Highly selective adsorption of phosphate from high-salinity water environment using MgO-loaded and sodium alginate-immobilized bentonite beads

The effective removal of phosphorus (P) has been a recent hot topic due to water eutrophication hazards and zero-liquid discharge (ZLD) regulations. In this study, MgO-loaded bentonite (Bt) was immobilized by sodium alginate (SA), and then freeze-dried to form beads (SA-MgO@Bt). For comparison, SA-immobilized pure Bt was freeze-dried to form another kind of bead (SA-Bt), and SA-immobilized Bt with MgO loading was oven-dried to form a third type of bead (OSA-MgO@Bt). The specific surface area (SSA) of SA-MgO@Bt was measured to be 59.5 m2/g, 2.2 and 1.4 times that of SA-Bt and OSA-MgO@Bt. The total pore volume of SA-MgO@Bt was also verified to be much higher than that of the other two beads. Overall, SA-MgO@Bt exhibited a maximum adsorption capacity of 70.5 mg/g, a high removal efficiency above 91% in a wide pH range from 3 to 10, and a high selectivity for phosphate from high-salinity water, surpassing those of SA-Bt and OSA-MgO@Bt, and related adsorbents reported in literature. For the fixed-bed column experiments with a water flow rate of 1.0 mL/min, the phosphate concentration could still be reduced from 50 mg/L to 0.5 mg/L even after 8 h. Based on the dynamics, isotherms, and microstructural analysis, the ligand exchange and Mg–P complexation formation, in addition to the electrostatic attraction mode, were also proposed to contribute to the adsorption performance of SA-MgO@Bt. Finally, the pot experiments also validated the feasibility of the as-obtained phosphate-adsorbed SA-MgO@Bt beads as a slow-release-fertilizer for the growth of mint, providing new possibilities of P resource utilization.