Mechanistic insights into electro-osmotic and osmotic water transport in electrodialysis for high-concentration sodium sulfate recovery

This study investigates electro-osmotic (EO) and osmotic (OS) water transport in electrodialysis (ED) for the recovery of concentrated sodium sulfate (Na 2 SO 4 ) solutions. EO and OS contributions were quantified by gravimetric monitoring of concentrate mass and conductivity measurements of the diluate under varying concentrations, current densities, and stack potentials. EO transport, driven by hydrated ion migration, was largely independent of current density, yielding a specific water transport of 2.36 g H 2 O/g Na2SO4 (18.6 mol H 2 O/mol Na 2 SO 4 ). From this value, the maximum achievable Na2SO4 concentration for the membrane used in this study was estimated at 29.8 wt% (384 g/L), exceeding its solubility limit (∼281 g/L). In contrast, OS water transport exhibited strong dependence on concentration gradients and operating parameters. Notably, the OS water flux, expressed per unit membrane area and time, decreased at higher current densities despite increasing concentration gradients, suggesting that osmotic transport in ED is influenced not only by thermodynamic driving forces but also by ion migration behavior under an applied electric field. Elevated stack potentials suppressed total water migration and improved concentrate salinity, but increased specific energy consumption. These findings demonstrate that controlling OS water transport is essential for enhancing ED performance and improving the efficiency of Na₂SO₄ recovery. • Electro-osmotic and osmotic water transport were quantified separately in ED. • Electro-osmotic transport remained independent of applied current density. • Osmotic water flux strongly depended on current density and concentration gradient. • Increased stack potential reduced water migration but raised energy consumption. • ED enabled Na 2 SO 4 recovery up to concentrations close to solubility.