Modeling Nanofiltration Separation Mechanisms in Covalent Organic Framework Membranes

Solute transport in nanofiltration (NF) membrane systems is described with the Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE), which couples size- and charge-based solute partitioning mechanisms into and out of membrane pores with flow through pores, as described by the Extended Nernst-Planck equation. If membrane structural and chemical characteristics are well defined, the DSPM-DE can theoretically be used to identify solute rejection mechanisms, predict NF performance, and guide membrane design. However, the presence of additional separation mechanisms, like adsorption, and the heterogeneous, convoluted characteristics of traditional NF membranes challenge these goals. In this work, we apply covalent organic frameworks (COFs) as model NF membrane materials to demonstrate control over partitioning and transport mechanisms. We experimentally isolate and quantify steric and non-steric contributions to solute partitioning and transport in NF via application of the DSPM-DE to COF membranes fabricated with tailored pore sizes and charge properties. We also demonstrate enhanced non-steric solute rejection by changing COF membrane chemistry, and we highlight the significant impact of adsorption on measured solute rejection by COF membranes. Next, the knowledge gained through this modeling work is applied to estimate the removal of environmentally persistent pharmaceutical pollutants from a simulated domestic wastewater effluent by COF NF membranes with different charge properties, a relevant separation for water recycling. Size-based rejection of uncharged pharmaceutical solutes is well-predicted by the DSPM-DE. Charge-based exclusion mechanisms are relevant for the negatively charged solute, and its rejection by the membrane with fixed negative charges (TpPa-SO3H COF) is accurately predicted. For the nominally uncharged membrane (TpPa-1 COF), however, rejection is underpredicted. Membrane characterizations reveal that the fixed negative charge of the TpPa-SO3H COF can be effectively screened by feed solution ionic strength. In contrast, the persistent negative charge of the TpPa-1 COF is not screened by solution ionic strength and results in enhanced non-steric rejection over model predictions. Quantifying relative contributions of solute separation mechanisms via the DSPM-DE allows for strategic NF membrane design. Combined with independent control over pore properties afforded by COF membranes, this knowledge drives the development of tunable, fit-for-purpose NF membranes with predictable performance for challenging separations.

Name thesis:degree_name

Doctor of Philosophy

Discipline thesis:degree_discipline

Environmental Engineering

Grantor

University of Houston

Year dc:date.issued

2024

Author dc:creator

• Flores, Dana Reed

Advisor dc:contributor.advisor

• Shaffer, Devin L.

Committee members dc:contributor.committeemember

• Rixey, William G
• Louie, Stacey M
• Venna, Surendar
• Getachew, Bezawit

• Transport modeling
• Adsorption
• Covalent organic frameworks
• Membranes
• Wastewater
• Nanofiltration

Language dc:language.iso

English

Handle dc:identifier.uri

https://hdl.handle.net/10657/18347

OAI identifier oai:identifier

oai:uh-ir.tdl.org:10657/18347