Does Tert-Butyl Alcohol Really Terminate the Oxidative Activity of •OH in Inorganic Redox Chemistry?

Overlooked Impacts of Alcohols in Electro-H2O2 and Fenton Chemistry

Alcohols are promising fuels for direct alcohol fuel cells and are common scavengers to identify reactive oxygen species (ROS) in electro-Fenton (EF) systems. However, the side impacts of alcohols on oxygen reduction reactions and ROS generation are controversial due to the complex interactions between electrodes and alcohol-containing electrolytes. Herein, we employed synchrotron-Fourier-transform infrared spectroscopy and electron paramagnetic resonance technologies to directly observe the changes of chemical species and electrochemical properties on the electrode surface. Our studies suggested that alcohols exhibited different limiting degrees on proton (H+) mass transfer toward the catalytic surface, following an order of methanol < ethanol < isopropanol < tert-butyl alcohol (TBA). In addition, the formation of hydrophobic TBA clusters at high concentrations (>400 mM) resulted in a significant reduction in ionic conductivity and an elevation in charge transfer resistance, which impedes H+ mass transfer and raises the energy barrier for 2e- oxygen reduction reaction processes. Moreover, the organic radical •CH2(CH3)2CH2OH produced by the interaction of Fe3+ and •OH with the alcohol in the EF system serves as a crucial intermediate in facilitating H2O2 regeneration, which complicates the quenching effect of alcohols on •OH identification. Therefore, it is recommended that methanol should be used as the scavenger instead of TBA and the concentration should be less than 400 mM in EF systems.

Solar Photodegradation of a Novel des-F(6)-Fluoroquinolone, Garenoxacin, and Ecotoxicity of Its Phototransformation Products

Garenoxacin (GRNX) is a novel des-F(6)-fluoroquinolone on the horizon; thus, its fate and risk in the aquatic environment deserve attention. This study systematically investigated, for the first time, the phototransformation of GRNX under simulated and natural sunlight and assessed the ecotoxicity of its photodegradation products. Phototransformation of GRNX was observed to depend strongly on its ionization state, with direct photolysis and self-sensitized photolysis having comparable contributions for the cationic and zwitterionic species, while the latter dominated for the anionic species. Singlet oxygen generated via the self-sensitized photolysis of GRNX was the major reactive oxygen species in its photodegradation. Phototransformation of GRNX in different ionization states followed distinct pathways, with defluorination of the difluoromethyl group occurring only for the zwitterionic and anionic species. GRNX photodegradation in natural water could be described by a simple kinetic model based on the measured steady-state concentrations of 1O2 and ·OH. Toxicity tests with Vibrio fischeri and Chlorella vulgaris consistently indicate that the generation of hydroxylation and decarboxylation products during photodegradation of GRNX increased the acute toxicity. These findings not only provide insights into the fate of GRNX in sunlit surface water but also reveal the potentially significant risk of its photodegradation products to the aquatic ecosystem.

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.

Algal Extracellular Organic Matter Induced Photochemical Oxidation of Mn(II) to Solid Mn Oxide: Role of Mn(III)-EOM Complex and Its Ability to Remove 17α-Ethinylestradiol

Photosensitizer-mediated abiotic oxidation of Mn(II) can yield soluble reactive Mn(III) and solid Mn oxides. In eutrophic water systems, the ubiquitous algal extracellular organic matter (EOM) is a potential photosensitizer and may have a substantial impact on the oxidation of Mn(II). Herein, we focused on investigating the photochemical oxidation process from Mn(II) to solid Mn oxide driven by EOM. The results of irradiation experiments demonstrated that the generation of Mn(III) intermediate was crucial for the successful photo oxidization of Mn(II) to solid Mn oxide mediated by EOM. EOM can serve as both a photosensitizer and a ligand, facilitating the formation of the Mn(III)-EOM complex. The complex exhibited excellent efficiency in removing 17α-ethinylestradiol. Furthermore, the complex underwent decomposition as a result of reactions with reactive intermediates, forming a solid Mn oxide. The presence of nitrate can enhance the photochemical oxidation process, facilitating the conversion of Mn(II) to Mn(III) and then to solid Mn oxide. This study deepens our grasp of Mn(II) geochemical processes in eutrophic water and its impact on organic micropollutant fate.

