Metal-Polyphenol Coordination at the Aqueous Contra-diffusion "Interface": A Green Way to High-Performance Iron(III)/Tannic Acid Thin-Film-Composite Nanofiltration Membranes.

Thin-film-composite (TFC) nanofiltration membranes have found wide uses in environment remediation and industrial separation. There is a growing trend to avoid the use of organic solvents and toxic chemicals during membrane fabrication. Therefore, the aqueous fabrication of TFC membranes receives considerable interest as a green and sustainable process. However, it remains challenging to construct a defect-free and ultrathin film in a homogeneous aqueous phase without the assistance of an interface. The contra-diffusion process provides a special "interface" to confine the film formation within a narrow space by regulating the competition between precursor diffusion and interfacial reactions. Herein, Fe 3+ /tannic acid (TA) TFC membranes were fabricated by a contra-diffusion process. The effects of fabrication parameters on the Fe 3+ /TA TFC membrane microstructure and performance were also investigated. The negatively charged membrane performs a competitive Na 2 SO 4 rejection of 95.6% with a permeation flux of 44.3 L m -2 h -1 under 0.6 MPa as well as more than 99.5% rejection to several anionic dyes. The as-prepared membranes perform superior nanofiltration performance compared to other reported Fe 3+ /TA-based membranes, owing to the thin and defect-free selective layers by self-regulation. Moreover, the membranes exhibit stable rejection during a long-term nanofiltration test.