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Adsorption: Journal of the International Adsorption Society (v.11, #5-6)
Modeling of Adsorption in Finite Cylindrical Pores by Means of Density Functional Theory by E. A. Ustinov; D. D. Do (pp. 455-477).
Adsorption of argon at its boiling point in finite cylindrical pores is considered by means of the non-local density functional theory (NLDFT) with a reference to MCM-41 silica. The NLDFT was adjusted to amorphous solids, which allowed us to quantitatively describe argon adsorption isotherm on nonporous reference silica in the entire bulk pressure range. In contrast to the conventional NLDFT technique, application of the model to cylindrical pores does not show any layering before the phase transition in conformity with experimental data. The finite pore is modeled as a cylindrical cavity bounded from its mouth by an infinite flat surface perpendicular to the pore axis. The adsorption of argon in pores of 4 and 5 nm diameters is analyzed in canonical and grand canonical ensembles using a two-dimensional version of NLDFT, which accounts for the radial and longitudinal fluid density distributions. The simulation results did not show any unusual features associated with accounting for the outer surface and support the conclusions obtained from the classical analysis of capillary condensation and evaporation. That is, the spontaneous condensation occurs at the vapor-like spinodal point, which is the upper limit of mechanical stability of the liquid-like film wetting the pore wall, while the evaporation occurs via a mechanism of receding of the semispherical meniscus from the pore mouth and the complete evaporation of the core occurs at the equilibrium transition pressure. Visualization of the pore filling and empting in the form of contour lines is presented.
Keywords: density functional theory; adsorption; capillary condensation; MCM-41
Multicomponent Adsorption of Pesticides onto Activated Carbon Fibers by Catherine Faur; Héléne Métivier-Pignon; Pierre Le Cloirec (pp. 479-490).
The adsorption equilibria of pesticides and metabolites (atrazine, deethylatrazine, deisopropylatrazine and simazine) are studied onto activated carbon fibers –ACF– with a broad pore size distribution (32% mesopore volume, 68% micropore volume). Mono-and multi-component isotherms have been determined for low concentrations, from 0.23×10−6 to 9.52×10−6 mol L−1. Single solute isotherms, modeled by Freundlich and Langmuir models, tend to prove the influence of the adsorbate's solubility in the adsorption capacity of activated carbon fibers. Binary solute isotherms confirm the strong influence of pesticide solubility on the competitive adsorption mechanism: the competition is higher in the case of adsorbates of different solubilities (atrazine and DEA or DIA for example). Multicomponent experimental data were modeled by extended Langmuir-based equations and the Ideal Adsorbed Solution theory. Whereas the first ones failed to model accurately binary adsorption due to restrictive hypothesis, the IAS model showed a good agreement between experimental and predicted data. It emphasised also the difficulty in satisfying the hypothesis of the model in the case of highly adsorbed compounds. Finally, the simultaneous adsorption of atrazine and NOM (in a natural water, DOC = 18.2 mg L−1) shows no adsorption competition effects between natural organic matter and atrazine. This is due to the presence of secondary micropores (0.8–2 nm) and mesopores in the ACF, which limit a pore blockage phenomenon by NOM.
Keywords: liquid multicomponent adsorption; activated carbon fibers; pesticides; natural organic matter; extended Langmuir equation; IAS modeling
Adsorption and Diffusion Behavior of Ethane and Ethylene in Sol-Gel Derived Microporous Silica by A. Conan Dewitt; K. W. Herwig; Stephen J. Lombardo (pp. 491-499).
High surface area silica (500 m2/g) was synthesized by the sol-gel method from tetraethyl orthosilicate. The total porosity of the sample was 37% and most of the pores were well below 2 nm in size. The adsorption characteristics of ethylene and ethane in the silica were measured from 300–350 K by gravimetry, and Langmuir adsorption constants and enthalpies and entropies of adsorption were determined. Quasielastic neutron scattering was used to determine the translation and rotational diffusivities of both adsorbates from 200–270 K. Based on the adsorption and translational diffusion characteristics of ethylene and ethane, separation factors of 1.1–2 for olefin to paraffin are predicted.
Keywords: gas separation; adsorbents; diffusion
Bridged Polysilsesquioxane Xerogels Functionalizated by Amine- and Thiol- Groups: Synthesis, Structure, Adsorption Properties by A. Dąbrowski; M. Barczak; A. Stolyarchuk (Shvaykovska); I. V. Melnyk; Yu. L. Zub (pp. 501-517).
