Aqueous Mn Research Articles

Band Engineering of Mn-P Alloy Enables HER-suppressed Aqueous Manganese Ion Batteries.

Aqueous manganese ion batteries hold potential for stationary storage applications owing to their merits in cost, energy density, and environmental sustainability. However, the formidable challenge is the instability of metallic manganese (Mn) anodes in aqueous electrolytes due to severe hydrogen evolution reaction (HER), which is more serious than the commonly studied Zn metal anodes. Moreover, the mechanism of HER side reactions has remained unclear. Herein, we design a series of Mn-P alloying anodes by precisely regulating their energy band structures to mitigate the HER issue. It is found that the serious HER primarily originates from the spontaneous Mn-H2O reaction driven by the excessively high HOMO energy level of Mn, rather than electrocatalytic water splitting. Owing to a reduced HOMO energy level and enhanced electron escape work function, the MnP anode achieves an evidently enhanced cycle durability (over 1000 hours at a high current density of 5 mA cm-2). The MnP||AgVO full cell with an N/P ratio of 4 exhibits better rate capability and extended cycle life (7000 cycles) with minimal capacity degradation than the cell using metallic Mn anode (less than 100 cycles). This study provides a practical approach for developing highly durable aqueous Mn ion batteries.

Band Engineering of Mn‐P Alloy Enables HER‐suppressed Aqueous Manganese Ion Batteries

Aqueous manganese ion batteries hold potential for stationary storage applications owing to their merits in cost, energy density, and environmental sustainability. However, the formidable challenge is the instability of metallic manganese (Mn) anodes in aqueous electrolytes due to severe hydrogen evolution reaction (HER), which is more serious than the commonly studied Zn metal anodes. Moreover, the mechanism of HER side reactions has remained unclear. Herein, we design a series of Mn‐P alloying anodes by precisely regulating their energy band structures to mitigate the HER issue. It is found that the serious HER primarily originates from the spontaneous Mn‐H2O reaction driven by the excessively high HOMO energy level of Mn, rather than electrocatalytic water splitting. Owing to a reduced HOMO energy level and enhanced electron escape work function, the MnP anode achieves an evidently enhanced cycle durability (over 1000 hours at a high current density of 5 mA cm‐2). The MnP||AgVO full cell with an N/P ratio of 4 exhibits better rate capability and extended cycle life (7000 cycles) with minimal capacity degradation than the cell using metallic Mn anode (less than 100 cycles). This study provides a practical approach for developing highly durable aqueous Mn ion batteries

Strategically Modulating Proton Activity and Electric Double Layer Adsorption for Innovative All-Vanadium Aqueous Mn2+/Proton Hybrid Batteries.

Aqueous Mn-ion batteries (MIBs) exhibit a promising development potential due to their cost-effectiveness, high safety, and potential for high energy density. However, the development of MIBs is hindered by the lack of electrode materials capable of storing Mn2+ ions due to acidic manganese salt electrolytes and large ion radius. Herein, the tunnel-type structure of monoclinic VO2 nanorods to effectively store Mn2+ ions via a reversible (de)insertion chemistry for the first time is reported. Utilizing exhaustive in situ/ex situ multi-scale characterization techniques and theoretical calculations, the co-insertion process of Mn2+/proton is revealed, elucidating the capacity decay mechanism wherein high proton activity leads to irreversible dissolution loss of vanadium species. Further, the Grotthuss transfer mechanism of protons is broken via a hydrogen bond reconstruction strategy while achieving the modulation of the electric double-layer structure, which effectively suppresses the electrode interface proton activity. Consequently, the VO2 demonstrates excellent electrochemical performance at both ambient temperatures and -20°C, especially maintaining a high capacity of 162mAhg-1 at 5Ag-1 after a record-breaking 20000 cycles. Notably, the all-vanadium symmetric pouch cells are successfully assembled for the first time based on the "rocking-chair" Mn2+/proton hybrid mechanism, demonstrating the practical application potential.

