Fine scale remobilisation of Fe, Mn, Co, Ni, Cu and Cd in contaminated marine sediment

Abstract

Contaminated sediment from a marine harbour was maintained for 16 months in two flumes that continuously re-circulated the overlying water, sustaining a smooth flow at the sediment surface. The sediment was placed in one flume intact, while for the other it was pre-homogenised. The concentrations of trace metals in the porewaters were measured at a vertical resolution of 2 mm using the technique of diffusive equilibration in thin-films (DET) and microelectrodes were used to measure concentration profiles of oxygen and pH. Separate experiments showed good agreement between metals measured by DET and those measured in porewaters extracted by centrifugation and filtration. Local mobilisation of metals was measured in 2-dimensional arrays by deploying DGT (diffusive gradients in thin-films) probes. There were high concentrations of Fe in the porewaters, which limited the concentration of sulphide to less than 0.3 μM. With both DET and DGT measurements, there were sharply defined maxima of Cu and Cd within 2 mm of the sediment water interface, consistent with their release from organic material as it is oxidised. There was a Co maximum about 5–8 mm lower than the Cu and Cd maxima, apparently coincidental with Mn mobilisation. While there were clear Ni maxima, their location appeared to vary from being coincident with Co to a few mm above the Co maxima. The remobilisation of metals could not be explained by the pH gradients in the near-surface sediments. As sulphate reduction rates were appreciable, the apparent lack of metal mobilisation at depth was attributed to the formation of metal sulphides. The DGT measurements in the same probe were well replicated horizontally. This, the replication of the same features between flumes and the reasonable correspondence between DGT and DET, showed that the localised mobilisation of metals was associated with recent diagenetic processes, rather than the depositional history. There were substantial fluxes of Cu and Cd to the overlying water. Even though there were steep gradients of Fe, Mn, Ni and Co within 1 cm of the sediment–water interface, there was no clear evidence for any substantial metal fluxes across the interface. Adsorption of Mn to Fe oxides, rather than oxidation, may be responsible for its removal from solution at the same depth as Fe.