Geochemistry of Extremely
Alkaline (pH > 12) Ground water
in Slag–Fill Aquifers
By Austin Krabbenhoft
11/29/10
Lake Calumet - Chicago
The Problem
• Ground water is among the most
degraded in Illinois
• Has a high pH (>12), high total dissolved
solids, and high ammonia (>50 mg/L)
• High levels of Ba, Cr, Mn
• Moderate levels of 15 other metals
including Pb, Hg, As, and Li
(Roadcap, Walton, & Bethke, 2005)
The Problem
• Why?
• Slag wastes used as fill
• Other harmful waste also used as fill
– Fly ash
– Solid industrial wastes
– Demolition debris
– Household trash
• 600 m3 of fill dumped on 150 km2
Other Sources of Contamination
• Leakage from Landfills
• Spills at hazardous waste-handling
facilities
• Road-salt runoff
• Illegal dumping
Sampling Site
• Former wetlands filled with steel slag.
• Water was sampled from an isolated pond
fed by diffuse ground water.
• Land surrounding the site is unvegetated
and had never been developed
Sample Collection
• Samples of precipitated calcite and slag
were taken
• Water was collected in the field using a
pump and a .45 micron high-capacity filter
Chemical Analysis - Slag
• Composed of Iron slag & Steel slag
– Iron slag
• Ca2MgSi2O7
• Contains little or no iron
• Uniform in composition
– Steel slag
• Composed of 50% calcium silicates
 Rakinaite Ca3Si2O7
 Larnite Ca2SiO4
Weathered Products
• Weathered down to:
– Rakinaite Ca3Si2O7 + 7H2O → 3Ca2+ + 2H4SiO4 + 6OH-
– Larnite Ca2SiO4 + 4H2O → 2Ca2+ + H4SiO4 + 4OH– Akermanite Ca2MgSi2O7 + 7H2O → 2Ca2+ + Mg2+ + 2H4SiO4 +
6OH-
Weathered Products
• Each reaction releases calcium ions and
uses protons
• Creates Ca-OH in the ground water
• This explains the high alkalinity of the
water
Calcium and Carbon Dioxide
• Carbonate from rainwater and underlying
sands and soils forms CO32• CO32- is the dominate species at a pH of
10
• When the alkaline water is exposed to
atmospheric CO2 the pH is reduced by 4
factors and calcite precipitates
Calcite Reactions
• At high pH
– CO2 + H2O→2H+ + CO32– Ca2+ + CO32-→CaCO3
•
At neutral pH
–
+
2+
H + CaCO3 → Ca + HCO3
Geochemical Model
•TITLE Before sparging
•
SOLUTION 1
•
pH 11.2 charge
•
temp 14.5
•
pe 4.075
•
units mmol/L
•
Al .012
•
Ba .00023
•
B .0037
•
Cd .00014
•
Ca .82
•
C .33 as CO3-2
•
Cl .093
•
Cu .00052
•
F .053
•
Fe .00016
•
Pb .00036
•
Li .0049
•
Mg .005
•
Mn .00005
•
N .047 as N03-
•
K .69
•
Si .061
•
Na .57
•
Sr .0015
•
S .14 as SO4-2
•
Zn .0089
•
END
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TITLE After sparging
SOLUTION 1
pH 11.2 charge
temp 14.5
pe 4.075
units mmol/L
Al .012
Ba .00023
B .0037
Cd .00014
Ca .82
C .33 as CO3-2
Cl .093
Cu .00052
F .053
Fe .00016
Pb .00036
Li .0049
Mg .005
Mn .00005
N .047 as N03K .69
Si .061
Na .57
Sr .0015
S .14 as SO4-2
Zn .0089
