Monolayer–multilayer adsorption phenomenological model: Kinetics, equilibrium and thermodynamics

Abstract

A phenomenological model for multilayer liquid phase adsorption in closed and batch system was developed to assess the diffusional resistances and the adsorption rate-limiting steps. Equilibrium, kinetic and thermodynamic results indicated a multilayer adsorption behavior, wherein two adsorption steps were identified: (i) monolayer adsorption, with chemisorption characteristic binding energy due to synergistically electrostatic and H-bond intermolecular interactions; (ii) multilayer adsorption, favored by high dye concentrations and temperatures, however presenting a physical biding energy, related to less intense interactions such as H-bond and van der Waals forces. Additionally, dye aggregation in the liquid phase was observed and investigated. The proposed adsorption model allowed to investigate physically the kinetics, equilibrium and thermodynamics through a unique approach and to elucidate the mechanisms in distinct operational conditions. Hence, this mechanistic and predictive model can be used for the optimization and scale-up of the multilayer adsorption processes.