Formation Of Mn Oxides Research Articles

Transformation of the phyllomanganate vernadite to tectomanganates with small tunnel sizes: Favorable geochemical conditions and fate of associated 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.

Differential manganese and iron recycling and transport in continental margin sediments of the Northern Gulf of Mexico

Pore water and solid phase geochemical profiles of sediment cores collected along two transects on the western and eastern sides of the Mississippi River mouth in the northern Gulf of Mexico were incorporated into a reactive transport model to determine the role of manganese and iron in the remineralization of carbon. Reactive transport model calculations indicate that sedimentation rates control the intensity of anaerobic carbon remineralization and select for the dominant anaerobic carbon remineralization pathways. Although sulfate reduction dominates the shelf station (65 m water depth), denitrification and microbial manganese reduction appear equally significant anaerobic respiration processes along the continental slope the closest to the Mississippi River, whereas microbial iron reduction does not represent an important process in these sediments. These findings suggest that the differential kinetics of manganese and iron redox transformations influence carbon remineralization processes on the continental slope. The fast kinetics of Fe2+ oxidation near the sediment-water interface and high sedimentation rates maintain Fe under the form of Fe(III) oxides and thermodynamically prevent sulfate reduction from dominating carbon remineralization processes on the slope, whereas the much slower Mn2+ oxygenation kinetics allows diffusion of Mn2+ across the sediment-water interface of the shelf station closest to the river mouth. Exposure to oxygenated bottom waters and entrainment within mobile muds typical of deltaic sediments during high riverine discharge likely promote the formation and downslope transport of Mn(III/IV) oxides within the nepheloid layer. This phenomenon appears to form a manganese ‘conveyor belt’ that selectively enriches Mn(III/IV) oxides relative to Fe(III) oxides in the deep sediment. In contrast, the intensity of anaerobic carbon remineralization processes along the eastern continental slope the farthest from the Mississippi River plume is much lower due to the low organic and lithogenic inputs, and denitrification dominates anaerobic respiration. Overall, these findings suggest that manganese cycling and its role in carbon remineralization processes in continental slope sediments exposed to large riverine inputs may be more important than previously considered.

Natural attenuation of lead by microbial manganese oxides in a karst aquifer

Lead is a toxic environmental contaminant associated with current and historic mine sites. Here we studied the natural attenuation of Pb in a limestone cave system that receives drainage from the ancient Priddy Mineries, UK. Extensive deposits of manganese oxides were observed to be forming on the cave walls and as coatings in the stream beds. Analysis of these deposits identified them as birnessite (δ-MnO2), with some extremely high concentrations of sorbed Pb (up to 56 wt%) also present. We hypothesised that these cave crusts were actively being formed by microbial Mn(II)-oxidation, and to investigate this the microbial communities were characterised by DNA sequencing, enrichment and isolation experiments. The birnessite deposits contained abundant and diverse prokaryotes and fungi, with ~5% of prokaryotes and ~ 10% of fungi closely related to known heterotrophic Mn(II)-oxidisers. A substantial proportion (up to 17%) of prokaryote sequences were assigned to groups known as autotrophic ammonia and nitrite oxidisers, suggesting that nitrogen cycling may play an important role in contributing energy and carbon to the cave crust microbial communities and consequently the formation of Mn(IV) oxides and Pb attenuation. Enrichment and isolation experiments showed that the birnessite deposits contained Mn(II)-oxidising microorganisms, and two isolates (Streptomyces sp. and Phyllobacterium sp.) could oxidise Mn(II) in the presence of 0.1 mM Pb. Supplying the enrichment cultures with acetate as a source of energy and carbon stimulated Mn(II)-oxidation, but excess organics in the form of glucose generated aqueous Mn(II), likely via microbial Mn(IV)-reduction. In this karst cave, microbial Mn(II)-oxidation contributes to the active sequestration and natural attenuation of Pb from contaminated waters, and therefore may be considered a natural analogue for the design of wastewater remediation systems and for understanding the geochemical controls on karst groundwater quality, a resource relied upon by billions of people across the globe.

