Degradation of oxamic acid using dimensionally stable anodes (DSA) based on a mixture of RuO2 and IrO2 nanoparticles

Efficient, electrochemical degradation of organic pollutants via nanofibrous Pt/Ir-RuO2 electrode with enhanced stability

A diverse range of surfactants and chelating agents are frequently used in industrial processes, especially in the decontamination of nuclear facilities for decommissioning. To treat and degrade these organic pollutants, electrooxidation (EO) has emerged as a cost-effective method. Along these lines, in this work, a nanofibrous electrode was constructed to facilitate efficient EO. Among various metal oxide for EO, RuO2 is known for its excellent electrochemical activity, which was fabricated into a nanofiber structure with a large specific surface area and doped with IrO2 to increase its stability. In addition to the use of nanofibrous Ir-RuO2, a Pt intermediate layer was incorporated to increase both structural stability and electrical conductivity. The nanofibrous electrode with a Ru:Ir composition of 9:1 showed an electrochemical activation area 1.7 times larger and a charge transfer resistance 4.7 times smaller than a flat-type electrode with the same composition. Efficient degradation (99%) of organic pollutants (sodium dodecyl benzene sulfonate, Triton X-100, ethylenediaminetetraacetic acid, and nitrilotriacetic acid (NTA)) was successfully performed using the nanofibrous electrode. Effective decomposition of radioactive waste (NTA combined with Co ions) with 99% degradation within 4 h was achieved.

Mathematical modeling and kinetics of batch and continuous electro-catalytic oxidation of pharmaceutical-contaminated wastewater

An accurate yet simple model is the key to the design and control of intricate electro-catalytic oxidation of pharmaceutical contaminated wastewater. For both batch and unsteady-state continuous flow stirred tank reactors (CSTR), batch reactor models have been used earlier. Further, these models do not correlate rate to the operating conditions, and consider pseudo-first/second-order kinetics. Here, first-principles models are proposed by formulating unsteady-state mass balances, modifying them to attain realistic final conditions, and incorporating fractional variable-order kinetics. Following integral analysis, analytical solutions are obtained. These are independently applicable to design, unlike a numerical solution. Nonlinear regression is performed to estimate the model parameters from the transient experimental data. The simulations yield markedly accurate model parameters together with a better fit to the experimental data of Ti/RuO2-mediated amoxicillin-trihydrate electro-oxidation, for CSTR and batch reactors. For the batch reactor, the operating conditions are varied one at a time. Their effects on the model parameters are elucidated based on the oxidant and transformation species formed. The computed optimum model parameters are: rate constant 3.318 × 10-3 mg-0.092 m1.276 min-1, order 1.092, initial rate 4.032 × 102 mg m-2 min-1, and final conversion 90.6% in 180 min. The corresponding operating conditions are: pH 2.0, feed 50 mg L-1, electrolyte 2 g L-1, and current 1 A. A simple generalized power-law correlation, associating rate to the operating conditions, is then estimated. Statistical analysis of these models using central composite design delivers R2 0.99, predicted R2 0.96, and optimum set close to the above. The corresponding sensitivity analysis and generalized correlation, both show applied current to be the most significant operating condition. The dynamic modeling approaches proposed here can be extended to model, control, and scale-up complex reaction systems.

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.

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.

Carbon Dioxide Anion Radicals Assisted Highly Efficient Photocatalytic H2O2 Production over Bi(C2O4)OH

Carbon dioxide anion radical (CO2•-) can act as a versatile single electron reductant, but its generation pathways are quite limited. Herein, we demonstrate that oxalic acid (OA) could be effectively and continuously utilized to produce CO2•- over Bi(C2O4)OH, a novel photocatalyst, under light irradiation. Bi(C2O4)OH would proceed with self-redox reactions under the light irradiation producing CO2•-, through the oxidation of C2O42-. OA in the solution could recoordinate with Bi3+, thus maintaining the structure of the photocatalysts and the stability of the reactions. Benefiting from the fast reaction between CO2•- and O2 in forming •O2-, hydrogen peroxide (H2O2) would be efficiently produced (219.0 μmol/h). This study proposes a novel approach for harnessing OA containing wastewater and explores its potential application in the efficient production of H2O2.

