Biogenic and Synthetic MnO2 Nanoparticles: Size and Growth Probed with Absorption and Raman Spectroscopies and Dynamic Light Scattering

Development of a New Permanganate/Chlorite Process for Water Decontamination

Development of new technologies with strong selectivity for target pollutants and low sensitivity toward a water matrix remains challenging. Herein, we introduced a novel strategy that used chlorite as an activator for Mn(VII) at pH 4.8, turning the inert reactivity of the pollutants toward Mn(VII) into a strong reactivity. This paved a new way for triggering reactions in water decontamination. By utilizing sulfamethoxazole (SMX) as a typical pollutant, we proposed coupled pathways involving electron transfer across hydrogen bonds (TEHB) and oxidation by reactive manganese species. The results indicated that a hydrogen bonding complex, SMX-ClO2-*, formed through chlorite binding the amino group of SMX initially in the TEHB route; such a complex exhibited a stronger reduction capability toward Mn(VII). Chlorite, in the hydrogen bonding complex SMX-ClO2-*, can then complex with Mn(VII). Consequently, a new reactive center (SMX-ClO2--Mn(VII)*) was formed, initiating the transfer of electrons across hydrogen bonds and the preliminary degradation of SMX. This is followed by the involvement of the generated Mn(V)-ClO2-/Mn(III) in the reduction process of Mn(VII). Such a process showed pH-dependent degradation, with a removal ratio ranging from 80% to near-stagnation as pH increased from 4.8 to 7. Combining with pKa analysis showed that the predominant forms of contaminants were crucial for the removal efficiency of pollutants by the Mn(VII)/chlorite process. The impact of the water matrix was demonstrated to have few adverse or even beneficial effects. With satisfactory performance against numerous contaminants, this study introduced a novel Mn(VII) synergistic strategy, and a new reactivity pattern focused on reducing the reduction potential of the contaminant, as opposed to increasing the oxidation potential of oxidants.

Reprogramming exosomes for immunity-remodeled photodynamic therapy against non-small cell lung cancer

Traditional treatments against advanced non-small cell lung cancer (NSCLC) with high morbidity and mortality continue to be dissatisfactory. Given this situation, there is an urgent requirement for alternative modalities that provide lower invasiveness, superior clinical effectiveness, and minimal adverse effects. The combination of photodynamic therapy (PDT) and immunotherapy gradually become a promising approach for high-grade malignant NSCLC. Nevertheless, owing to the absence of precise drug delivery techniques as well as the hypoxic and immunosuppressive characteristics of the tumor microenvironment (TME), the efficacy of this combination therapy approach is less than ideal. In this study, we construct a novel nanoplatform that indocyanine green (ICG), a photosensitizer, loads into hollow manganese dioxide (MnO2) nanospheres (NPs) (ICG@MnO2), and then encapsulated in PD-L1 monoclonal antibodies (anti-PD-L1) reprogrammed exosomes (named ICG@MnO2@Exo-anti-PD-L1), to effectively modulate the TME to oppose NSCLC by the synergy of PDT and immunotherapy modalities. The ICG@MnO2@Exo-anti-PD-L1 NPs are precisely delivered to the tumor sites by targeting specially PD-L1 highly expressed cancer cells to controllably release anti-PD-L1 in the acidic TME, thereby activating T cell response. Subsequently, upon endocytic uptake by cancer cells, MnO2 catalyzes the conversion of H2O2 to O2, thereby alleviating tumor hypoxia. Meanwhile, ICG further utilizes O2 to produce singlet oxygen (1O2) to kill tumor cells under 808 nm near-infrared (NIR) irradiation. Furthermore, a high level of intratumoral H2O2 reduces MnO2 to Mn2+, which remodels the immune microenvironment by polarizing macrophages from M2 to M1, further driving T cells. Taken together, the current study suggests that the ICG@MnO2@Exo-anti-PD-L1 NPs could act as a novel drug delivery platform for achieving multimodal therapy in treating NSCLC.

Cryo-EM Structure of the Mnx Protein Complex Reveals a Tunnel Framework for the Mechanism of Manganese Biomineralization

The global manganese cycle relies on microbes to oxidize soluble Mn(II) to insoluble Mn(IV) oxides. Some microbes require peroxide or superoxide as oxidants, but others can use O2 directly, via multicopper oxidase (MCO) enzymes. One of these, MnxG from Bacillus sp. strain PL-12, was isolated in tight association with small accessory proteins, MnxE and MnxF. The protein complex, called Mnx, has eluded crystallization efforts, but we now report the 3D structure of a point mutant using cryo-EM single particle analysis, cross-linking mass spectrometry, and AlphaFold Multimer prediction. The β-sheet-rich complex features MnxG enzyme, capped by a heterohexameric ring of alternating MnxE and MnxF subunits, and a tunnel that runs through MnxG and its MnxE3F3 cap. The tunnel dimensions and charges can accommodate the mechanistically inferred binuclear manganese intermediates. Comparison with the Fe(II)-oxidizing MCO, ceruloplasmin, identifies likely coordinating groups for the Mn(II) substrate, at the entrance to the tunnel. Thus, the 3D structure provides a rationale for the established manganese oxidase mechanism, and a platform for further experiments to elucidate mechanistic details of manganese biomineralization.

Artificial Manganese Metalloenzymes with Laccase-like Activity: Design, Synthesis, and Characterization

Laccase is an oxidase of great industrial interest due to its ability to catalyze oxidation processes of phenols and persistent organic pollutants. However, it is susceptible to denaturation at high temperatures, sensitive to pH, and unstable in the presence of high concentrations of solvents, which is a issue for industrial use. To solve this problem, this work develops the synthesis in an aqueous medium of a new Mn metalloenzyme with laccase oxidase mimetic catalytic activity. Geobacillus thermocatenulatus lipase (GTL) was used as a scaffold enzyme, mixed with a manganese salt at 50 °C in an aqueous medium. This leads to the in situ formation of manganese(IV) oxide nanowires that interact with the enzyme, yielding a GTL-Mn bionanohybrid. On the other hand, its oxidative activity was evaluated using the ABTS assay, obtaining a catalytic efficiency 300 times higher than that of Trametes versicolor laccase. This new Mn metalloenzyme was 2 times more stable at 40 °C, 3 times more stable in the presence of 10% acetonitrile, and 10 times more stable in 20% acetonitrile than Novozym 51003 laccase. Furthermore, the site-selective immobilized GTL-Mn showed a much higher stability than the soluble form. The oxidase-like activity of this Mn metalloenzyme was successfully demonstrated against other substrates, such as l-DOPA or phloridzin, in oligomerization reactions.

Mining the Carbon Intermediates in Plastic Waste Upcycling for Constructing C-S Bond

Postconsumer plastics are generally perceived as valueless with only a small portion of plastic waste being closed-loop recycled into similar products while most of them are discarded in landfills. Depositing plastic waste in landfills not only harms the environment but also signifies a substantial economic loss. Alternatively, constructing value-added chemical feedstocks via mining the waste-derived intermediate species as a carbon (C) source under mild electrochemical conditions is a sustainable strategy to realize the circular economy. This proof-of-concept work provides an attractive "turning trash to treasure" strategy by integrating electrocatalytic polyethylene terephthalate (PET) plastic upcycling with a chemical C-S coupling reaction to synthesize organosulfur compounds, hydroxymethanesulfonate (HMS). HMS can be produced efficiently (Faradaic efficiency, FE of ∼70%) via deliberately capturing electrophilic intermediates generated in the PET monomer (ethylene glycol, EG) upcycling process, followed by coupling them with nucleophilic sulfur (S) species (i.e., SO32- and HSO3-). Unlike many previous studies conducted under alkaline conditions, PET upcycling was performed over an amorphous MnO2 catalyst under near-neutral conditions, allowing for the stabilization of electrophilic intermediates. The compatibility of this strategy was further investigated by employing biomass-derived compounds as substrates. Moreover, comparable HMS yields can be achieved with real-world PET plastics, showing its enormous potential in practical application. Lastly, Density function theory (DFT) calculation reveals that the C-C cleavage step of EG is the rate-determining step (RDS), and amorphous MnO2 significantly decreases the energy barriers for both RDS and C-S coupling when compared to the crystalline counterpart.

