Prolonging the antibacterial activity of nanosilver-coated membranes through partial sulfidation

Biofouling is a major issue in membrane-based water treatment because it shortens membrane life and decreases the permeate flux. Silver, a known biocide, is often used for in situ formation of silver nanoparticles (Ag NPs) on membranes for biofouling mitigation. However, Ag NPs dissolve quickly in water, limiting their effectiveness over long periods of time. This study focuses on the modification of silver-functionalized reverse osmosis (RO) membranes with different concentrations of Na2S (10-1, 10-3, and 10-5 M) to identify the degree of sulfidation that limits Ag release while preserving the antibacterial effect. Sulfidized membranes decreased Ag release by > 85% depending on the extent of sulfidation. Antibacterial activity was assessed using P. aeruginosa and E. coli. Results showed the highest inactivation at 73% for P. aeruginosa and 57% for E. coli for 10-5 and 10-3 M Na2S-treated membranes, respectively, while the more sulfidized membrane treated with 10-1M Na2S treatment had the lowest antibacterial effect. Moreover, when tested in a dynamic cross-flow RO system for 24h, the flux declined by 24% for the Ag NPs and by 23%, 17%, and 19% as the extent of sulfidation increased. Additionally, the Ag remaining in the membrane was higher for the highest sulfidized membrane with 519 ng/cm2. Therefore, retention of the silver coating over time appears to be more important for biofilm control in RO systems than high antibacterial activity. Both 10-5 M and 10-3 M Na2S-treated membranes had the best balance between reduced dissolution rate and good antibacterial and anti-biofouling performance, respectively.