Effect of some parameters on the rate of the catalysed decomposition of hydrogen peroxide by iron(III)-nitrilotriacetate in water

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Nitrogen-Doped Carbon Materials for Persulfate Activation via Electron Transfer Pathways

The incorporation of nitrogen into carbon materials is a strategy that effectively boosts their catalytic potency. Herein, a nitrogen-enriched carbon substance, designated as CN0.6, was synthesized from melamine, serving as a precursor. This substance has been established to act as an efficient catalyst devoid of metals for the activation of peroxymonosulfate (PMS). At a temperature of 25 °C, a concentration of 0.05 g/L CN0.6 along with 1 mM PMS suffices to achieve the complete degradation of concentrated tetracycline hydrochloride (TC) in a short period of 4 min. This enhanced catalytic performance is attributed to the optimal level of nitrogen doping, which elevates the pyrrolic nitrogen content and introduces additional defects characterized by an ID/IG ratio of 1.02. These factors collectively augment the adsorptive capacity for PMS and create a greater number of active sites to facilitate its activation. The dominance of a nonradical electron transfer mechanism in the CN0.6/PMS system has been confirmed through a series of analyses, including radical identification, quenching tests, and electrochemical assessments. Employing high-resolution liquid chromatography coupled with tandem mass spectrometry (LC-MS), the investigation identified three potential degradation routes for TC. Furthermore, the intermediates produced are determined to possess reduced toxicity in comparison to TC. The findings of this study offer a approach to the synthesis of highly efficient nitrogen-doped, metal-free catalysts, presenting a promising strategy for the degradation of environmental pollutants.

Ranitidine degradation in layered double hydroxide activated peroxymonosulfate system: impact of transition metal composition and reaction mechanisms

Ranitidine, a competitive inhibitor of histamine H2 receptors, has been identified as an emerging micropollutant in water and wastewater, raising concerns about its potential impact on the environment and human health. This study aims to address this issue by developing an effective removal strategy using two types of layered double hydroxide (LDH) catalysts (i.e., CoFeLDH and CoCuLDH). Characterization results show that CoFeLDH catalyst has superior catalytic properties due to its stronger chemical bond compared to CoCuLDH. The degradation experiment shows that 100% degradation of ranitidine could be achieved within 20 min using 25 mg/L of CoFeLDH and 20 mg/L of peroxymonosulfate (PMS). On the other hand, CoCuLDH was less effective, achieving only 70% degradation after 60 min at a similar dosage. The degradation rate constant of CoFeLDH was 10 times higher than the rate constant of CoCuLDH at different pH range. Positive zeta potential of CoFeLDH made it superior over CoCuLDH regarding catalytic oxidation of PMS. The catalytic degradation mechanism shows that sulfate radicals played a more dominant role than hydroxyl radicals in the case of LDH catalysts. Also, CoFeLDH demonstrated a stronger radical pathway than CoCuLDH. XPS analysis of CoFeLDH revealed the cation percentages at different phases and proved the claim of being reusable even after 8 cycles. Overall, the findings suggest that CoFeLDH/PMS system proves to be a suitable choice for attaining high degradation efficiency and good stability in the remediation of ranitidine in wastewater.

Reactive layered hydroxide membrane for advanced water treatment: Micropollutant degradation and antifouling potential

The effective removal of micropollutants by water treatment technologies remains a significant challenge. Herein, we develop a CoFe layered double hydroxide (CoFeLDH) catalytic membrane for peroxymonosulfate (PMS) activation to achieve efficient micropollutant removal with improved mass transfer rate and reaction kinetics. This study found that the CoFeLDH membrane/PMS system achieved an impressive above 98% degradation of the probe chemical ranitidine at 0.1 mM of PMS including five more micropollutants (Sulfamethoxazole, Ciprofloxacin, Carbamazepine, Acetaminophen and Bisphenol A) at satisfactory level (above 80%). Moreover, significant improvements in water flux and antifouling properties were observed, marking the membrane as a specific advancement in the removal of membrane fouling in water purification technology. The membrane demonstrated consistent degradation efficiency for several micropollutants and across a range of pH (4-9) as well as different anionic environments, thereby showing it suitability for scale-up application. The key role of reactive species such as SO4•-, and O2• - radicals in the degradation process was elucidated. This is followed by the confirmation of the occurrence of redox cycling between Co and Fe, and the presence of CoOH+ that promotes PMS activation. Over the ten cycles, the membrane could be operated with a flux recovery of up to 99.8% and maintained efficient performance over 24 h continuous operation. Finally, the efficiency in degrading micropollutants, coupled with reduced metal leaching, makes the CoFeLDH membrane as a promising technology for application in water treatment.

Potassium peroxoborate: A sustained-released reactive oxygen carrier with enhanced PAHs contaminated soil remediation performance

Seeking for a safe, efficient, inexpensive, and eco-friendly oxidizer is always a big challenge for in-situ chemical oxidation (ISCO) technology. This study adopted the potassium peroxoborate (PPB), a novel peroxide, for soil remediation for the first time. PPB based chemical oxidation system (PPB-CO) could efficiently degrade polycyclic aromatic hydrocarbons (PAHs) without other reagents added, reaching 72.1 %, 64.2 %, and 50.0 % removal rates for naphthalene, phenanthrene, and pyrene after 24 h reaction, respectively. The superior total PAHs removal efficiency (60.6 %) was 3.6-4.7 times higher than that of other commercial peroxides (2Na2CO3•3H2O, CaO2, and H2O2). Mechanism analysis revealed that varieties of reactive oxygen species (ROS) can be generated by PPB through Fenton-like or non-Fenton routines, including H2O2, perborates species, O2•-, •OH, and 1O2. The sustainable generation of H2O2 reduced the disproportionation effect of H2O2 by 86 %, significantly improving the utilization rate. Moreover, sandbox experiments and actual contaminated soil remediation experiments verified the feasibility of PPB-CO in a real polluted site. This work provides a novel strategy for effectively soil remediation, highlighting the selection and application of new oxidants.

Organic cocatalysts improved Fenton and Fenton-like processes for water pollution control: A review

In recent times, organic compounds have been extensively utilized to mitigate the limitations associated with Fe(Ⅲ) reduction and the narrow pH range in Fenton and Fenton-like processes, which have garnered considerable attention in relevant studies. This review presents the latest advancements in the comprehensive analysis and applications of organic agents as assistant/cocatalysts during Fenton/Fenton-like reactions for water pollution control. The primary focus includes the following: Firstly, the mechanism of organic co-catalytic reactions is introduced, encompassing both complexation and reduction aspects. Secondly, these organic compounds are classified into distinct categories based on their functional group structures and applications, namely polycarboxylates, aminopolycarboxylic acids, quinones, phenolic acids, humic substances, and sulfhydryl compounds, and their co-catalytic functions and mechanisms of each category are discussed in meticulous detail. Thirdly, a comprehensive comparison is conducted among various types of organic cocatalysts, considering their relative merits, cost implications, toxicity, and other pertinent factors. Finally, the review concludes by addressing the universal challenges and development prospects associated with organic co-catalytic systems. The overarching objective of this review is to provide insights into potential avenues for the future advancement of organic co-catalytic Fenton/Fenton-like reactions in the context of water purification.

