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Formation rates for various products of hydrogen sulfide oxidation by δ-MnO2 were studied as a function of pH, temperature, concentration of the reactants, and ionic strength at the environmentally-relevant conditions. The main goals of this work were studying the effects of these parameters on speciation of zero-valent sulfur, including individual polysulfides and revealing the mechanism of its formation. A reaction between hydrogen sulfide and manganese dioxide is the fastest environmentally-relevant abiotic process of polysulfides formation, reactive sulfur species, which participate in complexation of metals, pyrite formation, sulfurization of organic matter and may serve as a substrate for microorganisms. In the pH range of 7.0 to 10.5, zero-valent sulfur accounted for > 55% of the sulfur in the products of hydrogen sulfide oxidation, while at pH ≥ 7.5, the formation rates of polysulfide zero-valent sulfur were higher than those of particulate zero-valent sulfur formation. Speciation of polysulfides on the initial stage of the reaction shows significant shift toward the higher polysulfides compared to their speciation calculated under assumption of thermodynamic equilibrium in both H2S - Sn2− - S8(aq) - α-S8 and H2S - Sn2− - S8(aq) - S8(coll) systems. We suggest that the shift in the speciation of polysulfide system results from the presence of a short-living highly reactive sulfur atoms, which are formed on the first stage of the reaction: two-electron transfer from hydrogen sulfide to manganese dioxide. Results of this study provide constraints on the rates of polysulfide formation at the redox interfaces of the stratified aquatic systems, and allows quantitative assessment of their role in sulfur cycling.

Author(s): Eldridge, DL; Kamyshny, A; Farquhar, J | Abstract: Inorganic polysulfur compounds (polysulfides, Sx2-; polysulfur radical ions, Sx·-; thiosulfate, S2O32-; polythionate, SxO62-; elemental sulfur, e.g. S8) participate in numerous geochemical processes related to the sulfur cycle. These include authigenic pyrite formation in sediments undergoing early stages of diagenesis, reactions associated with magmatic-hydrothermal processes, and numerous other aquatic sulfur redox processes (e.g., pyrite and sulfide oxidation). Sulfur isotope fractionations among many of these and associated compounds (e.g., H2S, HSO4-) are either unknown or unconstrained over wide ranges of temperatures. We present theoretical estimates of equilibrium sulfur isotope fractionation factors among aqueous polysulfur compounds (including select polysulfides, polysulfur radical anions, and polythionates) and select aqueous sulfide and sulfate compounds that correspond to all three stable isotope ratios of sulfur (33S/32S, 34S/32S, 36S/32S). Our estimates are based on electronic structure calculations performed at the B3LYP/6–31+G(d,p) level of theory and basis set implemented in concert with an explicit solvation model whereby molecules are encapsulated in water clusters of varying size (30–52 H2O) to simulate the aqueous solvation environment. These calculations yield relatively small magnitude fractionation factors between aqueous polysulfides, polysulfur radicals, and reduced sulfur moieties in polythionates relative to the aqueous sulfide compounds but reveal numerous crossovers that result in non-intuitive temperature dependencies. Our predictions of 34S/32S-based fractionation factors among aqueous sulfur compounds generally agree with previous experimental constraints where available within estimated uncertainties (e.g., HSO4-/H2S(aq), H2S(aq)/HS-, HSO4-/S0, H2S(aq)/S0). We use our calculations to explore equilibrium isotope fractionations among polysulfur and sulfide compounds that are precursors to authigenic pyrite in the framework of established mechanisms (e.g., the polysulfide mechanism). We examine possible explanations for why pyrite formation may be associated with relatively small isotope fractionation with respect to precursor aqueous sulfur compounds. We additionally use our theoretical calculations to constrain multiple sulfur isotope (33S/32S, 34S/32S, 36S/32S) mass balance models associated with the abiotic hydrolytic disproportionation of intermediate sulfur compounds (SO2, S8, S3·-) relevant to hydrothermal-magmatic-volcanic systems in order to illustrate the potential for subtle but potentially resolvable effects expressed in values of Δ33S and Δ36S associated with these processes. We apply a SO2 disproportionation mass balance model based on previous work but newly constrained by our theoretical calculations to (hyper-) acid crater lakes associated with active volcanoes, and newly highlight the potential for the utility of multiple sulfur isotope analyses in volcanic gas monitoring and constraining sulfur cycling processes in such systems.

Phosphorite deposits in southern Israel alternate between low phosphorous pristine phosphorites and high phosphorous reworked phosphorites. Deposition of pristine phosphorites occurs at high productivity and sedimentation rates under suboxic to anoxic conditions, whereas deposition of reworked phosphorites has been attributed to oxidizing conditions at low sedimentation rates during high-energy episodes accompanied by bioturbation. In this study, iron speciation in primary and reworked phosphates was used for investigating paleoenvironmental conditions of the two phosphate facies. Studied phosphates include low iron sections (Zin and Rotem synclines, North Negev) and iron-rich phosphates deposit (Ein-Ofarim, North Arava). Iron speciation in different synclines provides information on diagenetic processes during phosphogenesis and reworking processes.Pristine phosphorites are associated with substantially higher total iron (FeT) and highly reactive iron (FeHR) contents compared to the reworked phosphorites. On the other hand, the FeHR to FeT ratio is higher in reworked phosphorites. The pyrite Fe (Fepy) fraction to FeHR ratio is diagnostic for non-sulfidic water conditions for all studied phosphorites. The combination of FeHR/FeT and Fepy/FeHR paleoredox proxies points to possibly anoxic to anoxic-ferruginous sedimentary settings for pristine phosphorites while reworked phosphorites seem to be formed under more anoxic conditions. However, this observation opposes the general concept of reworked facies formation under aerated conditions. As ferruginous conditions are implausible, an alternative explanation for the observed iron speciation could be external input (possibly aeolian dust) of iron-rich particles during initial phosphate. During the reworking processes, removal of poorly reactive iron-rich smectites during reworking processes, although both fractions were depleted by up to 90%. Thus, the observed differences in FeHR to FeT ratio between pristine and reworked phosphates were likely caused by physical processes rather than changes in environmental redox conditions.