In this report, the results of our laboratory and field experiments are
summarized and discussed to show the influence of soil microbial activities on
the bioavailability and biotransport of transuranics in desert environments.
DISCUSSION
We reported previously that Aspergillus niger, a commonly occurring soil
fungus, took up plutonium from culture media and transported it to the fungal
spores (Au, 1974). The chemical form of the plutonium added to the culture
media, and the pH of the growth media, had a marked influence on plutonium
uptake and translocation (Au and Beckert, 1975a).
To better perceive the
concept of plutonium transport, we used the term transport factor (TF) which
is applicable for culture media where the distribution of nutrients and pollutants is uniform (Au et al., 1976a). The concentration-independent transport
factor is defined as the fraction of the total plutonium that is transported
from the medium to the tissue divided by the fraction of the total dry mass
transported from the medium to the tissue, or
TF
where,
- Pu, /Puy
ML/My
Pup
total plutonium content of tissue (e.g., mycelium, spores)
Puy
total plutonium originally present in the parent medium.
My
M
dry mass of tissue
dry mass originally present in the parent medium.
The TF shows immediately if accumulation of or discrimination against the
pollutant has occurred:
TF > 1 indicates accumulation, and TF < 1 defines
discrimination against the pollutant.
In general, it was found that for Aspergillus niger all transport factors
defining the movement of plutonium from culture media via the mycelium to the
spores were smaller than 1, indicating discrimination against the transport of
plutonium from the agar media to the spores. This was true for all chemical
forms of plutonium tested at pH 2.5 and 5.5. As required by the definition,
the transport factors derived from experiments using plutonium dioxide spheres
were fairly concentration-independent, although the plutonium concentrations
applied to the culture media varied by a factor of 100.
The TF values derived
from experiments using plutonium nitrate and plutonium citrate, respectively,
were reasonably close at pH 2.5, but showed a greater variation at pH 5.5
(Beckert and Au, 1975).
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