Mn Oxyhydroxides Research Articles

Effects of the combined use of lanthanum carbonate and activated carbon capping materials on phosphorus and dissolved organic matter in lake sediments

Lanthanum carbonate (LC) represents a novel material for the immobilization of internal phosphorus (P) in sediments. Activated carbon (AC) is a traditional adsorbent that has been employed in the remediation of sediments on a wide scale. The objective of this study is to examine the mechanisms and effects of the combined use of LC and AC capping materials on the immobilization of P and dissolved organic matter (DOM) in sediments, through a 90-day incubation experiment. The results of isotherm experiments showed that the adsorption mechanism of P on LC and AC was mainly chemisorption. The XPS analyses showed the adsorption mechanism of P on LC was mainly ligand exchange and inner-sphere complexation; while the adsorption mechanism of P on AC was mainly ligand exchange and electrostatic adsorption. The results demonstrated that the concentrations of soluble reactive phosphorus (SRP) and DOM in the 0 to −100 mm sediment layer were reduced by 69.79% and 33.93%, respectively, in comparison to the control group with the LC + AC group. Moreover, the HCl-P and Res-P (stable P) in the 0–5 cm sediment layer were increased by 50.07% and 21.04%, respectively, in the LC + AC group. This indicates that the combined application of LC and AC has the potential to reduce the risk of P release. Furthermore, the formation of Fe(III)/Mn(IV) oxyhydroxides by LC + AC treatment resulted in an increased adsorption of SRP and DOM. Moreover, the effect of LC + AC capping on microbial community was smaller than that of LC/AC capping alone. The findings of this study indicated that the combined use of LC and AC represents a novel approach to the effective treatment of internal P and DOM in eutrophic lake sediments.

The Geology, Geochemistry, and Mineralogy of the Moa Bay Ni Laterite Mining District, Cuba

Abstract The Moa Bay lateritic Ni-Co mining district (eastern Cuba) has total mineral resources of 198.54 million metric tonnes (Mt) at 1.07% Ni and 0.12% Co. Laterite profiles from this district are characterized by their oxide-dominated ore zones. Laterite profiles from the Yagrumaje Norte, Punta Gorda, and Yamanigüey deposits contain average Ni and Co concentrations in the oxide zone of 0.88 and 0.12%. Goethite is the most abundant mineral in the oxide zone and the most important Ni-Co-Sc–bearing mineral, with median NiO, CoO, and Sc contents of 0.78 wt %, 0.07 wt %, and 58 ppm, respectively, and up to 2.77 wt %, 0.26 wt %, and 117 ppm. Maghemite is also widely present (avg of 5% and up to 19% modal proportion) and represents an important but largely ignored Ni- and Co-bearing ore phase, with median NiO and CoO concentrations of 2.11 and 0.25 wt %, respectively, and maximum values of 13.9 and 1.84 wt % each. Nickel and Co substitute for ferric iron in the structure of maghemite. Manganese oxyhydroxides (lithiophorite and lithiophorite-asbolane intermediate), which are also significant Ni-Co–bearing phases, have median NiO and CoO contents of 10.6 and 6.41 wt %, respectively. Some Mn oxyhydroxides, which formed after replacing goethite, also contain significant amounts of Sc (up to 94 ppm). Although most deposits in the Moa Bay lateritic district are classified as oxide type, Yamanigüey (avg Ni grade of 1.98%) is characterized by well-developed saprolite horizons, with secondary serpentine (serpentine II) and garnierite being the main Ni-bearing phases.

Manganese oxidation states and availability in forest weathering profiles of contrasting climate

