Voltammetric determination of lead and cadmium in vegetables by graphene paste electrode modified with activated carbon from coconut husk

A multilayer graphene paste electrode (MGPE), consisting of graphene flakes and mineral oil, was modified with activated carbon (AC). The AC was produced from coconut husk using zinc chloride as the activating agent. The AC/MGPE was fabricated by substituting an amount of the multilayer graphene (MG) with the AC, and the resulting paste was packed in a Teflon syringe. The electrode was used to simultaneously detect lead (Pb²⁺) and cadmium (Cd²⁺) via anodic stripping voltammetry (ASV). The amounts of MG and AC in the electrodes were varied by substituting the amount of the former with the latter (by percent) and the best electrode was found to consist of 5% AC and 95% MG. The optimal parameters, accumulation time, deposition time, and deposition potential were determined. The optimized parameters were used on the said best electrode to determine the calibration curve and limit of detection. The peak current had a linear relation with the concentration of the heavy metals with a correlation value (R²) of 0.9956 and 0.9973 for Cd²⁺ and Pb²⁺, respectively. After which the electrode was used to detect heavy metals in vegetable samples, and the results were verified via atomic absorption spectroscopy. The heavy metal concentrations that were detected in most of the samples were above the maximum limit as dictated by the World Health Organization. (paper)