Bridged polysilsesquioxane xerogels containing amine (–NH2; –NH(CH2)2NH2; —NH) and thiol (–SH) groups were synthesized by hydrolytic polycondensation of 1,2-bis(triethoxysilyl)ethane, 1,4-bis(triethoxysilyl)benzene and appropriate trifunctionalized silanes in the presence of a fluoride-ion catalyst in an ethanol solution. 29Si CP/MAS NMR give indication of the molecular framework of these materials formed by structural T1, T2 and T3 units. 3-aminopropyl or 3-mercaptopropyl groups accessible to proton or metal ions are fixed to the xerogel surface by the siloxane bonds. IR and 13C CP/MAS NMR data clearly show that 3-aminopropyl groups form hydrogen bonds. The same data testify that all xerogels contain non-condensed silanol groups and some fraction of non-hydrolyzed ethoxygroups.Functionalized polysilsesquioxane xerogels obtained by means of organic spacers have a porous structure (500–1000 m2/g) and a high content of functional groups (1.0–2.7 mmol/g). AFM data indicate that xerogels are formed by aggregating primary particles—the size of such aggregates is in the range 30–65 nm. It was established that the main factors influencing the structure and adsorption properties considered hybrid materials are: the nature and geometrical size of the functional groups, spacer flexibility and, in some cases, the ratio of the reacting alkoxysilanes and the ageing time of the gel.
Keywords: sol-gel method; functionalized bridged; polysilsesquioxane xerogels; porosity; hybrid materials
Influence of Light Scattering by Residual Alumina Nanoparticles on the Analysis of Surfactants Adsorption Using Spectroscopy by Qi Kang; Bao Yu Gao; Jingtian Hu; Dazhong Shen (pp. 519-527).
The adsorption of a surfactant mixture, based on an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS) and a nonionic surfactant (Triton X-100, or TX100), on alumina nanoparticles was determined by solution depletion method combined with spectrometric measurement. It is shown that the light scattering, originated from the residual adsorbent alumina particles in the supernatant after centrifugation separation, interferes with the measurements of absorbance of the surfactant molecules, and therefore constitutes an error source for determination of the surfactant concentration in the supernatant by spectrometric means. The intensity of this light scattering, namely the influence of the residual alumina nanoparticles upon the surfactant adsorption, was related to the surfactant adsorption and its equilibrium concentration and varied among a batch. In this paper we report a Kalman filter method in order to eliminate the variational scattering background caused by non-separated residual alumina nanoparticles in each supernatant. This method is of interest as it is simple, easy to carry out and of high precision.
Keywords: adsorption; light scattering; surfactant mixture; Kalman filter
Surface Speciation Modeling of Heavy Metals in Kaolin: Implications for Electrokinetic Soil Remediation Processes by Ashraf Z. Al-Hamdan; Krishna R. Reddy (pp. 529-546).
The driving mechanisms for flux occur at interface between the soil particles and solution during electrokinetic soil remediation and the nature of this interface affects the electrokinetic response of the system. The pH-dependent adsorption of heavy metal contaminants by kaolin and the sensitivity of kaolin zeta potential to the aqueous phase properties are two important aspects that complicate the metal movement during electrokinteic soil remediation. This paper addresses these aspects and presents an electrostatic adsorption model that describes the behavior of kaolin surface for Cr(VI), Cr(III), Ni(II), and Cd(II) under various chemical conditions. This study showed that the aqueous properties: pH, ionic strength and the presence of the heavy metals Cr(VI), Cr(III), Ni(II), and Cd(II) in the system affect the zeta potential of kaolin surface. The zeta potential of kaolin shifts to a more negative value if the system pH increases. However, it shifts to a more positive value if the system ionic strength or metal concentration increases. It was found that the amount of the heavy metal adsorbed by kaolin has a pronounced sensitivity to the pH. As a result of the adsorption modeling, the constant capacitance protonation-dissociation intrinsic constants of kaolin are: pK $$_{+}^{{mathop{
m int}}}=-3.8pm 0.5$$ and $$pK_{-}^{{mathop{
m int}}}=9.4pm 0.5$$ . The ion-kaolin surface complexation constants (pK $$^{{
m int}}$$ ) of Cr(VI), Cr(III), Ni(II), and Cd(II) are: −12.5± 0.5, −5.0± 1.0, 2.6± 0.6, and 3.3± 1.0, respectively.
Keywords: kaolin; electrophoresis experiments; equilibrium constants; electrostatic adsorption models; electrokinetic remediation
Enhanced Dye Adsorption by Microemulsion-Modified Calcined Diatomite (μE-CD) by M. A. M. Khraisheh; M. S. Alg-Houti (pp. 547-559).
