HOMER · Hydrophobicity, lipophilicity and bio-availability of trace metals in coastal systems

Final Activity Report Summary - HOMER (Hydrophobicity, Lipophilicity and Bioavailability of Trace Metals in Coastal Systems)

The Platinum group elements (PGE) are platinum (Pt), palladium (Pd) and rhodium (Rh). These elements are increasingly used in the catalytic converters of motor vehicles, which consume approximately 50 to 60 % of the total world demand for platinum and palladium, and 100 % of the global demand for rhodium. This led to a widespread environmental contamination by the PGE, including the atmosphere and water pathways. However, even though the PGE are classified as 'emerging contaminants', knowledge of their environmental chemistry, fate and transport, particularly in the aquatic systems, is very limited. Therefore, the laboratory studies that were undertaken in this project focussed on gaining a better understanding of the hydrophobicity, reactivity and mobility of PGE in natural waters so that their behaviour could be predicted. The most important scientific achievements of the project were the following: 1. A major achievement was related to the validation of the conduct of laboratory experiments, so that they were truly representative of environmental processes. The PGE showed a tendency to adsorb to the surfaces of containers generally used for sample storage and experimentation and this was fully evaluated. The losses from solution followed the trend Pd greater than Rh and greater than Pt and borosilicate glass was the least reactive surface, whereas a number of plastic, i.e. polymeric, surfaces, such as polypropylene, Teflon and low-density polyethylene, showed higher removal of PGE. 2. Low solubility of Pd, particularly in freshwater and in the absence of organic matter, as well as of Rh in seawater was observed. The lower solubility of Rh in seawater was caused by salt-induced destabilisation of its polynuclear complexes. On the contrary, the presence of seawater increased the solubility of Pd due to complexation with chloride. Platinum demonstrated high solubility in all water types and was not dependent on the presence of seawater or pH. 3. The behaviour of Pd in natural waters was controlled by its rapid association with dissolved organic ligands, a significant fraction of which was hydrophobic and subject to salting out, e.g. removed from solution by seawater salts, on estuarine mixing. Even though complexation with dissolved organic ligands was generally alleged to stabilise and maintain trace elements in solution, association of Pd with neutral or near neutral organic matter conspired to accentuate its removal at high salinities through salting out. 4. The interaction of Pt and Rh with organic matter was kinetically hindered. Thus, in contrast to Pd, the speciation of Pt and Rh, and therefore their hydrophobicity, particle-reactivity and mobility, greatly depended on the residence time of these elements in a given system. 5. Overall, Pd showed a similar behaviour and reactivity to other transition elements whose speciation was known to be dominated by complexation with organic matter, e.g. copper (Cu), lead (Pb), zinc (Zn) and nickel (Ni), whereas the behaviour of Pt was similar to metals like cadmium (Cd), whose interaction with organic ligands was limited. Rhodium showed an intermediate behaviour. The results of this study improved our understanding and ability to predict the transport, reactivity and fate of PGE in coastal systems where these metals were mobilised or discharged.

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