Remarkable improved electro-Fenton efficiency by electric-field-induced catalysis of CeO2

High-efficiency removal of As(iii) from groundwater using siderite as the iron source in the electrocoagulation process

Electrocoagulation technology, due to its simplicity and ease of operation, is often considered for treating arsenic-contaminated groundwater. However, challenges such as anode wear have hindered its development and application. This study aims to develop a siderite-filled anode electrocoagulation system for efficient removal of As(iii) and investigate its effectiveness. The impact of operational parameters on the removal rate of As(iii) was analyzed through single-factor tests, and the stability and superiority of the device were evaluated. The response surface methodology was employed to analyze the interactions between various factors and determine the optimal operational parameters by integrating data from these tests. Under conditions where the removal rate of As reached 99.3 ± 0.37%, with an initial concentration of As(iii) at 400 μg L-1, current intensity at 30 mA, initial solution pH value at 7, and Na2SO4 concentration at 10 mM. The flocculant used was subjected to characterization analysis to examine its structure, morphology, and elemental composition under these optimal operational parameters. The oxidation pathway for As(iii) within this system relies on integrated results from direct electrolysis as well as ˙O2 -, ˙OH, and Fe(iv) mediated oxidation processes. The elimination of arsenic encompasses two fundamental mechanisms: firstly, the direct adsorption of As(iii) by highly adsorbent flocculants like γ-FeOOH and magnetite (Fe3O4); secondly, the oxidation of As(iii) into As(v), followed by its reaction with siderite or other compounds to generate a dual coordination complex or iron arsenate, thus expediting its eradication. The anodic electrocoagulation system employing siderite as a filler exhibits remarkable efficiency and cost-effectiveness, while ensuring exceptional stability, thereby providing robust theoretical underpinnings for the application of electrocoagulation technology in arsenic removal.

Recent developments on advanced oxidation processes for degradation of pollutants from wastewater with focus on antibiotics and organic dyes

The existence of various pollutants in water environment contributes to global pollution and poses significant threats to humans, wildlife, and other living beings. The emergence of an effective, realistic, cost-effective, and environmentally acceptable technique to treat wastewater generated from different sectors is critical for reducing pollutant accumulation in the environment. The electrochemical advanced oxidation method is a productive technology for treating hazardous effluents because of its potential benefits such as lack of secondary pollutant and high oxidation efficiency. Recent researches on advanced oxidation processes (AOPs) in the period of 2018-2022 are highlighted in this paper. This review emphasizes on recent advances in electro-oxidation (EO), ozone oxidation, sonolysis, radiation, electro-Fenton (EF), photolysis and photocatalysis targeted at treating pharmaceuticals, dyes and pesticides polluted effluents. In the first half of the review, the concept of the AOPs are discussed briefly. Later, the influence of increasing current density, pH, electrode, electrolyte and initial concentration of effluents on degradation are discussed. Lastly, previously reported designs of electrochemical reactors, as well as data on intermediates generated and energy consumption during the electro oxidation and Fenton processes are discussed. According to the literature study, the electro-oxidation technique is more appropriate for organic compounds, whilst the electro-Fenton technique appear to be more appropriate for more complex molecules.

Piezo-photocatalytic properties of BaTiO3/CeO2 nanoparticles with heterogeneous structure synthesized by a gel-assisted hydrothermal method

