(158d) Modification of Polycarbonate Membranes with EDC/NHS Coupling: The Impact of Electrostatic Particle-Pore Interactions on Rejection

It has long been recognized that electrostatic and other long range particle-pore interactions can play a dominant role in MF and UF separations involving biological systems. In this work, the impact of physicochemical interactions between particles and porous membranes has been examined from both experimental and theoretical perspectives. To facilitate interpretation of results in terms of fundamental membrane and pore characteristics, experimental work involved model systems comprised of polycarbonate track etch membranes with well-defined pore geometry and monodisperse polystyrene particles.

Commercially available polycarbonate track etched membranes and polystyrene particles both carry a strong negative surface charge. To create an experimental system with different particle-pore wall interactions, membranes were modified using an EDC/NHS coupling reaction with tetraethylpentamine, creating a set of membranes with reduced surface charges and no measurable change in pore size. Streaming potential measurements were used to determine pore surface characteristics. The rejection of negatively charged red dyed spherical polystyrene particles was measured using a 10 ml dead-end filtration cell with both modified and non-modified membranes. The impact of flow rate, rotational stir speed and feed concentration on measured rejections was examined for both modified and non-modified membranes.

When compared to particle rejections observed with non-modified membranes with the same pore size, larger rejections (up to 36% increase) were observed with the modified membranes. The pore surface modification reduces electrostatic repulsion between particle and membrane, resulting in an increase in particle deposition on the membrane surface. This explanation is supported by visual examination of the membranes which shows a stronger red color on the modified membranes at the end of filtration. Examination of the impact of flow rate and stir speed on rejections illustrate the complexity of the particle-pore wall interactions for these systems.

A macroscale model of the filtration cell (using the k-epsilon turbulent flow model) was developed and coupled with a Brinkman model of the membrane and a microscale model of a single pore. Model results were used to track particle trajectories as they flowed from the reservoir outside the membrane into the pore. In this novel modelling approach, the kinetic energy of a particle at the pore entrance was calculated and coupled with steric and electrostatic particle-pore wall repulsive forces to examine particle concentration profiles within the pore and to estimate the probability of particle deposition to the membrane surface. Model predictions were found to be consistent with experimentally measured rejection values.

The results from this study provide insight into the impact of electrostatic and hydrodynamic particle-pore wall interactions on particle trajectories and resulting rejections from porous membranes. The results as well as the impact of these interactions on membrane design considerations will be discussed in this presentation.