Peracetic acid activated by nickel cobaltite as effective oxidizing agent for BPA and its analogues degradation

The presented research concerns the use of nickel cobaltite nanoparticles (NiCo2O4 NPs) for the heterogeneous activation of peracetic acid and application of NiCo2O4-PAA system for degradation 10 organic micropollutants from the group of bisphenols. The bisphenols removal (initial concentration 1 μM) process was optimized by selecting the appropriate process conditions. The optimal amount of catalyst (115 mg/L), peracetic acid (PAA) concentration (7 mM) and pH (7) were determined using response surface analysis in the Design of Experiment. Then, NiCo2O4 NPs were used to check the possibility of reuse in subsequent oxidation cycles. The work also attempts to explain the mechanism of oxidation of the studied micropollutants. The participation of the sorption process on the catalyst was excluded and based on the experiments with radical scavengers it can be concluded that the oxidation proceeds in a radical pathway, mainly with participation of O2•- radicals. Experiments conducted in real water matrices exhibit low impact on degradation efficiency. Toxicity tests with green alga Acutodesmus obliquus and aquatic plant Lemna minor showed that post-reaction mixture influenced growth and the content of photosynthetic pigments in concentration dependent manner.

Mn(II) oxidation by the UV/chlorine system under near-neutral pH conditions: The important role of ClO· and ClO2

The oxidation kinetics of Mn(II) by free chlorine is relatively low under near-neutral pH conditions which limits the Mn removal efficiency in drinking water treatment. Therefore, this study investigated the oxidation efficiency of Mn(II) by the UV-enhanced chlorination (UV/chlorine) system and identified the responsible reactive radical species. The results show that the oxidation kinetic of Mn(II) was greatly enhanced by the UV/chlorine system under near-neutral pH or even acidic conditions. The pseudo-first-order reaction rate of Mn(II) at pH 8.0 (within the first 20 min) increased from 2.60 × 10-5 s-1 to 3.41 × 10-4 s-1. Based on the scavenging experiments and the steady-state kinetic modeling, ClO· and ClO2, whose steady-state concentration (∼10-10 M and ∼10-9 M, respectively at pH 8.0) was at least 4 orders of magnitude higher than that of HO· and Cl·, were recognized as the dominant reactive species contributing to the oxidation of Mn(II). Kinetic model calculations indicate that the contribution of ClO· to the oxidation of Mn(II) was consistently maintained above 70 %, and ClO2 also played an important role in the oxidation of Mn(II) especially under acidic and alkaline conditions. In addition, the background components of HCO3- and Cl- had negligible influence on the oxidation efficiency because they barely changed the concentration of the ClO· and ClO2. This study first demonstrates the important role of ClO2 in the oxidation of Mn(II) in the UV/chlorine system, and the possible role of ClO2 in the degradation of some organic pollutants needs to be carefully evaluated in the future.

Efficient Degradation of Intracellular Antibiotic Resistance Genes by Photosensitized Erythrosine-Produced 1O2

Intracellular antibiotic resistance genes (iARGs) constitute the important part of wastewater ARGs and need to be efficiently removed. However, due to the dual protection of intracellular DNA by bacterial membranes and the cytoplasm, present disinfection technologies are largely inefficient in iARG degradation. Herein, we for the first time found that erythrosine (ERY, an edible dye) could efficiently degrade iARGs by producing abundant 1O2 under visible light. Seven log antibiotic-resistant bacteria were inactivated within only 1.5 min, and 6 log iARGs were completely degraded within 40 min by photosensitized ERY (5.0 mg/L). A linear relationship was established between ARG degradation rate constants and 1O2 concentrations in the ERY photosensitizing system. Surprisingly, a 3.2-fold faster degradation of iARGs than extracellular ARGs was observed, which was attributed to the unique indirect oxidation of iARGs induced by 1O2. Furthermore, ERY photosensitizing was effective for iARG degradation in real wastewater and other photosensitizers (including Rose Bengal and Phloxine B) of high 1O2 yields could also achieve efficient iARG degradation. The findings increase our knowledge of the iARG degradation preference by 1O2 and provide a new strategy of developing technologies with high 1O2 yield, like ERY photosensitizing, for efficient iARG removal.

