Dissolved Sulfide Concentrations Research Articles

Reactive transport calculations to evaluate sulphide fluxes in the near-field of a SF/HLW repository in the Opalinus Clay

Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Copper coating of the steel disposal canister is considered as an option to ensure complete waste containment for a period of 100’000 years. Sulphide is a potential corroding agent to copper. While background sulphide concentrations in the OPA are very low (ca. 10−11 mol/L), it cannot be ruled out that sulphide reducing bacteria (SRB) thriving at discrete locations within the engineered barrier system (EBS) of the repository could generate significantly higher dissolved sulphide concentrations. The aim of this study is to conservatively evaluate the potential for SRB to generate and sustain elevated sulphide concentrations in the repository near-field by considering specific geochemical bottle-necks, to quantify the maximum sulphide fluxes towards the canister, and to assess their potential to corrode the copper canister coating. To address these objectives reactive transport calculations are performed, and main conceptual and parametric uncertainties of the model are assessed.

Screening of biological sulfate reduction conditions for sulfidogenesis promotion using a methanogenic granular sludge

Effluents containing great amounts of oxidized sulfur compounds, such as sulfate or sulfite, can be valorized as elemental sulfur from a sequential reduction-oxidation biological process. However, the most important, challenging step to be optimized is the reduction of sulfate. The present study aimed at seeking out the optimal conditions to promote sulfidogenesis instead of methanogenesis using waste carbon sources and a methanogenic granular sludge. Crude glycerol showed better results in terms of the consumed COD/S-Sulfate ratio compared with acetate, cheese whey, pig slurry, and vinasse. Then, the screening of several conditions (T, pH, and COD/S-Sulfate ratio) and the effects of air presence and dissolved sulfide inhibition on sulfate reduction was carried out. Sulfidogenesis was promoted at 35 °C, pH = 8.5, COD/S-Sulfate ratio above 7.0 g O2 g−1 S, microaerophilic conditions, and dissolved sulfide concentrations below 250 mg S2− L−1. These conditions were tested for nearly 3 months in the startup and operation of a 2 L UASB reactor. An inlet sulfate concentration of 220 mg S L−1 and an HRT of 2 h were set. Removal efficiencies of approximately 90% were obtained with less than 20% of organic matter destined for biogas production.

Numerical investigation into the restoration of ocean environments using steelmaking slag

Steelmaking slag has been utilized in shore protection and to improve ocean environments of sea bottom sediments in semi-enclosed areas and coastal regions. This is achieved by reducing dissolved sulphide concentration. In this study, a numerical model is proposed and developed using a Eulerian-Lagrangian model coupled with an ocean circulation model to compute advection-diffusion of dissolved sulphides and fluid-particle interactions between ocean circulation and steelmaking slag. We applied the model to the Fukuyama inner harbour in the Seto Inland Sea and Tokyo Bay and compared our results with field data. The numerical results show good agreement with the field results. We demonstrate that steelmaking slag can control advection-diffusion with regard to concentration of hydrogen sulphide. The steelmaking slag could be a useful material in restoration of ocean environments at enclosed sea areas.

Mercury transformations in resuspended contaminated sediment controlled by redox conditions, chemical speciation and sources of organic matter

