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

[Display omitted] •Adsorption of Indigotine blue dye (IBD) on the CoFe2O4/CMC was studied.•Adsorption mechanism was analyzed and interpreted with the multilayer statistical physics model.•Adsorption occurs by aggregation of IBD molecules on the CoFe2O4/CMC surface.•Adsorption of IBD onto CoFe2O4/CMC is endothermic, spontaneous and multilayer. 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 (Qsat) 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 (ΔE1a and ΔE2a), 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 (Sad) evolution allows confirming the occurrence of different adsorbed layers (NC = 1+N2), while the Gibbs adsorption free energy (Gad) confirmed that the process is spontaneous.