Diversity of Mn oxides produced by Mn(II)-oxidizing fungi

Manganese (Mn) oxides are environmentally abundant, highly reactive mineral phases that mediate the biogeochemical cycling of nutrients, contaminants, carbon, and numerous other elements. Despite the belief that microorganisms (specifically bacteria and fungi) are responsible for the majority of Mn oxide formation in the environment, the impact of microbial species, physiology, and growth stage on Mn oxide formation is largely unresolved. Here, we couple microscopic and spectroscopic techniques to characterize the Mn oxides produced by four different species of Mn(II)-oxidizing Ascomycete fungi (Plectosphaerella cucumerina strain DS2psM2a2, Pyrenochaeta sp. DS3sAY3a, Stagonospora sp. SRC1lsM3a, and Acremonium strictum strain DS1bioAY4a) isolated from acid mine drainage treatment systems in central Pennsylvania. The site of Mn oxide formation varies greatly among the fungi, including deposition on hyphal surfaces, at the base of reproductive structures (e.g., fruiting bodies), and on envisaged extracellular polymers adjacent to the cell. The primary product of Mn(II) oxidation for all species growing under the same chemical and physical conditions is a nanoparticulate, poorly-crystalline hexagonal birnessite-like phase resembling synthetic δ-MnO₂. The phylogeny and growth conditions (planktonic versus surface-attached) of the fungi, however, impact the conversion of the initial phyllomanganate to more ordered phases, such as todorokite (A. strictum strain DS1bioAY4a) and triclinic birnessite (Stagonospora sp. SRC1lsM3a). Our findings reveal that the species of Mn(II)-oxidizing fungi impacts the size, morphology, and structure of Mn biooxides, which will likely translate to large differences in the reactivity of the Mn oxide phases.     

On the role of Mn(IV) vacancies in the photoreductive dissolution of hexagonal birnessite

Photoreductive dissolution of layer type Mn(IV) oxides (birnessite) under sunlight illumination to form soluble Mn(II) has been observed in both field and laboratory settings, leading to a consensus that this process is a key driver of the biogeochemical cycling of Mn in the euphotic zones of marine and freshwater ecosystems. However, the underlying mechanisms for the process remain unknown, although they have been linked to the semiconducting characteristics of hexagonal birnessite, the ubiquitous Mn(IV) oxide produced mainly by bacterial oxidation of soluble Mn(II). One of the universal properties of this biogenic mineral is the presence of Mn(IV) vacancies, long-identified as strong adsorption sites for metal cations. In this paper, the possible role of Mn vacancies in photoreductive dissolution is investigated theoretically using quantum mechanical calculations based on spin-polarized density functional theory (DFT). Our DFT study demonstrates unequivocally that Mn vacancies significantly reduce the band-gap energy for hexagonal birnessite relative to a hypothetical vacancy-free MnO₂ and thus would increase the concentration of photo-induced electrons available for Mn(IV) reduction upon illumination of the mineral by sunlight. Calculations of the charge distribution in the presence of vacancies, although not fully conclusive, show a clear separation of photo-induced electrons and holes, implying a slow recombination of these charge-carriers that facilitates the two-electron reduction of Mn(IV) to Mn(II).     

Characterization of the manganese oxide produced by pseudomonas putida strain MnB1

Manganese oxides form typically in natural aqueous environments via Mn(II) oxidation catalyzed by microorganisms, primarily bacteria, but little is known about the structure of the incipient solid-phase products. The Mn oxide produced by a Pseudomonas species representative of soils and freshwaters was characterized as to composition, average Mn oxidation number, and N₂ specific surface area. Electron microscopy, X-ray diffraction, and X-ray absorption near edge structure spectroscopy were applied to complement the physicochemical data with morphological and structural information. A series of synthetic Mn oxides also was analyzed by the same methods to gain better comparative understanding of the structure of the biogenic oxide. The latter was found to be a poorly crystalline layer type Mn(IV) oxide with hexagonal symmetry, significant negative structural charge arising from cation vacancies, and a relatively small number of randomly stacked octahedral sheets per particle. Its properties were comparable to those of δ-MnO₂ (vernadite) and a poorly crystalline hexagonal birnessite (“acid birnessite”) synthesized by reduction of permanganate with HCl, but they were very different from those of crystalline triclinic birnessite. Overall, the structure and composition of the Mn oxide produced by P. putida were similar to what has been reported for other freshly precipitated Mn oxides in natural weathering environments, yielding further support to the predominance of biological oxidation as the pathway for Mn oxide formation. Despite variations in the degree of sheet stacking and Mn(III) content, all poorly crystalline oxides studied showed hexagonal symmetry. Thus, there is a need to distinguish layer type Mn oxides with structures similar to those of natural birnessites from the synthetic triclinic variety. We propose designating the unit cell symmetry as an addition to the current nomenclature for these minerals.     

