Page 87 - Soil and water contamination, 2nd edition

74 Soil and Water Contamination

minerals may have a large reactivity and are particularly relevant for interactions between the
solid and the liquid phase ; for this reason, the Fe and Al hydroxides and clay minerals will be
discussed separately.

4.2.2 Aluminium and iron oxides/hydroxides
In temperate climates, Al and Fe oxides/hydroxides occur mainly in the form of gibbsite
(γ-Al(OH) ) and goethite (α-FeOOH), respectively. The major form of iron oxide in tropical
3
soils is generally haematite (α-Fe O ). The mineral structure of these Al and Fe oxides/
2 3
hydroxides, commonly referred to as sesquioxides , is often relatively simple. They consist of
2-
-
a dense packing of oxygen (O ) and/or hydroxyl (OH ) anions held together in a specific
3+
3+
configuration of Fe or Al cations. The internal configuration may range from perfectly
regular, resulting in a crystalline structure, to rather irregular if impurities are present,
resulting in amorphous sesquioxides. The specific surface area of sesquioxides depends on
the conditions during formation and may vary from several tens of square metres per gram
to very low values for macroscopic crystals and concretions. In the interior of the crystals,
2-
3+
-
3+
electroneutrality occurs, i.e. the charge of the Fe /Al cations and of the O /OH anions
-
2-
balance each other out. However, at the surface the O /OH anions are not balanced against
the metal cations, because this is the terminal layer. Consequently, there is an excess of
anions, which causes the surfaces of sesquioxides to be generally negatively charged. At the
surface of the dry crystal, electroneutrality is maintained by adsorption of an appropriate
+
number of cations, generally H ions (protons ). In contact with the aqueous phase , these
protons may dissociate and be exchanged for other cations , depending on the pH of the
solution. In this case, the surface acts like an acid . Consequently, the negative charge of the
surfaces of the sesquioxides increases with increasing pH and so does the capacity to sorb
cations other than protons. At low pH values, the surface may adsorb a larger number of
protons than needed for neutralisation. In this case, the surface thus acts like a base and
attains a positive charge. These deprotonation and protonation reactions can be written as:
MO - + 2H + MOH + H + MOH + (4.1)
2
where ≡ MO = the surface oxygen . The pH at which the charge of the mineral surface is
neutral (zero charge) is called the Point of Zero Charge (PZC ). The PZC of gibbsite varies
between 5.0 and 6.5, the PZC of goethite is about 7.3, and the PZC of haematite is about
8.5 (Appelo and Postma, 1996). This implies that under acidic to neutral conditions
(pH < 7), goethite and haematite have a positive surface charge and, accordingly, are able
to adsorb anions . Note that sesquioxides also have affinity for specific anions, especially

phosphate , through specific adsorption . The adsorption phenomena specific to phosphate
will be discussed further in Section 4.3.
4.2.3 Clay minerals

At the geological time scale (i.e. thousands to millions of years), clay minerals are formed as
the chemical weathering products of relatively readily weatherable minerals such as micas ,
olivine, pyroxenes, amphiboles, and calcium -rich plagioclases. Examples of abundant clay
minerals are kaolinite , illite , montmorillonite , vermiculite, and chlorite. Figure 4.2 gives an
overview of the possible pathways of formation of various clay minerals. The clay minerals
in soil originate from either the parent sedimentary bedrock material or in-situ formation,
and their ultimate form depends on the composition of the parent material and on the
climate. Kaolinite is a residue of extensive weathering under humid and acid conditions.

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