Photochemical oxidation of thiols and copper complexing ligands in estuarine waters

The possible photolysis of organic copper complexing ligands in marine waters is investigated here by irradiation of estuarine waters with natural sunlight and with the simulated UV component of sunlight at wavelengths of 300-350nm, typical for intermediate latitudes, in an incubator. Copper complexing ligands and thiol compounds were determined by cathodic stripping voltammetry. Field experiments showed that photochemical oxidation of the copper complexing ligands and thiol compounds occurred but at variable rates due to variations in the sunlight intensity and salinity. Ligand concentrations were decreased by between 2% and 60%, and thiol concentrations by 30-50%, by a day exposure to sunlight. The incubator experiments confirmed that the thiol compounds occurring in the samples were broken down with a half-life of 3 h and natural copper complexing ligands with a half-life of 20-45 h, by the simulated daylight. After saturation with copper, the half-life of the thiols increased by nearly a decade. It is possible that L1-type ligands, which are nearly saturated with copper, are also broken down more slowly than the weaker L2-type ligands. The experiments confirm that solar radiation breaks down a significant fraction of copper complexing ligands and thiol compounds in surface waters. However, it is likely that this photochemical breakdown is much less for ligands stabilised by copper, and it is reduced by downward mixing of the surface waters and subsequent shading by dissolved organic matter and particulate matter in tidal estuarine conditions. (c) 2006 Elsevier B.V All rights reserved.