Reduction Of Elemental Sulfur Research Articles

Biosulfidogenesis Mediates Natural Attenuation in Acidic Mine Pit Lakes.

Acidic pit lakes are abandoned open pit mines filled with acid mine drainage (AMD)—highly acidic, metalliferous waters that pose a severe threat to the environment and are rarely properly remediated. Here, we investigated two meromictic, oligotrophic acidic mine pit lakes in the Iberian Pyrite Belt (IPB), Filón Centro (Tharsis) (FC) and La Zarza (LZ). We observed a natural attenuation of acidity and toxic metal concentrations towards the lake bottom, which was more pronounced in FC. The detection of Cu and Zn sulfides in the monimolimnion of FC suggests precipitation of dissolved metals as metal sulfides, pointing to biogenic sulfide formation. This was supported by microbial diversity analysis via 16S rRNA gene amplicon sequencing of samples from the water column, which showed the presence of sulfidogenic microbial taxa in FC and LZ. In the monimolimnion of FC, sequences affiliated with the putative sulfate-reducing genus Desulfomonile were dominant (58%), whereas in the more acidic and metal-enriched LZ, elemental sulfur-reducing Acidianus and Thermoplasma spp., and disproportionating Desulfocapsa spp. were more abundant. Furthermore, the detection of reads classified as methanogens and Desulfosporosinus spp., although at low relative abundance, represents one of the lowest pH values (2.9 in LZ) at which these taxa have been reported, to our knowledge. Analysis of potential biomarker lipids provided evidence that high levels of phosphocholine lipids with mixed acyl/ether glycerol core structures were associated with Desulfomonile, while ceramide lipids were characteristic of Microbacter in these environments. We propose that FC and LZ function as natural bioremediation reactors where metal sulfide precipitation is mediated by biosulfidogenesis starting from elemental sulfur reduction and disproportionation at an early stage (LZ), followed by sulfate reduction at a later stage (FC).

Elemental sulfur-driven sulfidogenic process under highly acidic conditions for sulfate-rich acid mine drainage treatment: Performance and microbial community analysis

Elemental sulfur-driven sulfidogenic process has been demonstrated to be more economical and energy-efficient than sulfate-driven sulfidogenic process when treating metal-laden wastewater. In previous studies, we observed that the polysulfide-involved indirect sulfur reduction ensured the superiority of sulfur over sulfate as the electron acceptor in the sulfidogenic process under neutral or weak-alkaline conditions. However, realizing high-rate sulfur reduction process for acid mine drainage (AMD) treatment without pH amelioration is still a great challenge because polysulfide cannot exist under acidic conditions. In this study, a laboratory-scale sulfur-packed bed reactor was therefore continuously operated with a constant sulfate concentration (~1300 mg S/L) and decreasing pH from 7.3 to 2.1. After 400 days of operation, a stable sulfide production rate (38.2 ± 7.6 mg S/L) was achieved under highly acidic conditions (pH 2.6–3.5), which is significantly higher than those reported in sulfate reduction under similar conditions. In the presence of high sulfate content, elemental sulfur reduction could dominate over sulfate reduction under neutral and acidic conditions, especially when the pH ≥ 6.5 or ≤ 3.5. The decreasing pH significantly reduced the diversity of microbial community, but did not substantially influence the abundance of functional genes associated with organic and sulfur metabolisms. The predominant sulfur-reducing genera shifted from Desulfomicrobium under neutral conditions to Desulfurella under highly acidic conditions. The high-rate sulfur reduction under acidic conditions could be attributed to the combined results of high abundance of Desulfurella and low abundance of sulfate-reducing bacteria (SRB). Accordingly, sulfur reduction process can be developed to achieve efficient and economical treatment of AMD under highly acidic conditions (pH ≤ 3.5).