BaTiO3/CeO2 nanoparticles with heterogeneous structure were successfully synthesized via a gel-assisted hydrothermal method. The molar ratio of Ti/Ce was set as 1 : 0, 0.925 : 0.075, 0.9 : 0.1; 0.875 : 0.125, and 0.85 : 0.15 in the dried gels. Affected by the values of Ti/Ce, the particle sizes of hydrothermal products decreased obviously, and the surface of nanoparticles became rough and even had small protrusions. XRD, SEM, HRTEM, XPS, DRS, ESR, and PFM were used to characterize the nanoparticle textures. We speculated that the main body and surface of nanoparticles were BaTiO3 and CeO2 protrusions, respectively. The catalytic performance of BaTiO3/CeO2 nanoparticles was characterized by their abilities to degrade RhB in water under different external conditions (light irradiation, ultrasonic oscillation, or both). In all test groups, BaTiO3/CeO2 nanoparticles with a Ti/Ce molar ratio of 0.875 : 0.125 in the initial dried gel exhibited the strongest catalytic ability when light irradiation and ultrasonication were applied simultaneously owing to the appropriate amount of Ce3+ and oxygen vacancies.

Degradation of dimethyl phthalate through Fe(II)/peroxymonosulphate heightened by fulvic acid: efficiency and possible mechanism

Ferrous iron (Fe(II)) reacts with peroxymonosulphate (PMS) to form active oxidants that can degrade refractory organic pollutants. However, the conversion rate of Fe(III) to Fe(II) is slow, which limits its actual application. In the study, the effect of fulvic acid (FA) on the degradation of dimethyl phthalate (DMP) by Fe(II)/PMS was investigated. Moreover, the degradation process of DMP was predicted by the preliminary identification of active free radicals and intermediates. As expected, FA gave rise to a higher concentration of Fe(II) than that in Fe(II)/PMS to enhance the removal of DMP in Fe(II)/PMS system. The precipitate, involved in FA and iron, was an important composite to promote the degradation of DMP in the system. Also, the response surface methodology (RSM) was applied to model and optimize the degradation conditions of DMP. The highest removal efficiency (85.70%) was obtained at pH = 3.86, [PMS] = 0.96 mM, [FA] = 11.44 mg/L and [DMP] = 5 µM. The results of free radical quenching experiments and EPR showed that •OH and SO₄^•- were the main active radicals in this system. The degradation intermediates of DMP were monomethyl phthalate (MMP), phthalic acid and benzoic acid. Discoveries of this study had raised the current understanding of the application of FA keeping the cycles of Fe(II)/Fe(III) for peroxymonosulphate activation, which could afford valuable information for the degradation of organic pollutants by FA/Fe(II)/PMS.

In-situ production of iron flocculation and reactive oxygen species by electrochemically decomposing siderite: An innovative Fe-EC route to remove trivalent arsenic

The removal of trivalent arsenic (As (III)) from water has received extensive attention from researchers. Iron electrocoagulation (Fe-EC) is an efficient technology for arsenic removal. However, electrode passivation hinders the development and application of Fe-EC. In this work, an innovative Fe-EC route was developed to remove As (III) through an electrochemical-siderite packed column (ESC). Ferrous ions were produced from siderite near the anode, and hydroxide was generated near the cathode during the electrochemical decomposition of siderite. As a result, an effect of Fe-EC-like was obtained. The results showed that an excellent removal performance of As (III) (>99%) was obtained by adjusting the parameters (As (III) concentration at 10 mg/L, pH at 7, Na₂SO₄ at 10 mM and the hydraulic retention time at 30 min) and the oxidation rate of As (III) reached 84.12%. The mechanism analysis indicated that As (III) was oxidized to As (Ⅴ) by the produced active oxide species and electrode, and then was removed by capturing on the iron oxide precipitates. As (III) was likely to be oxidized in two ways, one by the reactive oxygen species (possibly •OH, Fe(IV) and •O₂^- species), and another directly by the anode. The long-term effectiveness of arsenic removal demonstrated that ESC process based on the electrochemical-siderite packed column was an appropriate candidate for treating As (III) pollution.

