Initial Stage Of Adsorption Research Articles

Iodide adsorption from aqueous solutions by bis(trimethoxysilylpropyl)amine polycondensate/silver chloride composites

Composite adsorbents for iodide were prepared with AgCl as an adsorption active component and bis(trimethoxysilylpropyl)amine (TSPA) as a gel precursor. The prepared composite adsorbents were used to adsorb iodide from aqueous solutions by considering the effects of initial iodide concentration, temperature, initial solution pH, and coexisting NaCl. The loading of AgCl in the prepared composite adsorbents was 5.5 mmol/g, much higher than that in the reported silver or silver chloride-impregnated activated carbon (around 0.097 mmol/g). The high AgCl loading ensures that the prepared composite adsorbents have a high adsorption capacity. At the initial adsorption stage of 0–12 h, the adsorption rate increases with increasing initial iodide concentration from 2 to 8 mmol/L. When the initial iodide concentration further increases from 8 to 14 mmol/L, the adsorption rate does not have an obvious increase. Pseudo-second-order model fits the experimental kinetic data quite well. The initial adsorption rate increases greatly from 1.97 to 7.32 mmol g-1 h-1 as adsorption temperature rises from 25 to 55°C. The equilibrium adsorption amount is insensitive to solution pH and coexisting NaCl, but it increases slightly with the increasing adsorption temperature. The adsorption isotherms are of H2 type, indicating that the adsorption is chemical adsorption. The comparison of the energy dispersive spectroscopy (EDS) spectra and X-ray diffraction (XRD) patterns of the composite adsorbents before and after adsorption shows that after adsorption, all AgCl in the original adsorbents changes into AgI, confirming the chemical nature of the adsorption. The composite adsorbents prepared in this work have a high adsorption capacity, a good selectivity, and good chemical and mechanical stabilities. They are very suitable for the adsorption of iodide form aqueous solutions.

Surface reconstruction at the initial Ge adsorption stage on Si(114)-2 × 1

By combined investigation of scanning tunneling microscopy and synchrotron core-level photoemission spectroscopy on the structural and chemical evolution at the initial stage of Ge adsorption on Si(114)-2 × 1, it has been observed that one-dimensional (1D) sawtooth-like nanostructures composed of (113) and (117) facets and 1D trenches adjacent to the (113) facets are readily formed without any wetting layer. Due to the absence of chain structures on the reconstructed Si(114)-2 × 1, enhanced Ge interdiffusion detected from Ge/Si(5 5 12)-2 × 1 has not been found. Instead, Si atoms originating from etched surfaces and arriving Ge atoms form the alloy facets with Ge-rich surfaces. These experimental results prove that, if the direction of the Ge overlayer corresponding to that of the substrate is unstable like the present case, the arriving atoms prefer to form facets covered with the species of lower surface free energies rather than a uniform wetting layer.

Molecular Simulations of Water Adsorbed on Mesoporous Silica Thin Films

Grand canonical Monte Carlo and canonical ensemble molecular dynamics simulations were used to investigate water adsorption properties in mesoporous silica thin films. The effect of pore radius on the adsorption properties was assessed using two models of mesoporous silica thin films having different pore radius and film thickness (1.38 and 5.66 nm in Model 1, respectively, and 1.81 and 7.30 nm in Model 2, respectively). In the simulations, a water adsorption layer or water menisci were formed in a mesopore accompanying the growth or shrinkage of stable adsorption layers on the upper and lower surfaces. The thickness of a water adsorption layer immediately before the onset of capillary condensation and the curvature radius of a water meniscus prior to capillary evaporation were identified. In the initial stage of layer adsorption, the surface state and radius of the pore affected the coordination structure of water around silanol groups. In the pore-filling state, the hydrogen bond network of water in the pore was similar but not the same as that in the bulk liquid; the difference was because the pore wall affected the position of water molecules. This difference may confer unique dynamic properties upon the water in mesoporous silica.

