Application of sphere cap mercury microelectrodes and scanning electrochemical microscopy (SECM) for heavy metal monitoring at solid/solution interfaces.

In this paper, a combination of anodic stripping voltammetry (ASV) at mercury microelectrodes and scanning electrochemical microscopy (SECM) was employed to establish the concentration profiles of heavy metal ions at the soil/water interface of a natural sediment and bauxitic‐soil sample. Air dried soil samples were equilibrated with aqueous solutions containing 0.5 M NaCl plus acetate buffers or HClO4 at different concentrations to allow the achievement of different pH either in the aqueous phase or in the soil slurry. It was verified that mixing acidified aqueous solution and the soil without stirring led to concentration profiles displaying a maximum at the water/soil interface. This was explained as due to pH gradients that originated at the soil/water interface and within the soil slurries. On the other hand, mixing the soils and water solutions, under stirring for 24–48 h, provided almost uniform pH throughout the system. This allowed obtaining uniform metal ion concentrations within the soil slurries. For the evaluation of the concentrations, the anodic charge involved in the stripping step was utilized. The mercury microelectrode was standardized in a 0.5 M NaCl aqueous solution by performing calibration plots over the concentration range 0.1–500 μM. To evaluate the concentrations of the metal ions in the soil slurries, a procedure was developed to obtain the apparent diffusion coefficients of the metal ions, which depended on the physical properties of the soil such as porosity and tortuosity. The microanalytical methodology proposed here can be utilized for predicting the mobilization of trace elements from soils that comes into contact with acid rain.