Search Results (58)

Hydrogen generation has become a popular research subject in light of currently pressing issues, such as the rapidly increasing environmental pollution, the depleting fossil fuel reserves, and the looming energy crisis. Sustainable electrochemical water splitting is regarded as one of the most desirable methods for obtaining green hydrogen. Considering this state of affairs, the water splitting electrocatalytic activity of glassy carbon electrodes modified with birnessite-type K₂Mn₄O₈ and mixed-valence iron phosphate Fe₃(PO₃OH)₄(H₂O)₄ materials were evaluated in electrolyte solutions having different pH values. Both compounds were characterized by X-ray diffraction and FT-IR spectroscopy in order to analyze their phase purity and their structural features. The most catalytically active birnessite-type K₂Mn₄O₈-based electrode was manufactured using a catalyst ink containing only the electrocatalyst dispersed in ethanol and Nafion solution. In 0.1 M H₂SO₄, it exhibited an oxygen evolution reaction (OER) overpotential of 1.07 V and a hydrogen evolution reaction (HER) overpotential of 0.957 V. The Tafel slopes obtained in the OER and HER experiments were 0.180 and 0.142 V/dec, respectively. The most catalytically active mixed-valence iron phosphate Fe₃(PO₃OH)₄(H₂O)₄-based electrode was obtained with a catalyst ink containing the specified material mixed with carbon black and dispersed in ethanol and Nafion solution. In a strongly alkaline medium, it displayed a HER overpotential of 0.515 V and a Tafel slope value of 0.122 V/dec. The two electrocatalysts have not been previously investigated in this way, and the acquired data provide insights into their electrocatalytic activity and improve the scientific understanding of their properties and applicative potential. Full article

Different composites of manganese oxides (Mn_xO_y) containing sodium (Na) and silver (Ag) were synthesized by the molten salt method with various MnSO₄·H₂O/NaNO₃ (M/N) molar ratios (between 0.3 and 1), and different AgNO₃ and NaOH amounts, obtaining two groups of materials: without the addition of AgNO₃ (labeled as M/N) and with AgNO₃ (labeled as M/N-A). As for the M/N group, the system with the lowest M/N ratio yielded the highest specific capacitance (160.5 F ${\mathrm{g}}^{-1}$), attributed to the formation of Mn₃O₄ and sodium birnessite. In the M/N-A group, the 1 M/N-0.5A system, produced with M/N ratio of 1 and addition of 0.5 g of AgNO₃, exhibited the highest specific capacitance (229.1 F ${\mathrm{g}}^{-1}$), associated with the presence of Mn₂O₃, silver hollandite, and metallic Ag. This enhancement is attributed to the synergistic effects of Na⁺ and Ag⁺ ions, which improve charge transfer kinetics and electrochemical performance. It was demonstrated that decreasing the MnSO₄·H₂O/NaNO₃ ratio in the M/N group and increasing AgNO₃ content in the M/N-A group enhances the electrochemically active surface area. Galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques confirmed that the 1 M/N-0.5A system exhibited the best performance, characterized by high energy retention, stable cycling behavior, and low capacitance dispersion, indicating its strong potential as an active material for pseudocapacitor applications. Full article

Manganese (Mn) oxides exhibit significant potential to either stabilize or destabilize soil organic carbon (SOC) through the polymerization and/or oxidation of organic molecules via organo-mineral interactions. Birnessite (MnO₂) is known to strongly interact with soil dissolved organic matter (DOM), which is DOM composition-dependent. Humic acid (HA) and fulvic acid (FA) are commonly used as organic fertilizers in soils. In this study, the contrasting reaction of DOM with birnessite in flooded paddy soil with HA and FA amendment was investigated at a molecular level. The results demonstrated that HA amendment enhanced the reaction of phenolic compounds in soil DOM with birnessite, leading to the formation of condensed aromatic compounds and polymeric products (PP) with higher molecular weights and aromaticity. This suggests that HA amendment enhances the birnessite-induced polymerization of soil DOM. In contrast, FA facilitated the birnessite-induced oxidation of soil DOM, yielding dicarboxylic acids (DA), monocarboxylic acids (MA), and quinones products (QP). These findings demonstrate that the reactivity of soil DOM with birnessite is significantly influenced by the composition of DOM exogenously added. This study provides comprehensive understandings of the interactions among Mn and C and helps to predict behaviors of DOM molecules in flooded paddy soil, which is critical for optimizing sustainable soil management. Full article

