Characterization of Cu-complexes in smectite with different layer charge location: chemical, thermal and EXAFS studies

The retention of Cu and Cu-amino acid complexes by montmorillonite and beidellite, before and after repeated acidified aqueous solution treatments, was studied using X-ray diffraction, chemical and thermal analyses, mass spectrometry and synchrotron-based X-ray absorption spectroscopy (XAS).The results indicate that the extraction of metal complexes from smectites depends on the nature of the layer charge and on the kind of organic species. Cu-cysteine complexes are strongly retained in the interlayer position, whereas Cu-glycine complexes are mostly adsorbed in cationic form which can be easily removed from the silicate layer. The layer periodicity for Cu-smectites treated with glycine shows little or no layer expansion, whereas significant swelling of the layer periodicity is observed in smectites treated with cysteine.Thermal decomposition of both smectites with sorbed Cu-amino acid species shows the evolution of H₂O, NO, CH₃CH₃, and CO₂. In Cu-cysteine treated smectites, the release of H₂S, NO₂, SO₂, and N₂O₃ also occurs.X-ray absorption spectroscopy (XAS) was used to assess the relationships between the structure of the Cu complexes and their desorption from smectites. In Cu-exchanged smectites, the first coordination shell agrees with the hypothesis that the Cu coordinates to oxygen atoms to form monomer and/or dimer complexes. The first coordination shell of Cu in smectites treated with glycine shows four atoms at distances of ∼2 Å. Two of these bonds are with nitrogen and two with oxygen atoms. For copper-cysteine complexes XAS data are compatible with the existence of Cu-N clusters, thus suggesting that Cu links to the amino acid by the aminic group.     

X-ray absorption spectroscopy study of Cu and Zn adsorption complexes at the calcite surface: Implications for site-specific metal incorporation preferences during calcite crystal growth

We report results from in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Cu(II) and Zn(II) complexes forming at the calcite surface following adsorption from preequilibrated calcite-saturated solutions. Both Cu(II) and Zn(II) coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes. The Zn adsorption complexes are in tetrahedral coordination with first-shell O neighbors with R_Zn-O = 1.95 Å, and the Cu complexes are Jahn-Teller distorted, with equatorial R_Cu-O = 1.95 Å. Results from EXAFS data of dilute Cu- and Zn-calcite solid solutions confirm substitution of these metals in the Ca site of the calcite structure as octahedral complexes during coprecipitation. X-ray fluorescence microanalyses of calcite (101?4) hillocks grown in coprecipitation experiments show that divalent Cu and Zn, which have ionic radii smaller than Ca, are preferentially incorporated into the parallel arrays of <4?41>₊ steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, other divalent metals with sixfold ionic radii smaller than Ca (Co, Cd, Mn, Mg) have been shown to be preferentially incorporated into <4?41>₋ growth steps, which define the second pair of vicinal faces on the growth spirals, but which are symmetrically nonequivalent to the steps on the first pair. The distortion from octahedral symmetry observed for the Cu and Zn adsorption complexes likely plays a key role in the observed preference of Cu and Zn for incorporation into the <4?41>₊ steps.     

Electronic environments in carrollite, CuCo2S4, determined by soft X-ray photoelectron and absorption spectroscopy

Fracture surfaces of a natural carrollite specimen have been characterised by synchrotron and conventional X-ray photoelectron spectroscopy and near-edge X-ray absorption spectroscopy. For the synchrotron X-ray measurements, the mineral surfaces were prepared under clean ultra high vacuum and were unoxidised. The characterisation was undertaken primarily to establish unequivocally the oxidation state of the Cu in the mineral, but also to obtain information on the electronic environments of the Co and S, and on the surface species. Experimental and simulated Cu L_2,3-edge absorption spectra confirmed an oxidation state of Cu^I, while Co 2p photoelectron and Co L_2,3 absorption spectra were largely consistent with the Co^III established previously by nuclear magnetic resonance spectroscopy. S 2p photoelectron spectra provided no evidence for S to be present in the bulk in more than one state, and were consistent with an oxidation state slightly less negative than S^-II. Therefore it was concluded that carrollite can be best represented by Cu^ICo^III₂(S₄)^-VII. The Cu^I oxidation state is in agreement with that expected for Cu tetrahedrally coordinated by S, but is in disagreement with the Cu^II deduced previously from some magnetic, magnetic resonance and Cu L-edge X-ray absorption spectroscopic measurements. A significant concentration of S species with core electron binding energies both lower and higher than the bulk value were formed at fracture surfaces, and these entities were assigned to monomeric and oligomeric surface S species. The density of Cu d states calculated for carrollite differed from that previously reported but was consistent with the observed Cu L₃ X-ray absorption spectrum. The initial oxidation of carrollite in air under ambient conditions was confirmed to be congruent, unlike the incongruent reaction undergone by a number of non-thiospinel sulfide minerals.     

