Precipitation Of Mn Oxides Research Articles

Enhanced mobility of iron and manganese on Mars: Evidence from kinetic experiments and models

Several missions have reported complex alteration mineralogies on early Mars, which preserve environmental records of multiple water-rock-atmosphere interactions. The MSL and M2020 missions in Gale and Jezero have identified Fe and Mn-bearing secondary phases. These elements are used as tracers for the redox conditions on both Earth and Mars. However, to fully understand the short-lived and local-scale processes observed on Mars, it is necessary to go beyond thermodynamic models and experiments. Enhancing our ability to interpret the redox and hydrological conditions from the observed phase assemblage requires understanding the evolution of Fe and Mn during weathering. This study reports the results of kinetic alteration experiments and geochemical models conducted under Mars-like conditions. We tested variable pO2, pCO2, temperatures, and starting solutions. The results suggest that Fe is more mobile on Mars than on Earth, with a pseudo-equilibrium concentration that is kinetically controlled by dissolution and oxidation rates. Despite some initially modeled siderite precipitation, no siderite precipitation was observed in the altered powder. Solutions with higher acid concentrations were primarily controlled by dissolution kinetics, with both Fe and Mn being mobile, even when a minor amount of P, Fe, and S bearing secondary phases are formed. Based on our experimental results, we updated the model and conducted two large-scale sensitivity tests on our kinetic simulation. We confirmed that our experiments were too high in pO2 for siderite to form; however, we found that over a range of clearly oxidizing conditions from an equilibrium standpoint, Fe and Mn are mostly mobile, and siderite precipitation can occur. We were able to determine the pO2, pCO2 and the temporal space where Fe-oxide or siderite predominate or coexist, constraining the meaning of reducing or oxidizing conditions. Moreover, we also observed that siderite formation would require a much longer water residence time than Fe-oxide to precipitate, interpreted as higher weathering rates, or later evaporation required to effectively precipitate under any conditions. On ancient Mars, both Fe and Mn would be relatively mobile and prone to be leached from their host rock. Observing siderite or oxide would not primarily be a redox marker but would be a clue to a different hydrological regime. At the planetary scale, it would be challenging to form authigenic siderite during alteration. Although siderite would not indicate the presence of a particularly reducing atmosphere and that Mn-oxides are mainly pH controlled and do not require terrestrial-level amounts of oxygen, a collocated precipitation of siderite and Mn-oxides could also provide valuable information to constrain the redox environment of the ancient Mars.

Pliocene subsurface fluid flow driven by rapid erosional exhumation of the Colorado Plateau, southwestern USA

Abstract Erosion may modify the architecture of subsurface flow systems by removing confining units and changing topography to influence patterns of fluid circulation or by inducing gas exsolution from subsurface fluids, influencing compositional and buoyancy patterns in flow systems. Here, we examine the geologic record of subsurface flow in the sedimentary rocks of the Paradox Basin in the Colorado Plateau (southwestern USA), including the distribution and ages of Fe- and Mn-oxide deposits and bleached, former red-bed sandstones. We compare our results to those of previous geo- and thermochronology studies that documented as much as 2 km of erosional exhumation at ca. 3–4 Ma and Fe-and Mn-oxide precipitation at 3.6 Ma along fault zones in the region. We used (U-Th)/He and K-Ar dating to document two new records of subsurface flow of reduced fluids between 3 and 4 Ma. The first is precipitation of Mn-oxides along the Moab fault (Utah, USA) at 3.9 ± 0.2 Ma. The second is clay mineralization associated with laterally extensive bleaching in the Curtis Formation, which we dated using K-Ar illite age analysis to 3.60 ± 0.03 Ma. The coincidence of the timing of bleaching, Fe- and Mn-oxide formation in multiple locations, and erosional exhumation at 3–4 Ma raises the question of how surface erosion may have induced a phase of fluid flow in the subsurface. We suggest that recent erosion of the Colorado Plateau created steep topographic gradients that enhanced regional groundwater flow, whereby meteoric water circulation flushed reduced fluids toward discharge zones. Dissolved gases, transported from hydrocarbon reservoirs, also may have been exsolved by rapid depressurization.

