Sulfate Reduction Process Research Articles

FORMATION OF GEOCHEMICAL ANOMALIES IN HYDROCARBON MIGRATION IN THE PERMAFROST ZONE OF WESTERN SIBERIA

Climate warming can be caused by global changes due to emissions of the greenhouse gases, which are mainly carbon dioxide and methane. Although vertical migration of hydrocarbons (seepages) to the surface from oil and gas fields has been known for many years, this important environmental factor has not yet received due attention in the study of the Arctic and Subarctic regions. The major hydrocarbon-induced chemical and mineralogical changes within the permafrost stratum were investigated in the south of theTazPeninsulainWestern Siberia. The samples of frozen core from the deep (35 m) hole, which had been drilled from the top of hydrolaccolite, were examined to analyze the cryogenic texture of the frozen rock mass, to estimate the gas content in rock and ice, and to determine the authigenous mineral association using SEM and EDX spectroscopy analysis. It is shown that the migration of hydrocarbon gases through the permafrost stratum is caused by shear deformations with the formation of cryogenic crack-type textures on the sliding surfaces, which are characterized by the presence of gas-saturated ice crystallites and high jointing of quartz. It has been established that the migration of hydrocarbons, primarily methane, frozen in sedimentary strata causes significant changes of the pH/Eh parameters: local anaerobic conditions may be changed by microaerophilic ones through the formation of oxygen during crystallization of the water in the slide area; mainly neutral and weakly acidic conditions can locally be changed by the alkaline ones due to the cryogenic concentration of chlorides during freezing. It was found that the impulse character of hydrocarbon migration in permeation zones of frozen strata causes mosaic distribution of sulfate and iron reduction processes, which control the neogenesis (including as a result of microbiological processes) of various forms of iron compounds: sulfides – carbonates oxides.

Process integration for biological sulfate reduction in a carbon monoxide fed packed bed reactor

This study examined immobilized anaerobic biomass for sulfate reduction using carbon monoxide (CO) as the sole carbon source under batch and continuous fed conditions. The immobilized bacteria with beads made of 10% polyvinyl alcohol (PVA) showed best results in terms of sulfate reduction (84 ± 3.52%) and CO utilization (98 ± 1.67%). The effect of hydraulic retention time (HRT), sulfate loading rate and CO loading rate on sulfate and CO removal was investigated employing a 1L packed bed bioreactor containing the immobilized biomass. At 48, 24 and 12 h HRT, the sulfate removal was 94.42 ± 0.15%, 89.75 ± 0.47% and 61.08 ± 0.34%, respectively, along with a CO utilization of more than 90%. The analysis of variance (ANOVA) of the results obtained showed that only the initial CO concentration significantly affected the sulfate reduction process. The reactor effluent sulfate concentrations were 27.41 ± 0.44, 59.16 ± 1.08, 315.83 ± 7.33 mg/L for 250, 500 and 1000 mg/L of influent sulfate concentrations respectively, under the optimum operating conditions. The sulfate reduction rates matched well with low inlet sulfate loading rates, indicating stable performance of the bioreactor system. Overall, this study yielded very high sulfate reduction efficiency by the immobilized anaerobic biomass under high CO loading condition using the packed bed reactor system.

A novel aerobic sulfate reduction process in landfill mineralized refuse

It is thought that mineralized refuse could be excavated from almost-full landfill sites to provide space for the increasing burden of municipal solid waste. When excavating, however, the H2S emissions from the mineralized waste need to be considered carefully. In an attempt to understand how H2S emissions might change during this excavation process, we carried out a series of tests, including exposing anaerobic mineralized refuse to oxygen, isolating and determining possible functional bacteria, and characterizing the electron donors and/or acceptors. The results showed that H2S would be released when landfill mineralized refuse was exposed to oxygen (O2), and could reach concentrations of 6 mg m−3, which was 3 times the concentrations of H2S released from anaerobic mineralized refuse. Sulfur-metabolized microorganisms accounted for only 0.5% of the microbial functional bacteria (MFB) derived from the mineralized refuse when exposed to O2 for 60 days, and SRB were not present. The MFB maintained H2S production by aerobic sulfate reduction using SO42− and S2O32− as electron acceptors, and sulfate-reducing rates of 16% and 55%, respectively, were achieved. Lactate and S2O32− were the preferred electron donor and acceptor, respectively. By enhancing the carbon source and electron transfer, MFB may undergo strong aerobic sulfate reduction even at low abundances of sulfur-metabolized microorganisms.

