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Exposure to vanadium (V) results in adverse health outcomes in humans and a plethora of species in the environment across trophic levels. V is released from rock, slag and mine tailings, where it is able to oxidize to vanadate (VV) (HnVO4 −3+n), which poses the greatest risk to human health. Vanadate is highly soluble unlike its VIII or VIV, and is readily taken up by well systems, resulting in human exposure in areas where water treatment is minimal. However, the geochemical controls that determine the mobility of V in the environment are poorly understood, making the behavior of V in the subsurface difficult to predict. This dissertation addresses this knowledge gap by investigating the abiotic reactions of V with common iron and manganese mineral phases. These mineral phases are already known to play an important role in the fate and transport of similar contaminants, but many aspects of their specific reactivity towards V are unknown. To address this knowledge gap, their surface capacity, adsorption affinity and their ability to retain vanadate via surface complexation is investigated. The vanadate adsorption capacity and surface affinity are both found to be inversely proportional to the crystallinity of the oxide examined. One manganese oxide, birnessite is also known to promote the oxidation of many trace metal contaminants. The oxidative capacity of birnessite towards VIV is much greater than its surface capacity for VV. At the aggregate scale, birnessite, V and Fe oxides coexist in close spatial proximity where rates of diffusion limit the transport of solutes. In such an environment, birnessite effectively oxidizes VIV and retains more VV than adjacent Fe oxide phases. Further, FeII-bearing oxides can reduce VV back to VIV. However, this pathway is slow compared to the oxidation rate of VIV by birnessite. This suggests that surface complexation of vanadate, even by birnessite is a greater attenuation pathway than abiotic FeII-mediated reduction. The result of this research emphasizes the role of abiotic surface processes in constraining the mobility of V within the soil matrix which will improve the reliability of models implemented in the management and reclamation of V contaminated sites.

Volatile organic compounds (VOCs) have caused a serious threat to the atmosphere and human health. Therefore, it is of great significance to exploit effective catalytic materials for the safe and effective catalytic elimination of VOCs. Herein, Ag-MnOx-ACF composite catalysts were constructed via a two-step impregnation strategy and used for catalytic toluene degradation. A remarkable low-temperature catalytic activity (T100 = 50℃), excellent stability, as well as CO2 selectivity (80%) were achieved over the Ag-MnOx-ACF catalyst. A series of characterizations indicated that the unique manganese defects structure of birnessite phase manganese oxide played an essential role for toluene oxidation, which was conducive to generating surface adsorbed oxygen. The higher ratio of Mn3+/Mn4+, abundant surface adsorbed oxygen and highly dispersed Ag species were determined to significantly facilitate toluene degradation. The mechanism of efficient degradation of toluene at low temperature was proposed. O3 and H2O molecules were activated via surface hydroxyl and Mn defects on Ag-MnOx-ACF to produce sufficient •OH, enhancing the degradation performance of toluene. We provide a new idea for the catalytic oxidation of benzene VOCs at low even room temperatures.

MnO2-based aqueous Zn-ion batteries (ZIBs) hold great promising for large-scale energy storage applications owing to their safe and sustainable nature. However, rapid capacity decay under high depth of discharge limits the applications of MnO2 cathodes. In the meantime, the reaction chemistry and degradation process of MnO2 cathodes cannot be fully understood, leading to improvement of their cycling stability lacks of robust methods. Herein, ZIB performances of MnO2 polymorphs are investigated to disclose their detailed reaction chemistry and degradation mechanisms. Ex situ characterizations at different cycles exhibit evolution of active materials (original MnO2→Mn2+→birnessite→ZnMn2O4/Mn3O4) and coexisted reactions from co-insertion, dissolution/deposition and chemical conversion mechanisms. Variational contributions from these intermediate products and different reaction mechanisms cause fluctuated performance during cycling. Initial performance activation is from enhanced activity of birnessite, while the degradation is caused by its conversion to electrochemically inactive ZnMn2O4 and Mn3O4. By optimizing tunnel structures, it is found that R-MnO2 shows low manganese dissolution with its reaction mainly achieved by intercalation/extraction of Zn2+/H+, and theoretical calculations verify its low Jahn-Teller distortion at discharged state. This specific property circumvents conversion of R-MnO2 to metastable birnessite, giving rise to a stable capacity under high depth of discharge.

Organochlorines are important environmental contaminants that impact ecosystems, public health, and global warming. Organochlorines are generally regarded as anthropogenic compounds, but they also ...