The abundance and oxidation states (II, III and IV) of manganese (Mn) in a weathering profile encompassing both the soil layers (A and B horizons) and the underlaid saprolite (C horizons) determine the availability of Mn as a plant nutrient and regulate its role in cycles of other elements in Earth’s critical zone. However, it remains unclear how the abundance and oxidation states vary with depth under different climates, and how the soil forming processes and soil properties control the variations. We examined four forest granite weathering profiles developed under climates ranging from temperate to tropical climate. Regardless of climate types, all four profiles showed similar vertical variation patterns of Mn concentration and oxidation states. The major features of the patterns can be understood from the perspective of soil forming processes and soil properties. Climate affected the Mn oxidation states in the fine fraction (< 2 mm; i.e., the soil fraction) of the poorly weathered saprolite by controlling the weathering degree of Mn-bearing primary minerals. The weathering released Mn(II) and Mn(III) in the primary minerals to the circumneutral environment where it was subsequently oxidized by O2. In contrast, climate affected the Mn oxidation states in the soil layers poor in parent materials largely by controlling soil redox conditions and pH because most of the Mn in soils was reactive. As the climate became warmer/wetter, the weathering intensified and soils became more reducing and acidic, resulting in more reduced Mn in the soil layers but more oxidized Mn in the fine fraction of saprolite. Moreover, relative to Mn(II) and Mn(IV), Mn(III) preferentially accumulated in the subsoil (B horizons), likely as Mn(III) oxyhydroxides in the colder and drier climates, and as a substitute ion in well-crystallized Fe(III) oxides in the warmer and wetter climates. These findings improve our understanding of Mn availability and cycling and its role in biogeochemical cycles of other elements in Earth’s critical zone.

Observações de campo nos materiais lateríticos de Mosqueiro (Belém, Pará) como potenciais pigmentos minerais naturais

Back in the 1980s, when I began exploring the cliffs of Baía do Sol (Mosqueiro, Belém, Pará), carved into immature laterite profiles derived from clayey and sandy sedimentary rocks of the Barreiras Formation, I was very impressed by the colorful tones of the mottled horizon in these profiles. With my children (they were still “children”) we played painting each other and it was really fun. When reading and studying cave paintings in the Amazon and around the world, it was evident, obviously, that these paintings were made with different materials, mainly Fe and Mn oxyhydroxides, coal, ash and organic extracts.

Toward mending the marine mass balance model for nickel: Experimentally determined isotope fractionation during Ni sorption to birnessite

In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average Δ60/58Nidissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Nidissolved-sorbed ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance.

Unravelling arsenic bioavailability in floodplain soils impacted by mining activities

Gold mining not only introduces mercury (Hg) contamination to soils but also facilitates the mobilization of other toxic substances, including arsenic (As). This study assessed the total content, chemical species, and bioavailable fraction of As in surface soils impacted by mining residues during frequent flooding. Analysis of 207 soil samples across the floodplain region of La Mojana, Colombia, screened to 2 mm with polyethylene mesh, revealed significant correlations (p < 0.05) between inorganic As, the residual phase, sulphur (S), iron (Fe), manganese (Mn), and aluminum (Al), indicating associations with sulfides and oxyhydroxides of Fe and Mn. The origin of toxicity was linked to suspended materials transported by rivers during flooding in areas with intense mining activity. Sites with better oxidizing conditions exhibited a higher presence of phases associated with amorphous and crystalline oxides in non-flooded areas. Although the bioavailable fraction was minimal in flooded sites, reducing conditions facilitated As mobility, resulting in higher concentrations in deeper soil layers, particularly as As(III). The contamination factor (CF) ranged from 1.3 to 11.1, and the geochemical index (Igeo) ranged from −0.2 to 2.9, indicating a moderate to high As contamination level in soils. This poses potential health risks, considering the agricultural use of these soils.

Understanding arsenic and manganese enrichment in the aquifers of the Ghaghara river basin, Middle Gangetic Plain (MGP), India: A multivariate statistical, compositional data analysis (CoDA), and receptor model approach

The focus of this study was to comprehend the spatial distribution, source apportionment, evaluation of natural background level (NBL), and mobilization mechanisms of arsenic (As) and manganese (Mn) in the Ghaghara river basin, located in the middle Gangetic plain (MGP) of India. A combination of analytical tools was employed, including multivariate statistical analysis (MSA), correlation analysis, and receptor models such as positive matrix factorization (PMF) and principal component analysis-multi linear regression (PCA-MLR). Taking into consideration the compositional constraints of the geochemical data, compositional data analysis (CoDA) methods were applied on the raw data prior to the correlation analysis and MSA. The results of MSA, correlation analysis and receptor models showed that major ion chemistry and As, Mn enrichment in groundwater were largely controlled by carbonate weathering, Fe and Mn oxyhydroxides dissolution. Anthropogenic activities, such as the infiltration of dissolved organic matter-rich water from local surface water bodies used as dumping sites and the infiltration of fertilizer-rich water from agricultural lands, were found to notably impact the groundwater geochemistry and release of As in the research area. The lower NBL of As (5.92 μg/L) compared to its mean concentrations (17.9 μg/L) in the study area also indicated that the rate of As release in groundwater through natural processes was comparatively low but various anthropogenic activities operating in the study region possibly acted as a trigger for the mobility of As from the mineral phases of the subsurface sediments. This study also highlighted the significance of applying CoDA techniques on geochemical data prior to statistical analysis, and the importance of receptor models, to better understand the nature and contribution of various natural and anthropogenic processes governing the hydrochemistry of groundwater in the alluvial aquifers.

