New laser-based method for the synthesis of stable and active Ti/SnO2–Sb anodes

Scientometric analysis of electrocatalysis in wastewater treatment: today and tomorrow

Electrocatalytic methods are valuable tools for addressing water pollution and scarcity, offering effective pollutant removal and resource recovery. To investigate the current status and future trends of electrocatalysis in wastewater treatment, a detailed analysis of 9417 papers and 4061 patents was conducted using scientometric methods. China emerged as the leading contributor to publications, and collaborations between China and the USA have emerged as the most frequent partnerships. Primary article co-citation clusters focused on oxygen evolution reaction and electrochemical oxidation, transitioning towards advanced oxidation processes ("persulfate activation"), and electrocatalytic reduction processes ("nitrate reduction"). Bifunctional catalysts, theoretical calculations, electrocatalytic combination technologies, and emerging contaminants were identified as current research hotspots. Patent analysis revealed seven types of electrochemical technologies, which were compared using SWOT analysis, highlighting electrochemical oxidation as prominent. The technological evolution presented the pathway of electro-Fenton to combined electrocatalytic technologies with biochemical processes, and finally to coupling with electrocoagulation. Standardized evaluation systems, waste resource utilization, and energy conservation were important directions of innovation in electrocatalytic technologies. Overall, this study provided a reference for researchers to understand the framework of electrocatalysis in wastewater treatment and also shed light on potential avenues for further innovation in the field.

Characterization and electrochemical properties of TiO2-rNTs/SnO2-Sb/PbO2 electrodes for the mineralization of persistent organic pollutants using anodic oxidation coupled Electro-Fenton treatment: Effect of precursor selection

The present study compares the effect of using different solvents on the electrochemical properties of the reduced TiO2 nanotubes (TiO2-rNTs) layered Ti/TiO2-rNTs/SnO2-Sb/PbO2 anodes. The electrodes are prepared using three different solvent-based precursors: (i) isopropanol, (ii) ethylene glycol and citric acid (Pechini method), and (iii) 2-hydroxyethylammonium acetate (2HEAA) ionic liquid (IL) via the thermal decomposition route. The decomposition mechanism of precursor solutions was explored using the thermogravimetric (TGA) analysis. Further, the physicochemical properties of the electrodes are examined using Field emission Scanning Electron microscopy (FE-SEM), X-ray diffraction spectroscopy (XRD), and X-ray photoelectron emission spectroscopy (XPS). The results revealed that solvents with higher viscosity and slower decomposition rates support better film uniformity and higher stability of the electrode. The TiO2 -rNTs bottom layer and PbO2 top layer helped obtain higher film stability, increased working potential window (2.2 V vs. SHE) of the electrode, and the repeatability of the results. The performance of different electrodes based on the precursor solution is found as IL ≫ Pechini > Isopropanol. 4-chlorophenol (4-CP) is used as a model pollutant to test the performance of IL-Ti/TiO2-rNTs/SnO2-Sb/PbO2 anode in an anodic oxidation (AO) coupled electro-Fenton (EF) treatment. Further, the reliability of the electrode is evaluated by mineralizing other persistent organic pollutants (POPs) like tetracyclin, phenol, 2-chlorophenol (2-CP), and 2,4-dichlorophenol (2,4-DCP). Under the optimized conditions, the proposed system was able to mineralize the tetracyclin, phenol, 2-CP, 2,4-DCP, and 4-CP up to 78.91, 82.07, 74.96, 78.78, and 69.3 %, respectively. Moreover, the degradation mechanism of chlorophenols is proposed.

Electrochemically enhanced iron oxide-modified carbon cathode toward improved heterogeneous electro-Fenton reaction for the degradation of norfloxacin

In this work, different iron-based cathode materials were prepared using two different approaches: a novel one-step approach, which involved the incorporation of iron oxide with Printex® L6 carbon/PTFE (PL6C/PTFE) on bare carbon felt (CF) and a two-step approach, where iron oxide is deposited onto CF previously modified with PL6C/PTFE. The results obtained from the physical characterization indicated that the presence of iron oxide homogeneously dispersed on the felt fibers with the CF 3-D network kept intact in the one-step approach; whereas the formation of iron oxide aggregates between the felt fibers for material obtained using the two-step approach. Among the iron oxide-based cathodes investigated, the iron-incorporated electrode exhibited the greatest efficiency in terms of the removal and mineralization of norfloxacin (NOR) under neutral pH (complete NOR removal in less than 30 min with around 50% mineralization after 90 min). The findings of this study show that the low cost and simple-to-prepare iron-modified carbon-based materials in HEF process led to the enhanced degradation of organic contaminants in aqueous solutions.