Manganese (Mn) oxides, such as birnessite (δ-MnO2), are ubiquitous mineral phases in soils and sediments that can interact strongly with antimony (Sb). The reaction between birnessite and aqueous Mn(II) can induce the formation of secondary Mn oxides. Here, we studied to what extent different loadings of antimonate (herein termed Sb(V)) sorbed to birnessite determine the products formed during Mn(II)-induced transformation (at pH 7.5) and corresponding changes in Sb behavior. In the presence of 10 mM Mn(II)aq, low Sb(V)aq (10 μmol L-1) triggered the transformation of birnessite to a feitknechtite (β-Mn(III)OOH) intermediary phase within 1 day, which further transformed into manganite (γ-Mn(III)OOH) over 30 days. Medium and high concentrations of Sb(V)aq (200 and 600 μmol L-1, respectively) led to the formation of manganite, hausmannite (Mn(II)Mn(III)2O4), and groutite (αMn(III)OOH). The reaction of Mn(II) with birnessite enhanced Sb(V)aq removal compared to Mn(II)-free treatments. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that heterovalent substitution of Sb(V) for Mn(III) occurred within the secondary Mn oxides, which formed via the Mn(II)-induced transformation of Sb(V)-sorbed birnessite. Overall, Sb(V) strongly influenced the products of the Mn(II)-induced transformation of birnessite, which in turn attenuated Sb mobility via incorporation of Sb(V) within the secondary Mn oxide phases.

Thallium (Tl) is a highly toxic trace metal. It occurs mostly as soluble monovalent Tl(I) and less frequently as poorly soluble trivalent Tl(III). Laboratory studies have shown that vacancy-containing hexagonal birnessites can sorb Tl with a very high affinity via a mechanism that involves the oxidation of Tl(I) to Tl(III) and strong complexation of Tl(III), whereas other manganese (Mn) oxides bind Tl(I) non-oxidatively and with lower sorption affinity. Information on the mode of Tl uptake by natural Mn oxides in soils, on the other hand, is still limited. In this study, we characterized the association of Tl with Mn oxides and Tl (redox) speciation in a naturally Tl-rich soil using micro-focused synchrotron X-ray absorption near edge structure (XANES) spectroscopy and X-ray fluorescence (XRF) chemical imaging. The results show that most soil Tl was Tl(I) associated with micaceous clay minerals in the soil matrix. High levels of Tl in soil Mn concretions, on the other hand, were mostly identified as Tl(III), suggesting that oxidative Tl uptake by vacancy-containing hexagonal birnessite was the main process of Tl accumulation in soil Mn concretions. The spectroscopic results in combination with chemical extractions and published sorption isotherms for Tl on synthetic Mn oxides suggest that the formation and transformation of natural Mn oxides in soils and sorption competition of Tl with major and trace metal cations determine the extent and mode of Tl uptake by soil Mn oxides. Methodologically, this study compares classical micro-XRF element mapping combined with point XANES analyses for spatially-resolved element speciation with high-resolution chemical imaging of entire sample areas, which is of great interest for the geochemical community in light of diffraction-limited storage ring upgrades to many synchrotron lightsources.

Manganese (Mn) oxides have been prevalent on Earth since before the Great Oxidation Event and the Mn cycle is one of the most important biogeochemical processes on the Earth's surface. In sunlit natural environments, the photochemistry of Mn oxides has been discovered to enable solar energy harvesting and conversion in both geological and biological systems. One of the most widespread Mn oxides is birnessite, which is a semiconducting layered mineral that actively drives Mn photochemical cycling in Nature. The oxygen-evolving centre in biological photosystem II (PSII) is also a Mn-cluster of Mn4CaO5, which transforms into a birnessite-like structure during the photocatalytic oxygen evolution process. This phenomenon draws the potential parallel of Mn-functioned photoreactions between the organic and inorganic world. The Mn photoredox cycling involves both the photo-oxidation of Mn(II) and the photoreductive dissolution of Mn(IV/III) oxides. In Nature, the occurrence of Mn(IV/III) photoreduction is usually accompanied with the oxidative degradation of natural organics. For Mn(II) oxidation into Mn oxides, mechanisms of biological catalysis mediated by microorganisms (such asPseudomonas putidaandBacillusspecies) and abiotic photoreactions by semiconducting minerals or reactive oxygen species have both been proposed. In particular, anaerobic Mn(II) photo-oxidation processes have been demonstrated experimentally, which shed light on Mn oxide emergence before atmospheric oxygenation on Earth. This review provides a comprehensive and up-to-date elaboration of Mn oxide photoredox cycling in Nature, and gives brand-new insight into the photochemical properties of semiconducting Mn oxides widespread on the Earth's surface.

