Formation of todorokite from vernadite in Ni-rich hemipelagic sediments

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg²⁺ cations. The average structural formula of 10Å-vernadite is if the layers contain vacancy sites, or alternately , if they contain Mn³⁺. The average formula of todorokite is .A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg²⁺ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg²⁺ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg²⁺ (i.e., high layer charge), or Mn³⁺ and Mn⁴⁺ cations ordered following the Mn³⁺-Mn⁴⁺-Mn⁴⁺ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.     

Coupled biotic-abiotic Mn(II) oxidation pathway mediates the formation and structural evolution of biogenic Mn oxides

Manganese (Mn) oxides are among the strongest oxidants and sorbents in the environment, impacting the transport and speciation of metals, cycling of carbon, and flow of electrons within soils and sediments. The oxidation of Mn(II) to Mn(III/IV) oxides has been primarily attributed to biological processes, due in part to the faster rates of bacterial Mn(II) oxidation compared to observed mineral-induced and other abiotic rates. Here we explore the reactivity of biogenic Mn oxides formed by a common marine bacterium (Roseobacter sp. AzwK-3b), which has been previously shown to oxidize Mn(II) via the production of extracellular superoxide. Oxidation of Mn(II) by superoxide results in the formation of highly reactive colloidal birnessite with hexagonal symmetry. The colloidal oxides induce the rapid oxidation of Mn(II), with dramatically accelerated rates in the presence of organics, presumably due to mineral surface-catalyzed organic radical generation. Mn(II) oxidation by the colloids is further accelerated in presence of both organics and light, implicating reactive oxygen species in aiding abiotic oxidation. Indeed, the enhancement of Mn(II) oxidation is negated when the colloids are reacted with Mn(II) in the presence of superoxide dismutase, an enzyme that scavenges the reactive oxygen species (ROS) superoxide. The reactivity of the colloidal phase is short-lived due to the rapid evolution of the birnessite from hexagonal to pseudo-orthogonal symmetry. The secondary particulate triclinic birnessite phase exhibits a distinct lack of Mn(II) oxidation and subsequent Mn oxide formation. Thus, the evolution of initial reactive hexagonal birnessite to non-reactive triclinic birnessite imposes the need for continuous production of new colloidal hexagonal particles for Mn(II) oxidation to be sustained, illustrating an intimate dependency of enzymatic and mineral-based reactions in Mn(II) oxidation. Further, the coupled enzymatic and mineral-induced pathways are linked such that enzymatic formation of Mn oxide is requisite for the mineral-induced pathway to occur. Here, we show that Mn(II) oxidation involves a complex network of abiotic and biotic processes, including enzymatically produced superoxide, mineral catalysis, organic reactions with mineral surfaces, and likely photo-production of ROS. The complexity of coupled reactions involved in Mn(II) oxidation here highlights the need for further investigations of microbially-mediated Mn oxide formation, including identifying the role of Mn oxide surfaces, organics, reactive oxygen species, and light in Mn(II) oxidation and Mn oxide phase evolution.     

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.     

Sorption of ferric iron from ferrioxamine B to synthetic and biogenic layer type manganese oxides

Siderophores are biogenic chelating agents produced in terrestrial and marine environments that increase the bioavailability of ferric iron. Recent work has suggested that both aqueous and solid-phase Mn(III) may affect siderophore-mediated iron transport, but scant information appears to be available about the potential roles of layer type manganese oxides, which are relatively abundant in soils and the oligotrophic marine water column. To probe the effects of layer type manganese oxides on the stability of aqueous Fe-siderophore complexes, we studied the sorption of ferrioxamine B [Fe(III)HDFOB⁺, an Fe(III) chelate of the trihydroxamate siderophore desferrioxamine B (DFOB)] to two synthetic birnessites [layer type Mn(III,IV) oxides] and a biogenic birnessite produced by Pseudomonas putida GB-1. We found that all of these predominantly Mn(IV) oxides greatly reduced the aqueous concentration of Fe(III)HDFOB⁺ at pH 8. Analysis of Fe K-edge EXAFS spectra indicated that a dominant fraction of Fe(III) associated with the Mn(IV) oxides is not complexed by DFOB as in solution, but instead Fe(III) is specifically adsorbed to the mineral structure at multiple sites, thus indicating that the Mn(IV) oxides displaced Fe(III) from the siderophore complex. These results indicate that layer type manganese oxides, including biogenic minerals, may sequester iron from soluble ferric complexes. We conclude that the sorption of iron-siderophore complexes may play a significant role in the bioavailability and biogeochemical cycling of iron in marine and terrestrial environments.     

