Manganese oxide nanorods as a robust Fenton-like catalyst at neutral pH: Crystal phase-dependent behavior

Characteristics of biological manganese oxides produced by manganese-oxidizing bacteria H38 and its removal mechanism of oxytetracycline

Oxytetracycline (OTC) is widely used in clinical medicine and animal husbandry. Residual OTC can affect the normal life activities of microorganisms, animals, and plants and affect human health. Microbial remediation has become a research hotspot in the environmental field. Manganese oxidizing bacteria (MnOB) exist in nature, and the biological manganese oxides (BMO) produced by them have the characteristics of high efficiency, low cost, and environmental friendliness. However, the effect and mechanism of BMO in removing OTC are still unclear. In this study, Bacillus thuringiensis strain H38 of MnOB was obtained, and the conditions for its BMO production were optimized. The optimal conditions were determined as follows: optimal temperature = 35 °C, optimal pH = 7.5, optimal Mn(Ⅱ) initial concentration = 10 mmol/L. The results show that BMO are irregular or massive, mainly containing MnCO3, Mn2O3, and MnO2, with rich functional groups and chemical bonds. They have the characteristics of small particle size and large specific surface area. OTC (2.5 mg/L) was removed when the BMO dosage was 75 μmol/L and the solution pH was 5.0. The removal ratio was close to 100 % after 12 h of culture at 35 °C and 150 r/min. BMO can adsorb and catalyze the oxidation of OTC and can produce ·O2-, ·OH, 1O2, and Mn(Ⅲ) intermediate. Fifteen products and degradation pathways were identified, and the toxicity of most intermediates is reduced compared to OTC. The removal mechanism was preliminarily clarified. The results of this study are convenient for the practical application of BMO in OTC pollution in water and for solving the harm caused by antibiotic pollution.

Effect of MnxOy Nanoparticles Stabilized with Methionine on Germination of Barley Seeds (Hordeum vulgare L.)

The aim of this research was to study the effect of Mn_xO_y nanoparticles stabilized with L-methionine on the morphofunctional characteristics of the barley (Hordeum vulgare L.) crop. Mn_xO_y nanoparticles stabilized with L-methionine were synthesized using potassium permanganate and L-methionine. We established that Mn_xO_y nanoparticles have a diameter of 15 to 30 nm. According to quantum chemical modeling and IR spectroscopy, it is shown that the interaction of Mn_xO_y nanoparticles with L-methionine occurs through the amino group. It is found that Mn_xO_y nanoparticles stabilized with L-methionine have positive effects on the roots and seedling length, as well as the seed germination energy. The effect of Mn_xO_y nanoparticles on Hordeum vulgare L. seeds is nonlinear. At a concentration of 0.05 mg/mL, there was a statistically significant increase in the length of seedlings by 68% compared to the control group. We found that the root lengths of samples treated with Mn_xO_y nanoparticle sols with a concentration of 0.05 mg/mL were 62.8%, 32.7%, and 158.9% higher compared to samples treated with L-methionine, KMnO₄, and the control sample, respectively. We have shown that at a concentration of 0.05 mg/mL, the germination energy of seeds increases by 50.0% compared to the control sample, by 10.0% compared to the samples treated with L-methionine, and by 13.8% compared to the samples treated with KMnO₄.

Light and nitrogen vacancy-intensified nonradical oxidation of organic contaminant with Mn (III) doped carbon nitride in peroxymonosulfate activation

Recently, Mn-based materials have a great potential for selective removal of organic contaminants with the assistance of oxidants (PMS, H₂O₂) and the direct oxidation. However, the rapid oxidation of organic pollutants by Mn-based materials in PMS activation still presents a challenge due to the lower conversion of surface Mn (III)/Mn (IV) and higher reactive energy barrier for reactive intermediates. Here, we constructed Mn (III) and nitrogen vacancies (Nv) modified graphite carbon nitride (MNCN) to break these aforementioned limitations. Through analysis of in-situ spectra and various experiments, a novel mechanism of light-assistance non-radical reaction is clearly elucidated in MNCN/PMS-Light system. Adequate results indicate that Mn (III) only provide a few electrons to decompose Mn (III)-PMS* complex under light irradiation. Thus, the lacking electrons necessarily are supplied from BPA, resulting in its greater removal, then the decomposition of the Mn (III)-PMS* complex and light synergism form the surface Mn (IV) species. Above Mn-PMS complex and surface Mn (IV) species lead to the BPA oxidation in MNCN/PMS-Light system without the involvement of sulfate (SO₄^{• ̶}) and hydroxyl radicals (•OH). The study provides a new understanding for accelerating non-radical reaction in light/PMS system for the selective removal of contaminant.

