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

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 Fe₃O₄ 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: HCO₃⁻ > SO₄²⁻ > Cl⁻≈NO₃⁻. 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.