Geochemistry of core sediments from the southeastern Bay of Bengal: Inferences on weathering and early diagenetic changes

A sediment core (ABP24/05), collected at a water depth of 3520 ​m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition. High ratios of K/Rb, Ti/Al and Zr/Rb during ~45 to ~18 ​cal ​kyr B.P. in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period. This might have brought a higher quantity of unaltered minerals to the study area. Low ratios of K/Rb, Zr/Rb, and Ti/Al and increase of SiO2/TiO2, Rb/Al and Cs/Al from ~18 ​cal ​kyr B.P. to present may be indicating an increase in the rate of chemical weathering during this period. The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea. Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since ~18 ​cal ​kyr B.P. The low organic carbon (OC), high Mn/Al, Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during ~11 ​cal ​kyr B.P. in the core sediments, which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean. The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.

Effect of dew point on hot-dip galvanizing behavior of a high-manganese TWIP steel for automotive application

The effect of dew points (− 50, − 10 and + 10 °C) on the galvanizing properties of a high-manganese twinning-induced plasticity (TWIP) steel was studied. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray photoelectron spectroscopy (XPS) were used for microscopic observation and qualitative analysis of the interfacial layer between the steel surface and the zinc layer after hot-dip galvanizing. SEM analysis results show that three different morphologies of metallic oxides are formed on the interfacial layer under the different dew points. GDOES results show that Al is present in the molten zinc, reacting with Fe on the steel surface to form Fe2Al5, which can increase the galvanizing properties. XPS results show that the valence states of Mn in the interfacial alloy layer are Mn2+ and Mn4+, and the valence states of Fe are Fe0, Fe2+ and Fe3+. The experimental results show that the hot-dip galvanizing performance is the best at − 10 °C and the formation of Mn and Fe intermetallic oxides has a bad effect on hot-dip galvanizing behavior of the high-manganese TWIP steel. The types of the formed surface oxides (MnO, Mn3O4, Mn2O3, FeO, and Fe2MnO4) on the surface of the steel sheet are confirmed. It can obtain the best hot-dip galvanizing performance of the high-manganese TWIP steel by controlling the dew point from − 10 to − 5 °C.

Saccharide-mediated transformation of Ce during the formation of manganese (hydr)oxide

Ce anomaly is an important indicator for many geochemical processes. Although the role of strong ligand such as siderophore desferrioxamine B (DFOB) in facilitating negative Ce anomaly formation on Mn (hydr)oxide has been well studied, the effect of ubiquitous weak ligands such as polysaccharides on the mobility of Ce have not been considered. Here, we investigated the effects of polysaccharides with different molecular weights on Mn (hydr)oxide formation as well as the distribution coefficients of rare-earth elements (REEs) at the solid–liquid interface. In coprecipitation experiments, REEs distribution coefficients between Mn (hydr)oxide and the solution showed a negative Ce anomaly during coprecipitation of Mn in the presence of dextran with molecular weight of 5000 Dalton (Dextran 5 k). The molecular size of the saccharides affected the crystal growth of Mn (hydr)oxide. Mn (hydr)oxide was deposited on a matrix of polysaccharide molecules to form a metal–organic framework in the presence of dextran. The crystal size decreased with an increase in the length of the saccharide molecular chain. As a control test, when well crystallized δ-MnO2 was used as an adsorbent, the distribution coefficients of the REEs showed a positive Ce anomaly in the presence of dextran 5 k. Based on the results, we suggest that the mechanism by which saccharides affect the sorption behavior of REEs on Mn (hydr)oxide is different from the reported mechanisms of strong metal-binding ligands such as DFOB. Saccharide-mediated electron transfer may occur between Mn (hydr)oxides and Ce(III), resulting in the formation of an organic-Ce(IV) complex in solution. Our results imply that ubiquitous weak ligands such as polysaccharides may significantly affect the mobility of Ce largely due to the association with Mn oxides, which are widespread natural electron acceptors.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25–2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5–9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6–99.0 % occurred as increasing the pH 5–9, 25–95 °C and oxygen flow rate of 0–2.4 L min−1. NaOH or Ca(OH)2 as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O2 for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min−1 were optimal for As removal.