Determination and prediction of micro scale rare earth element geochemical associations in mine drainage treatment wastes

Acid mine drainage (AMD) has been proposed as a novel source of rare earth elements (REE), a group of elements that includes critical metals for clean energy and modern technologies. REE are sequestered in the Fe–Al–Mn-rich precipitates produced during the treatment of AMD. These AMD solids are typically managed as waste but could be a REE source. Here, results from AMD solids characterization and geochemical modeling are presented to determine the minerals/solid phases that are enriched in REE and identify the mechanism(s) of REE attenuation.AMD solids collected from limestone-based AMD treatment systems were subjected to sequential extraction and synchrotron microprobe analyses to characterize the binding nature of the REE. The results of these analyses indicated REEs were mainly associated with Al or Mn phases. Only selected REE (Gd, Dy) were associated with Fe phases, which were less abundant than Al and Mn phases in analyzed samples. The sequential extractions demonstrated that acidic and/or reducing extractions effectively mobilize REE from the AMD solids evaluated. The observed element associations in solids are consistent with geochemical model results that indicate dissolved REE can be effectively attenuated by adsorption on freshly precipitated Fe, Al, and Mn oxides/hydroxides. The model, which simulates dissolution of CaCO3 and the precipitation of Fe, Al, and Mn oxides with increased pH, accurately predicts the pH dependent accumulation of dissolved REE with Al, Mn, and Fe oxides/hydroxides in the studied AMD treatment systems.The methods and results presented here can be used to identify conditions favorable for accumulation of REE-enriched AMD solids and possible passive or active treatment(s) to extract REE from AMD. This information can be used to design AMD treatment systems for the recovery of REE and is an opportunity to transform the challenges of addressing polluted mine drainage into an environmental and economic asset.

Episodic ventilation of euxinic bottom waters triggers the formation of black shale-hosted Mn carbonate deposits

Many manganese (Mn) carbonate deposits over geologic history are often closely associated with black shales. However, the mechanism(s) by which the Mn entered the sediments remains controversial because on one hand oxygenated waters are required for primary Mn(IV) oxide formation but on the other hand black shales generally form under reducing conditions. China hosts some of the world’s largest black shale-hosted Mn deposits, for example, the Carboniferous Ortokarnash and Malkantu Mn deposits in the Malkansu region, offering a good opportunity to revisit the genesis of black shale-hosted Mn carbonate deposits. Here, we present the first coupled, systematic paleoredox and hydrographic reconstruction of those deposits with the aim of elucidating the redox-state of the depositional basin at the time of their initial deposition. The Mn ore beds in this region are hosted within laminated, organic-rich mudstones (i.e., black shales) reflective of a relatively deep-water depositional environment. The Mn ores are comprised of the Mn(II) carbonate minerals (rhodochrosite and Ca-rhodochrosite), with minor alabandite, pyrite, and monazite. Multiple independent lines of evidence, including positive shale-normalized Ce anomalies (average 3.0), negative δ13CVPDB values (average −11.55 ‰), and negative δ98MoNIST+0.25 values (average −1.07 ‰), indicate that the Mn(II) carbonate ores were formed during diagenesis via the coupled oxidation of organic matter and reduction of Mn(IV) oxides originally deposited from an oxygenated water column. However, in apparent contradiction, highly reactive iron to total iron (FeHR/FeT; average 0.66) and pyrite iron to highly reactive iron (FePy/FeHR; average 0.71) ratios, combined with a high abundance of small framboidal pyrites (mean diameter ∼5 μm) with narrow size ranges (standard deviations <2 μm), suggest that the associated black shales were deposited in euxinic (H2S-bearing) bottom waters. Euxinia resulted from sluggish water mass circulation, with sedimentary Mo/TOC ratios indicating a relatively strong hydrologic restriction. During this stage, dissolved Mn(II) would have accumulated in euxinic waters but not become permanently fixed into sediments, as indicated by higher degrees of enrichment for Mo over U and lower Mo isotope values than coeval seawater recorded in the black shales. These reveal an active Mn(IV) oxide shuttle across a redox-stratified water column. Combined, the Mn ore intervals document sharp benthic oxygenation of euxinic bottom waters, a process that might be induced by the periodic incursions of oxic seawater associated with eustatic sea level rises. This model is supported by the Mo isotopic characteristics of the Mn carbonate ores, which indicate original Mn oxide precipitation from an overlying water with Mo isotopic composition close to coeval open seawater (minimum δ98MoNIST+0.25 of 1.9 ‰). In contrast to the generalized “bathtub ring” model envisaging that the black shale-hosted Mn deposits were formed at shallow basin margins, our results highlight that they were more likely formed by in-situ ventilation of anoxic (especially euxinic) deep waters of basin center settings. In this regard, we suggest that the ventilation model better explains the close association of Mn carbonate ores with black shales, and has profound implications for understanding the paleoredox framework of otherwise Mn-rich black shale successions.

Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

The Mesoproterozoic era has long been considered a time of relative environmental and biological stasis. However, emerging insight suggests that this period may have been more dynamic than previously considered, both in terms of oxygenation and potential consequences for biological evolution. Nevertheless, our understanding of this immense period of time remains limited. To provide more detailed constraints on oxygenation dynamics, we report a multiproxy geochemical study of an early Mesoproterozoic (∼1600–1540 million years ago, Ma) carbonate-dominated succession from the North China craton. We include inorganic carbon isotope (δ13Ccarb), iron-speciation, and major and trace element data, in addition to molybdenum isotopic compositions (δ98/95Mo). These geochemical data support previous inferences of persistent anoxic and ferruginous deeper water conditions in the earliest Mesoproterozoic ocean, with limited oxygenation of surface waters. However, the behaviour of these redox-sensitive geochemical proxies reveals pulsed oxygenation events, with each event increasing the maximum depth of oxygenation, leading to overall progressive oxygenation of the ocean. During these pulsed oxygenation events we find the lightest Mo isotope signatures ever measured in the rock record, which we attribute to initial drawdown of isotopically light Mo in association with extensive Mn and Fe (oxyhydr)oxide precipitation, followed by diagenetic recycling. However, shallower water sediments deposited after the pulses of deeper water oxygenation more faithfully record the Mo isotopic composition of coeval seawater. For these samples, we utilise a single reservoir Mo cycling model, constrained by an updated estimate of Mesoproterozoic seawater Mo concentration, and scaled using a function associated with differential organic carbon flux between the shelf and basin. When scaled to modern rates of Mo accumulation under variable marine redox conditions, our modelling estimates suggest a minimum oxic seafloor area of ∼30% of the total seafloor area at ∼1540 Ma. It remains unclear whether the oxygenation observed across this ∼60 million year interval represents a progressive transition to a more persistently oxygenated ocean, or whether oceanic oxygen levels fluctuated considerably through the later Mesoproterozoic.

Evolution of Retisol impacted by artificial drainage: What can we learn from stable Fe isotope ratios?

Iron oxides are one of the most reactive mineral phases in soils. As a consequence, their transformations can considerably affect the dynamics of the adsorbed elements and of the associated soil ecosystem services. Understanding the dynamics of Fe oxides in soils is therefore a key issue for the evolution of soil and associated ecosystem services. A potentially powerful tool to study the transformations of Fe-oxides in soil is stable Fe isotopes. However, there are still important gaps in our knowledge of Fe isotope fractionations.In order to examine the Fe isotope fractionations related to each process occurring in soils, we focused on a drainage-influenced sequence of Retisols, a soil type characterized by clay translocation and subsequent degradation by redox processes inducing a strong spatial Fe segregation in contrasted soil volumes. We combined the isotopic approach at the scale of a bulk horizon and at the scale of the different soil volumes, with mineralogical analyses and mass balance calculations in order to investigate the consequences of the drainage on Fe isotope fractionation. We showed that while there were no Fe isotope fractionations at the profile scale, Fe isotopic signatures varied significantly among soil volumes (δ56Fe values from −0.49 ± 0.05 to 0.29 ± 0.06‰). These variations suggest that redox processes are the main mechanisms responsible for the Fe redistribution among the volumes, and particularly that Fe accumulation during Mn oxide precipitation makes a significant contribution to Fe isotopic fractionation, during these Retisol differentiation. In contrast, drainage-induced eluviation does not result in further Fe isotope fractionations in soil volumes in these Retisols. The isotopic signatures of the different mineral phases present in the volumes were calculated using the mass balance approach and suggest that the iron oxides (goethite, ferrihydrite) have δ56Fe values close to 0‰, while the clay minerals are enriched in heavy Fe isotopes and the Mn oxides in light Fe isotopes. This study provides insight into the dynamics of Fe minerals in hydromorphic soils and offers a new perspective on stable Fe isotope fractionation in soils.

