EXAFS Results Research Articles

Effect of Mg substitution on magnetic properties of Co 1−x Mg x Fe 2 O 4 nanoparticles investigated by EXAFS analysis

Abstract Co1−xMgxFe2O4 (x = 0.00, 0.25, 0.50, 0.75 and 1.00) nanoparticles were prepared by the hydrothermal method in Aloe vera extract solution. X-ray diffraction (XRD) results indicate a pure phase of all samples with the cubic spinel ferrite structure. TEM images reveal a decrease in average particle sizes from 37.03 ± 13.17 to 9.86 ± 2.63 nm with increasing Mg content. The X-ray absorption near edge structure (XANES) technique confirmed the existence of Co2+ and Fe2+, 3+ with partial substitutions of Mg cations in the structure. The magnetic properties of samples studied by vibrating sample magnetometer (VSM) indicated ferrimagnetism at room temperature. The saturation magnetizations (Ms) significantly decreased with increasing Mg content, relating to cation distribution revealed by the EXAFS results.

Spectroscopic and DFT study on the species and local structure of arsenate incorporated in gypsum lattice

The incorporation of arsenate (As(V)) in the gypsum structure is an important process for arsenic (As) fixation during industrial effluent treatment and may influence the mobility and bioavailability of As in surface environment. However, spectroscopic evidence is still lacking for its species and local structure. The species and local structure of As(V) in a gypsum lattice were investigated using Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT) modelling, and full-potential multiple scattering (FPMS) simulations. Ascorbic acid-treated As(V)-gypsum co-precipitates were used to avoid the influence of amorphous calcium arsenate on the characterization. The lack of the FTIR band in the range of 750–860cm−1 was an indicative that no AsO43− species was incorporated into the gypsum structure. DFT calculations proved that the incorporation of AsO43− was energetically much harder than HAsO42− species. The FPMS structural refinement yielded the optimal AsO interatomic distances of 1.77, 1.67, 1.65, and 1.66Å, with an average of 1.69±0.057Å, in agreement with the DFT and EXAFS results. Our work conclusively showed that HAsO42− dominated as the species of As(V) incorporated into the gypsum lattice, with the H atom in the HAsO42− group adjacent to water layer, regardless of pH.

Deoxygenation of palm kernel oil to jet fuel-like hydrocarbons using Ni-MoS2/γ-Al2O3 catalysts

In the current study, palm kernel oil was used as a renewable feedstock for production of jet fuel-like hydrocarbons via the deoxygenation over the Ni-MoS2/γ-Al2O3 catalyst. The dominant C12 fatty acid content in palm kernel oil makes it promising for jet fuel application. Synthesized by a liquid processing method with thiourea organosulfur agent, the catalyst revealed MoS2 structure with low stacking, while Ni substitution in the MoS2 structure and interaction with the Al2O3 support were determined based on the Ni K-edge XANES and EXAFS results. A high hydrodeoxygenation (HDO) activity, which as the major pathway in the deoxygenation, was observed upon application of a H2 pressure of 30–50bar over Ni-MoS2/γ-Al2O3. The optimum product yield of approximately 92% was obtained mainly from the HDO pathway (∼60%) with 58% selectivity to C10–C12 jet fuel hydrocarbons. The flow property of the jet fuel-like hydrocarbons was more desirable than those obtained from palm olein oil-derived fuel.

Novel highly ordered core–shell nanoparticles

Core–shell nanoparticles have potential for a wide range of applications due to the tunability of their magnetic, catalytic, electronic, optical, and other physicochemical properties. A frequent drawback in the design of core–shell nanoparticles and nanocrystals is the lack of control over an extensive, disordered, and compositionally distinct interface that occurs due to the dissimilarity of structural and compositional phases of the core and shell. In this work, we demonstrate a new hydrothermal nanophase epitaxy (HNE) technique to synthesize highly structurally ordered α-Cr2O3@α-Co0.38Cr1.62O2.92 inverted core–shell nanoparticles (CSNs) with evidence for the nanoscale growth of corundum structure beginning from the core and extending completely into the shell of the CSNs with minimal defects at the interface. The high-resolution TEM results show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms. The XPS and Co K-edge XANES analyses indicate the +2 oxidation state of cobalt is incorporated in the shell of the CSNs. Our XPS and EXAFS results are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the Co2+ ion for the Cr3+ ion in the α-Co0.38Cr1.62O2.92 shell. Furthermore, the CSNs exhibit the magnetic exchange bias effect, which is attributed to the exchange anisotropy at the interface made possible by the nanophase epitaxial growth of the α-Co0.38Cr1.62O2.92 shell on the α-Cr2O3 core of the nanoparticles. The combination of a well-structured, sharp interface and novel nanophase characteristics is highly desirable for nanostructures having enhanced magnetic properties.

