Mn Redox Cycling Research Articles

Thermochemical energy storage at high temperature via redox cycles of Mn and Co oxides: Pure oxides versus mixed ones

Development of thermal energy storage (TES) systems for concentrated solar power (CSP) is essential in order to match a variable electricity demand with an intermittent energy source supply, enhancing energy generation dispatchability. The high energy storage densities and the possibility of working at higher temperature ranges make thermochemical heat storage (TCS) via reduction–oxidation (redox) cycles of metal oxides a promising concept for energy storage. For this purpose, manganese and cobalt oxides have been selected as feasible candidates due to their favourable thermodynamic properties. In order to explore the potential of these materials, the capacity of both pure (Mn2O3 and Co3O4) and mixed oxides (Mn3−xCoxO4) to withstand several charge–discharge cycles was evaluated by thermogravimetrical analysis. Results showed better cyclability for the mixed oxides with low Mn content (x≥2.94) and, specially, for the corresponding pure oxides, confirming that these materials may be a viable option for TCS.

Oxidative UO2 Dissolution Induced by Soluble Mn(III)

The stability of UO2 is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO2 and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than that by a comparable concentration of dissolved O2. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO2. The ability of Mn(III) complexes to oxidize UO2 was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO2 surface to facilitate electron transfer. Systematic investigation into the kinetics of UO2 oxidative dissolution by the Mn(III)-PP complex suggested that Mn(III) could directly oxidize UO2 without involving particulate Mn species (e.g., MnO2). The expected 2:1 reaction stoichiometry between Mn(III) and UO2 was observed. The reactivity of soluble Mn(III) in oxidizing UO2 was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)-pyrophosphate complexes. Disproportionation of Mn(III)-PP occurred at pH 9.0, and the oxidation of UO2 was then driven by both MnO2 and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.

Impact of Microbial Mn Oxidation on the Remobilization of Bioreduced U(IV)

Effects of Mn redox cycling on the stability of bioreduced U(IV) are evaluated here. U(VI) can be biologically reduced to less soluble U(IV) species and the stimulation of biological activity to that end is a salient remediation strategy; however, the stability of these materials in the subsurface environments where they form remains unproven. Manganese oxides are capable of rapidly oxidizing U(IV) to U(VI) in mixed batch systems where the two solid phases are in direct contact. However, it is unknown whether the same oxidation would take place in a porous medium. To probe that question, U(IV) immobilized in agarose gels was exposed to conditions allowing biological Mn(II) oxidation (HEPES buffer, Mn(II), 5% O2 and Bacillus sp. SG-1 spores). Results show the oxidation of U(IV) to U(VI) is due primarily to O2 rather than to MnO2. U(VI) produced is retained within the gel to a greater extent when Mn oxides are present, suggesting the formation of strong surface complexes. The implication for the long-term stability of U in a bioremediated site is that, in the absence of competing ligands, biological Mn(II) oxidation may promote the immobilization of U(VI) produced by the oxidation of U(IV).

Elimination of Manganese(II,III) Oxidation in Pseudomonas putida GB-1 by a Double Knockout of Two Putative Multicopper Oxidase Genes

Bacterial manganese(II) oxidation impacts the redox cycling of Mn, other elements, and compounds in the environment; therefore, it is important to understand the mechanisms of and enzymes responsible for Mn(II) oxidation. In several Mn(II)-oxidizing organisms, the identified Mn(II) oxidase belongs to either the multicopper oxidase (MCO) or the heme peroxidase family of proteins. However, the identity of the oxidase in Pseudomonas putida GB-1 has long remained unknown. To identify the P. putida GB-1 oxidase, we searched its genome and found several homologues of known or suspected Mn(II) oxidase-encoding genes (mnxG, mofA, moxA, and mopA). To narrow this list, we assumed that the Mn(II) oxidase gene would be conserved among Mn(II)-oxidizing pseudomonads but not in nonoxidizers and performed a genome comparison to 11 Pseudomonas species. We further assumed that the oxidase gene would be regulated by MnxR, a transcription factor required for Mn(II) oxidation. Two loci met all these criteria: PputGB1_2447, which encodes an MCO homologous to MnxG, and PputGB1_2665, which encodes an MCO with very low homology to MofA. In-frame deletions of each locus resulted in strains that retained some ability to oxidize Mn(II) or Mn(III); loss of oxidation was attained only upon deletion of both genes. These results suggest that PputGB1_2447 and PputGB1_2665 encode two MCOs that are independently capable of oxidizing both Mn(II) and Mn(III). The purpose of this redundancy is unclear; however, differences in oxidation phenotype for the single mutants suggest specialization in function for the two enzymes.

