Detection of lead and cadmium in soil using composite nanomaterials modified screen-printed electrode

Lead Pb (Ⅱ) and cadmium Cd (Ⅱ) are toxic heavy metals, particularly difficult to be biodegraded in soil. Furthermore, lead and cadmium at trace levels can cause serious damage to brains, kidneys, blood, nerves, and other organs. A large amount of Pb (Ⅱ) and Cd (Ⅱ) can also be deposited in the soil environment after anthropogenic improper activities, such as sewage irrigation, the abuse of chemical fertilizers and pesticides, as well as the excessive discharge of industrial wastes. Pb (Ⅱ) and Cd (Ⅱ) can be absorbed by crops, and then accumulated in animals, finally enriched thousands of times into the human body under the biomagnification of the food chain, thereby causing economic losses and a great threat to human health. Therefore, it is highly urgent to accurately detect the accumulation of Pb (Ⅱ) and Cd (Ⅱ) in soil. An electrochemical Square Wave Anodic Stripping Voltammetry (SWASV) is widely utilized to combine with chemically modified working electrodes for the detection of heavy metal ions. Screen-Printed Electrodes (SPEs) have also been commonly used in recent years, due to easy preparation, disposable capability, and low cost. Particularly, SPEs with a small size are quite qualified as the sensing device of miniature electrochemical detection equipment, suitable for many detection scenarios, such as flow cells and microfluidics. In this study, electrochemical reduction and coating were applied to fabricate a modified SPE (Bi/Nafion/rGO/IL/SPE) with the bismuth film/Nafion/reduced graphene oxide/ionic liquid composite nanomaterials, to accurately, fast, and reliably detect trace Pb (Ⅱ) and Cd (Ⅱ) in soil with a low-cost. Cyclic Voltammetry (CV) was also selected to characterize the electron transport capability of the modified electrodes. It was found that the composite nanomaterials greatly improved the electron transport capability of SPE and the stripping voltammetry responses for Pb (Ⅱ) and Cd (Ⅱ) on the SPE. Moreover, Scanning Electron Microscopy (SEM) was utilized to characterize the morphology of the modified electrodes. It was found that the rGO greatly enhanced the specific surface area of bare SPEs, thereby obtaining much more active sites for the electro-deposition of heavy metal ions. Energy Dispersive Spectroscopy (EDS) was used to identify the deposition amount of heavy metal ions on the surface of different modified electrodes. The results demonstrated that the modification with the bismuth film, rGO, and Nafion gradually increased the deposition amount of Pb (Ⅱ) and Cd(Ⅱ) on the modified SPE surface. Additionally, the Pb(Ⅱ) and Cd(Ⅱ) standard solutions were selected to optimize the experimental parameters, including the pH value of support electrolyte, bismuth ions concentration, deposition potential, and potential time. There were linear responses of Bi/Nafion/rGO/IL/SPE to Pb(Ⅱ) and Cd(Ⅱ) in the concentration from 1 to 80 μg/L, with the critical values of 0.124 μg/L for Pb(Ⅱ) and 0.232 μg/L for Cd(Ⅱ) (S/N=3), under an optimal experimental condition. The determination coefficients (R2) of linear correction models were 0.993 and 0.985 for Pb(Ⅱ) and Cd(Ⅱ), respectively. In six SWASV measurements, the relative standard deviation (RSD) of Pb(Ⅱ) and Cd(Ⅱ) peak currents were 1.57% and 2.32%, respectively, indicating high stability and repeatability of Bi/Nafion/rGO/IL/SPE. Other heavy metal ions were also added to investigate the anti-interference performance of modified electrodes. It was found that there was no serious interference of other heavy metal ions (except for Cu(Ⅱ)) on the voltammetry responses of Pb(Ⅱ) and Cd(Ⅱ), where the changes of peak currents were all less than 5%. Since Cu(Ⅱ) inhibited the Pb(Ⅱ) and Cd(Ⅱ) peak currents by 40.02% and 62.85%, respectively, the Cu2+ interference could be alleviated by adding ferricyanide. Finally, actual soil samples under the standard addition were used to verify the practicability of modified SPEs. Results demonstrated that the average recovery rates of Pb(Ⅱ) and Cd(Ⅱ) were 98.71% and 98.93%, respectively, indicating that the Bi/Nafion/rGO/IL/SPE can be applied to detect the trace lead and cadmium in soil.