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Crystal truncation rod (CTR) X-ray diffraction is an invaluable tool for measuring mineral surface and adsorbate structures, and has been applied to several environmentally and geochemically important systems. Traditionally, the method has been restricted to single crystals with lateral dimensions >3 mm. Minerals that meet this size criterion represent a minute fraction of those that are relevant to interfacial geochemistry questions, however. Crystal screening, data collection, and CTR measurement methods have been developed for crystals of <0.3 mm in lateral size using the manganese oxide mineral chalcophanite (ZnMn3O7·3H2O) as a case study. This work demonstrates the feasibility of applying the CTR technique to previously inaccessible surfaces, opening up a large suite of candidate substrates for future study. [ABSTRACT FROM AUTHOR]

The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and Fe2+, simultaneous oxidation of the two progressed by MnO2 and the resultant As(V) and Fe3+ triggered the formation of crystalline scorodite (FeAsO4·2H2O). At 0.5% or 0.25% MnO2, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 « 0.01mg/L at 0.5% MnO2, 0.78 « 0.05mg/L at 0.25% MnO2. For the oxidation of As(III) and Fe2+, 54% (at 0.5% MnO2) or 14% (at 0.25% MnO2) of initially added MnO2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of Mn2+-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved Mn2+ as biogenic birnessite ((Na, Ca)0.5(MnIV,MnIII)2O4·1.5H2O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems. [ABSTRACT FROM AUTHOR]

Nanostructured birnessite is of interest as an electrode material for aqueous high power electrochemical energy storage as well as desalination devices. In neutral pH aqueous electrolytes, birnessite exhibits a capacitive response attributed to the adsorption of cations and protons at the outer surface and within the hydrated interlayer. Here, we utilize the understanding of proton‐coupled electron transfer (PCET) in buffered electrolytes to decouple the role of protons and cations in the capacitive charge storage mechanism of birnessite at neutral pH. We find that without buffer, birnessite exhibits primarily potential‐independent (capacitive) behavior with excellent cycling stability. Upon the addition of buffer, the capacity initially increases and the cyclic voltammograms become more potential‐dependent, features attributed to the presence of PCET with the birnessite. However, long‐term cycling in the buffered electrolyte leads to significant capacity fade and dissolution, which is corroborated through ex situ characterization. ReaxFF atomistic scale simulations support the observations that proton adsorption leads to birnessite degradation and that capacitive charge storage in birnessite is primarily attributed to cation adsorption at the outer surface and within the interlayer. [ABSTRACT FROM AUTHOR]

Purpose: Methionine is a redox-sensitive sulfur-containing amino acid and can be oxidized to sulfur compounds, including sulfoxide and other volatile sulfur compounds participating in the entire sulfur cycle. However, further definition of the kinetics and transformation pathways of methionine mediated by manganese oxide under environmental conditions is required. This study aimed to clarify the transformation mechanism of methionine mediated by birnessite to improve understanding of the important role of methionine in the sulfur cycle. Methods: The kinetics of birnessite-mediated degradation of methionine as a function of methionine concentration, birnessite loading, pH, and co-solutes were investigated. High-performance liquid chromatography (HPLC) and ultra-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS) were used to identify the degradation products of methionine. Results: Methionine was rapidly degraded by birnessite. An acidic solution facilitated the degradation reaction, and metal ions and anions markedly hindered it. The initial reaction kinetics indicated that the reaction orders with respect to methionine and birnessite were 1.3 and 1.5, respectively, at a pH of 5.0 and 0.7 for H+ when pH ranged from 5.0 to 7.0. A third-order initial rate constant was determined to be (3.29 ± 0.91) × 10−4 μM−2.5 min−1. Three functional groups of methionine played dominant roles in the oxidative transformation. Birnessite activated the thioether group of methionine to trigger the transformation of amino and carboxyl groups, during which the thioether group was oxidized to sulfoxide and then gradually to sulfone. The amino group was finally converted to NH4+ and carboxyl could be removed as CO2. Conclusions: Methionine can be transformed to a wide range of products by birnessite once released into the environment. The present study demonstrates a more comprehensive transformation mechanism of methionine mediated by birnessite and further clarifies the fate of methionine in soils and sediments. [ABSTRACT FROM AUTHOR]

[Display omitted] • NiFe-LDH/MnO 2 electronic heterojunction is successfully prepared. • NiFe-LDH/MnO 2 composites, as photocatalysts, are able to abate NO gases. • NiFe-LDH/MnO 2 heterojunction boosts the formation of superoxide radicals. • NiFe-LDH/MnO 2 exhibits a high NO removal efficiency under visible light. 2D/2D heterojunctions between ultrathin NiFe-CO 3 layered double hydroxide (LDH) and varying amounts of birnessite (δ-MnO 2) were prepared using an environmentally friendly and cost-effective self-assembly method. The prepared samples have proven to be efficient in their application, mitigating NOx emissions via a photooxidative process. The effective creation of 2D/2D NiFe–LDH/MnO 2 heterojunctions has been verified through a variety of characterisation techniques, such as XRD, FT-IR, TGA, XFR, N 2 adsorption–desorption isotherms, HRTEM and SEM images, EDX, and XPS. Photocatalytic measurements have indicated that the heterojunction formed between NiFe-LDH and MnO 2 modifies its photoconductive behaviour, significantly enhancing the visible light photocatalytic performance of the partners. Finally, the optimal relationship between NiFe-LDH and MnO 2 in the photocatalytic process was studied. It was found that a ratio of 10:1, respectively, exhibited superior properties compared to the other composites, highlighting the best performance in the degradation of NO under visible light and notable stability during the recycling process. [ABSTRACT FROM AUTHOR]

• The water oxidation activity of birnessite was significantly altered by the doping ions. • Ca-birnessite exhibits a smaller overpotential of 579 mV at 10 mA/cm2 compared to K-birnessite. • Doping-Ca enhanced the kinetic properties with a significant reduction of 100 mV/dec in the Tafel slope. Birnessite is an affordable and high-efficiency catalyst for water oxidation. In this study, we attempted to doping Ca into birnessite to investigate the influence on its photocatalytic oxidation activity. The study was carried out through a combination of experimental investigations and DFT calculations of the relevant crystal and electronic structures. The analysis of Raman spectra using 2T2D correlation and DFT indicates that Ca contributes more to the high frequency regions (400–600 cm−1) than K. DFT further reveals that the introduction of Ca leads to an increase in the number of localized electrons at the Fermi energy level and in the conduction band. Photo-electrochemical experiments demonstrate a significant reduction of 100 mV/dec in the Tafel slope, along with a 1.48-fold increase in photo-current density for Ca-birnessite when compared to K-birnessite. In general, the results demonstrate that Ca-birnessite can effectively heighten visible light absorption, reduce the band gap, and enhance water oxidation activity. [ABSTRACT FROM AUTHOR]

