Chitosan-based hydrogel beads with molecularly imprinted receptors on halloysite nanotubes for tetracycline separation in water and soil

Tetracycline (TC), a commonly utilized broad-spectrum antibiotic, is frequently detected in water and soil, posing a significant risk to the natural environment and human health. In the present study, the composite hydrogel beads based on chitosan (CS) and halloysite-supported molecularly imprinted polymers, synthesized by two procedures with significantly different solvent volumes (Hal@MIP^a(b)), were obtained and used to adsorb the antibiotic. The presence of Hal improved the thermal stability of the hydrogel beads. The system with a thinner polymer layer (CS_Hal@MIP^b), containing polymers produced under conditions of significantly higher reagent dilution, was more resistant to higher temperatures than CS_Hal@MIP^a. The adsorptive properties were compared with pure CS beads, those containing incorporated Hal, and free polymers obtained by different protocols (MIP^a(b)). In the optimized pH 5.0, the maximum adsorption capacities were 175.24 and 178.05 mg g⁻¹ for CS_Hal@MIP^a and CS_Hal@MIP^b, respectively. The values were slightly lower compared to the systems with free polymers, but the materials achieved equilibrium more rapidly (12 h). The adsorption process was spontaneous and exothermic. Freundlich isotherm and pseudo-second-order kinetic models most accurately described the experimental data. The hydrogel beads retained high selectivity in the presence of other antibiotics, and their high efficiency in the TC removal from real water samples was maintained. Their addition to soil enhanced adsorption capacities, surpassing that of chitosan-based beads containing free polymers. Significantly, the quantity of TC desorption diminished due to the halloysite's presence, which limited its penetration into groundwater. The primary mechanism of tetracycline adsorption on the hydrogel beads studied is pore filling, but other interactions (hydrogen bonding, π-π stacking, electrostatic attraction) are also involved.