Synthesis and optimization of spherical nZVI (20-60 nm) immobilized in bio-apatite-based material for efficient removal of phosphate: Box-Behnken design in a fixed-bed column.

In the present study, bio-apatite/nZVI composite was synthesized through Fe(III) reduction with sodium borohydride and was fully characterized by FTIR, XRD, SEM-EDX, TEM, BET, BJH, and pH. Column experiments were carried out for the removal of phosphate as a function of four operational parameters including initial phosphate concentration (100-200 mg L), initial solution pH (2-9), bed height (2-6 cm), and influent flow rate (2.5-7.5 mL min) using a response surface methodology (RSM) coupled with Box-Behnken design (BBD). 2D contour and 3D surface plots were employed to analyze the interactive effects of the four operating parameters on the column performance (e.g., uptake capacity and saturation time). According to ANOVA analysis, the influent flow rate and bed height are the most important factor on phosphate uptake capacity and saturation time, respectively. A quadratic polynomial model was excellently fitted to experimental data with a high coefficient of determination (> 0.96). The RSM-BBD model predicted maximum phosphate adsorption capacity of 85.71 mg g with the desirability of 0.995 under the optimal conditions of 135.35 mg L, 2, 2 cm, and 7.5 mL min for initial phosphate concentration, initial solution pH, bed height, and influent flow rate, respectively. The XRD analysis demonstrated that the reaction product between bio-apatite/nZVI composite and phosphate anions was Fe (PO). 8HO (vivianite). The suggested adsorbent can be effectively employed up to five fixed-bed adsorption-desorption cycles and was also implemented to adsorb phosphate from real samples.