How in situ fabricated polyzwitterionic hydrogel coating on nanofiltration and ultrafiltration membranes influences mass transfer processes and fouling development

To satisfy the demand of antifouling properties and efficient pollutant removal for membrane technologies during water treatment process, filtration-driven polyzwitterionic hydrogel coating was conducted on nanofiltration and ultrafiltration membranes in this study. Based on the concentration polarization-enhanced in situ "click" reaction between polyzwitterion with reactive methacrylate side groups and a cross linker with bis-thiol structure, the one-step method for modifying nanofiltration membranes was successfully established after the systematic optimization of coating process parameters (coating pressure) and coating solution compositions (polyzwitterion and cationic surface linker concentration). For cellulose ultrafiltration membranes, surface pre-functionalization to yield carboxylic acid groups that can bind to the cationic surface linker was necessary. The coating increased size-based solute rejection, and the stoichiometric ratio between thiol cross linker and methacrylate groups in the polyzwitterion had an influence onto the molecular weight cut-off of hydrogel coated ultrafiltration membranes that can be related to efficiency of the "click" network formation. On nanofiltration membranes, the coating could notably facilitate salt flux and suppress water flux, by promoting cake-enhanced concentration polarization. Hydrogel-coated membranes showed excellent anti-protein-fouling properties, but aggravated inorganic and humic acid fouling, because of attractive interactions between zwitterionic coating and pollutants with charge and/or smaller size.