Sulfur K-edge X-ray Absorption Near Edge Structure Research Articles

Contrasting molecular structures and photooxidation behaviors between dissolved organic sulfur released from rice straw-biochar and aerobically decomposed rice straw

Photooxidation of dissolved organic sulfur (DOS) in soils and natural waters plays an important role in the sulfur biogeochemical cycle. However, the structural-dependent photoliabilities of DOS from different sources remain unclear. Here, the molecular structures and photooxidation behaviors of DOS in pyrogenic dissolved black matter (PyDOM) derived from rice straw-pyrolyzed biochar (referred to as PyDOM-S and considered to be representative of black carbon from prairie fires) were thoroughly characterized and compared with those of DOS in leached dissolved organic matter (LDOM) derived from aerobically decomposed rice straw (referred to as LDOM-S and considered to be generally representative of organic-rich horizons in soils and peats). The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed that both PyDOM-S and LDOM-S were, respectively, dominated by aliphatic (60.4 % and 41.1 %) and lignin-like compounds (35.1 % and 40.1 %), followed by minimal aromatic and polyaromatic compounds (2.8 % and 11.9 % in total). As demonstrated by the sulfur K-edge X-ray absorption near-edge structure (S-XANES) analysis, PyDOM-S consisted mainly of organosulfate (80.4 %) contrasting to the diversified and mingled reduced sulfurs (62.7 %) and oxidized sulfurs (37.3 %) of LDOM-S. Under simulated sunlight irradiation, 74 % of sulfur in PyDOM-S was photomineralized to sulfate within 24 h and totaling 89 % after 168 h, but only 9 % and 42 % for LDOM-S given the same periods of time, confirming the much faster photomineralization of PyDOM-S. After 168-h irradiation, almost all molecules in PyDOM-S disappeared, whereas a large proportion (44.2 %) of LDOM-S molecules (mainly aliphatic and lignin-like compounds) were photo-resistant. Furthermore, the photomineralization of PyDOM-S was mainly contributed by the final and complete oxidation of organosulfate to sulfate; however, the photooxidation of LDOM-S was dominated by the sequential oxidation of exocyclic sulfur and heterocyclic sulfur to organosulfate prior to releasing sulfate. These results highlight that pyrogenic-sourced PyDOM-S and diagenesis-derived LDOM-S exhibit contrasting photooxidation behaviors due to the associated distinct molecular structures.

Determination of calcium and sulfate species in aerosols associated with the conversion of its species through reaction processes in the atmosphere and its influence on cloud condensation nuclei activation

Calcium (Ca) species in aerosols collected in Higashi–Hiroshima in southwestern Japan were determined by X-ray absorption near-edge structure (XANES) analysis, and their size distributions and seasonal variation were investigated. Calcium K-edge XANES results showed that gypsum (CaSO4⋅2H2O) and calcium oxalate (CaC2O4⋅2H2O) were the major Ca species that were formed by reactions of mineral particles in the atmosphere. These species exhibit similar hygroscopicity to that of calcite (CaCO3), which is an original Ca species in mineral particles. In addition, high hygroscopic Ca species, such as calcium nitrate [Ca(NO3)2⋅4H2O] and calcium chloride (CaCl2⋅2H2O) were considered minor Ca species. Moreover, the formation of CaSO4⋅2H2O by the reaction of mineral particles suppresses the formation of hygroscopic sulfate aerosol species, such as ammonium sulfate [(NH4)2SO4], reducing the number of cloud condensation nuclei (CCN). Sulfur K-edge XANES also confirmed that CaSO4⋅2H2O was a major species of sulfate in coarse particles. By contrast, high hygroscopic sulfate, such as (NH4)2SO4, was major species in fine particle. In addition, CaSO4⋅2H2O fraction to total sulfate increased during the sampling period, which was affected by a dust event. Consequently, the amount of hygroscopic sulfate in fine particles was decreased by the reaction, forming CaSO4⋅2H2O in coarse particles in the presence of mineral dust. Although these facts have been indicated mostly by modeling and laboratory studies, the present research suggested that the reaction of CaCO3 to form CaSO4⋅2H2O will reduce the amount of hygroscopic sulfate aerosols based on natural sample measurement in terms of the speciation of Ca and sulfate in aerosols using XANES spectroscopy.

A Sulfur K-Edge XANES and Raman Study on the Effect of Chloride Ion on Bacterial and Chemical Leaching of Chalcopyrite at 25 °C

In the present study, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray absorption near edge structure (XANES) spectroscopy, and Raman spectroscopy were used to investigate the effect of chloride ions on chalcopyrite bioleaching and chemical leaching at 25 °C. The leaching results show that the addition of chloride ions suppressed the dissolution of chalcopyrite in both chemical and bacterial leaching. In chemical leaching, the solution redox potential in the experiment without Cl− stayed in a range more suitable for chalcopyrite dissolution compared with experiments with Cl−. In bioleaching, the bacteria activity was suppressed by Cl− leading to low dissolution rates of Cu. Surface morphology and chemical speciation studies confirmed the formation of potassium jarosite on the surface of the chalcopyrite leach residues. In addition, S K-edge XANES indicated the formation of trace amount of elemental sulfur and covellite on the mineral surfaces.

