Sulfur K-edge X-ray Absorption Research Articles

In Solution Identification of the Lysine-Cysteine Redox Switch with a NOS Bridge in Transaldolase by Sulfur K-Edge X-ray Absorption Spectroscopy.

A novel covalent post-translational modification (lysine-NOS-cysteine) was discovered in proteins, initially in the enzyme transaldolase of Neisseria gonorrhoeae (NgTAL) [Nature 2021, 593, 460-464], acting as a redox switch. The identification of this novel linkage in solution was unprecedented until now. We present detection of the NOS redox switch in solution using sulfur K-edge X-ray absorption spectroscopy (XAS). The oxidized NgTAL spectrum shows a distinct shoulder on the low-energy side of the rising edge, corresponding to a dipole-allowed transition from the sulfur 1s core to the unoccupied σ* orbital of the S-O group in the NOS bridge. This feature is absent in the XAS spectrum of reduced NgTAL, where Lys-NOS-Cys is absent. Our experimental and calculated XAS data support the presence of a NOS bridge in solution, thus potentially facilitating future studies on enzyme activity regulation mediated by the NOS redox switches, drug discovery, biocatalytic applications, and protein design.

Speciation and Possible Origins of Organosulfur Compounds in Rice Paddy Soils Affected by Acid Mine Drainage.

Although sulfur cycling in acid mine drainage (AMD)-contaminated rice paddy soils is critical to understanding and mitigating the environmental consequences of AMD, potential sources and transformations of organosulfur compounds in such soils are poorly understood. We used sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy to quantify organosulfur compounds in paddy soils from five AMD-contaminated sites and one AMD-uncontaminated reference site near the Dabaoshan sulfide mining area in South China. We also determined the sulfur stable isotope compositions of water-soluble sulfate (δ34SWS), adsorbed sulfate (δ34SAS), fulvic acid sulfur (δ34SFAS), and humic acid sulfur (δ34SHAS) in these samples. Organosulfate was the dominant functional group in humic acid sulfur (HAS) in both AMD-contaminated (46%) and AMD-uncontaminated paddy soils (42%). Thiol/organic monosulfide contributed a significantly lower proportion of HAS in AMD-contaminated paddy soils (8%) compared to that in AMD-uncontaminated paddy soils (21%). Within contaminated soils, the concentration of thiol/organic monosulfide was positively correlated with cation exchange capacity (CEC), moisture content (MC), and total Fe (TFe). δ34SFAS ranged from -6.3 to 2.7‰, similar to δ34SWS (-6.9 to 8.9‰), indicating that fulvic acid sulfur (FAS) was mainly derived from biogenic S-bearing organic compounds produced by assimilatory sulfate reduction. δ34SHAS (-11.0 to -1.6‰) were more negative compared to δ34SWS, indicating that dissimilatory sulfate reduction and abiotic sulfurization of organic matter were the main processes in the formation of HAS.

Probing Substituent Effect on Nickel-Sulfur Bond Covalency in Ni(II)-Dithiophosphinate Complexes by Sulfur K-Edge XAS and DFT Calculations.

Complexes of Ni(II) with a series of aryl or alkyl substituent dithiophosphinic acids were characterized by crystallographic structure, sulfur K-edge X-ray absorption spectroscopy (XAS), and density functional theory (DFT). In these complexes, Ni(II) coordinates with four sulfur atoms from two dithiophosphinate anions form a well-defined square-planar structure. Despite the minor differences in the geometry parameters among the complexes, the electronic structure is affected significantly by the substituent group attached to dithiophosphinic acid. In particular, the addition of ortho-CF3 group to the aryl ring constrains the orientation of the aryl ring and enhances the conjugation between the aryl ring and the coordinating core. Sulfur K-edge XAS spectra help further reveal the electronic structure of the complexes. Both the pre-edge feature and rising-edge feature provide abundant information on the molecular orbitals and show a distinctive effect of the substituent groups on the electronic structure of the complexes, which is supposedly relevant to the ligand's performance in Ln(III)/An(III) separation.