Robust Self-Supported SnO2-Mn2O3@CC Electrode for Efficient Electrochemical Degradation of Cationic Blue X-GRRL Dye

Exploration of highly efficient and robust catalyst is pivotal for electrocatalytic degradation of dye wastewater, but it still is a challenge. Here, we develop a three-dimensional self-supported SnO₂-Mn₂O₃ hybrid nanosheets grown on carbon cloth (noted by SnO₂-Mn₂O₃@CC) electrode via a simple hydrothermal method and annealing treatment. Benefitting from the interlaced nanosheets architecture that enlarges the surface area and the synergetic component effect that accelerates the interfacial electronic transfer, SnO₂-Mn₂O₃@CC electrode exhibits a superior electrocatalytic degradation efficiency for cationic blue X-GRRL dye in comparison with the single metal oxide electrode containing SnO₂@CC and Mn₂O₃@CC. The degradation efficiency of cationic blue X-GRRL on SnO₂-Mn₂O₃@CC electrode can reach up to 97.55% within 50 min. Furthermore, self-supported architecture of nanosheets on carbon cloth framework contributes to a robust stability compared with the traditional electrode via the multiple dip/brush coating accompanied by the thermal decomposition method. SnO₂-Mn₂O₃@CC electrode exhibits excellent recyclability, which can still retain a degradation efficiency of 94.12% after six cycles. This work may provide a new pathway for the design and exploration of highly efficient and robust electrooxidation catalysts for dye degradation.

Electrochemical degradation of methylene blue by Pb modified porous SnO2 anode

A significant number of pollutants in wastewater can be electrocatalytically oxidized by SnO₂-Sb, a relatively inactive electrode. However, the arduous process of environmental remediation due to poor electrochemical performance and short service life of the traditional Ti/SnO₂-Sb electrode. In this work the SnO₂ electrode with a micron-sized sphere structure was prepared by in-situ hydrothermal. The results of the study that the electrode (Pb-10%) synthesized from the precursor solution in which the Pb:Sn molar ratio is 10% exhibits excellent electrooxidation properties. Impressiveing, the Pb-10% electrode displayed the small charge transfer resistance (10.71 Ω) and the high oxygen evolution potential (2.26 V vs. SCE). Thus, the electrochemical degradation experiment demonstrates that 100 mg L^-1 MB was degraded by Pb-10% electrode under the condition of initial pH = 5, and the decolorization rate reached 94.6%. Moreover, the influence of different parameters such as Pb doping amount, initial pH value of solution, initial concentration of MB and inorganic ions on degradation efficiency were also explored, in turn the practical application of electrodes in the field of purifying water resources is optimized. It is worth noting that the service life of the optimized electrode (100 mA cm^-2, 0.5 M H₂SO₄, 90 h) is about 12 times longer than that of the bare electrode (Sn-Sb). Therefore, the high-performance Ti/SnO₂-Sb electrode prepared in this work possesses vast application prospects in the electrocatalytic oxidation.

Research trends in the development of anodes for electrochemical oxidation of wastewater

The review focuses on the recent development in anode materials and their synthesis approach, focusing on their compatibility for treating actual industrial wastewater, improving selectivity, electrocatalytic activity, stability at higher concentration, and thereby reducing the mineralization cost for organic pollutant degradation. The advancement in sol–gel technique, including the Pechini method, is discussed in the first section. A separate discussion related to the selection of the electrodeposition method and its deciding parameters is also included. Furthermore, the effect of using advanced heating approaches, including microwave and laser deposition synthesis, is also discussed. Next, a separate discussion is provided on using different types of anode materials and their effect on active •OH radical generation, activity, and electrode stability in direct and indirect oxidation and future aspects. The effect of using different synthesis approaches, additives, and doping is discussed separately for each anode. Graphene, carbon nanotubes (CNTs), and metal doping enhance the number of active sites, electrochemical activity, and mineralization current efficiency (MCE) of the anode. While, microwave or laser heating approaches were proved to be an effective, cheaper, and fast alternative to conventional heating. The electrodeposition and nonaqueous solvent synthesis were convenient and environment-friendly techniques for conductive metallic and polymeric film deposition.

