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This study investigated the geochemical controls on selenium mobility by two forms of birnessite mineral, focusing on the influence of pH, concentration, and the presence of cations. The research aimed to understand the dominant reaction mechanism (redox vs. sorption) under environmentally relevant selenium concentrations. Experiments were conducted to study the interaction of selenium with the birnessite minerals under varying conditions. The results demonstrated a significant shift in the dominant reaction mechanism from slow redox reactions observed at artificially elevated selenium concentrations to fast sorption processes at environmentally relevant concentrations, with second order rates increasing from 7.63 x 10-7 to 0.0224 L mol-1 h-1, respectively. This finding highlights the crucial role of selenium concentration in determining its interaction with birnessite minerals and emphasizes the importance of conducting research under realistic conditions to accurately predict selenium mobility and develop effective remediation strategies. [ABSTRACT FROM AUTHOR]

The structure, composition, and specificity of accumulation of trace elements by rounded nodules in gleyic soddy brown-podzolic soils (Gleyic Luvisol (Manganiferric)) of nature reserves and a national park in the south of the Far East have been studied by advanced analytical methods and noninvasive techniques. The nodules are characterized by pronounced differentiation into external (brown and ocher-brown, Fe-enriched, and dense) and internal (dark brown, Mn-enriched, and loose) zones. According to the distribution of Mn compounds in the internal zone, two types of nodules are identified: with an undifferentiated internal zone and with a core (cores). The cores contain C-enriched microzones, which are centers of Fe and Mn precipitation. The stages of coprecipitation of Fe and Mn and the stages with predominant precipitation of one of the elements are identified in the nodules. The nodules consist of a complex of minerals inherited from soils, as well as of nodule-specific minerals (goethite, feroxyhyte, and birnessite). The Fe content in the nodules is, on average, four times higher than that in the soil, the Mn content is 21.9 times higher, and the C content is 3.6 times higher. The most intensive accumulation in nodules is typical for Pb (EF = 5.53–12.14), which is determined by the joint participation of C- and Mn-containing compounds in its binding. Nickel (EF 0.89–5.81) and Cr (EF 1.22–2.60) are less actively accumulated; and the accumulation of V (EF 0.85–1.88) and Sr (EF 0.58–1.43) is weak. The phases accumulating Ni, Cr, V, and Sr are represented by nodules containing Fe and C. Zinc does not accumulate in the nodules. A comparison of the concentrations of water-soluble forms of trace elements reflects a decrease in the mobility of Cr, Pb, Ni, V, and Sr in nodules as compared to soils. [ABSTRACT FROM AUTHOR]

The large molybdenum (Mo) isotope fractionation from seawater is caused by the adsorption of Mo on manganese oxides. However, the effects of the manganese oxide mineralogy (crystal structure) and surface Mo coverage on Mo isotope fractionation have not been investigated. In this study, the isotope fractionation of Mo by adsorption on synthetic todorokite, birnessite, and δMnO2 was investigated under a wide range of surface Mo coverages. The Mo isotope fractionation changed from Δ98/95Mo = 2.18 ± 0.05‰ to 2.61 ± 0.06‰ for todorokite; from 1.25 ± 0.05‰ to 2.10 ± 0.05‰ for birnessite; and from 2.19 ± 0.07‰ to 2.73 ± 0.08‰ for δMnO2. The Mo isotope fractionations of the three manganese oxides were negatively correlated with surface coverage normalized to the specific surface area. The independence of the obtained correlation of the manganese oxide species indicates that the Mo isotope fractionation depends on the surface coverage but not on the mineralogy of the manganese oxides. The experimentally observed Mo isotope fractionation (<2.7‰) in manganese oxides generally underestimates the isotope fractionation in natural ferromanganese oxides (~3‰). According to the dependency of the Mo isotope fractionation on the surface coverage, the underestimation relative to previous experimental studies can be attributed to the lower Mo surface coverage of natural ferromanganese oxides. [ABSTRACT FROM AUTHOR]

Electron transfer between birnessite and organic compounds (OC) plays a dominant role in the coupling cycle of manganese (Mn) and carbon across diverse environmental settings. While previous studies have extensively investigated individual processes of interface Mn reduction, surface Mn2+ adsorption, and surface-to-interior electron transfer, the dynamic interplay among these reactions and the mechanisms regulating subtle changes in surface and interior Mn states remained poorly understood. Additionally, existing models have not adequately captured electron transfer kinetics in multivariable systems involving pH, Mn2+ concentration, electron donor type, etc. In this study, we investigated the reduction kinetics of birnessite under the influence of multiple environmental variables by employing three typical OC: formic acid (HCOOH), formaldehyde (HCHO), and methanol (CH 3 OH). Time-series analysis revealed kinetic discrepancy and time lag between the alteration of the average Mn oxidation state (AMOS) within the solid and the release of Mn2+ from the reductive dissolution of birnessite, indicating a two-stage electron transfer mechanism occurring at the interface between birnessite and OC. X-ray absorption fine structure spectra revealed a rapid increase in corner-sharing MnO 6 octahedra and a decline in AMOS during the initial stage, followed by a slight decrease in AMOS and substantial mineral dissolution to release Mn2+ in the subsequent stage. The transition point between the two stages is primarily influenced by the concentration of surface MnII under pH regulation, as confirmed by soft X-ray absorption spectroscopy and density functional theory calculations. Based on these findings, the adsorption equilibrium and electron transfer rate were modeled by a machine learning framework (JAX), which is influenced by three main factors: pH, Mn2+ concentration, and OC types. The adsorption equilibrium constant for HCHO was one order of magnitude lower than for HCOOH, yet displayed a faster reaction rate due to higher electron transfer rates. Competitive adsorption of OC and Mn2+ on reactive sites was influenced by both pH and Mn2+ concentrations. Combining these parameters, we created a 3D surface plot that comprehensively considered the interplay between different elementary reactions, including competitive adsorption and redox reaction rates, thereby visualizing the kinetic regulation mechanisms in multivariable systems. Furthermore, a comprehensive rate equation for the reduction of birnessite by OC was developed to predict its behavior in natural settings. With an electron storage capacity of 2.7×1023 electrons/mol Mn before structural decomposition or dissolution, we propose that birnessite can act as a geobattery driving cryptic elemental biogeochemical cycling. Our findings also suggest that the highly reversible redox reactivity of birnessite and the kinetics discrepancy in multi-step electron transfer reactions enable it to facilitate energy conversion among OC, sunlight, and microbes across a variety of temporal and spatial scales. [ABSTRACT FROM AUTHOR]

