Geology Reference
In-Depth Information
responsible for the generation of this waste continues to be used in
countries such as China, Russia, Kazakhstan, India, and Pakistan.
92
In
Glasgow, the chromium content of this residue is high at 3-4% w/w but,
more significantly, elevated concentrations as high as
B
1% w/w of
highly toxic and carcinogenic hexavalent chromium, which takes the
form of chromate (Cr
VI
O
2
) at the prevailing high pH of 9-12 char-
acteristic of high-lime COPR, have been measured in the solid phase at
the contaminated sites. Furthermore, on sites in the Glasgow area where
COPR from a high-lime chemical works was last deposited in the 1960s,
Cr
VI
is still leaching out in such quantity that receiving groundwaters
and streams in the area are seriously contaminated at concentrations of
up to 100 mg L
1
, about 2000 times greater than Environmental Quality
Standards (EQSs).
93
There are many problems involved in the in situ application of
remediation-by-reduction methods, e.g. employing Fe, Fe
II
, or organic
compounds, to reduce Cr
VI
held in specific mineral phases within COPR
to the much less harmful trivalent Cr
III
. For example, the use of ferrous
iron, Fe
21
, from ferrous sulfate, rather than reduce the Cr
VI
O
2
held
within the COPR via
2CrO
2
4
þ
6Fe
21
þ
13H
2
O
Cr
2
O
3(s)
þ
6Fe(OH)
3(s)
þ
8H
1
-
(3.95)
actually increases the release of CrO
2
because of anion exchange of
SO
2
from the ferrous sulfate for CrO
2
held in the layered double
hydroxide mineral hydrocalumite within the COPR.
94
In principle,
many of the problems involved in the in situ application of methods
designed to reduce Cr
VI
held within COPR to Cr
III
can be avoided if the
emphasis is switched to treatment of the Cr
VI
released to groundwater
and surface water from COPR. Where the objective is to treat ground-
water either in a conventional pump and treat system or as it migrates
through a permeable reactive barrier, there is much less impediment to
chemical methods using Fe
II
or Fe
0
or S
2
:
95-97
2CrO
2
4
þ
3H
2
S
(aq)
þ
2H
2
O
2Cr(OH)
3(s)
þ
3S
(s)
þ
4OH
-
(3.96)
More generally for Cr
VI
-contaminated groundwater, which may also
arise from other industrial activities such as tanning and electroplating,
both in situ
98
and pump and treat methods could employ biological as
well as chemical methods for the reduction of Cr
VI
to Cr
III
, for example
in packed bed bioreactors.
99
Another generic method for the reduction of Cr
VI
to Cr
III
is the
addition of organic matter, for example as horse dung or molasses. Such
treatment, however, whether for direct chemical reduction (cf. Equation