Typical Soil Redox Processes in Pentachlorophenol Polluted Soil Following Biochar Addition.

Reductive dechlorination is the primary pathway for environmental removal of pentachlorophenol (PCP) in soil under anaerobic condition. This process has been verified to be coupled with other soil redox processes of typical biogenic elements such as carbon, iron and sulfur. Meanwhile, biochar has received increasing interest in its potential for remediation of contaminated soil, with the effect seldom investigated under anaerobic environment. In this study, a 120-day anaerobic incubation experiment was conducted to investigate the effects of biochar on soil redox processes and thereby the reductive dechlorination of PCP under anaerobic condition. Biochar addition (1%, w/w) enhanced the dissimilatory iron reduction and sulfate reduction while simultaneously decreased the PCP reduction significantly. Instead, the production of methane was not affected by biochar. Interestingly, however, PCP reduction was promoted by biochar when microbial sulfate reduction was suppressed by addition of typical inhibitor molybdate. Together with Illumina sequencing data regarding analysis of soil bacteria and archaea responses, our results suggest that under anaerobic condition, the main competition mechanisms of these typical soil redox processes on the reductive dechlorination of PCP may be different in the presence of biochar. In particularly, the effect of biochar on sulfate reduction process is mainly through promoting the growth of sulfate reducer (Desulfobulbaceae and Desulfobacteraceae) but not as an electron shuttle. With the supplementary addition of molybdate, biochar application is suggested as an improved strategy for a better remediation results by coordinating the interaction between dechlorination and its coupled soil redox processes, with minimum production of toxic sulfur reducing substances and relatively small emission of greenhouse gas (CH4) while maximum removal of PCP.

Effects of Fe-S-As coupled redox processes on arsenic mobilization in shallow aquifers of Datong Basin, northern China

High arsenic groundwater generally coexists with elevated Fe2+ concentrations (mg L−1 levels) under reducing conditions, but an explanation for the extremely high arsenic (up to ∼2690) concentrations at very low Fe2+ (i.e., μg L−1 levels) in groundwater of Datong Basin remains elusive. Field groundwater investigation and laboratory microcosm experiments were implemented in this study. The field groundwater was characterized by weakly alkaline (pH 7.69 to 8.34) and reducing conditions (Eh −221.7 to −31.9 mV) and arsenic concentration averages at 697 μg L−1. Acinetobacter (5.9–51.3%), Desulfosporosinus (4.6–30.2%), Brevundimonas (3.9–19%) and Pseudomonas (3.2–14.6%) were identified as the dominant genera in the bacterial communities. Bacterially mediated arsenate reduction, Fe(III) reduction, and sulfate reduction are processes occurring (or having previously occurred) in the groundwater. Results from incubation experiment (27 d) revealed that nitrate, arsenate, and Fe(III)/sulfate reduced sequentially with time under anoxic conditions, while Fe(III) and sulfate reduction processes had no obvious differences, occurring almost simultaneously. Moreover, low Fe2+ concentrations were attributed to initially high pH conditions, which relatively retarded Fe(III) reduction. In addition, arsenic behavior in relation to groundwater redox conditions, matrices, and solution chemistry were elaborated. Bacterial arsenate reduction process proceeded before Fe(III) and sulfate reduction in the incubation experiment, and the total arsenic concentration (dominated by arsenite) gradually increased from ∼7 to 115 μg L−1 as arsenate was reduced. Accordingly, bacterially mediated reductive desorption of arsenate is identified as the main process controlling arsenic mobility, while Fe(III) reduction coupled with sulfate reduction are secondary processes that have also contributed to arsenic enrichment in the study site. Overall, this study provide important insights into the mechanism controlling arsenic mobility under weakly alkaline and reducing conditions, and furnishes that arsenate reduction by bacteria play a major role leading to high accumulation of desorbed arsenite in groundwater.