Sorption of antimony (Sb) onto birnessite significantly influences the fate of Sb in oceanic and terrestrial environments and fractionates Sb isotopes. Nevertheless, little is known about Sb isotopic fractionation during its adsorption on birnessite. Here, we show the value of Δ123Sb adsorbed-aqueous increases from −0.398 to −0.332 ‰ in 1 h and then decreases and stabilizes at −0.384 ‰ in 72 h. The enrichment of the light Sb isotope is predominantly due to the distortion of the octahedral symmetry. X-ray absorption spectroscopy results indicate Sb first forms a double-corner-sharing complex on birnessite and then transforms to a double-edge-sharing complex during adsorption. The optimized bond distances for double-corner-sharing (3.37 Å) and double-edge-sharing (2.90 Å) complexes calculated using density functional theory (DFT) fits well with the structure (3.41 and 3.00 Å) revealed by X-ray absorption spectroscopy, respectively. The fractionation derived from reduced partition function ratios calculated using DFT aligns well with the experimental results. Therefore, the variation in Sb isotopic fractionation during adsorption is attributed to the evolving structure of Sb complexes on birnessite. Our results demonstrate the isotopic fractionation of Sb during adsorption on birnessite and provide a molecular-scale understanding of Sb behavior, contributing to the correct reconstruction of the Sb isotope composition of ancient seawater using ferromanganese crusts and nodules, and efforts to trace Sb migration in epigenetic mining environments. [ABSTRACT FROM AUTHOR]

Bir-MnO2 was synthesized through a hydrothermal method and modified into H-Bir by nitric acid acidification. The mechanism of its action in ozonation was explored by characterization. H-Bir possessed more oxygen vacancies and lower average oxidation state (AOS) than Bir-MnO2. The oxygen vacancies and surface hydroxyl groups are the main active sites of this catalyst. Under the reaction conditions of initial pH=7 , ozone dosage of 7 mg/L , initial pollutant concentration of 10 mg/L , and catalyst dosage of 0.5 g/L for 40 min , the degradation rates of H-Bir catalyzed ozonation of p-nitrobenzoic acid (PNBA) and 4-chlorobenzoic acid (PCBA) reached 82.68% and 85.83%, respectively. Differences in the degradation factors affecting PCBA and PNBA were found in the performance study of H-Bir catalyzed ozonation. The combination of density functional theory (DFT) and catalytic ozonation experiment results revealed the difference between the degradation of PNBA and PCBA in the H-Bir/O3 system. It was deduced that the nitro, chlorine and carboxyl groups were the main reaction sites. Possible degradation pathways for the two pollutants were also proposed. [ABSTRACT FROM AUTHOR]

Adsorptive fractionation and oxidation of dissolved organic matter (DOM) on manganese (Mn) oxide surfaces alter the molecular composition and stability of DOM, but the impact of iron (Fe) oxides on the coupled adsorption-oxidation processes of DOM by Mn oxides is largely unknown. In this study, the underlying mechanisms of molecular transformation of DOM on birnessite (Bir) in the presence of ferrihydrite (Fh), with varying Fh/Bir mass ratios, were investigated at both molecular levels and microscopic scales with a suite of characterization techniques, including Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). We found that higher Fh/Bir mass ratio impeded the adsorption of DOM by birnessite and the reductive dissolution of birnessite to release dissolved Mn, and the impediment on the reductive dissolution of birnessite was proportional to Fh/Bir mass ratio. Overall, during the interactions of DOM with ferrihydrite and birnessite, phenolic compounds were preferentially adsorbed by Fe and Mn minerals, and compounds with higher oxygen contents and polymeric substances were formed. FT-ICR-MS analysis further suggested that higher Fh/Bir mass ratio inhibited the occurrence of aromatic ring-opening and carboxylation of substituted groups on aromatic rings, but promoted polymerization of phenolic compounds. Cs-STEM analysis revealed that DOM distributions on ferrihydrite, birnessite, and their mixtures were regulated by their microscopic structures and reactivity. Compared with aromatic carbon (C), carboxylic and phenolic C were more likely to associate with birnessite. Our results highlighted the significance of organo-mineral associations with the mixed minerals in regulating the distribution and reactivity of organic C. This study has provided molecular evidences for molecular transformation of DOM mediated by both Mn and Fe oxides, which contributed to advancing our understanding on coupled reactions of organic C at the mineral–water interfaces in the environment. [ABSTRACT FROM AUTHOR]

This study investigated the geochemical controls on selenium mobility by two forms of birnessite mineral, focusing on the influence of pH, concentration, and the presence of cations. The research aimed to understand the dominant reaction mechanism (redox vs. sorption) under environmentally relevant selenium concentrations. Experiments were conducted to study the interaction of selenium with the birnessite minerals under varying conditions. The results demonstrated a significant shift in the dominant reaction mechanism from slow redox reactions observed at artificially elevated selenium concentrations to fast sorption processes at environmentally relevant concentrations, with second order rates increasing from 7.63 x 10-7 to 0.0224 L mol-1 h-1, respectively. This finding highlights the crucial role of selenium concentration in determining its interaction with birnessite minerals and emphasizes the importance of conducting research under realistic conditions to accurately predict selenium mobility and develop effective remediation strategies.

In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average Δ60/58Ni dissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Ni dissolved-sorbed ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance. [ABSTRACT FROM AUTHOR]

• The transformation of birnessite and its environmental implications were reviewed. • The relationship between transformation pathways and mechanisms of birnessite and environmental conditions was proposed. • The stabilization mechanisms of heavy metals in the transformation process were summarized. Birnessite is ubiquitous in the natural environment where heavy metals are retained and easily transformed. The surface properties and structure of birnessite change with the changes in external environmental conditions, which also affects the fate of heavy metals. Clarifying the effect and mechanism of the birnessite phase transition process on heavy metals is the key to taking effective measures to prevent and control heavy metal pollution. Therefore, the four transformation pathways of birnessite are summarized first in this review. Second, the relationship between transformation pathways and environmental conditions is proposed. These relevant environmental conditions include abiotic (e.g., co-existing ions, pH, oxygen pressure, temperature, electric field, light, aging, pressure) and biotic factors (e.g., microorganisms, biomolecules). The phase transformation is achieved by the key intermediate of Mn(III) through interlayer-condensation, folding, neutralization-disproportionation, and dissolution-recrystallization mechanisms. The AOS (average oxidation state) of Mn and interlayer spacing are closely correlated with the phase transformation of birnessite. Last but not least, the mechanisms of heavy metals immobilization in the transformation process of birnessite are summed up. They involve isomorphous substitution, redox, complexation, hydration/dehydration, etc. The transformation of birnessite and its implication on heavy metals will be helpful for understanding and predicting the behavior of heavy metals and the crucial phase of manganese oxides/hydroxides in natural and engineered environments. [Display omitted] [ABSTRACT FROM AUTHOR]

The layered δ‐MnO2 (dMO) is an excellent cathode material for rechargeable aqueous zinc‐ion batteries owing to its large interlayer distance (~0.7 nm), high capacity, and low cost; however, such cathodes suffer from structural degradation during the long‐term cycling process, leading to capacity fading. In this study, a Co‐doped dMO composite with reduced graphene oxide (GC‐dMO) is developed using a simple cost‐effective hydrothermal method. The degree of disorderness increases owing to the hetero‐atom doping and graphene oxide composites. It is demonstrated that layered dMO and GC‐dMO undergo a structural transition from K‐birnessite to the Zn‐buserite phase upon the first discharge, which enhances the intercalation of Zn2+ ions, H2O molecules in the layered structure. The GC‐dMO cathode exhibits an excellent capacity of 302 mAh g−1 at a current density of 100 mA g−1 after 100 cycles as compared with the dMO cathode (159 mAh g−1). The excellent electrochemical performance of the GC‐dMO cathode owing to Co‐doping and graphene oxide sheets enhances the interlayer gap and disorderness, and maintains structural stability, which facilitates the easy reverse intercalation and de‐intercalation of Zn2+ ions and H2O molecules. Therefore, GC‐dMO is a promising cathode material for large‐scale aqueous ZIBs. [ABSTRACT FROM AUTHOR]