Understanding atrazine elimination via treatment of the enzyme-based Fenton reaction: Kinetics, mechanism, reaction pathway, and metabolites toxicity

The degradation kinetics and mechanism of atrazine (ATZ) via an enzyme-based Fenton reaction were investigated at various substrate concentrations and pH values. Toxicological assessment was conducted on ATZ and its degradation products, and the associated reaction pathway was examined. The in situ production of hydrogen peroxide (H2O2) was monitored within the range of 3-15 mM, depending on the increase in glucose concentration, while decreasing the pH to 3.2-5.1 (initial pH of 5.8) or 6.5-7.4 (initial pH of 7.7). The degradation efficiency of ATZ was approximately 2-3 times higher at an initial pH of 5.8 with lower glucose concentrations than at an initial pH of 7.7 with higher substrate concentrations during the enzyme-based Fenton reaction. The apparent pseudo-first-order rate constant for H2O2 decomposition under various conditions in the presence of ferric citrate was 1.9-6.3 × 10-5 s-1. The •OH concentration ([•OH]ss) during the enzyme-based Fenton reaction was 0.5-4.1 × 10-14 M, and the second-order rate constant for ATZ degradation was 1.5-3.3 × 109 M-1 s-1. ATZ intrinsically hinders the growth and development of Arabidopsis thaliana, and its inhibitory effect is marginal, depending on the reaction time of the enzyme-based Fenton process. The ATZ transformation during this process occurs through dealkylation, hydroxylation, and dechlorination via •OH-mediated reactions. The degradation kinetics, mechanism, and toxicological assessment in the present study could contribute to the development and application of enzyme-based Fenton reactions for in situ pollutant abatement. Moreover, the enzyme-based Fenton reaction could be an environmentally benign and applicable approach for eliminating persistent organic matter, such as herbicides, using diverse H2O2-producing microbes and ubiquitous ferric iron with organic complexes.

Redox property of coordinated iron ion enables activation of O2 via in-situ generated H2O2 and additionally added H2O2 in EDTA-chelated Fenton reaction

The Fenton system was a generation system of reactive oxygen species via the chain reactions, which employed H2O2 and O2 as radical precursors and Fe2+/Fe3+ as electron-donor/acceptor for triggering or terminating the generation of radicals. Recent work mainly emphasized the Fe2+- activated H2O2 and the application of in-situ generated •OH, while neglecting other side-reactions. In this work, EDTA (Ethylene diamine tetraacetic acid) was employed as a chelating agent of iron ions, which simultaneously changed the redox property of coordinated iron. The Fe2+-EDTA complexes in the presence of dissolved oxygen enabled the two-electron transfer from Fe2+ to O2 and the in-situ production of H2O2, which further activate H2O2 for yielding •OH. Meanwhile, coordinated Fe3+ exhibited non-negligible reactivity toward H2O2, which was higher than that of free Fe3+ in the traditional Fenton system. The complexation of EDTA with Fe3+ could enhance the Fe2+ generation reaction by the H2O2, accompanied by the O2•- formation. The enhancement of O2•- formation and Fe2+-EDTA regeneration induced the subsequent H2O2 activation by Fe2+-EDTA, thus accelerating the Fe3+-EDTA/Fe2+-EDTA cycle for simultaneously producing O2•- and •OH. To sum up, the EDTA-chelated Fenton system extended the applicable pH range to circumneutral/alkaline level and tuned the redox property of coordinated iron for diversifying the •OH production routes. The research reinterpreted the chain reactions in the Fenton system, revealing another way to enhance the radical production or other property of the Fenton/Fenton-like system.

Winery and olive mill wastewaters treatment using nitrilotriacetic acid/UV-C/Fenton process: Batch and semi-continuous mode

In this work, both red and white winery wastewaters (WW) and olive mill wastewater (OMW) were submitted to a treatment by Fenton-based processes (FBPs). The main aim was to evaluate the most efficient and economic process. Initial tests, resorting to a batch reactor, demonstrated that UV-C/Fenton (λ = 254 nm) was the most effective process. Operational conditions such as pH, H2O2 and Fe2+ concentrations revealed to have a superior influence within dissolved organic carbon (DOC) removal as well as regarding the reactor's energy consumption. As a means to prevent iron precipitation, the addition of nitrilotriacetic acid (NTA) was tested. With experimental conditions pH = 3.0, [H2O2] = 194 mM, [Fe2+] = 1.0 mM, [NTA] = 1.0 mM, radiation UV-C (254 nm), time = 240 min, the kinetic rate related with DOC removal showed a kredWW = 0.0128 min-1 > kOMW = 0.0124 min-1 > kwhiteWW = 0.0104 min-1 and both the WW and OMW achieved the Portuguese legal limit values for wastewater discharge. Furthermore, comparative experiments were performed in a semi-continuous reactor, being that the results put in evidence that the concentration of H2O2 added and the flow rate of reagents' addition (F) had a significant effect on the efficiency of the reactor. Under an optimum experimental procedure pH = 3.0, [H2O2] = 97 mM, [Fe2+] = 1.0 mM, [NTA] = 1.0 mM, radiation UV-C (254 nm), F = 1 mL min-1, time = 240 min, there were observed higher DOC removal kinetic rates (kOMW = 15.20 × 10-3 min-1 > kredWW = 11.64 × 10-3 min-1 > kwhiteWW = 11.57 × 10-3 min-1) and a cost ranging between 0.0402 and 0.0419 €/g.DOC. These results showed that semi-continuous reactors have the potential to be applied to large scale treatments, with low reagents consumption and reduced energy requirements.

Mechanism and controlling factors on rapid methylmercury degradation by ligand-enhanced Fenton-like reaction at circumneutral pH

Methylmercury (MeHg), derived from industrial processes and microbial methylation, is still a worldwide environmental concern. A rapid and efficient strategy is necessary for MeHg degradation in waste and environmental waters. Here, we provide a new method with ligand-enhanced Fenton-like reaction to rapidly degrade MeHg under neutral pH. Three common chelating ligands were selected (nitriloacetic acid (NTA), citrate, and ethylenediaminetetraacetic disodium (EDTA)) to promote the Fenton-like reaction and degradation of MeHg. Results showed that MeHg can be rapidly degraded, with the following efficiency sequence: EDTA > NTA > citrate. Scavenger addition demonstrated that hydroxyl radical (^▪OH), superoxide radical (O₂^▪-), and ferryl (Fe^ⅣO²⁺) were involved in MeHg degradation, and their relative contributions highly depended on ligand type. Degradation product and total Hg analysis suggested that Hg(Ⅱ) and Hg⁰ were generated with the demethylation of MeHg. Further, environmental factors, including initial pH, organic complexation (natural organic matter and cysteine), and inorganic ions (chloride and bicarbonate) on MeHg degradation, were investigated in NTA-enhanced system. Finally, rapid MeHg degradation was validated for MeHg-spiked waste and environmental waters. This study provided a simple and efficient strategy for MeHg remediation in contaminated waters, which is also helpful for understanding its degradation in the natural environment.

Enhanced degradation of emerging contaminants by percarbonate/Fe(II)-ZVI process: case study with nizatidine

Pharmaceuticals have recently emerged as a significant environmental concern due to the growth of population, expansion of industry, and the shift in modern lifestyles. Herein, we present a Fe(II)/percarbonate (SPC) process with dramatically enhanced efficiency by the introduction of zerovalent iron (ZVI). After the addition of ZVI, the removal rate of nizatidine (NZTD) went up from 71.7 to 84.2%. The removal rate of NZTD decreases with rising pH and speeds up with increasing temperature. It was found that under the condition of pH = 7 and T = 25 °C, the molar ratio of the optimal concentration of NZTD degradation in the system was [NZTD]₀:[SPC]₀:[Fe(II)]₀:[ZVI]₀ = 1:8:24:16, with a degradation rate of 99.8%. At the same time, target pollutants can also be successfully eliminated from actual water bodies. Moreover, we test for toxicity using luminescent bacteria, and the results demonstrate that the system is capable of effectively decreasing the toxicity of NZTD. The research findings can contribute to the clarification of the migration and transformation law of NZTD in the oxidation process, thereby providing a scientific basis and technical support for the removal of NZTD in the tertiary water treatment for a water source.

