Showing papers on "Langmuir published in 1985"

Chemical modeling of anion competition on goethite using the constant capacitance model

Abstract: The constant capacitance model was used to describe phosphate, selenite and silicate adsorption on goethite (a-FeOOH) in single anion systems. The model assumes a ligand exchange mechanism and considers the charge on both adsorbate and adsorbent. It is therefore appropriate for describing the adsorption behavior of these specifically adsorbed anions. The computer program FITEQL, a program that fits equilibrium constants to experimental data using an optimization technique, was used to obtain optimal values for the anion surface complexation constants on goethite. The model represented adsorption of these anions well over the entire pH range studied (3-12). The main advantage of the model is its ability to represent changes in anion adsorption occurring with changes in pH. Extension of the model to describing anion competition in mixed systems using the anion surface complexation constants from single anion systems was qualitatively successful. In mixed anion systems the model predictions reproduced the shape of the adsorption isotherms well over the pH range 3 to 12. However, phosphate adsorption was overestimated while adsorption of the competing anion was underestimated. Additional Index Words: phosphate adsorption, selenite adsorption, silicate adsorption, ligand exchange, FITEQL, MICROQL. Goldberg, S. 1985. Chemical modeling of anion competition on goethite using the constant capacitance model. Soil Sci. Soc. Am. J. 49:851-856. T CONSTANT CAPACITANCE model of the Oxidewater interface was developed by Stumm, Schindler and their co-workers (Stumm et al., 1970; Schindler and Gamsjager, 1972; Stumm et al., 1976; Stumm et al., 1980). It is a chemical model which differs from the Langmuir and Freundlich adsorption equations in that it explicitly defines surface species and chemical reactions. The adjustable model parameters are the conditional equilibrium constants for the surface reactions as well as the capacitance—an empirical parameter. The effect of pH on the extent of adsorption and on the magnitude of surface charge is included in this model. The model successfully describes the amphoteric behavior of inorganic surface hydroxyl groups as well as the adsorption of specifically adsorbed anions. Anions become specifically adsorbed by undergoing ligand exchange with reactive surface hydroxyl groups. Specific adsorption of anions produces a shift in the zero point of charge (ZPC), of the oxide mineral to a more acid value. A shift in ZPC was observed for adsorption of selenite on goethite (Kingston et al., 1968) and on hydrous alumina (Rajan, 1979) and for silicate adsorption on goethite (Kingston et al., 1972; Sigg and Stumm, 1981) indicating that these ions are adsorbed specifically. For this reason, the constant capacitance model is appropriate for describing 1 Contribution from the U. S. Salinity Laboratory, USDA-ARS, 4500 Glenwood Drive, Riverside, CA 92501. Received 24 Sept. 1984. Approved 22 Jan. 1985. 2 Soil Scientist. the adsorption behavior of these anions. The adsorption behavior of various other anions has been described with the model, including adsorption of the aromatic compounds catechol, salicylic acid, benzoic acid, and phthalic acid on 7-Al2O3 (Kummert and Stumm, 1980), and F~, SO^, acetate, H2SiO^-, and PO]" adsorption on goethite (Sigg and Stumm, 1981). The ability of the model to describe phosphate adsorption on various x-ray amorphous as well as crystalline aluminum and iron oxides was recently demonstrated by Goldberg and Sposito (1984). Selenium is an essential element in animal nutrition being required in small amounts and producing toxicity symptoms at higher concentrations. Since the concentration range between deficiency and toxicity is narrow, reactions affecting selenium availability have been investigated. Wells (1967) postulated that selenium was retained by the clay size particles, especially gibbsite and iron oxides, since marked selenium accumulation occurred in soil horizons enriched in these colloids. Geering et al. (1968) and Gary and Allaway (1969) found evidence for the retention of selenite by adsorption onto soil iron oxides. Gary and Allaway (1969) indicated that the adsorption complex was a finely divided sesquioxide having a solubility equivalent to or less than freshly precipitated Fe(OH)3. Hamdy and Gissel-Nielsen (1977) found that selenite was fixed very rapidly by Fe2O3 and adsorbed strongly, indicating a major role of iron oxides in selenite retention in soils. Rajan (1979) found that selenite adsorption on hydrous alumina occurs specifically via a ligand exchange mechanism with surface hydroxyl ions. By determining the ratio of hydroxyl ions released to selenite ions adsorbed, Rajan (1979) proposed that selenite forms both monodentate and bridging complexes at low surface coverage but forms almost exclusively monodentate complexes near the experimental adsorption maximum. Dissolved silica in soil solution plays an important role in soil development and a beneficial one in plant growth. Silicate was chemisorbed on crystalline aluminum hydroxide (Kingston and Raupach, 1967) and was adsorbed with high affinity on iron and aluminum hydrous oxides (McPhail et al., 1972). McKeague and Cline (1963) found that freshly precipitated iron and aluminum hydroxides exhibited a much higher capacity for removing silicate from solution than did crystalline oxide minerals. They found that silicate adsorption on a bauxite sample containing mostly gibbsite increased with increasing pH over the pH range 4.8 to 9.7. Similar behavior was found for the soils studied by McKeague and Cline (1963) in that silicate adsorption increased over the pH range 4 to 9. Decreases in soil silicate sorption were found above pH 10. Beckwith and Reeve (1963) also found that silicate adsorption behavior on aluminum and iron oxides was very similar to that on soil samples in its pH dependence in the pH range 3 to 9. Obihara and Russell (1972) obtained silicate adsorption curves of soils

