Quantitative characterization of hydroxyl radical generation in a goethite-catalyzed Fenton-like reaction

Fast real-time monitoring of meat freshness based on fluorescent sensing array and deep learning: From development to deployment

A fluorescent sensor array (FSA) combined with deep learning (DL) techniques was developed for meat freshness real-time monitoring from development to deployment. The array was made up of copper metal nanoclusters (CuNCs) and fluorescent dyes, having a good ability in the quantitative and qualitative detection of ammonia, dimethylamine, and trimethylamine gases with a low limit of detection (as low as 131.56 ppb) in range of 5 ∼ 1000 ppm and visually monitoring the freshness of various meats stored at 4 °C. Moreover, SqueezeNet was applied to automatically identify the fresh level of meat based on FSA images with high accuracy (98.17 %) and further deployed in various production environments such as personal computers, mobile devices, and websites by using open neural network exchange (ONNX) technique. The entire meat freshness recognition process only takes 5 ∼ 7 s. Furthermore, gradient-weighted class activation mapping (Grad-CAM) and uniform manifold approximation and projection (UMAP) explanatory algorithms were used to improve the interpretability and transparency of SqueezeNet. Thus, this study shows a new idea for FSA assisted with DL in meat freshness intelligent monitoring from development to deployment.

The prospect of CuxO-based catalysts in photocatalysis: From pollutant degradation, CO2 reduction, and H2 production to N2 fixation

The topic of photocatalysis and CuxO-based materials has been intertwined for quite a long time. Its relatively high abundance in the earth's crust makes it an important target for researchers around the globe. One of the properties exploited by researchers is its ability to exist in different oxidation states (Cu0, Cu+, Cu2+, and Cu3+) and its implications on photocatalytic efficiency improvement. Recently, they have been extensively used as photocatalytic materials for dye and pollutant degradation. However, it has almost reached saturation levels, therefore, currently, they are being mostly utilized for CO2 reduction and H2 evolution. Hence, this review will discuss the evolution (in application) of CuxO-based photocatalysts, relating to their past, present, and future. Moreover, photocatalytic efficiency improvement strategies such as doping, heterojunction formation, and carbonaceous construction with other materials will also be touched upon. Finally, the prospect of Cu2O-based photocatalysts will be discussed in the field of photocatalytic N2 fixation to ammonia. The significance of N2 chemisorption on photocatalysts to maximize ammonia production will also be given importance.

Fenton Reaction in vivo and in vitro. Possibilities and Limitations

The review considers the problem of hydrogen peroxide decomposition and hydroxyl radical formation in the presence of iron in vivo and in vitro. Analysis of the literature data allows us to conclude that, under physiological conditions, transport of iron, carried out with the help of carrier proteins, minimizes the possibility of appearance of free iron ions in cytoplasm of the cell. Under pathological conditions, when the process of transferring an iron ion from a donor protein to an acceptor protein can be disrupted due to modifications of the carrier proteins, iron ions can enter cytosol. However, at pH values close to neutral, which is typical for cytosol, iron ions are converted into water-insoluble hydroxides. This makes it impossible to decompose hydrogen peroxide according to the mechanism of the classical Fenton reaction. A similar situation is observed in vitro, since buffers with pH close to neutral are used to simulate free radical oxidation. At the same time, iron hydroxides are able to catalyze decomposition of hydrogen peroxide with formation of a hydroxyl radical. Decomposition of hydrogen peroxide with iron hydroxides is called Fenton-like reaction. Studying the features of Fenton-like reaction in biological systems is the subject of future research.

Oxidative damage to βL-crystallin in vitro by iron compounds formed in physiological buffers

β_L-crystallin aggregation due to oxidative damage in the presence of H₂O₂ and ferric chloride was studied in-vitro under conditions close to physiological. It was shown that the protein aggregation characterized by the nucleation time and the aggregation rate significantly depended on the composition of the isoosmotic buffers used, and decreased in the series HEPES buffer > Tris buffer > PBS. Ferric chloride at neutral pH was converted into water-insoluble iron hydroxide III (≡Fe^IIIOH). According to the data of scanning electron microscopy the ≡Fe^IIIOH particles formed in HEPES buffer, Tris buffer, and PBS practically did not differ in structure. However, the sizes of ≡Fe^IIIOH floating particles measured by dynamic light scattering differed significantly and were 44 ± 28 nm, 93 ± 66 nm, 433 ± 316 nm (Z_aver ± SD) for HEPES buffer, Tris buffer, and PBS, respectively. It was found by the spin trap method that the ability of ≡Fe^IIIOH to decompose H₂O₂ with the formation of a ^•OH decreases in the series HEPES buffer, Tris buffer, and PBS. The authors suggest that the ability to generate ^•OH during the decomposition of H₂O₂ is determined by the total surface area of ≡Fe^IIIOH particles, which significantly depends on the composition of the buffer in which these particles are formed.

Effect of inorganic salt on the removal of typical pollutants in wastewater by RuO2/TiO2 via catalytic wet air oxidation

The treatment of high-salinity and high-organic wastewater is a tough task, with the removal of organic matter and the separation of salts often mutually restricting. Catalytic wet air oxidation (CWAO) coupled desalination technology (membrane distillation (MD), membrane bioreactor (MBR), ultrafiltration (UF), nanofiltration (NF), etc.) provides an effective method to simultaneously degrade the high-salinity (via desalination) and high-organic matters (via CWAO) in wastewater. In this work, five kinds of RuO₂/TiO₂ catalysts with different calcination temperatures were prepared for CWAO of maleic acid wastewater with a theoretical chemical oxygen demand (COD) value of 20,000 mg L^-1. RuO₂/TiO₂ series catalysts demonstrated prominent salt resistance, with more than 80% TOC removal rates in the CWAO system containing 5 wt% Na₂SO₄; while RuO₂/TiO₂-350 showed the best degradation performance in both non-salinity and Na₂SO₄-containing conditions. Multiple characterization techniques, such as XRD, BET, XPS, NH₃-TPD and TEM etc., verified the physicochemical structure of RuO₂/TiO₂ catalysts, and their influence on the degradation of pollutants. The calcination temperature was found to have a direct impact on the specific surface area, pore volume, oxygen vacancies and acid sites of catalysts, which in turn affected the ultimate catalytic activity. Furthermore, we also investigated the performance of the RuO₂/TiO₂-350 catalyst for the treatment of acids, alcohols and aromatic compounds with the addition of NaCl or Na₂SO₄, proving its good universality and excellent salt resistance in saline wastewater. Meanwhile, the relationship between the structure of three types of organic compounds and the degradation effect in the CWAO system was also explored.