Unraveling the Role of Humic Acid in the Oxidation of Phenolic Contaminants by Soluble Manganese Oxo-Anions

Humic acid (HA) is ubiquitous in natural aquatic environments and effectively accelerates decontamination by permanganate (Mn(VII)). However, the detailed mechanism remains uncertain. Herein, the intrinsic mechanisms of HA's impact on phenolics oxidation by Mn(VII) and its intermediate manganese oxo-anions were systematically studied. Results suggested that HA facilitated the transfer of a single electron from Mn(VII), resulting in the sequential formation of Mn(VI) and Mn(V). The formed Mn(V) was further reduced to Mn(III) through a double electron transfer process by HA. Mn(III) was responsible for the HA-boosted oxidation as the active species attacking pollutants, while Mn(VI) and Mn(V) tended to act as intermediate species due to their own instability. In addition, HA could serve as a stabilizer to form a complex with produced Mn(III) and retard the disproportionation of Mn(III). Notably, manganese oxo-anions did not mineralize HA but essentially changed its composition. According to the results of Fourier-transform ion cyclotron resonance mass spectrometry and the second derivative analysis of Fourier-transform infrared spectroscopy, we found that manganese oxo-anions triggered the decomposition of C-H bonds on HA and subsequently produced oxygen-containing functional groups (i.e., C-O). This study might shed new light on the HA/manganese oxo-anion process.

A facile cellulose finishing strategy through in-situ growth of sliver-doped manganese dioxide assisted by amine-quinone for improving indoor living quality

Nowadays, various harmful indoor pollutants especially including bacteria and residual formaldehyde (HCHO) seriously threaten human health and reduce the quality of public life. Herein, a universal substrate-independence finishing approach for efficiently solving these hybrid indoor threats is demonstrated, in which amine-quinone network (AQN) was employed as reduction agent to guide in-situ growth of Ag@MnO2 particles, and also acted as an adhesion interlayer to firmly anchor nanoparticles onto diverse textiles, especially for cotton fabrics. In contrast with traditional hydrothermal or calcine methods, the highly reactive AQN ensures the efficient generation of functional nanoparticles under mild conditions without any additional catalysts. During the AQN-guided reduction, the doping of Ag atoms onto cellulose fiber surface optimized the crystallinity and oxygen vacancy of MnO2, providing cotton efficient antibacterial efficiency over 90 % after 30 min of contact, companying with encouraging UV-shielding and indoor HCHO purification properties. Besides, even after 30 cycles of standard washing, the Ag@MnO2-decorated textiles can effectively degrade HCHO while well-maintaining their inherent properties. In summary, the presented AQN-mediated strategy of efficiently guiding the deposition of functional particles on fibers has broad application prospects in the green and sustainable functionalization of textiles.

Generalized Synthesis of Highly-Dispersed, Ultrafine Transition Metal Nanoparticles on Silica Spheres for Enhanced Optical Absorption

Robust synthesis of ultrafine metal nanoparticles (ufMNPs) below 5 nm with clean surfaces and strong optical absorption in the visible spectral range is challenging due to their instability originating from large surface-to-volume ratios. This work reports a general strategy involving two sequential steps: i) loading metal precursor ions onto the surface of silica nanospheres (SiOx NSs) by forming a uniform coating of metal oxyhydroxide [MOy(OH)z] through preferred surface acid-base reactions and ii) thermally reducing MOy(OH)z in forming gas at elevated temperatures to form ufMNPs evenly dispersed on the surface of SiOx NSs. The capability of this synthesis strategy is verified by loading ufMNPs of various transition metals and bimetallic combinations onto the SiOx NSs. The ufMNPs exhibit strong optical absorption enhanced by the optical scattering resonances in the SiOx NSs, which generate intense electric fields near the surface of the SiOx NSs. The SiOx NSs also support stabilizing the ufMNPs, which do not need additional organic capping reagents. The successful synthesis of SiOx-NS-supported ufMNPs with clean surfaces and enhanced optical absorption is promising for exploring the photocatalytic properties of ufMNPs.

Hydrothermally grown Cu doped NiMnO3 perovskite nanostructures suitable for optoelectronic, photoluminescent and electrochemical properties

Transition metal-based perovskites have emerged as highly promising and economically advantageous semiconductor materials due to their exceptional performance in optoelectronics, photovoltaic, photocatalysis, and photoluminescence. In this study, we employed a microwave-assisted hydrothermal process to produce a Cu-doped NiMnO3 nanocomposite electrode material. The appearance of a peak corresponding to the (110) plane with a 2θ value of 36.6° confirmed the growth of the rhombohedral NiMnO3 crystal structure. The presence of metal-oxygen bonds in NiMnO3 was confirmed through FTIR spectra. XPS validates the chemical composition, providing additional support for the results obtained from XRD and FT-IR analyses. FE-SEM affirmed the anisotropic growth of small sphere-like structures that agglomerated to form broccoli-like shapes. Cu doping modified the band gap, reducing it from 2.2 to 1.7 eV and enhancing its photoluminescent (PL) activity by introducing defects. The increase in PL intensity (visible light luminescent intensity) can be attributed to a concurrent rise in complex defects and the rate of recombination of electron-hole pairs. Finally, the electrochemical activity demonstrated the pseudo-capacitor behavior of the synthesized material, with capacitance values increasing as the copper (Cu) content in the parent lattice increased.

Characterizing Biogenic MnOx Produced by Pseudomonas putida MnB1 and Its Catalytic Activity towards Water Oxidation

Mn-oxidizing microorganisms oxidize environmental Mn(II), producing Mn(IV) oxides. Pseudomonas putida MnB1 is a widely studied organism for the oxidation of manganese(II) to manganese(IV) by a multi-copper oxidase. The biogenic manganese oxides (BMOs) produced by MnB1 and similar organisms have unique properties compared to non-biological manganese oxides. Along with an amorphous, poorly crystalline structure, previous studies have indicated that BMOs have high surface areas and high reactivities. It is also known that abiotic Mn oxides promote oxidation of organics and have been studied for their water oxidation catalytic function. MnB1 was grown and maintained and subsequently transferred to culturing media containing manganese(II) salts to observe the oxidation of manganese(II) to manganese(IV). The structures and compositions of these manganese(IV) oxides were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction, and their properties were assessed regarding catalytic functionality towards water oxidation in comparison to abiotic acid birnessite. Water oxidation was accomplished through the whole-cell catalysis of MnB1, the results for which compare favorably to the water-oxidizing ability of abiotic Mn(IV) oxides.