A fungus, Acremonium strictum KR21-2, produces biogenic manganese oxides (BMOs) that can oxidize exogenous Mn2+ ions to form different BMO phases. When other guest ions are present during the BMO formation, it can strongly affect the mineralogical characteristics of the resultant BMO phase. The impact of coexisting Ni2+ ions on the mineralogy of BMO phases formed through enzymatic Mn(II) oxidation and its sequestration ability is not yet fully understood. To better understand it, repeated sequestration experiments were conducted using BMOs in Ni2+/Mn2+ binary, single Ni2, and single Mn2+ solution systems with a pH range of 6.0 to 7.5. It was observed that simultaneous sequestration of Ni2+ and Mn2+ was efficient, with irreversible Ni2+ incorporation at pH values above 7.0. The resultant BMO phases showed that Ni2+-bearing Mn oxides resembling feitknechitite (β-MnOOH) were developed through enzymatic Mn(II) oxidation. At pH values below 6.5, the turbostratic birnessite structure was maintained even in Ni2+/Mn2+ binary solutions, and subsequently, the Ni2+ sequestration efficiency was low. The pseudo-first-order rate constants of enzymatically inactivated BMOs for Mn2+ sequestration were two orders of magnitude lower than those of active BMOs, indicating the crucial role of the enzymes in precipitating Ni2+-bearing Mn oxide phases. These findings provide new insights into the mechanism of Ni2+ interaction with Mn oxide through microbial activity under circumneutral pH conditions. [ABSTRACT FROM AUTHOR]

Anthropogenic emissions of vanadium (V) into terrestrial and aquatic surface systems now match those of geogenic processes, and yet, the geochemistry of vanadium is poorly described in comparison to other comparable contaminants like arsenic. In oxic systems, V is present as an oxyanion with a +5 formal charge on the V center, typically described as H x VO4 (3-x)-, but also here as V(V). Iron (Fe) and manganese (Mn) (oxy)hydroxides represent key mineral phases in the cycling of V(V) at the solid-solution interface, and yet, fundamental descriptions of these surface-processes are not available. Here, we utilize extended X-ray absorption fine structure (EXAFS) and thermodynamic calculations to compare the surface complexation of V(V) by the common Fe and Mn mineral phases ferrihydrite, hematite, goethite, birnessite, and pyrolusite at pH 7. Inner-sphere V(V) complexes were detected on all phases, with mononuclear V(V) species dominating the adsorbed species distribution. Our results demonstrate that V(V) adsorption is exergonic for a variety of surfaces with differing amounts of terminal -OH groups and metal-O bond saturations, implicating the conjunctive role of varied mineral surfaces in controlling the mobility and fate of V(V) in terrestrial and aquatic systems.

Manganese oxides are highly reactive minerals and influence the geochemical cycling of carbon, nutrients, and numerous metals in natural environments. Natural Mn oxides are believed to be dominantly formed by biotic processes. A marine Mn-oxidizing fungus Neoroussoella solani MnF107 was isolated and characterized in this study. SEM observations show that the Mn oxides are formed on the fungal hyphal surfaces and parts of the hypha are enveloped by Mn oxides. TEM observations show that the Mn oxides have a filamentous morphology and are formed in a matrix of EPS enveloping the fungal cell wall. Mineral phase analysis of the fungal Mn oxides by XRD indicates that it is poorly crystalline. Chemical oxidation state analysis of the fungal Mn oxides confirms that it is predominantly composed of Mn(IV), indicating that Mn(II) has been oxidized to Mn (IV) by the fungus. [ABSTRACT FROM AUTHOR]

Studies suggest that feitknechtite (β-MnOOH) is a prevalent, and perhaps necessary, intermediate phase during the synthesis of birnessite-like phases, the abiotic oxidation of Mn2+, and the transformation of biogenic hexagonal phyllomanganates to more complex Mn oxides in laboratory and natural systems. Researchers have generally described feitknechtite as consisting of pyrochroite-like (or cadmium iodide-like) Mn-O octahedral layers, but a detailed crystal structure has not been reported. We used TEM/SAED and powder XRD and Rietveld refinements to derive the unit cell and, for the first time, report a complete structure description for feitknechtite (β-MnOOH). Rietveld refinements were also completed for three natural feitknechtite/hausmannite samples, and time-resolved synchrotron XRD experiments were used to follow the thermal transformation of feitknechtite to hausmannite. Additionally, we identified and report the structure for a second, and perhaps novel, MnOOH polymorph (proposed designation ε-MnOOH), mixed with the synthetic feitknechtite, that is similar to β-MnOOH but with a different layer stacking. [ABSTRACT FROM AUTHOR]

Nanostructured first row transition metal (Mn, Fe, Co, Ni, and Cu) oxides (TMOs) have shown promise as catalysts for creation of new and ransformative technologies for manufacturing value-added chemicals that are energy- and atom-efficient. Most of the synthesis routes to TMOs involve harsh reaction conditions or prolonged preparation times. Herein, we use potassium superoxide (KO2), a commercially available stable salt of superoxide, as a viable oxidant for rapid but mild redox synthesis of birnessite type layered manganese dioxide (δ-MnO2) nanomaterials. These δ-MnO2 materials are synthesized in a fast (as fast as 5 min), ambient (room temperature), and convenient condition, employing a simple laboratory apparatus (grinding with mortar and pestle followed by washing with water). Characterization studies reveal a hierarchical porosity and sponge-like morphology for the δ-MnO2 nanomaterial, whereas the surface area of the material is tunable as a function of the adopted synthetic aspects. The δ-MnO2 materials deliver promising catalytic activity in the selective aerobic oxidation of 5-hydroxymethylfurfural alcohol (HMF) to 3,5-diformylfuran (DFF), an important probe reaction to transform biomass-derived feedstocks to useful chemicals. Density functional theory (DFT) is used to investigate the interaction of HMF with the catalyst surface and to chart out the energetics pathway of system relaxation, together showcasing various bond dissociations, intermediate steps, and rate limiting kinetics. [ABSTRACT FROM AUTHOR]

The symmetry in a polymorph is one of the most important elements for determining the inherent lattice nature. The MnO2 host tends to high‐symmetry MnO6 octahedra as a result of the electronic structure t2g3eg0 of Mn4+ ions, displaying an ordered structure accompanying with poor polarization loss and limiting its application toward high‐performance microwave absorbers. Here, a pseudo‐Jahn−Teller (PJT) distortion and PJT disordering design with abundant self‐forming interfaces and local magnetic domains in the monoclinic birnessite–MnO2 host is first reported. The PJT distortion can give rise to asymmetric MnO6 octahedra, inducing the formation of interfaces and increased electron spin magnetic moment in the lattice. The resultant birnessite with PJT distortions and PJT disordering delivers an outstanding reflection loss value of −42.5 dB at an ultralow thickness of 1.7 mm, mainly derived from the excited interfacial polarization and magnetic loss. This work demonstrates an effective approach in regulating the lattice structure of birnessite for boosting microwave absorption performance. [ABSTRACT FROM AUTHOR]

