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 (BC@Ca+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 uni-formly 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. BC@Ca+Zn exhibited the best P adsorption efficiency compared to BC@Ca and BC@Zn. The maximum adsorption capacity of BC@Ca+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 Lang-muir isotherm models, respectively. The effect of coexisting anions (Cl-, NO3-, SO42-) on P sorption by BC@Ca+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 BC@Ca+Zn. This study suggests that Ca/Zn modified biochar can effectively improve the P adsorption capacity for potential applications.