Preparation and characterization of PMT-TiO2-NTs@NiO-C/Sn-Sb composite electrodes by a two-step pulsed electrodeposition method for the degradation of crystalline violet

To overcome the limitations imposed by Sn-Sb electrodes, the titanium foam (PMT)-TiO₂-NTs@NiO-C/Sn-Sb composite electrodes with cubic crystal structure are synthesized by introducing NiO@C nanosheet arrays interlayer on the TiO₂-NTs/PMT matrix through hydrothermal and carbonization process. Then a two-step pulsed electrodeposition method is used to prepare the Sn-Sb coating. Benefiting from the advantages of stacked 2D layer-sheet structure, the obtained electrodes exhibit enhanced stability and conductivity. Synergy of inner and outer layers fabricated by different pulse times strongly influence the electrochemical catalytic properties of the PMT-TiO₂-NTs@NiO-C/Sn-Sb (Sn-Sb) electrode. Hence, the Sn-Sb (b0.5 h + w1 h) electrode is the optimal electrode to degrade the Crystalline Violet (CV). Next, the effect of the four experimental parameters (initial CV concentration, current density, pH value and supporting electrolyte concentration) on the degradation of CV by the electrode are investigated. The degradation of the CV is more sensitive to alkaline pH, and the rapid decolorization of CV when the pH is 10. Moreover, the possible electrocatalytic degradation pathway of CV is performed using HPLC-MS. Results from the tests show that the PMT-TiO₂-NTs/NiO@C/Sn-Sb (b0.5 h + w1 h) electrode is an interesting alternative material in industrial wastewater applications.

Investigation on the adverse impacts of electrochemically produced ClOx- on assessing the treatment performance of dimensionally stable anode (DSA) for Cl--containing wastewater

The presence of chloride ions can facilitate the COD removal efficiency due to the involvement of active chlorine species in the electro-oxidation process, but few attentions have been paid to the negative effect of the electro-generated oxychlorides on electro-oxidation performance. In this study, the effects of oxychlorides were investigated as functions of current density and phenol concentration using DSA anodes in terms of the evaluation of the COD removal performance and the biological toxicity. The results show that oxychlorides formed in the electro-oxidation system could result in the over-evaluation of the COD removal performance. Increasing current density (15-50 mA cm^-2) aggravated the over-evaluation of COD removal (4%-18%), owing to the enhancement in the electrochemical generation of oxychlorides. The increase of phenol concentration inhibited the production of oxychlorides, but the effect of oxychlorides on COD values at phenol concentration of 200 mg L^-1 (82 mg L^-1) was higher than that at 100 mg L^-1 (51 mg L^-1). The ClO₃^- was predominantly responsible for over-evaluation of the COD removal. In addition, bioassays with chlorella indicated that the electro-generated oxychlorides significantly increased the biological toxicity of the treated Cl^--containing wastewater. This work provides new guidance for the correct evaluation of COD treatment performance and highlight the importance of minimizing toxic inorganic chlorinated byproducts during electro-oxidation of Cl^--containing wastewater.

Complexation modelling and oxidation mechanism of organic pollutants in cotton pulp black liquor during iron salt precipitation and electrochemical treatment

Removal behavior of organic pollutants such as lignin in cotton pulp black liquor (CPBL) was investigated in precipitation followed by electrochemical oxidation (EO) using FeCl₃, Fe₂(SO₄)₃, FeCl₂ and FeSO₄ as precipitants, electrolyte and catalysts. Based on comparison of precipitation efficacy of iron salts, spectroscopic techniques, thermodynamic equilibrium calculations and molecular dynamics (MD) simulations were used to provide insight into the interaction between iron cations and lignin. The results showed that FeCl₃ achieved the highest removal of chemical oxygen demand (COD, 76.05%), UV₂₅₄ (69.21%) and lignin (78.28%). Iron cationic complexation with lignin was identified as the key mechanism in precipitation. Fe³⁺ was more active in binding to organic ligands mainly due to charge effect compared to Fe²⁺. The strong Fe-sulphate coordination affected the complexation with lignin. MD simulations showed the formation of inner sphere complexes of iron cations with deprotonated carboxyl and hydroxyl groups via bidentate and monodentate coordination. The removal efficiency of electrochemical oxidation (EO) as a post-treatment of the precipitation was dependent on iron salts. Removals of COD, UV₂₅₄ and color can achieve 98.88%, 98.9% and 99.97% by FeCl₃ precipitation and EO processes. The effluent reached the primary discharge standard specified in Integrated Wastewater Discharge Standard of China (GB8978-1996). FeCl₃ demonstrated significant advantages in the removal of organic pollutants from cotton pulp black liquor in the combined process of precipitation and electrochemical treatment and may have practical application potential.

