Application of Sphere Cap Mercury Microelectrodes and Scanning Electrochemical Microscopy (SECM) for Heavy Metal Monitoring at Solid/Solution Interfaces

Abstract

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 HClO 4 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 mM. 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.

In Situ Scanning Electrochemical Microscopy (SECM) Detection of Metal Dissolution during Zinc Corrosion by Means of Mercury Sphere‐Cap Microelectrode Tips

“…As is evident from Figure 1B, in the aerated solution two well-defined waves that correspond to Equations (1) and (2) [43] are observed. This supports the idea that Zn II ions are actually reduced in a locally basic medium.…”

“…Above the insulating substrate, negative feedback is observed, which reflects the decrease in the normalized current with the decrease of the tip-substrate distance due to the partial blockage of oxygen diffusion to the microelectrode. [6,43] Above the zinc surface, negative feedback is also recorded. However, in the latter case, oxygen depletion starts at higher tipsubstrate distances.…”

“…[34][35][36][37][38][39][40] More recently, mercury microelectrodes have also been successfully employed as SECM tips to approach solid/solution interfaces to monitor various metal ions, including Zn 2 + , Cd 2 + , Pb 2 + , Cu 2 + , Mn 2 + , and Tl + . [41][42][43][44][45][46][47] Moreover, the functions to predict diffusioncontrolled steady-state currents [48] and approach curves at sphere-cap tips for both negative and positive feedback conditions are available. [49] Mercury sphere-cap microelectrodes can be fabricated onto microdisks of platinum by electrodeposition from mercury-ion solutions; the deposits adopt sphere-cap geometry.…”

Diamond as Advanced Material for Scanning Probe Microscopy Tips

“…Usually, ultra‐microelectrodes (UMEs) used in SECM experiments are fabricated from noble metal microwires made from materials such as Au and Pt. However, also mercury thin film electrodes have been utilized in SECM experiments 65, 66. As in any other SPM technique, the integrity and well‐defined geometric dimensions of the probe electrode are a key factor for quantitative measurements, in particular if real world samples with unknown composition are examined.…”

Fractionation and Speciation of Trace Elements in Soil Solution

“…This resulted in a detection limit of 0.05 mg l À1 and a precision of 2.5 % relative standard deviation. Anodic stripping voltammetry with Hg-coated microelectrodes has recently also been used to measure concentration profiles of trace metal ions at the soil/water interface of a natural sediment and a bauxitic soil sample [139] and for the analysis of Cd, Cu and Pb in soil extracts [140]. Gold microelectrode arrays were developed for the analysis of Cu in soil extracts [141].…”