Sodium Adsorption Research Articles

Water Structure and Aqueous Uranyl(VI) Adsorption Equilibria onto External Surfaces of Beidellite, Montmorillonite, and Pyrophyllite: Results from Molecular Simulations

Molecular dynamics simulations were performed to provide a systematic study of aqueous uranyl adsorption onto the external surface of 2:1 dioctahedral clays. Our understanding of this key process is critical in predicting the fate of radioactive contaminants in natural groundwaters. These simulations provide atomistic detail to help explain experimental trends in uranyl adsorption onto natural media containing smectite clays. Aqueous uranyl concentrations ranged from 0.027 to 0.162 M. Sodium ions and carbonate ions (0.027-0.243 M) were also present in the aqueous regions to more faithfully model a stream of uranyl-containing groundwater contacting a mineral system comprised of Na-smectite. No adsorption occurred near the pyrophyllite surface, and there was little difference in uranyl adsorption onto the beidellite and montmorillonite, despite the difference in location of clay layer charge between the two. At low uranyl concentration, the pentaaquouranyl complex dominates in solution and readily adsorbs to the clay basal plane. At higher uranyl (and carbonate) concentrations, the mono(carbonato) complex forms in solution, and uranyl adsorption decreases. Sodium adsorption onto beidellite occurred both as inner- and outer-sphere surface complexes, again with little effect on uranyl adsorption. Uranyl surface complexes consisted primarily of the pentaaquo cation (85%) and to a lesser extent the mono(carbonato) species (15%). Speciation diagrams of the aqueous region indicate that the mono(carbonato)uranyl complex is abundant at high ionic strength. Oligomeric uranyl complexes are observed at high ionic strength, particularly near the pyrophyllite and montmorillonite surfaces. Atomic density profiles of water oxygen and hydrogen atoms are nearly identical near the beidellite and montmorillonite surfaces. Water structure therefore appears to be governed by the presence of adsorbed ions and not by the location of layer charge associated with the substrate. The water oxygen density near the pyrophyllite surface is similar to the other cases, but the hydrogen density profile indicates reduced hydrogen bonding between adsorbed water molecules and the surface.

Sodium adsorption on the TiO2(110) surface: an XAFS structural study

The adsorption site of sodium atoms deposited onto a clean TiO2(110) surface has been determined by EXAFS. The best result is obtained for an ‘in-between’ site where the sodium is bound to two bridging oxygen atoms at 2.25 Å and one in-plane one at 2.40 Å in full agreement with DFT calculations. At higher coverage the site becomes a hollow site where Na is equidistant to the three oxygen atoms at 2.30 Å.

Density functional analysis of field emission from metals

A fully self-consistent method suitable to evaluate field emission currents is newly developed on the basis of the density functional theory. This method does not require nearby counter electrode. By using this method, first we have investigated field emission currents from a biased metallic surface represented by the bare jellium model. Next, the present method has been applied successfully to the case where a sodium layer is adsorbed on the jellium surface of r s=2 bohr. We have confirmed reduction of the potential barrier and a considerable increase in field emission currents by the sodium adsorption. These analyses show the validity of our method.

Sodium adsorption on a Ge(100)-(2 × 1) surface

The adsorption of Na on a Ge(100)-(2 × 1) surface has been studied by means of AES, LEED, EELS, TPD and work-function measurements. In the submonolayer coverage region the coverage dependencies of the desorption activation energy E( Θ) and desorption frequency v( Θ) have been determined using the threshold TPD method. Our experimental data show that after the completion of the first Na layer, 3D crystallites develop on the Na Ge(100) surface (Stranski-Krastanov growth mode). For Θ > 1 ML, formation, followed by decomposition of a certain NaGe surface compound occurs in the temperature range 410–550 K.

