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The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO2+) generation has been reported as one of the primary oxidants. FeO2+ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO2+ in the Fenton reaction depends on factors such as pH and Fe: H2O2 ratio. Reaction mechanisms have been proposed to generate FeO2+, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO2+ (using xylidine as a selective substrate). The findings revealed that FeO2+ production is increased compared to the classical Fenton reaction and that FeO2+ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO2+ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO2+ through this pathway.

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Mid-high-frequency ultrasound (200-1000 kHz) eliminates organic pollutants and also generates H2O2. To take advantage of H2O2, iron species can be added, generating a hybrid sono-Fenton process (sF). This paper presents the possibilities and limitations of sF. Heterogeneous (a natural mineral) and homogeneous (Fe2+ and Fe3+ ions) iron sources were considered. Acetaminophen, ciprofloxacin, and methyl orange were the target organic pollutants. Ultrasound alone induced the pollutants degradation, and the dual competing role of the natural mineral (0.02-0.20 g L-1) meant that it had no significant effects on the elimination of pollutants. In contrast, both Fe2+ and Fe3+ ions enhanced the pollutants' degradation, and the elimination using Fe2+ was better because of its higher reactivity toward H2O2. However, the enhancement decreased at high Fe2+ concentrations (e.g., 5 mg L-1) because of scavenger effects. The Fe2+ addition significantly accelerated the elimination of acetaminophen and methyl orange. For ciprofloxacin, at short treatment times, the degradation was enhanced, but the pollutant complexation with Fe3+ that came from the Fenton reaction caused degradation to stop. Additionally, sF did not decrease the antimicrobial activity associated with ciprofloxacin, whereas ultrasound alone did. Therefore, the chemical structure of the pollutant plays a crucial role in the feasibility of the sF process.

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Diverse reducing mediators have often been used to increase the degradation of emerging pollutants (EPs) and dyes through the Fenton reaction (Fe2+ + H2O2 → Fe3+ + HO● + HO-). Adding reductants can minimize the accumulation of Fe3+ in a solution, leading to accelerated Fe2+ regeneration and the enhanced generation of reactive oxygen species, such as the HO● radical. The present study consisted in reviewing the effects of gallic acid (GA), a plant-extracted reductant, on the Fenton-based oxidation of several EPs and dyes. It was verified that the pro-oxidant effect of GA was not only reported for soluble iron salts as a catalyst (homogeneous Fenton), but also iron-containing solid materials (heterogeneous Fenton). The most common molar proportion verified in the studies was catalyst:oxidant:GA equal to 1:10-20:1. This shows that the required amount of both catalyst and GA is quite low in comparison with the oxidant, which is generally H2O2. Interestingly, GA has proven to be an effective mediator at pH values well above the ideal range of 2.5-3.0 for Fenton processes. This allows treatments to be carried out at the natural pH of the wastewater. The use of plant extracts or wood barks containing GA and other reductants is suggested to make GA-mediated Fenton processes easier to apply for treating real wastewater.

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The present work consisted in evaluating the effect of a natural plant reducer, gallic acid (GA), on the discolouration/oxidation of two azo dyes by Fenton processes (Fe3+/H2O2 and Fe2+/H2O2). A kinetic study was performed to better interpret the discolouration data at different temperatures. The 1st-order kinetic model presented the best fit for the experimental data of methyl orange discolouration, while the 2nd-order was better for chromotrope 2R. Due to the addition of GA and the temperature rise, there were increases in discolouration and in the reaction rate constant values. As a highlight, it was possible to verify the reduction of the apparent activation energy (Ea) due to the presence of GA. For example, Ea for discolouring methyl orange corresponded to 81.5 and 53.6 kJ.mol-1 by Fe2+/H2O2 and Fe2+/H2O2/GA, respectively. Thus, it can be inferred that the GA reduces the energy barrier to increase the oxidation of dyes by Fenton processes.

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Amino acid cysteine has been used as reducing mediator with the aim of improving dye degradation by homogeneous Fenton processes (Fe2+/H2O2 and Fe3+/H2O2). Through its known Fe3+-reducing activity, this amino acid can enhance the production of reactive oxygen species as HO• (hydroxyl radical) and its pro-oxidant properties have been verified while decolorizing diverse dyes in the present work. Its presence enhanced decolorization of Methyl Orange, Phenol Red, Safranin T, Rhodamine B, Reactive Black 5 and Reactive Yellow 2, mainly in reactions initially containing Fe3+ as a catalyst (Fe3+-reactions). E.g. Fe3+/H2O2 and Fe3+/H2O2/cysteine systems decolorized 27% and 44% of Phenol Red after 60 min, respectively. A kinetic modeling analysis has revealed that 1st-order and mainly 2nd-order kinetic models were well fitted to both Fe2+- and Fe3+-reactions data. Improvements in reaction rate constants have been observed by adding cysteine. In experiments performed at varied temperatures, it was found a decrease in activation energy (Ea) due to cysteine addition while decolorizing Safranin T: Ea decreased from 104.6 to 88.9 kJ mol-1 for Fe3+-reactions and from 81.0 to 52.2 kJ mol-1 for Fe2+-reactions. Therefore, it was found that cysteine decreases the energy barrier so as to improve Fenton-based decolorization reactions.