Multistage genesis of the late Cretaceous manganese karst-hosted Tasdremt deposit (High Atlas, Morocco)

The eastern part of the Souss Basin (Morocco) contains several Mn deposits in the Tasdremt district. Three Mn orebodies occur within the Cenomanian-Turonian dolostones and the Senonian (Coniacian to Maastrichtian) detrital series, the main orebody being located at the boundary between them. The Mn ores consist of coronadite group minerals, mostly coronadite and hollandite, in a karstified dolostone. New field observations, petrographic analyses, and geochemical data define the Tasdremt deposits as a karst-hosted accumulation (11–60 wt.% Mn), particularly enriched in Ba (1.5–8.2 wt.%) and Pb (1.0–5.0 wt.%) with poor contaminations in Al, Fe, and P. This study shows that the ore-forming process is similar to that occurring in the Imini C3 level, located ~ 100 km to the north-east. Such similarities with the high-grade pyrolusite-bearing ore suggest that the Tasdremt deposit is a lateral equivalent of the Imini deposits. However, the scarcity of pyrolusite in Tasdremt results in lower Mn grades, the Tasdremt ores being considered an aborted/incomplete system in comparison with the Imini deposits. 40Ar/39Ar geochronology of K-bearing Mn oxides yields late Cretaceous ages, defining three phases at ~ 91.5 Ma, ~ 77.5–82 Ma, and ~ 65–67 Ma. Although the source of metals remains hypothetical, mineralizing fluids were carried by O2-free groundwater that mixed with O2-rich shallow meteoric waters at the Tasdremt depositional site. The dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of Mn oxides, causing the coeval increase of pH and Eh, respectively. The Early Atlasic deformation during the Late Cretaceous is associated with mineralization events and was responsible for creation of low stand reliefs from Tasdremt to Imini. This period enabled karstification and mineralization. Connecting the Tasdremt deposits to other African Mn deposits is difficult since the latter consist of laterite resting above Paleoproterozoic Mn protores, and consequently formed under different conditions from karst-hosted deposits. It is likely that other Mn occurrences formed along the Atlas belt in similar settings.

Highly enhanced oxidation of arsenite at the surface of birnessite in the presence of pyrophosphate and the underlying reaction mechanisms

Manganese(IV) oxides, and more especially birnessite, rank among the most efficient metal oxides for As(III) oxidation and subsequent sorption, and thus for arsenic immobilization. Efficiency is limited however by the precipitation of low valence Mn (hydr)oxides at the birnessite surface that leads to its passivation. The present work investigates experimentally the influence of chelating agents on this oxidative process. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite was investigated using batch experiments and different arsenic concentrations at circum-neutral pH. In the absence of PP, Mn(II/III) species are continuously generated during As(III) oxidation and adsorbed to the mineral surface. Field emission-scanning electron microscopy, synchrotron-based X-ray diffraction and Fourier transform infrared spectroscopy indicate that manganite is formed, passivating birnessite surface and thus hampering the oxidative process. In the presence of PP, generated Mn(II/III) species form soluble complexes, thus inhibiting surface passivation and promoting As(III) conversion to As(V) with PP. Enhancement of As(III) oxidation by Mn oxides strongly depends on the affinity of the chelating agent for Mn(III) and from the induced stability of Mn(III) complexes. Compared to PP, the positive influence of oxalate, for example, on the oxidative process is more limited. The present study thus provides new insights into the possible optimization of arsenic removal from water using Mn oxides, and on the possible environmental control of arsenic contamination by these ubiquitous nontoxic mineral species.

