Effective removal of small colloidal nanoparticles (NPs, e.g., <20 nm), including rigid particles, macromolecules, organics, viruses, antibiotics, hormones, etc., by dead-end filtration, is very important in drinking water, chemical, biopharmaceutical, and semiconductor factories. Most existing operations focused on sieving mechanism-based filtration with small-pore membranes (e.g., <20 nm) to capture these tiny NPs, which unfortunately leads to high energy consumption. This study proposed an emerging method that is to use large pore membranes (i.e., ∼100 nm) to capture NPs down to 2.8 nm. The ultimate goal is to address the grand challenge presented in the United Nations Sustainable Development Goal 12 on energy saving. To capture (or adhere) small NPs by large-pore membranes, the surface electrostatic interactions between NP and membrane should be considered in addition to the sieving mechanism. To examine the feasibility of the proposed method, the retention efficiency of ∼100 nm rated polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polycarbonate track-etched (PCTE, as a model filter) membranes against 2.8 nm ZnS quantum dot (QD), 5 and 10 nm Au and 100 nm PSL in water and isopropanol (IPA, a representative organic solvent) were studied experimentally and theoretically. In the experiments, the electrospray-scanning mobility particle sizer (ES-SMPS) was used to measure the size distribution of these nanosized colloids before and after the membrane to determine the retention efficiency. In the theory, we combined the hydrodynamic particle transport, extended Derjaguin-Landau-Verwey-Overbeek (xDLVO), and Maxwell models to calculate the NP retention. Results showed that the data agreed with the theoretical model very well, and the NP retentions in IPA were higher than that in water due to a significant change of acid-base interaction from repulsion to attraction. The retention of the 10 nm Au NPs and the 2.8 nm NPs could be attained by ∼80 and 35%, respecively, in IPA by the PTFE. This study was the first to show the combined theoretical model can accurately predict the retention of sub-10 nm NPs by different membranes in an organic solvent and water. The model predicts the retention of 5 nm Au NPs can be increased to higher than 99% by increasing the zeta potential of the PTFE about 5 times, thus a sustainable UF cutting significant carbon footprint is foreseeable in near future.
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