Environmental Engineering Reference
In-Depth Information
instances where barium leaching is a concern, the waste can be treated with sodium
or calcium sulfate to form insoluble barium sulfate.
8.2.3
O
XIDATION
/R
EDUCTION
Oxidation/reduction can be used to treat metals, non-metals, organics, and organo-
metallic compounds. Reduction is usually performed prior to S/S, but adding reduc-
ing agents directly to the grout slurry has also been used.
16
The presence of strong oxidants or reductants can change the valence state of a
number of metals, affecting their chemical speciation and often drastically changing
their solubility. Metals with more than one oxidation state possible in aqueous
solutions include arsenic, chromium, iron, mercury, nickel, selenium, and techne-
tium. And even though they only have one oxidation state in aqueous systems, lead,
silver, copper, cadmium, and zinc can also be strongly influenced by redox pro-
cesses.
17
Ferrous sulfate, sodium bisulfite (NaHSO
3
), sodium metabisulfite
(Na
2
S
2
O
5
), metallic iron (zero valent iron), sodium hydrosulfite (Na
2
S
2
O
4
), sodium
hypochlorite, and potassium permanganate are commonly used to reduce or oxidize
metals to the desired valence state.
Ferrous sulfate is probably the most widely used. It is safe and inexpensive, and
often co-precipitates other metal contaminants. Its main drawbacks are that it can
require a low pH for acceptable treatment times and it can result in a large volume
increase. However, low pH does not always appear to be necessary. Rapid reduction
of Cr
6+
in solution up to pH 10 has been reported.
18
Sodium bisulfite and sodium metabisulfite (the anhydrous form of sodium
bisulfite) are effective reducing agents for Cr
6+
and require much less acid and alkali
than ferrous sulfate, and therefore generate less sludge. However, bisulfites can
generate sulfur oxides on contact with acids, and this can be a disadvantage to their
use. In general, shipping, handling, storage, and air pollution considerations make
it impractical for most treatment processes.
19
Ground granulated blast furnace slag, as described in detail in Chapter 4, is an
effective reducing agent because of the iron sulfide naturally present in the slag
20
(see Figure 8.3 regenerated from the data of Angus and Glasser
21-23
). A slag:cement
combination of 75:25 virtually eliminates calcium hydroxide as a hydration product;
i.e., the presence of excess slag prevents buildup of this cement hydration product.
24
Oak Ridge National Laboratory (ORNL) tested various mixtures of slags in combi-
nation with portland cement and flyash for the reduction and stabilization of radio-
active wastes containing technetium and nitrates.
25
This natural reducing capability
of slag has proven effective in stabilizing technetium by reducing the soluble +7
pertechnetate anion to the more insoluble +4 cation.
26
Westinghouse Savannah River
Company has been using slag formulations since 1984 for reduction of Tc
+7
and
Cr
+6
, as well as for improved nitrate retention and better durability.
27
Ferrous sulfide
decreased pertechnetate leaching from cement-flyash grouts, but increased leaching
of other anions (chromate, selenate, and nitrate).
28
Figure 8.4 gives the E
h
-pH dia-
gram for Tc(VII)-Tc(IV) calculated by HSC 5
28b
using NH
4
+
as the counterbalancing
cation in aqueous solution.
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