Speciation studies of copper in the Gulf of Elefsis: the role of the macroalgae Ulva rigida

Abstract

The Gulf of Elefsis, situated in the vicinity of Athens, is a high productivity area regarding the biota. One of the most abundant algae existing there is Ulva rigida. Controlled laboratory simulation experiments, as well as field investigations indicated that copper is complexed to a high extent with organic substances released by U. rigida and other naturally occurring organic ligands. A significant part of these organic ligands exhibit surface active properties, i.e. the accumulation on different phase boundaries (e.g. seawater/atmosphere, seawater/living organisms, seawater/particulate suspended matter, seawater/sediments). The complexing capacity values of samples collected from different microenvironments within the Gulf of Elefsis, determined by the electrochemical method of differential pulse anodic stripping voltammetry (DPASV), vary between 0.04 and 0.43 μM Cu 2+. At the same time, the complexing capacities of samples obtained from the laboratory tanks in which U. rigida has been placed in order to be decomposed under controlled conditions, show much higher complexing capacity values (up to 13 μM Cu 2+), especially during the final stage of the experiment. The method of linear sweep voltammetry (LSV) was applied for the determination of the concentration of sulfur species (S 0+S 2−) present in nonaerated tanks containing U. rigida. The total concentration of sulfur species in tank No. 2 was 300 nM while in tank No. 3 it was 500 nM. As a result of the presence of sulfur species, a part of the complexing capacity of the substances released during algae biomass decomposition in these two tanks could be attributed to the formation of copper sulfides. Furthermore, the surfactant activity (amount of surface active substances (SAS) expressed in equivalents of the nonionic surfactant Triton-X-100) was determined by A.C. out of phase voltammetry. Higher amounts of SAS were detected in the tanks containing U. rigida (up to 0.70 mg/l eq. T-X-100), in comparison to samples originating from natural microenvironments which had values ranging between 0.10 and 0.17 mg/l eq. T-X-100. These results show that U. rigida, although definitely not the only or necessarily the major contributor to the organic ligand pool in the study area, contributes considerably to the production of ligands with high metal complexing capacity and/or high surfactant activity.