Enzymatic microbial Mn(II) oxidation and Mn biooxide production in the Guaymas Basin deep-sea hydrothermal plume

Microorganisms play important roles in mediating biogeochemical reactions in deep-sea hydrothermal plumes, but little is known regarding the mechanisms that underpin these transformations. At Guaymas Basin (GB) in the Gulf of California, hydrothermal vents inject fluids laden with dissolved Mn(II) (dMn) into the deep waters of the basin where it is oxidized and precipitated as particulate Mn(III/IV) oxides, forming turbid hydrothermal “clouds”. Previous studies have predicted extremely short residence times for dMn at GB and suggested they are the result of microbially-mediated Mn(II) oxidation and precipitation. Here we present biogeochemical results that support a central role for microorganisms in driving Mn(II) oxidation in the GB hydrothermal plume, with enzymes being the primary catalytic agent. dMn removal rates at GB are remarkably fast for a deep-sea hydrothermal plume (up to 2 nM/h). These rapid rates were only observed within the plume, not in background deep-sea water above the GB plume or at GB plume depths (∼1750-2000 m) in the neighboring Carmen Basin, where there is no known venting. dMn removal is dramatically inhibited under anoxic conditions and by the presence of the biological poison, sodium azide. A conspicuous temperature optimum of dMn removal rates (∼40 °C) and a saturation-like (i.e. Michaelis-Menten) response to O₂ concentration were observed, indicating an enzymatic mechanism. dMn removal was resistant to heat treatment used to select for spore-forming organisms, but very sensitive to low concentrations of added Cu, a cofactor required by the putative Mn(II)-oxidizing enzyme. Extended X-ray absorption fine structure spectroscopy (EXAFS) and synchrotron radiation-based X-ray diffraction (SR-XRD) revealed the Mn oxides to have a hexagonal birnessite or δ-MnO₂-like mineral structure, indicating that these freshly formed deep-sea Mn oxides are strikingly similar to primary biogenic Mn oxides produced by laboratory cultures of bacteria. Overall, these results reveal a vigorous Mn biogeochemical cycle in the GB hydrothermal plume, where a distinct microbial community enzymatically catalyzes rapid Mn(II) oxidation and the production of Mn biooxides.     

Oxidation and Mineralization of Mn2+ Ions Mediated by Pseudomonas putida: Insights from an Experimental Study

The formation of manganese oxides in nature is commonly mediated by microorganisms.In this study,the mineralization of biogenic manganese oxidation mediated by Pseudomanas putida has been experimentally investigated by employing various characterization techniques,including SEM,FESEM,TEM,XRD,and STXM-NEXAFS.The results indicate that Mn~(2+) ions can be oxidized into Mn(Ⅳ) minerals(birnessite and pyrolusite) and Mn(Ⅲ) minerals(hausmannite and feitknechtite),successively.The primary products(birnessite and pyrolusite) further transformed into hausmannite and feitknechtite under Mn~(2+) ion-enriched conditions.However,birnessite and pyrolusite are the endproducts of the continuous microbial oxidation processes.These biogenic Mn oxides are poorly crystallized,which provides them with a high potential for usage in environmental restoration of contaminated soils and waters contaminated with heavy metals.The approaches employed in this study will also enrich genesis research of biological oxidation of Mn(Ⅱ) species in nature.     

