Geochemistry of Extremely Alkaline (pH > 12) Ground water

in Slag–Fill AquifersBy Austin Krabbenhoft

11/29/10

Lake Calumet - Chicago

The Problem

• Ground water is among the most degraded in Illinois (Roadcap, Walton, & Bethke, 2005)

• 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

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 CO3

2-

• 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+ + CO3

2-

– Ca2+ + CO32-→CaCO3

• At neutral pH

– H+

+ CaCO3 → Ca2+

+ HCO3-

Geochemical Model• 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 N03-• K .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

•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

Geochemical Model

• ----------------------------After----------------------------

• pH = 8.587 • pe = 12.965• Specific Conductance (uS/cm, 14 oC) = 226• Density (g/cm3) = 0.99937• Activity of water = 1.000• Ionic strength = 3.779e-003• Mass of water (kg) = 1.000e+000• Total alkalinity (eq/kg) = 2.502e-003• Total CO2 (mol/kg) = 2.385e-003• Temperature (deg C) = 14.500• Electrical balance (eq) = -7.321e-015• Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.00• Iterations = 14• Total H = 1.110145e+002• Total O = 5.551475e+001

•----------------------------Before------------------------

• pH = 11.573 • pe = 4.075 • Specific Conductance (uS/cm, 14 oC) = 410• Density (g/cm3) = 0.99930• Activity of water = 1.000• Ionic strength = 3.630e-003• Mass of water (kg) = 1.000e+000• Total alkalinity (eq/kg) = 2.549e-003• Total CO2 (mol/kg) = 3.300e-004• Temperature (deg C) = 14.500• Electrical balance (eq) = -7.323e-015• Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.00• Iterations = 9• Total H = 1.110145e+002• Total O = 5.550986e+001

Geochemical Model• ------------------------------Sample---------------------------• Phase SI log IAP log KT

• Calcite 1.34 -7.09 -8.43 CaCO3• CO2(g) -8.76 -10.09 -1.33 CO2• Dolomite 0.34 -16.49 -16.84

CaMg(CO3)2• Fe(OH)3(a) -0.84 4.05 4.89 Fe(OH)3• FeS(ppt) -111.47 -115.38 -3.92 FeS• O2(g) -24.29 -27.09 -2.81 O2• Pb(OH)2 1.14 9.66 8.52 Pb(OH)2• Zn(OH)2(e) -0.02 11.48 11.50 Zn(OH)2

• ------------------------------Sample after sparging------------• Phase SI log IAP log KT

• Calcite 0.69 -7.74 -8.43 CaCO3• CO2(g) -3.50 -4.83 -1.33 CO2• Dolomite -0.86 -17.69 -16.84 CaMg(CO3)2• Fe(OH)3(a) 1.43 6.32 4.89 Fe(OH)3• FeS(ppt) -156.37 -160.28 -3.92 FeS• O2(g) -0.67 -3.48 -2.81 O2• Pb(OH)2 -0.57 7.96 8.52 Pb(OH)2• Zn(OH)2(e) -0.55 10.95 11.50 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 CO3

2- 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• 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

• 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 N03-• K .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

Modeling Results

• ----------------------------Sparging with Air---------------------------

• pH = 8.587• pe = 12.965 • Specific Conductance (uS/cm, 14 oC) = 226• Density (g/cm3) = 0.99937• Activity of water = 1.000• Ionic strength = 3.779e-003• Mass of water (kg) = 1.000e+000• Total alkalinity (eq/kg) = 2.502e-003• Total CO2 (mol/kg) = 2.385e-003• Temperature (deg C) = 14.500• Electrical balance (eq) = -7.321e-015• Percent error, 100*(Cat-|An|)/(Cat+|An|) = -0.00• Iterations = 14• Total H = 1.110145e+002• Total O = 5.551475e+001

• ----------------------------Addition of Pyrite---------------------------

• pH = 7.074 • pe = 0.250 • Specific Conductance (uS/cm, 14 oC) = 263• Density (g/cm3) = 0.99938• Activity of water = 1.000• Ionic strength = 4.666e-003• Mass of water (kg) = 1.000e+000• Total alkalinity (eq/kg) = 3.082e-004• Total CO2 (mol/kg) = 3.301e-004• Temperature (deg C) = 14.500• Electrical balance (eq) = 3.705e-018• Percent error, 100*(Cat-|An|)/(Cat+|An|) = 0.00• Iterations = 16• Total H = 1.110130e+002• Total O = 5.551303e+001

Their Results My Results

• ------------------------------ Addition of Pyrite- ------------------------------

• Phase SI log IAP log KT

• Alunite 4.31 4.26 -0.06 KAl3(SO4)2(OH)6• Calcite -1.79 -10.22 -8.43 CaCO3• Fe(OH)3(a) -3.67 1.23 4.89 Fe(OH)3• Melanterite -7.68 -10.02 -2.35 FeSO4:7H2O• Pyrite -40.46 -59.24 -18.78 FeS2• Smithsonite -2.32 -12.21 -9.88 ZnCO3• Strontianite -3.68 -12.95 -9.28 SrCO3• Zn(OH)2(e) -2.60 8.90 11.50 Zn(OH)2

• ------------------------------ Sparging with Air- ------------------------------

• Phase SI log IAP log KT

• Alunite -6.11 -6.16 -0.06 KAl3(SO4)2(OH)6

• Calcite 0.69 -7.74 -8.43 CaCO3• Fe(OH)3(a) 1.43 6.32 4.89 Fe(OH)3• Melanterite -20.78 -23.13 -2.35

FeSO4:7H2O Pyrite -256.73 -275.51 -18.78 FeS2• Smithsonite -0.86 -10.74 -9.88 ZnCO3• Strontianite -1.19 -10.47 -9.28 SrCO3• Zn(OH)2(e) -0.55 10.95 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.