Evaluation of coal mine drainage and associated precipitates for radium and rare earth element concentrations

Coal mine drainage (CMD) and associated metal-rich precipitates have recently been proposed as unconventional sources of rare earth elements (REEs). However, the potential occurrence of radium (Ra), a known carcinogen, with the REE-bearing phases has not been investigated. We hypothesized that Ra may occur in solids that are precipitated from CMD as a “radiobarite” solid solution ((Ba,Sr,Ra)SO4) and/or adsorbed with hydrous metal oxides. REEs have been documented to sorb or co-precipitate with iron (Fe), manganese (Mn), and aluminum (Al) oxyhydroxide in CMD solids. Likewise, Ra has been documented to sorb to hydrous Fe and Mn oxides especially where sulfate (SO4) and/or barium (Ba) concentrations are insufficient to precipitate radiobarite. Thus, we conducted the first-ever survey of Ra concentrations in corresponding CMD water and solid samples in the United States. Samples were analyzed from 4 untreated and 9 treated CMD sites in both the bituminous and anthracite coal regions of Pennsylvania across a range of pH and SO4 concentrations. The dissolved Ra in CMD was relatively low (<0.5 Bq/L), consistent with radiobarite solubility; however, CMD solids were largely composed of amorphous Fe, Al, and Mn oxyhydroxide and silicate minerals. Ra was associated with Mn-enriched CMD solids, upwards of 875 Bq/kg. Total REE + yttrium (Y) content in the CMD solids was enriched upwards of 3600 mg/kg and was significantly correlated with Al content. These preliminary results suggest that REE extraction may target Al-rich solids to avoid Ra in Mn-rich solids.

A method for studying the influence of Mn oxyhydroxides on the trace element content of aquatic bryophytes

The main objective of this study was to develop and test a method of separating externally deposited Mn oxyhydroxides and co-precipitated elements from samples of aquatic moss (the moss Fontinalis antipyretica). The method, which uses 0.1 M hydroxylamine to dissolve the oxyhydroxides, was tested with samples collected in rivers with slightly acidic, well‑oxygenated waters, where high rates of Mn precipitation occur. The method was effective (it extracted up to 84 % of the Mn) and selective (Fe oxyhydroxides were not extracted). The elements Ba, Cd, Zn and Ni were associated with the Mn oxyhydroxides, while Al, As, Cr, Cu, Fe, Hg and Pb were not. Deposition of Mn therefore increased the concentration of some elements in the moss samples. However, as Mn precipitation depends on Eh and pH, which are independent of the concentrations of the elements in water, the relationship between water and moss element concentrations is not clear (i.e. the data are noisy). This is a problem in biomonitoring studies, which assume a close relationship between element concentrations in moss and water. The value of the proposed extraction method is that it can be used to correct the effect of Mn deposition. We present an example of this correction applied to the Cd concentrations in the test data. We found that the noise introduced by the Mn, including age-related effects (observed by comparing concentrations in 0–2.5 and 2.2–5.0 cm sections from the shoot apex), can be reduced. Additionally, the correction revealed recent increases in Cd concentrations in one site that were not observed in the uncorrected data. Another finding of interest was the low content of total Mn and different extractability (of most elements) observed in moss samples collected in alkaline waters. Finally, we discuss how future studies designed for different environmental scenarios can benefit from application of the proposed method.