Effects of gas saturation and sparging on sonochemical oxidation activity in open and closed systems, part II: NO2-/NO3- generation and a brief critical review

The sonochemical generation of NO₂^- and NO₃^- is considered to be one of the reasons for the low sonochemical oxidation activity in the presence of N₂ in the liquid phase. In this study, the generation characteristics of NO₂^- and NO₃^- were investigated using the same 28 kHz sonoreactor and the 12 gas conditions used in Part I of this study. Three gas modes, saturation/closed, saturation/open, and sparging/closed, were applied. N₂:Ar (25:75), N₂:Ar (50:50), and O₂:N₂ (25:75) in the saturation/closed mode generated the three highest values of NO₂^- and NO₃^-. Ar and O₂ were vital for generating relatively large concentrations of NO₂^- and NO₃^-. The absorption of N₂ from the air resulted in high generation of NO₂^- and NO₃^- for Ar 100 % and Ar/O₂ mixtures under the saturation/open mode. In addition, gas sparging enhanced the generation of NO₂^- and NO₃^- for N₂:Ar (25:75), O₂:N₂ (25:75), and N₂ significantly because of the change in the sonochemically active zone and the increase in the mixing intensity in the liquid phase, as discussed in Part I. The ratio of NO₃^- to NO₂^- was calculated using their final concentrations, and a ratio higher than 1 was obtained for the condition of Ar 100 %, Ar/O₂ mixtures, and O₂ 100 %, wherein a relatively high oxidation activity was detected. From a summary of the results and findings of previous studies, it was revealed that the observations of NO₂^- + NO₃^- could be more appropriate for investigating the NO₂^- and NO₃^- generation characteristics. In addition, H₂O₂/NO₂^-/NO₃^- related activity rather than H₂O₂ activity was suggested to quantify the OH radical activity more appropriately in the presence of N₂.

A pilot lab trial for enhanced oxidative transformation of procymidone fungicide and its aniline metabolite using heterogeneous MnO2 catalysts

In this study, the feasibility of two heterogeneous catalysis (non-Fenton heterogeneous catalysis and catalytic ozonation) was evaluated for the oxidative transformation of the fungicide procymidone and its major metabolite (3,5-dichloroaniline; 3,5-DCA) under a pilot lab experiment. Among the studied treatments, only H₂O₂ or O₃ significantly oxidized procymidone and 3,5-DCA. However, heterogeneous catalysis used with various types of MnO₂ catalysts was found to be an effective rapid strategy for transformation of procymidone and its aniline metabolite. Among the studied catalysts, δ-MnO₂ performed well in the enhanced oxidative transformation of procymidone and 3,5-DCA in MnO₂-mediator system assay. The optimal reaction parameters, such as reaction pH, and initial catalyst concentration were comparatively evaluated. However, heterogeneous catalysis and catalytic ozonation were revealed as the rapid strategy for oxidative transformation of investigated procymidone and 3,5-DCA as compared to single oxidation by peroxide/ozone. Finally, as a novel insight of this investigation, a postulated reaction mechanism underlying the accelerated transformation of aniline metabolites via heterogeneous catalysis was explored. The findings of this study will open new avenues for evaluating heterogeneous catalysis during oxidative transformation of non-phenolic pollutants in both lab trial and field applications. This study can be expanded for use in actual field settings, using environmental samples from contaminated areas exposed to non-phenolic pesticides and their metabolites.

Oxidative Roles of Polystyrene-Based Nanoplastics in Inducing Manganese Oxide Formation under Light Illumination

Every year, large quantities of plastics are produced and used for diverse applications, growing concerns about the waste management of plastics and their release into the environment. Plastic debris can break down into millions of pieces that adversely affect natural organisms. In particular, the photolysis of micro/nanoplastics can generate reactive oxygen species (ROS). However, their oxidative roles in initiating redox chemical reactions with heavy and transition metals have received little attention. In this study, we investigated whether the photolysis of polystyrene (PS) nanoplastics can induce the oxidation of Mn²⁺(aq) to Mn oxide solids. We found that PS nanoplastics not only produced peroxyl radicals (ROO^•) and superoxide radicals (O₂^•-) by photolysis, which both play a role in unexpected Mn oxidation, but also served as a substrate for facilitating the heterogeneous nucleation and growth of Mn oxide solids and controlling the formation rate and crystalline phases of Mn oxide solids. These findings help us to elucidate the oxidative roles of nanoplastics in the oxidation of redox-active metal ions. The production of ROS from nanoplastics in the presence of light can endanger marine life and human health, and affect the mobility of the nanoplastics in the environment via redox reactions, which in turn may negatively impact their environmental remediation.

Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production

Fe(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole-electron pairs, reactive radicals, and Fe(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Oxidation of organic micropollutant surrogate functional groups with peracetic acid activated by aqueous Co(II), Cu(II), or Ag(I) and geopolymer-supported Co(II)

Peracetic acid (PAA) in combination with transition metals has recently gained increasing attention for organic micropollutant abatement. In this study, aqueous Co(II), Cu(II), and Ag(I) were compared for their capacity to activate PAA. Co(II) outperformed Cu(II) or Ag(I) and the optimum conditions were 0.05 mM of Co(II), 0.4 mM of PAA, and pH 3. However, due to a wider applicability in water treatment, pH 7 (i.e., bicarbonate buffer) was selected for detailed investigations. The abatement of different micropollutant surrogates could be described with a second-order rate equation (observed second-order rate constants, k_obs were in the range of 42-132 M^-1 s^-1). For the para-substituted phenols, there was a correlation between the observed second-order rate constants of the corresponding phenolates and the Hammett constants (R² = 0.949). In all oxidation experiments, the reaction rate decreased significantly after 1-2 min, which coincided with the depletion of PAA but also with the deactivation of the Co(II) catalyst by oxidation to Co(III) and subsequent precipitation. It was demonstrated that Co(II) immobilized on a geopolymer-foam performed approximately similarly as aqueous Co(II) but without deactivation due to Co(III) precipitation. This provides a potential option for the further development of heterogeneous catalytic Co(II)/PAA advanced oxidation processes utilizing geopolymers as a catalyst support material.

Photochemical reactions of dissolved organic matter and bromide ions facilitate abiotic formation of manganese oxide solids

Manganese (Mn) oxide solids are ubiquitous in nature, acting as both electron donors and acceptors in diverse redox reactions in the environment. Reactions of Mn(III/IV) oxides with dissolved natural organic matter (DOM) are commonly described as reductive dissolutions that generate Mn²⁺(aq). In this study, we investigated the role of photochemical reactions of DOM in Mn²⁺(aq) oxidation and the resulting formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates can be generated, including excited triplet state DOM (³DOM*), hydroxyl radicals (^•OH), superoxide radicals (O₂^•-), hydrogen peroxide, and singlet oxygen. Among these, we found that O₂^•- radicals were mainly responsible for Mn oxidation. The solution pH controlled the formation of Mn oxide solids by affecting both Mn²⁺ oxidation by O₂^•- during photolysis of DOM and reductive dissolutions of Mn oxide solids by DOM. Further, with the addition of bromide ions (Br^-), reactions between ³DOM* and Br^-, together with reactions between ^•OH and Br^-, can form reactive bromide radicals. The formed Br radicals also promoted Mn oxide formation. In DOM with more aromatic functional groups, more Mn²⁺ was oxidized to Mn oxide solids. This enhanced oxidation could be the result of promoted pathways from charge-transfer state DOM (DOM^•+/•-) to O₂^•-. These new observations advance our understanding of natural Mn²⁺ oxidation and Mn(III/IV) oxide formation and highlight the underappreciated oxidative roles of DOM in the oxidation of metal ions in surface water illuminated by sunlight.

Aqueous Iron(IV)-Oxo Complex: An Emerging Powerful Reactive Oxidant Formed by Iron(II)-Based Advanced Oxidation Processes for Oxidative Water Treatment

High-valent iron(IV)-oxo complexes are of great significance as reactive intermediates implicated in diverse chemical and biological systems. The aqueous iron(IV)-oxo complex (Fe_aq^IVO²⁺) is the simplest but one of the most powerful ferryl ion species, which possesses a high-spin state, high reduction potential, and long lifetime. It has been well documented that Fe_aq^IVO²⁺ reacts with organic compounds through various pathways (hydrogen-atom, hydride, oxygen-atom, and electron transfer as well as electrophilic addition) at moderate reaction rates and show selective reactivity toward inorganic ions prevailing in natural water, which single out Fe_aq^IVO²⁺ as a superior candidate for oxidative water treatment. This review provides state-of-the-art knowledge on the chemical properties and oxidation mechanism and kinetics of Fe_aq^IVO²⁺, with special attention to the similarities and differences to two representative free radicals (hydroxyl radical and sulfate radical). Moreover, the prospective role of Fe_aq^IVO²⁺ in Fe_aq²⁺ activation-initiated advanced oxidation processes (AOPs) has been intensively investigated over the past 20 years, which has significantly challenged the conventional recognition that free radicals dominated in these AOPs. The latest progress in identifying the contribution of Fe_aq^IVO²⁺ in Fe_aq²⁺-based AOPs is thereby reviewed, highlighting controversies on the nature of the reactive oxidants formed in several Fe_aq²⁺ activated peroxide and oxyacid processes. Finally, future perspectives for advancing the evaluation of Fe_aq^IVO²⁺ reactivity from an engineering viewpoint are proposed.