LAUSR

Abstract Membrane fouling in forward osmosis (FO) process is more complicated than pressure-driven membrane process due to the phenomenon of reverse salt diffusion. Gel layer fouling due to reverse salt diffusion of three draw solutions (NaCl, MgCl2 and CaCl2) were investigated from the thermodynamic view in FO process. The reverse-diffused Mg2+/Ca2+ caused more serious gel fouling and flux decline than Na+ at permeate volume of 500 mL. Non-invasive inverted phase-contrast microscope observed that reverse-diffused Mg2+/Ca2+ induced the formation and cross-linking of long sodium alginate chains. Thermogravimetric analysis (TGA) revealed that Mg2+/Ca2+ enhanced the binding ability of bound water to gel chains. From the thermodynamic view, the above changes caused by Mg2+/Ca2+ resulted in that a higher driving force (osmosis pressure difference) was consumed to overcome the chemical potential difference between gel and permeate during FO process, which gave rise to the less driving force and the worse performance. This study provided a new perspective and insight to explain gel fouling in FO process.