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

## TL;DR

This paper explores using Fe–Mn biochar composites made from sugarcane bagasse to remove herbicides from water, showing effectiveness but also highlighting the need for safety checks.

## Contribution

The study introduces Fe–Mn biochar composites synthesized from sugarcane bagasse for sustainable herbicide removal and evaluates their adsorption mechanisms and safety.

## Key findings

- IME composites showed consistent herbicide removal across pH levels, while COP was pH-dependent.
- 2,4-D removal mainly involved Fe3+-carboxylate complexation, while picloram adsorption relied on weaker interactions.
- Metal leaching from IME exceeded Mn safety limits, and reuse performance declined significantly after three cycles.

## Abstract

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 Cucumis sativus 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.