Theoretical study of indigotine blue dye adsorption on CoFe2O4/chitosan magnetic composite via analytical model

An advanced model is applied to analytically explore the adsorption mechanism of Indigotine blue dye (IBD) onto a CoFe2O4/chitosan magnetic composite (CoFe2O4/CMC). Contrary to the classical adsorption models, the model is defined starting from the statistical physics theory and it is a generalized model containing numerous parameters having a defined physical meaning. Experimental evidences based on equilibrium data at four temperatures (298- 328 K) are analyzed for a useful analytical interpretation. The analysis of the data via the advanced model indicates that the number of IBD linked per CoFe2O4/CMC active site (n) varied from 3.65 to 3.83, also demonstrating that the orientation (adsorption geometry) of the IBD dye on CoFe2O4/CMC surface is angled and affected by the temperature. Based on these values, it can be deduced that the adsorption mechanism at all the investigated temperatures is a multi-molecular process. It is also demonstrated that an increment in the temperature leads to a positive evolution of the adsorption capacity (Q(sat)) from 224.81 mg/g at T = 298 K to 289.87 mg/g at T = 328 K, testifying that the adsorption process is overall endothermic. This theoretical evidence is corroborated by the estimation of the adsorption energies (Delta E-1(alpha) and Delta E-2(alpha)), which indicated that physical interactions between IBD molecules and the CoFe2O4/CMC receptor sites and among IBD-IBD molecules take place. Finally, three thermodynamic parameters are determined for a further insight into the interpretation of the adsorption mechanism. The adsorption configurational entropy (S-ad) evolution allows confirming the occurrence of different adsorbed layers (N-C = 1 + N-2), while the Gibbs adsorption free energy (G(ad)) confirmed that the process is spontaneous.