Sulfide and iron control on mercury speciation in anoxic estuarine sediment slurries

In order to understand the role of sulfate and Fe(III) reduction processes in the net production of monomethylmercury (MMHg), we amended anoxic sediment slurries collected from the Venice Lagoon, Italy, with inorganic Hg and either potential electron acceptors or metabolic byproducts of sulfate and Fe(III) reduction processes, gradually changing their concentrations. Addition of sulfide (final concentration: 0.2–6.3 mM) resulted in an exponential decrease in the sulfate reduction rate and MMHg concentration with increasing concentrations of sulfide. Based on this result, we argue that the concentration of dissolved sulfide is a critical factor controlling the sulfate reduction rate, and in turn, the net MMHg production at steady state. Addition of either Fe(II) (added concentration: 0–6.1 mM) or Fe(III) (added concentration: 0–3.5 mM) resulted in similar trends in the MMHg concentration, an increase with low levels of Fe additions and a subsequent decrease with high levels of Fe additions. The limited availability of dissolved Hg, associated with sulfide removal by precipitation of FeS, appears to inhibit the net MMHg production in high levels of Fe additions. There was a noticeable reduction in the net MMHg production in Fe(III)-amended slurries as compared to Fe(II)-amended ones, which could be caused by a decrease in the sulfate reduction rate. This agrees with the results of Hg methylation assays using the enrichment cultures of anaerobic bacteria: whereas the enrichment cultures of sulfate reducers showed significant production of MMHg (4.6% of amended Hg), those of Fe(III), Mn(IV), and nitrate reducers showed no production of MMHg. It appears that enhanced Fe(III)-reduction activities suppress the formation of MMHg in high sulfate estuarine sediments

Biogeochemical factors affecting mercury methylation in sediments of the Venice Lagoon, Italy

Mercury methylation and sulfate reduction rates, total Hg, and monomethyl Hg in the sediments of the Venice Lagoon(Italy) were measured in June 2005 in order to identify the factors affecting the methylation of inorganic Hg. While the rates of Hg methylation and sulfate reduction were generally higher in the surface layers (0–2.5 cm), the correlation between Hg methylation and sulfate reduction rates was not significant when considering all depths and sites. This discrepancy is discussed considering two factors: the activity of sulfate-reducing bacteria and Hg solubility. The former factor is important in determining the Hg methylation rate in comparable geochemical conditions as evidenced by similar vertical profiles of Hg methylation and sulfate reduction rates in each sediment core. The latter factor was assessed by comparing the Hg methylation rate with the particle–water partition coefficient of Hg. The Hg methylation rates normalized to sulfate reduction rates showed a negative linear correlation with the logarithm of the particle–water partition coefficient of Hg, suggesting that the availability of dissolved Hg is a critical factor affecting Hg methylation. Solid FeS seems to play an important role in controlling the solubility of Hg in Venice Lagoon sediments, where sulfate and iron reductions are the dominant electron-accepting processes. Overall, the production of monomethyl Hg in the Venice Lagoon is controlled by a fine balance between microbial and geochemical processes with key factors being the microbial sulfate reduction rate and the availability of dissolved Hg

Mercury speciation in marine sediments under sulfate-limited conditions

Sediment profiles of total mercury (Hg) and monomethylmercury (MMHg) were determined from a 30-m drill hole located north of Venice, Italy. While the sediment profile of total Hg concentration was fairly constant between 1 and 10 m, that of the MMHg concentration showed an unexpected peak at a depth of 6 m. Due to the limited sulfate content (<1 mM) at the depth of 6 m, we hypothesized that the methylation of inorganic Hg(II) at this depth is associated with the syntrophic processes occurring between methanogens and sulfidogens. To test this hypothesis, anoxic sediment slurries were prepared using buried Venice Lagoon sediments amended with HgCl2, and we monitored MMHg concentration in sediment slurries over time under two geochemical conditions: high sulfate (1-16 mM) and limited sulfate concentrations (<100 µM). After day 52 and onward from the addition of inorganic Hg(II), the MMHg concentrations were higher in sulfate-limited slurries compared to high sulfate slurries, along with methane production in both slurries. On the basis of these results, we argue that active methylation of inorganic Hg(II) occurs under sulfate-limited conditions possibly by syntrophic processes occurring between methanogens and sulfidogens. The environmental significance of syntrophic Hg(II) methylation should be further studied

