Adsorption of Eriochrome Black Ton Apatite Silicates: Steric and Energetic Interpretations via Theoretical Modelling

This work investigates the adsorption of Eriochrome Black T (EBT) on four apatite silicate adsorbents: SrMil CaMil, SrCal, and CaCal. The adsorption mechanisms are analyzed using theoretical parameters obtained from isothermal fitting based on a statistical model assuming a multilayer adsorption. The steric parameter (n) indicates the orientation and aggregation of EBT molecules on the adsorbents. The interaction of EBT molecules with adsorbent surface occurs via a multimolecular mechanism, suggesting that the molecules anchor via a non-parallel manner. A multilayer adsorption mechanism is identified for EBT, characterized by the formation of 2 to 3 layers of adsorbed dye molecules, variable on adsorbent type. The experimental adsorption capacities are in the range of 290 to 420 mg/g, with the highest adsorption capacity shown by SrMil. Adsorption energy calculations indicate that the adsorption mechanism is physical and mainly based on van der Waals forces and hydrogen bonding, with an adsorption energy (ΔE) of less than 40 kJ/mol. This reversible adsorption process highlights the potential of apatite silicate adsorbents, particularly SrMil, for effective EBT removal in environmental applications.