Application of Box-Behnken Design to Optimize Phosphate Adsorption Conditions from Water onto Novel Adsorbent CS-ZL/ZrO/Fe₃O₄: Characterization, Equilibrium, Isotherm, Kinetic, and Desorption Studies

Phosphate (PO₄^3-) is an essential nutrient in agriculture; however, it is hazardous to the environment if discharged in excess as in wastewater discharge and runoff from agriculture. Moreover, the stability of chitosan under acidic conditions remains a concern. To address these problems, CS-ZL/ZrO/Fe₃O₄ was synthesized using a crosslinking method as a novel adsorbent for the removal of phosphate (PO₄^3-) from water and to increase the stability of chitosan. The response surface methodology (RSM) with a Box-Behnken design (BBD)-based analysis of variance (ANOVA) was implemented. The ANOVA results clearly showed that the adsorption of PO₄^3- onto CS-ZL/ZrO/Fe₃O₄ was significant (p ≤ 0.05), with good mechanical stability. pH, dosage, and time were the three most important factors for the removal of PO₄^3-. Freundlich isotherm and pseudo-second-order kinetic models generated the best equivalents for PO₄^3- adsorption. The presence of coexisting ions for PO₄^3- removal was also studied. The results indicated no significant effect on PO₄^3- removal (p ≤ 0.05). After adsorption, PO₄^3- was easily released by 1 M NaOH, reaching 95.77% and exhibiting a good capability over three cycles. Thus, this concept is effective for increasing the stability of chitosan and is an alternative adsorbent for the removal of PO₄^3- from water.