Precipitation of Mn Oxides in Quaternary Microbially Induced Sedimentary Structures (MISS), Cape Vani Paleo-Hydrothermal Vent Field, Milos, Greece

Understanding microbial mediation in sediment-hosted Mn deposition has gained importance in low-temperature ore genesis research. Here we report Mn oxide ores dominated by todorokite, vernadite, hollandite, and manjiroite, which cement Quaternary microbially induced sedimentary structures (MISS) developed along bedding planes of shallow-marine to tidal-flat volcaniclastic sandstones/sandy tuffs, Cape Vani paleo-hydrothermal vent field, Milos, Greece. This work aims to decipher the link between biological Mn oxide formation, low-T hydrothermalism, and, growth and preservation of Mn-bearing MISS (MnMISS). Geobiological processes, identified by microtexture petrography, scanning and transmission electron microscopy, lipid biomarkers, bulk- and lipid-specific δ13Corganic composition, and field data, and, low-temperature hydrothermal venting of aqueous Mn2+ in sunlit shallow waters, cooperatively enabled microbially-mediated Mn (II) oxidation and biomineralization. The MnMISS biomarker content and δ13Corg signatures strongly resemble those of modern Mn-rich hydrothermal sediments, Milos coast. Biogenic and syngenetic Mn oxide precipitation established by electron paramagnetic resonance (EPR) spectroscopy and petrography, combined with hydrothermal fluid flow-induced pre-burial curing/diagenesis, may account for today’s crystalline Mn oxide resource. Our data suggests that MISS are not unique to cyanobacteria mats. Furthermore, microbial mats inhabited by aerobic methanotrophs may have contributed significantly to the formation of the MnMISS, thus widening the spectrum of environments responsible for marine Mn biometallogenesis.

Biotic and Abiotic Mechanisms of Manganese (II) Oxidation in Lake Erie

Manganese (Mn), an element sensitive to redox conditions in aquatic environments, plays a role in numerous biogeochemical cycles. Although the speciation of Mn in lakes largely tracks the availability of dissolved oxygen, direct oxidation of Mn by molecular oxygen occurs slowly and, instead, the majority of Mn oxidation occurs via biotic and abiotic mechanisms involving microbes, organic matter, light, reactive oxygen species, and mineral surfaces. While each of these mechanisms is either known or likely to occur in freshwater, the relative balance and interaction among these biogeochemical pathways in an intact plankton system remains uncertain. We investigated potential abiotic and biotic mechanisms contributing to Mn oxide formation in Lake Erie, which experiences seasonal hypoxia and accumulation of Mn during seasonal stratification. Overall, Mn oxidation rates were much higher in the shallow and highly productive western basin (up to 1.5 µmoles L-1 d-1) compared to the deeper and less productive central basin, where we observed very little Mn oxidation over seven days. Our experiments suggest that abiotic mechanisms involving mineral surfaces played a larger role than biotic mechanisms, particularly in the light where Mn oxidation was highest. Reactive oxygen species exhibited antagonistic roles: hydrogen peroxide acted as a net reductant for manganese oxides and completely masked oxidation of Mn by the superoxide free radical. These findings show that multiple mechanisms may exert control over the fate of Mn and suggest that Mn released from sediments during hypoxia can potentially remain dissolved in the water for an extended period of time. Since the severity of Lake Erie hypoxia has increased in recent years, and we are becoming increasingly aware of the health effects of Mn in drinking water sources, these findings should help inform efforts to predict when and where Mn will accumulate in the lake water.