Birnessite is a layered MnO2 mineral capable of intercalating nanometric water films in its bulk. With its variable distributions of Mn oxidation states (MnIV, MnIII, and MnII), cationic vacancies, and interlayer cationic populations, birnessite plays key roles in catalysis, energy storage solutions, and environmental (geo)chemistry. We here report the molecular controls driving the nanoscale intercalation of water in potassium-exchanged birnessite nanoparticles. From microgravimetry, vibrational spectroscopy, and X-ray diffraction, we find that birnessite intercalates no more than one monolayer of water per interlayer when exposed to water vapor at 25 °C, even near the dew point. Molecular dynamics showed that a single monolayer is an energetically favorable hydration state that consists of 1.33 water molecules per unit cell. This monolayer is stabilized by concerted potassium–water and direct water–birnessite interactions, and involves negligible water–water interactions. Using our composite adsorption–condensation–intercalation model, we predicted humidity-dependent water loadings in terms of water intercalated in the internal and adsorbed at external basal faces, the proportions of which vary with particle size. The model also accounts for additional populations condensed on and between particles. By describing the nanoscale hydration of birnessite, our work secures a path for understanding the water-driven catalytic chemistry that this important layered manganese oxide mineral can host in natural and technological settings. Originally included in thesis in manuscript form.

Biofiltration for Mn removal has not been proven in the tropics (18-29 °C). Also, the use of pumice as an alternative filter medium for Mn removal is still poorly known. In this study, a bench-scale biofiltration experiment, using virgin pumice, silica sand, and anthracite, was conducted for 107 days using tropical groundwater at 22 °C. Characterization of manganese oxide (MnOx) on filter media was carried out by Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The ability of culturable bacteria to oxidize Mn was verified by the leucoberbelin blue dye assay (LBB). The microbial activity on filter media was studied by ATP analyses. Identification of the MOB was performed by 16S rRNA sequencing. Results showed that the ripening time in each column was similar (∼80 days); therefore, the filter media and water temperature do not seem to accelerate the start-up period. The MnOx present on all ripened media was of the birnessite type. The MnOx morphology, influent water parameters, and microbiological activity suggest a start-up period likely assisted by biological oxidation. Therefore, biofiltration for manganese removal is feasible under tropical conditions. The similarity in the performance of pumice with the other media confirmed its suitability for biotic Mn removal. All materials presented similar known MOB at the genus level; however, different closest related species colonized selectively the filter media. Two strains of Pseudoxanthomonas sp., a not recognized genus on matured biofilters, look promising as inoculums in pumice and sand. Proper application of biofiltration in the tropics needs operational and bioaugmentation strategies.

Rechargeable zinc-ion batteries represent an emerging energy-storage technology that offers the advantages of low cost, use of abundant and nontoxic materials, and competitive energy content in lightly packaged forms. Nanoscale manganese oxides are among the most promising positive-electrode materials for zinc-ion cells, and their performance is further enhanced when these oxides are expressed as conformal deposits on porous carbon architectures, such as carbon nanfoam paper (CNF). We describe an “in-place” conversion of nanometric birnessite Na+-MnOx@CNF to crystalline spinel ZnMn2O4@CNF, a manganese oxide polymorph that nominally contains sites for Zn2+ insertion. The ZnMn2O4@CNF cathodes are electrochemically conditioned in two-terminal cells and ex situ characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. Despite specific Zn2+ insertion sites in ZnMn2O4, we demonstrate that the predominant discharge mechanism involves coupled insertion of protons and precipitation of Zn4(OH)6SO4·xH2O; upon recharge, protons deinsert and Zn4(OH)6SO4 dissolves.

Birnessite-type manganese oxides, consisting of stacked MnOx sheets, separated by charge-balancing metal ions and structural water are potential candidates for electrochemical applications. Due to their structural complexity, Raman spectroscopy is one of the most widely used techniques for studying this class of materials. However, the interpretation of the Raman spectra is still debated. In fact, the observed Raman bands are often intuitively assigned to either Mn–O or M–O vibrations, where M corresponds to charge balancing metal ions such as Na, Ca, and K-ions which are present in natural forms of birnessite. Here, we report a combined experimental and computational study that, opposite to the commonly accepted assignments, strongly suggests that many of the characteristic Raman bands can be attributed to vibrations related to the interlayer water. Our computational findings are compared with detailed ex situ and in situ Raman spectroscopy/X-ray diffraction, and cyclovoltammetry results for K–birnessite, allowing for a full explanation of potential-dependent changes in the Raman spectra. Furthermore, the excellent correlation between the intensity of a band positively assigned to water vibrations and the d-spacing, give support for the important influence of interlayer water on the Raman spectra. This study points to the crucial role of arrangement and content of structural water and deepens the current understanding of hydrated birnessites.

