An Introduction to Environmental Chemistry

of impenetrable siliceous-iron layers (laterite) inhibit plant growth. By contrast,

the low-altitude vertisols have high fertility on account of their high CEC. These

clay-rich soils do not suffer from metal-associated toxicity, although deep crack-

ing during dry periods can be a problem for agriculture.

The marked contrast in solubility (Box 4.12) between insoluble oxides or oxy-

hydroxides of aluminium and iron and other more soluble soil cations and H 4 SiO 4

The Chemistry of Continental Solids 107

Box 4.12 Solubility product, mineral solubility and saturation index

The dynamic equilibrium between a mineral

and its saturated solution (i.e. the point at

which no more mineral will dissolve), for

example:

eqn. 1

is quantified by the equilibrium constant (K),

in this case:

eqn. 2

Since the CaCO 3 is a solid crystal of calcite, it

is difficult to express its presence in terms of

activity (see Section 2.6). This is overcome by

recognizing that reaction between a solid

and its saturated solution is not affected by

the amount of solid surface presented to the

solution (as long as the mixture is well

stirred). Thus the activity of the solid is

effectively constant; it is assigned a value

of 1 or unity (see eqn. 3), and makes no

contribution to the value of Kin equation

3.

The equilibrium constant for a reaction

between a solid and its saturated solution is

known as the solubility product and is usually

given the notation Ksp. Solubility products

have been calculated for many minerals,

usually using pure water under standard

conditions (1 atm pressure, 25°C

temperature).

The solubility product for calcite (eqn. 1) is

thus:

eqn. 3

The solubility product can be used to

calculate the solubility (mol l-^1 ) of a mineral in

Ka asp=Ca^2 +- +-CO^223 ==¥a aCa CO^293 33 10- -mol l^22
1

...

K=aaa

Ca+-CO

CaCO

2 32

3

.

CaCO 3 ()calciteªCa()^2 aq++CO 32 ()-aq

pure water. The case for calcite is simple since

each mole of CaCO 3 that dissolves produces

one mole of Ca^2 +and one mole of CO 32 -.

Thus:

eqn. 4

and therefore:

eqn. 5

Thus:

eqn. 6

The degree to which a mineral has dissolved

in water can be calculated using the

saturation index, i.e.:

eqn. 7

IAP is the ion activity product, i.e. the

numerical product of ion activity in the

water. An Wvalue of 1 indicates saturation,

values greater than 1 indicate

supersaturation and values less than 1

indicate undersaturation.

For example, groundwater in the

Cretaceous chalk aquifer of Norfolk, UK, has

a calcium ion (Ca^2 +) activity of 1¥ 10 -^3 mol l-^1

and a carbonate ion (CO 32 - ) activity of 3.5¥

10 -^6 mol l-^1. The saturation state of the water

with respect to calcite is:

eqn. 8

i.e. the water is slightly supersaturated with

respect to calcite.

W= = ¥ ¥ =

¥

+- - -

() -

aa

K

Ca CO

sp calcite

2 3236

9

110 35 10 106

33 10

....

.

Degree of saturation IAP

sp

==W

K

Calcite so ility

mol l

lub.

.

=¥=

¥

• --

33 10

57 10

9

51

Calcite so ility Ca CO

sp mol l

lub.

.

( )==

=¥

+-

--

(^2232)
33 10^922
aa
K
Calcite solubility==aaCa^2 +-CO 32