Oxalic acid-crosslinked biochar–chitosan composite: a novel antibacterial material

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Abstract The rise of antibiotic-resistant pathogens highlights the need for safe, sustainable antimicrobial materials capable of inhibiting clinically relevant bacteria such as E. coli and S. aureus . This study reports the synthesis and comprehensive characterization of a biochar–chitosan composite crosslinked with oxalic acid (BC–OA–CH). X-ray diffraction revealed crystalline Sylvite (KCl) and Quartz (SiO₂) phases in biochar and a semi-crystalline polymeric profile for chitosan, while the composite exhibited newly formed OA dihydrate crystals, confirming successful crosslinking and structural reorganization. Fourier-transform infrared spectroscopy indicated strong intermolecular interactions driven by ionic complexation between oxalate groups and protonated chitosan amines, supported by extensive hydrogen bonding. Backscattered electron imaging further showed uniform biochar dispersion and the presence of distinct oxalate-rich crystalline inclusions within the composite matrix. Antibacterial evaluation using the Kirby–Bauer method demonstrated a synergistic enhancement of activity, with the BC–OA–CH composite producing inhibition zones of 21.12 ± 0.08 mm ( E. coli ) and 22.33 ± 0.06 mm ( S. aureus ), surpassing the individual effects of biochar, chitosan, and the BC–CH blend. The enhanced performance is attributed to combined mechanisms including localized acidification, chitosan-mediated membrane disruption, and improved bacterial contact facilitated by the biochar scaffold. These findings identify BC–OA–CH as a promising, low-toxicity antimicrobial material suitable for applications such as coatings, packaging, and water-treatment systems.

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