Comparative evaluation of free and immobilized Scenedesmus obliquus cells in desalination of brackish water.

Sustainable desalination has increased interest in microalgal biotreatment, yet immobilized microalgae performance under high salinity remains poorly characterized. This study compared free-cell suspensions and agar-immobilized Scenedesmus obliquus for NaCl removal from synthetic brackish water (10-30 g/L NaCl) over 12 days. Free-cell treatments achieved 0.66-2.92 g/L removal (3.61-11.72%). In contrast, the algae-agar bead system showed superior performance and temporal consistency, at 30 g/L NaCl, the 1:5 bead-to-solution ratio achieved the highest TDS-based removal of 4.51 g/L (18.36%), 1.54-fold higher than the 15-mL free-cell treatment (2.92 g/L). Blank agar beads showed no removal, confirming agar as an inert scaffold. While TDS trends were consistent, the 15-mL free-cell treatment achieved a slightly higher EC reduction (11.79%) than the 1:5 beads (11.00%) at 30 g/L. FTIR and SEM/EDX analyses revealed marked surface damage and salt encrustation in free cells, with Na signals increasing by 5.30% points and C signals decreasing by 6.25% points. These findings support biosorption and surface-associated salt deposition rather than intracellular sequestration. The apparent adsorption isotherms models indicated heterogeneous uptake in free cells (Freundlich, R² ≥ 0.999), while immobilized cells (1:5) showed moderate agreement with the Langmuir model (R² = 0.853), tentatively suggesting monolayer adsorption. Overall, immobilization improved TDS removal and consistency, though free cells remained competitive in EC reduction at high salinities. This study highlights the potential of immobilized systems for robust biodesalination in challenging saline environments.