Water-gas shift reaction over platinum/strontium apatite catalysts

Platinum supported on calcium hydroxy (CaHAP) and fluoroapatite (CaFAP) exhibits excellent water-gas shift (WGS) activity comparable to the best Pt WGS catalysts on reducible oxides. The high activity of these unusual WGS catalysts is attributed to the parallel activation of CO on the noble metal and H2O on the ionic phosphates with formate species apparently being the major reaction intermediates. We have studied Pt supported on strontium hydroxy and fluoroapatites with varying cation and noble metal contents now in order to elucidate the role of the apatite cations and anions on WGS activity. Initial Pt dispersions approached 100% for loadings up to 3% (weight) with mean particle sizes of 1–2nm according to H2 chemisorption and EXAFS results (coordination number of 6.6). TEM analyses show further that the Pt aggregates grew slightly during catalytic testing up to 723K with 1.5–2.5nm particles being most abundant afterwards. Moreover, Ptn+ reduction is much facilitated on these apatites as revealed by temperature programmed reduction coupled with in situ X-ray absorption spectroscopy (XAS). The WGS reaction rates in a reformate-type gas mixture increased strongly with Sr/P ratio in the apatite, with the rates over the most active Pt/SrHAP and Pt/SrFAP catalysts being among the highest reported to date. In particular, Pt/SrHAP and Pt/SrFAP rates surpassed the rates over analogous Ca2+ substituted catalysts by ca. 30% at 573K. This demonstrates that apatite cations have a strong impact on WGS conversion over these systems opening a route to catalysts with potentially even higher WGS activity.

Arsenic uptake and speciation in Arabidopsis thaliana under hydroponic conditions

Arsenic (As) uptake and species in Arabidopsis thaliana were evaluated under hydroponic conditions. Plant nutrient solutions were treated with arsenite [As(III)] or arsenate [As(V)], and aqueous As speciation was conducted using a solid phase extraction (SPE) cartridge. Arabidopsis reduced As(V) to As(III) in the nutrient solution, possibly due to root exudates such as organic acids or the efflux of As(III) from plant roots after in vivo reduction of As(V) to As(III). Arsenic uptake by Arabidopsis was associated with increased levels of Ca and Fe, and decreased levels of K in plant tissues. Arsenic in Arabidopsis mainly occurred as As(III), which was coordinated with oxygen and sulfur based on XANES and EXAFS results. The existence of As(III)O and As(III)S in EXAFS indicates partial biotransformation of As(III)O to a sulfur-coordinated form because of limited amount of glutathione in plants. Further understanding the mechanism of As biotransformation in Arabidopsis may help to develop measures that can mitigate As toxicity via genetic engineering.

Phase stability, EXAFS investigation and correlation between nanostructure and extrinsic magnetic properties of nanocrystalline Pr2(Co,Fe)7

Nanocrystalline Pr2 Co7−x Fex (x⩽3.5) powder was synthesized by high energy milling and was subsequently annealed at 973 K for 30 min. Their crystalline structures were investigated by means of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM). Magnetic properties were also studied at room temperature and 10 K as a function of the iron composition.This compounds structure depends on the iron content; for x⩽ 1 they crystallize in the Ce2 Ni7-type hexagonal structure (space group: P63/mmc). For 1 <x⩽ 3, we observe a mixture of three phases, Pr2(Co,Fe)7, Pr(Co,Fe)3 and Pr2(Co,Fe)17 while for larger values of x the Pr2(Co,Fe)7 phase disappears completely. Since the substitution of cobalt by iron in a pure Pr2 Co7−x Fex phase is limited to x⩽ 1, a site preference can be suggested. Unfortunately standard XRD cannot be used unambiguously because Co and Fe have too close normal X-ray atomic scattering factors. Among the local and Fe selective available experimental methods, we have chosen to explore the P63/mmc space group Wyckoff positions for iron by EXAFS at the Fe edge. This study shows that the local iron radial distribution function is much larger than expected for all the specific sites, at least two Fe–Pr distances, except for 12k which gives the best fit. A preferential substitution of cobalt by iron in Ref. 12k site is coherent with (i) the EXAFS result (ii) the substitution rate x<1 (iii) the anisotropic increase of the crystal cell parameters. A mixture of 12k, 6h and 2a cannot be excluded. However, if we assume a unique preferential site, 12k is the only solution.For x⩽ 1, the magnetocryslalline anisotropy is observed in Pr2 Co7−x Fex alloys with fairly strong anisotropy fields at room temperature in range from 123 to 136 kOe for x = 0 and x = 1, respectively. For x>1, the magnetocrystalline anisotropy field of the 2:17 phase is lower than that of the 2:7 phase, which causes a further reduction in anisotropy and coercivity.

