Geology Reference
In-Depth Information
Box 9.3 Hydrolysis
to a geochemist, the term
hydrolysis
refers to reactions in
which either (or both) of the O-h bonds in water is broken.
Consider the hydrolysis of atmospheric sulfur dioxide.
only in fairly acidic solutions. If an al-bearing solution
mixes with a less acidic solution, precipitation occurs as a
result of hydrolysis:
SO
+→ →+
HO
HSOHHSO
sulfurous
acid
+
−
+ →
(
)
+
3
Al
+
3
HO
Al OH
3
H
+
(9.3.3)
2
2
23
3
2
3
(9.3.1)
solution
insoluble
precipitate
this is one of the reactions contributing to the phenom-
enon of
acid rain
(O'Neill, 1998). hydrolysis of an acidic
oxide (SO
2
, NO
2
, CO
2
) generally gives an acidic solution.
1
the converse applies to the hydrolysis of a basic oxide:
Geochemists refer to such elements and their precip-
itates as
hydrolysates
(see Figure 9.3.1). For certain
elements like Fe and Mn, the first step in this process is
oxidation. Fe
2+
and Mn
2+
are readily dissolved by mildly
acid waters during weathering, but once dissolved they
are prone to oxidation to Fe
3+
and Mn
4+
, whose higher
ionic potentials lie in the hydrolysate field and which
consequently precipitate as minerals like goethite
[FeO(Oh)].
Na OHO aOH a H
alkaline
hydroxide
solution
+→ →+
2
2
+
2
−
2
2
(9.3.2)
hydrolysis reactions are important in weathering. elements
of intermediate ionic potential like al and Fe(III) are soluble
OXY-ANIONS
HYDROLYSATES
+6
S
n+
N
P
V
Nb(Ta)
+4
C
Si
Mn Ti
SnZr(Hf)
U Th
SOLUBLE
CATIONS
GaV
Sc
Cr
Fe
Al
iv
Al
vi
REE (Y)
B
MnMg
Co
Fe
NiCuZn
+2
Be
Ca
Eu Sr Pb
Ba
Li
Na
K
Rb
Cs
0
0
0.05
0.10
0.15
0.20
Ionic radius/nm
r
Figure 9.3.1
how metals behave in aqueous systems, in relation to nominal cation charge and ionic radius
(cf. Figure 9.1.1). high charge-to-radius elements like p, Si and B exist in solution as the oxy-anions
phosphate
pO
4
3−
,
metasilicate
SiO
3
2−
and
borate
BO
3
3−
. the lowest charge-to-radius elements occur as soluble cations. In between is the
hydrolysate field.
1
the acids involved are sulfurous (h
2
SO
3
), nitric (hNO
3
) and
carbonic (h
2
CO
3
- see Chapter 4) acids.
Search WWH ::
Custom Search