XANES Analysis Research Articles

Solubility and speciation of iron in hydrothermal fluids

Abstract Iron is among the most abundant elements in Earth's crust and is also a major aqueous solute in a variety of hydrothermal settings, yet major questions remain regarding the solubility and speciation of iron at hydrothermal conditions. Here, we conducted hydrothermal diamond-anvil cell experiments using synchrotron-radiation micro-XRF and XANES analyses as well as Raman spectroscopy, in situ at hydrothermal temperatures and pressures, to characterize the solubility and speciation of iron in fluids buffered by a variety of mineral assemblages. Our experiments included the assemblages hematite-magnetite (HM), fayalite-magnetite-quartz (FMQ), and magnetite-pyrite-pyrrhotite (MPP). Our results indicate highest solubilities of HM in HCl solutions. In sodium chloride solutions of similar molalities, FMQ shows higher Fe solubility than MPP. XANES data are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANES spectra of these solutions with calculated XANES spectra from the literature indicates octahedral FeClx(H2O)6−x2−x (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl42− or FeCl3(H2O)− at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl2(H2O)4+ at temperatures less than 100 °C, which transitions to FeCl4− between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H2O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl3(H2O)x (x = 0–3) species. All Raman spectra of ferrous iron solutions show a fairly broad band at about 280 cm−1, which can be interpreted to stem from Fe(II)–Cl vibrations of FeCl3(H2O)x− (x = 0–3) (point group D3h) species or, at low Cl-Fe ratios of about two or less, from octahedral FeClx(H2O)6−x2−x (x = 0–3) species. These results provide important constraints on the hydrothermal mobilization of iron and fluid-rock reactions involving iron- and chloride-bearing fluids.

The roles of pyrite for enhancing reductive removal of nitrobenzene by zero-valent iron

Zero-valent iron (ZVI) is a popular reductant that has been successfully applied for remediation of groundwater contaminated with various pollutants, but it still suffers from surface passivation and pH increase in the reaction media. In this study, pyrite, a ubiquitous sulfide mineral in anaerobic environment, was adopted to enhance the reactivity of ZVI for removal of nitrobenzene. The synergetic effect between pyrite and ZVI was observed for nitrobenzene reduction, and the rate constant kobs at the initial pH (pH0) 6.0 was enhanced by 8.55–23.1 folds due to the presence of pyrite with pyrite/ZVI mass ratio ranging from 1.0 to 6.0. Moreover, nitrobenzene could be removed effectively at pH0 ranging from 5.0 to 10.0 in the presence of pyrite, while negligible removal of nitrobenzene by ZVI (0.5 g/L) alone was observed at pH0 ≥7.0. ZVI sample recovered from the reacted ZVI/pyrite mixture was also more effective for nitrobenzene degradation than pristine ZVI. The mechanism study revealed that pyrite could suppress the pH increase in reaction media, boost the production of reactive Fe2+, and activate the ZVI surface through replacing partially the passive oxide film with iron sulfide (FeS). In particular, the formation of highly reactive FeS@Fe in the reaction system of ZVI/pyrite mixture was proved by XRD, Mössbauer, XANES, XPS and SEM-EDS analyses, which provides a facile way for in-situ sulfidation of ZVI and for enhancing the removal of contaminants with ZVI technology.

Synthesis of magnetic biochar composites for enhanced uranium(VI) adsorption

Magnetic biochar composites were successfully fabricated by pyrolysis of siderite and rice husk under N2 condition. The results of a variety of characterization implied magnetic biochar displayed porous structures with larger specific surface area. The batch adsorption experiments showed high adsorption properties of magnetic biochar composites toward U(VI) (52.63 mg/g at pH 4.0), whereas U(VI) adsorption was significantly influenced by Na2CO3 and HA. U(VI) adsorbed onto magnetic biochar was reduced to U(IV) by Fe3O4 according to XPS and XANES analyses. In addition, no significant effect of ionic strength of NaCl and EXAFS results, illustrated the inner-sphere surface complexation of U(VI) on magnetic biochar. Owing to the simple synthesis procedure, low cost, high adsorption efficiency, easy separation and environmental friendly, magnetic biochar can be considered as a potential adsorbent for the purification of U(VI)-bearing wastewater in environmental remediation.