Using the sol–gel method, by reacting solutions of maleic acid and potassium permanganate, samples have been obtained with a turbostat stacking of layers, which is characteristic of the birnessite structure. Using X-ray phase and thermogravimetric analysis, Raman spectroscopy, and low-temperature nitrogen adsorption, the dependence of the textural–structural characteristics of the synthesized materials on the temperature of thermal modification (150, 360°C) and sample composition (Sr-form, H-form) has been shown. It has been found that all samples are characterized by a mesoporous texture. It is shown that the water content in the samples is closely related to the quantitative determination of the specific surface area: the more water, the greater the SBET value. [ABSTRACT FROM AUTHOR]

A black substance exuding from fractures was observed in 2012 in Ytterby mine, Sweden, and identified in 2017 as birnessite with the composition Mx[Mn(III,IV)]2O4∙(H2O)n. M is usually calcium and sodium, with x around 0.5. The Ytterby birnessite is unique, with M being calcium, magnesium, and also rare earth elements (REEs) constituting up to 2% of the total metal content. The biogenic origin of the birnessite was established in 2018. Analysis of the microbial processes leading to the birnessite formation and the REE enrichment has continued since then. The process is fast and dynamic, as indicated by the depletion of manganese and of REE and other metals in the fracture water during the passage over the precipitation zone in the mine tunnel. Studies of the exchangeability of metals in the structure are the main objective of the present program. Exposure to solutions of sodium, calcium, lanthanum, and iron led to exchanges and altered distribution of the metals in the birnessite, however, generating phases with almost identical structures after the exchanges, and no new mineral phases were detected. Exchangeability was more efficient for trivalent elements (REE) over divalent (calcium) and monovalent (sodium) elements of a similar size (ionic radii 90–100 pm). [ABSTRACT FROM AUTHOR]

Manganese(III/IV) oxide is a promising pseudocapacitive material for supercapacitor electrodes due to favorable attributes such as its chemical resilience, high earth abundance and low specific cost. Herein, the morphological, compositional and electrochemical characteristics of co‐precipitated manganese(III/IV) oxide products, each described by the general formula NaxKyMnOz, are investigated to establish how these properties are influenced by synthesis conditions. NaxKyMnOz growths in low‐temperature (<100 °C) basic and acidic environments are shown to promote the formation of turbostratic birnessite and cryptomelane phases, respectively, with the latter polymorph containing a relatively low concentration of interstitial Na+ and K+ cations. It is demonstrated that K+ pre‐insertion during synthesis yields lower initial charge‐transfer resistances than equivalent Na+ intercalation, and that this parameter correlates strongly with storage performance. Accordingly, Na‐mediated storage initially delivers inferior specific capacitances and Coulombic efficiencies than K‐based mechanisms, but K+ intercalation/deintercalation causes faster capacitance decay during prolonged galvanostatic cycling. Furthermore, whilst crystallographic phase is shown to have a weaker effect on NaxKyMnOz storage properties than the choice of intercalating guest cations, cryptomelane electrodes are more susceptible to cycling‐induced capacitance and efficiency losses than their birnessite counterparts. In combination, these insights provide an instructive foundation for the optimization of NaxKyMnOz in high‐power storage applications. [ABSTRACT FROM AUTHOR]

This study examined effective removal methods for high amounts of manganese (Mn) and zinc (Zn) in acid mine drainage (AMD) by addition of different neutralizing agents (NaOH and NaClO) and synthesized birnessite (δ-MnO2) using two-type AMD samples which Mn and Zn concentrations were 778 and 410 mg L−1 for A mine and 18.0 and 5.51 mg L−1 for B mines, respectively. The precipitation mechanism of these metal ions was investigated by geochemical modeling (PHREEQC) and X-ray absorption near edge structure (XANES) analysis. Mn concentrations were below the effluent standard (10 mg L−1) at pH 9–10 with the NaOH neutralization, whereas it was accomplished at lower pH (6–7) condition with the NaClO addition; it could act as an oxidizing agent, resulting that most of Mn precipitated as δ-MnO2. Zn concentrations decreased below the effluent standard (2 mg L−1) at pH 8–9 using both neutralizing agents. XANES analysis results indicated Zn was removed by the surface complexation formation on manganite and δ-MnO2 surface. More effective removal of Mn and Zn from AMD was found around pH 6 when a sufficient amount of δ-MnO2 was added to both AMD before the NaOH neutralization; a geochemical model coupling charge distribution multisite ion complexation revealed the triple-corner-sharing on δ-MnO2 was the most reasonable mechanism. Our result suggests that the presence of sufficient δ-MnO2 was the most effective for high Mn and Zn contents AMD treatment; however, ferrous ion (Fe2+) can inhibit the adsorption reaction and decompose δ-MnO2. Thus, pre-precipitation of Fe2+ is required to enhance the effect of δ-MnO2 on Mn and Zn removals from AMD.

Abstract Manganese(III/IV) oxide is a promising pseudocapacitive material for supercapacitor electrodes due to favorable attributes such as its chemical resilience, high earth abundance and low specific cost. Herein, the morphological, compositional and electrochemical characteristics of co‐precipitated manganese(III/IV) oxide products, each described by the general formula NaxKyMnOz, are investigated to establish how these properties are influenced by synthesis conditions. NaxKyMnOz growths in low‐temperature (

Birnessite recharging processes in neutral (sodium sulfate) and alkaline (sodium hydroxide) solutions are compared. Birnessite is fabricated by cathodic deposition from alkaline permanganate bath on smooth carbon supports. The hypothesis is formulated and verified concerning the reason of much wider potential interval available for birnessite recharging in neutral solutions as compared to alkaline. Namely, an apparent width of this interval observed in neutral solutions is found to depend on birnessite loading and convection in solution. These observations can be explained by the changes of local pH in the course of recharging in neutral medium: seeming extension of the potential window results from the screened shift of the onset(s) of pH-dependent irreversible process(es). The effects of cathodic and anodic potential limits on birnessite recharging are addressed systematically. The total charges corresponding to reversible birnessite behavior in neutral (Na2SO4) and alkaline (NaOH) solutions are found to be very close, despite the potential interval is apparently wider for the former solution. The advantages and risks of recharging in neutral media are considered. [ABSTRACT FROM AUTHOR]

