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Adsorption: Journal of the International Adsorption Society (v.17, #6)
Estimation of pore size distribution in MCM-41-type silica using a simple desorption technique by Evgeny I. Shkolnikov; Elena V. Sidorova; Alexander O. Malakhov; Vladimir V. Volkov; Anne Julbe; André Ayral (pp. 911-918).
A recently developed dynamic desorption technique is used for obtaining vapor isotherms on porous materials. This gravimetric technique does not require any preliminary calibration and is based on analyzing the kinetics of liquid evaporation from a porous sample under quasi-steady state conditions. The crucial feature of the technique is concerned with the fact that no vapor pressure measurements are necessary. The technique is illustrated by desorption of benzene vapors from mesoporous silica MCM-41. To calculate the pore size distribution, the Derjaguin–Broekhoff–de Boer theory in its combination with the Wheeler model for capillary condensation is used. In the calculations, the reference data on benzene adsorption on a nonporous silica gel from two different sources (published by different authors) are applied. The mean mesopore sizes estimated from desorption isotherms are shown to be in a fair agreement with the calculations through the geometrical method based on the X-ray diffraction data. The dynamic desorption technique can serve as an additional tool for the characterization of a porous media.
Keywords: Desorption; Benzene; MCM-41-type silica; Isotherm; Pore size distribution
Hydrothermal modification of carbon adsorbents by J. Skubiszewska-Zięba; V. V. Sydorchuk; V. M. Gun’ko; R. Leboda (pp. 919-927).
The effects of high-pressure autoclave treatments on porous structure and surface properties were studied for a variety of activated carbons (AC, synthetic and produced from plum stones) treated with water vapour, hydrogen peroxide (10–50%) or 10% aqueous ammonia solution at relatively low temperatures (250, 350, 400○C). Surface and structural parameters of modified ACs were determined using nitrogen, water, ammonia and benzene adsorption isotherms. It was found that the effects of AC modification resulting in changes in their porous structure and surface chemistry depend on the kind of initial ACs, modifier type and concentration of modifier and treatment temperature. At the same conditions synthetic ACs are modified to a larger extent than ACs prepared using natural raw materials. Repeated treatment of a given carbon intensifies changes in its porous structure.
Keywords: Activated carbon; Hydrothermal treatment; Hydrogen peroxide; Porous structure; Slitshaped pores; Primary adsorption centres; Ammonia adsorption
Fixed bed phosphate adsorption by immobilized nano-magnetite matrix: experimental and a new modeling approach by Adva Zach-Maor; Raphael Semiat; Hilla Shemer (pp. 929-936).
Nano-sized magnetite impregnated charcoal granular activated carbon (nFe-GAC) was utilized for the removal of phosphate from aqueous solutions using a fixed bed column. The dynamic of the phosphate adsorption was analyzed using a new approach to the Thomas model based on a two-step differential sorption rate process. The initial adsorption was found to be external mass transfer controlled, while intra-particle diffusion was the predominant mechanism in the latter stage. Consequently, two kinetic coefficients were calculated for each breakthrough curve resulting in an excellent model prediction. By implementing this approach a transition point, at which diffusion becomes the predominant adsorption mechanism, can be accurately determined. The effect of varying parameters, such as feed flow rates, feed pH, initial phosphate concentrations and adsorbent bed height were examined and described using the modified Thomas model. Reaction rates increased with augmentation of the flow rates from 1 to 40 mL/min while the adsorption capacity and transition point decreased. Similar transition points were obtained for initial phosphate concentrations between 10 and 100 mg/L. The unique characteristics of the nFe-GAC were evident as it exhibited very high phosphate adsorption capacity, at a wide range of pH values (4–9) with negligible effect of competing ions and short critical bed depth.
Keywords: Composite adsorbent; Breakthrough curve; Thomas model; Effluent
Adsorption of naphthalene and pyrene from isooctane solutions on commercial activated carbons by F. Murilo T. Luna; Caio C. B. Araújo; Carolina B. Veloso; Ivanildo J. Silva Jr.; Diana C. S. Azevedo; Célio L. Cavalcante Jr. (pp. 937-947).
The adsorption of naphthalene and pyrene on two different types of commercial activated carbons was studied by batch and column experiments. Adsorption equilibrium was measured at three different temperatures. Heats of adsorption were estimated from the equilibrium results and compared to other previous reports. From the column experiments, using parameters obtained from the batch experiments and literature correlations, effective surface diffusivities were estimated for naphthalene and pyrene on both adsorbents in different feed concentrations. The corrected diffusivities, using Darken equation, appear to be almost constant for naphthalene (ca. 1.3⋅10−8 cm2/min), and for pyrene (ca. 2.3⋅10−10 cm2/min), in both activated carbons.