Coupled effects of iron (hydr)oxides and clay minerals on the heterogeneous oxidation of aqueous Mn(II) and crystallization of manganese (hydr)oxides

The formation of nanominerals and mineral nanoparticles (NMMNs) has drawn broad attention due to their high reactivity and omnipresence in the environment. While the heterogeneous formation of NMMNs on surfaces of various minerals has been extensively studied, little is known about how mineral heteroaggregates influence this process. Herein, we investigated how heteroaggregates of iron (hydr)oxides and clay minerals affect the heterogeneous oxidation of Mn(II) and crystallization of manganese (hydr)oxides (MnOx). Our results revealed that iron (hydr)oxides (ferrihydrite) and clay minerals (kaolinite or montmorillonite) in heteroaggregates can exert coupled effects on these processes, dictating the distribution of Mn and the morphology of MnOx. Specifically, ferrihydrite catalyzed gradual oxidative removal of Mn(II) and triggered MnOx nucleation; in contrast, kaolinite/montmorillonite rapidly adsorbed Mn(II) but hardly catalyzed its oxidation. These reactions collectively caused fast adsorption and gradual oxidation of Mn(II) on the heteroaggregates. Further, MnOx nanoparticles formed on ferrihydrite surfaces migrated to kaolinite/montmorillonite surfaces, which resulted in MnOx interacting with various component minerals of the heteroaggregates. This strongly altered the further growth pathways and the eventual morphology of MnOx. Consequently, while MnOx nanoparticles in the ferrihydrite-only system aggregated freely and formed well-extended nanowires, most of those in the ferrihydrite-kaolinite system became short nanorods due to the immobilization by kaolinite surfaces; in the ferrihydrite-montmorillonite system, considerable MnOx nanoparticles attached to montmorillonite surfaces due to strong electrostatic attraction, and further grew into blocky particles via particle attachment. These results illustrate that surface reactivities of heteroaggregated ferrihydrite and kaolinite/montmorillonite are coupled when they interact with Mn(II) or MnOx. Our work, for the first time, exemplify the cooperation between surfaces of various minerals during the heterogeneous formation of NMMNs. Findings from this study also help advance our understanding of MnOx formation on surfaces with diverse atomic structures, and also Mn cycling in the environment.

Unlocking the Charge-Storage Potential of a Phenanthraquinone-based Two-Dimensional Covalent Organic Framework (2D COF).

The high surface area, open pore-structure and atomic-level organization inherent in many covalent organic frameworks (COFs) make them an attractive polymer platform for developing functional materials. Herein, a chemically robust 2D COF (TpOMe-DAPQ COF) containing phenanthraquinone moieties was prepared by condensing 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde (TpOMe) and 2,7-diamino-9,10-phenanthraquinone (DAPQ) using the convenient mechanochemical method. The poor charge-storage capacity of the pristine TpOMe-DAPQ COF was substantially improved by first investigating its redox-site accessibility (RSA) using different conductivity-enhancement methods, and then optimizing the amount of EDOT needed to perform an in-situ polymerization. The resulting composite (0.4EDOT@TpOMe-DAPQ) was characterized and its enhanced charge-storage capabilities enabled it to be used as an anode material in an aqueous Mn beaker-cell battery capable of delivering 0.76 V. This work outlines the rational design approach used to develop a functional charge-storage material utilizing a COF-based polymerization platform.

Manganese exposure from spring and well waters in the Shenandoah Valley: interplay of aquifer lithology, soil composition, and redox conditions

Manganese (Mn) is of particular concern in groundwater, as low-level chronic exposure to aqueous Mn concentrations in drinking water can result in a variety of health and neurodevelopmental effects. Much of the global population relies on drinking water sourced from karst aquifers. Thus, we seek to assess the relative risk of Mn contamination in karst by investigating the Shenandoah Valley, VA region, as it is underlain by both karst and non-karst aquifers and much of the population relies on water wells and spring water. Water and soil samples were collected throughout the Shenandoah Valley, to supplement pre-existing well water and spring data from the National Water Information System and the Virginia Household Water Quality Program, totaling 1815 wells and 119 springs. Soils were analyzed using X-ray fluorescence and Mn K-Edge X-ray absorption near-edge structure spectroscopy. Factors such as soil type, soil geochemistry, and aquifer lithology were linked with each location to determine if correlations exist with aqueous Mn concentrations. Analyzing the distribution of Mn in drinking water sources suggests that water wells and springs within karst aquifers are preferable with respect to chronic Mn exposure, with < 4.9% of wells and springs in dolostone and limestone aquifers exceeding 100 ppb Mn, while sandstone and shale aquifers have a heightened risk, with > 20% of wells exceeding 100 ppb Mn. The geochemistry of associated soils and spatial relationships to various hydrologic and geologic features indicates that water interactions with aquifer lithology and soils contribute to aqueous Mn concentrations. Relationships between aqueous Mn in spring waters and Mn in soils indicate that increasing aqueous Mn is correlated with decreasing soil Mn(IV). These results point to redox conditions exerting a dominant control on Mn in this region.