Ultrastable 3V-PPh3 polymers supported single Rh sites for fixed-bed hydroformylation of olefins

The heterogenization of homogeneous catalysts is of critical importance for both fundamental research and practical applications. The key obstacle to industrial applications of heterogenized homogeneous catalysts is the long-term stability of the immobilized molecular complexes. We unambiguously demonstrate in this report that the vinyl-functionalized PPh3 polymers supported single Rh sites (Rh/POL–PPh3) catalyst provided not only excellent catalytic activity but also ultrahigh stability for fixed-bed olefins hydroformylation, which is one of the most important industrial homogeneous catalytic processes for the synthesis of aldehydes and alcohols. HAADF-STEM and EXAFS results indicated that the Rh atoms were present in single Rh sites which were strongly coordinated with the exposed phosphorous atoms of the POL–PPh3 support. The Rh/POL-PPh3 catalyst provided high activity and selectivity because it maintained the catalytic functionalities similar to the homogeneous HRh(CO)(PPh3)3 complex, as demonstrated both by 31P MAS NMR and in situ FT-IR experiments. Thus a reaction mechanism, similar to that over the homogeneous HRh(CO)(PPh3)3 complex, was proposed for olefin hydroformylation over the novel Rh/POL–PPh3 catalyst.

YCeZrO Ternary Oxide Solid Solution Supported Nonplatinic Lean-Burn NOx Trap Catalysts Using LaCoO3 Perovskite as Active Phase

A series of nonplatinic ceria-based oxides supported catalysts LaCoO3/K2CO3/S (S=CeO2, Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) were prepared by successive impregnation and employed for lean-burn NOx trapping. It is found that the doping of Zr or YZr into CeO2 facilitates the formation of CeZrO binary or YCeZrO ternary solid solutions, increasing the specific surface area and improving the redox property of ceria. The results of EXAFS and H2-TPR reveal that the LaCoO3 in the solid solution supported catalysts possesses higher dispersion and better reducibility than that in CeO2 supported one. The result of O2-TPD shows that the surface active oxygen species are remarkably increased after loading LaCoO3 on the supports. The YCeZrO ternary solid solution supported catalyst containing 5 wt % K2CO3 exhibits the best performance for NO oxidation and reduction at 350 °C, showing a high NO-to-NO2 conversion (66.5%) at lean condition, and a very high NOx reduction percentage (98.2%) and an extremely high NOx-to-N2 s...

XAFS study on nanostructured (La;Sr)CoO and (La;Sr)(Co;Fe)O IT-SOFC cathodes

In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of nanostructured La0.6Sr0.4CoO3(LSCO) and La0.6Sr0.4(Co;Fe)O3(LSCFO) cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 10000C in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions by X-ray absorption spectroscopies. For this purpose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder diffraction investigations.

Interaction of Cm(iii) and Am(iii) with human serum transferrin studied by time-resolved laser fluorescence and EXAFS spectroscopy

The complexation of Cm(III) with human serum transferrin was investigated in a pH range from 3.5 to 11.0 using time-resolved laser fluorescence spectroscopy (TRLFS). At pH ≥ 7.4 Cm(III) is incorporated at the Fe(III) binding site of transferrin whereas at lower pH a partially bound Cm(III) transferrin species is formed. At physiological temperature (310 K) at pH 7.4, about 70% of the partially bound and 30% of the incorporated Cm(III) transferrin species are present in solution. The Cm(III) results obtained by TRLFS are in very good agreement with Am(III) EXAFS results, confirming the incorporation of Am(III) at the Fe(III) binding site at pH 8.5.

