The detection of heavy metal ions (HMI) is an essential step in water treatment.
Due to their adverse effect on human health and the environment, various agencies have set guidelines for the concentration of HMI in drinking water. However, conventional HMI detection techniques, such as atomic absorption spectroscopy (AAS) and inductively coupled plasma mass spectroscopy (ICP-MS), are often too expensive and not practical for on-site real-time monitoring. Electrochemical methods, including anodic stripping voltammetry (ASV) and electrochemical impedance spectroscopy (EIS), appear as alternatives for detecting HMI. Among these techniques, EIS possesses great potential in providing more information about water composition with modelling and data analysis. However, past research on the physical interpretation of the impedance response and the differentiation of different types of HMI in a mixed aqueous solution is inadequate. In the first part of the research, the electrochemical cell for HMI detection was constructed with pure platinum electrodes and the impedance response was collected for two types of HMI solutions, ZnSO4 (aq) and Pb(NO3)2(aq), and their mixed solution. The data was collected repetitively in a wide range of settings to understand the influence of varying parameters, including concentration, types of HMI, temperature, mixing ratio, and designs of the electrochemical cells. The reproducibility of results was quantified by the calculation of standard deviation. It was found that each HMI solution has its own characteristic impedance value. In addition, the change of impedance response follows a linear trend line in the Bode and Nyquist impedance plot when varying parameters such as concentration, temperature, and mixing ratio. In addition to controlled experiments, the electrochemical cell was tested in tap water and industrial water samples provided by the project’s collaborators. In the second part of the research, various techniques of surface analysis were
conducted on the surface of the platinum working electrode, including scanning
electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and Xray photoelectron spectroscopy (XPS). Since the HMI-related adsorbates were presented in the results, a hypothesis was proposed, which suggests that the adsorption/desorption process of HMIs has taken place during the EIS measurement, and this phenomenon attributes to the inductive behavior in the impedance response. Finally, equivalent circuit models were proposed based on the result of EIS measurements, surface analysis, and literature studies. Data fitting for the impedance data was carried out with Zview software version 3.5h. Charge transfer resistance and other equivalent circuit elements’ impedance values were compared to specify the differences between the two HMI.