Detection of Heavy Metals By Anodic Stripping Voltammetry Using Nanocarbon Thin Film Electrodes in an Electrochemical Flow Cell
Authors: Miyao Yuka , Niwa Osamu , Kurita Ryoji , Kamata Tomoyuki , Hachiya Hiromitsu 
Published: February 2020

It is important to detect and quantify toxic heavy metals such as Pb in order to prevent damaging human health by their toxicities. A very small amount of heavy metals are included in food or water. The contamination of heavy metals into air, drinking waters, wastewaters and foods sometimes poses a serious threat to the problems on global health and environment. Conventional methods for the detection of heavy metals, such as atomic adsorption spectrometry, inductively coupled plasma mass spectrometry, and neutron activation analysis, require expensive and sophisticated instruments with the complicated sample preparation. Therefore, a simple, inexpensive and high performance analyzer for heavy metals is strongly desired.

Anodic stripping voltammetry (ASV) is a standard methods for the analysis of heavy metals, because of several advantages, including high cost performance, high sensitivity resulted by its preconcentrating process, and suitability for a miniaturized system. In ASV, mercury film electrodes and the hanging mercury drop electrode had been widely used as the working electrode. However, more environmentally friendly electrode materials for use in ASV working electrodes has been strongly required, because of the increasing toxic risks on the used/disposed mercury.

Here, an ASV-based Pb detection method using a nanocarbon thin film electrode with an electrochemical flow cell has been developed. The nanocarbon thin films were formed by unbalanced magnetron (UBM) sputtering, which is an exceptionally versatile technique for the deposition of high quality, well adhered films. UBM sputtering can be used to fabricate the hard films on silicon wafers that indicate the abrasion resistance and the sliding properties.

The nanocarbon thin films comprise two types of carbon structure, diamond (sp3) and graphite (sp2), which have excellent properties including a wide potential window, the good electrode activity, and the superior stability. The sp2/sp3 ratios of nanocarbon thin films were adjusted by the tuning of the ion irradiation energy between a silicon substrate and a carbon target. The surface fouling of the nanocarbon films were significantly suppressed because of extremely flat surface. It is the very suitable material for the ASV measurement of heavy metals such as Pb.

A flow-type ASV-based Pb-detection method was studied. The flow cell was equipped with three electrodes consisted of nanocarbon film working electrode, an Ag/AgCl reference electrode with 3M NaCL inner solution and a SUS-type counter electrode. The flow cell was designed in smaller size that has a micrometer sized thin layer around the working electrode in order to increase the concentration efficiency. Measuring samples were adjusted to pH 5.0 by the addition of acetic acid solution, and s introduced into flow cell at a flow rate of 0.03 ml min-1 by using a syringe pump. The limits of detection was 5 µg l-1 for Pb in a concentrating time of 3 min.

Although this nanocarbon film electrode can be used in ASV measurement, quantitative measurement is difficult when the interfering components such as Cu are existed in sample. Since Cu is the most serious interfering component in the ASV measurement for Pb, Cu ions must be removed in a pretreatment process by using chelating resin column method or electrochemical method. However, the chelating resin method had the problems including the complicated apparatus or the frequently maintenance.

The flow cell system for removing the effects by the coexisting Cu was developed. The removal cell was preferably a simple structure to prevent the fouling and clogging, and equipped with a working electrode and a counter electrode by using the nanocarbon thin films. Since it was possible to concentrate only Cu at more positive potential than Pb, the coexisting Cu was deposited on the removal working electrode. In order to increase the removal rate of Cu, it is important to design a cell to efficiently deposit copper on the electrode.

The removal flow cell was designed to have the thin layer around the working electrode similar to the measurement flow cell. For the solution including 1 mg l-1 of Cu, we have succeeded in Cu removal more than 90 % by selecting the appropriate potential. It was confirmed that the removal of Cu to a concentration does not affect the measurement of Pb by using a simple mechanism. The Cu removal before measuring showed the good performances in Pb measurement.