Initial stages of benzotriazole adsorption on the Cu(111) surface

Benzotriazole (BTAH) has been used as a copper corrosion inhibitor since the 1950s; however, the molecular level detail of how inhibition occurs remains a matter of debate. The onset of BTAH adsorption on a Cu(111) single crystal was investigated via scanning tunnelling microscopy (STM), vibrational spectroscopy (RAIRS) and supporting DFT modelling. BTAH adsorbs as anionic (BTA(-)), CuBTA is a minority species, while Cu(BTA)2, the majority of the adsorbed species, form chains, whose sections appear to diffuse in a concerted manner. The copper surface appears to reconstruct in a (2 × 1) fashion.

Calorimetric study and simulation of the adsorption of methanol and propanol onto activated carbon fibers

Adsorption isotherms and differential heats of adsorption of methanol, ethanol, 1- and 2-propanol, and 1- and 2-butanol onto two types of activated carbon fiber (ACF) were measured at 298.0K to investigate the mechanism for the adsorption of lower alcohols onto ACFs. The dependence of the differential heat at the initial stage of adsorption on the type of alcohol can be quantitatively explained by a dispersion pore potential. However, it is necessary to take into account the effect of surface functional groups on the total amount of differential heat. Therefore, a grand canonical Monte Carlo (GCMC) simulation was applied to the adsorption of methanol and propanol onto ACF using simple slit pore models with and without carbonyl functional groups. Large differences in the amount of alcohol adsorbed and the differential heat were observed for different carbonyl configurations. A comparison of the experimental and simulated isotherms and differential heats revealed that a surface without functional groups does not reproduce the experimental isotherm and differential heat. The positioning of five carbonyls close together on the carbon surface makes a very active site that provides similar features to those obtained experimentally. For the pore model with active sites, 1-propanol molecules form a bilayer structure similar to methanol molecules, whereas 2-propanol molecules do not form such a definite layer structure. This is a typical isomer effect that originates from the steric hindrance of the alkyl group on 2-propanol.

Adsorption of atomic oxygen on HfC and TaC (1 1 0) surface from first principles

We investigated the initial adsorption of oxygen atom on the HfC (110) and TaC (110) surface using first principles. Both the carbides have the same crystal structure, a sodium–chloride structure. The (110) surfaces of the carbides were modeled with (2×1) supercells. Every supercell is composed of five atomic planes. Our results demonstrate that the preferred site for oxygen atom is the C–Hf bridge site and the Ta–Ta bridge site on the HfC (110) and TaC (110) surface, respectively. The adsorption sites are different from the one on the (100) surfaces of the carbides. For the carbides, the adsorption energies of oxygen on the (110) surfaces are larger than that on the (100) surfaces. There exists the CO bond in O/HfC (110), while no CO bond is found in O/TaC (110), indicating that the adsorption mechanism of atomic oxygen on the TaC (110) surface is different from that on the HfC (110) surface at initial adsorption stage.

Scanning tunneling microscopy/spectroscopy study of adsorption of C60F36 molecules on the 7 × 7-Si(111) surface

Spatially resolved images of an individual C60F36 fluorofullerene molecules on Si(111)-7 × 7 surface have been obtained by means of scanning tunneling microscopy/spectroscopy (STM/STS). The presence of isomers with different symmetry (T, C3, C1) has been revealed in STM investigation of initial adsorption stage of C60F36 on silicon surface Si(111)-(7 × 7). The adsorbed fluorofullerene molecule can occupy any adsorption site of silicon surface (corner site, faulted half, unfaulted half) that indicates for strong molecule-substrate interaction. The HOMO-LUMO gap of the adsorbed C60F36 molecules have been estimated from current image tunneling spectroscopy (CITS) and z(V) with engaged feedback measurements. The value of HOMO-LUMO gap observed experimentally was 3 eV. The C60F36 molecules adsorption on Si(111)-(7 × 7) surface was stable and kept equilibrium configuration during several hours.