Influence of Organic Matter Thermal Maturity on Rare Earth Element Distribution: A Study of Middle Devonian Black Shales from the Appalachian Basin, USA

This study focuses on understanding the association of rare earth elements (REE; lanthanides + yttrium + scandium) with organic matter from the Middle Devonian black shales of the Appalachian Basin. Developing a better understanding of the role of organic matter (OM) and thermal maturity in REE partitioning may help improve current geochemical models of REE enrichment in a wide range of black shales. We studied relationships between whole rock REE content and total organic carbon (TOC) and compared the correlations with a suite of global oil shales that contain TOC as high as 60 wt.%. The sequential leaching of the Appalachian shale samples was conducted to evaluate the REE content associated with carbonates, Fe–Mn oxyhydroxides, sulfides, and organics. Finally, the residue from the leaching experiment was analyzed to assess the mineralogical changes and REE extraction efficiency. Our results show that heavier REE (HREE) have a positive correlation with TOC in our Appalachian core samples. However, data from the global oil shales display an opposite trend. We propose that although TOC controls REE enrichment, thermal maturation likely plays a critical role in HREE partitioning into refractory organic phases, such as pyrobitumen. The REE inventory from a core in the Appalachian Basin shows that (1) the total REE ranges between 180 and 270 ppm and the OM-rich samples tend to contain more REE than the calcareous shales; (2) there is a relatively higher abundance of middle REE (MREE) to HREE than lighter REE (LREE); (3) there is a disproportionate increase in Y and Tb with TOC likely due to the rocks being over-mature; and (4) the REE extraction demonstrates that although the OM has higher HREE concentration, the organic leachates contain more LREE, suggesting it is more challenging to extract HREE from OM than using traditional leaching techniques.

Manganese reductive dissolution coupled to Sb mobilization in contaminated shooting range soil

A “redox-stat” RMnR bioreactor was employed to simulate moderately reducing conditions (+ 420 mV) in Sb-contaminated shooting range soils for approximately 3 months, thermodynamically favoring Mn(IV) reduction. The impact of moderately reducing conditions on elemental mobilization (Mn, Sb, Fe) and speciation [Sb(III) versus Sb(V); Fe2+/Fe3+] was compared to a control bioreactor RCTRL without a fixed redox potential. In both bioreactors, reducing conditions were accompanied by an increase in effluent Sb(V) and Mn(II) concentrations, suggesting that Sb(V) was released through microbial reduction of Mn oxyhydroxide minerals. This was underlined by multiple linear regression analysis showing a significant (p < 0.05) relationship between Mn and Sb effluent concentrations. Mn concentration was the sole variable exhibiting a statistically significant effect on Sb in RMnR, while under the more reducing conditions in RCTRL, pH and redox potential were also significant. Analysis of the bacterial community composition revealed an increase in the genera Azoarcus, Flavisolibacter, Luteimonas, and Mesorhizobium concerning the initial soil, some of which are possible key players in the process of Sb mobilization. The overall amount of Sb released in the RMnR (10.40%) was virtually the same as in the RCTRL (10.37%), which underlines a subordinate role of anoxic processes, such as Fe-reductive dissolution, in Sb mobilization. This research underscores the central role of relatively low concentrations of Mn oxyhydroxides in influencing the fate of trace elements. Our study also demonstrates that bioreactors operated as redox-stats represent versatile tools that allow quantifying the contribution of specific mechanisms determining the fate of trace elements in contaminated soils.Key points• “Redox-stat” reactors elucidate Sb mobilization mechanisms• Mn oxyhydroxides microbial reductive dissolution has a major role in Sb mobilization in soils under moderately reducing conditions• Despite aging the soil exhibited significant Sb mobilization potential, emphasizing persistent environmental effects

Early Diagenetic Processes in the Sediments of the Krka River Estuary

To study the processes that govern the post-depositional mobility of metals in the estuarine sediment, five sediment cores were sampled in the Krka River estuary (Croatia). The obtained concentration ranges in the pore water were 0.057–49.7 μM for Fe, 0.310–100 μM for Mn, 0.068–26.8 nM for Co, 0.126–153 nM for Cu, 11.5–2793 nM for Zn, 0.222–31.3 nM for Pb, 4.09–59.4 nM for U, 38.8–2228 nM for Mo, and 0.065–2.29 nM for As. The vertical distribution of metals in the dissolved and solid fraction of the sediment, coupled with other diagenetic tracers (e.g., dissolved sulphide), demonstrate the importance of early diagenetic reactions, in particular Fe and Mn oxyhydroxide and sulphate reduction, for the cycling of metals in the sediment. The redox zonation in the sediment was compressed, and the suboxic zone occurs immediately below the sediment–water interface. The estimated benthic fluxes in the estuary were 5220 kg y−1 for Fe, 27,100 kg y−1 for Mn, 6.00 kg y−1 for Co, 20.5 kg y−1 for Cu, 5.16 kg y−1 for Pb, 111 kg y−1 for Mo, and 87.3 kg y−1 for As. The riverine input was more important than the benthic flux, except in the case of Mn and Fe.

