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PubMed1998

Pore Size Analysis of MCM-41 Type Adsorbents by Means of Nitrogen and Argon Adsorption.

Methods of nonlocal density functional theory (NLDFT), proposed recently for predictions of adsorption equilibrium and calculations of pore size distributions in micro- and mesoporous materials, were tested on reference MCM-41 materials. Five newly synthesized MCM-41 adsorbents with presumably uniform pore channels varying from 32 to 45 Å were characterized by X-ray diffraction (XRD), nitrogen adsorption at 77 K, and argon adsorption at 77 and 87 K. New sets of intermolecular interaction parameters of the NLDFT model for N2 and Ar adsorption on MCM-41 were determined. The parameters were specified to reproduce the bulk liquid-gas equilibrium densities and pressures, liquid-gas interfacial tensions, and standard adsorption isotherms on nonporous surfaces in the multilayer adsorption region. The pore size distributions calculated from the desorption branches of the experimental isotherms measured at three different temperatures were consistent with each other. Comparison of the NLDFT-calculated pore sizes with XRD data showed that the thickness of pore walls in the MCM-41 samples under consideration varied from ca. 6 to 12 Å. We found no correlation between the pore size and the pore wall thickness. The results obtained support the NLDFT model as a suitable tool for characterizing nanoporous materials and predicting adsorption equilibrium. The MCM-41 samples studied can be used as references for adsorption measurements. Copyright 1998 Academic Press.

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PubMed1999

Surface Characters and Adsorption Behavior of Pb(II) onto a Mesoporous Titanosilicate Molecular Sieve.

Removal of Pb2+ ions from aqueous solution using the adsorption process onto Ti-MCM-41 has been investigated. A simplified surface complexation model was used to calculate the conditional binding constants for surface complexation of Pb2+ ions onto Ti-MCM-41. Dynamic modeling of the adsorption showed that the first-order reversible kinetic model held for the adsorption process. The overall rate constant k', the adsorption rate constant k1, the desorption rate constant k2, and the equilibrium constant Ke for the adsorption process were calculated from the results of the thermodynamic analysis, and standard free energy DeltaG degrees, standard enthalpy DeltaH degrees, and standard entropy DeltaS degrees of the adsorption process were also calculated. Equilibrium modeling of the adsorption showed that the adsorption of Pb2+ ions was fitted to a Freundlich isotherm. Copyright 1999 Academic Press.

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PubMed1999

Adsorption of NOM onto Activated Carbon: Effect of Surface Charge, Ionic Strength, and Pore Volume Distribution.

Adsorption of natural organic matter (NOM) onto seven activated carbons with a wide range of surface properties was studied at high and low ionic strength over a range of pH values. From adsorption isotherm studies it was found that, for six of seven carbons, at low surface concentrations, increased ionic strength decreased NOM adsorption. As the surface concentration increased, the adsorption isotherms converged and intersected, after which the addition of salt resulted in increased adsorption. This "crossover point" marked a change in the adsorption mechanism from the "screening reduced" to the "screening enhanced" adsorption regimes. The adsorption mechanisms are extremely complicated and appear attributable to various factors, including electrostatic forces, pore volume distribution, and chemical interactions between the NOM and the surface functionalities on the carbon surfaces. Copyright 1999 Academic Press.

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PubMed1999

Adsorption of Phosphonates onto the Goethite-Water Interface.

The adsorption of one phosphonate, two hydroxyphosphonates, and five aminophosphonates onto the iron (hydr)oxide goethite (alpha-FeOOH) has been studied as a function of pH. At phosphonate concentrations significantly lower than the total number of available surface sites, nearly 100% adsorption is observed below pH 8.0. Adsorption decreases to negligible levels as the pH is increased to 12.0. Under the conditions just described, adsorption of nitrilotris(methylenephosphonic acid) as a function of pH is nearly independent of the ionic strength (from 1 mM to 1 M). At phosphonate concentrations close to the total number of available surface sites, adsorption decreases over a broader range in pH and reflects the number of phosphonate groups. At pH 7.2, the maximum extent of adsorption decreases as the number of phosphonate groups increases from one to five. Adsorption is modeled using a 2-pK constant capacitance model that postulates formation of a 1:1 surface complex involving one surface site and one phosphonate functional group. Denoting the fully deprotonated phosphonate ligand as La-, different protonation levels for adsorbed phosphonate species are represented by a series of equilibrium constants of the form For a phosphonate of charge a-, there are (a - 1) possible surface protonation levels. For a surface protonation level n, log betan,surf values derived from this modeling approach are related to the surface complex charge Z through the following linear relationship: Using this approach, adsorption as a function of phosphonate concentration and pH can be fully accounted for. Copyright 1999 Academic Press.

