Interaction and mobility of cobalt-60 between water and sediments in marine environments possible effects by acid rain

Abstract

In Japan, all nuclear power stations have been constructed facing sea-shores and the operating radioactive wastes have been either stored at the sites or discharged into marine environments, if permissible. The purpose of this investigation was to understand the behavior of a radioactive cobalt ( 60Co), one of the typical radionuclides produced by operating nuclear reactors, in marine environments, namely cobalt interaction between water and sediments. Great emphasis was focused on demonstrating a quantitative difference in behavior of cobalt between marine and freshwater environments. Under natural conditions, cobalt remained in the water phase in marine environments more than 5 times as much as in freshwater environments. In freshwater, most cobalt (over 95%) is absorbed by and permanently retained within bed sediments under the normal environmental conditions. However, in marine environments cobalt gains mobility. Furthermore, this mobility of cobalt once established did not diminish nor disappear when surrounding conditions were changed. In short, cobalt in marine environments is generally mobile. This considerable mobility was greatly influenced by the environmental factors: (e.g. pH, the redox potential, the ionic strength, the type of sediments and the length of reaction time) but, it was primarily determined by the initial environmental conditions. The result of this experiment suggests that the treatment and the storage of radioactive cobalt waste should be done in freshwater environments rather than in a marine environment if we try to reduce the mobility. Because the mobility depends on pH, acid rain is, therefore, to be expected to strongly influence the mobility of 60Co in aquatic environments.