Enhanced adsorption of phosphate on orange peel-based biochar activated by Ca/Zn composite: Adsorption efficiency and mechanisms

Biochar (BC) is a good adsorbent with high porosity, high specific surface area, and abundant functional groups. However, it exhibits poor phosphate (P) adsorption due to its negative surface charge. Metal (hydr)oxide modification is an effective approach to promote P sorption on biochar. Hence, Ca/Zn composite (CaCO3/ZnO) loaded biochar ([email protected]+Zn) derived from orange peel was prepared through precipitation of Zn2+/Ca2+ on the surface of orange peel followed by carbonization at 700 °C. SEM-EDS, FTIR, XRD, BET, and XPS were used to characterize the biochar before and after modification. The results demonstrated that CaCO3/ZnO were uniformly loaded on the surface of biochar. More reactive sites and oxygen-containing groups were observed on biochar after decoration, which significantly enhanced P sorption. An optimal adsorption capacity was achieved with a metal loading of 3 mM/g. [email protected]+Zn exhibited the best P adsorption efficiency compared to [email protected] and [email protected] The maximum adsorption capacity of [email protected]+Zn was 52.96 mg/g, which was 8-fold higher compared with raw BC. The adsorption kinetics and isotherm were fitted better with the pseudo-second-order and Langmuir isotherm models, respectively. The effect of coexisting anions (Cl-, NO3-, SO42-) on P sorption by [email protected]+Zn was negligible, whereas humic acid significantly inhibited P adsorption. Phosphate was adsorbed by carbonyl, hydroxyl, and other oxygen-containing functional groups on the surface of [email protected]+Zn. This study suggests that Ca/Zn modified biochar can effectively improve the P adsorption capacity for potential applications.