Assessment of copper, cadmium and zinc remobilization in Mediterranean marine coastal sediments

Abstract

The remobilization of copper, cadmium and zinc in sediments of three selected coastal microenvironments of the Aegean Sea (Eastern Mediterranean) is assessed. Various analytical methods and techniques were employed providing concentrations, profiles and forms of metals and organic matter in sediments and pore waters. At Loutropyrgos, a non-industrial site located, however, within an intensively industrialized enclosed gulf, an intense resupply of zinc in pore water from sediment was recorded, correlating with the highest value of weakly bound fraction of zinc determined at this area. The comparatively high zinc concentrations measured in the pore waters (394 nM), exceed considerably those in the overlying seawater (12.5 nM determined by DGT; 13.5 nM total), resulting in the formation of a strong concentration gradient at the sediment–water interface. Potential zinc flux at the sediment–water interface at Loutropyrgos (based on 0.4 mm DGT profile) was calculated equal to 0.8 mmol.m −2.d −1. The half lives of trace metals at Loutropyrgos site, based on the aforementioned DGT profiles, amount to 0.1 y (Zn), 2.8 y (Cd), 4.5 y (Cu), 2.2 y (Mn) and 0.4 y (Fe) pointing out to the reactivity of these metals at the sediment–water interface. The concentration of dissolved organic carbon (DOC) in pore waters of the three selected sites (2.7–5.2 mg/L) was up to four times higher compared to that of the corresponding overlying seawater. Similarly, the concentrations of carbohydrates in pore waters (0.20–0.91 mg/L monosaccharides; 0.71–1.6 mg/L polysaccharides) are an order of magnitude higher than those of seawater, forming a concentration gradient at the sediment–water interface. Total carbohydrates contribute between 34 and 48% of the organic carbon of the pore waters, being significantly higher than those of seawater from the corresponding areas, which were in the range of 15–21%. The complexing capacity as for copper ions (CCu) determined in pore water ranges widely, from 0.03 μM at Kalamos to 3.76 μM at Molos, whereas the corresponding values for cadmium ions (CCd) were non detectable, except for Kalamos site, where the value for CCd was equal to 0.03 μM. A significant increase in the values of CCu, normalized as for DOC, was observed in pore waters in relation to those of overlying seawater. This indicates an ‘enrichment’ of pore waters in dissolved organic ligands for copper ions per unit of DOC. Up to 72% of DOC could be present as ligands capable to complex copper ions.