Advances in magnetic materials for microplastic separation and degradation

The widespread use of plastics in modern human society has led to severe environmental pollution with microplastics (MP/MPs). The rising consumption of plastics raises the omnipresence of microplastics in aquatic environments, which carry toxic organic matter, transport toxic chemicals, and spread through the food chain, seriously threatening marine life and human health. In this context, several advanced strategies for separating and degrading MPs from water have been developed recently, and magnetic materials and their nanostructures have emerged as promising materials for targeting, adsorbing, transporting, and degrading MPs. However, a comprehensive review of MP remediation using magnetic materials and their nanostructures is currently lacking. The present work provides a critical review of the recent advances in MP removal/degradation using magnetic materials. The focus is on the comparison and analysis of the MP's removal efficiencies of different magnetic materials, including iron/ferrite nanoparticles, magnetic nanocomposites, and micromotors, aiming to unravel the underlying roles of magnetic materials in different types of MP degradation and present the general strategies for designing them with optimal performance. Finally, the review outlines the forthcoming challenges and perspectives in the development of magnetic nanomaterials for MP remediation.

Performance of electro-Fenton process for the treatment of synthetic sulphidic spent caustic waste stream generated from petroleum refineries

Sulphidic spent caustic (SSC) is an alkaline waste stream which is generated during caustic scrubbing of liquefied petroleum gas and ethylene products. Due to presence of high concentrations of sulphides and phenols, the waste stream requires proper treatment before mixing with the low strength wastewater streams produced from other refinery operations. Electrochemical process is an emerging treatment method that can work efficiently at ambient conditions. The present study reports performance of electro-Fenton (EF) process for the treatment of synthetic SSC wastewater (sulphides = 10 g L-1, phenol = 2 g L-1 and pH = 12.9). The EF runs were carried out for 2 h duration in a reactor equipped with iron electrodes. The effects of H2O2 dose (0.26-1.3 M), current density (1-20 mA cm-2), pH (4.5-12.9) and stirring speed (100-1000 rpm) were investigated on removal of pollutants. The H2O2 was rapidly consumed in initial 30 min during which the significant fraction of the pollutants was degraded or removed. The optimum conditions for EF process were found to be as follows: pH = 4.5, H2O2 dose = 1.05 M, current density = 5 mA cm-2 and stirring speed = 500 rpm. At these conditions, the maximum sulphide and phenol removals from the wastewater were 98% and 91%, respectively. The results will be helpful to the wastewater treatment plant operators worldwide dealing with high concentrations of such pollutants.

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

Accelerated electron transfer process via MOF-derived FeCo/C for enhanced degradation of antibiotic contaminants towards heterogeneous electro-Fenton system

The Fe(III) to Fe(II) process limits the rate of the electro-Fenton system. In this study, MIL-101(Fe) derived porous carbon skeleton-coated FeCo bimetallic catalyst Fe₄/Co@PC-700 was prepared as a heterogeneous electro-Fenton (EF) catalytic process. The experimental results showed its good performance in catalytic removal of antibiotic contaminants, the rate constant of tetracycline (TC) degradation catalyzed by Fe₄/Co@PC-700 was 8.93 times higher than that of Fe@PC-700 under the pH conditions of raw water (pH = 5.86), exhibited good removal of TC, oxytetracycline (OTC), hygromycin (CTC), chloramphenicol (CAP) and ciprofloxacin (CIP). It was shown that the introduction of Co promoted more Fe⁰ production, allowing the material to exhibit faster Fe(III)/Fe(II) cycling rates. ¹O₂ and high-priced metal oxygen species were identified as the main active species of the system, in addition to the analysis of possible degradation pathways and toxicity of intermediates of TC. Finally, the stability and adaptability of Fe₄/Co@PC-700 and EF systems to different water matrices were evaluated, showing that Fe₄/Co@PC-700 was easy to recover and could be applied to different water matrices. This study provides a reference for the design and system application of heterogeneous EF catalysts.

Natural iron minerals in an electrocatalytic oxidation system and in situ pollutant removal in groundwater: Applications, mechanisms, and challenges

Natural iron-bearing minerals are widely distributed in the environment and show prominent catalytic performance in pollutant removal. This work provides an overview of groundwater restoration technologies utilizing heterogeneous electro-Fenton (HEF) techniques with the aid of different iron forms as catalysts. In particular, applications of natural iron-bearing minerals in groundwater in the HEF system have been thoroughly summarized from either the view of organic pollutant removal or degradation. Based on the analysis of the catalytic mechanism in the HEF process by pyrite (FeS₂), goethite (α-FeOOH), and magnetite (Fe₃O₄) and the geochemistry analysis of these natural iron-bearing minerals in groundwater, the feasibility and challenges of HEF for organic degradation by using typical iron minerals in groundwater have been discussed, and natural factors affecting the HEF process have been analyzed so that appropriate in situ remedial measures can be applied to contaminated groundwater.

