INSOLUBLE SOURCE TERMS
MO, +L
ML
SOLUBLE SOURCE TERMS
HYDROLYZABLE
M(NO,), + L + HO -
(1)
MO, : nH,0
OR
+ML
M(OH}, |
M(NO,)x + L + H,O
MO; + ML
ORGANIC COMPLEXES
(3)
Fig. 2.
Further generalizations of transuranic behavior on the basis of source
terms are complicated by the overwhelming fmportance of soil properties and
processes in influencing transuranic behavior on a regional and local basis.
This review will consider, in detail, the influence of soil properties and
abiotic and biotic processes on the long-term solubility of the transuranics entering soils.
Principal emphasis will be directed toward the
role of soil microorganisms in this phenomenon. Microorganisms, in intimate
association with soil particles, are known to play an important role in
effecting solubilization of elements considered insoluble in soils strictly
on the basis of their inorganic chemistry.
To date, studies of the microbiology of the transuranic elements have been limited principally to Pu.
This review will emphasize Pu, but, where possible, the avatlable information will be used as a framework for broader discussions encompassing the
long term behavior of other transuranic elements.
TRANSURANIC CHEMISTRY IN SOIL
NONHYDROLYZABLE
(2)
substitution by major competing fons, such as Ca and H (Lahav and Hochberg,
1976; Lindsay, 1972; Norvell, 1972) and the stability of the organic ligand
to microbial decomposition (Wildung and Garland, 1975). The disruption of
the complex may lead to marked reduction in transuranic solubility through
hydrolysis and precipitation reactions as described for acid solutions on
dilution, A portion of the ion released may react with other, perhaps more
Stable, ligands in soil. The mobility of the intact complexes, in turn,
will be principally a function of their chemical and microbiological
stability and the charge on the complex which will govern the degree of
sorption on soil particulates.
ML, +L, +H,O
ML, + ML, + ML,»
ML, +L, + HO
AS IN (1) OR (2)
Influence of source term on transuranitec behavior in soil.
Transuranic elements are represented by M. and L represents
inorganic and organic ligands capable of reacting with transuranic elements and forming soluble or insoluble products,
Plutonium
The principal chemical reactions likely influencing Pu behavior in soil are
summarized in Fig. 3.
Plutonium fons may commonly exist in aqueous solution
in valence states ILI, IV, V(Pud,*) and VI (Put). Other valence states
are known (II, VIT) and predicted (VIII) but these occur under unique
conditions (Cleveland, 1970).
Disproportionation reactions are common, and
due to kinetic factors, Pu is unique among the chemical elements in that it
may simultaneously exist in all of the common valence states.
The tendency
of Pu to hydrolyze in aqueous solutions of low acidity follows the order
Pu" > Pud2t? > Put? > pud.+ (Cleveland, 1970).
Hydrolysis, which occurs in
a stepwise fashion, is likely the major mechanism whereby Pu is insolublized
in the environment. At high (g/1) Pu concentrations, hydrolysis of Put’,
may lead to the formation of a collodial Pu polymer. At these concentrations,
the polymer is characterized by a distinct absorption spectrum,
Although
the polymer has not been fully characterized, it is generally thought to be
an intermediate hydrolysis product of Put* containing oxide or hydroxide
bridges, with an absorption spectrum different than Pu{OH),.
However,
studies by Lloyd and Haire (1973) have indicated that the polymer may be
aggregates of emall, discrete, amorphous or crystalline, primary partictes
of 5 to 20 A in diameter.
It is of interest that x-ray diffraction patterns
of the polymeric Pu and that of Pu(OH), (Ockenden and Welch, 1956) both
showed a pattern characteristic of the cubic PuO> lattice, suggesting that
the polymer and the hydroxide of Put* may be hydrated PuO2 with differences occurring in primary particle size and crystallinity (Lloyd and
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