Coupled effects of montmorillonite incorporation and freeze – thaw-induced structural modification on tetracycline adsorption and reusability of alginate/PVA composite beads

Tetracycline (TC) is a widely detected antibiotic contaminant in aquatic environments and poses potential ecological risks. This study investigated the coupled effects of montmorillonite (MMT) incorporation and drying method, oven-drying versus freeze – thaw treatment, on the physicochemical properties and TC adsorption performance of alginate/poly(vinyl alcohol) (Alg/PVA) composite beads. Four bead formulations were prepared: CA, CAF, APM, and APMF and characterized by SEM, XRD, FTIR, pHpzc analysis. Freeze – thaw treatment significantly increased swelling capacity and water uptake, suggesting improved accessibility of the internal bead structure. Among all adsorbents, APMF exhibited the highest swelling percentage (353.55 ± 11.87%), water uptake (83.85 ± 4.18%), and TC removal efficiency. Adsorption performance was influenced by solution pH, adsorbent surface charge, and TC speciation. The adsorption kinetics were best described by the pseudo-second-order model, while the Freundlich isotherm provided the best fit to equilibrium data, suggesting multilayer adsorption on heterogeneous surfaces. Thermodynamic analysis confirmed a spontaneous, endothermic process. Regeneration studies demonstrated that APMF maintained the highest adsorption retention and lowest weight loss over four consecutive cycles. These findings suggest that the combined effects of MMT incorporation and freeze – thaw treatment contribute to enhanced adsorption performance and improved reusability of the composite beads.