Anomalous molybdenum isotope trends in Upper Pennsylvanian euxinic facies: Significance for use of δ98Mo as a global marine redox proxy

The use of molybdenum isotope data (δ98Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H2S]aq>11μM) water column to the sediment, thus preserving the seawater δ98Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ98Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ98Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ98Mo values (+1.1 to +1.8‰). These results contrast with the pattern of redox — δ98Mo covariation in modern marine environments, in which the heaviest δ98Mo values are found in the most intensely euxinic facies.We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater–freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ98Mo by water-column redox cycling of Mn, with particulate Mn-oxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ98Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ98Mo trends within the Hushpuckney Shale, seawater δ98Mo must have varied by ~1.2‰ at a ~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was <100kyr. Although both the second and third hypotheses are viable based on the present limited δ98Mo dataset, we discuss how each model might be tested through additional Mo isotope data.

Redox conditions in sediments and during sedimentation in the Ontong Java Plateau, west equatorial Pacific

Redox-sensitive elements in sediments, such as manganese (Mn), vanadium (V), molybdenum (Mo), and uranium (U), are promising indicators of past redox conditions during sedimentation and early diagenesis. However, in the Ontong Java Plateau, west equatorial Pacific, there are sparse datasets of redox-sensitive elements in sediment cores. Here, we present a 250 ka record of redox sensitive elements from a 460 cm gravity core at site WP7 (3 degrees 56'S, 156 degrees E, water depth 1 800 m), which was recovered from the southwest Ontong Java Plateau during the 1993 cruise of R/V Science I of the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). Relative to the Post-Archean Australian Shale (PAAS), authigenic Mn, cobalt (Co), nickel (Ni), Mo, V, U, and cadmium (Cd) were found at constantly low levels except when peaks occurred at several depth intervals. Manganese, Co, Ni, and Mo concentrations were elevated at 25-35 cm due to Mn redox cycling. The core was divided into three distinct sections, the top 0-25 cm being oxic, a suboxic section at 25-35 cm and from 35-460 cm which was anoxic. Differential authigenic enrichments of Co, Ni, Mo, V, U, and Cd at the same depth intervals were observed indicating that the enrichments happened during sedimentation or diagenesis and suffered no post settlement redox changes. Therefore, no significant changes in redox conditions during sedimentation must have happened. The water at depth on the Ontong Java Plateau during past 250 ka must have been well oxygenated, possibly resulted from the more or less continuous presence of oxygen-rich deep water like the modern Antarctic Intermediate Water (AAIW) and Antarctic Circumpolar Water (ACW); while it's slightly less oxygenated in glacial intervals, possibly due to ventilation weakening and/or the surface productivity increase.

Rapid Reaction of Nanomolar Mn(II) with Superoxide Radical in Seawater and Simulated Freshwater

Superoxide radical (O2-) has been proposed to be an important participant in oxidation-reduction reactions of metal ions in natural waters. Here, we studied the reaction of nanomolar Mn(II) with O2- in seawater and simulated freshwater, using chemiluminescence detection of O2- to quantify the effect of Mn(II) on the decay kinetics of O2-. With 3-24 nM added [Mn(II)] and <0.7 nM [O2-], we observed effective second-order rate constants for the reaction of Mn(II) with O2- of 6×10(6) to 1×10(7) M(-1)·s(-1) in various seawater samples. In simulated freshwater (pH 8.6), the effective rate constant of Mn(II) reaction with O2- was somewhat lower, 1.6×10(6) M(-1)·s(-1). With higher initial [O2-], in excess of added [Mn(II)], catalytic decay of O2- by Mn was observed, implying that a Mn(II/III) redox cycle occurred. Our results show that reactions with nanomolar Mn(II) could be an important sink of O2- in natural waters. In addition, reaction of Mn(II) with superoxide could maintain a significant fraction of dissolved Mn in the +III oxidation state.

