaction.

Robust and Self-Cleaning Electrochemical Production of Periodate

Periodate, a platform oxidizer, can be electrochemically recycled in a self-cleaning process. Electrosynthesis of periodate is well established at boron-doped diamond (BDD) anodes. However, recovered iodate and other iodo species for recycling can contain traces of organic impurities from previous applications. For the first time, it was shown that the organic impurities do not hamper the electrochemical re-oxidation of used periodate. In a hydroxyl-mediated environment, the organic compounds form CO2 and H2 O during the degradation process. This process is often referred to as "cold combustion" and provides orthogonal conditions to periodate synthesis. To demonstrate the strategy, different dyes, pharmaceutically active ingredients, and iodine compounds were added as model contaminations into the process of electrochemical periodate production. UV/Vis spectroscopy, NMR spectroscopy, and mass spectrometry (MS) were used to monitor the degradation of organic molecules, and liquid chromatography-MS was used to control the purity of periodate. As a representative example, dimethyl 5-iodoisophthalate (2 mm), was degraded in 90, 95, and 99 % while generating 0.042, 0.054, and 0.082 kilo equiv. of periodate, respectively. In addition, various organic iodo compounds could be fed into the periodate generation for upcycling such iodo-containing waste, for example, contrast media.

Bibliometric analysis of global research progress on electrochemical degradation of organic pollutants

As a result of anthropogenic action, an increasing amount of toxic organic compounds has been released into the environment. These pollutants have adverse effects on human health and wildlife, which has motivated the development of different types of technologies for the treatment of effluents and contaminated environments. The electrochemical degradation of organic pollutants has attracted the interest of research centers around the world for its environmental compatibility, high efficiency, and affordable cost. In the present study, a bibliometric analysis was performed using the Web of Science database in order to assess the progress of publications related to electrochemical degradation of organic pollutants between the years 2001 and 2021. The data retrieved showed a significant increase in publications related to the topic in the last 20 years. Electrochimica Acta was the magazine responsible for the largest number of publications (291, 6.52%). The studies mainly included the areas of engineering, chemistry, and environmental science ecology. China with a total of 1472 (32.96%) publications dominated research in this area, followed by Spain (436, 9.76%) and Brazil (345, 7.72%). The institutions with the highest number of contributions were the University of Barcelona and the Chinese Academy of Sciences, and the most productive authors were Brillas E. and Oturan M. A. The results of this study provide important references and information on possible research directions for future investigations on electrochemical degradation of organic pollutants.

Wastewater Treatment Using a Photoelectrochemical Oxidation Process for the Coffee Processing Industry Optimization of Chemical Oxygen Demand (COD) Removal Using Response Surface Methodology

The elimination of organic compounds in coffee processing effluent utilizing electrochemical oxidation (ECO) as well as a combination of electrochemical oxidation (ECO) and ultraviolet and hydrogen peroxide (UV/H2O2) was explored. Then, the percentage reduction of chemical oxygen demand (COD) was investigated. The effect of different experimental factors such as solution pH, sodium chloride (NaCl) concentration, calcium chloride (CaCl2) concentration, electric current, electrolysis duration, and hydrogen peroxide dosage on the percent removal efficiency of the hybrid electrochemical oxidation (ECO) with the ultraviolet and hydrogen peroxide (UV/H2O2) process has been investigated. The response surface methodology (RSM) based on central composite design (CCD) was used to organize the trial runs and optimize the results. The hybrid electrochemical oxidation (ECO) with the ultraviolet and hydrogen peroxide (UV/H2O2) process removed 99.61% of the chemical oxygen demand (COD) with a low power usage of 1.12 kWh/m3 compared to the other procedures, according to the experimental data analysis. These findings were obtained with a pH of 7, a current of 0.40 A, 1.5 g of CaCl2, and a total electrolysis period of 40 minutes. When it came to eliminating organic compounds from coffee manufacturing effluent, CaCl2 outperformed NaCl. Analysis of variance (ANOVA) with 95% confidence limits was used to examine the significance of independent variables and their interactions.

Inactivation behavior and intracellular changes in Escherichia coli during electro-oxidation process using Ti/Sb-SnO2/PbO2 anode: Elucidation of the disinfection mechanism

This study investigates the behavior and intracellular changes in Escherichia coli (model organism) during electro-oxidation with Ti/Sb-SnO₂/PbO₂ anode in a chlorine free electrochemical system. Preliminary studies were conducted to understand the effect of initial E. coli concentration and applied current density on disinfection. At an applied current density 30 mA cm^-2, 7 log reduction of E. coli was achieved in 75 min. The role of reactive oxygen species' (ROS) in E.coli disinfection was evaluated, which confirmed hydroxyl (•OH) radical as the predominant ROS in electro-oxidation. Observations were carried out at cell and molecular level to understand E.coli inactivation mechanism. Scanning electron microscopy images confirmed oxidative damage of the cell wall and irreversible cell death. Intracellular and extracellular protein quantification and genetic material release further confirmed cell component leakage due to cell wall rupture and degradation due to •OH radical interaction. Change in cell membrane potential suggests the colloidal nature of E. coli cells under applied current density. Plasmid deoxyribonucleic acid degradation study confirmed fragmentation and degradation of released genetic material. Overall, effective disinfection could be achieved by electro-oxidation, which ensures effective inactivation and prevents regrowth of E. coli. Disinfection of real wastewater was achieved in 12 min at an applied current density 30 mA cm^-2. Real wastewater study further confirmed that effective disinfection is possible with a low cost electrode material such as Ti/Sb-SnO₂/PbO₂. Energy consumed during disinfection was determined to be 4.978 kWh m^-3 for real wastewater disinfection at applied current density 30 mA cm^-2. Cost of operation was estimated and stability of the electrode was studied to evaluate the feasibility of large scale operation. Relatively low energy and less disinfection time makes this technology suitable for field scale applications.