Efficient upcycling of iron scrap and waste polyethylene terephthalate plastic into Fe3O4@C incorporated MIL-53(Fe) as a novel electro-Fenton catalyst for the degradation of salicylic acid

The current research demonstrates the efficiency of a low-cost MIL-53(Fe)-metal-organic framework (MOF) derived Fe₃O₄@C (MIL-53(Fe)@Fe₃O₄@C) electrocatalyst in a batch-scale electro-Fenton (EF) process for the degradation of salicylic acid (SA) from wastewater. The electrocatalyst was prepared from the combination of polyethylene terephthalate (PET) and iron scrap wastes. The result showed 91.68 ± 3.61% degradation of 50 mg L^-1 of SA under optimum current density of 5.2 mA cm^-2, and pH of 7.0 during 180 min of electrolysis time. The degradation of SA from waste catalyst was similar to the chemical-based MIL-53(Fe)-derived Fe₃O₄@C (cFe) cathode catalyst. The presence of chloride ions (Cl^-) in the water matrix has shown a strong inhibitory effect on the elimination of SA, followed by nitrate (NO₃^-), and bicarbonate (HCO₃^-) ions. The multiple cyclic voltammetry (CV) analysis and reusability test of waste cathode catalyst showed only 8.03% drop of current density at the end of the 20th cycle and 5% drop of degradation efficiency after 6th cycle with low leaching of iron. The radical scavenging experiment revealed that the HO^• generated via electrochemical generation of H₂O₂ had a prominent contribution in the removal of SA compared to HO₂^•/O₂^•-. Besides, possible catalysis mechanism and degradation pathways were deduced. Furthermore, a satisfactory performance in the treatment of SA spiked in real water matrices was also observed by waste-derived Fe₃O₄@C cathode catalyst (wFe). Additionally, the total operating cost and toxicity analysis showed that the as-synthesized wFe cathode catalyst could be appropriate for removing organic pollutants from wastewater in the large-scale application.

Facile Synthesis of n-Fe3O4/ACF Functional Cathode for Efficient Dye Degradation through Heterogeneous E-Fenton Process

In order to put forward an efficient and eco-friendly approach to degrade dye-containing industrial effluents, an n-Fe3O4/ACF nanocomposite was synthesized using the facile precipitation method and applied as a functional cathode for a heterogeneous electro-Fenton (E-Fenton) process. In particular, optimal initial pH value, current density, pole plate spacing, and electrode area were confirmed through systematical experiments as 5.73, 30 mA/cm2, 3 cm, and 2 × 2 cm2, respectively. Under such optimal reaction conditions, 98% of the methylene blue (MB) was degraded by n-Fe3O4/ACF after 2 h of E-Fenton treatment. In addition, n-Fe3O4/ACF could still decolor about 90% of the methylene blue (MB) for five rounds with some reductions in efficiency. Furthermore, results of electrochemical impedance spectroscopy and heterogeneous E-Fenton performance tests indicated that the loading of metal material Fe3O4 could enhance the overall electron transport capacity, which could accelerate the whole degradation processes. Moreover, the rich pores and large specific surface area of n-Fe3O4/ACF provided many active sites, which could greatly improve the efficiency of O2 reduction, promote the generation of H2O2, and shorten the reaction length between •OH and the pollutant groups.

Fe3O4/Mn3O4/ZnO-rGO hybrid quaternary nano-catalyst for effective treatment of tannery wastewater with the heterogeneous electro-Fenton process: Process optimization

This study investigated chemical oxygen demand (COD) removal from tannery wastewater (TWW) with a novel Fe₃O₄/Mn₃O₄/ZnO-rGO heterogeneous electro Fenton (HEF) hybrid magnetically-separable nano-catalyst. The graphite cathode and Ti/IrO₂/RuO₂ anode were used in the HEF process. With aeration (2 L/min), in-situ H₂O₂ generation occurred. The nano-catalyst was characterized by XRD, XPS, DLS, FT-IR, ζ potential, SEM, TEM, and BET techniques in detail. The system was modelled with a central composite design and optimized numerically. The established model was adequate, valid, reliable, and reproducible to predict the COD removal efficiency. OH and O₂^- were the oxidative species responsible for organic matter degradation. The effect of different processes was investigated, and efficiency was ranked from high to low as; HEF > anodic oxidation-H₂O₂ > anodic oxidation > adsorption. Under the optimum conditions; pH: 3.5, current density: 7.37 mA/cm², reaction time: 79.43 min, and catalyst dose: 0.06 g/L, COD removal efficiency reached a maximum of 97.08%. The energy consumption and cost to remove 1 kg COD were 10.87 kWh and $1.41. The degradation of COD fitted the pseudo-first-order model. The nano-catalyst was stable and reusable with a minimum yield of 78.12% after 5 cycles.

Improved Magnetite Nanoparticle Immobilization on a Carbon Felt Cathode in the Heterogeneous Electro-Fenton Degradation of Aspirin in Wastewater

Toward the improvement of the application of heterogeneous electro-Fenton in water treatment, we report a new strategy of enhancing the immobilization of a magnetite nanoparticle catalyst on a carbon felt cathode. Exploiting the intrinsic ferrimagnetic properties of magnetite nanoparticles, magnet bars were used to attach the magnetite into the void spaces of the porous carbon felt (CF) cathode. The magnetite nanoparticles were prepared by coprecipitation with variations in the molar ratios of Fe²⁺/Fe³⁺. The magnetite was characterized, attached onto the CF electrode with magnetic bars, and used in the heterogeneous electro-Fenton (EF) degradation of aspirin. The effects of the following on the degradation were studied: Fe²⁺/Fe³⁺, pH, catalyst loading concentration, and voltage. The heterogeneous EF degradation of aspirin in wastewater improved by 23% when magnetic bars were used to enhance the immobilization of the magnetite catalysts. The 1:4 Fe²⁺/Fe³⁺ ratio resulted in the highest hetero-EF catalytic degradation of aspirin with complete degradation (100%) achieved after 140 min. For a mixture of pharmaceuticals, degradation percentages of 94.3% (aspirin), 88% (ciprofloxacin), and 80% (paracetamol) in 3 h were obtained. The magnetized magnetite on the cathode was reusable for 10 cycles. Thus, the use of magnets shows a promising strategy to avoid the leaching of ferrimagnetic nanoparticle catalysts embedded in the cathode for heterogeneous EF processes.

