The influence of surface-bound Fe(II) on uranium oxidation state and speciation was studied as a function of time (6 min-72 h) and pH (6.1-8.5) in a U(VI)-Fe(II)-montmorillonite (Ca-montmorillonite, MONT) system under CO(2)-free, anoxic (O(2) <1 ppmv) conditions. The results show a rapid removal of U(VI) from the aqueous solution within 1 h under all pH conditions. U L(III)-edge X-ray absorption near-edge structure spectroscopy shows that 96% of the total sorbed U(VI) is reduced at pH 8.5. However, the extent of reduction significantly decreases at lower pH values as specifically sorbed Fe(II) concentration decreases. The reduction kinetics followed by X-ray photoelectron spectroscopy during 24 h at pH 7.5 demonstrates the presence of partially reduced surface species containing U(VI) and U(IV). Thermodynamically predicted mixed valence solids like U(3)O(8)/beta-U(3)O(7)/U(4)O(9) do not precipitate as verified by transmission electron microscopy and extended X-ray absorption fine-structure spectroscopy. This is also supported by the bicarbonate extraction results. The measured redox potentials of Fe(II)/Fe(III)-MONT suspensions are controlled by the Fe(II)/hydrous ferric oxide [HFO(s)] couple at pH 6.2 and by the Fe(II)/lepidocrocite [gamma-FeOOH(s)] couple at pH 7.5. The key finding of our study is the formation of a sorbed molecular form of U(IV) in abiotic reduction of U(VI) by sorbed Fe(II) at the surface of montmorillonite.

Competition between selenium (IV) and silicic acid for the hematite (α-Fe2O3) surface has been studied during this work. Single batch experiments have been performed to study separately the sorption of selenium (IV) and silicic acid as a function of the pH. With the help of the 2-pK surface complexation model, experimental data have been fitted using the FITEQL 4.0 program. Two monodentate inner-sphere surface complexes have been used to fit selenite ions retention, FeSeO3- and FeHSeO3. In order to fit sorption of silicic acid, the two following surface complexes, namely FeH3SiO4, and FeH2SiO4-, have been used. Using the surface complexation constants coming from these two binary systems, prediction curves of the effect of silicic acid on the retention of selenium (IV) onto hematite have been obtained. Finally, performed experiments showed a competition between selenium (IV) and silicic acid for the surface sites of hematite. Experimental data matched DDLM predictions, confirming the ability of the surface complexation model to predict quantitatively and qualitatively the ternary system selenium (IV)/H4SiO4/hematite.

Sorption of selenium(IV) and silicic acid onto magnetite (Fe(3)O(4)) was investigated in binary systems, with concentrations of silicic acid under the solubility limit of amorphous silica. Using the double diffuse layer model (DDLM), surface complexation constants of selenium(IV) and H(4)SiO(4) onto magnetite were extracted using Fiteql 4.0. Then, prediction curves of the sorption of selenium(IV) in the presence of silicic acid onto magnetite were obtained, using the calculated surface complexation constants. Finally, laboratory experiments were performed and showed a competition between selenium(IV) and silicic acid for the surface sites of magnetite. Experimental results matched the model predictions, confirming its ability to model qualitatively and quantitatively the ternary system.