Microbiomes in a manganese oxide producing ecosystem in the Ytterby mine, Sweden: impact on metal mobility.

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

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.

Remediation of Manganese-Contaminated Coal-Mine Water Using Bio-Sorption and Bio-Oxidation by the Microalga Pediastrum duplex (AARLG060): A Laboratory-Scale Feasibility Study.

Acidification occurs as a result of acid mine drainage after the oxidative weathering of metal sulfides. The acidic condition corrodes other toxic elements from the soil and becomes distributed around the operating site. Although coal mines go through a process of rehabilitation, water samples in the rehabilitated reservoir still reveal high concentrations of certain metals, for example, manganese (Mn). Both living and non-living biomass substances were used in Mn remediation. However, using non-living biomass as a sorbent may be inappropriate for the purposes of upscaling in high-volume water bodies. Thus, living microalga, Pediastrum duplex AARLG060, has become of significant interest for this type of application. The Mn remediation of microalga was performed by biosorption and bio-oxidation. The aim of this study was to evaluate the potential of microalgal Mn remediation of the water obtained from a rehabilitated coal-mine reservoir. The equilibrium and isotherm values of the remediation process were also studied. The microalga was used to remediate Mn in water under three different water conditions, including filtrated water obtained from the rehabilitated site, non-filtrated water that was sterilized with an autoclave, and non-treated water. Remediation was performed by culturing microalga with modified medium consisting of N, P, C, and Mg nutrients. The remediated Mn concentration present in the cultures was detected by atomic absorption spectroscopy. The precipitated Mn was collected as a result of bio-oxidation, and EDTA was used to wash Mn from the biomass. This was designated as an adsorption result. Characterization of biosorption was evaluated by employing the Langmuir and Freundlich models. The results demonstrated that all treatments of living microalga could support Mn bio-oxidation. The Mn remediation was successfully performed at over 97% in every treatment. The adsorption characteristics revealed a close similarity to the Langmuir isotherm of monolayer adsorption. The scanning electron microscope–energy dispersive spectroscopy (SEM–EDS) indicated precipitation of Mn oxide on the cell surface, while transmission electron microscopy (TEM) revealed that the nanoparticles of Mn were scattered mainly in the chloroplast and throughout the vacuoles of the cells.

Evaluation and application of foraminiferal element/calcium ratios: Assessing riverine fluxes and environmental conditions during sapropel S1 in the Southeastern Mediterranean

Abstract Paleostudies often rely on foraminiferal calcite chemistry, which reflect past sea water condition through so-called proxy relationships. One way to evaluate robustness of these proxy relationships is to test them in well-studied and during well-constrained climate transitions. The southeastern (SE) Mediterranean is a perfect natural laboratory with a large range of past environmental conditions. These range from low productivity well-ventilated waters like they are at present, to poorly ventilated, high productivity conditions during sapropels. We here explore the reliability of recently developed foraminiferal-based proxies (Ba/Ca, Mn/Ca, Na/Ca) as tracers for changes in productivity, oxygenation and salinity during the most recent sapropel S1. We use laser ablation ICP-MS analyses of the planktonic G. ruber and six benthic species (B. alata, G. affinis, G. altiformis, G. orbicularis, H. boueana, U. peregrina). Our results show that planktonic Ba/Ca is a reliable tracer for Ba2+-enriched Nile outflow, where benthic Ba/Ca traces enhanced paleo(export) productivity relatively well. The interpretation of Mn/Ca data is less straightforward, and the low values may suggest a lower precipitation of Mn-oxides under prevailing hypoxia. The decrease in planktonic and benthic Na/Ca is coherent with excess Nile runoff lowering salinities in the

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.