Vanadium-embedded mesoporous carbon microspheres as effective catalysts for selective aerobic oxidation of 5-hydroxymethyl-2-furfural into 2, 5-diformylfuran

Vanadium dioxide (VO2) embedded mesoporous carbon spheres (V-CS) were synthesized by a facile hydrothermal method followed by post thermal treatments in air. The V-CS materials were used as catalysts in the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) using atmospheric oxygen as the oxidant. The effect of reaction time, temperature, catalyst loading and solvents on the reaction were investigated. Under optimized reaction conditions, a DFF yield of 99% can be achieved. The physical and chemical properties of the catalysts were studied by FESEM, BET, XRD, FTIR, Raman, XANES and element analysis. The characterization results evidenced that the micro/mesoporous structure of the carbon spheres along with the highly dispersed vanadium dioxide species is responsible for the excellent catalytic performance. The post thermal treatment was the key step to afford the superior V-CS catalysts successfully. In addition, the catalyst did not have any significant loss in activity during the recycling test, showing good reusability.

XRF Microscopy and Boron K-edge XANES Analysis of Bulk Rock Samples Associated with Uranium Deposits

XRF Microscopy and Boron K-edge XANES Analysis of Bulk Rock Samples Associated with Uranium Deposits

Pt3mn Intermetallic Catalysts for Oxygen Reduction Reaction in Pemfcs with Improved Activity and Durability

Polymer electrolyte membrane fuel cells (PEMFCs) are energy conversion devices that change chemical energy into electric energy. PEMFCs have high power density and efficiency wiht no pollution due to hydrogen as a fuel [1]. However, their high cost and low stability hindered PEMFCs from full-commercialization. This issue is originated from large amount of noble platinum catalyst in the cathode electrode to overcome the sluggish oxygen reduction reaction (ORR) [2]. To increase ORR activity with small amount of Pt, Pt-M alloy (M: Fe, Ni, Co, etc) catalysts have been studied over the past years. Particularly, Pt-M intermetallic catalysts showed significant enhancement of ORR activity and stability because of their abundant active sites and strong interactions between Pt and metal [3]. In this report, Pt3Mn intermetallic nanoparticles on carbon support (Pt3Mn/C) for ORR in PEMFCs was produced by using a facile synthesis. By using commercial Pt/C and Mn precursor, Pt3Mn/C catalyst could be easily synthesized. Total synthesis can be separated into 2 steps; first, Pt/C was sonicated in hexane, and Mn(acac)2, HDD, diphenyl ether was mixed and heated for 30 min. at 250 oC. Secondly, As-synthesized Pt3Mn/C was heat-treated under 700 oC for 4 hours in H2/Ar atmosphere to form Pt3Mn intermetallic nanoparticles. Pt3Mn intermetallic catalyst demonstrated a high mass activity compared to commercial Pt/C. While initial mass activity of Pt/C was 0.125 A/mgPt, Pt3Mn/C showed 0.386 A/mgPt, which was more than 3 times higher activity. Furthermore, Pt3Mn/C not only exhibited a high initial ORR activity, but also showed a great stability in a voltage-cycling test. After accelerated degradation test for 10k cycles, mass activity of Pt/C was reduced into 0.028 A/mgPt (77.6 % decrease), when Pt3Mn/C showed 0.276 A/mgPt (28.5% decrease). Additionally, Pt3Mn intermetallic catalyst showed a fast kinetic in alcohol oxidation reactions compared to commercial Pt/C. In methanol oxidation reaction, mass activity of Pt/C was about 0.10 A/mgPt, when Pt­3Mn/C showed 0.15 A/mgPt, which was 1.5 times higher. In ethanol oxidation reaction, Pt/C showed 0.035A/mgPt, when Pt3Mn/C showed 0.070 A/mgPt, which was 2 times higher than Pt/C. Further details of Pt3Mn intermetallic catalysts was characterized by adopting XAS analysis and DFT calculations to verify their high catalytic activity and stability. Through XAFS data, heat-treated Pt3Mn intermetallic nanoparticle showed higher coordination number, which implies its higher ordering than as-synthesized Pt3Mn. From XANES analysis, stability of catalysts can be compared. DFT calculation was also performed to compare ORR dissociative mechanism between Pt3Mn(111) and Pt(111). While ORR spontaneously occurs Pt3Mn(111) under potential of 0.87 V, Pt(111) needs potential of 0.53 V, which indicates high ORR activity of Pt3Mn. Finally, we demonstrated opportunity of utilizing in actual applications by the single-cell test. These results verified that intermetallic Pt3Mn/C catalyst can be used as an active and stable ORR catalyst for PEMFCs. Catalyst loading was 0.15 mgPt/cm2 for both anode and cathode. Initial performance of MEA with Pt3Mn/C cathode was 551 mA/cm2, when Pt/C showed 420 mA/cm2 @ 0.7 V. After long term durability test of each MEA, single cell performance of Pt3Mn catalyst decreased into 486 mA/cm2, which was 88% of initial performance. However, Pt/C catalyst decreased into 308 mA/cm2, which was 73% of initial performance, larger degradation than Pt3Mn/C.