Uranyl sorption onto birnessite: A surface complexation modeling and EXAFS study

This work investigates the mechanism of the uranyl interaction with birnessite, one of the most common layer-type MnO2 mineral at the Earth's surface, by coupling macroscopic (surface complexation experiments) and microscopic (EXAFS measurements) approaches. The sorption of uranyl on synthetic hexagonal birnessite, the low-pH birnessite form, was studied under various conditions of pH (3–6), electrolyte backgrounds (0.1M NaClO4, NaNO3 and Na2CO3), and solid/liquid ratios (from 0.27 to 4.5g/L). Sorption isotherms exhibit a complex form indicative of at least two types of sorption sites. EXAFS data reveal the presence of two equatorial O shells at ca. 2.32Å and 2.46Å for all the samples, and a Mn shell at ca. 3.38Å in the low-pH (≤5) samples only. No U–U pair was detected, despite the presence of polynuclear dissolved species in some of the samples.From the combination of the sorption isotherms and EXAFS results, a structural model for the sorption of uranyl onto hexagonal birnessite is proposed, in which two energetically different sites are involved. At low pH (≤5) a bidentate edge-sharing complex with Mn octahedra of the mineral edges can be inferred, whereas bidentate corner-sharing and/or monodentate complexation to layer vacancies would most likely describe EXAFS features of higher pH samples. A diffuse double layer model of surface complexation was developed for describing within the same framework the uranyl sorption against pH, involving both high-affinity (Mn octahedra edge) and low-affinity (above layer vacancies) sites.The comparison of the uranyl sorption onto hexagonal birnessite and various related environmental minerals shows that the affinity of uranyl for birnessite largely exceeds the sorption observed on montmorillonite and zeolite and turns out to be comparable to iron oxides, confirming the potential role of phyllomanganates to the control of uranyl mobility in post-oxic acidic environments.

Effect of Cobalt Particle Size on the Catalyst Intrinsic Activity for Fischer–Tropsch Synthesis

The active sites for Fischer–Tropsch synthesis are surface cobalt metal atoms. In order to obtain a high dispersion for reaction, ultra-small cobalt clusters (e.g., 1 nm) were supported on KL-zeolite using a chemical vapor deposition procedure. A Pt promoter was added to facilitate reduction of the cobalt oxide species. Upon activation in hydrogen, a high dispersion was obtained, but after exposure to realistic FT conditions in a pressurized continuously stirred tank reactor reactor, XANES and EXAFS results indicate that the fine cobalt clusters were susceptible to reoxidation, resulting in adverse activity and selectivity (30–37 % CH4 and 3.5–4.0 % CO2 on a C% basis).

Fischer–Tropsch synthesis: Comparisons between Pt and Ag promoted Co/Al2O3 catalysts for reducibility, local atomic structure, catalytic activity, and oxidation–reduction (OR) cycles

For economic reasons, Ag as a substitute for Pt promoter for FT Co/Al2O3 catalysts was advocated, due to its satisfactory ability to facilitate cobalt oxide reduction, its good catalytic performance in improving the CO conversion and selectivity and, especially, its much lower price compared to that of Pt (i.e., $23.31/Troy oz Ag. vs $1486.0/Troy oz Pt (May 10, 2013)). A comparative study between Pt and Ag promoters at several equivalent atomic loadings was performed in this work. While either Pt or Ag significantly facilitates cobalt oxide reduction supplying additional Co metal active sites compared to the unpromoted Co/Al2O3 catalysts, the total metal site density increased with increasing Pt loading, but become attenuated at high Ag loading. The EXAFS results indicate isolated Pt atoms interact with cobalt clusters to form Pt–Co bonds, without evidence of Pt–Pt bond formation, even at levels as high as 5wt% Pt. In Ag promoted Co/Al2O3 catalyst, not only were Ag–Co bonds observed, but Ag–Ag bonds were present, even at levels as low as 0.276% Ag. The degree of Ag–Ag coordination increased as a function of Ag loading, while decreases in BET surface area and a shift to wider average pore size suggests some pore blocking by Ag at high loadings, which likely restricted access of reactants to internal cobalt sites. Therefore, although both promoters initially facilitate reduction of cobalt oxides, their local atomic structures are fundamentally different. Either Pt or Ag can significantly improve the CO conversion rate on a per gram catalyst basis of Co/Al2O3. Slightly adverse effects on selectivity (i.e., increased CH4 and CO2, at detriment to C5+) were found with Pt, especially at higher loading, while Ag provides some benefits (i.e., slightly decreases CH4 and CO2, and increases C5+) at all loadings tested in this work. Moreover, TPR and chemisorption/pulse reoxidation results show that Pt and Ag continue to be in proximity with Co following oxidation–reduction (OR) cycles to continue to facilitate reduction. Additional reaction tests are required to determine the impact of regeneration on performance.