Synthesis of the MnO2-Fe3O4 catalyst support on amorphous silica: a new Fenton's reagent in the degradation of the reactive blue-19 in aqueous solution

In this study, a new Fenton's reagent was synthesized via two steps: (1) the dispersed Fe₃O₄ nanoparticles were immobilized on the surface of the SiO₂ carrier via the precipitation process, and (2) the MnO₂ nano-sheets were coated on the surface of Fe₃O₄/SiO₂ via hydrothermal method. The SiO₂ carrier has been synthetically utilized from Vietnamese rice husk. The successful formation of the MnO₂-Fe₃O₄/SiO₂ composite has been analytically characterized by the XRD (X-ray diffraction), SEM (scanning electron microscope), EDS (energy dispersive spectrometry)-mapping, FTIR (Fourier transform infrared), S_BET (Brunauer-Emmett-Teller specific surface area), and adsorption/desorption isotherms. This Fenton system was employed to catalyze degradation process of the reactive-blue 19 (RB19) with approximately 100% of removal efficiency after 25 min at the optimal condition of 0.15 g/100 mL of catalyst dosage, pH = 3, and the H₂O₂ concentration of 3 mL/100 mL. Moreover, the catalyst could be reused at least six times with high catalytic activity that was more than 90%. In conclusion, this study showed that the mesoporous MnO₂-Fe₃O₄/SiO₂ composite has a great potential for the removal application of dyes from wastewater, and the application of Vietnam rice husk in environmental treatment was developed.

The role of MnO2 crystal morphological scale and crystal structure in selective catalytic degradation of azo dye

MnO₂, as a representative manganese-based catalyst with many kinds of crystal forms, has been widely used to activate PMS. However, the role of morphological scale and crystal structures on the catalytic capability of MnO₂ still lacks further study. In this study, four different crystal forms of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, and δ-MnO₂) are succeeded in being fabricated via hydrothermal processes and evaluated by activating PMS for the removal of Reactive Yellow X-RG, typical azo dye. Experiment results indicate that α-MnO₂ with a one-dimensional structure exhibits the best catalytic performance among the four as-prepared MnO₂, which can be attributed to its broadest crystal interplanar distance (0.692), the highest portion of Mn (III)/Mn (IV) (4.194), and lowest value of average oxidation state AOS (2.696). Correlation analysis confirms that interplanar distance is the most relative factor with the catalytic activity of MnO₂ among the three studied factors (R² = 0.99715). Meanwhile, the morphological scale structure of α-MnO₂ can also account for its highest catalytic ability among the four as-prepared MnO₂, including its large specific area and advantageous one-dimensional nanostructure. Furthermore, according to the response surface methodology, when the dosage of PMS is 2.369 g/L, the dosage of α-MnO₂ is 0.991 g/L, and the initial dye concentration is 1025 mg/L, the maximum removal rate of Reactive Yellow X-RG is up to 97.38%.

Sustained production of superoxide radicals by manganese oxides under ambient dark conditions

Manganese (Mn) oxides are ubiquitous in the environment and have strong reactivity to induce the transformation of various contaminants. However, whether reactive oxygen species contribute to their surface reactivity remains unclear. Here, sustainable production of superoxide radicals (O₂^•-) by various MnO₂ polymorphs in the dark was quantified and the mechanisms involved were explored. The results confirm that O₂^•- was produced through one-electron transfer from surface Mn(III) to adsorbed O₂. In contrast, no H₂O₂ was detected due to its decomposition by Mn oxides to form O₂^•- and Mn(III), leading to the sustained production of O₂^•- on Mn oxide surfaces. In addition, the production of O₂^•- was found to make a clear contribution (4 - 28%) to the transformation of a series of halophenols by MnO₂, suggesting that the O₂^•--mediated surface reaction is an important supplement to the direct electron-transfer mechanism in the reactivity of Mn oxides. These findings advance our understanding of the surface reactivity of Mn oxides and also reveal an important but hitherto unrecognized abiotic source of O₂^•- in the natural environment.