Hydrogeological and multi-isotopic approach to define nitrate pollution and denitrification processes in a coastal aquifer (Sardinia, Italy)

Agricultural coastal areas are frequently affected by the superimposition of various processes, with a combination of anthropogenic and natural sources, which degrade groundwater quality. In the coastal multi-aquifer system of Arborea (Italy)—a reclaimed morass area identified as a nitrate vulnerable zone, according to Nitrate Directive 91/676/EEC—intensive agricultural and livestock activities contribute to substantial nitrate contamination. For this reason, the area can be considered a bench test for tuning an appropriate methodology aiming to trace the nitrate contamination in different conditions. An approach combining environmental isotopes, water quality and hydrogeological indicators was therefore used to understand the origins and attenuation mechanisms of nitrate pollution and to define the relationship between contaminant and groundwater flow dynamics through the multi-aquifer characterized by sandy (SHU), alluvial (AHU), and volcanic hydrogeological (VHU) units. Various groundwater chemical pathways were consistent with both different nitrogen sources and groundwater dynamics. Isotope composition suggests a mixed source for nitrate (organic and synthetic fertilizer), especially for the AHU and SHU groundwater. Moreover, marked heterotrophic denitrification and sulfate reduction processes were detected; although, for the contamination related to synthetic fertilizer, the attenuation was inefficient at removing NO3− to less than the human consumption threshold of 50 mg/L. Various factors contributed to control the distribution of the redox processes, such as the availability of carbon sources (organic fertilizer and the presence of lagoon-deposited aquitards), well depth, and groundwater flow paths. The characterization of these processes supports water-resource management plans, future actions, and regulations, particularly in nitrate vulnerable zones.

Effect of selected 8-hydroxyquinoline-2-carboxanilides on viability and sulfate metabolism of Desulfovibrio piger

An increased number of sulfate-reducing bacteria is often isolated from faeces of patients with gastrointestinal diseases, which can be the cause of the development of bowel inflammation. Frequent use of antibiotics causes the resistance of intestinal microorganisms and ineffective treatment of these diseases. The antimicrobial activity and biological properties of the selected ring-substituted 8-hydroxyquinoline-2-carboxanilides against Desulfovibrio piger Vib-7 were studied. The addition of these compounds in the cultivation medium inhibited the bacterial growth and the process of sulfate reduction dose-dependently. A significant cytotoxic activity under the influence of ring-substituted 8-hydroxyquinoline-2-carboxanilides was determined. The strongest cytotoxic effect of the derivatives was observed for compounds 8-hydroxy-N-(3-methoxyphenyl)quinoline-2-carboxamide and 8-hydroxy-N-(3-trifluoromethylphenyl)quinoline-2-carboxamide that caused a low survival of D. piger Vib-7 in concentration 17μM and high toxicity rates.