Enhanced HCB removal using bacteria from mangrove as post-treatment after electrochemical oxidation using a laser-prepared Ti/RuO2-IrO2-TiO2 anode

The environmental persistence of hexachlorobenzene (HCB) is a challenge that promotes studies for efficient treatment alternatives to minimize its environmental impact. Here, we evaluated the HCB removal by electrochemical, biological, and combined approaches. The electrochemical treatment of 4 μM HCB solutions was performed using a synthesized Ti/RuO₂-IrO₂-TiO₂ anode, while the biological treatment using mangrove-isolated bacteria was at 24, 48, and 72 h. The HCB degradability was assessed by analyzing chemical oxygen demand (COD), microbial growth capacity in media supplemented with HCB as the only carbon source, gas chromatography, and ecotoxicity assay after treatments. The synthesized anode showed a high voltammetric charge and catalytic activity, favoring the HCB biodegradability. All bacterial isolates exhibited the ability to metabolize HCB, especially Bacillus sp. and Micrococcus luteus. The HCB degradation efficiency of the combined electrochemical-biological treatment was evidenced by a high COD removal percentage, the non-HCB detection by gas chromatography, and a decrease in ecotoxicity tested with lettuce seeds. The combination of electrochemical pretreatment with microorganism degradation was efficient to remove HCB, thereby opening up prospects for in situ studies of areas contaminated by this recalcitrant compound.

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

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

Improved 4-nitrophenol removal at Ti/RuO2-Sb2O4-TiO2 laser-made anodes

In this study, binary and ternary mixed metal oxide anodes of Ti/RuO₂-Sb₂O₄ and Ti/RuO₂-Sb₂O₄-TiO₂ were prepared using two different heating methods: conventional furnace and alternative CO₂ laser heating. The produced anodes were physically and electrochemically characterized by using different techniques. The main difference found in the laser-made anodes was their more compact morphology, without the common deep cracks found in anodes made by typical thermal decomposition, which showed an important correlation with the prolonged accelerated service life. The correlation between the physicochemical properties of the anodes with their performance towards the 4-nitrophenol oxidations is discussed. The results demonstrated that the ternary anode (Ti/RuO₂-Sb₂O₄-TiO₂) is very promising, presenting a kinetic 5.7 times faster than the respective binary anode and the highest removal efficiency when compared with conventionally made anodes. Also, the lowest energy consumption per unit of mass of contaminant removed is seen for the laser-made Ti/RuO₂-Sb₂O₄-TiO₂ anode, which evidences the excellent cost-benefit of this anode material. Finally, some by-products were identified, and a degradation route is proposed. Graphical abstract.

Preparation and Recent Developments of Ti/SnO2-Sb Electrodes

Ti/SnO2-Sb electrode, which is one of the dimensionally stable anode (DSA) electrodes, offers high specific conductivity, excellent electrocatalytic performance, and great chemical stability. For these reasons, Ti/SnO2-Sb electrode has been extensively studied in the fields of wastewater treatment. This review covers essential research work about the advanced oxidation technology and related DSA electrodes. It gives an overview of preparation methods of SnO2 electrodes, including sol-gel method, dip-coating method, electrodeposition method, chemical vapor deposition method, thermal decomposition method, magnetron sputtering method, and hydrothermal method. To extend service life and improve electrocatalytic efficiency, the review provides comprehensive details about the modification technologies of Ti/SnO2-Sb electrode, such as doping modification, composite modification, and structural modification. In addition, the review discusses common problems in industrial applications of Ti/SnO2-Sb electrode and highlights the promising outlook of Ti/SnO2-Sb electrode.

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.

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

Dimensionally stable anodes (DSA) have been widely used to degrade organic compounds because these surfaces promote the electrogeneration of active chlorine species in the bulk of the solution, as well as in the vicinity of the anode when NaCl is used as supporting electrolyte. In this work, the nanoparticles synthesis of IrO₂ and RuO₂ was performed to obtain two types of DSA electrodes named Class I and II to degrade oxamic acid. For Class I and II DSA, the nanoparticles used were synthesized separately and in the same reaction medium, respectively. Electrolysis were carried out in an open cylindrical cell without division at 25 °C, DSAs were used as anodes and a stainless-steel electrode as cathode, both elements have a geometric area of 2.8 cm² immersed in 0.05 mol L^-1 of NaCl or Na₂SO₄ and a current density of 3 mA cm^-2 was applied for 6 h. Active chlorine species generated in the absence of oxamic acid in NaCl were also detected and quantified through ion chromatography. In Na₂SO₄ there was no degradation of the compound, but in NaCl the oxamic acid concentration reaching 85% with Class I DSA. The same tendency is observed in mineralization, in which Class I DSA allowed reaching a CO₂ transformation close to 73%. The difference in the results occurs because with Class I DSA, more hypochlorite is generated than with Class II and therefore there is a larger amount of oxidizing species in the solution that enables the degradation and mineralization of oxamic acid.