QSAR modelling of organic dyes for their acute toxicity in Daphnia magna using 2D-descriptors

The present study reports quantitative structure-activity relationship (QSAR) models for 22 organic dyes spanning a broad chemical domain to predict their toxicity in Daphnia magna [log (1/EC₅₀)]. Only two-dimensional descriptors with clear physicochemical meaning were used to construct the QSAR models. The process of development, validation, and interpretation of models adheres to the stringent recommendations of the Organization for Economic Cooperation and Development (OECD) guidelines. In this study, the multi-layered stepwise regression method and linear discriminant analysis (LDA) method were employed for the deployment of regression - and classification-based models respectively; however, the final regression-based QSAR models were obtained through the partial least squares (PLS) regression. Additionally, the applicability domain of the developed models was verified. The constructed models should be applicable in the absence of toxicity data of new or untested dye structures, particularly when the compounds fall within the developed models' scope, and also implementable to develop more environmentally friendly alternatives.

Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water

The hunt for efficient and environmentally friendly degradation processes has positioned the heterogeneous advanced oxidation processes as an alternative more interesting and economical rather than homogenous processes. Hence, the current study lies in investigating the efficiency of different heterogeneous catalysts using transition metals in order to prevent the generation of iron sludge and to extend the catalogue of possible catalysts to be used in advanced oxidation processes. In this study, nickel and zinc were tested and the ability for radical-generation degradation capacity of both ions as homogeneous was evaluated in the electro-Fenton-like degradation of 2-phenylphenol. In both cases, the degradation profiles followed a first-order kinetic model with the highest degradation rate for nickel (1 mM) with 2-phenylphenol removal level of 90.12% and a total organic reduction near 70% in 2 h. To synthesise the heterogeneous nickel catalyst, this transition metal was fixed on perlite by hydrothermal treatment and in a biochar or carbon nanofibers by adsorption. From the removal results using the three synthesized catalysts, it is concluded that the best catalysts were obtained by inclusion of nickel on biochar or nanofibers achieving in both with removal around 80% before 1 h. Thus, to synthetize a nickel electrocatalyst, nickel doped nanofibers were included on carbon felt. To do this, the amount of carbon black, nickel nanofibers and polytetrafluoroethylene to add on the carbon felt was optimized by Taguchi design. The obtained results revealed that under the optimised conditions, a near-complete removal was achieved after 2 h with high stability of the nickel electrocatalyst that open the applicability of this heterogeneous system to operate in flow systems.

Prussian blue modified CeO2 as a heterogeneous photo-Fenton-like catalyst for degradation of norfloxacin in water

The heterogeneous photo-Fenton-like process is emerging as a promising treatment of antibiotics-containing wastewater. The preparation of new efficient and stable catalysts is one of the research fields. A composite catalyst, prussian blue (PB) modified CeO₂ was prepared, characterized, and applied for photo-Fenton oxidation of norfloxacin (NOR) in this study. It was found that chemical doping of PB leaded to more oxygen vacancies and increased the surface area of CeO₂ obviously. PB/CeO₂ with more Ce³⁺ facilitated electron transfer between Fe³⁺/Fe²⁺ with Ce³⁺/Ce⁴⁺. PB could also improve the separation rate of photoexcited electron-hole pairs in CeO₂ nanostructures. When the doping ratio of PB and CeO₂ was 10%, PB/CeO₂ show the highest catalytic degradation ability and 88.93% of NOR could be degraded within 30 min. PB/CeO₂ composite showed well reactivity at the wide pH value range of 3-8. The reusable experiments and low iron dissolution with less than 1 mg/L indicated that PB/CeO₂ could be employed as an efficient heterogeneous photo-Fenton-like catalyst in organic contaminants degradation.

A comparison study of applying natural iron minerals and zero-valent metals as Fenton-like catalysts for the removal of imidacloprid

Natural iron minerals and zero-valent metals have been widely tested as catalysts for the Fenton-like process, but the systematical comparison study about their catalytic performance was rarely conducted, and the risk of the secondary pollution of toxic heavy metals was still not uncertain. In this paper, a comparison study of applying pyrite, ilmenite, vanadium titano-magnetite (VTM), zero-valent iron (ZVI), and zero-valent copper (ZVC) as Fenton-like catalysts for the removal of imidacloprid was performed. The results showed that ZVI exhibited the highest activity among the recyclable solid catalysts with a removal rate of 96.8% at initial pH 3 using 10.78 mmol/L H₂O₂, due to iron corrosive dissolution. Vanadium titano-magnetite (VTM) exhibited the best activity at first use among tested minerals but with low reusability. Pyrite with stable morphology showed a medium but sustainable ability to degrade imidacloprid, achieving a removal rate of 10.5% in the fifth use. The reaction much favored the acidic condition of initial pH around 2 or 3. Meanwhile, there was a significant positive correlation between removal efficiency and dissolved Fe or Cu concentration. Pyrite was considered to be a promising catalyst in Fenton-like reaction. It was suggested that the system proceeded predominantly through a homogeneous route via dissolved Fe or Cu ions. Except ZVC and VTM, other tested catalysts showed the low possibility of causing secondary pollution of toxic metals in the application of Fenton-like process.