Insight on the heterogeneously activated H2O2 with goethite under visible light for cefradine degradation: pH dependence and photoassisted effect

The iron mineral-catalyzed degradation of cephalosporin antibiotics with H₂O₂ occurs ubiquitously in nature. Despite numerous studies, the effects of environmental conditions on reactive species production and degradation processes of cephalosporins remain unclear. Here, we report the iron mineral of goethite as the efficient and heterogenous catalyst for the degradation of cefradine (CRD) via H₂O₂ activation under different conditions involving pH and visible light irradiation. Results show that the CRD removal rate is highly dependent on pH and visible light irradiation. Interestingly, when the pH ranges from 4.0 to 7.0, the degradation intermediates of CRD under dark are the same as under visible light conditions in the goethite/H₂O₂ system. And, the ratio of CRD degradation rate constant (k_Light/k_Dark) reaches a maximum at pH 5.0, suggesting that CRD existing as zwitterion species is preferable for its removal with photoassistance. The mechanism investigation reveals that both •OH and ≡[Fe^IVO]²⁺ oxidants are generated during the reaction process, and •OH is the major oxidant at acidic pH, while ≡[Fe^IVO]²⁺ is more likely to be formed with photoassistance at near-neutral pH. According to UPLC-MS/MS analysis, CRD degradation likely happens via hydrogen atom abstraction from cyclohexadienyl by •OH, thioether and olefin oxidation by ≡[Fe^IVO]²⁺, and Fe^III-catalyzed hydrolytic cleavage of β-lactam ring. These findings highlight the vital roles of pH and photoassistance in the heterogeneously activated H₂O₂ with goethite for CRD degradation.

Iron oxide nanoparticles embedded in organic microparticles from Yerba Mate useful for remediation of textile wastewater through a photo-Fenton treatment: Ilex paraguariensis as a platform of environmental interest - Part 1

Seven composites of iron oxide nanoparticles embedded in organic microparticles mediated by Cu(II) were synthesized using yerba mate (Ilex paraguariensis) dry leaf extract as precipitant, capping agent, and dispersant medium, using different Cu/Fe molar ratios. A thorough characterization of the particles by transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis-mass spectrometry (TGA-MS), Fourier transform infrared spectrometer (FTIR), and atomic absorption-spectrometry (AA) indicates that all materials have spheric-like morphology with nanoparticles composed by metal oxide phases embedded into organic microparticles. Interestingly, this organic matter is proposed to play an important role in the solids' photocatalytic activity in a photo-Fenton reaction, in which iron photo-leaching was elucidated, and a mechanism through ligand-to-metal charge transfer processes was proposed. All materials showed catalytic activity in the methyl orange elimination, achieving discolorations up to 96% in 2 h under UV irradiation at 375 nm. An experimental correlation between all samples' UV/Vis spectra and their performances for methyl orange discoloration was observed. This process opens a landscape very interesting for the use of agroindustrial residues for green synthesis of metal oxide nanomaterials and their use and understanding of organo-metallic systems participation in Fenton-based processes.

Oxalate regulate the redox cycle of iron in heterogeneous UV-Fenton system with Fe3O4 nanoparticles as catalyst: Critical role of homogeneous reaction

The redox cycle of iron is a well-known rate-determining step for hydroxyl radical generation in photo-Fenton system. In this study, oxalate was employed as regulator to enhance the degradation of Orange II in Fe₃O₄ magnetic nanoparticles (NPs)-catalyzed heterogeneous UV-Fenton system. Results showed that the oxalate could interact with the surface ≡Fe^III species of catalyst, which weakened the bond of ≡Fe^III-O and promoted the leaching of iron ions. Then the redox cycle of iron and generation of HO· would be accelerated via the homogeneous UV-Fenton reaction. The degradation rate constant of Orange II reached 0.220 min^-1 when additional oxalate concentration was 0.4 mM, which was 2.5 times as high as that without oxalate in heterogeneous UV-Fenton system. In this case, the removal efficiencies of color and TOC were 99.3% and 92.0% after 30 and 120 min treatment, respectively. In addition, based on the results of XRD and XPS characterization, it could be deduced that the crystal structure and elemental configuration of Fe₃O₄ magnetic nanoparticles could be maintained after reaction. Besides, the results of FTIR and magnetization characterization indicated that the C₂O₄^2- on surface of catalyst could be degraded and the catalyst could be easily separated from aqueous by applying an external magnetic field. The Fe₃O₄ magnetic nanoparticles showed high catalytic stability and reusability under the regulation of oxalate due to the fact that the leached iron ions could be re-adsorbed on the catalyst after treatment.

Efficient naphthalene degradation in FeS2-activated nano calcium peroxide system: Performance and mechanisms

Naphthalene (NAP) has received increasing concern due to frequent detection in groundwater and harm to humans. In this study, FeS₂ was selected as a novel catalyst to activate nano calcium peroxide (nCP) for NAP degradation. Batch experiments were conducted in a 250 mL glass reactor containing 0.1 mM NAP solution to investigate the effect of reagents dosage, pH, air conditions (with or without N₂ purge), and different solution matrixes on NAP degradation. Scavenging tests, electron paramagnetic resonance (EPR) spectrum, and radical probe tests were conducted to identify the main radicals. Results indicated that over 96% NAP was removed in a wide pH range (3.0-9.0) within 180 min at optimal dosage of nCP = 1.0 mM and FeS₂ = 5.0 g L^-1 in nCP/FeS₂ system. Aerobic condition was more beneficial to NAP degradation and the system could tolerate complex solution conditions. Moreover, HO• was determined to be responsible for NAP degradation. NAP degradation intermediates were detected by gas chromatography-mass spectrometry (GC-MS) and the possible degradation pathways were revealed. Finally, the efficient degradation of other organic pollutants confirmed the broad-spectrum reactivity of the nCP/FeS₂ system. Overall, these findings strongly demonstrated the potential applicability of nCP/FeS₂ system in remediating organic contaminated groundwater.