Alkali and earth alkali modified CuOx/SiO2 catalysts for propylene partial oxidation: What determines the selectivity?

In this work, CuOx/SiO2 catalysts were investigated in the propylene partial oxidation reaction. Ordered mesoporous silica (KIT-6) was used to deposit 1–10 wt. % copper and subsequently modified with Na, K and Ca. The synthesized materials were characterized by N2 physisorption, XRD, TEM-EDS, CO2-TPD, operando UV/Vis DRS, operando XANES and pyridine DRIFT spectroscopy. Regardless of the CuOx loading, catalyst deactivation was observed during propylene oxidation reaction in non-modified catalysts, which was related to sintering of oligomeric [Cu-O-Cu]n species. Sintering of CuOx is strongly promoted under a reducing propylene atmosphere and related to the presence of Cu+1. The resulting bulk CuOx promotes acrolein selectivity. We produced modified catalysts with finely dispersed alkali metal cations, associated with the subnanometer CuOx phase, resulting in a greatly stabilized morphology and catalytic activity. Operando XANES analysis revealed that a substantial fraction of Cu2+ is transformed to Cu+ during the propylene oxidation reaction (52–68%, depending on the modifying atom). Also, the dynamics of reaching the quasi steady oxidation state differ strongly. The kinetics of oxygen abstraction and replenishment are substantially different, indicative of modified chemistry of the nucleophilic oxygen species, present in 5CuNa catalyst in contrast to others (5Cu and 5CuCa). We propose that Cu+ is not crucial for PO formation. Instead the electropositive Na+ and K+ decrease the nucleophilic strength of oxygen in CuOx, by attracting its electrons. Consequently, the catalytic action of oxygen changes from oxidative attack on the allylic hydrogen to oxygen insertion into the CC bond of propylene. This results in a noticeable selectivity shift from acrolein to propylene oxide. The effect of calcium on decreasing the nucleophilic character of O species in CuOx is negated by charge compensation by strongly adsorbed hydroxyl groups and Ca modification for PO selectivity is inefficient. Additionally we found, that further oxidation of propylene oxide is, most likely, the main factor determining high selectivity for COx products. The alkali modification which increases the PO selectivity does not function via elimination of LAS, but exclusively through attenuation of nucleophilic character of oxygen species.

Detoxification of U(VI) by Paecilomyces catenlannulatus investigated by batch, XANES and EXAFS techniques