Limited oxygen production in the Mesoarchean ocean

The Archean Eon was a time of predominantly anoxic Earth surface conditions, where anaerobic processes controlled bioessential element cycles. In contrast to "oxygen oases" well documented for the Neoarchean [2.8 to 2.5 billion years ago (Ga)], the magnitude, spatial extent, and underlying causes of possible Mesoarchean (3.2 to 2.8 Ga) surface-ocean oxygenation remain controversial. Here, we report δ15N and δ13C values coupled with local seawater redox data for Mesoarchean shales of the Mozaan Group (Pongola Supergroup, South Africa) that were deposited during an episode of enhanced Mn (oxyhydr)oxide precipitation between ∼2.95 and 2.85 Ga. Iron and Mn redox systematics are consistent with an oxygen oasis in the Mesoarchean anoxic ocean, but δ15N data indicate a Mo-based diazotrophic biosphere with no compelling evidence for a significant aerobic nitrogen cycle. We propose that in contrast to the Neoarchean, dissolved O2 levels were either too low or too limited in extent to develop a large and stable nitrate reservoir in the Mesoarchean ocean. Since biological N2 fixation was evidently active in this environment, the growth and proliferation of O2-producing organisms were likely suppressed by nutrients other than nitrogen (e.g., phosphorus), which would have limited the expansion of oxygenated conditions during the Mesoarchean.

Powdered activated carbon enhanced Manganese(II) removal by chlorine oxidation

Chlorine is not effective in the oxidative removal of soluble manganese(II) ions at neutral pH. Powdered activated carbon (PAC) also has a very limited capacity for Mn(II) removal through adsorption in drinking water treatment practice. This study explored the combined use of PAC and chlorine for Mn(II) removal and found that PAC dramatically catalyzed Mn(II) oxidation by chlorine under diverse conditions. At a dose as low as 5.0 mg/L, two different commercial PACs increased Mn(II) oxidation rate by two orders of magnitude respectively and reduced Mn(II) concentration from 200 μg/L to < 10 μg/L in tens of minutes. First-order kinetics with respect to aqueous Mn(II) concentration were observed. Typically, homogeneous Mn(II) oxidation by chlorine depends strongly on alkaline pH. In the presence of PAC, however, the reaction was still rather fast at pH 6.0. Increasing PAC doses linearly increased Mn(II) oxidation rate, indicating that the reaction was highly PAC surface active sites dependent. The efficacy of PAC was further corroborated in removing Mn(II) from natural water. SEM-EDS and XPS demonstrated that a MnO2 coating was formed on PAC surface after reaction, which resulted from heterogeneous oxidation of Mn(II) on PAC surface rather than the precipitation of Mn oxides formed through homogeneous oxidation in solution. Adsorption of free Mn(II) ions onto PAC surface was proved to directly correlate with Mn(II) oxidation rate. Two kinds of electron transfer pathways from adsorbed Mn(II) species to chlorine, enhanced by surface-complexation and electrically-conductive carbon surface respectively, were hypothesized.

Electron microscopy study on the formation of ferromanganese crusts, western Pacific Magellan Seamounts

Variations in mineralogy and chemical composition, layer structures, redox states of Fe and Mn, and microbial diversity are closely linked to the biogeochemical process when a ferromanganese crust layer forms. A sample collected from the Magellan Seamount (OSM11), in the western Pacific, was characterized in five well-defined layers, top to bottom (L1–5). Fe-vernadite occurs in all layers, compared to detritus quartz, feldspar, goethite, and hematite in L1 and L3, and carbonate fluorapatite (CFA) in L4–5. The relatively high concentrations of Ca and P in L4–5, and Fe, Co, and Si in L1 and L3 correspond to the mineralogical variations in the crust layer. Disappearance of elongated voids along the convex growth line is likely due to void filling precipitation of CFA in L4–5, resulting in the void reduction (31.6 to 6.0%). The oxidation states of Fe in Fe-vernadite measured by electron energy loss spectroscopy (EELS) ranges from 36 to 63% of Fe3+/Fetot, and a layer where CFA appeared (L4) contains a more reductive form of Fe (Fe3+/Fetot = 36–48%). Presence of Fe- (coxC) and Mn-oxidizing gene (cumA), particularly displaying a strong PCR band of coxC in L2–3, indicates a dominant oxidizing condition. Direct evidence of microbial activity in Fe - and Mn-oxide precipitation with various redox conditions was identified in the focused ion beam-sectioned microfossil. Particularly, a spectrum image displaying a more reducing form of Fe around the voids previously occupied by microorganisms, strongly supports that microorganisms play an important role in the redox reaction in the growth of a crust.