Singlet oxygen-dominated electrocatalytic oxidation treatment for the high-salinity quaternary ammonium compound wastewater with Ti/(RuxIry)O2 anode

The widespread application of quaternary ammonium compounds (QAC) has posed a serious hazard to the environment and human being, and high concentration of Cl^- in QAC wastewater may further increase the difficulty of pollutants elimination. In this study, such a QAC wastewater under high salinity conditions was chosen as the target, the prepared Ti/(Ru_xIr_y)O₂ anode exhibited favorable catalytic performance for the oxidation and mineralization of QAC under high salinity conditions. Increasing the Ru/Ir ratio of Ti-based electrode coating also slightly promoted the inner catalytic capacity. The combination of electron paramagnetic resonance (EPR) and quenching experiments indicates that ¹O₂ served as a main reactive specie in the Ti/(Ru_xIr_y)O₂ electrooxidation system. The increase of pH could decrease the removal efficiency of QAC for the reduced ¹O₂ yield, and the rise of Cl^- concentration could favor the QAC oxidation, and Cl^- was a better electrolyte to promote the oxidation of organic contaminants when compared to Na₂SO₄ or Na₂CO₃. Additionally, the conversion pathway of the model pollutant was tentatively investigated, the results demonstrated that there were almost no halogenated final products residual by electrocatalytic oxidation with Ti/(Ru_xIr_y)O₂ anode. This study not only elucidate the reaction mechanism of Ti/(Ru_xIr_y)O₂ anode electrocatalytic oxidation of high salinity QAC wastewater, but also may provide an efficacious and eco-friendly method for the treatment of high salinity QAC wastewater.

Modification of a Brazilian natural clay and catalytic activity in heterogeneous photo-Fenton process

The catalytic activity of a Brazilian natural clay modified with the immobilization of iron oxide was applied in the heterogeneous Fenton process for the degradation of the antibiotic sulfathiazole (STZ). The clay without any treatment indicated a lamellar type material with mesoporous distribution that presents a heterogeneous mixture of phases (type 1:1 and 2:1 structures), with a predominance of quartz, montmorillonite, gibbsite and kaolinite, and with SiO₂, Al₂O₃, Fe₂O₃, K₂O, TiO₂, MgO as major oxides. Its high absorption in the UV-Vis ranges with a bandgap energy of 1.9 eV was attributed to the presence of hematite. It was observed that the effects of the addition of starch before heat treatment, and impregnation with iron, modified the clay surface. F rom the X-ray photoelectron spectroscopy analysis it was concluded that a structural reorganization is related to the conversion of the various iron oxide phases into hematite, as well as promoting an increase in Fe²⁺/ Fe³⁺ redox reactions allowing rapid degradation of STZ. The catalyst impregnated with iron and treated at 600 °C showed to be an economical and versatile catalyst with high catalytic efficiency (>97% STZ degradation after 60 min), with small differences according to the type of LED device used. Furthermore, it presented high stability and reusability reaching 93% degradation of STZ after four cycles of reuse with low consumption of H₂O₂.

Degradation of Reactive Brilliant Red X-3B by Photo-Fenton-like Process: Effects of Water Chemistry Factors and Degradation Mechanism

Azo dye wastewater belongs to the highly concentrated organic wastewater, which is difficult to be treated by traditional biological processes. The oxidation efficiency of a single physicochemical method is not considerable. Recent research indicated that the advanced oxidation processes (AOPs) based on the highly reactive hydroxyl radical (∙OH) became one of the preferred methods in dealing with such dye wastewater. In this paper, the typical azo dye, reactive brilliant red X-3B, was employed as the target pollutant, and the transition metal Mn and hydrogen peroxide as the catalysts. A photo-Fenton-like process, UV/Mn2+-H2O2 system, was established, which enables a combination of various technologies to improve azo dye degradation efficiency while reducing disposal costs. The results indicated that the UV/Mn2+-H2O2 system had the synergism of Mn2+/H2O2 and UV/H2O2, which was 2.6 times greater than the sum of the two individual effects. And the degradation of X-3B reached the optimum under the conditions of 0.59 mmol/L of the Mn2+, 10 mmol/L of the H2O2, pH = 6 and a high level of DO. The ∙OH, generated from chem-catalytic and photocatalytic decomposition of H2O2, played the predominant role in the decolorization of X-3B and mineralization of its intermediates. The ∙OH tended to attack and break the chromophore group, resulting in the rapid decolorization of X-3B. The azo bond in X-3B was easy to be decomposed in the form of N2, while the triazinyl group was recalcitrant for ring opening. The degradation process of the UV/Mn2+-H2O2 system preferred to be conducted at an acidic condition and appropriate concentrations of Mn2+ and H2O2. The alkaline condition would decrease the utilization of H2O2, and excessive H2O2 would also quench the ∙OH.

pH Dependence of Hydroxyl Radical, Ferryl, and/or Ferric Peroxo Species Generation in the Heterogeneous Fenton Process

The heterogeneous Fenton process in the presence of Fe-containing minerals is ubiquitous in nature and widely deployed in wastewater treatment. While there have been extensive relevant studies, the dependence on pH of the nature and extent of oxidant generation and key reaction pathways remain unclear. Herein, the adsorption and decomposition of formate and H₂O₂ were quantified in the presence of ferrihydrite within the pH range of 3.0-6.0, and experiments with methyl phenyl sulfoxide were conducted to distinguish between HO^• and weaker oxidant(s) which react via oxygen atom transfer including ferryl ion ([Fe^IVO]²⁺) and/or ferric hydroperoxo intermediates (≡Fe^III(O₂H)). Both HO^• and [Fe^IVO]²⁺/≡Fe^III(O₂H) are concurrently produced on the surface over the acidic to near-neutral pH range. Despite the simultaneous formation of both oxidants, HO^• is the major oxidant responsible for substrate oxidation in the interfacial boundary layer with [Fe^IVO]²⁺/≡Fe^III(O₂H) exhibiting limited exposure to substrates. With an increase of pH, the yield of both oxidants is inhibited by the decreasing availability of surface sites due to ferrihydrite particle aggregation. Increasing pH also favors the nonradical decay of H₂O₂ as evident from the consistent oxidant production rate relative to the surface area (SSA) despite an accelerated H₂O₂ decay rate relative to SSA with pH increase.

