Potentials of emergent plant residue derived biochar to be alternative carbon-based phosphorus fertilizer by Fe(II)/Fe(III) magnetic modification

Abstract Emergent plants have been remarkably effective in reducing phosphorus (P) discharge from ecological ditches; however, the treatment and recycling of these residues is a great challenge. In this study, magnetic biochars (MB s , i.e., MB- A , MB- C , and MB- T ) were fabricated from three emergent plant residues ( Acorus calamus L., Canna indica L., and Thalia dealbata Fraser, respectively) and modified with Fe(II)/Fe(III). Scanning electron microscopy-energy dispersive spectroscopy and X-ray diffraction spectra confirmed the successful loading of Fe 3 O 4 and FeO(OH) onto the surfaces of the MB s . Batch adsorption experiments showed that MB s exhibited a higher P adsorption capacity than that of the raw biochars. Within the range of 0.8–43.0 mg L −1 in solution, the adsorption capacities of P by MB- A , MB- C , and MB- T were 304.6–5658.8, 314.9–6845.6, and 292.8–5590.0 mg kg −1 , with adsorption efficiencies of 95.2–32.9%, 98.4–39.8%, and 91.5–32.5%, respectively. The primary mechanisms that caused P to adsorb onto the MB s were inner-sphere complexation and electrostatic attraction. Low pH conditions were more beneficial for the P adsorption of the MB s , while co-existing anions had a negative impact with the following order: HCO 3 − > SO 4 2− > Cl − ≈NO 3 − . The P-31 nuclear magnetic resonance results further demonstrated that the main adsorbed P species on the MB s was orthophosphate, followed by orthophosphate monoesters and DNA. Overall, MB s offer a resource utilization strategy for emergent plant residues and P-laden MB s are promising alternative P fertilizers. Graphical Abstract