A Simple Elemental Sulfur Reduction Method for Isotopic Analysis and Pilot Experimental Tests of Symmetry‐Dependent Sulfur Isotope Effects in Planetary Processes

AbstractRecent insights into fundamental mechanisms underlying quadruple stable sulfur isotope (32S, 33S, 34S, and 36S) mass‐independent fractionation (S‐MIF) chemistry and potential implications for planetary processes highlight the urgent need of conducting laboratory experiments to delineate the chemical physics of S‐MIF. Elemental sulfur (S0), a ubiquitous component in the atmospheres of early Earth and Mars, is a major product or reactant in most experiments. Developing different chemical protocols for isotopic analysis is consequently of utility. The reduction of S0 to hydrogen sulfide (H2S) is the first step of the fluorination method for isotopic analysis, but existing S0 reduction methods require relatively long chemical reagent preparation time. Here we present an operationally simple and rapid method for reducing S0 to H2S directly by “Thode solution.” External uncertainties of our method for δ34S, Δ33S, and Δ36S measurements (associated with reduction, fluorination, purification, and mass spectrometer analysis) are 0.3‰, 0.01‰, and 0.2‰, respectively, comparable with traditional methods. This new technique was used to determine quadruple stable sulfur isotope compositions of product S0 in a set of pilot experiments designed to investigate possible symmetry‐dependent isotope effects in planetary‐relevant elemental sulfur recombination reactions. Differences in Δ33S and Δ36S between initial and produced S0 are slightly larger than analytical errors, shedding new light into the role of sulfur recombination reactions in S‐MIF. Our study offers a practical approach for measuring multiple isotopic compositions of S0 in laboratory and natural samples and creates new opportunities for deepening understanding of S‐MIF signatures in Archean rocks and Martian meteorites.

Distribution of Acidophilic Microorganisms in Natural and Man-made Acidic Environments.

Acidophilic microorganisms can thrive in both natural and man-made environments. Natural acidic environments comprise hydrothermal sites on land or in the deep sea, cave systems, acid sulfate soils and acidic fens, as well as naturally exposed ore deposits (gossans). Man-made acidic environments are mostly mine sites including mine waste dumps and tailings, acid mine drainage and biomining operations. The biogeochemical cycles of sulfur and iron, rather than those of carbon and nitrogen, assume centre stage in these environments. Ferrous iron and reduced sulfur compounds originating from geothermal activity or mineral weathering provide energy sources for acidophilic, chemolithotrophic iron- and sulfur-oxidizing bacteria and archaea (including species that are autotrophic, heterotrophic or mixotrophic) and, in contrast to most other types of environments, these are often numerically dominant in acidic sites. Anaerobic growth of acidophiles can occur via the reduction of ferric iron, elemental sulfur or sulfate. While the activities of acidophiles can be harmful to the environment, as in the case of acid mine drainage, they can also be used for the extraction and recovery of metals, as in the case of biomining. Considering the important roles of acidophiles in biogeochemical cycles, pollution and biotechnology, there is a strong need to understanding of their physiology, biochemistry and ecology.

Removal of heavy metals using a novel sulfidogenic AMD treatment system with sulfur reduction: Configuration, performance, critical parameters and economic analysis

A novel sulfidogenic acid mine drainage (AMD) treatment system with a sulfur reduction process was developed. During the 220-d operation, >99.9% of 380-mg/L ferric, 150-mg/L aluminum, 110-mg/L zinc, 20-mg/L copper and 2.5-mg/L lead ions, and 42.6–44.4% of 100-mg/L manganese ions in the synthetic AMD were step-by-step removed in the developed system with three pre-posed metal precipitators and a sulfur reduction reactor. Among them, zinc, copper and lead ions were removed by the biogenic hydrogen sulfide that produced through elemental sulfur reduction; while ferric, aluminum and manganese ions were removed by the alkali precipitation. Compared with the reported sulfate reduction reactors, the sulfur reduction reactor significantly reduced the chemical cost by 25.6–78.9% for sulfide production, and maintained a high sulfide production rate (1.12 g S2-/L-d). The pH level in the sulfidogenic reactor driven by sulfur-reducing bacteria posed a significant effect on the sulfide production rate. Under a nearly neutral condition (pH 7.0–7.5), elemental sulfur dissolved into polysulfide to increase the bioavailability of S0. At acidic conditions (pH < 6.0), polysulfide formation was limited and sulfate reduction became dominant. Therefore, maintaining the sulfidogenic reactor driven by sulfur-reducing bacteria at neutral condition is essential to realize high-rate and low-cost AMD treatment. Moreover, the escape of residual hydrogen sulfide from the system was eliminated by employing a 17% recirculation from effluent to the sulfidogenic reactor.