Molecular interactions of SO2 with carbonate minerals under co-sequestration conditions: A combined experimental and theoretical study

We present a combined experimental and theoretical study investigating the reactivity between select and morphologically important surfaces of carbonate minerals with supercritical CO2 and co-existing H2O and SO2. Trace amounts of SO2 cause formation of CaSO3 in the form of hannebachite in the initial stages of SO2 adsorption and transformation. Atomistic simulations based on density functional theory of these initial steps indicate accumulation of water over the magnesium sites, and suggest depletion of Mg over the Ca from the mineral surface. Under co-sequestration conditions with wet scCO2, water is not likely to cause carbonate dissolution of a perfect surface, however, it stabilizes pre-existing low coordination oxygen atoms by creating surface hydroxyl groups on the CO2-defect sites. Formation of bisulfites (surface-SO2OH) occurs with a low barrier of ca 0.5eV, estimated by the climbing image nudged elastic band method (CI-NEB). Estimates of the effective transformation rates are in the range of 4.0×101 to 4.0×104 s−1. The sulfur-containing species bind preferentially on surface calcium atoms creating the first nucleation sites. Molecular dynamics simulations also show dynamic tautomerization of the adsorbed bifulfites (s-SO2OH ⇌ s-S(H)O3), which is likely to slow down further oxidation to sulfates in less oxidative environments. From the same simulations, we extract local geometries of the resulting CaSO3H···OH species, similar to the crystallographic structure of hannebachite. Collectively, the experimental results and ab initio molecular dynamics simulations suggest potential of carbonate reservoirs for in situ chemical scrubbing of CO2 captured from fossil fuel sources, which could be stored permanently for sequestration purposes or extracted and utilized for enhanced oil recovery (EOR).

Lateral Distribution of Li Atoms at the Initial Stage of Adsorption on TiO2(110) Surface

Adsorption of Li atoms on a titanium dioxide (TiO2) (110)-(1×1) surface was examined by a scanning tunneling microscope (STM). Lithium adatoms were observed as protrusions in empty state images. Three different adsorption sites for Li atoms, a 4-fold hollow site of in-plane O atoms, a 3-fold hollow site composed of one bridging O atom and two in-plane O atoms, and a bridging O vacancy, were identified. The Li adatoms were placed predominantly on the 4-fold hollow site. Time-lapse STM observation revealed that the Li adatoms at the three sites were mobile at room temperature. The number density of the Li adatoms on the terraces was reduced around the steps, which was attributed to the perturbation of the surface electronic state by the Li adatoms concentrated at the steps.

Facilitation of Water Penetration through Zero-Dimensional Gates on Rolled-up Graphene by Cluster–Chain–Cluster Transformations

We demonstrate a water penetration mechanism through zero-dimensional nanogates of a single-walled carbon nanohorn. Water vapor adsorption via the nanogates is delayed in the initial adsorption stage but then proceeds at a certain rate. The mechanism is proposed to be a water cluster–chain–cluster transformation via the nanogates. The growth of water clusters in internal nanospaces facilitates water penetration into these nanospaces, providing an intrinsic mechanism for zero-dimensional water.

Adsorption of Alizarin Red from Aqueous Solution Using Ultra-Fine Fly Ash

Alizarin Red (AR) has been shown to be effectively removed from aqueous solution using the ultrafine fly ash (UFA) prepared by ball milling from raw fly ash (FA), a low-cost industrial solid waste. The maximum removal rate was 91.04% in a solution of initial AR concentration of 700 mg/L, adsorption time of 60 min at pH 5.0 and temperature of 25°C. Compared with FA, the adsorption capacity of UFA is higher for AR removal. Effects of important parameters, contact time, adsorbate concentration, pH and temperature, were investigated. The UFA and AR-loaded UFA were characterized by FT-IR. The equilibrium, kinetics and thermodynamics of AR adsorption onto UFA were evaluated. The AR uptake process followed the pseudo-second-order rate equation well, but the pseudo-first-order rate equation and intra-particle diffusion equation could only be applied to describe the initial stage of adsorption, furthermore, intra-particle diffusion might be the rate-controlling step of fast adsorption process. The sorption decreased with increasing temperature and the adsorption apparent activation energy was 8.28kJ/mol. Langmuir isotherm equation could better describe the adsorption equilibrium at different temperatures, compared with Freundlich model. Thermodynamic parameters, ΔG, ΔH and ΔS, were also calculated. The results inferred that the adsorption of AR/UFA system was feasible, spontaneous and exothermic nature of the process which was mainly controlled by physical adsorption.