Mercury (Hg) contaminated sediments can be significant sources of Hg in aquatic ecosystems and, through re-emission processes, to the atmosphere. Transformation and release of Hg may be enhanced by various sediment perturbation processes, and controlling biogeochemical factors largely remain unclear. We investigated how rates of Hg transformations in pulp-fiber enriched sediment contaminated by Hg from chlor-alkali industry were controlled by (i) transient redox-changes in sulfur and iron chemistry, (ii) the chemical speciation and solubility of Hg, and (iii) the sources and characteristics of organic matter (OM). Sediment-bottom water microcosm systems were exposed to four combinations of air and nitrogen gas for a total time of 24 h. The treatments were: 24 h N2, 0.5 h air + 23.5 h N2, 4 h air + 20 h N2 and 24 h of air exposure. As a result of these treatments, microcosms spanned a wide range of redox potential, as reflected by the dissolved sulfide concentration range of ≤0.3–97 µM. Four different chemical species of inorganic divalent Hg (HgII) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to the microcosms: 201Hg(NO3)2(aq), 202HgII adsorbed to OM (202HgII-OM(ads)), 198HgII as microcrystalline metacinnabar (β-198HgS(s)) and Me204HgCl(aq). Microcosm systems were composed of bottom water mixed with sediment taken at 0–2, 0–5 and 0–10 cm depth intervals. The composition of OM varied with sediment depth such that compared to deeper sediment, the 0–2 cm depth-interval had a 2-fold higher contribution of labile OM originating from algal and terrestrial inputs, serving as metabolic electron-donors for microorganisms. The potential methylation rate constant (kmeth) of Hg tracers and net formation of ambient MeHg (MeHg/THg molar ratio) increased up to 50% and 400%, respectively at intermediate oxidative conditions, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria with a capability of methylating HgII. Due to differences in HgII water-sediment partitioning, kmeth varied by a factor of 11–70 for the different isotope-enriched Hg tracers. The chemical form of HgII was a major controlling factor for kmeth and its response to the resuspension-oxidation of the system. The β-198HgS(s) tracer had the lowest kmeth and it was mainly constrained by redox-driven HgII solubility. The 202HgII-OM(ads) tracer showed an intermediate value on kmeth. It was controlled by both HgII solubility and availability of electron donors and acceptors, regulating bacterial activity. The 201Hg(NO3)2(aq) tracer had the highest value on kmeth which was limited mainly by bacterial activity. The kmeth was up to a factor of 3 higher in the 0–2 cm sediment depth-interval than in 0–5 and 0–10 cm intervals due to a larger contribution of labile OM in the 0–2 cm sediment. Reduction of HgII to Hg0 followed by volatilization exclusively occurred at high sulfidic conditions in the top 0–2 cm sediment. Aromatic moieties of terrestrial OM, present mainly in the top sediment, is suggested to control the reduction of HgII. The Hg0 volatilization rate constant for the 202HgII-OM(ads) tracer exceeded that for β-198HgS(s) by one order of magnitude. Our results suggest that contaminated sediments posing a high risk for reactivation of legacy Hg following transient redox resuspension events are characterized by depletion of sulfate in the sediment porewater prior to resuspension, predominance of HgII species with solubility exceeding that of crystalline β-HgS(s), and conditions promoting in situ formation and/or import of labile OM from algal and terrestrial sources.

Microbial sulfate-reducing activities in anoxic sediment from Marine Lake Grevelingen: screening of electron donors and acceptors

Sulfate-reducing bacteria in marine sediments mainly utilize sulfate as a terminal electron acceptor with different organic compounds as electron donors. This study investigated microbial sulfate-reducing activity of coastal sediment from Marine Lake Grevelingen (MLG), the Netherlands using different electron donors and electron acceptors. All four electron donors (ethanol, lactate, acetate and methane) showed sulfate-reducing activity with sulfate as electron acceptor, suggesting the presence of an active sulfate-reducing bacterial population in the sediment, even at dissolved sulfide concentrations exceeding 12 mM. Ethanol showed the highest sulfate reduction rate of 55 µmol gVSS−1 day−1 compared to lactate (32 µmol gVSS−1 day−1), acetate (26 µmol gVSS−1 day−1) and methane (4.7 µmol gVSS−1 day−1). Sulfide production using thiosulfate and elemental sulfur as electron acceptors and methane as the electron donor was observed, however, mainly by disproportionation rather than by anaerobic oxidation of methane coupled to sulfate reduction. This study showed that the MLG sediment is capable of performing sulfate reduction by using diverse electron donors, including the gaseous and cheap electron donor methane.

Release of hydrogen sulfide in a sewer system under intermittent flow conditions: the Eiceira case study, in Portugal.

The presence and fate of hydrogen sulfide in wastewater systems were studied in two stretches of an intercepting sewer system located in a coastal village, in Portugal. A range of hydraulic parameters were obtained and liquid and gas phase measurements were carried out, both continuously and through intensive sampling campaigns. Upstream, where flow rates were relatively low, dissolved sulfide concentrations around 12 mg S L-1 and hydrogen sulfide gas concentrations above 250 ppm were observed, along with limited corrosion damage. It is believed this is due to the low relative humidity detected along the atmosphere of the studied sewer system. Downstream, gas concentrations were always below 40 ppm. Despite that, high signs of corrosion were detected, particularly in manholes with drop structures. It is thought that condensation of spray produced by the fall is the main cause of the phenomenon. Another relevant observation was the rapid decline in dissolved sulfide contents along gravity trunk sewers following the discharge of rising mains, with loss rates as high as 40 mg S L-1 h-1. Air-flow velocities corresponded to 15-50% of wastewater flows, an observation which agrees with other authors' publications addressing relatively small pipes and moderate water flows.

Remediation of arsenic-contaminated groundwater by in-situ stimulating biogenic precipitation of iron sulfides

Severe health problems due to elevated arsenic (As) in groundwater have made it urgent to develop cost-effective technologies for As removal. This field experimental study tested the feasibility of in-situ As immobilization via As incorporation into newly formed biogenic Fe(II) sulfides in a typical As-affected strongly reducing aquifer at the central part of Datong Basin, China. After periodic supply of FeSO4 into the aquifer for 25 d to stimulate microbial sulfate reduction, dissolved sulfide concentrations increased during the experiment, but the supplied Fe(II) reacted quickly with sulfide to form Fe(II)-sulfides existing majorly as mackinawite as well as a small amount of pyrite-like minerals in sediments, thereby restricting sulfide build-up in groundwater. After the completion of field experiment, groundwater As concentration decreased from an initial average value of 593 μg/L to 159 μg/L, with an overall As removal rate of 73%, and it further declined to 136 μg/L adding the removal rate up to 77% in 30 d after the experiment. The arsenite/Astotal ratio gradually increased over time, making arsenite to be the predominant species in groundwater residual As. The good correlations between dissolved Fe(II), sulfide and As concentrations, the increased abundance of As in newly-formed Fe sulfides as well as the reactive-transport modeling results all indicate that As could have been adsorbed onto and co-precipitated with Fe(II)-sulfide coatings once microbial sulfate reduction was stimulated after FeSO4 supply. Under the strongly reducing conditions, sulfide may facilitate arsenate reduction into arsenite and promote As incorporation into pyrite or arsenopyrite. Therefore, the major mechanisms for the in-situ As-contaminated groundwater remediation can be As surface-adsorption on and co-precipitation with Fe(II) sulfides produced during the experimental period.

Comparison of geochemical data obtained using four brine sampling methods at the SECARB Phase III Anthropogenic Test CO2 injection site, Citronelle Oil Field, Alabama

The chemical composition of formation water and associated gases from the lower Cretaceous Paluxy Formation was determined using four different sampling methods at a characterization well in the Citronelle Oil Field, Alabama, as part of the Southeast Regional Carbon Sequestration Partnership (SECARB) Phase III Anthropogenic Test, which is an integrated carbon capture and storage project. In this study, formation water and gas samples were obtained from well D-9-8 #2 at Citronelle using gas lift, electric submersible pump, U-tube, and a downhole vacuum sampler (VS) and subjected to both field and laboratory analyses. Field chemical analyses included electrical conductivity, dissolved sulfide concentration, alkalinity, and pH; laboratory analyses included major, minor and trace elements, dissolved carbon, volatile fatty acids, free and dissolved gas species. The formation water obtained from this well is a Na–Ca–Cl-type brine with a salinity of about 200,000mg/L total dissolved solids. Differences were evident between sampling methodologies, particularly in pH, Fe and alkalinity. There was little gas in samples, and gas composition results were strongly influenced by sampling methods. The results of the comparison demonstrate the difficulty and importance of preserving volatile analytes in samples, with the VS and U-tube system performing most favorably in this aspect.

Basin-scale controls on the molybdenum-isotope composition of seawater during Oceanic Anoxic Event 2 (Late Cretaceous)

It is well established that the burial of organic carbon in marine sediments increased dramatically at a global scale at the Cenomanian–Turonian boundary (Oceanic Anoxic Event 2: OAE-2, ∼94Myr ago, Late Cretaceous). Many localities containing chemostratigraphic expressions of this event are not, however, enriched in organic carbon, and point to a heterogeneous set of oceanographic and environmental processes operating in different ocean basins. These processes are difficult to reconstruct because of the uneven geographical distribution of sites recording OAE-2, thus limiting our understanding of the causes and palaeoceanographic consequences of the environmental changes that occurred at this time. A new, highly resolved molybdenum-isotope dataset is presented from the Cape Verde Basin (southern proto-North Atlantic Ocean) and a lower resolution record from the Tarfaya Basin, Morocco. The new data reveal periodic oscillations in the Mo-isotope composition of proto-North Atlantic Ocean sediments, from which coupled changes in the dissolved sulphide concentration and Mo inventories of the basin seawater can be inferred. The cyclic variations in sedimentary Mo-isotope compositions can be hypothetically linked to regional changes in the depth of the chemocline, and in the rate of seawater exchange between basinal waters and global seawater. The new data suggest that a global seawater Mo-isotope composition of ∼1.2‰ was reached very soon after the onset of OAE-2, implying a rapid expansion of marine deoxygenation coeval with, or slightly preceding, enhanced global rates of organic-carbon burial. During OAE-2, the modelled flux of Mo into anoxic sediments is likely to have been ∼60–125 times greater than at the present day, although the spatial extent of anoxia is unlikely to have been greater than 10% of the total seafloor.

