(280g) MF and UF Coated Membranes for Selective Separation of Organic Anions-Pfas

Per- and polyfluoroalkyl substances (PFAS) have been found in both drinkable water and human blood. Due to the stable and bio-accumulative properties of these manmade chemicals, concerns about human health and environmental impact have been raised and the EPA has set a 70 ppt PFAS limit for drinkable water. In order to remain under this limit and potential future, more restrictive limits, scientific advances for the separation processes of this fluorinated compounds are critical. Functionalized and coated micro- and ultrafiltration membranes, as is the case of NF membranes, present an energy-efficient platform through which selective separation can optimize the use of the driving force applied into the system to target desired compounds. To make this technology more competitive to current alternatives for the separation of PFAS, however, the full potential of the membrane platform must be deployed and proper understanding of the physicochemical interactions of PFAS in solution and their partitioning into the polymeric membranes will be needed.

This work presents the creation of a single exclusion/sorption platform for PFAS separation, allowing for both a highly concentrated downstream for easier post-treatment of these compounds, as well as purer produced water. A commercial PVDF-400 microfiltration membrane (about 100 nm pores) was successfully pore-functionalized with poly (N-isopropylacrylamide) (PNIPAAm) and used for further formation of a polyaromatic amide nanofiltration (NF) layer. This PNIPAAm pore-functionalized NF membrane displays a temperature-responsive behavior allowing the regeneration of the sorption capacity of the PNIPAAm by swinging the temperature below the lower critical solution temperature (LCST) of 32 °C as shown in our group’s previous studies. Additionally, to enable higher separation (exclusion) of the NF layer, PFAS and model organic anions, such as their hydrogenated counterpart, among others, were systematically studied. Bringing up molecular properties and their display in aqueous solution allowed a comprehensive understanding of the separation of these compounds. Meanwhile, some PFAS had more stable high retentions, while other organic anions, such as oxalate, transitioned from approximately 100% rejection down to 0% rejection by increasing the hydrogen concentration of the solution by 4 orders of magnitude. Rejection of PFOA using similarly modeled pore size NF membranes (0.43 and 0.44 nm) but with a different surface charge significantly influenced its rejection, from ~95% down to ~70% in a more positively charged NF membrane. X-ray photoelectron spectroscopy (XPS) showed strong long-term adsorption into positively charged membranes (~3.5% atomic) compared to the negatively charged counterpart (<1% atomic), which may relate to the PFAS partitioning and its reduction in rejection performance. This research is funded by NIEHS-SRP.