Electrochemical Reduction and Oxidation of Chlorinated Aromatic Compounds Enhanced by the Fe-ZSM-5 Catalyst: Kinetics and Mechanisms

Devising cost-effective electrochemical catalyst system for the efficient degradation of chlorinated aromatic compounds is urgently needed for environmental pollution control. Herein, a Fe-ZSM-5 zeolite was used as a suspended catalyst to facilitate the degradation of lindane as a model chlorinated pesticide in an electrochemical system consisting of the commercial DSA (Ti/RuO₂-IrO₂) anode and graphite cathode. It was found that the Fe-ZSM-5 zeolite greatly accelerated the degradation of lindane, with the degradation rate constant more than 8 times higher than that without Fe-ZSM-5. In addition, the Fe-ZSM-5 zeolite widened the working pH range from 3 to 11, while efficient degradation of lindane in the absence of Fe-ZSM-5 was only obtained at pH ≤ 5. The degradation of lindane was primarily due to reductive dechlorination mediated by atomic H* followed by ^•OH oxidation. Fe-ZSM-5 zeolite could enrich lindane, H*, and ^•OH on its surface, thus provided a suitable local environment for lindane degradation. The Fe-ZSM-5 zeolite exhibited high stability and reusability, and reduced the energy consumption. This research provides a potential reduction-oxidation strategy for removing organochlorine compounds through a cost-efficient Fe-ZSM-5 catalytic electrochemical system.

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.

Electrocatalytic oxidation of low concentration cefotaxime sodium wastewater using Ti/SnO2-RuO2 electrode: Feasibility analysis and degradation mechanism

In this research, Ti/SnO₂-RuO₂ stable anode was successfully prepared by thermal decomposition method, and low concentration cefotaxime sodium (CFX) was degraded by green and sustainable electrocatalytic oxidation technology. The electrocatalytic activity and stability of the Ti/SnO₂-RuO₂ coating electrode were studied according to the polarization curve of oxygen and chlorine evolution. The effects of current density, initial concentration, pH, electrolyte concentration, and other technological parameters on the degradation efficiency were discussed. Orthogonal experiment results indicated that when the current density was 25 mA cm^-2, concentration of electrolyte was 5 mM and the pH value was 7, the best CFX removal rate of 86.33% could be obtained. The degradation efficiency of electrocatalytic oxidation was discussed through electrochemical analysis. Fourier transform infrared spectroscopy was used to analyze the different inlet and outlet stages before and after the degradation of CFX, and the possible degradation process was discussed. Therefore, the electrocatalytic oxidation of Ti/SnO₂-RuO₂ electrode was a clean and efficient technology, which could be widely used in the treatment of CFX wastewater.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H₂O₂ production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m² g^-1), enhanced the mesopore/micropore volume distribution and increased the activity (E_1/2 = 0.609 V) and yield of H₂O₂ (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H₂O₂, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of β-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm^-2. Up to 14 degradation products were identified in the Cl^--containing medium.

Electrocatalysis degradation of coal tar wastewater using a novel hydrophobic benzalacetone modified lead dioxide electrode

Coal tar wastewater is hard to degrade by traditional methods because of its toxic pollutant constituents and high concentration of aromatic hydrocarbons, especially phenolic substances. A new type of hydrophobic benzacetone modified PbO₂ anode (BA-PbO₂ electrodes) was used for the electrocatalytic treatment of coal tar wastewater in a continuous cycle reactor. The surface morphology, structure, valences of elements, hydrophobicity, hydroxyl radical (·OH) produced capacity, electrochemical properties and stability of BA-PbO₂ electrodes were characterized by SEM (scanning electron microscopy), XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), contact angle, a fluorescence probe test, an electrochemical workstation and accelerated life test, respectively. The BA-PbO₂ electrodes exhibited a compact structure and finely dispersed crystallize size of 4.6 nm. The optimum degradation conditions of coal tar wastewater were as follows: current density of 90 mA cm^-2, electrode gap of 1 cm and temperature at 25 °C with flow velocity of 80 L h^-1. The chemical oxygen demand (COD) removal efficiency reached 92.39% after 240 min of degradation under the optimized conditions and the after-treatment COD value was 379.51 mg L^-1 which was lower than the centralized emission standard (less than 400 mg L^-1). These findings demonstrated the feasibility and efficiency of electrocatalytically degrading coal tar wastewater by BA-PbO₂ electrodes. The possible mechanism and pathway for phenol a specific pollutant in coal tar wastewater were investigated by quantum chemistry calculations (Multiwfn) and gas chromatography-mass spectrometry (GC-MS). The toxicity of each intermediate was predicted by the ECOSAR program.

