A simple isotherm equation for describing gas adsorption on heterogeneous microporous solids

A simple isotherm equation for describing gas adsorption on solids showing heterogeneity of microporous structure is proposed. It is shown that this equation gives a good representation of the experimental data of argon, nitrogen and benzene adsorption on different types of activated carbons. Its parameters may be used to characterize heterogeneity of microporous structure of the solids.

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Eine einfache Isotherme zur Beschreibung der Gas-Adsorption an heterogenen mikroporösen Feststoffen

Zusammenfassung Es wird eine einfache Isotherme zur Beschreibung der Gasadsorption an Feststoffen mit heterogener mikroporöser Struktur vorgeschlagen. Es wird gezeigt, daß diese Gleichung die experimentellen Daten für die Adsorption von Argon, Stickstoff und Benzol an verschiedenen Typen von Aktivkohle gut beschreibt. Die Parameter der Isotherme können zur Charakterisierung der Heterogenität der mikroporösen Struktur von Festkörpern herangezogen werden.

     

A simple adsorption model for ionic surfactants

In the past, few theoretical attempts have been made to describe quantitatively the adsorption of ionic surfactants at liquid interfaces. Well-known adsorption isotherms due to Frumkin or Hill–de Boer cannot respond to the specific electrostatic and geometric properties of the surfactant molecules. Our approach is based on a combination of the Gouy–Chapman theory with a modified Frumkin isotherm. The modification implies that the system is free to choose an optimal head group area and an optimal arrangement of the surfactant molecules in the interface as a function of bulk concentration. Interaction energies between neighbouring adsorbed surfactant molecules and between surfactant and water molecules are taken into consideration. The minimum of the Gibbs free energy of the system is equivalent to a minimal interfacial tension. Thus, the thermodynamically stable isotherm can be obtained as the lower envelope of the family of σ versus ln c isotherms resulting from different choices of the model parameters, including the area per molecule. According to the Gibbs equation, the Γ versus ln c adsorption isotherm is obtained as the derivative of this envelope. By variation of the model parameters, the envelope of the calculated adsorption isotherms can be fitted to experimental data of the interfacial tension versus bulk concentration. A computer program is used to calculate the σ versus c and the Γ versus ln c curves as well as to fit the parameters. Received: 28 October 1999/Accepted: 8 February 2000     

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.     

Removal of cationic methylene blue and malachite green dyes from aqueous solution by waste materials of Daucus carota

In present study adsorption capacity of waste materials of Daucus carota plant (carrot stem powder: CSP and carrot leaves powder: CLP) was explored for the removal of methylene blue (MB) malachite green (MG) dye from water. The morphology and functional groups present were investigated by scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy. The operating variables studied were pH, adsorbent dose, ionic strength, initial dye concentration, contact time and temperature. Equilibrium data were analysed using Langmuir and Freundlich isotherm models and monolayer adsorption capacity of adsorbents were calculated. Kinetic data were studied using pseudo-first and pseudo-second order kinetic models and the mechanism of adsorption was described by intraparticle diffusion model.Various thermodynamic parameters such as enthalpy of adsorption ΔH°, free energy change ΔG° and entropy ΔS° were estimated. Negative value of ΔH° and negative values of ΔG° showed that the adsorption process was exothermic and spontaneous. Negative value of entropy ΔS° showed the decreased randomness at the solid–liquid interface during the adsorption of MB and MG onto CSP and CLP.     

Comparative of the removal of Pb2+, Cd2+ and Ni2+ by nano crystallite hydroxyapatite from aqueous solutions: Adsorption isotherm study

Release of heavy metal onto the water and soil as a result of agricultural and industrial activities may pose a serious threat to the environment. In this study, the adsorption behavior of nano hydroxyapatite with respect to Pb²⁺, Cd²⁺ and Ni²⁺ has been studied in order to consider its application to purity metal finishing wastewater. The batch method has been employed, using metal concentrations in solution ranging from 100 to 400 mg/L. The uptake capacity and distribution coefficients (K_d) were determined for the adsorption system as a function of sorbate concentration. The Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich (DKR) isotherms applied for sorption studies showed that the amount of metal sorbed on nano hydroxyapatite. It was found that the adsorption phenomena depend on charge density and hydrated ion diameter. According to the equilibrium studies, the selectivity sequence can be given as Pb²⁺ > Cd²⁺ > Ni²⁺. These results show that nano hydroxyapatite holds great potential to remove cationic heavy metal species from industrial wastewater.     