Metal island growth and dynamics on molybdenite surfaces

In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS₂) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface.The strength and the nature of the Ag-S(MoS₂) bond were further investigated by using molecular orbital calculations, ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). By applying quantum mechanical approaches using a two-dimensional periodic molybdenite slab and hexagonal MoS₂ clusters of different sizes with metal atoms adsorbed to them, it is possible to calculate the electron transfer between the mineral surface and the metal atom as well as the adsorption energy as a function of surface coverage. In addition, we used the results from the quantum mechanical runs to derive empirical potentials that model the characteristics of the forces within the crystal, within the adsorbed islands, and the metal and mineral surface. The combination of quantum mechanical calculations and empirical force field calculations explain the electronic structure and the highest stability of Ag islands that have seven atoms in diameter, which exactly agrees with the size of experimentally observed islands. UPS results also suggest that a specific new state is formed (approximately 4.5 eV into the valence band) which may describe the Ag-S bond because it does not occur in pure silver or molybdenite.This study shows how the combination of microscopic (STM), spectroscopic (STS, UPS), compositional (X-ray photoelectron spectroscopy, XPS) and molecular modeling (quantum mechanical and empirical) techniques is a useful approach to understand the nature of the metal to sulfide bond. Further insights may be gained concerning the natural association of certain metals with sulfides.     

Catalytic oxidation of arsenite and reaction pathways on the surface of CuO nanoparticles at a wide range of pHs

Recently, the wide application of CuO nanoparticles (NPs) in engineering field inevitably leads to its release into various geologic settings, which has aroused great concern about the geochemical behaviors of CuO NPs due to its high surface reactivity and impact on the fate of co-existing contaminants. However, the redox transformation of pollutants mediated by CuO NPs and the underlying mechanism still remain poorly understood. Here, we studied the interaction of CuO NPs with As(III), and explored the reaction pathways using batch experiments and multiple spectroscopic techniques. The results of in situ quick scanning X-ray absorption spectroscopy (Q-XAS) analysis verified that CuO NPs is capable of catalytically oxidize As(III) under dark conditions efficiently at a wide range of pHs. As(III) was firstly adsorbed on CuO NPs surface and then gradually oxidized to As(V) with dissolved O₂ as the terminal electron acceptor. As(III) adsorption increased to the maximum at a pH close to PZC of CuO NPs (~?pH 9.2), and then sharply decreased with increasing pH, while the oxidation capacity monotonically increased with pH. X-ray photoelectron spectroscopy and electron paramagnetic resonance characterization of samples from batch experiments indicated that two pathways may be involved in As(III) catalytic oxidation: (1) direct electron transfer from As(III) to Cu(II), followed by concomitant re-oxidation of the produced Cu(I) by dissolved O₂ back to Cu(II) on CuO NPs surface, and (2) As(III) oxidation by reactive oxygen species (ROS) produced from the above Cu(I) oxygenation process. These observations facilitate a better understanding of the surface catalytic property of CuO NPs and its interaction with As(III) and other elements with variable valence in geochemical environments.     