Langmuir‐blodgett films from preformed polymers

Abstract: Recent work on Langmuir films and Langmuir-Blodgett films prepared from derivatives of styrene-maleic anhydride and octadec-1-ene-maleic anhydride copolymers is reviewed.

Novel effects of counterions on langmuir films of 22-tricosenoic acid

Abstract: It has long been known that metallic counterions at very low concentrations have a significant effect on the isotherm, lateral cohesion and viscosity of a fatty-acid Langmuir monolayer on a water surface. The evidence for four new effects is presented. Counterions are shown to impair crystallinity, inhibit epitaxial deposition, reduce initial drainage speed, and cause a variation of drainage speed with time, all phenomena of great importance in the deposition of Langmuir-Blodgett films. On the basis of the time elapsed between monolayer spreading and the onset of the phenomenon, the counterion effects are seen to be divided into two categories. Gel and phase-change models are discussed, while the growth of water-surface crystallites is shown to be a secondary effect.

Pressure swing adsorption. Part II: Experimental study of a nonlinear trace component isothermal system

Abstract: The recently developed mathematical model for simulation of a PSA system (Raghavan et al., 1985) has been extended to a nonlinear Langmuir equilibrium system. The pressure swing adsorption separation of a small concentration of ethylene from helium has been studied experimentally on 4A and 5A molecular sieves. In the 4A system mass transfer is controlled by intracrystalline diffusion and is slow, whereas in the 5A system mass transfer is very much faster and is controlled by macropore diffusion. With rate and equilibrium parameters derived from experimental breakthrough curves, the theoretical model provides a good representation of the experimentally observed behavior of both PSA systems.

Alveolar liquid lining: Langmuir method used to measure surface tension in bovine and canine lung extracts.

Abstract: The Langmuir trough has been used to study monolayers of surfactant from beef lung extracts, dog lavage and the principal component (dipalmitoyl lecithin; DPL) in which surface tension has been simultaneously monitored on each film by the Wilhelmy method and the original Langmuir method whose readings are independent of contact angle. Readings on the Wilhelmy balance at 80% film compression reached the near-zero values recorded in previous studies, averaging 2.0 dyn cm-1, but Langmuir readings on the same films averaged 13.5 dyn cm-1 for DPL with no reading below 7.0 dyn cm-1. Similar differences were found for bovine extracts and dog lung lavage and have been attributed to a serious contact-angle artifact in the Wilhelmy method widely used in studying pulmonary surfactant. Higher minimum values of surface tension determined by the Langmuir method are shown to be incompatible with normal physiological function in the smaller mammals on the basis of the traditional bubble model of the alveolus. When Langmuir or Wilhelmy values are determined under more physiological conditions of area change, pH, temperature, humidity etc., values for surface tension are even higher and less compatible with the concept of a continuous liquid lining for the alveolar diameter of any mammal.

Why the close‐packed Ni(111) surface dissociatively chemisorbs oxygen and nucleates oxide faster than the more open Ni(100) surface

Abstract: The 300 K kinetics of dissociative chemisorption and oxide island nucleation and growth are compared for Ni(100) and Ni(111). We conclude that the behavior of oxygen sticking probability as a function of coverage can be understood in terms of direct Langmuir adsorption (no molecular diffusion) and the availability of active empty clusters of adsorption sites of certain sizes and configurations.