Mn accumulation in a submerged plant Egeria densa (Hydrocharitaceae) is mediated by epiphytic bacteria.

Many aquatic plants act as biosorbents, removing and recovering metals from the environment. To assess the biosorbent activity of Egeria densa, a submerged freshwater macrophyte, plants were collected monthly from a circular drainage area in Lake Biwa basin and the Mn concentrations of the plants were analysed. Mn concentrations in these plants were generally above those of terrestrial hyperaccumulators, and were markedly higher in spring and summer than in autumn. Mn concentrations were much lower in plants incubated in hydroponic medium at various pH levels with and without Mn supplementation than in field-collected plants. The precipitation of Mn oxides on the leaves was determined by variable pressure scanning electron microscopy-energy dispersive X-ray analysis and Leucoberbelin blue staining. Several strains of epiphytic bacteria were isolated from the field-collected E. densa plants, with many of these strains, including those of the genera Acidovorax, Comamonas, Pseudomonas and Rhizobium, found to have Mn-oxidizing activity. High Mn concentrations in E. densa were mediated by the production of biogenic Mn oxide in biofilms on leaf surfaces. These findings provide new insights into plant epidermal bacterial flora that affect metal accumulation in plants and suggest that these aquatic plants may have use in Mn phytomining.

Critical control of flooding and draining sequences on the environmental risk of Zn-contaminated riverbank sediments

PurposeDiffuse pollution emanating from metal mining-impacted sediment could serve as a barrier to achieving European Union Water Framework Directive and US Clean Water Act requirements. UK climate projections (UKCP09) predict increases in rainfall and aridity that will influence river stage alternately exposing and submersing contaminated riverbank sediment. Research focuses on the environmental contaminant dissolved Zn and investigates patterns of release, key geochemical mechanisms controlling Zn mobilisation and the environmental risk of sediment subjected to these perturbations.Materials and methodsUsing two laboratory mesocosm experiments, metal mining-contaminated sediment was subjected to alternate wet and dry sequences of different duration and frequency. The first experiment was run to determine the influence of submersion and exposure of contaminated sediment on releases of Zn and to establish environmental risk. The second experiment utilised diffusional equilibration in thin film (DET) to observe the patterns of Zn release, with depth, in the sediment. Pore water chemical analysis at the sediment-water interface enabled elucidation of key geochemical mechanisms of control of Zn mobilisation.Results and discussionPatterns of Zn release were found to be different, depending on the length of wet and dry period. High concentrations of dissolved Zn were released at the start of a flood for runs with longer dry periods. A buildup of soluble Zn sulphate minerals over long dry periods followed by dissolution on first flood wetting was a key geochemical mechanism controlling Zn release. For longer wet runs, increases in dissolved Mn and Zn were observed over the flood period. Key geochemical mechanisms controlling Zn mobilisation for these runs were: (i) reductive dissolution of Mn (hydr)oxides and release of partitioned Zn over prolonged flood periods followed by (ii) oxidation and precipitation of Mn (hydr)oxides and sorption of Zn on exposure to atmospheric conditions.ConclusionsMesocosm experiments were a first step in understanding the effects of UK climate projections on the riverbank environment. Contaminated sediment was found to pose a significant environmental risk in response to hydrological perturbations. The ‘transient’ nature of dissolved Zn release could make identifying the exact sources of pollution a challenge; therefore, further field studies are advised to monitor contaminant releases under a range of hydrological conditions and account for complex hydrology at mining sites.

