Converting softening nanofiltration brine into high-solubility liquid salts (HSLS) via electrodialysis metathesis: Effect of membrane type

Abstract

A fundamental aspect of developing electrodialysis-metathesis (EDM) process involves the selection of an appropriate membrane for converting softening nanofiltration (SNF) brine into high-solubility liquid salts (HSLS). Three types of ion-exchange membranes (heterogeneous, Ss; semi-homogeneous, Lh; and homogeneous, Yl, Tb, Aj) were investigated herein by assessing the overall performances (running resistance, salt removal ratio, energy consumption, etc.) and the transport of ions and water during the EDM process. The results showed that increasing the fixed ion density promoted counter-ions migration, while decreasing the permselectivity or increasing the water uptake or ion-exchange capacity enhanced the co-ion flux. The membrane thickness constitutes an efficient barrier for water osmosis, which could lead to more concentrated HSLS. As a result, the investigated membranes could be ranked using the flux of counter-ions and water as indices: Aj > Tb > Yl ≈ Lh > Ss. The Lh membrane exhibited similar energy consumption characteristics to the Yl and Tb membranes. Regardless of the membrane type, the Cl-type HSLS obtained had higher purity, while the Na-type HSLS had higher concentration. Overall, the membranes with higher fixed ion density and permselectivity, and lower resistance and water uptake, were best suited for the zero liquid discharge desalination of typical high-hardness brine using the described EDM process. Finally, considering the performance and cost of the semi-homogeneous Lh membrane, this membrane type is recommended for application in EDM to convert SNF brine into HSLS.