Octithiophene on Cu(111) and Au(111): Formation and Electronic Structure of Molecular Chains and Films

Adsorption and electronic structure of octithiophene (8T) molecules on Cu(III) and Au(III) surfaces are investigated using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. We find a large difference in adsorption behavior of 8T molecules on the two surfaces. At the initial stage of adsorption, 8T molecules are stabilized in the form of molecular chain on a terrace of Cu(III), whereas neither such chain structure nor isolated 8T molecules have been observed on a terrace of Au(III). By increasing the amount of adsorbed molecules, a disordered monolayer film is formed on Cu(III) while a well-ordered monolayer film is formed on Au(III). From the spectroscopic investigations using bias-dependent STM images and STS spectra and by comparing the data with theoretical calculations, it is found that the electronic property of 8T molecules in the molecular chain on Cu(III) is different from that of a free-standing 8T molecule while that in the monolayer film on Au(III) keeps original character of the free-standing 8T molecule. The present study shows that adsorption of 8T molecules on Cu(III) results in a formation of adsorption-induced states near the Fermi level.

Adsorption of Pyrazine on a Si(001) Surface Partially Covered with an Indium Dimer Structure

Pyrazine forms a molecular one-dimensional (1D) line on a bare Si(001)-2×1 surface across the Si dimer row. Indium also forms a 1D line across the Si dimer row. In this study, the adsorption of pyrazine on a Si(001)-2×1 surface on which the indium parallel dimer structure is formed was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). It was found by STM that pyrazine adsorbates were predominantly located at the lateral neighboring sites of the indium line structure during the initial stage of adsorption. This preferential adsorption of pyrazine can enhance the formation of long linear chains of pyrazine along the indium lines.

Effect of sodium dodecyl sulfate on dynamic surface properties of lysozyme solutions

The surface dilatation rheological properties of lysozyme/sodium dodecyl sulfate (SDS) mixed solutions are studied by the oscillating ring method. At the initial stage of adsorption, the rate of variations in the surface properties depends nonmonotonically on SDS concentration due to the reversal of the lysozyme/SDS complex charge with increasing degree of surfactant binding. The nonmonotonic kinetic dependences of the dynamic surface elasticity indicate the breakage of the secondary and tertiary structures of the protein in the surface layer. For lysozyme/SDS solutions, the denaturing effect of the interface appears to be stronger than for previously studied systems, namely, bovine serum albumin/dodecyltrimethylammonium bromide and β-lactoglobulin/dodecyltrimethylammonium bromide.

Origin of enhanced Ge interdiffusion at the initial stage of Ge deposition on Si(5 5 12)-2 × 1: Tensile stress induced by substrate chain structures

By combined investigation of STM and synchrotron PES on Ge/Si(5 5 12)-2×1 at 530°C, it has been found that, in addition to the upward-relaxed surface Si atoms, a subsurface Si atom is also readily replaced by an arriving Ge atom at the initial adsorption stage. Such enhanced interdiffusion is due to a unique character of one-dimensional chain structures of the reconstructed substrate, such as π-bonded and honeycomb chains not existing on other low-index Si surfaces such as Si(001)-c(4×2) and Si(111)-7×7, applying a tensile surface stress to the neighbouring subsurface atoms. Interdiffusion of Ge having lower surface energy induces adsorption of the displaced Si atoms on the surface to form sawtooth-like facets composed of (113)/(335) and (113)/(112) with arriving Ge atoms until the surface is filled with those facets. Such displacive adsorption is the origin of high Si concentration of formed facets.

Adsorptive Removal of Antimony (III) Using Modified Montmorillonite: A Study on Sorption Kinetics

The adsorptive removal of antimony (III) has been successfully obtained on montmorillonite (MMT) and modified MMT from synthetic solution. The adsorption behavior of the modified and unmodified MMT has been investigated as a function of initial concentration of metal ion in the solution, pH of the solution and contact time using a batch extraction process. The optimized process can be applied for the adsorption, detection and estimation of antimony from 0.006 μg/ml (6.0 ppb) to 100.00 μg/ml (100.0 ppm) in aqueous solution. It has been observed that almost 99% of antimony (III) can be successfully extracted from a solution containing 100 μg/ml of the metal ion at pH 6.0 at 25?C ± 2?C. The investigation of the kinetics of sorption of antimony (III) on MMT/modified MMTshows intraparticle diffusion to be the rate limiting step during the initial stages of adsorption followed by chemisorption.