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PubMed1999

Surface Chemical Studies on the Competitive Adsorption of Poly(acrylic acid) and Poly(vinyl alcohol) onto Alumina.

The adsorption of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) onto alumina has been studied as a function of pH, both individually and in the presence of each other. The adsorption density of PAA is found to decrease with an increase of pH while that of PVA shows the opposite trend. In a binary system containing PAA and PVA, the presence of PVA does not affect the adsorption of PAA onto alumina, but the addition of PAA diminishes the adsorption of PVA in the pH range investigated. The adsorption isotherm of PAA at acidic pH exhibits high-affinity Langmuirian behavior. The isotherms for PVA appear rounded and are of the low-affinity type. Once again the adsorption isotherms of PAA remain unaltered in the presence of PVA whereas those of PVA are significantly affected resulting in a lowering of the adsorption density consequent to PAA addition. A variation in the sequence of addition of PAA and PVA does not affect the adsorption behavior of either of the polymers. The electrokinetic behavior of alumina with PAA is hardly influenced by the addition of PVA. On the other hand, the electrophoretic mobility of alumina in the presence of PVA is significantly altered in the presence of PAA and closely resembles the trend observed with PAA alone. Desorption studies reveal that over 80% of PVA could be desorbed in the pH range 3-9 whereas in the case of PAA, the percent desorption increases from 20 to about 70% as the pH is increased from about 3 to 8. Solution conductivity tests confirm interaction of aluminum species and PAA in the bulk solution. FTIR spectroscopic data provide evidence in support of hydrogen bonding and chemical interaction in the case of the PAA-alumina system and hydrogen bonding with respect to the PVA-alumina interaction. Copyright 1999 Academic Press.

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PubMed1999

Effect of Charge and Hydrophobicity on Adsorption of Modified Starches on Polyester.

Polyester fabric (poly(ethylene terephthalate)) is a hydrophobic polymer. Its hydrophobic nature can be a disadvantage for certain applications like dyeing, finishing, detergency, etc. Physical or chemical modification of the polyester to make it more hydrophilic is therefore desirable for certain performance characteristics. Surface modification of polyester to make it hydrophilic can be achieved by adsorbing polymers on the polyester surface. Starch is a commonly available, hydrophilic polymer used in many textile applications that can be used to modify polyester. However, it needs to be chemically modified so that it can adsorb on the polyester fabric and physically modify the fabric characteristics. The polymers used in this study are two different modified starches-cationic and anionic starches and mixtures of the two. The adsorption kinetics on a polyester substrate was studied. The effect of charge and hydrophobicity on adsorption was investigated. Cationic starches were shown to readily adsorb on polyester and this was attributed to electrostatic interactions. Hydrophobic substituents on the cationic moiety resulted in increased adsorption. This was attributed to the weak hydrophobic interaction between the polymer chains which could result in a more coiled polymer conformation. It is hypothesized that more starch molecules are required for surface coverage of the polyester, resulting in an increase in adsorption. Anionic starch was adsorbed on the substrate but at a slower rate than the cationic starches. It is likely that there is a H bonding between acid groups on the starch and the ester groups of the polyester. However, the anionic starch is desorbed when the polyester is placed in an aqueous medium. When a blend of cationic starch and anionic starch was used, a low concentration of anionic starch was seen to increase adsorption, indicating that the polyelectrolyte complex itself may be adsorbing on the substrate. Further increases cause a decrease in adsorption as no sites may be available on the complex for adsorption. When hydrophobic substituents are present, addition of the anionic starch causes a decrease in adsorption at all concentrations. This was attributed to the "crosslinking" between the hydrophobically modified starch and the anionic polymer. Copyright 1999 Academic Press.

