Comparison of Chemical Speciation of Copper in the Oosterschelde and Westerschelde Estuaries, The Netherlands

Abstract

The aim of this investigation was to assess the chemical speciation of copper. Although the Cu concentrations in both the dissolved and particulate phases differ strongly between the Oosterschelde (OS) and Westerschelde (WS) estuaries, The Netherlands, the Cu content of the bivalve Macoma balthicafrom both sea arms is comparable. Therefore, chemical speciation of Cu was examined during 1 year at two sites, one in each sea arm. Two dissolved organic ligand groups could be distinguished. For both sea arms, a relatively weak ligand group with logK′=9·27 and a mean ligand concentration of 250 neq Cu l −1was determined. Moreover, a relatively strong group was detected with logK′=13·04 and a mean ligand concentration of 180 neq Cu l −1in the WS, and with logK′=13·7 and a mean ligand concentration of 48 neq Cu l −1in the OS. In both sea arms, the calculated free concentration of Cu 2+is extremely low (<10 −14 M). The strong ligand group is related to salinity and to dissolved organic carbon (DOC) indicating that the river Scheldt and estuarine/marine DOC supply this material. The weak ligand group is related to salinity and marine chlorophyll ain the WS, and to DOC in the OS. The regulating mechanisms of the distribution of Cu over the chemical species are different for the two sea arms. In the WS, there seems to be no equilibrium between dissolved and particulate Cu. Hence the free Cu concentration is determined by complexation with dissolved organic ligands. In the OS, adsorption on particulate organic matter is the key factor. It is even possible to make a good estimate of free Cu in the OS if only dissolved and particulate Cu and POC are known. It is concluded that Macomadoes not accumulate Cu from the dissolved phase since the free Cu concentration is too low; food must be the source. Since desorption of Cu from particulate matter in the polluted WS is slow in contrast to desorption in the relatively clean OS, kinetics of particulate Cu seem to be the reason for the relatively high Cu content of Macomain the OS.