X-ray Adsorption Fine Structure Spectroscopy Research Articles

Regulation of SiO2 Nanoparticles on the Adsorptive Fractionation of Dissolved Organic Matter by Goethite.

SiO2 nanoparticles (SiO2NPs) are most widely available and coexisting with DOM at the mineral-water interface; however, the role of SiO2NPs in DOM fractionation and the underlying mechanisms have not been fully understood. Using Fourier transform ion cyclotron resonance mass spectrometry, combined with Fourier transform infrared spectroscopy and X-ray adsorption fine structure spectroscopy, was employed to investigate the adsorptive fractionation of litter layer-derived DOM on goethite coexisting with SiO2NPs under different pH conditions. Results indicated that the inhibitory effect of the coexisting SiO2NPs on OM sorbed by goethite was waning as environmental pH increased due to the reduced steric interactions and the concurrent elevated hydrogen bonding/hydrophobic partitioning interactions on the goethite surface. We observed the coexisting SiO2NPs inhibited the adsorption of high carboxylic-containing condensed aromatic/aromatics compounds on goethite under different pH conditions while improving the adsorption of highly unsaturated aliphatic/phenolic and carbohydrate-like compounds in an alkaline and/or circumneutral environment. More nitrogen-containing structures may favor the adsorption of phenolic and nonaromatic compounds to goethite by counteracting the negative effect of SiO2NPs. These findings suggest that DOM sequestration may be significantly regulated by the coexisting SiO2NPs at the mineral-water interface, which may further influence the carbon-nitrogen cycling and contaminant fate in natural environments.

Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and non-paddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-microaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

Characterization and differentiation of rock varnish types from different environments by microanalytical techniques

We investigated rock varnishes collected from several locations and environments worldwide by a broad range of microanalytical techniques. These techniques were selected to address the challenges posed by the chemical and structural complexity within the micrometer- to nanometer-sized structures in these geological materials. Femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS), scanning transmission X-ray microscopy-near edge X-ray adsorption fine structure spectroscopy (STXM-NEXAFS) in combination with scanning electron microscopy (SEM) of focused ion beam (FIB) ultra-thin (100–200nm) sections, conventional and polarization microscopy, as well as electron paramagnetic resonance (EPR) measurements were used to obtain information about these rock varnishes.Rock varnishes from different environments, which cannot readily be distinguished based on their macroscopic appearance, differ significantly in their constituent elemental mass fractions, e.g., of Mn, Fe, Ni, Co, Ba, and Pb, and their rare earth element (REE) patterns. Structural characteristics such as the particle sizes of embedded dust grains, internal structures such as layers of Mn-, Fe-, and Ca -rich material, and structures such as cavities varied between varnishes from different environments and regions in the world. The EPR spectra were consistent with aged biogenic Mn oxides in all samples, but showed subtle differences between samples of different origin.Our observations allow us to separate rock varnishes into different types, with differences that might be indicators of distinct geneses. Five different types of rock varnish could be distinguished, Type I–V, of which only Type I might be used as potential paleoclimate archive. Each varnish type has specific characteristics in terms of their elemental composition, element distribution, and structures. The combination of element ratios (Mn/Ba, Al/Ni, Mn/REY, Mn/Ce, Mn/Pb, LaN/YbN, and Ce/Ce*), total REE contents, and structures can be used to separate the different types of rock varnish from each other.

Formation and Stability of Phenylphosphonic Acid Monolayers on ZnO: Comparison of In Situ and Ex Situ SAM Preparation.