One-step electrosynthesis of CdS quantum dots stabilized by babassu oil and luminescent films deposited by DoD technology

The electrochemical synthesis of CdS quantum-dots was carried out in a one-step electrochemical procedure using babassu oil (BO) as stabilizer. The electrochemical reduction of elemental sulfur in DMSO was investigated by cyclic voltammetry, showing the formation of polysulfides and S2−, such results are in agreement to literature. Thus, the CdS-BO electrosynthesis was realized in DMSO solvent and 30 mA constant current in cavity cell. The cadmium and sulfide ions were locally generated by simultaneous oxidation of a cadmium bar (sacrificial anode), and reduction of elemental sulfur in a graphite powder macroelectrode. The CdS-BO quantum-dots showed a UV–Vis absorption band at λmax = 387 nm with symmetrical emission band at λem = 547 nm, with a full width at half maximum (FWHM) of 124 nm. Lauric (C12H24O2, 48.9%) and myristic (C14H28O2, 20.4%) acids were identified as major components of the BO, in addition, 2.4% of free fatty acids were identified in the BO composition by titration, which explains the good stabilization of CdS QDs synthesized. The CdS-BO colloidal system was used as photonic ink and deposited on silica sheets by inkjet printing by drop on demand technology, without waste generation at all stages. The nanoparticles preserved luminescence after printing and drying, showing that the BO promotes an effective stabilization of the quantum dots.

A Fundamental Understanding of Sulphur Speciation in Ionic Liquids By in-Situ UV-Vis Method

Ionic liquids (ILs) are liquids at room temperature which consist of a large variety of cation and anion. The dipolar and van der waals interactions between the cationic and anionic species govern the physical properties of ILs and their applications. Since last few decades, the potential application of ILs in lithium based battery has increased rapidly as some of the ILs offer wide electrochemical stability window, lithium redox compatibility, fast ionic diffusivity and low flammability [1]. Among other Li based battery systems, lithium sulphur (Li-S) battery is one of the most promising candidates for the next generation high energy density battery with high theoretical capacity of 1675 mAh/g [2]. However, the electrochemical redox reaction in Li-S battery is a complex phenomenon. The reduction of elemental sulphur (S8) proceeds via the formation of various intermediate (Sn 2- , n>2) polysulphides before reaching the final product S2-. The stability of these polysulphides are highly dependent on the electrolyte medium [3]. Hence the selection of appropriate electrolyte components play an important role in the overall performance of Li-S battery. The electrochemistry of sulphur-polysulphide system has been well studied in organic solvents, however very less literature is reported on the sulphur reduction mechanism in ILs. In this study, we have investigated the sulphur reduction and speciation in three ionic liquids: N-methyl-N-propylpyrrolidinium bis(fluorosulphonyl)imide (Pyr13 FSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide (Pyr13 TFSI) , trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4 FSI). The electrochemical redox reactions of sulphur in these ILs have been studied through cyclic voltammetry (CV) and the polysulphide intermediates are simultaneously investigated by in-situ UV Vis method. These studies showed two reductions and one oxidation peak and the plausible pathways have also been identified. The redox behaviour of sulphur speciation in P111i4 FSI is observed to be very similar to Pyr13 TFSI where glassy carbon is used as working electrode in a three electrode electrochemical cell. On the other hand, the redox process of Pyr13 FSI is found to be distinctively different than Pyr13 TFSI. From the in operando UV-vis study, it is found that Pyr13 FSI electrolyte is responsible for the formation of S3.- radical anion, whereas the Pyr13 TFSI and P111i4FSI electrolyte do not assist the radical formation during the reduction process. Additionally it can also be stated that, in case of P111i4 FSI, the polysulphides are stabilised by small phosphonium cation which protects the FSI anion from the nucleophilic attack of polysulphides, resulting in the stable sulphur redox cycling in P111i4FSI. Thus, based on molecular reactivity, CV and in-situ UV-vis, we propose that a novel phosphonium cation based ionic liquid (P111i4 FSI) is capable of using in Li-S battery. Therefore, this study brings forward a new concept that both of the cation and anion of an IL cumulatively affect the redox behaviour and sulphur speciation in Li-S battery systems. [1] Hallett, J. P.; Welton, T., Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chemical Reviews 2011, 111 (5), 3508-3576. [2] Ji, X.; Nazar, L. F., Advances in Li-S batteries. Journal of Materials Chemistry 2010, 20 (44), 9821-9826. [3] Zhang, S. S., Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions. Journal of Power Sources 2013, 231 (Supplement C), 153-162.

