Removal of Anions PO43- and Methyl Orange Using Fe-Modified Biochar Derived from Rice Straw

A novel magnetic biochar (Fe3O4-biochar) using rice straw as the raw material and magnetite (Fe3O4 nanoparticles) as the objective magnetic medium was successfully synthesized under high-temperature and oxygen-free conditions. Several techniques and methodologies (SEM/EDX, FT-IR, N2 adsorption-desorption isotherms, and pHpzc measurements) were used to determine the surface functional groups and physicochemical properties of Fe3O4-biochar, which showed that the Fe3O4-biochar was successfully synthesized and deposited on the surface of the pristine biochar. The surface area of the Fe3O4-biochar was measured as 337.77 m²/g and 0.227 cm³/g pore volume. Then the adsorption behavior of phosphate (PO43-) and methyl orange (MO) from the aqueous solution onto the Fe3O4-biochar was investigated. The influence of variables including pH, initial concentration of PO43-/MO, adsorbent dosage, and contact time was studied in detail. The optimal adsorption amount of PO43- (189.2 mg/g) was obtained with 0.1 Fe3O4-biochar g/L, at pH of 2 for 240 min; whereas the optimal adsorption amount of MO (37.31 mg/g) was obtained with 0.03 Fe3O4-biochar g/L, at pH of 2 for 240 min. The equilibrium data were fitted to both Langmuir and Freundlich isotherms (R² 0.92 for PO43-, R²  0.96 for MO). Besides, the pseudo-second-order exhibited a better fit for the kinetic studies (R² 0.79 for PO43-, R² 0.88 for MO). This study showed that Fe3O4-biochar could be utilized as an efficient, magnetically separable adsorbent for the removal of anions PO43 and MO from the aqueous mediums.