Self-assembled monolayers (SAMs) enable an electronic interface tailoring of conductive metal oxides and offer an alternative to common transparent electrodes in optoelectronic devices. Here, the influence of surface orientation and pretreatment on the formation and stability of SAMs has been studied for the case of phenylphosphonic acid (PPA) on ZnO single crystals. Using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) and density-functional theory (DFT) calculations, the thermal stability and orientational ordering of PPA-SAMs on the polar and mixed-terminated ZnO surfaces were analyzed. On all surfaces, PPA-SAMs remain stable up to 550 K, while at higher temperatures a C-P bond cleavage and dissociative desorption takes place yielding two distinct desorption peaks. Based on DFT calculations, these desorption channels are attributed to protonated and deprotonated chemisorbed PPA molecules, which can be related to tri- and bidentate species, hence allowing to determine their relative abundance from the intensity ratio. Beside immersion, an alternative monolayer preparation based on vacuum deposition in combination with controlled desorption of excess multilayers is demonstrated. This enables a SAM preparation on bare ZnO surfaces without any precoating due to exposure to ambient air, which is further compared with SAM formation on intentionally hydroxylated substrates. Corresponding TDS data indicate that initial hydroxylation favors the formation of tridentate and deprotonated bidentate, while the OMBD preparation on bare surfaces yields a larger fraction of protonated bidentate species. The orientation of PPA molecules adopted in the SAMs was determined from the dichroism of K-edge NEXAFS measurements and reveals an almost upright orientation for the deprotonated species, while a slight tilting is obtained for monolayer films with a large fraction of protonated bidentate molecules.

Catalyst Chemical State during CO Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy and Near Edge X-ray Adsorption Fine Structure Spectroscopy.

The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O2 pressure range of 10-300 mTorr at 298-413 K was studied in situ using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O2:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (∼1 nm) Cu2O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu2O film increases in thickness and exceeds the escape depth (∼3-4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO2(δ-), with a coverage that correlates with the amount of Cu2O, suggesting that this phase is the most active for CO oxidation.

Support Effects of Ni2P Catalysts on the Hydrodeoxygenation of Guaiacol: In Situ XAFS Studies

The hydrodeoxygenation (HDO) of guaiacol was investigated over SiO2, ZrO2, and active carbon (AC) supported Ni2P catalysts. The physical properties of the catalysts were analyzed by temperature-programmed reduction (H2-TPR), CO-uptake chemisorption, and N2 physisorption. X-ray diffraction and extended X-ray adsorption fine structure spectroscopy were used to obtain structural properties for the supported Ni2P catalysts. The HDO was tested in a batch reactor at 573 K and 30 atm. The Ni2P/SiO2 catalyst underwent a decrease in the HDO conversion from 87 to 30 % for the first and second run of reaction. However, the Ni2P/ZrO2 and Ni2P/AC catalysts showed a little low but stable HDO conversions of 72 and 46 %, respectively. The in situ XAFS analysis revealed that differently from the cases of Ni2P/ZrO2 or and Ni2P/AC catalysts, the local structure of the Ni2P on SiO2 support underwent an oxidation to form nickel phosphate during the reaction, demonstrating that the SiO2 based Ni2P was vulnerable to the water or hydroxyl group of the reactant due to the hydrophilic nature of SiO2 support.

