Kinetic modeling of the oxidation of p-hydroxybenzoic acid by Fenton's reagent: implications of the role of quinones in the redox cycling of iron

Were Persulfate-Based Advanced Oxidation Processes Really Understood? Basic Concepts, Cognitive Biases, and Experimental Details

Persulfate (PS)-based advanced oxidation processes (AOPs) for pollutant removal have attracted extensive interest, but some controversies about the identification of reactive species were usually observed. This critical review aims to comprehensively introduce basic concepts and rectify cognitive biases and appeals to pay more attention to experimental details in PS-AOPs, so as to accurately explore reaction mechanisms. The review scientifically summarizes the character, generation, and identification of different reactive species. It then highlights the complexities about the analysis of electron paramagnetic resonance, the uncertainties about the use of probes and scavengers, and the necessities about the determination of scavenger concentration. The importance of the choice of buffer solution, operating mode, terminator, and filter membrane is also emphasized. Finally, we discuss current challenges and future perspectives to alleviate the misinterpretations toward reactive species and reaction mechanisms in PS-AOPs.

Optical Bioassays Based on the Signal Amplification of Redox Cycling

Optical bioassays are challenged by the growing requirements of sensitivity and simplicity. Recent developments in the combination of redox cycling with different optical methods for signal amplification have proven to have tremendous potential for improving analytical performances. In this review, we summarized the advances in optical bioassays based on the signal amplification of redox cycling, including colorimetry, fluorescence, surface-enhanced Raman scattering, chemiluminescence, and electrochemiluminescence. Furthermore, this review highlighted the general principles to effectively couple redox cycling with optical bioassays, and particular attention was focused on current challenges and future opportunities.

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.

Nanoconfinement-triggered oligomerization pathway for efficient removal of phenolic pollutants via a Fenton-like reaction

Heterogeneous Fenton reaction represents one of the most reliable technologies to ensure water safety, but is currently challenged by the sluggish Fe(III) reduction, excessive input of chemicals for organic mineralization, and undesirable carbon emission. Current endeavors to improve the catalytic performance of Fenton reaction are mostly focused on how to accelerate Fe(III) reduction, while the pollutant degradation step is habitually overlooked. Here, we report a nanoconfinement strategy by using graphene aerogel (GA) to support UiO-66-NH2-(Zr) binding atomic Fe(III), which alters the carbon transfer route during phenol removal from kinetically favored ring-opening route to thermodynamically favored oligomerization route. GA nanoconfinement favors the Fe(III) reduction by enriching the reductive intermediates and allows much faster phenol removal than the unconfined analog (by 208 times in terms of first-order rate constant) and highly efficient removal of total organic carbon, i.e., 92.2 ± 3.7% versus 3.6 ± 0.3% in 60 min. Moreover, this oligomerization route reduces the oxidant consumption for phenol removal by more than 95% and carbon emission by 77.9%, compared to the mineralization route in homogeneous Fe2++H2O2 system. Our findings may upgrade the regulatory toolkit for Fenton reactions and provide an alternative carbon transfer route for the removal of aqueous pollutants.

Persulfate-based remediation of organic-contaminated soil: Insight into the impacts of natural iron ions and humic acids with complexation/redox functionality

The use of persulfate (PDS) for in-situ chemical oxidation of organic contaminants in soils has garnered significant interest. However, the presence of naturally occurring iron-containing substances and humic acid (HA) in environmental compartments can potentially influence the effectiveness of soil remediation. Thus, this study aimed to investigate the role of key functional groups (adjacent phenolic hydroxyl (Ar-OH) and carboxyl groups (-COOH)) in HA that interact with iron. Modified HAs were used to confirm the significance of these moieties in iron interaction. Additionally, the mechanism by which specific functional groups affect Fe complexation and redox was explored through contaminant degradation experiments, pH-dependent investigations, HA by-products analysis, and theoretical calculations using six specific hydroxybenzoic acids as HA model compounds. The results showed a strong positive correlation between accessible Ar-OH and -COOH groups and Fe3+/Fe2+ redox. This was attributed to HA undergoing a conversion process to a semiquinone-containing radical form, followed by a quinone-containing intermediate, while Fe3+ acted as an electron shuttle between HA and PDS, with Fe3+ leaching facilitated by generated H+ ions. Although the stability of HA-Fe3+ complexes with -COOH as the primary binding sites was slightly higher at neutral/alkaline conditions compared to acidic conditions, the buffering properties of the soil and acidification of the PDS solution played a greater role in determining the Ar-OH groups as the primary binding site in most cases. Therefore, the availability of Ar-OH groups on HA created a trade-off between accelerated Fe3+/Fe2+ redox and quenching reactions. Appropriate HA and iron contents were found to favor PDS activation, while excessive HA could lead to intense competition for reactive oxygen species (ROS), inhibiting pollutant degradation in soil. The findings provide valuable insights into the interaction of HA and Fe-containing substances in persulfate oxidation, offering useful information for the development of in-situ remediation strategies for organic-contaminated soil.

Challenges Relating to the Quantification of Ferryl(IV) Ion and Hydroxyl Radical Generation Rates Using Methyl Phenyl Sulfoxide (PMSO), Phthalhydrazide, and Benzoic Acid as Probe Compounds in the Homogeneous Fenton Reaction

Ferryl ion ([FeIVO]2+) has often been suggested to play a role in iron-based advanced oxidation processes (AOPs) with its presence commonly determined using the unique oxidation pathway from methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO2). However, we show here that the oxidation products of PMSO, formed on reaction with hydroxyl radical, enhance PMSO2 formation as a result of their complexation with Fe(III) leading to the changes in the reactivity of Fe(III) species in the homogeneous Fenton reaction. As such, PMSO should be used with caution to investigate the role of [FeIVO]2+ in iron-based AOPs with these insights suggesting the need to reassess the findings of many previous studies in which this reagent was used. The other common target compounds, phthalhydrazide and hydroxybenzoic acids, were also found to modify the rate and extent of iron cycling as a result of complexation and/or redox reactions, either by the probe compound itself and/or oxidation products formed. Overall, this study highlights that these confounding effects of the aromatic probe compounds on the reactivity of iron species should be recognized if reliable mechanistic insights into iron-based AOPs are to be obtained.