Ultrathin MnO2 with strong lattice disorder for catalytic oxidation of volatile organic compounds

Catalytic oxidation proves the most promising technology for volatile organic compounds (VOCs) abatement. Lattice disorder plays a crucial role in the catalytic activity of catalysts due to the exposure of more active sites. Inspired by this, we successfully prepared a series of ε-MnO2 with different lattice disorder defects via several simple methods and applied them to the catalytic oxidation of two typical VOCs (toluene and acetone). Various characterizations and performance tests confirm that the ultrathin (1.4-1.8 nm) structure and strong lattice disorder can enhance the low temperature reduction and reactive oxygen species, so that MnO2-R exhibits excellent toluene and acetone oxidation activities. In-situ DRIFTS tests were carried out to detect reaction intermediates in the toluene and acetone oxidation process on the catalyst surface. Moreover, we propose a possible synergistic mechanism for toluene and acetone mixtures catalytic oxidation. This work reveals the important role of lattice disorder defects in the catalytic oxidation of VOCs on Mn-based catalysts, and deepens the insights of the reaction path in toluene and acetone catalytic oxidation.

An Organic Coordination Manganese Complex as Cathode for High-Voltage Aqueous Zinc-metal Battery

Aqueous Zn-Mn battery has been considered as the most promising scalable energy-storage system due to its intrinsic safety and especially ultralow cost. However, the traditional Zn-Mn battery mainly using manganese oxides as cathode shows low voltage and suffers from dissolution/disproportionation of the cathode during cycling. Herein, for the first time, a high-voltage and long-cycle Zn-Mn battery based on a highly reversible organic coordination manganese complex cathode (Manganese polyacrylate, PAL-Mn) was constructed. Benefiting from the insoluble carboxylate ligand of PAL-Mn that can suppress shuttle effect and disproportionationation reaction of Mn3+ in a mild electrolyte, Mn3+ /Mn2+ reaction in coordination state is realized, which not only offers a high discharge voltage of 1.67 V but also exhibits excellent cyclability (100 % capacity retention, after 4000 cycles). High voltage reaction endows the Zn-Mn battery high specific energy (600 Wh kg-1 at 0.2 A g-1 ), indicating a bright application prospect. The strategy of introducing carboxylate ligands in Zn-Mn battery to harness high-voltage reaction of Mn3+ /Mn2+ well broadens the research of high-voltage Zn-Mn batteries under mild electrolyte conditions.

Enhanced uranium separation by charge enabling γ-MnO2 with oxygen vacancies

Numerous efforts have been devoted to understanding the electron transfer process of uranium (UO22+) on adsorbent materials, whereas the potential oxygen vacancies (OVs) in metal oxides have long been overlooked. Once these interactions are taken into account, the emerging molecular orbital effects undoubtedly affect the adsorption process. Here, we synthesized CC/γ-MnO2 by growing MnO2 on carbon cloth (CC), followed by the creation of oxygen vacancies (OVs) through electrochemical methods to form CC/γ-MnO2-OVs. The CC/γ-MnO2-OVs shows significantly enhanced selectivity and durability for UO22+, with the maximum adsorption capacity increasing from 456.8 to 1648.1 mg/g (by a factor of 3.6). Theoretical calculations suggest that the generation of OVs leads to an increase in charge transfer and a decrease in adsorption energy between UO22+ and CC/γ-MnO2, due to the interaction between Mn 3d orbital in CC/γ-MnO2 and O 2p orbital in UO22+. The OVs in CC/γ-MnO2 provide a spatial structure for anchoring the OU=O moiety of UO22+, while the surface van der Waals forces and the formation of chemical bonds between Mn-U contribute to charge interactions. This synergistic effect allows CC/γ-MnO2-OVs to exhibit favorable selectivity, a large adsorption capacity, and rapid adsorption kinetics towards uranyl ions. This work achieves enhanced UO22+ separation by introducing OVs in CC/γ-MnO2 through a facile electrochemical strategy, highlighting the great potential for nuclear waste processing.

Unraveling the intrinsic mechanism behind the selective oxidation of sulfonamide antibiotics in the Mn(II)/periodate process: The overlooked surface-mediated electron transfer process

Mn(II) exhibits a superb ability in activating periodate (PI) for the efficient degradation of aqueous organic contaminants. Nevertheless, ambiguous conclusions regarding the involved reactive species contributing to the removal of organic contaminants remain unresolved. In this work, we found that the Mn(II)/PI process showed outstanding and selective reactivity for oxidizing sulfonamides with the removal ranging from 57.1% to 100% at pH 6.5. Many lines of evidence suggest that the in-situ formed colloidal MnO2 (cMnO2) served as a catalyst to mediate electron transfer from sulfonamides to PI on its surface via forming cMnO2-PI complex (cMnO2-PI*) for the efficient oxidation of sulfonamides in the Mn(II)/PI process. Experimental results and density functional theory (DFT) calculations verify that the inclusive aniline moiety was the key site determining the electron transfer-dominated oxidation of sulfonamides. Furthermore, DFT calculation results reveal that the discrepancies in the removal of sulfonamides in the Mn(II)/PI process were attributed to different kinetic stability and chemical reactivity of sulfonamides caused by their heterocyclic substituents. In addition, a high utilization efficiency of PI was achieved in the Mn(II)/PI process owing to the surface-mediated electron transfer mechanism. This work provides deep insights into the surface-promoted mechanism in the cMnO2-involved oxidation processes.

In Situ Regulation of MnO2 Structural Characteristics by Oxyanions to Boost Permanganate Autocatalysis for Phenol Removal

Oxyanions, a class of constituents naturally occurring in water, have been widely demonstrated to enhance permanganate (Mn(VII)) decontamination efficiency. However, the detailed mechanism remains ambiguous, mainly because the role of oxyanions in regulating the structural parameters of colloidal MnO2 to control the autocatalytic activity of Mn(VII) has received little attention. Herein, the origin of oxyanion-induced enhancement is systematically studied using theoretical calculations, electrochemical tests, and structure-activity relation analysis. Using bicarbonate (HCO3-) as an example, the results indicate that HCO3- can accelerate the degradation of phenol by Mn(VII) by improving its autocatalytic process. Specifically, HCO3- plays a significant role in regulating the structure of in situ produced MnO2 colloids, i.e., increasing the surface Mn(III)s content and restricting particle growth. These structural changes in MnO2 facilitate its strong binding to Mn(VII), thereby triggering interfacial electron transfer. The resultant surface-activated Mn(VII)* complexes demonstrate excellent degrading activity via directly seizing one electron from phenol. Further, other oxyanions with appropriate ionic potentials (i.e., borate, acetate, metasilicate, molybdate, and phosphate) exhibit favorable influences on the oxidative capability of Mn(VII) through an activation mechanism similar to that of HCO3-. These findings considerably improve our fundamental understanding of the oxidation behavior of Mn(VII) in actual water environments and provide a theoretical foundation for designing autocatalytically boosted Mn(VII) oxidation systems.

Permanganate activation by glucose-derived carbonaceous materials for highly efficient degradation of phenol and p-nitrophenol: Formation of hydroxyl radicals and multiple roles of carbonaceous materials

Owing to its inertness toward refractory organic pollutants and the release of Mn²⁺, the use of permanganate was limited in soil and groundwater remediation. The present study proposed an improvement strategy based on glucose-derived carbonaceous materials, which enhanced the potential of permanganate degrading organic pollutants. The glucose-derived carbonaceous material with 1000 °C charring temperature was named C1000, which was exploited in activating KMnO₄ for the elimination of refractory organic contaminants. The addition of C1000 in the KMnO₄ system triggered the degradation of refractory p-nitrophenol and quicken phenol degradation. Unlike the detection of Mn(III) species in a solo KMnO₄ system, the presence of C1000 facilitated the formation of ^•OH in the KMnO₄ system, which was confirmed by the use of quenchers such as methanol, benzoic acid, tertiary butanol, and carbonate. Additionally, the glucose-derived carbonaceous material played multiple roles in improving the performance of permanganate, including the enrichment of organic pollutants, donation of electrons to permanganate, and acting as an electron shuttle to facilitate the oxidation of organic pollutants by permanganate. The study's novel findings have the potential to expand the use of permanganate in the remediation of organic pollutants.