Understanding the nature of Fenton processes in soil matrices: The role of iron forms and organic matter

Understanding the processes of pollutants removal in soil remediation practices is crucial to apply the appropriate treatment method. Although widely employed in soil contamination events, the mechanisms of the Fenton reaction are still debatable. To investigate the catalytic performance of soils towards the degradation of p-xylene in Fenton reactions, we performed a series of experiments employing two soil samples with different physical-chemical properties, Oxisol and Alfisol. These soils were subjected to extraction procedures that separated the different types of pedogenic iron oxides (amorphous and crystalline) and produced soil fractions with different organic matter contents. We observed that Oxisol, which contains high amounts of amorphous pedogenic iron oxides, performed better in hydrogen peroxide decomposition and radical generation but worse in p-xylene degradation. These results originated from the presence of hematite in Oxisol, which has a lower catalytic activity than goethite, the pedogenic oxide present in Alfisol. Samples containing high concentrations of organic matter performed better in decomposing hydrogen peroxide but worse in degrading p-xylene due to the scavenging of active species by labile organic matter compounds.

Heterogeneous Fenton Chemistry Revisited: Mechanistic Insights from Ferrihydrite-Mediated Oxidation of Formate and Oxalate

The heterogeneous Fenton process has been widely applied though some aspects of this process are still poorly understood. In this study, we simultaneously quantify the adsorption and decomposition of formate and H₂O₂ at pH 4.0 in the presence of freshly formed ferrihydrite and provide new insights into the ferrihydrite-induced heterogeneous Fenton mechanism with the aid of kinetic and reactive-transport modeling. Our results show that the decomposition of H₂O₂ and formate is controlled by surface-initiated reactions. Adsorbed formate occupies the surface sites otherwise available for reaction with H₂O₂ and therefore hampers the surface Fenton reactions despite the negligible accumulation of H₂O₂ on the surface. The minimal impact of methanol (an effective HO^• scavenger) on formate oxidation as well as the poor oxidation of fully adsorbed oxalate compared with the ready oxidation of partially adsorbed formate demonstrates that oxidation mainly occurs in the solid-liquid boundary layer, rather than in bulk or on the surface. This is suggested to be due to the diffusion of surface-generated HO^•, rather than surface Fe(II), to the boundary layer with the results of kinetic and reactive-transport modeling supporting this conclusion. The new findings are critical to our understanding of the removal behavior of more complex organic target species and to the design of more effective heterogeneous Fenton technologies.

Nontronite mineral clay NAu-2 as support for hematite applied as catalyst for heterogeneous photo-Fenton processes

This study describes the characterization of Nontronite, a clay mineral with high content of structural iron, before and after iron incorporation and 600 °C heat treatment. The Nontronite was classified as a mesoporous material, with high absorption in the UV-Vis range and band gap energy of 1.9 eV, indicative of the presence of superficial hematite, also verified in XRD analysis. The heat treatment promoted a structure rearrangement and the conversion of other iron phases to hematite, allowing the formation of surface irregular sites on Nontronite and facilitating the access for the decomposition of H₂O₂ into HO. Its catalytic activity in heterogeneous photo-Fenton process was evaluated during the degradation of the antibiotic sulfathiazole (STZ) and showed high activity achieving undetectable levels of STZ after 20 min under UV-LED irradiation and solar irradiation, and showing no iron leaching under controlled pH = 3. The degradation intermediates identified indicated hydroxylation as the main degradation route.

Fe-based Fenton-like catalysts for water treatment: Preparation, characterization and modification

Fenton reaction based on hydroxyl radicals () is effective for environment remediation. Nevertheless, the conventional Fenton reaction has several disadvantages, such as working at acidic pH, producing iron-containing sludge, and the difficulty in catalysts reuse. Fenton-like reaction using solid catalysts rather than Fe²⁺ has received increasing attention. To date, Fe-based catalysts have received increasing attention due to their earth abundance, good biocompatibility, comparatively low toxicity and ready availability, it is necessary to review the current status of Fenton-like catalysts. In this review, the recent advances in Fe-based Fenton-like catalysts were systematically analyzed and summarized. Firstly, the various preparation methods were introduced, including template-free methods (precipitation, sol gel, impregnation, hydrothermal, thermal, and others) and template-based methods (hard-templating method and soft-templating method); then, the characterization techniques for Fe-based catalysts were summarized, such as X-ray diffraction (XRD), Brunauer, Emmett and Teller (BET), SEM (scanning electron microscopy)/TEM (transmission electron microscopy)/HRTEM (high-resolution TEM), FTIR (Fourier transform infrared spectroscopy)/Raman, XPS (X-ray photoelectron spectroscopy), ⁵⁷Fe Mössbauer spectroscopy etc.; thirdly, some important conventional Fe-based catalysts were introduced, including iron oxides and oxyhydroxides, zero-valent iron (ZVI) and iron disulfide and oxychloride; fourthly, the modification strategies of Fe-based catalysts were discussed, such as microstructure controlling, introduction of support materials, construction of core-shell structure and incorporation of new metal-containing component; Finally, concluding remarks were given and the future perspectives for further study were discussed. This review will provide important information to further advance the development and application of Fe-based catalysts for water treatment.

Catalytic enrichment of plasma with hydroxyl radicals in the aqueous phase at room temperature

Iron oxide on mesoporous silica gave a synergy with plasma jet for HO˙ radical production at neutral pH.