Tailoring crystalline Mn5O8 nanostructures: Kinetic control and stabilization for enhanced electrocatalytic O2 evolution

The stabilization of manganese oxide nanostructures with tunable oxidation states and morphologies is crucial for regulating catalytic properties. In this study, we aim to stabilize the metastable monoclinic Mn5O8 phase and control the morphology of the Mn5O8 nanostructures using a hydrothermal assisted calcination process in the presence of different precipitating agents. Raman experiments confirmed that the interaction of aqueous Mn2+ salts with different precipitating agents such as NaOH, ammonia, and oleylamine, resulted in the formation of Mn5O8. On the other hand, the precipitation of Mn2+ salt using diammonium oxalate monohydrate as a precipitating agent led to complete oxidation forming Mn2O3. Detailed phase and microstructural analyses were conducted using PXRD, XPS, and FESEM. The analyses revealed the formation of the monoclinic Mn5O8 phase with various morphologies, including hexagonal nanoplates, nanorods, and random nanoplates, when using NaOH, ammonia, and oleylamine as precipitating agents. The synthesized Mn5O8 phase was deposited onto an iron sheet as a substrate. Furthermore, we demonstrated the effect of the structure, morphology, and size of the Mn5O8 material on O2 evolution activity. The catalytic performance of irregularly shaped Mn5O8 nanoplates showed significantly lower η10 (270 mV) and η50 (350 mV) compared to Mn5O8 hexagonal nanoplates, Mn5O8 nanorods, and Mn2O3 nanostructure. Additionally, an insight into the catalytic performance was investigated in detail by evaluating electrochemical active sites, impedance spectroscopy, and hydrophilicity/hydrophobicity of the Mn5O8 material.

Analysis of the Impact of Operating Conditions on Performance of H2 Gas – Aqueous Manganese Redox Flow Batteries

In recent years the need to move to renewable energy sources as a means of combating climate change has become increasingly clear. Long duration energy storage (LDES) is essential for the incorporation of renewable energy generation into grid-scale energy systems. Redox Flow Batteries (RFB) offer a promising solution due to scalability, safety, and cost. The hybrid H2 gas – aqueous Mn RFB has been identified as a candidate1.Recognising the absence of an established protocol for measurement of RFB system performance, this study uses charge- discharge polarisations to explore the sensitivity of the H2/Mn performance to changes in temperature. When designing a RFB for commercial usage it is important to balance improved performance with the necessary stability that is required for any potential LDES system. In this work we use a developed stress test to demonstrate the impact of temperature on the systems stability.Varying temperature showed improvement of 2.8 mV/ oC in charging overpotentials of and 5.7 mV/ oC in discharging overpotential. However, this must be weighed against the impact on the stability of the cell. These findings serve as inputs for the development of the control system essential for the successful operation of the H2/Mn RFB system. 1Rubio-Garcia, J. et al. Hydrogen / manganese hybrid redox flow battery. J. Phys. Energy (2018)1015006.

Manganese cycling and transport in boreal estuaries impacted by acidic Mn-rich drainage