FeIIFeIII layered double hydroxide modified carbon felt cathode for removal of ciprofloxacin in electro-Fenton process

The disadvantages of limited working pH range and poor stability have hindered the practical application of traditional electro-Fenton process. In this research, a novel heterogeneous electro-Fenton (HEF) process with Fe^IIFe^III layered double hydroxide/carbon felt (Fe^IIFe^III LDH/CF) as cathode was developed for the rapid destruction of ciprofloxacin (CIP) in bulk solution. Effects of crucial influencing factors (initial pH, current intensity) on CIP degradation were investigated. Results indicated that Fe^IIFe^III LDH/CF cathode was efficient for CIP degradation (88.11%). Furthermore, CIP degradation performance in HEF could remain stable over wide range of pH (pH 3-9). The catalytic degradation of CIP in HEF process might be a combined effect of homogeneous EF reaction, anodic oxidation, and surface catalysis process via≡Fe^II/≡Fe^III cycle. Possible degradation pathways were proposed. The results suggested that Fe^IIFe^III LDH/CF cathode showed great application potential for CIP degradation.

Heterogeneous Fenton catalysts: A review of recent advances

Heterogeneous Fenton catalysts are emerging as excellent materials for applications related to water purification. In this review, recent trends in the synthesis and application of heterogeneous Fenton catalysts for the abatement of organic pollutants and disinfection of microorganisms are discussed. It is noted that as the complexity of cell wall increases, the resistance level towards various disinfectants increases and it requires either harsh conditions or longer exposure time for the complete disinfection. In case of viruses, enveloped viruses (e.g. SARS-CoV-2) are found to be more susceptible to disinfectants than the non-enveloped viruses. The introduction of plasmonic materials with the Fenton catalysts broadens the visible light absorption efficiency of the hybrid material, and incorporation of semiconductor material improves the rate of regeneration of Fe(II) from Fe(III). A special emphasis is given to the use of Fenton catalysts for antibacterial applications. Composite materials of magnetite and ferrites remain a champion in this area because of their easy separation and reuse, owing to their magnetic properties. Iron minerals supported on clay materials, perovskites, carbon materials, zeolites and metal-organic frameworks (MOFs) dramatically increase the catalytic degradation rate of contaminants by providing high surface area, good mechanical stability, and improved electron transfer. Moreover, insights to the zero-valent iron and its capacity to remove a wide range of organic pollutants, heavy metals and bacterial contamination are also discussed. Real world applications and the role of natural organic matter are summarised. Parameter optimisation (e.g. light source, dosage of catalyst, concentration of H2O2 etc.), sustainable models for the reusability or recyclability of the catalyst and the theoretical understanding and mechanistic aspects of the photo-Fenton process are also explained. Additionally, this review summarises the opportunities and future directions of research in the heterogeneous Fenton catalysis.

Iron-based persulfate activation process for environmental decontamination in water and soil

Sulfate radical based advanced oxidation processes have been extensively studied for the degradation of environmental contaminants. Iron-based materials such as ferrous, ferric, ZVI, iron oxides, sulfides etc., and various natural iron minerals have been explored for activating persulfate to generate sulfate radicals. In this review, an overview of different iron activated persulfate systems and their application in the removal of organic pollutants and metals in water and soil are summarised. The chemistry behind the activation of persulfate by homogenous and heterogeneous iron-based materials with/without the assistance of electrochemical techniques are also discussed. Besides, the soil decontamination by iron persulfate system and a brief discussion on the ability of the persulfate system to reduce metals presence in wastewater are also summarised. Finally, future research prospects, believed to be useful for all researchers in this field, based on up to date research progress is also given.

Removal of aniline from water by an Fe(II)-nano-Fe3O4@PAC heterogeneous catalyst in a Fenton-like process

Aniline is a toxic chemical, and in many industries it is degraded by Fenton processes. In this study, an Fe(II)-nano-Fe₃O₄@PAC heterogeneous Fenton catalyst (MFC) was prepared with a coprecipitation and impregnation method, which is simple, efficient and cost effective. The results of the magnetic performance tests showed that the MFC has typical ferromagnetism properties. Nano-Fe₃O₄ was found both on the surface and inside the pores of the powdered activated carbon (PAC). MFC was found to be an efficient catalyst in the Fenton-like process for aniline degradation. The optimal conditions were obtained by the orthogonal experimental method. The results showed that under the optimal conditions (pH = 3.00, temperature = 20°C, concentration of MFC = 1.0 g/L, concentration of H₂O₂ = 0.27 g/L), the 5 mg/L aniline solution degradation ratio reached 91.2% and the mineralization ratio reached 75.77% in 30 min. In addition, kinetics studies indicated that the aniline degradation process follows a pseudo-first-order kinetics model. No refractory intermediate such as azobenzene, was found during the reaction. The pH value is an important factor in aniline solution degradation. This result indicates that in addition to the surface catalytic reaction, the Fenton reaction also occurs in solution. Fe²⁺/Fe³⁺ on the MFC surface and Fe²⁺/Fe³⁺ in solution both affect aniline degradation. This catalyst has the advantage of being easily magnetically separated from the aqueous phase. It has useful application prospects in solving organic industrial wastewater pollutions in developing countries because of its cost-effectiveness.

Electro-Fenton treatment of a complex pharmaceutical mixture: Mineralization efficiency and biodegradability enhancement

Combination of the electro-Fenton process with a post-biological treatment could represent a cost-effective solution for application of electrochemical advanced oxidation processes. The objective of this study was to assess this treatment strategy in the case of a complex pharmaceutical mixture. First, main operating parameters ([Fe²⁺] and current) of the electro-Fenton process were optimized. An optimal concentration of 0.2 mM of Fe²⁺ was obtained for mineralization of the pharmaceutical mixture. An optimal current of 400 mA was also obtained for degradation of caffeine and 5-fluorouracil in the mixture. However, mineralization of the effluent was continuously improved when increasing the current owing to the promotion of mineralization of organic compounds at the BDD anode. Besides, energy efficiency was decreased at prolonged treatment time because of mass transport limitation. Interestingly, it was observed a strong biodegradability enhancement of the solution after short treatment times (<3 h) at 500 and 1000 mA, which can be related to the degradation of parent compounds into more biodegradable by-products. The need for an acclimation time of the biomass to the pre-treated effluent was also emphasized, most probably because of the formation of some toxic by-products as observed during acute toxicity tests. Therefore, a biological post-treatment could represent a cost-effective solution for the removal of biodegradable residual organic compounds as well as for the removal of nitrogen released from mineralization of organic compounds under the form of NO₃^- and NH₄⁺ during electro-Fenton pre-treatment.