Paecilomyces catenlannulatus (P. catenlannulatus) as a genus of entomogenous fungus presented a variety of surface reactive groups by batch characterizations. The detoxification of U(VI) by P. catenlannulatus was investigated under different water chemistry (pH, incubation time, foreign anions and U(VI) concentration) by batch techniques. Approximately 75% of U(VI) was reduced to U(IV) (i.e., U(IV)O2(s)) by P. catenlannulatus at pH 5.5 and 7 days under glovebox conditions, therefore the formation of precipitates decreased the toxicity of U(VI) for P. catenlannulatus. In addition, phosphate facilitate the U(VI) reduction, whereas carbonate and sulfate inhibited the U(VI) reduction. The activities of catalase (CAT), superoxide dismutase (SOD) and glutathione (GSH) level were stimulated exposure to 1-30 mg/L U(VI), indicating that CAT, SOD and GSH were antagonized for the oxidant stress derived from U(VI) at low concentrations. According to XPS and XANES analysis, the occurrence of U(IV) revealed the reduction of adsorbed U(VI) to U(IV) by P. catenlannulatus. The results of EXAFS analysis indicated that the fitting of U-O and U-U shell for U-loaded P. catenlannulatus was similar to that of U(IV)O2(s)). The formation of U-bearing precipitates decreased the toxicity of U(VI) for P. catenlannulatus. These findings indicated that P. catenlannulatus is capable to detoxify U(VI) by extracellar/intracellar defense systems. Therefore, P. catenlannulatus can be utilized as a promising bioadsorbents for remediation of uranium-contaminated wastewater in environmental cleanup.

The role of yttrium and titanium during the development of ODS ferritic steels obtained through the STARS route: TEM and XAS study

Oxide Dispersion Strengthened Ferritic Steels (ODS FS) are candidate materials for structural components in future fusion reactors. Their high strength and creep resistance at elevated temperatures and their good resistance to neutron radiation damage is obtained through extremely fine microstructures containing a high density of nanometric precipitates, generally yttrium and titanium oxides.This work shows transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) characterization of Fe-14Cr-2W-0.3Ti-0.24Y ODS FS obtained by the STARS route (Surface Treatment of gas Atomized powder followed by Reactive Synthesis), an alternative method to obtain ODS alloys that avoids the mechanical alloying to introduce Y2O3 powder particles. In this route, FS powders already containing Ti and Y, precursors of the nanometric oxides, are obtained by gas atomization. Then, a metastable Cr- and Fe-rich oxide layer is formed on the surface of the powder particles. During consolidation by HIP at elevated temperatures, and post-HIP heat treatments above the HIP temperature, this oxide layer at Prior Particle Boundaries (PPBs) dissociates, the oxygen diffuses, and Y-Ti-O nano-oxides precipitate in the ferritic matrix.TEM characterization combined with XAFS and XANES analyses have proven to be suitable tools to follow the evolution of the nature of the different oxides present in the material during the whole processing route and select appropriate HIP and post-HIP parameters to promote profuse and fine Y-Ti-O nanometric precipitates.

XANES and EXAFS investigation of uranium incorporation on nZVI in the presence of phosphate

Effect of phosphate on the reduction of U(VI) on nZVI was determined by batch, XPS, XANES and EXAFS techniques. The batch experiments showed that nZVI was quite effective for the removal of uranium under the anaerobic conditions, whereas the addition of phosphate enhanced uranium removal over wide pH range. At low pH, the reduction of U(VI) to U(IV) significantly decreased with increasing phosphate concentration by XPS and XANES analysis. According to EXAFS analysis, the occurrence of UU shell at 10 mg/L phosphate and pH 4.0 was similar to that of U(IV)O2(s), whereas the UP and UFe shells were observed at 50 mg/L phosphate, revealing that reductive co-precipitate (U(IV)O2(s)) and precipitation of uranyl-phosphate were observed at low and high phosphate, respectively. The findings are crucial for the prediction of the effect of phosphate on the speciation and binding of uranium by nZVI at low pH, which is significant in controlling the mobility of U(VI) in contaminated environments.

Microwave Assisted Synthesis and Oxidation Resistance of Sm3+ Doped Fe3O4 Nanoparticles

In this study, pure and Sm3+ doped Fe3O4 nanoparticles were prepared via a microwave assisted synthesis in water solution. The samples were characterized by TEM, XRD, XANES, and VSM techniques. The structure and morphology, Fe and Sm oxidation states, the oxidation resistance and magnetic properties of pure and Sm doped Fe3O4 nanoparticles were studied. TEM and XRD results show that Sm doped Fe3O4 nanoparticles have a good dispersion and are smaller than pure Fe3O4 nanoparticles. The Fe and Sm oxidation states of pure and Sm doped Fe3O4 nanoparticles were studied by means of XANES analysis. The magnetic properties of Sm doped Fe3O4 nanoparticles determined by VSM exhibit superparamagnetic behavior with high saturation magnetization. It was found that Sm doping in a small amount makes it possible not only to reduce the sizes of magnetite nanoparticles but also to improve their oxidation resistance and increase their saturation magnetization value. These pure and Sm doped Fe3O4 magnetic nanoparticles are expected to be used in a number of biomedical applications.