Sulfate reduction processes in salt marshes affected by phosphogypsum: Geochemical influences on contaminant mobility

Sulfate reduction and its associated contaminant immobilization in marsh soils supporting a phosphogypsum stack was examined by pore-water and solid analysis, selective extractions, microscopy and sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The negative impact of this stack on estuarine environments is a concerning problem. In the weathering profile, total concentrations of most pollutants increase with depth; instead, dissolved contents in pore-waters increase to middle of the saturated zone but then decrease drastically down to reach the marsh due to sulfide precipitation. Excess of acid-volatile sulfide plus pyritic sulfur over metals bound to the oxidizable fraction indicates that sulfide precipitation is the main mechanism responsible for metal removal in the marsh. Thus, abundant pyrite occurred as framboidal grains, in addition to other minor sulfides of As, Zn and Cu as isolated particles. Moreover, high contents of elemental sulfur were found, which suggest partial sulfide oxidation, but marsh may have capacity to buffer potential release of contaminants. The importance of sulfur species was quantitatively confirmed by XANES, which also supports the accuracy of selective extraction schemes. Accordingly, managing pore-water quality through organic carbon-rich amendments over phosphogypsum stacks could lead to a decrease in contaminant loading of leakages resulting from weathering.

Free ammonia enhances dark fermentative hydrogen production from waste activated sludge

Ammonium and/or free ammonia (the unionized form of ammonium) are generally thought to inhibit the activities of microbes involved in anaerobic digestion of waste activated sludge. It was found in this work, however, that the presence of ammonium (NH4+-N) largely enhanced dark fermentative hydrogen production from alkaline pretreated-sludge. With the increase of initial NH4+-N level from 36 to 266 mg/L, the maximal hydrogen production from alkaline (pH 9.5) pretreated-sludge increased from 7.3 to 15.6 mL per gram volatile suspended solids (VSS) under the standard condition. Further increase of NH4+-N to 308 mg/L caused a slight decrease of hydrogen yield (15.0 mL/g VSS). Experimental results demonstrated that free ammonia instead of NH4+-N was the true contributor to the enhancement of hydrogen production. It was found that the presence of free ammonia facilitated the releases of both extracellular and intracellular constituents, which thereby provided more substrates for subsequent hydrogen production. The free ammonia at the tested levels (i.e., 0–444 mg/L) did not affect acetogenesis significantly. Although free ammonia inhibited all other bio-processes, its inhibition to the hydrogen consumption processes (i.e., homoacetogenesis, methanogenesis, and sulfate-reducing process) was much severer than that to the hydrolysis and acidogenesis processes. Further investigations with enzyme analyses showed that free ammonia posed slight impacts on protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase activities but largely suppressed the activities of coenzyme F420, carbon monoxide dehydrogenase, and adenylyl sulfate reductase, which were consistent with the chemical analyses performed above.

Effect of COD/SO42- Supply Ratio Variations of Sulfate-Reducing Bacteria of Sulphood Raise in Acid Mine Drainage

Sulphur dioxide gas is one of most contaminating gas in the air. Sulphur gas can be produced by mining activities. Sulphur gas will be harmful if bond with CO2 to form as Sulphur Dioxide. To reduce the Sulphur Dioxide gas concentration we must inhibite the sulphur gas formation from mining activities. The inhibition of sulphur gas could be done by reduce the sulphate concentration in acid mine drainage. One of important factor that influencing the reduce of sulphate is COD/SO42- ratio. The effect of COD/SO42- ratio on bacterial growth and sulfate removal process can be investigated with anaerobic batch reactor. The laundry septic tank sediments were inoculated on an anaerobic batch reactor which were contacted with artificial coal acid mine water wastes with 1000 sulfate concentrations and 2000 mg SO42- /L. In an anaerobic batch reactor there are five reactors with variations of COD / SO42-1.0, 1.5, 2.0, 4.0, and 8.0 ratios. Efficiency ratio and the best sulfate removal rate is in reactor ratio 2.0 with value efficiency of 46.58% and a reduction rate of 29.128 mg / L.day in an anaerobic batch reactor. The efficiency of the removal rate decreased when the COD / SO42->2.0 ratio decreased. The fastest pH decline was in the COD/SO42-8.0 ratio variation in the anaerobic batch reactor and. The COD / SO42-ratio can help the sulfate reduction process in the optimum value by affecting the sulfate-reducing bacterial metabolism in the balance of the acceptor and the electron donor.

