(40a) Rejection Mechanisms of ZnS Quantum Dots and Au Nanoparticles and Selection of Membrane Filters for Ultrafiltration and Nanofiltration

Dramatic increase in demands of engineered nanomaterials can increase their release [S1] into water environments. Efforts on improving water treatment techniques to prevent the release of hazards to the environments should be made. Membrane processes are considered as a very effective and promising method for water treatments. However, particle removal[S2]mechanisms have not been fully elucidated due to complex interactions between colloids and membranes, especially for very small colloidal particles. In this study, a series of systematic filtration tests using eight different types of membrane filters against 1.7 nm ZnS quantum dots (QDs) and 5, 10 ad 20 nm Au nanoparticles (NPs) was performed to understand the rejection mechanisms. By calculating retention, recovery and adsorption efficiencies, we found that adsorption, electrostatic repulsion and reentrainment of NPs attached on membrane surfaces are the three dominant rejection mechanisms for NPs smaller than pore sizes. Results showed that each test filter had different rejection mechanisms for removing small NPs. Nylon membrane filters had a greater adsorption affinity with Au NPs compared to other membranes with even smaller nominal pore sizes. Moreover, we found electrostatic repulsion played a significant role in removing NPs, but the efficiency was affected by flux conditions, or filtration flow rate, due to hydrodynamic drags. NP and membrane properties such as zeta potential and Hamaker constant did not correlate well with membrane rejection abilities. Different membrane filters with the same nominal pore size of 0.1 µm showed different NP removal efficiencies. Our study provides the detailed insights of rejection mechanisms of various membranes and suggests a way of selecting membrane filters in different filtration purposes.