Thermodynamic analysis of single and binary adsorption of Food Yellow 4 and Food Blue 2 on CC-chitosan: Application of statistical physics and IAST models

The paper is a complementary study of Food Yellow 4 and Food Blue 2 adsorption in single-compound and binary systems on modified chitosan (CC-chitosan), using statistical physics and Ideal Adsorbed Solution Theory (IAST) models. In order to provide new macroscopic interpretations of adsorption mechanisms, three thermodynamic functions were determined, such as configurationally entropy, free enthalpy and internal energy. The configurationally entropy trend showed that the dye molecules have plenty possibilities/positions to be docked on empty receptor sites at low concentrations, but limited possibilities at high equilibrium concentrations. The study of the free enthalpy and internal energy confirmed that the adsorption process in single-compound and binary systems is spontaneous. The second theoretical approach is based on the IAST model to predict the multicomponent adsorption isotherms using single-compound isotherm data. The IAST model provides a very good prediction of binary adsorption data, for both the adsorbates. Best results were obtained for Food Yellow 4 adsorption data; while for Food Blue 2 an increasing deviation between experimental and modeling binary adsorption data can be observed along with increasing concentration. Finally, the IAST model correctly described the decrease in adsorption capacity of both the dyes from single-compound to binary system, due to competition effects.