Sulfur X-ray Absorption Near Edge Structure Research Articles

Selective Photochemical Oxidation of Reduced Dissolved Organic Sulfur to Inorganic Sulfate.

The chemical nature and stability of reduced dissolved organic sulfur (DOSRed) have implications on the biogeochemical cycling of trace and major elements across fresh and marine aquatic environments, but the underlying processes governing DOSRed stability remain obscure. Here, dissolved organic matter (DOM) was isolated from a sulfidic wetland, and laboratory experiments quantified dark and photochemical oxidation of DOSRed using atomic-level measurement of sulfur X-ray absorption near-edge structure (XANES) spectroscopy. DOSRed was completely resistant to oxidation by molecular oxygen in the dark and underwent rapid and quantitative oxidation to inorganic sulfate (SO42-) in the presence of sunlight. The rate of DOSRed oxidation to SO42- greatly exceeded that of DOM photomineralization, resulting in a 50% loss of total DOS and 78% loss of DOSRed over 192 h of irradiance. Sulfonates (DOSSO3) and other minor oxidized DOS functionalities were not susceptible to photochemical oxidation. The observed susceptibility of DOSRed to photodesulfurization, which has implications on carbon, sulfur, and mercury cycling, should be comprehensively evaluated across diverse aquatic environments of differing DOM composition.

Evaluation of mercury stabilization mechanisms by sulfurized biochars determined using X-ray absorption spectroscopy

The application of biochar to treat mercury (Hg) in the environment is being proposed on an increasing basis due to its widespread availability and cost effectiveness. However, the efficiency of Hg removal by biochars is variable due to differences in source material composition. In this study, a series of batch tests were conducted to evaluate the effectiveness of sulfurized biochars (calcium polysulfide and a dimercapto-related compound, respectively) for Hg removal; Hg-loaded biochars were then characterized using synchrotron-based techniques. Concentrations of Hg decreased by >99.5% in solutions containing the sulfurized biochars. Sulfur X-ray absorption near-edge structure (XANES) analyses indicate a polysulfur-like structure in polysulfide-sulfurized biochar and a thiol-like structure (shifted compared to dimercapto) in the dimercapto-sulfurized biochar. Micro-X-ray fluorescence (μ-XRF) mapping and confocal X-ray micro-fluorescence imaging (CXMFI) analyses indicate Hg is distributed primarily on the edges of sulfurized biochar and throughout unmodified biochar particles. Hg extended X-ray absorption fine structure (EXAFS) analyses show Hg in enriched areas is bound to chlorine (Cl) in the unmodified biochar and to S in sulfurized biochars. These results indicate that Hg removal efficiency is enhanced after sulfurization through the formation of strong bonds (Hg-S) with S-functional groups in the sulfurized biochars.

Mechanisms of mercury removal by biochars produced from different feedstocks determined using X-ray absorption spectroscopy

Thirty-six biochars produced from distinct feedstocks at different temperatures were evaluated for their potential to remove mercury (Hg) from aqueous solution at environmentally relevant concentrations. Concentrations of total Hg (THg) decreased by >90% in batch systems containing biochars produced at 600 and 700°C and by 40–90% for biochars produced at 300°C. Elevated concentrations of SO42− (up to 1000mgL−1) were observed in solutions mixed with manure-based biochars. Sulfur X-ray absorption near edge structure (XANES) analyses indicate the presence of both reduced and oxidized S species in both unwashed and washed biochars. Sulfur XANES spectra obtained from biochars with adsorbed Hg were similar to those of washed biochars. Micro-X-ray fluorescence mapping results indicate that Hg was heterogeneously distributed across biochar particles. Extended X-ray absorption fine structure modeling indicates Hg was bound to S in biochars with high S content and to O and Cl in biochars with low S content. The predominant mechanisms of Hg removal are likely the formation of chemical bonds between Hg and various functional groups on the biochar. This investigation provides information on the effectiveness and mechanisms of Hg removal that is critical for evaluating biochar applications for stabilization of Hg in surface water, groundwater, soils, and sediments.