The role of geometric frustration of water molecules in the rate of water oxidation in the nanoconfined interlayer of manganese-oxide layered materials (birnessite, buserite) is examined in a well-controlled experiment. Calcium buserite is prepared, and used in a split-batch synthetic protocol to prepare calcium birnessite, sodium buserite, and sodium birnessite, and partially dehydrated sodium birnessite. Thus, four samples are prepared in which features effecting catalytic efficiency (defect density, average manganese oxidation state) are controlled, and the main difference is the degree of hydration of the interlayer (two layers of water in buserites vs. one layer of water in birnessite). Molecular dynamics simulations predict birnessite samples to exhibit geometric water frustration, which facilitates redox catalysis by lowering the Marcus reorganization energy of electron transfer, while buserite samples exhibit traditional intermolecular hydrogen bonding among the two-layer aqeuous region, leading to slower catalytic behavior akin to redox reactions in bulk water. Water oxdiation activity is investigated using chemical and electrochemical techniques, demonstrating and quantifying the role of water frustration in enhancing catalysis. Calculation and experiment demonstrate dehydrated sodium birnessite to be most effective, and calcium buserite the least effective, with a difference in electrocatlytic overpotential of ∼750 mV and a ∼20-fold difference in turnover number.

Ceria doped with manganese in various amounts was prepared via hydrothermal synthesis. XRD revealed that beside ceria, monoclinic birnessite (Na0.55Mn2O4 × 1.5H2O) appears in the samples. The occurrence of birnessite was achieved using different synthesis conditions and precursors. SEM imaging revealed birnessite layered formations surrounded with ceria nanoparticles, while EDS confirmed the presence of both phases. Weight percentage, lattice constants and crystallite size of ceria, as well as birnessite, were obtained through whole powder pattern decomposition. Pure and Mn doped ceria samples with crystallite sizes between 3.1 and 3.4 nm, and specific surface area values between 183 and 212 m2 g−1 were obtained. It was established that the amount of Mn entering the ceria lattice or forming birnessite varies with nominal composition. Introduction of Mn in ceria is evident from the decrease of ceria lattice constant (from 5.4185 A to 5.4002 A). The entrance of manganese in the ceria crystal lattice was confirmed via EDS spectroscopy. Thermally induced changes were monitored via DTA/TGA, XRD and FTIR. Birnessite decomposes below 200 °C, while thermal treatment at 500 °C for 2 h yields Na2Mn5O10. Weight percentage of birnessite was confirmed through calculations based on the loss of interlayer water during TGA analysis. HRTEM revealed nanorod morphology of Na2Mn5O10, while EELS gave insight into the composition of this phase.

The mineral composition of ferromanganese crusts from Belyaevsky Volcano in the Central Basin of the Sea of Japan has been studied. The crusts have diverse structures and a complex polymineral composition. Along with the previously known todorokite, birnessite, manganite, pyrolusite, and vernadite, we determined rhodochrosite, quenselite, manganosite, hetaerolite, manganotantalite, and tungomelane for the first time. The group of accessory minerals includes magnetite, titanomagnetite, hematite, titanohematite, pyrite, pyrrhotite, galena, monazite, cerianite, and baddeleyite. Copper gold was recognized for the first time in manganese breccia. The origin of manganese minerals was caused by low-temperature hydrothermal fluids, which supplied metals and defined local conditions and intermittent growth of manganese crusts. Sapropel organic matter of siliceous–clayey carbonaceous sediments in the Cenozoic sequences of the deep basin could be a source of metals and the reason for the suboxidizing environments of crystallization of some manganese (manganite, todorokite, and birnessite) and accessory (intermetallics, sulfides, and native elements) minerals.

Various manganese oxides are considered as rather active ORR catalysts and/or supercapacitor materials for operation in alkaline medium. However all these oxides are thermodynamically unstable at high pH and undergo more or less slow transformations to non-stoichiometric oxides containing alkali metal cations M (MxMnO2*nH2O, known also as birnessite mineral). We report a possibility to deposit birnessite thin layers by means of permanganate cathodic reduction. We consider ORR activities and reversible recharging data for birnessite deposited at various potentials with account for XRD information on birnessite cell disordering and microscopic data on the morphology of the deposits. Deposition is complicated by formation of soluble Mn(V) and Mn(VI) species and their disproportionation. However fortunately these by-side reactions appear to be strongly inhibited by forming birnessite solid, and the resulting current efficiencies are high enough. We compare deposition efficiency and adhesion for Pt, Ni, and glassy carbon supports. The best adhesion is found for Ni support. Deposits morphology strongly depends on deposition potential. Thin deposits formed at low overvoltage demonstrate homogeneously distributed pores, and only starting from sub-mkm thickness branched flakes appear, which are typical for natural birnessite morphology. Contrary, at higher overvoltage the deposits start to branch from the very beginning, and consist from thin flakes exclusively. This morphology is unfavorable for electrode behavior because of too high material resistance. Deposition transients allow to monitor the morphology. For flakes formation, oscillatons at deposition transients are observed, which can be associated with layer-by-layer growth. The recharging of all birnessite materials is reversible, but the specific capacity strongly depends on deposition potential and deposit thickness. The best results (ca. 500 mkF/cm2 at scan rates up to 0.1 V/s) are found for thin deposits fabricated at low overvoltage. ORR mass activity is comparable with observed earlier for MnO2, which is believed to undergo slow transformation to birnessite in alkaline solutions. The important advantage of electrodeposited birnessite as compared to the majority of chemically synthesized manganese oxides is a possibility to operate in the absence of conducting binder.