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.     

黄骅坳陷古近系沙河街组生物灰岩分布特征及其形成条件

黄骅坳陷古近系沙河街组的生物灰岩发育于沙一段下部的"特殊岩性段"内,其分布具有北区零星、中区集中、南区广泛的特点。沙一段生物灰岩在坳陷北区仅在板桥地区的唐家河一带零星见到;在中区主要见于港西凸起和孔店凸起之间,沿着大中旺-周清庄-王徐庄-扣村一带分布;在南区则分布广泛,主要有4个发育带:沧州发育带、舍女寺发育带、盐山发育带和集北头-灯明寺发育带。综合分析认为,沙一段生物灰岩分布的控制因素主要有古气候、古水体介质条件、古水深、古地形坡度以及陆源碎屑的供给等。     

The formation of todorokite and birnessite in sea water pumped from under ground

Manganese oxides precipitated from aerated well sea water at the Marine Science Museum, Tokai University, have been analyzed chemically and mineralogically. The $\text{O}\text{Mn}$ ratios are lower in todorokite than in birnessite but these minerals have similar contents of minor transition metals, which can be taken up additionally from sea water after the precipitation of Mn oxides. On the basis of these results, the genesis of Mn minerals is discussed in relation to marine Mn nodules.     

The surface chemistry of sediments from the Panama Basin: The influence of Mn oxides on metal adsorption

The solid Mn content of sediments at a site in the Panama Basin (5°21′N 81°56′W) decreases from 3.9% in the interfacial sediment to 1% at 1.5 cm and <0.2% below 5 cm. These conditions provide an opportunity to examine the influence of Mn oxides on the metal adsorption characteristics of natural marine sediments.The adsorption of 14 metals on interfacial sediment and sediment from depths of 0.5 to 3 cm and 15 to 19 cm from the Panama Basin site was studied, and distribution coefficients (K_D) were determined. A comparison of the K_D values for a variety of samples containing different Mn contents (i.e., Panama Basin sediments, MANOP site H interfacial sediment, red clay, and buserite) indicates that an increase in the solid Mn content enhances the ability of the particles to bind certain metals (e.g., Zn, Pb, Co, Cd, and Ba) while the binding ability for other metals (e.g., Cs, Be, Sc, Pu, Sn, and Fe) is not significantly affected. For the Panama Basin sediment, the K_D values for Ni, Co, Cd, Ba, and Mn for the Mn enriched interfacial sediment are 5 to 23 times greater than the K_D values for the Mn depleted deep sediment. The K_D values for Cs, Be, Sc, Pu, and Fe for the two types of sediment are essentially the same. The correlation between the Mn content and the binding ability of the sediment for particular metals coincides with the Mn-metal correlations observed in bulk compositional data for ferromanganese nodules and sediments. This implies that the observed metal enrichments in nodules or hemipelagic sediments are most likely caused by preferential adsorption of the metals by Mn enriched phases.     

Structure of Mn and Fe oxides and oxyhydroxides: A topological approach by EXAFS

The structure of Mn and Fe oxides and oxyhydroxides has been probed by EXAFS. It is shown that EXAFS spectroscopy is sensitive to the nature of interpolyhedral linkages relying on metal-two nearest metal distances. Spectra recorded at 290 K and 30 K indicate that intercationic distances can be determined by EXAFS with a good accuracy (0.02 Å) assuming a purely Gaussian distribution function, even at room temperature. Although the accuracy on atomic numbers determination is fair for these disordered systems, EXAFS can differentiate structures with contrasted edge- over corner-sharing ratio like pyrolusite, ramsdellite, todorokite and lithiophorite or lepidocrocite and goethite. A direct application of this result has shown that the proportion of pyrolusite domains within the lattice of nsutite from Ghana is equal to 35±15 percent. The systematic study of Mn dioxides also put forward the sensitivity of EXAFS to the presence of corner-sharing octahedra, with a detection limit found to be less than 8 percent. In spite of their similar XRD patterns, the EXAFS study of todorokite and asbolane confirms that they possess a distinct structure; that is, a tunnel structure for the former and a layered structure for the second. Such a topological approach has been used to probe the structure of ferruginous vernadite; a highly disordered iron-bearing Mn oxide. Fe and Mn K-edges EXAFS spectra are very dissimilar, traducing a different short range order. The Mn phase is constituted by MnO₂ layers. Its large local structural order contrasts with the short range disorder of the iron phase. This hydrous Fe oxyhydroxide is constituted by face-, edge- and corner-sharing octahedra. This iron phase possesses the same local order as feroxy-hyte, but is long range disordered. The presence of face-sharing Fe(O,OH)₆ octahedra prevents its direct solid-state transformation into well crystallized oxyhydroxides, and explains the necessary dissolution-reprecipitation mechanism generally invoked for the hydrous ferric gel → goethite transformation.     