Paraben Compounds—Part II: An Overview of Advanced Oxidation Processes for Their Degradation

Water scarcity represents a problem for billions of people and is expected to get worse in the future. To guarantee people’s water needs, the use of “first-hand water” or the reuse of wastewater must be done. Wastewater treatment and reuse are favorable for this purpose, since first-hand water is scarce and the economic needs for the exploration of this type of water are increasing. In wastewater treatment, it is important to remove contaminants of emerging concern, as well as pathogenic agents. Parabens are used in daily products as preservatives and are detected in different water sources. These compounds are related to different human health problems due to their endocrine-disrupting behavior, as well as several problems in animals. Thus, their removal from water streams is essential to achieve safe reusable water. Advanced Oxidation Processes (AOPs) are considered very promising technologies for wastewater treatment and can be used as alternatives or as complements of the conventional wastewater treatments that are inefficient in the removal of such contaminants. Different AOP technologies such as ozonation, catalytic ozonation, photocatalytic ozonation, Fenton’s, and photocatalysis, among others, have already been used for parabens abatement. This manuscript critically overviews several AOP technologies used in parabens abatement. These treatments were evaluated in terms of ecotoxicological assessment since the resulting by-products of parabens abatement can be more toxic than the parent compounds. The economic aspect was also analyzed to evaluate and compare the considered technologies.

MnO2 Nanoflowers Induce Immunogenic Cell Death under Nutrient Deprivation: Enabling an Orchestrated Cancer Starvation-Immunotherapy

MnO2 nanoparticles have been widely employed in cancer immunotherapy, playing a subsidiary role in assisting immunostimulatory drugs by improving their pharmacokinetics and/or creating a favorable microenvironment. Here, the stereotype of the subsidiary role of MnO2 nanoparticles in cancer immunotherapy is challenged. This study unravels an intrinsic immunomodulatory property of MnO2 nanoparticles as a unique nutrient-responsive immunogenic cell death (ICD) inducer, capable of directly modulating immunosurveillance toward tumor cells. MnO2 nanoflowers (MNFs) constructed via a one pot self-assembly approach selectively induce ICD to nutrient-deprived but not nutrient-replete cancer cells, which is confirmed by the upregulated damage associated molecular patterns in vitro and a prophylactic vaccination in vivo. The underlying mechanism of the MNFs-mediated selective ICD induction is likely associated with the concurrently upregulated oxidative stress and autophagy. Built on their unique immunomodulatory properties, an innovative nanomaterials orchestrated cancer starvation-immunotherapy is successfully developed, which is realized by the in situ vaccination with MNFs and vascular disrupting agents that cut off intratumoral nutrient supply, eliciting potent efficacy for suppressing local and distant tumors. These findings open up a new avenue toward biomedical applications of MnO2 materials, enabling an innovative therapeutics paradigm with great clinical significance.

Mechanisms and pathways of ethidium bromide Fenton-like degradation by reusable magnetic nanocatalysts

Ethidium bromide (3,8-diamino-6-phenyl-5-ethylphenanthridinium bromide, EtBr) is a carcinogenic compound widely used for staining nucleic acids that is difficult to treat. In this study, magnetic nanocatalysts (MNCs) were synthesized for the heterogeneous Fenton-like degradation of EtBr. The initial pH, MNC content, and H₂O₂ concentration were the key factors affecting the EtBr degradation performance and dynamics. An EtBr removal efficiency of 98.97% was achieved within 4 h under optimal conditions (initial pH, 3.0; MNC content, 1 g/L; H₂O₂ concentration, 50 mM), and the degradation followed the ring-open pathway with (2E,4Z,8E)-3-amino-N-ethyl-7,9-dihydroxynona-2,4,8-trienamide as an intermediate, as determined by liquid chromatography and mass spectrometry (LC/MS). Unexpected and satisfactory Fenton-like oxidation of EtBr occurred under basic conditions, which was explained by a novel denitration pathway with 2-[nitro(phenyl)methyl]-(1,1'-biphenyl)-4,4'-diamine as an intermediate. The MNCs retained 62.17% of their degradation efficiency after five consecutive reaction and harvest cycles. Our work elucidated the mechanisms and pathways of EtBr removal in a Fenton-like reaction using MNCs, and comprehensively discussed the optimal reaction conditions and its potential for re-use.

