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Adsorption: Journal of the International Adsorption Society (v.8, #1)
Professor Sergey Petrovich Zhdanov: 90th Birthday on April 18, 2002
by Sergey S. Khvoshchev; Nina N. Feoktistova; Martin Bülow (pp. 5-7).
Complex Sorption Kinetics of Carbon Dioxide in NaX-Zeolite Crystals by Martin Bülow (pp. 9-14).
Ad-/desorption-kinetic measurements were performed for carbon dioxide on a mono-disperse crystal sample of NaX-type zeolite (mean crystal diameter: 2R ≅ (100 ± 6) μm) by a constant-volume–variable-pressure method in a “differential” concentration mode over ranges of temperature, (193–293) K, and pressure, (20–26,700) Pa. They revealed a complex sorption process in sorbate uptake and release during experiments of ad- and desorption, respectively, which is comprised of a slow, hitherto unknown rate mechanism superimposed on very fast intracrystalline diffusion of carbon dioxide. IR-spectroscopic data that evidence formation of chemisorption species in CO2-NaX sorption systems may allow for a tentative interpretation of this finding. Circumstantial evidence for the correctness of this conclusion follows from sorption–kinetic experiments for carbon dioxide on BaKX-type zeolite crystals (mean crystal diameter: 2R ≅ (120 ± 6) μm).
Keywords: NaX and BaKX zeolites; carbon dioxide; sorption kinetics; complex rate mechanism; IR spectroscopy; chemisorption
Diffusion and Immobilization Mechanisms in Zeolites Studied by ZLC Chromatography by Mladen Eić; André Micke; Milan Kočirík; Mohamed Jama; Arlette Zikánová (pp. 15-22).
Zero Length Column chromatography was used to study mass transfer in zeolites involving coupled diffusion and immobilization mechanisms. A modeling based on Volterra integral equation technique was utilized to simulate sorption and desorption kinetic curves and compare results of the simulations with experimentally obtained curves. This approach was applied to analyze sorption kinetics in the model system: toluene/silicalite-1 (75°C–178°C). The system generally shows a non-Fickian behavior and can be described by diffusion coupled with immobilization.
Keywords: Fickian diffusion; immobilization; ZLC chromatography; zeolites; Volterra integral equation
The Langmuirian Adsorption Kinetics Revised: A Farewell to the XXth Century Theories? by Wladyslaw Rudzinski; Tomasz Panczyk (pp. 23-34).
A brief historical review of the development of the theoretical approaches to the kinetics of gas adsorption/desorption on/from the solid surfaces is presented. The attention is focused on new approaches, challenging the classical theories based on the ideas of Absolute Rate Theory (ART). These new approaches relate the adsorption/desorption kinetics to the chemical potentials of the molecules in the gas and adsorbed states. Among them the so-called Statistical Rate Theory (SRT) has the most rigorous theoretical foundations. That new approach predicts that depending on experimental conditions one can have a variety of kinetic equations corresponding to the Langmuir equilibrium adsorption isotherm.
Keywords: gas adsorption; adsorption kinetics; langmuirian kinetics; statistical rate theory
An Analytical Study of Molecular Transport in a Zeolite Crystallite Bed by Papson N'Gokoli-Kekele; Marie-Anne Springuel-Huet; Jacques Fraissard (pp. 35-44).
Analytical solutions of the diffusion equations to obtain the diffusant concentrations in the macro- and micropores which constitute the pore system of a zeolite bed are presented. The parameter which determines the influence of each pore type on the evolution of the adsorbate/adsorbant system towards the equilibrium state is described. Examples are given to illustrate a qualitative and quantitative study based on the curves obtained from these equations.
Keywords: mathematical models; transport diffusion; intracrystallite diffusion; intercrystallite diffusion; zeolites
On the Correct Use of the Dubinin-Astakhov Equation to Study the Mixed-Gas Adsorption Equilibria by Krzysztof Nieszporek (pp. 45-57).
