UCLA

Surface tethering of hydrophilic polymer brush layers is a popular approach for membrane surface modification with the target of overcoming and mitigating various challenges such as perm-selectivity tradeoff, removal of specific contaminants, and membrane fouling and mineral scaling. In the present study, a systematic investigation of membrane surface structuring with tethered polyacrylic acid (PAA) layers was conducted to tune both reverse osmosis (RO) and ultrafiltration (UF) membranes performance in terms of water permeability, solute rejection, molecular weight cutoff (MWCO), fouling resistance, scaling propensity and cleaning efficacy. Surface nano-structured (SNS) PAA brush layers were synthesized onto the base polysulfone (PSf) UF and polyamide (PA) thin-film composite (TFC) RO membranes via membrane surface activation with different atmospheric pressure plasma (APP) types (i.e., Air, He/O2, and He), followed by graft polymerization (GP) of acrylic acid (AA). Effective tuning of SNS-PAA-PSf UF membrane performance in terms of hydraulic permeability and molecular weight cutoff (MWCO) was feasible by adjustments of the APP and graft polymerization conditions. It was shown, for the first time, that SNS-PAA-PSf membranes can be synthesized with a range of hydraulic permeability (spanning a factor of 1.1-2.6 in magnitude) for a given MWCO, or a range of MWCO (spanning a factor of 1.5-2.3 in magnitude) for a given hydraulic permeability, thereby overcoming the hydraulic permeability-MWCO tradeoff. The SNS-PAA-PSf membrane characteristics (surface hydrophilicity, intrinsic membrane resistance, and PEG MWCO) were responsive to pH and ionic strength due to the conformational change (i.e., swelling/collapse) of the surface tethered PAA chains. Within the tested range of pH (3-11) and ionic strength (0-547 mM), the SNS-PAA-PSf membrane demonstrated self-regulated membrane performance (i.e., Rm 0.74 - 2.29�1013 m-1, and MWCO 1.8 - 15.0 kDa) and surface hydrophilicity (i.e., surface energy -114.5 to -139.2 mJ/m2). UF fouling stress tests with bovine serum album (BSA) and alginic acid in high salinity water and post-cleaning with D.I. water demonstrated reduced flux decline (by ~11.3%) and improved permeability recovery (by ~34%) for the SNS-PAA-PSf membrane relative to the native PSf membrane. The surface tethered PAA chains also improved polyamide (PA) RO membrane removal of nitrate, boron, As (III), and As (V), with rejection of 98.0%, 90.7%, 96%, and 99.6%, respectively, relative to 76.8-84.9%, 87.3-92.1%, and 94.5-97.2% for the tested commercial RO membranes. The increased membrane removal of the specific contaminants is attributed to the surface tethered PAA layer sealing of microscopic defects in the polyamide membrane active layer. The SNS-PAA-PA membrane also exhibited lower flux decline for both gypsum and calcium carbonate scaling tests compared to the tested commercial RO membranes and 100% and 94% permeability recovery post D.I. water flushing, respectively. Scale up of the membrane surface nano-structuring approach, atmospheric pressure plasma-induced graft polymerization (APPIGP), was developed for SNS-PAA-PA membrane sheets of size sufficient for fabrication of 2.5 inch � 21 inch spiral-wound RO elements. Laboratory testing of 18 membrane coupons (~2” x 4”) extracted from different locations of the SNS-PAA-PA membrane sheet, in terms of water and salt permeability coefficients and intrinsic membrane rejection, demonstrated the similar or higher performance uniformity level compared to Base-PA. SNS-PAA-PA spiral wound elements, fabricated with the above SNS-PAA-PA membrane sheets, outperformed the commercial Dow SW30 element exhibiting lower flux decline and 100% permeability recovery in fouling tests of both BSA and sodium alginate model foulant solutions. Results of the present study suggest that the APPIGP approach can be scaled up to fabricate commercial scale spiral-wound RO elements of superior antifouling properties.