Environmental controls on biomineralization and Fe-mound formation in a low-temperature hydrothermal system at the Jan Mayen Vent Fields

Diffuse low-temperature hydrothermal vents on the seafloor host neutrophilic microaerophilic Fe-oxidizing bacteria that utilize the Fe(II) supplied by hydrothermal fluids and produce intricate twisted and branching extracellular stalks. The growth behavior of Fe-oxidizing bacteria in strongly opposing gradients of Fe(II) and O2 have been thoroughly investigated in laboratory settings to assess whether extracellular stalks and aligned biomineralized fabrics may serve as biosignatures of Fe-oxidizing bacteria and indications of palaeo-redox conditions in the rock record. However, the processes controlling the growth of biogenic Fe-oxyhydroxide deposits in natural, modern hydrothermal systems are still not well constrained. In this study, we aimed to establish how variations in the texture of stratified hydrothermal Fe-oxyhydroxide deposits are linked to the physicochemical conditions of the hydrothermal environment. We conducted 16S rRNA gene analyses, microscopy and geochemical analyses of laminated siliceous Fe-mounds from the Jan Mayen Vent Fields at the Arctic Mid-Ocean Ridge. Chemical analyses of low- and high-temperature hydrothermal fluids were performed to characterize the hydrothermal system in which the Fe-deposits form. Our results reveal synchronous inter-laminar variations in texture and major and trace element geochemistry. The Fe-deposits are composed of alternating porous laminae of mineralized twisted stalks and branching tubes, Mn-rich horizons with abundant detrital sediment, domal internal cavities and thin P- and REE-enriched lamina characterized by networks of ≪1μm wide fibers. Zetaproteobacteria constitute one third of the microbial community in the surface layer of actively forming mounds, indicating that microbial Fe-oxidation is contributing to mound accretion. We suggest that Mn-oxide precipitation and detrital sediment accumulation take place during periodically low hydrothermal fluid discharge conditions. The elevated concentrations of P and REE in distinct laminae suggest Fe-cycling and accumulation of diagenetic species at depth in the deposits during hydrothermal quiescence and co-precipitation of these species with Fe-oxyhydroxides at the mound surface with reinitiated hydrothermal discharge. The origin of the low-temperature hydrothermal source fluid and the Fe-deposits are evident by low LREE/HREE ratios and negative Eu-anomalies, which clearly differ from the LREE and Eu enrichment of nearby high-temperature white smoker venting fluids. Our study demonstrates that hydrothermal fluctuations exert the primary control on the formation of laminae and the activity of Fe-oxidizing bacteria in marine hydrothermal Fe-deposits and indicates that REE-patterns may be used to distinguish high-temperature plume fallout and biomineralized low-temperature Fe-deposits in the rock record.

Structural Match of Heterogeneously Nucleated Mn(OH)2(s) Nanoparticles on Quartz under Various pH Conditions.

The early nucleation stage of Mn (hydr)oxide on mineral surfaces is crucial to understand its occurrence and the cycling of nutrients in environmental systems. However, there are only limited studies on the heterogeneous nucleation of Mn(OH)2(s) as the initial stage of Mn (hydr)oxide precipitation. Here, we investigated the effect of pH on the initial nucleation of Mn(OH)2(s) on quartz. Under various pH conditions of 9.8, 9.9, and 10.1, we analyzed the structural matches between quartz and heterogeneously nucleated Mn(OH)2(s). The structural matches were calculated by measuring the lateral and vertical dimensions using grazing incidence small angle X-ray scattering and atomic force microscopy (AFM), respectively. We found that a poorer structural match occurred at a higher pH than at a lower pH. The faster nucleation under a higher pH condition accounted for the poorer structural match observed. By fitting the structural match using classical nucleation theory, we also calculated the interfacial energy between Mn(OH)2(s) and water: γnf = 71 ± 7 mJ/m2. The calculated m values and γnf provided the variance of interfacial energy between quartz and Mn(OH)2(s): γsn = 262-272 mJ/m2. This study provides new qualitative and quantitative information on heterogeneous nucleation on an environmentally abundant mineral surface, quartz, and it offers important underpinnings for understanding the fate and transport of trace ions in environmental systems.