As critical transition zones between the land and the sea, estuaries are not only hotspots of hydrogeochemical and microbial processes/reactions, but also play a vital role in processing and transferring terrestrial fluxes of metals and nutrients to the sea. This study focused on three estuaries in the Gulf of Bothnia. All of them experience frequent inputs of acidic and Mn/metal-rich creek waters due to flushing of acid sulfate soils that are widespread in the creekś catchments. Analyzing existing long-termwaterchemistrydata revealed a strong seasonal variation of Mn loads, with the highest values in spring (after snow melt) and autumn (after heavy rains). We sampled surface waters, suspended particulate matter (SPM), and sediments from the estuarine mixing zones and determined the loads and solid-phase speciation of Mn as well as the composition and metabolic potentials of microbial communities. The results showed that the removal, cycling, and lateral transport of Mn were governed by similar phases and processes in the three estuaries. Manganese X-ray absorption spectroscopy data of the SPM suggested that the removal of Mn was regulated by silicates (e.g., biotite), organically complexed Mn(II), and MnOx (dominated by groutite and phyllomanganates). While the fractional amounts of silicate-bound Mn(II) were overall low and constant throughout the estuaries, MnOx was strongly correlated with the Mn loadings of the SPM and thus the main vector for the removal of Mn in the central and outer parts of the estuaries, along with organically complexed Mn(II). Down estuary, both the fractional amounts and average Mn oxidation state of the MnOx phases increased with (i) the total Mn loads on the SPM samples and (ii) the relative abundances of several potential Mn-oxidizing bacteria (Flavobacterium, Caulobacter, Mycobacterium, and Pedobacter) in the surface waters. These features collectively suggested that the oxidation of Mn, probably mediated by the potential Mn-oxidizing microorganisms, became more extensive and complete towards the central and outer parts of the estuaries. At two sites in the central parts of one estuary, abundant phyllomanganates occurred in the surface sediments, but were converted to surface-sorbed Mn(II) phases at deeper layers (>3–4 cm). The occurrence of phyllomanganates may have suppressed the reduction of sulfate in the surface sediments, pushing down the methane sulfate transition zone that is typically shallow in estuarine sediments. At the outermost site in the estuary, deposited MnOx were reduced immediately at the water–sediment interface and converted most likely to Mn carbonate. The mobile Mn species produced by the Mn reduction processes (e.g., aqueous Mn(II) and ligand complexed Mn(III)) could partly diffuse into the overlying waters and, together with the estuarine Mn loads carried by the surface waters, transfer large amounts of reactive Mn into open coastal areas and subsequently contribute to Mn shuttling and inter-linked biogeochemical processes over the seafloor. Given the widespread occurrence of acid sulfate soils and other sulfidic geological materials on many coastal plains worldwide, the identified Mn attenuation and transport mechanisms are relevant for many estuaries globally.

Improving conventional household greensand treatment for efficient Mn(II) removal from drinking water

Geogenic contaminants pose a global threat to ensuring access to safe drinking water. Manganese (Mn) is a naturally-occurring redox-active element which in its reduced form, Mn(II), is a widespread groundwater contaminant. Prolonged consumption of water containing high levels of Mn has been linked to adverse effects on memory, attention, motor skills, and nervous system function, particularly in vulnerable groups including pregnant individuals and young children. In addition, Mn can lead to aesthetic issues such as altered taste, clogging, and damage to plumbing systems. While Mn has historically been regulated as an aesthetic concern, a mounting body of evidence linking health issues to Mn exposure through drinking water has heightened the challenges of using groundwater to meet drinking water needs. Recently, Health Canada established a health guideline which set a maximum acceptable concentration for total Mn in drinking water of 120 μg/L. Greensand (GS) filters are commonly used in conventional drinking water treatment systems for Mn(II) removal due to their cost-effectiveness and high exchange capacity. However, under certain conditions conventional GS systems may have a low Mn(II) removal efficiency (Galangashi et al. 2021) and encounter challenges related to Mn leaching (Outram et al. 2018), resulting in failure to meet health-related standards. Furthermore, GS filters typically undergo regeneration using potassium permanganate (KMnO4), a mild oxidant that results in the release of additional Mn waste byproducts during the regeneration of the filter. Recent research suggests that Mn-containing materials can effectively activate peroxymonosulfate (PMS) and produce reactive oxygen species (ROS) that facilitate contaminant degradation. This study aims to investigate whether PMS may be used to improve greensand treatment systems and enhance Mn(II) removal from drinking water. Batch experiments were performed to test the Mn(II) removal efficiency across a range of PMS concentrations (0-500 μM) and GS mass (0.1-3 g). Aqueous Mn concentrations were measured over time using inductively coupled plasma optical emission spectrometry (ICP-OES). Solid-phase reaction products were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The mechanisms of Mn oxidation were identified by quenching experiments and electron paramagnetic resonance (EPR) spectroscopy. The activation of PMS by greensand significantly increased the removal efficiency of Mn(II) compared to the conventional method (e.g., 96.83(±3.77)% at PMS = 500 µM vs. 5.77(±11.1)% at PMS = 0 µM). Our results attribute the mechanism underlying increased Mn removal in our improved treatment method to advanced oxidation processes that involve free radicals (e.g., hydroxyl radical (·OH) and sulfate radical (SO4·-)) and non-free radical pathways, with Mn oxides as the main oxidation products. This study provides a new treatment method for more efficient Mn(II) removal while simultaneously reducing Mn wastes produced during the regeneration cycle which allows for a more sustainable and holistic management of invaluable groundwater resources.