An XPS and SEM study of gold deposition at low temperatures on sulphide mineral surfaces: Concentration of gold by adsorption/reduction

Using X-ray photoelectron spectroscopy (XPS or ESCA) and scanning electron microscopy (SEM), we have examined the mechanism of adsorption and reduction of gold solutions on sulphides. KAu^IIICl₄ solutions are quickly adsorbed on the sulphide surfaces, and the Au(III) is quickly reduced to Au(0). Monolayer Au coverage is attained within one minute. The reduction is auto-catalyzed and Au metal grows on the surface. SEM photographs clearly show agglomerates of gold unevenly distributed on the surface. Specific sites of abnormally high Au concentration are found. We propose possible mechanisms for the adsorption and reduction. Our results suggest that adsorption of Au(III) and Au(I) by sulphide minerals, followed by reduction, could play an important role in the deposition of gold in natural systems especially at low temperature and low gold solution concentrations.     

生物纳米FeS包覆层对灰岩中和能力的影响

本文采用硫酸盐还原菌(SRB)和嗜酸铁还原菌(JF-5)合成纳米FeS,并将其包覆在灰岩表面,采用溶解动力学实验研究了不同纳米FeS包覆层对灰岩溶解和中和能力的影响。结果表明,X射线衍射表面包覆层矿物为纳米的四方硫铁矿,光电子能谱(XPS)结果进一步显示包覆层中Fe的价态为+2,S的价态为-2;包覆层对灰岩的溶解有明显的钝化影响,中和能力随厚度的增加而降低,最厚包覆层的存在能够使最终中和p H值降低1.5个单位。利用Frick第一定律推导了包覆层存在下灰岩的溶解过程公式,建立了包覆层溶解动态模型。     

Protective mineral layers formed from granite–dry steam interaction

This work focuses on interaction between granite–stainless steel (SUS) pipe–dry steam in the presence of Cu. Three types of hydrothermal experiments were conducted: (1) SUS–Cu–granite, (2) SUS–Cu, (3) SUS–granite. It was found that a high protective amorphous AlSi_1,6O₄ layer (thickness about 5 μm) was formed on the supporting pipe surface only in the case of SUS–Cu–granite interaction. The Al silicate layer formed during the experiment was characterized by X-ray diffraction (XRD) techniques and scanning electron microscopy (SEM) with EDX. According to kinetic data this layer has high protective properties.     

Precipitation of amorphous CaCO3 (aragonite-like) by cyanobacteria: A STXM study of the influence of EPS on the nucleation process

An amorphous or nanocrystalline calcium carbonate (ACC) phase with aragonite-like short-range order was found to be a transient precursor phase of calcite precipitation mediated by cyanobacteria of the strain Synechococcus leopoliensis PCC 7942. Using scanning transmission X-ray microscopy (STXM), different Ca-species such as calcite, aragonite-like CaCO₃, and Ca adsorbed on extracellular polymers were discriminated and mapped, together with various organic compounds, at the 30 nm-scale. The nucleation of the amorphous aragonite-like CaCO₃ was found to take place within the tightly bound extracellular polymeric substances (EPS) produced by the cyanobacteria very close to the cell wall. The aragonite-like CaCO₃ is a type of ACC since it did not show either X-ray or electron diffraction peaks. The amount of aragonite-like CaCO₃ precipitated in the EPS was dependent on the nutrient supply during bacterial growth. Higher nutrient concentrations (both N and P) during the cultivation of the cyanobacteria resulted in higher amounts of precipitation of the aragonite-like CaCO₃, whereas the amount of Ca²⁺ adsorbed per volume of EPS was almost independent of the nutrient level. After the onset of the precipitation of the thermodynamically stable calcite and loss of supersaturation the aragonite-like CaCO₃ dissolved whereas Ca²⁺ remained sorbed to the EPS albeit at lower concentrations. Based on these observations a model describing the temporal and spatial evolution of calcite nucleation on the surface of S. leopoliensis was developed. In another set of STXM experiments the amount of aragonite-like CaCO₃ precipitated on the cell surface was found to depend on the culture growth phase: cells in the exponential growth phase adsorbed large amounts of Ca within the EPS and mediated nucleation of ACC, while cells at the stationary/death phase neither adsorbed large amounts of Ca²⁺ nor mediated the formation of aragonite-like CaCO₃. It is suggested that precipitation of an X-ray amorphous CaCO₃ layer by cyanobacteria could serve as a protection mechanism against uncontrolled precipitation of a thermodynamically stable phase calcite on their surface.     