Abstract The aim of this work was to study the effect of the microwave radiation on the thermal and spectroscopic features, as well as about arrangement (order-disorder) and morphological properties, of the layered manganese oxide with birnessite-type structure. The route employed to obtain Na-birnessite matrix was redox precipitation. The products were characterized by X-ray diffraction, thermal analysis (TG-DTG-DSC), infrared (FTIR) and Raman spectroscopy, scanning electron microscopy (SEM) and nitrogen adsorption-desorption technique. The results showed that microwave radiation influenced in a short time (5 min) the octahedral ordering of birnessite, as well as in increasing the crystallite size. Thermal analysis showed that the thermal behavior of the lamellar matrix was different from that of birnessite under microwave radiation. After microwave-assisted hydrothermal treatment, FTIR and Raman spectroscopy investigations were used to differentiate ordered and disordered birnessites. Otherwise, there were no changes in SEM morphology of the lamellar-type phases, but the particle size changed.

Birnessite nanoflowers composed of layers have been proven to be the strongest adsorbent and oxidant in the surface environment. However, the current synthesis methods of birnessite nanoflowers are suffering from long reaction time and high reaction temperature. Based on these, this paper explores a new method for the rapid and controlled synthesis of layered manganese oxides. The method relies on the molar ratios of KMnO4 and H2O2 redox reacting species to drive the production of birnessite nanoflowers under acidic conditions. The molar ratios of KMnO4 and H2O2 are the key to the crystal structure of the as-prepared. It was found that when the molar ratios of KMnO4 and H2O2 is from 1:1.25 to 1:1.90, the sample is birnessite nanoflowers, and when the ratio is increased to 1:2.0, the sample is a mixture of birnessite nanoflowers and feitknechtite nanoplates. Among the as-prepared samples, BF-1.85 (molar ratios of KMnO4 and H2O2 is 1:1.85) shows the highest capacity for Pb2+ adsorption (2,955 mmol/kg) and greatest degradation efficiency of phenol and TOC. The method proposed herein is economical and controllable, and it yields products with high efficiency for the elimination of inorganic and organic pollutants.

Studies on oxidation kinetics of sulfadiazine (SDZ) using δ−MnO2 (birnessite) and natural MnO2 are limited. Reaction order at different SDZ speciation was determined based on the effects of initial H+, MnO2 and SDZ concentrations using initial rate method, which would be useful to determine the optimum pH and MnO2 concentration. Birnessite and natural MnO2 with different physico-chemical properties such as BET surface area, pHPZC, d-spacing, and crystal size similarly showed good efficiencies in oxidizing neutral SDZ (pH 5) and anionic SDZ (pH 8). Activation energy (Ea) and thermodynamic parameters indicated the similar oxidation efficiencies in the temperature range of 10–40°C. The S O 4 2 − was produced from the SDZ oxidation coupled to the reduction of MnO2 to Mn2+. The effect of co-solute ciprofloxacin (CIP) on the oxidation kinetics of SDZ was also studied. The rates of SDZ oxidation by both birnessite and natural MnO2 were reduced by the presence of CIP due to competition in oxidation between SDZ and CIP. The SDZ was more rapidly oxidized than CIP in both single- and bi-solute systems, as indicated by the presence of CIP intermediate, whereas the intermediate of SDZ was not detected. [ABSTRACT FROM AUTHOR]

Raman spectra were collected for an extensive set of well-characterized layer-structure Mn oxide mineral species (phyllomanganates) employing a range of data collection conditions. We show that the application of various laser wavelengths, such as 785, 633, and 532 nm, at low power levels (30–500 μW) in conjunction with the comprehensive database of standard spectra presented here, makes it possible to distinguish and identify the various phyllomanganate minerals. The Raman mode relative intensities can vary significantly as a function of crystal orientation relative to the incident laser light polarization direction as well as incident laser light wavelength. Consequently, phase identification success is enhanced when using a standards database that includes multiple spectra collected for different crystal orientations and with different laser light wavelengths. The position of the highest frequency Raman mode near 630–665 cm–1 shows a strong linear correlation with the fraction of Mn3+ in the octahedral Mn sites. With the comprehensive Raman database of well-characterized Mn oxide standards provided here (and available online as Online Material Deposit item AM-21-37666, Online Material. Deposit items are free to all readers and found on the MSA website, via the specific issue's Table of Contents (go to http://www.minsocam.org/MSA/AmMin/TOC/2021/Mar2021%5fdata/Mar2021%5fdata.html).), and use of appropriate data collection conditions, micro-Raman is a powerful tool for identification and characterization of biotic and abiotic Mn oxide phases from diverse natural settings, including on other planets, as well as for laboratory and industrial materials. [ABSTRACT FROM AUTHOR]

• pH conditions during tectomanganate formation control tunnel size. • Acidic conditions favor the formation of tectomanganates with small tunnel sizes. • Co remains structurally incorporated in tectomanganates with large tunnel sizes. • Under acidic conditions, increased content of corner-sharing octahedra releases Co. The present work uncovers the geochemical control on the nature (tunnel size) of the tectomanganates formed from layered precursors, and thus provides insights into the formation of Mn oxides in natural environments. Large tunnel sizes are favored under circum-neutral conditions, whereas low pH conditions favor the formation of tectomanganates with smaller tunnel sizes. Both the increased proportions of Mn(III) in vernadite/birnessite layers resulting from low pH conditions and the subsequent enhancement of Mn(III) disproportionation during subsequent transformation contribute to the formation of tectomanganates with smaller tunnel sizes. The fate of foreign elements during the phyllomanganate-to-tectomanganate mineral transformation is another important aspect of this mineral transformation, together with the impact of these elements on the transformation. Layered and tunnel Mn oxides have indeed a pivotal influence on the geochemical cycling of transition metals, including Co, that possess a strong affinity for these mineral species. The present experimental work shows that the formation of todorokite (3 × 3 tunnel size), hollandite (2 × 2), or nsutite (intergrown 1 × 1 and 1 × 2 fragments) is essentially unaffected by limited Co-enrichment (≤5 at.%) of the initial phyllomanganate structure. Higher Co contents reduce the content of Jahn-Teller distorted Mn(III) octahedra in layered precursor and hamper the phyllomanaganate-to-tectomanganate transformation. Finally, Co is retained in the structure of todorokite and hollandite during their formation under circum-neutral conditions whereas part (∼20%) of the Co present in layered precursors is expelled out of the framework and/or sorbed to nsutite formed under acidic conditions. This effect is induced by the reduced stability of Co(III) octahedra when the relative proportion of corner-sharing linkages increases. In turn, this effect influences Co structural incorporation in different Mn oxides and its potential release to solution. [ABSTRACT FROM AUTHOR]