Enhanced removal of humic acid from piggery digestate by combined microalgae and electric field

Microalgae technology is a promising method for treating piggery digestate, while its removal ability of humic acids (HAs) is poor. Here, an electric field-microalgae system (EFMS) was used to improve the removal of HAs from the piggery digestate. Results indicated that the removal of HAs by EFMS relied on the initial concentration of HAs, electrical intensity, the initial inoculation concentration of microalgae and pH. Values of these parameters were optimized as electrical intensity of 1.2 V/cm, microalgae initial inoculation concentration of 0.1 g/L and pH 5.0. The HAs removal efficiency by EFMS (55.38%) was 13% and 38% higher than that by single electric field and microalgal technology. It was observed that oxidation, coagulation and assimilation contributed to the removal of HAs, suggesting that EFMS could serve as an attractive and cost-effective technique for the removal of HAs from the piggery digestate.

Cu2O/CeO2 Photoelectrochemical Water Splitting: A Nanocomposite with an Efficient Interfacial Transmission Path under the Co-action of p-n Heterojunction and Micro-mesocrystals

Cu 2 O is an ideal p-type material for photoelectrochemical (PEC) hydrogen evolution, while serious electron-hole recombination and photocorrosion restrict its further improvement of the PEC activity. In this work, CeO 2 nanoparticles (NPs) self-assemble on the Cu 2 O octahedra surface, successfully constructing Cu 2 O/CeO 2 structure in which p-n heterojunction and micro-mesocrystals (m-MCs) structure work together. The optimum Cu 2 O/CeO 2 composite without the use of any cocatalyst exhibits 5 times higher photocurrent density (4.63 mA cm -2 at 0 V versus the reversible hydrogen electrode) than that of Cu 2 O octahedra, which is better than most Cu 2 O-based photocathodes without cocatalyst and even comparable with the advanced Cu 2 O-based photocathodes. The hydrogen production of the optimal Cu 2 O/CeO 2 (Faradaic efficiency of ~100%) is 17.5 times higher than that of pure Cu 2 O octahedra and the photocurrent shows almost no decay under the 12 h stability test. The delicately designed Cu 2 O/CeO 2 structure in this work provides reference and inspiration for the design of cathodes materials.

Reduction-oxidation series coupling degradation of chlorophenols in Pd-Catalytic Electro-Fenton system

Organochlorine pesticides are widespread in soils, sediments and even in groundwater, causing great concern to human health because of its toxicity and carcinogenic effects. The remarkable mineralization and lowered toxicity are particularly important during the removal of organochlorine pesticides. In this study, Pd/CeO₂ was prepared and employed as a bifunctional catalyst, to construct the reduction-oxidation series coupling Electro-Fenton (EF) system. The removal of chlorophenols (CPs) reached over 95% within 10 min at pH 3.0 and a current density of 25 mA/cm² in Pd/CeO₂-EF system. The second-order rate constant of CPs degradation was 10.28 L mmol^-1min^-1 in Pd/CeO₂-EF system, which was 29 times as fast as the sum of electrolysis with Pd/CeO₂ (0.24 L mmol^-1min^-1) and EF (0.11 L mmol^-1min^-1). Dehydrochlorination by Pd [H] contributed to the removal of CPs in Pd/CeO₂-EF system. The generated reactive oxygen species, mainly OH was also confirmed by ESR to contribute to the removal of CPs. The reduction-oxidation series coupling degradation of CPs in Pd/CeO₂-EF system increased the TOC removal to 70% in 360 min. The analysis of intermediate products further revealed the reductive and oxidative products in Pd/CeO₂-EF. Moreover, the system of Pd/CeO₂-EF exhibited an excellent performance treatment for CPs in actual groundwater. This study provides a new stratagem to eliminate organochlorine pesticides in groundwater environments rapidly and thoroughly.

