Tailor-Made Polysulfone Ultrafiltration Membranes for the Study of the Effects of Metal Oxide Nanoparticles on Fouling

TiO2 and ZnO nanoparticles are currently used in many consumer products, such as cosmetics, sunscreens, paint and many other applications. Therefore, with this increasing manufacture and application, it is inevitable that large amounts of nanoparticles are eventually discharged into the environment. Due to their size and high capacity to convey toxic substances, nanoparticles can have adverse effects on human health and the environment. Therefore, nanoparticles are an emerging class of contaminants that water purification technologies must be designed to remove when present.Membranes are capable of separating species as a function of their physical and chemical properties when a driving force is applied, which enables filtration for removal of colloids, cells and molecules. Filtration processes, such as ultrafiltration (UF) and microfiltration (MF), play major roles in filtration of fresh waters, brackish and saline waters, as well as wastewaters. A concern for membranes is the ability of nanoparticles to deposit within the membrane pores as well as on the membrane surface to foul the surface. Particles smaller than membrane pores can accumulate at the pore entrance to ultimately block the pore entrance and/or enter the membrane material matrix and adsorb to the membrane interior. Like larger colloids, fouling by nanoparticles is influenced by the solution chemistry (ionic strength, pH) and may therefore be controllable. However, the impacts of solution chemistry on the aggregation state and deposition mechanisms of nanoparticles have not yet been fully addressed.Therefore in this work, hydrophobic flash-sheet polysulfone (PSf) membranes, chosen due to polysulfone’s high mechanical strength and thermostability, were casted via phase inversion to investigate the effects of two metal oxide nanoparticles on membrane fouling. First, membranes were cast using different casting dope concentrations in the presence/absence of a pore former (i.e. lithium chloride) with different thicknesses for characterization with respect to flux and salt (i.e. sodium chloride) rejection. The results revealed that with increasing dope concentration and membrane thickness, permeability decreased and subsequently salt rejection increased. Furthermore, compared to LiCl-free membranes, LiCl-containing ones were more permeable and displayed higher salt rejection, which was due to more porosity and smaller pore size caused by LiCl additives in membranes.In the second portion of the study, the effects of two metal oxide nanoparticles, TiO2 and ZnO, on membrane flux and salt rejection were determined. In this work, TiO2 and ZnO were added to the filtration solution, also known as feed solution. Interestingly, when TiO2 was a part of filtration solution, higher filtration fluxes than pure water fluxes were obtained; on the other hand, ZnO presence in the filtration solution decreased the filtration flux. In other words, TiO2 presence in the filtration solution increased the flux, while ZnO behaved, as expected, almost the same as filtration of only sodium chloride (NaCl) solution. The increase in flux was possibly due to TiO2 agglomeration (despite sonication) and flocculation of TiO2 nanoparticles in the presence of increasing ionic strength. It is hypothesized that at high ionic strength, zeta potentials would decrease due to double layer compression, which caused TiO2 to tend to flocculate to form a more permeable fouling cake. The larger particles would also experience more hydrodynamic lift, which would lead to a thinner cake layer and thus higher flux during filtration.