The layered manganese dioxide (δ-type MnO2) film electrodeposited on carbon cloth (CC) functioned as a cathode in aqueous zinc-ion batteries. Battery performance was notably improved by doping MnO2...

Birnessite-type manganese oxides and their magnetic composites with iron oxides are known due to their effective properties in water oxidation catalysis (WOC). The present work demonstrates how transition metal ion doping influences the WOC performances of magnetic and non-magnetic MnOx catalysts in a comparative manner. Transition metal cation (TM = Cr3+, Co2+, Ni2+, Cu2+, Zn2+) doped layered birnessite and birnessite-manganese ferrite composite nanocatalysts were synthesized and characterized. Core-shell type, magnetic composite catalyst particles of similar to 150 nm diameter were obtained by in situ deposition of birnessite shells onto 50-60 nm sized manganese ferrite spheres by reducing MnO4- ions with propionic acid in the presence of the TM ions. The resulting TM-doped catalyst materials were analyzed by XRD, SEM, TEM, IR, TGA and AAS to verify their compositions, structures and morphologies. WOC activities and stabilities were comparatively examined in both chemical (using Ce4+ as sacrificial oxidant at pH similar to 1-2) and electrochemical (pH 7, screen-printed particles on FTO) test setups. In both types of experiments, the highest catalytic activity was observed for catalysts doped with Co2+ ions, followed by Ni2+-doped samples. In comparison to the K-birnessite reference material, the Co2+- and Ni2+-doped analogs were additionally found to be more stable and more active in the chemical and long-term electrocatalysis measurements, demonstrating the potential of TM-doping to improve the performance of magnetic-MnOx-based WOC materials. In addition, the formation of a magnetic core-shell structure converts the micron-sized amorphous MnO2 particles to spherical at the nanoscale and thus facilitates the mass-energy transfer throughout the entire catalyst surface.

Anaerobic oxidation of methane (AOM) is a major biological process that reduces global methane emission to the atmosphere. Anaerobic methanotrophic archaea (ANME) mediate this process through the coupling of methane oxidation to different electron acceptors, or in concert with a syntrophic bacterial partner. Recently, ANME belonging to the archaeal family Methanoperedenaceae (formerly known as ANME-2d) were shown to be capable of AOM coupled to nitrate and iron reduction. Here, a freshwater sediment bioreactor fed with methane and Mn(IV) oxides (birnessite) resulted in a microbial community dominated by two novel members of the Methanoperedenaceae, with biochemical profiling of the system demonstrating Mn(IV)-dependent AOM. Genomic and transcriptomic analyses revealed the expression of key genes involved in methane oxidation and several shared multiheme c-type cytochromes (MHCs) that were differentially expressed, indicating the likely use of different extracellular electron transfer pathways. We propose the names “Candidatus Methanoperedens manganicus” and “Candidatus Methanoperedens manganireducens” for the two newly described Methanoperedenaceae species. This study demonstrates the ability of members of the Methanoperedenaceae to couple AOM to the reduction of Mn(IV) oxides, which suggests their potential role in linking methane and manganese cycling in the environment.

Manganese oxides (MnO2) are a class of promising electrode materials for high-performance supercapacitors. Among various possible crystal phases of MnO2, the two-dimensional layered birnessite (δ-MnO2) is considered to facilitate cation intercalation/deintercalation with little structural rearrangement during the charging and discharging process. Here, we report a facile strategy for synthesis of δ-MnO2 with significantly enhanced electrochemical capacitive energy storage performance via phase transformation of orthorhombic MnO2. The resultant δ-MnO2 delivers a specific capacitance of 251.4 F g−1 at a current density of 1 A g−1, which is almost three times higher than that of the original orthorhombic phase. The solid-state flexible asymmetric supercapacitor based on birnessite MnO2 as the positive electrode possesses an operating potential window as high as 2.0 V as well as high energy density and power density. This work demonstrates the post crystal engineering of MnO2 is an effective way to optimize their electrochemical properties.

Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 μg L-1 due to mounting evidence that VV exposure can lead to adverse health outcomes. Groundwater V concentration exceeds the notification level in many locations, yet geochemical controls on its mobility are poorly understood. Here, we examined the redox interaction between VIV and birnessite (MnO2), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble VIV species such as haggite [V2O3(OH)2] into highly mobile and toxic vanadate (HnVO4(3-n)-) in continuously stirred batch reactors under neutral pH conditions. Synchrotron X-ray absorption spectroscopic (XAS) analysis of in situ and ex situ experiments showed that oxidation of VIV occurs in two stages, which are both rapid relative to the measured dissolution rate of the VIV solid. Concomitantly, the reduction of birnessite during VIV oxidation generated soluble MnII, which led to the formation of the MnIII oxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between VV and birnessite, although retention of VV was minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of VIV in the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.

Novel nanocomposite adsorbent based on manganese dioxide-loaded polyacylonitrile propylene fibers was synthesized for selective removal of strontium ions from contaminated waters by in situ formation of manganese dioxide with layered structure (birnessite) nanoparticles on the fibers surface. Data of scanning electron microscopy and X-ray diffraction confirmed the formation of birnessite homogeneous layer on the fibers surface, which consisted of rounded nanoparticles (50 to 70 nm). The efficiency of the synthesized adsorbent for removal of strontium ions was studied under various experimental conditions. It has demonstrated a rapid adsorption process, high adsorption efficiency over a wide pH range, and selectivity in Sr ions removal from model solutions with high salt content.

Dissolved organic matter (DOM) varies widely across natural and engineered systems, but little is known about the influence of DOM composition on its reactivity with manganese oxides. Here, we investigate bulk and molecular transformations of 30 diverse DOM samples after reaction with acid birnessite (MnO2), a strong oxidant that may react with DOM in Mn-rich environments or engineered treatment systems. The reaction of DOM with acid birnessite reduces Mn and forms DOM that is generally more aliphatic and lower in apparent molecular weight. However, the extent of reaction depends on the water type (e.g., wastewater, rivers) and highly aromatic DOM undergoes greater changes. Despite the variability in reactivity due to the DOM composition, aqueous products attributable to the oxidation of phenolic precursors are identified in waters analyzed by high-resolution mass spectrometry. The number of matched product formulas correlates significantly with indicators of DOM aromaticity, such as double-bond equivalents (p = 2.43 × 10-4). At the molecular level, highly aromatic, lignin-like carbon reacts selectively with acid birnessite in all samples despite the variability in initial DOM composition, resulting in the formation of a wide range of aqueous products. These findings demonstrate that DOM oxidation occurs in diverse waters but also suggest that reactivity with acid birnessite and the composition of the resulting aqueous DOM pool are composition-dependent and linked to the DOM source and initial aromaticity.

Phenylarsonic acid compounds could be oxidized by manganese oxides in surface soils, resulting in quick release of inorganic arsenic. This study investigated the structure–reactivity relationships ...

Insight into the influence of hydration energy of structural cations within birnessite-type layered MnO2 on charge storage mechanisms via redox reaction and intercalation/deintercalation processes ...

Relative contribution of microbially mediated and purely abiotic processes during Mn(II) oxidation is evaluated by investigating microbially colonized manganese deposits at the Sambe hot spring in Japan. Microscopic observations and DNA analyses suggest that manganese-oxidizing bacteria are present in the deposits at all of the sites investigated, while thick microbial mats, which consist mainly of phototrophic microorganisms, are developed at only one site. Microelectrode measurements (pH, O2 concentration, and Mn(II) concentration) near the deposit surface indicate that microbially mediated processes are dominant at all sites. These microbially mediated processes exceed the purely abiotic process by more than 2–24 times. At sites without abundant phototrophs, the major microbially mediated process that is occurring is light-independent oxidation (e.g., direct/indirect oxidation via manganese-oxidizing bacteria/fungi). In contrast, the site with abundant phototrophs is characterized by the occurrence of light-independent oxidation (mostly indirect oxidation by oxygenic phototrophs), and the contribution of light-independent oxidation is subordinate. Chemical gradients, which are established within a diffusive boundary layer and microbial mat, drive indirect oxidation by oxygenic phototrophs, and the O2 concentration gradient is particularly important when the concentration of dissolved inorganic carbon is higher than ca. 3 mM. X-ray diffraction and X-ray absorption fine-structure spectrometry analyses indicate that microbially mediated processes yield poorly-crystalline birnessite (i.e., vernadite) regardless of their relative contribution. The observations of this study imply that (1) photosynthetic increases in the O2 concentration drive indirect oxidation by oxygenic phototrophs at high concentrations of dissolved inorganic carbon like the Precambrian ocean, and that (2) the contribution of indirect oxidation by oxygenic phototrophs to ancient manganese oxides can be evaluated via geochemical redox proxies.