模拟表生环境水钠锰矿亚结构转化及钙锰矿的形成

水钠锰矿是土壤与沉积物中最为常见的氧化锰矿物, 依据其MnO₆层对称特点分为六方和三斜两种亚结构类型.六方水钠锰矿在表生环境中可通过Mn2+的化学或生物氧化形成, 而环境中三斜水钠锰矿的形成及进一步转化为钙锰矿的途径尚不清楚.以两种六方水钠锰矿(酸性水钠锰矿和水羟锰矿)为前驱物, 采用X射线吸收光谱(EXAFS)、X射线衍射(XRD)、电镜(FESEM/TEM)及化学组成分析等技术方法模拟表生环境研究了水钠锰矿从六方向三斜的亚结构转化及生成钙锰矿的化学条件和矿物学机制.结果表明, 适当Mn(Ⅱ)浓度和弱碱性条件(pH≥8)可使六方水钠锰矿逐渐转化为三斜水钠锰矿, 继而经Mg2+交换、常压回流得到了长纤维状的钙锰矿, 其晶体生长以溶解-结晶为主.Mn(Ⅱ)与六方水钠锰矿MnO₆八面体层内的Mn(Ⅳ)反应生成Mn(Ⅲ)并填充层内空位, 使水钠锰矿对称型由六方向三斜转变.与酸性水钠锰矿相比, 水羟锰矿结晶弱、层状堆积混乱度高, 与Mn(Ⅱ)反应迅速, 层结构向三斜水钠锰矿转化快.pH升高, 促进六方水钠锰矿对Mn(Ⅱ)的吸附和Mn(Ⅱ)与Mn(Ⅳ)间的反应, 六方水钠锰矿转化为三斜水钠锰矿的速率加快."六方水钠锰矿→三斜水钠锰矿"可能是环境中三斜水钠锰矿的重要来源, 及进一步形成钙锰矿的重要化学生成机制.      

Structural characterization of terrestrial microbial Mn oxides from Pinal Creek, AZ

The microbial catalysis of Mn(II) oxidation is believed to be a dominant source of abundant sorption- and redox-active Mn oxides in marine, freshwater, and subsurface aquatic environments. In spite of their importance, environmental oxides of known biogenic origin have generally not been characterized in detail from a structural perspective. Hyporheic zone Mn oxide grain coatings at Pinal Creek, Arizona, a metals-contaminated stream, have been identified as being dominantly microbial in origin and are well studied from bulk chemistry and contaminant hydrology perspectives. This site thus presents an excellent opportunity to study the structures of terrestrial microbial Mn oxides in detail. XRD and EXAFS measurements performed in this study indicate that the hydrated Pinal Creek Mn oxide grain coatings are layer-type Mn oxides with dominantly hexagonal or pseudo-hexagonal layer symmetry. XRD and TEM measurements suggest the oxides to be nanoparticulate plates with average dimensions on the order of 11 nm thick × 35 nm diameter, but with individual particles exhibiting thickness as small as a single layer and sheets as wide as 500 nm. The hydrated oxides exhibit a 10-Å basal-plane spacing and turbostratic disorder. EXAFS analyses suggest the oxides contain layer Mn(IV) site vacancy defects, and layer Mn(III) is inferred to be present, as deduced from Jahn-Teller distortion of the local structure. The physical geometry and structural details of the coatings suggest formation within microbial biofilms. The biogenic Mn oxides are stable with respect to transformation into thermodynamically more stable phases over a time scale of at least 5 months. The nanoparticulate layered structural motif, also observed in pure culture laboratory studies, appears to be characteristic of biogenic Mn oxides and may explain the common occurrence of this mineral habit in soils and sediments.     

氧化锰矿物的生物成因及其性质的研究进展

土壤中的氧化锰矿物是原生矿物风化和成土过程的产物,是最具反应活性的一类矿物,决定着环境中许多物质的形态、迁移和转化,在元素生物地球化学循环中起着重要的作用,其形成机制和环境效应备受关注。已有的研究表明,环境中氧化锰的形成与微生物作用紧密相关,微生物作用可使自然环境中的Mn(Ⅱ)氧化速率提高105倍。参与Mn(Ⅱ)氧化的微生物在环境中广泛存在,已知的典型锰氧化细菌分布在变形菌门、放线菌门或厚壁菌门,它们均通过胞外聚合物中的多铜氧化酶来催化氧化Mn(Ⅱ)。细菌氧化Mn(II)成Mn(Ⅳ)是酶催化的两个连续的快速单电子传递过程,Mn(Ⅲ)在溶液中以与酶结合的瞬时中间态出现。生物形成氧化锰的最初形态为层状锰矿物,与δ-MnO2或酸性水钠锰矿很类似,且结晶弱,粒径小,锰氧化度高,结构中的八面体空穴多,因而比化学形成的氧化锰具有更强的吸附、氧化等表面活性。环境中Mn(Ⅱ)微生物氧化及形成的Mn(Ⅲ)中间体与碳、氮、硫等生命元素的地球化学循环的关系令人关注。     

