Regulating the concentration of dissolved oxygen to achieve the directional transformation of reactive oxygen species: A controllable oxidation process for ciprofloxacin degradation by calcined CuCoFe-LDH

Applying molecular oxygen for organic pollutant degradation: Strategies, mechanisms, and perspectives

Molecular oxygen (O2) is an environmentally friendly, cost-effective, and non-toxic oxidant. Activation of O2 generates various highly oxidative reactive oxygen species (ROS), which efficiently degrade pollutants with minimal environmental impact. Despite extensive research on the application of O2 activation in environmental remediation, a comprehensive review addressing this topic is currently lacking. This review provides an informative overview of recent advancements in O2 activation, focusing on three primary strategies: photocatalytic activation, chemical activation, and electrochemical activation of O2. We elucidate the respective mechanisms of these activation methods and discuss their advantages and disadvantages. Additionally, we thoroughly analyze the influence of oxygen supply, reactive temperature, and pH on the O2 activation process. From electron transfer and energy transfer perspectives, we explore the pathways for ROS generation during O2 activation. Finally, we address the challenges faced by researchers in this field and discuss future prospects for utilizing O2 activation in pollution control applications. This detailed analysis enhances our understanding and provides valuable insights for the practical implementation of organic pollutant degradation.

Revealing the vital role of sulfur site on the surface of pyrite in 1O2 formation for promoting ciprofloxacin degradation via peracetic acid activation

Pyrite has been widely utilized to activate oxidants for water treatment, yet the regulation of reactive oxygen species (ROS) by sulfur sites on its surface has been overlooked. In this study, the surface sulfur sites were regulated by thermal modification of natural pyrite in the N2 atmosphere (denoted as P-X, where X represented pyrolysis temperatures ranging from 400 to 700 °C), and these modified pyrites were employed to activate peracetic acid (PAA) for ciprofloxacin (CIP) degradation. The results revealed that the degradation rate of CIP increased as the reduced sulfur content increased, with the P600/PAA system achieving the highest apparent degradation rate (kobs = 0.0999 min-1). Quenching experiments and electron paramagnetic resonance (EPR) analysis identified various ROS involved in the P-X/PAA system, with hydroxyl radical (·OH) and singlet oxygen (1O2) identified as dominant reactive species responsible for CIP degradation. The reduced sulfur sites served as the primary active sites facilitating the conversion of organic radicals (·CH3C(O)OO) into superoxide radicals (·O2-) and 1O2. Furthermore, the P600/PAA system demonstrated robust adaptability under both acidic and neutral pH conditions, efficiently degrading CIP even in the presence of complex matrices such as Cl-, NO3-, SO42-, NH4+, or humic acid (HA) in water bodies, although HCO3- was found to inhibit CIP degradation. This study significantly enhances our understanding of the interaction between reduced sulfur sites and ROS in PAA-based advanced oxidation processes (AOPs), offering a promising technology for efficient antibiotic treatment in water purification.

Elimination of representative antibiotic-resistant bacteria, antibiotic resistance genes and ciprofloxacin from water via photoactivation of periodate using FeS2

The propagation of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) induced by the release of antibiotics poses great threats to ecological safety and human health. In this study, periodate (PI)/FeS2/simulated sunlight (SSL) system was employed to remove representative ARB, ARGs and antibiotics in water. 1 × 107 CFU mL-1 of gentamycin-resistant Escherichia coli was effectively disinfected below limit of detection in PI/FeS2/SSL system under different water matrix and in real water samples. Sulfadiazine-resistant Pseudomonas and Gram-positive Bacillus subtilis could also be efficiently sterilized. Theoretical calculation showed that (110) facet was the most reactive facet on FeS2 to activate PI for the generation of reactive species (·OH, ·O2-, h+ and Fe(IV)=O) to damage cell membrane and intracellular enzyme defense system. Both intracellular and extracellular ARGs could be degraded and the expression levels of multidrug resistance-related genes were downregulated during the disinfection process. Thus, horizontal gene transfer (HGT) of ARB was inhibited. Moreover, PI/FeS2/SSL system could disinfect ARB in a continuous flow reactor and in an enlarged reactor under natural sunlight irradiation. PI/FeS2/SSL system could also effectively degrade the HGT-promoting antibiotic (ciprofloxacin) via hydroxylation and ring cleavage process. Overall, PI/FeS2/SSL exhibited great promise for the elimination of antibiotic resistance from water.

