Cadmium sensing with bentonite-modified carbon paste electrode: electrochemical insights

Electrochemical detection of cadmium cations in aqueous solutions using a bentonite-modified carbon paste as working electrode was investigated. For this purpose, cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy techniques were used. The findings demonstrated that the electrode composed of 20 mass% bentonite manifested high detection sensitivity, and hydrochloric acid was the best suitable supporting electrolyte. Under these operating conditions, the electron transfer process was quasi-reversible. Also, it was found that the increase of pH resulted in the decrease of the anodic current density, likely because of partial immobilization of the cadmium-derivative species (Cd2+, Cd(OH)(+)) by negatively charged clay particles. Furthermore, a linear relationship was observed between the initial concentration of Cd2+ (up to 5 x 10(-5) M) and the anodic current density. The latter signal markedly declined as the Cd2+ ion concentration approached the cation exchange capacity of the electrode (about 6 x 10(-6) M). The use of high potential scan rate (> 50 mV/s) led to the cathodic peak vanishing presumably because of the decline in the electron transfer between Cd2+ ions and the electrode. The used electrode was regenerated by using salt solution and successfully reused. Moreover, the co-presence of cadmium and lead ions did not alter its electrochemical performance. The electrode-solution interface was equivalent to an electrical circuit comprising a constant phase element in parallel with a resistance and a Warburg diffusion regime.