Microbial community structure and dynamics in restored subtropical seagrass sediments

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 74:43-57 (2015) - DOI: https://doi.org/10.3354/ame01725 Microbial community structure and dynamics in restored subtropical seagrass sediments Amanda S. Bourque1,2,*, Rebecca Vega-Thurber3, James W. Fourqurean2 1Habitat Restoration Program, Biscayne National Park, National Park Service, 9700 SW 328 Street, Homestead, Florida 33033, USA 2Marine Science Program, Department of Biological Sciences and Southeast Environmental Research Center, Florida International University, Miami, Florida 33199, USA 3Department of Microbiology, Oregon State University, Corvallis, Oregon 97331, USA *Corresponding author: amanda_bourque@nps.gov ABSTRACT: Microorganisms in seagrass sediments facilitate many key ecosystem processes, yet current knowledge of microbial facilitation of seagrass community recovery following disturbance or restoration is limited. We studied microbial community responses to restoration of a subtropical seagrass meadow disturbed by vessel groundings in south Florida, USA, and relationships between microbial communities and sediment properties at the study sites using terminal restriction fragment length polymorphism. Two restoration methods were evaluated: the installation of bird roosting stakes as a means to provide a nutrient source, and placement of sand fill into excavations to prevent erosion. Both disturbed and restoration sites had less complex microbial community structure than undisturbed reference seagrass sediments. Microbial community structure varied little between disturbed and fertilized sites, but was distinct in filled sites. Sediment bulk density, sediment organic matter and total phosphorus content, porewater ammonium, soluble reactive phosphorus, and dissolved sulfide concentrations were important environmental predictors of microbial community structure across the restoration treatments. We show that community structure and diversity varied with sediment depth, among restoration treatments, and through time. Our results indicate that microbial communities in seagrass meadows are changed by physical disturbance of the rhizosphere, and that common restoration techniques lead to the formation of distinct microbial communities during the first year of recovery. KEY WORDS: Microbial diversity · Restoration · Disturbance · Soil structure · Biscayne National Park Full text in pdf format PreviousNextCite this article as: Bourque AS, Vega-Thurber R, Fourqurean JW (2015) Microbial community structure and dynamics in restored subtropical seagrass sediments. Aquat Microb Ecol 74:43-57. https://doi.org/10.3354/ame01725 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 74, No. 1. Online publication date: January 12, 2015 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2015 Inter-Research.

Biotreatment of As-containing simulated acid mine drainage using laboratory scale sulfate reducing upflow anaerobic sludge blanket reactor

Heavy metal contamination of water sources can occur from the discharge of acid mine drainage (AMD). This study assessed sulfidogenic treatment of As-, Fe-, Zn-, Ni- and Cu-containing AMD in an upflow anaerobic sludge blanket (UASB) reactor, operated for approximately 500days. Sulfate reducing granules were successfully enriched with synthetic wastewater and sulfate concentration decreased from 2000mg/L in the influent to 100–200mg/L in the effluent. The pH increased from 3–4 to 6–8 as a result of biogenic alkalinity production. Arsenic removal was not detected in the absence of heavy metals, possibly due to the high dissolved sulfide concentration. In the presence of heavy metals, and at low dissolved sulfide concentrations, As removal efficiency increased to 98–100% likely due to the formation of arsenopyrite (FeAsS) or the adsorption of As on metal sulfide precipitates. Fe, Cu, Ni and Zn removal efficiencies approached 99% in the presence of dissolved sulfide. When hydrogen sulfide generation was insufficient to precipitate all of the metals, Fe was detected in the UASB effluent. The results showed that As-, Fe-, Zn-, Ni- and Cu-containing AMD can be effectively treated by sulfate reducing granules in UASB reactors.