Multiple sulfur isotope constraints on microbial sulfate reduction below an Archean seafloor hydrothermal system.

Microbial sulfate reduction is among the most ubiquitous metabolic processes on earth. The oldest evidence of microbial sulfate reduction appears in the ca. 3.5Ga Dresser Formation in the North Pole area of Pilbara Craton in Western Australia. That evidence was found through analysis of quadruple sulfur isotopes of sulfate and sulfide minerals deposited on the seafloor. However, the activity of microbial sulfate reduction below the Archean seafloor remains poorly understood. Here, we report the quadruple sulfur isotopic compositions of sulfide minerals within hydrothermally altered seafloor basalt and less altered basaltic komatiite collected from the North Pole Dome area. The Δ33 S values of the sulfide minerals were nonzero negative, suggesting that sulfate reduction occurred below the Archean seafloor. To constrain the substrate sulfate sources and sulfate reduction processes, we constructed a numerical model. Comparing the modeled and observed sulfur isotopes, we show that the substrate sulfate comprises seawater sulfate with a negative Δ33 S anomaly and 34 S-enriched sulfate with no anomalous Δ33 S. The latter component probably represents sulfate produced by local hydrothermal processes. The maximum sulfur isotopic fractionation between the putative substrate sulfate and the observed sulfide minerals within the altered basalt and basaltic komatiite is 35‰, which is consistent with a microbial origin. Alternatively, thermochemical sulfate reduction may also produce sulfide. However, considering the hydrothermal temperature inferred from the metamorphic grade of the altered basalt, the sulfur isotopic fractionation produced by inorganic sulfate reduction is probably below 20‰. Collectively, larger fractionations imply the involvement of biological sulfate reduction processes, both in the hydrothermal system below the seafloor and in less altered subsurface settings.

Bacterial community structure in response to environmental impacts in the intertidal sediments along the Yangtze Estuary, China

This study was designed to investigate the characteristics of bacterial communities in intertidal sediments along the Yangtze Estuary and their responses to environmental factors. The results showed that bacterial abundance was significantly correlated with salinity, SO42− and total organic carbon, while bacterial diversity was significantly correlated with SO42− and total nitrogen. At different taxonomic levels, both the dominant taxa and their abundances varied among the eight samples, with Proteobacteria being the most dominant phylum in general. Cluster analysis revealed that the bacterial community structure was influenced by river runoff and sewerage discharge. Moreover, SO42−, salinity and total phosphorus were the vital environmental factors that influenced the bacterial community structure. Quantitative PCR and sequencing of sulphate-reducing bacteria indicated that the sulphate reduction process occurs frequently in intertidal sediments. These findings are important to understand the microbial ecology and biogeochemical cycles in estuarine environments.

Oxygenation in carbonate microbialites and associated facies after the end-Permian mass extinction: Problems and potential solutions