Flood effects on phosphorus immobilisation in a river water filled pit lake—Case study Lake Goitsche (Germany)

Between 1999 and 2002, a former open-cast mine was filled with river water forming the recent Lake Goitsche. During filling initially acid water was neutralised. Phosphorus (P) imported from Mulde River was nearly completely removed from the water column by co-precipitation with iron (Fe) and aluminium (Al) and deposited in the sediment. During extremely high waters of the Mulde River in 2002, a dike breach facilitated a second high import of P into Lake Goitsche with suspended and dissolved matter. The analysis of total phosphorus (TP), however, showed that P again had been eliminated from the water body a few months after the flood event. Sediment investigations before filling with river water, during filling, and after the flood event were used to analyse the process of P immobilisation in a lake with acid mine drainage history. The ratios of Fe to soluble reactive P (SRP) of sediment pore water were up to three orders of magnitudes higher than in natural lakes and can serve as an indicator for potential internal P loading from sediments. The SRP concentrations at the oxic/anoxic boundary were near or below the limit of quantification (< 0.2 μmol/L). Fe and manganese (Mn) redox cycling were responsible for hindering P dissolution from sediment to lake water. Finally it can be stated, that the risk of eutrophication for such a lake seems to be low.

PbO2(s, Plattnerite) Reductive Dissolution by Aqueous Manganous and Ferrous Ions

Pb(IV)O2(s, plattnerite) nanoparticle aggregrates in aqueous suspension are readily reduced by Mn2+(aq) and Fe2+(aq) between pH 3.0 and 8.5, yielding pb2+(aq) and adsorbed Pb". Fe2+(aq) oxidation generates Fe(III) (hydr)oxides that impede Pb(IV) reduction, especially at pH > or =5. Under acidic conditions, production of dissolved Fe(III) may also be significant. Mn2+(aq) oxidation generates mixed Mn(III)/Mn(IV) (hydr)oxides that are less of an impediment to Pb(IV) reduction. Adding both Fe2+(aq) and Mn2+(aq) can set in motion a Mn redox cycle that catalyzes PbO2(s) reduction by Fe2+(aq). Reaction with Fe2+(aq) and Mn2+(aq) can affect subsequent reactions with natural organic matter. Hydroquinone was employed as a representative organic reductant. Both hydroquinone and its two-electron oxidation product p-benzoquinone are readily quantified by HPLC. Adding Fe2+(aq) before hydroquinone greatly diminishes p-benzoquinone production. Adding Mn2+(aq) before hydroquinone has little effect on p-benzoquinone production. Free chlorine residual variations in premise plumbing can setthe stage for the reactions documented here.

Protein Oxidation Implicated as the Primary Determinant of Bacterial Radioresistance

In the hierarchy of cellular targets damaged by ionizing radiation (IR), classical models of radiation toxicity place DNA at the top. Yet, many prokaryotes are killed by doses of IR that cause little DNA damage. Here we have probed the nature of Mn-facilitated IR resistance in Deinococcus radiodurans, which together with other extremely IR-resistant bacteria have high intracellular Mn/Fe concentration ratios compared to IR-sensitive bacteria. For in vitro and in vivo irradiation, we demonstrate a mechanistic link between Mn(II) ions and protection of proteins from oxidative modifications that introduce carbonyl groups. Conditions that inhibited Mn accumulation or Mn redox cycling rendered D. radiodurans radiation sensitive and highly susceptible to protein oxidation. X-ray fluorescence microprobe analysis showed that Mn is globally distributed in D. radiodurans, but Fe is sequestered in a region between dividing cells. For a group of phylogenetically diverse IR-resistant and IR-sensitive wild-type bacteria, our findings support the idea that the degree of resistance is determined by the level of oxidative protein damage caused during irradiation. We present the case that protein, rather than DNA, is the principal target of the biological action of IR in sensitive bacteria, and extreme resistance in Mn-accumulating bacteria is based on protein protection.