Toward a conceptual model of the calcite surface: hydration,hydrolysis, and surface potential

Because of a recent increase in interest in the properties of the calcite surface, there has also been an increase in activity toward development of mathematical models to describe calcite’s surface behaviour, particularly with respect to adsorption and precipitation. For a mathematical model to be realistic, it must be based on a sound conceptual model of atomic structure at the interface. New observations from high resolution techniques have been combined with previously published data to resolve the apparent conflict with results from electrokinetic studies and to present a picture of what the calcite surface probably looks like at the atomic scale.In ultra-high vacuum (10⁻¹⁰ mbar), a cleaved surface remains unreacted for at least an hour, but the unreacted surface does not remain as a termination of the bulk structure. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) show that the outer-most atomic layer relaxes and the surface slightly restructures. In air, dangling bonds are satisfied by hydrolysed water. XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) reveal the presence of adsorbed OH and H. In AFM images, the features so typical of calcite, namely, alternate-row offset, pairing and height difference, as well as the consistent dependence of these features on the force and direction of tip scanning, are best explained by OH filling of the vacant O sites created during cleavage on the Ca octahedra. Thus there is solid evidence to indicate the presence of OH and H chemi-sorbed at the termination of the bulk calcite structure.Wet chemical studies, however, show that calcite’s pH_pzc (zero point of charge) varies with sample history and solution composition. Electrophoretic mobility measurements indicate that the potential-determining ions are not H⁺ and OH⁻, but rather Ca²⁺ and CO₃²⁻ (or HCO₃⁻ or H₂CO₃⁰). This apparent conflict is resolved by a slight modification of the electrical double layer (EDL) model. At the bulk termination, hydrolysis species are chemi-bonded. At the Stern layer, adsorption attaches Ca²⁺ and CO₃²⁻ (or other carbonate species), but the hydrolysis layer keeps them in outer-sphere coordination to the surface. With dehydration, loss of the hydrolysis species results in direct contact between adsorbed ions and the bulk termination, therefore, inner-sphere sorption is equivalent to extension of the three dimensional bulk network, which is precipitation. Attachment of ions with size and charge compatible with Ca and CO₃ likewise results in coprecipitation and solid–solution formation.     

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.     

Mechanism of feldspar weathering—II. Observations of feldspars from soils

Examination of the surface morphology (via scanning electron microscopy) and surface composition (via X-ray photoelectron spectroscopy) of sodic plagioclase and potash feldspar grains taken from four different soils, provides little or no evidence for the existence of a tightly adhering protective surface layer of altered composition on the feldspar surface. Grains, from which all adhering clay has been removed by ultrasonic cleaning, exhibit the same chemical composition in the outermost few tens of angstroms as the underlying feldspar. Aluminum-rich ‘clay’ coatings which continue to adhere to the grains after ultrasonic treatment are patchy, highly hydrous, and unlikely to act as major diffusion-limiting, and thus protective, barriers. Attack by dissolution of the feldspar surface is non-uniform and follows a definite etching sequence characterized by the development and growth of distinctive etch pits. This dissolution sequence can be reproduced by treating fresh feldspars in the laboratory with strong HF-H₂SO₄^? solutions and, thus, the sequence is unaffected by the composition of the attacking solution. All of our results suggest that the dissolution of feldspar during weathering is controlled by selective chemical reaction at the feldspar-solution interface and not by uniform diffusion through a protective surface layer.     