Three-dimensional interconnected cobalt sulfide foam: Controllable synthesis and application in supercapacitor

Transition metal sulfides have been considered as the promising electroactive materials for the supercapacitors with high specific capacitances. Herein, cobalt sulfide foam (CSF) with three-dimensional (3D) porous hierarchical structure was controllably synthesized on the electrode by the facile electrochemical reduction of elemental sulfur in the precursor of sulfur incorporated polypyrrole (S/PPy) composite with a electrochemical reduction method followed with in-situ reacting with Co2+ ions in the electrolyte, and used for high-performance supercapacitors. Since such a CSF material provided smooth ion diffusion pathways for the fast ion transfer between electrode and electrolyte and simultaneously the anchoring of conductive PPy to the cross-linked cobalt sulfide (CoS) nanosheets enabled good electrical contact between the current collector and the active material for the fast electron transfer. By synergetic combination of these advantages, the optimum electrode delivered exceptional rate capability (84.8% capacity retention from 1 to 20 mA cm−2) and excellent cycling stability (86.8% capacity retention after 10000 cycles). An asymmetric supercapacitor assembled with the CSF as the positive electrode and activated carbon as the negative electrode displayed outstanding electrochemical performance. It is expected to provide a green and simple method for the preparation of transition metal sulfide-based electrodes for the practical energy storage.

One-step electrodeposition of cauliflower-like CozNiySx@polypyrrole electrocatalysts on carbon fiber paper for hydrogen evolution reaction

Abstract Transition-metal chalcogenides as the promising alternatives to noble-metal-based electrocatalysts for hydrogen evolution reaction (HER) with high activity and durability in water splitting have attracted extensive attention in recent years. Herein, CozNiySx@PPy composites with three-dimensional (3D) cauliflower-like were firstly prepared on carbon fiber paper (CFP) via a simple and efficient electrochemical reduction of elemental sulfur in the precursor of S@PPy composite coated on CFP to react with Co and Ni ions in the electrolyte. The optimum electrode, i.e., CozNiySx@PPy/CFP-6 (A-6) prepared by using an electrolyte with a Co/Ni molar ratio of 0/6, showed excellent catalytic activity (with an overpotential of 185 mV@10 mA cm−2 and a small Tafel slope of 78.13 mV dec−1) as well as long-term stability (at least 100 h) in 1 M KOH solutions. This work provides a novel way to fabricate effective and non-noble-metal electrodes for HER in water splitting.

Purification of a Crenarchaeal ATP Synthase in the Light of the Unique Bioenergetics of Ignicoccus Species.

In this study, the ATP synthase of Ignicoccus hospitalis was purified, characterized, and structurally compared to the respective enzymes of the other Ignicoccus species, to shed light on energy conservation in this unique group of archaea. The crenarchaeal genus Ignicoccus comprises three described species, i.e., I. hospitalis and Ignicoccus islandicus from hot marine sediments near Iceland and Ignicoccus pacificus from a hydrothermal vent system in the Pacific Ocean. This genus is unique among all archaea due to the unusual cell envelope, consisting of two membranes that enclose a large intermembrane compartment (IMC). I. hospitalis is the best studied member of this genus, mainly because it is the only known host for the potentially parasitic archaeon Nanoarchaeum equitansI. hospitalis grows chemolithoautotrophically, and its sole energy-yielding reaction is the reduction of elemental sulfur with molecular hydrogen, forming large amounts of hydrogen sulfide. This reaction generates an electrochemical gradient, which is used by the ATP synthase, located in the outer cellular membrane, to generate ATP inside the IMC. The genome of I. hospitalis encodes nine subunits of an A-type ATP synthase, which we could identify in the purified complex. Although the maximal in vitro activity of the I. hospitalis enzyme was measured around pH 6, the optimal stability of the A1AO complex seemed to be at pH 9. Interestingly, the soluble A1 subcomplexes of the different Ignicoccus species exhibited significant differences in their apparent molecular masses in native electrophoresis, although their behaviors in gel filtration and chromatography-mass spectrometry were very similar.IMPORTANCE The Crenarchaeota represent one of the major phyla within the Archaea domain. This study describes the successful purification of a crenarchaeal ATP synthase. To date, all information about A-type ATP synthases is from euryarchaeal enzymes. The fact that it has not been possible to purify this enzyme complex from a member of the Crenarchaeota until now points to significant differences in stability, possibly caused by structural alterations. Furthermore, the study subject I. hospitalis has a particular importance among crenarchaeotes, since it is the only known host of N. equitans The energy metabolism in this system is still poorly understood, and our results can help elucidate the unique relationship between these two microbes.