Effect of chromium on the spatial distribution of sulfur during pyrolysis of coal with high organic sulfur

The selective removal of stable sulfur-configuration makes high-sulfur coking coal reserves available in coking process. However, stable organic sulfur is difficult to remove effectively and is easily immobilized by secondary reactions in the volatile phase migration pathway. In this work, Cr commonly found in steelmaking wastewater was used as additive for the desulfurization of high organic sulfur coking coal, and its effect on spatial distribution and conversion of sulfur forms during the high-temperature coking process was studied by sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. The results show that the chromium additives effectively promote sulfur removal during pyrolysis. In the vertical direction, the addition of Cr2O3 captures sulfur radicals to form sulfide and promotes the breaking of sulfoxide OS bonds at the bottom of the coke. Furthermore, in the horizontal direction, the addition of CrCl3 by impregnation generates sulfide by combining with sulfur radicals after breaking down thiophenic CS bonds and promotes the breaking of sulfoxide O-S bonds. This results in a significant decrease of the desulfurization rate as the escape of volatiles from the bottom to top and from inside to outside of the coke. The results not only provide guidance for selective sulfur removal of coal during pyrolysis with high-sulfur coking coal but also for the conversion of polluting steelmaking wastewater into available resources.

Organic matrix-entrapped methylene blue as a photochemical reactor applied in chemical synthesis and nanotechnology

Aggregation modulates the photochemical and photophysical properties of methylene blue (MB+). MB+ adsorbed by the anionic polymer Chelex® 100 was characterized by UV–visible absorption spectroscopy and sulfur K-edge X-ray absorption near edge structure (XANES) as H- and J-aggregates. Nuclear magnetic resonance (NMR) indicates that dye molecules bind to the unprotonated carboxylic groups of the polymer via electrostatic interactions and are bridged by water molecules. MB+ also stacks with phenyl rings appended in the Chelex® 100 chain, leading to a gain in the mobility of ordered water displaced around the aromatic rings. Like the blue bottle experiment, MB+ in Chelex® 100 was entirely photoconverted for the leuco form (LMB) by visible and red light and reoxidized by molecular oxygen. Electron paramagnetic resonance (EPR) of MB+ in Chelex® 100 demonstrated the formation of a sulfur-centered free radical during photobleaching. The free radical involvement in MB+ photobleaching is consistent with a type I mechanism in which the stabilization of the products MB2+• and MB• in dye aggregates allows the formation of LMB by MB• dismutation or a second electron abstraction. Chelex® 100 beds loaded with MB+ solutions prepared in H2O and D2O produce H2O2 and D2O2, respectively, after successive cycles of photobleaching and recycling by O2 dissolved in the liquid phase. The formation of hydrogen peroxide is attributed to spontaneous superoxide ion disproportionation in the microenvironment of the polymer lattice. The LMB produced in Chelex® 100 was also able to act as a reducing agent for the synthesis of anisotropic iridescent gold nanoparticles (GNPs). The LMB/Chelex system allows the modulation of the shape and size of GNPs using different dye/Au3+ ratios. GNPs produced using LMB as a reducing agent are also efficient cell delivery systems. The specific properties of LMB confined in solid hosts LMB/Chelex also have potential applications in new solid-state optoelectronic devices such as sensing and imaging.

Simulated sulfur K-edge X-ray absorption spectroscopy database of lithium thiophosphate solid electrolytes

X-ray absorption spectroscopy (XAS) is a premier technique for materials characterization, providing key information about the local chemical environment of the absorber atom. In this work, we develop a database of sulfur K-edge XAS spectra of crystalline and amorphous lithium thiophosphate materials based on the atomic structures reported in Chem. Mater., 34, 6702 (2022). The XAS database is based on simulations using the excited electron and core-hole pseudopotential approach implemented in the Vienna Ab initio Simulation Package. Our database contains 2681 S K-edge XAS spectra for 66 crystalline and glassy structure models, making it the largest collection of first-principles computational XAS spectra for glass/ceramic lithium thiophosphates to date. This database can be used to correlate S spectral features with distinct S species based on their local coordination and short-range ordering in sulfide-based solid electrolytes. The data is openly distributed via the Materials Cloud, allowing researchers to access it for free and use it for further analysis, such as spectral fingerprinting, matching with experiments, and developing machine learning models.

Tetrathiafulvalene-2,3,6,7-tetrathiolate linker redox-state elucidation via S K-edge X-ray absorption spectroscopy.