Kinetics of Indirect UO2 Oxidation by Mn(II)-Oxidizing Bacillus Spores

Reductive immobilization of U(VI) as U(IV)O2 has been widely explored as a feasible approach for remediating uranium contaminated sites. Many soil bacteria, including Bacillus sp., oxidize Mn(II) to Mn(IV) oxides which, in turn, chemically oxidize UO2 to U(VI), there by mobilizing uranium. We are investigating UO2 oxidation coupled to bacterially-catalyzed Mn(II) oxidation in order to better understand the environmental constraints controlling the stability of UO2 and the kinetics of UO2 oxidation. We have conducted experiments using spores of the Bacillus sp. strain SG-1 to investigate changes in the oxidation rate of synthetic and biogenic UO2 with varying concentrations of synthetic and biogenic MnO2, In addition, we also measured the changes in O2 uptake and the key parameters of the MichaelisMenten kinetics (Km and Vmax) associated with bacterial Mn(II) oxidation as concentrations of synthetic and biogenic UO2 varied. Biogenic MnO2 produced by Bacillus sp. SG-1 oxidized biogenic UO2 up to four times faster than synthetic UO2 and more effectively than synthetic MnO2. The rates of enzymatic Mn(II) oxidation as a function of Mn(II) concentration conformed to the Michaelis-Menten kinetics; rates of Mn(II) oxidation in the presence of different concentrations of U(IV) indicated a competitive type of inhibition where the Vmax values were unaffected by UO2 concentration, but the Km values increased with increasing UO2. This inhibition does not appear to be directly related to the properties of UO2 itself but rather to the formation of soluble UO2 2+ which inhibits the enzymatic Mn(II) oxidation

Fundamental Controls on the Environmental Reactivity of Transition-and Heavy-Metal Bacteriogenic Oxides

Transition- and heavy-metal oxides, abundant in natural waters and subsurface sediments, drive rich trace-element chemistries through sorption, dissolution, redox, heterogeneous catalytic, and photochemical reactions. Often, the production of these reactive natural materials is mediated by microorganisms, and in some systems biogenic oxides may dominate over abiotic oxides in volume and overall importance. Biogenic transition/heavy metal oxides are often nanoparticulate and can interact readily with their chemical environments via ongoing synthesis, aging, and dissolution/reprecipitation reactions. Through these processes, co-contaminant cations may be readily sorbed/ incorporated. The compositions and molecular-scale structures of biogenic transition/heavy metal oxides thus are influenced by ground water solute chemistry. As these factors may significantly moderate the Gibbs energy of the materials, this behavior is of significance as a moderator or predictor of the reactivity of the oxide materials. Understanding these structure/function relationships in bacteriogenic oxides is a fundamental prerequisite to understanding the roles of these reactive solid phases in the environment and in harnessing them for engineered applications such as in-situ waste forms for the stabilization of subsurface contaminants. This talk will discuss geochemical factors that control the environmental reactivity of bacteriogenic Mn and U oxides, with emphasis on molecular-scale structures, morphology, and the mechanisms by which they drive or moderate linked elemental cycles

Kinetics of Indirect UO2 Oxidation by Mn(II)-Oxidizing Bacillus Spores

Reductive immobilization of U(VI) as U(IV)O2 has been widely explored as a feasible approach for remediating uranium contaminated sites. Many soil bacteria, including Bacillus sp., oxidize Mn(II) to Mn(IV) oxides which, in turn, chemically oxidize UO2 to U(VI), there by mobilizing uranium. We are investigating UO2 oxidation coupled to bacterially-catalyzed Mn(II) oxidation in order to better understand the environmental constraints controlling the stability of UO2 and the kinetics of UO2 oxidation. We have conducted experiments using spores of the Bacillus sp. strain SG-1 to investigate changes in the oxidation rate of synthetic and biogenic UO2 with varying concentrations of synthetic and biogenic MnO2, In addition, we also measured the changes in O2 uptake and the key parameters of the MichaelisMenten kinetics (Km and Vmax) associated with bacterial Mn(II) oxidation as concentrations of synthetic and biogenic UO2 varied. Biogenic MnO2 produced by Bacillus sp. SG-1 oxidized biogenic UO2 up to four times faster than synthetic UO2 and more effectively than synthetic MnO2. The rates of enzymatic Mn(II) oxidation as a function of Mn(II) concentration conformed to the Michaelis-Menten kinetics; rates of Mn(II) oxidation in the presence of different concentrations of U(IV) indicated a competitive type of inhibition where the Vmax values were unaffected by UO2 concentration, but the Km values increased with increasing UO2. This inhibition does not appear to be directly related to the properties of UO2 itself but rather to the formation of soluble UO2 2+ which inhibits the enzymatic Mn(II) oxidation

Dissolved Mn(III) in Water Treatment Works: Prevalence and Significance

Dissolved Mn(III) has been identified at all stages throughout a Water Treatment Works (WTW) receiving inflow from a peaty upland catchment in NE England. Ninety percent of the influent total manganese into the WTW is particulate Mn, in the form of Mn oxide (>0.2 μm). Approximately 9% (mean value, n = 22, range of 0–100%) of the dissolved (<0.2 μm) influent Mn is present as dissolved Mn(III). Mn(III) concentrations are highest (mean of 49% of total dissolved Mn; n = 26, range of 17–89%) within the WTW where water comes into contact with the organic-rich sludges which are produced as waste products in the WTW. These Mn(III)-containing wastewaters are recirculated to the head of the works and constitute a large input of Mn(III) into the WTW. This is the first report of Mn(III) being identified in a WTW. The ability of Mn(III) to act as both an oxidant and a reductant is of interest to the water industry. Understanding the formation and removal of Mn(III) within may help reduce Mn oxide deposits in pipe networks. Further understanding how the ratio of Mn(III) to Mn(II) can be used to optimise dissolved Mn removal would save the water industry significant money in reducing discoloration ‘events’ at the customers' tap

Nanoscale Structure and Dissolution Kinetics of Abiotic and Bacteriogenic Uraninite

No abstract available