Stability and Selectivity in Nanofiltration: Overcoming Limitations of Polyamide with Polyelectrolyte and Oligomer-Seeded Membrane Architectures

Abstract

Nanofiltration (NF) is a pressure-driven filtration process used to perform separations critical to resource recovery and wastewater treatment efforts. Most NF membranes are composed of polyamide and prepared using interfacial polymerization (IP). Although polyamide NF membranes are the state of the art, their suitability for emerging applications is constrained chemical instability beyond the pH range of 3-10 and the inherent lack of kinetic control over conventional IP (C-IP) processes. This dissertation aims to advance NF technology by developing polyelectrolyte and oligomer-seeded membrane architectures that enhance alkaline resistance, ion/ion selectivity, and water/salt selectivity. In the first objective, we demonstrate the promise of NF membranes for treatment of alkaline adsorption regeneration solutions. Commercial polyamide NF membranes achieve > 95% phosphate rejection while producing virtually pure hydroxide streams. However, prolonged exposure to caustic solutions reveals gradual performance degradation, highlighting the critical need for chemically robust NF membranes. We then develop polyelectrolyte-based NF membranes (PE-NF) for base recovery separations in the second objective. Our PE-NF membranes removed >90% of carbonates and phosphates from alkaline streams for at least four weeks, far outlasting commercial polyamide NF membranes. In the third objective, we introduced oligomer-seeded IP (OS-IP) as a strategy to overcome the structural and performance limitations of C-IP membranes. By introducing trifunctional reactive oligomers into the aqueous phase, OS-IP membranes formed ultrathin (< 25 nm) selective layers with water permeances and water/salt selectivities far surpassing both commercial and lab-made NF membranes. Expanding OS-IP to alternate chemistries further enhances alkaline stability, yielding membranes that both maintained performance stability during multi-week exposure to pH 13 solutions while achieving competitive or superior performance to commercial and lab-made alkaline resistant NF membranes. In summary, our findings highlight the remarkable OH-/carbonate and OH-/phosphate selectivities of PE-NF membranes and introduces OS-IP as a breakthrough platform for overcoming long-standing limitations of conventional polyamide chemistry.