Mass transfer model for the adsorption of basic dyes on woodmeal in agitated batch adsorbers

Abstract: The adsorption of three basic dyes, Basic Blue 3, Basic Blue 69 and Basic Orange 2, onto woodmeal has been studied in batch adsorbers. The effects of initial dye concentration and mass of woodmeal have been studied. A two-resistance mathematical model has been developed based on external mass transfer, pore diffusion and the assumption of an irreversible isotherm for operating conditions lying on the monolayer of observed Langmuir isotherms. The model, which has an analytical solution, predicts theoretical concentration decay curves which show good agreement with observed experimental decay curves.

Removal of Tritiated Water Vapor by Adsorption on Molecular Sieves - Effects of Co-Existing H2O

Abstract: Effect of co-existing H/sub 2/O (isotopic swamping effect) in the adsorption of tritiated water vapor on molecular sieves was experimentally studied by two means. One was by breakthrough experiment. It was found that H/sub 2/O-HTO two components Langmuir type adsorption isotherm is satisfactory to explain the HTO breakthrough curve. The other was by using a divisible column, where behavior of HTO at much lower concentration than at the inlet and the axial distribution of tritiated water on the adsorbent were studied. Concentration of tritiated water vapor at the column outlet was found to decrease with the increase of H/sub 2/O concentration of the input. However the improvement of decontamination factor was little.

Langmuir and Scatchard Parameters Do Not Describe the Binding of Actinomyces viscosus to Saliva-treated Hydroxyapatite

Abstract: Summary: The binding of Actinomyces viscosus T14V to saliva-treated spheroidal hydroxyapatite (SHA) beads was studied. The association constant (K) and the total number of binding sites (N) obtained from the Langmuir plots were in good agreement with those reported by other workers (approx. 3 X 10−8 and 3 X 108, respectively). The values for N obtained from Scatchard plots differed from those obtained from Langmuir plots by factors of 106 or more. These results suggest that either these equations are inappropriate to describe binding or certain assumptions regarding this system are not being met. The use of these models requires, among other constraints, that the process be reversible and that measurements be taken at equilibrium. A method was developed which allowed a close examination of the equilibrium dynamics without perturbation of the system. The results suggest that the adsorption process is only poorly reversible. Adsorption to SHA was not at equilibrium after 1.5 h. Even when bacteria were allowed to adsorb for longer periods, and the system appeared to approach equilibrium, the increased time of adherence did not significantly alter the derived K or N values. Our results suggest that the use of Scatchard and Langmuir plots is inappropriate to describe binding of A. viscosus to SHA.

Iodine Doping-Dedoping of Thin Layers of (AlpcF)n

Abstract: The polyphthalocyariine(AIPcF)n was deposited on various substrates as layers a few hundred A thick. An increase of the conductivity is observed when such layers are submitted to a I2+N2 gaseous mixture. The dedoping is performed by sweepingwith pure N2. The partly reversible and I2 pressure dependent doping-dedoping process can be analysed in terms of Langmuir's theory.

The multiple binding of heavy metals by digested sludge

Abstract: The multiple interactions of heavy metals with anaerobically digested sludges and with digested sludge that had been washed with EDTA have been examined by equilibrating the sludge solids with solutions containing mixtures of zinc acetate, lead acetate and chromic chloride. This equilibration has been studied at 20°C and Langmuir and Freundlich isotherms were used to describe the metal binding behaviour of the solids. The rates of adsorption were also measured. This showed that while the rate constants were independent of the added metal concentration for untreated sludge, the binding constant for washed sludge was related to the initial metal concentration. A comparison of the adsorption isotherms suggests that the binding of zinc is different from that of lead and chromium.

Estimation of heat of adsorption from energy distribution on heterogeneous surfaces by a theoretical analysis of adsorption process

Abstract: An attempt was made to calculate theq—θ function (experimental heat function) from thee—θ function (energy level function) of a heterogeneous surface. In order to obtain the heat of adsorption, adsorption was assumed to take place on each energy level of thee—θ function. In reversible adsorption, Langmuir's monolayer model was extended to determine the occupation of energy levels by molecules. On this basis, the heat of adsorption could be calculated. Irreversible adsorption was also examined on the basis of chemical kinetics. Molecules were supposed to be held on sites to which they first collided. These calculations were useful for estimating the extent of deviation of theq—θ function from thee—θ function.