Spinel-structured hausmannite (Mn(II)Mn(III)₂O₄) is a vital intermediate in Mn mineralogy and a key player in redox chemistry in the environment. Its transformation into other Mn oxides is a critical factor in controlling its environmental occurrence and reactivity. Yet structural impurities and solution pH, as well as the fate of impurities during transformation, which influence hausmannite transformation processes and products, remain largely unknown. In the present work, we address this knowledge gap by investigating pristine and metal-substituted hausmannite, specifically nickel (Ni) or cobalt (Co), equilibrated at two time periods (8 h and 30 days) and three different pH levels (4, 5, and 7). Solution chemistry data revealed that both the equilibration period and pH had a significant impact on hausmannite dissolution rates and the concomitant repartitioning of Ni or Co. Hausmannite with Ni or Co substitution exhibited lower dissolution rates than pristine mineral under acidic conditions. Mineralogy and crystal chemistry data indicated that hausmannite was the major host phase after 30-day equilibration, followed by minor transformed products, including birnessite and manganite. Although minor, birnessite became more abundant than manganite at low pHs. Analytical high-resolution transmission electron microscopy (HRTEM) analyses revealed a poorly crystalline, nano-scaled MnO₂ formed from hausmannite and the majority of metal impurities remaining in the host hausmannite. Yet Co was associated with both hausmannite and the newly formed birnessite, whereas Ni was only found with hausmannite, indicating the strong sequestration of Co by Mn(II/III) and Mn(IV) mineral phases. This study highlights the significant impacts of metal impurities and pH on the stability of hausmannite and its transformation into birnessite, as well as the control of Mn-oxide minerals on the solubility and sequestration of transition metals in the environment. Full article

Current research focuses more on redox of toxic Cr(VI), with less attention to Cr(III) changes in flooded soil. First, the structure of acid birnessite and cryptomelane was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and other test methods. This study investigated farmland soil in Yuxi, Yunnan Province, under flooding stress induced by the addition of two distinct concentrations of manganese oxides. Throughout the experiment, key physicochemical properties of the soil—including pH, redox potential (Eh), Cr(VI) concentration, and chromium speciation—were systematically measured and analyzed. Structural characterization demonstrated distinct morphological and surface area properties. Specifically, acid birnessite, with petal-like stacked spheres, has a specific surface area of 103.76 m²/g, while cryptomelane, strip-shaped, has an area of 95.92 m²/g. The submergence experiment yielded the following phenomena: (1) During the 60-day flooding experiment, soil amended with 0.5% or 1% acid birnessite or cryptomelane exhibited an increase in Eh compared to the control group. (2) At the end of the 60-day submergence period, the Cr(VI) concentration in the soil treated with 1% acid birnessite increased by 2.4 times compared to the control group. In addition, after 60 days, Cr(VI) concentrations in the soil exceeded 5 mg/L in soils with manganese oxide added to them. This study evaluates how manganese oxides oxidize Cr(III), aiding in assessing their environmental risks and long-term impacts on metal transformation. The findings help predict chromium behavior in farm soils and guide remediation strategies. Full article

This study developed a semi-passive treatment system for manganese (Mn)- and zinc (Zn)-containing mine water by repurposing a neutralization tank into a biologically active stirred reactor. Laboratory-scale experiments demonstrated efficient removal of Mn²⁺ (>97%) and Zn²⁺ (>80%) with hydraulic retention times (HRTs) as short as 6 h—significantly faster than traditional passive systems. XRD and XANES analyses identified the predominant formation of birnessite, a layered Mn oxide, during Mn²⁺ oxidation, with Zn co-treatment promoting the precipitation of Zn-containing carbonates. Despite decreasing crystallinity of birnessite over time, microbial activity, dominated by Mn-oxidizing genera, such as Sphingomonas, Pseudonocardia, Sphingopyxis, Nitrospira, and Rhodobacter, persisted in the presence of Zn²⁺, ensuring system stability. Importantly, the low leachability of Mn and Zn from the resulting sludge in TCLP tests confirmed its environmental safety and potential for reuse. By leveraging existing infrastructure and microbial biomineralization, this system bridges the gap between passive and active treatments, significantly reducing treatment footprints and operational costs. These findings highlight the potential of repurposing mine water treatment tanks as a scalable, cost-effective solution for sustainable mine water remediation. Full article

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 Δ^98/95Mo = 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. Full article

The Quaternary Mercure basin is a complex fault structure located in the Pollino region of the southern Apennines (Italy). A persistent seismic gap makes the Mercure basin structure one of Italy’s highest seismic risk zones. The southernmost termination of the Mercure basin is the Timpa della Manca fault. The fault’s mirror is characterised by distinctive, lineated, black-coloured striae decorating a cataclasite made of carbonate clasts. These black-coloured striae consist of a mixture of Mn phases, including hollandite, todorokite, birnessite, and orientite, which are associated with goethite and hematite along with minor amounts of phyllosilicates (chlorite, muscovite), quartz, and sursassite. This mineral association and their phase stability suggest that hydrothermal circulating fluids may have mobilised and re-precipitated low-temperature Mn hydrous phases within the shear zone, leaving remnants of higher-temperature minerals. Oceanic crust remnant blocks within the Frido Unit appear to be the most likely source of the Mn. The uniqueness of the Mn striae on the Timpa della Manca fault offers intriguing insights into fluid circulation within the Mercure basin tectonic system, with potential implications for the seismotectonic characteristics of the Pollino region. Full article