Analytical models for describing cation adsorption/desorption kinetics as considering the electrostatic field from surface charges of particles

Surface charges of particles together with the adsorbed counter ions in diffuse layer can set up a strong electrostatic field around the particles in aqueous solution. The existent kinetic models for describing cation exchange on solid/liquid interface were either empirical or semi-empirical, and in which the electrostatic field is not considered. In this paper, as considering the important effect of electrostatic field around particles on cations adsorption/desorption, for the first time the dynamic distribution equations of cations in diffuse layer for adsorption and desorption processes in both flow method and batch technique have been established. Those equations clearly show how the cation concentration changes with time in different position of diffuse layer during the cation exchange process, and the corresponding new kinetic models have been obtained upon them. The new models indicate that, in both flow method and batch technique, for the adsorption process, experimental results should appear zero order kinetic process caused by the strong force adsorption in the initial stage of adsorption, and then transform to the first order kinetic process of the weak force adsorption; and for the desorption process, however, only first order kinetic process may exist. The new models are essentially different from the classic apparent or empirical kinetic models since all the parameters have their defined physical meanings in the new models, thus the rate parameters in the new models have the potential to theoretically predict. Theoretical analyses also indicated that, the adsorption/desorption rate in flow method experiment will be much higher than that in batch technique experiment.

Adsorption kinetics under the influence of barriers at the subsurface layer

At the initial stage of surfactant adsorption (when the layer is relatively diluted), the kinetics may be dominated by factors related to the transfer of molecules through the subsurface region and onto the interface. We consider two independent physical effects: (1) diffusion through a subsur- face layer with nanometer thickness, where structuring or molecular interactions can impose substantial changes on the transfer rate, as compared with the bulk diffusion and (2) hindrance to the act of adsorption itself, when the molecules hit the interface from a place directly adjacent to it. These two effects are taken into account by formulating a model which includes the balance of fluxes in the subsurface layer. This model allows one to find analytical solution for the adsorption asafunctionoftime.Applicationofthetheoryisillustratedby analyzing experimental data for two proteins which adsorb on air/water interface. Attention is paid to the particular case when the resistance to adsorption is relatively small but is still significant as compared with the bulk diffusion. Then, the theoretical fit ofthe adsorptionvs. timecanbeimplemented in a specific linear scale. The overall resistance of the interfacial zone comprises additive contributions from the hindrance to the act of adsorption and from the (retarded) diffusion through the subsurface layer. They are incorporated into one physical parameter (or characteristic time), which influences the kinetics.

Ultrasound enhanced adsorption and desorption of chromium (VI) on activated carbon and polymeric resin

The effect of ultrasound on the adsorption and desorption of Cr(VI) on activated carbon and PVC–PPA resin were investigated. The results indicated that, on resin, the adsorption rate increased and the equilibrium time decreased obviously in the presence of ultrasound, while on activated carbon, ultrasound only enhanced the adsorption rate slightly at the initial stage of adsorption. Ultrasound didn't influence the desorption rates of Cr(VI) from both activated carbon and resin in distilled water. The addition of NaOH to the system caused an enhancement of Cr(VI) desorption, especially on activated carbon in the presence of ultrasound. After acid-treatment, the ultrasonic regenerated activated carbon had a good re-adsorption capacity.

Initial stage of adsorption of octithiophene molecules on Cu(111)

The initial stage of the adsorption of octithiophene (8T) molecules on a Cu(111) surface is investigated using a scanning tunneling microscope at room temperature. We find a characteristic molecular chain structure of 8T molecules on a terrace of the Cu(111) surface, which has not been reported so far for adsorption of oligothiophene molecules on metal surfaces. Up to the adsorption of 0.26 monolayer (ML), 8T molecules in the molecular chain align with their long axis parallel to the Cu<11-2> direction. With increasing coverage, there appear 8T molecules that align with their long axis parallel to the Cu<110> direction. The appearance of different molecular orientations is understood by the decrease of the number of the adsorption sites for extending the molecular chains. Fragments of 8T molecules, such as single thiophene molecules, are also observed in this work. They are trapped only at the step edges of the Cu(111) surface at the beginning and later trapped in a small Cu(111) region surrounded by 8T molecules.