Kinetics of Decomposition of Thiocyanate in Natural Aquatic Systems.

Rates of thiocyanate degradation were measured in waters and sediments of marine and limnic systems under various redox conditions, oxic, anoxic (nonsulfidic, nonferruginous, nonmanganous), ferruginous, sulfidic, and manganous, for up to 200-day period at micromolar concentrations of thiocyanate. The decomposition rates in natural aquatic systems were found to be controlled by microbial processes under both oxic and anoxic conditions. The Michaelis-Menten model was applied for description of the decomposition kinetics. The decomposition rate in the sediments was found to be higher than in the water samples. Under oxic conditions, thiocyanate degradation was faster than under anaerobic conditions. In the presence of hydrogen sulfide, the decomposition rate increased compared to anoxic nonsulfidic conditions, whereas in the presence of iron(II) or manganese(II), the rate decreased. Depending on environmental conditions, half-lives of thiocyanate in sediments and water columns were in the ranges of hours to few dozens of days, and from days to years, respectively. Application of kinetic parameters presented in this research allows estimation of rates of thiocyanate cycling and its concentrations in the Archean ocean.

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotopes.

It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.

Annual sulfur cycle in a warm monomictic lake with sub-millimolar sulfate concentrations.

BACKGROUND: We studied the annual variability of the concentration and isotopic composition of main sulfur species and sulfide oxidation intermediates in the water column of monomictic fresh-water Lake Kinneret. Sulfate concentrations in the lake are <1 mM and similar to concentrations that are proposed to have existed in the Paleoproterozoic ocean. The main goal of this research was to explore biogeochemical constrains of sulfur cycling in the modern low-sulfate fresh-water lake and to identify which processes may be responsible for the isotopic composition of sulfur species in the Precambrian sedimentary rocks. RESULTS: At the deepest point of the lake, the sulfate inventory decreases by more than 20% between March and December due to microbial sulfate reduction leading to the buildup of hydrogen sulfide. During the initial stages of stratification, sulfur isotope fractionation between sulfate and hydrogen sulfide is low (11.6 ‰) and sulfur oxyanions (e.g. thiosulfate and sulfite) are the main products of the incomplete oxidation of hydrogen sulfide. During the stratification and at the beginning of the lake mixing (July-December), the inventory of hydrogen sulfide as well as of sulfide oxidation intermediates in the water column increases and is accompanied by an increase in sulfur isotope fractionation to 30 ± 4 ‰ in October. During the period of erosion of the chemocline, zero-valent sulfur prevails over sulfur oxyanions. In the terminal period of the mixing of the water column (January), the concentration of hydrogen sulfide decreases, the inventory of sulfide oxidation intermediates increases, and sulfur isotope fractionation decreases to 20 ± 2 ‰. CONCLUSIONS: Sulfide oxidation intermediates are present in the water column of Lake Kinneret at all stages of stratification with significant increase during the mixing of the water column. Hydrogen sulfide inventory in the water column increases from March to December, and sharply decreases during the lake mixis in January. Sulfur isotope fractionation between sulfate and hydrogen sulfide as well as concentrations of sulfide oxidation intermediates can be explained either by microbial sulfate reduction alone or by microbial sulfate reduction combined with microbial disproportionation of sulfide oxidation intermediates. Our study of sulfur cycle in Lake Kinneret may be useful for understanding the range of biogeochemical processes in low sulfate oceans over Earth history.

Microbial consumption of zero-valence sulfur in marine benthic habitats.

Zero-valence sulfur (S°) is a central intermediate in the marine sulfur cycle and forms conspicuous accumulations at sediment surfaces, hydrothermal vents and in oxygen minimum zones. Diverse microorganisms can utilize S°, but those consuming S° in the environment are largely unknown. We identified possible key players in S° turnover on native or introduced S° in benthic coastal and deep-sea habitats using the 16S ribosomal RNA approach, (in situ) growth experiments and activity measurements. In all habitats, the epsilonproteobacterial Sulfurimonas/Sulfurovum group accounted for a substantial fraction of the microbial community. Deltaproteobacterial Desulfobulbaceae and Desulfuromonadales were also frequently detected, indicating S° disproportionation and S° respiration under anoxic conditions. Sulfate production from S° particles colonized in situ with Sulfurimonas/Sulfurovum suggested that this group oxidized S°. We also show that the type strain Sulfurimonas denitrificans is able to access cyclooctasulfur (S8), a metabolic feature not yet demonstrated for sulfur oxidizers. The ability to oxidize S°, in particular S8 , likely facilitates niche partitioning among sulfur oxidizers in habitats with intense microbial sulfur cycling such as sulfidic sediment surfaces. Our results underscore the previously overlooked but central role of Sulfurimonas/Sulfurovum group for conversion of free S° at the seafloor surface.