Intensification of the photodegradation efficiency of an emergent water pollutant through process conditions optimization by means of response surface methodology

Heterogeneous photocatalysis has been increasingly investigated during the past years and has been recognized as a promising technique for clean and safe water purification. The current study exploits the advantage of this technique demonstrating that the removal of a biorefractory water pollutant named clofibric acid can be really improved by photocatalysis through a parametric comprehensive investigation and optimization study based on response surface methodology. Its novelty comes from the approach used to enhance the efficiency of the photocatalytic degradation of clofibric acid. A custom central composite design consisting of 49 trials was applied for process modeling and a quadratic robust model was derived based on the analysis of variance for the optimization of the process parameters. The effective removal of the target molecule with about 70% carbon mineralization was achieved under optimal photocatalytic conditions: 1.5 mg/L as the initial concentration of pollutant, 0.61 g/L catalyst, and an irradiation time of 190 min. Further, it was provided that nitrates play a positive role in the removal of this pollutant, while hydrogenocarbonates slow down its elimination. The ecotoxicity evaluation at different trophic levels confirmed the low toxicity of photodegradation by-products. Data analysis demonstrated that response surface methodology is a reliable approach for the optimization of the interactive effects of photocatalytic process parameters and is able to enhance their performance for the complete elimination of this hardly removed water pollutant.

Coupling the effect of Co and Mo on peroxymonosulfate activation for the removal of organic pollutants

Although heterogeneous cobalt-based catalysts have been widely studied and used in SO₄^•- based advanced oxidation processes, the efficiencies were still not high enough due to the limiting step of Co(III)/Co(II) cycle in the system. In this study, a bimetallic oxide composed of Co and Mo was designed and used for enhancing the performance of peroxymonosulfate activation on organic pollutants removal. The CoMoO₄ nanorods exhibited superior catalytic activity for methylene blue (MB) degradation than Co₃O₄, MoO₃, and their mechanical mixture, which was attributed to the synergetic effect between Co and Mo. CoMoO₄ nanorods were able to efficiently degrade MB under a wide pH range of 3-11 and could maintain high efficiency in 5 cycles with less leakage of metal ions. Moreover, CoMoO₄ nanorods displayed broad spectrum applicability to the different water matrix and a variety of pollutants such as phenol and Congo red. The Co(II) was proved to be the main active site of the catalyst, while Mo played an important role in promoting the Co(III)/Co(II) cycle. Surface free radicals are the main active species in the degradation process. This work provides new insights into the design of cobalt-based bimetallic catalyst and the improvement on PMS activation.

Dye mineralization under UV/H2O2 promoted by chloride ion at high concentration and the generation of chlorinated byproducts

Chloride ion (Cl^-) may promote or inhibit the oxidation of specific organic compounds treated by hydroxyl radical based advanced oxidation processes (HR-AOPs) depending on the reactivity of chlorine radicals towards the organics. However, the effects of high contents of Cl^- on the removal of total organic compounds (TOC) in high salinity organic wastewater treated by HR-AOPs were unclear. The removal and mineralization of azo dye Orange II (OrgII) by UV/H₂O₂ process with Cl^- at high contents under various pH conditions were investigated. As the pH conditions increased higher than pH 5, TOC removal rates increased slightly possibly related to the increase of O₂^- production and the reduce of futile decomposition of H₂O₂ into O₂. Cl^- at relative high concentration (1000 and 2000 mM) significantly promoted the mineralization of dyes with TOC removal increasing by 10 %-40 % under both acid and alkaline conditions. The proposed mechanism is that the reaction of Cl^- with OH would decline the decomposition of H₂O₂ into O₂ by inhibiting the reaction between OH and H₂O₂, and the generated chlorine species (Cl and Cl₂^-) could further promote the oxidation of dye molecules into intermediates and be helpful for the subsequent mineralization process. In addition, H₂O₂ and Cl^- can slowly react to give HClO and ClO^-, which may partly contribute to the decolorization and mineralization of OrgII. Meanwhile, an appropriate relative proportion between Cl₂^- and OH depending on Cl^- contents and pH conditions is important to enhance the TOC removal. However, the formation of various chlorinated byproducts especially under alkaline condition may increase the risk of environmental pollution accidents. The results demonstrate the promotion of TOC removal by UV/H₂O₂ under certain high contents of Cl^- and provide new insight into the application of HR-AOPs to the pretreatment of high salinity organic wastewater.

Graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) for antimony removal: The role of active oxygen species and reaction mechanism

Sulfidated nano zero-valent iron (S-nZVI) was used to remove various pollutants from wastewater. However, the instability, poor dispersibility, and low electron transfer efficiency of S-nZVI limit its application. Herein, graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) was successfully synthesized using graphene oxide (GO) as a carrier. The properties of S-nZVI@GO were characterized by scanning electron microscopy coupled to X-ray photoelectron spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) concerning the surface morphology, crystalline structure, and elemental components. S-nZVI@GO displayed an excellent capacity for antimony (Sb) removal under aerobic conditions (96.7%), with a high adsorption capacity (Q_max = 311.75 mg/g). It maintained a high removal rate (over 90%) during a wide pH range (3-9). More importantly, S-nZVI@GO activated the molecular oxygen in water via a single-electron pathway to produce •O₂－ and H₂O₂, and then oxidized trivalent antimony (Sb(III)) to pentavalent antimony (Sb(V)) and further separated it by synergistic adsorption and co-precipitation. Therefore, S-nZVI@GO shows excellent potential for Sb contamination remediation.

Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process

During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe²⁺/Fe³⁺), as well as surface-adsorbed Fe²⁺/Fe³⁺, have been widely recognized to dominate hydroxyl radical (^•OH) generation, while this study reveals that the secondary solid iron species also play non-negligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fe_coat (iron-containing coating on the pyrite surface) and Fe_dep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fe_coat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fe_dep and Fe_coat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fe_coat and Fe_dep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fe_dep, originated from the hydrolyzation of dissolved Fe³⁺, accelerates the nonradical decay of hydrogen peroxide (H₂O₂); as a result, the accumulation of Fe_dep always decreases the ^•OH production during the pyrite oxidation process. However, part of Fe_dep adsorbs on the pyrite surface through electrostatic attraction and converts into Fe_coat. The electron conduction between Fe_coat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors ^•OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

Textile Dye Removal by Acacia dealbata Link. Pollen Adsorption Combined with UV-A/NTA/Fenton Process

The decolourization of an aqueous solution of the textile dye Acid Red 88 (AR88) and the control of the invasive plant species Acacia dealbata Link. (ADL) were addressed in this work. The aims of the study were (1) characterization of the ADL pollen, (2) application of the pollen powder in adsorption processes, (3) selection of the best operational conditions for nitriloacetic acid (NTA)-UV-A-Fenton process and (4) assess the efficiency of the combined treatment adsorption and NTA-UV-A-Fenton in AR88 decolourization. In a first step, ADL pollen was used as a AR88 bioadsorbent. Fourier-transform infrared spectroscopy (FTIR) analysis were performed and revealed the presence of proteins, fatty acids, carbohydrates and lignin in the pollen. Afterwards, trough scanning electron microscopy (SEM), it was possible to verify that ADL pollen has several empty spaces that can be used for dye adsorption. Biosorption results showed higher adsorption of AR88 with application of pH 3.0 and [pollen] = 3.0 g/L with 18.8 mg/g of dye adsorbed. The best fitting was observed with Langmuir, SIPS and Jovanovic isotherms (0.993, 0.996 and 0.994, respectively). To complement the biosorption, a UV-A-Fenton process was applied, and results showed a higher AR88 removal with (NTA) addition. Higher irradiance power favored the oxidation process with high Ф photodegradation value and low Electric Energy per Order ($$E_{\text{EO}}$$ E EO ) and Specific Applied Energy ($$E_{\text{SAE}}$$ E SAE ). The combination of biosorption with NTA-UV-A-Fenton was the most efficient system with an AR88 decolourization of 98.5% and a total organic carbon (TOC) removal of 83.5%.