Fabrication,characterization and in vivo evaluation of dexpanthenol sustained-release nanofibers for wound healing

In this study, wound dressings consisting of dexpanthenol (Dex)-loaded electrospun nanofibers were fabricated using polyvinyl alcohol (PVA)/sodium alginate (SA), and chitosan as the core and the shell, respectively. Considering the remarkable properties of chitosan, it was used as a shell against drug release and to improve the thermal stability, and tensile strength of the scaffold. By comparing the thermogravimetric, and tensile strength results of nanofibers with and without shell, it was revealed that the presence of chitosan in the shell side could improve the thermal stability and increased the tensile strength by about three times. The isotherm models of dexpanthenol release from the PVA/SA/Dex-CS scaffold was best described by the Langmuir model. Besides, Fourier transform infrared, scanning electron microscopy, and X-ray diffraction techniques were performed to characterize nanofibers. Furthermore, an in vivo investigation of a wound dressing with dexpanthenol showed better healing compared to the wound dressings without dexpanthenol.     

Studying the uptake of aniline vapor by active alumina through in-line monitoring a differential adsorption bed with near-infrared diffuse reflectance spectroscopy

Through non-invasive monitoring the uptake of aniline vapor by active alumina in a differential adsorption bed (DAB) with near-infrared diffuse reflectance spectroscopy (NIR-DRS), we have studied several features of the adsorption, including isotherm, kinetics and the chemical state of aniline molecules in aniline-alumina system. What is perhaps more important, since the information above is obtained synchronously, the proposed methodology could provide information about the type of adsorption (chemical or physical adsorption), the change of chemical state of aniline in the system during the adsorption process, whether the chemical adsorption and physical adsorption took place simultaneously, the rate of the chemical and physical adsorption, and so on.     

Evaluation of selective composite cryogel for bromate removal from drinking water

Bromate, which is a potential carcinogen, should be removed from drinking water to levels of less than 10 μg/L. A chitosan‐based molecularly imprinted polymer (MIP) and a sol–gel ion‐exchange double hydrous oxide (Fe₂O₃·Al₂O₃·xH₂O) adsorbent (inorganic adsorbent) were prepared for this purpose. The sorption behavior of each adsorbent including sorption kinetics, isotherms, effect of pH and selective sorption were investigated in detail. Sorption experimental results showed that the MIP adsorbents had better selectivity for bromate, even in the presence of high concentrations of nitrate, as compared to the inorganic adsorbent. It was found that pH does not affect the adsorption of bromate when using the inorganic adsorbent. Additionally, both adsorbents were immobilized in a polymeric cryogel inside plastic carriers to make them more practical for using in larger scale. Regeneration of the cryogels either containing MIP or inorganic adsorbents were carried out by 0.1 M NaOH and 0.1 M NaCl, respectively. It was found that the regenerated MIP and inorganic adsorbents could be used at least three and five times, respectively, without any loss in their sorption capacity.     

吸附式空调系统的原理及添加剂强化吸附工质性能的研究

氯化锂晶体具有很高的吸水性能，在吸附式空调制冷系统中常常被用作吸附剂。本文介绍了吸附式空调制冷系统的原理，研究了吸附剂（氯化锂）、吸附质（水汽）的性质；采用高真空吸附重量法测定氯化锂的等温吸附曲线及氯化锂吸附水汽的速率，并通过添加一种既有吸附增稠作用，又能强化氯化锂吸附性能的添加剂,改善氯化锂的等温吸附特性,降低氯化锂的再生温度。采用氯化锂混合吸附剂-水的开式旋转吸附式空调系统，具有显著的节能效果和双重杀菌功能，提供人们所喜爱的舒适空调域（室内温湿度26．7℃／50%RH），同时改善室内空气品质。