Characterization of manganese oxide precipitates from Appalachian coal mine drainage treatment systems

The removal of Mn(II) from coal mine drainage (CMD) by chemical addition/active treatment can significantly increase treatment costs. Passive treatment for Mn removal involves promotion of biological oxidative precipitation of manganese oxides (MnO_x). Manganese(II) removal was studied in three passive treatment systems in western Pennsylvania that differed based on their influent Mn(II) concentrations (20–150 mg/L), system construction (±inoculation with patented Mn(II)-oxidizing bacteria), and bed materials (limestone vs. sandstone). Manganese(II) removal occurred at pH values as low as 5.0 and temperatures as low as 2 °C, but was enhanced at circumneutral pH and warmer temperatures. Trace metals such as Zn, Ni and Co were removed effectively, in most cases preferentially, into the MnO_x precipitates. Based on synchrotron radiation X-ray diffraction and Mn K-edge extended X-ray absorption fine structure spectroscopy, the predominant Mn oxides at all sites were poorly crystalline hexagonal birnessite, triclinic birnessite and todorokite. The surface morphology of the MnO_x precipitates from all sites was coarse and “sponge-like” composed of nm-sized lathes and thin sheets. Based on scanning electron microscopy (SEM), MnO_x precipitates were found in close proximity to both prokaryotic and eukaryotic organisms. The greatest removal efficiency of Mn(II) occurred at the one site with a higher pH in the bed and a higher influent total organic C (TOC) concentration (provided by an upstream wetland). Biological oxidation of Mn(II) driven by heterotrophic activity was most likely the predominant Mn removal mechanism in these systems. Influent water chemistry and Mn(II) oxidation kinetics affected the relative distribution of MnO_x mineral assemblages in CMD treatment systems.     

Oxidation of manganese by spores of a marine bacillus: Kinetic and thermodynamic considerations

The catalytic properties of spores of a marine Bacillus known to oxidize divalent manganese were used to perform laboratory Mn(II) oxidation experiments at environmental conditions of pH and Mn(II) concentration. We found that at pH 7.8 the initial kinetics of Mn(II) oxidation facilitated by the spores was four orders of magnitude greater than that which would be expected for abiotic autocatalysis on a colloidal MnO₂ surface. The rate progressively decreased as the spores became coated with manganese oxide, eventually becoming very near that predicted for abiotic surface catalysis. Transmission electron microscopic observations and oxidation state measurements of solids precipitated at pH 7.5 and [Mn(II)] < 50 nM indicated that the initial oxidation product was hausmannite (Mn₃O₄ or MnO_x where x = 1.33) which aged to more highly oxidized MnO₂ (x = 1.9) in the time scale of weeks. By utilizing spores to catalyze the oxidation rate, we were able to maintain our experimental system within the seawater range of pH and Mn(II) where highly oxidized manganese oxide precipitates are thermodynamically stable. In doing so we obtained, for the first time, laboratory precipitates with oxidation states similar to that found in marine particulate material. These results suggest that the concentration of manganese in seawater and the oxidation state of marine manganese oxides are controlled by the rapid precipitation of Mn₃O₄, which can be microbially mediated, followed by the disproportionation to MnO₂.     

Oxidation of arsenite to arsenate on birnessite in the presence of light

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h^?1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.     