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 as Pseudomonas putida and Bacillus species) 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. [ABSTRACT FROM AUTHOR]

The hydrothermal treatment of neutral and acidified KMnO4 solutions at concentration of 0.037–0.093 mol/L and temperature of 120, 170, and 220°С for 24 h leads to selective synthesis of three different manganese dioxide modifications: α-MnO2, δ-MnO2, and β-MnO2. The temperature of hydrothermal treatment and medium acidity has been shown to affect considerably the phase composition of KMnO4 reduction products and reaction yield. The obtained samples of MnO2 have been characterized by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and diffuse reflectance spectroscopy. [ABSTRACT FROM AUTHOR]

The removal of lincomycin (LIN) from aqueous solution by birnessite was investigated by batch experiments. When the dosage of birnessite is 500 mg L−1 and the initial concentration of LIN is 15.5 μmol L−1, more than 90% of LIN was removed within 240 min at pH 4.90. Under different conditions, the reactions were well fitted with the second-order model (R2 > 0.95). The removal kinetics and the reaction mechanism were described. The presence of cations (e.g., K+, Ca2+, Mg2+, Fe2+, and Mn2+) inhibited the removal of LIN by birnessite, following the order: Mn2+ > Fe2+ > Ca2+ > Mg2+ > K+ ≈ Na+, due to the sorption of cations on birnessite, companying with the electron transfer and precipitation of oxides (for Mn2+ and Fe2+). The addition of Cu2+, SO42−, or NO3− improved the reactions. The presence of Cu2+ could oxidize antibiotics, and the repulsion between SO42−or NO3− and birnessite might disperse the birnessite suspensions during the reactions. Mn(IV) and Mn(III) were the core Mn species that play an important role in LIN removal. These findings will help to understand the removal process of LIN and illustrate the influence of cations and anions on the removal of similar pollutants by birnessite. [ABSTRACT FROM AUTHOR]

Manganese oxide is perspective material for selective removal of strontium ions from multicomponent solu-tions. Birnessite-type manganese oxide with the hexagonal disordered structure has been synthesized in the form of round microparticles. The synthesized samples have been tested for the removal of strontium ions from mono - and multicomponent solutions. It was found that birnessite can effectively remove strontium ions from mono-component solutions. The presence of alkaline metal ions (up to ~ 0.3 M) in the multicomponent solutions has no significant effect on adsorption; however, calcium ions (Ca/Sr ~ 30/1) lead to the considerable decrease in ad-sorption. The rather high adsorption parameters received in the experimental work allow to consider the synthe-sized birnessite as a sorbent with high selectivity towards strontium ions.

Ceria doped with manganese in various amounts was prepared via hydrothermal synthesis. XRD revealed that beside ceria, monoclinic birnessite (Na 0.55 Mn 2 O 4 × 1.5H 2 O) 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 Å to 5.4002 Å). 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 Na 2 Mn 5 O 10. Weight percentage of birnessite was confirmed through calculations based on the loss of interlayer water during TGA analysis. HRTEM revealed nanorod morphology of Na 2 Mn 5 O 10 , while EELS gave insight into the composition of this phase. [ABSTRACT FROM AUTHOR]

Birnessite not only oxidizes arsenite into arsenate but also interacts with organic matter in various ways. However, effects of organic matter on interaction between As and birnessite remain unclear. This study investigated effects of citrate and EDTA (3.12 and 2.05 mM, respectively) on oxidation of As(III) (1.07 mM) and adsorption of As(V) (0.67 mM) on birnessite (5.19 mM as Mn) at near-neutral pH. We found that As(V) adsorption on birnessite was enhanced by citrate and EDTA, which resulted from the increase in active adsorption sites via dissolution of birnessite. In comparison with citrate batches, more As was adsorbed on birnessite in EDTA batches, where dissolved Mn was mainly presented as Mn(III)-EDTA complex. Citrate or EDTA-induced dissolution of birnessite did not decrease the As(III) oxidation rate in the initial stage where As(III) oxidation rate was rapid. Afterwards, As(III) oxidation was conspicuously suppressed in citrate-amended batches, which was mainly attributed to the decrease in adsorption sites by adsorption of citrate/Mn(II)-citrate complex. This suppression was enhanced by the increase in concentrations of dissolved Mn(II). Citrate inhibited As adsorption after As(III) oxidation due to the strong competitive adsorption of citrate/Mn(II)-citrate complex. However, the As(III) oxidation rate was increased in EDTA-amended batches in the late stage, which mainly derived from the increase in the active sites via birnessite dissolution. The strong complexation ability of EDTA led to formation of Mn(III)-EDTA complex. Arsenic adsorption was not affected due to the limited competitive adsorption of the complex on the solid. This work reveals the critical role of low molecular weight organic acids in geochemical behaviors of As and Mn in aqueous environment. [ABSTRACT FROM AUTHOR]

• Oxidative-coupling reaction took place during diclofenac (DCF) transformation. • Mn(IV) in birnessite was reduced to Mn(III) accompanied by DCF oxidization. • Hydroxylation and dimerization pathways of DCF were proposed. Diclofenac (DCF), a widely used non-steroidal anti-inflammatory, reacted readily with birnessite under mild conditions, and the pseudo first order kinetic constants achieved 8.84 × 10−2 hr−1. Five products of DCF including an iminoquinone product (2,5-iminoquinone-diclofenac) and four dimer products were observed and identified by tandem mass spectrometry during the reaction. Meanwhile, 2,5-iminoquinone-diclofenac was identified to be the major product, accounting for 83.09% of the transformed DCF. According to the results of spectroscopic Mn(III) trapping experiments and X-ray Photoelectron Spectroscopy, Mn(IV) contained in birnessite solid was consumed and mainly converted into Mn(III) during reaction process, which proved that the removal of DCF by birnessite was through oxidation. Based on the identified products of DCF and the changes of Mn valence state in birnessite solid, a tentative transformation pathway of DCF was proposed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

