Fe–Mn Biochar Composites from Sugarcane Bagasse for Herbicides Removal: Structure, Mechanisms, and Safety Relationships toward Sustainable Water Treatment

Fe-Mn biochar composites were synthesized from sugarcane bagasse through prepyrolytic impregnation with FeCl₃ and MnCl₂, using immersion (IME) and coprecipitation (COP) methods, followed by pyrolysis at 600 °C for 2 h. Their characterization revealed distinct differences in surface chemistry and oxide dispersion. Both composites contained mixed Fe₃O₄, Fe₂O₃, MnO, and Mn₃O₄ phases, but IME exhibited a amorphous carbon matrix, while COP displayed greater crystallinity (∼41%). In aqueous adsorption studies, IME maintained nearly constant removal efficiency across pH 2-10, whereas COP was strongly pH-dependent, leading to IME's selection for subsequent studies. Adsorption isotherms of 2,4-dichlorophenoxyacetic acid (2,4-D) and picloram (25 °C; 2 g L^-1) were well fitted by the Sips model, with maximum adsorption capacities of 18.1 and 8.1 mg g^-1, respectively. X-ray photoelectron spectroscopy of IME revealed Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ species and indicated that 2,4-D removal occurred mainly by Fe³⁺-carboxylate complexation, while picloram adsorption involved weaker polar and van der Waals interactions. Reuse tests showed a decline in performance after three cycles (∼97% → 29%), suggesting active-site blockage. Metal leaching from IME at pH 5 was limited (0.025 mg L^-1 for Fe and 2.94 mg L^-1 for Mn). Fe complied with drinking-water limits, whereas Mn exceeded them, highlighting the need for safety evaluation. Phytotoxicity assays using <i>Cucumis sativus</i> confirmed no adverse effects from residual 2,4-D, demonstrating effective detoxification. Overall, Fe-Mn biochar composites present a promising, sustainable approach for herbicide removal, but the environmental safety of treated effluents should be ensured.