Study of polarized activated carbon filters as simultaneous adsorbent and 3D-type electrode materials for electro-Fenton reactors

Electro-Fenton (EF) based water treatment processes using activated carbon (AC) packed beds constitute an attractive approach for the development of competitive degradation technology of persistent pollutants in aqueous effluents. In this work, the results of a study aimed to assess the effect on the EF performance of different parameters of the reactor's operation are presented. By means of a factorial experimental design, the influence of the AC source (lignitic or vegetal), AC acid pre-treatment, particle size distribution and the amount of Fe loaded resin in the reactor were analyzed. From the resulting data it was found that the most influential parameter in the EF performance of the reactor is the AC source. Modest effects were observed for AC acid pre-treatment, which limits Fe ion adsorption on the AC substrate. The use of a wide particle distribution of AC particles was also found to improve inter-particle electrical contact, thus favoring the electrochemical processes that take place inside the reactor. An investigation on the effect of the amount of Fe in the reactor as well as its distribution dynamics, also revealed that an excess of Fe ions in the reactor decreases the EF performance of the system since Fe ions efficiently adsorb on the AC substrate, particularly in non- acid treated samples. The best operation conditions consisted on using un-meshed vegetable AC, without acid pretreatment in an EF reactor loaded with 0.25 g of Fe, which allowed to reach full color removal of bright blue FCP model dye in 70 min.

Comparison of aniline removal by UV/CaO2 and UV/H2O2: Degradation kinetics and mechanism

The instability and rapid consumption of H₂O₂ limit the application of UV/H₂O₂ in water treatment. Recently, calcium peroxide (CaO₂) has been demonstrated as an effective source of H₂O₂. However, the performance and mechanism of UV/CaO₂ are still unknown. Herein, UV/CaO₂ and UV/H₂O₂ were compared for degradation of aniline. The removal efficiency of aniline by UV/CaO₂ was slightly lower than that by UV/H₂O₂, which could be attributed to the light scavenger by CaO₂ suspended particles. HO‧ was identified to participate in aniline degradation in both UV/CaO₂ and UV/H₂O₂, while O₂^-· was only involved in UV/CaO₂. The efficiency of aniline degradation in UV/CaO₂ was affected by the released H₂O₂ in the system. The release and decomposition rate of H₂O₂ in UV/CaO₂ system were influenced by the CaO₂ dosage and reaction pH, but slightly related with water matrix. Excessive CaO₂ would scavenge aniline degradation through the released H₂O₂ to react with HO‧. Acidic condition would enhance the concentration of H₂O₂ in UV/CaO₂ and promote the degradation of aniline. Cl^- showed slight and almost no effect on aniline degradation in UV/CaO₂ and UV/H₂O₂ systems, respectively, while HCO₃^- scavenged aniline degradation in UV/CaO₂. NO₃^- inhibited aniline degradation in both UV/CaO₂ and UV/H₂O₂. Compared to UV/H₂O₂, UV/CaO₂ shows the similar efficiency on organics removal but conquers the limitations in UV/H₂O₂, which is a promising alternative choice in water treatment.

Synergistic degradation of 4-chlorophenol by persulfate and oxalic acid mixture with heterogeneous Fenton like system for wastewater treatment: Adaptive neuro-fuzzy inference systems modeling

The 4-chlorophenol (4-CP) is known to be a highly toxic compound having harmful effects on human health and the environment. Due to adverse effect of 4-CP, a new combination of persulfate (PS) and oxalic acid (OA) with heterogeneous Fenton like (HFL) system was developed and applied for 4-CP degradation as an emerging contaminant from synthetic wastewater. The individual (OA, PS, and HFL) and combined (HFL/OA, HFL/PS, and HFL/OA/PS) systems were investigated under various conditions to synergistic effects verification and determination of degradation mechanism of 4-CP. Compared to individual and combined systems, significant synergetic of 4-CP degradation efficiency was observed by HFL/OA/PS system. The highest 4-CP degradation efficiency by HFL/OA/PS system under optimal conditions (solution pH: 6, H₂O₂ dose: 275 mg/L, goethite dose: 125 mg/L, OA dose: 50 mg/L and PS dose: 100 mg/L) with an initial 4-CP concentration of 30 mg/L was 99.6 ± 4.9% after 35 min reaction time. 4-CP degradation by HFL/OA/PS system was followed with the first-order kinetic. The application of radical scavengers including ethanol (EtOH) and tert-butyl alcohol (TBA) revealed that the SO₄^•- radical was determined as primary produced radical species. The Cl- ions release was measured during degradation reaction at various 4-CP concentrations and indicating the complete 4-CP degradation. The developing of the adaptive neuro-fuzzy inference system (ANFIS) for 4-CP degradation efficiency prediction was revealed. These results show that prediction of 4-CP degradation efficiency using HFL/OA/PS system is possible by the ANFIS model with a high accuracy (R²: 0.98).

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.

Surface composition and catalytic activity of an iron mining residue for simultaneous degradation of sulfonamide antibiotics

Iron mining residue was evaluated as a potential catalyst for heterogeneous Fenton/photo-Fenton degradation of sulfonamide antibiotics. The residue contained 25% Fe₂O₃ and 8% CeO₂, as determined by X-ray fluorescence spectroscopy, as well as other minor phases such as P₂O₅, SiO₂, and TiO₂. X-ray photoelectron spectroscopy analysis revealed a lower content of iron oxides on the surface, which restricted interaction of the residue with H₂O₂. Despite this limitation and the relatively low specific surface area (26 m² g^-1) of the crude iron mining residue (without any pretreatment), the material presented high catalytic activity for Fenton degradation of sulfonamide antibiotics. The degradation was strongly dependent on the initial pH, showing the highest efficiency at pH 2.5. For this condition, a concentration of sulfathiazole below the detection limit was obtained within 30 min, under black light irradiation and using 0.3 g L^-1 residue, with low H₂O₂ consumption (0.2 mmol L^-1). The residue also provided highly efficient sulfathiazole degradation in the dark, with the concentration of the antibiotic decreasing to an undetectable level after 45 min. Simultaneous degradation of two sulfonamide antibiotics revealed higher recalcitrance of sulfamethazine, compared to sulfathiazole, but the levels of both antibiotics decreased to below the detection limit after 45 min. The residue was very stable, since no significant concentration of soluble iron was detected after the degradation process. Furthermore, high catalytic activity was maintained during up to five cycles, showing the potential of this material for use as a low-cost and environmentally compliant catalyst in Fenton processes.