Sulfur K-edge XAS data provide a unique tool to examine oxidation states and covalency in electronically complex S-based ligands. We present sulfur K-edge X-ray absorption spectroscopy on a discrete redox-series of Ni-based tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a 1D coordination polymer (CP), NiTTFtt. Experiment and theory suggest that Ni-S covalency decreases with oxidation which has implications for charge transport pathways.

An on-the-fly deep neural network for simulating time-resolved spectroscopy: predicting the ultrafast ring opening dynamics of 1,2-dithiane.

Revolutionary developments in ultrafast light source technology are enabling experimental spectroscopists to probe the structural dynamics of molecules and materials on the femtosecond timescale. The capacity to investigate ultrafast processes afforded by these resources accordingly inspires theoreticians to carry out high-level simulations which facilitate the interpretation of the underlying dynamics probed during these ultrafast experiments. In this Article, we implement a deep neural network (DNN) to convert excited-state molecular dynamics simulations into time-resolved spectroscopic signals. Our DNN is trained on-the-fly from first-principles theoretical data obtained from a set of time-evolving molecular dynamics. The train-test process iterates for each time-step of the dynamics data until the network can predict spectra with sufficient accuracy to replace the computationally intensive quantum chemistry calculations required to produce them, at which point it simulates the time-resolved spectra for longer timescales. The potential of this approach is demonstrated by probing dynamics of the ring opening of 1,2-dithiane using sulphur K-edge X-ray absorption spectroscopy. The benefits of this strategy will be more markedly apparent for simulations of larger systems which will exhibit a more notable computational burden, making this approach applicable to the study of a diverse range of complex chemical dynamics.

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex, Western Iran

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH2O controls on the liquidus phases, the estimated oxygen fugacity (logfO2) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S6+/∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S6++S4++S2−). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO2 from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calc-alkaline.

Analysis of Cimetidine Crystal Polymorphs by X-ray Absorption Near-Edge Spectroscopy.

Sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) measurements were performed to characterize the crystal polymorphs of the active pharmaceutical ingredients (APIs) containing sulfur atoms. Cimetidine (CIM) was used as a model API. Each crystal form of CIM has its own XANES spectrum, so we can discriminate the crystal form by its spectrum. The analysis of the crystal structure of CIM revealed that the difference in the shape of XANES spectra was ascribable to the difference in the C-S-C bond angle of CIM molecules and the intermolecular hydrogen bonds, such as C-H···S and N-H···S, and S-S interaction. It was found that the peak shape of the XANES spectrum is gentle when the C-S-C bond angle is large, while the peak shape can be steep when the C-S-C bond angle is small. Furthermore, it was found that the peak energy values varied depending on the hydrogen bonds and S-S interaction. By linear combination fitting using XANES spectra, it was possible to quantify the ratio of CIM form A crystal in mixed powders of form A and monohydrate crystals. These results indicate that XANES measurements can be a useful technique to evaluate the crystal polymorphism of APIs containing S atom in pharmaceutical formulation.

Insight into the spatial distribution of sulfur during pyrolysis of coal with high organic sulfur

The complicated mass transfer during coke making process leads to the uneven spatial distribution of sulfur forms in coke, which is of great importance for the sulfur transformation and desulfurization efficiency. In this work, Yanjiahe (YJH) coal with high sulfur content was selected to investigate the spatial distribution and conversion of different sulfur forms during the high temperature coking process by sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. In consideration of volatile matters evolution pathway, the content and transformation of different sulfur forms at different positions of the coke during pyrolysis were determined. The results show that the desulfurization rate of coke obtained at 500 °C gradually increases with the migration direction of volatiles. The sulfur-containing gases generated at the bottom of coke is difficult to escape due to the resistance of coke at the top, so the secondary reaction with coke in the bottom causes the increase of total sulfur content. In addition, the thiophenic sulfur content at the top is lower than that of bottom or center. This indicates that the volatiles generated by the decomposition of thiophenic sulfur at the top more easily escape from coke than that of bottom or center. When the pyrolysis temperature continues to rise, the desulfurization rate decreases with the migration direction of volatiles. The sulfoxide at the top can be converted into sulfonate and sulfate, so the sulfoxide content at the top is less than that in the bottom while the sulfonate and sulfate content is the highest at the top than that in the bottom. High temperature is promotive to the decomposition of organic sulfur or inorganic sulfur, and to promote the combination of the –OH groups or -H groups and sulfur radicals, which results in release of SO2 or H2S. The various sulfur forms perform different migration pathway with temperatures from the bottom to the top of the coke.

