Salinity-dependent ion selectivity in monovalent-selective electrodialysis

Monovalent-selective electrodialysis (MSED) is increasingly applied for targeted ion removal, yet the interplay between solution composition, salinity, and operating configuration is not fully understood at the stack level. This study systematically evaluates how NaCl–MgSO 4 mixtures spanning brackish to seawater salinities (0.02–0.5 M NaCl) influence monovalent selectivity, transport behavior, and energy consumption in a commercial CMS/ACS electrodialysis stack. Additional experiments varied water recovery (50–95%) to assess the robustness of selectivity under practical desalination conditions. Divalent interference was found to be strongest at low salinity, where high Mg 2+ and SO 4 2− activities and limited concentration polarization allow these ions to participate more readily in charge transport. As salinity increased, suppressed divalent activities sharply reduced their transport, enabling the selective layers to express a much stronger intrinsic preference for monovalent ions. Equal-charge experiments confirmed that membrane structure, rather than feed imbalance, governs monovalent preference. Higher MgSO 4 concentrations cause energy consumption to increase, because the membranes strongly rejected Mg 2+ and SO 4 2− which then accumulate near the membrane surfaces, intensifying concentration polarization leading to increasing transport resistance. Water recovery up to 95% did not compromise selectivity, and concentrate profiles remained linear across recoveries, indicating minimal back-diffusion or water transport. These findings clarify how composition, salinity, and recovery jointly control MSED performance and provide guidance for optimizing selective desalination and brine-management strategies. • High divalent activity at low salinities results in strong divalent interference. • Increased salinity sharply suppresses divalent activity, reducing transport. • Membrane structure governs monovalent preference, not feed imbalance. • MgSO 4 leads to a systematically earlier rise in voltage, especially at 0.1 M NaCl. • Water recovery up to 95% did not compromise membrane selectivity.