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The use of advanced oxidative processes (AOPs) is an efficient alternative for the treatment of textile wastewaters. The aim of this study was to assess the dye removal efficiency of a Fenton-based degradation process followed by a polishing step using biochar prepared from rice husk. Six recalcitrant textile dyes - Reactive Red 195 (D1), Synolon Brown S2 (D2), Orange Remazol RGB (D3), Yellow Synozol K3 (D4), Reactive Orange (D5), and Reactive Black 5 (D6) - were treated with Fenton and photo-Fenton processes (with and without biochar polishing) under optimized conditions. The results showed a general efficiency ranking: photo-Fenton + biochar ≈ Fenton + biochar > photo-Fenton ≈ Fenton. The Fenton process was also efficient for the regeneration of the dye-saturated biochar. The photo-Fenton + biochar process achieved the following color removal percentages: D1 (98.8%), D2 (99.7%), D3 (98.9%), D4 (96.3%), D5 (94.2%) and D6 (94.8%). This process was applied to a real conventionally-treated textile wastewater and analysis showed a reduction in BOD (87.5% degradation), COD (62.5% degradation) and color (93.5% mean removal). These results reveal the possibility for the reuse of the treated water for non-potable industrial uses, for example, floor washing or the cleaning of machines and toilet areas.

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Abstract Agave cocui vinasse was physicochemically characterized with reference to the relevant environmental regulations. The following results were obtained: COD: 71,000 mg.L-1, total solids: 21,000 mg.L-1, dissolved solids: 17,000 mg.L-1; pH: 4.06, conductivity: 9.45 mhoscm-1, total Fe: 48.83 mg.L-1, total phenols: 8.66 mg.L-1; BOD: 30,000 mg.L-1. Fenton and photo-Fenton reactions were applied to treat the wastewater produced. For the Fenton process, the optimal oxidation conditions found were pH = 3.48, [COD]:[H2O2] mass ratio = 1:5, and [Fe+2]: [H2O2] mass ratio = 1:6. For the photo-Fenton process, the optimal parameters found were: pH = 3.98, [COD]:[H2O2] = 1:7.86, and [Fe+2]: [H2O2] = 1:5. The experimental data were adjusted to fit second order polynomial models with R2 = 0.88 for the Fenton process and R2 = 0.91 for the photo-Fenton process, respectively. The sludge produced featured the following characteristics: average COD: 41,000 mg.L-1, total Fe: 296,000 mg.L-1, pH: 7.7. The variables with the greatest influence in both processes were [Fe+2]:[H2O2] and [COD]:[H2O2].

Resumen Se caracterizó fisicoquímicamente la vinaza de Agave cocui con base en regulaciones ambientales de referencia. Se obtuvieron los siguientes resultados: DQO: 71 000 mg.L-1, sólidos totales: 21 000 mg.L-1, sólidos disueltos: 17 000 mg.L-1; pH: 4,06, conductividad: 9,45 mhos.cm-1, hierro total: 48,83 mg.L-1, fenoles totales: 8,66 mg.L-1 ; DBO: 30 000 mg.L-1. Se aplicaron las reacciones Fenton y foto Fenton para tratar las aguas residuales producidas. Para el proceso Fenton, las condiciones de oxidación óptimas encontradas fueron pH = 3,48, relación de masa [DQO]:[H2O2] = 1:5 y relación de masa [Fe+2]:[H2O2] = 1:6. Para el proceso foto Fenton, los parámetros óptimos encontraron fueron: pH = 3,98, [DQO]:[H2O2] = 1:7,86 y [Fe+2]:[H2O2] = 1:5. Los datos experimentales fueron ajustados a modelos de segundo orden polinomial con R2 = 0,88 para el proceso Fenton y R2 = 0,91 para el proceso foto Fenton, respectivamente. El lodo producido presentó las siguientes características: DQO promedio: 41 000 mg.L-1, hierro total: 296 000 mg.L-1, pH: 7,7. Las variables con mayor influencia en ambos procesos fueron [Fe+2]:[H2O2] and [DQO]: [H2O2].

Resumo A vinhaça de Agave cocui foi caracterizada físico-quimicamente de acordo com as normas ambientais de referência. Os seguintes resultados foram obtidos: DQO: 71.000 mg.L-1, sólidos totais: 21.000 mg.L-1, sólidos dissolvidos: 17.000 mg.L-1; pH: 4,06, condutividade: 9,45 mhos.cm-1, Fe total: 48,83 mg.L-1, fenóis totais: 8,66 mg.L-1; DBO: 30,000 mg.L-1. As reações de Fenton e foto Fenton foram aplicadas para tratar a água residual produzida. Para o processo Fenton, as condições de oxidação ideais encontradas foram pH = 3,48, razão de massa [DQO]:[H2O2] = 1:5 e razão de massa [Fe+2]:[H2O2] = 1:6. Para o processo de foto Fenton, os parâmetros ideais encontrados foram: pH = 3,98, [DQO]:[H2O2] = 1:7,86 e [Fe+2]:[H2O2] = 1:5. Os dados experimentais foram ajustados para se adequar modelos polinomiais de segunda ordem com R2 = 0,88 para o processo Fenton e R2 = 0,91 para o processo foto Fenton, respectivamente. O lodo produzido apresentou as seguintes características: DQO média: 41.000 mg.L-1, Fe total: 296.000 mg.L-1, pH: 7,7. As variáveis com maior influência em ambos os processos foram [Fe+2]:[H2O2] e [DQO]:[H2O2].