BACKGROUND Microbial fuel cell (MFC) power production is limited by the cathodic oxygen reduction reaction (ORR). The quest for platinum‐free materials for improving the cathodic ORR in MFCs is a challenging task. Birnessite‐type MnO2/carbon cathodes in MFCs have been rarely reported so far. In this work, a birnessite/C cathode was tested in a MFC. RESULTS: A birnessite/C cathode was synthesized (using a rapid, facile and low‐cost method) and its structural and morphological characteristics were assessed. The ORR on such a cathode was investigated using linear sweep voltammetry in a catholyte of 0.8 mol L–1 Na2SO4, pH 2 (same conditions as those of experimental MFCs). The current density values were higher than those for a plain C cathode. The volumetric power density improved from 224 to 6201 mW m−3 on replacing the plain C cathode with the birnessite/C cathode in a MFC. Deltaproteobacteria were found at the C anode and they were associated with the high power densities obtained. CONCLUSIONS: Birnessite/C is a promising cathodic electrode because its synthesis is straightforward, it is not expensive and it could promote the ORR and improve the power density in MFCs. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Microbe-mediated precipitation of Mn-oxides enriched in rare earth elements (REE) and other trace elements was discovered in tunnels leading to the main shaft of the Ytterby mine, Sweden. Defining the spatial distribution of microorganisms and elements in this ecosystem provide a better understanding of specific niches and parameters driving the emergence of these communities and associated mineral precipitates. Along with elemental analyses, high-throughput sequencing of the following four subsystems were conducted: (i) water seeping from a rock fracture into the tunnel, (ii) Mn-oxides and associated biofilm; referred to as the Ytterby Black Substance (YBS) biofilm (iii) biofilm forming bubbles on the Mn-oxides; referred to as the bubble biofilm and (iv) fracture water that has passed through the biofilms. Each subsystem hosts a specific collection of microorganisms. Differentially abundant bacteria in the YBS biofilm were identified within the Rhizobiales (e.g. Pedomicrobium), PLTA13 Gammaproteobacteria, Pirellulaceae, Hyphomonadaceae, Blastocatellia and Nitrospira. These taxa, likely driving the Mn-oxide production, were not detected in the fracture water. This biofilm binds Mn, REE and other trace elements in an efficient, dynamic process, as indicated by substantial depletion of these metals from the fracture water as it passes through the Mn deposit zone. Microbe-mediated oxidation of Mn(II) and formation of Mn(III/IV)-oxides can thus have considerable local environmental impact by removing metals from aquatic environments. [ABSTRACT FROM AUTHOR]

Birnessite structured MnO x with tuned potassium, copper and water content in the interlayer spacing is produced by a simple and cost-effective method. The new structures are investigated by XRD, Raman spectroscopy, SEM, EDX, HRTEM, DLS, TG, and DSC. Our study demonstrates a successful intercalation process to produce birnessites with mixed interlayer cations. Both Birnessite and Cu2+/Birnessite structure have a nanosheet-like morphology where the sizes of the copper-treated birnessite nanoparticles are drastically decreased compared to the copper ion free structures. The specific surface area is increased from 21.6 m2/g to 77.8 m2/g in the presence of copper as a result of a longer ageing process. Our study reveals that the electron transfer numbers of Birnessite and Cu2+/Birnessite are about 3.40 and 3.65, respectively in the oxygen reduction reaction. Both as-synthesized pristine Birnessite and copper tuned Birnessite are a promising candidate for a cheap, noble metal-free electrocatalyst for fuel cell applications. • K-, Cu-Birnessite nanostructures by cheap oone pot method for ORR. • Birnessite showed high activity in ORR with the 4-electron pathway. • Cu effecting the Mn3+/Mn4+ ratio resulting in good ORR activity. [ABSTRACT FROM AUTHOR]

With a focus on a large set of natural birnessites collected from terrestrial, freshwater systems, we applied and compared the capabilities of X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to characterize crystal structure and chemistry. Using XRD, we successfully identified 3 of the 11 natural birnessite samples as hexagonal ranciéite -like phases, but the remaining samples yielded less interpretable "3-line" diffraction patterns with broad, asymmetrical peaks at d-spacings of ~7.2, ~2.4, and ~1.4 Å. EXAFS analysis suggested that many of these samples had characteristics of both triclinic and hexagonal birnessite. However, application of EXAFS to the ranciéite-like phases yielded unreasonably high concentrations of triclinic birnessite as an intergrowth, calling into question the use of synthetic hexagonal H -birnessite as an appropriate standard in the linear combination fitting of EXAFS data for natural birnessites. FTIR spectroscopy of the "3-line" birnessite samples success- fully distinguished triclinic and hexagonal constituents, and analyses of peak positions suggested that natural birnessites occur as a full spectrum of triclinic and hexagonal intergrowths. XPS analysis of these samples revealed that higher Mn3+ concentrations relative to Mn2+ and Mn4+ are correlated to increased proportions of triclinic birnessite. [ABSTRACT FROM AUTHOR]

Enrichments of REY (rare earth + yttrium) and other trace metals (Co and Ni) in deep-sea ferromanganese (Fe Mn) micronodules have received increasing attention in both deep-sea research and mineral exploration. Due to the presence of multiple, easily-crushed and poorly-crystallized phases in micronodules, the genesis of micronodules and their adsorption of various trace elements are poorly understood. To address this gap, we examined the spatial distributions of elements in cross-sections of micronodules from the western tropical North Pacific Ocean using high-resolution (HR) LA-ICP-MS raster mapping coupled with laser Raman and X-ray photoelectron spectroscopy (XPS). The ferromanganese micronodules we studied are dominated by Fe and Mn oxides with minor carbonate minerals, such as siderite, rhodochrosite and calcite. LA-ICP-MS maps show that these micronodules consist of a Mn-rich core and a Fe-rich rim. The Fe-enriched rim is enriched in As and surrounds a Mg, Mn, Cu, Co and Ni concreted core. Laser Raman maps show that the micronodule core contains more birnessite, an important scavenger of trace metals in deep sea sediments, than the rim. The birnessite filled core of these micronodules does not have elevated REY. Indeed, birnessite line channels may feed metal-rich fluid containing REY to adjacent minerals, including well-crystallized bio-apatite and zeolite, as high Ce and Y levels are spatially correlated with these minerals. The observed element profiles and XPS observations showing the coexistence of multiple oxidation states of Mn (+2, +3 and +4), Fe (+2 and +3) and Ce (+3, +4) demonstrate that the Fe Mn phases of these micronodules are of a diagenetic origin and that they are sites of redox-driven metal enrichment in deep-sea sediment. Unlabelled Image • First high frequency/resolution LA-ICP-MS element maps of deep sea Fe-Mn nodules • Results reveal uptake and adsorption patterns of trace metals in micronodules. • Micronodules are shown to be carriers of valuable metals (Co, Ni) and REY. • Element concentrations demonstrate a diagenetic genesis for micronodules. [ABSTRACT FROM AUTHOR]