Paraquat Degradation by Biological Manganese Oxide (BioMnO x ) Catalyst Generated From Living Microalga Pediastrum duplex AARL G060

Paraquat is a non-selective fast-acting herbicide used to control weeds in agricultural crops. Many years of extensive use has caused environmental pollution and food toxicity. This agrochemical degrades slowly in nature, adsorbs onto clay lattices, and may require environmental remediation. Studies have shown that biosynthesized manganese oxide (BioMnOx) successfully degraded toxic synthetic compounds such as bis-phenol A and diclofenac, thus it has potential for paraquat degradation. In this experiment, P. duplex AARL G060 generated low (9.03 mg/L) and high (42.41 mg/L) concentrations of BioMnOx. The precipitated BioMnOx was observed by scanning electron microscopy (SEM), and the elemental composition was identified as Mn and O by energy-dispersive x-ray spectroscopy (EDS). The potential for BioMnOx to act as a catalyst in the degradation of paraquat was evaluated under three treatments: (1) a negative control (deionized water), (2) living alga with low BioMnOx plus hydrogen peroxide, and (3) living alga with high BioMnOx plus hydrogen peroxide. The results indicate that BioMnOx served as a catalyst in the Fenton-like reaction that could degrade more than 50% of the paraquat within 72 h. A kinetic study indicated that paraquat degradation by Fenton-like reactions using BioMnOx as a catalyst can be described by pseudo-first and pseudo-second order models. The pH level of the BioMnOx catalyst was neutral at the end of the experiment. In conclusion, BioMnOx is a viable and environmentally friendly catalyst to accelerate degradation of paraquat and other toxic chemicals.

Recent developments in MnO2-based photocatalysts for organic dye removal: a review

The textile industry consumes a large volume of organic dyes and water. These organic dyes, which remained in the effluents, are usually persistent and difficult to degrade by conventional wastewater treatment techniques. If the wastewater is not treated properly and is discharged into water system, it will cause environmental pollution and risk to living organisms. To mitigate these impacts, the photo-driven catalysis process using semiconductor materials emerges as a promising approach. The semiconductor photocatalysts are able to remove the organic effluent through their mineralization and decolorization abilities. Besides the commonly used titanium dioxide (TiO₂), manganese dioxide (MnO₂) is a potential photocatalyst for wastewater treatment. MnO₂ has a narrow bandgap energy of 1~2 eV. Thus, it possesses high possibility to be driven by visible light and infrared light for dye degradation. This paper reviews the MnO₂-based photocatalysts in various aspects, including its fundamental and photocatalytic mechanisms, recent progress in the synthesis of MnO₂ nanostructures in particle forms and on supporting systems, and regeneration of photocatalysts for repeated use. In addition, the effect of various factors that could affect the photocatalytic performance of MnO₂ nanostructures are discussed, followed by the future prospects of the development of this semiconductor photocatalysts towards commercialization.

Mineralization Enhancement of Pharmaceutical Contaminants by Radical-Based Oxidation Promoted by Oxide-Bound Metal Ions

While the use of transition metal oxides in catalyzing advanced oxidation reactions has been widely investigated, very few reports have focused on how the preliminary contact of oxides with target compounds may affect the succession of reaction. In this study, we examined the adsorption and electron transfer reactions of two fluoroquinolones, flumequine (FLU), and norfloxacin (NOR), with goethite (α-FeOOH) or manganese (Mn) oxide, and their impact on the subsequent mineralization of target compounds using H₂O₂ or S₂O₈^2- under UVA irradiation. Intriguingly, higher total organic carbon (TOC) removal was achieved when antibiotics and metal oxides were allowed for preequilibration before starting the oxidation reaction. The rate and extent of TOC removal are strongly dependent on the molecule structure and the redox-active mineral used, and much less on the preequilibration time. This high efficiency can be ascribed to the presence of reduced metal ions, chemically or photochemically generated during the first stage, onto oxide minerals. Oxide-bound Mn^II plays a crucial role in catalyzing oxidant decomposition and then producing greater amounts of radical species through a photoassisted redox cycle, regardless of the underlying surface, Mn^IVO₂ or Mn^IIIOOH. This finding will be of fundamental and practical significance to Mn-based oxidation reactions and wastewater treatment processes.

