Showing papers on "Langmuir published in 1996"

Adsorption Isotherms of Phenolic Compounds from Aqueous Solutions onto Activated Carbon Fibers

Abstract: Liquid-phase adsorption equilibria of eight phenolic compounds onto activated carbon fibers were measured in the concentration range 40−500 g·m-3 at 303 K. High adsorption capacities were observed for the chlorinated phenols compared to the methyl-substituted phenols. Several two- and three-parameter isotherm equations were tested. Among the equations tried, the three-parameter equation of Jossens et al. based on a heterogeneous surface adsorption theory was found to be the most satisfactory over the entire range of concentration. The widely used two-parameter equations of Langmuir and Freundlich were not applicable to the present adsorption systems.

Removal of lead ions from aqueous solution using sphagnum moss peat as adsorbent

Abstract: The use of sphagnum moss peat for the removal of lead from aqueous solution has been studied in batch experiments. Investigations include the effect of pH and temperature, and kinetics of adsorption. The adsorption equilibria data were found to follow Langmuir models. It was found that the efficiency of lead removal using sphagnum moss peat as adsorbent depended very little either on the reaction temperatures (10 to 40°C) or on the initial pH values (4.0 to 6.0). Kinetic data suggested that the adsorption process was endothermic and pore diffusion was not the only rate-determining step.

Use of chitin and chitosan in lobster shell wastes for color removal from aqueous solutions

Abstract: Chitin and chitosan prepared from lobster shell wastes were used as adsorbents for the removal of various dyes from aqueous solutions. It was shown that high adsorption capacities were observed for reactive dyes. The adsorption equilibrium data could be well described by Langmuir equation under the concentration range investigated (50–500 mg/L). On the basis of Langmuir adsorption capacity, chitosan obtained in this work was found to be more effective compared to chitin and a commercial chitosan.

Modeling the Effect of Temperature on Adsorption of Lead(II) and Zinc(II) onto Goethite at Constant pH

Abstract: The influence of temperature and adsorbate concentration on the sorption of Zn(II) and Pb(II) by goethite was studied at fixed pH. Proton stoichiometry, measured by direct titration, was found to depend on the identity of the metal ion, the pH, and the temperature. For Zn(II) the proton stoichiometry ranged from 1.55 at pH 6.5 and 10°C to 1.95 at pH 7.5 and 70°C, while for Pb(II) at pH 5.50 the value varied from 1.05 at 10°C to 1.4 at 70°C. Three adsorption models—the Langmuir two-site model, the surface precipitation model of Farley, Dzombak, and Morel (FDM), and the BET model—were applied to the data, and the fitted parameters were used to determine the enthalpy and entropy of adsorption. Although the models assume rather different sorption reactions, and therefore generate correspondingly different equilibrium constants, the estimated enthalpies of adsorption were all positive, between 4 and 30 kJ/mol. There was also fair agreement between the various estimates of the adsorption capacities. However, the models yield very different estimates of entropies of adsorption, these being positive (and generally in the range 100 to 160 J K −1 mol −1 ) for the Langmuir and BET models but negative (in the range −80 to −160 J K −1 mol −1 ) for the FDM model.

Second harmonic electromagnetic emission via Langmuir wave coalescence

Abstract: The coalescence of Langmuir waves to produce electromagnetic waves at twice the plasma frequency is considered. A simplified expression for the rate of production of second harmonic electromagnetic waves is obtained for a broad class of Langmuir spectra. In addition, two different analytic approximations are considered. The validity of the commonly used head‐on approximation is explored, in which the two coalescing Langmuir waves are assumed to approach from opposite directions. This approximation breaks down at low Langmuir wavenumbers, and for narrow Langmuir wave spectra. A second, more general, approximation is introduced, called the narrow‐spectrum approximation, which requires narrow spectral widths of the Langmuir spectra. The advantages of this approximation are that it does not break down at low Langmuir wavenumbers, and that it remains valid for relatively broad Langmuir wave spectra. Finally, the applicability of these approximations in treating harmonic radiation in type III solar radio bursts is discussed.

