Voltammetric determination of heavy metals in environmental samples by carbon electrodes

Abstract

Quantification of heavy metals such as lead and cadmium is a significant pursuit to accurately assess the contamination levels in the environment. Usually, they are emitted from anthropological and industrial sources and can be found in the environment at trace levels. Nevertheless, heavy metals tend to accumulate both in the environmental media and the living organisms, which in turn causes more significant harm. These pollutants are most commonly determined via spectrometric methods like atomic absorption spectrometry (AAS) and inductively coupled plasma (ICP) due to their sensitivity and accuracy. However, thanks to the recent developments in the field, electrochemical methods can generate results much faster with comparable sensitivity and accuracy, when suitable methods and electrodes are chosen. Electrochemical methods not only save valuable time for the analyst but also allow for on-site measurements with lower instrumental costs than the abovementioned spectrometric methods. There are plenty of electroanalytical methods to employ. Among them, voltammetric methods, especially anodic stripping voltammetry (ASV) and adsorptive stripping voltammetry (AdSV), offer a significant advantage in the determination of heavy metals as they allow ppb-level analysis in a relatively short time with good reproducibility. Achieved analytical performances can be improved by coupling ASV or AdSV with square wave (SWASV/SWAdSV) or differential pulse (DPASV/DPAdSV) techniques. Regardless of the quantification method, the choice of the working electrode is a crucial factor for the overall analytical performance. In the last decade, promising results were reported in the literature with carbon-based electrodes (CBE) in various electroanalytical applications. They have become one of the main alternatives to hanging mercury drop electrodes because they can easily be modified with numerous materials to achieve an outstanding performance without having any toxic effects. Other than being non-toxic, CBE offer many advantages like having an outstanding surface structure, versatile electrochemical properties, and low costs. There are different types of CBE with various properties that are fit for different purposes. Some of the popular ones for heavy metal analysis are glassy carbon electrodes (GCE), carbon paste electrodes (CPE), graphite electrodes (GrE), and screen-printed carbon electrodes (SPCE). They all have unique properties, and they allow the user to conduct multi-element analysis in diverse matrices such as wastewaters, food samples, and environmental water samples. In the scope of this thesis, two CBE with two particularly distinct structures were studied: pencil graphite electrodes (PGE) and SPCE. Both electrodes were utilized in different ways to detect several metals of interest: with PGE lead, cadmium, and zinc were determined via SWASV in water samples collected from the Ergene basin, and with SPCE copper and iron were determined via DPAdSV in certified wastewater reference material. Additionally, some future aspects and possibilities were considered after some studies conducted with the SPELEC instrument, which presents a brand new technique that allows for simultaneous spectrometric and electrochemical measurements. The studies related to PGE were carried out in Turkey in the laboratories of Istanbul Technical University and Mir Research and Development company, while the studies related to SPCE were conducted in the laboratories of the University of Barcelona, Spain. When the PGE was employed as the working electrode, two main modification strategies were applied: dip-coating for multi-wall carbon nanotube (MWCNT) and Nafion, and in-situ plating for bismuth. Once the dip-coating process was complete, the electrodes were left to dry at room temperature and then used in the analysis. Bismuth was collected on the electrode surface during the stripping step along with the analytes. Moreover, when using bare SPCE, copper and iron ions were determined simultaneously in the presence of ortho-phenanthroline and K4(Fe)CN6 at a large linear range with DPAdSV technique. This method displayed a greater sensitivity toward the copper(I) o-phenanthroline complex compared to iron(II) o-phenanthroline complex. The studies carried out for this thesis also includes a literature survey, which mainly focuses on the four most preferred CBE in voltammetric heavy metal analysis: glassy carbon electrodes, carbon paste electrodes, graphite electrodes, and screen-printed carbon electrodes. The flow of this thesis includes firstly an introduction of the thesis subject and the relevant analytical techniques. Following the introduction, an in-depth liteture review is presented in Chapter 4. After the review, in Chapter 6, optimization and validation studies conducted with Nafion, MWCNT, and bismuth modified PGE to determine lead and cadmium in contaminated waters is revealed. In Chapter 7, the analytical performance of PGE modified with only MWCNT and bismuth was investigated. For this purpose method optimization and validation studies were conducted with three analytes: lead, cadmium and zinc. In Chapter 8, iron and copper determination with bare SPCE in the presence of K4(Fe)CN6 and o-phenanthroline was studied and obtained satisfying results were presented. Finally, in Chapter 9, by using the iron and o-phenanthroline complex as a model substance, perspectives on spectroelectrochemistry instrument applications are discussed. Employed methods and specific conditions are displayed separately for each study.