Sustainable valorization of sugarcane bagasse into biochar-metal oxides and lignin for herbicide removal

Abstract

The generation of agro-industrial residues and water contamination by pesticides represent environmental challenges that require sustainable and innovative strategies. In this context, sugarcane bagasse was explored through two approaches: (i) pyrolysis for the production of biochar-metal oxide composites based on Mn and Fe, and (ii) fractionation using deep eutectic solvents (DES), both aiming at waste valorization and the development of new adsorbent materials. The produced materials were characterized using different techniques and evaluated for the removal of synthetic herbicides from aqueous media, including 2,4- dichlorophenoxyacetic acid (2,4-D), 4-chlorophenoxyacetic acid (4-CPA), and picloram. Mnbased biochar–metal oxide composites were obtained from bagasse modified with MnCl₂ and KMnO₄ via immersion, followed by pyrolysis at 400 °C, assessing the effect of different Mn precursors on composite formation and their role in 2,4-D adsorption. Fe-based biochar–metal oxide composites, using FeCl₃ as the metal precursor and produced at different pyrolysis temperatures (400 and 600 °C), were applied to the removal of 2,4-D and 4-CPA, evaluating the influence of the adsorbate molecular structure on the removal process. Bimetallic Fe/Mn composites were prepared through pre-pyrolytic modification by immersion and coprecipitation and applied to the removal of 2,4-D and picloram. Fractions obtained from the optimized delignification process of sugarcane bagasse were also applied to 2,4-D removal. Experimental assays investigated the effects of pH and ionic strength, adsorption kinetics and isotherms, as well as reuse potential. Theoretical studies based on Density Functional Theory (DFT) contributed to understanding the interaction mechanisms between herbicides and the monometallic composites. For the bimetallic materials, leaching tests evaluated metal behavior and distribution under adsorption conditions. Adsorption mechanisms were elucidated by XPS analysis, and phytotoxicity tests using seeds assessed the environmental safety of the materials. The results demonstrated that modulation of the physicochemical properties of the composites determines herbicide removal efficiency, material stability, and environmental safety, with the type of metal precursor, modification route, and pyrolysis temperature being key variables. Additionally, the lignin fractions obtained showed potential for 2,4-D removal. The proposed strategies for producing innovative remediation materials enabled the valorization of agroindustrial residues, contributing to the protection of water resources and, consequently, to the preservation of terrestrial life.