Caffeic acid accelerated the Fe(II) reinvention in Fe(III)/PMS system for bisphenol A degradation: Oxidation intermediates and inherent mechanism

Fe(II)-catalyzed PMS process was widely used in the degradation of refractory pollutants in wastewater, while its performance was restricted by the slow regeneration efficiency of Fe(II). Herein, caffeic acid (CFA), a representative of hydroxycinnamic acids, was introduced into Fe(III)/PMS system to accelerate the transformation of Fe(III) to Fe(II) and promote the removal of bisphenol A (BPA). Under optimum condition of 0.1 mM CFA, 0.05 mM Fe(III), and 0.5 mM PMS, almost complete removal of BPA can be achieved within 20 min, which was roughly 6.2 times higher than that in Fe(III)/PMS system. As the addition of CFA into Fe(III)/PMS system, pH application range was widened from acidic to alkaline conditions. The reduction and chelation of CFA expedited the Fe(III)/Fe(II) cycle by forming CFA-Fe chelate, thereby facilitating the PMS activation. Based on LC-MS analysis and DFT calculation, the intermediate products of CFA were found to play a decisive role in boosting the regeneration of Fe(II), and the toxicity of these intermediates towards organisms was evaluated by ECOSAR. The alcohol-scavenging and chemical probe tests certified that hydroxyl radical (•OH), sulfate radical (SO4•-), and Fe(IV) coexisted in Fe(III)/CFA/PMS system, and the second-order reaction rate constants of •OH and SO4•- reacted with CFA were calculated to be 3.16✕109 and 2.30✕1010 M-1 s-1, respectively. Two major degradation pathways of BPA, •OH addition and SO4•- induced hydroxylation reaction, were proposed. This work presented a novel green phenolic compound that expedited the Fe(II)-catalyzed PMS activation process for the treatment of organic contaminants.

Synergetic Oxidation of the Hydroxyl Radical and Superoxide Anion Lowers the Benzoquinone Intermediate Conversion Barrier and Potentiates Effective Aromatic Pollutant Mineralization

Regulation of the free radical types is crucial but challenging in the ubiquitous heterogeneous catalytic oxidation for chemosynthesis, biotherapy, and environmental remediation. Here, using aromatic pollutant (AP) removal as a prototype, we identify the massive accumulation of the benzoquinone (BQ) intermediate in the hydroxyl radical (•OH)-mediated AP degradation process. Theoretical prediction and experiments demonstrate that BQ is both a Lewis acid and base because of its unique molecular and electronic structure caused by the existence of symmetrical carbonyl groups; therefore, it is hard to be electrophilically added by oxidizing •OH as a result of the high reaction energy barrier (ΔG = 1.74 eV). Fortunately, the introduction of the superoxide anion (•O2-) significantly lowers the conversion barrier (ΔG = 0.91 eV) of BQ because •O2- can act as the electron donor and acceptor simultaneously, electrophilically and nucleophilically add to BQ synchronously, and break it down. Subsequently, the breakdown products can then be further oxidized by •OH until completely mineralized. Such synergistic oxidation based on •OH and •O2- timely eliminates BQ, potentiates AP mineralization, and inhibits electrode fouling caused by high-resistance polymeric BQ; more importantly, it effectively reduces toxicity, saves energy and costs, and decreases the environmental footprint, evidenced by the life cycle assessment.

Constant oxidation of atrazine in Fe(III)/PDS system by enhancing Fe(III)/Fe(II) cycle with quinones: Reaction mechanism, degradation pathway and DFT calculation

Quinones are potential pollutants and redox active compounds widely distributed in environmental media. In this study, methyl-p-benzoquinone (MBQ) was introduced into Fe(III)/peroxydisulfate system (Fe(III)/PDS) to expedite the conversion of Fe(III) to Fe(II) and the degradation of atrazine (ATZ), ultimately establishing an environmentally friendly system of "treating pollution with pollution". MBQ/Fe(III)/PDS system showed superior performance to traditional Fe(II)/PDS system in pH range of 2-7. Sulfate radical (SO₄^•-) and hydroxyl radical (^•OH) were confirmed to exist in MBQ/Fe(III)/PDS system according to alcohol quenching experiments and ESR tests. Meanwhile, stable 80% of η[PMSO₂] (i.e., the molar ratio of PMSO₂ generation to PMSO consumption) was achieved and manifested that highly reactive substance Fe(IV) also participated in MBQ/Fe(III)/PDS system. The spontaneous transformation of MBQ and methyl-hydroquinone (MHQ) drove Fe(III)/Fe(II) cycle, during which MHQ induced Fe(III) reduction and Fe(II) regeneration. Transformation pathways of ATZ were proposed based on HPLC-MS detection and DFT calculation and ATZ degradation could be initiated by lateral chain oxidation and dechlorination-hydroxylation. The acute toxicity, bioaccumulation factor, developmental toxicity and mutagenicity of ATZ and its degradation intermediates were evaluated by Toxicity Estimation Software Tool, and the luminescent bacteria test was conducted to investigate the acute toxicity variation of the reaction solution. Cl^- obviously inhibited ATZ degradation and three main by-products generation, while humic acid (HA) had little effect on them probably due to the established balance between inhibition (some components in HA competed to consume reactive species) and acceleration (quinone units in HA also facilitated Fe(III)/Fe(II) cycle).

Activated Carbon Assisted Fenton-like Treatment of Wastewater Containing Acid Red G

The Fenton reaction as an effective advanced oxidation technology has been widely used in wastewater treatment for its stable effluent quality, simple operation, mild condition, and higher organic degradation with non-selectivity. However, the traditional Fenton reaction is limited by the sluggish regeneration of Fe2+, resulting in a slower reaction rate, and it is necessary to further increase the dosage of Fe2+, which will increase the production of iron sludge. Activated carbon (AC) has a strong adsorption property, and it cannot be ignored that it also can reduce Fe3+. In this study, the degradation of acid red G (ARG) by adding AC to the Fe3+/H2O2 system, the role of the reducing ability, and the reason why AC can reduce Fe3+ were studied. By adding three kinds of ACs, including coconut shell-activated carbon (CSPAC), wood-activated carbon (WPAC), and coal-activated carbon (CPAC), the ability of ACs to assist the Fe3+/H2O2 Fenton-like system to degrade ARG was clarified. Through the final treatment effect and the ability to reduce Fe3+, the type of AC with the best promotion effect was CSPAC. The different influence factors of particle size, the concentration of CSPAC, concentration of H2O2, concentration of Fe3+, and pH value were further observed. The best reaction conditions were determined as CSPAC powder with a particle size of 75 μm and dosage of 0.6 g/L, initial H2O2 concentration of 0.4 mmol/L, Fe3+ concentration of 0.1 mmol/L, and pH = 3. By reducing the adsorption effect of CSPAC, it was further observed that CSPAC could accelerate the early reaction rate of the degradation process of ARG by the Fe3+/H2O2 system. FT-IR and XPS confirmed that the C-O-H group on the surface of CSPAC could reduce Fe3+ to Fe2+. This study can improve the understanding and role of AC in the Fenton reaction, and further promote the application of the Fenton reaction in sewage treatment.