Vetiver roots have been utilized for the preparation of activated carbon (AC) by chemical activation with different impregnation ratios of phosphoric acid, XP (g H3PO4/g precursor): 0.5:1; 1:1 and 1.5:1. Textural characterization, determined by nitrogen adsorption at 77 K shows that mixed microporous and mesoporous structures activated carbons (ACs) with high surface area (>1000 m2/g) and high pore volume (up to 1.19 cm3/g) can be obtained. The surface chemical properties of these ACs were investigated by X-ray photoelectron spectroscopy (XPS) and Boehm titration. Their textural and chemical characteristics were compared to those of an AC sample obtained by steam activation of vetiver roots. Classical molecules used for characterizing liquid phase adsorption, phenol and methylene blue (MB), were used. Adsorption kinetics of MB and phenol have been studied using commonly used kinetic models, i.e., the pseudo-first-order model, the pseudo-second-order model, the intraparticle diffusion model and as well the fractal, BWS (Brouers, Weron and Sotolongo) kinetic equation. The correlation coefficients (R2) and the normalized standard deviation Δq (%) were determined showing globally, that the recently derived fractal kinetic equation could best describe the adsorption kinetics for the adsorbates tested here, indicating a complex adsorption mechanism. The experimental adsorption isotherms of these molecules on the activated carbon were as well analysed using four isotherms: the classical Freundlich, Langmuir, Redlich–Peterson equations, but as well the newly published deformed Weibull Brouers–Sotolongo isotherm. The results obtained from the application of the equations show that the best fits were achieved with the Brouers–Sotolongo equation and with the Redlich–Peterson equation. Influence of surface functional groups towards MB adsorption is as well studied using various ACs prepared from vetiver roots and sugar cane bagasse. Opposite effects governing MB and phenol adsorption mechanism on ACs are demonstrated. The various effects involved in adsorption mechanisms of each molecule are demonstrated.

In the case of a hypothetical severe accident in a nuclear power plant, interactions of gaseous RuO4 with reactor containment building surfaces (stainless steel and epoxy paint) could possibly lead to a black Ru-containing deposit on these surfaces. Some scenarios include the possibility of formation of highly radiotoxic RuO4(g) by the interactions of these deposits with the oxidizing medium induced by air radiolysis, in the reactor containment building, and consequently dispersion of this species. Therefore, the accurate determination of the chemical nature of ruthenium in the deposits is of the high importance for safety studies. An experiment was designed to model the interactions of RuO4(g) with samples of stainless steel and of steel covered with epoxy paint. Then, these deposits have been carefully characterised by scanning electron microscopy (SEM/EDS), electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). The analysis by XPS of Ru deposits formed by interaction of RuO4(g), revealed that the ruthenium is likely to be in the IV oxidation state, as the shapes of the Ru 3d core levels are very similar with those observed on the RuO2·xH2O reference powder sample. The analysis of O 1s peaks indicates a large component attributed to the hydroxyl functional groups. From these results, it was concluded that Ru was present on the surface of the deposits as an oxyhydroxide of Ru(IV). It has also to be pointed out that the presence of “pure” RuO2, or of a thin layer of RuO3 or Ru2O5, coming from the decomposition of RuO4 on the surface of samples of stainless steel and epoxy paint, could be ruled out. These findings will be used for further investigations of the possible revolatilisation phenomena induced by ozone.

Sorption of H4SiO4 (including experiments as a function of time, Kd measurement with different m/v ratios and sorption edges) onto different iron (hydro)oxides as goethite (α-FeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4) has been studied with concentration of silicates under solubility limit. A surface complexation model has been used to account for sorption edge of silicates onto these iron oxide surfaces. It reveals that two types of surface complex namely FeH3SiO4 and FeH2SiO−4, are needed to describe properly the experimental observations.

We report a new SWNT functionalization method through 4-methoxyphenyl free radical addition. The 4-methoxyphenyl radicals are generated by air oxidation of 4-methoxyphenylhydrazine hydrochloride. Absorption and Raman spectroscopy show the covalent nature of the bond between the nanotubes and the functional groups. The XPS and TGA data furnish quantitative information on the degree of functionalization.

The efficiencies of two methods of functionalizing single wall carbon nanotubes (SWCNTs) are compared, either through a radical addition of 4-methoxyphenylhydrazine hydrochloride by a classical thermally activated procedure, or via a microwave-assisted method. X-ray photoelectron spectroscopy and thermal gravimetric analysis clearly indicate the efficiency of both methods. Raman and absorption spectroscopy further confirm the functionalization and reveal the covalent nature of the bonds created at the carbon nanotube surface. For the microwave-assisted reaction, 5–15min is enough to functionalize the SWCNTs. Longer microwave exposure times reduce the functionalization yield and lead to a removal of groups which were bonded in a previous stage. An optimal choice of microwave irradiation time allows reducing the reaction time from days to minutes.