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PubMed2000

Effects of Cosolvents and pH on Protein Adsorption on Polystyrene Latex: A Dynamic Light Scattering Study.

Dynamic light scattering was used to study the adsorption of two proteins with different surface properties (IgG and HSA) on negatively charged polystyrene latex. The proteins were adsorbed from water and from water/methanol and water/glycerol mixtures at various pH. Some striking differences between the adsorption behaviors of the proteins were observed. Whereas the thickness of the adsorbed layer of HSA was extremely sensitive to pH and solvent composition, that of IgG was not. IgG mainly showed an end-on orientation on polystyrene whereas several different surface orientations are suggested for HSA under different conditions. The addition of methanol inhibited the adsorption of HSA on the latex, but it did not affect the adsorption of IgG. In contrast, the addition of glycerol increased the thickness of the adsorbed layers of both proteins. So, the orientation of IgG on the latex is insensitive to pH but is a function of the kind of solvent whereas both pH and solvent strongly affect the adsorption of HSA. This is a puzzling result since both cosolvents should equally affect the adsorption of both proteins if the dominant forces for adsorption are the same. Therefore, we concluded that, whereas hydrophobic interactions are the dominant force in the adsorption behavior of HSA, van der Waals forces are the main forces involved in the attachment of IgG to the lattices. Copyright 2000 Academic Press.

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PubMed2000

Characteristics of Nonafluorobutyl Methyl Ether (NFE) Adsorption onto Activated Carbon Fibers and Different-Size-Activated Carbon Particles.

The characteristics of adsorption of 1,1,1,2,2,3,3,4,4-nonafluorobutyl methyl ether (NFE), a chlorofluorocarbon (CFC) replacement, onto six different activated carbon; preparations (three activated carbon fibers and three different-sized activated carbon particles) were investigated to evaluate the interaction between activated carbon surfaces and NFE. The amount of NFE adsorbed onto the three activated carbon fibers increased with increasing specific surface area and pore volume. The amount of NFE adsorbed onto the three different-sized-activated carbon particles increased with an increase in the particle diameter of the granular activated carbon. The differential heat of the NFE adsorption onto three activated carbon fibers depended on the porosity structure of the activated carbon fibers. The adsorption rate of NFE was also investigated in order to evaluate the efficiency of NFE recovery by the activated carbon surface. The Sameshima equation was used to obtain the isotherms of NFE adsorption onto the activated carbon fibers and different-sized-activated carbon particles. The rate constant k for NFE adsorption onto activated carbon fibers was larger for increased specific surface area and pore volume. The rate of NFE adsorption on activated carbons of three different particle sizes decreased with increasing particle diameter at a low initial pressure. The adsorption isotherms of NFE for the six activated carbons conformed to the Dubinin-Radushkevich equation; the constants BE(0) (the affinity between adsorbate and adsorbent) and W(0) (the adsorption capacity) were calculated. These results indicated that the interaction between the activated carbon and NFE was larger with the smaller specific surface area of the activated carbon fibers and with the smaller particle diameter of the different-sized-activated carbon particles. The degree of packing of NFE in the pores of the activated carbon fibers was greater than that in the pores of the granular activated carbons. Copyright 2000 Academic Press.

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PubMed2001

Modeling Competitive Adsorption of Molybdate, Sulfate, and Selenate on gamma-Al(2)O(3) by the Triple-Layer Model.

Competitive adsorption of molybdate, sulfate, and selenate onto gamma-Al(2)O(3) was investigated in the present study. Binary solute systems of MoO(2-)(4)+SO(2-)(4), MoO(2-)(4)+SeO(2-)(4), and SO(2-)(4)+SeO(2-)(4) and a ternary solute system of MoO(2-)(4)+SO(2-)(4)+SeO(2-)(4) were evaluated to determine their relative effects on competitive adsorption on the gamma-Al(2)O(3) surface. Anionic competitive adsorption efficiency was pH dependent. The higher the pH, the lower the efficiency of MoO(2-)(4) in preventing SO(2-)(4) and SeO(2-)(4) adsorption; similar results were found in SeO(2-)(4) depressing SO(2-)(4) adsorption. This research found that more sites are occupied in mixed anionic adsorbate systems than when either ion is present alone. The results suggest that the gamma-Al(2)O(3) surface is composed of many groups of binding sites. Because of the heterogeneity of adsorption sites, the triple-layer model (TLM) predicted the competitive effects qualitatively but not quantitatively. TLM gave reasonable descriptions of molybdate adsorption in the presence of sulfate and selenate, indicating that the model may be useful in predicting molybdate adsorption on gamma-Al(2)O(3). Copyright 2001 Academic Press.

