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Dyes released from different industries are one of the major pollutants in the water system. So removal of dye before disposal to the main water stream is necessary. In this study, adsorption technique was adopted for removal of the cationic dye from aqueous system using reduced graphene oxide (rGO) as adsorbent material. The kinetic study of adsorption shows that the present system follows pseudo second order kinetic (linear) model. Adsorption isotherm investigation indicates that the adsorption of methyl green onto rGO nanosheets follows the isotherm models in the order Toth > Sips > Dubinin--Radushkevich (D--R) > Scatchard > Langmuir > Temkin > Freundlich model in the pH range 4--6. However, the order changes to D--R > Scatchard > Toth > Sips > Langmuir > Temkin > Freundlich model at pH 7 and 9. The adsorption capacity of methyl green onto the rGO nanosheets increases with increasing pH of the medium due to the change in the surface properties. Presence of other ions like Na+, Ca2+, Mg2+ and SO42- also influenced on adsorption due to the change in the surface properties. The magnitude of ΔG° values are found to be in the characteristic range of physisorption nature of adsorption. Further, the mechanism of the adsorption process was investigated by FTIR spectroscopy and it indicates that the adsorption is due to electrostatic interaction between methyl green dye molecule and rGO nanosheets. X-ray radial distribution function (RDF) analysis revealed the formation regular molecular packets of graphene. X-ray diffraction (XRD) analysis of the rGO nanosheets before and after adsorption of the dye molecule indicates that some textural changes occur in the rGO nanosheets upon adsorption of the methyl green dye molecule. [ABSTRACT FROM AUTHOR]

Abstract: The Azizian–Volkov (AV)-isotherm is a general isotherm for gas adsorption with two different states of an adsorbate. It has been derived previously by a classical thermodynamic approach. The new AV-isotherm gives the possibility to analyze multi and single centered adsorption and the interfacial structure in a general case. The AV-isotherm takes into account the interaction between adsorbed molecules and adsorbent and provides information about the orientation of adsorbates. Traditional isotherms (Henry, Langmuir, and Frumkin) for homogeneous adsorption can be derived from the AV-isotherm. In the present work the AV-isotherm is derived using statistical thermodynamics. This derivation provides a stronger basis for this isotherm. A new concept of the important parameter (η) of AV-isotherm is obtained. Finally, the AV-isotherm was simplified and its application was justified by analysis of the equilibrium data of a numerically generated and experimental systems. [Copyright &y& Elsevier]

Abstract: In the present study, natural sepiolite was used as an adsorbent for the investigation of the adsorption kinetics, isotherms and thermodynamic parameters of an acid dye (Acid Blue 193, AB193) from aqueous solution at various pHs, temperatures and concentrations. Two simplified kinetic models, first-order and pseudo-second-order, were used to predict the adsorption rate constants. It was found that the kinetics of the adsorption of AB193 onto natural sepiolite at different operating conditions was the best described by the pseudo-second-order model. The rate parameters of the intraparticle diffusion model for adsorption were also evaluated and compared to identify the adsorption mechanisms. Adsorption isotherms and equilibrium adsorption capacities were determined by the fittings of the experimental data to three well-known isotherm models including Langmuir, Freundlich and Dubinin-Radushkevich (D-R). The results showed that the D-R model appears to fit the adsorption better than other adsorption models for the adsorption of AB193 onto natural sepiolite. The equilibrium constants were used to calculate thermodynamic parameters, such as the change of free energy, enthalpy and entropy. [Copyright &y& Elsevier]

Hexane adsorption on mesoporous silicas has been studied in the temperature range 260 up to 350K. Classical models for the textural characterization of MCM-41 by hexane were employed and confirmed analyses using nitrogen as a probe whereas energetic parameters such as CBET were found to be dramatically low. This has been discussed in terms of balance of wall/adsorptive interaction and dispersive interaction leading to unusual adsorption processes, capillary condensation commencing before the monolayer completion. Isosteric heats of adsorption could be derived. Specific confinement effects could be evidenced and the heat of adsorption was found to be strongly dependent on the pore size. [Copyright &y& Elsevier]

Adsorption isotherms of dimethyldodecylamine-N-oxide (DDAO) onto laponite were determined by potentiometry, using a surfactant-specific electrode. The isotherms were obtained at pH 7; 8 and 9, and at various NaCl concentrations. The stability of laponite in these conditions was evidenced. All isotherms are mingled in these conditions and the level of adsorption of the non-ionic surfactant is found to be much higher than that of its cationic homologue, dodecyltrimethylammonium (DTA+). [Copyright &y& Elsevier]