Purpose: The adsorption-oxidation of Sb on manganese oxide plays an important role in controlling Sb mobility and fate in soils and sediments. Widespread organic substances such as microbes may greatly affect this process, and deserve a careful investigation. This study examines the adsorption and oxidation of Sb(III, V) on birnessite, a typical manganese oxide, with and without Bacillus cereus cells. Materials and methods: Adsorption isotherms were conducted to explore the adsorption capacity of Sb to the birnessite–bacteria composite. X-ray photoelectron spectroscopy (XPS) was applied to determine the valence state of Mn and the adsorbed Sb species. Results and discussion: The SEM results show that birnessite adheres to the outer surface of bacterial cells, and the aggregation of minerals occurs to a lesser extent in the presence of cells. Batch adsorption results show a much larger Sb adsorption on individual birnessite than on bacteria, and the measured Sb adsorption to the birnessite–bacteria composite is larger than that predicted assuming additive, i.e., the sum of the end-member metal adsorptivities. On birnessite, Sb(III) is predominately oxidized to Sb(V) according to the XPS analysis, and the presence of bacteria hinders this oxidation reaction. Conclusions: We propose that microbe−birnessite association favors the immobilization of Sb on solid phases, but can inhibit the oxidation of Sb(III) to Sb(V), which is of great significance for evaluating the toxicity, bio-availability, and mobility of Sb in both natural and contaminated environments. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ceramic membrane loaded with PAC-MnOx enable to remove Mn2+. • Microorganisms multiply using residual organic matter during the "recovery period". • Strong oxidation dissolves EPS in the pores of PAC-MnOx. • Disinfection enabled to improve flux without changing the crystal structure of MnOx. • The removal of Mn2+ could be realized via the catalytic oxidation of birnessite in the later stage. Gravity-Driven Ceramic Membrane (GDCM) technology has gained popularity and recognition in water purification on account of its low energy consumption and convenient operation. This study aimed to comprehensively assess the effects of different disinfectants, including hydrogen peroxide (H 2 O 2) and sodium hypochlorite (NaClO), on GDCM performance. It focused on the change in manganese removal and structure of the powdered activated carbon (PAC) - manganese oxides (MnOx) pre-deposited at the membrane interface and as well membrane fouling mitigation. The results showed that after the first two disinfection cycles, the flux recovery was 15 LMH for H 2 O 2 , while using NaClO only achieved an average recovery of 2 LMH and increased the effluent manganese, exceeding 0.1 mg/L within a short period of time. During the disinfection process, H 2 O 2 exhibited better performance in mitigating membrane fouling compared to NaClO in the GDCM system. Additionally, by observing the recovery time of Mn2+ after disinfection, it can be deduced that the initial manganese removal mainly resulted from biological action of manganese-oxidizing bacteria (MnOB), while the later stage relied primarily on the catalytic oxidation action of birnessite. Fluorescence excitation–emission matrix analysis (EEM) indicated that most organic compounds dissolved in the disinfectant solution after disinfection, with some permeating through the membrane, while the remaining portion continued to accumulate in the filter cake layer. This accumulation was further confirmed by the enhanced absorbance peaks of functional groups (–NH and –OH) in the Fourier-transform infrared spectroscopy (FTIR). Scanning electron microscope - energy dispersive x-ray spectroscopy (SEM-EDS) mapping analysis revealed that MnOx formed more lamellar structures between nano-flower balls after disinfection. This increased the number of active sites and opened up water flow channels, contributing to improved flux. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that Mn3+ as the dominant valence in MnOx exhibited excellent catalytic oxidation ability. Raman analysis and X-ray diffraction (XRD) indicated that MnOx showed consistent Raman spectra with birnessite before and after disinfection, and the oxidative effect of disinfection did not alter the crystal structure of MnOx. This result demonstrates the importance of applying effective disinfection in the GDCMs. [ABSTRACT FROM AUTHOR]

[Display omitted] • FBCMBR coupled with intermittently fluidized filters was built for manganese removal. • Regular swabbing of membrane fouling by fluidized filters suppressed TMP increase. • Exfoliative active film from filters was retained by membranes for manganese oxidation. • Autocatalytic chemical oxidation cycle predominated in manganese removal in FBCMBR. • FBCMBR ripening time benefited from particular startup strategies about flux and filters. Groundwater has been urgently confronted with extensive manganese contamination nowadays. In this study, a novel fixed-bed ceramic membrane bioreactor (FBCMBR) coupled with intermittently fluidized ceramsite-PAC-MnOx filters by high-pressure gas was developed for efficient manganese removal. The results indicated FBCMBR 1# with full-filled ceramsite-PAC-MnOx filters reduced transmembrane pressure by 61.2 %, compared to FBCMBR 2# with less-filled filters, and achieved an average manganese removal of 82.8 % during 55-day filtration. Confocal laser scanning microscope (CLSM) images evidenced regular swabbing of apoptotic microorganisms and metabolites from membranes by fluidized filters eliminated reversible fouling. Dispersed active film was retained by membranes and still responsible for manganese removal. Stimulated microbial activities to 25.7 μmol/L adenosine triphosphate and enriched manganese oxide bacteria (i.e, Pseudomonas) to 2 × 105 MPN/mL by abundant ceramsite promoted biological oxidation of manganese. SEM-EDS mapping, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) spectra demonstrated filters were covered by three-dimensional flower-like structure birnessite, in which Mn(Ⅲ) contributed to catalytic oxidation of Mn(Ⅱ) as dominated valence and Mn(Ⅳ) was responsible for concentration, facilitating the formation of birnessite and autocatalytic oxidation cycle. Sterilization experiments confirmed catalytic oxidation rather than biological oxidation predominated in manganese removal in FBCMBR, which also showed great self-repair ability with exposure to extremely bio-adverse conditions. For further FBCMBR optimization, flux was stepwise increased or exogenous mature birnessite was introduced during startup period. Both remarkably reduced ripening time (more than 50 days earlier) and reliably stabilized effluent manganese concentration at less than the threshold value of China's standard for drinking water quality (0.1 mg/L). [ABSTRACT FROM AUTHOR]

