Phospho-modified polyamide nanofiltration membranes with high permeability by using alendronate sodium as additives

The conventional in-situ interfacial polymerization (IP) process, characterized by its rapid reaction rate, is known to be an uncontrollable process that can lead to defects in the performance of nanofiltration (NF) membranes. Herein, a novel NF membrane with a phosphorylated polyamide (PA) network was prepared through a controlled IP process with the addition of alendronate sodium (AS). The phosphoric acid groups of AS can restrain the diffusion of PIP, due to the synergistic interactions with piperazine (PIP) including electrostatic interaction and hydrogen bonding. Moreover, the amine groups of AS could react with trimesoyl chloride (TMC) in organic phase, which incorporated AS into the PA networks. Consequently, a more negatively-charged ionic PA layer with looser, rougher and more hydrophilic structure was obtained. The prepared membrane exhibited a water permeance of 25.0 L·m−2·h−1·bar−1 which was almost two-fold higher than that of NF membranes prepared without additives, while keeping an excellent salt rejection performance (Na2SO4 rejection >97 %), even in a high salt concentration (7.0 g/L). Furthermore, the membrane demonstrated high rejections for different dyes, long-term operation stability and comparable anti-fouling ability against typical contaminants. This work provided a facile and rational approach to preparing ionic NF membranes with high permeability.