EQUILIBRIUM_PHASES 1
O2(g) -0.670976998
CO2(g) -3.5
END
Geochemical Model
•----------------------------Before------------------------
•
•
pH
•
pe
•
Specific Conductance (uS/cm, 14 oC)
•
Density (g/cm3)
•
Activity of water
•
Ionic strength
•
Mass of water (kg)
•
Total alkalinity (eq/kg)
•
Total CO2 (mol/kg)
•
Temperature (deg C)
•
Electrical balance (eq)
• Percent error, 100*(Cat-|An|)/(Cat+|An|)
•
Iterations
•
Total H
•
Total O
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=
=
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=
=
=
11.573
4.075
410
0.99930
1.000
3.630e-003
1.000e+000
2.549e-003
3.300e-004
14.500
-7.323e-015
-0.00
9
1.110145e+002
5.550986e+001
----------------------------After----------------------------
pH
pe
Specific Conductance (uS/cm, 14 oC)
Density (g/cm3)
Activity of water
Ionic strength
Mass of water (kg)
Total alkalinity (eq/kg)
Total CO2 (mol/kg)
Temperature (deg C)
Electrical balance (eq)
Percent error, 100*(Cat-|An|)/(Cat+|An|)
Iterations
Total H
Total O
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
8.587
12.965
226
0.99937
1.000
3.779e-003
1.000e+000
2.502e-003
2.385e-003
14.500
-7.321e-015
-0.00
14
1.110145e+002
5.551475e+001
Geochemical Model
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------------------------------Sample---------------------------•
Phase
SI log IAP log KT
•
Calcite
CO2(g)
Dolomite
Fe(OH)3(a)
FeS(ppt)
O2(g)
Pb(OH)2
Zn(OH)2(e)
1.34
-7.09
-8.76 -10.09
0.34 -16.49
-0.84
4.05
-111.47 -115.38
-24.29 -27.09
1.14
9.66
-0.02
11.48
-8.43
-1.33
-16.84
4.89
-3.92
-2.81
8.52
11.50
CaCO3
CO2
CaMg(CO3)2
Fe(OH)3
FeS
O2
Pb(OH)2
Zn(OH)2
•
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------------------------------Sample after sparging-----------Phase
SI log IAP log KT
Calcite
CO2(g)
Dolomite
Fe(OH)3(a)
FeS(ppt)
O2(g)
Pb(OH)2
Zn(OH)2(e)
0.69
-7.74
-3.50
-4.83
-0.86 -17.69
1.43
6.32
-156.37 -160.28
-0.67
-3.48
-0.57
7.96
-0.55
10.95
-8.43
-1.33
-16.84
4.89
-3.92
-2.81
8.52
11.50
CaCO3
CO2
CaMg(CO3)2
Fe(OH)3
FeS
O2
Pb(OH)2
Zn(OH)2
Possible Solutions
• As an experimental
solution atmospheric air
was bubbled through 900
mL of site water that
contained 100 g of
precipitate.
• The water was sparged
with a glass gas
dispersion tube at a
constant rate until pH
stabilized
• Mortality rate went from
100% in the extremely
alkaline water to <10%
Possible Solutions
•
Alternatives:
– Sparge the water with 1 atm of
CO2
– Mix a strong acid like HCl with the
water
– Pros:
• Drops the pH 100 times faster
than with atmospheric air
• Any additional CO32- or HCl
beyond 7 would dissolve the
calcite and not affect the pH
– Cons:
• Those systems can be expensive
and labor intensive to set up and
monitor
• Reduced pH does not necessarily
mean more livable.