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. Full article

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. Full article

Ferromanganese (Fe-Mn) polymetallic nodules are significant marine mineral resources containing various metal elements of substantial economic and scientific research value. Previous studies have primarily focused on the mineralogy and geochemistry of the nodules, while research on their nano-mineralogy is still lacking. In this study, we conducted scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) porous structure gas adsorption/desorption, and specific surface area analysis to examine the nano-mineralogy and mineralization of the polymetallic nodules from the interbasin of seamounts in the western Pacific Ocean (IBSWP). The results indicate that the growth profiles of the IBSWP polymetallic nodules exhibit microstructural features such as laminated, stromatolithic, columnar, and mottled structures. The mineral compositions are primarily composed of Fe-Mn minerals and detritus, including quartz; minerals from the feldspar group; and minerals from the clay group. The Fe-Mn phase minerals are relatively poorly crystallized. The Mn-phase minerals contain vernadite and small amounts of todorokite, buserite, and birnessite, while the Fe-phase minerals are mainly comprised of amorphous FeOOH. The main ore-forming minerals consist of nano-minerals, and the nanostructures of the polymetallic nodules endow them with unusually large specific surface areas and pore volumes, resulting in strong adsorption properties. The unique nano-properties and surface/interface adsorption effects of Fe-Mn minerals play a crucial role in controlling the enrichment of ore-forming elements. Full article

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. Full article

Many regions of the United States contain manganese deposits economically valuable in New England, Appalachian, and Piedmont regions in the Eastern United States, in Northern Arkansas, and, to a small extent, in Central–Western California. Mn oxide/hydroxide (commonly referred to as Mn oxide minerals) are found in a wide variety of geological settings and occur as fine-grained aggregates, veins, marine and freshwater nodules and concretions, crusts, dendrites, and coatings on rock surfaces (e.g., desert varnish). How manganese oxides form and what mechanisms determine which oxides are likely to form are limited and still debated. This paper focuses on Mn oxides collected at the southern bound of the abandoned open-pit site called Crimora Mine (Augusta County, Virginia). This study uses mineralogical and chemical features to shed light on the origin of manganese deposits in Crimora along the western foot of the Blue Ridge in South–West Virginia. We report the first detailed study on the genesis of the Crimora manganese deposit conducted since the mine was closed in the 1950s. Crimora Mine sample is dark black fine- to medium-grained round and oblong nodules coated with a fine-grained intermix of yellowish earthy limonite, clays, and quartz. Scanning electron microscopy (SEM) revealed that the Crimora Mn-oxides exhibit concentric layering, breccia-like matrices, and veins. X-ray powder diffraction (XRPD) identified the set of Mn minerals as hollandite and birnessite. The concentration and range of dissolved chemical species in freshwater, seawater, and hydrothermal depositional fluids impart a geochemical signature to the Mn-oxides, providing a diagnostic tool to shed light on their genetic origin. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis of the Crimora manganese oxides shows Mn, Fe, and Ti, as well as trace elements such as Co, Ba, Y, Zn, Cr, Ni, Tl, La, V, and Li. A bivariate analysis based on the geochemical correlation of Mn and other common substituting cations (e.g., Fe, Co, Ti) shows a mixed genesis in different environments with varying biological and sedimentary supergene (freshwater and marine) conditions. These data suggest that the Mn-rich deposit in Crimora, VA, was formed in a continental margin environment of surficial deposits and reprecipitated in mixed biogenic and supergene conditions. Full article

Rechargeable magnesium batteries are an attractive alternative to lithium batteries because of their higher safety and lower cost, being spinel-type materials promising candidates for their positive electrode. Herein, MgMn₂O₄ with a tetragonal structure is synthesized via a simple, low-cost Pechini methodology and tested in aqueous media. Electrochemical measurements combined with in-situ Raman spectroscopy and other ex-situ physicochemical characterization techniques show that, in aqueous media, the charge/discharge process occurs through the co-intercalation of Mg²⁺ and water molecules. A progressive structure evolution from a well-defined spinel to a birnessite-type arrangement occurs during the first cycles, provoking capacity activation. The concomitant towering morphological change induces poor cycling performance, probably due to partial delamination and loss of electrical contact between the active film and the substrate. Interestingly, both MgMn₂O₄ capacity retention and cyclability can be increased by doping with nickel. This work provides insights into the positive electrode processes in aqueous media, which is vital for understanding the charge storage mechanism and the correlated performance of spinel-type host materials. Full article

A black substance exuding from fractures was observed in 2012 in Ytterby mine, Sweden, and identified in 2017 as birnessite with the composition M_x[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). Full article