ENGINEERING REACTIVE MEMBRANES: SCALABLE FABRICATION AND STABLE COATING TECHNIQUES

Abstract

Membrane technologies play a central role in wastewater treatment systems due to their efficiency, modularity, and low energy consumption. However, conventional polymeric membranes face significant limitations, particularly membrane fouling, which reduces performance, increases operational costs, and shortens membrane lifespan. Additionally, due to their reliance on physicochemical separation mechanisms, low-pressure membranes are often ineffective at removing small, neutral emerging contaminants. Electrically conductive membranes (ECMs) have emerged as a promising multifunctional alternative. By enabling simultaneous filtration and electrochemical reactions, ECMs offer electro-assisted antifouling and enhanced contaminant removal capabilities. Typically, ECMs are fabricated by coating porous polymer membranes with conductive nanomaterials. However, this approach often reduces membrane permeance by up to 90% due to pore blockage and results in unstable conductive surfaces caused by the detachment of conductive materials. Moreover, the scalability of ECM manufacturing remains a challenge, as current coating-based synthesis methods are incompatible with roll-to-roll production. This thesis addresses the stability and scalability challenges in ECMs fabricated from carbon nanotubes (CNTs) and polymeric substrates. First, we introduced the use of a new chemistry based on polydopamine (PDA) and polyethyleneimine (PEI) to enhance CNT adhesion on poly(vinylidene fluoride) (PVDF) membranes using conventional coating methods. Four PDA/PEI-based fabrication strategies were evaluated, with the most effective involving CNT deposition on PEI-crosslinked PDA-coated PVDF. This method produced highly stable and conductive membranes, achieving up to 99% electrochemical removal of model organic dyes. We then employed response surface methodology (RSM) to optimize the PDA/PEI formulation, revealing that low concentrations of PDA and PEI combined with high PEI molecular weight yielded membranes with exceptional permeance (~900 L/m²·h·bar) and conductivity (~30,000 S/m). To address the scalability limitations of ECMs, we developed a sequential casting (SC) process for fabricating membranes using carbon nanotubes (CNTs) and polyethersulfone (PES) substrates. ECMs synthesized by the SC process demonstrated 2.7 times higher pure water permeance (572.9 ± 84.4 vs. 210.1 ± 22.3 L.m-2.h-1.bar-1) at comparable electrical conductivities (1169 ± 172 vs. 1385 ± 157 S/m) as compared to ECMs made via conventional coating-based synthesis. We systematically investigated key parameters influencing membrane performance, including CNT type and concentration, casting thickness, resting time, and shear rate. Notably, combining high-aspect-ratio CNTs with high casting shear rates (i.e., 125 s⁻¹) resulted in membranes with 1.7 times higher conductivity and 1.9 times higher permeance compared to ECMs made at low shear rates (12.5 s⁻¹). Overall, this work presents a suite of scalable, tunable, and high-performance ECMs that address critical limitations of conventional synthesis methods. These findings lay the groundwork for practical ECM design in advanced wastewater treatment, with applications in fouling control, contaminant degradation, and electrochemical separations.