Organic buffers act as reductants of abiotic and biogenic manganese oxides

Proton activity is the master variable in many biogeochemical reactions. To control pH, laboratory studies involving redox-sensitive minerals like manganese (Mn) oxides frequently use organic buffers (typically Good's buffers); however, two Good's buffers, HEPES and MES, have been shown to reduce Mn(IV) to Mn(III). Because Mn(III) strongly controls mineral reactivity, avoiding experimental artefacts that increase Mn(III) content is critical to avoid confounding results. Here, we quantified the extent of Mn reduction upon reaction between Mn oxides and several Good's buffers (MES, pK_a = 6.10; PIPES, pK_a = 6.76; MOPS, pK_a = 7.28; HEPES, pK_a = 7.48) and TRIS (pK_a = 8.1) buffer. For δ-MnO₂, Mn reduction was rapid, with up to 35% solid-phase Mn(III) generated within 1 h of reaction with Good's buffers; aqueous Mn was minimal in all Good's buffers experiments except those where pH was one unit below the buffer pK_a and the reaction proceeded for 24 h. Additionally, the extent of Mn reduction after 24 h increased in the order MES < MOPS < PIPES < HEPES << TRIS. Of the variables tested, the initial Mn(II,III) content had the greatest effect on susceptibility to reduction, such that Mn reduction scaled inversely with the initial average oxidation number (AMON) of the oxide. For biogenic Mn oxides, which consist of a mixture of Mn oxides, bacterial cells and extracelluar polymeric substances, the extent of Mn reduction was lower than predicted from experiments using abiotic analogs and may result from biotic re-oxidation of reduced Mn or a difference in the reducibility of abiotic versus biogenic oxides. The results from this study show that organic buffers, including morpholinic and piperazinic Good's buffers and TRIS, should be avoided for pH control in Mn oxide systems due to their ability to transfer electrons to Mn, which modifies the composition and reactivity of these redox-active minerals.

High-Voltage-Enabled Stable Cobalt Species Deposition on MnO2 for Water Oxidation in Acid

The design and maintenance of highly active sites in an acidic environment is vital and challenging for the oxygen evolution reaction (OER). In this work, it is found that the obtained CoO₂ under high applied potential can be stable on MnO₂ host in acidic environment, which may act as an effective means to solve the instability of cobalt-based electrocatalyst. The significant improvement of acidic OER activity (6.9 times) and stability (46.4 times) of 90-Co-MnO₂ (treated by molten salt with more Co deposition sites) demonstrates the advantages of this approach. In situ Raman and the Pourbaix diagram suggest that the enhanced performance derives from the stable presence of CoO₂ at the voltage >1.8 V versus reversible hydrogen electrode (RHE). However, when the potential is <1.8 V, the corresponding other cobalt species is too unstable to facilitate the OER. Density functional theorycalculations reveal that the deposited cobalt oxides can act as active sites, thus effectively reducing the reaction energy barrier of the rate-determining step. This work provides a new perspective for enhancing the stability of cobalt-based electrocatalyst. In the future, the dual consideration of applied potential and stable species of active element in the Pourbaix diagram may be a new direction for developing acid-stable electrocatalysts.

Vertical Graphene Film Enables High-Performance Quasi-Solid-State Planar Zinc-Ion Microbatteries

With the advantages of low cost, high safety, and environmental friendliness, quasi-solid-state zinc-ion microbatteries (ZIMBs) have received widespread attention in the field of flexible wearable devices and on-chip integratable energy storage. However, hysteresis Zn-ion transport kinetics and inhomogeneous growth of the zinc anode result in the poor capacity reversibility and cycling stability. Herein, a quasi-solid-state planar zinc-ion cell was developed by employing a vertical graphene (VG) film as an effective conductive modification layer for both the cathode and anode. The VG distinctly induces uniform Zn deposition/stripping, accelerates the charge transport, and enhances the adhesion between the active materials and current collectors. As a result, planar Zn@VG//MnO2@VG exhibits a high areal capacity of 159 μAh cm-2, a remarkably high areal energy/power density of 201.5 μWh cm-2/67.16 μW cm-2, and a high capacity retention of 95.6% at a bending angle of 180°. The proposed facile strategy for electrode modification provides a new insight into the design of high-performance flexible and planar ZIMBs.

Direct observation of Mn distribution/speciation within and surrounding a basidiomycete fungus in the production of Mn-oxides important in toxic element containment

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn²⁺ absorption by a basidiomycete fungus, Coprinopsis urticicola, isolated from Ningyo-toge mine water samples, was investigated in the laboratory under controlled conditions utilizing electron microscopy, synchrotron-based X-ray analysis, and fluorescence microscopy with a molecular pH probe. The fungus' growth was first investigated in an agar-solidified medium supplemented with 1.0 mmol/L Mn²⁺, and Cu²⁺ (0-200 μM), Zn²⁺ (0-200 μM), or diphenyleneiodonium (DPI) chloride (0-100 μM) at 25 °C. The results revealed that Zn²⁺ has no significant effects on Mn oxide formation, whereas Cu²⁺ and DPI significantly inhibit both fungal growth and Mn oxidation, indicating superoxide-mediated Mn oxidation. Indeed, nitroblue tetrazolium and diaminobenzidine assays on the growing fungus revealed the production of superoxide and peroxide. During the interaction of Mn²⁺ with the fungus in solution medium at the initial pH of 5.67, a small fraction of Mn²⁺ infiltrated the fungal hyphae within 8 h, forming a few tens of nm-sized concentrates of soluble Mn²⁺ in the intracellular pH of ∼6.5. After 1 day of incubation, Mn oxides began to precipitate on the hyphae, which were characterized as fibrous nanocrystals with a hexagonal birnessite-structure, these forming spherical aggregates with a diameter of ∼1.5 μm. These nanoscale processes associated with the fungal species derived from the Ningyo-toge mine area provide additional insights into the existing mechanisms of Mn oxidation by filamentous fungi at other U mill tailings sites under circumneutral pH conditions. Such processes add to the class of reactions important to the sequestration of toxic elements.

Yttrium-preintercalated layered manganese oxide as a durable cathode for aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) are regarded as competitive alternatives for large-scale energy storage on account of cost-effectiveness and inherent safety. In particular, rechargeable Zn-MnO₂ batteries have drawn increasing attention due to high manufacturing readiness level. However, obtaining MnO₂ with high electrochemical activity and high cyclic stability toward Zn²⁺/H⁺ storage still remains challenging. Herein, we reveal that incorporating yttrium ions (Y³⁺) into layered MnO₂ can regulate the electronic structure of the MnO₂ cathode by narrowing its band gap (from 3.25 to 2.50 eV), thus boosting the electrochemical performance in RAZIBs. Taking advantage of this feature, the optimized Y-MnO₂ (YMO) sample exhibits greater capacity (212 vs. 152 mA h g^-1 at 0.5 A g^-1), better rate capability (94 vs. 61 mA h g^-1 at 8 A g^-1), reduced charge-transfer resistance (79 vs. 148 Ω), and promoted mass transfer kinetics (3.13 × 10^-11vs. 2.37 × 10^-11 cm² s^-1) in comparison with Y-free MnO₂ (MO). More importantly, compared to MO, YMO-0.1 exhibits enhanced energy storage capability by nearly 40% (309 vs. 222 W h kg^-1) and stable cycle performance (94 vs. 52 mA h g^-1 after 3000 cycles). In situ Raman microscopy further reveals that the presence of Y³⁺ endows MnO₂ with remarkable electrochemical reversibility during charge/discharge processes. This work highlights the importance of the Y³⁺ preintercalation strategy, which can be further developed to obtain better cathode materials for aqueous batteries.