An improved pyrite pretreatment protocol for kinetic and isotopic studies.

BACKGROUND: Pyrite is one of the most abundant and widespread of the sulfide minerals with a central role in biogeochemical cycles of iron and sulfur. Due to its diverse roles in the natural and anthropogenic sulfur cycle, pyrite has been extensively studied in various experimental investigations of the kinetics of its dissolution and oxidation, the isotopic fractionations associated with these reactions, the microbiological processes involved, and the effects of pyrite on human health. Elemental sulfur (S0) is a common product of incomplete pyrite oxidation. Preexisting S0 impurities as unaccounted reaction products are a source of experimental uncertainty, as are adhered fine grains of pyrite and its oxidation products. Removal of these impurities is, therefore, desirable. A robust standardized pretreatment protocol for removal of fine particles and oxidation impurities from pyrite is lacking. Here we describe a protocol for S0 and fine particle removal from the surface of pyrite by rinsing in acid followed by repeated ultrasonication with warm acetone. RESULTS: Our data demonstrate the presence of large fractions of S0 on untreated pyrite particle surfaces, of which only up to 60% was removed by a commonly used pretreatment method described by Moses et al. (GCA 51:1561-1571, 1987). In comparison, after pretreatment by the protocol proposed here, approximately 98% S0 removal efficiency was achieved. Additionally, the new procedure was more efficient at removal of fine particles of adhered pyrite and its oxidation products and did not appear to affect the particle size distribution, the specific surface area, or the properties of grain surfaces. CONCLUSIONS: The suggested pyrite pretreatment protocol is more efficient in removal of impurities from pyrite grains, and provides multiple advantages for both kinetic and isotopic investigations of pyrite transformations under various environmental conditions.

Multiple sulfur isotopes fractionations associated with abiotic sulfur transformations in Yellowstone National Park geothermal springs.

BACKGROUND: The paper presents a quantification of main (hydrogen sulfide and sulfate), as well as of intermediate sulfur species (zero-valent sulfur (ZVS), thiosulfate, sulfite, thiocyanate) in the Yellowstone National Park (YNP) hydrothermal springs and pools. We combined these measurements with the measurements of quadruple sulfur isotope composition of sulfate, hydrogen sulfide and zero-valent sulfur. The main goal of this research is to understand multiple sulfur isotope fractionation in the system, which is dominated by complex, mostly abiotic, sulfur cycling. RESULTS: Water samples from six springs and pools in the Yellowstone National Park were characterized by pH, chloride to sulfate ratios, sulfide and intermediate sulfur species concentrations. Concentrations of sulfate in pools indicate either oxidation of sulfide by mixing of deep parent water with shallow oxic water, or surface oxidation of sulfide with atmospheric oxygen. Thiosulfate concentrations are low (<6 µmol L(-1)) in the pools with low pH due to fast disproportionation of thiosulfate. In the pools with higher pH, the concentration of thiosulfate varies, depending on different geochemical pathways of thiosulfate formation. The δ(34)S values of sulfate in four systems were close to those calculated using a mixing line of the model based on dilution and boiling of a deep hot parent water body. In two pools δ(34)S values of sulfate varied significantly from the values calculated from this model. Sulfur isotope fractionation between ZVS and hydrogen sulfide was close to zero at pH < 4. At higher pH zero-valent sulfur is slightly heavier than hydrogen sulfide due to equilibration in the rhombic sulfur-polysulfide - hydrogen sulfide system. Triple sulfur isotope ((32)S, (33)S, (34)S) fractionation patterns in waters of hydrothermal pools are more consistent with redox processes involving intermediate sulfur species than with bacterial sulfate reduction. Small but resolved differences in ∆(33)S among species and between pools are observed. CONCLUSIONS: The variation of sulfate isotopic composition, the origin of differences in isotopic composition of sulfide and zero-valent sulfur, as well as differences in ∆(33)S of sulfide and sulfate are likely due to a complex network of abiotic redox reactions, including disproportionation pathways.

Eutrophication leads to the formation of a sulfide-rich deep-water layer in Lake Sevan, Armenia.

