Peanut shell biochar for plastic electrodes: Green E-sensors for sensitive heavy metal detection

• Upcycling of peanut shells for green biochar production. • Biochar porosity: a large surface area for electroanalytical sensing. • Chitosan and NaOH to tune the surface features and the electrode functionality. • Biochar-chitosan electrode enable in situ Cd and Pb detection by simple treatment. • Bismuth electrodeposition to improve the detection of heavy metals in sea water. The upcycling of agricultural waste into high-value functional materials is a key aspect of sustainable material development and the circular bioeconomy. This study investigates the fabrication and characterization of biochar-based electrodes (E-sensors) derived from peanut shells, an abundant agro-industrial by-product with emerging potential for valorization. To enhance electrochemical performance, biochar (Bc) was modified with chitosan CS under to alkaline treatment, improving hydrophilicity, porosity, and charge transfer properties. Structural and surface analyses, including FTIR, Raman spectroscopy, SEM, and XPS, confirmed the successful introduction of functional groups while preserving the hierarchical pore structure of Bc. Electrochemical evaluation, using cyclic voltammetry and square-wave stripping anodic voltammetry, revealed satisfactory results for Pb²⁺and Cd²⁺detection in buffer and real sea water samples. Additionally, to improve the analytical performance bismuth was successfully electrodeposited at the unconventional plastic electrodes surface. This contributed to enhance the Pb and Cd limits of detection in sea water (14.27 ng/mL, RSD 10% and 20.74 ng/mL, RSD 7% respectively), with results well below WHO and EPA regulatory thresholds. These findings demonstrate the potential of biochar-based electrodes as sustainable, cost-effective alternatives for heavy metal detection, underscoring the role of Bc in advancing green sensor technologies and environmental monitoring.