In nature the pH of surface water mostly lies
between 4 and 9. Rainwater is frequently slightly
acid due to dissolved CO 2 , which gives an acid
reaction:
H 2 O þ CO 2 ¼ H 2 CO 3 carbonic acid
ð
Þ
H 2 CO 3 ¼ H
þ
þ HCO
À
3 ¼ 2H
þ
þ CO
2À
3
Humic acids may give the water in lakes and rivers
a low pH. Sulphur pollution from burning oil and coal
gives SO 2 , which is oxidised in water to sulphuric
acid:
2SO 2 þ O 2 þ 2H 2 O ¼ 2H 2 SO 4
In areas with calcareous rocks or soils this sulphuric
acid is immediately neutralised and the water becomes
basic, as is the case across much of Europe. By contrast, in areas with acidic granitic rocks as in the south
of Norway and large areas of Sweden, the rock does
not have sufficient buffer capacity to counteract acid
rain or acidic water produced by vegetation (due to
humic acids). Organic material also contains a certain
amount of sulphur, and drainage of bogs, or drought,
can produce an acidic reaction. This is because H 2 S
from organic material is oxidised to sulphate when the
water table is lowered, allowing oxygen to penetrate
deeper in these organic deposits.
The water near the surface of large lakes and the sea
can have a high pH because CO 2 is consumed due to
high organic production (photosynthesis). If the
organic material decomposes (oxidises) on its way to
the bottom, CO 2 is released again, causing the pH to
decrease with depth since the solubility of the CO 2
increases with the increasing pressure.
CO 2 is also less soluble in the warm surface water
than in the colder water at greater depth.
Seawater is a buffered solution, with a typical pH
close to 8, though this varies somewhat with temperature, pressure and the degree of biological activity.
Eh and pH are important parameters for describing
natural geochemical environments, and the diagram
obtained by combining these two parameters is particularly useful.
The lower limit for Eh in natural environments is
defined by the line Eh ¼ À0:059 pH, because otherwise we would have free oxygen, and the upper limit
corresponds to Eh ¼ 1:22 À 0:059 pH, beyond which
free oxygen would be released from the water. If we
also set pH limits at 4 and 9 in natural environments,
we can divide the latter into four main categories:
1. Oxidising and acidic
2. Oxidising and basic
3. Reducing and acidic
4. Reducing and basic
Variations of pH and Eh are the major factors
involved in chemical precipitation mechanisms in sedimentary environments where there is not strong evaporation (evaporite environments).
The stability of some important compounds in
porewater may be presented in an Eh - pH diagram
(Fig. 3.4).
The solubility of many elements is highest in the
reduced state and they are precipitated by oxidation.
This is particularly characteristic of iron and manganese, whereas others such as uranium and vanadium
are least soluble in the reduced state.
3.3.1 Distribution Coefficients
When a mineral crystallises out of solution, the composition of the mineral will be a function of the composition of the solution and the temperature and
pressure. Trace elements which are incorporated in
the mineral structure are particularly sensitive to
variations of these factors. With constant temperature
and pressure, the concentration of a trace element
within a mineral which is being precipitated, is proportional to its concentration in the solution. The ratio
between the concentration of an element in the mineral
and its concentration in the solution (water) is called
the distribution coefficient (Kd).
A number of elements substitute for Ca
++ in the
calcite lattice: Mn
++ , Fe
++ and Zn
++ have distribution
coefficients ðKd >1Þ. This means that they will be
captured, so that the mineral becomes enriched in
these elements relative to the solution.
Mn
þþ
=Ca
þþ
ðmineralÞ ¼ k Á Mn
þþ
=Ca
þþ
ðsolutionÞ
k ¼ Kd Mn
ð Þ=KdðCaÞ
3 Sedimentary Geochemistry
95
between 4 and 9. Rainwater is frequently slightly
acid due to dissolved CO 2 , which gives an acid
reaction:
H 2 O þ CO 2 ¼ H 2 CO 3 carbonic acid
ð
Þ
H 2 CO 3 ¼ H
þ
þ HCO
À
3 ¼ 2H
þ
þ CO
2À
3
Humic acids may give the water in lakes and rivers
a low pH. Sulphur pollution from burning oil and coal
gives SO 2 , which is oxidised in water to sulphuric
acid:
2SO 2 þ O 2 þ 2H 2 O ¼ 2H 2 SO 4
In areas with calcareous rocks or soils this sulphuric
acid is immediately neutralised and the water becomes
basic, as is the case across much of Europe. By contrast, in areas with acidic granitic rocks as in the south
of Norway and large areas of Sweden, the rock does
not have sufficient buffer capacity to counteract acid
rain or acidic water produced by vegetation (due to
humic acids). Organic material also contains a certain
amount of sulphur, and drainage of bogs, or drought,
can produce an acidic reaction. This is because H 2 S
from organic material is oxidised to sulphate when the
water table is lowered, allowing oxygen to penetrate
deeper in these organic deposits.
The water near the surface of large lakes and the sea
can have a high pH because CO 2 is consumed due to
high organic production (photosynthesis). If the
organic material decomposes (oxidises) on its way to
the bottom, CO 2 is released again, causing the pH to
decrease with depth since the solubility of the CO 2
increases with the increasing pressure.
CO 2 is also less soluble in the warm surface water
than in the colder water at greater depth.
Seawater is a buffered solution, with a typical pH
close to 8, though this varies somewhat with temperature, pressure and the degree of biological activity.
Eh and pH are important parameters for describing
natural geochemical environments, and the diagram
obtained by combining these two parameters is particularly useful.
The lower limit for Eh in natural environments is
defined by the line Eh ¼ À0:059 pH, because otherwise we would have free oxygen, and the upper limit
corresponds to Eh ¼ 1:22 À 0:059 pH, beyond which
free oxygen would be released from the water. If we
also set pH limits at 4 and 9 in natural environments,
we can divide the latter into four main categories:
1. Oxidising and acidic
2. Oxidising and basic
3. Reducing and acidic
4. Reducing and basic
Variations of pH and Eh are the major factors
involved in chemical precipitation mechanisms in sedimentary environments where there is not strong evaporation (evaporite environments).
The stability of some important compounds in
porewater may be presented in an Eh - pH diagram
(Fig. 3.4).
The solubility of many elements is highest in the
reduced state and they are precipitated by oxidation.
This is particularly characteristic of iron and manganese, whereas others such as uranium and vanadium
are least soluble in the reduced state.
3.3.1 Distribution Coefficients
When a mineral crystallises out of solution, the composition of the mineral will be a function of the composition of the solution and the temperature and
pressure. Trace elements which are incorporated in
the mineral structure are particularly sensitive to
variations of these factors. With constant temperature
and pressure, the concentration of a trace element
within a mineral which is being precipitated, is proportional to its concentration in the solution. The ratio
between the concentration of an element in the mineral
and its concentration in the solution (water) is called
the distribution coefficient (Kd).
A number of elements substitute for Ca
++ in the
calcite lattice: Mn
++ , Fe
++ and Zn
++ have distribution
coefficients ðKd >1Þ. This means that they will be
captured, so that the mineral becomes enriched in
these elements relative to the solution.
Mn
þþ
=Ca
þþ
ðmineralÞ ¼ k Á Mn
þþ
=Ca
þþ
ðsolutionÞ
k ¼ Kd Mn
ð Þ=KdðCaÞ
3 Sedimentary Geochemistry
95