Benzoquinone-assisted heterogeneous activation of PMS on Fe3S4 via formation of active complexes to mediate electron transfer towards enhanced bisphenol A degradation

Benzoquinone (BQ) is of great significance for enhancement of contaminants degradation in the homogeneous oxidation system of peroxymonosulfate (PMS). However, the role of BQ in the heterogeneous activation of PMS for contaminants oxidation is still not clear. Herein, this work reported that the addition of BQ into the Fe₃S₄/PMS system could effectively enhance the degradation and mineralization of bisphenol A (BPA). Mechanistic study uncovered that the BQ and PMS would form active complexes (BQ-PMS*) on the surface of Fe₃S₄ and the excited BQ-PMS* can oxidize the BPA. To be specific, the electron of BPA was extracted by BQ-PMS* and then transfer to the surface of Fe₃S₄. The surface electron can induce the change of valence state of S and Fe elements, which can trigger the degradation of BPA and inhibit the decomposition of BQ itself. To the best of our knowledge, it is the first time to unveil the positive role of BQ in the heterogeneous activation of PMS, which may shed new light on the establishment of high-efficient PMS-based oxidation technology for remediation of organic pollutant.

Mechanical Insights into Activation of Peroxides by Quinones: Formation of Oxygen-Centered Radicals or Singlet Oxygen

In this work, the mechanism of the activation of peroxides by quinones has been investigated through quantum chemical calculations. Hydrogen peroxide (H₂O₂), peroxomonosulfate (PMS), peracetic acid (PAA), and CH₃OOH were chosen as the model peroxides and p-benzoquinone (p-BQ) and tetrachloro-1,4-benzoquinone (TCBQ) as the model quinones. The nucleophilic attack of peroxides can occur on the carbonyl and olefinic carbons of quinones. For p-BQ, the nucleophilic attack of HO₂^-, CH₃OO^-, PMS, and PAA might prefer to occur on the carbonyl carbons, which have more positive atomic charges. Then, further transformation could not be induced from the addition of HO₂^- and CH₃OO^- to p-BQ. Comparatively, singlet oxygen (¹O₂) could be generated in the cases of PMS and PAA. For TCBQ, the chlorine atoms cause the olefinic carbons to carry more positive atomic charges, and then, HO₂^- preferred to add to the olefinic carbons, which might induce the formation of the hydroxyl radical (^•OH). The activation of PMS by TCBQ was similar to that by p-BQ, with the kinetical feasibility of ¹O₂ formation. These findings may provide some theoretical insights into the reaction of peroxides with quinones, especially into the interconnection between the substitutes and the formation of oxygen-centered radicals (e.g., ^•OH) and ¹O₂.

Hydroxyl radicals induced mineralization of organic carbon during oxygenation of ferrous mineral-organic matter associations: Adsorption versus coprecipitation

The iron (Fe) phases have been widely proposed to preserve organic carbon (OC) via adsorption or coprecipitation pathways, however, such role of Fe phases might be largely reversed under redox-fluctuation conditions, especially for Fe(II) minerals-protected OC. In this study, we synthesized the Fe(II)-OC associations via adsorption and coprecipitation using FeCO₃ and three types of low-molecular-weight organic compounds (LMWOCs) at different C/Fe molar ratios, and investigated the OC mineralization induced by hydroxyl radicals (OH) during oxygenation processes. Abundant OH can be produced upon oxygenation of FeCO₃-LMWOCs associations within 96 h, giving values of 28.49-151.36 μM in adsorption and 12.63-76.41 μM in coprecipitation treatments depended on types of LMWOCs and C/Fe molar ratios. Fe(II) species in coprecipitates with hydroquinone (HQ) mainly transformed into Goethite-like phases after oxygenation, while adsorption samples induced more formation of lower-crystalline Fe phase (e.g., ferrihydrite). The surface-Fe(II) was the primary electron donors to O₂, which further induced hydrogen peroxide (H₂O₂) formation via one- and two-electron transfer pathways. Finally, the produced OH removed 0.55-9.65 and 0.16-85.54 mg L^-1 total OC in adsorption and coprecipitation treatments after oxygenation. Collectively, this study highlights that OC associated with Fe(II) minerals might be labile due to the oxidation of formed OH, and the role of Fe phases in OC sequestration may be further re-evaluated under redox fluctuation conditions.

Hydroxylamine promoted Fe(III) reduction in H2O2/soil systems for phenol degradation

Production of hydroxyl radicals (•OH) upon the oxidation of solid Fe(II) by O₂ or H₂O₂ in soils and sediments has been confirmed, which benefits in situ remediation of contaminants. However, Fe(III) reduction by H₂O₂ is rate-limiting. Accelerating the Fe(III)/Fe(II) cycle could improve the efficiency of remediation. This study intended to use hydroxylamine to promote Fe(III)/Fe(II) cycle during 100 g/L soil oxidation by H₂O₂ for phenol degradation. The removal of phenol was 76% in 3 h during soil oxidation with 1 mM H₂O₂ in the presence of 1 mM hydroxylamine but was negligible in the absence of hydroxylamine. Fe(III) in the soil was reduced to 0.21 mM Fe(II) by 1 mM hydroxylamine in 30 min. The accelerated cycle of Fe(III)/Fe(II) in the soil by hydroxylamine could effectively decompose H₂O₂ to produced •OH, which was responsible for the effective enhancement of phenol degradation during soil oxidation. Under the conditions of 1 mM H₂O₂ and 100 g/L soil, the pseudo-first-order kinetic constant of phenol degradation increased proportionally from 0.0453 to 0.0844 min^-1 with the increase of hydroxylamine concentrations from 0.5 to 1 mM. The kinetic constant also increased from 0.0041 to 0.0111 min^-1 with H₂O₂ concentration increased from 0.5 to 2 mM, while it decreased from 0.0100 to 0.0051 min^-1 with soil dosage increased from 20 to 200 g/L. In addition, column experiments showed that phenol (10 mg/L) degradation ratio kept at about 48.7% with feeding 2 mM hydroxylamine and 2 mM H₂O₂ at 0.025 PV/min. Column experiments suggested an optional application of hydroxylamine and H₂O₂ for in situ remediation. The output of this study provides guidance and optional strategies to enhance contaminant degradation during soil oxidation.

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 comprehensive kinetic model for phenol oxidation in seven advanced oxidation processes and considering the effects of halides and carbonate

As one of the most powerful approaches to mechanistically understanding complex chemical reactions and performing simulations or predictions, kinetic modeling has been widely used to investigate advanced oxidation processes (AOPs). However, most of the available models are built based on limited systems or reaction mechanisms so they cannot be readily extended to other systems or reaction conditions. To overcome such limitations, this study developed a comprehensive model on phenol oxidation using over 540 reactions, covering the most common reaction mechanisms in nine AOPs-four peroxymonosulfate (PMS), four peroxydisulfate (PDS), and one H₂O₂ systems-and considering product formation and the effects of co-existing anions (chloride, bromide, and carbonate). Existing models in the literature were first gathered and then revised by correcting inaccurately used reactions and adding other necessary reactions. Extensive model tuning and validation were next conducted by fitting the model against experimental data from both this study and the literature. The effects of anions were found to follow PDS/CuO > H₂O₂/UV > other PDS or PMS systems. Halogenated organic byproducts were mainly observed in the PMS systems in the presence of halides. Most of the 543 reactions were found to be important based on the sensitivity analysis, with some anions-involved reactions being among the most important, which explained why these anions substantially altered some of the reaction systems. With this comprehensive model, a deep understanding and reliable prediction can be made for the oxidation of phenol (and likely other phenolic compounds) in systems containing one or more of the above AOPs.