Iron and manganese have been studied as a proposal for new materials that could be used for the adsorption of arsenic (V) (As (V)), in order to remove this contaminant. The objective of this work was to study the effect of the molar ratio of three Mn/Fe + Mn composites (X = 0.17, 0.32, 0.47) on the properties of adsorbent media, and to determine their influence on arsenic removal by comparing them with two metallic oxyhydroxides, that are commonly used as adsorbents of As(V) in aqueous solution (goethite and birnessite). These media were synthesized by chemical precipitation while controlling particle size. They were characterized using X-ray diffraction (XDR), scanning electron microscopy with X-ray dispersive energy spectrometry (SEM-EDS), surface area analysis by the BET method, Fourier transform infrared spectroscopy (FTIR), and isoelectric point analysis (IEP). The surface area (286 m2/g) of the composite with a molar ratio of X = 0.17 was larger than that of the other media. The adsorption kinetics and isotherms were fitted to the mathematical models, specifically, the pseudo-second order and Langmuir, respectively. The X = 017 composite had an adsorption capacity of 3.28 mg/g and removed 99% of As(V) with an initial concentration of 0.5 and 97% with an initial concentration of 10 mg/L, at 180 min, 25 °C, and pH 7. The five adsorbent media were tested with well water with an initial As(V) concentration of 0.075 mg/L, and the best behavior was exhibited with a molar ratio of X = 0.17 at 90 min, resulting in 100% removal of As(V). The results suggest that this material is an effective and viable alternative to remove this contaminant from water.

High selectivity and ion-exchange character of birnessite-type sorbents make these materials promising for Sr-90 removal from radioactive wastes containing seawater. However, it is still debatable how their functional properties are related to the nature of active phases and structure and, therefore, routes to improve birnessite performance are virtually unknown. Here we study the evolution of structure, surface morphology, and redox and sorption properties of birnessite-type sorbents in sorption/regeneration cycles and after reductive treatment with hydrazine and hydrogen to enhance birnessite uptake of Sr-90 from highly mineralized solutions. The influence of various treatment procedures on physico-chemical characteristics of birnessite has been evaluated using XRD, SEM, low temperature nitrogen sorption, and temperature-programmed reduction. Temperature-programmed reduction was implemented to reveal changes in birnessite’s active sites distribution and morphology, which play a crucial role in sorption behavior of the material. The sorption activity towards Sr-90 has been evaluated for the birnessites under static and dynamic conditions. The original approach used for characterization of birnessite sorption materials has helped to reveal how these sorbents change under operating conditions, and provided routes to the targeted modification of these manganese-oxide materials.

Unusual bedded manganese deposits occur within hemipelagic carbonates, dated as early Messinian ( 5.97 Ma) based on calcareous nannofossils. These sediments accumulated during a time of increased isolation of the Mediterranean Sea from the Atlantic Ocean but prior to major sea-level fall and evaporite precipitation during the Messinian salinity crisis. The manganese deposits form six main laterally continuous beds (up to 70 cm thick). Compositionally, they are dominated by poorly crystalline manganese oxide (birnessite), with associated mild enrichment in some trace elements (e.g., Ba, Ni, Cu, Mo, As, U). Background minerals include magnesian carbonate, kaolinite, chlorite, mixed-layer clays and talc (serpentinite-derived). A tectonically controlled deep-water basin existed to the south of a topographic ridge (future Kyrenia Range) in which seawater alternated between oxidising and oxygen-depleted at depth. Orbital precession influenced the weathering of continental source areas that included basic igneous rocks and some pre-existing hydrothermal deposits. After entering the basin via land runoff, manganese was retained in solution below the chemocline during stagnant low-oxygen periods. Manganese oxide and associated trace elements (e.g., Mo, U) precipitated in response to cyclical seawater oxidation to form manganese-rich layers up to layers with up to with up to c. 30% MnO. To reach these very high concentrations, soluble manganese may have fluxed upwards from an anoxic environment represented by early Messinian sapropels. Manganese precipitation events diminished with time giving way to well-oxygenated hemipelagic marl accumulation with a shelly infauna. Gypsum appears abruptly upwards above the marls in adjacent sections. The Mn deposition mainly represents an interplay between tectonically-controlled basin formation, chemically favourable source climatic conditions, possible sea-level change effects and likely diagenetic manganese enrichment. The regional tectonic setting relates to early-stage collision of the African and Tauride (Eurasian) plates.

