1
GEOL390 Environmental Applications of Geochemical Modeling: HW#2 Answer Key
2) a) Calculate the total concentration of dissolved silica (as ppm SiO2) in equilibrium with quartz at (a) pH 9.0,
(b) pH 10.0, and (c) pH 11.0. Use GWB React and add 0.1 molal KCl to the solution for charge balance.
# React script, saved Wed Apr 10 2002 by Dabieshan
data = "e:\program files\gwb\gtdata\thermo.dat" verify
work_dir = E:\Users\Ayers\GEO320\GWB
temperature = 25
swap Quartz for SiO2(aq)
1 kg free H2O
free gram Quartz = 500
molality K+ = .1
molality Cl- = .1
pH = 9
slide pH to 11
Solubility SiO2(aq) in ppm: a) 7.0 at pH = 9 b) 17.7 at pH = 10 c) 126.8 at pH = 11
b) Use Act2 to construct a solubility diagram for SiO2(aq) from
pH = 4 to 14. How do the results compare to those you
obtained in (a)? Explain why the solubility of quartz depends
so strongly on pH.
2
----
H4(H2SiO4)4
0
log a SiO2(aq)
Quartz
–2
# Act2 script, saved Fri Feb 28 2003 by ayersj
data = "c:\program files\gwb\gtdata\thermo.dat" verify
–4
H SiO
diagram SiO2(aq) on SiO2(aq) vs pH
H SiO
x-axis from 4 to 14 increment .5
–6
SiO (aq)
y-axis from -8 to 2 increment 1
Below pH = 9.9 the solubility of quartz is independent of
25°C
–8
4
5
6
7
8
9
10
11
12
13
14
pH and H4SiO4 is the dominant species. Between pH=9.9 and
pH
~12 H3SiO4 is the dominant species. Quartz solubility
depends on pH because it dissolves in water to form a weak acid, silicic acid = H4SiO4. Dissociation of H4SiO4
to form H3SiO4- increases the solubility by LeChatlier’s Principle.
3
-4
Diagram SiO 2(aq), T = 25 °C , P = 1.013 bars, a [H 2O] = 1
2
4
2
ayersj Fri Feb 28 2003
3) Kaolinite is often present in soils and sediments affected by acid mine drainage, in which it is the least
soluble of the clay minerals. It is probably often the chief
source of the dissolved aluminum in acid mine drainage. Use
React to calculate the solubility of kaolinite as a function of
pH. Assume Na+ = Cl- = SO42- = 0.01 molal, SiO2(aq) = 17
mg/L from pH 4 to 9. Make plots of fluid composition,
minerals, and Al species as a function of pH. At what pH is the
0
Kaolinite
Paragonite
solubility of kaolinite expressed as ΣAl at a minimum? pH 66.25
# React script, saved Wed Apr 03 2002 by Ayers
data = "c:\program files\gwb\gtdata\thermo.dat" verify
Minerals (log moles)
–1
–2
–3 Quartz
–4
–5
–6
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
pH
ayersj Fri Feb 28 2003
2
work_dir = C:\
temperature = 25
swap Kaolinite for Al+++
1 kg free H2O
free gram Kaolinite = 100
total molality SO4-- = .01
pH = 4
total mg/l SiO2(aq) = 17
total molality Cl- = .1
total molality Na+ = .1
slide pH to 9
0
0
–1
Sodium
Chlorine
-
Silicon
–4
Al
(log molal)
+++
–3 Aluminum
Species with Al
Fluid composition (log moles)
Al(SO4 )2
Sulfur
–2
–5
4
+++
+
Al(OH)2
Al(OH)
++
AlOH
++++
Al2 (OH)2
+
Al3 (OH)4 (5 )
+
AlSO4
Al(OH)3
–10
–6
–7
–20
–8
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
4
9
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
pH
pH
ayersj Fri Feb 28 2003
ayersj Fri Feb 28 2003
4) Choose a rare earth phosphate (remember to load the extended thermodynamic database
thermo.com.v8.r6+.dat). Construct a solubility diagram using Act2 that illustrates the effect of pH on solubility.
# Act2 script, saved Fri Feb 28 2003 by ayersj
data = "C:\Program Files\Gwb\Gtdata\thermo.com.v8.r6+.dat" verify
diagram La+++ on La+++ vs pH
log activity HPO4-- = -1
x-axis from 0 to 14 increment 1
y-axis from -15 to 0 increment 1
0
–2
–4
-4
, T = 25 °C , P = 1.013 bars, a [H O] = 1, a [HPO ] = 10
–1
–8
++
2
–10 LaH PO
2
4
–12
---
La(PO4)2
-
La(HPO4 )2
+++
–14
Diagram La
log a La
+++
LaPO4:10H2O
–6
25°C
0
2
4
6
8
pH
10
12
14
ayersj Fri Feb 28 2003