Degradation of refractory organics in dual-cathode electro-Fenton using air-cathode for H2O2 electrogeneration and microbial fuel cell cathode for Fe2+ regeneration

The electrogeneration of H₂O₂ and electro-regeneration of ferrous are conflicting matters in electro-Fenton system. In this research, the degradation of Rhodamine B, methyl orange (MO) and 4-chlorophenol (4-CP) was investigated using a novel dual-cathode microbial fuel cell (MFC) electro-Fenton (EF) hybrid system. An air-cathode of an EF system was used for H₂O₂ electrogeneration and a carbon felt cathode of a MFC was used to accelerate Fe²⁺ regeneration. Synergistic improvement of MFC power generation and the degradation of the above refractory organics through EF reaction was achieved. The EF air-cathode was fabricated by adopting activated carbon/graphite powder mixture and PVDF binder, which showed higher H₂O₂ generation but slower Fe³⁺ reduction rate than MFC carbon felt cathode. The Rhodamine B removal rate constant and mineralization current efficiency of the MFC coupled EF were 64% and 42% higher than that of uncoupled EF, respectively. The MFC-EF coupled system also exhibited significantly higher removal efficiency for MO and 4-CP than that of un-coupled EF system. Moreover, the power density of MFC was greatly enhanced by coupling EF due to higher Fe³⁺/Fe²⁺ redox potential than oxygen reduction.

Ferric iron reduction reaction electro-Fenton with gas diffusion device: A novel strategy for improvement of comprehensive efficiency in electro-Fenton

Applying the optimal 2-electron oxygen reduction reaction potential in electro-Fenton (2e^-ORR-EF) for degradation has become a common strategy because of the highest H₂O₂ generation rate in such condition. However, in 2e^-ORR-EF system, the Fe(III) ions crystallize on the surface of cathode and form a layer of film according to SEM, XPS, XRD and Mössbauer spectrum resulting in poor reaction rate of EF. Hence, we propose FRR-EF, which is operated by applying the optimal potential of ferric iron reduction reaction (FRR) rather than that of 2e^-ORR on cathode for EF. Gas diffusion device was also carried out to ensure the H₂O₂ generation rate. In this novel strategy, only - 0.1 V was applied on cathode. High H₂O₂ production rate (0.021 ± 0.002 mmol L^-1 min^-1 cm^-2), and slow Fe(II) consumption rate (0.03 min^-1) were achieved. The EIS result showed that at this potential, the formation of the Fe film was effectively alleviated, thus prolonging the degradation life of the cathode. This new strategy can balance both 2e^-ORR and FRR, thus improving the comprehensive efficiency of EF, which provides essential references to the EF not only in potential operation but also in the design of reaction device.

Enhanced visible-light photocatalysis of clofibric acid using graphitic carbon nitride modified by cerium oxide nanoparticles

Recently, the emerging pharmaceutical micropollutants have become an environmental concern. Herein, we report an efficient elimination of clofibric acid (CA) using visible light-driven g-C₃N₄/CeO₂ prepared by hydrothermal method. Among the catalysts with different compound ratios, g-C₃N₄/CeO₂-3 (1.2 g g-C₃N₄ with 3 mmol Ce(NO₃)₃∙6H₂O) exhibited the best photocatalytic performance. The effect of catalyst dosage was investigated and the optimal value was determined as 0.5 g L^-1. The effect of initial pH (pH₀) showed CA elimination decreased with increasing pH₀. The underlying mechanism for CA oxidation was proposed based on synthetical analysis of photoluminescence emission spectra, transient photocurrent responses, electron paramagnetic resonance, chemical quenching experiments and band edge potential of g-C₃N₄ and CeO₂. Photogenerated hole was primarily responsible for CA elimination while singlet oxygen played an auxiliary role. The products of CA oxidation were detected using liquid chromatography mass spectrometry (LC-MS) method and a possible pathway was put forward. Various organics were used as target contaminants to assess photocatalytic performance of g-C₃N₄/CeO₂ heterojunction under acidic and alkaline pH conditions. The analysis of relationship between the oxidation peak potential (E_OP) and the reaction rate constant indicated that photocatalysis using as prepared g-C₃N₄/CeO₂-3 heterojunction is apt to oxidize contaminants with electron withdrawing group under acid condition.