Birnessite minerals help control the fate of Zn in surface environments and readily fractionate Zn isotopes through adsorption reactions, yet little is known about the role played by various reactive sites in stable isotopic fractionation. Here we present the Zn isotope fractionation data cause by adsorption on birnessite under different reaction times, pH values, and Zn concentrations. We observe that isotopic equilibrium of Zn is attained after ∼120 h of reaction time at pH 6. At pH 3–5 and Zn concentrations of 0.05–0.3 mM, the isotopic fractionation (Δ66Zn adsorbed-aqueous) is around −0.46 ± 0.04‰, and gradually increases to −0.09 ± 0.05‰ at pH 6–8 and Zn concentrations of 0.2 mM. The change in Zn isotopic compositions as a function of pH and Zn concentration is well described using the surface complexation model, where two binding sites are involved: external edge sites and interlayer vacancies. According to this model, two different isotopic fractionation factors of Zn are calculated: Δ66Zn adsorbed-aqueous = −0.46 ± 0.04‰ for adsorption on vacancy sites and Δ66Zn adsorbed-aqueous = 0.52 ± 0.04‰ for binding to edge sites. Extended X-ray absorption fine structure spectroscopy (EXAFS) demonstrates that Zn forms triple-corner-sharing (TCS) octahedral complex on birnessite vacancies at pH 3 and Zn concentrations of 0.05–0.2 mM, where Zn is coordinated on one side to three oxygen atoms of the Mn vacancy (∼2.03 Å) and to three water molecules on the other side (∼2.15 Å), suggesting the formation of distorted Zn O octahedra (average bond length: ∼2.09 Å). At pH 6 and 8, double-corner-sharing (DCS) complexes on layer edges formed in addition to the TCS octahedral complex on vacancies. Density functional theory (DFT) optimisations suggest that DCS Zn complex exist in tetrahedral coordination. Based on EXAFS spectroscopy, DFT optimisations and surface complexation modeling, the distinct isotopic fractionation of Zn is related to the differences in Zn local structure at different reactive sites of birnessite. Our results provide a molecular-scale understanding of Zn isotopic fractionation in natural birnessite-containing settings, as well as new insights into predicting the links between adsorption and fractionation of other similar metals. [ABSTRACT FROM AUTHOR]

Birnessite has been widely used for electrochemical removal of heavy metals due to its high pseudocapacitance. Incorporation of carbon-based materials into birnessite can enhance its conductivity and stability, and synergistically improve the electrochemical adsorption capacity due to the double-layer capacitor reaction derived from carbon-based materials. In this study, biochar was successfully incorporated with birnessite at various ratios to synthesize composites (BC-Mn) for effective electrochemical removal of cadmium (Cd(II)) from water. The effects of cell voltage, initial pH, and recycling performance of BC-Mn were evaluated. As a result, the electrosorption capacity of BC-Mn for Cd(II) exhibited gradual increases with increasing birnessite content and reached equilibrium at a Mn content of 20% (BC-Mn20). The Cd(II) adsorption capacity of BC-Mn20 rose at higher cell voltage, and reached the maximum at 1.2 V. At pH 3.0–6.0, the electrosorption capacity initially rose until pH 5.0 and then approached equilibrium with a further increase in pH value. The Cd(II) electrochemical adsorption capacity of BC-Mn20 in the solution could reach 104.5 mg g−1 at pH 5.0 for 8 h at 1.2 V. Moreover, BC-Mn20 exhibited excellent reusability with a stability of 95.4% (99.7 mg g−1) after five cycles of reuse. Due to its superior heavy metal adsorption capacity and reusability, BC-Mn20 may have a promising prospect in the remediation of heavy metal polluted water. [ABSTRACT FROM AUTHOR]

Birnessite MxMnO2 (M = Na+, K+, etc.) has emerged as a promising alternative to the classical MnO2 material owing to its improved pseudocapacitive performance for energy storage. Understanding their structure–property correlation is essential for the development and application of advanced supercapacitors. Herein, we adopt the crystal field theory and density functional simulation to reveal the structural dependence of the pseudocapacitive property of KxMnO2. Attributing to the Jahn–Teller effect of Mn3+, the bandgap of KxMnO2 can be tuned by changing the x value (i.e., the Mn(III)/Mn(IV) ratio). Then, we design a narrow‐bandgap K0.25MnO2 (0.84 eV), which affords a high capacitance of 415 F g−1 at 1 A g−1 and a desirable rate capability of 293 F g−1 at 20 A g−1. Operando Raman spectroscopy confirms that the Jahn–Teller induced structure evolution of [MnO6] octahedron accounts for the superior pseudocapacitive behavior of K0.25MnO2. This finding offers theoretical guidance to the design and application of birnessite materials for pseudocapacitors. [ABSTRACT FROM AUTHOR]

Defect strategy is one of the most effective approaches for enhancing the efficiency of Mn-based catalysts in removing formaldehyde (HCHO) under ambient conditions. Herein, the interlayer Mn-O octahedra in birnessite synergistically form with Mn vacancies through proton exchange in acidic solutions, creating surface Frenkel defects. As evidenced by highly combined physicochemical characterization and density functional theory calculations, surface Frenkel defects are highly active sites on the surface of birnessite. The surface Frenkel defects can induce surface electronic reconstruction, generating strong electrophilicity, which promotes the evolution of superoxide radicals (O 2 •−) into singlet oxygen (1O 2). In-situ quenching DRIFTS indicates that 1O 2 is the most important ROS in the oxidation process of HCHO. Additionally, the surface Frenkel defects enhance the adsorption of HCHO, promoting the dehydrogenation of HCHO to form CO, which is then rapidly oxidized to CO 2 and H 2 O under the action of 1O 2. This provide a more effective kinetic and thermodynamic catalytic pathway. The defect-rich birnessite (R-Bir) achieved complete HCHO removal in the dynamic test at 10 ppm within 10 hours, significantly outperforming defect-free birnessite (F-Bir) and most previously reported catalysts. This study precisely reveals the catalytic mechanism of HCHO over R-Bir and provides valuable insights for the design of high-performance environmental catalysts. [Display omitted] • Frenkel defects enhance birnessite's catalytic activity for HCHO oxidation. • Frenkel defects promote formation of reactive singlet oxygen 1O 2. • 1O 2 plays a crucial role in the catalytic oxidation of HCHO. • Electrophilic defects improve HCHO adsorption and conversion efficiency. [ABSTRACT FROM AUTHOR]

[Display omitted] • The electron transfer of Mn(III) and O 2 drives the generation of ·O 2 –. • Oxygen vacancies replenish surface hydroxyls, maintaining reactivity. • Findings reveal Mn(Ⅲ) and oxygen vacancies' synergistic interaction. Birnessite exhibits a significant oxidative effect on formaldehyde. Mn(Ⅲ) and oxygen vacancies are critical indicators for assessing the activity of birnessite; however, their mutual interaction remains unclear. Herein, birnessite samples were synthesized to oxidize formaldehyde by adjusting the molar ratio of Mn(Ⅶ) to Mn(Ⅱ). The results indicated that this ratio influences the Mn(Ⅲ) content and oxygen vacancy concentration in birnessite. Mn(Ⅲ) can generate superoxide radicals through electron transfer with oxygen, thereby dominating the oxidation of formaldehyde. Surface hydroxyl groups facilitate the conversion of intermediate species, specifically dioxymethylene (DOM) and formate, during the oxidation of formaldehyde. Oxygen vacancies compensated for the surface hydroxyl groups consumed during the reaction, thereby maintaining the reactivity of birnessite. Ultimately, formaldehyde is converted to CO 2 and H 2 O through the synergistic action of Mn(Ⅲ) and oxygen vacancies. The degradation rate of formaldehyde by birnessite, with a Mn ratio of 0.6 (0.6Bir), remained stable at 100 % when tested at room temperature, with a WHSV of 600 L/g·h for 10 h. These findings provide valuable insights into the synergistic interaction between Mn(III) and oxygen vacancies during the removal of formaldehyde by birnessite. [ABSTRACT FROM AUTHOR]