Graphical Abstract

Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes

Peroxidase-like nanozymes are nanoscale materials that can closely mimic the activity of natural peroxidase for a range of oxidation reactions. Surface coating with polymer nanogels has been considered to prevent the aggregation of nanozymes. For a long time, the understanding of polymer coating has been largely limited to its stabilization effect on the nanozyme in aqueous media, while little is known about how polymer coating plays a role in interaction with substrates and primary oxidants to dictate the catalytic process. This work reported a facile sequential modification of Fe3O4 nanoparticles to polyacrylamide coated nanozymes, and as low as 112 mg/L samples with only 5 mg/L Fe3O4 could nearly quantitatively (99%) remove a library of organic dyes with either H2O2 or Na2S2O8 as primary oxidants. The catalytic results and molecular simulation provide both experimental and computational evidence that the hydrogen bonding interaction between the reactant and nanozymes is key for the high local concentration hence catalytic efficiency. We envision that this work, for the first time, provides some insights into the role of polymer coating in enhancing the catalytic activity of nanozyme apart from the well-known water dispersity effect.

Modulated construction of Fe-based MOF via formic acid modulator for enhanced degradation of sulfamethoxazole：Design, degradation pathways, and mechanism

Metal-organic frameworks (MOFs) have attracted more attention because of their excellent environmental catalytic capabilities. Modulation approach as an advanced assistant strategy is vital essential to enhancing the performance of MOFs. In this study, the modulated method was used to successfully synthesize a group of Fe-based MOFs, with formic acid as the modulator on the synthesis mixture. The most modulated sample Fe-MOFs-2 exhibit high specific surface areas and higher catalytic activity, which could effectively degrade SMX via PS activation, with almost 95% removal efficiency within 120 min. The results revealed that the % RSE of modulated Fe-MOFs-2 increased from 2.31 to 3.27 when compared with the origin Fe-MOFs. This may be due to the addition of formic acid induces the formation of more coordinatively unsaturated metal sites in the catalyst, resulting in structural defects. In addition, the quenching experiment and EPR analysis verified SO₄^-·and·OH as the major active free radicals in the degradation process. Modulated Fe-MOFs-2 demonstrated good reusability and stability under fifth cycles. Finally, four possible degradation pathways and catalytic mechanism of Fe-MOFs-2 was tentatively proposed. Our work provides insights into the rational design of modulated Fe-MOFs as promising heterogeneous catalysts for advanced wastewater treatment.

The principle of detailed balancing, the iron-catalyzed disproportionation of hydrogen peroxide, and the Fenton reaction

The iron-catalyzed disproportionation of H₂O₂ has been investigated for over a century, as has been its ability to induce the oxidation of other species present in the system (Fenton reaction). The mechanisms of these reactions have been under consideration at least since 1932. Unfortunately, little or no attention has been paid to ensuring the conformity of the proposed mechanisms and rate constants with the constraints of the principle of detailed balancing. Here we identify more than 200 publications having mechanisms that violate the principle of detailed balancing. These violations occur through the use of incorrect values for certain rate constants, the use of incorrect forms of the rate laws for certain steps in the mechanisms, and the inclusion of illegal loops. A core mechanism for the iron-catalyzed decomposition of H₂O₂ is proposed that is consistent with the principle of detailed balancing and includes both the one-electron oxidation of H₂O₂ by Fe(III) and the Fe(II) reduction of HO₂˙.

A novel diagnostic method for distinguishing between Fe(IV) and •OH by using atrazine as a probe: Clarifying the nature of reactive intermediates formed by nitrilotriacetic acid assisted Fenton-like reaction

In this work, we found that the distribution of two specific atrazine (ATZ) oxidation products (desethyl-atrazine (DEA) and desisopropyl-atrazine (DIA)) was different in oxidation processes involving aqueous ferryl ion (Fe(IV)) species and ^•OH. Specifically, the molar ratio of produced DEA to DIA (i.e., [DEA]/[DIA]) increased from 7.5 to 13 with increasing pH from 3 to 6 when ATZ was oxidized by Fe(IV), while the treatment of ATZ by ^•OH led to the [DEA]/[DIA] value of 2 which was independent of pH. Moreover, ATZ showed high reactivity towards Fe(IV) over a wide pH range, especially at near-neutral pH, at which ATZ oxidation in Fe(II)/peroxydisulfate system was even much faster than another well-defined Fe(IV) scavenger, the sulfoxides. By using this approach, it was obtained that the [DEA]/[DIA] value remained at 2 during ATZ transformation by the nitrilotriacetic acid (NTA) assisted Fenton-like (Fe(III)/H₂O₂) system, which was independent of solution pH and reactants dosage. This result clarified that ^•OH was the primary reactive intermediate formed in the NTA assisted Fe(III)/H₂O₂ system. This study not only developed a novel sensitive diagnostic tool for distinguishing Fe(IV) from ^•OH, but also provided more credible evidence to the nature of reactive intermediate in a commonly controversial system.

Application of Chelating Agents to Enhance Fenton Process in Soil Remediation: A Review

Persistent organic contaminants affecting soil and groundwater pose a significant threat to ecosystems and human health. Fenton oxidation is an efficient treatment for removing these pollutants in the aqueous phase at acidic pH. However, the in-situ application of this technology for soil remediation (where pHs around neutrality are required) presents important limitations, such as catalyst (iron) availability and oxidant (H2O2) stability. The addition of chelating agents (CAs), forming complexes with Fe and enabling Fenton reactions under these conditions, so-called chelate-modified Fenton process (MF), tries to overcome the challenges identified in conventional Fenton. Despite the growing interest in this technology, there is not yet a critical review compiling the information needed for its real application. The advantages and drawbacks of MF must be clarified, and the recent achievements should be shared with the scientific community. This review provides a general overview of the application of CAs to enhance the Fenton process for the remediation of soils polluted with the most common organic contaminants, especially for a deep understanding of the activation mechanisms and influential factors. The existing shortcomings and research needs have been highlighted. Finally, future research perspectives on the use of CAs in MF and recommendations have been provided.

New insights into the mechanisms of tartaric acid enhancing homogeneous and heterogeneous copper-catalyzed Fenton-like systems

The specific roles of tartaric acid (TA), as an eco-friendly ligand, in homogeneous and heterogeneous copper-catalyzed systems were systematically revealed and new mechanisms of TA enhancing the three Fenton-like processes were proposed to provide a theoretical significance in overcoming the deficiency of conventional Fenton processes. The results identified hydroxyl radical (•OH) as the main species responsible for the simultaneous decomposition of TA and metronidazole (MNZ) according to TOC removal. The ESR technique was used to detect superoxide radicals (•O₂^-), carbon-centered radical (•R) and hydrogen radical (•H) in the Cu²⁺/TA/H₂O₂ system, which contributed to the acceleration of the Cu²⁺/Cu⁺ redox cycle. The enhancing effect of TA on the homogeneous process was ascribed to the formation of a soluble complex with Cu²⁺, which favored the pH range extension, Cu⁺ oxidation, and radical generation. Moreover, the adsorption of TA on the catalysts surface promoted the consumption of H₂O₂, inducing •OH generation. The formed surface complex (≡Cu²⁺-TA) also accelerated the regeneration of ≡Cu⁺, which was confirmed by density functional theory (DFT) calculation and surface characterization analysis (SEM, XRD, and XPS). The possible degradation pathways of MNZ in TA-modified Fenton-like system were also clarified.