Efficient removal of organic compounds in eutrophic water via a synergy of cyanobacterial extracellular polymeric substances and permanganate

This study provides a new thinking for the efficient utilization of permanganate (Mn (VII)) in eutrophic water treatment. Eutrophic water contained a large amount of extracellular polymeric substances (EPS) with reduction and chelation; this study used phenol as typical organic matter and cyanobacteria EPS as a representative EPS to explore the mechanism by which EPS influences the oxidation of phenol by Mn(VII) at pH 5.0-9.0. The results showed that under the condition of pH 5.0-7.0, adding 0.2-10 mg/L EPS to the Mn(VII) system could effectively improve the oxidation efficiency of Mn(VII) for phenol. EPS promoted the continuous formation and stability of in situ EPS-MnO₂ colloids and significantly enhanced the oxidation of Mn(VII). EPS also combined with phenol and increased the electron cloud density to promote the oxidation of phenol by Mn(VII).

Interfacial tension driven adsorption of MnO2 nanoparticles at the liquid/liquid interface to tailor ultra-thin polypyrrole sheets

An emerging aspect of research is designing and developing fully tunable metamaterials for various applications with fluid interfaces. Liquid/liquid interface-assisted methods represent an efficient and facile route for synthesizing two-dimensional (2-D) thin films of potential materials. The underlying mechanism behind thin film formation at the liquid/liquid interface involves the preferential adsorption of nano-sized particles at the interface to minimize high interfacial tension. Here, a water/chloroform interface-assisted method is employed for the one-pot synthesis of highly crystalline polypyrrole/manganese dioxide (PPy/MnO₂) sheets. The temporal evolution in the dynamic interfacial tension (from 32 mN m^-1 to 17 mN m^-1) observed in pendant drop tensiometry proved the preferential adsorption of MnO₂ atttached PPy oligomers at the water/chloroform interface. An ultra-thin sheet-like morphology and uniform distribution of ∼6 nm highly crystalline MnO₂ nanoparticles are evidenced by transmission and atomic force microscopy techniques. The predominance of interfacial polymerization in retaining the electrochemical activity of the PPy/MnO₂ sheets is elucidated for the electrochemical detection of nicotine. This study opens a new avenue for the realization of ultra-thin sheets of polymer-nanomaterial hybrids, enabling applications ranging from new classes of sensors to optics.

Nanoparticle classification, physicochemical properties, characterization, and applications: a comprehensive review for biologists

Interest in nanomaterials and especially nanoparticles has exploded in the past decades primarily due to their novel or enhanced physical and chemical properties compared to bulk material. These extraordinary properties have created a multitude of innovative applications in the fields of medicine and pharma, electronics, agriculture, chemical catalysis, food industry, and many others. More recently, nanoparticles are also being synthesized ‘biologically’ through the use of plant- or microorganism-mediated processes, as an environmentally friendly alternative to the expensive, energy-intensive, and potentially toxic physical and chemical synthesis methods. This transdisciplinary approach to nanoparticle synthesis requires that biologists and biotechnologists understand and learn to use the complex methodology needed to properly characterize these processes. This review targets a bio-oriented audience and summarizes the physico–chemical properties of nanoparticles, and methods used for their characterization. It highlights why nanomaterials are different compared to micro- or bulk materials. We try to provide a comprehensive overview of the different classes of nanoparticles and their novel or enhanced physicochemical properties including mechanical, thermal, magnetic, electronic, optical, and catalytic properties. A comprehensive list of the common methods and techniques used for the characterization and analysis of these properties is presented together with a large list of examples for biogenic nanoparticles that have been previously synthesized and characterized, including their application in the fields of medicine, electronics, agriculture, and food production. We hope that this makes the many different methods more accessible to the readers, and to help with identifying the proper methodology for any given nanoscience problem.

Methyl silicate promotes the oxidative degradation of bisphenol A by permanganate: Efficiency enhancement mechanism and solid-liquid separation characteristics

Permanganate (Mn (VII)) is an environmentally-friendly mild oxidant in the field of advanced oxidation treatment, however, manganese colloids are produced as byproducts, which is difficult to separate from water, resulting in secondary pollution. This study used potassium methyl silicates (PMS) as surface modifiers to improve the aggregation of colloidal particles by increasing the hydrophobicity of the colloidal surface, and then explored the oxidation of bisphenol A (BPA) by Mn (VII) under the influence of potassium methyl silicate and the solid-liquid separation performance of the reaction system. The results showed that PMS and sodium silicate (SS) substantially enhanced the degradation of BPA by Mn (VII), and the promotion effect of potassium methyl silicate was greater than that of sodium silicate. PMS provided not only enough adsorption sites for MnO₂ colloidal particles formed in the reaction process, but also reaction space for Mn (VII) to catalyze the oxidation of BPA. PMS combined with the hydroxyl group of MnO₂ through hydrogen bonds and forms hydrophobic PMS-MnO₂ complexes which accelerated sedimentation by polycondensation. The strong adsorption ability of in situ formed MnO₂ colloids also accelerated the deposition of PMS-MnO₂ complex. This study solved the low efficiency problem of Mn (VII) oxidation degradation of organic pollutants and difficult separation of manganese containing colloids and provided a new strategy for the efficient utilization of Mn (VII).

A ratiometric fluorescence platform composed of MnO2 nanosheets and nitrogen, chlorine co-doped carbon dots and its logic gate performance for glutathione determination

Illustration of the principle of a dual-emission ratiometric fluorescence strategy for the selective detection of GSH based on an N, Cl-CD-assisted MnO2 nanosheet–OPD system.

Coupled Mn(II) and Cr(III) Oxidation Mediated by Ascomycete Fungi

Manganese (Mn) oxides are considered as the primary oxidant of trivalent chromium [Cr(III)] in the environment. Microbial activities are responsible for the majority of Mn oxide formation in nature, thus likely influencing Cr(III) oxidation. Previous studies have been limited to Cr(III) oxidation by bacterial Mn oxides. Herein, we report coupled Mn(II) and Cr(III) oxidation in the presence of three Mn(II)-oxidizing Ascomycete fungi. In contrast to the previously reported inhibitory effect of Cr(III) on bacterial Mn(II) oxidation, varying effects of Cr(III) on fungal Mn(II) oxidation were observed, which may be linked to their Mn(II)-oxidation mechanisms. Under the concentrations of Mn(II) and Cr(III) applied in this study, Cr(III) promoted Mn(II) oxidation if it was mediated by hyphae-associated processes, but inhibited Mn(II) oxidation if it was achieved via extracellular enzymes/metabolites. The Cr(III) oxidation rate and extent were affected by Cr(III) speciation, Cr(VI) removal capacity (i.e., adsorption/reduction) of fungi, and organic content. The morphology and spatial relationship of Mn oxides with fungi varied, depending on fungal species and Cr(III) presence. Our findings highlight the importance of Mn(II)-oxidizing fungi in biogeochemical cycles of Mn and Cr and have significant implications for the origin of geogenic Cr(VI) and stability of reduced chromium in contaminated environments.