Assessment of the potential of microbial consortium for the reclamation of mine tailings containing potentially toxic elements

Bioremediation using microorganisms is an emerging green technology for the remediation of potentially toxic elements (PTEs) in soils and sediments. However, such technology can differently impact PTEs dynamics (e.g., immobilization and mobilization), ultimately affecting the efficiency of the remediation programs. In this study, we aimed to assess different microbial remediation mechanisms triggered by a microbial consortium to bioremediate Fe-rich mining tailings. The tailings were incubated in a mesocosm system for 35 days with increasing colony-forming units (CFU) of a specific microbial consortium (Azospirillum sp., Pseudomonas sp., Saccharomyces sp., and Rhizobium sp.). At the end of the experiment, we determined the geochemical fractionation of Fe and PTEs in the solid phase to assess the effect of treatments on PTE’s bioavailability. Increasing the CFU resulted in higher Fe (15%) and Mn (37%) reductive dissolution compared to the control. As a result, the Fe and Mn concentrations in water increased by 9-fold. In addition, microbial consortium decreased the contents of Fe and Mn associated with oxides (-59% and −79%, respectively) and increased the more bioavailable solid fractions. The microbial consortium also efficiently decreased PTEs pseudo total contents in the mine tailings (Cr: −85%, Cd: −61%, Pb: −55%, and Cu: −49%). In addition, lower CFUs increased PTEs dissolved in the drainage water, indicating a potential for assisting other remediation strategies. Lower CFU also induced high Cr biomineralization (94%). In conclusion, our work provides novel evidence of a microbial consortium for remediating Fe mine tailings through different strategies (biodissolution and biomineralization). In view of the effects of the microbial consortium over Fe and Mn oxyhydroxide dissolution rates, further research should test it on microbially assisted phytoremediation protocols.

Spatial Distribution, Ecological Risk Assessment, and Source Identification of Metals in Sediments of the Krka River Estuary (Croatia)

To evaluate the level of contamination and predict the potential toxicity risk, selected metal concentrations (Cd, Pb, Cr, Mn, Co, Ni, Cu, Zn, and As) were determined in 40 surface sediment samples from the stratified karstic Krka River estuary (Croatia). In addition, diffusive gradients in thin films (DGT) probes were deployed in situ to understand the mobilization mechanisms and bioavailability of metals in the sediment. The results show significant spatial differences between the upper and lower estuary, with the latter being more affected by anthropogenic pollution. The pollution assessment using the enrichment factor (EF), the geoaccumulation index (Igeo), and the pollution load index (PLI) showed a strong enrichment of metals in the lower part of the estuary, especially of Mn, Cu, Zn, Pb, and As. The statistical analysis (PCA) revealed the former ferromanganese factory and the port as major sources of pollution in the area. Nickel, Co, and Cr, although slightly elevated, may be attributed to the natural origin. The metal mobility in the estuarine sediment was primarily governed by early diagenetic processes (aerobic organic matter mineralization, Fe and Mn oxyhydroxide reduction), which caused the release of metals from the sediment into the pore water and subsequently into the overlying water column.

The combined effects of lanthanum-modified bentonite and Vallisneria spiralis on phosphorus, dissolved organic matter, and heavy metal(loid)s

The use of lanthanum-modified bentonite (LMB) combined with Vallisneria spiralis (V∙s) (LMB + V∙s) is a common method for controlling internal phosphorus (P) release from sediments. However, the behaviors of iron (Fe) and manganese (Mn) under LMB + V∙s treatments, as well as the associated coupling effect on P, dissolved organic matter (DOM), and heavy metal(loid)s (HMs), require further investigations. Therefore, we used in this study a microelectrode system and high-resolution dialysis technology (HR-Peeper) to study the combined effects of LMB and V∙s on P, DOM, and HMs through a 66-day incubation experiment. The LMB + V∙s treatment increased the sediment DO concentration, promoting in-situ formations of Fe (III)/Mn (IV) oxyhydroxides, which, in turn, adsorbed P, soluble tungsten (W), DOM, and HMs. The increase in the concentrations of HCl-P, amorphous and poorly crystalline (oxyhydr) oxides-bound W, and oxidizable HMs forms demonstrated the capacity of the LMB + V∙s treatment to transform mobile P, W, and other HMs forms into more stable forms. The significant positive correlations between SRP, soluble W, UV254, and soluble Fe (II)/Mn, and the increased concentrations of the oxidizable HMs forms suggested the crucial role of the Fe/Mn redox in controlling the release of SRP, DOM, and HMs from sediments. The LMB + V∙s treatment resulted in SRP, W, and DOM removal rates of 74.49, 78.58, and 54.78 %, which were higher than those observed in the control group (without LMB and V∙s applications). On the other hand, the single and combined uses of LMB and V·s influenced the relative abundances of the sediment microbial communities without exhibiting effects on microbial diversity. This study demonstrated the key role of combined LMB and V∙s applications in controlling the release of P, W, DOM, and HMs in eutrophic lakes.