Lake Sevan is a meso-eutrophic water body, which was severely impacted by anthropogenic level decrease, pollution and eutrophication during the last century. Starting in the 1970s, these processes resulted in the formation of an oxygen-depleted hypolimnion during summer-autumn stratification of the lake. In this work, we demonstrate for the first time that eutrophication of the lake leads not only to the full depletion of oxygen and nitrate in the hypolimnion but as well to the presence of sulfate-reducing microorganisms and toxic hydrogen sulfide. Concentrations of hydrogen sulfide in the hypolimnion of Major and Minor Sevan in October were as high as 9 and 39 µM, respectively. In October 2019, 66 % of lake's bottom was covered by sulfidic waters, while the fraction of sulfidic water volume reached 19 %. Values of δ34S for hypolimnetic sulfide are lower by only 7-12 ‰ compared to epilimnetic sulfate, while δ33S values of sulfide are similar to the δ33S values of sulfate. These isotopic fingerprints are not consistent with microbial sulfate reduction as the sole source of hydrogen sulfide in the hypolimnion. We attribute the formation of a sulfidic deep-water layer to a combination of microbial sulfate reduction in the water column and diffusion of hydrogen sulfide from the sediments.

Iron and sulfide nanoparticle formation and transport in nascent hydrothermal vent plumes.

Deep-sea hydrothermal vents are a significant source of dissolved metals to the global oceans, producing midwater plumes enriched in metals that are transported thousands of kilometers from the vent source. Particle precipitation upon emission of hydrothermal fluids controls metal speciation and the magnitude of metal export. Here, we document metal sulfide particles, including pyrite nanoparticles, within the first meter of buoyant plumes from three high-temperature vents at the East Pacific Rise. We observe a zone of particle settling 10-20 cm from the orifice, indicated by stable sulfur isotopes; however, we also demonstrate that nanoparticulate pyrite (FeS2) is not removed from the plume and can account for over half of the filtered Fe (≤0.2 µm) up to one meter from the vent orifice. The persistence of nanoparticulate pyrite demonstrates that it is an important mechanism for near-vent Fe stabilisation and highlights the potential role of nanoparticles in element transport.

Turnover Rates of Intermediate Sulfur Species ([Formula: see text], S0, S2[Formula: see text], S4[Formula: see text], [Formula: see text]) in Anoxic Freshwater and Sediments.

The microbial reduction of sulfate to sulfide coupled to organic matter oxidation followed by the transformation of sulfide back to sulfate drives a dynamic sulfur cycle in a variety of environments. The oxidative part of the sulfur cycle in particular is difficult to constrain because the eight electron oxidation of sulfide to sulfate occurs stepwise via a suite of biological and chemical pathways and produces a wide variety of intermediates ([Formula: see text], S0, S2[Formula: see text], S4[Formula: see text], and [Formula: see text]), which may in turn be oxidized, reduced or disproportionated. Although the potential processes affecting these intermediates are well-known from microbial culture and geochemical studies, their significance and rates in the environment are not well constrained. In the study presented here, time-course concentration measurements of intermediate sulfur species were made in amended freshwater water column and sediment incubation experiments in order to constrain consumption rates and processes. In sediment incubations, consumption rates were [Formula: see text] [Formula: see text] [Formula: see text] S4[Formula: see text] S2[Formula: see text], which is consistent with previous measurements of [Formula: see text], S4[Formula: see text], and S2[Formula: see text] consumption rates in marine sediments. In water column incubations, however, the relative reactivity was [Formula: see text] [Formula: see text] [Formula: see text] S2[Formula: see text] S4[Formula: see text]. Consumption of thiosulfate, tetrathionate and sulfite was primarily biological, whereas it was not possible to distinguish between abiotic and biological polysulfide consumption in either aqueous or sediment incubations. [Formula: see text] consumption in water column experiments was biologically mediated, however, rapid sedimentary consumption was likely due to reactions with iron minerals. These experiments provide important constraints on the biogeochemical reactivity of intermediate sulfur species and give further insight into the diversity of biological and geochemical processes that comprise (cryptic) environmental sulfur cycling.

Impact of Aeolian Dry Deposition of Reactive Iron Minerals on Sulfur Cycling in Sediments of the Gulf of Aqaba.

The Gulf of Aqaba is an oligotrophic marine system with oxygen-rich water column and organic carbon-poor sediments (≤0.6% at sites that are not influenced by anthropogenic impact). Aeolian dust deposition from the Arabian, Sinai, and Sahara Deserts is an important source of sediment, especially at the deep-water sites of the Gulf, which are less affected by sediment transport from the Arava Desert during seasonal flash floods. Microbial sulfate reduction in sediments is inferred from the presence of pyrite (although at relatively low concentrations), the presence of sulfide oxidation intermediates, and by the sulfur isotopic composition of sulfate and solid-phase sulfides. Saharan dust is characterized by high amounts of iron minerals such as hematite and goethite. We demonstrated, that the resulting high sedimentary content of reactive iron(III) (hydr)oxides, originating from this aeolian dry deposition of desert dust, leads to fast re-oxidation of hydrogen sulfide produced during microbial sulfate reduction and limits preservation of reduced sulfur in the form of pyrite. We conclude that at these sites the sedimentary sulfur cycle may be defined as cryptic.

