Unraveling effects of multivalent salts on internal fouling by proteins in NF-like forward osmosis

Multivalent salts are preferentially used as the draw solute for forward osmosis (FO) with a nanofiltration (NF)-like membrane, which promises advantages in the treatment of protein-containing feeds. When assessing the phenomenon of back diffusion, particular concerns are raised about the complex interplay between the protein foulants and the polymeric network of the substrate in the presence of various multivalent salts. This study employed a composite membrane (fabricated by the Layer-by-Layer assembly of polyelectrolytes on a hydrolyzed polyacrylonitrile substrate) to filter a feed solution containing proteins having a distinct isoelectric point (i.e., bovine serum albumin and lysozyme), while varying the cationic valence and the acidity/basicity of the draw solution (using sodium sulfate, magnesium sulfate, ammonium sulfate, and sodium citrate). Individual effects on the proteins and the substrate were examined by ex-situ characterization based on model-predicted concentrations of the multivalent salts back-diffused into the porous network. Optical coherence tomography was exploited to noninvasively detect the spatiotemporal evolution of protein-polymer interactions, which was quantified by evaluating the surface-averaged intensity. In terms of the characterization results, the protein-polymer interactions were varied by the back-diffusion of neutral and acidic salts such that the internal fouling by proteins could evolve in a bottom-up or top-down way; in contrast, the use of basic salt would mitigate the internal fouling probably owing to the stronger electrostatic repulsion. This study provides a clearer picture of how the substrate network could be fouled by proteins, thereby shedding light on the development of anti-fouling strategies for NF-like FO.