Investigations on Pt-Pd Aerogels for Oxygen Reduction Reaction

Carbon supported Pt nanoparticles are widely used as electrocatalysts for polymer electrolyte fuel cells (PEFCs). However, the carbon corrosion associated with particle detachment leads to a strong loss of electrochemically active surface area (ECSA) under operating fuel cell conditions. In order to increase the long-term performance, a promising strategy is the utilization of unsupported high surface area metallic electrocatalysts. For instance, nanostructured thin films (NSTF) showed significantly improved activity and durability for the sluggish kinetics of oxygen reduction reaction (ORR). [1] The challenge, therefore, is the development of unsupported high surface area metallic electrocatalysts for the ORR. Recently, we reported that multimetallic aerogels show considerable catalytic performance and durability over the time. They consist of a three-dimensional network of well-connected nanoparticles, having high porosity, high conductivity and high surface area of up to 180 m2 g-1. [2-3] The benefits for ORR activity and durability have recently been published. [4 - 5]In this work, we show two approaches for the preparation of Pt-Pd aerogel: the spontaneous one-step gelation route and the two-step gelation route. Both methods lead to the formation of the aerogel structure. However, the spatial distribution of the dissimilar metals and the homogeneity within the aerogels are expected to be strongly different. Ex-situ Extended X-ray Adsorption Fine Structure (EXAFS) spectroscopy was performed to provide chemical and structural information about homogeneity and distribution for the differently synthesized Pt-Pd aerogel materials. Afterwards, electrochemical investigations on the differently synthesized Pt-Pd aerogels were performed by using RDE technique. An electrochemical protocol was developed to establish the ECSA for the highly complex Pt-Pd aerogels. Based on the previous work, the durability of the Pt-Pd aerogels were traced out by monitoring of the ECSA and ORR activity over time combined with the changes of particle size, network structure and chemical composition.Our data show that the chemical and structural homogeneity results from the two differently used synthetic approaches and have strong influences on the catalytic performance and long-term stability. The one-step gelation process forms almost alloy particles, while non-alloyed connected particles are generated by using two-step gelation process. The homogeneously alloyed Pt-Pd aerogels exhibit higher activity and durability compared to non-alloyed systems. In-situ electrochemical XAS studies were performed to point out the changes of structure and composition during the electrochemical cycling.Our work provides new insights into the relationship between structure, composition, reactivity and robustness for unsupported high surface area bimetallic aerogels.[1]Debe, M..K., Schmoeckel, A. K., Vernstrom, G. D., Atanasoski R. J. Power Sources. 2006, 161(2), 1002-11.[2] Liu, W. et al., Angew. Chem. Int. Ed. 2012, 51, 5743-5747.[3] Herrmann, A.-K. et al., Chem. Mat., 2014, 26(2), 1074–1083.[4] Liu W. et al., Angew. Chem. Int. Ed. 2013, 52, 9849–985.[5] Schmidt, T. J. et al.; European Patent Application EP 12177908, 2012

Converting leguminous green manure into biochar: changes in chemical composition and C and N mineralization

Leguminous green manure is an important source of nitrogen (N) and carbon (C) in cropping systems. The fast turnover of leguminous green manure enables it to release N quickly, but limits its effectiveness in maintaining soil organic C content. Converting leguminous green manure into biochar facilitates its use as a soil amendment. In this study, we assessed how the conversion of leguminous green manure (Sesbania roxburghii) into biochar altered its chemical composition and subsequent C and N mineralization. Biomass was charred along a temperature gradient from 200 to 500°C. Using nuclear magnetic resonance and near-edge X-ray adsorption fine structure spectroscopy, we found that both C and N became enriched in aromatic and heterocyclic aromatic structures in biochar, and this structural change led to a reduction in C and N mineralization rates. The mineralized C decreased from 32.7% of the added C of raw biomass to <0.5% of that of biochar at charring temperatures above 400°C. N release shifted from N mineralization in raw biomass to N immobilization at charring temperatures at 500°C. As such, soil amended with biochar produced at charring temperatures exceeding 400°C demonstrated a 25% decrease in dry shoot biomass compared with unamended soil. The results indicated that the C stability of leguminous green manure can be achieved by converting raw material into biochar, but that the charring process may limit it to providing N.

Carbon adsorption onto Fe oxyhydroxide stalks produced by a lithotrophic iron‐oxidizing bacteria