Equilibrium distribution of polysulfide ions in aqueous solutions at different temperatures by rapid single phase derivatization.

Polysulfides are abundant form of reduced sulfur compounds whose distribution in aquatic systems continues to pose environmental challenge. The Gibbs free-energy of formation, enthalpy of formation, and standard entropy of inorganic polysulfides were derived based on measurements of the temperature-dependent distribution of inorganic polysulfides in supersaturated aqueous polysulfide solutions. The data complements the relevant Gibbs free-energy data that were derived in our recent publication. The thermodynamic data show that the average polysulfide length is increased and polysulfides dissolve better at elevated temperatures, though the extent of this increase is pH dependent. At high pH (pH > 10) increasing the temperature from 25 to 80 degrees C results in a 5.6% increase in the concentration of polysulfide bound sulfur (i.e., dissolved zerovalent sulfur) and increases the average chain length (n) by 0.2 sulfur atoms, whereas at pH 8.2 the n increases by 0.25, and the dissolved polysulfide sulfur increases threefold.

Sulfur stable isotope distribution of polysulfide anions in an (NH4)2Sn aqueous solution.

Methylation of polysulfides [(NH4)2Sn)] by reaction with CF3SO3CH3 followed by separation of the produced dimethylpolysulfides by liquid chromatography and subsequent highly accurate stable isotope analysis by a continuous-flow isotope ratio mass spectrometer shows that polysulfide anions in an aqueous solution exchange isotopes with the other sulfur species in the system. It demonstrates for the first time that polysulfide anions are 34S-enriched in equilibrium relative to total sulfur as a function of their sulfur chain length.

Method for the determination of inorganic polysulfide distribution in aquatic systems.

Inorganic polysulfides have significant technological importance, and their environmental role is gradually being unraveled. But despite their importance, there is still no method for quantification of the individual members of the polysulfide family in nonsynthetic samples. The method is based on fast, single-phase derivatization with methyl trifluoromethanesulfonate followed by one of three modes of sample treatment depending on polysulfide concentration. Under the most aggressive preconcentration treatment involving liquid-liquid extraction, solvent evaporation to dryness, dissolution in n-dodecane, and finally HPLC-UV analysis of the dimethylpolysulfane distribution, the minimum detection limits of the individual polysulfides are in the range 15-70 nM. The method was demonstrated for the analysis of synthetic solutions, natural groundwater, polysulfide fortified seawater, and surface water and for time tracing of the distribution of the individual polysulfides during the oxidation of hydrogen sulfide by hydrogen peroxide. The observed speciation was evaluated by comparison with the theoretical distribution of polysulfides at equilibrium with sulfur precipitate showing that the dominant polysulfides' (i.e., tetra- to hexasulfide) concentrations agree well with the predicted distribution (90% of the results fall within less than 30% deviation from the predicted values), whereas up to 3-fold deviation was observed for the less abundant trisulfide and octasulfide species.

Equilibrium distribution of polysulfide ions in aqueous solutions at 25 degrees C: a new approach for the study of polysulfides' equilibria.

A new approach based on rapid, chemical derivatization in a single phase was used to determine the disproportionation constants and the underlying thermodynamics of inorganic polysulfides in aqueous solutions. This method resolves the dispute over the existence of hexasulfide in aqueous solutions and establishes the presence of even higher polysulfide chains in water. The Gibbs free energies of formation (G(Sn)(o)2-) for the polysulfide species are 77.4, 71.6, 67.4, 66.1, 67.2, 70.5, and 73.6 kJ/mol for n = 2-8, respectively. Our approach is based on single phase, fast methylation of polysulfides with methyl trifluoromethanesulfonate (methyl triflate) and subsequent determination of the dimethylpolysulfides by HPLC. Two independent methods were used in order to confirm quantitative equivalence between the observed distribution of dimethylpolysulfides and the polysulfide distribution in the water: (i) Kinetic studies of each competing reaction step showed that the kinetics of the derivatization are faster than each of the competing reactions that may lead to disproportionation and deviation of the observed distribution of dimethylpolysulfides from that of the aqueous polysulfides. (ii) Determination of isotope mixing during the derivatization of a mixture of two solutions, one containing polysulfide of natural isotopic distribution and the second containing 34S-rich polysulfide revealed that polysulfide mixing during derivatization is rather low. The systematic error due to redistribution of pentasulfide during derivatization is 3% based on isotope dilution tests and less than 5% of total zero-valent sulfur based on kinetic considerations.