Efficiency and Quantitative Structure-Activity Relationship of Monoaromatics Oxidation by Quinone-Activated Persulfate

Quinones and quinone-containing organics have potential of activating persulfate to produce sulfate radical. In this work, the optimal condition for quinone activation of persulfate was investigated. It was found representative monoaromatics were degraded fastest in alkaline environment (pH 10.0), but excessive alkalinity restrained the reaction instead. The mechanisms to explain this phenomenon were speculated. The effect of initial quinone concentration on persulfate oxidation was also investigated at pH 10.0. In addition, a quantitative structure-activity relationship model was established with 15 kinds of monoaromatics, which revealed the most negative atomic net charges on carbon atom played an important role on degradation rates. Chemicals with a smallerq C - were easier oxidized in quinone-activate system. This finding helps further exploration of effective activator in alkaline environment.

Redox cycling of copper mediated by hydrazine: efficient catalyst systems for oxidative degradation of rhodamine B

Catalytic processes based on Fenton-like reactions on the degradation of organic pollutants have been improved by accelerating the redox cycling of metal ions. This work presents, at first, the results obtained for the heterogeneous degradation of rhodamine B (RhB) by copper ferrite (CuFe₂O₄) in presence of hydrogen peroxide (H₂O₂) and hydrazine (N₂H₄) as redox cycle accelerator. Atomic absorption spectroscopy showed small amounts of Cu²⁺ are leached from ferrite highlighting the influence of the homogeneous catalysis in the whole process. The data obtained for the homogeneous process using Cu²⁺ in solution containing both N₂H₄ and H₂O₂ indicated such system is highly efficient mineralizing 73% of RhB within only 10 min of reaction and having H₂O and CO₂ as major products. Using tert-butyl alcohol as radical scavenger, it was confirmed hydroxyl radical (HO^•) is the active oxidant species regarding the RhB degradation. The homogeneous catalyst was applied to a real sample of textile effluent spiked with RhB and showed reasonable efficiency, although lower than that obtained for the standard solutions of RhB. This result was assigned to the interference of salts in the medium that react with HO^• thus acting as radical scavenger.

Degradation of norfloxacin by hydroxylamine enhanced fenton system: Kinetics, mechanism and degradation pathway

This study demonstrated a fast and efficient degradation of a typical antibiotic norfloxacin (NOR) in a hydroxylamine enhanced Fenton (HA-Fenton) system, which showed a higher catalytic activity over a wider pH range (3.0-9.0). The removal efficiency of NOR was 96% at following conditions: 10 mg/L NOR, 10 μM Fe²⁺, 1.0 mM H₂O₂, 0.4 mM HA and pH 5.0. The degradation rate of NOR in the HA-Fenton system (0.23 min^-1) was 10.9 times of that (0.021 min^-1) in Fenton system. The addition of HA to Fenton system accelerated the conversion of Fe³⁺ to Fe²⁺, leading to the high concentration of ·OH in the HA-Fenton system. Ten degradation transformation products were detected by ultra-performance liquid chromatography tandem quadrupole time of flight mass spectrometer (UPLC-QTOF-MS), consequently, three main degradation steps were proposed, including defluorination, quinolone group transformation, and defluorination and piperazinyl ring opening. Further analyses of NO₃^-, NO₂^- and F^- after the reaction indicated that defluorination process was the crucial degradation step. The HA-Fenton system might offer an efficient alternative for degradation of antibiotics in wastewater.

Aqueous-phase oxidation of syringic acid emitted from biomass burning: Formation of light-absorbing compounds

Syringic acid is a methoxyphenol model compound derived from biomass burning, and its photooxidation processes have important effects on atmospheric chemistry. However, its aqueous-phase photochemistry remains unclear. In this study, we systematically report the photooxidation of syringic acid induced by OH radicals in the aqueous phase. Employing the relative rate technique, the bimolecular rate constant for syringic acid reaction with OH radicals was acquired to be (1.1 ± 0.3) × 10¹⁰ M^-1 s^-1. Notably, colored products were formed as the reaction progressed. Furthermore, the UV-vis and fluorescence spectra confirmed the formation of light-absorbing organic species, and the results agreed well with previous results on atmospheric and natural humic-like substances (HULIS). The photooxidation products were detected by high performance liquid chromatography mass spectrometry (HPLC/MS), and a possible reaction mechanism was proposed. The aqueous-phase reaction of syringic acid would undergo functionalization process forming a hydroxylation product that enhances the degree of oxidation of aqueous secondary organic aerosol (aqSOA), and goes through dimerization process by C-C or C-O coupling of phenoxy radicals which may conduce to the formation of HULIS. These findings suggest that the photooxidation of syringic acid is an important pathway for highly oxygenated phenolic aqSOA formation, providing a secondary source for HULIS in a liquid phase or in deliquescent particles surrounded by a layer of water.

Stable Fe3O4 submicrospheres with SiO2 coating for heterogeneous Fenton-like reaction at alkaline condition

In the traditional Fenton process, the efficient generation of hydroxyl radical (HO) strongly relies on an acidic circumstance and the iron ions would precipitate and form large amounts of hazardous iron-containing sludge at alkaline pH. To realize stable heterogeneous Fenton-like catalytic degradation at alkaline condition, Fe₃O₄ submicrospheres with SiO₂ coating were successfully synthesized by using water glass as the silica sources via a facile ultrasound assisted method. The as-obtained Fe₃O₄@SiO₂ spheres were further used as catalysts for the Fenton-like degradation of tetracycline hydrochloride (TC). The Fe₃O₄@SiO₂ submicrospheres exhibited superior catalytic activity in higher pH environment (pH value = 11), and the degradation efficiency toward TC was ca. 80% after ten successive runs. The kinetics for the catalytic degradation of TC were agreed well with the second-order kinetic model. The reaction rate constant (k) over the Fe₃O₄@SiO₂ submicrospheres at a pH value of 11 was 7.69 times greater than that at a pH value of 3. Reactive species scavenging experiments revealed that HO and superoxide radical (O₂^- / HO₂^-) played a dominant role during the Fenton-like degradation of TC at pH 3 and pH 11, respectively. Possible Fenton-like degradation pathways of TC were proposed through the identification of intermediates using the high performance liquid chromatography coupled with mass spectrometry (HPLC-MS), which involved cleavage of methyl groups, N-dimethyl group, and hydroxy groups, ring-opening reaction, etc. The degradation efficiency of TC was close to 91.5% and total organic carbon (TOC) in solution was eliminated by about 41.4% at the optimized conditions. In a word, with the unique acidic surface properties and abundant Si-OH bonds, the Fe₃O₄@SiO₂ submicrospheres exhibited well dispersion, good catalytic activity, strong alkali resistance and excellent recyclability in an ultrasonic-Fenton-like system.