Novel manganese cycling at very low ionic strengths in the Columbia River Estuary

Mixing of waters of different ionic strengths induces the geochemical cycling of reactive elements. The most reactive zone is where the gradient in ionic strength is steepest. In oxygenated systems, the redox-active metal manganese cycles between soluble and particulate fractions through three oxidation states, manganese(II), manganese(III) and manganese(IV). This cycling strongly affects the mobility of inorganic and organic chemicals. The most accessible environmental system where waters with different ionic strengths mix are estuaries. During six Eulerian studies in the Columbia River Estuary, each up to 26 h, we measured manganese speciation and concentration across a salinity (S_P) gradient centred around S_P = 0.06-6, equivalent to a seawater ionic strength (I_Sp) of 1.2-120 mM. This zone, representing the region between freshwater and the more intensively studied estuarine turbidity maximum, presents a highly dynamic geochemical environment in which the manganese cycle propagates through four steps as I_Sp increases due to mixing: 1. Before a measurable change in I_Sp, manganese, as particulate manganese(III/IV) oxides (MnO_x), undergoes reduction, independent of photochemical processes, to soluble manganese(III) stabilized in organic complexes (Mn(III)-L) and manganese(II); 2. As I_Sp increases between 5 and 80 mM, Mn(III)-L reduction continues and manganese(II) adsorbs onto particle surfaces; 3. As I_Sp increases further, though remaining below 80 mM (S_P ≈ 4), adsorbed manganese(II) desorbs and/or is oxidized and is released as Mn(III)-L or oxidises further to MnO_x; 4. The breakdown of Mn(III)-L complexes leads to higher manganese(II) and MnO_x, which at Mid-Estuary-Salinities (I_Sp = 320-480 mM) precipitates. This manganese cycling in low I_Sp waters directly affects a system's redox chemistry and provides a window into understanding the extensive, yet hidden, freshwater/saline water interface in aquifers, soils, sediments and estuaries.

Structural characterization and magnetic response of poly(p-xylylene)-MnSb and MnSb films deposited at cryogenic temperature

In this study, we employed several experimental techniques to investigate structure and magnetic properties of poly(p-xylylene)–MnSb composites synthesized by low-temperature vapor deposition polymerization technique and MnSb films deposited at various temperatures. The presence of MnSb nanocrystallites in the studied films was verified by the results of X-ray diffraction, electron microscopy and Raman spectroscopy studies. The obtained data revealed the formation of Sb-rich sublayer with well-oriented Sb grains near the susbtrate, which seems to act as a buffer for the consequent poly(p-xylylene)–MnSb or MnSb layer growth. Increasing the polymer content results in qualitative change of surface morphology of studied films. At high polymer content the hybrid nanocomposite with MnSb nanoparticles embedded into poly(p-xylylene) matrix is formed. All investigated samples demonstrated detectable ferromagnetic response at room temperature, while the parameters of this response revealed a complex correlation with nominal composition, presented crystal phases and surface morphology of studied films. Estimated values of the Curie temperature of the samples are close to that of bulk MnSb.

Metallo-inhibition of Mnx, a bacterial manganese multicopper oxidase complex

The manganese oxidase complex, Mnx, from Bacillus sp. PL-12 contains a multicopper oxidase (MCO) and oxidizes dissolved Mn(II) to form insoluble manganese oxide (MnO₂) mineral. Previous kinetic and spectroscopic analyses have shown that the enzyme's mechanism proceeds through an activation step that facilitates formation of a series of binuclear Mn complexes in the oxidation states II, III, and IV on the path to MnO₂ formation. We now demonstrate that the enzyme is inhibited by first-row transition metals in the order of the Irving-Williams series. Zn(II) strongly (K_i ~ 1.5 μM) inhibits both activation and turnover steps, as well as the rate of Mn(II) binding. The combined Zn(II) and Mn(II) concentration dependence establishes that the inhibition is non-competitive. This result is supported by electron paramagnetic resonance (EPR) spectroscopy, which reveals unaltered Mnx-bound Mn(II) EPR signals, both mono- and binuclear, in the presence of Zn(II). We infer that inhibitory metals bind at a site separate from the substrate sites and block the conformation change required to activate the enzyme, a case of allosteric inhibition. The likely biological role of this inhibitory site is discussed in the context of Bacillus spore physiology. While Cu(II) inhibits Mnx strongly, in accord with the Irving-Williams series, it increases Mnx activation at low concentrations, suggesting that weakly bound Cu, in addition to the four canonical MCO-Cu, may support enzyme activity, perhaps as an electron transfer agent.

Remarkable MnO2 structure-dependent H2O promoting effect in HCHO oxidation at room temperature

H₂O is often critical in determining the activity and stability of metal oxide catalysts for HCHO oxidation; however, synthesis of metal oxide catalysts with super resistance to H₂O remains a challenging. Herein, we synthesized Akhtenskite-type MnO₂ catalyst with Mn-O-Mn stretching along MnO₆ octahedra layers, which promotes the utilization of the associatively adsorbed H₂O. The activity and stability of formaldehyde oxidation at room temperature enhanced in humid air. Diffuse-reflectance infrared Fourier transform (DRIRFT) spectroscopy was used to characterize the H₂O adsorption and intermediate species. The associatively adsorbed H₂O promotes the oxidation of formaldehyde to CO₂ via the formic acid intermediate. The service life of MnO₂ is prolonged due to formic acid generation. MnO₂ gradually deactivates when formic acid accumulates and forms formate and hydrogen carbonate species. This study provides significant insights into the development of a high-efficiency MnO₂ catalyst for formaldehyde oxidation in humid air, and the developed MnO₂ catalyst is a promising candidate for application in practical formaldehyde elimination.

Enzymatically synthesised MnO2 nanoparticles for efficient near-infrared photothermal therapy and dual-responsive magnetic resonance imaging

Manganese dioxide (MnO2) nanoparticles (NPs) are highly attractive for biomedical applications due to their biocompatibility, stimuli-responsive magnetic resonance imaging (MRI) properties and capability to modulate the hypoxic tumour microenvironment (TME). However, conventional MnO2 NPs do not possess photothermal therapy (PTT) functions except for hybrids with other photothermal materials. Herein, we first reveal the extraordinary photothermal conversion efficiency (44%) of enzymatically synthesised MnO2 NPs (Bio-MnO2 NPs), which are distinct from chemically synthesised MnO2 NPs. In addition, the Bio-MnO2 NPs revealed high thermal recycling stability and solubility as well as dual pH- and reduction-responsive MRI enhancement for tumour theragnosis. These NPs were prepared through a facile MnxEFG enzyme-mediated biomineralization process. The MnxEFG complex from Bacillus sp. PL-12 is the only manganese mineralization enzyme that could be heterologously overexpressed in its active form to achieve Bio-MnO2 NPs without a bacterial host. The hexagonal layer symmetry of the Bio-MnO2 NPs is the key feature facilitating the high photothermal conversion efficiency and TME-responsive T1-weighted MRI. Evaluations both in vitro at the cellular level and in vivo in a systematic tumour-bearing mouse xenograft model demonstrated the high photothermal ablation efficacy of the Bio-MnO2 NPs, which achieved complete tumour eradication with high therapeutic biosafety without obvious reoccurrence. Moreover, stimuli-responsive MR enhancement potentially allows imaging-guided precision PTT. With their excellent biocompatibility, mild synthesis conditions and relatively simple composition, Bio-MnO2 NPs hold great translational promise.

