Ethanol-regulated synthesis of aluminum−tannic acid network nanofiltration membranes with improved performance for antibiotic removal

Metal-polyphenol networks, formed by the coordination between metal ions and polyphenolics, have been applied for nanofiltration membrane fabrications. However, the current reported MPN membranes were not dense enough to remove smaller molecules. To obtain nanofiltration membranes with low molecular weight cut-offs (MWCOs), the membranes were constructed from tannic acid (TA) and metal ions through ethanol regulation in this work. The influences of the type of metal ions and the solvent of the coating solutions on the membrane performance were investigated. Al ion have smaller size, higher charge number, and stronger binding energy with polyphenol, resulting in dense membranes compared with Cu2+, Co2+, and Ni2+ ions. The phenolic hydroxyl groups in TA molecules are less ionized in ethanol than in water, and the π-π* interaction of TA in ethanol is stronger than in water. The TA ethanol solution coating resulted in membranes with a thicker selective layer. The membranes prepared in TA ethanol solution were denser, with lower permeances and higher rejections. The optimized Al3+-TA/PSF membrane had a permeance of around 14.3 Lm−2 h−1 bar−1, with a MWCO down to 188 Da, corresponding to a small pore size of around 0.45 nm. Compared with the reported MPN membranes, this membrane had a much smaller MWCO and a higher permeance, exhibiting the advantages of this method. The performance of antibiotic removal was comparable with commercial polyamide membrane NF270, indicating the membrane might have the potential for micro-pollutant (antibiotics) removal.