Efficient electrochemical removal of 5-fluorouracil pharmaceutical from wastewater by mixed metal oxides via anodic oxidation process

Nowadays, the entry of organic compounds into water resources is one of the leading global concerns due to the lack of water resources and rapid population growth. In this research, anodic oxidation (AO) method was used to remove 5-fluorouracil (5-FU) from aqueous solutions via Ni/RuO₂ and Ti/IrO₂-TiO₂-RuO₂ electrodes as cathode and anode, respectively. For this purpose, the characterization analysis of the electrodes, including X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, and atomic force microscopy were performed. The electrochemical performance of the anode was investigated via cyclic voltammetry analysis. Then, the effect of operational variables, including applied current (mA), initial pH of the solution, initial 5-FU concentration (mg/L), and process time (min) on the 5-FU removal efficiency under the AO process was evaluated via artificial neural network (ANN) modeling. The results revealed that the maximum 5-FU removal efficiency was 96.96%. The applied current intensity, pH, initial 5-FU concentration, and process time were 300 mA, 5, 20 mg/L, and 140 min, respectively. Moreover, the investigation of 5-FU removal by-products and mineralization efficiency of the AO process was carried out via gas chromatography-mass spectrometry and total organic carbon analysis, respectively. The total organic carbon mineralization efficiency was 84.80% after 6 h of reaction time. The reusability and stability of the Ti/IrO₂-TiO₂-RuO₂ anode on 5-FU removal efficiency were measured and showed an approximately 5% decay in 5-FU removal efficiency after eight consecutive runs. The overall results and analysis confirmed this method is capable of removing 5-FU through Ti/IrO₂-TiO₂-RuO₂ anode and Ni/RuO₂ cathode from aqueous medium.

Green Biosynthesis of Tin Oxide Nanomaterials Mediated by Agro-Waste Cotton Boll Peel Extracts for the Remediation of Environmental Pollutant Dyes

The sustainable synthesis of metal oxide materials provides an ecofriendly and more exciting approach in the domain of a clean environment. Besides, plant extracts to synthesize nanoparticles have been considered one of the more superior ecofriendly methods. This paper describes the biosynthetic preparation route of three different sizes of tetragonal structure SnO₂ nanoparticles (SNPs) from the agro-waste cotton boll peel aqueous extract at 200, 500, and 800 °C for 3 h and represents a low-cost and alternative preparation method. The samples were characterized by X-ray diffraction, Fourier transform infrared spectrophotometry, ultraviolet-visible absorption spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy. Surface area and porosity size distribution were identified by nitrogen adsorption-desorption isotherms and Brunauer-Emmett-Teller analysis. The photocatalytic properties of the SNP samples were studied against methylene blue (MB) and methyl orange (MO), and the degradation was evaluated with three different size nanomaterials of 3.97, 8.48, and 13.43 nm. Photocatalytic activities were carried out under a multilamp (125 W Hg lamps) photoreactor. The smallest size sample exhibited the highest MB degradation efficiency within 30 min than the most significant size sample, which lasted 80 min. Similarly, in the case of MO, the smallest sample showed a more superior degradation efficiency with a shorter period (40 min) than the large-size samples (100 min). Therefore, our studies suggested that the developed SNP nanomaterials could be potential, promising photocatalysts against the degradation of industrial effluents.

Photocatalytic Recovery of Gold from a Non-Cyanide Gold Plating Solution as Au Nanoparticle-Decorated Semiconductors

In this work, a photocatalytic process was carried out to recover gold (Au) from the simulated non-cyanide plating bath solution. Effects of semiconductor types (TiO₂, WO₃, Nb₂O₃, CeO₂, and Bi₂O₃), initial pH of the solution (3-10), and type of complexing agents (Na₂S₂O₃ and Na₂SO₃) and their concentrations (1-4 mM each) on Au recovery were explored. Among all employed semiconductors, TiO₂ exhibited the highest photocatalytic activity to recover Au from the simulated spent plating bath solution both in the absence and presence of complexing agents, in which Au was completely recovered within 15 min at a pH of 6.5. The presence of complexing agents remarkably affected the size of deposited Au on the TiO₂ surface, the localized surface plasmon effect (LSPR) behavior, and the valence band (VB) edge position of the obtained Au/TiO₂, without a significant change in the textural properties or the band gap energy. The photocatalytic activity of the obtained Au/TiO₂ tested via two photocatalytic processes depended on the common reduction mechanism rather than the textural or optical properties. As a result, the Au/TiO₂ NPs obtained from the proposed recovery process are recommended for use as a photocatalyst for the reactions occurring at the conduction band rather than at the valence band. Notably, they exhibited good stability after the fifth photocatalytic cycle for Au recovery from the actual cyanide plating bath solution.