Incorporation and subsequent diagenetic alteration of sulfur in Arctica islandica

Biogenic carbonates are important archives for reconstructing the marine sulfur cycle. However, uncertainties exist about the exact location of carbonate associated sulfate (CAS) and a possible biological control on sulfate incorporation. The behavior of CAS in biogenic carbonates during diagenetic alteration is even more poorly constrained. To investigate the mechanisms of sulfur incorporation and the effects of alteration on sulfur in biogenic carbonates, modern marine bivalve shells of Arctica islandica species were hydrothermally altered at 100 degrees C and 175 degrees C. Fluorescence microscopy, element mapping via NanoSIMS and mu-XRF, sulfur XANES analyses, and delta S-34 measurements were performed on the experimentally altered shell segments. Changes in elemental compositions and delta S-34 of sulfate in the post-alteration solutions were also determined. Results indicate clear differences between the delta S-34 values of the CAS (+ 21 parts per thousand, V-CDT) that reflects ambient seawater sulfate and the organically bound sulfur that is isotopically lighter (+ 14.8 parts per thousand, V-CDT or less). Carbonate associated sulfate is primarily incorporated in the mineral phase of the shell, whereas reduced sulfur phases are mainly found within the intrashell organic matter. Hydrothermal alteration experiments at 100 degrees C resulted in minimal changes of sulfur within the bivalve shells. In contrast, the 175 degrees C experiments triggered decomposition of intrashell organic matrices which then led to extensive diagenetic alteration in both the shell microstructure and chemistry. Changes in total concentration, speciation, and spatial distribution of sulfur reflect the diagenetic processes that occurred within the shells. Preferential incorporation of CAS in a neomorphic calcite phase with Mg/Ca ratios of 0.13-0.21 was observed. Due to its presence in both organic and inorganic phases and its multiple oxidation states with different isotopic compositions we conclude that sulfur is a useful and sensitive proxy for diagenetic alteration in biogenic aragonite.

Redox behavior and solubility of plutonium under alkaline, reducing conditions

Abstract The solubility and redox behavior of hydrous Pu(IV) oxide was comprehensively investigated by an experimental multi-method approach as a function of different redox conditions in 0.1 M NaCl solutions, allowing a detailed characterization of Pu(IV) and Pu(III) solubility and solid phase stability in these systems. Samples were prepared at ~3≤pHm≤~6 (pHm=–log m H + ) ${{\text{m}}_{{{\text{H}}^{\text{ + }}}}})$ and ~8≤pHm≤~13 at T=(22±2)°C under Ar atmosphere. No redox buffer was used in one set of samples, whereas mildly and strongly reducing redox conditions were buffered in two series with hydroquinone or SnCl2, respectively, resulting in (pe+pHm)=(9.5±1) and (2±1). XRD, XANES and EXAFS confirmed the predominance of Pu(IV) and the nanocrystalline character of the original, aged PuO2(ncr,hyd) solid phase used as a starting material. Rietveld analysis of the XRD data indicated an average crystal (domain) size of (4±1) nm with a mean cell parameter of (5.405±0.005) Å. The solubility constant of this solid phase was determined as log ∗ ​ K ° s , 0 $^ * K{^\circ _{{\text{s}},0}}$ =–(58.1±0.3) combining solubility data in acidic conditions and redox speciation by solvent extraction and CE–SF–ICP–MS. This value is in excellent agreement with the current thermodynamic selection in the NEA-TDB. Synchrotron-based in-situ XRD, XANES and EXAFS indicate that PuO2(ncr,hyd) is the solid phase controlling the solubility of Pu in hydroquinone buffered samples. Under these redox conditions and ~8≤pHm≤~13, the solubility of Pu is very low (~10−10.5 m) and pH-independent. This is consistent with the solubility equilibrium PuO2(am,hyd)+2H2O(l)⇔ Pu(OH)4(aq). Although in-situ XRD unequivocally shows the predominance of PuO2 in Sn(II)-buffered systems, XANES analyses indicate a significant contribution of Pu(III) (30±5%) in the solid phases controlling the solubility of Pu at (pe+pHm)=(2±1). For this system, EXAFS shows a systematic shortening of Pu–O and Pu–Pu distances compared to the starting Pu material and hydroquinone-buffered systems. The solubility of Pu remains very low (~10−10.5 m) at pHm>9, but shows a very large scattering (~10−9–10−10.5 m) at pHm=8. Experimental observations collected in Sn(II) buffered systems can be explained by the co-existence of both PuO2(ncr,hyd) and Pu(OH)3(am) solid phases, but also by assuming the formation of a sub-stoichiometric PuO2−x (s) phase. This extensive study provides robust upper limits for Pu solubility in alkaline, mildly to strongly reducing conditions relevant in the context of nuclear waste disposal. The potential role of Pu(III) in the solid phases controlling the solubility of Pu under these conditions is analysed and discussed in view of the current NEA-TDB thermodynamic selection, which supports the predominance of PuO2(am,hyd) and constrains the formation of Pu(OH)3(am) at pHm>8 outside the stability field of water.