Tissue-level distribution and speciation of foliar manganese in Eucalyptus tereticornis by µ-SXRF and µ-XANES shed light on its detoxification mechanisms

This study is the first to investigate the speciation and spatial distribution patterns of manganese (Mn) accumulated at elevated concentrations in Eucalyptus leaves by X-ray fluorescence (µ-XRF) and absorption near-edge spectroscopy (µ-XANES). Eucalyptus tereticornis is a tree species with great economic value and potential to accumulate and tolerate high Mn despite not being considered a hyperaccumulator. Seedlings grown under glasshouse conditions were irrigated with two Mn treatments: control Mn (9 µM) and high Mn solution (1000 µM). Biomass and total nutrient concentrations were assessed in roots, stems and leaves. Manganese, calcium (Ca) and potassium (K) spatial patterns were imaged by µ-SXRF in different foliar structures, and Mn speciation was conducted in these compartments by µ-XANES. Under high supply, Mn was distributed across the leaf mesophyll suggesting vacuolar sequestration in these cells. High Mn decreased cytosolic Ca by almost 50% in mesophyll cells, but K remained unaltered. Speciation suggests that a majority of the Mn fraction was complexed by organic ligands modeled as Mn-bound malate and citrate, instead of as free aqueous Mn2+ or oxidised forms. These two detoxification mechanisms: effective vacuolar sequestration and organic acid complexation, may be responsible for the impressively high Mn tolerance found in eucalypts.

Oxidative dissolution of Cr(OH)3 and mixed Fe-Cr(III) phases by aqueous Mn(III)-pyrophosphate complex

The key role of manganese (Mn) in the biogeochemical cycle of trace elements has been of great interest in recent years. Nevertheless, the redox properties of aqueous Mn(III) have been studied to a lesser extent. Mn(III) is not stable in solution by itself. However, when complexed with inorganic ligands, it has shown potential to oxidize and reduce trace elements. In the present study, we are exploring the redox characteristics of the complex Mn(III)-Pyrophosphate (Mn(III)-PP). This complex is stable over a wide range of pH values but requires the ratio of Mn:PP to be less than 1:6. Specifically, the redox reaction of chromium (Cr(III)) and Mn(III)-PP is investigated. A solid, Cr(OH)3, is used as a source of Cr(III). For this reaction, environmentally relevant parameters, such as pH, ionic strength, ratio Mn(III)/Cr(III), and excess of ligand, were assessed. Results showed that Mn(III) can effectively oxidize Cr(III) to Cr(VI), taking about 15 days for the reaction to complete. This reaction occurs only under acidic conditions (pH 4), and with a low excess of Pyrophosphate. The initial Mn(III) concentration decreases as the Cr(VI) is produced, and Cr(VI) can be adsorbed back into the Cr(OH)3 surface, limiting the mobility of this toxic species. Despite this adsorption, significant amounts of Cr(VI) are release in the aqueous phase. This study shows the importance of a mobile species (Mn-PP complex) in the oxidation of Cr(III) and the release of Cr(VI) to the environment.

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, pKa = 6.10; PIPES, pKa = 6.76; MOPS, pKa = 7.28; HEPES, pKa = 7.48) and TRIS (pKa = 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 pKa 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.

Siderophore-Mediated Mobilization of Manganese Limits Iron Solubility in Mixed Mineral Systems.

Recent laboratory and field studies show the need to consider the formation of aqueous Mn(III)-siderophore complexes in manganese (Mn) and iron (Fe) geochemical cycling, a shift from the historical view that aqueous Mn(III) species are unstable and thus unimportant. In this study, we quantified Mn and Fe mobilization by desferrioxamine B (DFOB), a terrestrial bacterial siderophore, in single (Mn or Fe) and mixed (Mn and Fe) mineral systems. We selected manganite (γ-MnOOH), δ-MnO2, lepidocrocite (γ-FeOOH), and 2-line ferrihydrite (Fe2O3·0.5H2O) as relevant mineral phases. We found that DFOB mobilized Mn(III) as Mn(III)-DFOB complexes to varying extents from both Mn(III,IV) oxyhydroxides but reduction of Mn(IV) to Mn(III) was required for the mobilization of Mn(III) from δ-MnO2. The initial rates of Mn(III)-DFOB mobilization from manganite and δ-MnO2 were not affected by the presence of lepidocrocite but decreased by a factor of 5 and 10 for manganite and δ-MnO2, respectively, in the presence of 2-line ferrihydrite. Additionally, the decomposition of Mn(III)-DFOB complexes through Mn-for-Fe ligand exchange and/or ligand oxidation led to Mn(II) mobilization and Mn(III) precipitation in the mixed-mineral systems (∼10% (mol Mn/mol Fe)). As a result, the concentration of Fe(III) mobilized as Fe(III)-DFOB decreased by up to 50% and 80% in the presence of manganite and δ-MnO2, respectively, compared to the single mineral systems. Our results demonstrate that siderophores, through their complexation of Mn(III), reduction of Mn(III,IV), and mobilization of Mn(II), can redistribute Mn to other soil minerals and limit the bioavailability of Fe in natural systems.