Use of pristine CNTs to measure H2S in the solution containing bovine serum albumin and gamma-globulin by confocal laser scanning microscopy

It is reported that the pristine carbon nanotube (pCNT) can be used to measure H2S in water solutions and in the solution containing hemoglobin using confocal laser scanning microscopy at a special wavelength. Under the motivation of further understanding the specificity of measuring H2S in blood, in this paper, we report the measurement of H2S in the solution containing bovine serum albumin (BSA) and bovine gamma-globulin (γ-Globulin), which are two major proteins in blood, using Raman spectroscopy, sulfur K-edge X-ray absorption near-edge spectroscopy, and confocal laser scanning microscopy. The results show that although these two proteins can bind to the sidewall of the pCNTs, the binding does not affect the H2S measurement by confocal laser scanning microscopy. The results thus suggest that CNTs be readily used in H2S measurement under the presence of proteins.

C-H▪▪▪S Hydrogen Bonds in Ampicillin and Amoxicillin Crystals Investigated by Sulfur K-Edge X-Ray Absorption Near-Edge Structure Spectroscopy and Single-Crystal X-Ray Structure Analysis.

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy was evaluated for its ability to detect non-conventional C-H▪▪▪S hydrogen bonds in crystals of the sulfur-containing penam antibiotics ampicillin and amoxicillin. The XANES spectra of the nearly isomorphous crystals of ampicillin trihydrate and amoxicillin trihydrate were very similar, whereas that of ampicillin anhydrate displayed unique features. Single-crystal X-ray structure analyses revealed that the C-H▪▪▪S hydrogen bond geometries and the chemical types of the hydrogen donors differed between the isomorphous trihydrate crystals and ampicillin anhydrate crystal. These observations demonstrate that the shapes of the sulfur K-edge XANES spectra are dependent on the nature of the C-H▪▪▪S hydrogen bonds. Sulfur K-edge XANES spectroscopy shows promise for use in the detection and analysis of non-covalent interactions, including hydrogen bonds to sulfur atoms, within active pharmaceutical ingredients.

Changes in the sulfur forms of subbituminous coals during oxidation with hydrogen peroxide and peracetic acid

We have investigated chemical desulfurization of subbituminous coals to prepare ultra-low sulfur coal for the converter process in the steel industry. Herein, we report the oxidative desulfurization of thiophene contained in two subbituminous coals using two peroxides, hydrogen peroxide and peracetic acid. Sulfur K-edge X-ray absorption near edge structure analysis was carried out to follow the change in sulfur form during oxidation. Sulfur form distribution was determined quantitatively by deconvolution of the pyrite, thiophene, sulfoxide, sulfone, and sulfate peaks in the Sulfur K-edge XANES spectra of the raw and oxidized samples using the least-squares method. In the presence of hydrogen peroxide, oxidation was promoted only at 60 °C. Furthermore, an oxidation treatment was carried out at 20 °C using peracetic acid generated by mixing hydrogen peroxide and acetic anhydride, which selectively reacted with the sulfur atoms. Treatment with peracetic acid promoted the oxidation of thiophene and improved the desulfurization extent with low carbon loss. Sulfone-type sulfur formed by the oxidation of thiophene could be completely volatilized by heat treatment at 400 °C under a N2 gas stream. By combining pre-oxidation with peracetic acid treatment and pyrolysis, 74 % of total sulfur and 72 % of thiophene could be removed while recovering approximately 90 % of total carbon as char.

Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory.

