Eh = E° + 0.059 log aity4
(Pu2*)
Q)
Eh = 1.182 - 0.236 pH - 0.059 log (Pu’*)
(4)
or
1.0
Equation (4) is taken from Pourbaix (1966), who used E° = 1,182 volts for
Pu({OH), as the solid phase. Using the assumption that ion activity approximates
don concentration, equation 4 can be used to establish the Eh-pH line at which
a given concentration of Pu? will be in equilibrium with solid-phase Pu(OH),.
Equation 3 and 4 predict that as pH increases (Eh constant}, the equilibrium
{Pu3") will decrease. Conversely, when pH is constant, lowering Eh increases
the maximum concentration of Pu({II1) stable in the presence of Pu(0#H),.
0.8
0.6
,
+
Figure 3 is an Eh-pH diagram comparing the Pus? - Pu(OH), and Fe? - Fe(OH) 4
redox system.
he diagram was constructed using applicable equations (Pourbaix,
1966).
The Fe? - Fe(OH), line was calculated at 10-4 M {Fe7 ) using the
equation:
(5)
The pu3t - Pu(OH), lines at 107" and 10-8 (Pu3*) were calculated using equation
4 above.
This approach suggests that under environmental conditions where
Fe(III) is reduced to Fe(II), Pu(IV) may be susceptible to reduction to Pu({III).
This reduction would occur more readily under acidic conditions.
The relatively
greater abundance of Fe over Pu in the environment also suggests that the redox
reactions of Fe may have an influence on Pu chemistry.
This illustration was
specifically used to point out that in the environment, even if equilibrium was
presumed to be established, other substances like Fe can become important
relative to Pu.
Thus at dilute Pu concentrations,
environmental Fe 2
concent ra-
tions could be sufficiently in excess of Pu to influence Pu chemistry in a manner
not obvious when Pu calculations are considered alone.
0.4
En (volts)
zh = 1.05? - 0.1773 pH - 0.0581 log (Fe*)
3+
Pu(OH),
‘
ruze
.
Fe(OH),
0.2
\
\
\
—
‘
-0.2
}
H,0
VL
\ 4
‘
‘KL
1
+
Ferrous tron is, of course, a reductant towards Pu"
n acidic solutions
(Connick, 1954) as Figure 1 shows (fi.e., at pH 1, Fe? is a stronger reductant
than Pu? }.
ot
Besides Fe*
,
other
al. (1976) discussed
states and suggested
humic substances and
Teductants are present in the environment.
Bondietti et
the influence of organic substances of Pu oxidation
that Pu(VI) or Pu(V) appear. unstable in the presence of
polysaccharides.
The use of stability field diagrams and mineral stabtlity calculations has one
very important limitation which must be kept in mind.
The results of such
analyses are valid only for the conditions considered.
Reactions which are
not considered may aiso occur, adding additional complexities to the real
world behavior of Pu in soils, sediments, or natural waters. Stability field
diagrams also do not consider kinetic parameters of the given reactions.
Hs
“AL
I
AL
se
|
Fe(OH),
= ~s,
a
i
———
|
|
2
|
|
Pu(OH), it
L4
8
6
4
|
10
12
14
pH
Fig. 3. Comparison of the stability relationships for soluble
pu2*(qo74 and 1078 M) and Fe@*(107* M) in equilibrium
with Pu(OH), and Fe(OH), .» respectively. One atmosphere
total pressure and 25°C. Plutonium (V) and (VI) states
not included in analysis.
454
455