Removal of aqueous hydrogen sulfide by granular ferric hydroxide—Kinetics, capacity and reuse

Aqueous hydrogen sulfide causes a number of sulfide-related problems in sediment/aqueous environments. This paper investigates the use, regeneration and reuse of granular ferric hydroxide (GFH) for removal of aqueous hydrogen sulfide in batch experiments simulating water environments. The sulfide removal by GFH can be described by pseudo-first-order reaction kinetics with respect to dissolved sulfide concentrations and the removal rate was proportional to the GFH dosage. The sulfide removal rate almost tripled as pH decreased from 9.0 to 7.2. An increasing ionic strength (in NaCl solution) and the presence of SO42− in simulated seawater decreased the removal rate while Ca2+ and Mg2+ in seawater hardly had any influence. The aqueous sulfide was mainly oxidized to elemental sulfur with the concurrent reduction of solid Fe(III) to Fe(II). The accumulation of the products (elemental sulfur, iron sulfide and surface-associated Fe(II)) on the surface of GFH eventually led to the latter’s exhaustion. By mixing with water containing dissolved oxygen, the exhausted GFH was able to recover with the simultaneous oxidation of Fe(II) to ferric (hydr)oxides and of solid sulfide to elemental sulfur and sulfur of higher valence states. The recovery in removal capacity could be attributed to the formation of amorphous or less ordered ferric (hydr)oxides on the GFH surface and the reduction in GFH granule size. This study suggests that GFH is a promising renewable material for removal of aqueous hydrogen sulfide in sediment/aqueous systems.

Dosing free nitrous acid for sulfide control in sewers: Results of field trials in Australia

Intermittent dosing of free nitrous acid (FNA), with or without the simultaneous dosing of hydrogen peroxide, is a new strategy developed recently for the control of sulfide production in sewers. Six-month field trials have been carried out in a rising main sewer in Australia (150 mm in diameter and 1080 m in length) to evaluate the performance of the strategy that was previously demonstrated in laboratory studies. In each trial, FNA was dosed at a pumping station for a period of 8 or 24 h, some with simultaneous hydrogen peroxide dosing. The sulfide control effectiveness was monitored by measuring, on-line, the dissolved sulfide concentration at a downstream location of the pipeline (828 m from the pumping station) and the gaseous H2S concentration at the discharge manhole. Effective sulfide control was achieved in all nine consecutive trials, with sulfide production reduced by more than 80% in 10 days following each dose. Later trials achieved better control efficiency than the first few trials possibly due to the disrupting effects of FNA on sewer biofilms. This suggests that an initial strong dose (more chemical consumption) followed by maintenance dosing (less chemical consumption) could be a very cost-effective way to achieve consistent control efficiency. It was also found that heavy rainfall slowed the recovery of sulfide production after dosing, likely due to the dilution effects and reduced retention time. Overall, intermittent dose of FNA or FNA in combination with H2O2 was successfully demonstrated to be a cost-effective method for sulfide control in rising main sewers.

Selective precipitation of metals from synthetic spent refinery catalyst leach liquor with biogenic H2S produced in a lactate-fed anaerobic baffled reactor

This work evaluated the feasibility of using biogenic H2S and NaOH to selectively precipitate molybdenum (Mo), nickel (Ni), cobalt (Co) and vanadium (V) from synthetic spent refinery catalyst leach liquor. A lactate-fed sulfate-reducing anaerobic baffled reactor (ABR) was operated at room temperature to generate the required H2S. The average sulfate reduction rate in the ABR was 130mgL−1d−1 and the average dissolved sulfide concentration was 190mgL−1. Biogenic H2S facilitated the selective precipitation of Mo at pH2 with recoveries of 36–72%. Vanadium precipitation of 64–70% was achieved with NaOH at pH6. The purity indices of Mo and V precipitates were 0.97 and 0.90 at pH2 and pH6, respectively. Percent Ni and Co precipitations were only up to 23 and 16%, respectively at pH3.5, and the purity indices of these metals were low due to their simultaneous precipitation. After the optimization of the Ni and Co precipitations, the mixed Ni–Co sulfide solids could be sold to smelters or hydrometallurgical processing to recover the metals. Moreover, using biogenic H2S to selectively precipitate Mo at pH2 as sulfide and NaOH to precipitate V at pH6 as hydroxide/oxide could be a viable option for recovering these metals from spent catalysts leachate.