Intraparticle diffusivity of Cd ions in a new biosorbent material

Abstract: Cadmium equilibrium sorption isotherms were determined for formaldehyde crosslinked Sargassum fluitans, establishing that an effective regeneration of the new biosorbent material is possible by an acid wash. Batch desorption kinetics were investigated at pH values of 1.0 and 2.0. By incorporating the linear and non-linear Langmuir equilibrium isotherm relationships into the rate equations, a mathematical model was proposed for modeling the metal desorption process. The model was solved numerically and a MATLAB computer program was used to curve-fit the experimental data. The model successfully predicted the Cd 2+ elution concentration profile in a batch reactor. The average values of the intraparticle diffusivity of Cd 2+ in the algal biosorbent calculated from the model were 3.40 x 10 -6 and 1.65 x 10 -6 cm 2 s -1 , 0.473 and 0229 times the molecular diffusivity of Cd 2+ in water, at pH values of 1.0 and 2.0, respectively. These values agreed well with theoretical predictions.

Sorptive Removal of Cesium-137 and Strontium-90 from Water by Unconventional Sorbents.

Abstract: It has been shown that coal fly ash is a good adsorbent for both radionuclides of 137Cs and 90Sr. Radiocesium adsorption is maximal around the neutral region whereas radiostrontium adsorption increases with pH, especially above pH 8. Cesium retention sharply drops with ionic strength while strontium adsorption increases sharply and steadily at low and moderate concentrations of the inert electrolyte, respectively. The suggested mechanisms of radionuclide retention by fly ash is specific adsorption of Cs+ and irreversible ion-exchange uptake of Sr2+. The isotherm of adsorption is a Langmuir approximation of the B.E.T. multi-layered sorption. Acid pretreatment of fly ash, though not increasing radionuclide sorption capacity, may be useful in preventing the leach-out of other contaminants from the sorbent into water during the adsorption process.

Application of the kinetic-thermodynamic model for inhibition of acid corrosion of steel by inhibitors containing sulfur and nitrogen

Abstract: Corrosion inhibition characteristics of four selected S-alkylisothioronium halides on steel in sulfuric acid (H2SO4) media were investigated at 30°C, 40°C, 50°C, and 60°C. The number of active sites on the metal occupied by a molecule of the inhibitor (1/y) and the binding constants (K) of the inhibitor to the active sites were calculated at the different temperatures. Activation free energies, enthalpies, and entropies for the inhibition processes were determined from rate-constant data measured at 30°C, 40°C, 50°C, and 60°C at two fixed inhibitor concentrations. Data were compared and fitted to a published kinetic-thermodynamic model. Results were compared withfits obtained from application of Langmuir, Frumkin, and Flory-Huggins isotherms. Results also were correlated to the chemical structure of the inhibitors, the length of the alkyl chain of the cation, and the type of the anion.

Effect of Dye Nature on Its Adsorption from Aqueous Solutions onto Activated Carbon

Abstract: Liquid-phase adsorption of two dyes on activated carbon was measured in the 283–323 K temperature range. Higher capacity and faster kinetics of adsorption were obtained for a basic dye than for an acidic dye under comparable conditions, probably due to the different ionic natures of the dyes. The equilibrium data for acidic dye could be well described by the Freundlich equation, but the best-fit model for basic dye changed from the Langmuir to the Freundlich equations when the temperature was raised. In addition, the thermodynamic functions were determined. A plot of the fraction of adsorption against (time)1/2 was adopted to describe the adsorption process. It was shown that for an acidic dye the amount of adsorbent used played an important role in the adsorption mechanism.