MnO2-UV联用光化学降解苯酚废水的初步研究

探讨了人工合成的高价锰氧化物与紫外光(UV)联用时降解苯酚废水的特性。结果表明,氧化锰矿物在无UV时对苯酚的降解能力差异大,1 g/L的氧化锰4 h对200 mg/L苯酚废水的降解率和COD去除率分别为:锰钾矿97.51%、酸性水钠锰矿89.07%、碱性水钠锰矿11.36%、钙锰矿9.67%;锰钾矿87.79%、酸性水钠锰矿53.11%、碱性水钠锰矿6.42%、钙锰矿1.43%。UV光照下,氧化锰矿物对苯酚的降解率有不同程度的提高,且表现出显著的表面光催化性质,增加了苯酚的深度降解,COD去除率显著提高。UV下氧化锰4 h对苯酚的降解率分别为:锰钾矿99.48%、酸性水钠锰矿91.86%、碱性水钠锰矿40.15%、钙锰矿35.95%);COD的去除率分别为:锰钾矿98.11%、酸性水钠锰矿68.45%、钙锰矿27.57%、碱性水钠锰矿24.27%。MnO2-UV联用时降解苯酚可能包括两种主要作用机制:氧化锰矿物的直接化学氧化降解和UV下MnO2的表面光催化降解。     

Oxidation of As^Ⅲ by Several Manganese Oxide Minerals in Absence and Presence of Goethite

Oxidation of As^Ⅲ by three types of manganese oxide minerals affected by goethite was investigated by chemical analysis, equilibrium redox, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. Three synthesized Mn oxide minerals of different types, birnessite, todorokite, and hausmannite, could actively oxidize As^Ⅲ to Asv, and greatly varied in their oxidation ability. Layer structured birnessite exhibited the highest capacity of As^Ⅲ oxidation, followed by the tunnel structured todorokite. Lower oxide hansmannite possessed much low capacity of As^Ⅲ oxidation, and released more Mn^2＋ than birnessite and todorokite during the oxidation. The maximum amount of Asv produced during the oxidation of As^Ⅲ by Mn oxide minerals was in the order： birnessite （480.4 mmol/kg） 〉 todorokite （279.6 mmol/kg） 〉 hansmannite （117.9 mmol/kg）. The oxidation capacity of the Mn oxide minerals was found to be relative to the composition, crystallinity, and surface properties. In the presence of goethite oxidation of As^Ⅲ by Mn oxide minerals increased, with maximum amounts of Asv being 651.0 mmol/kg for birnessite, 332.3 mmol/kg for todorokite and 159.4 mmol/kg for hansmannite. Goethite promoted As^Ⅲ oxidation on the surface of Mn oxide minerals through adsorption of the Asv produced, incurring the decrease of Asv concentration in solutions. Thus, the combined effects of the oxidation （by Mn oxide minerals）-adsorption （by goethite） lead to rapid oxidation and immobilization of As in soils and sediments and alleviation of the As^Ⅲ toxicity in the environments.     

Surface complexation of Pb(II) by hexagonal birnessite nanoparticles

Natural hexagonal birnessite is a poorly crystalline layer type Mn(IV) oxide precipitated by bacteria and fungi which has a particularly high adsorption affinity for Pb(II). X-ray spectroscopic studies have shown that Pb(II) forms strong inner-sphere surface complexes mainly at two sites on hexagonal birnessite nanoparticles: triple corner-sharing (TCS) complexes on Mn(IV) vacancies in the interlayers and double edge-sharing (DES) complexes on lateral edge surfaces. Although the TCS surface complex has been well characterized by spectroscopy, some important questions remain about the structure and stability of the complexes occurring on the edge surfaces. First-principles simulation techniques such as density functional theory (DFT) offer a useful way to address these questions by providing complementary information that is difficult to obtain by spectroscopy. Following this computational approach, we used spin-polarized DFT to perform total-energy-minimization geometry optimizations of several possible Pb(II) surface complexes on model birnessite nanoparticles similar to those that have been studied experimentally. We first validated our DFT calculations by geometry optimizations of (1) the Pb-Mn oxyhydroxide mineral, quenselite (PbMnO₂OH), and (2) the TCS surface complex, finding good agreement with experimental structural data while uncovering new information about bonding and stability. Our geometry optimizations of several protonated variants of the DES surface complex led us to conclude that the observed edge-surface species is very likely to be this complex if the singly coordinated terminal O that binds to Pb(II) is protonated. Our geometry optimizations also revealed that an unhydrated double corner-sharing (DCS) species that has been proposed as an alternative to the DES complex is intrinsically unstable on nanoparticle edge surfaces, but could become stabilized if the local coordination environment is well-hydrated. A significant similarity exists in the structural parameters for the TCS complex and those for a DCS edge-surface complex that is protonated in the same manner as the optimal DES complex, which could complicate detecting the DCS complex in X-ray absorption spectra.     