This paper presents the possibilities of Integral Equation (IE) approach to study the mixed-gas adsorption equilibria. As a result, the generalizations of Dubinin-Astakhov equation for the case of mixed-gas adsorption are presented. These new equations are examined using a few adsorption systems recently published in literature.
Keywords: adsorption; isotherms; integral equation approach; heterogeneity; Dubinin-Astakhov equation
CVD-Titania on Mesoporous Silica Gels by V.M. Gun'Ko; A.G. Dyachenko; M.V. Borysenko; J. Skubiszewska-Zięba; R. Leboda (pp. 59-70).
Different amounts of CVD-titania ( $$C_{{ ext{TiO}}_{ ext{2}} } $$ from 2.3 to 19.2 wt%) with amorphous and crystalline (anatase) phases were synthesized on silica gels (Kieselgel 40, 60, and 100) and characterized by means of XRD, IR, DTG, and adsorption methods. The amounts of titania depend strongly on the pore size distribution of the support, as the narrower the pores, the lower the deposit concentration due to diminution of the accessibility of narrower pores and deceleration of titania grafting in them. A portion of CVD-titania filling matrix pores is rather amorphous than that forming on the outer (external) surfaces of silica gel grains, as anatase crystallites have the average size of 70 nm for KG 40/TiO2 ( $$C_{{ ext{TiO}}_{ ext{2}} } $$ = 6.5 wt% including 26% of anatase and 74% of amorphous titania), 21 nm (KG 60/TiO2 at $$C_{{ ext{TiO}}_{ ext{2}} } $$ = 11 wt%, 16% anatase) and 16 nm (KG 100/TiO2 at $$C_{{ ext{TiO}}_{ ext{2}} } $$ = 19.2 wt%, 29% anatase), which are larger than the average pore size of the silica gels. The crystallite size decreases with increasing average pore diameter.
Keywords: CVD-titania/silica gel; adsorption; XRD; IR; pore size distribution; amorphous titania; anatase
Effect of pH on Competitive Adsorption of Cu(II), Ni(II), and Zn(II) from Water onto Chitosan Beads by Ruey-Shin Juang; Huey-Jen Shao (pp. 71-78).
The amounts of adsorption of Cu2+, Ni2+, and Zn2+ from single, binary, and tertiary nitrate solutions onto glutaraldehyde cross-linked chitosan beads were measured. The beads had an average particle size and pore volume of 2 mm and 0.06 cm3/g, respectively, and had a BET surface area of 60 m2/g. All experiments were performed at 298 K as a function of initial pH (2.0–5.0), total metal concentration (0.77–17.0 mol/m3), and molar concentration ratio (0.25–4) in the aqueous phase. It was shown that the amount of metal adsorption generally increased with increasing solution pH. Competitive adsorption was significant in binary and tertiary systems when Cu2+ was present. The selectivity factor reached maximum in an equilibrium pH range of 5.1–5.3 and 4.5–4.9 for the Cu-Ni and Cu-Zn binary systems, respectively. This adsorbent provided a possibility for selective separation of Cu2+ from such multi-component solutions.
Keywords: competitive adsorption; selectivity; chitosan beads; Cu(II); Ni(II); Zn(II)
Adsorption of Nitrogen on Silica Gel Over a Large Range of Temperatures by Li Zhou; Shupei Bai; Yaping Zhou; Bin Yang (pp. 79-87).
To study the mechanism of physical adsorption of supercritical gases, the adsorption equilibria of N2 on silica gel for 103–298 K using 20 K increments and pressures up to 10 MPa were measured. A transition of the adsorption mechanism was proven on crossing the critical temperature, but the transition way observed is different from that observed with activated carbon. This causes a difference in the locations of the linear section of the n-ρ g isotherm at the near-critical temperature. Although the isotherm type is different on silica gel and on activated carbon in the sub-critical region, all isotherms in the supercritical region can be well modeled by a single model. It leads to the argument that the adsorption mechanism of supercritical gases is identical no matter what kind of adsorbent is used.
Keywords: supercritical adsorption; adsorption mechanism; isotherm modeling; adsorption equilibria; gas/solid adsorption