In this article, adsorption modelling was presented to describe the sorption of textile dye, Direct Red 75 (DR75), from coloured wastewater onto the natural and modified adsorbent, Posidonia oceanica. The formulation of the double-layer model with two energy levels was based on statistical physics and theoretical considerations. Thanks to the grand canonical ensemble in statistical physics some physico-chemical parameters related to the adsorption process were introduced in the analytical model expression. Fitting results show that the dye molecules are adsorbed in parallel position to the adsorbent surface. The magnitudes of the calculated adsorption energies show that the DR75 dye is physisorbed onto Posidonia. Both Van der Waals and hydrogen interactions are implicated in the adsorption process. Despite its simplicity, the model fits a wide range of experimental data, thereby supporting the underlying data that the grafted groups facilitate the parallel anchorage of the anionic dye molecule. Thermodynamic parameters, such as adsorption energy, entropy, Gibbs free adsorption energy and internal energy were calculated according to the double-layer model. Results suggested that the DR75 adsorption onto Posidonia was a spontaneous and exothermic process.
Experimental adsorption isotherm of two basic dyes: Basic Blue 3 and Basic Red 24 from aqueous solution onto modified nylon 6,6 were analyzed by using a double layer adsorption model with two energy levels. Such model is based on statistical physics treatment. The parameters involved in the analytical expression of this model such as the fraction or the number of adsorbed dye molecule(s) per site, n, the receptor sites density, N M , and the energetic parameters, c 1 and c 2 , were determined by fitting the experimental adsorption isotherms at four temperatures between 293 and 353 K with different degrees of grafting between 20 and 80%. The evolution of these parameters versus temperature and the grafting percent allows us to interpret and better understand this adsorption process at molecular level. Two different behaviors of the two dye molecules were highlighted according to their localized and non localized charges. The configurational entropy at various temperatures has also been studied. This parameter allowed to deduce some results related to the evolution of the disorder at the adsorption surface.
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