Novel molecular level insights into forward osmosis membrane fouling affected by reverse diffusion of draw solutions based on thermodynamic mechanisms

Abstract

Forward osmosis membrane bioreactor (FOMBR) has a more intricate membrane fouling mechanism than MBR because of the special existence of the reverse diffusion of draw solution. The mechanisms of membrane fouling affected by reverse diffusion of draw solution and ion content in gel layer were investigated from thermodynamic perspective and molecular level in this study. Phase-contrast microscopy non-invasively observed that the molecular chain structure of the gel layer containing low content of reverse diffused solute (NaCl) was sparse, while the alginate molecular chains with high NaCl content were parallel and compact. Density functional theory (DFT) further simulated the formation mechanism of different alginate chain structures, that is, the reverse diffused solute shortened the distance between hydrogen bonds and reduced the interaction energy at the terminals of alginate chains. This proved that the NaCl trapped in alginate layer directly affected its structure. Heating experiments indicated that gel layer acted as a “chemical potential barrier” that prevented bound water from turning into free water. Accordingly, the “chemical potential barrier” described by Flory-Huggins theory was proposed, which was confirmed to account for the filtration of alginate gels affected by reverse diffusion of draw solution. To the best of our knowledge, this is the first membrane fouling study that explores the effect of reverse diffusion of draw solution on the “chemical potential barrier” of gel layer from perspective of molecular chain structure. This study investigated FOMBR fouling mechanism at molecular level and provided a new strategy for FOMBR system fouling quantification.