Kinetics of elemental sulfur reduction by petroleum hydrocarbons and the implications for hydrocarbon thermal chemical alteration

Abstract Although sulfur-containing compounds are known to play a significant role in the diagenic and catagenic processes that generate oil and gas, relatively little is known about the kinetics of reactions between elemental S and petroleum hydrocarbons. To investigate this subject, a series of closed-system pyrolysis experiments using paraffin, a low-sulfur oil, and a high-sulfur oil with and without elemental S were conducted, and first-order chemical kinetics were fit to the experimental results. The average value for the activation energy required to reduce elemental S to H2S and to thermochemically alter higher molecular weight hydrocarbons to methane was calculated to be 193 kJ mol−1 (46 kcal mol−1). The results of this study demonstrate that under typical geologic conditions the rate of reduction of elemental S to H2S by petroleum hydrocarbons is quite rapid. The maximum time for substantial amounts of elemental S to persist in contact with petroleum hydrocarbons is estimated to be no more than a few million years in cool reservoirs (e.g., 120 °C) the half-life of elemental S may be as short as hundreds of years. Additionally, the presence of elemental S substantially lowers the onset temperature of hydrocarbon thermal chemical alteration (TCA). The activation energy for TCA of a low-sulfur oil to generate methane is estimated to be lowered by 92 kJ mol−1 (22 kcal mol−1) due to the presence of elemental sulfur. Consequently, the presence of elemental S in petroleum reservoirs is expected to lower the thermal stability of oil and decrease the maximum depth at which oil occurs within a basin (thermal deadline). The observed acceleration of hydrocarbon TCA is possibly due to organic sulfur compounds (e.g., thiols and sulfides) that form through the reaction of H2S or polysulfides with hydrocarbons and subsequently thermally degrade leading to the formation of sulfur radicals that in turn enhance TCA reactions.

Titanocene Selenide Sulfides Revisited: Formation, Stabilities, and NMR Spectroscopic Properties.

[TiCp2S5] (phase A), [TiCp2Se5] (phase F), and five solid solutions of mixed titanocene selenide sulfides [TiCp2SexS5−x] (Cp = C5H5−) with the initial Se:S ranging from 1:4 to 4:1 (phases B–E) were prepared by reduction of elemental sulfur or selenium or their mixtures by lithium triethylhydridoborate in thf followed by the treatment with titanocene dichloride [TiCp2Cl2]. Their 77Se and 13C NMR spectra were recorded from the CS2 solution. The definite assignment of the 77Se NMR spectra was based on the PBE0/def2-TZVPP calculations of the 77Se chemical shifts and is supported by 13C NMR spectra of the samples. The following complexes in varying ratios were identified in the CS2 solutions of the phases B–E: [TiCp2Se5] (51), [TiCp2Se4S] (41), [TiCp2Se3S2] (31), [TiCp2SSe3S] (36), [TiCp2SSe2S2] (25), [TiCp2SSeS3] (12), and [TiCp2S5] (01). The disorder scheme in the chalcogen atom positions of the phases B–E observed upon crystal structure determinations is consistent with the spectral assignment. The enthalpies of formation calculated for all twenty [TiCp2SexS5−x] (x = 0–5) at DLPNO-CCSD(T)/CBS level including corrections for core-valence correlation and scalar relativistic, as well as spin-orbit coupling contributions indicated that within a given chemical composition, the isomers of most favourable enthalpy of formation were those, which were observed by 77Se and 13C NMR spectroscopy.

Elemental sulfur reduction in the deep-sea vent thermophile, Thermovibrio ammonificans.

The reduction of elemental sulfur is an important energy-conserving pathway in prokaryotes inhabiting geothermal environments, where sulfur respiration contributes to sulfur biogeochemical cycling. Despite this, the pathways through which elemental sulfur is reduced to hydrogen sulfide remain unclear in most microorganisms. We integrated growth experiments using Thermovibrio ammonificans, a deep-sea vent thermophile that conserves energy from the oxidation of hydrogen and reduction of both nitrate and elemental sulfur, with comparative transcriptomic and proteomic approaches, coupled with scanning electron microscopy. Our results revealed that two members of the FAD-dependent pyridine nucleotide disulfide reductase family, similar to sulfide-quinone reductase and to NADH-dependent sulfur reductase (NSR), respectively, are over-expressed during sulfur respiration. Scanning electron micrographs and sulfur sequestration experiments indicated that direct access of T. ammonificans to sulfur particles strongly promoted growth. The sulfur metabolism of T. ammonificans appears to require abiotic transition from bulk elemental sulfur to polysulfide to nanoparticulate sulfur at an acidic pH, coupled to biological hydrogen oxidation. A coupled biotic-abiotic mechanism for sulfur respiration is put forward, mediated by an NSR-like protein as the terminal reductase.