Reduction of manganese porphyrins by flavoenzymes and submitochondrial particles: A catalytic cycle for the reduction of peroxynitrite

The reduction of manganese(III) meso-tetrakis(( N-ethyl)pyridinium-2-yl)porphyrin (MnTE-2-PyP) to manganese(II) was catalyzed by flavoenzymes such as xanthine oxidase and glucose oxidase, and by Complex I and Complex II of the mitochondrial electron transport chain. The reduced manganese porphyrin has been previously shown to react rapidly with superoxide and carbonate radical anion. Herein, we describe the reaction of a reduced manganese porphyrin with peroxynitrite that proceeds as a two-electron process, has a rate constant greater than 7 × 10 6 M −1 s −1 (at pH 7.25 and 37°C), and produces nitrite and the Mn(IV)Porphyrin. The Mn(II)/Mn(IV) redox cycle was used to divert peroxynitrite from the inactivation of succinate dehydrogenase. In a typical experiment, 5 μM MnTE-2-PyP in the presence of excess succinate was able to protect the succinate dehydrogenase and succinate oxidase activities of submitochondrial particles challenged with a cumulative dose of 140 μM peroxynitrite infused in the course of 2 h. Other MnPorphyrins that are reduced more slowly do not provide as much protection underscoring the rate limiting character of the reduction step. The data presented here serve to rationalize the pharmacological action of MnPorphyrins as peroxynitrite reduction catalysts in vivo and opens avenues for the development of MnPorphyrins to protect mitochondria from oxidative damage.

Enrichment of Excess 210Po in Anoxic Ponds

We have investigated the cycling of naturally occurring 210Po in waters from seasonally anoxic Pond B (South Carolina) and permanently anoxic Jellyfish Lake (Palau Islands, western Pacific Ocean). The maximum 210Po activity in Pond B was about 14 mBq L(-1) during summer. This activity was much higher than its parent 210Pb activity, indicating excess 210Po inputs from bottom sediments. The summertime excess 210Po activity was accompanied by increases in Fe and Mn, suggesting 210Po diffusion from bottom sediments under reducing conditions. Activity of 210Po was much lower underwinter oxic conditions, most likely a consequence of efficient coprecipitation with Fe and Mn oxides that occurs with destruction of Pond B stratification. In permanently anoxic Jellyfish Lake, the maximum activity of 210Po was 133 mBq L(-1), among the highest reported from any surface aqueous environment. A box model suggests that the release of only 2% of 210Po, produced from the 210Pb in the bottom sediments, would account for the observed excess. Our results suggest that 210Po can be naturally enriched in anoxic environments to a high level, perhaps in concert with the Fe and Mn redox cycles.

Resolving and modeling the effects of Fe and Mn redox cycling on trace metal behavior in a seasonally anoxic lake

Vertical profiles of the dissolved and particulate (>0.45 μm) concentrations of Fe, Mn, Co, Ni, Cu, Pb, Al and Ba were determined on two occasions (14 and 22 August 1996) during summer stratification in a seasonally anoxic lake (Esthwaite Water, UK). The results were combined with contemporaneous in situ measurements of water-column remobilization of the metals from settling particles at the base of the suboxic zone and other ancillary measurements. The combined data were interpreted with the aid of an equilibrium speciation model (WHAM6), incorporating metal-humic interactions and a surface-complexation description of binding to Fe and Mn oxides. The behavior of all the metals was related in different ways to the position of the O 2-H 2S interface and to Fe and Mn redox cycling. In the region of the O 2-H 2S interface the behavior of Co and to a lesser degree Ni was dominated by Mn redox cycling. Ba behavior was dominated by the biogenic precipitation and dissolution of barite and to a lesser degree by Mn redox cycling. The behavior of Al was linked to both Mn and Fe redox cycling, although the extent of binding to the oxides and to humic substances was poised with respect to pH. Unlike the other metals, the profiles of Pb and Cu showed little variation above the dissolved sulfide maximum, but modeling indicated that binding of Pb was significant to both Mn and Fe oxides. The featureless nature of the Cu profiles in the upper part of the water column was linked to its overriding association with dissolved humic substances. Below the dissolved sulfide maximum, Co, Ni, Ba, Cu, Pb and Mn were all affected by sulfide precipitation, probably through a common association with FeS. In the case of Co, Ni, Cu and Pb, inverse relationships between the measured dissolved and particulate concentrations were attributed to the coexistence of both filterable and nonfilterable FeS particles and associated mass balance effects. The observed behavior of the metals in relation to the role played by Fe and Mn oxides was generally consistent with WHAM6 predictions. The model predictions highlighted the fact that trace metal speciation in general, and binding to Mn and Fe oxides in particular, can be highly sensitive to the variations in solution conditions found in freshwater systems.