Elucidating Gold-MnO2 Core-Shell Nanoenvelope for Real Time SERS-Guided Photothermal Therapy on Pancreatic Cancer Cells

Pancreatic cancer represents one of the most aggressive in nature with a miserable prognosis that warrants efficient diagnostic and therapeutic interventions. Herein, a MnO₂ overlaid gold nanoparticle (AuNPs) based photothermal theranostic nanoenvelope (PTTNe:MnO₂@AuNPs) was fabricated to substantiate surface-enhanced Raman spectroscopy (SERS) guided real-time monitoring of photothermal therapy (PTT) in pancreatic cancer cells. A sharp enhancement of the fingerprint Raman signature of MnO₂ at 569 cm^-1 exhibited as a marker peak for the first time to elucidate the intracellular PTT event. In this strategic design, the leftover bare AuNPs after the degradation of the MnO₂ layer from the nanoenvelope in the presence of intracellular H₂O₂ enabled real-time tracking of biomolecular changes of Raman spectral variations during PTT. Moreover, the surface of the as-synthesized nanoenvelope was functionalized with a pancreatic cancer cell targeting peptide sequence for cholecystokinin fashioned the PTTNe with admirable stability and biocompatibility. Finally, the precise cell death mechanism was explicitly assessed by SERS spectral analysis as a complementary technique. This targeted phototheranostic approach demonstrated in pancreatic cancer cells presented a therapeutically viable prototype for futuristic personalized cancer nanomedicine.

Immobilized Co2+ and Cu2+ induced structural change of layered double hydroxide for efficient heterogeneous degradation of antibiotic

In this study, MgMn-layered double hydroxide (MgMnLDH) exhibited excellent remediation functionality for heavy metals-antibiotics combined pollution. On the one hand, Co²⁺ and Cu²⁺ was efficiently immobilized on MgMnLDH with maximum quantity of 4.30 and 10.65 mmol g^-1, respectively. A series of characterizations reflected the changes in structure and physicochemical properties of MgMnLDH after the immobilization. Density functional theory calculations (DFT) confirmed that the binding modes were lattice substitution for Co²⁺ and surface precipitation for Cu²⁺. On the other hand, the immobilized heavy metals enhanced the heterogeneous degradation for sulfamethoxazole (SMX) by peroxymonosulfate (PMS) activation. Complete degradation was achieved within 10 min in MgMnLDH-Co-4/PMS system and 60 min in MgMnLDH-Cu/PMS system, while only 20% in MgMnLDH/PMS system. The pH adaptability, reusability, stability and activation mechanism of two systems were systematically compared. The superior degradation performance of MgMnLDH-Co-4 benefited from the intense Co/Mn synergism and abundant oxygen vacancies, which could accelerate electron transfer during PMS activation process. The applicability of two catalysis system was confirmed in purifying other antibiotics and actual wastewater. The results highlight the importance of structural control in heterogeneous catalysis and provide new idea for environmental remediation.

Lead Particle Size Fractionation and Identification in Newark, New Jersey's Drinking Water

Following a pH reduction in their drinking water over a span of more than 20 years, the City of Newark, New Jersey, has struggled with elevated lead (Pb) release from Pb service lines and domestic plumbing in the zone fed by the Pequannock Water Treatment Plant. In response, Newark initiated orthophosphate addition and provided faucet-mounted point-of-use (POU) filters and pitcher filters certified for Pb and particulate reduction under NSF/ANSI Standards 53 and 42 to residential homes in that zone. Water chemistry analysis and size fractionation sampling were performed at four of these houses. Analysis of the particulate material retained by the fractionation filters revealed that Pb was dominantly present in the water as fine Pb(II) orthophosphate particles. A considerable amount of the particulates occurred as a nanoscale fraction that sometimes passed through the POU faucet or pitcher filtration units. Scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy analyses showed that the nanoparticles (<100 nm) and their aggregates were composed of Pb, phosphorus, and chlorine, which are consistent with pyromorphite, Pb₅(PO₄)₃Cl. Electron diffraction and X-ray analyses supported the presence of hydroxypyromorphite and chloropyromorphite nanoparticles and the size range estimates from the imaging. This research confirmed that nonadherent Pb(II)-orthophosphate nanoparticles were an important form of Pb in drinking water in the Pequannock water quality zone of Newark.

Dissolved Silicate Enhances the Oxidation of Chlorophenols by Permanganate: Important Role of Silicate-Stabilized MnO2 Colloids

Dissolved silicate is an important background constituent of natural waters, but there is little clarity regarding the effect of silicate on the oxidizing capability of permanganate (Mn(VII)) and on its efficiency for remediation applications. In the present study, we found that dissolved silicate, metasilicate or disilicate (DS), could significantly promote the oxidation of 2,4-dichlorophenol (2,4-DCP) by Mn(VII), and the extent of the promoting effect was even more evident than that of pyrophosphate (PP). The experiments showed that, unlike PP, DS was not capable of coordinating with Mn(III) ions, and the promoting effect of DS was not due to the oxidizing capability of complexed Mn(III). Instead, DS ions, as a weak base, could combine with the hydroxyl groups of MnO₂ via hydrogen bonding to limit the growth of colloidal MnO₂ particles. The DS-stabilized colloidal MnO₂ particles, with hydrodynamic diameters less than 100 nm, could act as catalysts to enhance the oxidation of 2,4-DCP by Mn(VII). The best promoting effect of DS on the performance of Mn(VII) oxidant was achieved at the initial solution pH of 7, and the coexisting bicarbonate ions further improved the oxidation of 2,4-DCP in the Mn(VII)/DS system. Sand column experiments showed that the combined use of Mn(VII) and DS additive could mitigate the problem of permeability reduction of sand associated with the retention of MnO₂ particles. This study not only deepens our understanding on the role of dissolved silicate in a Mn(VII) oxidation process but also provides an effective and green method to enhance the oxidizing capacity of Mn(VII)-based treatment systems.

An innovative method for manganese (Mn2+) and ammonia nitrogen (NH4+-N) stabilization/solidification in electrolytic manganese residue by basic burning raw material

High concentrations of manganese (Mn²⁺) and ammonia nitrogen (NH₄⁺-N) in electrolytic manganese residue (EMR) have seriously hindered the sustainable development of electrolytic manganese industry. In this study, an innovative basic burning raw material (BRM) was used to stabilize/solidify Mn²⁺ and NH₄⁺-N in EMR. The characteristics of EMR and BRM, stabilize mechanism of NH₄⁺-N and Mn²⁺, and leaching test were investigated. The concentrations of NH₄⁺-N and Mn²⁺ were 12.8 mg/L and 0.1 mg/L, respectively, when the solid liquid ratio was 1.5:1, and the mass ratio of EMR and BRM was 100:10, at the temperature of 20 °C reacting for 12 h Mn²⁺ was mostly solidified as bustamite ((Mn,Ca)Si₂O₆), groutite (MnOOH) and ramsdellite (MnO₂). NH₄⁺-N was mostly recycled by (NH₄)₂SO₄ and (NH₄)₃H(SO₄)₂. Leaching test results indicated that the concentrations of heavy metals were within the permitted level for the integrated wastewater discharge standard (GB8978-1996). Economic evaluation revealed that the cost of EMR treatment was $ 10.15/t by BRM. This study provided a new research idea for EMR harmless disposal.