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PubMed2001

The Adsorption of Gemini and Conventional Surfactants onto Some Soil Solids and the Removal of 2-Naphthol by the Soil Surfaces.

The adsorption of two cationic gemini surfactants, [C(n)H(2n+1) N(+)(CH(3))(2)-CH(2)CH(2)](2).2Br(-), where n=12 and 14, on limestone, sand, and clay (Na-montmorillonite) from their aqueous solution in double-distilled water and the effect of this adsorption on the removal of 2-naphthol have been studied. Compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (C(n)H(2n+1)N(+)(CH(3))(3).Br(-), where n=12 and 14), the molar adsorptions of the gemini and the conventional surfactants on Na-montmorillonite are almost identical and very close to their cation exchange capacities. On sand and limestone, the molar adsorption of the cationic gemini surfactants is much larger than that of their corresponding conventional surfactants. Adsorption studies of the pollutants onto the three kinds of solids treated by either the gemini or the conventional surfactants show that the former are both more efficient and more effective at removing 2-naphthol from the aqueous phase. On all three soil solids, the addition of KBr increases the efficiency of the adsorption of both types of cationics and for most cases increases also the maximum amount adsorbed, but decreases slightly the efficiency of removal of 2-naphthol. On limestone, the anionic gemini adsorbs with one hydrophilic group oriented toward the Ca(2+) sites on the surface and its second hydrophilic group oriented toward the aqueous phase. The conventional anionic surfactant forms a double layer. The gemini anionic is more efficient and more effective than the conventional anionic in the removal of 2-naphathol from the aqueous phase. Both anionic conventional and gemini surfactants have no adsorption on sand. The adsorption mechanisms for all the surfactants on the three soil solid surfaces are discussed. Copyright 2001 Academic Press.

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PubMed2002

Adsorption of oleic acid at sillimanite/water interface.

The interaction of oleic acid at sillimanite-water interface was studied by adsorption, FT-IR, and zeta potential measurements. The isoelectric point (IEP) of sillimanite obtained at pH 8.0 was found to shift in the presence of oleic acid. This shift in IEP was attributed to chemisorption of oleic acid on sillimanite. Adsorption experiments were conducted at pH 8.0, where the sillimanite surface is neutral. The adsorption isotherm exhibited a plateau around 5 micromol/m2 that correspond to a monolayer formation. Adsorption of oleic acid on sillimanite, alumina, and aluminum hydroxide was studied by FT-IR. Chemisorption of oleic acid on the above substrates was confirmed by FT-IR studies. Hydroxylation of mineral surface was found to be essential for the adsorption of oleic acid molecules. These surface hydroxyl sites were observed to facilitate deprotonation of oleic acid and its subsequent adsorption. Thus protons from oleic acid react with surface hydroxyl groups and form water molecules. Based on the experimental results, the mechanism of oleic acid adsorption on mineral substrate was proposed. Free energy of adsorption was estimated using the Stern-Graham equation for a sillimanite-oleate system.

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PubMed2002

Adsorption of a diverse set of organic vapors on the bulk water surface.

The bulk water surface is of fundamental interest to physical as well as environmental chemistry. As there is a lack of wide-ranging adsorption data from the air to the bulk water surface, a large and diverse data set of adsorption coefficients of nonionic, organic compounds has been produced with inverse gas chromatography. The 61 compounds were chosen to cover a large range of properties, considering the intermolecular interactions between the compounds and the bulk water surface, i.e., van der Waals and electron-donor/acceptor interactions. The data set gained in this work was interpreted with a linear free energy relationship (LFER) based on these intermolecular interactions. From this LFER, a general adsorption model is derived, including compound (i) and surface (surf) properties: log K(i surf/air)(m(3)/m(2)) = 0.135(+/-0.003) log K(i hexadecane/air)(gamma(surf)(vdW))(0.5) + 5.11(+/-0.15)Sigma beta(i2)(H)EA(surf) + 3.60(+/-0.28)Sigma alpha(i2)(H)ED(surf) - 8.47. This adsorption model can be used for the characterization of adsorption to any other surface. The adsorption model for bulk water surface adsorption as well as the general adsorption model can be used as prediction tools in natural or technical systems.