Full insights into the roles of pH on hydroxylation of aromatic acids/bases and toxicity evaluation

Advanced oxidation processes (AOPs) based on hydroxyl radicals (•OH) are the most important technologies for the removal of bio-recalcitrant organic pollutants in industrial wastewater. The pH is one of the crucial environmental parameters that affect the removal efficiency of pollutants in AOPs. In this work, the mechanistic and kinetic insights into the roles of pH on the hydroxylation of five aromatic acids and bases in UV/H₂O₂ process have been investigated using theoretical calculation methods. Results show that the reactivity of •OH towards the twelve ionic/neutral species is positively correlated with electron-donating effect of substituents, which contributes to the positively pH-dependent reactivity of aromatic acids and bases towards •OH. The hydroxylation apparent rate constants (k_app, M^-1 s^-1) (at 298 K) increase as the pH values increase from about 1 to 10, but they decrease as the pH values increase from about 10 to 14. However, the best pH values for degradation are not around 10 because the [•OH] decreases continuously with the increasing pH values from 3 to 9.5. Combining the factors of k_app and [•OH], the best degradation pH values are around 5.5~7.5 for p-hydroxybenzoic acid, p-aminophenol, aniline and benzoic acid, 3.0~7.5 for phenol and 5.5~7.5 for mixed pollutants of these aromatic acids/bases in UV/H₂O₂ process. Moreover, a significant number of hydroxylation by-products are more toxic or harmful to aquatic organisms and rat (oral) than their parental pollutants. Altogether, this work provides comprehensive understanding of the roles of pH on •OH-initiated degradation behavior of aromatic acids and bases.

Characterization of UV and chlorine contributions to transformation of 2,3,4-trihydroxybenzophenone under combined UV-chlorine treatment

Combined UV-chlorine treatment is a promising disinfection technology providing synergistic effects on bacteria-killing. The interaction between UV and chlorine would affect pollutants removal and disinfection by-products formation, while little is known about how UV and chlorine respectively contribute to pollutants transformation under combined UV-chlorine treatment. In this study, UV filter 2,3,4-trihydroxybenzophenone (2,3,4-THBP) was selected as a model compound to investigate the transformation characteristics and acute toxicity variation under combined UV-chlorine treatment. Especially, separative UV and chlorination treatments were conducted to illustrate their respective contribution in combined UV-chlorine treatment. It was found that the optimal removal percentage of 2,3,4-THBP under combined UV-chlorine treatment was 85.3% within 5 min and kept stable until 3 h at 3-equivalent (equiv.) of free available chlorine (FAC) and 1 mW/cm² of irradiation intensity. Correspondingly, acute toxicity of reaction mixture at 3 h increased twice as high as that of 2,3,4-THBP itself. Four transformation products were tentatively identified, and their formation possibly involved the reactions of chlorine substitution, oxidation, hydroxylation, and hydrolysis. FAC initiated the preliminary transformation of 2,3,4-THBP, and the synergistic effects of UV and chlorine promoted the further transformation of intermediates from chlorination treatment. Most important was that, 2,3,4-THBP could form some toxic products in the real ambient water matrix under solar irradiation, and acute toxicity of reaction mixture was 1.84 times higher than that of 2,3,4-THBP. This study would provide a better understanding on the transformation characteristics of pollutants under combined UV-chlorine treatment, and provide a reference for optimizing disinfection treatment.

In situ organic Fenton-like catalysis triggered by anodic polymeric intermediates for electrochemical water purification

Organic Fenton-like catalysis has been recently developed for water purification, but redox-active compounds have to be ex situ added as oxidant activators, causing secondary pollution problem. Electrochemical oxidation is widely used for pollutant degradation, but suffers from severe electrode fouling caused by high-resistance polymeric intermediates. Herein, we develop an in situ organic Fenton-like catalysis by using the redox-active polymeric intermediates, e.g., benzoquinone, hydroquinone, and quinhydrone, generated in electrochemical pollutant oxidation as H₂O₂ activators. By taking phenol as a target pollutant, we demonstrate that the in situ organic Fenton-like catalysis not only improves pollutant degradation, but also refreshes working electrode with a better catalytic stability. Both ¹O₂ nonradical and ·OH radical are generated in the anodic phenol conversion in the in situ organic Fenton-like catalysis. Our findings might provide a new opportunity to develop a simple, efficient, and cost-effective strategy for electrochemical water purification.

A novel peroxymonosulfate (PMS)-enhanced iron coagulation process for simultaneous removal of trace organic pollutants in water

Conventional coagulation process is not effective to eliminate trace organic compounds (TrOCs). This study proposed a novel peroxomonosulfate (PMS) amended iron coagulation process and found its effectiveness in eliminating TrOCs in synthetic and natural waters. In synthetic waters containing hydroquinone (HQ) or benzoquinone (BQ), Fe(III)/PMS effectively degraded carbamazepine (CBZ), a representative of resistant TrOCs. The step of reduction of Fe(III) to form Fe(II) governed the degradation rate of CBZ as PMS activation by Fe(II) was the dominant reaction to generate SO₄^•-, which was the major reactive oxidant in the system. Meanwhile, HQ was quickly transformed to BQ in the Fe(III)/PMS system and BQ acted as an electron shuttle to induce Fe(III)/Fe(II) redox cycle and accelerate PMS activation. Natural organic matter (NOM) bearing phenolic and quinone moieties played similar roles as HQ and BQ and fast CBZ degradation was also observed. Finally, two surface waters spiked CBZ were subjected to bench-scale experimentation to simulate coagulation/flocculation/sedimentation processes in water treatment plants. Compared to the conventional iron coagulation process, the PMS-amended iron coagulation process increased the removal percentage of CBZ by 50%-80%. Overall, this study demonstrates that PMS enhanced iron coagulation is a promising process for the abatement of TrOCs in water treatment.

