526 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



seawater did occur. Near equilibrium was reached in the exoskeleton and gills after 50 

 days of exposure. At 220 days the gut and hepatopancreas concentration factors 

 approached that of the shell and gills (^100) but gave evidence of still being far from 

 equilibrium. Flesh at day 220 showed concentration factors of ==3, but the shape of the 

 uptake curve suggests that higher values would have been reached had the experiment 

 been continued. Approximately 90% of the total plutonium taken up by the lobster was 

 found in the exoskeleton; some 4.6 and 1.2% was retained by the hepatopancreas and 

 flesh, respectively. As would be expected from these results, the major portion of the 

 plutonium accumulated by the lobster is lost during molting. 



Between 1966 and 1975 very little information was pubhshed in the open literature 

 deahng with the actual rates of uptake and loss of plutonium in marine organisms. Zlobin 

 (1966; 1971), Zlobin and Mokanu (1970), and Zlobin and Perlyuk (1971) presented data 

 on the uptake of ^^^'^''Opu in marine algae, principally the brown a\gae Ascophyllum 

 nodosum, which suggested that the element was actually assimilated by the plant rather 

 than simply adsorbed to it. However, the subsequent work of Wong, Hodge, and Folsom 

 (1972), Hodge, Hoffman, and Folsom (1974), Folsom, Hodge, and Gurney (1975), and 

 Folsom and Hodge (1975) using other macroalgae suggests that adsorption is the more 

 likely mechanism for accumulation and that plutonium might be attached to large 

 macromolecules or micelles, which have slow diffusivities but great affinity for a variety 

 of surfaces. In any case evidence now exists that marine plants (phytoplankton and 

 free-floating and rooted algae) do accumulate transuranics to a relatively high level and 

 that the rate of accumulation is rapid. This process applies to both fallout-derived 

 plutonium (Noshkin, 1972) and that introduced from fuel reprocessing plants (Hethering- 

 ton, Jefferies, and Lovett, 1975; Hetherington et al., 1976; Fraizier and Guary, 1976; 

 1977). For phytoplankton equilibrium between the algae and water can be established in 

 as Httle as 5 to 10 days (Gromov, 1976). 



The first laboratory experiments using ^^"^Pu to determine plutonium biokinetics in 

 marine organisms appear to be those of Fowler, Heyraud, and Beasley (1975). Using 

 mussels (Mytilus gallop) ovincialis), shrimp (Lysmata seticaudata), and marine worms 

 (Nereis diversicolor), they followed both uptake and loss of plutonium in the organisms 

 after direct uptake from seawater and, in the case of mussels and shrimp, from labeled 

 food as well. The valence state of ^^'^Pu tracer was chemically adjusted to either the 

 quadrivalent (-1-4) or hexavalent (+6) state before the isotope was introduced into the 

 experimental aquaria. No subsequent attempt was made to determine the valence state of 

 the isotope during the course of the experiment. In all three organisms direct uptake from 

 seawater occurred quite readily. For mussels exposed to filtered seawater containing 

 Pu(+6), concentration factors ranged from 20 to 60 after 26 days of exposure, and a large 

 percentage of the plutonium taken up resided in the shell and byssus threads. In those 

 cases where the byssus was removed from the mussel, greater than 80% of the activity was 

 associated with the shell. The activity was firmly bound to the shell material, and the 

 shell showed only minor losses even when rinsed for as long as 8 hr in 0.17V HCl solution. 

 Mussels that accumulated Pu(+6) directly from seawater showed a two-component loss 

 when placed in unlabeled seawater; the biological half-life (T^i^) for the fast pool 

 containing 35% of the total plutonium was 7 days; that for the slower turnover pool, 

 which contained 65% of the total plutonium, was 776 days (>2yr). Mussels that had 

 accumulated Pu(+4) from both food and water showed more rapid turnover owing both 

 to a shorter labeling time and presumably to a more rapid clearance of labeled material 