The surface charge of colloidal particles is usually determined by potentiometric titration. These acid-base titrations make it possible to measure the pH of point-of-zero charge (pzc) for oxide minerals. This macroscopic property is the most important parameter used in surface complexation modeling to reproduce experimental data. The pzc values of goethite reported in the literature vary between 7.0 and 9.5. Carbonate adsorption and/or surface morphology are thought to account for this wide range.We demonstrate a procedure for the removal of the carbonate ions that initially adsorb on goethite and strongly affect the titration curves and pzc determination. We also investigated the crystal-face-specific reactivity of two morphologically different goethites. The z-profiles obtained from atomic force microscopy (AFM) images showed that the goethite with the smallest specific surface area (S = 49 m(2)/g, denoted G49) exhibits 70% of the (001) face, whereas this value is only 30% for the goethite with largest specific surface area (S = 95 m(2)/g, denoted G95). This morphologic difference results in slightly different pzc values: 9.0 for G49 goethite and 9.1 for G95 geothite. These experimental pzc values have been correlated with multisite complexation calculations using both the full-site and the 1-pK approaches. We used the full-site approach to consider all of the configurations of hydrogen bond interactions with surface site. The resulting mean charges gave estimated pzc values of 8.9 and 9.2 for the (001) and (101) faces, respectively. Considering these theoretical pzc values for individual faces and the face distributions obtained from AFM analysis, the calculated pzc values are in full agreement with the experimental pzc values. However, this morphologic difference is more expressed in surface charge values than in the pzc values. Indeed, the surface charge of G49 goethite is much higher than that of G95 goethite, and the 1-pK calculations make it possible to fit the titration data satisfactorily. Copyright (C) 2003 Elsevier Science Ltd.

The interactions of mercury on cleaved and polished galena PbS(0 0 1) were studied at room temperature. The surface of the samples was characterised by X-ray photoelectron spectroscopy (XPS), RHEED and X-ray absorption spectroscopy (EXAFS) before and after interaction with a pressure of 2.5 x 10(-3) mbar of mercury vapours. Sorbed mercury was oxidised to Hg(II), as revealed by the chemical shift observed on the Hg 4f(7/2) levels. Since the adsorption of mercury is completely inhibited when the oxidation products of galena cover the surface, the formation of Hg-S bonding is most likely to occur. The amount of adsorbed mercury at saturation was estimated to be about one monolayer, i.e. one atom of mercury per surface atom of sulphur. A (I x 1) structure was observed by RHEED for the longest exposures. The analysis of the EXAFS oscillations at the L-III edge of mercury allowed an estimate of the Hg-S bond length of 2.62 Angstrom and a co-ordination number of 3. A model is proposed for the structure of the chemisorbed layer of mercury. (C) 2001 Elsevier Science B.V. All rights reserved.

In this study, the sorption of Hg(II) onto pyrite was investigated as a function of the aqueous solution pH, X-ray photoelectron spectroscopy (XPS) was used to characterize the surfaces, and to identify the adsorbed species when possible. After 12 h of equilibration at a given initial pH (between 6 and 10,5), slabs of pyrite were in contact with Hg(II) solution (210 mu M, constant ionic strength fixed with 0.01 M NaNO3) for 24 h, The final pH ranged between 4.1 and 3.6, The solid samples were then dried and transferred into the XPS machine for surface analyses (Hg 4f, S 2p, O 1s and Fe 2p levels).Taking advantage of the differential charge effect when the surface was partly covered with Fe(III) oxyhydroxides, it was shown that Hg could be sorbed onto both pyritic zones and oxidized patches. On pyritic zones where no charge effect was observed, the Hg 4f(7/2) level was pointed out at 100.7 eV, ruling the presence of Hg(O) at the surface out. The S 2p level analyses clearly showed that the main component was S-2(2-), with some traces of polysulphur, Neither S(II) nor S(VI) were detected in any experiment, excluding the formation of HgS, sulphate and thiosulphate, Most observations were understood as the formation of two surface complexes between mercury and either the pyritic functional groups or the oxyhydroxide sites. Copyright (C) 2000 John Wiley & Sons, Ltd.