Anodic Oxidation of Industrial Winery Wastewater Using Different Anodes

Winery wastewater represents the largest waste stream in the wine industry. This deals with the mineralization of the organic matter present in winery wastewater using anodic oxidation and two types of anodes—namely, a boron-doped diamond electrode (BDD) and two mixed metal oxides (MMO), one with the nominal composition Ti/Ru0.3Ti0.7O2 and the other with Ti/Ir0.45Ta0.55O2. To conduct the study, the variability of different quality parameters for winery wastewater from the Chilean industry was measured during eight months. A composite sample was treated using anodic oxidation without the addition of supporting electrolyte, and the experiments were conducted at the natural pH of the industrial wastewater. The results show that this effluent has a high content of organic matter (up to 3025 ± 19 mg/L of total organic carbon (TOC)), which depends on the time of the year and the level of wine production. With MMO electrodes, TOC decreased by 2.52% on average after 540 min, which may be attributed to the presence of intermediate species that could not be mineralized. However, when using a BDD electrode, 85% mineralization was achieved due to the higher generation of hydroxyl radicals. The electrolyzed sample contained oxamic, acetic, and propionic acid as well as different ions such as sulfate, chloride, nitrate, and phosphate. These ions can contribute to the formation of different species such as active species of chlorine, persulfate, and perphosphate, which can improve the oxidative power of the system.

Electrocatalytic Degradation of Levofloxacin, a Typical Antibiotic in Hospital Wastewater

Presently, in the context of the novel coronavirus pneumonia epidemic, several antibiotics are overused in hospitals, causing heavy pressure on the hospital’s wastewater treatment process. Therefore, developing stable, safe, and efficient hospital wastewater treatment equipment is crucial. Herein, a bench-scale electrooxidation equipment for hospital wastewater was used to evaluate the removal effect of the main antibiotic levofloxacin (LVX) in hospital wastewater using response surface methodology (RSM). During the degradation process, the influence of the following five factors on total organic carbon (TOC) removal was discussed and the best reaction condition was obtained: current density, initial pH, flow rate, chloride ion concentration, and reaction time of 39.6 A/m², 6.5, 50 mL/min, 4‰, and 120 min, respectively. The TOC removal could reach 41% after a reaction time of 120 min, which was consistent with the result predicted by the response surface (40.48%). Moreover, the morphology and properties of the electrode were analyzed. The degradation pathway of LVX was analyzed using high-performance liquid chromatography–mass spectrometry (LC–MS). Subsequently, the bench-scale electrooxidation equipment was changed into onboard-scale electrooxidation equipment, and the onboard-scale equipment was promoted to several hospitals in Dalian.

Thermal decomposition based fabrication of dimensionally stable Ti/SnO2-RuO2 anode for highly efficient electrocatalytic degradation of alizarin cyanin green

In this work, Ti/SnO₂-RuO₂ dimensionally stable anode has been successfully fabricated via thermal decomposition method and further used for highly efficient electrocatalytic degradation of alizarin cyanin green (ACG) dye wastewater. The morphology, crystal structure and composition of Ti/SnO₂-RuO₂ electrode are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF), respectively. The result of accelerated life test suggests that as-prepared Ti/SnO₂-RuO₂ anode exhibits excellent electrochemical stability. Some parameters, such as reaction temperature, initial pH, electrode spacing and current density, have been investigated in detail to optimize the degradation condition of ACG. The results show that the decolorization efficiency and chemical oxygen demand removal efficiency of ACG reach up to 80.4% and 51.3% after only 40 min, respectively, under the optimal condition (reaction temperature 25 °C, pH 5, electrode spacing 1.0 cm and current density 3 mA cm^-2). Furthermore, the kinetics analysis reveals that the process of electrocatalytic degradation of ACG follows the law of quasi-first-order kinetics. The excellent electrochemical activity demonstrates that the Ti/SnO₂-RuO₂ electrode presents a favorable application prospect in the electrochemical treatment of anthraquinone dye wastewater.