Macroscopic and microscopic behaviors of Mn(II) (ad)sorption to goethite with the effects of dissolved carbonates under anoxic conditions

Manganese and its compounds have been extensively studied because of their far-reaching roles in a wide range of biogeochemical processes in natural systems. The (ad)sorption behavior of Mn(II), however, is poorly understood despite its important role as the primary reaction step for surface-catalyzed Mn(II) oxidation that is the principal abiotic process forming various Mn (oxyhydr)oxides in nature. Here, we systematically examined Mn(II) (ad)sorption to one of the most common natural sorbents, goethite, in oxygen- and carbonate-free systems. Traditional sorption edge and isotherm experiments were conducted by varying sorbate-to-sorbent ratio (0.027–15 μmol∙m−2) and solution pH (pH 5.0–9.0). The effects of dissolved carbonates on Mn(II) sorption were also assessed in a range of naturally prevalent concentrations (0.5–10 mM NaHCO3). The Mn(II) uptake on goethite followed a typical Langmuir isotherm in the absence of dissolved carbonates, with increasing maximum adsorption capacities (Γmax) from 0.19 at pH 6.5 to 3.4 μmol∙m−2 at pH 9.0. The presence of dissolved carbonates raised the extent of Mn(II) adsorption, which appeared to be directly correspondent to that of the adsorption of dissolved carbonates. Extended X-ray absorption fine structure (EXAFS) analysis indicated that Mn(II) predominantly formed inner-sphere binuclear bidentate surface complexes. Mn(II) uptake became deviated from the Langmuir model and showed a clear indication of surface precipitation when the Mn(II) sorption density (Γ) exceeded a threshold value in a given solution composition. This secondary Mn(II) phase was identified as rhodochrosite using X-ray diffraction (XRD) and transmission electron microscope (TEM). Additionally, it would be plausible that a minor fraction of adsorbed Mn(II) coexisted with the secondary rhodochrosite according to a linear combination fitting (LCF) of the X-ray absorption near edge structure (XANES) spectra of Mn reference and samples. These systematic investigations of the macroscopic and microscopic behaviors of Mn(II) (ad)sorption to goethite provide a critical avenue for disentangling surface-catalytic Mn(II) oxidation processes, which ultimately lead to the formation of diverse Mn (oxyhydr)oxides in the environment.

Effect of Cu(II) on Mn(II) Oxidation by Free Chlorine To Form Mn Oxides at Drinking Water Conditions.

The chemical oxidation of dissolved Mn(II) to Mn(III/IV) oxides (MnOx) can lead to the accumulation of Mn deposits in drinking water distribution systems. However, Mn(II) oxidation by free chlorine is quite slow under mild conditions (e.g., pH 7.7 and 1.0 mg/L Cl2). This study found a significant role for Cu(II) in Mn(II) oxidation under conditions relevant to the supply of chlorinated drinking water. At pH 7.7, dissolved Cu(II) accelerated Mn(II) oxidation more than 10 times with a dose of 20 μg/L. Solid characterization revealed that during Mn(II) oxidation, Cu(II) adsorbed to freshly formed MnOx and produced Mn-Cu mixtures (denoted as MnOx-Cu(II)). An autocatalytic model for the reaction kinetics suggested that the freshly formed MnOx-Cu(II) had a much higher catalytic activity than that of pure MnOx. Solid CuO also catalyzed Mn(II) oxidation, and kinetic modeling indicated that after an initial oxidation of Mn(II) facilitated by the CuO surface, the freshly formed MnOx-Cu(II) on CuO surface played the dominant role in accelerating further Mn(II) oxidation. This study indicates a high potential for the formation of Mn oxides at locations in a drinking water distribution system or in premise plumbing where both Mn(II) and Cu(II) are available. It provides insights into the co-occurrence of other metals with Mn deposits that is frequently observed in distribution systems.