Formation of nano-crystalline todorokite from biogenic Mn oxides

Todorokite, as one of three main Mn oxide phases present in oceanic Mn nodules and an active MnO₆ octahedral molecular sieve (OMS), has garnered much interest; however, its formation pathway in natural systems is not fully understood. Todorokite is widely considered to form from layer structured Mn oxides with hexagonal symmetry, such as vernadite (δ-MnO₂), which are generally of biogenic origin. However, this geochemical process has not been documented in the environment or demonstrated in the laboratory, except for precursor phases with triclinic symmetry. Here we report on the formation of a nanoscale, todorokite-like phase from biogenic Mn oxides produced by the freshwater bacterium Pseudomonas putida strain GB-1. At long- and short-range structural scales biogenic Mn oxides were transformed to a todorokite-like phase at atmospheric pressure through refluxing. Topotactic transformation was observed during the transformation. Furthermore, the todorokite-like phases formed via refluxing had thin layers along the c^∗ axis and a lack of c^∗ periodicity, making the basal plane undetectable with X-ray diffraction reflection. The proposed pathway of the todorokite-like phase formation is proposed as: hexagonal biogenic Mn oxide → 10-Å triclinic phyllomanganate → todorokite. These observations provide evidence supporting the possible bio-related origin of natural todorokites and provide important clues for understanding the transformation of biogenic Mn oxides to other Mn oxides in the environment. Additionally this method may be a viable biosynthesis route for porous, nano-crystalline OMS materials for use in practical applications.     

Kinetics of Mn(II) oxidation by Leptothrix discophora SS1

The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O₂, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O₂ and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria.     

A specific Ce oxidation process during sorption of rare earth elements on biogenic Mn oxide produced by Acremonium sp. strain KR21-2

Sorption of rare earth elements (REEs) and Ce oxidation on natural and synthetic Mn oxides have been investigated by many researchers. Although Mn(II)-oxidizing microorganisms are thought to play an important role in the formation of Mn oxides in most natural environments, Ce oxidation by biogenic Mn oxide and the relevance of microorganisms to the Ce oxidation process have not been well understood. Therefore, in this study, we conducted sorption experiments of REEs on biogenic Mn oxide produced by Acremonium sp. strain KR21-2. The distribution coefficients, K_d(REE), between biogenic Mn oxide (plus hyphae) and 10 mmol/L NaCl solution showed a large positive Ce anomaly and convex tetrad effect variations at pH 3.8, which was consistent with previous works using synthetic Mn oxide. The positive Ce anomaly was caused by oxidation of Ce(III) to Ce(IV) by the biogenic Mn oxide, which was confirmed by analysis of the Ce L_III-edge XANES spectra. With increasing pH, the positive Ce anomaly and convex tetrad effects became less pronounced. Furthermore, negative Ce anomalies were observed at a pH of more than 6.5, suggesting that Ce(IV) was stabilized in the solution (<0.2 μm) phase, although Ce(III) oxidation to Ce(IV) on the biogenic Mn oxide was confirmed by XANES analysis. It was demonstrated that no Ce(III) oxidation occurred during sorption on the hyphae of strain KR21-2 by the K_d(REE) patterns and XANES analysis. The analysis of size exclusion HPLC-ICP-MS showed that some fractions of REEs in the filtrates (<0.2 μm) after sorption experiments were bound to organic molecules (40 and <670 kDa fractions), which were possibly released from hyphae. A line of our data indicates that the negative Ce anomalies under circumneutral pH conditions arose from Ce(III) oxidation on the biogenic Mn oxide and subsequent complexation of Ce(IV) with organic ligands. The suppression of tetrad effects is also explained by the complexation of REEs with organic ligands. The results of this study demonstrate that the coexistence of the biogenic Mn oxide and hyphae of strain KR21-2 produces a specific redox chemistry which cannot be explained by inorganic species.     