The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character inthese orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

An ab-initio study on the thermodynamics of disulfide, sulfide, and bisulfide incorporation into apatite and the development of a more comprehensive temperature, pressure, pH, and composition-dependent model for ionic substitution in minerals

AbstractThe mineral apatite, Ca10(PO4)6(F,OH,Cl)2, incorporates sulfur (S) during crystallization from S-bearing hydrothermal fluids and silicate melts. Our previous studies of natural and experimental apatite demonstrate that the oxidation state of S in apatite varies systematically as a function of oxygen fugacity (fO2). The S oxidation states –1 and –2 were quantitatively identified in apatite crystallized from reduced, S-bearing hydrothermal fluids and silicate melts by using sulfur K-edge X-ray absorption near-edge structure spectroscopy (S-XANES) where S 6+/ΣS in apatite increases from ~0 at FMQ-1 to ~1 at FMQ+2, where FMQ refers to the fayalite-magnetite-quartz fO2 buffer. In this study, we employ quantum-mechanical calculations to investigate the atomistic structure and energetics of S(-I) and S(-II) incorporated into apatite and elucidate incorporation mechanisms.One S(-I) species (disulfide, S22−) and two S(-II) species (bisulfide, HS−, and sulfide, S2−) are investigated as possible forms of reduced S species in apatite. In configuration models for the simulation, these reduced S species are positioned along the c-axis channel, originally occupied by the column anions F, Cl, and OH in the end-member apatites. In the lowest-energy configurations of S-incorporated apatite, disulfide prefers to be positioned halfway between the mirror planes at z = 1/4 and 3/4. In contrast, the energy-optimized bisulfide is located slightly away from the mirror planes by ~0.04 fractional units in the c direction. The energetic stability of these reduced S species as a function of position along the c-axis can be explained by the geometric and electrostatic constraints of the Ca and O planes that constitute the c-axis channel.The thermodynamics of incorporation of disulfide and bisulfide into apatite is evaluated by using solid-state reaction equations where the apatite host and a solid S-bearing source phase (pyrite and Na2S2(s) for disulfide; troilite and Na2S(s) for sulfide) are the reactants, and the S-incorporated apatite and an anion sink phase are the products. The Gibbs free energy (ΔG) is lower for incorporation with Na-bearing phases than with Fe-bearing phases, which is attributed to the higher energetic stability of the iron sulfide minerals as a source phase for S than the sodium sulfide phases. The thermodynamics of incorporation of reduced S is also evaluated by using reaction equations involving dissolved disulfide and sulfide species [HnS(aq)(2−n) and HnS(aq)(2−n); n = 0, 1, and 2] as a source phase. The ΔG of S-incorporation increases for fluorapatite and chlorapatite, and decreases for hydroxylapatite, as these species are protonated (i.e., as n changes from 0 to 2). These thermodynamic results demonstrate that the presence of reduced S in apatite is primarily controlled by the chemistry of magmatic and hydrothermal systems where apatite forms (e.g., an abundance of Fe; solution pH). Ultimately, our methodology developed for evaluating the thermodynamics of S incorporation in apatite as a function of temperature, pH, and composition is highly applicable to predicting the trace and volatile element incorporation in minerals in a variety of geological systems. In addition to solid-solid and solid-liquid equilibria treated here at different temperatures and pH, the methodology can be easily extended to different pressure conditions by just performing the quantum-mechanical calculations at elevated pressures.

Electronic structure of (MePh3P)2[NiII(bdtCl2)2]·2(CH3)2SO and (MePh3P)[NiIII(bdtCl2)2] (bdtCl2 = 3,6-dichlorobenzene-1,2-dithiolate)

High-resolution X-ray diffraction experiments, theoretical calculations and atom-specific X-ray absorption experiments were used to investigate two nickel complexes, (MePh3P)2[NiII(bdtCl2)2]·2(CH3)2SO [complex (1)] and (MePh3P)[NiIII(bdtCl2)2] [complex (2)]. Combining the techniques of nickel K- and sulfur K-edge X-ray absorption spectroscopy with high-resolution X-ray charge density modeling, together with theoretical calculations, the actual oxidation states of the central Ni atoms in these two complexes are investigated. Ni ions in two complexes are clearly in different oxidation states: the Ni ion of complex (1) is formally NiII; that of complex (2) should be formally NiIII, yet it is best described as a combination of Ni2+ and Ni3+, due to the involvement of the non-innocent ligand in the Ni—L bond. A detailed description of Ni—S bond character (σ,π) is presented.