The environmental impacts of As mobilization and nitrous oxide (N 2 O) emission in flooded paddy soils are serious issues for food safety and agricultural greenhouse gas emissions. Several As immobilization strategies utilizing microbially-mediated nitrate reducing-As(III) oxidation (NRAO) and birnessite (δ-MnO 2)-induced oxidation/adsorption have proven effective for mitigating As bioavailability in flooded paddy soils. However, several inefficiency and unsustainability issues still exist in these remediation approaches. In this study, the effects of a combined treatment of nitrate and birnessite were assessed for the simultaneous suppression of As(III) mobilization and N 2 O emission from flooded paddy soils. Microcosm incubations confirmed that the combined treatment achieved an effective suppression of As(III) mobilization and N 2 O emission, with virtually no As(T) released and at least a 87% decrease in N 2 O emission compared to nitrate treatment alone after incubating for 8 days. When nitrate and birnessite are co-amended to flooded paddy soils, the activities of denitrifying enzymes within the denitrification electron transport pathway were suppressed by MnO 2. As a result, the majority of applied nitrate participated in nitrate-dependent microbial Mn(II) oxidation. The regenerated biogenetic MnO 2 was available to facilitate subsequent cycles of As(III) immobilization and concomitant N 2 O emission suppression, sustainable remediation strategy. Moreover, the combined nitrate-birnessite amendment promoted the enrichment of Pseudomonas , Achromobacter and Cupriavidu, which are known to participate in the oxidation of As(III)/Mn(II). Our findings document strong efficacy for the combined nitrate/birnessite treatment as a remediation strategy to simultaneously mitigate As-pollution and N 2 O emission, thereby improving food safety and reducing greenhouse gas emissions from flooded paddy soils enriched with NH 4 + and As. [Display omitted] • Combined nitrate-birnessite treatment decreased As(III) mobilization and N 2 O emission from flooded paddy soils. • Birnessite stimulates anoxic oxidation of ammonia and suppresses denitrification. • Nitrate facilitates regeneration of MnO 2 , thereby suppressing As(III) mobilization and N 2 O emission. • The combined treatment has good prospects for remediating As pollution and N 2 O emission from paddy soils. [ABSTRACT FROM AUTHOR]

• Highly active Sm/Mn catalysts were prepared at low temperature (150 °C). • 0.05Sm/Mn catalyst achieved 100% NO X conversion at low temperatures (<50 °C). • Sm doping promoted the acidic sites on the Sm/Mn catalyst surface. • The introduction of Sm improved low temperature H 2 O/SO 2 resistance of catalysis. To develop novel NH 3 -SCR catalysts with low-temperature and high-efficency, a series of birnessite (δ-MnO 2) catalysts with different Sm doping by methanol reduction are synthesized. The results showed that the introduction of Sm successfully inhibited the crystallization of MnO X , promoted the specific surface area, the contents of chemisorbed oxygen species and the formation of surface acid sites. It also increased the relative content of (Mn4+ + Mn3+)/Mn, which was beneficial to low-temperature SCR activity. The catalytic performance of xSm/Mn (δ-MnO 2 based with xSm doping) catalysts was evaluated by NH 3 -SCR performances. Among them, 0.05 Sm/Mn showed 100% NO X conversion and high N 2 selectivity compared to the other Sm addition catalysts within the operating low-temperature window (25–200 °C), at a high GHSV (gas hourly space velocity) of 60,000 h−1. The modified 0.05 Sm/Mn catalysts followed the L-H and E-R reaction mechanisms and were dominated by L-H, as revealed by in-situ DRIFTs analysis. [ABSTRACT FROM AUTHOR]

Glyphosate is the most commonly used herbicide in the United States. In the environment, glyphosate residues can either degrade into more toxic and persistent byproducts such as aminomethylphosphonic acid (AMPA) or environmentally benign species such as sarcosine or glycine. In this research, the birnessite-catalyzed degradation of glyphosate was studied under environmentally relevant temperatures (10–40 °C) using high-performance liquid chromatography, inductively coupled plasma mass spectrometry, nuclear magnetic resonance, and theoretical calculations. Our results show a temperature-dependent degradation pathway preference for AMPA and glycine production. The AMPA and glycine pathways are competitive at short reaction times, but the glycine pathway became increasingly preferred as reaction time and temperature increased. The measured free energy barriers are comparable for both the glycine and AMPA pathways (93.5 kJ mol−1 for glycine and 97.1 kJ mol−1 for AMPA); however, the entropic energy penalty for the AMPA pathway is significantly greater than the glycine pathway (-TΔS‡ = 26.2 and 42.8 kJ mol−1 for glycine and AMPA, respectively). These findings provide possible routes for biasing glyphosate degradation towards safer products, thus to decrease the overall environmental toxicity. [Display omitted] • Kinetics of degradation of glyphosate at higher temperatures produced more glycine. • Free energy barrier caused the glycine pathway to be more favorable than AMPA. • Suppressing AMPA production is favored for protecting the environment and ecology. [ABSTRACT FROM AUTHOR]

Birnessite has been considered as a potential thallium (Tl)-oxidizing mineral and Tl scavenger in natural and engineered systems. However, the birnessite aging that occurs during Tl sorption may severely limit the long-term effectiveness of Tl immobilization on birnessite. In this study, kinetic experiments were performed in batch coupled stirred-flow systems to investigate the effects of Tl(I) loading and pH on Tl(I) retention during short- and long-term Tl-birnessite interactions. Spectral analysis, microscopic observations, and theoretical calculations revealed a change in the Tl(I) binding mode to birnessite with time. In the short term, low levels of Tl(I) could be effectively immobilized through adsorption to birnessite vacancies. Structural disruption and vacancy occupation in birnessite resulted in the migration of adsorbed Tl(I) toward the edges of the mineral during a 24-d reaction period. Under acidic to neutral conditions, the migration of vacancy-bound Tl(I) induced sustained Tl(I) release (10–45%) from birnessite over the long-term. Under alkaline conditions, hydroxide ions contributed to the formation of Tl(I)–OH species with double edge sharing, which enhanced the binding ability (α ≈ 7 × 10−4 min−1) and stable retention of Tl(I) on birnessite. Understanding Tl(I) retention dynamics during long-term Tl-birnessite interactions elucidates the Tl cycle in Mn oxide-containing systems. [Display omitted] • Nonoxidative adsorption dominates the massive immobilization of Tl on birnessite. • 10–45% of the adsorbed Tl(I) is released under acidic and neutral environments. • Structural destruction and vacancy occupation lead to migration of adsorbed Tl(I). • TCS-Tl(I) on Mn(III)-vacancies is converted to DES-Tl(I) on edges in the reaction. • OH− enhances the Tl(I) binding ability and long-term immobilization on birnessite. [ABSTRACT FROM AUTHOR]

