Molecular level insights into the dynamic evolution of forward osmosis fouling via thermodynamic modeling and quantum chemistry calculation: Effect of protein/polysaccharide ratios

Abstract

While forward osmosis (FO) shows broad application prospects in wastewater reuse and desalination, it is still hindered by membrane fouling issues. From the perspective of molecular structure and energy, this work investigated the effects of protein (PN)/polysaccharide (PS) ratios of organic foulants on the dynamic evolution of membrane fouling in FO. Fouling tests showed two interesting membrane fouling phenomena: Ⅰ) when PN content increased from 0.2 to 0.8 g/L with fixed PS content of 0.2 g/L, membrane fouling decreased for filtration of the same volume of foulants; Ⅱ) with the increase of filtration volume, the membrane fouling caused by 1:1 PN/PS foulants increased almost linearly, while the membrane fouling caused by 4:1 PN/PS foulants increased significantly at first, and then rapidly reached a stable fouling state. Thermodynamic analyses and quantum chemical calculations revealed that the attractive interaction energy between 1:1 PN/PS foulants and membrane was 1.90 times greater than that for 4:1 PN/PS foulants, and the protein and polysaccharide molecules with a ratio of 1:1 were more likely to form a tight cross-linked structure. Furthermore, the reverse solute diffusion significantly reduced the electrostatic repulsion energy among the polymer chains for 4:1 PN/PS foulants, which induced the transformation of foulant morphology from gel to floc and reduced the adhesion energy to membrane surface by 61%. As a result, in the later fouling stage, few 4:1 PN/PS foulants were newly attached to the membrane, leading to a pseudo-stable flux. Based on the aforementioned findings, this study reveals a unified thermodynamic framework for the effects of the PN/PS ratios of organic foulants on the dynamic evolution of FO membrane fouling. The proposed mechanism is beneficial in formulating “tailored” fouling mitigation strategies and elucidating the appropriate application fields for FO technology.