Geology Reference
In-Depth Information
K
SP
¼
{Ca
21
}{SO
2
4
}
¼
10
4.58
(3.26)
A solution is considered to be undersaturated, saturated, or oversaturated
with respect to a solid phase, for example gypsum, if K
SP
4
{Ca
21
}{SO
2
}
observed
, K
SP
¼
{Ca
21
}{SO
2
}
observed
,orK
SP
o
{Ca
21
}
{SO
2
}
observed
, respectively.
3.2.2.3 Influence of Organic Matter. Natural organic matter is pre-
sent in most natural systems. With respect to weathering processes, its
importance in freshwaters and indeed soils and sediments in contact
with freshwaters can be attributed to the presence of organic acids,
which include low molecular weight compounds, e.g. oxalic acid, and
extremely complex, high molecular weight coloured compounds de-
scribed as humic substances - a heterogeneous mixture of polyfunctional
macromolecules ranging in size from a few thousand to several hundred
thousand Daltons (Da). In addition to inorganic acids, e.g. H
2
CO
3
,
these provide hydrogen ions for the acid hydrolysis of minerals. As well
as promoting dissolution, natural organic matter can influence the
formation of new mineral phases.
31
3.2.3 Complexation Reactions in Freshwaters
In this section, as a starting point, it is assumed that all species in
solution are in hydrated form, e.g. M
III
(H
2
O)
3
6
, where the six water
molecules form the first co-ordination sphere of the metal ion, M
III
. The
hydrated form is often represented as, for example, M
3
þ
(aq)
.
3.2.3.1 Outer and Inner Sphere Complexes. Outer sphere complexa-
tion involves interactions between metal ions and other solute species in
which the co-ordinated water of the metal ion and/or the other solute
species are retained. For example, the initial step in the formation of ion
pairs, where ions of opposite charge approach within a critical distance
and are then held together by coulombic attractive forces, is described as
outer sphere complex formation.
Mg
2
þ
(aq)
þ
SO
2
4(aq)
(Mg
21
(H
2
O)(H
2
O)SO
2
"
4(aq)
)
(3.27)
The formation of ion pairs is influenced by the nature of the oppositely
charged ions, the ionic strength of solution, and ion charge. Ion pairs are
generally formed between hard (low polarizability) metal cations, e.g.
Mg
21
,Fe
31
, and hard anions, e.g. CO
2
3
,SO
2
4
, and ion pair formation
is generally most significant in high ionic strength aqueous phases. In