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This article presents a study on the degradation of a residual textile mixture composed of cationic surfactant cetyltrimethylammonium bromide (CTAB) and the remazol yellow gold RNL-150% and reactive blue BF-5G textile dyes. This was carried out by employing the photo-peroxidation and photo-Fenton processes in LED and UV-C photoreactors. The photo-Fenton process was the most efficient as regards the degradation of the CTAB and dye mixture, for both types of radiation. In the kinetic study, degradations of 99% were obtained in 180 min for the chromophore groups using both types of radiation. The degradation of the CTAB and aromatic groups was, meanwhile, an average of 25% when employing LED radiation. The behavior of the degradation reaction was pseudo-first-order. Toxicity tests indicated that the solutions were better able to grow seeds and bacteria after treatment with the photo-Fenton process, using both types of radiation. The photo-Fenton processes carried out by employing LED and UV-C photoreactors were able to degrade the CTAB and dye mixture, thus highlighting the efficiency of LED radiation when its power (three times smaller) is compared to that of UV-C radiation. This process, therefore, represents an alternative for use in textile wastewater treatment systems.

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Formation of oxygen-based free radicals from photochemical decomposition of hydrogen peroxide (H2O2) on Mars may be a key factor in the potential survival of terrestrial-like organisms on the red planet. Martian conditions that generate reactive oxygen species involve the decomposition of H2O2 at temperatures of around 278 K under relatively high doses of C-band ultraviolet radiation (UVC). This process is further amplified by the presence of iron oxides and perchlorates. Photosynthetic organisms exhibit a number of evolutionary traits that allow them to withstand both oxidative stress and UVC radiation. Here, we examine the effect of free radicals produced by the decomposition of H2O2 under emulated martian conditions on the viability of Scenedesmus dimorphus, a unicellular alga that is resistant to UVC radiation and varying levels of perchlorate and H2O2, both of which are present on Mars. Identification and quantification of free radicals formed under these conditions were performed with Electron Paramagnetic Resonance spectroscopy. These results were correlated with the viability of S. dimorphus, and the formation of oxygen-based free radicals and survival of the alga were found to be strongly dependent on the amount of H2O2 available. For H2O2 amounts close to those present in the rarefied martian environment, the products of these catalytic reactions did not have a significant effect on the algal population growth curve.

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BACKGROUND: Wood-decay basidiomycetes are effective for the degradation of highly lignified and recalcitrant plant substrates. The degradation of lignocellulosic materials by brown-rot strains is carried out by carbohydrate-active enzymes and non-enzymatic Fenton mechanism. Differences in the lignocellulose catabolism among closely related brown rots are not completely understood. Here, a multi-omics approach provided a global understanding of the strategies employed by L. sulphureus ATCC 52600 for lignocellulose degradation. RESULTS: The genome of Laetiporus sulphureus ATCC 52600 was sequenced and phylogenomic analysis supported monophyletic clades for the Order Polyporales and classification of this species within the family Laetiporaceae. Additionally, the plasticity of its metabolism was revealed in growth analysis on mono- and disaccharides, and polysaccharides such as cellulose, hemicelluloses, and polygalacturonic acid. The response of this fungus to the presence of lignocellulosic substrates was analyzed by transcriptomics and proteomics and evidenced the occurrence of an integrated oxidative-hydrolytic metabolism. The transcriptomic profile in response to a short cultivation period on sugarcane bagasse revealed 125 upregulated transcripts, which included CAZymes (redox enzymes and hemicellulases) as well as non-CAZy redox enzymes and genes related to the synthesis of low-molecular-weight compounds. The exoproteome produced in response to extended cultivation time on Avicel, and steam-exploded sugarcane bagasse, sugarcane straw, and Eucalyptus revealed 112 proteins. Contrasting with the mainly oxidative profile observed in the transcriptome, the secretomes showed a diverse hydrolytic repertoire including constitutive cellulases and hemicellulases, in addition to 19 upregulated CAZymes. The secretome induced for 7 days on sugarcane bagasse, representative of the late response, was applied in the saccharification of hydrothermally pretreated grass (sugarcane straw) and softwood (pine) by supplementing a commercial cocktail. CONCLUSION: This study shows the singularity of L. sulphureus ATCC 52600 compared to other Polyporales brown rots, regarding the presence of cellobiohydrolase and peroxidase class II. The multi-omics analysis reinforces the oxidative-hydrolytic metabolism involved in lignocellulose deconstruction, providing insights into the overall mechanisms as well as specific proteins of each step.