A novel elemental sulfur reduction and sulfide oxidation integrated process for wastewater treatment and sulfur recycling

Sulfidogenic processes have been successfully used in saline wastewater treatment for sludge minimization, but are inapplicable in treating sulfate-deficit wastewater. This study reported a novel internal sulfur cycling (ISC) process for sulfate-deficit wastewater treatment. The process consisted of a sulfur-reducing reactor (SRR) for organics removal, followed by a sulfide-oxidizing reactor (SOR) and sedimentation tank for sulfur recovery. Under different hydraulic retention times and organic loading rates, the performance of ISC system was evaluated. The lab-scale ISC system removed 94% of chemical oxygen demand (COD), of which 81% were accomplished in the SRR by sulfur reducers without excessive sludge withdrawal throughout the 200 days of operation. The produced sulfide were mainly re-oxidized back to elemental sulfur in the SOR by sulfide-oxidizing bacteria, and sulfur balance demonstrates that 76% of sulfur recycling were achieved. Cost-benefit analysis reveals that the ISC process is a more cost-effective sludge-minimized biotechnology for sulfate-deficit wastewater treatment compared to conventional activated sludge processes.

Obligate sugar oxidation in Mesotoga spp., phylum Thermotogae, in the presence of either elemental sulfur or hydrogenotrophic sulfate-reducers as electron acceptor.

Mesotoga prima strain PhosAc3 is a mesophilic representative of the phylum Thermotogae comprising only fermentative bacteria so far. We show that while unable to ferment glucose, this bacterium is able to couple its oxidation to reduction of elemental sulfur. We demonstrate furthermore that M. prima strain PhosAc3 as well as M. prima strain MesG1 and Mesotoga infera are able to grow in syntrophic association with sulfate-reducing bacteria (SRB) acting as hydrogen scavengers through interspecies hydrogen transfer. Hydrogen production was higher in M. prima strain PhosAc3 cells co-cultured with SRB than in cells cultured alone in the presence of elemental sulfur. We propose that the efficient sugar-oxidizing metabolism by M. prima strain PhosAc3 in syntrophic association with a hydrogenotrophic sulfate-reducing bacterium can be extrapolated to all members of the Mesotoga genus. Genome comparison of Thermotogae members suggests that the metabolic difference between Mesotoga and Thermotoga species (sugar oxidation versus fermentation) is mainly due to the absence of the bifurcating [FeFe]-hydrogenase in the former. Such an obligate oxidative process for using sugars, unusual within prokaryotes, is the first reported within the Thermotogae. It is hypothesized to be of primary ecological importance for growth of Mesotoga spp. in the environments that they inhabit.

Elemental sulfur reduction to H2S by Tetrahymena thermophila

Eukaryotic nucleocytoplasm is believed to be descended from ancient Archaea that respired on elemental sulfur. If so, a vestige of sulfur reduction might persist in modern eukaryotic cells. That was tested in Tetrahymena thermophila, chosen as a model organism. When oxygenated, the cells consumed H2S rapidly, but when made anoxic they produced H2S mostly by amino acid catabolism. That could be inhibited by adding aminooxyacetic acid, and then H2S production from elemental sulfur became more evident. Anoxic cell lysates produced H2S when provided with sulfur and NADH, but not with either substrate alone. When lysates were fractionated by centrifugation, NADH-dependent H2S production was 83% in the soluble fraction. When intact cells that had just previously oxidized H2S were shifted to anoxia, the cells produced H2S evidently by re-using the oxidized sulfur. After aerobic H2S oxidation was stopped, the oxidation product remained available for H2S production for about 10 min. The observed H2S production is consistent with an evolutionary relationship of nucleocytoplasm to sulfur-reducing Archaea. Mitochondria often are the cellular site of H2S oxidation, suggesting that eukaryotic cells might have evolved from an ancient symbiosis that was based upon sulfur exchange.