Crystal chemistry of clay-Mn oxide associations in soils, fractures, and matrix of the Bandelier Tuff, Pajarito Mesa, New Mexico

The upper 25 m of Bandelier Tuff at Pajarito Mesa, New Mexico, include soils, shallow fractures, deeper fractures, and tuff matrices in which clays provide a record of transport and alteration. The principal pathways within this system are fractures that penetrate the tuff. Large fractures that host deep root penetration provide a setting in which clay deposits accumulate through particulate or colloidal migration from the soil zone. Clays throughout the system are predominantly expandable interstratified illite/smectites (I/S), but clays of the tuff matrix at depth are distinctly Fe-rich and are not mixed with clays transported from the surface into fractures. Chemical alteration superimposed on clay particles transported into fractures results in clays with lower Al : Si ratios, higher Na, and higher lanthanide content with increasingly negative Eu anomalies with depth. These changes are accompanied by invasion and precipitation of Mn oxides, principally birnessite, within clay bodies. Investigation of the Mn oxides by synchrotron X-ray fluorescence (SXRF) shows that Mn is associated with Ba, Ce, Ni, and Pb. In addition, synchrotron X-ray absorption near-edge structure (XANES) spectra show that Ce in Mn oxides occurs as Ce³⁺ and Ce⁴⁺, with average Ce oxidation state of ∼3.75. The Mn oxides intergrown with clays actively participate in removal of Ce from solution, accompanied by oxidation of Ce³⁺ to Ce⁴⁺. Other lanthanides are accumulated by the clays but are not concentrated along with Ce in the Mn oxides. Extraction of Ce from solution by Mn oxides is more effective than lanthanide accumulation in clay, a process that is variable and likely influenced by defects, extent of recrystallization, and particle sizes. This dichotomy in lanthanide interaction results in locally constant Ce content but either negative or positive Ce anomalies in the clay-Mn oxide system as a consequence of variability in the abundance of the other lanthanides. Nevertheless, the net lanthanide pattern for the sum of all clay-Mn oxide samples in either shallow or deep fractures has no Ce anomaly, indicating that other lanthanides segregated from Ce are not transported beyond the range of either the shallow or deep fracture systems. Evidence from Eu anomalies indicates that lanthanides accumulated in the fracture clays are acquired from the local tuff. The clay-Mn oxide assemblage is more effective than clay alone in accumulating of a wide variety of heavy metals.     

Chromite oxidation by manganese oxides in subseafloor basalts and the presence of putative fossilized microorganisms

Chromite is a mineral with low solubility and is thus resistant to dissolution. The exception is when manganese oxides are available, since they are the only known naturally occurring oxidants for chromite. In the presence of Mn(IV) oxides, Cr(III) will oxidise to Cr(VI), which is more soluble than Cr(III), and thus easier to be removed. Here we report of chromite phenocrysts that are replaced by rhodochrosite (Mn(II) carbonate) in subseafloor basalts from the Koko Seamount, Pacific Ocean, that were drilled and collected during the Ocean Drilling Program (ODP) Leg 197. The mineral succession chromite-rhodochrosite-saponite in the phenocrysts is interpreted as the result of chromite oxidation by manganese oxides. Putative fossilized microorganisms are abundant in the rhodochrosite and we suggest that the oxidation of chromite has been mediated by microbial activity. It has previously been shown in soils and in laboratory experiments that chromium oxidation is indirectly mediated by microbial formation of manganese oxides. Here we suggest a similar process in subseafloor basalts.     

PseudomonasputidaMnB1氧化分解菱锰矿的实验研究

自然界中,菱锰矿氧化形成锰的氧化物矿物是非常普遍的现象,在菱锰矿被氧化分解发生物相转变的过程中,碳酸 盐溶解和锰的氧化往往同时发生,微生物可能起着催化作用。选取锰氧化模式菌株PseudomonasputidaMnB1和广西梧州菱 锰矿,通过菱锰矿在该细菌作用下发生转变的实验,利用场发射扫描电镜、扫描透射X射线显微成像等分析方法,研究了 矿石表面形貌变化以及锰元素在细胞上的分布特征。结果表明细菌显著促进的菱锰矿的溶解,在此基础上,进一步探讨了 细菌在菱锰矿氧化过程中的贡献,本实验结果丰富了次生锰矿床的微生物成因研究。