In Situ Modification of Desalination Membranes for Improved Performance

Katie Baransi Karkaby, Wolfson Department of Chemical Engineering, Technion, Haifa, Israel
Maria Bass, Wolfson Department Of Chemical Engineering, Technion, Haifa, Israel
Viatcheslav Freger, Wolfson Department Of Chemical Engineering, Technion, Haifa, Israel

Nanofiltration (NF) and reverse osmosis (RO) are widely used today for desalination and water treatment. In these membranes nanoscale defects and non-uniformities of the top polyamide layer may reduce overall selectivity. This study examines “caulking” such defects via concentration polarization-enhanced graft-polymerization and tuning selectivity - by filtering the solution of appropriate monomers and initiators through the membrane.1,2 A novel improvement of this technique reported here is based on addition of a surfactant to the reactive solution, which appears to enhance polarization (via solubilisation of monomer in surfactant micelles) and adherence of graft-polymer to the membrane surface, which facilitates grafting and reduces monomer consumption.

The effect was particularly notable for a hydrophobic monomer glycidyl methacrylate (GMA) combined with a non-ionic surfactant. Surface characterization by ATR-FTIR, HR-SEM and contact angle indicated a significantly increased degree of grafting and uniformity of the coating, compared to a procedure with no surfactant added. Modified ESPA1-2521 elements had permeability comparable to commercial BWRO elements with enhanced rejection of boric acid and representative endocrine-disrupting contaminants (EDCs) compared to their rejection before modification.

The procedure was however less suitable for grafting hydrophilic zwitter-ionic polymers, which has the potential to improve resistance to fouling. For grafting of a zwitter-ionic polymer along with GMA for imparting membrane with both selectivity and antifouling properties, we are currently exploring a two-step procedure whereby GMA with added surfactant is grafted first and zwitter-ionic polymer is added on top of the poly-GMA. Tests of performance, fouling with humic acid, and surface characterization of modified membranes are underway and will be reported