Environmental Engineering Reference
In-Depth Information
Simulation of Species in the Multi-metal Solutions
Simulations of the metals' speciation in the test beaker solutions were carried out using
CHEAQS Pro software
[12]
. The goal was to identify whether or not any of the metals
were precipitated as salts in the test solutions. The program calculates the metals' spe-
cies in a solution with equilibrium constants, drawn from various data bases and with
input in the form of solution ion concentrations. The following beaker solutions were
selected for the simulation: pH 7.0 with a phosphate buffer and pH ~9 without a buffer.
The results were evaluated by comparison with the calculated removal efficiencies in
the beaker tests with furnace sludge added. The XRD results revealed that the sludge
contained several minerals comprising iron and carbonate. Therefore a second set of
simulations was performed where the input concentration of ferric iron (Fe
3+
) was 40 mg/L
and the carbonate (CO3
2−
) concentration 0.1, 10 or 1,000 mg/L. These simulations were
only carried out for the multi-metal solutions at pH 7.0.
Result and Discussion
The results of the XRD qualitative analysis revealed that the sludge is mainly com-
posed of different iron hydr/oxides, carbonates and silicates. The following minerals
were found:
Iron hydroxide-oxide: Fe
1.833
(OH)
0.5
O
2.5
Hematite: Fe
2
O
3
Magnetite: Fe
3
O
4
Wuestite: FeO
Iron sulphate: Fe(SO
4
)(H
2
O)
Quarts: SiO
2
Calcium silicate: CaSi
2
O
5
Calcite: CaCO
3
The results are in agreement with quantitative XRD analysis of blast-furnace slag
[10]
. The only difference was that kaolinite Al
2
Si
2
O
5
(OH)
4
was one of the main
minerals in the latter study. The active surfaces of the iron oxides in the sludge may
explain the good sorption capacity for metals. The mineral goethite FeO(OH),
closely related to the Fe
1.833
(OH)
0.5
O
2.5
, has well documented sorption capacities for
metals
[13]
. The other iron oxides detected could also be rearranged to active surface
OH groups in contact with water, thereby acting as effective sorbents for metals
through complex binding and surface precipitation
[14]
. Kaolinite and carbon, such
as coke and graphite, may also enhance the sorption capacity of the material
[10]
,
but these components were not screened for in this study. The carbonates endow
the material with a high buffer capacity, and the risk of a decrease in pH for water
solutions in contact with the sludge material is low. The sludge had a buffer capacity
of 0.32 mmol H +/g for decreasing pH to 6.0. The old furnace sludge had a higher
buffer capacity of 1.2 mmol H +/g. The alkalinity of the old sludge was 64 mg/L,
and the concentration of carbonate was calculated to 0.47 mg/L.