Birnessite, a layered manganese (Mn) oxide, possesses extraordinary metal adsorption and oxidation activity, and thus imposes impacts on many biogeochemical processes. The reactivity of birnessite strongly relies on its Mn oxidation state composition (the proportions of Mn II, III and IV), particularly by the Mn(III) proportion. Partial reduction of birnessite transforms birnessite to be Mn(II, III)-rich or to MnOOH and Mn3O4, and thus strongly affects birnessite reactivity. As a metal scavenger, naturally occurring birnessite contains abundant transition and alkali and alkaline earth metal cations in its structure; however, the effects of these metal cations on the partial reduction-induced transformation of birnessite remain unknown. We examined the effects of Zn2+, Mg2+, Ca2+ and ionic strength (controlled by NaCl) on transformation of birnessite (δ-MnO2) and adsorption and oxidation of natural organic matter during partial reduction by fulvic acid (FA) at pH 8 and FA/MnO2 mass ratios (R) of 0.1 or 1 over 600 h under anoxic conditions. Results showed that low ionic strength (0 versus 50 mM NaCl) disfavored FA adsorption, fractionation and oxidation, and thus disfavored formation of Mn(III) in the reacted birnessite. Compared to the 50 mM NaCl system, all divalent cations (Mg2+, Ca2+ and Zn2+) favored FA adsorption and fractionation. Both Mg2+ and Ca2+ significantly enhanced FA oxidation at the early stage but barely at the late stage, whereas Zn2+ strongly suppressed FA oxidation during the entire experimental period. Due to adsorption competition, the presence of the divalent cations resulted in low concentration of Mn(II) adsorbed on vacancies of birnessite. Both Ca2+ and Mg2+ favored Mn(III) production in MnO6 layers, while Zn2+ inhibited it. A small portion of birnessite also transformed to feitknechtite and hausmannite, and the transformation seemed faster in the presence of Ca2+ or Mg2+ than in NaCl solution. In the presence of Zn2+ at the high FA/MnO2 ratio (R = 1), Zn-substituted hansmannite formed extenstively. The formation of Mn(III) in the reacted birnessite can be ascribed to comproportionation between Mn(IV) and Mn(II) adsorbed on either vacancies or edge sites of birnessite. The low-valence Mn oxide phases likely formed via the comproportionation on the edges. The divalent cations affected Mn(III) concentrations of birnessite and formation of the low-valence Mn oxides by competing with Mn(II) for adsorption on edge/vacancy sites or stabilizing Mn(III) in the layers. This work indicates that divalent metal cations strongly influence reactivity and transformation of birnessite in the coupled Mn and carbon redox cycles, and that birnessite containing divalent cations can be an important adsorbent for natural organic carbon in Mn-rich environments. Overall, this study provides insights into the coupled cycles of Mn, trace metals and organic carbon in alkaline and saline environments.

Long-term cycling stability is a main issue for the application of manganese-based electrodes in pseudocapacitors. Herein, we demonstrate a facile approach to enhance the cycling stability of birnessite electrodes by pre-adding slight Mn2+ into the aqueous electrolyte. The cycling performance of the electrode and the morphology of birnessite change with Mn2+ concentration, which are ascribed to the electrochemical oxidation of Mn2+ or the reduction of birnessite during the charge/discharge cycling process. By controlling a low concentration of Mn2+, the birnessite electrode exhibits enhanced capacitance and cycling stability, while a hierarchical nanosheets assembled nanowall array architecture is achieved after the prolonged cycling. The resulting electrode delivers a superior rate capability (60% retention at 20 A g−1) and a high cycling stability (93% retention after 10000 cycles). This work opens up a new strategy for the development of manganese-based electrodes with high rate performance and long-term cycling stability.

Manganese(III/IV) oxides are naturally occurring oxidants of arsenic (As) and can transform the more mobile and toxic arsenite [As(III)] to the less mobile and less toxic arsenate [As(V)]. However,...

In this work the influence of cobalt doping on both properties and catalytic activity of cryptomelane and birnessite in soot combustion was investigated in detail. The investigated samples have been synthesized by a microwave-assisted hydrothermal method and cobalt was introduced in two ways—by impregnation or by addition to the precursor mixture before hydrothermal treatment. The obtained catalysts have been characterized by XRF, BET, XRD, RS, XPS and ATR-IR. Surprisingly, no distinct effect of cobalt presence on catalytic activity has been found. Most probably cobalt was localized in the position of manganese within the cryptomelane/birnessite structure or within segregated manganese-cobalt complex oxides, where its oxidation level was fixed. A series of reference manganese-cobalt mixed oxides (100–0% Mn) have been synthesized to elucidate this effect. These samples have been structurally characterized and thermal evolution of their structures was profoundly investigated. A high tendency of cobalt incorporation into the manganese oxide matrix and its subsequent stabilization in the spinel forms was proposed as an explanation of the observed lack of a promotional effect of this additive in soot combustion.