Exploring Ce³⁺ adsorption on clay minerals to propel the frontier of heavy metal sensing with clay-CeO₂ modified carbon paste electrode

A novel approach was employed to craft innovative carbon-paste electrodes using cerium(IV) oxide (CeO₂)-doped clay, aiming to modify graphite electrodes. The introduction of Ce involved the adsorption of Ce³⁺ ions onto an activated carbon (AC) clay mixture, showcasing an impressive adsorption capacity of 40.0 mg g⁻¹, which surpasses that of clay alone. This step was undertaken to enhance the conductivity of raw clay and increase its electrochemical surface area. Sintering clay-CeO₂ at 1000 °C resulted in the maximum electrochemical area when utilized as a 10 wt% dopant for graphite electrodes. Heavy metal ion analysis (HMI) involving Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ was conducted using five electrochemical techniques: Cyclic Voltammetry (CV), Square Wave Voltammetry (SWV), Normal Pulse Voltammetry (NPV), Differential Pulse Voltammetry (DPV), and Linear Sweep Voltammetry (LSV). SWV emerged as the optimal technique and was termed SWAdSV after the stripping process, displaying peak current responses for Cd²⁺ (74.7 µA), Pb²⁺ (42.4 µA), Cu²⁺ (36.0 µA), and Hg²⁺ (22.8 µA). Calibration curves were utilized to determine the Limit of Detection (LOD) and Limit of Quantification (LOQ) values for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺. The repeatability, expressed as %RSD, for the same electrode was found to be 1.70%, 2.04%, 1.14%, and 3.61% for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺. While the reproducibility, also expressed as %RSD, for different electrodes was determined to be 1.98%, 2.31%, 1.52%, and 1.89% for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, respectively.