Selective ion migration in seawater freeze desalination under varying freezing conditions

• Higher temperatures improve ion exclusion and desalination efficiency. • Decreasing temperature shifts salinity–crystallinity from exponential to logarithmic. • Low temperatures reduce selective ion rejection capability. • Na⁺ shows higher rejection rate than Cl⁻. • Na⁺ rejection is higher in synthetic brine than NaCl. Salt entrapment remains a major challenge in freeze desalination, and understanding ion migration during freezing is essential for improving salt rejection. However, this phenomenon has not been sufficiently studied. In this work, bottom-to-top directional freezing of synthetic seawater was conducted at three temperatures (−15 °C, −20 °C, and −35 °C) to investigate ion trapping in ice. Ion concentrations were measured in ice layers corresponding to crystallinity levels from 0% to 100%. Results show that higher freezing temperatures (−15 °C) with slower cooling rates enhance desalination efficiency by promoting ion exclusion and reducing salt incorporation into ice. The salinity–crystallinity relationship follows an exponential trend, indicating efficient separation during early ice formation. In contrast, lower freezing temperatures (−35 °C) lead to rapid crystallization and uniform ion trapping, producing a logarithmic trend in salinity–crystallinity and reducing overall salt rejection. At −15 °C, Na⁺ exhibited significantly higher rejection than Cl⁻, but this difference diminishes at lower temperatures due to limited ion mobility and insufficient diffusion time. Ion-specific rejection efficiencies at −15 °C follow the sequence Cl⁻ < Na⁺ < SO₄²⁻ < K⁺ < Ca²⁺ < Mg²⁺, with Mg²⁺ showing the highest exclusion due to its strong hydration. Complementary molecular dynamics (MD) simulations quantified ion-specific hydration energies and revealed pronounced Na⁺–SO₄²⁻ ion pairing in multicomponent brine, explaining the enhanced Na⁺ exclusion observed experimentally. This work provides new insights into temperature-dependent ion behavior and highlights the role of hydration energetics in freeze desalination. The findings support the use of dynamic temperature control and molecular-level understanding to improve desalination efficiency and ice purity in sustainable water treatment systems.