Experimental studies of the transfer of neptunium from sea water to sediments and organisms (annelids and molluscs)

This paper deals with the physico-chemical behaviour of neptunium in sea water and with its capacity for transfer to coastal or deep-sea sediments and for uptake by two benthic species, Arenicola marina (L.) and Cerastoderma edule (L.). We used the 239Np isotope with mass concentrations in sea water of about 2.5 x 10-10 - 5.4 x 10-12 g litre-1, which are close to the concentrations observed in the environment for 237Np. The experimental results show that Np occurs in solution mainly as the neptunyl ion NpO2+, but soluble anionic and neutral complexes and particulate forms were also detected. The distribution coefficients for the sediments varied from 350 to 6500. The carbonate phase plays a decisive role in neptunium transfer but other fixation processes are also possible. Concentration factors after 13 days were found to have values of 40 and 14 for the shells and soft parts of cockles, respectively, while polychaetes had a concentration factor of 2. Thus Np has a smaller probability of transfer to sediments and organisms than have plutonium and americium. (author)