Sodium Adsorption onto Goethite as a Function of pH and Ionic Strength

Sodium Adsorption onto Goethite as a Function of pH and Ionic Strength

Sodium Adsorption onto Goethite as a Function of pH and Ionic Strength

Sodium Adsorption onto Goethite as a Function of pH and Ionic Strength

Sodium Adsorption and Reaction on NiO(111)/Ni(111)

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSodium Adsorption and Reaction on NiO(111)/Ni(111)M. Bender, K. Al-Shamery, and H.-J. FreundCite this: Langmuir 1994, 10, 9, 3081–3085Publication Date (Print):September 1, 1994Publication History Published online1 May 2002Published inissue 1 September 1994https://doi.org/10.1021/la00021a035RIGHTS & PERMISSIONSArticle Views177Altmetric-Citations31LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (635 KB) Get e-Alerts Get e-Alerts

Standard enthalpies of ion adsorption onto oxides from aqueous solutions and mixed solvents

Adsorption of sodium from 0.001–0.1 M NaCl solutions in water and mixed solvents (aqueous methanol and dioxane up to 30%) on silica and alumina at given pH and ionic strength increases with the temperature. However, at given surface charge density and ionic strength, sodium adsorption in the above systems is practically independent of temperature over the range 15–35°C; thus the apparent positive (endothermic) enthalpy, calculated from iso-pH data, reflects the increase in the negative surface charge. This suggests that cation adsorption in the studied systems is purely electrostatic in character. In contrast, adsorption of Cl − ions on alumina at constant pH and ionic strength is independent of the temperature. This observation is in line with adsorption of various anions on goethite at high fractional surface coverage. Cobalt adsorption on silica and alumina in the presence of 0.1 M NaCl at constant surface charge density also increases with the temperature. Thus the tendency found for other (hydr)oxide—multivalent cation adsorption systems has been confirmed.

Thick sodium overlayers on GaAs(110).

We report density-functional theory calculations of the electronic structure, total energy, and forces for the Na adsorption on GaAs(110) using the local-density approximation of the exchange-correlation functional and ab initio pseudopotentials. Results are presented for coverages ranging from one adatom per substrate surface cell up to the thick overlayer limit. The atomic and electronic structure of the substrate is locally changed by the sodium adsorption on GaAs(110), depending on the coverage. In particular, we analyze the wave-function character of the states at the Fermi level, how it changes with sodium coverage, and we identify the formation of metal induced gap states (MIGS) at the interface. These MIGS are found to have mostly Ga dangling-bond character for all coverages. The calculated values of the p-type Schottky barrier and of the variation of photothreshold as a function of coverage are in good agreement with experimental data.

Scanning Tunneling Microscopy of Oxygen and Sodium Adsorption on the Si(111) Surface

The surface structure and the insulator-metal transition of the Na overlayer on the Na-induced Si(111)3×1 surface was studied using scanning tunneling microscopy and spectroscopy. The results suggested that this reconstruction was caused by the conversion of the 7×7 structure to the bulk-terminated 1×1 structure and a monolayer of Na adatoms arranged in the 3×1 periodicity at a coverage of 2/3 monolayer. When oxygen was adsorbed on this surface the surface morphology hardly changed up to several hundred Langmuir. This surface passivation arose from the removal of Si dangling bonds by the Na adatoms. Upon further Na evaporation onto the Si(111)3×1-Na surface at room temperature we could grow an epitaxial Na double layer and a multilayer through an island formation. Oxygen adsorption on the Na double layer, and a multilayer resulted in cluster formation. This result indicated that more than two layers of alkali overlayer was needed to promote the chemical activity on a silicon surface.

Sodium adsorption on a Si(001)-(2 × 1) surface

The adsorption of Na on a Si(001)-(2 × 1) surface has been studied by means of AES, LEED, EELS, Temperature Programmed Desorption (TPD), and work function (Δφ) measurements. Four peaks are observed in the Na TPD spectra. The lowest temperature peak corresponds to a multilayer Na structure. The observed weak coverage dependence of the Na desorption energy suggests a predominantly covalent character of the NaSi bonding. Our TPD data show that with Na in the first layer ( θ Na = 0.68), no metallization of the Na overlayer takes place. The AES, LEED, EELS and Δφ data give strong evidence of 3D clustering (Stranski-Krastanov growth mode), which at 300 K occurs for θ Na ⩾ 0.5.