It is widely accepted that soil organic carbon cycling depends on the presence and catalytic functionality of extracellular enzymes. Recent reports suggest that combusted and autoclaved soils may have the capacity to degrade organic test substrates to a larger extent than the living, enzyme-bearing soils. In search of the underlying mechanisms, we adsorbed Beta-Glucosidase (BG) and Bovine Serum Albumin (BSA) on the phyllosilicate kaolinite and the manganese oxide birnessite at pH 5 and pH 7. The protein-mineral samples were then subjected to gradual energy inputs of a magnitude equivalent to naturally occurring wildfire events. The abundance and molecular masses of desorbed organic compounds were recorded after ionization with tunable synchrotron vacuum ultraviolet radiation (VUV). The mechanisms controlling the fate of proteins varied with mineralogy. Kaolinite adsorbed protein largely through hydrophobic interactions and, even at large energy inputs, produced negligible amounts of desorption fragments compared to birnessite. Acid birnessite adsorbed protein through coulombic forces at low energy levels, became a hydrolyzing catalyst at low energies and low pH, and eventually turned into a reactant involving disintegration of both mineral and protein at higher energy inputs. Fragmentation of proteins was energy dependent and did not occur below an energy threshold of 0.20 MW cm−2. Neither signal abundance nor signal intensity were a function of protein size. Above the energy threshold value, BG that had been adsorbed to birnessite at pH 7 showed an increase in signal abundance with increasing energy applications. Signal intensities differed with adsorption pH for BSA but only at the highest energy level applied. Our results indicate that proteins adsorbed to kaolinite may remain intact after exposure to such energy inputs as can be expected to occur in natural ecosystems. Protein fragmentation and concomitant loss of functionality must be expected in surface soils replete with pedogenic manganese oxides. We conclude that minerals can do both: protect enzymes at high energy intensities in the case of kaolinite and, in the case of birnessite, substitute for and even exceed the oxidative functionality that may have been lost when unprotected oxidative enzymes were denatured at high energy inputs.

A very low concentration of manganese (Mn) in water is a critical issue for municipal and industrial water supply systems. Mn removal technology is based on the use of manganese oxides (MnOx), especially manganese dioxide (MnO2) polymorphs, under different conditions of pH and ionic strength (water salinity). The statistical significance of the impact of polymorph type (akhtenskite ε-MnO2, birnessite δ-MnO2, cryptomelane α-MnO2 and pyrolusite β-MnO2), pH (2–9) and ionic strength (1–50 mmol/L) of solution on the adsorption level of Mn was investigated. The analysis of variance and the non-parametric Kruskal–Wallis H test were applied. Before and after Mn adsorption, the tested polymorphs were characterized using X-ray diffraction, scanning electron microscope techniques and gas porosimetry analysis. Here we demonstrated the significant differences in adsorption level between MnO2 polymorphs' type and pH; however, the statistical analysis proves that the type of MnO2 has a four times stronger influence. There was no statistical significance for the ionic strength parameter. We showed that the high adsorption of Mn on the poorly crystalline polymorphs leads to the blockage of micropores in akhtenskite and, contrary, causes the development of the surface structure of birnessite. At the same time, no changes in the surfaces of cryptomelane and pyrolusite, the highly crystalline polymorphs, were found due to the very small loading by the adsorbate. [ABSTRACT FROM AUTHOR]

Detailed mineralogical and textural studies, combined with sequential X-ray diffraction and geochemical modeling, helped to solve the "copper pitch/wad" enigma in the Exótica deposit located downstream of the Chuquicamata porphyry copper deposit. Copper pitch and copper wad are essentially chrysocolla with co-precipitated Mn oxides, mainly birnessite, as well as pseudo-amorphous Mn oxide/oxyhydroxides. Linking the mineralogical, geochemical, and textural evidences with the geological, tectonic, and climatic evolution of the Chuquicamata–Calama area, a four-step genetic model for the evolution of the Exótica deposit is presented: (A) formation of a mature supergene enrichment profile at Chuquicamata (~ 30–25 Ma to ~ 15 Ma) during an erosion-dominated regime (∼900 m of erosion) which was accompanied by acidic (pH ∼2–4) Cu-Mn-Si-dominated rock drainage (ARD) with fluid flow southwards through the Exótica valley towards the Calama Basin, resulting in a strongly kaolinized and chrysocolla/copper wad-impregnated bedrock of the Exótica deposit; (B) deposition of the Fortuna gravels in the Exótica valley (starting ∼19 Ma) intercepted the Cu-Mn-Si-dominated ARD, triggering the main chrysocolla, copper pitch/wad mineralization as syn-sedimentary mineralization by chiefly surficial flow in strongly altered gravels; (C) tectonic freezing and onset of hyper-aridity (∼15–11 Ma) exposed the enriched chalcocite blanket of Chuquicamata to oxidation, resulting in acidic (pH ~ 2–4) and Cu-Si-dominated solutions with less Mn. These solutions percolated in a slightly more reducing groundwater flow path and mineralized relatively unaltered gravels with pure chrysocolla; and (D) ingression of confined chloride-rich groundwater in the upper oxidation zone of Chuquicamata, most likely between 6 and 3 Ma, is responsible for the atacamite/brochantite mineralization (pH ~ 5.5–7) of mainly unaltered gravels in the northern and central part of the Exótica deposit. [ABSTRACT FROM AUTHOR]

Mn(II)-oxidizing organisms promote the biomineralization of manganese oxides with specific textures, under ambient conditions. Controlling the phases formed and their texture on a larger scale may offer environmentally relevant routes to manganese oxide synthesis, with potential technological applications, for example, for energy storage. In the present study, we sought to use biofilms to promote the formation of electroactive minerals and to control the texture of these biominerals down to the electrode scale (i.e., cm scale). We used the bacterium Pseudomonas putida strain MnB1 which can produce manganese oxide in a biofilm. We characterized the biofilm–mineral assembly using a combination of electron microscopy, synchrotron-based X-ray absorption spectroscopy, X-ray diffraction, thermogravimetric analysis and electron paramagnetic resonance spectroscopy. Under optimized conditions of biofilm growth on the surface of current collectors, mineralogical characterizations revealed the formation of several minerals including a slightly crystalline MnOx birnessite. Electrochemical measurements in a half-cell against Li(0) revealed the electrochemical signature of the Mn4+/Mn3+ redox couple indicating the electroactivity of the biomineralized biofilm without any post-synthesis chemical, physical or thermal treatment. These results provide a better understanding of the properties of biomineralized biofilms and their possible use in designing new routes for one-pot electrode synthesis. [ABSTRACT FROM AUTHOR]

