Environmental Engineering Reference
In-Depth Information
A cheap precipitant forming hydroxides is lime (CsÖvári et al. 2002). CaO
was tested in lysimeter experiments with a duration of 3 years. 400 kg of
uranium-containing waste from heap leaching (70 mg U/kg) was treated
with a 1:20 mixture of CaO. The resulting uranium concentration was below
1 mg/L. Promising results were also achieved in subsequent field tests
using horizontal barriers (1.5 kg CaO/t). Uranium concentrations lower than
1 mg/L, sometimes less than 0.1 mg/L are achievable.
The key advantage of PRBs is their low energy consumption and it is there-
fore important that the precipitates are not changed back by any means to
soluble forms. Unlike in wastewater treatment plants, the precipitated mate-
rial remains in the barrier for the whole period of operation. Spent barrier
materials can be replaced by placing the reactive matrix in a double-walled
structure of prefabricated elements (Beitinger and Fischer 1994). Stability of
the resulting precipitates is therefore an important issue for the application
of precipitation reactions in PRBs.
9.2.1.1 Uranium
Uranium is the heaviest naturally occurring element. All isotopes are radio-
active; the half-lives of the two relevant isotopes
238
U and
235
U are 4.5 × 10
9
and 7.0 × 10
8
years, respectively (Seelmann-Eggebert et al. 1981). Uranium
present in groundwater, for example, from mine tailings, is dangerous not
because of its radioactivity but because of its toxicity as a heavy metal. Based
on the German Radiation Protection Act, a 0.3 mg/L limit can be calculated
from the radiation limit (7.0 Bq/L of the natural mixture of isotopes). Due to
its toxicity an upper limit of 0.015 mg/L is being considered by the World
Health Organization (WHO) (Birke et al. 2009).
Uranium occurs mainly in the oxidation states +4 and +6. The hexavalent
uranium, that is, the uranyl ion
UO
2+
and respective hydroxo- and carbonato-
complexes are more mobile than U(IV) compounds, similarly to chromium
where Cr(VI) has a higher mobility than Cr(III). The speciation of uranium
is a complex system dependent on pH and carbonate concentration as can be
seen in Figure 9.4 (calculated using MinteqA2 [Allison et al. 1991]). For more
details on the complex uranium solution equilibria see Langmuir (1978).
Removal from groundwater is possible by reduction of U(VI) to U(IV) in a
reducing environment, for example, by elemental iron (Cantrell et al. 1995):
2
+
2
+
Fe
+
UO
()
aq
→
Fe
+
UO
()
s
(9.4)
2
2
The solubility of uraninite UO
2
is in the range of 10
−8
mol/L in a pH range
between 4 and 14. Below a pH of 4, uranium becomes soluble. A measure-
ment on the dissolution of spent nuclear fuel in deionized water under non-
oxidizing conditions provided results in the range of 10
−9
-10
−5
mol/L (Bickel
et al. 1996). Under oxidizing conditions where UO
2
can be transformed
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