Peroxymonosulfate Activation by Fe-Co-O-Codoped Graphite Carbon Nitride for Degradation of Sulfamethoxazole

Graphite carbon nitride (g-C₃N₄) has a stable structure but poor catalytic capability for activating peroxymonosulfate (PMS). In this study, the codoping of g-C₃N₄ with bimetallic oxides (iron and cobalt) and oxygen was investigated to enhance its catalytic capability. The results showed that iron, cobalt, and oxygen codoped g-C₃N₄ (Fe-Co-O-g-C₃N₄) was successfully prepared, which was capable of completely degrading sulfamethoxazole (SMX) (0.04 mM) within 30 min, with a reaction rate of 0.085 min^-1, indicating the superior catalytic activity of Fe-Co-O-g-C₃N₄. The mineralization efficiency of SMX was 22.1%. Sulfate radicals and singlet oxygen were detected during the process of PMS activation. However, the role that singlet oxygen played in degrading SMX was not obvious. Surface-bound reactive species and sulfate radicals were responsible for SMX degradation, in which sulfate radicals contributed to 46.6% of SMX degradation. The superior catalytic activity was due to the synergistic effect of metal oxides and O-g-C₃N₄, in which O-g-C₃N₄ could act as a carrier and an activator as well as an electron mediator to promote the conversion of Fe(III) to Fe(II) and Co(III) to Co(II). Four main steps of SMX degradation were proposed, including direct oxidation of SMX, bond fission of N-C, bond fission of N-S, and bond fission of S-C. The effect of the pH, temperature, PMS concentration, chloridion, bicarbonate, and humic acids on SMX degradation was investigated. Cycling experiments demonstrated the good stability of Fe-Co-O-g-C₃N₄. This study first reported the preparation of bimetallic oxide and oxygen codoped g-C₃N₄, which was an effective PMS activator for degradation of toxic organic pollutants.

Superoxide-driven autocatalytic dark production of hydroxyl radicals in the presence of complexes of natural dissolved organic matter and iron

We introduced superoxide as potassium superoxide (KO₂) to artificial lake water containing dissolved organic matter (DOM) without or with introduced ferric iron complexes (DOM-Fe(III)), and monitored the production rate of hydroxyl radicals as well as changes in the absorption and fluorescence properties of DOM. The introduction of KO₂ decreased the absorption by DOM but increased the spectral slope coefficient of DOM more with complexed ferric Fe than without it. The introduction of KO₂ increased the fluorescence of humic-like components in DOM without introduced ferric Fe but resulted in the loss of fluorescence in DOM with introduced ferric Fe. A single introduction of 13 μmol L^-1 KO₂ produced 10 μmol L^-1 and 104 μmol L^-1 hydroxyl radicals during a week-long experiment without and with the introduced DOM-Fe(III) complexes, respectively. The production rate of hydroxyl radicals decreased exponentially with time but levelled off and continued several days in DOM with introduced ferric Fe. These findings suggest that in the presence of DOM-Fe complexes, superoxide can trigger an autocatalytic Fenton reaction that produces hydroxyl radicals and breaks down DOM.

Enhanced heterogeneous Fenton-like degradation of nuclear-grade cationic exchange resin by nanoscale zero-valent iron: experiments and DFT calculations

Nanoscale zero-valent iron (nZVI) was prepared and used as a heterogeneous Fenton-like catalyst for the degradation of nuclear-grade cationic exchange resin. The properties of nZVI before and after reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) surface area analysis. The results showed that nZVI-H₂O₂ system exhibited the enhanced degradation of cationic resins, compared with Fe²⁺-H₂O₂, Cu⁰-H₂O₂, and Fe⁰/Cu⁰-H₂O₂ systems. The effects of initial temperature, nZVI dose, and H₂O₂ concentration were studied, and the higher temperature and nZVI dose with relatively low H₂O₂ concentration brought faster degradation rate. The degradation of cationic resins followed the pseudo-first-order kinetics with the apparent activation energy of 53.29 kJ/mol. According to the experimental and calculated infrared and UV-visible spectra, the carbon skeleton of cationic resins was broken with the detachment of benzene ring and the desulfonation of resin polymer by hydroxyl radicals (•OH), generating long-chain alkenes. These intermediates were further oxidized through the hydroxyl substitution, hydrogen abstraction, ring cleavage, or carbonylation reactions, finally forming carboxylic acids remained in solution.

Fenton-like degradation of dimethyl phthalate enhanced by quinone species

Fenton-like degradations of dimethyl phthalate (DMP) and phenolic compounds (phenol, catechol, resorcinol, and hydroquinone) in single and binary systems were investigated by focusing on the Fe(III)/Fe(II) redox cycle during the reaction processes. Quinone-like substances were generated and found to be responsible for the autocatalytic transformation of Fe(III) to Fe(II) in the Fenton-like process with DMP or phenolics. Moreover, phenolic compounds could accelerate the Fenton-like degradation of DMP, with an increased efficiency of H₂O₂ utilization. The effect of phenolic compounds on the degradation of DMP followed the order: catechol ≈ hydroquinone > resorcinol > phenol, which could be attributed to the interaction between quinone-like substances and iron ions. Hydroquinone-like substances accelerated the Fe(III)/(II) redox cycle. The formation of iron complexes between catechol-like substances and iron ions facilitated the release of H⁺ and regeneration of Fe(II). In addition, a plausible mechanism for enhanced Fenton-like degradation of DMP by phenolics was proposed.

Unraveling the interaction of hydroxylamine and Fe(III) in Fe(II)/Persulfate system: A kinetic and simulating study

Hydroxylamine showed an outstanding performance on enhancing the oxidation of pollutants in Fe(II) involved advanced oxidation processes, while the detailed reaction schemes have not been fully revealed. Specific functions of hydroxylamine in the oxidation of benzoic acid with Fe(II)/persulfate (PDS) system were explored. With the addition of hydroxylamine, degradation kinetics of benzoic acid deviated from both two-stage kinetics and pseudo first order kinetics, but could be interpreted well with binomial regression analysis. Degradation rate constant (k_obs) of benzoic acid was calculated and showed the same variation trend with [hydroxylamine][Fe(III)]²/([Fe(II)][H⁺])², the value of which was changed during reaction processes. A detailed kinetic model for simulating the degradation profile of benzoic acid with hydroxylamine acceleration was proposed for the first time and indicated that interactions of hydroxylamine and Fe(III) were fast equilibrium reactions, which was a dominant factor influencing the oxidation kinetics of benzoic acid in Fe(II)/hydroxylamine/PDS system. Comparative study showed that when 1.4 mM of ascorbic acid was added into Fe(II)/PDS system, degradation kinetics of benzoic acid was similar to that enhanced by hydroxylamine. However, when 0.6 mM or 1.0 mM of ascorbic acid was added, oxidation kinetics still presented as the two-stage profile. Kinetic simulations indicated that Fe(II) was produced slower from Fe(III)-ascorbic acid complexes than that with hydroxylamine, which caused the difference in oxidation kinetics. This study could improve our understanding about the effect of hydroxylamine and other reductants in promoting pollutants elimination in Fe(II)/PDS system.