Hexavalent chromium, a byproduct of many industrial processes, is toxic and produces mobile aqueous oxyanions, whereas Cr(III) is relatively immobile in the environment and, moreover, essential in human glucidic metabolism. For this reason, Fe(II)-Fe(III) layered double hydroxysalt green rusts, recently identified as a mineral in hydromorphic soils, were evaluated as potential Fe(II)-bearing phases for hexavalent chromium reduction. Both considered synthetic varieties, the hydroxysulfate GR(SO42-) and the hydroxychloride GR(Cl-), proved to be very reactive; their interaction with potassium chromate solutions leads to the rapid and complete reduction of Cr(VI) into Cr(III). The Cr(III)-bearing solid phase, studied by X-ray diffraction, Mossbauer, X-ray photoelectron, and Raman spectroscopies, was determined to be a poorly ordered Cr(III)-Fe(III) oxyhydroxide, similar to the "2 the line ferrihydrite". The comparison between the experimental redox potential and pH values for a theoretical equilibrium diagram bearing Cr and Fe phases indicated that the solubility of this solid solution, which may govern the behavior of chromium in the environment, is of the same order as that of pure Cr(OH)(3).

The sorption mechanism of Ni(II) on pyrite (FeS2) and on arsenopyrite (FeAsS) has been investigated at pH 10, using complementary methods for surface characterization such as x-ray photoelectron spectroscopy (XPS), Fourier transform mass spectrometry (FTMS), high-frequency dielectric measurements and electrophoretic mobility measurements. At this pH, the surfaces of the minerals are rapidly covered by an oxidation layer composed mainly of Fe(III) oxides on FeS2 and Fe(III) arsenite and arsenate on FeAsS and presenting negative zeta potentials of −40 and −60 mV, respectively. The first step of the interaction with Ni(II) is the formation of a hydroxylated surface complex through proton exchange with the surface hydroxyl groups ( Fe OH). Then, electrostatic interactions between colloidal Ni(II) and the surface produce a heterogeneous coating of an average thickness of a few nanometres. This film inhibits significantly the oxidation by H2O2 of these two minerals, particularly FeAsS. Copyright © 1999 John Wiley & Sons, Ltd.

X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively) and scanning tunneling microscopy (STM) were used. to observe the initial oxidation of pyrite surfaces in air. The results show the growth of oxide-like oxidation products, with minor contributions from sulfate, UPS shows a decrease in the density of electronic slates in the uppermost valence band of pyrite, corresponding to oxidation of surface Fe2+. This allows reliable interpretation of STM images, which show that initial surface oxidation of Fe2+ proceeds by growth of oxidized patches. The borders of oxidized patches contain small segments oriented in the [110] and [100] directions. STM of as-received pyrite cube surfaces, oxidized in air for years, also show the importance of the [110] crystallographic directions, on the surface, in controlling reaction progress.A model in which oxidation probabilities for Fe2+ surface sites are proportional to the number of nearest-neighbor oxidized (Fe3+) sites was tested using a Monte Carlo approach and reproduces the surface patterns observed in STM.An oxidation mechanism consistent with the XPS, UPS, STM, and Monte Carlo results is proposed. The rate constant for electron transfer from surface-exposed pyrite Fe2+ to O-2 is small. Electron transfer is more rapid from pyrite Fe2+ to Fe3+ present on the surface as an oxidation product, such as in the patches we observed. Fe2+ in oxide is a better reductant than Fe2+ in pyrite, so electron transfer to O-2 from the oxide is also fast. However, this two-step mechanism is faster overall only if electron transfer to the surface oxide patches is irreversible (e.g., because of S-2 oxidation or electron hopping within the surface oxide patches). Cycling of Fe between the Fe2+ and Fe3+ forms, particularly along borders between oxidized and unoxidized areas, is thus a key feature of the pyrite oxidation mechanism. An understanding of the surface electronic and band structure aids definition of the redox potentials of electrons in various surface states. Rates of electron transfer from these states to O-2 are estimated using a kinetic theory of elementary heterogeneous electron transfer.