Mn-based catalysts for sulfate radical-based advanced oxidation processes: A review

Sulfate radical-based advanced oxidation processes (AOPs) have drawn increasing attention during the past two decades, and Mn-based materials have been proven to be effective catalysts for activating peroxymonosulfate (PMS) and peroxydisulfate (PDS) to degrade many contaminants. This article presents a comprehensive review of various Mn-based materials to activate PMS and PDS. The activation mechanisms of different Mn-based catalysts (i.e., Mn oxides MnO_x, MnO_x hybrids, and MnO_x‑carbonaceous material composites) were first summarized and discussed in detail. Besides the commonly reported free radicals (SO₄^-• and ^•OH), non-radical mechanisms such as singlet oxygen and direct electron transfer have also been discovered for selected materials. The effects of pH, inorganic ions, natural organic matter (NOM), dissolved oxygen content, temperature, and the crystallinity of the materials on the catalytic reactivity were also discussed. Then, important instrumentations and technologies employed to characterize Mn-based materials and to understand the reaction mechanisms were concisely summarized. Three common overlooks in the experimental designs for examining the PMS/PDS-MnO_x systems were also discussed. Finally, future research directions were suggested to further improve the technology and to provide a guidance to develop cost-effective Mn-based materials to activate PMS/PDS.

Ultrasound-assisted heterogeneous Fenton-like process for bisphenol A removal at neutral pH using hierarchically structured manganese dioxide/biochar nanocomposites as catalysts

Bisphenol A (BPA) is an important emerging contaminant with endocrine-disrupting potential that has frequently been detected in aquatic environments. In this study, two types of hierarchically structured manganese dioxide/biochar nanocomposites (MnO₂/BCs) were prepared for the first time via facile hydrothermal synthesis. The hydrothermal reaction was maintained at 100 °C for 6 h or 12 h, after which an ultrasound-assisted heterogeneous Fenton-like process was used to catalyze the removal of BPA under neutral pH condition. The characterization results indicated that MnO₂ nanoparticles were successfully formed on the nanocomposite surfaces and had flower-like (δ-MnO₂, 6 h) and urchin-like (α-MnO₂, 12 h) morphology. This enabled a significant improvement in the catalytic activity of BPA removal by the reversible redox reaction. A series of experiments confirmed that the crystalline properties of the nanocomposites affected their catalytic activity. In particular, the α-MnO₂/BCs exhibited catalytic activity in the ultrasound-assisted heterogeneous Fenton-like process and completely removed BPA within 20 min under the following conditions: [BPA]₀ = 100 μM; [H₂O₂]₀ = 10 mM; [catalyst]₀ = 0.5 g/L; ultrasound = 20 kHz (130 W) at 40% amplitude; pH = 7.0 ± 0.1; and temperature = 25 ± 1 °C. This efficiency may have been due to the synergistic effect of ultrasound and α-MnO₂/BCs, which simultaneously induce the effective generation of reactive free radicals and increase the mass transfer rate at the solid-liquid interface. Overall, these results demonstrated that hierarchical urchin-like α-MnO₂/BCs have significant potential as an efficient and low-cost catalyst in ultrasound-assisted heterogeneous Fenton-like systems.

Efficient electrochemical degradation of tetrabromobisphenol A using MnO2/MWCNT composites modified Ni foam as cathode: Kinetic analysis, mechanism and degradation pathway