Geochemical characteristics of oil sands fluid petroleum coke

The geochemical characteristics of fluid petroleum coke from the Athabasca Oil Sands Region (AOSR) of northern Alberta, Canada were investigated. Continuous core samples were collected to 8 m below surface at several locations (n = 12) from three coke deposits at an active oil sands mine. Bulk elemental analyses revealed the coke composition was dominated by C (84.2 ± 2.3 wt%) and S (6.99 ± 0.26 wt%). Silicon (9210 ± 3000 mg kg−1), Al (5980 ± 1200 mg kg−1), Fe (4760 ± 1200 mg kg−1), and Ti (1380 ± 430 mg kg−1) were present in lesser amounts. Vanadium (1280 ± 120 mg kg−1) and Ni (230 ± 80 mg kg−1) exhibited the highest concentrations among potentially-hazardous minor and trace elements. Sequential extractions revealed potential for release of these metals under field-relevant conditions. Synchrotron powder X-ray diffraction revealed the presence of Si and Ti oxides, organically-complexed V and hydrated Ni sulfate, and provided information about the asphaltenic carbon matrix. X-ray absorption near edge structure (XANES) spectroscopy at the V and Ni K-edges revealed that these metals were largely hosted in porphyrins and similar organic complexes throughout coke grains. Minor differences among measured V and Ni K-edge spectra were largely attributed to slight variations in local coordination of V(IV) and Ni(II) within these organic compounds. However, linear combination fits were improved by including reference spectra for inorganic phases with octahedrally-coordinated V(III) and Ni(II). Sulfur and Fe K-edge XANES confirmed that thiophenic coordination and pyritic-ilmenitic coordination are predominant, respectively. These results provide new information on the geochemical and mineralogical composition of oil sands fluid petroleum coke and improve understanding of potential controls on associated water chemistry.

Products of SO2 Adsorption on Fuel Cell Electrocatalysts by Combination of Sulfur K-Edge XANES and Electrochemistry

Electrochemical adsorption of SO(2) on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO(2) adsorption products on catalyst coated membranes (CCMs) at different electrode potentials using a combination of in situ sulfur K-edge XANES (X-ray absorption near-edge structure) spectroscopy and electrochemical techniques. CCMs employed platinum nanoparticles supported on Vulcan carbon (Pt/VC). SO(2) was adsorbed from a SO(2)/N(2) gas mixture while holding the Pt/VC-electrode potential at 0.1, 0.5, 0.7, and 0.9 V vs a reversible hydrogen electrode (RHE). Sulfur adatoms (S(0)) are identified as the SO(2) adsorption products at 0.1 V, while mixtures of S(0), SO(2), and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO(2) adsorption products at 0.5 and 0.7 V. At 0.9 V, SO(2) is completely oxidized to (bi)sulfate ions. The identity of adsorbed SO(2) species on Pt/VC catalysts at different electrode potentials is confirmed by modeling of XANES spectra using FEFF8 and a linear combination of experimental spectra from sulfur standards. Results on SO(2) speciation gained from XANES are used to compare platinum-sulfur electronic interactions for Pt(3)Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO(2) contamination.

Speciation of sulfur from filamentous microbial mats from sulfidic cave springs using X-ray absorption near-edge spectroscopy

Most transformations within the sulfur cycle are controlled by the biosphere, and deciphering the abiotic and biotic nature and turnover of sulfur is critical to understand the geochemical and ecological changes that have occurred throughout the Earth's history. Here, synchrotron radiation-based sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to examine sulfur speciation in natural microbial mats from two aphotic (cave) settings. Habitat geochemistry, microbial community compositions, and sulfur isotope systematics were also evaluated. Microorganisms associated with sulfur metabolism dominated the mats, including members of the Epsilonproteobacteria and Gammaproteobacteria. These groups have not been examined previously by sulfur K-edge XANES. All of the mats consisted of elemental sulfur, with greater contributions of cyclo-octasulfur (S8) compared with polymeric sulfur (Smicro). While this could be a biological fingerprint for some bacteria, the signature may also indicate preferential oxidation of Smicro and S8 accumulation. Higher sulfate content correlated to less S8 in the presence of Epsilonproteobacteria. Sulfur isotope compositions confirmed that sulfur content and sulfur speciation may not correlate to microbial metabolic processes in natural samples, thereby complicating the interpretation of modern and ancient sulfur records.

