Enhanced aqueous phosphate removal using chitosan-modified zirconium-loaded cork biochar

The adsorption of phosphate (PO₄-P) is essential for controlling and reducing eutrophication. This study reports the synthesis of a new adsorbent material: Zr(IV)-loaded chitosan-modified used cork stopper biochar composite (CS-CBC-Zr) beads. The suitability of CS-CBC-Zr beads for PO₄-P removal was assessed using the batch method. The effects of various parameters were investigated, including zirconium loading level, adsorbent dosage, pH, co-anions, contact time, and the initial concentration of PO₄-P. The experimental data were thoroughly analyzed using adsorption kinetic and isotherm models. Virgin beads and P-adsorbed beads were characterized using FTIR, SEM, and XPS analyses. The results demonstrated that the combination of Zr coating and CS-CBC beads exhibited superior adsorption performance compared to individual CS-CBC beads. The CS-CBC-Zr beads exhibited a maximum adsorption capacity of 33.89 mg/g, as predicted using the Langmuir-Freundlich (Sips) model. The CS-CBC-Zr beads removed PO₄-P with an efficiency of 95% at an initial pollutant concentration of 50 mg/L and reached adsorption equilibrium within 120 min of contact time, outperforming some comparable adsorbents. Moreover, the beads achieved excellent PO₄-P removal performance over a wide pH range of 4-10, making them highly versatile. The experiment on the effect of coexisting anions demonstrated the excellent selectivity of CS-CBC-Zr for PO₄-P. Phosphate adsorption on CS-CBC-Zr fitted well with the pseudo-second-order kinetic and Sips models. Kinetic data closely fitted the pseudo-second-order model, suggesting that adsorption was primarily governed by chemisorption.