Sorption and oxidation of Co(II) at the surface of birnessite: Impacts of aqueous Mn(II)

The sorption of aqueous Mn(II) (1 mM) and Co(II) (50 and 200 μM) onto hexagonal birnessite (0.1 g L−1) was studied under anoxic conditions at pH 6.5 and 7.5 in binary and ternary experiments using batch kinetic experiments and XRD, ATR-FTIR, and Co K-edge EXAFS analyses. In the binary systems, sorption of Co(II) was accompanied by partial oxidation to Co(III) yielding a mixture of corner-sharing Co(II) and edge-sharing Co(III) complexes at both pH values, while Mn(II)-birnessite interaction resulted in coordination of Mn(II/III) at layer vacancy sites at pH 6.5, and led to reductive transformation of birnessite into feitknechtite at pH 7.5. In the ternary systems, strong mutual interferences between Co(II) and Mn(II) co-sorbates reduced the rate and extent of sorption relative to the binary experiments. The introduction of Mn(II)aq into the high-Co systems (200 μM) halted the slow sorption of Co(II) that was attributed to the incorporation of Co(III) into layer vacancies, while Co(II)aq prevented Mn(II)-driven transformation of birnessite into feitknechtite at pH 7.5. In the low-Co system (50 μM), reductive transformation of birnessite by Mn(II)aq at pH 7.5 produced Co(II)-substituted feitknechtite, a conversion that was accompanied by reduction of sorbed Co(III) to Co(II) which was partially released to solution. The strong effects of co-sorption are attributed to the similarity in sorption mechanisms of Co(II) and Mn(II), which both sorb as a mixture of di- and trivalent species. The results of this work demonstrate that aqueous Mn(II) may significantly affect the reactivity of phyllomanganate sorbents towards dissolved Co(II), and therefore impact the speciation and solubility of this trace metal in anoxic and suboxic geochemical environments.

Impacts of sulfonic acids on fungal manganese oxide production

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+,□)O2nH2O], 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+2O3) 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.

New method for selective recovery of manganese from NCM-based cathode material of spent Li-ion batteries

ABSTRACT This study provides a new leaching method for selective recovery of manganese from the spent NCM-based cathode active material of lithium-ion secondary batteries (LIBs). The recovery method involves the use of aqueous Mn(II) phase spontaneously generated in sulfate media as a reducing agent, in which Ni(III) and Co(III) are reduced to soluble species while Mn(II) precipitates as Mn(IV) oxides simultaneously. To optimize the process, the effects of time, temperature, and acid concentration on the leaching behavior were investigated. As a result, two-step successive leaching at an optimal condition was proposed, by which the dissolution efficiency of Co and Ni reached up to 99% and ~100%, respectively while Mn leaching was restricted to 15.5%. The leaching residue mainly consisted of manganese oxide (MnO2), containing Mn ~40%. The amount of Mn in the residue increased to ~70% by calcinating at 650 °C, which converted its phase into Mn2O3 and Mn3O4. Therefore, our two-step leaching technique will lead to the recovery of nearly all the contents of Li, Co, and Ni in addition to ~70% purity Mn as supplemental benefit from spent LIB materials without using any oxidants or reductants.

Issues and Opportunities Facing Aqueous Mn2+ /MnO2 -based Batteries.