New advances on the carbon isotope and rare earth elements chemostratigraphy of the late Ediacaran Tamengo Formation (Corumbá Group, Brazil)

The Tamengo Formation (Corumbá Group, midwest Brazil) is a carbonate-dominated succession of major importance to unravel the environmental and biological changes during the Ediacaran–Cambrian transition in Gondwana. Although it has been extensively studied in terms of sedimentology, isotope geochemistry, biostratigraphy, and geochronology, these studies are constrained to the Corumbá region. This work presents new sedimentological, C and O isotope chemostratigraphy, and rare earth elements (REE) plus yttrium (REY) data of four sections of the Tamengo Formation in the Serra da Bodoquena region, approximately 200 km south of Corumbá, discussing how these new data are related to the type sections. The sections present ooid grainstones, with hummocky, swaley, and wavy structures, interbedded with mudstones and shales, indicating deposition of fine particles by suspension fallout in-between periods of high-energy, with reworking by storm waves. The δ13Ccarb curves show positive plateaus with minor, short-lived negative excursions linked to facies variations, which are related to a large δ13Ccarb depth gradient in a redox-stratified water column. The REY profiles present middle REE (MREE)-bulge patterns and absent to slightly positive Ce anomalies, consistent with MREE adsorption onto Fe–Mn oxyhydroxides in the water column followed by release in pore waters. Subsequently, REY remobilization during anoxic diagenetic stages resulted in the MREE-bulge pattern and overprinted some of the original seawater REY features, including the Ce anomalies. The Tamengo Formation in the Serra da Bodoquena region represents a storm-dominated carbonate ramp, as previously interpreted for this unit in the Corumbá region. Nevertheless, there is a significant shift of 2 ‰ in the peaks of δ13Ccarb data between both localities, which may be related to differences in the overall sector of the carbonate ramp. There are also substantial variations in the δ13Ccarb record of the Tamengo Formation and other coeval Gondwana basins, such as the Nama and Itapucumi groups, with different peak values, which may be related to latitudinal differences on the inorganic carbon isotope composition or to the degree of basin restriction.

Surface reactivity of the iron and manganese-oxidizing bacterium Leptothrix cholodnii SP-6

Surfaces of prokaryotic cells play a significant role in the adsorption of metals from aqueous solution and the formation of authigenic minerals (Konhauser 2006). Although most studies focus on the cell wall, it is known that many bacteria synthesise an extracellular layer of polysaccharides and proteins, including what are known as sheaths. It has been shown that the cyanobacterium Calothrix sp. produces as sheath which is neutrally charged at circumneutral pH values, and it was hypothesized that such a sheath might allow the cyanobacterium to survive in geothermal settings with high silicification rates (Phoenix et al. 2002). Specifically, the dominance of hydroxyl sites on Calothrix’s sheath surface facilitates hydrogen bonding with aqueous silica species, inducing the precipitation of amorphous silica on the sheath and thus protecting the underlying cell (Phoenix et al. 2002). Leptothrix cholodnii is a sheathed, iron and manganese-oxidizing bacterium that frequently inhabits minerals seeps, where Fe2+ and Mn2+ discharge into oxygenated surface waters (Spring et al. 1996). As a result, the sheath becomes encrusted with Fe(III) and Mn(IV) oxyhydroxides while the underlying cells are protected from mineralization (Emerson and Ghiorse 1992, Emerson et al. 2010). However, unlike Calothrix, Leptothrix’s sheath composition suggests that it might behave differently at circumneutral pH (Emerson and Ghiorse 1993). To investigate the surface reactivity of Leptothrix's sheath and cell wall we analyzed isolated sheaths, sheathless cells, and intact filaments of L. cholodnii SP-6. We studied these components using potentiometric titration, zeta-potential, Cd-adsorption, and Fourier transform infrared (FTIR) spectroscopy to elucidate changes in surface charge between the cell wall and sheath. For the isolated sheaths and intact filaments, titration data were fit using a two-site protonation model, resulting in the following pKa values: 6.05 (±0.29) and 9.34 (±0.11); and 7.77 (±0.17) and 10.50 (±0.20), respectively. For the sheathless cells, the best fit was obtained by using a three-site protonation model, resulting in the following pKa values: 5.40 (±0.59), 8.11 (±1.64) and 10.73 (±0.45). Total proton-active site concentrations were lower in isolated sheaths compared to intact filaments. Additionally, at circumneutral pH, net negative charge was lower for sheathless cells compared to intact filaments and isolated sheaths (Fig. 1). This information agrees with the Cd adsorption behaviour found for the three materials (Fig. 2). Thus, our preliminary results suggest that Leptothrix’s sheath is less reactive than the intact filaments at circumneutral pH, leading us to hypothesize that the outermost layer would sequester relatively lower amounts of cations, including Mn2+, from solution and potentially would protect the underlaying cell from deleterious mineralization. In addition to that, the less reactive sheath’s surface would also contribute to cell attachment, which is important for a species commonly found in streams (Phoenix et al. 2002, Emerson et al. 2010).