Three-dimensional heterogeneous Electro-Fenton system with a novel catalytic particle electrode for Bisphenol A removal

Herein, a novel three-dimensional (3D) heterogeneous Electro-Fenton (EF) system with improved gas diffusion electrode (GDE) as cathode and magnetic nitrogen doped/reduced graphene oxide (Fe₃O₄/N-rGO) as catalytic particle electrodes (CPEs) was built for Bisphenol A (BPA) removal. The Fe₃O₄/N-rGO served as both particle electrodes and heterogeneous catalyst. The study concluded that BPA could be effectively removed via this hybrid system. The synergistic effect between the 3D electrode and EF system was discussed by comparing the performance of different functional particle electrodes. The 3D electrode system exhibited a larger specific surface area electrode, which improved the mass transfer of pollutants to electrode, and also accelerated the regeneration of Fe^Ⅱ due to faster electron transfer, thereby enhancing the efficiency of EF catalysis. The EF process promotes the regeneration rate of particle electrodes and thus accelerates the 3D electrode reaction course. The parameters affecting degradation behavior of BPA were optimized. As a result, optimal removal rate of BPA and TOC was 93% and 60.5%, respectively within 90 min. The CPEs showed high catalytic performance (86.5% for BPA and 50.3% for TOC) and low catalyst loss (less than 9.5%) after 5 cycles, indicating its excellent stability and reusability. The possible mechanism of 3D heterogeneous EF was investigated by comparing the catalytic activity and •OH production capacity of homogeneous EF and Fenton-like. Built on the analysis of intermediates, a possible decomposition pathway of BPA was proposed.

Iron oxide nanoparticles as heterogeneous electro-Fenton catalysts for the removal of AR18 azo dye

Heterogeneous electro-Fenton mineralization of Acid Red 18 (AR18) in aqueous solution was studied with magnetite Fe₃O₄ (MNPs) and haematite Fe₂O₃ (HNPs) nanoparticles as catalysts. High mineralization yields of AR18 were obtained with magnetite, 81% TOC removal after 180 min of electrolysis in 40 mg L^-1 Fe₃O₄, pH 3.0, at 50 mA of current intensity and in 50 mM Na₂SO₄. In order to explain the obtained mineralization yield achieved with MNPs, the quantification of hydrogen peroxide (H₂O₂), hydroxyl radical (•OH) and iron leaching were performed at 50 and 100 mA. From the high iron concentration found in the bulk solution and the slight impact of the catalyst mass concentration on TOC removal, the formation of hydroxyl radicals occurs mainly through homogeneous process. In the presence of hydroxyl radical scavenger, degradation remained total after 15 min showing the involvement of a direct electrochemical oxidation of the dye at the electrode surface. The hydroxyl radical oxidation is responsible for at least 50% of mineralization.

Nafcillin degradation by heterogeneous electro-Fenton process using Fe, Cu and Fe/Cu nanoparticles

Heterogeneous electro-Fenton (HEF) is as an alternative to the conventional electro-Fenton (EF) process. HEF uses a solid phase catalyst, whereas EF employs a solubilized one. This implies that in HEF, material can be recovered through a simple separation process such as filtration or magnetic separation in HEF. HEF also has the advantage of not requires a previous pH adjustment, which facilitates working in a higher pH range. In this work, Fe, Cu and Fe/Cu bimetallic nanoparticles (Fe/Cu NPs) were synthesized, characterized and used for the degradation of Nafcillin (NAF). The effect of the adsorption and the anodic oxidation (AO-H₂O₂) process was tested to assess their influence on HEF. NAF adsorption did not exceed 24% of antibiotic removal and the AO-H₂O₂ process eliminated the total NAF after 240 min of electrolysis. Through the HEF process, the antibiotic was completely removed using Fe/Cu NPs after 7.0 min of electrolysis, while these NPs, mineralization reached 41% after 240 min. In this case, NAF degradation occurs mainly due to the generation of hydroxyl radicals in the BDD electrode, and the Fenton reaction with Fe and Cu NPs. The main organic intermediates produced during the degradation of NAF by HEF were identified allowing the proposal of degradation pathway. Finally, the antibiotic was also completely eliminated from a wastewater from slaughterhouse after 15 min of treatment by HEF and using Fe/Cu bimetallic NPs.

Stabilized landfill leachate treatment by zero valent aluminium-acid system combined with hydrogen peroxide and persulfate based advanced oxidation process

The toxic leachate generated from landfills is becoming a major nuisance to the environment and has vital role in groundwater contamination. This study evaluated the potential of zero valent aluminium (ZVAl) based advanced oxidation processes (AOPs) for stabilized landfill leachate treatment. Hydrogen peroxide (HP) and persulfate (PS) were used to generate additional radicals in aerated ZVAl acid process. ZVAl-acid system achieved 83% COD removal efficiency under optimized conditions such as acid washing time of 20 min, ZVAl dose of 10 g L^-1 at initial pH 1.5. The highest exclusion efficiencies in terms of TOC, COD as well as color were 83.52%, 96% and 63.71% respectively in treatment systems showing the following order: ZVAl/H⁺/Air/HP/PS > ZVAl/H⁺/Air/PS > ZVAl/H⁺/Air/HP > ZVAl/H⁺/Air > ZVAl/H⁺. The involvement of other metals such as Fe and Cu in the process has been found. The reusability study revealed that ZVAl powder can be effectively used up to three cycles. The 28.48 mg/l of Al³⁺ residue was observed in this process which has to be removed before discharge of effluent. The study indicated that the ZVAl based AOPs is stable and active for the degradation of organic pollutants present in landfill leachate and a promising solution except for the aluminium discharge which has to be given special care.