Efficient removal of bisphenol A with activation of peroxydisulfate via electrochemically assisted Fe(III)-nitrilotriacetic acid system under neutral condition

In this work, an innovative electrochemically assisted Fe(III)-nitrilotriacetic acid system for the activation of peroxydisulfate (electro/Fe(III)-NTA/PDS) was proposed for the removal of bisphenol A (BPA) at neutral pH with commercial graphite electrodes. The efficient BPA decay was mainly originated from the continuous activation of PDS by Fe(II) reduced from Fe(III)-NTA complexes at the cathode. Scavenger experiments and electron paramagnetic resonance (EPR) measurements confirmed that the removal of BPA occurred through graphite adsorption, direct electron transfer (DET) and radical oxidation. Sulfate and hydroxyl radicals were primarily responsible for the oxidation of BPA while graphite adsorption and DET played a minor role in BPA removal. The influence of Fe(III) concentration, PDS dosage, input current, NTA to Fe(III) molar ratio as well as coexisting inorganic anions (Cl^-, NO₃^-, H₂PO₄^- and HCO₃^-) on BPA elimination was explored. The BPA removal efficiency reached 93.5 % after 60 min reaction in the electro/Fe(III)-NTA/PDS system under the conditions of initial pH 7.0, 0.30 mM Fe(III), 0.15 mM NTA, 5 mM PDS and 5 mA constant current. Overall, this research provided a novel perspective and potential for remediation of organic wastewater using NTA in combination with electrochemistry in the homogeneous Fe(III)/persulfate system.

Are iron chelates suitable to perform photo-Fenton at neutral pH for secondary effluent treatment?

There have been concerns about which iron chelate is most suitable for application in the photo-Fenton process as well as the fate of these chelates after application. In this study, five chelating agents, i.e. citric acid (CA), oxalic acid (OA), ethylenediamine disuccinic acid (EDDS), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), representing the most used iron chelates were assessed for suitability of application in homogeneous photo-Fenton-like process at pH of 7. The efficiency of the iron chelates were assessed in the disinfection of secondary effluent. The results for the disinfection and bacteria regrowth followed the order EDTA>OA>NTA>CA>OA. All the iron chelates were observed to have increased the COD of the effluent with EDDS having the highest COD contribution. The ability of the chelates to remove aromaticity was measured by the UV₂₅₄ measurement. The efficiency of the chelates to remove aromaticity decreased in the order CA>EDDS>NTA>CA>OA. To determine the fate of the chelates, toxicity tests were conducted on the chelates before and after irradiation and the results revealed a decrease in toxicity after photoirradiation, implying the chelates were degraded and the products/intermediates produced were of less toxicity as compared to the parent compounds.

Efficient degradation of lomefloxacin by Co-Cu-LDH activating peroxymonosulfate process: Optimization, dynamics, degradation pathway and mechanism

In this study, bimetal layered double hydroxides (Co_xCu_y-LDHs) containing a carbonate interlayer were synthesized using coprecipitation with a variety of Co/Cu mole ratios. Meanwhile, the corresponding layered double oxides (Co_xCu_y-LDOs) were prepared as controls. In this study, Electrical energy per order was performed to evaluate economic analysis. Correspondingly, we found that Co_xCu_y-LDHs possessed a significantly better PMS activation capability than the corresponding metal oxide composite (Co₃O₄/CuO). Compared with other Co_xCu_y-LDHs, Co₂Cu₁ LDH possessed the best PMS activation capability for LOM degradation and the lowest electrical energy per order (EE/O) value during the reaction. Additionally, Co₂Cu₁ LDH presented an excellent stability and worked over a wide pH range. The hydroxide states of Co(III), Co(II), Cu(I) and Cu(II) were all able to activate PMS, indicating that there were many active sites on the surface of Co₂Cu₁ LDH. The involvement of radicals in this reaction system was determined via scavenger experiments and electron paramagnetic resonance (EPR). Meanwhile, it's worth noting that a mathematical model was developed to quantify the involvement of SO₄^- and OH. Subsequently, we determined PMS activation mechanism and LOM decomposition pathway for the PMS/Co₂Cu₁ LDH system.

Kinetic and mechanism study of UV/pre-magnetized-Fe0/oxalate for removing sulfamethazine

In this study, UV irradiated photochemical reactions of oxalate (Ox) with premagnetized-Fe⁰ (pre- Fe⁰) as the catalyst was used to degrade sulfamethazine (SMT). Magnetic field promoted the release of iron ion from Fe⁰ thus enhanced SMT and Ox removal in UV/pre- Fe⁰/Ox process. X-ray photoelectron spectroscopy demonstrated that the presence of UV and Ox promoted the transformation of Fe³⁺ to Fe²⁺ on Fe⁰, which enhanced the surface bound •OH (•OH_surf) generation. Ox inhibited the formation of iron (hydro)xides and enhanced the hydroxylation of Fe⁰ surface. •OH_surf was mainly responsible for SMT removal (44%), while UV direct photolysis and •OH in the solution both caused around 28% SMT removal. The process with Ox exhibited much higher efficiency in SMT degradation than that added with H₃PO₄, citric acid and ethylenediaminetetraacetic acid, which greatly expanded the chelate-modified Fenton processes and their treatment efficiency.

Oxidation degradation of 2,2',5-trichlorodiphenyl in a chelating agent enhanced Fenton reaction: Influencing factors, products, and pathways

The sodium pyrophosphate (SP)-enhanced Fenton reaction has been proven to have promising potential in remediation of polychlorinated biphenyls in soils by keeping iron ions soluble at high pH and minimizing the useless decomposition of H₂O₂. However, little information can be obtained about the effect of environmental factors on its remediation performance. Thus, the effect of environmental factors on the degradation of 2,2',5-trichlorodiphenyl (PCB18), one of the main PCB congeners in Chinese sites, was investigated in this study. PCB18 degradation was sensitive to pH, which ranged from 39.8% to 99.5% as increased pH from 3.0 to 9.0. ·OH was responsible for PCB18 degradation at pH 5.0, while both ·OH and O₂^- resulted in PCB 18 degradation at pH 7.0 with the calculated reaction activation energy of 73.5 kJ mol^-1. Bivalent cations and transition metal ions decreased PCB18 degradation markedly as their concentrations increased. The addition of humic acid had an inhibitory on PCB18 degradation, but no obvious inhibition of PCB18 removal was observed when the same concentration of fulvic acid was added. The addition of 1 and 10 μM model humic constituents (MHCs) promoted PCB18 degradation, but the addition of 100 μM MHCs decreased PCB18 removal. Biphenyl, two dichlorobiphenyl, and two hydroxy trichlorobiphenyl derivatives were identified as the major degradation products of PCB18 in the Fe²⁺/SP/H₂O₂ system at pH 7.0. Thus, an oxidative pathway contributed by OH and a reductive pathway induced by O₂^- were proposed as the main mechanisms for PCB18 degradation in the SP-enhanced Fenton reaction.