Boosting Efficient Alkaline Hydrogen Evolution Reaction of CoFe-Layered Double Hydroxides Nanosheets via Co-Coordination Mechanism of W-Doping and Oxygen Defect Engineering

While surface defects and heteroatom doping exhibit promising potential in augmenting the electrocatalytic hydrogen evolution reaction (HER), their performance remains unable to rival that of the costly Pt-based catalysts. Yet, the concurrent modification of catalysts by integrating both approaches stands as a promising strategy to effectively address the aforementioned limitation. In this work, tungsten dopants are introduced into self-supported CoFe-layered double hydroxides (LDH) on nickel foam using a hydrothermal method, and oxygen vacancies (Ov) are further introduced through calcination. The analysis results demonstrated that tungsten doping reduces the Ov formation energy of CoFeW-LDH. The Ov acted as oxophilic sites, facilitating water adsorption and dissociation, and reducing the barrier for cleaving HO─H bonds from 0.64 to 0.14 eV. Additionally, Ov regulated the electronic structure of CoFeW-LDH to endow optimized hydrogen binding ability on tungsten atoms, thereby accelerating alkaline Volmer and Heyrovsky reaction kinetics. Specifically, the abundance of Ov induced a transition of tungsten from a six-coordinated to highly active four-coordinated structure, which becomes the active site for HER. Consequently, an ultra-low overpotential of 41 mV at 10 mA cm-2, and a low Tafel slope of 35 mV dec-1 are achieved. These findings offer crucial insights for the design of efficient HER electrocatalysts.

Enhanced degradation of reactive black 5 via persulfate activation by natural bornite: influencing parameters, mechanism and degradation pathway

Reactive black 5 (RBk5) is a refractory azo dye that constitutes a serious threat to the environment and humans. Herein, natural bornite (Nbo) was utilized to activate persulfate (PDS) for the RBk5 removal. The particle size of the Nbo catalyst was optimized and the RBk5 degradation rate constant that responded positively to the particle size of the Nbo catalyst was exhibited. Then, the operational factors affecting RBk5 removal were comprehensively investigated. With the addition of 1.5 g L-1 Nbo and 1.5 mM PDS, 99.05% of the RBk5 (20 mg L-1) was removed in 150 min compared with 0.46% removal with PDS only, which was caused by the additional reactive oxygen species (ROS) produced by the synergistic action of Fe-Cu bimetallic metal and reductive sulfur species. The Nbo catalyst presented high stability and reusability toward RBk5 removal. Identification of reactive oxygen species revealed that SO 4 ⋅ - , ·OH, O 2 ⋅ - and 1O2 collectively participated in RBk5 removal. Additionally, a possible degradation pathway for RBk5 was proposed, including cleavage of the azo, C-S and S-O bonds, hydroxylation, hydrolyzation, direct oxidation and other pathways. This work developed a highly effective and low-cost natural mineral-based bimetallic sulfide material for PDS activation for the degradation of contaminants and environmental remediation.

Crystal phase-driven performance of MnO2 in aqueous phase low-temperature thermal catalysis: Synergistic interactions between Mn3+ and surface lattice oxygen

Catalytic oxidation at mild conditions is crucial for mitigating the high pressure and high temperature challenges associated with current catalytic wet air oxidation (CWAO) technologies in wastewater treatment. Among potential materials for catalytic oxidation reactions, polycrystalline MnO2 existed in natural minerals holds considerable promise. However, the relationships between different crystal phases of MnO2 and their catalytic activity sources in aqueous phase remain uncertain and subject to debate. In this research, we synthesized various MnO2 crystal phases, comprising α-, β-, δ-, γ-, ε-, and λ-MnO2, and assessed their catalytic oxidation efficiency during low-temperature heating for treatment of organic pollutants. Our findings demonstrate that λ-MnO2 exhibits the highest catalytic activity, followed by δ-MnO2, γ-MnO2, α-MnO2, ε-MnO2, and β-MnO2. The variations in catalytic activity among different MnO2 are attributed to variances in their oxygen vacancy abundance and redox activity. Furthermore, we identified the primary active species, which include Mn3+ and superoxide radicals (•O2-) generated by surface lattice oxygen of MnO2. This research highlights the critical role of crystal phases in influencing oxygen vacancy content, redox activity, and overall catalytic performance, providing valuable insights for the rational design of MnO2 catalysts tailored for effective organic pollutant degradation in CWAO applications.

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

Oxygen Vacancy-Laden Confinement Impact on Degradation of Metal Complexes

Oxygen vacancies (Vo) have been recognized as the superior active site for PS-mediated environmental remediation; however, the formation and activation of Vo associated with the effects of chemical and spatial environments remain ambiguous. Herein, attributing to the low defect-formation energy of Vo in the presence of sulfonate groups, an in situ nucleating Vo-laden CuO nanosheet was deliberately fabricated inside the phase of a sulfonated mesoporous polystyrene substrate (Vo-CuO@SPM). The as-prepared nanocomposite demonstrated an excellent treatment efficiency toward metal complexes [Cu-EDTA as a case] with ignorable Cu(II) leaching, and it can be repeatedly employed for 25 recycles (not limited). Mechanistically, the electron transfer and the mass transport for PDS nonradical activation were proved to be substantially enhanced by the delocalized electrons and with the assistance of the microchannel environment. This work not only establishes insight into the formation of oxygen vacancies but also reveals the PS activation mechanism in the spatially confined sites.