Carbonate sediments deposited in normally-oxygenated shallow ocean waters of the latest Permian period, immediately prior to the end-Permian mass extinction, contain well-developed diverse shelly faunas. After the extinction of these skeletal metazoans, the sediments commonly comprise microbialites (regarded by most authors as benthic) and associated facies bearing evidence interpreted by many authors to indicate reduced oxygenation of the shallow ocean waters. However, the evidence of oxygenation state is inconsistent and the sequences have gaps, indicated in the following 5 points:1) Shelly fossils occur commonly in post-extinction shallow marine limestones, and likely to have been aerated in contact with the atmosphere. Nevertheless, although the largest mass extinction in Earth history may have caused reduced body size in shelly organisms, such reduction is arguably due to environmental stress of lowered oxygenation. Discriminating between these controls remains a challenge.2) Abundant pyrite framboids in many post-extinction limestones are interpreted by several authors as indicating dysoxic contemporaneous waters, so the organisms that lived there, now shelly fossils, were dysaerobic. However, verification is problematic because pyrite framboids scattered amongst shelly fossils cannot have formed amongst living organisms, which need at least some oxygen; synsedimentary framboid formation requires anoxic conditions in the redox boundary where sulphate-reducing processes work. Thus, framboids and shelly fossils found together means taphonomic mixing of sediments, destroying original depositional relationships so that it is not possible to determine whether the shells were aerobic or dysaerobic prior to sediment mixing. Furthermore, diagenetic growth of framboids is possible, as is import of previously-formed framboids from deeper water during upwelling. Thus, there is no proof of an environmental link between framboid size and occurrence, and contemporaneous oxygenation in these post-extinction shallow water facies, so we question the validity of this model in those facies, but consider that the model is valid for deeper water facies.3) Some publications provide evidence of oxygenation, from redox-sensitive elements in post-extinction limestones, while others indicate low oxygen conditions. Redox-sensitive geochemistry requires further work to explore this issue at higher resolution of sampling than has been so far applied.4) Biomarkers recorded in some post-extinction facies contain evidence of anoxic conditions (including green sulphur bacteria) but other examples lack these, which may be indicate fluctuations of water oxygenation. However, a key issue that has not yet been resolved is determination of whether biomarkers were imported into the sites of deposition, for example by upwelling currents, or formed where they are found. Thus, there is currently no verification that biomarkers of low oxygen organisms in shallow water settings actually formed in the places where they are sampled.5) The common occurrence of small erosion surfaces and stylolites represents loss of evidence, and must be accounted for in future studies.The oxygenation state of post-end-Permian extinction shallow marine facies continues to present a challenge of interpretation, and requires high-resolution sampling and careful attention to small-scale changes, as well as loss of rock through pressure solution, as the next step to resolve the issue.

Sulfur Isotopes in the Upper Part of the Black Sea Anoxic Zone

New data are reported on the sulfur isotope composition and concentration of sulfide and sulfate in the upper part of the Black Sea anoxic zone as a function of the potential water density. The observations were performed at a station with the coordinates 44.489° N and 37.869° E three times a week every two days. A local negative deficiency in sulfate concentration up to 1.7% related to the sulfate reduction processes was recorded. This anomaly in sulfate concentration was short-lived and did not affect the sulfur isotope composition. In the upper part of the anaerobic zone, the δ34S(SO4) value varied from 21.2 to 21.5‰, which could have occurred from mixing of water masses from the oxic zone (21.1‰) and the Bottom Convective Layer (23.0 ± 0.2‰). The sulfur isotope composition of sulfide ranged from −40.8% at a depth of 250 m to −39.4‰ at the upper boundary of the anoxic zone with a H2S content of only 2.7 μM. Two models (mass balance and fractionation of sulfur isotopes using the Rayleigh equation) are considered to explain the differences in δ34S(H2S) values observed.

Finite Time Estimation for Switched Nonlinear Systems: Application to Stirred Tank Bioreactor

Abstract This work presents the design of a class of finite time observer applied to a nonlinear switched system. The proposed observer is applied to a stirred batch anaerobic bioreactor, described by classic mass balances, where a sulfate-reducing process took place and kinetic regimen alteration is induced by a change in the carbon source. The proposed observer has a simple structure and under an adequate choosing of the observer´s gain the proposed methodology cancels the upper bounds of the system under the different kinetic regimens inducing the required finite time convergence. The performance of the proposed observer is showed via numerical simulations and for comparison purposes a sliding-mode observer is also implemented, both of them are conducted and experimentally corroborated. The convergence of the observer is done with a simple analysis of the estimation error dynamic equations for all the corresponding subspaces and is showed that the convergence of the estimator is reached under the required conditions.

