BIOLOGICAL AVAILABILITY
While the primary purpose of this paper is not to review biological uptake of
transuranium elements, it is appropriate to examine those experiments or field
observations where the reported data indicate differences in biological uptake
which can be ascribed to chemical state.
In particular, the dominant oxidation
state of Pu might be deduced from availability to plants in cases where the
state in the soil is not known. Jacobson and Overstreet (1948) conducted clay
adsorption and plant uptake experiments with different oxidation states of Pu.
Plutonium which was added initially as Pu(VI) was assimilated by barley
plants to a greater degree than Pu added in the tri- or tetravalent states.
Price (1973) compared the plant uptake of Np(V), Pu{IV), Am(II1) and Cm(IIT)
and observed that Np was assimilated the greatest. Americium and Cm were
taken up about equally, as might be expected since they are both trivalent and
chemically very similar, while Pu was assimilated the least.
Cummings and
Bankert (1971) compared Pr, Ce, and Pu and found that two lanthanides, which
are characteristically trivalent, were about ten times more available to oats
than Pu. The Pu oxidation state was not given; however, the lower uptake of
Pu would suggest the tetravalent state dominated since the trivalent state of
the actinides and lanthanides are similar chemically. Adams et aZ. (1975)
compared the uptake of Pu, U, and Am and observed the same effect; Am was taken
up more readily than Pu, while U was similar to Am.
Snap Bean
0.01
0.002
0.002
Soybean
0.01
0.004
0.003
Millet
0.01
0.0001
0.0001
Tomato
0.02
0.007
0.006
Differences,in ppant gvailability of the different oxidation state species
*#S5011] concentrations based on & M HNO, extractable.
(MD. , MO>2°, M3, M*’) may be related to cheir relative reactivity with soil
components or their relative insolubilities. Plant uptake appears to be
consistent with the relative tendencies of these oxidation states to hydrolyze
(Dahlman et al., 1976).
Table 4 presents the relative pliant uptake of Th, Pu, and U from soil contaminated in 1944 by trace Pu. A description of this site was discussed in this
symposium by Dahlman and McLeod.
Representative concentration ratios (C.R.)
are presented for snap beans, millet, soybeans, and tomatoes. These CR's are
based on the 8 M acid extractable soil concentrations of the three elements.
This extraction removes all of the soil Pu; consequently, natural U and Th
were compared using the same extraction conditions.
As can be observed, the
relative uptake of Pu and Th are similar, indicating that plant roots appear
to be mobilizing both elements in a similar manner.
Uranium is more readily
assimilated, which is consistent with the observed biogeochemical behavior of
U and Th; U being more available than Th. This provides indirect evidence
that Pu may behave like Th, a concept consistent with the thesis that Pu
should be tetravalent in aerobic environments (Bondietti et al., 1976).
Table 5 presents additional extraction data for the three elements from this
soil. When 1 M HNO; or 10% Na gCO3-5% NaHCO, solutions were used as extractants,
the ratios of Th to Pu were similar co the 8M acid values. While weak nitric
acid or carbonate did not substantially differ in their extraction of U (73
and 71%, respectively), dilute nitric acid was not effective in extracting Pu
or Th and carbonate was only intermediate between the strong (8 M) and weak (1
M) acid. This behavior is probably due to the fact that in 1 M acid, Pu and
468
TABLE 4, COMPARATIVE PLANT UPTAKE OF ACTINIDES FROM ORNL FLOODPLAIN
Plant
230y
Concentration Ratio*, **
2427p
239py
*Typical values; C.R. = [plant]/[soil].