Isotherm, thermodynamic, and kinetic studies of dye adsorption on graphene oxides with varying oxidation degrees

Abstract

The adsorption mechanism was analyzed based on equilibrium isotherms, kinetic studies, and thermodynamic parameters. Adsorption followed the Freundlich model, indicating heterogeneous surface interactions, with GOHOS exhibiting the highest adsorption capacity for MO (215.3 mg/g) and ACNS for MB (78.0 mg/g). The kinetic studies revealed that adsorption proceeded via a pseudo-second-order mechanism, with intraparticle diffusion playing a significant role in MO adsorption, whereas boundary layer effects were more prominent for MB.

The adsorption mechanism involves electrostatic interactions, π-π stacking, hydrogen bonding, and surface complexation, which vary depending on the oxidation degree of GO and the functional groups of the adsorbents. GOHOS, with a higher oxidation state, favored electrostatic interactions due to its negatively charged surface, whereas GOLOS exhibited stronger π-π stacking interactions with MB due to retained sp² hybridized domains. Thermodynamic studies confirmed the spontaneity and endothermic nature of the process. These findings provide critical insights into the role of oxidation levels in GO-based materials and their influence on adsorption efficiency. The study highlights the potential of these materials for dye removal from aqueous solutions, emphasizing their environmental and industrial relevance.