Capping of contaminated marine sediments : Redox reactions and trace metal mobility

Abstract

The risk associated with exposure to contaminants, has drawn attention to the fate and transport of contaminants in marine sediments. Remediation of contaminated marine sediment has been necessary at several locations to reduce the risk of exposure. Capping is a widely used remediation strategy that aims to isolate the contaminants from the surrounding environment.

Toxic trace metals are often found co-precipitated with iron sulphides in sediment. Interactions between oxic minerals in capping material and anoxic minerals in sediment can cause redox reactions leading to degradation of iron sulphides with a subsequent release of toxic trace metals. A series of leaching tests were performed to investigate the possible leaching of redox sensitive elements such as Fe and Mn, and toxic trace metals such as Zn, Cu, Pb, Hg and Cd. The tests were performed with crushed calcite, crushed gneiss and sediment from Oslo Harbour. The results of these investigations suggest that there will be no increase in leaching of trace metals from mixing calcite and gneiss with anoxic sediment. The results also suggest that possible leaching of elements from capping material alone into the water column should be considered when selecting capping material. Gneiss is leaching more Mn, Co and Cd than the sediment alone, which suggest that calcite should be preferred over gneiss as capping material.

Reactions between capping material and sediment were also investigated in the geochemical model PHREEQC-2 and compared to the results from the leaching tests. Models could be used as a supplement to laboratory experiment to enhance the understanding of the behaviour of capped marine sediments, if they give appropriate results. The use of models could be an advantage as they can easily be changed to include different parameters that can be used to investigate further scenarios. The interactions between capping material and sediment were investigated by equilibrium phases and the results show that the concentration of elements such as Fe, Ni and Zn shows quite good consistency between leaching tests and PHREEQC-2. To improve the performance of models, solid solutions or a combination of kinetics and equilibrium phases could be utilized, rather than equilibrium phases alone.