Chlorine incorporation into dye degradation by-product (coumarin) in UV/peroxymonosulfate process: A negative case of end-of-pipe treatment

Recently, UV/peroxymonosulfate (PMS) seems as a panacea for the treatment of recalcitrant organic pollutants; however, the presence of high concentration of chloride in saline wastewater indeed complicates this end-of-pipe technology. Here a negative case of UV/PMS for the treatment of one of secondary degradation byproducts of dyes (coumarin, COU) is demonstrated. The removal rate of COU is reduced by addition of Cl^- (0-10 mM). Further increase in Cl^- content favors a rapid COU degradation, whereas Cl^- involvement seems to open a "Pandora's box": 1) a variety of chlorinated organic intermediates such as 4-chloroisocoumarin and 5-chloro-2-hydroxy-benzaldehyde are identified; 2) Accumulation and relative increase of absorbable organic halogen (AOX) with reaction time in the presence of high levels of chloride are observed; 3) the acute toxicity of the treated COU solution increases; 4) mineralization rate of COU decreases with the increasing [Cl^-]. The fluorescence intensity in the UV/PMS/COU system declines with the addition of Cl^-, implying the scavenging effects of chloride on hydroxyl radicals. The possible reaction pathways of COU are discussed. These findings highlight the imperativeness of minimizing auxiliary salt dosages in dyeing processes (i.e., source reduction) and developing new end-of-pipe technologies that can work in a saline environment.

Photocatalytic Degradation of 2,4,6-Trichlorophenol by MgO–MgFe2O4 Derived from Layered Double Hydroxide Structures

In recent years, the search for solutions for the treatment of water pollution by toxic compounds such as phenols and chlorophenols has been increasing. Phenols and their derivatives are widely used in the manufacture of pesticides, insecticides, paper, and wood preservers, among other things. Chlorophenols are partially biodegradable but not directly photodegradable by sunlight and are extremely toxic—especially 2,4,6-trichlorophenol, which is considered to be potentially carcinogenic. As a viable proposal to be applied in the treatment of water contaminated with 2,4,6-trichlorophenol, this paper presents an application study of the thermally activated Mg/Fe layered double hydroxides as photocatalysts for the mineralization of this contaminant. Activated Mg/Fe layered double hydroxides were characterized by X-ray diffraction, thermal analysis, N2 physisorption, and scanning electron microscopy with X-ray dispersive energy. The results of the photocatalytic degradation of 2,4,6-trichlorophenol in aqueous solution showed good photocatalytic activity, with an efficiency of degradation of up to 93% and mineralization of 82%; degradation values which are higher than that of TiO2-P25, which only reached 18% degradation. The degradation capacity is attributed to the structure of the MgO–MgFe2O4 oxides derived from double laminate hydroxide Mg/Fe. A path of degradation based on a mechanism of superoxide and hollow radicals is proposed.

Insight on the generation of reactive oxygen species in the CaO2/Fe(II) Fenton system and the hydroxyl radical advancing strategy

Calcium peroxide (CaO₂) is a stable hydrogen peroxide (H₂O₂) carrier, and the CaO₂/Fe(II) system has been applied for treatment of various pollutants. It is commonly reported in the literature that hydroxyl radical (HO^●) and superoxide radical anions (O₂ ^●-) are the two main reactive oxygen species (ROSs) generated in the CaO₂/Fe(II) system. However, many of the reported results were deduced from degradation performance rather than specific testing of radical generation. Thus, the specific generation of ROSs and the influence of system conditions on ROSs yield is still unclear. To our knowledge, this is the first study specifically focusing on the generation of HO^● and O₂ ^●- in the CaO₂/Fe(II) system. Experimental conditions were optimized to investigate the production of HO^● and O₂ ^●-. The results showed the influences of CaO₂, Fe(II), and solution pH on HO^● and O₂ ^●- generation, and the HO^● generation efficiency was reported for the first time. In addition, the ROSs generation pathways in the CaO₂/Fe(II) system were elucidated. A strategy for enhancing HO^● yield is developed, based on the continuously dosing Fe(II). This proposed strategy has implications for the effective application of in situ chemical oxidation employing CaO₂/Fe(II) for groundwater remediation.

Generation of hydroxyl radicals from reactions between a dimethoxyhydroquinone and iron oxide nanoparticles

The hydroxyl radical (·OH) is a powerful oxidant that is produced in a wide range of environments via the Fenton reaction (Fe²⁺  + H₂O₂ → Fe³⁺  + ·OH + OH^-). The reactants are formed from the reduction of Fe³⁺ and O₂, which may be promoted by organic reductants, such as hydroquinones. The aim of this study was to investigate the extent of ·OH formation in reactions between 2,6-dimethoxyhydroquinone (2,6-DMHQ) and iron oxide nanoparticles. We further compared the reactivities of ferrihydrite and goethite and investigated the effects of the O₂ concentration and pH on the generation of ·OH. The main finding was that the reactions between 2,6-DMHQ and iron oxide nanoparticles generated substantial amounts of ·OH under certain conditions via parallel reductive dissolution and catalytic oxidation reactions. The presence of O₂ was essential for the catalytic oxidation of 2,6-DMHQ and the generation of H₂O₂. Moreover, the higher reduction potential of ferrihydrite relative to that of goethite made the former species more susceptible to reductive dissolution, which favored the production of ·OH. The results highlighted the effects of surface charge and ligand competition on the 2,6-DMHQ oxidation processes and showed that the co-adsorption of anions can promote the generation of ·OH.