Boosting the catalytic activity of natural magnetite for wet peroxide oxidation

This work explores the modification of naturally occurring magnetite by controlled oxidation (200-400 °C, air atmosphere) and reduction (300-600 °C, H₂ atmosphere) treatments with the aim of boosting its activity in CWPO. The resulting materials were fully characterized by XRD, XPS, TGA, TPR, SEM, and magnetization measurements, allowing to confirm the development of core-shell type structures. The magnetite core of the solid remained unchanged upon the treatment whereas the Fe(II)/Fe(III) ratio of the shell was modified (e.g. 0.42, 0.11 and 0.63 values were calculated for pristine Fe₃O₄, Fe₃O₄-O400, and Fe₃O₄-R400, respectively). The performance of the catalysts was tested in the CWPO of sulfamethoxazole (SMX) (5 mg L^-1) under ambient conditions and circumneutral pH (pH₀ = 5), using the stoichiometric dose of H₂O₂ (25 mg L^-1) and a catalyst load of 1 g L^-1. The key role of the ferrous species on the mineral shell was evidenced. Whereas the oxidation of magnetite led to significantly slower degradation rates of the pollutant, its reduction gave rise to a dramatic increase, achieving the complete removal of SMX in 1.5 h reaction time with the optimum catalyst (Fe₃O₄-R400) compared to the 3.5 h required with the pristine mineral. A reaction mechanism was proposed for SMX degradation, and a kinetic equation based on the Eley-Rideal model was accordingly developed. This model successfully fitted the experimental results. The stability of Fe₃O₄-R400 was evaluated upon five sequential runs. Finally, the versatility of the catalytic system was proved in real environmentally relevant water matrices.

Tris-Co(II)-H2O2 System-Mediated Durative Hydroxyl Radical Generation for Efficient Anionic Azo Dye Degradation by Integrating Electrostatic Attraction

The development of simple Fenton/Fenton-like systems with durative hydroxyl radical (^•OH) generation characteristics is significant to rapid organic pollutant degradation and cost-effective water treatment. In this study, a tris(hydroxymethyl)aminomethane (Tris)-incorporated Co(II)-H₂O₂ Fenton-like system has been successfully constructed for efficient Sunset Yellow (SY, a typical anionic azo dye) degradation under alkaline conditions. The mechanism of the enhanced degradation consists of two parts: first, the Tris-Co(II) complex triggers the durative generation of highly oxidized hydroxyl radicals; second, electrostatic attraction between SY and the Tris-Co(II) complex shortens the radical-SY interaction time and facilitates the degradation of SY. With the introduction of Tris to this proposed system, the decolorization rate of SY can be increased from 37.0 to 98.0% after 50 min and efficient SY degradation with a high total organic carbon removal efficiency (>59.0%) is achieved under a wide initial pH from 8.7 to 12.0. Moreover, the universality of the designed system for anionic azo dye degradation is verified with reactive red and congo red.

The effect and mechanism of organic pollutants oxidation and chemical energy conversion for neutral wastewater via strengthening reactive oxygen species

Toxic and refractory organic pollutants are continually discharged into the water environment, which has become the crisis for the human living and sustainable development. However, organic pollutants also contain large amounts of chemical energy. In this paper, we studied the effect and mechanism of organic pollutants oxidation and chemical energy conversion for neutral wastewater via strengthening reactive oxygen species (ROS) of HO and O₂^- in a photocatalytic fuel cell (PFC) system, since ROS has the power to oxidize or even mineralize the organics and is environment-friendly to treat refractory organic pollutants. In our PFC system, the HO was enhanced by the cyclic radical chain reaction via the addition of Fe²⁺ and tetrapolyphosphate (TPP), while O₂^- was enhanced by setting an additional bias voltage at the anode which was favorable to O₂ production. The results show that the HO and O₂^- concentration are highly enhanced, showing 8.28 and 8.99 times those of traditional PFC, respectively. Meanwhile, the degradation rate constant is remarkably increased by 6.52 times when methylene blue is used as a model pollutant. Furthermore, the performance of wastewater PFC is so improved that the short-circuit current density (J_sc) and maximum power density (JV_max) have been increased by a factor of 9.05 and 12.67 times in the same experiment, respectively.

Is Sulfate Radical Really Generated from Peroxydisulfate Activated by Iron(II) for Environmental Decontamination?

It is well documented that the traditional Fenton reagent (i.e., the combination of Fe(II) and H₂O₂) produces hydroxyl radical (^•OH) under acidic conditions, while at near-neutral pH the reactive intermediate converts to ferryl ion (Fe(IV)) that can oxidize sulfoxides to produce corresponding sulfones, markedly differing from their ^•OH-induced products. However, it remains unclear whether Fe(IV) is generated in the Fe(II) activated peroxydisulfate (PDS) process, where sulfate radical (SO₄^•-) is long recognized as the dominant intermediate in literature. Here we demonstrated that SO₄^•- oxidized methyl phenyl sulfoxide (PMSO, a model sulfoxide) to produce biphenyl compounds rather than methyl phenyl sulfone (PMSO₂). Interestingly, the formation of PMSO₂ was observed when PMSO was treated by the Fe(II)/PDS system over a wide pH range, and the yields of PMSO₂ were quantified to be ∼100% at acidic pH 3-5. The identification of Fe(IV) in the Fe(II)/PDS system could also reasonably explain the literature results on alcohol scavenging effect and ESR spectra analysis. Further, a Fe(IV)-based kinetic model was shown to accurately simulate the experimental data. This work urges re-evaluation of the Fe(II)/PDS system for environmental decontamination, given that Fe(IV) would have different reactivity toward environmental contaminants compared with SO₄^•- and/or ^•OH.

Atmospheric HULIS and its ability to mediate the reactive oxygen species (ROS): A review

Atmospheric humic-like substances (HULIS) are not only an unresolved mixture of macro-organic compounds but also powerful chelating agents in atmospheric particulate matters (PMs); impacting on both the properties of aerosol particles and health effects by generating reactive oxygen species (ROS). Currently, the interests of HULIS are intensively shifting to the investigations of HULIS-metal synergic effects and kinetics modeling studies, as well as the development of HULIS quantification, findings of possible HULIS sources and generation of ROS from HULIS. In light of HULIS studies, we comprehensively review the current knowledge of isolation and physicochemical characterization of HULIS from atmospheric samples as well as HULIS properties (hygroscopic, surface activity, and colloidal) and possible sources of HULIS. This review mainly highlights the generation of reactive oxygen species (ROS) from PMs, HULIS and transition metals, especially iron. This review also summarized the mechanism of iron-organic complexation and recent findings of OH formation from HULIS-metal complexes. This review will be helpful to carry out the modeling studies that concern with HULIS-transition metals and for further studies in the generation of ROS from HULIS-metal complexes.

