Extending the applicability of modified electrodialysis metathesis for high-recovery desalination by improving concentrate separation and monovalent ion retention

This study presents a two-step desalination process integrating monovalent-selective electrodialysis (mED) and modified electrodialysis metathesis (mEDM) to achieve high-recovery, chemical-free desalination, even in waters with high scaling potential. In the first step, mED extracts NaCl from the feedwater. The extracted NaCl is then used in the mEDM step to recombine the Na + with multivalent anions (SO₄ 2− ) from the feed water into a separate concentrate, and the Cl − with multivalent cations from (Ca 2+ , Mg 2+ ) the feed water into another concentrate. This way the feed gets desalted, and scaling of sparingly soluble salts such as CaSO₄ in the concentrates is prevented. Experiments with synthetic NaCl–MgSO₄ solutions showed that this process scheme outperformed conventional electrodialysis (ED), reverse osmosis (RO), and previously tested stand-alone mEDM, with scaling tendencies remaining far below saturation obtained by the other technologies. The system was then validated using brackish groundwater and (saline) oilfield produced water. For groundwater, it achieved 95 % water recovery with no scaling risk and reasonable energy consumption (∼1.32 kWh/m 3 ), potentially reaching 97.5 % recovery via theoretical simulation. For produced water with a 2:1 monovalent/divalent ion ratio—near the stoichiometric limit—the process maintained performance, although NaCl depletion and impurity buildup in the intermediate solution emerged as limiting factors. To support continuous operation, we propose some simple interventions to enable long-term stability and scalability. Overall, this two-step mED–mEDM configuration offers a robust, modular, and fully electrically-driven approach for chemical-free desalination, with a minimum of extra energy consumption. It holds strong potential for integration into existing systems—such as treating RO retentates—and could support high-recovery, low-footprint water treatment applications. • mED-mEDM two-step achieves 95 % water recovery without chemical additives. • Scaling prevention outperforms the stand-alone mEDM (previous work). • mED as pretreatment lowers the monovalent ion requirement of mEDM. • Energy consumption of mED-mEDM is around 1.32 kWh/m 3 at 95 % water recovery, comparable tp the stand-alone mEDM. • mED-mEDM offers a scalable, chemical-free solution for high-recovery desalination.