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PubMed1990

Adsorption-desorption behaviour of zinc(II) at iron(III) hydroxide-aqueous solution interface as influenced by pH and temperature.

Batch studies were carried out to investigate the adsorption of zinc(II) from fresh waters on an iron(III) hydroxide surface maintained at the pH of zero point of charge of hydroxide (ZPC, 6.85) and also on both the acidic (5.5) and alkaline (8.2) sides of pH of ZPC, at 15 and 35 degrees C. Zinc(II) adsorption on iron(III) hydroxide increased with an increase in pH. The rise in temperature from 15 to 35 degrees C increased zinc(II) adsorption at pH 5.5 and 6.85, but decreased it at alkaline pH (8.2). In none of the cases did adsorption attain a maximum adsorption density. The results indicate the presence of heterogeneous sites of varying affinity on the adsorbent. Zinc(II) adsorption followed Langmuir behaviour only at small adsorption densities (less than 10(-2.95) M Zn/kg at pH 5.5) and at higher adsorption densities, the availability of strongest binding sites decreased. Nonspecifically adsorbed zinc(II) (reversible to Ba(II)) decreased with the increase in pH and temperature. Sequential desorption experiments also revealed that desorption of adsorbed zinc(II) decreased with an increase in pH.

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PubMed2004

Investigating the adsorption of 2-mercaptothiazoline on activated carbon from aqueous systems.

2-Mercaptothiazoline (2-MT) is widely used as an organic corrosive as well as a diffusion inhibitor due to its high ability to form metal-chelate Schiff base complexes. This study investigated the elimination of 2-MT from aqueous systems with adsorption process to reach the goal of sustainable use of water resources. The Freundlich and Langmuir adsorption isotherms were adopted to examine the adsorption behavior of two types of 2-MT (i.e., 2-MT molecule and 2-MT complex) on activated carbon (AC) Chemviron Filtrasorb 400 (F 400) in three different solutions. The results of adsorption isotherm data showed that the adsorption ability of 2-MT molecule is much higher than that of 2-MT complex, whose ability may be reduced due to the electrostatic repulsion. The good applicability of Langmuir adsorption isotherm to experimental data indicated that the adsorption of 2-MT complexes on F 400 might be limited to a monolayer. Higher ionic strength and lower pH value of the solutions promoted the uptake of 2-MT onto F 400 from the solutions. In addition, not only 2-MT molecule and complex but also Cu(II) can be adsorbed on the surface of F 400, which was demonstrated by energy dispersive analysis of X-ray (EDAX). Effects of the two major interactions, chemical and physical interactions, on the adsorption of 2-MT on F 400 were compared as well in this study.

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PubMed2004

The effect of carbon surface chemical composition on the adsorption of acetanilide.

The study of acetanilide adsorption-desorption performed at three temperatures (300, 310, and 320 K) and at two pH levels (7.0 and 1.5) on the series of D43/1 carbons (initial and modified with HNO3, fuming H2SO4, and gaseous NH3) is reported. Sorption data are additionally supplemented with the results of thermal analysis and calorimetric and kinetic measurements. It is shown that, generally, acetanilide adsorption at the neutral pH level is reversible (only on the more acidic carbons and at the lowest temperature does hysteresis occur due to the formation of hydrogen bonds with surface OH groups), and it decreases for the chemically modified carbons. In contrast, at the acidic pH level acetanilide adsorption is irreversible. A mechanism of irreversibility is proposed and it is shown that hysteresis is caused by the chemical reaction between the nucleophile (carbon) and the protonized acetanilide molecules. For all studied carbons, at the acidic pH level, adsorption increases and this is caused by the weakly basic character of acetanilide molecule. Adsorption results are described applying adsorbability and Dubinin-Astakhov, quasi-Freundlich and solution analogue of the Toth adsorption isotherm equations. Using the kinetic data, the effective diffusion coefficients and the energy of diffusion are calculated. It is shown that the diffusion is mainly a surface process, and the contribution of the pore diffusion increases with the rise in temperature. By applying different correlations between the parameters obtained from the theoretical description of experimental data and those characterizing the chemical composition of the studied carbons, the role of the latter in the adsorption and kinetics of acetanilide adsorption is determined.