Impact of sulfate pollution on anaerobic biogeochemical cycles in a wetland sediment

The impact of sulfate pollution is increasingly being seen as an issue in the management of inland aquatic ecosystems. In this study we use sediment slurry experiments to explore the addition of sulfate, with or without added carbon, on the anaerobic biogeochemical cycles in a wetland sediment that previously had not been exposed to high levels of sulfate. Specifically we looked at the cycling of S (sulfate, dissolved and particulate sulfide – the latter measured as acid volatile sulfide; AVS), C (carbon dioxide, bicarbonate, methane and the short chain volatile fatty acids formate, acetate, butyrate and propionate), N (dinitrogen, ammonium, nitrate and nitrite) and redox active metals (Fe(II) and Mn(II)). Sulfate had the largest effects on the cycling of S and C. All the added S at lower loadings were converted to AVS over the course of the experiment (30 days). At the highest loading (8 mmol) less than 50% of consumed S was converted to AVS, however this is believed to be a kinetic effect. Although sulfate reduction was occurring in sediments with added sulfate, dissolved sulfide concentrations remained low throughout the study. Sulfate addition affected methanogenesis. In the absence of added carbon, addition of sulfate, even at a loading of 1 mmol, resulted in a halving of methane formation. The initial rate of formation of methane was not affected by sulfate if additional carbon was added to the sediment. However, there was evidence for anaerobic methane oxidation in those sediments with added sulfate and carbon, but not in those sediments treated only with carbon. Surprisingly, sulfate addition had little apparent impact on N dynamics; previous studies have shown that sulfide can inhibit denitrification and stimulate dissimilatory nitrate reduction to ammonia. We propose that because most of the reduced sulfur was in particulate form, levels of dissolved sulfide were too low to interfere with the N cycle.

Effect of COD/SO ratio on anaerobic treatment of landfill leachate during the start-up period

This study investigates the performance of an anaerobic baffled reactor (ABR) during the start-up period of raw young landfill leachate treatment at two chemical oxygen demand (COD) to SO ratios of 20 and 4. The reactor was operated at ambient temperature and low organic loading rates (0.52, 0.76 and 1.05 kg COD/m3 per day). During the study, sulfate-reducing bacteria (SRB) activity increased at the lower ratio of COD/SO producing higher levels of sulfide and alkalinity. The dissolved sulfide concentration reached an inhibitory level above 250 mg/L, which caused a sharp reduction in the total COD removal efficiency from 77–80% to 32%. Total volatile fatty acid (TVFA) production proceeded at a constant level despite increased organic loading. As the effluent total and organic COD concentrations increased, the inhibitory effect of the inborn sulfide was correlated to the limitation experienced in the hydrolysis/acidogenesis stages, and thus VFA production and organic matter removal.

Vent fluid chemistry of the Rainbow hydrothermal system (36°N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes

The Rainbow hydrothermal field is located at 36°13.8′N–33°54.15′W at 2300 m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10–15 active chimneys that emit high-temperature (∼365 °C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5–2 mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H 2, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH ( T,P ) (400 °C, 500 bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH ( T,P ) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH ( T,P ) and dissolved H 2 are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H 2 defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite + magnetite + talc ± tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.

Hydrothermalism in the Tyrrhenian Sea: Inorganic and microbial sulfur cycling as revealed by geochemical and multiple sulfur isotope data

