Membrane surfaces immobilized with ionic or reduced silver and their anti-biofouling performances

Abstract

Silver was immobilized onto the surface of a chitosan membrane to examine the anti-biofouling performance of the membrane surface. The chitosan base membrane (denoted as CS) was first immobilized with ionic silver (denoted as CS_Ag +) and then the CS_Ag + membrane surface was chemically treated to obtain the membrane surface with reduced or metallic silver (denoted as CS_Ag 0). The oxidation states of the immobilized silver on CS_Ag + and CS_Ag 0 and the interaction between silver and CS were investigated with X-ray photoelectron spectroscopy (XPS). The stability of the immobilized silver on the two types of membrane surfaces was evaluated through a leaching test. It was found that silver was effectively immobilized onto CS through surface complexation and the immobilized silver on CS_Ag 0 was at a reduced or lower oxidation state and was more stable than that on CS_Ag +. Anti-bacterial and anti-biofouling experiments for CS, CS_Ag + and CS_Ag 0 were conducted with two types of typical bacteria, i.e., E. coli and Pseudomonas sp . From the disk diffusion tests (for 24 h), it was found that, as compared to CS, both CS_Ag + and CS_Ag 0 showed significantly improved anti-bacterial performance, even though the CS_Ag + membrane surface seemed to exhibit slightly stronger anti-bacterial effect than the CS_Ag 0 membrane surface. In the longer time immersed experiments in bacteria suspensions for anti-biofouling performance (up to 10 d), both CS_Ag + and CS_Ag 0 showed much less biofouling than CS and they behaved almost equally good in their anti-biofouling behavior initially (24 h), but the CS_Ag 0 membrane surface gradually exhibited more stable and eventually better anti-biofouling performance than the CS_Ag + membrane surface afterwards. The results in this study demonstrated that the immobilization of silver onto membrane surfaces can be an effective method to improve a membrane's anti-biofouling property. The study was also the first of its kind that evaluated the relative anti-biofouling performance by immobilized silver in ionic and reduced states on a membrane surface.