Iron (Fe)-oxidizing bacteria have the potential to produce morphologically unique structures that may be used as biosignatures in geological deposits. One particular example is Mariprofundus ferrooxydans, which produces extracellular twisted ribbon-like stalks consisting of ferrihydrite, co-located with organic and inorganic elements. It is currently thought that M.ferrooxydans excrete and co-precipitate polysaccharides and Fe simultaneously; however, the cellular production of these polysaccharides has yet to be confirmed. Here, we report on a time-series study that used scanning transmission X-ray microscopy and C 1s and Ca 2p near-edge X-ray adsorption fine structure spectroscopy to investigate production of polysaccharides over the growth cycle of M.ferrooxydans. The production and morphology of twisted iron stalks were consistent with previous observations, but unexpectedly, in the log phase, the carbon content of the stalks was extremely low. It was not until stationary growth phase that a significant component of carbon was detected on the stalks. During the log phase, low levels of carbon, only detectable when the stalks were thin, suggested that M.ferrooxydans produce an extracellular polysaccharide template onto which the Fe precipitates. By stationary phase, the increased carbon association with the stalks was a result of adsorption of organic compounds that were released during osmotic shock post-stalk production. In the environment, elevated concentrations of DOC could adsorb onto the Fe stalks as well as a number of other elements, for example, Si, P, Ca, which, by preventing chemical interactions between the Fe nanoparticles, will prevent structural deformation during recrystallization and preserve the structure of these filaments in the rock record.

Pyridine on Si(001)-(2×1): Density functional theory simulations compared with spectroscopic measurements

The adsorption of pyridine at 200 and 300 K on single-domain Si(100)-2 x 1 surfaces at saturation coverage was investigated by means of x-ray photoelectron spectroscopy and angular-dependent near-edge x-ray adsorption fine-structure spectroscopy (NEXAFS). Spectroscopic results are compared with density functional theory calculations. The N 1(s) NEXAFS spectroscopic signatures of three chemical moieties (dative bond, C-C=N-C and Si-N sp(3)) are identified. At 200 K all surface products present Si-N bonds (dative bond and Si-N sp(3)), while at 300 K, C-C= N-C moiety is also detected probing minor species without a Si-N bond. At both temperatures the coexistence of these various species are dominated by 1,4-di-sigma butterflies (Si-N sp(3)).

Probing the Orientation of Electrostatically Immobilized Protein G B1 by Time-of-Flight Secondary Ion Spectrometry, Sum Frequency Generation, and Near-Edge X-ray Adsorption Fine Structure Spectroscopy

To fully develop techniques that provide an accurate description of protein structure at a surface, we must start with a relatively simple model system before moving to increasingly complex systems. In this study, X-ray photoelectron spectroscopy (XPS), sum frequency generation spectroscopy (SFG), near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to probe the orientation of Protein G B1 (6 kDa) immobilized onto both amine (NH(3)(+)) and carboxyl (COO(-)) functionalized gold. Previously, we have shown that we could successfully control orientation of a similar Protein G fragment via a cysteine-maleimide bond. In this investigation, to induce opposite end-on orientations, a charge distribution was created within the Protein G B1 fragment by first substituting specific negatively charged amino acids with neutral amino acids and then immobilizing the protein onto two oppositely charged self-assembled monolayer (SAM) surfaces (NH(3)(+) and COO(-)). Protein coverage, on both surfaces, was monitored by the change in the atomic % N, as determined by XPS. Spectral features within the SFG spectra, acquired for the protein adsorbed onto a NH(3)(+)-SAM surface, indicates that this electrostatic interaction does induce the protein to form an oriented monolayer on the SAM substrate. This corresponded to the polarization dependence of the spectral feature related to the NEXAFS N(1s)-to-π* transition of the β-sheet peptide bonds within the protein layer. ToF-SIMS data demonstrated a clear separation between the two samples based on the intensity differences of secondary ions stemming from amino acids located asymmetrically within Protein G B1 (methionine: 62 and 105 m/z; tyrosine: 107 and 137 m/z; leucine: 86 m/z). For a more quantitative examination of orientation, we developed a ratio comparing the sum of the intensities of secondary-ions stemming from the amino acid residues at either end of the protein. The 2-fold increase in this ratio, observed between the protein covered NH(3)(+) and COO(-) SAMs, indicates opposite orientations of the Protein G B1 fragment on the two different surfaces.

