Book Description

The speciation of trace metals plays a very important role in conditioning their biogeochemical cycles in the marine system. Their species are found to be strongly related to organic matters such as humic acids (HA) and fulvic acids (FA). This thesis aims to develop appropriate techniques to improve our understanding of the speciation of some trace metals, mainly in marine waters. Lead (Pb), cadmium (Cd) and chromium (Cr) which are widely used in industry were analytzed. The concentration of lead in uncontaminated seawater is between 10 and 100 pM, whilst in coastal waters it is higher at up to low nanomolar levels. Little is known about the chemical speciation of lead since it is difficult to be determined at picomolar levels. The work in this dissertation was to develop a suitable electrode and procedure to determine lead in seawater, with the objective that the electrode could be applied for in-situ measurement (without reagents) and for speciation. Several electrodes and materials were tested, including a solid bismuth rod and different microwire materials. Vibration was used to enhance mass transport, instead of solution stirring, to decrease detection limits and facilitate in-situ monitoring. The bismuth electrode was found to be suitable for monitoring lead in coastal waters: it is mercury-free and therefore environmentally friendly, but insufficiently sensitive for oceanic lead concentrations. Comparison of microwires of carbon, gold and silver showed that all of these bare electrodes give a signal for lead, but they suffer variable interference from cadmium, although this is minor if the cadmium concentration is lower than lead. A large improvement was obtained by coating the electrodes with mercury, which gave good peak resolution between cadmium and lead, and good sensitivity. The carbon and gold electrodes had good reproducibility and sensitivity when the mercury was renewed for each measurement. Once coated with mercury, the silver electrode formed amalgam with the silver which could not be completely removed. This electrode was thus not suitable to be used as a renewable mercury film electrode. However, as a permanently amalgamated silver electrode, it was found to have good sensitivity for lead, good separation from cadmium, and was stable for long time usage. This electrode, the silver amalgam microwire (SAM) electrode, was selected for further speciation study of Pb in oceanic waters. Optimum conditions for using the SAM electrode for trace lead detection in seawater involve the use of conditioning potentials. The limit of detection for lead was 4 pM lead in acidified seawater and 12 pM lead in seawater of pH 8. The higher limit of detection at pH 8 is due to the inorganic speciation at that pH. The SAM electrode was successfully used for pseudopolarography of lead to determine its organic complexation. This method was calibrated using model compounds and applied for the first time to estuarine, coastal and oceanic samples at natural concentrations of Pb. It was found that lead occurs mostly as a labile (reactive, organic) species, with a smaller fraction as a strongly organically bound species. The SAM electrode was also tested for chromium analysis using cathodic stripping voltammetry (CSV) in natural waters. Its detection limit is as good as the mercury drop electrode but reduces consumption of mercury.