Mn oxide formation by phototrophs: Spatial and temporal patterns, with evidence of an enzymatic superoxide-mediated pathway

Manganese (Mn) oxide minerals influence the availability of organic carbon, nutrients and metals in the environment. Oxidation of Mn(II) to Mn(III/IV) oxides is largely promoted by the direct and indirect activity of microorganisms. Studies of biogenic Mn(II) oxidation have focused on bacteria and fungi, with phototrophic organisms (phototrophs) being generally overlooked. Here, we isolated phototrophs from Mn removal beds in Pennsylvania, USA, including fourteen Chlorophyta (green algae), three Bacillariophyta (diatoms) and one cyanobacterium, all of which consistently formed Mn(III/IV) oxides. Isolates produced cell-specific oxides (coating some cells but not others), diffuse biofilm oxides, and internal diatom-specific Mn-rich nodules. Phototrophic Mn(II) oxidation had been previously attributed to abiotic oxidation mediated by photosynthesis-driven pH increases, but we found a decoupling of Mn oxide formation and pH alteration in several cases. Furthermore, cell-free filtrates of some isolates produced Mn oxides at specific time points, but this activity was not induced by Mn(II). Manganese oxide formation in cell-free filtrates occurred via reaction with the oxygen radical superoxide produced by soluble extracellular proteins. Given the known widespread ability of phototrophs to produce superoxide, the contribution of phototrophs to Mn(II) oxidation in the environment may be greater and more nuanced than previously thought.

Reduction behaviors of permanganate by microbial cells and concomitant accumulation of divalent cations of Mg2+, Zn2+, and Co2+

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 KMnO4 solution is effective not only for disinfection of microorganisms, but also for the elimination of metal cations from surface water.

Thermodynamic and Experimental Evaluation of La1−xSrxMnO3±δ Cathode in Presence of Cr-Containing Humidified Air

Thermodynamic predictions were carried out on the phase stability of the lanthanum manganite (LSM)/yttria-stabilized zirconia (YSZ) system in presence and absence of Cr-containing dry and humidified atmosphere over broad ranges of temperature, oxygen partial pressure, and humidity. Formation of Mn oxides, La2Zr2O7, SrZrO3, Mn(Mn,Cr)2O4 (spinel), and zirconia was predicted, in agreement with previous experimental efforts. Humidity enhances the perovskite degradation by increasing the kinetics of secondary phase formation reactions. The impact of Cr6+ poisoning is more significant in presence of humidity in the atmosphere, as volatile Cr6+ species, in the form of CrO3 and CrO2(OH)2, may diffuse into the perovskite structure and change its phase stability and properties.

Rich growth medium promotes an increased on Mn(II) removal and manganese oxide production by Serratia marcescens strains isolates from wastewater

This study investigated the performance of two different strains of Serratia marcescens on the manganese (II) removal and the effects of the nutrient-rich (NB medium) and nutrient-poor culture (K medium) media on this process. In the NB medium, the pigmented LG1 strain and the non-pigmented CL11 strain exhibited improved growth and higher Mn(II) tolerance (0–2000 mg L−1). In addition, improved Mn(II) removal (64.25%) and increased formation of Mn oxides were observed in this medium, especially for the LG1 isolate. EELS analysis revealed Mn inside the cell for LG1 and EDX analysis revealed Mn outside of the cell for CL11, indicating that the two strains remove Mn(II) via distinct mechanisms. The biooxidation of Mn by the CL11 strain may involve indirect mechanisms which alter the pH of the medium, whereas the LG1 strain may use a direct mechanism for the biooxidation mediated by cellular components such as intracellular proteins. These results demonstrate the biotechnological potential of the two strains. In addition, is the first time the high potential of the NB medium for Mn removal is demonstrated. This medium was found to greatly influence the Mn removal and oxide formation, especially in the case of the pigmented LG1 isolate.

Effect of water pressure and soaking time on the selective oxidation of DP980 advanced high strength steel

Due to the limited heating power in non-oxidation furnace of continuous strip processing facilities of heating stage, selective oxidation with higher oxygen pressure is practically inevitable, which will greatly influence the quality of coating during immersion into the zinc bath. In this paper, the oxidation behaviors of DP980 advanced high strength steel are studied during annealing in a pure H2 atmosphere with controlled water pressure ranging from 0.033 to 0.094 bar (PH2O/PH2) and soaking time from 30 s to 2 h. The oxidation proceeds as a combination of external formation of Mn oxides and internal formation of Mn2SiO4, MnSiO3 and SiO2 from the subsurface of the steel into the matrix. As the water pressure increases from 0.033 to 0.094 bar, it can be seen that the amount of oxide particles and coverage on the surface of the steel seems to decrease initially and increase afterwards. The sequence of oxides determined by oxygen partial pressure was calculated and illustrated using thermodynamics with alloys activity. Oxide growth mechanism during annealing and the effect of water pressure on the amount of oxide particles were also discussed.

Oxidative Formation and Removal of Complexed Mn(III) by Pseudomonas Species.

The observation of significant concentrations of soluble Mn(III) complexes in oxic, suboxic, and some anoxic waters has triggered a re-evaluation of the previous Mn paradigm which focused on the cycling between soluble Mn(II) and insoluble Mn(III,IV) species as operationally defined by filtration. Though Mn(II) oxidation in aquatic environments is primarily bacterially-mediated, little is known about the effect of Mn(III)-binding ligands on Mn(II) oxidation nor on the formation and removal of Mn(III). Pseudomonas putida GB-1 is one of the most extensively investigated of all Mn(II) oxidizing bacteria, encoding genes for three Mn oxidases (McoA, MnxG, and MopA). P. putida GB-1 and associated Mn oxidase mutants were tested alongside environmental isolates Pseudomonas hunanensis GSL-007 and Pseudomonas sp. GSL-010 for their ability to both directly oxidize weakly and strongly bound Mn(III), and to form these complexes through the oxidation of Mn(II). Using Mn(III)-citrate (weak complex) and Mn(III)-DFOB (strong complex), it was observed that P. putida GB-1, P. hunanensis GSL-007 and Pseudomonas sp. GSL-010 and mutants expressing only MnxG and McoA were able to directly oxidize both species at varying levels; however, no oxidation was detected in cultures of a P. putida mutant expressing only MopA. During cultivation in the presence of Mn(II) and citrate or DFOB, P. putida GB-1, P. hunanensis GSL-007 and Pseudomonas sp. GSL-010 formed Mn(III) complexes transiently as an intermediate before forming Mn(III/IV) oxides with the overall rates and extents of Mn(III,IV) oxide formation being greater for Mn(III)-citrate than for Mn(III)-DFOB. These data highlight the role of bacteria in the oxidative portion of the Mn cycle and suggest that the oxidation of strong Mn(III) complexes can occur through enzymatic mechanisms involving multicopper oxidases. The results support the observations from field studies and further emphasize the complexity of the geochemical cycling of manganese.

Changes in morphology and metabolism enable Mn-oxidizing bacteria from mid-oceanic ridge environment to counter metal-induced stress.