Hexane Aromatization: Analysis of the K-Edges of S and K Provides New Insight into H2S Poisoning of Pt/KL

The purpose of this investigation was to examine the effect of sulfur impurity on 1%Pt/KL-zeolite catalyst by co-feeding 500 ppbv hydrogen sulfide (H2S) during hexane aromatization under industrially relevant conditions using a plug flow reactor. Product selectivity and hexane conversion were measured with time-on-stream and compared to a clean run carried out under otherwise identical conditions. Sulfur addition to the feed accelerated the rate of deactivation as observed by rapid declines in both hexane conversion and benzene selectivity; hexene selectivity, the product of the less structurally sensitive dehydrogenation reaction, increased significantly. After 20 h, which was enough time to observe sufficient deactivation, the reaction was stopped. For the purpose of catalyst characterization, after cooling to 150 °C, the catalyst was preserved in Polywax 725 to prevent catalyst oxidation. XANES analysis at the potassium K-edge suggests that the local environment for potassium was not significantly altered by sulfur addition, while sulfur K-edge results indicate that sulfur bound to platinum to form platinum sulfide (PtS, not PtS2). Platinum sulfide is likely responsible for accelerating Pt growth, as observed in DRIFTS of adsorbed CO and HR-TEM/STEM micrographs.

Local atomic structure of mono- and binuclear metal complexes based on 3-formylpyrone and 3-formylcoumarin bis-azomethines

A series of Cu, Ni, and Mn complexes based on formylpyrone and formylcoumarin azomethynes with 1,3- diaminepropanol-2. The analysis of XANES and EXAFS spectra of the Cu, Ni, and Mn complexes, which were processed by both Fourier and wavelet transforms, enables the determination of local atomic structural parameters and the unambiguous evidence of the formation of both dinuclear and mononuclear structures of these coordination compounds. The possibility of forming binuclear compounds is shown to depend mainly on the nature of the bridging ligand and be independent of the type of the polydentate azomethyne ligand: pyrone or coumarin. The obtained structural results are well consistent with the magnetochemical data for the complexes.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by~0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Selenium speciation in phosphate mine soils and evaluation of a sequential extraction procedure using XAFS

