Complexation of copper and nickel in the dissolved phase of marine sediment slurries

Abstract

Speciation of copper and nickel in the water phase of incubated marine slurries under aerobic conditions was performed with MnO 2 and Sep—Pak C 18 cartridges. Changes in time during the incubations of concentrations of dissolved organic carbon (DOC), dissolved copper and nickel and inorganic nitrogen were followed. The influence of organic complexation on the dissolved concentrations of copper and nickel was investigated as well as competition between copper and nickel for dissolved organic ligands. Two pools of dissolved organic ligands could be distinguished. With the MnO 2 method a relatively strong ligand group was determined that was subjected to degradation. The conditional stability constant for copper with the relatively strong ligand was 10 11.1. The conditional stability constant for the relatively strong nickel ligand was difficult to determine due to saturation of the ligand sites; it was found to be around 10 10. However, it could not be ascertained whether nickel was reversibly com-plexed with the organic ligands. With Sep—Pak a relatively weak Hgand group was detected that was probably more resistant to degradation. The conditional stability constant of the weaker ligand could not be estimated, an approximation revealed that it was weaker than the ligand group determined with the MnO 2 method. For copper the difference between binding strength of the ligand groups was at least 100, for nickel the difference was less. Competition between copper and nickel for the ligands could not be detected. Only during the first day of the experiment, when the system was not in equilibrium was competition suspected. However, the replacement of nickel by copper from the ligand sites was not straightforward and could not be accounted for by our model. The concentration of total dissolved copper during the first week of the experiment was found to be controlled on the one hand by release from the sediment of copper already associated with dissolved organic matter (DOM) and on the other hand by concentration of the strongest ligand. The calculated free copper concentration increased from 10 −12 to 10 −9mol l −1 due to the oxidation of the strongest ligand. After saturation of the strongest ligand the relatively weak ligand controlled the free copper concentration. A continuing release of copper from the sediment by degradation of particulate organic matter (POM) will not increase the free copper concentration until the ligand sites of the weaker ligands get saturated. The total dissolved nickel concentration seemed only to be determined by the sum of the concentrations of the organic ligands. A degradation of ligands resulted in a decrease of the total dissolved nickel concentration. The calculated free nickel concentration did not change with time.