Langmuir probing studies of UV‐photoionized CO2 laser mixtures

Abstract: A Langmuir probe incorporated with a boxcar averager is used to measure the temporally and spatially resolved electron density and electron temperature in a UV‐photoionization device for several gases and gas mixtures. The predominant ionization mechanism is determined. The data provides information to optimize the effectiveness of the UV preionization operation in high‐pressure lasers.

Static and dynamic studies of protein sorption by porous alkylsilica

Abstract: Porous alkylsilicas are important stationary phases (sorbents) for separating proteins by high performance liquid chromatography and have great potential for large scale isolations. We have employed batch experiments, frontal chromatography, and surface energy measurement techniques to study the factors which influence protein sorption to these materials. Proteins such as bovine serum albumin are sorbed irreversibly from phosphate buffer at pH 7 and pH 2. Addition of alcohol at sufficient concentration to reduce the surface tension of the solvent to lesis than 34 ergs/cm2 effected desorption at pH 2 but not at pH 7. Proteins were desorbed by solutions of nonionic surfactant at pH 7. The chain-lengths of alkyl groups on the silica had little influence on sorption isotherms which, in the batch studies, were of the general Langmuir shape. The isotherms derived fron the frontal experiments were indicative of two-stage desorption process.

Ion Interaction: The Energetics and Mechanism of The Competitive Behavior Between Two Similarly Charged Molecules. 1. The Effect of Ionic Strength, Acetonitrile and Surfactant Concentration

Abstract: The competition between two molecules of similar polarity for adsorption sites on the stationary phase is discussed in light of the effects of acetonitrile, surfactant (cyclohexylaminopropane sulfonic acid, CAPS) and salt concentrations on the retention of the thyroid hormones (3,5-diiodo-thyronine, T2; 3,3′,5-diiodo-thyronine, T3 and thyroxine, T4). The Kinetics of adsorption and desorption for the surfactant are fast which allows for actual competition, otherwise an electrostatic “wall” would be seen by the analyte leadiny to exclusion. A three-parameter equation relates the surfactant concentration and ionic strength to the retention of the hormones, which suggests that the retention and selectivity arise from the structure and “density” of the outer layer in a Stern-Gouy-Chapman model and confirms that only the outer electrical layer significantly contributes. The data are analyzed with both Langmuir and Freundlich equations which describe an intermediary state where the marginal heat of adso...

Empirical representation of ammonium adsorption in two soils

Abstract: Two models for ammonium adsorption, employing the Langmuir and Freundlich isotherms and not representing other cations, were fitted to adsorption data on two different soils. The Langmuir model gave a slightly better fit to the data and had the added advantage that one of the parameters appeared to

Zeolites as Adsorbents for Alcohols from Aqueous Solutions

Abstract: Zeolites with hydrophobic character, such as silicalites, were synthesized, and their structures studied by X-ray diffraction. Their potential use as adsorbents for the removal of organic molecules from water was investigated by series of experiments with aqueous solutions of lower alcohols. Adsorption isotherms of the Langmuir type were applied, and calculations showed that the amount of propanol adsorbed on silicalite corresponded to approximately 70% of the pore volume. The amounts of alcohols adsorbed per unit weight of zeolite decreased in the order n-propanol > ethanol > methanol, and the alcohol was more efficiently removed with silicalites than with other zeolites, such as ZSM5, AlPO4-zeolites and others, due to the more hydrophobic and less polar character of silicalite. F-silicalite, which contains small amounts of fluorine, also showed interesting properties, but had lower adsorption capacity than the regular silicalite. Adsorption experiments with aqueous acetone showed that silicalite had approximately the same adsorption capacity for acetone as for npropanol. Heats of adsorption were determined calorimetrically.

Adsorption of Zinc in Selected Soils from Denmark

Abstract: The amount of specifically adsorbed Zn on some Danish surface soils was mainly influenced by the soil reaction. The adsorption can be described by Langmuir and Freundlich isotherms. The reaction proceeded spontaneously, but with positive values for enthalpy and entropy indicating that the adsorption was an endothermic process.