Deposition of Mo, Re and U under contrasting redox conditions; assessment of the [Re/Mo]sw redox proxy

We have analyzed concentrations of Mo, Re and U in the suspended particles, water column, underlying carbonate-dominated sediments and pore waters of two contrasting marine lakes situated on the Adriatic Coast of Croatia. Our data are the first for Re in these lakes. Rogoznica Lake is mainly euxinic, but subject to occasional oxygenation events, whereas Mljet Island’s Malo Jezero is currently oxic year around but historically was seasonally euxinic. These characteristics make the lakes useful testing grounds for redox proxies. We devote particular attention to Re/Mo ratios, which have great promise as redox proxies. In marine sediments, the range of log[Reauth/Moauth] exceeds 2, and typical analytical uncertainty is ±0.05, providing resolving power similar to δ98Mo. Authigenic [Res/Mos] ratios in sediments depend on varying supplies of three components: FeMo,Re)S4 coprecipitates, Fe(III) oxyhydroxides (plus Mn(III,IV) oxyhydroxides in some cases) and a still-uncharacterized Re-rich, Mo-poor component possibly of biological origin. Removal of the latter component explains a 50% depletion of Reaq without Moaq from Rogoznica’s oxic waters. Water column concentrations of Moaq and Reaq, as well as [Res/Mos] ratios in Rogoznica’s suspended particles are predicted successfully by a model that attributes these elements’ removal from sulfidic seawater solely to coprecipitation in Fe(Mo,Re)S4, assuming a partition coefficient of 0.6. However, to predict sediment compositions, additional authigenic Mo supplied by mainly Fe(III) oxyhydroxides during sporadic oxygenation events must be considered. In Malo Jezero, sulfide concentrations are too low even in pore waters for precipitation of Fe(Mo,Re)S4. Consistent with earlier work, Uaq profiles are best explained by eddy diffusive transport to the sediment rather than capture by a precipitate in the water column. In Rogoznica pore waters, Moaq extracted through 0.45 μm pore size filters exceeds by 50-fold concentrations predicted by the model and measured previously with 0.15 μm filters. This observation implies presence of ferrothiomolybdate colloids, which could be important, overlooked participants in the sedimentary geochemistry of Mo.