Selenium is a trace element found in western US soils, where ingestion of Se-accumulating plants has resulted in livestock fatalities. Therefore, a reliable understanding of Se speciation and bioavailability is critical for effective mitigation. Sequential extraction procedures (SEP) are often employed to examine Se phases and speciation in contaminated soils but may be limited by experimental conditions. We examined the validity of a SEP using X-ray absorption spectroscopy (XAS) for both whole and a sequence of extracted soils. The sequence included removal of soluble, PO4-extractable, carbonate, amorphous Fe-oxide, crystalline Fe-oxide, organic, and residual Se forms. For whole soils, XANES analyses indicated Se(0) and Se(-II) predominated, with lower amounts of Se(IV) present, related to carbonates and Fe-oxides. Oxidized Se species were more elevated and residual/elemental Se was lower than previous SEP results from ICP-AES suggested. For soils from the SEP sequence, XANES results indicated only partial recovery of carbonate, Fe-oxide and organic Se. This suggests Se was incompletely removed during designated extractions, possibly due to lack of mineral solubilization or reagent specificity. Selenium fractions associated with Fe-oxides were reduced in amount or removed after using hydroxylamine HCl for most soils examined. XANES results indicate partial dissolution of solid-phases may occur during extraction processes. This study demonstrates why precautions should be taken to improve the validity of SEPs. Mineralogical and chemical characterizations should be completed prior to SEP implementation to identify extractable phases or mineral components that may influence extraction effectiveness. Sequential extraction procedures can be appropriately tailored for reliable quantification of speciation in contaminated soils.

New Synthesis of nZVI/C Composites as an Efficient Adsorbent for the Uptake of U(VI) from Aqueous Solutions.

New nanoscale zerovalent iron/carbon (nZVI/C) composites were successfully prepared via heating natural hematite and pine sawdust at 800 °C under nitrogen conditions. Characterization by SEM, XRD, FTIR, and XPS analyses indicated that the as-prepared nZVI/C composites contained a large number of reactive sites. The lack of influence of the ionic strength revealed inner-sphere complexation dominated U(VI) uptake by the nZVI/C composites. Simultaneous adsorption and reduction were involved in the uptake process of U(VI) according to the results of XPS and XANES analyses. The presence of U-C/U-U shells demonstrated that innersphere complexation and surface coprecipitation dominated the U(VI) uptake at low and high pH conditions, respectively. The uptake behaviors of U(VI) by the nZVI/C composites were fitted well by surface complexation modeling with two weak and two strong sites. The maximum uptake capacity of U(VI) by the nZVI/C composites was 186.92 mg/g at pH 4.0 and 328 K. Additionally, the nZVI/C composites presented good recyclability and recoverability for U(VI) uptake in regeneration experiments. These observations indicated that the nZVI/C composites can be considered as potential adsorbents to remove radionuclides for environmental remediation.

Insights into structural properties of Cu species loaded on Bi2O3 hierarchical structures for highly enhanced photocatalysis

Pristine Bi2O3 is known to possess poor photocatalytic activity due to its low conduction band potential. Here, we attempt to enhance the photocatalytic activity of Bi2O3 by loading Cu species through a very simple impregnation method. After Cu loading at the optimum amount, we observe significant enhancements of photocatalytic activity for degradation and mineralization of atrazine under visible light by maximum factors of 9.4 (0.2wt.% Cu) and 11.7 (0.1wt.% Cu), respectively. Microstructural properties of Bi2O3 are not altered upon Cu loading, yet its light absorption ability is greatly improved. On the basis of EXAFS and XANES analysis, we suggest that Cu species exists in the +2 oxidation state, coordinated with four O atoms with bond lengths of ∼1.93Å in the form of amorphous nanoclusters.

Synergistic mechanism of U(VI) sequestration by magnetite-graphene oxide composites: Evidence from spectroscopic and theoretical calculation

Graphene oxide (GO) supported uniform magnetite nanoparticle composites (M-GO) were prepared via in situ chemical precipitation method. The reaction mechanisms between M-GO and U(VI) under anoxic conditions were clarified based on macroscopic, spectroscopic, and theoretical techniques. Incorporation of magnetite nanoparticle into GO matrix facilitated removal rate and reduction of U(VI) to U(IV), as indicated by the kinetics model. According to XPS and XANES analysis, high efficient removal of U(VI) by M-GO was attributed to the abundant oxygen-containing groups of GO and reduction potential of magnetite dispersed on the GO surface uniformly. Based on DFT calculation, the high binding energy of [GO-O…UO2]+ (23.5kcal/mol) and [GO-COO…UO2]+ (51.1kcal/mol) (51.1kcal/mol) revealed that the hydroxyl and carboxyl groups were responsible for the high removal of U(VI) on M-GO, which further facilitated the reduction of U(VI) to U(IV) due to the vicinity of structural Fe(II) of M-GO.