Adsorption of Co and selected actinides by Mn and Fe oxides in soils and sediments

Iron and Mn oxides and associated radionuclides in soils and sediments from the radioactive waste burial grounds at Oak Ridge National Laboratory have been selectively extracted using wet chemical techniques. Product-moment-correlation analyses have demonstrated that ⁶⁰Co and various actinides, principally ²⁴⁴Cm, ²⁴¹Am and ²³⁸Pu are dominantly associated with Mn oxides. Correlation coefficients between these radionuclides and Fe oxides and organic C are generally very low. The important role of Mn oxides in radionuclide adsorption is attributed to their unique surface and colloidal properties. The data illustrate the importance of the Mn oxide component of soils and sediments in controlling transition metal and actinide solubility.These results suggest two major implications for the disposal of radioactive waste. First, in order to minimize future ⁶⁰Co and actinide mobilization from disposal sites, a chemical environment in which Mn oxides are least soluble should be maintained. Second, the liberal use of Mn oxides in waste management operations might improve long-term retention of these radionuclides. Deep-sea Mn modules, which may in the future be mined for their trace metal contents, could serve as a ready supply of Mn oxide for waste disposal applications.     

Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations

The association of rare earth and other trace elements with Fe and Mn oxides was studied in Fe-Mn-nodules from a lateritic soil from Serra do Navio (Northern Brazil). Two improved methods of selective dissolution by hydroxylamine hydrochloride and acidified hydrogen peroxide along with a classical Na–citrate–bicarbonate–dithionite method were used. The two former reagents were used to dissolve Mn oxides without significant dissolution of Fe oxides, and the latter reagent was used to dissolve both Mn and Fe oxides. Soil nodules and matrix were separated by hand. Inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry after fusion with lithium metaborate, and X-ray diffraction were used to determine the elemental and mineralogical composition of the nodules and soil matrix. The latter was composed of kaolinite, gibbsite, goethite, hematite, and quartz. In the nodules, lithiophorite LiAl₂(Mn^IV₂Mn^III)O₆(OH)₆ was detected in addition to the above-mentioned minerals. The presence of hollandite (BaMn₈O₁₆) and/or coronadite (PbMn₈O₁₆) in the nodules is also possible. In comparison to the matrix, the nodules were enriched in Mn, Fe, K, and P, and relatively poor in Si, Al, and Ti. The nodules were also enriched in all trace elements determined. Phosphorus, As and Cr were associated mainly with Fe oxides; Cu, Ni, and V were associated with both Fe and Mn oxides; and Ba, Co, and Pb were associated mainly with Mn oxides. Distribution of rare earth elements indicated a strong positive Ce-anomaly in the nodules, compared to the absence of any anomaly in the matrix. Some of Ce was associated with Mn oxides. The improved methods achieved almost complete release of Mn from the sample without decreasing the selectivity of dissolution, i.e., without dissolving significant amounts of Fe oxides and other minerals, and provided reliable information on associations of trace elements with Mn oxides. These methods are thus proposed to be included in sequential extraction schemes for fractionation of trace elements in soils and sediments.     

Inter-laboratory comparison of oxygen isotope compositions from biogenic silica

Several techniques have been introduced in the last decades for the dehydration and release of O₂ from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ¹⁸O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ¹⁸O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ¹⁸O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ¹⁸O analyses, available on request through the relevant laboratories.     