Methyl contributes to the directed phosphorus doping of g-C3N4: pH-dependent selective reactive oxygen species enable customized degradation of organic pollutants

In the photocatalytic degradation process, constructing a controllable composite oxidation system with radicals and nonradicals to meet the requirement for efficient and selective degradation of diverse pollutants is significant. Herein, a methylated and phosphorus-doped g-C3N4 (NPEA) can exhibit selective radical and nonradical species formation depending on the pH values. The NPEA can spontaneously switch the production of active species according to the pH value of the reaction system, exhibiting steady-state concentrations of ·O2- for 11.83 × 10-2 µmol L-1 s-1 (with 92.7 % selectivity) under alkaline conditions (pH = 11), and steady-state concentrations of 1O2 for 5.18 × 10-2 µmol L-1 s-1 (with 88.7 % selectivity) under acidic conditions (pH = 3). The NPEA exhibits stability and universality in the degradation of pollutants with rate constant for sulfamethazine (k = 0.261 min-1) and atrazine (k = 0.222 min-1). Moreover, the LC-MS and Fukui function demonstrated that the NPEA can tailor degradation pathways for pollutants, achieving selective degradation. This study offers a comprehensive insight into the mechanism of the photocatalytic oxidation system, elucidating the intricate interplay between pollutants and reactive oxygen species.

Activation of peroxydisulfate via BiCoFe-layered double hydroxide for effective degradation of aniline

The sulfate radical-based advanced oxidation processes (SR-AOPs) is a promising method for the degradation of pollutants, with the development of highly efficient catalysts for persulfate activation has been widely concerned. The novel BiCoFe-LDH (BCF-x) was synthesized successfully by coprecipitation method, which can activate peroxydisulfate (PDS) efficiently to degrade aniline. Comparative analysis with pure CoFe-LDH revealed a remarkable increase in reaction rate constant by approximately 14.66 times; the degradation rate of aniline (10 mg/L) was 100% in 60 min with the condition of 0.5 g/L BCF-1.5 and 0.5 g/L PDS, due to BCF-1.5 which was characterized as a complex of CoFe-LDH and Bi2O2CO3, promoting electron transport to improve the efficiency of activated PDS. In the reaction system, SO4•-, ·OH, and 1O2 were responsible for the aniline degradation and ·OH was the primary one. Furthermore, this work proposes a reaction electron transfer catalytic mechanism, which provided a new insight and good application prospect for efficient activation of PDS for pollutant degradation.

Self-acclimation mechanism of pyrite to sulfamethoxazole concentration in terms of degradation behavior and toxicity effects caused by reactive oxygen species

Pyrite has been extensively tested for oxidizing contaminants via the activation of water molecule or dissolved oxygen, while the changing of oxidation species induced by contaminant's concentration has been largely underestimated. In this study, we revealed a self-acclimation mechanism of pyrite in terms of •OH conversion to 1O2 during the sulfamethoxazole (SMX) degradation process under oxic conditions. Two reaction stages of SMX degradation by pyrite were observed. The SMX concentration decreased by 70% rapidly in the first 12 h after the reaction was initiated, then, the removal rate began to decrease as the SMX concentration decreased. Importantly, •OH and O2•- were the dominant oxidizing species in stage one, while 1O2 was responsible for the further degradation of SMX in stage two. The self-acclimated mechanism of pyrite was proven to be caused by the conversion of oxidative species at the surface of pyrite. This process can overcome the shortages of •OH such as ultrashort lifetime and limited effective diffusion in the decontamination of micropollutant. Moreover, different reactive oxygen species will lead to different degradation pathways and environmental toxicity while degrading pollutants. This finding of oxidizing species' self-acclimation mechanism should be of concern when using pyrite for water treatment.

Hydroxylamine enhanced the degradation of diclofenac in Cu(II)/peracetic acid system: Formation and contributions of CH3C(O)O•, CH3C(O)OO•, Cu(III) and •OH

The slow reduction of Cu(II) into Cu(I) through peracetic acid (PAA) heavily limited the widespread application of Cu(II)/PAA system. Herein, hydroxylamine (HA) was proposed to boost the oxidative capacity of Cu(II)/PAA system by facilitating the redox cycle of Cu(I)/Cu(II). HA/Cu(II)/PAA system was quite rapid in the removal of diclofenac within a broad pH range of 4.5-9.5, with a 10-fold increase in the removal rate of diclofenac compared with the Cu(II)/PAA system at an optimal initial pH of 8.5. Results of UV-Vis spectra, electron paramagnetic resonance, and alcohol quenching experiments demonstrated that CH3C(O)O•, CH3C(O)OO•, Cu(III), and •OH were involved in HA/Cu(II)/PAA system, while CH3C(O)OO• was verified as the predominant reactive species of diclofenac elimination. Different from previously reported Cu-catalyzed PAA processes, CH3C(O)OO• mainly generated from the reaction of PAA with Cu(III) rather than CH3C(O)O• and •OH. Four possible elimination pathways for diclofenac were proposed, and the acute toxicity of treated diclofenac solution with HA/Cu(II)/PAA system significantly decreased. Moreover, HA/Cu(II)/PAA system possessed a strong anti-interference ability towards the commonly existent water matrix. This research proposed an effective strategy to boost the oxidative capacity of Cu(II)/PAA system and might promote its potential application, especially in copper-contained wastewater.