This study attempted to improve the adsorption performance of calcined diatomite for the removal of colour from wastewaters through modification with microemulsions. The surface area, pHZPC, Fourier transform infrared (FTIR) of calcined diatomite and μE-CD were studied. It is found that an increase in the BET surface area, pHZPC and total pore volume after modification was obtained. A decrease in average pore volume was observed after modification. This suggests that the pore opening deceased to be in the micro- and meso-pore region hence the adsorption capacity for the modified adsorbent will be enhanced by reducing the escaping of dye molecules. The influence of concentration, pH and particle size on the adsorption capacities of methylene blue (MB), hydrolysed reactive black (RB) and hydrolysed reactive yellow (RY) was investigated. It was concluded from FTIR and pH analysis that the predominant mechanism of RY molecules onto μE-CD is by an electrostatic attraction between the carboxylate anion and the dye. In the case of MB and RB, the adsorption mechanism could be a combination of different mechanisms such as electrostatic attraction, capturing by microemulsion micelles in the pores of the calcined diatomite or the hydrophobic attraction. The adsorption capacities were higher than unmodified calcined diatomite. In the case of adsorption of MB molecules, a high adsorption capacity onto μE-CD was obtained.
Keywords: calcined diatomite; microemulsion; adsorption; methylene blue and reactive dye; modified adsorbents
Optimization of Parameters for Cr(VI) Adsorption on Used Black Tea Leaves by Mohammad Abul Hossain; Mikio Kumita; Yoshimasa Michigami; Shigeru Mori (pp. 561-568).
Dynamic characteristics of Cr(VI) sorption on used black tea leaves (UBTLs) as a low-cost adsorbent are studied. Batch experiments were conducted to evaluate the effects of Cr(VI) concentration, solution pH and temperature on the removal process. Both of adsorption and reduction, involved in the process, are affected by the processing parameters. The adsorption kinetics is described successfully using pseudo-second order rate equation and the rate constant decreases with increasing the initial concentration of Cr(VI) up to 150 mg/L (for 0.1 g/L UBTLs) then becomes slow. Experimental and calculated kinetic data for equilibrium are well expressed by Langmuir isotherm. The solution pH has a profound effect on the adsorption rate. The rate constant increases linearly with an increase in temperature, and the low value of activation energy of adsorption, 16.3 kJ/mol, indicates that Cr(VI) is easily adsorbed on UBTLs. The maximum Cr(VI) adsorptive conditions, with a minimum reduction, were achieved from the dynamics of operational parameters: the initial Cr(VI) concentration < 150 mg/L (for 0.1 g/L UBTLs); the initial solution pH = 1.54–2.00 and the processing temperature < 50 ∘C, for the possibility of its practical application.
Keywords: chromium(VI); used black tea leaves; adsorption and reduction; sorption dynamics
Physical Adsorption of H2S Related to the Conservation of Works of Art: The Role of the Pore Structure at Low Relative Pressure by António João Cruz; João Pires; Ana P. Carvalho; M. Brotas De Carvalho (pp. 569-576).
The adsorption isotherms of H2S in selected adsorbents were determined at 298 K, at relative pressures up to about 0.005, aiming the use of these materials in the removal of that pollutant from the museums atmosphere. The Dubinin-Astakhov equation adjusts very well the experimental results, although one cannot interpret the pre-exponential factor w0 as the limiting adsorbed amount. The parameter E, related with the adsorption energy, and the parameter n, that can be associated with the surface heterogeneity of the adsorbents, are correlated and the first is also correlated with the adsorbed amounts. It was not found any expectable relationship between the adsorbed amounts and textural parameters of the adsorbents such as the specific surface area or the microporous volume. This points out that the adsorption of H2S is highly specific. In general, 13X and Y sodium zeolites seem to be the most effective adsorbents, but at lowest tested pressures, near the concentrations found at museums, a pillared clay prepared from a Wyoming montmorillonite seems to be more efficient.
Keywords: adsorption; zeolite; activated carbon; pillared clay; hydrogen sulfide; museum; conservation
Surface P and Al Distribution in P-modified ZSM-5 Zeolites by Mauri José Baldini Cardoso; Danielle De Oliveira Rosas; Lam Yiu Lau (pp. 577-580).
The surface composition of two commercial ZSM-5 zeolites with and without PO4 modification was examined by X-ray photoelectron spectroscopy. Both zeolites were slightly enriched in aluminum on the surface. The surface silica/alumina ratio (SAR) was about half of that of the bulk. Calcination at 600∘C decreased slightly this enrichment. After drying with H3PO4 followed by calcination, both zeolites showed surface enrichment of PO4. The surface of the zeolite with large crystal was more enriched. Steaming resulted in enrichment of aluminum on the surface, in agreement with migration of extra-framework alumina. Increasing phosphate content decreased this enrichment.