Mesoproterozoic biogenic thrombolites from the North China platform

Thrombolites are abundant in the subtidal dolostones of the Mesoproterozoic Wumishan Formation (ca 1.50–1.45 Ga) in the North China platform. Three major components are identified within the thrombolites: irregular mesoclots, micritic matrix and spar-filled voids. The mesoclot generally comprises a relatively organic-rich micritic core and a microsparitic outer layer that consists of fibrous aragonite (pseudocrystals) with less organic matter. In the core of mesoclots, abundant fossilized organic remnants, such as putative coccoidal and filamentous bacteria and mucus- to film-like extracellular polymeric substance (EPS), are closely associated with organominerals including nanoglobules and submicron-scale polyhedrons. In exceptionally well-preserved mesoclots, their outer layers commonly contain micropores displaying as bacterial molds and filamentous bacteria fossils. The matrix of mesoclots consists mainly of micropeloids (20–30 μm in diameter) and minor terrigenous detritus. Some mesoclots have denticulate edges and their matrix shows growth laminations that envelope the outlines of mesoclots. These features indicate that the mesoclots are primary and they were mineralized earlier than the surrounding matrix. The mineralization of mesoclots may have proceeded in two stages: (1) organomineralization of the cores through replacement of organic matter by minute organominerals resulting from anaerobic degradation of bacteria and EPS and (2) inorganic precipitation of the outer layers fostered by an increase in carbonate alkalinity in micro-environment due to organic matter decomposition. The thrombolites from the Mesoproterozoic Wumishan Formation may have formed through complex interactions between microbes and environments and represent the earliest known Precambrian biogenic thrombolites.     

Mineralogy of manganese oxides from Groote Eylandt

The manganese ores on Groote Eylandt occur in a flat-lying horizon in a series of Lower Cretaceous sands and clays which overly unconformably the Middle Proterozoic sandstone basement. The ore horizon exhibits a variety of textural and compositional ore types. Textural types include pisolites and concretions, either free or cemented into massive, bouldery or pebbly horizons. Compositional variations result from varying degrees of admixture of the manganese materials with the sands and clays of the formation. — The ore minerals are pyrolusite and cryptomelane, with minor amounts of lithiophorite, psilomelane, nsutite and todorokite. Gangue minerals include kaolinite, quartz and goethite. — Mineralogical studies indicate that three microtypes of cryptomelane occur. Low levels of cobalt, nickel and zinc in certain of the ore types are associated with the mineral lithiophorite.     

Use of normalized metal concentrations in the Mn oxides/hydrous oxides extraction phase of stream sediments to enhance the difference between a landfill emission plume and background

Normalized concentrations of Fe, Zn, Ba and Co bound to the Mn phase were determined using a ratio of metal concentration to Mn concentration. Metal concentrations were taken from a previous study which found elevated concentrations of heavy metals in the stream sediments in the vicinity of 2 landfills using a HN0₃ extraction of the whole sediment sample and geochemical phases. Results from the normalized metal concentrations for Zn, Fe and Ba showed a clearer distinction between background and emission plume regions along the stream compared to the same for the HN0₃ and Mn phase extractions. Lack of elevated concentrations and normalized concentrations for Co indicated the concentrations represented the background. The small distance used in the study could explain the lack of a decreasing trend of metal concentrations and normalized metal concentrations downstream in the emission plume of the sediments. Although Mn⁺² 2concentrations are known to be incorporated into the natural state of the Mn phase, these concentrations were thought to be small enough not to influence the results from the normalized concentrations. Non-selectivity of extractants and metal redistribution among chemical phases during extraction procedures are thought to be small and did not invalidate the results.     

高岭土吸附剂去除含锰废水中锰离子的实验研究

高岭土吸附剂处理含锰废水中锰离子的实验表明:高岭土的最佳粒度为0.177 mm,pH值控制在7.5～ 8.5间,常温搅拌30 min,吸附剂与水量比为12 g∶1 L,对锰离子质量浓度为100 mg/L的废水的处理效果最好,使锰离子质量浓度由100 mg/L降至0.1 mg/L,锰离子的去除率超过90%,达到GB8978-1996工业废水排放的一级标准.高岭土对锰离子吸附的等温吸附曲线符合Freundlich模型,其吸附机理主要是吸附作用和沉淀作用.     

湿法冶锰废渣中锰的形态分析与应用

将湿法冶锰废渣经过烘干、研细等预处理,按其所使用的原材料进行形态研究,通过筛选和实验,确定了废渣中的锰可用形态分析方法。在弄清废渣中锰的各种形态及含量的基础上,将其成果进行转化,用于指导工业生产、环境评价和废渣处理,具有很好的经济效益和社会效益。     