Multi-wavelength spectrophotometric determination of hydrogen peroxide in water with peroxidase-catalyzed oxidation of ABTS

In this study, a new spectrophotometric method was proposed for the measurement of hydrogen peroxide (H₂O₂) in aqueous solutions. The method was based on the peroxidase (POD)-catalyzed reaction in which 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) was oxidized to form the stable green radical (ABTS⁺). The generated ABTS⁺ could be determined spectrophotometrically. The absorbance of the generated ABTS⁺ at 415 nm, 650 nm, 732 nm and 820 nm were linear with H₂O₂ concentrations in the range of 0-40 μM. The sensitivities of the proposed ABTS method for H₂O₂ determination at 415 nm, 650 nm, 732 nm and 820 nm were 6.29 × 10⁴ M^-1 cm^-1, 2.00 × 10⁴ M^-1 cm^-1, 2.54 × 10⁴ M^-1 cm^-1 and 1.89 × 10⁴ M^-1 cm^-1, respectively. The oxidation of ABTS to generate ABTS⁺ in POD-catalyzed H₂O₂ system at pH 6.0 was so fast that the determination time of the ABTS method was as short as 0.5 min. The stoichiometry of the reaction of H₂O₂ and ABTS in the presence of POD was calculated as nearly 1:2 (1:1.92). The residual absorbance of the generated ABTS⁺ was also found to be stable within 30 min in natural waters. Low H₂O₂ concentration in rainwater could be both measured by the ABTS method and the DPD method with high accuracy, however, H₂O₂ concentration in wastewater contained rhodamine B could only be accurately measured with the ABTS method at 732 nm. Moreover, waste solutions after H₂O₂ analysis with the proposed ABTS method were non-hazardous towards E. coli.

F, Walter Z. T, Xochitl DB, Mika S. Towards reliable quantification of hydroxyl radicals in the Fenton reaction using chemical probes

Quantification of hydroxyl radical concentration using two chemical probes was assessed through the Fenton reaction. The probes were 1,2-benzopyrone (coumarin) for fluorescence and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) for electron spin resonance (ESR). The corresponding hydroxylated species, namely 7-hydroxycoumarin (7HC) and 2-hydroxy-5,5-dimethyl-1-pyrroline-N-oxide (DMPO-OH adduct), were monitored by fluorescence and ESR-spin trapping techniques, respectively. The experiments were designed according to the theoretical conditions determined for stable fluorescence and EPR signals. The results demonstrate that: the optimal [chemical probe] : [H₂O₂] ratio predicted by a simplified quasi-steady-state model was in good agreement with the optimal [chemical probe] : [H₂O₂] ratio observed experimentally for [H₂O₂] : [Fe²⁺] = 10, and the proper adjustment of the [chemical probe] : [H₂O₂] ratio at a given concentration of the Fenton's reagent improves the detected amount of hydroxyl radicals. Finally, using DMPO required a higher concentration compared to coumarin to yield the same amount of ˙OH detected but resulted in a more reliable probe for detecting ˙OH under the consideration of this study.

Quantification of hydroxyl radical concentration using two chemical probes was assessed through the Fenton reaction.

Natural soil mediated photo Fenton-like processes in treatment of pharmaceuticals: Batch and continuous approach

This paper manifests the potential viability of soil as a cost-free catalyst in photo-Fenton-like processes for treating pharmaceuticals at large scale. Naturally available soil without any cost intensive modification was utilized as a catalyst to degrade pharmaceuticals, specifically ornidazole (ORZ) and ofloxacin (OFX). Soil was characterized and found enriched with various iron oxides like hematite, magnetite, goethite, pyrite and wustite, which contributes toward enhanced dissolution of Fe³⁺ than Fe²⁺ in the aqueous solution resulting in augmented rate of photo-Fenton reaction. The leached iron concentration in solution was detected during the course of experiments. The degradation of ORZ and OFX was assessed in solar induced batch experiments using H₂O₂ as oxidant and 95% ORZ and 92% OFX removal was achieved. Elevated efficiencies were achieved due to Fe²⁺/Fe³⁺ cycling, producing more hydroxyl radical leading to the existence of homogeneous and heterogeneous reactions simultaneously. The removal efficiency of solar photo-Fenton like process was also compared to photo-Fenton process with different irradiation sources (UV-A and UV-B) and were statistically analysed. Continuous-scale studies were conducted employing soil either in the form of soil beads or as a thin layer spread on the surface of baffled reactor. Soil beads were found to have satisfactory reusability and stability. 84 and 79% degradation of ORZ and OFX was achieved using soil as thin layer while with soil beads 71 and 68% degradation, respectively. HPLC and TOC study confirmed the efficient removal of both the compounds. Toxicity assessment demonstrates the inexistence of toxic intermediates during the reaction.

Optimization of a textile dye degradation in a recirculating fluidized-bed reactor using magnetite/S2O82- process

Optimization of Acid Orange 7 (AO7) treatment using heterogeneous Fenton-like method in a recirculating fluidized-bed reactor (FBR) was investigated by using central composite design (CCD). Natural magnetite (NM) as Fenton-like catalyst was characterized using scanning electron microscopy. A nonlinear CCD model was obtained for the prediction of dye degradation as a function of experimental variables such as peroxydisulfate concentration (0.1-0.5 mmol/L), initial AO7 concentration (5-25 mg/L), pH (3-9) and NM dosage (0.25-1.25 g/L) after 105 min of treatment. The calculated results by the model were consistent with the experimental results (R² = 0.959). Furthermore, the model is suitable to estimate the optimum operational conditions and determine the effects of the parameters for maximum AO7 degradation. Eventually, gas chromatography-mass spectroscopy was used for the recognition of the dye degradation by-products.

One-step preparation of nanostructured martite catalyst and graphite electrode by glow discharge plasma for heterogeneous electro-Fenton like process

Natural Martite ore particles and graphite were modified by alternating current (AC) glow discharge plasma to form nanostructured catalyst and cathode electrode for using in the heterogeneous-electro Fenton-like (Het-EF-like) process. The performance of the plasma-treated martite (PTM) and graphite electrode (PTGE) was studied for the treatment of paraquat herbicide in a batch system. 85.78% degradation efficiency for 20 mg L^-1 paraquat was achieved in the modified process under desired operational conditions (i.e. current intensity of 300 mA, catalyst amount of 1 g L^-1, pH = 6, and background electrolyte (Na₂SO₄) concentration of 0.05 mol L^-1) which was higher than the 41.03% for the unmodified one after 150 min of treatment. The ecofriendly modification of the martite particles and the graphite electrode, no chemical needed, low leached iron and milder operational pH were the main privileges of plasma utilization. Moreover, the degradation efficiency through the process was not declined after five repeated cycles at the optimized conditions, which proved the stability of the nanostructured PTM and PTGE in the long-term usage. The archived results exhibit this method is the first example of high efficient, cost-effective, and environment-friendly method for generation of nanostructured samples.