Enhanced Fe(III)-mediated Fenton oxidation of atrazine in the presence of functionalized multi-walled carbon nanotubes

In this study we reported that the presence of functionalized multi-walled carbon nanotubes (FCNT-H) would greatly enhance the degradation of atrazine (ATZ), a model contaminant, in the Fe(III)-mediated Fenton-like system. Efficient ATZ degradation (>90%) was achieved within 30 min in the presence of 20 mg.L^-1 FCNT-H, 2.0 mg.L^-1 Fe(III), and 170 mg.L^-1 H₂O₂, whereas negligible ATZ degradation occurred in FCNT-H free system. The structure and surface chemistry of FCNT-H and other CNTs were well characterized. The formed active species were determined based on ESR analysis, and the mass balance of Fe species during the reaction was monitored. In particular, a new method based on ferrozine complexation was proposed to track the formed Fe(II). The results indicated that ATZ was mainly degraded by the generated hydroxyl radical (HO·), and Fe(III)/Fe(II) cycling was still the rate-limiting step. Besides a small fraction of Fe(III) reduced by FCNT-H, a new pathway was revealed for fast reduction of most Fe(III), i.e., reaction of FCNT-H-Fe(III) complexes with H₂O₂. Comparison of different CNTs-mediated Fe(III)/H₂O₂ systems indicated that such enhanced effect of CNTs mainly resulted from the surface carboxyl group instead of hydroxyl and carbonyl group. Combined with X-ray photoelectron spectroscopy (XPS) analysis, the electron density migration from Fe(III) to FCNT-H possibly resulted in the fast reduction of FCNT-H-Fe(III) complexes by H₂O₂. This study enables better understanding the enhanced Fe(III)-mediated Fenton-like reaction in the presence of MWCNTs and thus, will shed new light on how to develop more efficient similar Fenton systems via Fe(III) complexation.

Efficient degradation of sulfamethoxazole by the Fe(II)/HSO5- process enhanced by hydroxylamine: Efficiency and mechanism

Fenton or Fenton-like processes have been regarded as feasible methods to degrade a wide variety of contaminants by generating reactive species, but the efficiency is still challenged by the slow transformation from Fe(III) to Fe(II) and pH. This study employed hydroxylamine (HA) to improve the oxidation efficiency of Fe(II)/HSO₅^- (Fe(II)/PMS) process, by selecting sulfamethoxazole (SMX) as the target compound. The degradation efficiency and mechanism of SMX by the HA/Fe(II)/PMS process were elucidated for the first time. Compared with Fe(II)/PMS process, the HA/Fe(II)/PMS process showed about 4 times higher degradation efficiency of SMX at pH 3.0. The analysis of steady-state concentration of Fe species indicated that HA enhanced the transformation of Fe(III) to Fe(II), sustaining the rapid Fenton-like reactions. Both sulfate radicals and hydroxyl radicals accounted for the degradation of SMX, with the latter regarded as the dominant reactive species. Degradation intermediates of SMX were further analyzed, and three main transformation pathways were thus proposed. The HA/Fe(II)/PMS process was also effective in the removal of SMX and total organic carbon from real pharmaceutical wastewater. This work would broaden the scope of application of Fenton and Fenton-like processes enhanced by HA in contaminants treatment.

Application of Fenton oxidation to reduce the toxicity of mixed parabens

The aims of the present work were to assess the application of a chemical process to degrade a mixture of parabens and determine the influence of a natural river water matrix on toxicity. Model effluents containing either a single compound, namely methylparaben, ethylparaben, propylparaben, butylparaben, benzylparaben or p-hydroxybenzoic acid, or to mimic realistic conditions a mixture of the six compounds was used. Fenton process was applied to reduce the organic charge and toxic properties of the model effluents. The efficiency of the decontamination has been investigated using a chemical as well as a toxicological approach. The potential reduction of the effluents' toxicity after Fenton treatment was evaluated by assessing (i) Vibrio fischeri luminescence inhibition, (ii) lethal effects amongst freshwater Asian clams (Corbicula fluminea), and (iii) the impact on mammalian neuronal activity using brain slices. From the environmental point of view such a broad toxicity analysis has been performed for the first time. The results indicate that Fenton reaction is an effective method for the reduction of chemical oxygen demand of a mixture of parabens and their toxicity to V. fischeri and C. fluminea. However, no important differences were found between raw and treated samples in regard to mammalian neuronal activity.

Accelerated Catalytic Fenton Reaction with Traces of Iron: An Fe-Pd-Multicatalysis Approach

An accelerated catalytic Fenton (ACF) reaction was developed based upon a multicatalysis approach, facilitating efficient contaminant oxidation at trace levels of dissolved iron. Beside the Fe(II)/H2O2 catalyst/oxidant pair for production of OH-radicals, the ACF system contains Pd/H2 as catalyst/reductant pair for fast reduction of Fe(III) back to Fe(II) which accelerates the Fenton cycle and leads to faster contaminant degradation. By this means, the concentration of the dissolved iron catalyst can be reduced to trace levels (1 mg L(-1)) below common discharge limits, thus eliminating the need for iron sludge removal, which is one of the major drawbacks of conventional Fenton processes. ACF provides fast degradation of the model contaminant methyl tert-butyl ether (MTBE, C0 = 0.17 mM) with a half-life of 11 min with 1 mg L(-1) dissolved iron, 500 mg L(-1) H2O2, 5 mg L(-1) Pd (as suspended Pd/Al2O3 catalyst) and 0.1 MPa H2, pH 3. The effects of pH, H2 partial pressure and H2O2 concentration on MTBE degradation rates were studied. Results on kinetic deuterium isotope effect and quenching studies are in conformity with OH-radicals as main oxidant. The heterogeneous Pd/Al2O3 catalyst was reused within six cycles without significant loss in activity.

Activation of Peroxymonosulfate by Benzoquinone: A Novel Nonradical Oxidation Process

The reactions between peroxymonosulfate (PMS) and quinones were investigated for the first time in this work, where benzoquinone (BQ) was selected as a model quinone. It was demonstrated that BQ could efficiently activate PMS for the degradation of sulfamethoxazole (SMX; a frequently detected antibiotic in the environments), and the degradation rate increased with solution pH from 7 to 10. Interestingly, quenching studies suggested that neither hydroxyl radical (•OH) nor sulfate radical (SO4•-) was produced therein. Instead, the generation of singlet oxygen (1O2) was proved by using two chemical probes (i.e., 2,2,6,6-tetramethyl-4-piperidinol and 9,10-diphenylanthracene) with the appearance of 1O2 indicative products detected by electron paramagnetic resonance spectrometry and liquid chromatography mass spectrometry, respectively. A catalytic mechanism was proposed involving the formation of a dioxirane intermediate between PMS and BQ and the subsequent decomposition of this intermediate into 1O2. Accordingly, a kinetic model was developed, and it well described the experimental observation that the pH-dependent decomposition rate of PMS was first-order with respect to BQ. These findings have important implications for the development of novel nonradical oxidation processes based on PMS, because 1O2 as a moderately reactive electrophile may suffer less interference from background organic matters compared with nonselective •OH and SO4•-.