ATR-FTIR spectroscopic characterization of coexisting carbonate surface complexes on hematite

The speciation of carbonate adsorbed to hematite in air-equilibrated aqueous solutions has been studied using ATR-FTIR spectroscopy. Samples were measured over a range of pH conditions, at 0.1 M NaCl and at low ionic strength, and in H₂O and D₂O solutions to permit a multispecies analysis of the data. Second-derivative analyses and fits to the spectra indicate the presence of two major and two minor surface-bound carbonate species. The two major complexes coexist at near-neutral pH and low ionic strength. One of these two complexes is relatively sensitive to ionic strength, being displaced at 0.1 M NaCl, whereas the other is not. Comparison of experimental to DFT/MO-calculated frequencies suggest these two major species to be (a) a monodentate binuclear inner-sphere carbonate surface complex, and (b) a fully or partially solvated carbonate (CO₃²⁻) species that is symmetry broken and appears to reside in the structured vicinal water layers at the hematite-water interface, retained by hydrogen bonding and/or other forces. Minor carbonate complexes include diffuse layer CO₃²⁻ and an unidentified inner-sphere species. Both of the dominant species observed here are likely to be significant controls of the surface charge and sorptive properties of Fe-oxides.     

Fast ion conduction character and ionic phase-transitions in disordered crystals: the complex case of the minerals of the pearceite–polybasite group

The minerals of the pearceite–polybasite group, general formula (Ag,Cu)₁₆ M ₂S₁₁ with M = Sb, As, have been recently structurally characterized. On the whole, all the structures can be described as a regular succession of two module layers stacked along the c axis: a first module layer (labeled A), with general composition [(Ag,Cu)₆(As,Sb)₂S₇]²⁻, and a second module layer (labeled B), with general composition [Ag₉CuS₄]²⁺. In detail, in the B layer of the pearceite structure silver cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. We have shown for the first time that the observed structural disorder in the B layer is strongly related to the fast ion conduction character exhibited by these minerals. This paper reports an integrated XREF, DSC, CIS and EPMA study on all the members of the pearceite–polybasite group. DSC and conductivity measurements pointed out that the 222 members show ionic-transitions at 340 K (arsenpolybasite-222) and 350 K (polybasite-222), whereas the 221 members have transitions at lower temperature, that is, 310–330 K (arsenpolybasite-221) and 335 K (polybasite-221). For the 111 members (pearceite and antimonpearceite), the transition occurs below room temperature at 273 K. In situ single-crystal X-ray diffraction experiments showed that these minerals present the same high temperature structure and are observed at room temperature either in their high temperature (HT) fast ion conductivity form or in one of the low temperature (LT) fully ordered (222), partially ordered (221) or still disordered (111) forms, with transition temperatures slightly above or below room temperature. The pearceite–polybasite group of minerals can be considered as a homogeneous series with the same aristotype, fast ion conducting form at high temperature. Depending upon the Cu content, an ordering occurs with transition temperatures related to that content: the lower the Cu content, the higher the transition temperature from the fast ion conducting HT form to the non-ion conducting form.Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Magnesia mixture as a regulator in the separation of pyrite from chalcopyrite and arsenopyrite

The aim of this paper is to find an effective method for the separation of the undesirable constituents, namely, chalcopyrite and arsenopyrite from pyrite used for the production of H₂SO₄. A new effective method is developed for co-depressing chalcopyrite with arsenopyrite by AsI₃, followed by the addition of magnesia mixture. This method has been shown to be based on the fact that iron sites exist in the three minerals, whereas copper and arsenic sites exist only in chalcopyrite and arsenopyrite, respectively. This is coupled with the ability of both Cu(I) and Cu(II) to precipitate As(III) in the form of insoluble copper arsenides, namely Cu₃As, Cu₃As₂. In contrast, neither Fe(II) nor Fe(III) form stable arsenides. Consequently, As³⁺ ions are selectively adsorbed onto the surface of chalcopyrite. The facility for oxidizability of As(III) is well known and hence it adsorbs oxygen from the pulp and changes to As(V) of higher valency and smaller size, with ionic potential over 10. Accordingly, it yields a stable complex anion with covalent bonding, namely, [AsO₄]^3?. These newly created arsenate sites on the surface of chalcopyrite, as well as the corresponding original arsenate sites on the surface of arsenopyrite combine with magnesia mixture to form cations leading to the formation of tightly abutting strongly hydrophilic layers of … AsO₄NH₄Mg.6H₂O. The spread of this hydrophilic film on arsenopyrite and chalcopyrite surfaces leads to the screening of their surfaces, making them difficult of access for the collector, ethyl xanthate. Since the pK_a of xanthic acid occurs at pH below 3, xanthate species predominate at pH above 8 and are adsorbed selectively on the pyrite surface in sufficient quantity for its selective flotation and hence for its separation to take place in the pH range 8–9.     