EXAFS and XANES structural characterization of bimetallic AuPd vapor derived catalysts

Using an innovative procedure known as metal vapor synthesis (MVS) to prepare bimetallic catalysts, starting from Au and Pd vapors, [AuPd] co-evaporated and [Au][Pd] separately evaporated bimetallic catalysts were achieved. After being tested, the catalytic activity and selectivity of the [AuPd] catalyst turned out to be higher than the [Au][Pd] ones. Using EXAFS spectroscopy it was shown that, in the [AuPd] samples, small bimetallic AuPd nanoparticles were present, having an Au rich core surrounded by an AuPd alloyed shell while in the [Au][Pd] sample there was the presence of monometallic Au and Pd nanoparticles showing some alloying only in the boundary regions. The EXAFS results were also qualitatively confirmed by the XANES spectra.

Formation of Ag clusters in zeolite X studied by in situ EXAFS and infrared spectroscopy

The in situ investigation of local structures by means of Ag K-edge EXAFS and infrared spectra for fully and partially Ag+ exchanged zeolite X (Agx–X; x=86, 66 and 12, where x is number of Ag atoms in unit cell) were carried out from room temperature to 300°C under vacuum. By evacuation at room temperature the coordination number of oxygen around Ag, NAg–O, decreases due to the removal of water molecules. By heating samples under vacuum, Ag clusters grew up via step that Ag–Ag interaction was weakened for Ag86–X and Ag66–X. On the other hand, for Ag12–X, the process with weak Ag–Ag interaction did not exist but Ag cluster was formed just by heating in a vacuum. These differences may have depended on the amount of residual Na+ ions in the zeolite cavity. From the infrared and EXAFS results it was suggested that the mobile Na+ ions in the cavity accelerated to produce Ag clusters for Ag12–X and Ag66–X in low temperature. On the other hand, Ag+ ions were only thermally excited in Ag86–X because there was no Na+ ion in the zeolite cavity. EXAFS result indicated the two types of Ag clusters which were characterized by the Ag coordination number, NAg–Ag=3.0±0.4 (cluster I) and 4.7±0.4 (cluster II). The Ag cluster formation proceeded step by step as these two types of clusters. It was contrasted to Ag zeolite 4A that only the middle size Ag clusters that NAg–Ag=3.8±0.4 exist.

H2PtCl6-derived Pt nanoparticles on USY zeolite: A qualitative and quantitative electron tomography study

The structural details of a bifunctional Pt/USY catalyst were studied using electron tomography. As a part of a fundamental study on the synthesis of Pt/USY catalyst, H2PtCl6 precursor was used for the impregnation of USY zeolite support as a source of negatively charged PtCl62− ions in order to restrict interaction with acidic sites of USY zeolite. After heat treatments, it was visualized in 3D that some Pt particles were 3–4nm in size, while the majority maintained the size of 1.2–1.5nm which corresponds to the diameter of the zeolite Y micropore cavities (1.2nm). Electron tomography further revealed that 3–4nm Pt particles seem to be entrapped in the zeolite crystal, without preferential location in mesopores or at the surface of the crystals. Quantitative image analysis provided the diameters (compared with EXAFS results) and the nearest neighbor distances of hundreds of imaged Pt particles.

