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Title: Speciation of trace metals and metalloids in natural waters using the vibrating gold microwire electrode
Author: Gibbon-Walsh, Kristopher Bryant
ISNI:       0000 0004 2737 8871
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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This work reports the use of the vibrating gold microwire electrode, with new methods developed for the speciation of Zn, Cu, Mn and As at natural levels in waters of neutral pH. Trace metals and metalloids can be distributed as different species in the environment, which can control mobility, toxicity and bioavailability and in turn depends on many complex factors. Analysis of this distribution (speciation) can provide an understanding of the relationship between such elements and their relationship with organisms in marine environments and humans through contaminated drinking water supplies. Such speciation can be analysed using a vibrating gold microwire electrode (VGME), which is easily prepared and maintained at minimal cost. High sensitivity is found for trace metals: Mn; Cu and Zn; and the metalloid As, resulting from a very low diffusion layer (~0.8 urn for a Sum gold wire) means that they can be measured at trace levels in natural waters. This combined with the VGME's portability, reliability and stability for long term measurement (repeated measurements over several days) and its capability to distinguish between distinct forms ofthe above trace elements mean that speciation methods could be successfully developed and validated in natural waters, with no, or minimal sample preparation. Such methods made it possible to analyse speciation on-site, which decreases the potential problems inherent in maintaining sample speciation during storage. Contamination of groundwater with As is a major health risk through contamination of drinking and irrigation water supplies. In geochemically reducing conditions As is mostly present as ASIII, which is why a method that uses cathodic stripping voltammetry (CSV) to determine reactive AS"I was developed. The ASIII is detected after adsorptive deposition of As(OHho, followed by a potential scan to measure the reduction current from AsIII to Aso. The method is suitable for waters of pH 7-12. The CSV method was successfully applied to groundwaters from Severn Trent, UK, however speciation using this method is severely hampered by high levels of iron and manganese. Experiments showed that the interference is eliminated by addition of EDTA, making it possible to determine the arsenic speciation on-site by CSV in West Bengal, India. The VGME is also used to detect nanomolar levels of dissolved Mn by anodic stripping chronopotentiometry (ASC) and sub-nanomolar levels of dissolved Zn by anodic stripping voltammetry (ASV) in neutral pH seawater. The limits of detection for Mn (1.4 nM) and for Zn (0.3 nM) in seawater with a 300 s plating time, are better than achieved using other non-mercury based electrodes and nearly as good as a mercury film electrode for Zn. Deposition of Mn at the VGME was further utilised to catalyse the reduction of Asv to ASIII, enabling for the first time the direct electrochemical determination of Asv in natural waters of neutral pH including seawater by ASV using a manganese coated gold microwire electrode. Direct electrochemical determination of Asv in neutral pH waters is impossible due to its electro-inactivity. Therefore Mn is added to excess (~1 JlM Mn) to the water leading to a Mn coating during the deposition of Asv on the electrode, when depositing at -1.3 V. The detection limit was 0.2 nM Asv using a deposition time of 180 s. Speciation of Cu is determined without the need for sample preparation, using scanned stripping techniques for the first time at natural levels in seawater. A desorption potential (-1.2 V) and a conditioning interval between scans make the VG ME suitable for on-site and potentially in-situ copper speciation. The resulting pseudopolarograms are analysed using an experimentally constructed 'chelate scale' to determine the strength of copper ligand interactions in real seawater samples.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral