Geoscience Reference
In-Depth Information
The remediation wells have been installed downstream of the CWA storage bunker having a depth
of 15 m (well 1) and 19 m (well 2) with a screen length of 4 m each. The distance between the
wells is 30 m. The pumps have been installed 2 m above the filter screens. In order to reduce
the risk of iron incrustations at the pumps, additional infiltration pipes for Fe-enriched water
were installed ending one meter below the pumps. Thus, the wells can both pump and infiltrate
water. t-As-concentration of 1.3 mg L
−
1
and 2.5 mg L
−
1
were measured at well 1 and in well 2,
respectively after installing the wells.
7.3
REMEDIATION METHOD
7.3.1
Precipitation and sorption by metals
Since i-As adsorbs to iron oxides (
Chapter 3
) it is possible to reduce the concentration in the
aquatic phase by adding iron salts. The removal of i-As from water has been thoroughly inves-
tigated in batch experiments (Bissen and Frimmel, 2003; Dixit and Hering, 2003; Hering
et al
.,
1996; Pierce and Moore, 1982; Rott and Meyer, 2000; Scott
et al
., 1995; Violante
et al
., 2006;
2007; Wilkie and Hering, 1996). It could be demonstrated that iron (Fe) was suitable to remove
i-As from a solution. The needed amount of applied Fe was always greater than the amount
of As in the solution (8:1-40:1). The adsorption of i-As is strongly influenced by availability
of competitors for sorption spots such as sulfate, phosphate, silica, ammonia and bicarbonate.
Thus the efficiency of the remediation method is dependent on the hydrogeochemical environ-
ment. However, the concentrations of potential competitors were rather low (SO
4
42 mg L
−
1
,PO
4
∼
0mgL
−
1
, and SiO
2
3mgL
−
1
). Holländer
et al
. (2008) and Krüger
et al
. (2008) confirmed the
literature results with the ambient groundwater from the military site. They removed more than
90% of i-As.
Daus
et al
. (2008) found that the CWA decomposition product phenylarsonic acid was removed
from a solution by precipitating Fe. Holländer
et al
. (2008) and Krüger
et al
. (2008) showed that
organic arsenicals from CWA were precipitated by adding different Fe compounds. They added
50 mg zero-, bi- or trivalent Fe to 250 mL of water containing 9 mg L
−
1
t-As (66% org-As) and
precipitated the iron by aeration. Under those conditions, they could demonstrate that the t-As-
concentration can be reduced in all experiments. However, bivalent ferrous chloride (FeCl
2
) and
ferrous sulfate (FeSO
4
) showed the best efficiency of about 80% in removal of t-As. Further batch
experiments showed that adding FeCl
2
to solutions at different As compositions and concentra-
tions resulted in removal efficiencies over 90% for i-As and 50-75% for org-As. Additional soil
column experiments showed that 1 g t-As could be removed using of 5 to 20 g Fe (Krüger
et al
.,
2008).
7.3.2
Remediation technique
Subterranean deferrification and demanganesation (Rott
et al
., 1996) has proven to decrease
As-concentrations in treated water. While oxidizing the Fe compounds,
in-situ
precipitated Fe
hydroxide coats the grain surface in the subsurface. As adsorbs on these Fe compounds and thus
is removed from the liquid phase (Rott and Meyer, 2000; Rott
et al
., 1996). Various experi-
ments showed the possibility to remove As cost-efficiently by subterranean deferrification and
demanganesation (Kauffmann, 2008; Rott and Meyer, 2000; Rott
et al
., 1996; van Halem
et al
.,
2010). Since the removal of As always requires a higher concentration of Fe(II) than As, this
method cannot be applied at sites where the Fe concentration is too low. Thus the subterranean
deferrification and demanganesation has been enhanced by an additional Fe dosage to overcome
these limitations. Ferric incrustations are an emerging problem for this technique if the bivalent
Fe reacts especially within the well with dissolved oxygen.
For building an effective subterranean reaction zone, the resulting pilot plant for the military
site comprises two bidirectional wells and a container with measurement and control technology.
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