Enhanced Organic Contaminant Sorption on Soil Treated with Cationic Surfactants

Abstract: Batch sorption and column transport laboratory experiments were performed to investigate the use of cationic surfactants to enhance sorption of dissolved organic contaminants in aquifer material with low organic carbon content. The experiments used sand from the Borden aquifer in Ontario, Canada, treated with hexadecyltrimethylammonium (HDTMA+) surfactant. Test results indicate that organic cations are strongly adsorbed by cation exchange reactions on the solid surface. Further adsorption occurs in admicelles, which form by hydrophobic interactions between adsorbed and mobile HDTMA+ monomers. The nonlinear adsorption isotherm may be described by Langmuir adsorption theory. The retardation factor for HDTMA+ on Borden aquifer material is approximately 460 when aqueous surfactant concentrations are less than 100 mg/l, while HDTMA+ is relatively mobile at higher concentrations. Column experiments suggest that the adsorption reaction is very rapid, and thus independent of ground-water flow velocity. A column treated with HDTMA+ equivalent to 33% of CEC and flushed with 325 pore volumes of surfactant-free water showed that although the HDTMA+ modified zone was smeared downgradient, all HDTMA+ mass remained adsorbed within the column. The results indicate that an essentially stationary surfactant-modified zone can be created. Adsorbed HDTMA+ mass increased foc from 0.02 wt% on natural Borden aquifer material to 0.18 wt% on the treated sediments. Organic matter derived from the adsorbed HDTMA+ is an effective partitioning medium for dissolved HOC, and results indicate that it may sorb HOC more efficiently than natural foc on a unit weight basis. Batch sorption tests showed that sorption coefficients (Kd) for trichloroethylene (TCE), perchloroethylene (PCE), and 1,2,4-trichlorobenzene (TCB) were 13 times higher on HDTMA+ treated Borden aquifer material compared to the natural sediments. Retardation factors estimated from column transport tests confirmed these results, and suggested that sorption of moderately hydrophobic organic contaminants on the HDTMA+ modified sediments was a rapid and reversible process. Data from these laboratory-scale experiments indicate that it may be feasible to develop an enhanced sorption zone in situ in a low foc aquifer as part of a ground-water remediation scheme for dissolved organic pollutants.

Color Removal from Dye Wastewater Using Vermiculite

Abstract: The purpose of this laboratory study was to investigate the effectiveness of vermiculite extract solutions (VES) as a coagulant and of vermiculite as an adsorbent for removal of color from dye wastewater. In coagulation experiments, it was found that VES could be applied to the treatment of dye wastewater with its contents of various cations (e.g. Mg, Fe, Al, Ca, Si etc.) useful for coagulation. Coagulation with VES produced less sludge at a lower coagulant dosage in comparison with to conventional inorganic coagulants. For evaluation of adsorption characteristics of vermiculite on basic dye wastewater, adsorption parameters for Langmuir and Freundlich isotherms were determined. The adsorption of the basic blue dye on vermiculite was found to conform with both isotherms. The adsorption capacity of vermiculite was found to increase with the decreasing particle size and with the increase of temperature and of intensity of agitation. The results indicate that vermiculite is an excellent adsorbent for basic b...

Removal of hexavalent chromium from aqueous solutions by granular activated carbon

Abstract: Batch adsorption trials were used to assess the effectiveness of using activated carbon as a sorbent for the removal of hexavalent chromium from solution. The results show that, although the pH for maximum efficiency varied slightly with the concentration of Cr(VI) being applied, the most effective pH was in the range 2.5 to 3.0. Based on linear regression analysis, the data obtained from the batch studies showed a good compliance with both the Langmuir and the Freundlich equations. The values obtained for the isotherm constants showed that the maximum adsorption capacity, X m , was 145 mg/g. An examination of the adsorption kinetics was also made and an analysis of the data shows that pore diffusion is not the only factor governing the sorption rate.

Heterogeneous/homogeneous reaction and transport coupling during flame-wall interaction

Abstract: The effect of surface chemistry and transport modeling on the head-on quenching process of laminar propane/air and methane/air flames is investigated numerically. The fully compressible, one-dimensional Navier-Stokes equations with detailed mechanisms for diffusion and chemical kinetcs (gas-phase and surface) are solved using a sixth-order finite-difference scheme in space and a third-order Runge-Kutta scheme in time. Important applications are in the modeling of flame-wall interaction processes in piston engines, their related experimental measurements, and general surface chemistry modeling. The latter is due to the interaction being unsteady in contrast to typical boundary layer or stagnation point flow studies over reactive surfaces. Also the surface response takes place in the high coverage zone. It is found that in order to simulate quenching at higher wall temperatures accurately, the process of diffusion, adsorbtion, and surface recombination especially for the free radical H and OH have to be described. This results in an increasing wall heat flux with wall temperature in excellent agreement with existing experimental data If the participation of the surface in the reaction channels of the near-wall gas-phase species is excluded (either because of the physical surface properties or because of the boundary treatment in the simulations), the quenching structure is completely changed with strongly exothermic radical recombination reactions dominating the interaction. The developed detailed surface mechanism contains 10 reactions involving four surface species, and seven gas-phase species. It takes into account the breakdown of the Langmuir adsorption concept for high surface coverages and includes a precursor-state model for the adsorption of H2, H, and OH.

