|M.Sc Student||Segev Mark Naama|
|Subject||Colloidal Deposition Polymer-Brush-Coated|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Guy Ramon|
|Full Thesis text|
Fouling control remains a challenge in membrane separation. Recent works have shown the potential of surface modification, e.g. polymer brush coatings, as a fouling control strategy. Such modifications are aimed at changing the surface physical-chemical characteristics and may also respond to external stimuli to actively release foulants from the membrane surface. Responsive polymers have the ability to respond to stimuli such as pH, temperature, ionic strength, etc. PNIPAM (poly-N-iso-propylacrylamide) exhibits a hydrophobic-hydrophilic phase transition at its lower critical solution temperature (LCST) ~32°C, which reduces to 20°C at high-ionic strength2. Microscopic techniques have contributed greatly to our understanding of particle deposition; however, very little is known in the case of brush-coated membranes. In the present study, we employ confocal microscopy to investigate, in-situ, the deposition and release patterns and kinetics of colloidal fouling on NF membranes with and without a PNIPAM brush layer.
A custom-built cross-flow filtration cell with an optical window enabled live observation of particles depositing on the membrane surface. Images of fluorescent-labelled particles and hexadecane droplets were acquired and analyzed for surface coverage with image processing software. Experiments began with 40 mins of deposition followed by cleaning cycles with pressure shut-off to eliminate permeation and then crossflow rinsing with 1.5M NaCl solution and DI water, alternately, to induce phase transition of the PNIPAM.
Surface coverage rate constants, ksc, were calculated from surface area coverage curves and their values illustrated that amine-functionalized particles deposit faster than carboxyl particles, and that the PNIPAM coating can significantly lower the deposition kinetics as well as the overall coverage. This may be attributed to the steric repulsion imparted by the brush layer on approaching particles. However, deposition was irreversible, even upon induction of the phase transition. In contrast to particles, droplets were partially released from the surface upon salt-activation. Oil droplets, unlike polystyrene beads, deform under shear and respond to alterations in surface morphology and hydrophobicity, which could explain their release.
The specific structure and chemistry of the membrane and foulant surfaces will determine the deposition propensity and its reversibility. For some foulants (rigid polystyrene particles) steric repulsion of the PNIPAM chains plays a key role in preventing deposition, while for others (oil droplets) a different trend is observed. However, the deposition observed here is largely irreversible, with the exception of oil droplets that are largely removed from the unmodified membrane, and are partially released from the PNIPAM-modified membrane with each activation cycle.
These results offer steps towards much-needed mechanistic insight into the function of brush-coatings as fouling-resistant and/or self-cleaning coatings for membranes.