Degradation of diiodoacetamide in water by UV/chlorination: Kinetics, efficiency, influence factors and toxicity evaluation

The formation and control of haloacetamides (HAcAms) in drinking water have raised high attention due to their high genotoxicity and cytotoxicity, especially the most cytotoxic one, diiodoacetamide (DIAcAm). In this study, the degradation of DIAcAm by UV/chlorination was investigated in terms of degradation kinetics, efficiency, influencing factors, oxidation products and toxicity evaluation. Results revealed that the degradation of DIAcAm by UV/chlorine process followed pseudo-first-order kinetics, and the rate constant between DIAcAm and OH radicals was determined as 2.8 × 10⁹ M^-1 s^-1. The contribution of Cl to DIAcAm degradation by UV/chlorine oxidation was negligible. Increasing chlorine dosage and decreasing pH significantly promoted the DIAcAm degradation during UV/chlorine oxidation, but the presence of bicarbonate (HCO₃^-) and natural organic matter (NOM) inhibited it. The mass balance analysis of iodine species was also evaluated during UV/chlorine oxidation of DIAcAm. In this process, with DIAcAm decreasing from 16.0 to 0.8 μM-I in 20 min, IO₃^-, I^- and HOI/I₂ increased from 0 to 6.3, 6.1 and 0.5 μM-I, respectively. The increase of CHO cell viability during DIAcAm degradation indicated that the toxicity of DIAcAm could be decreased by chlorination, UV irradiation and UV/chlorine oxidation treatments, in which UV/chlorine oxidation was more effective on toxicity reduction than chlorination and UV irradiation alone.

Kinetic study of the degradation of rhodamine B using a flow-through UV/electro-Fenton process with the presence of ethylenediaminetetraacetic acid

An UV enhanced electro-Fenton (EF) process was conducted in a flow-through system to remove rhodamine B (RhB) in the presence of ethylenediamine tetraacetate (EDTA). The process was denoted as UV/EDTA/EF where EDTA formed complexes with iron ions, thus keeping them soluble at high pH values. The process was very efficient as it could initiate the fast reduction of Fe^III to Fe^II and thus the decomposition of H₂O₂. The influence of Fe dose, the ratio of EDTA:Fe, aeration rate, flow rate, current, initial RhB concentration and pH on the RhB removal in the UV/EDTA/EF process was investigated. The best RhB removal was obtained as 89.9% at [Fe]₀ = [EDTA]₀ = 0.2 mM, current = 50 mA, aeration rate = 20 mL min^-1, flow rate = 7 mL min^-1, pH = 7 and [Na₂SO₄]₀ = 0.05 M. The degradation of EDTA during the process was also studied. Radical scavenging experiments indicated that OH was the dominant radical for RhB removal. While, the photolysis of Fe^IIIEDTA was mainly responsible for EDTA degradation. RhB and EDTA removal in different systems was compared. The stability test proved that in the presence of EDTA, the UV/EF process could remove RhB with high efficiency in the first two runs. While, the efficiency dropped remarkably after EDTA's complete depletion. The mechanisms of the UV/EDTA/EF process were proposed. UV/EDTA/EF conducted in the flow-through system was able to efficiently remove RhB as well as EDTA in a wide pH range and proposed as a promising approach for wastewater treatment.

A highly efficient flow-through electro-Fenton system enhanced with nitrilotriacetic acid for phenol removal at neutral pH

Low pH requirement is one of the biggest limitations of the application of traditional Fenton and electro-Fenton (EF) process because Fe^II/Fe^III would precipitate at high pH. In this study, a flow-through EF system operated in batch recirculation mode was constructed. Nitrilotriacetic acid (NTA) was used as a chelating agent in the EF system (NTA/EF) to keep iron soluble at high pH values, producing OH by reaction of H₂O₂ generated in situ with Fe^IINTA that obtained by the reduction of Fe^IIINTA at the cathode. This flow-through NTA/EF system accelerated the mass transfer of target molecules to the electrode surface and showed high efficiency for phenol removal at pH 5-8 with rate constants (k) at around 0.26 min^-1, higher than that of the batch test (k = 0.15 min^-1) and EF process without NTA (k = 0.16 min^-1). The influences of aeration rate, current, flow rate, Fe dose, the ratio of NTA to Fe, pH, and initial phenol concentration on the phenol removal were investigated. The system could be used for at least 3 times for phenol removal without obvious efficiency decline. The flow-through NTA/EF system is promising for the removal of organic contaminants in a wide pH range.

Efficient degradation of pharmaceutical micropollutants in water and wastewater by FeIII-NTA-catalyzed neutral photo-Fenton process

Ferric-nitrilotriacetate complex (Fe^III-NTA) has been adopted to catalyze the photo-Fenton degradation of emerging pharmaceutical micropollutants in water and wastewater at neutral pH. The generation of hydroxyl radicals (HO) in UVA/Fe^III-NTA/H₂O₂ was identified by using electron spin resonance (ESR) trapping technique. The effects of critical parameters (e.g., NTA:Fe^III molar ratio, Fe^III-NTA and H₂O₂ dosages) on the steady-state HO concentrations were studied in terms of the degradation of carbamazepine (CBZ, as a model compound) in Milli-Q water. In addition, the degradation of pharmaceuticals mixtures (including CBZ, crotamiton (CRMT) and ibuprofen (IBP)) in wastewater effluents from a biological aerated filter (BAF) by UVA/Fe^III-NTA/H₂O₂ was studied in continuous-flow mode. The results showed that the efficacies of Fe^III-NTA in catalyzing photo-Fenton degradation of pharmaceuticals in wastewater effluents were comparable to those obtained by Fe^III-ethylenediamine-N,N'-disuccinic acid (Fe^III-EDDS), and far exceeded other Fe^III-L complex (e.g., citric acid, malonic acid, oxalic acid and tartaric acid). More than 92% degradation efficiencies of CBZ, CRMT and IBP were obtained in continuous-flow mode under the given conditions of 0.178 mM Fe^III-NTA (1:1), 4.54 mM H₂O₂, UVA intensity 4.05 mW cm^-2, hydraulic retention time (HRT) 2 h, influent pH 7.6 (±0.2) and temperature 20 °C. The results presented herein suggest that Fe^III-NTA-catalyzed neutral photo-Fenton reaction can be an alternative tertiary process for the treatment of pharmaceutical micropollutants in secondary wastewater effluents.

Polyaniline: A New Metal-Free Catalyst for Peroxymonosulfate Activation with Highly Efficient and Durable Removal of Organic Pollutants

Metal-free heterogeneous catalysts are receiving more and more attention for wastewater remediation by activating peroxymonosulfate (PMS) due to their environmental benign. However, carbon-based materials as the most typical metal-free heterogeneous always suffer from poor durability. Inspired by the fact that a conjugated system may facilitate the electron transfer during PMS activation, we innovatively select polyaniline (PANI) as a new PMS activator and investigate its catalytic performance in detail. It is found that PANI can display better catalytic performance than traditional metal-based catalysts and popular N-doped carbocatalysts in methyl orange (MO) degradation. More importantly, PANI is not only universal for various pollutants degradation but also maintains its catalytic performance in repeated degradation experiments. The stable N sites in the conjugated chains and the oxidation-resistance benzene rings as the building units are considered to be responsible for such an excellent durability. In addition, the influences of some routine factors and actual water backgrounds are comprehensively checked and analyzed. The quenching experiments and electron paramagnetic resonance confirm that MO degradation is achieved through both radical and nonradical pathways, where SO₄^•- and ¹O₂ are primary reactive species. The reaction mechanism is also proposed with the assistance of X-ray photoelectron spectroscopy.