The formation of manganese (Mn) oxides is influenced by environmental conditions and, in some red soils, Mn oxides occur as coatings on the surface of kaolinite particles in the form of colloidal films or fine particles. The present study aimed to explore the types of formation mechanisms of Mn oxide minerals on the surface of kaolinite. Mn oxide minerals synthesized by reducing the Mn in KMnO4 with a divalent Mn salt (MnSO4) were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of various initial molar ratios of Mn2+/Mn7+ (R = 1:0.67, 1:1, 1:2, and 1:4), cationic species (Na+ or Mg2+), synthesis temperatures (30, 60, and 110°C), and amount of added kaolinite (0.25, 0.5, 1.0, 2.0, and 5.0 g) on the formation of Mn oxides were studied. The results showed that Mn oxide mineral types were affected by the initial R value and the background cation. With decreases in the initial R value, the synthesized minerals transformed from cryptomelane to birnessite. The relative mass ratios of kaolinite to Mn oxide were calculated as 1:0.92, 1:0.63, 1:1.15, and 1:1.63. The sodium cation (Na+) had a greater role than Mg2+ in promoting the dissolution–recrystallization of birnessite to cryptomelane. The synthesis temperature had no effect on mineral types, but Mn content increased as temperature increased. When the amount of added kaolinite was increased from 0.25 to 5.0 g, Mn oxide minerals formed gradually and transformed from birnessite to cryptomelane. This work revealed a possible formation process and reaction mechanism on the surface of kaolinite particles in some red soils. [ABSTRACT FROM AUTHOR]

Microbial manganese (Mn) oxidation plays a critical role in Mn(III/IV) oxide formation in modern day environmental systems. These oxidation processes and resulting biominerals are sensitive to aqueous conditions, such as pH and dissolved constituent concentrations. With Mn and sulfur (S) biogeochemical cycling closely linked in many environmental systems, and dissolved organic sulfur comprising a substantial pool of total sulfur in several environments, the impact of dissolved organic sulfur compounds on Mn redox processes is important to consider. Sulfonic acids, environmentally ubiquitous organosulfur compounds, play substantial roles in S cycling in many natural and contaminated systems. Research to assess the effects of these abundant sulfonic acids on Mn biogeochemical cycling, microbial Mn oxidation processes, and Mn biominerals is needed for understanding and predicting the impact of coupled S and Mn biogeochemical cycles, particularly in environments with dynamic redox gradients or in anthropogenically contaminated systems. Further, with research on microbial and abiotic Mn oxidation processes often using aminosulfonic acids to control pH, understanding the impact of such sulfonic acids on microbial Mn oxidation processes is critical. Several recent studies found that commonly used zwitterionic N substituted aminosulfonic acids, known as Good's buffers, such as HEPES and MES, can alter abiotic birnessite sheet structures. Here we investigate the impact of two sulfonic acids with broad applications to natural and contaminated sites as well as laboratory settings (HEPES and MES) on fungal Mn oxidation relative to a carbonate buffer and a buffer-free control by three Ascomycete fungi known to oxidize Mn(II): Stagonospora sp. SRC1lsM3a, Paraphaeosphaeria sporulosa AP3s5–JAC2a, and Plectosphaerella cucumerina DS2psM2a2. Structural analyses of the products show that sulfonic acids promote Mn oxidation by P. cucumerina , producing hexagonally symmetric phyllomanganates analogous to hexagonal birnessite or c–disordered H+ birnessite [(Ca,Na,K)(Mn4+,Mn3+,□)O 2 n H 2 O], with solid–associated Mn(II) bound to vacancy sites and biomass, while in their absence almost all Mn remains as either aqueous Mn(II) or solid–associated Mn(II) bound to biomass. In contrast, sulfonic acids exert the opposite effect on Mn oxidation by P. sporulosa , with their presence suppressing Mn(II) oxidation to Mn(IV), likely leading to the formation of mycogenic bixbyite (Mn3+ 2 O 3) while the buffer–free control forms a poorly crystalline phyllo- or tectomanganate. Meanwhile, all treatments exert a minimal effect on Mn(II) uptake from solution and Mn oxidation with Stagonospora sp., with all experimental systems and controls forming poorly crystalline, hexagonally symmetric phyllomanganates. The fact that the sulfonic acids here studied exert similar effects on Mn oxidation, but substantially different effects for each fungus, suggests they affect Mn oxidation via mechanistically similar pathways that are likely dependent on interactions with fungal exudates (which vary from species to species) or specific fungal Mn oxidation processes. Interestingly, for all fungi, MES increases Mn(III) in the resulting biominerals, while the carbonate buffer consistently decreases Mn(III). These results clearly demonstrate that sulfonic acids not only alter Mn oxide structures, as has been previously noted in abiotic studies, but can interfere with Mn oxidation reactions themselves, highlighting the incredible sensitivity of both Mn oxide structures and the Mn oxidation process to the aqueous environment. [ABSTRACT FROM AUTHOR]

Integrating MnO 2 with carbon is a reliable strategy to improve capacitive deionization (CDI) performance by leveraging the unique properties of both components (i.e., MnO 2 and carbon). However, the influences of preliminary functionalization of carbon (e.g., nitrogen doping, KOH activation) and pairing of cathodes and anodes on the CDI performance have yet to be systematically explored. Herein, we prepared a group of MnO 2 -decorated mesoporous carbon composites with nitrogen as a dopant (i.e., MK-NMCS, K-NMCS, NMCS, and CS), and systematically evaluated the desalination performance of various cathode//anode pairs in a hybrid capacitive deionization (HCDI) for capturing Na + , Cu 2+ , and Pb 2+ , respectively. Of all electrodes, the MK-NMCS//K-NMCS pair demonstrates the optimum desalination performance based on salt adsorption capacity (SAC) and cycling stability, offering a SAC of 25.4 mg g -1 and a SAC retention of 102.4% after 50 consecutive charge-discharge cycles at 1.2 V in 500 ppm of NaCl solution. In addition, the MK-NMCS//K-NMCS electrodes also show the maximum ion adsorption capacity (IAC) toward Cu 2+ and Pb 2+ ions compared to other cathode//anode pairs, attaining an IAC of 37.0 and 30.0 mg Cu 2+ per gram electrode materials at 1.2 V in 500 and 200 ppm of Cu 2+ solutions, respectively (cf. 32.2 mg of Pb 2+ per gram of electrode materials in 200 ppm of Pb 2+ solution). Besides, these electrodes exhibit excellent cycling stability when applied in removing each heavy metal ion separately, with IAC retentions of 90.0 and 98.5% after 50 cycles toward Cu 2+ and Pb 2+ ions, respectively. Mechanical analysis reveals that both heavy metals are likely to be sequestered via capacitive electrosorption by carbon, intercalation with MnO 2 , and surface complexation at the external surface of the [MnO 6 ] octahedral layers. Our results demonstrated a great potential of the MnO 2 -decorated N-doped carbon//prefunctionalized carbon pairs, in particular, the MK-NMCS//K-NMCS electrode pair for capturing heavy metal ions via HCDI platforms. Such prefunctionalization and pairing strategies are very promising for screening high-performance composite electrodes for wastewater remediation.