Flyash augmented Fe3O4 as a heterogeneous catalyst for degradation of stabilized landfill leachate in Fenton process

In this study magnetite (Fe₃O₄) was augmented over coal flyash and analyzed for the effectiveness as a catalyst in heterogeneous Fenton process for the degradation of persistent organic pollutant present in stabilized landfill leachate. Fe₃O₄ and flyash augmented Fe₃O₄ was prepared by simple chemical precipitation method and both had magnetic nature. XRD, FTIR and SEM with EDX characterization were consummated for both catalysts. The Fenton experiments were performed in batch mode and to identify the optimal operating condition for effective COD removal the leachate pH, catalysts and H₂O₂ dosages were varied. The reusability of the catalysts was studied. To understand the degradation mechanism adsorption study, Fenton oxidation of benzoic acid and scavenging experiments with KI and NaF were performed. It was witnessed that flyash augmented Fe₃O₄ exhibited 84.7% of COD degradation which was 12.3% of higher removal efficiency than Fe₃O₄ at optimum pH 3, 0.05 M H₂O₂ and 1000 mg/L of catalyst dosage in 100 min reaction time. This flyash augmented Fe₃O₄ showed 68% of TOC removal and good increment in biodegradability. Poor NH₃-N removal was observed in the Fenton treatment process. Decrease in aromaticity was found based on SUVA₂₅₄ value and also indicated the removal of organic matter. Similarly, reusability and stability were higher than Fe₃O₄. The results indicate that flyash augmented Fe₃O₄ is a competent catalyst in heterogeneous Fenton process for treatment of mature leachate. The usage of waste material flyash with Fe₃O₄ decreases the co-aggregation of Fe₃O₄ and improves the catalytic performance.

Methylene Blue Catalytic Degradation Using Silver and Magnetite Nanoparticles Functionalized with a Poly(ionic liquid) Based on Quaternized Dialkylethanolamine with 2-Acrylamido-2-methylpropane Sulfonate-co-Vinylpyrrolidone

Catalytic degradation of organic water pollutants has emerged as a cost- and energy-effective technique to treat wastewater. In this work, new silver and magnetite nanoparticles (NPs) were prepared with a protic poly(ionic liquid) (PIL) based on a quaternized diethylethanolamine cation combined with 2-acrylamido-2-methylpropane sulfonate-co-vinylpyrrolidone (QAMPSA/VP) as a capping and a reducing agent. The morphology, particle size, surface charge, thermal stability, and magnetic properties of QAMPS/VP-Ag and Fe₃O₄ NPs were investigated to determine the efficiency of the PIL as a reducing and a capping agent to protect the produced NPs from oxidation or thermal degradation. The activation energy, enthalpy, and entropy of the catalytic degradation of the cationic methylene blue (MB) dye in the presence of QAMPS/VP-Ag and Fe₃O₄ NPs were determined. The data elucidated that MB was completely degraded in 8 min in the presence of QAMPS/VP-Fe₃O₄ NPs as a Fenton oxidation catalyst. Moreover, their good magnetic properties allowed their easy separation and reuse for five cycles without losing their magnetic or catalytic properties.

Sono-photo-assisted heterogeneous activation of peroxymonosulfate by Fe/CMK-3 catalyst for the degradation of bisphenol A, optimization with response surface methodology

The present study examined the removal of bisphenol A (BPA) and total organic carbon (TOC) from aqueous solutions by the Fe/CMK-3 as peroxymonosulfate activator used in the sono-photo-catalytic process. The synthesis of Fe/CMK-3 was carried out using the co-precipitation method, and it was characterized by FTIR, XRD, BET, EDX, and TEM. The results showed that the iron nanoparticles were uniformly embedded in the CMK-3 pores. The effect of factors affecting on the removal of BPA and TOC was evaluated by response surface methodology (RSM) with center composite design (CCD). The analysis of variance of the quadratic model showed that the model is significant (p value < .0001 and R²  > 99.4%) and can be used to optimize the removal efficiency of BPA. Optimization results showed that the highest removal efficiency of BPA (100%) and TOC (80.6%) was achieved in optimum conditions of pH 7.8, catalyst dose 0.33 g/L, PMS dose 3.35 mmol/L, BPA concentration 39.3 mg/L, and 78.5 min. In addition, statistical analysis of the data showed that, in the studied range, the initial concentration of BPA was the most influential factor, followed by pH and PMS dose. Highest catalytic stability of Fe/CMK-3 showed the potential applicability of catalyst in the treatment of BPA-containing solutions. The quenching test showed that sulfate radical was the main responsible for the removal of BPA. The decrease in I_OUR value after the 75-min reaction time indicates that this process has a high ability for oxidation of the pollutant and its intermediates. Generally, the observed results suggest that the Fe-CMK-3/UV/US/PMS system can be a promising procedure for the removal of persistent pollutants such as BPA from aqueous media. PRACTITIONER POINTS: Fe/CMK-3 exhibited prominent catalytic activity and high stability for peroxymonosulfate activation. Effective degradation of bisphenol A was achieved in the Fe-CMK-3/UV/US/PMS system. The effect of five factors at five levels and their interactions during the removal of BPA was evaluated by RSM method coupled with central composite design (CCD). The analysis of variance of the quadratic model showed that the model is very significant (p value < .0001) and can be used to optimize the removal efficiency of BPA. The quenching test showed that sulfate radical was the main responsible for the removal of BPA. Reducing I_OUR value after the 75-min reaction time indicates that toxicity of the solution was significantly decreased in this system.