黔东松桃南华系大塘坡组锰矿层物源:来自Sr同位素的证据

锰矿床的物质来源是锰矿床研究的难点问题之一.辨别黔东松桃地区南华系大塘坡组锰矿沉积的物质来源有助于加深对锰矿成矿过程的理解.对黔东松桃地区南华系大塘坡组锰矿沉积的Sr同位素研究显示,15个锰矿石、锰质页岩及炭质页岩样品87Sr/86Sr同位素比值变化范围为0.705 727~0.732 536,其中炭质页岩样品具有最高的Sr同位素比值0.732 536,含锰岩系样品87Sr/86Sr同位素比值平均值为0.711 128.样品中87Sr/86Sr比值随着Al含量的上升,分别出现87Sr/86Sr比值上升与下降的两个分异趋势.87Sr/86Sr比值随Mn含量的上升总体呈现下降的趋势,但该趋势无显著相关性,残差分析显示这主要是由于样品中87Sr/86Sr比值随着Mn含量上升出现收敛性波动造成.上述现象是由于陆源碎屑成分和海底热液成分混合输入造成.通过与大塘坡组同时代(约660 Ma)古海水Sr同位素组成,世界范围内不同时代锰矿沉积以及现代红海沉积物的Sr同位素结果对比,发现黔东松桃地区南华系锰矿层中Sr同位素比值分布范围较宽,部分锰矿样品87Sr/86Sr比值低于古海水87Sr/86Sr比值,与典型大洋成因的锰矿层或铁锰结核具有不同的Sr同位素特征.联系黔东南华系大塘坡组锰矿层形成时期的特殊地质背景,认为锰质积累过程与沉淀过程为不同阶段产物——锰质的积累过程在Sturtian冰期盆地缺氧水体中完成,可能主要以海底热液喷溢系统完成;而锰矿的沉淀过程则是在间冰期伊始古海洋化学条件动荡的水体中完成.      

Oxidative Ce3+ sequestration by fungal manganese oxides with an associated Mn(II) oxidase activity

Sequestration of Ce³⁺ by biogenic manganese oxides (BMOs) formed by a Mn(II)-oxidizing fungus, Acremonium strictum strain KR21-2, was examined at pH 6.0. In anaerobic Ce³⁺ solution, newly formed BMOs exhibited stoichiometric Ce³⁺ oxidation, where the molar ratio of Ce³⁺ sequestered (Ce_seq) relative to Mn²⁺ released (Mn_rel) was maintained at approximately two throughout the reaction. A similar Ce³⁺ sequestration trend was observed in anaerobic treatment of BMOs in which the associated Mn(II) oxidase was completely inactivated by heating at 85 °C for 1 h or by adding 50 mM NaN₃. Aerobic Ce³⁺ treatment of newly formed BMO (enzymatically active) resulted in excessive Ce³⁺ sequestration over Mn²⁺ release, yielding Ce_seq/Mn_rel > 200, whereas heated or poisoned BMOs released a significant amount of Mn²⁺ with lower Ce³⁺ sequestration efficiency. Consequently, self-regeneration by the Mn(II) oxidase in newly formed BMO effectively suppressed Mn²⁺ release and enhanced oxidative Ce³⁺ sequestration under aerobic conditions. Repeated treatments of heated or poisoned BMOs under aerobic conditions confirmed that oxidative Ce³⁺ sequestration continued even after most Mn oxide was released from the solid phase, indicating auto-catalytic Ce³⁺ oxidation at the solid phase produced through primary Ce³⁺ oxidation by BMO. From X-ray diffraction analysis, the resultant solid phases formed through Ce³⁺ oxidation by BMO under both aerobic and anaerobic conditions consisted of cerianite with crystal sizes of 5.00–7.23 Å. Such nano-sized CeO₂ (CeO_2,BMO) showed faster auto-catalytic Ce³⁺ oxidation than that on well-crystalized cerianite under aerobic conditions, where the normalized pseudo-first order rate constants for auto-catalytic Ce³⁺ oxidation on CeO_2,BMO was two orders of magnitude higher. Consequently, we concluded that Ce³⁺ contact with BMOs sequesters Ce³⁺ through two oxidation paths: primary Ce³⁺ oxidation by BMOs produces nano-sized crystalline cerianite, and subsequent auto-catalytic Ce³⁺ oxidation efficiently occurs using dissolved oxygen as the oxidizing agent. Pretreatment of newly formed BMOs with La³⁺ solution resulted in decreased rate constants for primary Ce³⁺ oxidation by BMO due to site blocking by La³⁺ sorption. The results presented herein increase our understanding of the role of BMO in oxidative Ce³⁺ sequestration process(es) through enzymatic and abiotic paths in natural environments and provide supporting evidence for the potential application of BMOs towards the recovery of Ce³⁺ from contaminated waters.