Fermentation and dissimilatory manganese (Mn) reduction are inter-related metabolic processes that microbes can perform in anoxic environments. Fermentation is less energetically favorable and is often not considered to compete for organic carbon with dissimilatory metal reduction. Therefore, the aim of our study was to investigate the outcome of the competition for lactate between fermentation and Mn oxide (birnessite) reduction in a mixed microbial community. A birnessite reducing enrichment culture was obtained from activated sludge with lactate and birnessite as the substrates. This enrichment was further used to test how various birnessite activities (0, 10, 20, and 40 mM) affected the rates of fermentation and metal reduction, as well as community composition. Increased birnessite activity led to a decrease of lactate consumption rate. Acetate and propionate were the main products. With increasing birnessite activity, the propionate/acetate ratio decreased from 1.4 to 0.47. Significant CO2 production was detected only in the absence of birnessite. In its presence, CO2 concentrations remained close to the background since most of the CO2 produced in these experiments was recovered as MnCO3. The Mn reduction efficiency (Mn(II) produced divided by birnessite added) was the highest at 10 mM birnessite added, where about 50% of added birnessite was reduced to Mn(II), whereas at 20 and 40 mM approximately 21 and 16% was reduced. The decreased birnessite reduction efficiency at higher birnessite activities points to inhibition by terminal electron acceptors and/or its toxicity which was also indicated by retarded lactate oxidation and decreased concentrations of microbial metabolites. Birnessite activity strongly affected microbial community structure. Firmicutes and Bacteroidetes were the most abundant phyla at 0 mM of birnessite. Their abundance was inversely correlated with birnessite concentration. The relative sequence abundance of Proteobacteria correlated with birnessite concentrations. Most of the enriched populations were involved in lactate/acetate or amino acid fermentation and the only previously known metal reducing genus detected was related to Shewanella sp. The sequencing data confirmed that lactate consumption coupled to metal reduction was only one of the processes occurring and did not outcompete fermentation processes.

The heterogeneous oxidation of Cr(III) to Cr(VI), a toxic inorganic anion, by a synthetic birnessite (δ-MnO2) was investigated in batch reactions using a combination of analytical techniques including UV–Vis spectrophotometry, microwave plasma–atomic emission spectrometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), to evaluate both the solution speciation of Cr(III)/Cr(VI) and the surface of the reacted δ-MnO2. The formation of dissolved Mn(II) was determined during the batch reactions to evaluate the extent and stoichiometry of the Cr(III) oxidation reaction. A stoichiometric 3:2 Mn(II):Cr(VI) molar relationship was observed in the reaction products. The reductive dissolution of the δ-MnO2 by Cr(III) resulted in a surface alteration from the conversion of Mn(IV) oxide to reduced Mn(II) and Mn(III) hydroxides. The results of this investigation show that naturally occurring Cr(III) will readily oxidize to Cr(VI) when it comes in contact with MnO2, forming a highly mobile and toxic groundwater contaminant.

Permanganate treatment is widely used for disinfection of bacteria in surface-contaminated water. In this paper, the fate of the dissolved permanganate in aqueous solution after contact with cells of Pseudomonas fluorescens was studied. Concomitant accumulation of divalent cations of Mg2+, Zn2+, and Co2+ during precipitation of Mn oxides was also studied. The time course of the Mn concentration in solution showed an abrupt decrease after contact of Mn(VII) with microbial cells, followed by an increase after ~ 24 hr. XRD analysis of the precipitated Mn oxides, called biomass Mn oxides, showed the formation of low-crystalline birnessite. Visible spectroscopy and X-ray absorption near edge structure (XANES) analyses indicated that dissolved Mn(VII) was reduced to form biomass Mn oxides involving Mn(IV) and Mn(III), followed by reduction to soluble Mn(II). The numbers of electron transferred from microbial cells to permanganate and to biomass Mn oxides for 24 hr after the contact indicated that the numbers of electron transfer from microbial cell was approximately 50 times higher to dissolved permanganate than to the biomass Mn oxides in present experimental conditions. The 24 hr accumulation of divalent cations during formation of biomass Mn oxides was in the order of Co2+ > Zn2+ > Mg2+. XANES analysis of Co showed that oxidation of Co2+ to Co3+ resulted in higher accumulation of Co than Zn and Mg. Thus, treatment of surface water by KMnO 4 solution is effective not only for disinfection of microorganisms, but also for the elimination of metal cations from surface water. Unlabelled Image [ABSTRACT FROM AUTHOR]

Strontium adsorption was studied by acid–base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75°C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid–base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H+–>MnOH–Sr2+ heterogeneous system. At a low ionic strength and negatively charged surface, Sr2+ ions compete with electrolyte ions and form both inner- and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr2+, >MnOSr+, >MnOSrOH0) and outer-sphere ([>MnO–Sr2+]+) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75°C. [ABSTRACT FROM AUTHOR]

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A template‐free, one‐step synthesis of nano‐IrO2 on a layered Mn−Co birnessite support was developed to induce the catalyst‐support interaction. The IrO2 nanoparticles were prepared with inherent crystal lattice strain, which is derived from the Ir‐O−Mn mismatch at the interface. The oxidation state of iridium is higher than IrIV, revealing the electron transfer to the substrate. On account of the crystal lattice distortion and electronic manipulations being favorable to the oxygen evolution reaction (OER), the as‐synthesized composite exhibited excellent OER activity and stability. The improved catalytic nature was followed by enhancement in the electrochemical active surface area, mass specific activity, and intrinsic activity. Moreover, the Tafel slope of 42 mV dec−1 reveals better reaction kinetics for IrO2/Mn−Co (2 : 1)‐birnessite than many previously reported catalysts despite the low precious noble metal content in the as‐synthesized composite. Conclusively, we have proven that the strain engineering holds vital importance in forming catalyst‐support interaction and rational design of catalysts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A layered potassium–manganese oxide, birnessite (OL: KMn4O8) was subjected to long-term post-calcination thermal treatment aimed at stimulating its activity in catalytic soot oxidation. The 12 h thermal treatment at several temperatures (425, 500, 550 and 650 °C) in atmospheric conditions resulted in the swift transformation of the layered material into the tunnel-shaped octahedral molecular sieve cryptomelane (OMS-2: KMn8O16). The obtained materials were thoroughly characterized using powder X-ray diffraction, Raman spectroscopy, N2-BET specific surface area analysis, and transmission electron microscopy techniques. With increasing temperature of thermal treatment, the concentration of OMS-2 increased, which was confirmed by Rietveld analysis, and dominated the material properties, notably the lowering of work function of the catalysts (Δϕ = 0.2 and 0.4 eV in air and vacuum, respectively). The obtained results reveal the high catalytic activity of the OMS-2 formed from the thermal transformation of the OL material compared to both the parent OL material and the uncatalyzed soot oxidation reaction. The achieved catalytic activity showed direct correlation with the temperature of pre-treatment, with the most active catalyst being calcined at 550 °C/12 h, and lowering the temperature of 50% soot conversion (T50%) by spectacular 150 °C in loose contact. The catalytic activities were found to correlate well with the work function (low work function–high catalytic activity), confirming the electron transfer from the catalyst surface to oxygen molecule as the important step in the soot oxidation mechanism over mixed potassium–manganese oxides. The beneficial effect of thermal pre-treatment was found to last over multiple runs, maintaining a T50% lower by 100 °C compared to the untreated parent material. The obtained results indicate the importance of the temperature treatment for the catalytic performance of potassium promoted transition metal oxides as catalytically active phases for efficient soot removal in the conditions present in combustion engine exhaust gases. [ABSTRACT FROM AUTHOR]

