Environmental Engineering Reference
In-Depth Information
of existing structures. Mobility is strongly related
to the physicochemical state and the location of
pollutants. If elements or organic compounds
(pesticides) become trapped within the structure of
minerals or humic substances, they are neither mo-
bile nor bioavailable and, particularly in the case
of organics, they are physically protected and not
accessible to microorganisms that might be able
to transform them (Abdel-Sabour
2007
). “Clay
and clay minerals,” a mixture of clay minerals and
natural zeolites with calcium compounds can also
be used as liner material at solid waste disposal
sites (particularly soils polluted by Cr(VI)) as their
impermeability and sorption properties prevent
migration of toxic metals from waste sites (Minato
and Shibue
1998
). “Organic matter” content and
bioactivity are considered as important factors in
reducing almost 96 % of the added Cr(VI) under
aerobic, field moist conditions (Losi et al.
1994
).
Similarly, Cifuentes et al. (
1996
) added easily de-
gradable organic substances of a very narrow C:N
ratio and found marked Cr(VI) reduction.
The third technology, “
chemical reduction,
”
can also be used to convert Cr(VI) to the trivalent
valence state, which is generally less toxic and
less soluble (Patterson
1985
). Reducing agents
(such as ferrous sulfate, ferrous ammonium sul-
fate) can be delivered to the soil subsurface by
injection wells or in situ soil mixing equipment.
James et al. (
1997
) stated that effective remedia-
tion of Cr(VI)-contaminated soils by reduction
depends on: (1) reduction of Cr(VI) to Cr(III)
which is inert toward reoxidation; (2) absence of
undesirable reaction products; and (3) establish-
ment or maintenance of soil pH and Eh condi-
tions that favor the reduction of Cr(VI) and disfa-
vor oxidation of Cr(III). The extent of oxidation
of Cr(III) in soils amended with wastes is based
on four interacting parameters: (1) solubility and
form of Cr(III) related to oxidation waste oxida-
tion potential; (2) reactive soil Mn(I, IV) hydrox-
ide levels (soil oxidation potential for Cr(III); (3)
soil potential for Cr(VI) reduction (soil reduction
potential); and (4) soil waste pH as a modifier
of the first three parameters (pH modification
value). Each of these four parameters can be
quantified with laboratory tests and ranked nu-
merically; the sum of which is the potential Cr
oxidation score (PCOS) for assessing the relative
hazard of a waste-soil combination as proposed
by James et al. (
1995
). Patterson and Fendorf
(
1998
) reported that the reduction of Cr(VI) to
Cr(III) decreases the toxicity and mobility of Cr
contaminants in soils and water. In addition, the
formation of a highly insoluble Cr(III) product
would decrease the likelihood of future Cr(III)
reoxidation. They noticed that amorphous iron
sulfide minerals like mackinawite (FeS1-x) have
the potential to reduce large quantities of Cr(VI)
and in the process form very stable [Cr, Fe](OH)
3
solids. In their study the effectiveness of amor-
phous FeS as a reductant of Cr(VI) was assessed
by identifying the solution and solid phase prod-
ucts of the reaction between FeS suspensions
and chromate. Results showed that iron sulfide
removed all of the added Cr(VI) from solution
for the reaction conditions studied and reduced
between 85 and 100 % of the Cr(VI) to Cr(III).
Chromate reduction occurred dominantly at the
FeS surface and resulted in [Cr
0.75
,Fe
0.25
](OH)
3
;
while less extensive, reduction of Cr(VI) by
Fe(II) (aq) was noted and produced a solid with
the opposite Cr:Fe ratio, [Cr
0.25
,Fe
0.75
](OH)
3
(Abdel-Sabour
2007
).
6.8
Current Challenges and Future
Directions
Heavy metal stress is one of the major problems
affecting agricultural productivity of plants. Nat-
ural flora show relative differences in their heavy
metal tolerance capacity. Some plants grow well
in a soil enriched with toxic levels of heavy met-
als while others cannot. The roles of several ef-
fectors in amelioration of Cr have been discussed
above. Additionally, several natural plant species
have been identified showing heavy metal accu-
mulator behaviors. These natural heavy metal ac-
cumulators could be a potential source for genetic
manipulation of other important agricultural crop
plants. However, this needs a further detailed ac-
count of experimental validation. Metal chelation
proves to be of high importance as a generalized
means of heavy metal removal from soil and
water. However, in situ application of chelat-
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