Duet Fe3C and FeNx Sites for H2O2 Generation and Activation toward Enhanced Electro-Fenton Performance in Wastewater Treatment

Heterogeneous electro-Fenton (HEF) reaction has been considered as a promising process for real effluent treatments. However, the design of effective catalysts for simultaneous H₂O₂ generation and activation to achieve bifunctional catalysis for O₂ toward •OH production remains a challenge. Herein, a core-shell structural Fe-based catalyst (FeNC@C), with Fe₃C and FeN nanoparticles encapsulated by porous graphitic layers, was synthesized and employed in a HEF system. The FeNC@C catalyst presented a significant performance in degradation of various chlorophenols at various conditions with an extremely low level of leached iron. Electron spin resonance and radical scavenging revealed that •OH was the key reactive species and Fe^IV would play a role at neutral conditions. Experimental and density function theory calculation revealed the dominated role of Fe₃C in H₂O₂ generation and the positive effect of FeN_x sites on H₂O₂ activation to form •OH. Meanwhile, FeNC@C was proved to be less pH dependence, high stability, and well-recycled materials for practical application in wastewater purification.

Preparation of CeO2-doped carbon nanotubes cathode and its mechanism for advanced treatment of pig farm wastewater

The effluent from conventional treatment process (including anaerobic digestion and anoxic-oxic treatment) for pig farm wastewater was difficult to treat due to its low ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD_Cr) (<0.1). In the present study, electro-Fenton (EF) was used to improve the biodegradability of the mentioned effluent and the properties of self-prepared CeO₂-doped multi-wall carbon nanotubes (MWCNTs) electrodes were also studied. An excellent H₂O₂ production (165 mg L^-1) was recorded, after an 80-min electrolysis, when the mass ratio of MWCNTs, CeO₂ and pore-forming agent (NH₄HCO₃) was 6:1:1. Results of scanning electron microscopy (SEM), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS) showed that addition of NH₄HCO₃ and the doping of CeO₂ could increase the superficial area of the electrode as well as the oxygen reduction reaction (ORR) electro-catalytic performance. The BOD₅/COD_Cr of the wastewater from the first stage AO process increased from 0.08 to 0.45 and COD_Cr reduced 71.5% after an 80-min electrolysis, with 0.3 mM Fe²⁺ solution. The non-biodegradable chemical pollutants from the first stage AO process were degraded by EF. The non-biodegradable pollutants identified by LC-MS/MS in the effluent from AO process including aminopyrine, oxadixyl and 3-methyl-2-quinoxalinecarboxylic acid could be degraded by EF process, with the removal rates of 81.86%, 34.39% and 7.13% in 80 min, and oxytetracycline with the removal rate of 100% in 20 min. Therefore, electro-Fenton with the new CeO₂-doped MWCNTs cathode electrode will be a promising supplement for advanced treatment of pig farm wastewater.

Engineered FeVO4/CeO2 nanocomposite as a two-way superior electro-Fenton catalyst for model and real wastewater treatment

FeVO₄/CeO₂ was applied in the electro-Fenton (EF) degradation of Methyl Orange (MO) as a model of wastewater pollution. The results of the characterization techniques indicate that FeVO₄ with triclinic structure and face-centered cubic fluorite CeO₂ maintained their structures during the nanocomposite synthesis. The effect of applied current intensity, initial pollutant concentration, initial pH, and catalyst weight was investigated. The MO removal reached 96.31% and chemical oxygen demand (COD) removal 70% for 60 min of the reaction. The presence of CeO₂ in the nanocomposite plays a key role in H₂O₂ electro-generation as a significant factor in the electro-Fenton (EF) system. The metal leaching from FeVO₄/CeO₂ was negligible (cerium 4.1%, iron 4.3%, and vanadium 1.7%), which indicates that the active species in the nanocomposite are strongly interacting with each other and are stable. The performance of the nanocatalyst in real wastewaters, salty, and binary systems was acceptable and the pollutions were removed efficiently. The synergistic effect between V, Fe, and Ce could be account as the reason for the respectable function of FeVO₄/CeO₂. The electron transfer proceeds via Haber-Weiss mechanism. A degradation pathway was proposed through by-products analysis using gas chromatography-mass spectrometry (GC-MS) technique. The pseudo-first-order kinetic model described the obtained experimental results (R² = 0.9906). The electro-Fenton system efficiency was improved by adding persulfate. The nanocomposite preserved almost its efficiency after six cycles. The obtained results demonstrate that the synergistic catalyst (FeVO₄/CeO₂) has the capability to introduce as a promising replacement of conventional catalysts in the electro-Fenton processes with brilliant proficiency.

