A novel Solution-Diffusion-Flory-Rehner model to predict flux behavior during forward osmosis with thermo-responsive hydrogel as draw agent

Abstract

Although hydrogels as excellent draw agents have been extensively investigated, the relationship between the driving force of hydrogel and mass transfer behavior during forward osmosis (FO) remains unclear. The hydrogel-driven FO requires a mass transfer model to clarify its driving mechanism, yet no model was available at present. In the current study, a water flux model driven by thermal-responsive hydrogel was first developed by coupling solution-diffusion model and Flory-Rehner model (SDFR). Considering the diffusion characteristics of hydrogel, a redefined parameter DH corresponding to water diffusion coefficient in hydrogel matrix was introduced to the SDFR model. The validity of the model and DH were verified by comparing experimental water flux data for different concentrations of feed solution with the corresponding simulating results from SDFR model. The SDFR model successfully predicted the flux behavior of FO under wide varieties of operating conditions. Furthermore, the existence of internal concentration polarization phenomenon in hydrogel-driven FO was unveiled by modeling the pressure distribution across FO membrane, and its effect on flux behavior was quantified. The SDFR model was also employed to design a novel ionic thermal-responsive hydrogel with better FO performance. The new insights gained from the SDFR model in this study will help to develop more efficient FO applications with hydrogel-based draw agents.