Dissolution and
Precipitation
Lecture 23
Carbonate Solubility
•
•
Carbonate is the most
common kind of chemical
sediment and carbonate
components (Ca, Mg, CO3) are
often the dominant species in
natural waters.
Using equilibrium constants, we
can calculate calcite solubility
as a function of PCO2:
1/3
ìï
K1K sp-cal K CO2 üï
[Ca 2+ ] = í PCO2
ý
2
K
g
2+ g
2
ï
Ca
HCO3 þ
îï
•
•
•
Thus calcite solubility increases
with 1/3 power of PCO2.
One consequence is that
calcite shells tend to dissolve in
deep ocean water.
A second is that calcite will
dissolve out of soils when
microbial activity is present.
Carbonate Solubility
1/3
ìï
K1K sp-cal K CO2 üï
[Ca 2+ ] = í PCO2
ý
2
K 2g Ca2+g HCO
ïî
3 ï
þ
• Another interesting
feature is because
of this non-linearity,
mixing of two
saturated waters
can produce an
undersaturated
water.
Carbonate Solubility
• Open system solutions,
those in equilibrium
with CO2 gas in the
atmosphere or soil,
can dissolve more
calcite than closed
systems waters.
• In a sense, this is
because dissolving
CO2 in water lowers
pH, resulting in greater
dissolution.
Mg Solubility
•
Several Mg-bearing minerals can
precipitate from solution:
o
o
o
•
ΣCO2 = 10-2.5 M
brucite, Mg(OH)2
dolomite CaMg(CO3)2
magnesite MgCO3
We can use equilibrium constant
expressions, such as:
2
-11.6
K bru = aMg2+ aOH
- = 10
•
•
to construct a predominance
diagram showing which phase will
precipitate under a given set of
conditions.
We construct these in the same way
we constructed pe-pH diagrams,
namely manipulate the log
equilibrium constant expressions to logaMg2+ = - pK bru + 2 pKW - 2 pH = 16.4 - 2 pH
get [Mg2+] on one side of the
equation and pH on the other.
o
We simplify things by calculating equilibrium with
only 1 carbonate species at a time and ignoring
the others.
logaMg+ = - log aCO2- - pK mag
3
Mg solubility as a function of
CO2 & pH
[Mg2+] = 10-4 M
Mg solubility as a function of
CO2 & Ca/Mg
Mg2+ = 10-4 M
Constructing stability
diagrams
ΣCO2 = 5x 10-2 M
• This diagram shows the stability
of ferrous iron minerals as a
function of pH and sulfide for
fixed total Fe and CO2.
• Procedure: manipulate
equilibrium constant
expressions to obtain and
expression for ΣS in terms of pH.
For example:
[Fe2+] = 10-6 M
(Pyrrhotite)
(Siderite)
o FeCO3 + H+ ⇋ Fe2+ + HCO3–
o FeCO3 + 2H2O ⇋ HCO3- H+ + Fe(OH)2
o FeS + 2H2O ⇋ Fe(OH)2 + H+ + HS-
• Trick: simplify by ignoring
pH = log K FeCO - log[HCO3- ]- log[Fe2+ ] = 7.5
②
species present at low conc.
(e.g., CO32- at low pH).
③ pH = log[HCO3 ]+13.0
3
- pH
➄ log SS = pH - pK FeS + pK Fe(OH )2 + log(K S +10 )
Solubility of SiO2
• Silica forms silicic acid
(H4SiO4) in solution, which
can then dissociate through
a series of reactions, e.g.,
• H4SiO4 ⇋ H3SiO4– + H+
K1
• H3SiO4– ⇋ H2SiO42- + H+ K2
• Solubility can be expressed
as:
ìï K1 K1K 2 üï
[SiO2 ]T = [H 2 SiO4 ] í1+ 2 ý
a
aH + þï
+
H
îï
• where [H4SiO4] is controlled
by solubility of either quartz
or amorphous silica.
• As a consequence, its
solubility is a function of pH:
high only at high pH.
Solubility of Hydroxides
•
•
The hydroxide is the least soluble salt of many metals. Therefore,
it is the solubility of their hydroxides that controls the solubility of
these metals in natural waters.
Since these dissolution reactions involve OH–, they are pHdependent, and the slope of the solubility curve depends on
the valence of the metal (e.g., -3 for Fe3+, -2 for Fe2+, -1 for Ag+).
Solubility of Al(OH)3
• Solubility of gibbsite:
o
Al(OH)3 + 3H+ ⇋ Al3+ + H2O
K gib =
aAl 3+
aH +
= 10 -8.1
• However, Al forms hydroxide
complexes, e.g.:
• Al3+ + H2O ⇋ Al(OH)2+ + H+
• The total dissolved Al will be
the sum of all Al species in
solution:
ìï K
K
K
K üï
aAl 3+T = aAl 3+ í1+ 1 + 2 2 + 3 3 + 4 4 ý
ïî aH + aH + aH + aH + ïþ
• A consequence of this is that
acid rain leads to Al
poisoning.
Solubility of Ferric Iron
Silicate Solubility
• We’ve looked at the solubility of Si, Al, Fe and other
cations in the isolation of simple laboratory-like systems.
• The real world is usually more complex. Silicate rocks
predominate at the surface of the Earth, thus Si, Al, and
other cations will generally all be present.
• In addition to gibbsite and SiO2, some of the more
common weathering products of silicate rocks include:
• gibbsite: Al(OH)3
• kaolinite: Al2Si2O5(OH)4
• pyrophyllite: Al2Si4O10(OH)2
• illite (muscovite): KAl3Si3O10(OH)2
Silicate Solubility
Gibbsite will precipitate from Al-bearing solution only at lowest
concentrations of SiO2. Occurrence generally restricted to highly
weathered soils where all the SiO2 has washed out.
Silicate Solubility
Clay Minerals
• Review section 6.5 to become familiar with clay
minerals, but we will not cover it in class.