Degradation of mixed dye via heterogeneous Fenton process: Studies of calcination, toxicity evaluation, and kinetics

In this study, the degradation of mixed dye (mixture of Azure B and Congo red) was investigated using iron-loaded black soil as a catalyst via Fenton process. Iron-loaded black soil catalyst was prepared by the wet impregnation method, calcined at different temperatures with varying of iron loading on black soil. Their behavior was compared through characterization techniques (FTIR and XRD). Separately, the effect of calcination and aging of catalyst was investigated on the degradation of mixed dye with optimized conditions. Significant degradation (>90% only in 10 min) was observed in optimum conditions. Toxicity measurement was done by a seed germination test which gave significant results. In the kinetic study, it was found that Behnajady-Modirshahla-Ghanbery (BMG) model was the best suited for this process compared to other models. In addition, thermodynamic properties (Gibbs free energy [∆G], activation energy [E_a ], activation enthalpy [∆H], and activation entropy [∆S]) were also calculated. The stability of synthesized catalyst was found to be satisfactory. PRACTITIONER POINTS: Application of Iron-loaded black soil catalyst Mixed dye degradation and toxicity measurement Thermodynamic property studies.

Efficient electrochemical degradation of tetrabromobisphenol A using MnO2/MWCNT composites modified Ni foam as cathode: Kinetic analysis, mechanism and degradation pathway

In this study, MnO₂/MWCNT hybrids, prepared using a solvothermal method, were coated onto Ni foam and then used as a cathode for tetrabromobisphenol A (TBBPA) degradation. The reaction was confirmed to exhibit the pseudo first-order kinetics. Compared with the original Ni foam cathode, the fabricated electrode exhibited higher catalytic activity, attributed to its strong cross-linking and ability to produce catalytic free radicals. Radical scavenger experiments revealed that O₂^- and OH were involved in the decomposition of TBBPA. The effects of current density, pH, catalyst dosage, and initial TBBPA concentration on removal efficiency were further studied. An optimal removal rate of 98.3% was achieved while the rate constant reached values up to 0.07293 min^-1 and the debromination rate was more than 75.4% within 60 min. The electrode showed high catalytic performance and low catalyst loss after 10 cycles, indicating its excellent stability and reusability. The probable mechanism and pathway of TBBPA degradation were suggested based on the analysis of intermediate products. It could be inferred that the decomposition of TBBPA involved CC bond breaks (oxidation) and debromination (reduction). The MnO₂/MWCNT-Ni foam could be a promising cathode material for electrochemical degradation of halogenated organic compounds.

Decolorization and Mineralization of Rhodamine B in Aqueous Solution with a Triple System of Cerium(IV)/H2O2/Hydroxylamine

Hydroxylamine (HA) can react with hydrogen peroxide (H₂O₂) to generate hydroxyl radical (HO^•), but the reaction rate between them is very slow (2.2 × 10^-4 M^-1 s^-1). We propose a new system to accelerate the formation of aminoxyl radical (NH₂O^•) by the addition of cerium [Ce(IV)] to induce the continuous production of HO^• through reaction with H₂O₂. We also investigate the decolorization and mineralization of rhodamine B (RhB) and mechanism in the Ce(IV)/H₂O₂/HA system. The initial pH plays a significant role in decolorization of RhB. In this work, observation of the rapid decolorization process after 60 min revealed that approximately 80% of RhB was degraded at the initial pH of 4.0. The HO^• radicals were considered as the primary reactive oxidant in the system, during its investigation through coumarin capturing, benzoic acid capturing, and radical quenching experiments. The results of the present study suggest that the addition of Ce(IV) can greatly enhance the production of HO^•, and the rapid decolorization and mineralization of RhB can occur through the Ce(IV)/H₂O₂/HA system at acidic pH conditions.

Iron impregnated biochars as heterogeneous Fenton catalyst for the degradation of acid red 1 dye

In the present work, Acid Red 1 (AR1) dye degradation by two heterogeneous Fenton catalysts, namely iron loaded rice husk biochar (Fe-RHB) and coir pith biochar (Fe-CPB) are studied. Biochar prepared from RHB and CPB were sonicated in the presence of ferric nitrate for the synthesis of Fe-RHB and Fe-CPB by incipient impregnation method. Effect of operational parameters such as pH, the dosage of catalyst, H₂O₂ concentration and temperature were examined. Characterization of the synthesized Fenton catalyst, Fe-RHB and Fe-CPB were analysed by SEM, EDS, XRD and XPS techniques. In Fe-RHB Fenton system, maximum dye removal efficiency of 97.6% and TOC removal efficiency of 84.2% were obtained at pH 3 for 50 mg L^-1 of AR1 concentration, with 16 mM of H₂O₂ and 5 g L^-1 of catalyst dosage within 120 min reaction time. Similarly, for Fe-CPB, maximum dye removal efficiency of 99.1% and TOC removal efficiency of 86.7% were obtained with 16 mM of H₂O₂ and 4 g L^-1 of dosage for 50 mg L^-1 of initial dye concentration at pH 3. The prepared catalysts can be reused for successive cycles as the catalyst materials are highly stable and have very less iron leaching property.

