Importance of Surface Carboxyl Groups on Biofouling Development and Control for Thin Film Composite (TFC) Polyamide Membranes

Biofouling is a major challenging issue to be addressed for the successful application of thin-film composite (TFC) polyamide (PA) membranes. In this work, we demonstrated that the presence of divalent cations, especially Ca2+, can intensively aggravate the biofouling on the TFC PA membrane using Escherichia coli and Staphylococcus aureus as model bacteria. To figure out the key surface characteristics of the TFC membrane facilitating the biofouling upon Ca2+ addition, the PA layer was modified by in situ addition of carboxylated chitosan (CCTS) during interfacial polymerization (IP), which induced the increase of surface carboxyl groups, improvement of hydrophilicity, and reduction of surface roughness. By conducting biofouling tests, the importance of carboxyl groups on the membrane surface and their interaction with bacteria via Ca2+-carboxyl bridging in biofouling was verified. The water flux decreased to 49.5% (TFC-control) and 41.6% (TFC-CS (0.3)) of the initial water flux, respectively. Furthermore, by preoccupying carboxyl groups on the PA layer via in situ Ca2+ addition during IP reaction, it was found that reducing the carboxyl density of the PA layer can markedly inhibit the adhesion of bacterial on TFC membrane surface, the water flux only decreased by only 35.6% relative to the initial water flux, thus effectively mitigating biofouling of TFC membranes. This work provides a new pathway for better understanding and control of biofouling in the design and application of TFC PA membranes.