State Estimation Based on Nonlinear Observer for Hydrogen Production in a Photocatalytic Anaerobic Bioreactor

Abstract Hydrogen concentration in a photocatalytic continuous bioreactor was estimated. For the above system, a novel kinetic model of the sulfate-reducing process for hydrogen production was proposed and experimentally confirmed. In addition, we present the design of an estimator based on nonlinear observer, which is robust against modeling errors, to estimate the observable states of the bioreactor. Sulfate, biomass, sulfide, carbon dioxide, cadmium in liquid, cadmium sulfide, and hydrogen concentrations were estimated in spite of errors in the evaluation of the parameters using sulfate concentration as measurable output. The convergence of the proposed observer was analyzed using Lyapunov stability theory. Finally, maximum hydrogen production was 225 mL and 175 mL for batch and continuous processes, respectively.

Tracing sulfate recycling in the hypersaline Pétrola Lake (SE Spain): A combined isotopic and microbiological approach

Sulfur (S) plays a significant role in saline environments, and sulfate (SO42−) is an important component of the biogeochemical S-cycle since it acts as the main electron acceptor in anoxic sediments. The purpose of this paper is to evaluate the fate of S, its origin, and processes affecting sulfate outcome in the hypersaline Pétrola Lake in the Castilla-La Mancha region (High Segura Basin, SE Spain). The lake is the terminal discharge zone of an endorheic basin with considerable anthropogenic pressures. Anthropogenic activities (mainly agricultural inputs and wastewater discharge), together with bedrock leaching of sulfate and sulfide-rich sediments, increase dissolved SO42− in surface and groundwater up to 123,000mg/L. The source and fate of sulfate in this environment was investigated coupling hydrochemistry, including hydrogen sulfide (H2S) microprofiles, isotopic analyses (δ34S, δ18OSO4, δ2HH2O, δ18OH2O, and tritium), mineralogical determinations, and molecular biology tools (16S rDNA amplification and sequencing). The origin of dissolved SO42− in water is related to pyrite oxidation from Lower Cretaceous sediments, and secondary gypsum dissolution. Under the lake, dissolved SO42− decreases with depth, controlled by three main processes: (1) seasonal evaporation cycles, (2) hydrodynamic instability caused by the different density-driven groundwater flow, and (3) sulfate-reduction processes, i.e. dissimilatory bacterial sulfate reduction (BSR). These processes control the continuous recycling of sulfur in the system. Lake water and groundwater are in hydraulic connection, and a density-driven flow (DDF) is able to transport reactive organic matter and dissolved SO42− towards the underlying aquifer. Hydrochemical evolution in depth, H2S production (up to 0.024nmol/cm3·s) and the presence of sulfate-reducing bacteria suggest the existence of BSR processes. However, isotope techniques are insufficient to elucidate BSR processes since their isotopic effect is masked by low isotope fractionation and high SO42− concentrations. The pattern here described may be found in other saline basins worldwide.

Geological and geochemical constraints on the origin of the Early Cambrian Kalaizi Fe–Ba deposit in Western Kunlun, NW China

The Kalaizi deposit in the Bulunkuole Group of Taxkorgan area, Western Kunlun (NW China) is a unique regional Fe–Ba deposit accompanied with sporadic Fe–Pb–Zn sulfide mineralization within a series of iron deposits. All the ore bodies are stratabound and hosted by the Early Cambrian metamorphosed argillaceous clastic rocks. Here, we analyzed C–S–O–Sr isotopes and trace elements in individual minerals (e.g., magnetite, barite, ankerite) from the Kalaizi deposit to investigate the origin of this deposit for the first time. The S isotope fractionation (average of 21‰) between sulfate and sulfide minerals provided clear evidence that the deposit was formed in a semi-closed marine depositional environment with moderate replenishment of seawater sulfate. The distinct Post-Archean Australian Shale (PAAS)-normalized positive Eu anomalies (2.02–11.03) and lower Y/Ho ratios (27.2–33.4) relative to modern seawater in all individual magnetite minerals separated from magnetite–barite ores and the δ18O isotope signatures (10.0–13.0‰) in individual sulfate minerals suggested that hydrothermal activity played an important role in the formation of the deposit. Barium was likely derived from the dissolution of marine barite with a contribution from more radiogenic terrigenous sediments considering the slight deviations (within ±0.0013) of 87Sr/86Sr ratios of Kalaizi barites from the Early Cambrian seawater. The sulfate and sulfide minerals in the ore deposit should evolve from coeval seawater containing residual SO42− and resultant H2S by means of bacterial sulfate reduction process because (i) δ34S values (37.8–42.8‰) of anhydrite and barite minerals were slightly higher than those of penecontemporaneous seawater, (ii) pyrite and galena minerals owned δ34S values (16.3–23.5‰) were lower than coeval seawater, and (iii) 13C isotope values (−10.0 to −12.1‰) of single ankerite minerals were depleted. To summarize, the Kalaizi bedded Fe–Ba deposit is interpreted as a cold seep barite deposit overprinted by hydrothermal fluids.