Stabilized landfill leachate treatment using heterogeneous Fenton and electro-Fenton processes

In the present study, stabilized landfill leachate treatment by heterogeneous Fenton and electro-Fenton (EF) was carried out. Iron-manganese binary oxide loaded zeolite (IMZ) was used as a catalyst for generating hydroxyl radicals in the acidic medium. Heterogeneous Fenton process was capable of removing 88.6% COD from landfill leachate at the optimal conditions, while 87.5% COD removal was observed at optimal EF treatment conditions. Biodegradability of landfill leachate was increased significantly from 0.03 to 0.52 after Fenton treatment. The prepared heterogeneous catalyst was found reusable with a reduction in COD removal rate. Even though, both the processes are efficient for leachate treatment, the low catalyst dosage requirement in case of EF process justifies that it is more feasible than Fenton process.

Efficient catalytic degradation of bisphenol A by novel Fe0- vermiculite composite in photo-Fenton system: Mechanism and effect of iron oxide shell

Novel Fe⁰-vermiculite (Fe-Ver-C-H₂) composite was synthesized by thermal reduction and acted as catalysts to remove bisphenol A (BPA) in photo-Fenton system. In term of activation ability toward H₂O₂, separation ability and stability, Fe-Ver-C-H₂ presented obvious advantages over other kinds of Fe⁰-vermiculite composite (Fe-Ver-NaBH₄), which obtained by traditional liquid reduction. The reason was that iron oxide shells on the surface of Fe⁰ were α-Fe₂O₃ and Fe₃O₄ for Fe-Ver-NaBH₄ and Fe-Ver-C-H₂, respectively. And for Fe-Ver-C-H₂, the synergistic effect between iron core (Fe⁰) and iron oxide shell (Fe₃O₄) is beneficial to catalytic performance. The mechanism and plausible pathway of BPA degradation were also proposed according to the results of radical scavenger studies and gas chromatography-mass spectrometry (GC-MS), respectively. In addition, factorial effects for Fe-Ver-C-H₂ in photo-Fenton system were also investigated and optimized as: pH of 5, dosage of 0.2 g L^-1 and H₂O₂ concentration of 20 mM. This study presented a facile method to synthesize novel Fe⁰-vermiculite composite and provided a new sight to investigate the effect of iron oxide shell on the catalytic performance when Fe⁰-vermiculite composite acted as catalyst to remove contaminants from the environment in photo-Fenton system.

Ferrocene functionalized graphene based electrode for the electro-Fenton oxidation of ciprofloxacin

Ferrocene functionalized graphene based graphite felt electrode was firstly investigated for heterogeneous electro-Fenton oxidation of ciprofloxacin in neutral pH condition. Electrochemical reduction of Ferrocene functionalized graphene oxide (Fc-ErGO) was performed by cyclic voltammetry technique. At neutral pH condition, Fc-ErGO electrode (0.035 min^─1) exhibited ∼3 times and ∼9 times higher removal rates in comparison with plane ErGO (0.010 min^─1) and plane graphite felt (0.004 min^─1) electrodes respectively. The effect of pH and applied potential were studied for the degradation of ciprofloxacin in Fc-ErGO based electrode. Higher removal rate was observed at acidic pH (0.222 min^─1), whereas alkaline pH showed lower removal efficiency (0.014 min^─1). > 99% removal of ciprofloxacin was achieved with in 15 min and 120 min of reactions period at pH 3.0 and pH 7.0, respectively. H₂O₂ generation was found to be high in plane ErGO electrode system in all of the pH conditions. Owing to the high redox ability of ferrocene, Fc-ErGO electrode generated high concentration of OH radicals (426 μM pH 3.0; 247 μM pH 7.0; 210 μM pH 9.0) than ErGO and plane graphite felt electrodes; The electrode reusability study was performed to understand the electrode stability. There was no significant change in removal efficiency even after the 5th cycle of reusability study at both acidic and neutral conditions. The possible mechanism of oxidation in Fc-ErGO based electro-Fenton process was also proposed based on the continuous monitoring of H₂O₂ and OH radicals generated in the system.

Removal of rhodamine B dye from aqueous solution by electro-Fenton process using iron-doped mesoporous silica as a heterogeneous catalyst

In the current study, Rhodamine B (RhB) dye was removed by electro-Fenton (EF) process using iron-doped SBA-15 (Fe-SBA-15; SBA: Santa Barbara Amorphous) mesoporous silica as a heterogeneous catalyst. This catalyst was prepared with the help of ferric nitrate nonahydrate as a forerunner by wet impregnation method. Various techniques of characterization such as XRD and N₂ adsorption-desorption isotherms were performed to confirm the presence of iron particles in the pores of the catalyst. These characterization methods were also used to examine the morphological properties and textural arrangement of the synthesized material. In the batch study of EF process, 750 mL working volume of RhB dye was taken. Anode and cathode used in the process were graphite electrodes respectively with effective area of 25 cm² each. To maximise the process efficiency, the effect of initial pH, applied voltage, electrode spacing, the concentration of supporting electrolyte and Fe-SBA-15 dosage were investigated and optimized. The optimum conditions obtained were pH of 2, voltage of 8 V, an electrode spacing of 3 cm and Fe-SBA-15 dosage of 15 mg L^-1. At the end of 3 h electrolysis, maximum RhB removal of 97.7% and TOC removal of 35.1% were achieved for 10 mg L^-1 RhB concentration. In a batch study with real wastewater, 97% of color and 39% of TOC were removed at optimum conditions. Utilization of EF heterogeneous catalyst Fe-SBA-15 is an alternative technique for the elimination of dyes from solution.