Synthesis and physico-chemical study of complexation of glyoxylic acid aroylhydrazones with Cu(II) in solution and solid phase

A series of copper(II) binuclear metallochelates with glyoxylic acid aroylhydrazones have been synthesized. The composition and structure were established with infrared, UV-Vis, 1H NMR, extended X-ray adsorption fine structure spectroscopy as well as magnetochemical studies and potentiometry. The proton-donor and proton-acceptor features of the ligand systems play a key role in the formation of the complexes.

Temporal Evolution of Benzenethiolate SAMs on Cu(100)

The structure and thermal stability of self-assembled monolayers (SAMs) of benzenethiolate (BT) on Cu(100) have been studied by means of thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), UV photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray adsorption fine structure spectroscopy (NEXAFS). Vapor deposition at room temperature yields a well-ordered, densely packed c(6 × 2) saturation structure. At room temperature, this film is, however, metastable and transforms via partial decomposition by cleavage of the S-C bond into a less densely packed layer that reveals a coexisting p(2 × 2) phase. Such a transition occurs on a time scale of several days and is accompanied by a reduction of the work function change with respect to the bare Cu(100) surface from Δϕ = -0.9 eV for a freshly prepared saturated layer to -0.5 eV for an aged film. TDS experiments exhibit the presence of two distinct desorption channels (dissociative and intact desorption) occurring at different temperatures that reflects a variation of the local Cu-S interaction strength of BT at differently coordinated adsorption sites. Heating to above room temperature causes a rapid degradation and continuous thinning of BT films whereas above 500 K all thiolate species have desorbed or dissociated, leaving a sulfide overlayer behind that is accompanied by a substrate reconstruction. Interestingly, the upright orientation of BT adopted in the saturated monolayer remains almost identical upon heating and demonstrates the absence of downward tilting upon thermally induced thinning of the film.

Self-assembled aggregates formed by single-molecule magnets on a gold surface

The spontaneous ordering of molecules into two-dimensional self-assembled arrays is commonly stabilized by directional intermolecular interactions that may be promoted by the addition of specific chemical side groups to a molecule. In this paper, we show that self-assembly may also be driven by anisotropic interactions that arise from the three-dimensional shape of a complex molecule. We study the molecule Mn(12)O(12)(O(2)CCH(3))(16)(H(2)O)(4) (Mn(12)(acetate)(16)), which is transferred from solution onto a Au(111) substrate held in ultrahigh vacuum using electrospray deposition (UHV-ESD). The deposited Mn(12)(acetate)(16) molecules form filamentary aggregates because of the anisotropic nature of the molecule-molecule and molecule-substrate interactions, as confirmed by molecular dynamics calculations. The fragile Mn(12)O(12) core of the Mn(12)(acetate)(16) molecule is compatible with the UHV-ESD process, which we demonstrate using near-edge X-ray adsorption fine-structure spectroscopy. UHV-ESD of Mn(12)(acetate)(16) onto a surface that has been prepatterned with a hydrogen-bonded supramolecular network provides additional control of lateral organization.

Probing Solid Catalysts under Operating Conditions: Electrons or X‐rays?

Seeing is believing: In light of recent advances, the pros and cons of using electrons and X-rays for in situ studies of catalysts are analyzed: by using X-rays the structure of bound reactants at steady state are obtained from extended X-ray adsorption fine structure spectroscopy (EXAFS) data (see graph), thereby affording mechanistic insights.

Adsorption Geometry of Pyridine on the Single Domain Si(100)-2 × 1 Surface: Fully Polarization Resolved NEXAFS

The adsorption of pyridine at room temperature on single-domain Si(100)-2 × 1 surfaces at saturation coverage has been investigated by means of fully polarization resolved near-edge X-ray adsorption fine-structure spectroscopy (NEXAFS) experiments. The major product of pyridine chemisorption has been identified as the 1,4,5,6-tetra-σ-bonded species that is grafted on two successive dimers keeping a C2═C3 bond parallel to the dimer axis.