Bridging spatially segregated redox zones with a microbial electrochemical snorkel triggers biogeochemical cycles in oil-contaminated River Tyne (UK) sediments

Marine sediments represent an important sink for a number of anthropogenic organic contaminants, including petroleum hydrocarbons following an accidental oil spill. Degradation of these compounds largely depends on the activity of sedimentary microbial communities linked to biogeochemical cycles, in which abundant elements such as iron and sulfur are shuttled between their oxidized and reduced forms. Here we show that introduction of a small electrically conductive graphite rod (“the electrochemical snorkel”) into an oil-contaminated River Tyne (UK) sediment, so as to create an electrochemical connection between the anoxic contaminated sediment and the oxygenated overlying water, has a large impact on the rate of metabolic reactions taking place in the bulk sediment. The electrochemical snorkel accelerated sulfate reduction processes driven by organic contaminant oxidation and suppressed competitive methane-producing reactions. The application of a comprehensive suite of chemical, spectroscopic, biomolecular and thermodynamic analyses suggested that the snorkel served as a scavenger of toxic sulfide via a redox interaction with the iron cycle. Taken as a whole, the results of this work highlight a new strategy for controlling biological processes, such as bioremediation, through the manipulation of the electron flows in contaminated sediments.

Speciation and Transformation of Sulfur in Freshwater Sediments: a Case Study in Southwest China

Sulfur (S) is one of the most redox-sensitive elements and has a marked impact on the geochemical cycling of biogenic elements in freshwater sediments. Current understanding of the speciation of sedimentary S, and of the processes regulating it, is insufficient. In this study, the speciation and spatial variations of S and iron (Fe) in sediments (soils) from Lake Hongfeng, one of the largest freshwater lakes in Southwest China, were investigated using X-ray absorption near-edge structure (XANES) spectroscopy and diffusive gradient in thin film technique (DGT). The results show that S in sediments and soils was composed of seven fractions in different electronic oxidation states (EOSs), including (i) reduced S (R-S, G1, EOS = − 1), (ii) lowly oxidized S (LO-S, including G2-G5; EOS = 0, 0.5, 2, and 3.7), and (iii) highly oxidized S (HO-S, including G6 and G7; EOS = 5 and 6). Proportional differences of S speciation in sediments and soils indicated that HO-S is largely reduced to LO-S and R-S during depositional processes. The HO-S fraction decreased in the top surface sediments and then increased in the deeper layers, whereas the R-S fraction showed the opposite trend, suggesting that sulfate reduction and re-oxidation processes occurred in the sediments. High ratios of soluble Fe/S provided a favorable foundation for the reduction and burial of sedimentary S. The speciation and spatial variations of S in freshwater sediments are controlled by complex environmental factors, including terrigenous material discharges, water redox conditions, and porewater chemistry (such as for pH, Eh, and reactive Fe). Our study will help to deepen the understanding of the geochemical dynamics of S in the sediments of freshwater ecosystems.