Sodium adsorption on a [formula omitted] surface

Sodium adsorption on a [formula omitted] surface

Prediction of exchangeable sodium percentage in soils by mechanistic and layer models

An experiment was conducted in Plexiglass columns ( i d . 5.6 cm) to study depth distribution of adsorbed Na+ in Ca-saturated loamy sand, sandy loam and clay loam soils on percolating different volumes of 0.1M NaCl solution. The exchangeable sodium percentage (ESP) decreased gradually with depth and increased with increase in volume of solution percolated. The depth development of ESP of the soils was computed by using a numerical solution of a one-dimensional convective-dispersive partial differential equation (model I) and a conceptual layer model (model II). To account for the effect of adsorption on cation flow, the slopes of the nonlinear adsorption functions fitted to the normalized adsorption isotherms of Na+ were employed in model I , and the Gapon equation was employed in model II. As the Gapon selectivity coefficient (Kg) is dependent on Na saturation, its values were calculated from the regression equation: Kg = 0.22 + 0.0043SAR (where SAR is the sodium adsorption ratio). Model I overpredicted ESP of loamy sand and sandy loam soils throughout the column depth; the overprediction was more pronounced in the former. The coefficient of determination, calculated on the basis of pooled data of three soils and two solution volumes showed that the accuracy of prediction of model II was considerably higher (r2 = 0.80) than that of model 1 (r2 = 0.54).

Elastic scattering and charge exchange in He + ion scattering from alkali metal overlayers

Low energy (1 keV) He + ion scattering data from clean Cu(110) reveal simple binary collision spectra expected from a surface which readily neutralizes the ions by an Auger process. Experiments with sodium, potassium and caesium overlayers show that scattering from these species leads to energy loss features characteristic of reionization by electron promotion; similar features in the copper scattering peak on alkali adsorption are thought to be due to inelastic losses in the overlayer. Sodium adsorption in particular also causes a high background of inelastically scattered ions to be observed, implying reionization of He + ions which penetrate the copper substrate; the mechanism for this reionization is unclear.

Salts in sandy desert soils, southwestern Egypt

Different sandy soils,derived from mesozoic sandstone and pleistocene dune sands in the southwestern desert of Egypt, have been examined referring to the distribution of water soluble salts. Despite a low groundwater table (>25 m) and a very low mean annual rainfall (<1 mm) the salt contents are maximal in the subsoil. The salt composition is clearly related to soil depth and temperature and influenced by sodium adsorption. With soil depth the solubility of the salts increase, so that enrichment and distribution of salts here are both caused by rainfall. The results have been discussed for present and former climatic conditions and it was assumed that this type of salinization is caused by episodic rainstorms of the subrecent time.

EELS studies of oxygen and sodium adsorption and coadsorption on Ru(001)

EELS studies of oxygen and sodium adsorption and coadsorption on Ru(001)

EELS studies of oxygen and sodium adsorption and coadsorption on Ru(001)

Electronic excitations of sodium and of sodium coadsorbed with oxygen on Ru(001) have been investigated by electron energy loss spectroscopy in N(E) and d2n/dE2 mode. For sodium adsorbed on Ru(001) a characteristic, sharp low-energy loss feature at 3–4 eV loss energy is observed, which shifts in energy as a function of Na coverage. This loss is discussed in terms of a one-electron excitation involving a Na(3s)−Ru(4d) interface state at low Na coverages, but is interpreted as a plasmon loss of the Na layer as the Na coverage approaches one monolayer. Both one-electron transition and plasmon excitation are strongly influenced and quenched by coadsorption of oxygen. The disappearance of the one-electron excitation is interpreted in terms of a modification of the interface state by coadsorbed oxygen, and the destruction of the Na plasmon resonance is associated with the loss of free-electron character in the Na layer as a result of oxidation.

Sodium-induced modifications in the electronic structure of the W(100) surface

Adsorption of sodium on the W(100) surface is studied at room temperature by angular resolved ultraviolet photoemission spectroscopy (ARUPS) and LEED. A reconstruction of the W(100) surface induced by sodium adsorption is observed at least at low coverage. The high-lying tungsten surface states situated at 0.4 eV below the Fermi level at normal emission ( Gamma ) suffer a continuous shift towards higher binding energies when sodium coverage increases. With a monolayer of sodium, the shift attains 2 instead of 1 eV for caesium. The low-lying surface state located at 4.2 eV below the Fermi level at Gamma shifts towards higher binding energies in contrast with caesium adsorption. These effects reveal a stronger sodium atom-substrate interaction than that which occurs in caesium. The description of the Cs/W(100) system may be extended to Na/W(100): the d(z2)-like surface states of W(100) hybridise with the sodium 3s valence states to form a polarised covalent bond.

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED, Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to 1 2 monolayer on both surfaces, and 1 3 monolayer on Al(111). A hexagonal pattern corresponding to 7 8 monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at 1 2 monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.

Abstract: Sodium adsorption on aluminum (100) and (111) surfaces: ELEED, Auger, and contact potential measurements

First Page