Although birnessite‐type manganese dioxide (δ‐MnO2) with a large interlayer spacing (≈7 Å) is a promising cathode candidate for aqueous Zn/MnO2 batteries, the poor structural stability associated with Zn2+ intercalation/deintercalation limits its further practical application. Herein, δ‐MnO2 ultrathin nanosheets are coupled with reduced graphene oxide (rGO) via van der Waals (vdW) self‐assembly in a vacuum freeze‐drying process. It is interesting to find that the presence of vdW interaction between δ‐MnO2 and rGO can effectively suppress the layered‐to‐spinel phase transition in δ‐MnO2 during cycling. As a result, the coupled δ‐MnO2/rGO hybrid cathode with a sandwich‐like heterostructure exhibits remarkable cycle performance with 80.1% capacity retained after 3000 cycles at 2.0 A g−1. The first principle calculations demonstrate that the strong interfacial interaction between δ‐MnO2 and rGO results in improved electron transfer and strengthened layered structure for δ‐MnO2. This work establishes a viable strategy to mitigate the adverse layered‐to‐spinel phase transition in layered manganese oxide in aqueous energy storage systems. [ABSTRACT FROM AUTHOR]

Biogenic Mn oxides (BMOs) have become captivating with regard to elemental sequestration, especially at circumneutral pH conditions. The interaction of BMOs with oxyanions, such as vanadate (V), molybdate (VI), and tungstate (VI), remains uncertain. This study examined the sequestration of V(V), Mo(VI), and W(VI) (up to ~1 mM) by BMOs formed by the Mn(II)-oxidizing fungus, Acremonium strictum KR21-2. When A. strictum KR21-2 was incubated in liquid cultures containing either Mo(VI) or W(VI) with soluble Mn2+, the oxyanions were sequestered in parallel with enzymatic Mn(II) oxidation with the maximum capacities of 8.8 mol% and 28.8 mol% (relative to solid Mn), respectively. More than 200 μM V(V) showed an inhibitory effect on growth and Mn(II) oxidizing ability. Sequestration experiments using preformed primary BMOs that maintained the enzymatic Mn(II) oxidizing activity, with and without exogenous Mn2+, demonstrated the ongoing BMO deposition in the presence of absorbent oxyanions provided a higher sequestration capacity than the preformed BMOs. X-ray diffraction displayed a larger decline of the peak arising from (001) basal reflection of turbostratic birnessite with increasing sequestration capacity. The results presented herein increase our understanding of the role of ongoing BMO formation in sequestration processes for oxyanion species at circumneutral pH conditions. [ABSTRACT FROM AUTHOR]

Manganese oxides (MnO x) have attracted much attention due to abundant resource, low cost and eco-friendliness. In this study, birnessite type manganese dioxide/expanded graphite composites (KMO/EG) with a reduced lattice spacing of nanoflower and nanowire heterostructure KMO have been synthesized by a one-step hydrothermal method. The morphology of KMO has transformed from nanoflower to nanowire with a reduced lattice spacing due to nucleation sites on the surface of EG. The KMO/EG achieves a specific capacity of 444.5mAh g −1 and remains at 387.9mAh g −1 after 100 cycles at 0.1 A g −1 for Zn-ion battery. The enhanced specific capacity of KMO/EG is mainly attributed to the capacity contribution of EG and the good stability is related to the more stable structure of KMO caused by reduced lattice spacing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Self-supported titanium-doped nanosheet arrays (Ti-MnO 2 /CC) were obtained. • The obtained electrocatalysts exhibit excellent catalytic activities and stability. • Possible enhanced mechanism has been convinced. Efficient and sustainable hydrogen production via water electrolysis relies on the utilization of highly active electrocatalysts for the oxygen evolution reaction (OER). Birnessite (δ-MnO 2), a type of manganese oxide with a local atomic structure similar to the oxygen-evolving complex in photosystem II, has emerged as a highly promising OER catalyst. Despite considerable efforts have been dedicated to enhancing its activity, the crucial aspect of stability has been often overlooked. Herein, Ti ions have been selectively incorporated into the in-layered lattice of δ-MnO 2 nanosheet arrays using a hydrothermal method, aiming to enhance the stability without reducing its activity. The resulting optimal Ti-MnO 2 catalyst demonstrates enhanced OER activity in alkaline electrolyte, exhibiting an impressive 85 mV reduction in overpotential at 10 mA cm−2, surpassing the initial MnO 2 (495 mV reduced to 410 mV). Remarkably, it shows exceptional durability, with negligible performance decrease observed over the entire 50-hour testing period. It has been demonstrated that the incorporation of Ti serves a dual purpose: it adjusts the electronic structure around Mn, enhancing activity, and simultaneously stabilizes the Mn3+ active sites, resulting in exceptional durability. Furthermore, density functional theory (DFT) calculations provide additional confirmation of the optimized electronic structure of MnO 2 achieved through Ti doping, leading to a reduction in the free energy of adsorbed intermediates and expediting the kinetics of the OER process. Our findings open a pathway for enhancing OER stability and activity by doping electrocatalytic inert ions into the lattice of MnO 2 , applicable not only to Mn-based oxides but also other non-precious metal-based electrocatalysts. [ABSTRACT FROM AUTHOR]

Birnessite is a phyllomangante mineral widely found in surficial environments. Its morphology not only affects its adsorption and oxidation properties but also indicate changes in the climatic conditions of ancient Earth. This study explored how preparative parameters affect the morphology and reactivity birnessite. Synthesized at boiling temperatures, birnessite nanoflowers measured 265 nm with a specific surface area (SSA) of 34.87 m2/g, while at 65 °C, they increase to 2251 nm with a similar SSA of 30.33 m2/g. Lowering KMnO 4 or HCl concentrations increased nanoflower size. Reduced concentration of KMnO 4 hindered Coulombic forces, fostering a parallel petal arrangement and a higher SSA (59.11 m2/g). Lower concentration of HCl led to perpendicular petals and a reduced SSA (12.33 m2/g). Decreased concentrations of both KMnO 4 and HCl reduced initial δ-MnO 2 concentration, allowing time for edge-to-edge assembly and nanoflake substrate formation. Subsequently, δ-MnO 2 vertically assembled on substrates to form microwalls with SSA of 85.39 m2/g. Organic acids as capping agents disrupted this assembly. Adsorption tests for Cd2+ revealed microwalls achieved 7102 mmol/kg, surpassing original birnessite nanoflowers at 2114 mmol/kg. These results provide insights into the crystallization processes and reactivity of natural birnessite, as well as methods for the controllable synthesis of nanoflowers. [Display omitted] In this study, natural birnessite analogs were synthesized under various conditions, including temperature, oxidizing and reducing agents, ionic strengths, particle concentrations, and capping agents. • Characterization revealed the formation of birnessite nanoflowers with a diameter of 3 μm and no significant change in specific surface area (SSA, 30.3 3 m2/g) when the synthesis temperature was 60 °C. • A lower KMnO 4 concentration resulted in a parallel petal arrangement and an increased SSA of 59.11 m2/g. • Reduced concentrations of both KMnO 4 and HCl led to nano-wall morphology with an SSA of approximately 85.39 m2/g. • The addition of organic acids as capping agents hindered (001) plane assembly. • Adsorption tests for Cd2+ revealed microwalls achieved 7102 mmol/kg, surpassing original birnessite nanoflowers at 2114 mmol/kg. [ABSTRACT FROM AUTHOR]

Manganese oxides with layer and tunnel structures occur widely in nature and inspire technological applications. Having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the their thermodynamic stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn3O4, Mn2O3, and MnO2), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing average manganese oxidation state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aqueous solution is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodynamics and the energetics of the aqueous phase drive cryptomelane formation from birnessite, perhaps aided by oxidation-state differences. Transformation among birnessite phases of increasing surface area favors compositions with lower surface energy. These quantitative thermodynamic findings explain and support qualitative observations of phase-transformation patterns gathered from natural and synthetic manganese oxides.