– The toxicity rates were four times
higher than in air-sparging
– Due to the release of metals as
the calcite dissolved
My Solution
• Add pyrite to the slag fill and through the
following reaction it will make the water
more acidic
• 2 FeS2 (s) + 7 O2 + 2 H2O → 2 Fe2+ (aq) +
4 SO4 (aq) + 4 H+
• Need .3022 g of FeS2 to neutralize 1 L of
sample water
When Modeled
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TITLE Addition of atmospheric air
SOLUTION 1
pH 11.2 charge
temp 14.5
pe 4.075
units mmol/L
Al .012
Ba .00023
B .0037
Cd .00014
Ca .82
C .33 as CO3-2
Cl .093
Cu .00052
F .053
Fe .00016
Pb .00036
Li .0049
Mg .005
Mn .00005
N .047 as N03K .69
Si .061
Na .57
Sr .0015
S .14 as SO4-2
Zn .0089
EQUILIBRIUM_PHASES 1
O2(g) -0.670976998
CO2(g) -3.5
END
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TITLE Addition of pyrite
SOLUTION 1
pH 11.2 charge
temp 14.5
pe .25
units mmol/L
Al .012
Ba .00023
B .0037
Cd .00014
Ca .82
C .33 as CO3-2
Cl .093
Cu .00052
F .053
Fe .00016
Pb .00036
Li .0049
Mg .005
Mn .00005
N .015 as NH4+
O(0) .55
K .69
Si .061
Na .57
Sr .0015
S 1.26 as SO4-2
Zn .0089
END
Modeling Results
Their Results
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My Results
----------------------------Sparging with Air------------------•
--------pH
pe
Specific Conductance (uS/cm, 14 oC)
Density (g/cm3)
Activity of water
Ionic strength
Mass of water (kg)
Total alkalinity (eq/kg)
Total CO2 (mol/kg)
Temperature (deg C)
Electrical balance (eq)
Percent error, 100*(Cat-|An|)/(Cat+|An|)
Iterations
Total H
Total O
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
8.587
12.965
226
0.99937
1.000
3.779e-003
1.000e+000
2.502e-003
2.385e-003
14.500
-7.321e-015
-0.00
14
1.110145e+002
5.551475e+001
•
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----------------------------Addition of Pyrite-------------------------pH
pe
Specific Conductance (uS/cm, 14 oC)
Density (g/cm3)
Activity of water
Ionic strength
Mass of water (kg)
Total alkalinity (eq/kg)
Total CO2 (mol/kg)
Temperature (deg C)
Electrical balance (eq)
Percent error, 100*(Cat-|An|)/(Cat+|An|)
Iterations
Total H
Total O
=
7.074
=
0.250
= 263
=
0.99938
=
1.000
= 4.666e-003
= 1.000e+000
= 3.082e-004
= 3.301e-004
= 14.500
= 3.705e-018
=
0.00
= 16
= 1.110130e+002
= 5.551303e+001
•
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------------------------------ Sparging with Air- ----------------------------Phase
Alunite
Calcite
Fe(OH)3(a)
Melanterite
Pyrite
Smithsonite
Strontianite
Zn(OH)2(e)
SI log IAP
log KT
-6.11
-6.16
-0.06 KAl3(SO4)2(OH)6
0.69
-7.74
-8.43 CaCO3
1.43
6.32
4.89 Fe(OH)3
-20.78 -23.13
-2.35 FeSO4:7H2O
-256.73 -275.51 -18.78 FeS2
-0.86 -10.74
-9.88 ZnCO3
-1.19 -10.47
-9.28 SrCO3
-0.55
10.95
11.50 Zn(OH)2
•
------------------------------ Addition of Pyrite- -----------------------------
•
Phase
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Alunite
Calcite
Fe(OH)3(a)
Melanterite
Pyrite
Smithsonite
Strontianite
Zn(OH)2(e)
SI log IAP
4.31
-1.79
-3.67
-7.68
-40.46
-2.32
-3.68
-2.60
4.26
-10.22
1.23
-10.02
-59.24
-12.21
-12.95
8.90
log KT
-0.06 KAl3(SO4)2(OH)6
-8.43 CaCO3
4.89 Fe(OH)3
-2.35 FeSO4:7H2O
-18.78 FeS2
-9.88 ZnCO3
-9.28 SrCO3
11.50 Zn(OH)2
Problems with my modeling
• Could not make it work if I added the
aqueous Fe I would need to.
• Doesn’t specify how much FeS2 should be
added to the soil .
• A pH –below 8.1 may have dissolved
some calcite and brought more heavy
metals into solution.
Citations
•
Roadcap, S. G., Walton, R. K., Bethke, M. C. (2005). Geochemistry of extremely alkaline (pH > 12) ground water
in slag-fill aquifers Ground Water, 43 (6), 806-816.