Keywords: Adsorption; Naphthalene; Pyrene; Activated carbon; Equilibrium; Dual-resistance diffusion model; Surface diffusivity
Adsorption of natural gas and its whole components on adsorbents by Xingcun Li; Jinfu Chen (pp. 949-954).
Adsorption of each component of natural gas on adsorbent prepared from petroleum coke was studied. At 25 °C and 3.5 MPa, adsorption capacity of the components of natural gas are as follows: C3H8, H2S(0.980) > CO2(0.691) > C2H6(0.160) > CH4(0.136) > N2(0.096) (g/g). For natural gas, adsorption capacity is 145.2 (mL/mL) and delivery capacity is 105.7 (mL/mL). One equation between adsorption capacity and boiling point of adsorbed gas was firstly generalized. The adsorption capacity of different component like O2, N2, CH4, C2H6, CO2, H2S on adsorbents were predicted using the equation. The results fit well with the experimental data. The equation has significance in predicting the adsorption capacity for any component of natural gas. Charge-discharge tests were conducted 10 times, the result indicates that natural gas has significantly worse reversibility in adsorption and desorption in the adsorbent than that of CH4. The contents of the components after 10 charge-discharge show that the adsorption capacity drop of natural gas is due to the irreversible adsorption of heavy or polar components like C3H8, H2S.
Keywords: Natural gas; Methane; Adsorption; Desorption; Adsorbent
A revisit to the Gibbs dividing surfaces and helium adsorption by L. Herrera; Chunyan Fan; D. D. Do; D. Nicholson (pp. 955-965).
This paper addresses the long-standing problem of the so-called Gibbs dividing surface and the use of helium as a “non-adsorbing” gas for the determination of the “helium”-void volume and thence the Gibbs excess. Using helium is subject to some uncertainty because helium does adsorb (to call it a non-adsorbing gas is misleading) and it is able to access pore spaces that other larger adsorbates cannot. On the other hand, even helium atoms can not physically probe all the space described by the helium-void volume. To avoid these difficulties, we suggest an alternative to the formulation of the Gibbs dividing surface and the definition of the excess amount. We illustrate this with the two common tools to study adsorption—the volumetric and gravimetric techniques, and justify our new analysis with a computer simulation of a number of model adsorption systems. Furthermore, we also show that by using the correct accessible volume and inaccessible volume the excess amount obtained from a volumetric experiment is exactly the same as that obtained from a gravimetric experiment.
Keywords: Porous solids; Characterization; Gibbs diving surface; Accessible volume; Helium expansion
Effect of the synthesis temperature of sodium nonatitanate on batch kinetics of strontium-ion adsorption from aqueous solution by Aurélie Merceille; Evelyne Weinzaepfel; Yves Barré; Agnès Grandjean (pp. 967-975).
Sodium titanate materials are promising inorganic ion exchangers for the adsorption of strontium from aqueous solutions. Sodium nonatitanate exhibits a layered structure consisting of titanate layers and exchangeable sodium ions between the layers. The materials used in this study include samples synthesized by a hydrothermal method at temperatures between 60 °C and 200 °C. Their structure, composition, and morphology were investigated with X-Ray diffraction measurements; thermogravimetric, compositional and surface area analyses, and scanning electron microscopy. The structure, composition, and morphology depended on the synthesis temperature. Batch kinetics experiments for the removal of strontium from aqueous solutions were performed, and the data were fitted by a pseudo-second-order reaction model and a diffusive model. The strontium extraction capacity also depended on the synthesis temperature and exhibited a maximum for samples synthesized at 100 °C. The sorption process occurs in one or two diffusion-controlled steps that also depend on the synthesis temperature. These diffusion-limited steps are the boundary-layer diffusion and intra-particle diffusion in the case of pure nonatitanate synthesized at temperatures lower than 170 °C, and only intra-particle diffusion in the case of nonatitanate synthesized at 200 °C.
Keywords: Sorption; Kinetics; Ion exchange; Sodium nonatitanate; Strontium removal
Adsorption of water on Grace Silica Gel 127B at low and high pressure by Douglas B. Riffel; Ferdinand P. Schmidt; Francisco A. Belo; Antonio P. F. Leite; Farid B. Cortés; Farid Chejne; Felix Ziegler (pp. 977-984).
An application of the original Dubinin-Radushkevitch equation modified with based on Polanyi theory, which taking into consideration the self-associating molecules in heterogeneous microporous structures at low and high uptake was presented. The Polanyi-Dubinin models were applied to predicting the behavior of water adsorption on Grace Silica Gel 127B at pressures not yet reported in the literature. The heat of adsorption, differential entropy, henry ratio was obtained from adsorption isotherm. The resulting extensions are experimentally and analytically presented. The coefficient of multiple correlation (R 2) between the thermogravimetric measurements and the MDR equation was 0.9924, which is 8.13% better than the best Dubinin-Astakhov fit.
Keywords: Adsorption isotherm; Henry’s law; DR and DA equations