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The adsorption kinetics, isotherms and thermodynamics of Cr(III) on graphene oxide (GO) were studied. The adsorption kinetic data were well described with pseudo-second-order model and the equilibrium data were well fitted by Langmuir model. The calculated thermodynamic parameters indicated that the adsorption of Cr(III) on GO was spontaneous and endothermic. The maximum adsorption capacity of Cr(III) on GO at pH 5.0 and T = 296 K was about 92.65 mg g-1, which was higher than other reported adsorbents. It was found that the adsorption of Cr(III) on GO was strongly dependent on solution pH, but weakly dependent on ionic strength. Fourier transform infrared (FTIR) spectra suggested that Cr(III) was adsorbed on GO mainly through the formation of inner-sphere complexes with the O-containing functional groups on GO surface. Results in this study suggested that GO was a suitable material for the preconcentration and removal of Cr(III) from water. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] Activated carbon treated by KOH exhibited better adsorption properties than untreated one. [•] Thermodynamic parameters demonstrated the adsorption process was spontaneous and favorable. [•] It was probable to deduce adsorption mechanisms and design adsorption systems by the study of adsorption kinetics, isotherms and thermodynamics. [ABSTRACT FROM AUTHOR]

Abstract: Removal of silica from aqueous solution by industrial solid ‘waste’ Fe(III)/Cr(III) hydroxide was investigated. Initial pH has no effect on the adsorption of silica in the pH range 4.0–10.0 studied. First, second order and Elovich kinetic models were used to study the adsorption process. The adsorption system followed second order and Elovich kinetics. Langmuir and Freundlich isotherms have been employed to analyze the adsorption equilibrium data. Adsorption process followed Langmuir isotherm. The Langmuir adsorption capacity (Q 0) was found to be 12.25mgg−1. Freundlich constants k f and n were found to be 2.44 and 2.74, respectively. Silica adsorption in the presence of vanadate, phosphate and selenite was decreased whereas molybdate, thiocyanate, chloride, nitrate and sulfate have shown negligible effect. [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]

Abstract: Kinetics of adsorption of benzoate and salicylate onto the natural hematite surfaces have been carried out at pH 5 and at a fixed ionic strength (I=1×10-4moldm-3). The state of equilibrium was attained at 144 and 70h for benzoate and salicylate, respectively. Adsorption behaviour of benzoate and salicylate onto the natural hematite was studied over a wide range of pH at a fixed I=5×10-4moldm-3 NaCl and at different temperatures. The adsorption isotherms for both the adsorbates were Langmuir type up to pH 8. The Γmax for benzoate is ≉2.7–13.7 times more than that of salicylate depending on the pH of the suspension. Unlike benzoate, salicylate is not only adsorbed at one surface site but also covers around three or more surface sites of hematite surface. The rate constant for adsorption, K1 for salicylate onto the natural hematite is greater than that of benzoate and increases with the increase in temperature and ≉4 times greater at a fixed temperature. The activation energy for the adsorption of salicylate onto the natural hematite is 2.9 times more than that of benzoate on the same adsorbent resulting in lower Γmax. [Copyright &y& Elsevier]