Statistical thermodynamics of adsorption of dye DR75 onto natural materials and its modifications: double-layer model with two adsorption energies.

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.

Removal of lead and yellow 44 acid dye in single and binary component systems by raw Posidonia oceanica and the cellulose extracted from the raw biomass.

This study examined the sorption behaviour of Pb(II) and C.I. Acid Yellow 44 on Posidonia oceanica, an abundant Mediterranean biomass. A comparison with sorption onto cellulose extracted from the raw material was carried out to identify those site characteristics that play a predominant role in the adsorption of both dye and metal ions. Kinetic and equilibrium studies were performed for single and binary component systems, and the experimental data were analyzed by a non-linear method. The pseudo second-order kinetic model was successfully applied for both dye and metal retention onto sorbents in single and binary systems. Both sorbents were found to be more effective for lead than for Yellow 44 removal. However, lead sorption was more effective on raw P. oceanica suggesting that the metal ions are sorbed into the whole available biomass sites (cellulose and lignin). An opposite behaviour was reported for Yellow 44 sorption, which was found to be more effective on the extracted cellulose than raw P. oceanica. This finding proves that the only available sites for dye sorption are the cellulosic ones. The binary component experimental studies indicated competition between dye and lead for the available sites of raw P. oceanica. However, this competition was found to be less prevalent for sorption by cellulose extracted from raw P. oceanica suggesting that, in binary component systems, the cellulosic sites are equally available for both pollutants, the only limiting parameter being the size of the molecular sorbate. Langmuir and Freundlich isotherms were used to fit the experimental data using the non-linear method for parameter determination.