A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values

To overcome the drawback of low pH requirement of the classical Fenton reaction, researchers have applied chelating agents to form complexes with Fe and enable Fenton reaction at high pHs, which is reviewed in this article. The chelating agents reviewed include humic substances, polycarboxylates, aminopolycarboxylic acids, and polyoxometalates. Ligands affect the reactivity of Fe-complexes by changing their redox potentials, promoting their reaction with H₂O₂, and competing with target contaminants for the oxidative species. Fe(III)-complexes are reduced to Fe(II)-complexes by O₂^- not H₂O₂, as indicated by their redox potentials. The stability constants of Fe-complexes increase with increasing pK_a values of the corresponding ligands and also with increasing charge density of the metal ions. A higher stability constant of Fe(III)-complex indicates higher reaction rate of corresponding Fe(II)-complex with H₂O₂ and lower reduction rate of Fe(III)-complex to Fe(II)-complex. OH, O₂^-, and ferryl species were reported to be the reactive species on the contaminant removal in the chelate-modified Fenton process. The generation of these species depends on the chelating agents and reaction conditions. The process is very efficient in degrading contaminants, indicating a potential treatment approach for the pollution remediation at natural pH.

Degradation of geosmin and 2-methylisoborneol in water with UV/chlorine: Influencing factors, reactive species, and possible pathways

Geosmin (GSM) and 2-methylisoborneol (2-MIB) are two common odor compounds in drinking water. In this paper, the performance of UV/chlorine was compared with that of chlorine and UV to degrade GSM (100 ng L^-1) and 2-MIB (100 ng L^-1) in water. UV/chlorine was further exploited, and a steady-state kinetic model was used to conduct a detailed study on efficiency, rate, reactive species and pathway. The results showed that UV/chlorine greatly could improve the removal ratio to 90% within 5 min, from approximately 20% with only UV or dark chlorine in 60 min. The removal ratio and rate depended on UV light intensity, free chlorine dosage, reaction time and water quality parameters (e.g. pH, concentrations of HCO₃^- and Cl^-). Among these factors, the first two obviously could accelerate the rate and increase the ratio. The degradation was also significantly improved in an acidic condition, while alkaline conditions and HCO₃^- had inhibitory effects, and Cl^- created no difference. Contributions of OH and Cl to the degradation of 2-MIB and GSM were further revealed, and OH was found to be the most important reactive species. In the UV/chlorine process, 6 degradation byproducts of 2-MIB, including 1 alcohol, 2 ketones, and 3 olefins, were identified, and 14 degradation byproducts of GSM, including 6 ketones, 1 aldehyde, 2 alcohols, 3 naphthenes, and 2 olefins, were found by using solid-phase microextraction coupled to gas chromatography-mass spectrometry. The possible degradation pathways of GSM and 2-MIB in UV/chlorine thus were proposed.

Degradation behavior of triclosan by co-exposure to chlorine dioxide and UV irradiation: influencing factors and toxicity changes

This study investigated the transformation of triclosan (TCS) following co-exposure to UV irradiation and ClO₂. Special attention was given to understand the influencing of water quality parameters and toxicity changes during the co-exposure process. The results show that the co-exposure process prompted TCS elimination quickly and effectively, with more than 99% of TCS degraded under the experimental conditions. The molar yield ratios of 2,4-dichlorophenol/TCS (2,4-DCP/TCS) were calculated to be 35.81-74.49%; however, the by-product of 2,8-dichlorodibenzop-dioxin (2,8-Cl₂DD) was not detected. The TCS degradation was sensitive to ClO₂ dosage, pH, H₂O₂, and natural organic matter (NOM), but not to the carbonate (CO₃^2-) concentration. Neutral and slightly alkaline condition were favorable to TCS elimination. The TCS removal rate increased from 85.33 to 99.75% when the ClO₂ concentration increased from 0.25 to 1.5 mg L^-1. TCS degradation can be promoted at low NOM level (1, 3, and 5 mg L^-1), whereas was inhibited at high NOM concentrations of 7 and 9 mg L^-1. While adding H₂O₂, the degradation rate of TCS increased with increasing H₂O₂ concentration from 1 to 3 mg L^-1; however, too low or overdosed H₂O₂ (0.5 and 5 mg L^-1) hindered TCS degradation. Based on the results of a microtox bioassay, the toxicity did not change following the co-exposure process.

TCE degradation in groundwater by chelators-assisted Fenton-like reaction of magnetite: Sand columns demonstration

Trichloroethylene (TCE) degradation in sand columns has been investigated to evaluate the potential of chelates-enhanced Fenton-like reaction with magnetite as iron source for in situ treatment of TCE-contaminated groundwater. The results showed that successful degradation of TCE in sand columns was obtained by nitrilotriacetic acid (NTA)-assisted Fenton-like reaction of magnetite. Addition of ethylenediaminedisuccinic acid (EDDS) resulted in an inhibitory effect on TCE degradation in sand columns. Similar to EDDS, addition of ethylenediaminetetraacetic acid (EDTA) also led to an inhibition of TCE degradation in sand column with small content of magnetite (0.5 w.t.%), but enhanced TCE degradation in sand column with high content of magnetite (7.0 w.t.%). Additionally, the presence of NTA, EDDS and EDTA greatly decreased H₂O₂ uptake in sand columns due to the competition between chelates and H₂O₂ for surface sites on magnetite (and sand). Furthermore, the presented results show that magnetite in sand columns remained stable in a long period operation of 230 days without significant loss of performance in terms of TCE degradation and H₂O₂ uptake. Moreover, it was found that TCE was degraded mainly to formic acid and chloride ion, and the formation of chlorinated organic intermediates was minimal by this process.

ZIF-8 derived nitrogen-doped porous carbon as metal-free catalyst of peroxymonosulfate activation

Nitrogen-doped porous carbon (NPC) is synthesized through a direct pyrolysis of zeolitic imidazolate framework (ZIF)-8 under N₂ flow followed by acid washing process. It is found that NPC-800 pyrolyzed at 800 °C can inherit the perfect rhombic dodecahedron morphology of ZIF-8 crystals and achieve the considerable nitrogen-doping content of 15.20%. When NPC-800 is applied as the heterogeneous catalyst in peroxymonosulfate (PMS) activation for the degradation of Rhodamine B (RhB) and phenol, NPC-800 will exhibit its better performance than some conventional transition metal-based oxides and common carbon materials. The active sites can be primarily ascribed to nitrogen modification and sp ²-hybridized carbon frameworks. Besides, the influence of several parameters such as the dosage of catalyst, the concentration of oxidant, and reaction temperature is conducted systematically. More importantly, NPC-800 can maintain its considerable degradation in the presence of some anions and natural organic matters, even under some actual water background conditions. Although NPC-800 displays mild deactivation in repeated experiments, its catalytic performance can be easily recovered through heat treatment at 350 °C in air. Radical quenching tests reveal that both sulfate and hydroxyl radicals are responsible for the removal of organic pollutants. This research may provide a new way for the application of novel metal-free carbocatalysts in terms of PMS activation.