Surface oxidation of pyrite under ambient atmospheric and aqueous (pH = 2 to 10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy

The nature of the surface oxidation phase on pyrite, FeS₂, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur L_III edge, and iron L_II,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS₂ can be detected in the X-ray absorption spectra. In a 1.667 × 10⁻³ mol/L Fe³⁺ solution with ferric iron present as FeCl₃ in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.     

Surface Typochemistry of Native Gold

Using the methods of electron spectroscopy of the surface and SEM–EDS, it is shown that native gold of the deposit related to the epithermal Au–Ag ore formation contains oxidized gold with an oxidation degree of Au (I) or higher on the surface. A thin layer (~15 nm) with high concentrations of Ag and S and an underlying SiO₂-bearing layer with a thickness of ~30–60 nm play a protective role providing preservation of Ag and Au sulfides in the surface parts of the Au–Ag grains under the oxidizing conditions. S-rich marginal parts of native gold particles may be represented by solid solutions Ag_2–xAu _x S or (with a lack of S) by agglomerates of Ag _n Au _m S clusters. The formation of surface zoning in the nanoscale on the surface of native Au is abundant in nature and may be applied in prospecting.     

Interaction of muscovite (0 0 1) with Pu bearing solutions at pH 3 through ex-situ observations

The interaction of Pu³⁺ bearing solutions with the muscovite (0 0 1) basal plane is explored using a combination of ex-situ approaches including alpha-counting, to determine the Pu³⁺ adsorption isotherm, and X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR), to probe the interfacial structure and Pu-specific distribution, respectively. Pu uptake to the muscovite (0 0 1) surface from Pu³⁺ solutions in a 0.1 M NaClO₄ background electrolyte at pH 3 follows an approximate Langmuir isotherm with an apparent adsorption constant, K_app = 5 × 10⁴ M⁻¹, and with a maximum coverage that is consistent with the amount needed to fully compensate the surface charge by trivalent Pu. The XR results show that the muscovite surface reacted with a 10⁻³ M Pu³⁺ solution (at pH 3 with 0.1 M NaClO₄) and dried in the ambient environment, maintains a 30-40 Å thick layer, indicating the presence of a residual hydration layer (possibly including adventitious carbon). The RAXR results indicate that Pu sorbs on the muscovite surface with an intrinsically broad distribution with an average height of 18 Å, substantially larger than heights expected for any specifically adsorbed inner- or outer-sphere complexes. These results are discussed in the context of recent studies of cation adsorption trends on muscovite and the possible roles of Pu hydrolysis species in controlling the Pu-muscovite interactions.     

Interaction of finely ground galena and potassium amylxanthate in relation to flotation, 3. Influence of acid and neutral grinding

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Hallimond tube flotation and microelectrophoresis have been utilized to investigate the reactions in the adsorption-abstraction of K-amylxanthate on finely ground galena. The mineral was ground in a laboratory stainless steel rod mill under controlled conditions (pH 4.0 and 7.0) using HCl as a pH regulator. X-ray photoelectron spectroscopic (XPS) studies have been carried out in order to characterize the surface oxidation products after grinding (weak amounts of S_n and PbS₂O₃). The two-stage adsorption process discovered in previous studies was confirmed. For low concentrations or submonolayer capacity, the layer is formed with 1:1 monocoordinated lead xanthate and dixanthogen. For higher values of surface coverage, it is composed of lead xanthate (stoichiometric at pH 7 and non-stoichiometric at pH 4), amyldixanthogen and amylcarbonate disulphide. In the second stage mainly dixanthogen is formed. This stage corresponds to complete flotation and to a sharp decrease in zeta potentials.