Mn-oxidizing potential of two metal-tolerant bacterial strains - Halomonas meridiana and Marinobacter algicola isolated from the South West Indian Ridge waters were compared at varying concentrations of Mn (II), i.e., 1, 10, and 100 μmol and mmol L-1 . Accompanying changes in their morphology and metabolism were also determined. At concentrations >1 mmol L-1 Mn (II), Mn-oxidizing potential of M. algicola was 2-7 times greater than that of H. meridiana. Scanning electron microscopy revealed that exposure to elevated metal content prompted bacterial cells especially those of M. algicola to been enveloped in exopolymeric material and form aggregates. Energy dispersive spectrometric analysis showed that exopolymeric material acts as a nucleation site for Mn deposition and oxide formation which occurs in the form of microspherical aggregates. These features show striking resemblance to biogenically produced Fe-Mn oxide deposits from Lau Basin. Surprisingly, diffractograms of auto-oxidized and bacterially formed Mn-oxide showed similarities to the hydrothermal vein mineral Rhodochrosite indicating that it can also be produced biotically. Elongation of cells by up to 4× the original size and distortion in cell shape were evident at Mn (II) concentrations >100 μmol L-1 . Marked differences in C-substrate utilization by the test strains were also observed in presence of Mn (II). A shift in use of substrates that are readily available in oceanic waters like N-acetyl-d-glucosamine to those that can be used under changing redox conditions (d-cellobiose) or in the presence of metal ions (d-arabinose, l-asparagine) were observed. These findings highlight the significant role of autochthonous bacteria in transforming reduced metal ions and aiding in the formation of metal oxides. Under natural or laboratory conditions, the mode of bacterially generated Mn-oxide tends to remain the same.

Formation of diverse Mn oxides: a review of bio/geochemical processes of Mn oxidation

Mn (oxyhydr)oxides, widely distributed in most geological settings, are found in more than 30 different types with different atomic arrangements, a wide range of crystallinity, and mixed valences of structural Mn. Each type of Mn (oxyhydr)oxides is characterized by its crystal structure and the valence state of structural Mn, which are likely the primary factors controlling its distinct reactivity. It is well known that varying pathways of Mn(II) oxidation, including homogeneous, surface catalyzed, or microbial processes, are responsible for the diversity of Mn (oxyhydr)oxide formation. However, the investigation of Mn(II) oxidation products through various oxidation processes have not been fully understood yet partly because the chemical and mineralogical identification of nanosized and poorly crystalline Mn (oxyhydr)oxides has been difficult at times. In this work, the previous studies on the formation of diverse Mn (oxyhydr)oxides through varying geochemical or microbial oxidation processes are thoroughly reviewed. This work may provide general background information for understanding the complex bio/geochemical processes controlling the formation of Mn (oxyhydr)oxides under various conditions of the natural environment.

High Manganese Tolerance and Biooxidation Ability of Serratia marcescens Isolated from Manganese Mine Water in Minas Gerais, Brazil.

Manganese is an important metal for the maintenance of several biological functions, but it can be toxic in high concentrations. One of the main forms of human exposure to metals, such as manganese (Mn), is the consumption of solar salt contaminated. Mn-tolerant bacteria could be used to decrease the concentration of this metal from contaminated sites through safer environmental-friendly alternative technology in the future. Therefore, this study was undertaken to isolate and identify Mn resistant bacteria from water samples collected from a Mn mine in the Iron Quadrangle region (Minas Gerais, Brazil). Two bacterial isolates were identified as Serratia marcescens based on morphological, biochemical, 16S rDNA gene sequencing and phylogeny analysis. Maximum resistance of the selected isolates against increasing concentrations of Mn(II), up to 1200 mg L-1 was determined in solid media. A batch assay was developed to analyze and quantify the Mn removal capacities of the isolates. Biological Mn removal capacities of over 55% were detected for both isolates. Whereas that mechanism like biosorption, precipitation and oxidation could be explaining the Mn removal, we seek to give an insight into some of the molecular mechanisms adopted by S. marcescens isolates. For this purpose, the following approaches were adopted: leucoberbelin blue I assay, Mn(II) oxidation by cell-free filtrate and electron microscopy and energy-dispersive X-ray spectroscopy analyses. Overall, these results indicate that S. marcescens promotes Mn removal in an indirect mechanism by the formation of Mn oxides precipitates around the cells, which should be further explored for potential biotechnological applications for water recycling both in hydrometallurgical and mineral processing operations.