Keywords: synthesis techniques; characterization of structure; properties; measurement methods
Optimization of the Conditions for the Cr (III) Adsorption on Activated Carbon by Svetlana B. Lyubchik; Irene I. Perepichka; Olga L. Galushko; Andrey I. Lyubchik; Elena S. Lygina; Isabel M. Fonseca (pp. 581-593).
In order to understand the patterns of the adsorption equilibrium of Cr (III) on activated carbon, the adsorption process was studied by two different ways: classical batch experiments on commercial Norit and Merck activated carbons and their oxidized forms in a wide range of pHs; and extended time-based tests at the same pH values on the same adsorbents. This approach allowed us to understand the role of texture, chemical carbon surface functionality and experimental conditions (initial pH of the solution, contact time and adsorbate/adsorbent ratio) on the effectiveness of Cr (III) removal. The adsorption process of Cr (III) at (24 ± 1∘C) on Merck and Norit activated carbons and their oxidized forms were studied at pH values between 1.5 and 5 (either adjusted or buffered). Chromium concentration was fixed at 200 ppm. The carbon loading ranged from 1.2 to 20 g/l. The carbon/Cr (III) solution contact time was varied from 0.5–1 month to 5 months, to ensure that the saturation of the carbon level was reached. According to the data obtained, the presence of carboxylic groups on carbon surface seems to enhance Cr (III) uptake at initial pH of the solution in the range between 2 and 4. Depending on the nature of the adsorbent surface chemistry, the contact time to reach equilibrium may range from 3 to 5 months. There is an optimum carbon loading which limits the Cr (III) uptake/removal at given pH value. In order to understand the adsorption process, an ion exchange, surface complex and surface precipitation were considered.
Keywords: heavy metals; adsorption; activated carbon; surface chemistry
Synthesis and Characterization of Silica Gel Particles Functionalized with Bioactive Materials by M. A. Rodrigues; M. P. Bemquerer; D. B. Tada; E. L. Bastos; M. S. Baptista; M. J. Politi (pp. 595-602).
Bioactive materials (having an amino acid, Ac-Tryptophan, A or a peptide, Ac-Trp-Ala-Ala, B) were anchored onto silica particles. A photoactive linker (N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (DPN)) was initially attached to the particles and next the amino acids were bound by using both Zr/P chemistry and 1,3-diisopropylcarbodiimide/1-hydroxybenzotriazole coupling. In A derivative extensive complexation of Trp with DPN was observed. Photolysis studies showed the presence of excited state reactions on the silica particles, moreover the radical species (DPN.−, TrpH.+, Trp.) remained alive ∼100 times longer on the particle surface than in solution. These studies show that the formation rate of these radicals is a function of the supramolecular structures of the peptide and of the amino acid derivative on the silica particle.
Evaluation of the Thermodynamic Parameters for the Adsorption of Some Hydrocarbons on Chemically Treated-Bentonites by Inverse Gas Chromatography by R. Gürkan; H. Ceylan; T. Şahan; A. Savran; Ş. Kubilay (pp. 603-612).
Inverse gas chromatography has been used to evaluate the adsorption parameters (ΔHa, ΔHst, ΔSa and ΔGa) of some probe molecules, each representing a class of organic (n-hexane, cyclohexane, benzene, n-octane, 1-octene and isooctane) on bentonite and chemically treated-bentonites. The adsorption parameters of the probes on the bentonite samples were determined in infinite dilution region. Adsorption of the organic species was investigated in the temperature range of 200–275∘C, using a flame ionization detector, and nitrogen as a carrier gas. The net retention volumes (Vn) of the probes were determined by the help of the retention times (tR) observed on gas chromatograms for each probe. Injection was made at least three times for each probe, obtaining reproducible results of ± 0.5%. It was found that benzene exhibits more negative ΔH than for n-hexane and cyclohexane on all of the adsorbents. In addition, it was found that 1-octene exhibits more negative ΔH than for n-octane and isooctane on the chemically treated-bentonites, whereas n-octane exhibits more negative ΔH than for 1-octene and isooctane on the natural bentonite. Also, interactions of benzene with the natural- and chemically treated-bentonites were found to be stronger than those of n-hexane and cyclohexane with the same carbon number. Again, interactions of the 1-octene with the chemically treated-bentonites were found to be stronger those of n-octane and isooctane with the same carbon number. On the contrary, interactions of n-octane with the untreated-bentonite were found to be stronger than those of 1-octene and isooctane.
Keywords: inverse gas chromatography; thermodynamic parameters; adsorption; bentonite; hydrocarbons