The preparation of nanometer birnessite was prepared by inverse emulsion method using potassium permanganate as single manganese source. The morphology and structure of birnessite was characterized by SEM, XRD, IR, TG-DTG, energy spectrum. The removal of Cd2+in water using the birnessite was studied. The results show that the prepared birnessite is sheet and pure. For the Cd2+ in solution,the adsorption capacity is about 120 mg/g under the conditions of adsorption time of 120 min and temperature of 313 K. The adsorption behavior of birnessite for Cd2+ corresponds to Langmuir isotherm,the adsorption process follows the pseudo second order kinetic equations. [ABSTRACT FROM AUTHOR]

Mn carbonate is the main MnII mineral phase that precipitates in suboxic to anoxic environments. The coupled processes of MnIV oxide bioreduction and organic oxidation serve as dominant factors leading to Mn carbonate precipitation. This study examined the simultaneous respiration of oxygen and birnessite by a facultatively anaerobic bacterium, the Dietzia strain DQ12-45-1b (45-1b), and discussed the possible mechanism of rhodochrosite precipitation under general oxic environments. Compared to anaerobic experiments, the more rapid growth of 45-1b under aerobic conditions caused faster oxidation of acetate (1.0 × 103 μM h−1) and accumulation of HCO 3 − (5.5 × 102 μM h−1) within 72 h, which was coupled to a dramatic increase in pH from 7.0 to more than 9.2. By virtue of the higher biomass and bioactivity in the aerobic condition, the bioreduction of MnIV was accelerated and it caused a higher accumulating rate of soluble reduced Mn (4.0 μΜ h−1) than that in the anaerobic condition (2.0 μΜ h−1). Those rates indicated that an anaerobic-aerobic sub-interface was present in the aerobic system, in which anaerobic and aerobic respiration co-occurred to give rise to sufficient Mn(II) and alkalinity, thus, increased the supersaturation index (SI) for rhodochrosite. The mineral intermediates and products were identified by time-course XRD, SEM, and Raman spectra. Manganite (MnOOH) was found as the transient intermediate, which suggested the stepwise one-electron transfer mechanism of birnessite reduction. The dialysis tube, lysed cells, dead cells and two-compartment experiments suggested that the living 45-1b not only carried out a direct extracellular electron transfer for birnessite reduction but also provided necessary nucleation sites for rhodochrosite precipitation. Furthermore, both the isotope experiments and Raman analysis showed that the carbon source in rhodochrosite was mainly 13C isotope-labeled acetate, which corresponded well with the geological isotopic records. Finally, a conceptual model of Mn carbonate precipitation at oxic-suboxic/anoxic interfaces that could be possibly present in soil and sedimentary environments was proposed based on three prerequisites: (i) sufficient Mn(II) produced on an aerobic-anaerobic sub-interface, (ii) adequate alkalinity, and (iii) nucleation sites provided by cell surfaces. This model highlights the role of aerobic respiration in Mn(IV) reduction and Mn-carbonate formation, and may suggest a realistic way for inorganic carbon storage. [ABSTRACT FROM AUTHOR]

Over the last decade, increasing nickel concentrations presumably due to pyrite oxidation have been detected in aquifers in some areas of northwestern Germany. Maximum values of 114 µg L−1 exceeded the amended German drinking water guideline by nearly sevenfold. Hence, MnCl2·H2O is added during water treatment; however, a complete removal of nickel failed. The objective of this study was to evaluate the removal of nickel from groundwater by iron and manganese oxides from waterworks. Therefore, oxides were taken from the pre-filter and secondary filter and investigated in terms of their chemical and physical characteristics as well as their nickel adsorption behavior. Additionally, three further adsorbents (goethite, ferrihydrite and birnessite) were evaluated concerning their applicability for nickel removal. Adsorption experiments were carried out in a batch system as a function of time (1 min to 28 days) and pH (2 to 7.5). Furthermore, nickel adsorption was investigated as a function of the equilibrium concentration (0.00315–50 mmol L−1), and the data were evaluated with the Freundlich and Langmuir equations. The adsorption of nickel was strongly dependent on time, where a time span of ± 10 min changed the nickel adsorption by ± 2–8%. Nickel adsorption showed high sensitivity to pH and reached a maximum at pH 7.5. The results indicated that manganese oxides have a higher affinity for nickel adsorption and that adsorption by iron oxides is negligible. Small increases in contact time and an increase in pH during water treatment can promote nickel adsorption. [ABSTRACT FROM AUTHOR]

Sunlight drives photosynthesis and associated biological processes, and also influences inorganic processes that shape Earth's climate and geochemistry. Bacterial solar-to-chemical energy conversion on this planet evolved to use an intricate intracellular process of phototrophy. However, a natural nonbiological counterpart to phototrophy has yet to be recognized. In this work, we reveal the inherent "phototrophic-like" behavior of vast expanses of natural rock/soil surfaces from deserts, red soils, and karst environments, all of which can drive photon-to-electron conversions. Using scanning electron microscopy, transmission electron microscopy, micro- Raman spectroscopy, and X-ray absorption spectroscopy, Fe and Mn (oxyhydr)oxide-rich coatings were found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded an in situ photocurrent micromapping of the coatings, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solarresponsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe- and Mn (oxyhydr)oxide-mineral coatings. The native semiconducting Fe/Mn-rich coatings may play a role similar, in part, to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth's surfaces. [ABSTRACT FROM AUTHOR]