On pressure dependence of the parameters of the dual-mode sorption model

Abstract: Sensitivity of the parameters of the dual-mode sorption (DMS) model on the pressure range, in which sorption of gases in polymers have been studied, was analyzed. Different “gas-polymer” systems were considered but the most detailed analysis was performed for sorption of argon and nitrogen in poly[5,5-difluoro-6,6-bis(trifluoromethyl)] norbornene and polysulfone. It was shown that the model parameters depend upon the range of gas pressure studied. Expanding of the pressure range (0-pi) results in an increase in the Langmuir adsorption capacity C′H and in reduction of Henry's law solubility coefficient kD and Langmuir affinity parameter b. These behaviors does not depend on a choice of an experimental apparatus or software and procedure of nonlinear least squares treatment of the data. As statistical analysis indicated, a systematic error of the measurement cannot call forth the observed dependencies of the model parameters. Different physical reasons of these behaviors were considered, among them: the pressure dependence of the affinity parameter, and the dilation of a polymer. The results obtained showed that although the DMS model, as a rule, gives an excellent fit of the experimental curves, and, hence, can be used as a form of compact storage of information on gas sorption in polymers, one should be careful in using it outside the pressure range in which its parameters have been determined. © 1996 John Wiley & Sons, Inc.

Removal of Toxic Metals by Biosorption onto Nonliving Sewage Sludge

Abstract: Toxic metals, such as zinc, nickel, and cadmium, can be removed from dilute aqueous solutions by sorption onto nonliving sewage sludge applied as finely dispersed biosorbent particles after sterilization and drying. A comparison between the suggested method and precipitation, the most common method used for metals removal, was conducted in parallel. The main parameters examined in single component systems include initial metal concentration, temperature, and inhibition of the removal process by the existence of soluble constituents. Adsorption isotherms were employed to describe the metals uptake; the Langmuir types were found to fit the experimental data better than the Freundlich ones. Moreover, the desorption of metals from metal-loaded biomass was investigated. The possible selective separation of metals from binary and ternary mixtures by biosorption, which could lead to the recovery and recycling of the removed metals, was also examined by introducing an illustrative selectivity factor. ∗ P...

Water adsorption and desorption kinetics on silica insulation

Abstract: Silica is an important component of thermal insulation used in aerospace transport vehicles. Although normally hydrophilic and requiring rewaterproofing after flight, silica insulation has been reported to become hydrophobic on heating and cooling in a vacuum and, therefore, potentially hydrophobic after reentry. To understand and model these processes, the rates of desorption and adsorption of water on silica were studied by thermogravimetry, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Silica powders with a high surface area were heated in vacuo in the temperature range 200–1000°C, infrared absorbance spectra were obtained at various temperatures, and surface structure was examined by X-ray photoelectron spectroscopy. A kinetic sorption model was developed based on a Langmuir rate expression. Activation energies for adsorption and desorption were determined by fitting the experimental results numerically. The study found adsorption of water on the heat-treated silica occurs within seconds, implying hydrophobicity is lost rapidly after the heat-treated silica is exposed to water.

Modeling and Simulation of Transient Adsorption and Reaction in Vacuum Using the Temporal Analysis of Products Reactor

Abstract: For determining rate constants of catalytic processes (adsorption, desorption, and reaction) from the evaluation of transient experiments in vacuum applying the temporal-analysis-of-products (TAP) reactor, suitable models were developed taking into account the gas-phase transport of the molecules and the surface processes. On the basis of simulation results, it is shown that by applying the TAP technique absolute values for rate constants of adsorption and desorption can be derived within a “window” of their order of magnitudes. It is shown that different types of adsorption (Langmuir, Temkin, Freundlich, and dissociative) result in different simulated response signals. Models were developed accounting for reactions including the simulation of experiments with sequential pulsing of two gases in order to study the reaction of adsorbed and gaseous species. The specific position of the catalyst between two inert layers in the TAP reactor has to be considered because it influences the response in the case of ...