Tannic acid: an underexplored fraction of natural organic matter in ultrafiltration membrane processes

Abstract

Ultrafiltration (UF) membranes are effective for removing natural organic matter (NOM), a known precursor of carcinogenic disinfection byproducts. Most previous studies have used humic acid (HA) as a representative NOM in UF processes, whereas research on tannic acid (TA), a naturally occurring polyphenolic compound, remains limited. In this study, the performance (i.e., water flux and NOM removal) of UF membranes treating TA-containing water was evaluated and compared with that of UF membranes treating HA-containing water. Under different pH conditions, both HA and TA showed the highest removal efficiencies at pH 3 (80-84% for both HA and TA), followed by pH 7 (71-78% for HA and 46-79% for TA), and then pH 10 (62-69% for HA and 18-38% for TA). However, TA removal dropped significantly at pH 10 owing to the hydrolysis of TA molecules, whereas the decrease in HA removal was relatively minor. Under all the tested conditions, TA consistently caused a greater flux decline than HA, and TA fouling was more irreversible. Substantial reduction in the pore size of the TA-fouled membranes was observed, indicating severe fouling. This pronounced fouling was also observed when TA was added to the HA solutions and natural river water. Overall, our results highlighted that the presence of TA in UF processes can lead to more severe fouling than HA, a commonly used NOM surrogate. These findings suggest that greater attention should be paid to the presence of TA in feed water during water treatment processes.