Trace element and REE composition of a low-temperature ridge-flank hydrothermal spring

Warm (25°C) hydrothermal springs have been sampled on Baby Bare, a basaltic outcrop on 3.5-Ma-old crust ∼100-km east of the Endeavor Segment of the Juan de Fuca Ridge. The source for these springs is a 62 to 64°C formation water that has cooled conductively as it ascends to feed the springs. This water originated as bottom seawater that probably descended into basement ∼52 km to the southwest at another, much larger outcrop called Grizzly Bare. As this seawater flows towards Baby Bare, it is heated and altered by reactions within basaltic basement and by diffusive fluxes to and from the overlying sediment. Concentrations of Mn, Co, Ni, Zn, Cd, and Mo in the spring waters are greater than in bottom seawater, indicating that the oceanic crust is a source for these elements to the oceans. At least a portion of this increase probably results from the redox cycling of Mn in sedimentary sources near the basement interface that produces a diffusive flux to basement formation waters. Additional removal of Mo and inputs of the other five elements to two of the three springs are observed locally near sites of venting, where density gradients can form shallow circulation cells within the sediment and diffusive exchange occurs. Concentrations of Cu, U, V, Y, and the rare earth elements (REEs, excluding Ce) in these samples are less than in bottom seawater, indicating that the oceanic crust is a net sink for these elements in this environment. Copper is probably removed into newly formed carbonate and/or sulfide phases. Removal of the oxyanions U and V is consistent with a net removal of phosphate demonstrated previously for ridge-flank hydrothermal systems. Similarly, removal of Y and the REEs is associated with carbonate, phosphate-rich, and oxide phases. Calculated maximum global chemical fluxes from “warm” ridge-flank hydrothermal systems such as Baby Bare are insignificant relative to riverine fluxes for these elements, except possibly for Mn and Mo. The impact on global geochemical budgets for these elements from lower temperature (<25°C) alteration on ridge flanks is still unknown, but it may well be larger than for warm ridge flanks.

Trace metal partitioning in Fe–Mn nodules from Sicilian soils, Italy

Concentrations of 20 elements (Mn, Fe, Na, Mg, Al, K, Ca, V, Co, Ni, Cu, Zn, Rb, Sr, Cd, Cs, Ba, La, Ce, Pb) were determined in 18 Fe–Mn nodules from two alfisols, which are subject to periodic waterlogging. The nodules are significantly enriched in most trace metals relative to the host soil, with Mn, Co, Ce, Pb, Ba, Cd, Ni more enriched than Fe, V, La, Cu, Sr, Zn. Cesium and rubidium show poor or no enrichment in the nodules, consistent with their aluminosilicate affiliation. Scanning electron microscopy (SEM) examination and energy dispersive X-ray spectrometer (EDS) microprobe analysis have shown different microstructures (an undifferentiated fabric and a distinctive Fe–Mn banded structure) in the nodules from the two soils. A clearer partitioning of trace elements between Mn and Fe oxide phases is observed in the nodules showing the Fe–Mn banded structure, with Ba, Sr, Ni, Cu, and Cd correlated with Mn, V, and Pb with Fe. The nodules with undifferentiated fabric exhibit lower element/Mn molar ratios, which could reflect a minor adsorption of trace elements by MnO 2 in relation to a more rapid growth of the nodules. As shown by elemental associations, the Cd and Ce distribution appears to be closely linked to the redox cycling of Mn.