Application of three methods for determining mercury speciation in mine waste

Three methods, pyrolytic and chemical extractions (PCE), extended X-ray adsorption fine structure spectroscopy (EXAFS) and solid-phase-Hg-thermo-desorption (SPTD) were applied to determine mercury speciation in amended substrates and mine waste samples. Although these three methods determine Hg speciation by fundamentally different processes, comparison of the results are useful for validation of the three methods. PCE uses pyrolysis and weak leaches to determine relative percentages of volatile, ‘soluble’ and residual Hg in substrate. The results are operationally defined and specific species cannot be determined with this method. EXAFS is a nondestructive method which uses high energy synchrotron-sourced X-ray radiation to identify specific species based on scattering patterns. Least squares data analysis is done to link patterns to a database of model compounds. This method is most useful for identification of specific species, given that they are included in the model database. Identification of Hg 0 is difficult using EXAFS. SPTD identifies Hg species by incremental heating and comparison of thermal release patterns to a database of compounds. SPTD allows the identification of a more limited number of specific species than EXAFS, but is the best of the three methods for the identification of Hg 0 . Overlapping release patterns make the identification of species, such as HgS and some forms of matrix-bound Hg, difficult. Results of PCE analyses indicate that volatile and leachable forms of Hg in mine waste are low relative to the total Hg concentration. This was supported by EXAFS and SPTD analysis which identified HgS as the primary component of mine waste. In contrast, analysis of tailings from mills that utilized Hg to amalgamate Au and Ag from ores yielded conflicting results. The results of this study illustrate the importance of using multiple analytical methods for the evaluation of Hg in the substrate.

Cesium adsorption on clay minerals: an EXAFS spectroscopic investigation.

Cesium adsorption on the clay minerals vermiculite and montmorillonite is described as a function of surface coverage using extended X-ray adsorption fine structure spectroscopy (EXAFS). Cesium (Cs) possessed a variable coordination environment consisting of Cs-O distances between 3.2 and 4.3 A; however, disorder typical of the Cs coordination environments prevented the resolution of all oxygen shells. On the basis of the influence of Cs loading and exchangeability on this structural arrangement, we could recognize both inner-sphere and outer-sphere adsorption complexes. The shorter Cs-O bond distance belongs to outer-sphere complexes typical of hydrated ions. In inner-sphere complexes, partially or fully dehydrated Cs coordinates directly to siloxane groups of the clay minerals forming longer Cs-O bonds. The inner-sphere adsorption complexes may have occurred within the interlayer or at frayed edge sites and were less extractable than the outer-sphere complexed Cs. Both coordination number ratios and linear combination fitting of EXAFS spectra were useful in estimating the fractions of inner-sphere and outer-sphere adsorption complexes. Our results show that X-ray absorption spectroscopy (XAS), and particularly EXAFS, is a valuable technique for exploring the type of Cs binding in environmental samples.

Sodium-induced ordering of the benzoate species on Si(100)-2×1: a combined HREELS, XPS and NEXAFS study

The structure of benzoate on Na/Si(100)-2×1 has been studied by high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy and near edge X-ray adsorption fine structure spectroscopy. At room temperature, benzoic acid (C 6H 5COOH) chemisorbs on Na/Si(100)-2×1 through a cleavage of the OH bond in the carboxylic group. The benzoate molecules formed are bonded exclusively to the sodium atoms in a bidentate coordination, in which the oxygen atoms are equivalent. At room temperature, benzoate saturation on Na/Si(100)-2×1 is reached at a coverage of one benzoate species for each Na atom or silicon dimer. At this coverage, the molecules are tilted in polar direction by 62°±4° to the surface plane and azimuthally rotated by 41°±4° with respect to the [01 1 ] surface azimuth. We propose an adsorbate structure, in which the benzoate molecules are oriented parallel to each other in densely packed rows.

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.