Full geometry optimizations of the mixed‐valence CaMn4O4X(H2O)4 (X=OH or O) cluster in OEC of PS II: Degree of symmetry breaking of the labile Mn‐X‐Mn bond revealed by several hybrid DFT calculations

AbstractSeveral hybrid DFT methods were applied to full geometry optimizations of the CaMn4O4X(H2O)4 (XOH1− (1) or O2− (2)) cluster in the oxygen evolving complex (OEC) of photosystem II (PSII) to elucidate Mn‐Mn, Mn‐Ca, and Mn‐O distances on a theoretical ground. The computed Mn‐Mn distances were compared with previous (London and Berlin) X‐ray diffraction (XRD), and Berkeley and Berlin EXAFS results, together with the recent high‐resolution XRD structure by Umena and coworkers. Present computational results by the hybrid DFT methods have elucidated several differences among these accumulated results. These DFT results led us to reassign the Mn‐Mn and Mn‐Ca distances by the EXAFS experiments, which became consistent with the results obtained by the high‐resolution XRD structure. A characteristic feature revealed via the optimized Mn‐O distances was that the degree of symmetry breaking of the Mn1‐O(57)‐Mn4 bond is not so remarkable under the UBHandHLYP approximation but it can be large by other hybrid DFT methods. The computational results for 2 indicated reduction of the Mn3‐Mn4 distance with the deprotonation of the bridging oxo group. The hybrid DFT results for 1 are not inconsistent with an experimental proposal based on the new XRD structure, namely a protonated μ3‐oxygen at the internal O(57) site of the cluster in the S1 state. On the other hand, the reduction of Mn ions (not degradation of whole cluster structure) by the X‐ray irradiation still remains an important issue for refinements of the XRD structure. The computational results are discussed in relation to those of the electron spin echo envelope modulation (ESEEM) and possible pathways for water splitting reaction. Implications of the present DFT structures are discussed in relation to the previous DFT and related computational results, together with recent XRD results for cubane‐like model clusters for OEC of PSII. © 2012 Wiley Periodicals, Inc.

Uranyl adsorption on solvated edge surfaces of pyrophyllite: a DFT model study

In a computational study we addressed the adsorption of uranyl UO(2)(2+) on solvated (110) and (010) edge surfaces of pyrophyllite, applying a density functional approach to periodic slab models. We explored bidentate adsorption complexes on various partially deprotonated adsorption sites: octahedral Al(O,OH), tetrahedral Si(O,OH), and mixed AlO-SiO. Aluminol sites were determined to be most favorable on the (110) surface of pyrophyllite, while on the (010) surface mixed AlO-SiO sites are preferred. The structural parameters of all low-energy complexes on both surfaces agree rather well with EXAFS results for the structurally similar mineral montmorillonite. We calculate the average U-O distance to surface and aqua ligand oxygen atoms to increase with the increasing coordination number of uranyl whereas EXAFS results indicate the opposite trend. According to our results, several adsorption species, with different coordination numbers on different edge faces, may coexist on clay minerals. This computational finding rationalizes why earlier spectroscopic studies indicated the existence of more than one adsorption species, whereas a single type of adsorption complex was suggested from most EXAFS results.

Adsorption of Cu(II) to Bacillus subtilis: A pH-dependent EXAFS and thermodynamic modelling study

Bacteria are very efficient sorbents of trace metals, and their abundance in a wide variety of natural aqueous systems means biosorption plays an important role in the biogeochemical cycling of many elements. We measured the adsorption of Cu(II) to Bacillus subtilis as a function of pH and surface loading. Adsorption edge and XAS experiments were performed at high bacteria-to-metal ratio, analogous to Cu uptake in natural geologic and aqueous environments. We report significant Cu adsorption to B. subtilis across the entire pH range studied (pH ∼2–7), with adsorption increasing with pH to a maximum at pH ∼6. We determine directly for the first time that Cu adsorbs to B. subtilis as a (CuO5Hn)n−8 monodentate, inner-sphere surface complex involving carboxyl surface functional groups. This Cu–carboxyl complex is able to account for the observed Cu adsorption across the entire pH range studied. Having determined the molecular adsorption mechanism of Cu to B. subtilis, we have developed a new thermodynamic surface complexation model for Cu adsorption that is informed by and consistent with EXAFS results. We model the surface electrostatics using the 1pK basic Stern approximation. We fit our adsorption data to the formation of a monodentate, inner-sphere RCOOCu+ surface complex. In agreement with previous studies, this work indicates that in order to accurately predict the fate and mobility of Cu in complex biogeochemical systems, we must incorporate the formation of Cu-bacteria surface complexes in reactive transport models. To this end, this work recommends logKRCOOCu+=7.13 for geologic and aqueous systems with generally high B. subtilis-to-metal ratio.