Aqueous Mn2+ /MnO2 -based batteries have attracted enormous attentions in aqueous energy storage fields, owing to their high working voltage and theoretical capacity (616 mAh g-1 ) brought by the two-electron reaction (Mn2+ /Mn4+ ). However, there are currently several tricky challenges facing Mn2+ /MnO2 -based batteries: their complicated working mechanisms, existing issues, and optimization strategies. This Perspective aims to provide a mechanistic understanding and an overview of the insufficiency, optimization, and future development for Mn2+ /MnO2 -based batteries. The existing issues and deficiency in Mn2+ /MnO2 -based batteries have been systematically analyzed, and optimization strategies have also been rationally summarized and discussed with deep insights. Also, the often-overlooked optimized objects and aspects have been highlighted with unique perspectives. The proposals of testing methods and performance assessment are presented, containing different degradation mechanisms. Based on the above points, this Perspective will provide guidance and contribute to the further development of aqueous Mn2+ /MnO2 -based batteries.

Selective and rapid degradation of organic contaminants by Mn(V) generated in the Mn(II)-nitrilotriacetic acid/periodate process

The rapid generation of Mn(V) as the dominant reactive species for the selective and efficient degradation of trace organic contaminants (TrOCs) remains still challenging. In this work, aqueous Mn(II) complexed with nitrilotriacetic acid (Mn(II)-NTA complex) was employed for the first time to activate periodate (PI) (Mn(II)-NTA/PI process) to degrade TrOCs. Experiments showed that TrOCs could be degraded in the Mn(II)-NTA/PI process within 30 s. Mn(V), existing as Mn(V)-NTA complex, was identified as the active oxidant in this process by using methyl phenyl sulfoxide as a probe, scavenging experiments, and mass spectrometry analysis. Three-dimensional UV–vis spectra indicated that Mn(III)-NTA complex was firstly generated through oxidation of Mn(II)-NTA complex by PI and then oxidized to Mn(V)-NTA complex via two-electron transfer. The generation of Mn(V)-NTA complex was dependent on NTA concentration, PI concentration, and solution pH, whereas the reactivity of Mn(V)-NTA complex toward TrOCs strongly relied on the functional groups of TrOCs. A high PI utilization efficiency could be achieved in this process. Moreover, due to the catalyst-like role of Mn(II)-NTA complex in activating PI, Mn(V)-NTA complex could be continuously generated and thus the sustainable and efficient degradation of TrOCs occurred in this process even when the concentration of Mn(II)-NTA complex was low. This work advances the understanding of Mn(V) generation from Mn(II) commonly present in aquatic environments.

Mineral paragenesis in Paleozoic manganese ore deposits: Depositional versus post-depositional formation processes

Unlike modern Mn(IV)-dominated manganese nodules, ancient manganese deposits primarily contain Mn(II) (manganous) mineral phases. The general assumption is that these manganous minerals formed post-depositionally from a Mn(IV) precursor phase during diagenesis and metamorphism. However, recent studies have shown that aqueous Mn(II) may combine with either CO32–(aq) or H4SiO4(aq) to nucleate and precipitate as primary manganous phases within anoxic waters. These findings thus make it difficult to assess what comprised the primary mineral assemblage of ancient Mn deposits, which in turn, has implications for understanding the redox fabric of the past marine water column. Here, we investigate the paragenesis and oxidation state of various Mn minerals (oxides, carbonates, and silicates) from the late Devonian Xialei and middle to late Permian Zunyi manganese deposits in South China. Through characterization of the micro-scale distribution, composition, and speciation of different Mn-bearing mineral phases, we provide evidence to support an oxidative mechanism during Mn cycling at the time of deposition. We document the presence of a residual oxide phase (hausmannite; Mn3O4) that represents a primary to early formed Mn(III)-bearing oxide preserved in Mn(II)-carbonate nodules. Additionally, we observe spherulitic textures comprised of mixed-valence Mn(II,III,IV)-silicates enclosed in diagenetic Mn(II)-carbonate laminations. Our results allow for the construction of a paragenetic model for these deposits beginning with the reduction of a primary Mn(IV) phase and the subsequent precipitation of intermediate Mn(III)-oxide or -silicate mineral phases. Following continued exposure to reducing conditions during early diagenesis, Mn(III)-oxides were then replaced by Mn(II)-carbonates. Similarly, Mn(III)-silicates were transformed into various Mn(II)-silicates during late-stage diagenesis and metamorphism. This model suggests that the accumulation of various divalent Mn mineral phases in ancient sediments may, therefore, be indicative of deposition under a water column with a sufficient oxidizing potential to induce Mn(II) oxidation.