Abstract The Palinuro volcanic complex and the Panarea hydrothermal field, both located in the Tyrrhenian Sea (Italy), are associated with island arc magmatism and characterized by polymetallic sulfide mineralization. Dissolved sulfide concentrations, pH, and Eh measured in porewaters at both sites reveal a variable hydrothermal influence on porewater chemistry. Multiple sulfur isotopic measurements for disseminated sulfides (CRS: chromium reducible sulfur) extracted from sediments at Palinuro yielded a broad range in δ34S range between −29.8 and + 10.2‰ and Δ33S values between + 0.015 and + 0.134‰. In contrast, sediments at Panarea exhibit a much smaller range in δ34SCRS with less negative values between −11.3 and −1.8‰. The sulfur isotope signatures are interpreted to reflect a mixture between hydrothermal and biogenic sulfide, with a more substantial biogenic contribution at Panarea. Multiple sulfur isotope measurements were performed on sulfides and elemental sulfur from drill core material from the Palinuro massive sulfide complex. δ34S and Δ33S values for pyrite between −32.8 and −1.1‰ and between −0.012 to + 0.042‰, respectively, as well as for elemental sulfur with δ34S and Δ33S values between −26.7 and −2.1‰ and between + 0.035 and + 0.109‰, respectively, point to a microbial origin for much of the sulfide and elemental sulfur studied. Moreover, data suggest a coupling of bacterial sulfate reduction, sulfide oxidation and sulfur disproportionation. In addition, δ34S values for barite between + 25.0 and + 63.6‰ are also in agreement with high microbial turnover of sulfate at Palinuro. Although a magmatic SO2 contribution towards the formation of the Palinuro massive sulfide complex is very likely, the activity of different sulfur utilizing microorganisms played a fundamental role during its formation. Thus, porewater and multiple sulfur isotope data reveal differences in the hydrothermal activity at Palinuro and Panarea drill sites and underline the importance of microbial communities for the origin of massive sulfide mineralizations in the hydrothermal subsurface.

Mercury and methylmercury cycling in sediments of the mid‐Atlantic continental shelf and slope

We present a detailed study of the biogeochemical factors controlling mercury (Hg) distribution, methylmercury (MeHg) production, and MeHg efflux in sediments of the mid‐Atlantic continental shelf and slope. The mildly reduced surface sediments of the shelf and slope provide ideal conditions for MeHg production. They are sufficiently reduced to support microbial sulfate reduction, but contain very low dissolved sulfide concentrations. The redox zonation of sediments determined the depth distribution of MeHg production, whereas the bioavailability of inorganic Hg for methylation appeared to be the dominant driver of spatial patterns across the shelf and slope. Sediment total Hg concentrations were well predicted by sediment organic matter (SOM) content, with the highest concentrations of Hg and MeHg in the fine‐grained organic clays of the slope. However, SOM‐normalized Hg concentrations decreased with distance from shore. The changing character of organic matter with distance from shore appeared to affect Hg partitioning and bioavailability for methylation. The percentage of Hg in sediments as MeHg was well predicted by measured methylation rates, but not by demethylation rates. On the basis of measured concentrations in bottom waters and surficial pore waters, the average diffusive efflux of Hg(II) and MeHg from sediments to coastal waters was estimated to be 26 and 0.8 pmol m−2 d−1, respectively. Extrapolated globally, the diffusive input of MeHg from shelf and slope sediments is estimated to be 0.01 Mmol per year. As the actual fluxes can be substantially higher than diffusive fluxes, we suggest that shelf and upper slope sediments are a major source of MeHg to the coastal ocean.

Kinetic analysis of biological sulphate reduction using lactate as carbon source and electron donor: Effect of sulphate concentration

This study investigated the effect of feed sulphate concentration on the kinetics of anaerobic sulphate reduction by a mixed SRB culture, using lactate as the sole carbon source and electron donor. Chemostat cultures were operated across a range of residence times (0.5–5 d) and feed sulphate concentrations (1.0–10.0 g l −1). Similar phenomena were observed at feed sulphate concentrations of 1.0 and 10.0 g l −1 with the volumetric sulphate reduction rate increasing linearly with increasing volumetric sulphate loading rate. These reactors were characterised by higher specific volumetric sulphate reduction rates with maximum values of 0.24 and 0.20 g h −1 g −1. Contrastingly, the reactors fed with sulphate concentrations of 2.5 and 5.0 g l −1 showed distinctly different trends in which the volumetric sulphate reduction rate passed through a maximum at the dilution rates of 0.014 and 0.021 h −1, respectively, followed by a decline with further increase in sulphate loading rate. The maximum specific volumetric sulphate reduction rates observed were 2–6-fold lower than those observed at 1.0 and 10.0 g l −1 feed sulphate concentrations. Profiles of specific volumetric sulphate reduction rate and biomass concentration suggested a shift in lactate utilisation from oxidation to fermentation at high dilution rates, implying a change in the dominant components of the microbial consortium. The data suggest that population structure was influenced by lactate affinity and dissolved sulphide concentration. The trends observed were attributed to the greater ability of lactate oxidisers to scavenge lactate under limiting concentrations of the substrate and their greater resilience to dissolved sulphide species in comparison to lactate fermenters .