Experimental Confirmation of Isotope Fractionation in Thiomolybdates Using Ion Chromatographic Separation and Detection by Multicollector ICPMS

Molybdenum ⁹⁸Mo/⁹⁵Mo isotope ratios are a sediment paleo proxy for the redox state of the ancient ocean. Under sulfidic conditions, no fractionation between seawater and sediment should be observed if molybdate (MoO₄^2–) is quantitatively transformed to tetrathiomolybdate (MoS₄^2–) and precipitated. However, quantum mechanical calculations previously suggested that incomplete sulfidation could be associated with substantial fractionation. To experimentally confirm isotope fractionation in thiomolybdates, a new approach for determination of isotope ratios of individual thiomolybdate species was developed that uses chromatography (HPLC-UV) to separate individual thiomolybdates, collecting each peak and analyzing isotope ratios with multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially available MoO₄^2– and MoS₄^2– standards, the method was evaluated and excellent reproducibility and accuracy were obtained. For species with longer retention times, complete chromatographic peaks had to be collected to avoid isotope fractionation within peaks. Isotope fractionation during formation of thiomolybdates could be experimentally proven for the first time in the reaction of MoO₄^2– with 20-fold or 50-fold excess of sulfide. The previously calculated isotope fractionation for MoS₄^2– was confirmed, and the result for MoO₂S₂^2– was in the predicted range. Isotopic fractionation during MoS₄^2– transformation with pressurized air was dominated by kinetic fractionation. Further optimization and online-coupling of the HPLC-MC-ICPMS approach for determination of low concentrations in natural samples will greatly help to obtain more accurate species-selective isotope information

Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic

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 (<i>Gola di Lago</i>, 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 Fe_solid 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 (As_aq = 0.45–7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of As_aq. Our findings show that freshwater flocs from the <i>Gola di Lago</i> 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

Occurrence of Surface Polysulfides during the Interaction between Ferric (Hydr)Oxides and Aqueous Sulfide

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

Thioarsenate Toxicity and Tolerance in the Model System Arabidopsis thaliana

Thioarsenates form from arsenite under sulfate-reducing conditions, e.g., in rice paddy soils, and are structural analogues of arsenate. Even though rice is one of the most important sources of human arsenic intake, nothing is published about uptake, toxicity, or tolerance of thioarsenates in plants. Experiments using the model system Arabidopsis thaliana showed that monothioarsenate is less toxic than arsenite, but more toxic than arsenate at concentrations ≥25 μM As, reflected in stronger seedling growth inhibition on agar plates. Despite higher toxicity, total As accumulation in roots was lower upon exposure to monothioarsenate compared to arsenate, and a higher root efflux was confirmed. Root–shoot translocation was higher for monothioarsenate than for arsenate. Compared to the wild type (Col-0), both arsenate and monothioarsenate induced higher toxicity in phytochelatin (PC)-deficient mutants (<i>cad1–3</i>) as well as in glutathione biosynthesis (<i>cad2</i>) and PC transport (<i>abcc12</i>) mutants, demonstrating the important role of the PC pathway, not only for arsenate, but also for monothioarsenate detoxification. In Col-0, monothioarsenate induced relatively higher accumulation of PCs than arsenate. The observed differences in plant uptake, toxicity, and tolerance of thioarsenate vs oxyarsenate show that studying the effects of As on plants should include experiments with thiolated As species