Coatings for protecting cellulosic ultrafiltration membranes from degradation during humic surface water purification

Abstract

Improvement of cellulosic ultrafiltration membrane durability was studied. With no protective modifications, the commercial regenerated cellulose membrane showed notable signs of degradation when in contact with lake water for several days. The most evident signs of degradation were increasing permeate fluxes and decreasing model substance retentions. Differing degrees of protection were achieved through TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) −mediated oxidation of the membranes and charge-adhered coatings. Mere TEMPO-oxidation and consecutive coating with either poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDADMAC/PSS) or microfibrillar cellulose slightly improved the durability, whereas the poly(vinyl amine)/poly(acrylic acid) (PVAm/PAA) coating system made the membrane stable enough that no signs of degradation were observed during the testing period. The performed modifications did not alter the substrate membrane filtration properties significantly. The most notable changes were attributed to the [PVAm/PAA]₁ coating that reduced both pure water permeance and molecular weight cut-off value by approximately 10%. It is suggested that the [PVAm/PAA]₁ coating was sufficiently tight to block the cellulose-degrading substances that were present. Nevertheless, the structure was loose enough to sustain the desired properties of the substrate membrane. In concentrating filtrations, said coating mitigated flux decrease and the coated membrane ultimately had better filtration capacity than the non-coated alternatives.