Environmental Engineering Reference
In-Depth Information
TABLE 6.5.3
Effect of Various Solutes on the Water Solubility of Pure Sodium Nitrate-
Cancrinite
pH/cond
(µS/cm)
ppm
Na
ppm
Al
ppm
Si
ppm
NO
3
-
Leachant
Pure water
9.3/426
100
52
64
118
Same powder, fresh water
9.2/366
86
65
77
107
DI water + 0.25 g dry metakaolin
9.0/218
55
34
38
13
Silica-saturated water (initially 38 ppm SiO
2
)
9.0/459
111
46
96
133
0.1 M NaCl
8.5/dnd
dnd
6.8
3.5
15
0.05 M MgCl
2
8.1/dnd
77
1.5
3.2
36
0.05 M CaCl
2
8.1/dnd
133
4.8
0.7
18
0.002 M Ca(HCO
3
)
2
7.1/440
63
0.5
6.3
15
0.1 M LiOH
dnd
633
66
120
421
0.015 M Al(NO
3
)
3
4.3/dnd
810
dnd
280
dnd
0.13 M KNO
3
8.8/dnd
271
13.8
4.1
dnd
INEEL tap water
8.5/477
72
1.9
5.9
20
* dnd = did not determine
3.
Dissolved silica alone (i.e., silica present as H
4
SiO
4
— not as an anion
and thereby accompanied by an equivalent number of cations) does not
affect cancrinite (HC) solubility.
This, in turn, suggests that dissolution (leaching) of cancrinite (HCs) involves
both ion exchange and hydrolysis. Like most silicate soil minerals, cancrinite par-
ticles behave rather like sodium-form “weak acid” cation exchange resin; i.e., exhib-
iting substantial cation exchange capacity and having a high affinity for the hydro-
nium (H
+
) ion. Consequently, when cancrinite is exposed to pure water, hydrolysis
ensues to provide H
+
, which displaces an equivalent amount of sodium ion into the
solution (100 ppm Na 0.004 M). The majority of the hydroxide ion simultaneously
produced attacks the mineral's backbone to displace an equivalent amount of anions
(silicate, aluminate, and nitrate) — the remainder serves to slightly boost the solu-
tion's pH (to ~9.3, [OH
-
] 0.00002 M). When the water contains salts, the mass
action effect of the cations suppresses hydrolysis (and, consequently, dissolution of
the mineral) by occupying its cation exchange sites. The most important conclusion
to be drawn from this is that the solubility of HC waste forms would be considerably
lower in typical desert groundwater (like that under Yucca Mountain) than in the
pure water leachants specified by most leach protocols.
6.5.4.2
Standard Leach Protocols
6.5.4.2.1 Product Consistency Test (PCT)
Most current WACs specify use of the PCT.
11
For “high” waste forms, the Yucca
Mountain WAC decrees that a candidate material must lose a lesser fraction of its
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