Molecular Cloning and Heterologous Expression of Manganese(II)-Oxidizing Enzyme from Acremonium strictum Strain KR21-2

Diverse ascomycete fungi oxidize manganese(II) [Mn(II)] and produce Mn(III, IV) oxides in terrestrial and freshwater environments. Although multicopper oxidase (MCO) is considered to be a key catalyst in mediating Mn(II) oxidation in ascomycetes, the responsible gene and its product have not been identified. In this study, a gene, named mco1, encoding Mn(II)-oxidizing MCO from Acremonium strictum strain KR21-2 was cloned and heterologously expressed in the methylotrophic yeast Pichia pastoris. Based on the phylogenetic relationship, similarity of putative copper-binding motifs, and homology modeling, the gene product Mco1 was assigned to a bilirubin oxidase. Mature Mco1 was predicted to be composed of 565 amino acids with a molecular mass of 64.0 kDa. The recombinant enzyme oxidized Mn(II) to yield spherical Mn oxides, several micrometers in diameter. Zinc(II) ions added to the reaction mixture were incorporated by the Mn oxides at a Zn/Mn molar ratio of 0.36. The results suggested that Mco1 facilitates the growth of the micrometer-sized Mn oxides and affects metal sequestration through Mn(II) oxidation. This is the first report on heterologous expression and identification of the Mn(II) oxidase enzyme in Mn(II)-oxidizing ascomycetes. The cell-free, homogenous catalytic system with recombinant Mco1 could be useful for understanding Mn biomineralization by ascomycetes and the sequestration of metal ions in the environment

Role of pyrophosphate on the degradation of sulfamethoxazole by permanganate combined with different reductants: Positive or negative

Activating permanganate with reductants has gained increasing attention recently for efficient organic contaminants abatement via reactive intermediate Mn species. However, few studies have been conducted to explore the role of pyrophosphate (PP), a typical complexing agent for intermediate Mn species, in activated permanganate systems. In this study, taking sulfamethoxazole (SMX) as a probe compound, the influences of PP on SMX degradation by permanganate/thiosulfate and permanganate/hydroxylamine were extensively studied. It was found that both thiosulfate and hydroxylamine were able to activate permanganate for oxidation of SMX in the absence of PP. However, upon the introduction of PP, opposite effects were observed in the two systems where PP further improved the activation of permanganate by thiosulfate but dampened the performance of permanganate/hydroxylamine markedly. For permanganate/hydroxylamine system, MnO₂ was determined to be the only reactive oxidative species accounting for SMX degradation in the absence of PP, and its generation could be completely inhibited by PP. While in permanganate/thiosulfate system, both Mn(V) and MnO₂ were responsible for SMX oxidation, and the introduction of PP could strengthen the oxidative ability of Mn(V). These results could shed some insights on the suitability of applying PP to explore the kinetics and mechanisms of manganese involved redox reactions. PRACTITIONER POINTS: Both Na₂ S₂ O₃ and NH₂ OH·HCl can activate KMnO₄ for SMX removal without PP. MnO₂ is the reactive oxidative species involved in KMnO₄ /NH₂ OH·HCl system. Mn(V) and MnO₂ account for the SMX oxidation by KMnO₄ /Na₂ S₂ O₃ system. PP could inhibit the formation of MnO₂ but enhance the oxidative ability of Mn(V).

Bismuth for Controlled Assembly/Disassembly of Transition-Metal Oxo Clusters, Defining Reaction Pathways in Inorganic Synthesis and Nature

Trivalent bismuth is a unique heavy p-block ion. It is highly insoluble in water, due to strong hydrolysis tendencies, and known for low toxicity. Its lone pair is structure-directing, providing framework materials with structural flexibility, leading to piezoelectric and multiferroic function. The flexibility it provides is also advantageous for dopants and vacancies, giving rise to conductivity, luminescence, color, and catalytic properties. We are exploiting Bi³⁺ in a completely different way, as a knob to "tune" the solubility and stability of transition-metal oxo clusters. The lone pair allows capping and isolation of metastable cluster forms for solid-state and solution characterization. With controlled release of the bismuth (via bismuth oxyhalide metathesis), the metal oxo clusters can be retained in aqueous solution, and we can track their reaction pathways and conversion to related metal oxyhydroxides. Here we present isolation of a bismuth-stabilized Mn^IV cluster, fully formulated [Mn^IV₆Bi₂KO₉(CH₃COO)₁₀(H₂O)₃(NO₃)₂] (Mn₆Bi₂). In addition to characterization by single-crystal X-ray diffraction, solution characterization in acetonitrile and acetonitrile-acetic acid by small-angle X-ray scattering (SAXS) and electrospray ionization mass spectrometry shows high stability and the tendency of Mn₆Bi₂ to link into chains by bridging the bismuth (and potassium) caps with nitrate and acetate ligands. On the other hand, the dissolution of Mn₆Bi₂ in water, with and without metathesis of the bismuth, leads to the precipitation of related oxyhydroxide phases, which we characterized by transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy, and the conversion pathway by SAXS. Without removal of bismuth, amorphous manganese/bismuth oxyhydroxides precipitate within a day. On the other hand, metathesis of BiOBr yields a solution containing soluble manganese oxyhydroxide prenucleation clusters that assemble and precipitate over 10 days. This allows tracking of the reaction pathway via SAXS. We observe one-dimensional growth of species, followed by the precipitation of nanocrystalline hollandite (identified by TEM). The hollandite is presumably templated by the K⁺, originally in the crystalline lattice of Mn₆Bi₂. In this Forum Article that combines new results and prospective, we compare these results to prior studies in which we first introduced the use of capping Bi³⁺ to stabilize reactive clusters, followed by destabilization to understand reaction pathways in synthesis and low-temperature geochemistry.

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.

MnO2 flowery nanocomposites for efficient and fast removal of mercury(ii) from aqueous solution: a facile strategy and mechanistic interpretation

MnO2 flowery nanocomposites were explored as a novel and cost effective nanoadsorbent for the fast and efficient extraction of toxic inorganic contaminants from aqueous solution.

Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)

The detection of soluble Mn(III) is typically accomplished using strong complexing agents to trap Mn(III), but the generation of soluble Mn(III) induced by strong complexing agents has seldom been considered. In this study, pyrophosphate (PP), a nonredox active ligand, was chosen as a typical Mn(III) chelating reagent to study the influence of ligands on soluble Mn(III) formation in reactions involving Mn oxides and Mn(VII). The presence of excess PP induced the generation of soluble Mn(III)-PP from α- and δ-MnO₂ and led to the conproportionation reaction of α-, β-, δ-, or colloidal MnO₂ with Mn(II) at pH 7.0. Compared to MnO₂ minerals, colloidal MnO₂ showed much higher reactivity toward Mn(II) in the presence of PP and the conproportionation rate of colloidal MnO₂ with Mn(II) elevated with increasing PP dosage and decreasing pH. The generation of Mn(III) was not observed in MnO₄^-/S₂O₃^2- or MnO₄^-/NH₃OH⁺ system without PP while the introduction of excess PP induced the generation of Mn(III)-PP. Thermodynamic calculation results were consistent with the experimental observations. These findings not only provide evidence for the unsuitability of using strong ligands in quantification of soluble Mn(III) in manganese-involved redox reactions, but also advance the understanding of soluble Mn(III) generation in aquatic environment.