S-XANES analysis of thermal iron sulfide transformations in a suite of Argonne Premium Coals: A study of particle size effects during pyrolysis

A suite of four bituminous Argonne Premium Coal Samples, namely Pittsburgh#8 (P8), Blind Canyon (BC), Upper Freeport (UF), and Illinois #6 (IL6), were pyrolyzed according to the Easy Ro kinetic model (Burnham and Sweeney, 1989) to Ro=4.3 and iron sulfide thermal transformations were tracked by the use of S-XANES (Sulfur X-ray Absorption Near Edge Structure.) It was shown that the pyrite transformed first to pyrrhotite by Ro=1.5, and then started to transform to troilite by Ro=2.4. Some Argonne Coals displayed evidence of structural instability. In addition, particle size effects were examined. Pyrolysis was performed on not-ground (large-particled) coal samples, which were subsequently ground to micron-size particles before data collection. S-XANES was also collected for the not-ground post-pyrolysis IL6 coal to show the effect of the extent of reaction on the surface of the particles as opposed to the bulk. It was found that the pyrite-to-pyrrhotite transformation in large particles of IL6 coal proceeded from the surface of the particle and progress inward, consistent with the shrinking core model. A scheme for determining particle size based on organic sulfur content was also developed for a coal model consisting of a 50/50mol% mixture of pyrite and Maya petroleum vacuum resid asphaltene for a range of known particle sizes. Lastly, the behavior of both marcasite (a polymorph of pyrite) and pyrite in a coal model was investigated for large (~100μm) and small (~5μm) particles. The marcasite proved to be less structurally stable than pyrite for the large particles, with an abrupt transformation to a mixture of pyrrhotite and troilite, and an abrupt drop in aliphatic sulfur content, indicating consequent H2S generation at Ro=2.4. This transformation is much less pronounced for pyrite at the same point in pyrolysis.

Transfer characterization of sulfur from coal-burning emission to plant leaves by PIXE and XANES

The impact of coal-burning emission on sulfur in camphor leaves was investigated using Proton Induced X-ray Emission (PIXE) and synchrotron radiation technique X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The PIXE results show that the sulfur concentrations in the leaves collected at the polluted site are significantly higher than those in controls. The Sulfur XANES spectra show the presence of organic (disulfides, thiols, thioethers, sulfonates and sulfoxides) and inorganic sulfur (sulfates) in the leaves. The inorganic sulfur in the leaves of camphor tree polluted by coal combustion is 15% more than that of the control site. The results suggest that the long-term coal-burning pollution resulted in an enhanced content of the total sulfur and sulfate in the leaves, and the uptake of sulfur by leaves had exceeded the metabolic requirement of plants and the excess of sulfur was stored as SO2–4. It can monitor the sulfur pollution in atmosphere.

Sulfur: Not a “Silent” Element Any More

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Direct Characterization of Kerogen by X-ray and Solid-State 13C Nuclear Magnetic Resonance Methods

A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS) and sulfur X-ray absorption near edge structure (S-XANES) techniques are used to characterize organic oxygen, nitrogen, and sulfur species and carbon chemical/structural features in kerogens. The kerogens studied represent a wide range of organic matter types and maturities. A van Krevelen plot based on elemental H/C data and XPS derived O/C data shows the well established pattern for type I, type II, and type III kerogens. The anticipated relationship between the Rock−Eval hydrogen index and H/C is independent of organic matter type. Carbon structural and lattice parameters are derived from solid-state 13C NMR analysis. As expected, the amount of aromatic carbon, measured by both 13C NMR and XPS, increases with decreasing H/C. The correlation between aromatic carbon and Rock−Eval Tmax, an indicator of maturity, is linear for types II and IIIC kerogens, but each organic matter type follows a different relationship. The average al...

XANES SPECTROSCOPY OF SULFUR IN EARTH MATERIALS

X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.