Influence of composition and concentration of saline water on cation exchange behavior in forward osmosis desalination

Abstract

Thin film composite (TFC) membranes are the gold standard for the application of forward osmosis (FO) in seawater desalination and wastewater treatment. However, bidirectional mass transport phenomena in this process still hinder its wider application. A deeper understanding of ion transport behavior in various solution conditions is critical for the fabrication of FO membrane and future development of FO process. In this work, we systematically investigated the influences of solution chemistry on ion mass transport in TFC FO membranes. Our results suggested that the mass transports of cations were much larger than those of anions in FO processes, and monovalent cations with smaller hydrated radius could induce greater cation exchange than divalent cations. Though anions did not participate in the cation exchange processes directly, cations paired with nitrate could transport through the TFC membrane more readily than those paired with chloride or sulfate. Additionally, a set of experiments with series solution concentrations confirmed the existence of cation transport bottleneck for one TFC FO membrane. We found that the cation transport capacity of TFC membranes was mainly determined by the concentration of feed solution and the generation of cation exchange behavior was independent of water transport. Engineering implications of our findings for FO application were also discussed.

Keywords: Cation exchange; Forward osmosis; Solute species; Solution concentration; Thin-film composite membrane.