Mechanistic Exploration of Estrone Adsorption onto Chitosan Hydrogel Beads: Classical and Fractal-like Adsorption Kinetics and Isotherms Analysis

Abstract This study investigates the mechanistic behaviour of estrone (E1) adsorption onto chitosan hydrogel beads through the assessment of adsorption kinetics, equilibrium isotherms, and adsorbent regeneration. Batch adsorption experiments were conducted to investigate the influence of pH, contact time, initial (E1) concentration, adsorbent load, and agitation speed on E1 uptake. Classical kinetic models, viz., pseudo-first order (PFO), pseudo-second order, intraparticle diffusion models and fractal-like kinetic models were applied to elucidate rate-controlling mechanisms. The PFO model best statistical fit (i.e., R2 = 0.9939, adj.R2 = 0.9918, χ^2 = 5.25E-4) with calculated qe1 = 4.8891 mg.g-1, and qexp = 4.95 mg.g-1 indicating film-diffusion dominated physisorption. Although fractal-like kinetics captured heterogeneous and anomalous diffusion associated with the hydrogel’s irregular structure, they did not outperform the classical PFO model. Equilibrium data were fitted to Langmuir, Freundlich, Langmuir-Freundlich, and Hill isotherms. The Langmuir model gave the best statistical performance (i.e., R2 = 0.9170, adj.R2 = 0.8754, QL = 6.7766 mg.g-1, and KL = 1.0277 L.mg-1) suggesting predominant monolayer adsorption, whereas the other models demonstrated underlying surface heterogeneity and mild cooperative effects. Regeneration studies demonstrated that the hydrogels retained a substantial portion of their adsorption capacity over eight cycles, confirming structural stability and practical reusability.