Manganese-Iron Phosphate Nodules at the Groken Site, Gale Crater, Mars

The MSL Curiosity rover investigated dark, Mn-P-enriched nodules in shallow lacustrine/fluvial sediments at the Groken site in Glen Torridon, Gale Crater, Mars. Applying all relevant information from the rover, the nodules are interpreted as pseudomorphs after original crystals of vivianite, (Fe2+,Mn2+)3(PO4)2·8H2O, that cemented the sediment soon after deposition. The nodules appear to have flat faces and linear boundaries and stand above the surrounding siltstone. ChemCam LIBS (laser-induced breakdown spectrometry) shows that the nodules have MnO abundances approximately twenty times those of the surrounding siltstone matrix, contain little CaO, and have SiO2 and Al2O3 abundances similar to those of the siltstone. A deconvolution of APXS analyses of nodule-bearing targets, interpreted here as representing the nodules’ non-silicate components, shows high concentrations of MnO, P2O5, and FeO and a molar ratio P/Mn = 2. Visible to near-infrared reflectance of the nodules (by ChemCam passive and Mastcam multispectral) is dark and relatively flat, consistent with a mixture of host siltstone, hematite, and a dark spectrally bland material (like pyrolusite, MnO2). A drill sample at the site is shown to contain minimal nodule material, implying that analyses by the CheMin and SAM instruments do not constrain the nodules’ mineralogy or composition. The fact that the nodules contain P and Mn in a small molar integer ratio, P/Mn = 2, suggests that the nodules contained a stoichiometric Mn-phosphate mineral, in which Fe did (i.e., could) not substitute for Mn. The most likely such minerals are laueite and strunzite, Mn2+Fe3+2(PO4)2(OH)2·8H2O and –6H2O, respectively, which occur on Earth as alteration products of other Mn-bearing phosphates including vivianite. Vivianite is a common primary and diagenetic precipitate from low-oxygen, P-enriched waters. Calculated phase equilibria show Mn-bearing vivianite could be replaced by laueite or strunzite and then by hematite plus pyrolusite as the system became more oxidizing and acidic. These data suggest that the nodules originated as vivianite, forming as euhedral crystals in the sediment, enclosing sediment grains as they grew. After formation, the nodules were oxidized—first to laueite/strunzite yielding the diagnostic P/Mn ratio, and then to hematite plus an undefined Mn oxy-hydroxide (like pyrolusite). The limited occurrence of these Mn-Fe-P nodules, both in space and time (i.e., stratigraphic position), suggests a local control on their origin. By terrestrial analogies, it is possible that the nodules precipitated near a spring or seep of Mn-rich water, generated during alteration of olivine in the underlying sediments.

Estuarine flocculation dynamics of organic carbon and metals from boreal acid sulfate soils

Abstract. Flocculation of riverine dissolved organic matter to the particulate form in estuaries is an important mechanism for capturing dissolved metals to newly formed organic particles, regulating the metal transport from land to sea. The process is particularly relevant for rivers draining boreal acid sulfate soils of western Finland, which are known to deliver large amounts of trace metals with detrimental environmental consequences for the recipient estuaries in the eastern Gulf of Bothnia in the northern Baltic Sea. This is the first study to investigate dissolved metal (Al, Fe, Mn, Co, and Cu) association with flocculating organic particles in the laboratory, by mixing of acidic metal-rich water from acid-sulfate-soil-impacted rivers and particle-free artificial seawater. Water samples were collected in April 2021 from the Laihianjoki and Sulvanjoki rivers in western Finland. Experiments with an in situ laser-diffraction-based particle size distribution sensor and a multiparameter water quality sonde were run to continuously monitor the development of a suspended particle pool over the salinity gradient from 0 to 6, corresponding to the salinity range observed in these estuarine systems. Flocculator experiments with discrete salinity treatments were carried out to investigate metal behaviour with the collection of flocculated material on glass fibre filters. Filtrate was analysed for coloured dissolved-organic-matter absorbance and fluorescence for the characterization of potential changes in the organic matter pool during the flocculation process. Retentate on the filter was subjected to persulfate digestion of organic particles and metal oxyhydroxides (pH &lt; 2.3), and the digestion supernatants were analysed for metal concentrations. The laboratory experiments showed strong transfer of Al and Fe already at a salinity of 0–2 to newly formed organically dominated flocs that were generally larger than 80 µm. Very strong coupling between the decrease in humic fluorescence and the increase in organically bound Al demonstrated that Al transfer to the flocs was stronger than that of Fe. The flocs in the suspended particle pool were complemented by a smaller population of Al- and Fe-oxyhydroxide-dominated flocculi (median size of 11 µm) after pH exceeded ca. 5.5. Cobalt and Mn transfer to the particle pool was weak, although some transfer to Mn oxyhydroxides as well as Co association with the flocs took place. Up to 50 % of Cu was found to be bound to humic substances in the flocs in the river waters, and this proportion did not significantly change during mixing with seawater. The findings of this study demonstrate that salinity and pH are important independent but connected controls of the flocculation behaviour of dissolved metals from boreal acid sulfate soils and the seaward transport and environmental consequences of the metals in the marine environment.