Structural Design of PES-CS-MMT Composite Membrane by Layer-by-Layer Self-Assembly for the Removal of Antibiotic Wastewater

A multilayer gradient composite membrane was fabricated on a PES ultrafiltration substrate through layer-by-layer assembly of chitosan (CS) and montmorillonite (MMT), followed by Ca²⁺ crosslinking. The designed architecture forms a multi-layer gradient composite membrane through successive self-assembly, aiming to balance adsorption, interfacial transport and structural stability. SEM observations showed a clear stratified configuration with relatively uniform thickness distribution, including a relatively dense MMT-rich surface layer and a porous PES support that preserved mass-transfer channels. FTIR confirmed the introduction of hydroxyl/amino-containing CS and aluminosilicate-related MMT species onto the membrane surface, indicating successful incorporation of both organic and inorganic components. TG–DTG results further suggested enhanced thermal stability arising from the cooperative effect of the inorganic lamellae and the polymer framework. In dynamic tests, the membrane displayed concentration-responsive adsorption behavior toward gatifloxacin, ciprofloxacin and ofloxacin, and different pollutants reached equilibrium or quasi-steady states at different rates. Comparative kinetic results at the same initial concentration showed that diclofenac, gatifloxacin and ciprofloxacin approached stable plateaus much faster, whereas ofloxacin increased slowly and did not reach an obvious plateau within the tested period. These results indicate that pollutant removal was jointly governed by interfacial interactions, gradient-layer diffusion resistance and overall transport behavior rather than by concentration alone. Overall, the layer-by-layer strategy provided a controllable route for constructing gradient functional layers on PES membranes, demonstrating potential for advanced treatment of antibiotic-containing wastewater and related pharmaceutical effluents.