Ce-based catalysts used in advanced oxidation processes for organic wastewater treatment: A review

Refractory organic pollutants in water threaten human health and environmental safety, and advanced oxidation processes (AOPs) are effective for the degradation of these pollutants. Catalysts play vital role in AOPs, and Ce-based catalysts have exhibited excellent performance. Recently, the development and application of Ce-based catalysts in various AOPs have been reported. Our study conducts the first review in this rapid growing field. This paper clarifies the variety and properties of Ce-based catalysts. Their applications in different AOP systems (catalytic ozonation, photodegradation, Fenton-like reactions, sulfate radical-based AOPs, and catalytic sonochemistry) are discussed. Different Ce-based catalysts suit different reaction systems and produce different active radicals. Finally, future research directions of Ce-based catalysts in AOP systems are suggested.

Efficient degradation of AO7 by ceria-delafossite nanocomposite with non-inert support as a synergistic catalyst in electro-fenton process

CuFeO₂/CeO₂ as a novel catalyst was synthesized and its catalytic performance was evaluated for electro-Fenton degradation of acid orange 7 (AO7). It was demonstrated from the characterization results that the rhombohedral structure of CuFeO₂ and face-centered cubic fluorite structure of CeO₂ remained stable after nanocomposite construction. The impact of such operating parameters as pH, current intensity and, catalyst amount was investigated and the optimum conditions (100 mgL^-1 AO7, pH 3, 150 mgL^-1 CuFeO₂/CeO₂, I: 150 mA) determination led to 99.3% AO7 removal and 79.1% COD removal in 60 min. The introduction of CeO₂ as non-inert support had a significant impact on H₂O₂ electro-generation as an important step in AO7 removal. CuFeO₂/CeO₂ presented negligible metal leaching (iron 4.13%, copper 2.4%, and cerium 0.33%) which could be due to the strong interaction between active species and support. The nanocomposite performed efficiently in salty systems and two samples of real wastewaters due to Brønsted acidity character of ceria, which makes it a potential choice in industrial applications. The good performance of nanocomposite could be the result of the synergistic effect between Fe, Cu, and Ce. Regarding scavenging measurements results, the electro-Fenton process followed the Haber-Weiss mechanism. The by-products detection was performed using GC-MS analysis to propose an acceptable pathway for EF degradation of AO7. The BMG kinetics model (1/b = 0.969 (min) and 1/m = 0.269 (min^-1)) was matched with the experimental data and described the kinetics of reaction very well. The catalytic activity of CuFeO₂/CeO₂ almost remained after six cycles. Based on the obtained results, CuFeO₂/CeO₂ using the benefit of the synergistic effect of Ce³⁺ with Fe²⁺ and Cu⁺can be introduced as a promising novel catalyst for the electro-Fenton reaction in wastewater treatment.

Ce3+ self-doped CeOx/FeOCl: an efficient Fenton catalyst for phenol degradation under mild conditions

Herein, a novel Ce³⁺ self-doped CeO_x/FeOCl composite was successfully prepared by a facile method for the first time, which showed remarkable catalytic activity as a Fenton catalyst in the degradation of phenol under the conditions of a neutral solution, room temperature and natural light.