Enhancing Stability of Tannic Acid-Fe Nanofiltration Membrane for Water Treatment: Intercoordination by Metal-Organic Framework.

Tannic acid (TA)-Fe nanofiltration (NF) membrane has been demonstrated to possess more favorable removal of trace organic contaminants (TrOCs) over the conventional polyamide NF membrane. However, the drawback of acid instability severely hinders the practical application of TA-Fe NF membrane in the treatment of (weak) acidic wastewater containing TrOCs (e.g., pharmaceutical wastewater, surface water, and drinking water). Herein, we introduced the MIL-101(Cr) nanoparticle, a kind of metal-organic framework (MOF), into the TA-Fe selective layer to enhance the membrane acid stability. The acid-tolerance parameter of MIL-101(Cr)-stabilized TA-Fe membrane (TA-Fe-MOF membrane, 12,000 ppm/s) was two orders of magnitude larger than that of the TA-Fe membrane (50 ppm/s), and the TA-Fe-MOF membrane can withstand acid treatment at pH = 4 for more than 30 days. Meanwhile, the TA-Fe-MOF membrane displayed increased water permeance from 9.5 to 12.7 L/(m·h·bar) after the MOF addition, without compromising the selectivity. The enhanced acid stability for the TA-Fe-MOF membrane was ascribed to an intercoordination mechanism, where Fe centers (from TA-Fe complex) coordinated with -COOH groups (from terephthalic acid of MOF) and Cr centers (from MOF) coordinated with -OH groups (from TA of TA-Fe complex), which was verified by the density functional theory calculation. This study highlights a new approach for the development of a TA-Fe-based NF membrane with markedly enhanced acid stability, which is important for its real application in wastewater treatment and water reuse.