A high-rate sulfidogenic process based on elemental sulfur reduction: Cost-effectiveness evaluation and microbial community analysis

Biological sulfur reduction is an attractive technology for the treatment of metal-laden wastewater, but its efficiency and cost have been questioned due to the insolubility of sulfur. In this study, a laboratory-scale sulfur-reducing bioreactor was constructed to investigate the long-term feasibility of high-rate sulfur reduction. Our results show that 316±46mg S/L sulfide was produced within 3h, corresponding to a high sulfide production rate of 32±5mg S/L-h, significantly higher than those reported in sulfate-reducing systems. Sulfur reduction processes can significantly reduce the total operational cost compared to sulfate reduction processes. Moreover, long-term sulfur feeding significantly shaped the microbial communities. The predominance of sulfate-reducing genera (24.1%) diminished continuously during the 110days of operation, and sulfur reducers such as Geobacter and Clostridium became dominant at the end of experiment. All of these findings suggest that high-rate sulfur reduction processes driven by sulfur reducers can be a cost-effective alternative for metal-laden wastewater treatment.

(Invited) Multi-Electron Reduction of Perfluorinated Gases: The 8-Electron Sulfur Hexafluoride System As a Model Reaction

High-potential, high-capacity redox reactions are of fundamental interest in the development of novel primary and secondary battery chemistries. To date, the most widely explored “advanced” nonaqueous cathode reactions for Li or Na batteries involve two-electron transfers: nonaqueous O2 reduction in Li-O2 batteries (2 e-/O2) and reduction of elemental sulfur (16 e-/S8 or a net 2 e-/S), both of which are based on redox with the chalcogen family [1-3]. In this talk, we report on redox of molecules from a different family: those containing halogen ligands, with a particular focus on the model perfluorinated gas, sulfur hexafluoride (SF6) as an illustrative example. Perfluorinated gases in general, and SF6 in particular, are conventionally described as highly stable [4-5], yet can yield high thermodynamic potentials vs. Li metal with large theoretical numbers of electrons transferred (e.g. SF6 + 8Li --> 6LiF + Li2S, Eo r = 3.69 V vs. Li/Li+) [6]. In recent work, we have developed a Li-SF6 primary battery that employs a gas-to-solid reaction at the positive electrode, analogous in some aspects to a Li-O2 battery that similarly operates on a dissolved-gas cathode reaction. In such a configuration, we have demonstrated that SF6, which is virtually chemically inert against Li metal at room temperature, can be activated at an electrified interface consisting of a simple carbon electrode in a nonaqueous electrolyte (0.3 M LiClO4 in tetraethyleneglycol dimethyl ether (TEGDME)). Upon activation, SF6 readily undergoes continued reduction and reaction with Li+ ions, resulting in the hypothesized 8-electron reduction of the central S6+ to Li2S and the concurrent expulsion of all fluoride ligands to form stoichiometric LiF. The reaction stoichiometry has been validated using a suite of techniques, including pressure-coupled discharge measurements, solid phase analysis (XRD, FTIR and XPS), liquid-phase analysis (19F and 1H NMR), and electrochemical characterization. The reduction potential, which varies from ~2.0 – 2.4 V vs. Li/Li+depending on the electrolyte solvent, reflects significant overpotentials (~1 V) which are unusual among the currently studied “advanced” multi-electron reactions. In this talk, we will examine the behavior of the Li-SF6 reduction reaction as a function of parameters including electrolyte solvent, anion composition, and salt concentration, in order to gain mechanistic insight into the dramatic SF6 gas-to-solid phase transformation. Specifically, we characterize the electrolyte-dependence of the reduction onset potential, electron consumption and Coulombic efficiency, capacity, and nature of the solid phase (chemistry and morphology) and discuss the degree of “tunability” achievable by tailoring the reaction environment. Possible descriptors governing the growth process of the solid phase (LiF), resulting morphology, and corresponding capacity will be examined and compared to the theory developed in a sister field, that of O2-to-Li2O2 transformations in Li-O2 batteries [7-8]. In addition, we discuss current understanding of the origin of such large overpotentials involving “inert” molecules in electrochemical systems, and outline challenges to be addressed in the development of a practical system. The possibility to broaden this reaction class to include potentially reversible reactions based on halogenated molecules will be discussed. [1] Lu, Y. C., Gallant, B. M., Kwabi, D. G., Harding, J. R., Mitchell, R. R., Whittingham, M. S. & Shao-Horn, Y. Energ Environ Sci 6, 750-768, (2013). [2] Aurbach, D., McCloskey, B. D., Nazar, L. F. & Bruce, P. G. Nature Energy 1, 16128-16139 (2016). [3] Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J. M. Nat Mater 11, 19-29, (2012). [4] Seppelt, K. Chem Rev 115, 1296-1306, (2015). [5] Zamostna, L., Braun, T. & Braun, B. Angew Chem Int Edit 53, 2745-2749, (2014). [6] Groff, E. G. & Faeth, G. M. Journal of Hydronautics 12, 63-70 (1978). [7] Aetukuri, N. B., McCloskey, B. D., Garcia, J. M., Krupp, L. E., Viswanathan, V. & Luntz, A. C. Nat Chem 7, 50-56, (2015). [8] Johnson, L., Li, C. M., Liu, Z., Chen, Y. H., Freunberger, S. A., Ashok, P. C., Praveen, B. B., Dholakia, K., Tarascon, J. M. & Bruce, P. G. Nat Chem 6, 1091-1098, (2014). Figure 1