Comparison of bioleaching and electrokinetic remediation processes for removal of heavy metals from wastewater treatment sludge

Heavy metals prevent the growing amount of sewage sludge from being disposed as fertilizeron land. The electrokinetic remediation and bioleaching technology are the promising methods to remove heavy metals. In recent years, some innovation has been made to achieve better efficiency, including the innovation of processes and agents. This paper reviews the development of the electrokinetic remediation and bioleaching technology and analyses their advantages and limitation, pointing out the need of the future research for the heavy metals-contaminated sewage sludge.

Heterogeneous sono-Fenton-like process using martite nanocatalyst prepared by high energy planetary ball milling for treatment of a textile dye

High energy planetary ball milling was applied to prepare sono-Fenton nanocatalyst from natural martite (NM). The NM samples were milled for 2-6h at the speed of 320rpm for production of various ball milled martite (BMM) samples. The catalytic performance of the BMMs was greater than the NM for treatment of Acid Blue 92 (AB92) in heterogeneous sono-Fenton-like process. The NM and the BMM samples were characterized by XRD, FT-IR, SEM, EDX and BET analyses. The particle size distribution of the 6h-milled martite (BMM₃) was in the range of 10-90nm, which had the highest surface area compared to the other samples. Then, the impact of main operational parameters was investigated on the process. Complete removal of the dye was obtained at the desired conditions including initial pH 7, 2.5g/L BMM₃ dosage, 10mg/L AB92 concentration, and 150W ultrasonic power after 30min of treatment. The treatment process followed pseudo-first order kinetic. Environmentally-friendly modification of the NM, low leached iron amount and repeated application at milder pH were the significant benefits of the BMM₃. The GC-MS was successfully used to identify the generated intermediates. Eventually, an artificial neural network (ANN) was applied to predict the AB92 removal efficiency based upon the experimental data with a proper correlation coefficient (R²=0.9836).

Heterogeneous Fenton-like degradation of phenanthrene catalyzed by schwertmannite biosynthesized using Acidithiobacillus ferrooxidans

Heterogeneous Fenton-like degradation of phenanthrene in aqueous solution was investigated using schwertmannite biosynthesized by Acidithiobacillus ferrooxidans LX5 as a catalyst.

Preparation of zeolite nanorods by corona discharge plasma for degradation of phenazopyridine by heterogeneous sono-Fenton-like process

The plasma-modified clinoptilolite (PMC) nanorods were prepared from natural clinoptilolite (NC) utilizing environmentally-friendly corona discharge plasma. The PMC and NC were characterized by XRD, FT-IR, SEM, EDX, XPS and BET, which confirmed the nanocatalyst formation. The catalytic performance of the PMC in the heterogeneous sono-Fenton-like process was greater than the NC for treatment of phenazopyridine (PhP). The desired amounts were obtained for experimental parameters including initial pH (5), PMC dosage (2g/L), K2S2O8 concentration (2mmol/L), ultrasonic power (300W) and PhP concentration (10mg/L). Reactive oxygen species scavengers decreased the removal efficiency of the PhP. The treatment process followed pseudo-first order kinetic and seven degradation intermediates were identified by the GC-MS technique.

Central composite design optimization of pilot plant fluidized-bed heterogeneous Fenton process for degradation of an azo dye

Optimization of Acid Yellow 36 (AY36) degradation by heterogeneous Fenton process in a recirculated fluidized-bed reactor was studied using central composite design (CCD). Natural pyrite was applied as the catalyst characterized by X-ray diffraction and scanning electron microscopy. The CCD model was developed for the estimation of degradation efficiency as a function of independent operational parameters including hydrogen peroxide concentration (0.5-2.5 mmol/L), initial AY36 concentration (5-25 mg/L), pH (3-9) and catalyst dosage (0.4-1.2 mg/L). The obtained data from the model are in good agreement with the experimental data (R(2 )= 0.964). Moreover, this model is applicable not only to determine the optimized experimental conditions for maximum AY36 degradation, but also to find individual and interactive effects of the mentioned parameters. Finally, gas chromatography-mass spectroscopy (GC-MS) was utilized for the identification of some degradation intermediates and a plausible degradation pathway was proposed.

Heterogeneous sono-Fenton process using pyrite nanorods prepared by non-thermal plasma for degradation of an anthraquinone dye

Natural pyrite (NP) was treated using oxygen and nitrogen non-thermal plasmas to form modified catalysts. Cleaning effect of the O2 plasma by chemical etching leads to removal of impurities from catalyst surface and sputtering effect of the N2 plasma results in formation of pyrite nanorods. The mentioned plasmas were applied separately or in the order of first O2 and then N2, respectively. The catalytic performance of the plasma-modified pyrites (PMPs) is better than the NP for treatment of Reactive Blue 69 (RB69) in heterogeneous sono-Fenton process (US/H2O2/PMP). The NP and the most effective modified pyrite (PMP4) samples were characterized by XRD, FT-IR, SEM, EDX, XPS and BET analyses. The desired amounts were chosen for operational parameters including initial pH (5), H2O2 concentration (1mM), PMP4 dosage (0.6g/L), dye concentration (20mg/L), and ultrasonic power (300W). Moreover, the effects of peroxydisulfate and inorganic salts on the degradation efficiency were investigated. Gas chromatography-mass spectrometry (GC-MS) method was applied to identify the generated intermediates and a plausible pathway was proposed for RB69 degradation. Environmentally-friendly modification of the NP, low amount of leached iron and repeated reusability at milder pH are the significant privileges of the PMP4. The phytotoxicity test using Spirodela polyrrhiza verified the remarkable toxicity removal of the RB69 solution after the treatment process.