Ultra thin platinum films evaporated on Cu(111) at 100 K and at room temperature have been investigated by AES, RHEED, temperature programmed desorption (TPD) of carbon monoxide and photoemission of adsorbed xenon (PAX). A layer-by-layer growth mechanism was evidenced up to at least 5 ML Pt. In the first Pt monolayer, the PtPt bond distances are strained ∼7% beyond the equilibrium bond distances found for bulk platinum. CO TPD is very sensitive to the coverage of deposited Pt: during the first Pt monolayer built-up the disappearance of Cu adsorption sites is observed and new features related to platinum are observed; at monolayer coverage, it appears that molecular CO is more weakly bound than on bulk Pt, the maximum of the main desorption peak is lowered by about 120–150 K; a 2 ML Pt deposit gives rise to CO TPD spectra looking like CO TPD spectra of bulk platinum. The binding energy of the Xe5p12 levels used in PAX experiments as a local probe for the work function shows that the surface properties during the built-up of the first monolayer of platinum remain close to those of Cu(111). Adsorption of xenon and of carbon monoxide on 1 ML Pt present behaviors very close to those observed on the surface of a bulk PtCu alloy. An interpretation in terms of strong modifications of the electronic structure due to PtCu interactions is proposed.

Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been studied by spectroscopic (XPS, UPS, AES) and structural (RHEED) methods. Eu atoms of the first two deposited monolayers are divalent and form a p(2 X 2) arrangement on the Pd(111) surface. During the completion of the third Eu monolayer, this ordered structure smears out in connection with the appearance of trivalent Eu atoms at the interface. The fine structure modifications of Pd and valence band XPS spectra indicate the 4d band filling, via a charge transfer mechanism, of the Pd atoms involved in the interface. For thicker Eu coverages a diffusion process leads to the formation of disordered divalent Eu-Pd alloys.After annealing the sample at 820 K, RHEED diffraction patterns reappear (p(2 X 2)) and, simultaneously, an increase in the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trivalent Eu-Pd compound which forms at the interface, This behaviour occurs whatever the thickness of the Eu deposit is.

The electronic properties of europium layers deposited on a Pd(111) single crystal (up to 190 \AA{} of europium) have been studied by ultraviolet and x-ray photoemission spectroscopies on both the valence bands and core levels (Eu and Pd). It is shown that, except for very low europium thicknesses, the formation of the Eu/Pd(111) interface is dominated by diffusion processes between europium and palladium. A systematic comparison between the electronic properties of this diffusive interface and the ones of well-defined intermetallic compounds (${\mathrm{EuPd}}_{5}$, ${\mathrm{EuPd}}_{3}$, ${\mathrm{EuPd}}_{2}$, and EuPd) and amorphous ${\mathrm{Eu}}_{\mathit{x}}$${\mathrm{Pd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ alloys, allows one to specify what kind of alloys or compounds formed at the interface as a function of the Eu thickness and/or temperature. Here, benefit is taken from the well-known sensitivity of the Eu valence to local environment. The most interesting result of this study is that these highly disordered Eu/Pd interfaces can be crystallized by heating the layers at moderate temperatures (800--1000 K), or by performing the evaporations of Eu on the Pd(111) substrate held at similar temperatures. In all cases and for all Eu thicknesses, reflection high-energy electron diffraction, x-ray diffraction, and photoemission experiments show evidence of the epitaxy of a trivalent, most likely ${\mathrm{EuPd}}_{3}$, intermetallic compound on the Pd(111) surface. Moreover, the epitaxial growth of Pd(111) on this trivalent compound is possible. It opens the possibility of building a metallic superlattice such as Pd/${\mathrm{EuPd}}_{3}$.

Xenon adsorption on Pt(110)(1\ifmmode\times\else\texttimes\fi{}2) has been studied between 70 and 97 K by photoemission of adsorbed xenon, thermal desorption spectroscopy, and low-energy electron diffraction. In this temperature range the first layer is formed and several states have been observed depending on coverage. They have been identified in relation with their adsorption site, valley, (111) microfacets, and ridge sites.