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Fic. 11—The variation in the [Th}]/[Ca] ratio with the degree of calcification of the algae
(10-7 pg/1°)), there is no question of the solubility
product being exceeded for RaCOs, or Ra(OQH)>.
The behavior of thorium appears to parallel that of
radium, The greater rate of increase in the value of
[Th]/[Ca] ratio with decreasing calcium content in the
organism as compared to that for radium (Figures 10
and 11) is in agreement with the greater stability of
thorium complexes with organic ligands over those of
radium. However, the picture is not as simple as this,
as it is highly unlikely that the free thorium ion
(Th*+) exists at the pH of seawater,@® and there-
fore the organism is concentrating thorium either as a
hydrated oxide or a charged colloidal particle.
havior of these two ions in seawater. Unlike radium
at the pH, pCOe and Ey of seawater, uranium is
most likely to occur as the anionic species
[UO2(COs)3]*-.4” Therefore, while radium is available for either complexing by the organic fraction o1
coprecipitation in the inorganic fraction, the urany!
ion, being in the anionic form, is not readily available
for the formation of organic complexes, unless they
are much more stable than the carbonate complex.
Uranium is, therefore, coprecipitated as the anionic
carbonate with calcium carbonate (or undergoes an
anion exchange reaction) in those species of algae in
On the other hand, the results for uranium suggest
that the effect of the organic fraction on the complexation of uranium is secondary to the coprecipitation or
which calcification occurs. When the fraction of calclum carbonate is low, then the formation of organic
complexes becomes more important.
The differences in behavior found in the concentra-
is entirely consistent with the differences in the be-
are of considerable interest, particularly as the trend
to a greater concentration of radium in theleast cal-
ion exchange with the calcium carbonate matrix. This
difference in behavior between uranium and radium
tion of radium by membersof the three phyta studied