In this study, MnO₂/MWCNT hybrids, prepared using a solvothermal method, were coated onto Ni foam and then used as a cathode for tetrabromobisphenol A (TBBPA) degradation. The reaction was confirmed to exhibit the pseudo first-order kinetics. Compared with the original Ni foam cathode, the fabricated electrode exhibited higher catalytic activity, attributed to its strong cross-linking and ability to produce catalytic free radicals. Radical scavenger experiments revealed that O₂^- and OH were involved in the decomposition of TBBPA. The effects of current density, pH, catalyst dosage, and initial TBBPA concentration on removal efficiency were further studied. An optimal removal rate of 98.3% was achieved while the rate constant reached values up to 0.07293 min^-1 and the debromination rate was more than 75.4% within 60 min. The electrode showed high catalytic performance and low catalyst loss after 10 cycles, indicating its excellent stability and reusability. The probable mechanism and pathway of TBBPA degradation were suggested based on the analysis of intermediate products. It could be inferred that the decomposition of TBBPA involved CC bond breaks (oxidation) and debromination (reduction). The MnO₂/MWCNT-Ni foam could be a promising cathode material for electrochemical degradation of halogenated organic compounds.

Glucose-induced fabrication of Bi/α-FeC2O4·2H2O heterojunctions: a bifunctional catalyst with enhanced photocatalytic and Fenton oxidation efficiency

A series of novel bifunctional heterojunctions with outstanding photocatalytic and Fenton oxidation activities were synthesized via a one-pot solvothermal method in which glucose served as a reductant and carbon source.

Metal-Oxide-Based Microjets for the Simultaneous Removal of Organic Pollutants and Heavy Metals

Water contamination from industrial and anthropogenic activities is nowadays a major issue in many countries worldwide. To address this problem, efficient water treatment technologies are required. Recent efforts have focused on the development of self-propelled micromotors that provide enhanced micromixing and mass transfer by the transportation of reactive species, resulting in higher decontamination rates. However, a real application of these micromotors is still limited due to the high cost associated to their fabrication process. Here, we present Fe₂O₃-decorated SiO₂/MnO₂ microjets for the simultaneous removal of industrial organic pollutants and heavy metals present in wastewater. These microjets were synthesized by low-cost and scalable methods. They exhibit an average speed of 485 ± 32 μm s^-1 (∼28 body length per s) at 7% H₂O₂, which is the highest reported for MnO₂-based tubular micromotors. Furthermore, the photocatalytic and adsorbent properties of the microjets enable the efficient degradation of organic pollutants, such as tetracycline and rhodamine B under visible light irradiation, as well as the removal of heavy metal ions, such as Cd²⁺ and Pb²⁺.

Core–shell microspheres for the ultrafast degradation of estrogen hormone at neutral pH

In the past few years there has been growing concern about human exposure to endocrine disrupting chemicals. This kind of pollutants can bioaccumulate in aquatic organisms and lead to serious health problems, especially affecting child development. Many efforts have been devoted to achieving the efficient removal of such refractory organics. In this regard, a novel catalyst based on the combination of α-FeOOH and MnO₂@MnCO₃ catalysts has been developed by up-scalable techniques from cheap precursors and tested in the photo-Fenton-like degradation of an endocrine disruptor. Almost total degradation of 17α-ethynylestradiol hormone was achieved after only 2 min of simulated solar irradiation at neutral pH. The outstanding performance of FeOOH@MnO₂@MnCO₃ microspheres was mainly attributed to a larger generation of hydroxyl radicals, which are the primary mediators of the total oxidation for this hormone. This work contributes to the development of more cost-effective systems for the rapid and efficient removal of persistent organic pollutants present in sewage plant effluents under direct solar light.

Ultrafast photo-Fenton degradation of 17α-ethynylestradiol under simulated solar irradiation.

Microhydrangeas with a high ratio of low valence MnOx are capable of extremely fast degradation of organics

Low valence manganese oxides are essential to directly produce abundant ˙OH radicals for extremely fast catalytic degradation of dye pollutants.

α-Ferrous oxalate dihydrate: an Fe-based one-dimensional metal organic framework with extraordinary photocatalytic and Fenton activities

An Fe(ii)-based metal organic framework, α-ferrous oxalate dihydrate (α-FOD), with remarkable n-type semiconductor characteristics has been successfully prepared for use as a bifunctional catalyst in both photocatalysis and Fenton oxidation.

ZnMn2O4 nanorods: an effective Fenton-like heterogeneous catalyst with t2g3eg1 electronic configuration

The t_2g³e_g¹ electron configuration of Mn benefits the Fenton-like spinel ZnMn₂O₄ nanorod catalyst by promoting the creation of oxygen-containing radicals.