Polysulfides are often referred to as key reactants in the sulfur cycle, especially during the interaction of ferric (hydr)oxides and sulfide, forming ferrous-sulphide minerals. Despite their potential relevance, the extent of polysulfide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulfur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulfide at different initial Fe/S molar ratios under anoxic conditions at neutral pH. The higher reactivity of lepidocrocite leads to faster and higher electron turnover compared to goethite. We were able to demonstrate for the first time the occurrence of surface-associated polysulfides being the main oxidation products in the presence of both minerals, with a predominance of disulfide (S2(2-)(surf)), and elemental sulfur. Concentrations of aqueous polysulfide species were negligible (<1%). With prior sulfide fixation by zinc acetate, the surface-associated polysulfides could be precipitated as zerovalent sulfur (S°), which was extracted by methanol thereafter. Of the generated S°, 20-34% were associated with S2(2-)(surf). Varying the Fe/S ratio revealed that surface polysulfide formation only becomes dominant when the remaining aqueous sulfide concentration is low (<0.03 mmol L(-1)). We hypothesize these novel surface sulfur species, particularly surface disulfide, to act as pyrite precursors. We further propose that these species play an overlooked role in the sulfur cycle.

Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low-flow stream conditions, flocs may settle, become buried, and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study, we reacted freshwater flocs (704-1280 mg As/kg) from a minerotrophic peatland (Gola di Lago, Switzerland) with sulfide (5.2 mM, S(-II)spike/Fe = 0.75-1.62 mol/mol) at neutral pH and studied the speciation changes of Fe, S, and As at 25 ± 1 °C over 1 week through a combination of synchrotron X-ray techniques and wet-chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52-81% of Fesolid at day 7) as well as solid-phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite, although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide-reacted flocs contained primarily arsenate (47-72%) which preferentially adsorbed to Fe(III)-(oxyhydr)oxides, despite abundant mackinawite precipitation. At higher S(-II)spike/Fe molar ratios (≥1.0), the formation of an orpiment-like phase accounted for up to 35% of solid-phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III)-(oxyhydr)oxides, mobilization of As was recorded in all samples (Asaq = 0.45-7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of Asaq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate-reducing conditions and emphasize the pivotal role Fe-rich organic freshwater flocs play in trace metal(loid) cycling in S-rich wetlands characterized by oscillating redox conditions.

A novel chemolithotrophic metabolism based on a mixed arsenic-sulfur species has been discovered for the anaerobic deltaproteobacterium, strain MLMS-1, a haloalkaliphile isolated from Mono Lake, California, U.S. Strain MLMS-1 is the first reported obligate arsenate-respiring chemoautotroph which grows by coupling arsenate reduction to arsenite with the oxidation of sulfide to sulfate. In that pathway the formation of a mixed arsenic-sulfur species was reported. That species was assumed to be monothioarsenite ([H2As(III)S(-II)O2](-)), formed as an intermediate by abiotic reaction of arsenite with sulfide. We now report that this species is monothioarsenate ([HAs(V)S(-II)O3](2-)) as revealed by X-ray absorption spectroscopy. Monothioarsenate forms by abiotic reaction of arsenite with zerovalent sulfur. Monothioarsenate is kinetically stable under a wide range of pH and redox conditions. However, it was metabolized rapidly by strain MLMS-1 when incubated with arsenate. Incubations using monothioarsenate confirmed that strain MLMS-1 was able to grow (μ = 0.017 h(-1)) on this substrate via a disproportionation reaction by oxidizing the thio-group-sulfur (S(-II)) to zerovalent sulfur or sulfate while concurrently reducing the central arsenic atom (As(V)) to arsenite. Monothioarsenate disproportionation could be widespread in nature beyond the already studied arsenic and sulfide rich hot springs and soda lakes where it was discovered.

Microbial disproportionation of elemental sulfur to sulfide and sulfate is a poorly characterized part of the anoxic sulfur cycle. So far, only a few bacterial strains have been described that can couple this reaction to cell growth. Continuous removal of the produced sulfide, for instance by oxidation and/or precipitation with metal ions such as iron, is essential to keep the reaction exergonic. Hitherto, the process has exclusively been reported for neutrophilic anaerobic bacteria. Here, we report for the first time disproportionation of elemental sulfur by three pure cultures of haloalkaliphilic bacteria isolated from soda lakes: the Deltaproteobacteria Desulfurivibrio alkaliphilus and Desulfurivibrio sp. AMeS2, and a member of the Clostridia, Dethiobacter alkaliphilus. All cultures grew in saline media at pH 10 by sulfur disproportionation in the absence of metals as sulfide scavengers. Our data indicate that polysulfides are the dominant sulfur species under highly alkaline conditions and that they might be disproportionated. Furthermore, we report the first organism (Dt. alkaliphilus) from the class Clostridia that is able to grow by sulfur disproportionation.

Molybdenum precipitates preferentially under reducing conditions; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with <4% transformation over more than two months. High ionic strengths matrices (>2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters.