Importance of Sulfide, Polysulfides, and Elemental Sulfur for Abiotic and Biotic Redox Processes in Sulfur-Metal(loid)Systems
Sulfur, an ubiquitous element in the environment, occurs in different oxidation states from +6 to -2. Between the thermodynamically stable end members sulfate and sulfide, a variety of intermediate sulfur species exist, examples of which are sulfite, polythionates, thiosulfate, elemental sulfur, and polysulfides. Polysulfides are highly reducing and nucleophilic sulfur chains of the general structure Sn2-(n ≥ 2). Due to their high reactivity and instability, inorganic polysulfide analysis is challenging. Currently, the most reliable analytical approach is derivatization of inorganic polysulfides to form more stable organic polysulfanes, which can be analyzed chromatographically. Intermediate sulfur species in general and polysulfides in particular are assumed to be decisive for a variety of redox transformation processes of metal(loid)s but are only rarely analyzed. The aim of the present study was to investigate the role of sulfide, elemental sulfur, and especially polysulfides for abiotic and biotic redox processes in sulfur-metal(loid) systems. Open questions resulting from previous investigations concerning the interaction of different sulfur species with iron, arsenic, and molybdenum were addressed with special focus on the sulfur speciation. Elemental sulfur disproportionation is among the oldest metabolic pathways in earth’s history and still raises many questions. Growth of microorganisms by elemental sulfur disproportionation was found to depend on the presence of a sulfide scavenger such as ferric iron. In the present study, growth of haloalkaliphilic bacteria by elemental sulfur disproportionation was shown for the first time and was observed both, in the presence of iron, which is in accordance to previous studies, but also in the absence of iron. This was possible due to substantial formation of polysulfides under anoxic and alkaline conditions, which decreased free sulfide concentrations in solution and consequently rendered elemental sulfur disproportionation thermodynamically favorable. The reaction of dissolved sulfide with ferric (oxyhydr)oxides can result in the formation of thermodynamically stable pyrite, the most commonly occurring sulfide-bearing mineral. In former studies, different reaction pathways of pyrite formation were determined to occur in the aqueous phase. In the present study, polysulfides were found at the mineral surface during sulfidation of ferric (oxyhydr)oxides. Concentrations of disulfide, the dominating polysulfide species, increased with the reactivity of the iron minerals, which is also positively correlated to the kinetics of pyrite formation. Overall, it was concluded that surface-associated polysulfides play a decisive role as pyrite precursors. The reductive dissolution of ferric (oxyhydr)oxides is crucial with regard to the release of adsorbed nutrients or contaminants. It can be mediated indirectly by sulfur-reducing bacteria. Previously, thiosulfate, elemental sulfur, or polysulfides were proposed to serve as electron shuttles between bacteria and ferric minerals. We found elemental sulfur, attached to the mineral surface, as predominant sulfur oxidation product. Besides thiosulfate, tetrathionate, sulfite, and sulfide, polysulfides could initiate the electron shuttling process but were of minor importance for the shuttling process itself. Overall, the present study revealed a detailed insight into the role of different sulfur species during microbially mediated ferric mineral reduction. Soluble arsenic-sulfur species are crucial for the cycling of arsenic under sulfidic conditions. In former studies, trivalent thioarsenites were found to form by the reaction of arsenite with sulfide and to rapidly oxidize to pentavalent thioarsenates. The latter were suggested to form directly by the reaction of arsenite with polysulfides. In the present study, polysulfides were found to react with arsenite to form monothioarsenate. Moreover, the higher nucleophilicity of polysulfides in comparison to sulfide seemed to accelerate the formation of higher thiolated arsenates. The formation of polysulfides and monothioarsenate was also observed in biotic systems during growth of an anaerobic haloalkaliphile, which couples arsenate reduction with sulfide oxidation. Additionally, monothioarsenate was microbially disproportionated to arsenite and polysulfides. Confirming previous suggestions, polysulfides were found to play a crucial role for thioarsenate formation. Evidence for substantial microbial acceleration of thioarsenate transformation processes was found earlier. In the present study, monothioarsenate transformation was considerably faster in the presence of a hyperthermophile bacterium in comparison to abiotic conditions. Abiotically, monothioarsenate was determined to be desulfidized to form arsenate and sulfide, which in turn was oxidized to elemental sulfur and thiosulfate under high temperature and oxic conditions. The bacteria accelerated monothioarsenate transformation mainly by oxidizing the abiotically formed intermediate sulfur species to sulfate. In general, sulfur redox chemistry was found to be decisive for thioarsenate transformation processes. The formation of soluble thiomolybdate species was assumed to be crucial for molybdenum burial in sediments, an important indicator for reconstructing paleoredox conditions. However, up to now there is no evidence about thiomolybdate occurrence in the environment. In the laboratory, we found that rate and extent of thiomolybdate formation increased with increasing sulfide to molybdate excess and a pH of 7 was determined to be most favorable for the nucleophilic substitution reaction. Polysulfides did not have any influence on thiomolybdate formation. We optimized ion-pair chromatographic separation of thiomolybdates for coupling to an inductively coupled plasma-mass spectrometer to be able to analyze nanomolar thiomolybdate concentrations. Using this new method, spontaneous formation of thiomolybdates could be observed in euxinic marine waters after addition of a molybdate spike. Moreover, natural occurrence of thiomolybdates was detected for the first time in sulfidic geothermal waters. Overall, sulfide, elemental sulfur, and especially polysulfides were found to be a significant factor for a variety of abiotic and biotic transformation processes of the metal(loid)s iron, arsenic, and molybdenum. In general, sulfur speciation has a large impact on the speciation, reactivity, and mobility of the respective metal(loid)s. Extensive knowledge about the occurrence of different sulfur species and sulfur redox processes thus helps to understand the biogeochemical cycling of metal(loid)s in the environment.
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