Perfluoroalkyl substances (PFASs) have been found to be widespread and persistent in surface water, groundwater, and wastewater systems due to their strong resistance to environmental degradation processes. To tackle this issue, nanofiltration (NF) and reverse osmosis (RO) membranes have emerged as effective separation technologies for removing PFASs from groundwater. However, despite their superior removal efficiency, conventional NF and RO membranes often reject high levels of divalent ions commonly found in groundwater/surface water, such as calcium, magnesium, and sulfate, leading to membrane scaling. Such a scaling can deplete mineral content in treated water and limits water recovery during remediation. Additionally, attaining high water recovery with these membranes requires high pressure operation, resulting in increased energy costs. Herein, we report the use of covalent organic framework (COF) membranes for selective and effective removal of PFASs. Thin and continuous COF selective layer was fabricated on polymer substrate using the scalable interfacial polymerization method to form a thin film composite membrane. The pore size and charge of the COF membranes were tailored through rational selection of the monomers as structural building blocks to achieve high rejection of PFASs but low salt rejection. We found that COF membranes with an average pore diameter less than 1.5 nm and a highly negatively charged surface exhibited high rejection of PFOS (95.7 ± 3.5 %) and PFOA (93.5 ± 2.6 %), while allowing selective passage of divalent salts, with low rejection rates for CaCl₂ (0.1-4.9%) and MgCl₂ (0.1 - 2.1%). These salt rejection rates are significantly lower than those of commercial membranes, such as NF270 membrane, which has a CaCl₂ rejection of ~56.1% and MgCl₂ rejection of ~68.8%. This demonstrates the potential of COF membranes to enhance water recovery and reduce membrane scaling. Furthermore, the COF membranes exhibited a high water permeance of ~30 LMH/bar, which is more than 100% greater than the fluxes of commercial NF membranes. The effect of ionic strength on the membrane performance under high water recovery conditions will also be discussed in the presentation.

Xiaopeng Min

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