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PubMed2004

Adsorption of acid dyes from aqueous solution on activated bleaching earth.

In the present study, activated bleaching earth was used as clay adsorbent for an investigation of the adsorbability and adsorption kinetics of acid dyes (i.e., acid orange 51, acid blue 9, and acid orange 10) with three different molecular sizes from aqueous solution at 25 degrees C in a batch adsorber. The rate of adsorption has been investigated under the most important process parameters (i.e., initial dye concentration). A simple pseudo-second-order model has been tested to predict the adsorption rate constant, equilibrium adsorbate concentration, and equilibrium adsorption capacity by the fittings of the experimental data. The results showed that the adsorbability of the acid acids by activated bleaching earth follows the order: acid orange 51 > acid blue 9 > acid orange 10, parallel to the molecular weights and molecular sizes of the acid dyes. The adsorption removals (below 3%) of acid blue 9 and acid orange 10 onto the clay adsorbent are far lower than that (approximately 24%) of acid orange 51. Further, the adsorption kinetic of acid orange 51 can be well described by the pseudo-second-order reaction model. Based on the isotherm data obtained from the fittings of the adsorption kinetics, the Langmuir model appears to fit the adsorption better than the Freundlich model. The external coefficients of mass transfer of the acid orange 51 molecule across the boundary layer of adsorbent particle have also been estimated at the order of 10(-4)-10(-5) cm s(-1) based on the film-pore model and pseudo-second-order reaction model.

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PubMed2004

Zn adsorption by different fractions of Galician soils.

To evaluate the contribution of organic matter, oxides, and clay fraction to Zn adsorption in six soils from Galicia (Spain), after soil characterization, adsorption isotherms were obtained by adding nine solutions containing between 20 and 500 mg L(-1) concentrations of Zn(NO(3))(2). Distribution coefficients were obtained from the data of adsorption isotherms. Zn adsorption isotherms corresponding to untreated soil and to the organic matter removed samples and organic matter and oxides removed samples were compared with curves pattern and adjusted to Langmuir and Freundlich empirical models. Untreated soils described L-curves whereas when soils were deprived of any component, the curves described were S-type. Distribution coefficients allowed knowing the Zn adsorption capacity of the untreated soil, and of the organic matter, oxides, and clay fraction. Soil organic matter is the main component that affects Zn adsorption as long as soil pH is near neutrality. At acid pH, the oxides are the main component that affects Zn adsorption, although to a much smaller extent than the organic matter near neutral conditions. So soil pH is the main soil factor that determines Zn adsorption, before any other soil property.

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PubMed2005

Thermodynamic and kinetic investigations of PO3-4 adsorption on blast furnace slag.

The kinetics of adsorption of PO(3-)(4) by blast furnace slag were found to be fast, reaching equilibrium in 20 min and following a pseudo-second-order rate equation. The adsorption behavior of PO(3-)(4) on blast furnace slag has been studied as a function of the solution agitation speed, pH, and temperature. Results have been analyzed by Freundlich, Langmuir, BET, and Dubinin-Radushkevich (D-R) adsorption isotherms. The mean energy of adsorption, 10.31 kJ mol(-1), was calculated from the D-R adsorption isotherm. The rate constants were calculated for 293, 298, 303, and 308 K using a pseudo-second-order rate equation and the activation energy (E(a)) was derived using the Arrhenius equation. Thermodynamic parameters such as DeltaH(0), DeltaS(0), and DeltaG(0) were calculated from the slope and intercept of linear plot of lnK(D) against 1/T. The DeltaH(0) and DeltaG(0) values of PO(3-)(4) adsorption on the blast furnace slag show endothermic heat of adsorption. But there is a negative free energy value, indicating that the process of PO(3-)(4) adsorption is favored at high temperatures.

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