Bonding of methyl mercury to reduced sulfur groups in soil and stream organic matter as determined by x-ray absorption spectroscopy and binding affinity studies

We combined synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and binding affinity studies to determine the coordination, geometry, and strength of methyl mercury, CH 3Hg (II), bonding in soil and stream organic matter. Samples of organic soil (OS), potentially soluble organic substances (PSOS) from the soil, and organic substances from a stream (SOS) draining the soil were taken along a short “hydrological transect.” We determined the sum of concentrations of highly reduced organic S groups (designated Org-S RED), such as thiol (RSH), disulfane (RSSH), sulfide (RSR), and disulfide (RSSR), using sulfur K-edge XANES. Org-S RED varied between 27% and 64% of total S in our samples. Hg L III-edge EXAFS analysis were determined on samples added CH 3Hg (II) to yield CH 3Hg (II)/Org-S RED ratios in the range 0.01–1.62. At low ratios, Hg was associated to one C atom (the methyl group) at an average distance of 2.03 ± 0.02 Å and to one S atom at an average distance of 2.34 ± 0.03 Å, in the first coordination shell. At calculated CH 3Hg(II)/Org-S RED ratios above 0.37 in OS, 0.32 in PSOS, and 0.24 in SOS, the organic S sites were saturated by CH 3Hg +, and O (and/or N) atoms were found in the first coordination shell of Hg at an average distance of 2.09 ± 0.01 Å. Based on the assumption that RSH (and possibly RSSH) groups take part in the complexation of CH 3Hg +, whereas RSSR and RSR groups do not, approximately 17% of total organic S consisted of RSH (+ RSSH) functionalities in the organic soil. Corresponding figures for samples PSOS and SOS were 14% and 9%, respectively. Competitive complexation of CH 3Hg + by halide ions was used to determine the average binding strength of native concentrations of CH 3Hg (II) in the OS sample. Using data for Org-S RED, calculated surface complexation constants were in the range from 10 16.3 to 10 16.7 for a model RSH site having an acidity constant of mercaptoacetic acid. These values compare favorably with identically defined stability constants (log K 1) for the binding of methyl mercury to thiol groups in well-defined organic compounds.

Investigation of S-H bonds in biologically important compounds by sulfur K-edge X-ray absorption spectroscopy

X-ray Absorption Near Edge Structure (XANES) spectroscopy, often provides a direct correlation between observed resonances in the spectrum and molecular bonds in the sample. This can be used as a fingerprint for the presence of a given molecular environment of the absorber atom in a sample. As the white line is found at similar energy positions for S-C and S-H bonds, this approach is impossible when both types of bond are present simultaneously, as often in biological systems. To develop a criterium for the presence of S-H bonds in such samples, reduced glutathione, reduced coenzyme A, cysteine and their corresponding oxidized forms were investigated using sulfur K-edge XANES, revealing a unique feature at 2 475.8 eV in the respective difference spectra. To correlate this structure to S-H bonds, H2S and H2S2 were measured, whose difference spectrum also shows a structure at this energy position, whereas it is not present throughout a variety of C-S-C/C-S-S-C environments. Theoretical investigations suggest its correlation to a Rydberg transition occurring in the case of a S-H bond. Using this criterium, the presence of S-H bonds is in the purple sulfur bacterium Allochromatium vinosum during oxidation of intracellular accumulated sulfur, is proved, as expected from biological considerations.

Effects of sulfur impregnation temperature on the properties and mercury adsorption capacities of activated carbon fibers (ACFs).

Laboratory studies were conducted to determine the role of sulfur functional groups and micropore surface area of carbon-based adsorbents on the adsorption of Hg0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated with elemental sulfur between 250 and 650 degrees C. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulfur impregnated ACF samples decreased with increasing impregnation temperatures from 250 and 500 degrees C and then remained constant to 650 degrees C. Results from sulfur K-edge X-ray absorption near-edge structure (S-XANES) spectroscopy showed that sulfur impregnated on the ACF samples was in both elemental and organic forms. As sulfur impregnation temperature increased, however, the relative amounts of elemental sulfur decreased with a concomitant increase in the amount of organic sulfur. Thermal analyses and mass spectrometry revealed that sulfur functional groups formed at higher impregnation temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg0 adsorption capacity when compared to the raw ACF sample. The ACF sample treated with sulfur at 400 degrees C had a surface area of only 94 m2/g compared to the raw ACF sample's surface area of 1971 m2/g, but at least 86% of this sample's surface area existed as micropores and it had the largest equilibrium Hg0 adsorption capacities (2211-11,343 micrograms/g). Such a result indicates that 400 degrees C is potentially an optimal sulfur impregnation temperature for this ACF. Sulfur impregnated on the ACF that was treated at 400 degrees C was in both elemental and organic forms. Thermal analyses and CS2 extraction tests suggested that elemental sulfur was the main form of sulfur affecting the Hg0 adsorption capacity. These findings indicate that both the presence of elemental sulfur on the adsorbent and a microporous structure are important properties for improving the performance of carbon-based adsorbents for the removal of Hg0 from coal combustion flue gases.