Integrated nano-composite membranes for biological and organic fouling prevention

Our goal is to modify commercially available polyethersulfone (PES) membranes with a pore size of 0.1 microm in order to achieve both organic fouling and biofouling prevention. The PES membranes were modified by the standard polyelectrolyte multilayer modification method; layers of polystyrenesulfonate (PSS), poly(diallyldimethylammonium chloride) (PDADMAC) and silver nanoparticles capped with PSS were alternatively assembled by electrostatic deposition. Multilayers of PDADMAC and PSS (PSS as the top layer) create a negative charge on the membrane surface, thereby potentially increasing the electrostatic repulsion between the membrane surface and foulants. The hydrophilic nature of the PSS increases the hydrophilicity of the membrane, while capping silver nanoparticles onto multilayers of PDADMAC and PSS imparts biocidal characteristics to the virgin membrane. Films were kept thin (1.5 bi-layers of polyelectrolyte) in order to minimize the flux decline resulting from the increased thickness of the modified membranes. Each layer added a thickness of about 10 nm resulting in an added thickness of 30 nm for each modified membrane. Aqueous solutions of humic acid (20 mg/L) and of Escherichia coli ( E.coli suspension (106 CFU/mL) were filtered separately through both the virgin and modified PES membranes under stirred, batch microfiltration conditions. Filtration and cleaning studies confirm that the modification reduces the organic and biological fouling of the commercially available membranes and increases the flux recovery by 6.4 and 16.5 percent respectively for the membrane modified with and without nanoparticles after cleaning with 0.2 M of sodium hydroxide (NaOH) for the organic fouling and by 8.5 and 11.8 percent respectively for the membranes modified with and without nanoparticles after cleaning with 200 ppm of sodium hypochlorite (NaOCl) for the biological fouling. The results show an increase in inherent membrane resistance (R m), as evidenced by the lower initial permeability and flux of the modified membranes. Zeta potential measurements confirm the increased charge at the surface of the modified membranes; such an increase is indirectly proportional to the reduction in fouling with the more negatively charged membranes fouling less. SEM characterization confirmed that the silver nanoparticles are evenly embedded on the polyelectrolyte film surface with diameter on the order of 50nm; FTIR spectra confirmed the chemical modification of the membranes.