Highly efficient chitosan–Cu₂O nanocomposite thin films for dual antibacterial activity and heavy metal removal via green synthesis

Water contamination by heavy metals, such as lead and manganese, remains a serious global concern, particularly in regions with insufficient industrial regulation and limited wastewater-treatment infrastructure, in parallel, the rapid emergence of antibiotic-resistant bacteria has intensified the demand for alternative antimicrobial materials capable of preventing microbial growth on surfaces, packaging, and water-treatment systems. Composite materials that combine metal oxides with natural polymers have attracted considerable interest because they can simultaneously address pollutant removal and microbial inhibition. This study reports a sustainable approach for synthesizing chitosan–waste-derived cuprous oxide (Chi–Cu₂O) nanocomposite thin films using a bio-inspired chemical reduction process at room temperature. Copper recovered from spent lithium-ion batteries (SLIBs) was leached with sulfuric acid to produce Cu (II) sulfate, which was then in situ reduced to Cu(I) using ascorbic acid, an environmentally friendly reducing and protecting agent. The resulting Cu(I) ions coordinated with the functional groups of chitosan, enabling uniform Cu₂O nanoparticle formation within the polymer matrix without toxic reagents or high-temperature treatments. The synthesized nanocomposites were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), and zeta-potential analysis. The characterization results confirmed the successful formation of crystalline Cu₂O nanoparticles, their homogeneous distribution within the chitosan matrix, and the strong interaction between Cu₂O and chitosan functional groups responsible for enhanced stability. Chitosan concentration strongly influenced both nanoparticle yield and functional performance. Cu₂O alone achieved only 8.35% removal of Mn²⁺, while composites containing 0.75–1 wt% chitosan reached 97.57–98.06%, demonstrating enhanced stabilization and dispersion. The 1% Cu–chitosan composite also showed the highest heavy-metal adsorption, removing 60.98% of Pb²⁺, compared to 9.15% for Cu₂O alone, highlighting the synergistic effect between chitosan functional groups and Cu₂O nanoparticles. Antibacterial activity increased with chitosan content, with the 0.75–1% composites producing inhibition zones of 10–10.8 mm ( E. coli) , 10.3–10.8 mm ( P. aeruginosa ), 10–10.5 mm ( S. aureus ), and 19–19.5 mm ( B. pumilus ), corresponding to ∼52–95% of the activity of Gentamicin, while B. cereus remained resistant. Overall, the 1% Chi–Cu₂O nanocomposite exhibits the optimal combination of high nanoparticle yield, effective heavy-metal removal, and broad-spectrum antibacterial activity. Its green, low-cost synthesis from recycled copper, along with room-temperature processing, makes it a promising candidate for further investigation toward water purification and antimicrobial coating applications.