Investigating and significantly improving the stability of tannic acid (TA)-aminopropyltriethoxysilane (APTES) coating for enhanced oil-water separation

Tannic acid(TA)-aminopropyltriethoxysilane(APTES) coating possesses hierarchical structure and superhydrophilicity, and can be facilely coated on various porous membranes for oil-water separation. However, the TA-APTES coating will lose its superhydrophilicity and underwater superoleophobicity after being exposed to the air for more than one month. Besides, TA-APTES coating on membrane surface decomposes when the coating is immersed in acidic solution (pH <= 3) or alkaline solution (pH >= 12). In this study, the reasons for the instability of TA-APTES coating have been studied, and an effective strategy for significantly improving the stability of the coating has been developed via simple treatment with ferric ions. The coordination effect between ferric ions and the TA-APTES coating can not only restrain the oxidation of the phenolic hydroxyl groups, but also decrease the electrostatic repulsion and enhance the interactions between the chain segments. The resultant (TA-APTES)-Fe(III) coating possesses long-term superhydrophilicity and excellent acid-base resistance property, while the distinct and hierarchical structure of the TA-APTES coating can be reserved. Thanks to the outstanding stability of (TA-APTES)-Fe(III) coating, the PVDF-(TA-APTES)-Fe(III) membrane can maintain its outstanding anti-oil-fouling property and good oil/water separation performance even it is being exposed in air for one month or being immersed in strong acidic or alkaline solutions for 48 h, which is much better than the pristine PVDF-(TA-APTES) membrane. This study will accelerate the practical application of TA-APTES coating.