Enhanced Fenton-like degradation of TCE in sand suspensions with magnetite by NTA/EDTA at circumneutral pH

The present study investigated the degradation of trichloroethylene (TCE) in sand suspensions by Fenton-like reaction with magnetite (Fe₃O₄) in the presence of various chelators at circumneutral pH. The results showed that ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) greatly improved the rate of TCE degradation, while [S,S]-ethylenediaminedisuccinic acid (s,s-EDDS), malonate, citrate, and phytic acid (IP6) have minimal effects on TCE degradation. Quenching tests suggested that TCE was mainly degraded by hydroxyl radical (HO^·) attack, with about 90% inhibition on TCE degradation by the addition of HO^· scavenger 2-propanol. The presence of 0.1-0.5% Fe₃O₄/sand (w/w) contributed to 40% increase in TCE degradation rates. In particular, the use of chelators can avoid high concentrations of H₂O₂ required for the Fenton-like reaction with Fe₃O₄, and moreover improve the stoichiometric efficiencies of TCE degradation to H₂O₂ consumption. The suitable concentrations of chelators (EDTA and NTA) and H₂O₂ were suggested to be 0.5 and 20 mM, respectively. Under the given conditions, degradation rate constants of TCE were obtained at 0.360 h^-1 with EDTA and 0.526 h^-1 with NTA, respectively. Enhanced degradation of TCE and decreased usage of H₂O₂ in this investigation suggested that Fenton-like reaction of Fe₃O₄ together with NTA (or EDTA) may be a promising process for remediation of TCE-contaminated groundwater.

Comparison of classical fenton, nitrilotriacetic acid (NTA)-Fenton, UV-Fenton, UV photolysis of Fe-NTA, UV-NTA-Fenton, and UV-H2O2 for the degradation of cyclohexanoic acid

The treatment of a naphthenic acid model compound, cyclohexanoic acid, with classical Fenton, UV-H₂O₂, UV-Fenton, nitrilotriacetic acid (NTA)-Fenton, UV-NTA-Fenton, and UV photolysis of Fe-NTA processes at pHs 3 and 8 was investigated. At 1.47 mM H₂O₂, 0.089 mM Fe, and 0.18 mM NTA, the UV-NTA-Fenton process at pH 3 exhibited the highest H₂O₂ decomposition (100% in 25 min), CHA removal (100% in 12 min) with a rate constant of 0.27 ± 0.025 min^-1, and NTA degradation (100% in 6 min). Due to the formation of H₂O₂-Fe(III)NTA adduct, the total Fe concentration in the UV-NTA-Fenton system (0.063 mM at the end of the reaction) at pH 8 was much higher than that in the UV photolysis of Fe(III)NTA process (0.024 mM). The co-complexing effect of borate buffer helped to keep iron soluble; however, it imposed a negative influence on the CHA degradation in the UV-NTA-Fenton process (68% CHA removal in 60 min in the borate buffer compared to 92% in MilliQ water). The results demonstrated that the most efficient process for the CHA degradation under the experimental conditions was the UV-NTA-Fenton process at pH 3.

Coagulation-flocculation sequential with Fenton or Photo-Fenton processes as an alternative for the industrial textile wastewater treatment

In this study, the industrial textile wastewater was treated using a chemical-based technique (coagulation-flocculation, C-F) sequential with an advanced oxidation process (AOP: Fenton or Photo-Fenton). During the C-F, Al₂(SO₄)₃ was used as coagulant and its optimal dose was determined using the jar test. The following operational conditions of C-F, maximizing the organic matter removal, were determined: 700 mg/L of Al₂(SO₄)₃ at pH = 9.96. Thus, the C-F allowed to remove 98% of turbidity, 48% of Chemical Oxygen Demand (COD), and let to increase in the BOD₅/COD ratio from 0.137 to 0.212. Subsequently, the C-F effluent was treated using each of AOPs. Their performances were optimized by the Response Surface Methodology (RSM) coupled with a Box-Behnken experimental design (BBD). The following optimal conditions of both Fenton (Fe²⁺/H₂O₂) and Photo-Fenton (Fe²⁺/H₂O₂/UV) processes were found: Fe²⁺ concentration = 1 mM, H₂O₂ dose = 2 mL/L (19.6 mM), and pH = 3. The combination of C-F pre-treatment with the Fenton reagent, at optimized conditions, let to remove 74% of COD during 90 min of the process. The C-F sequential with Photo-Fenton process let to reach 87% of COD removal, in the same time. Moreover, the BOD₅/COD ratio increased from 0.212 to 0.68 and from 0.212 to 0.74 using Fenton and Photo-Fenton processes, respectively. Thus, the enhancement of biodegradability with the physico-chemical treatment was proved. The depletion of H₂O₂ was monitored during kinetic study. Strategies for improving the reaction efficiency, based on the H₂O₂ evolution, were also tested.

Activation of persulfates by natural magnetic pyrrhotite for water disinfection: Efficiency, mechanisms, and stability

This study introduces natural occurring magnetic pyrrhotite (NP) as an environmentally friendly, easy available, and cost-effective alternative catalyst to activate persulfate (PS) of controlling microbial water contaminants. The E. coli K-12 inactivation kinetics observed in batch experiments was well described with first-order reaction. The optimum inactivation rate (k = 0.47 log/min) attained at a NP dose of 1 g/L and a PS dose of 1 mM, corresponding to total inactivation of 7 log₁₀ cfu/mL cells within 15 min. Measured k increased > 2-fold when temperature increased from 20 to 50 °C; and > 4-fold when pH decreased from 9 to 3. Aerobic conditions were more beneficial to cell inactivation than anaerobic conditions due to more reactive oxygen species (ROS) generated. ROS responsible for the inactivation were identified to be SO₄^- > OH > H₂O₂ based on a positive scavenging test and in situ ROS determination. In situ characterization suggested that PS effectively bind to NP surface was likely to form charge transfer complex (≡Fe(II)⋯O₃SOOSO₃^-), which mediated ROS generation and E. coli K-12 oxidation. The increased cell-envelope lesions consequently aggravated intracellular protein depletion and genome damage to cause definite bacterial death. The NP still maintained good physiochemical structure and stable activity even after 4 cycle. Moreover, NP/PS system also exhibited good E. coli K-12 inactivation efficiency in authentic water matrices like surface water and effluents of secondary wastewater.

Benzene oxidation by Fe(III)-catalyzed sodium percarbonate: matrix constituent effects and degradation pathways

Complete degradation of benzene by the Fe(III)-activated sodium percarbonate (SPC) system is demonstrated. Removal of benzene at 1.0 mM was seen within 160 min, depending on the molar ratios of SPC to Fe(III). A mechanism of benzene degradation was elaborated by free-radical probe-compound tests, free-radical scavengers tests, electron paramagnetic resonance (EPR) analysis, and determination of Fe(II) and H2O2 concentrations. The degradation products were also identified using gas chromatography-mass spectrometry method. The hydroxyl radical (HO.) was the leading species in charge of benzene degradation. The formation of HO. was strongly dependent on the generation of the organic compound radical (R.) and superoxide anion radical (O.). Benzene degradation products included hydroxylated derivatives of benzene (phenol, hydroquinone, benzoquinone, and catechol) and aliphatic acids (oxalic and fumaric acids). The proposed degradation pathways are consistent with radical formation and identified products. The investigation of selected matrix constituents showed that the Cl and HCO₃ had inhibitory effects on benzene degradation. Natural organic matter (NOM) had accelerating influence in degrading benzene. The developed system was tested with groundwater samples and it was found that the Fe(III)-activated SPC has a great potential in effective remediation of benzene-contaminated groundwater while more further studies should be done for its practical application in the future because of the complex subsurface environment.

Enhanced degradation of gaseous benzene by a Fenton reaction

A wet scrubbing process coupled with advanced oxidation processes (AOP) has raised great interest for the abatement of volatile organic compounds (VOCs) owing to its strong oxidation capacity and few byproducts.