An Introduction to Environmental Chemistry

where R denotes aliphatic or aromatic hydrocarbon groups (see Section 2.7). The

acidity generated by organic matter decomposition is used to break down most

silicate minerals by the process of acid hydrolysis.

4.4.3 Acid hydrolysis

Continental water contains dissolved species that render it acidic. The acidity

comes from a variety of sources: from the dissociation of atmospheric CO 2 in

rainwater—and particularly from dissociation of soil-zone CO 2 (Section 4.4.2)—

to form H 2 CO 3 , and natural and anthropogenic sulphur dioxide (SO 2 ) to form

H 2 SO 3 and H 2 SO 4 (see Boxes 3.7 & 3.8). Reaction between a mineral and acidic

weathering agents is usually called acid hydrolysis. The weathering of CaCO 3

demonstrates the chemical principle involved:

eqn. 4.11

The ionic Ca–CO 3 bond in the calcite crystal is severed and the released CO 32 -

anion attracts enough H+away from the H 2 CO 3 to form the stable bicarbonate

ion HCO 3 -. Note that the second HCO 3 - formed in equation 4.11 is left over when

H+is removed from H 2 CO 3. Bicarbonate is a very weak acid, since it dissociates

very slightly into H+and CO 32 - , but it is not quite dissociated enough to react

with carbonate. Overall, the reaction neutralizes the acid contained in water. The

reaction is dependent on the amount of CO 2 available: adding CO 2 causes the

formation of more H 2 CO 3 (eqn. 4.7), which dissolves more CaCO 3 (forward reac-

tion in eqn. 4.11); conversely, lowering the amount of CO 2 encourages the reverse

reaction and precipitation of CaCO 3. Stalactites and stalagmites forming in caves

are an example of CaCO 3 precipitation induced by the degassing of CO 2 from

groundwater. This response to varying CO 2 is a clear example of Le Chatelier’s

Principle (see Box 3.2).

Acid hydrolysis of a simple silicate, for example the magnesium-rich olivine,

forsterite, is summarized by:

eqn. 4.12

Note that the dissociation of H 2 CO 3 forms the ionized HCO 3 - , which is a slightly

stronger acid than the neutral molecule (H 4 SiO 4 ) released by the destruction of

the silicate.

Mg SiO 24 ()sa+Æ++ 424 H CO 23 ()qMg^2 ()aq+ HCO 344 - ()aq H SiO()aq

CaCO 323 ()sa++H CO()qªCa()^2 aq+ 2 HCO 3 - ()aq

The Chemistry of Continental Solids 83

The relationship between extent of

dissociation and pH is important because pH

can vary sizeably in natural environments. For

example, variability in soil pH is marked,

suggesting that the behaviour of ionizable

species within soils will be different also.

This is the Henderson-Hasselbalchequation; it
indicates the relationship between pH and
pKa. Notice that where the concentration of
undissociated acid cHA and the concentration
of its dissociated base cA-are equal then pH
=pKa. Thus, the pKavalue is the pH at which
there is an equal proportion of dissociated
and undissociated acid.