Polysulfide Trapping Agent (PTA) Coated Directly Doped Sulfur Architectures (DDSA) – New Platforms for High Capacity Li – S Batteries

Increasing demands for high energy and high power storage systems for use in electric vehicles and electronic devices has made it imperative to develop high performance rechargeable batteries. This stringent need has attracted widespread attention worldwide. Lithium – sulfur (Li – S) batteries has been consequently, extensively studied recently as a potential rechargeable electrochemical power sources due to its high theoretical capacity (1672 Ah kg−1 sulfur)1 and high energy density (2500 Wh kg−1 sulfur)2 based on the complete reduction of elemental sulfur to form the end product of lithium sulfide (Li2S). However, commercialization pathway for Li – S batteries is still fraught with high self – discharge and the short cycle life problems of the Li – S rechargeable system. These limitations are largely related to the poor ionic and electronic conductivity of elemental sulfur3, formation of a series of sulfur reduction intermediates during discharge (Li2Sx, x > 2)4 , 5 that are highly soluble in organic electrolyte and the high reactivity of the dissolved polysulfides with the Li anode4. Several approaches including the use of a conductive additive6, encasing sulfur into conductive porous host materials7 and replacing the organic electrolyte with a polymer electrolyte8 has been shown to improve the performance by improving the ionic conductivity of sulfur and reducing polysulfide dissolution by avoiding direct contact with the liquid electrolyte. However, despite these advances, complete prevention of polysulfide dissolution has not yet been accomplished due to lack of fundamental understanding about the mechanism of polysulfide dissolution. In this work, directly doped sulfur architectures (DDSA) with sulfur loadings greater than 16mg/cm2 were created using simple electrochemical techniques. The cathodes were studied chemically and electrochemically to understand the mechanism of polysulfide dissolution in these structures. The DDSA electrodes are then coated with a polysulfide trapping agent (PTA) to chemically prevent the dissolution of polysulfide species. These PTA coated DDSA showed a high initial capacity of 1305 mAhg-1 with stable performance of 1208 mAhg-1 for over 30 cycles (Figure 1). Mechanism of polysulfide dissolution by PTA will be presented and discussed. Figure 1

In operando infrared spectroscopy of lithium polysulfides using a novel spectro-electrochemical cell

A new in operando spectro-electrochemical Li-S cell has been demonstrated. The novel design allows investigations of the liquid electrolyte phase, in a commercial coin cell geometry, at C rates much higher than conventional in situ cells. We use ATR FT-IR spectroscopy, coupled with a previously developed polysulfide diagnostic to quantify the evolution of lithium polysulfides during the discharge and charge cycles of a Li-S cell. The trends observed in the polysulfide order and concentration with respect to state of charge are consistent with prevailing understanding of the electrochemical mechanisms of Li-S battery operation. During discharge, we observe the reduction of elemental sulfur to dissolved Li2S8 polysulfides, and their cascading conversion to smaller polysulfides until insoluble species (Li2S2 and Li2S) are formed. During cell charging, we observe the oxidation of insoluble polysulfides to larger, soluble polysulfides (Li2Sn, n>3), and infer an eventual recovery of crystalline sulfur, from changes in polysulfides. Long-term evolution of polysulfides is observed over 7 discharge/charge cycles. Capacity fading is evident in the decay of polysulfide order and concentration at the same state of charge between cycles. Sulfur is not recovered by charging the cell in the latter cycles, and the active material is lost as solid Li2S.