Spectroscopic study on the local structure of sulfate (SO42−) incorporated in scorodite (FeAsO4·2H2O) lattice: Implications for understanding the Fe(III)-As(V)-SO42−-bearing minerals formation

Abstract The incorporation of sulfate (SO42−) into the scorodite (FeAsO4·2H2O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of SO42− speciation and local structure in scorodite lattice is still lacking. In this study, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), sulfur K-edge X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses in combination with density functional theory (DFT) calculations were used to determine the local coordination environment of SO42− in the naturally and hydrothermally synthesized scorodite. The SO42− retention in natural scorodite and the effect of pH value and initial Na+ concentration on the incorporation of SO42− in synthetic scorodite were investigated. The results showed that trace amounts of SO42− were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that SO42− was homogeneously distributed inside the natural and synthetic scorodite particles, and its content in the synthetic scorodite increased slightly with the initial Na+ concentration at pH of 1.2 and 1.8. The FTIR features and XANES results indicated that the coordination number (CN) of FeO6 octahedra around SO42− in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 Å with an average of ~1.47 Å, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 Å with an average of ~3.33 Å. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) Å with a CNS-Fe = 4 for SO42− in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that SO42− in the scorodite lattice may be in the form of a Fe2(SO4)3-like local structure. The present study is significant for understanding the formation mechanism of scorodite in natural environments and hydrometallurgical unit operations for waste sulfuric acid treatment.

All-Solid-State Lithium–Sulfur Batteries Enhanced by Redox Mediators

Redox mediators (RMs) play a vital role in some liquid electrolyte-based electrochemical energy storage systems. However, the concept of redox mediator in solid-state batteries remains unexplored. Here, we selected a group of RM candidates and investigated their behaviors and roles in all-solid-state lithium-sulfur batteries (ASSLSBs). The soluble-type quinone-based RM (AQT) shows the most favorable redox potential and the best redox reversibility that functions well for lithium sulfide (Li2S) oxidation in solid polymer electrolytes. Accordingly, Li2S cathodes with AQT RMs present a significantly reduced energy barrier (average oxidation potential of 2.4 V) during initial charging at 0.1 C at 60 °C and the following discharge capacity of 1133 mAh gs-1. Using operando sulfur K-edge X-ray absorption spectroscopy, we directly tracked the sulfur speciation in ASSLSBs and proved that the solid-polysulfide-solid reaction of Li2S cathodes with RMs facilitated Li2S oxidation. In contrast, for bare Li2S cathodes, the solid-solid Li2S-sulfur direct conversion in the first charge cycle results in a high energy barrier for activation (charge to ∼4 V) and low sulfur utilization. The Li2S@AQT cell demonstrates superior cycling stability (average Coulombic efficiency 98.9% for 150 cycles) and rate capability owing to the effective AQT-enhanced Li-S reaction kinetics. This work reveals the evolution of sulfur species in ASSLSBs and realizes the fast Li-S reaction kinetics by designing an effective sulfur speciation pathway.

Ground electronic state description of thiourea coordination in homoleptic Zn2+, Ni2+ and Co2+ complexes using sulfur K-edge X-ray absorption spectroscopy.

Sulfur K-edge X-ray absorption spectroscopy (XAS) was employed to experimentally characterize the coordinative bond between the thiourea (TU) or thiocarbamide ligand and transition metal (TM) ions Zn2+, Co2+ and Ni2+ in distorted tetrahedral and octahedral homoleptic coordination environments. Comparisons of XAS spectra of the free TU ligand and [Zn(TU)4]2+, [Co(TU)4]2+ and [Ni(TU)6]2+ complexes clearly identify spectral features unique to TM2+-S(TU) bonding. Quantitative analysis of pre-edge intensities describes the covalency of Ni2+-S(TU) and Co2+-S(TU) bonding to be at most 21% and 9% as expressed by the S 3p contributions per TM 3d electron hole. Using relevant Ni2+ complexes with dithiocarbamate and thioether ligands, we evaluated the empirical S 1s → 3p transition dipole integrals developed for S-donor ligands and their dependence on heteroatom substitutions. With the aid of density functional theory-based ground electronic state calculations, we found evidence for the need of using a transition dipole that is dependent on the presence of conjugated heteroatom (N) substitution in these S-donor ligands.