|Ph.D Student||Baransi Karkaby Katie|
|Subject||In-Situ Modification of Desalination Membranes for Improved|
|Department||Department of Chemical Engineering||Supervisor||Professor Viatcheslav Freger|
|Full Thesis text|
Nanofiltration and reverse osmosis membrane processes are widely used today for desalination and water treatment. The top polyamide selective layer of such composite membranes has a fascinatingly complex structure, which combines high permeability and good salt rejection, yet nanoscale non-uniformities inherently present in polyamide layer may reduce the selectivity to small uncharged solutes. Moreover, the relative hydrophobicity of the polyamide can lead to high affinity toward hydrophobic foulants.
This study systematically examines modifying the membrane surface by grafting a thin polymer layer on top of it, such grafting may cause “defect plugging” of the less selective and more permeable areas in the polyamide active layer, thus improve the rejection of wide range of small uncharged contaminants. Surface modification was done using concentration polarization-enhanced grafting. In this method monomers solution with freshly added initiators is filtered through the membrane and surface grafting is enhanced due to concentration polarization effect of monomer next to the membrane surface.
Feasibility of this modification approach was demonstrated by using the monomer glycidyl methacrylate for improving removal of uncharged small solutes combined with grafting of a zwitterionic acrylate, for imparting the membrane with low-fouling properties. Results from dead-end experiments showed that grafting by combination of both monomers significantly reduces passage of selected micropollutants as well as boric acid, with only a moderate loss in permeability (45% decrease in water permeability) of low pressure reverse osmosis membranes. Moreover, addition of non-ionic surfactant Triton X-100 to slightly hydrophobic glycidyl methacrylate solution was shown to facilitate grafting and produce a more uniform coating.
In addition, it was shown that the uniformity of the grafted layer depends greatly on the physical properties and interactions of the grafted polymer with the membrane surface. Therefore in order to enhance zwitterionic acrylate grafting, two sequential modification steps were applied, first by glycidyl methacrylate utilizing its strong adhesion to polyamide, then by zwitterionic acrylate for increasing its surface coverage.
Results from cross-flow experiments revealed enhanced fouling resistance (to humic acid as a model of organic foulant) of membranes modified by sequential modifications. Eventually, scaling-up the modification procedure to commercial low pressure reverse osmosis elements, showed that the use of both glycidyl methacrylate and zwitterionic monomers was inefficient for improving rejection. However, grafting by glycidyl methacrylate alone resulted in a superior rejection of boric acid and representative micropollutants with water permeability comparable to commercial brackish water reverse osmosis membranes.
The present study demonstrate that the proposed in situ grafting approach offers a facile and up-scalable method for enhancing rejection of wide range of contaminants both hydrophilic and hydrophobic, with many potential benefits for water desalination, purification, and reclamation.