Absorbed Mn&lt;SUP&gt;2+&lt;/SUP&gt; and Mn redox cycling in Iberian continental margin sediments (northeast Atlantic Ocean)

Although Mn 2+ sorption has been investigated extensively in the laboratory, the role of Mn 2+ sorption in natural marine sediments remains speculative. Our objectives were to study (1) the role of Mn 2+ sorption in the redox cycling of Mn, (2) to quantify Mn cycling and (3) to identify its rate-determining factors at the Iberian margin. Profiles of pore water Mn 2+ , adsorbed Mn 2+ and solid phase Mn were measured together with benthic oxygen fluxes along three transects across the margin from the shelf to the deep sea as well as in the Nazere Canyon. In the profiles, peaks of adsorbed Mn 2 were observed in-between those of solid phase Mn and pore water Mn 2+ , We propose that upon Mn reduction, the produced Mn 2+ is adsorbed onto adjacent Mn oxide or oxyhydroxide surfaces. Available adsorption-sites diminish and/or saturate as Mn reduction continues, upon which Mn 2+ is released into the pore water. Mn redox chemistry is controlled by the organic carbon flux to the sediment. A simple steady state model was formulated that includes Mn 2+ sorption as a combination of an instantaneous reversible equilibrium process and a first-order kinetic reaction. Model derived, depth integrated rates of Mn reduction as well as Mn 2+ desorption and oxidation rates range between 1 and 35 pmoles m -2 d -1 . Mn cycling is most intense at moderate carbon fluxes. Moreover, Mn cycling is enhanced at higher deposition fluxes of Mn oxide in the canyon. Budgets based on the model indicate that adsorbed Mn 2+ is an important redox intermediate between Mn oxide and pore water Mn 2+ in the reduced sediment layer. Adsorption of Mn 2+ restrains the efflux of dissolved Mn 2+ into the water column, by lowering the pore water gradient at stations with a thin oxidation zone. There, adsorbed Mn 2+ enhances the retention of Mn 2+ in the sediment column.

Factors controlling temporal variation in methyl mercury levels in sediment and water in a seasonally stratified lake

Total mercury (total Hg) levels were low in the water (2.5–7.5 pM) and in the sediment (150–300 pmol g (DW)−1), indicating an absence of significant local sources of Hg. During summer stratification, methyl mercury (MeHg) levels increased below the thermocline, reaching 2.5 pM in the anoxic hypolimnion, whereas in the epilimnion levels remained low (0.25–0.6 pM) throughout the study period (late April–late October 1993). On October 27 (the last sampling date), the lake was totally mixed and MeHg in the entire water column had returned to low levels. Within the part of the basin turning anoxic during summer stratification, the MeHg levels in the surficial sediment (0–0.5 cm) and those in the overlying water were negatively correlated (r = −0.87, P &lt; 0.05). Sediment MeHg decreased during summer stratification until August and then increased. In the sediment, total organic carbon was significantly correlated with total Hg (r = 0.86, P &lt; 0.05), MeHg (r = 0.90, P &lt; 0.01), and total Mn (r = 0.88, P &lt; 0.01). A mechanism for the partitioning of MeHg between water and sediment is discussed, involving the Mn redox cycle. Budget calculations indicated that exchanges of a fixed amount of MeHg between the water and sediment could not explain the observed seasonal variation in MeHg levels. Other processes that may have contributed to the variation in MeHg levels in the sediment and water are methylation, demcthylation, mass dilution, and uptake of MeHg in the biota.

Manganese behavior in the sediments of diverse Scottish freshwater lochs

Mn concentrations were determined in sediments and pore waters of five freshwater lochs in Scotland. The aim was to establish whether the geochemical behavior of Mn differed in a range of limnological conditions: oligotrophic/mesotrophic Loch Lomond; eutrophic unstratified Loch Leven; seasonally stratified eutrophic Balgavies Loch; oligotrophic, acidified Round Loch of Glenhead; and oligotrophic, acid‐sensitive Loch Coire nan Arr.Although redox‐driven diagenesis of Mn was evident in all lochs, the diversity of conditions in the lochs resulted in some cases in modifications to behavior predicted by conceptual models for transport of Mn at a redox boundary. These differences were reflected in the chemical associations and trends in concentration of Mn. A classic example of Mn redox cycling in the sediments of Loch Lomond provided a basis for comparison with other systems. Higher inventories of Mn in sediment collected from eutrophic Loch Leven during the summer, compared with sediment from autunm and winter sampling, were explained by the effects of enhanced primary productivity (high pH and dissolved oxygen concentrations) during a summer algal bloom. In Balgavies Loch, where the bottom water was hypoxic, at the time of sampling, previous diagenetic enrichment of Mn in the surface sediment was subject to modification by dissolution and release from the solid phase to pore water, which was elevated in Mn. The increase in solid‐phase Mn concentration with depth and lack of significant surface enhancement of Mn in Round Loch contrasted markedly with results from other lochs. This was attributed mainly to acidification, producing a pH‐related decrease in Mn sedimentation. Mn partitioning and porewater data for Loch Coire nan Arr were in some respects similar to those at Round Loch, with both lochs having peaty sediment. However, Mn concentration patterns attributed to acidification effects at Round Loch were not found in Loch Coire nan Arr.

Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling

Concentrations of dissolved Y and rare-earth elements (REE) are reported for oxic eastern Mediterranean seawater from the western Levantine Basin, and for anoxic hypersaline brine and overlying oxic seawater from the Tyro sub-basin, the latter data allowing a comparative study of Y and REE behaviours during redox-cycling. All oxic waters show shale-normalized Rare-Earths and Yttrium (REYSN; Y inserted between Dy and Ho) patterns with HREE enrichment, negative CeSN anomalies, and positive anomalies of LaSN, EuSN, and, most pronounced, YSN. The anoxic brine in the Tyro sub-basin displays the highest REY concentrations and the least HREE enrichment of all samples, and positive anomalies of LaSN, EuSN, GdSN, YSN, and, in marked contrast to the oxic samples, CeSN. Compared to overlying oxic water, the anoxic brine shows enrichment factors that decrease from La (12.1) to Yb (2.7), with a pronounced positive deviation for Ce (51.2) and a strong negative deviation for Y (2.4, compared to 4.3 for Dy and 3.7 for Ho). The YHo molar ratio decreases from 102 above to only 67 below the seawater-brine interface, due to preferential sorption of Ho with respect to Y on Fe- and Mn-oxyhydroxide particles that eventually dissolve under anoxic conditions. The pronounced Y-Ho fractionation during redox-cycling of Mn and Fe is further evidence for the considerably lower marine particle reactivity of Y compared to Ho, resulting from lower stabilities of surface complexes of Y relative to those of its REE neighbours. Hence, the YHo ratio appears to be a sensitive indicator of the impact of particles on the distribution of dissolved trace elements in natural waters.

Mercury pathways in the carson river—lahontan reservoir system, nevada, usa

AbstractConcentrations of mercury (Hg) were determined in surface waters and associated river bank sediment samples in a river—reservoir system contaminated by mine wastes. The distribution of total and methyl Hg in surface waters along the Carson River was similar to that measured in river bank sediments and influenced by flow regimes. High levels of Hg (up to 7,585 and 7.2 ng Hg/L for total and methyl Hg, respectively) determined on surface water samples were in large part discharged from Hg‐contaminated tailings, distributed in the river bank sediments. Once introduced into the river during the spring snowmelt runoff, Hg was transported downstream and accumulated in the lacustrine part of the system. Elemental Hg (Hg0) increased from 0.02 ng/L in the noncontaminated region to about 2 ng/L in the reservoir. The vertical distribution of total methylmercury (MeHgT) in water of the reservoir differs from that observed elsewhere, in both Hg‐contaminated and noncontaminated lakes. The highest levels of MeHgT (&lt;1 ng/L as Hg) and acid‐reactive Hg (4 ng/L) were observed in the alkaline and oxic surface waters. The decrease of pH with depth and the absence of oxygen in depth &gt;10 m did not enhance MeHg production. In the anoxic hypolimnion of the reservoir, the recycling of MeHgT was more influenced by the redox cycling of Mn. The addition of group VI anions (SeO2−4, MoO2−4, and WO2−4) in the range of concentrations of oxyanion‐forming elements found in the Carson River system to anoxic sediment slurry spiked with SO2−4 resulted in the reduction of rates of MeHg production. Their negative effect on MeHg production was enhanced by increasing pH. Group VI anions, analogous to SO2−4 are inhibitory to sulfate‐reducing bacteria, which are known to play a key role in MeHg production in anoxic sediments. Accordingly, the particular water geochemistry of the Carson River system could partly explain the observed low levels of MeHg where one would expect higher concentrations.