Chapter 20
Potentiometry for the Study of Acid-Base Properties
of Sediments
V. Zelano . M. Gulmini
20.1
Introduction
The term sediments generally refers to both deposited solids and suspended ones: the
latter are separated from dissolved components by filtration through a 0.45 Jlm membrane (Forstner 1989).
A sediment can be considered as a heterogeneous mixture of dissimilar particles,
which are themselves a complex assemblage of different inorganic and organic components. Contaminants can be transferred from the water column onto the sediment
through chemical and biological processes; sediment particles can then undergo a
series of transportation, settling, resuspension and deposition, each accompanied by
several chemical reactions which can result in a release of the associated pollutants.
The sediment can therefore act both as a sink and as a non-point source for potentially toxic compounds or ions.
As a consequence, the need to understand ilie interaction that can take place between
contaminants and sediments in natural water systems was brought to the attention of
environmental chemists as a tool to determine pollutants' availability and to study the
possibility of their release due to physicochemical modifications of the sediment sphere.
The general removal of a solute from the solution phase is called sorption. For heavy
metals, the incorporation into the sediment is not generally due to precipitation, since
their concentration is very low, and their solubility product is not reached. Therefore,
their sorption onto the sediment is due to chemical and physical processes, which can
be summarised as follows (Tessier and Campbell 1988):
• Adsorption and ionic exchange of ions bonded to particle surface, particularly onto
clays, iron and manganese oxides and organic particles. This process is promoted
by an increase of the pH value and a decrease of the ionic strength.
• Incorporation of cations into the crystal reticulate of secondary minerals such as
carbonates and sulfides. This process is due to sulfate biological reduction and natural
carbonation; it is promoted by an increase of pH and by favourable environmental
conditions for biological activity.
• Coprecipitation with iron and manganese hydrous oxides. A decrease in the acidity
and the oxidating conditions increase this possibility because the most insoluble
oxides are the cations with an increased state of oxidation [Fe(III) and Mn(IV)].
• Incorporation in the crystal of the primary minerals, especially silicates created in
the diagenesis processes.
• Complexation with inorganic and organic particles of biological or aniliropical origin.
Potentiometry for the Study of Acid-Base Properties
of Sediments
V. Zelano . M. Gulmini
20.1
Introduction
The term sediments generally refers to both deposited solids and suspended ones: the
latter are separated from dissolved components by filtration through a 0.45 Jlm membrane (Forstner 1989).
A sediment can be considered as a heterogeneous mixture of dissimilar particles,
which are themselves a complex assemblage of different inorganic and organic components. Contaminants can be transferred from the water column onto the sediment
through chemical and biological processes; sediment particles can then undergo a
series of transportation, settling, resuspension and deposition, each accompanied by
several chemical reactions which can result in a release of the associated pollutants.
The sediment can therefore act both as a sink and as a non-point source for potentially toxic compounds or ions.
As a consequence, the need to understand ilie interaction that can take place between
contaminants and sediments in natural water systems was brought to the attention of
environmental chemists as a tool to determine pollutants' availability and to study the
possibility of their release due to physicochemical modifications of the sediment sphere.
The general removal of a solute from the solution phase is called sorption. For heavy
metals, the incorporation into the sediment is not generally due to precipitation, since
their concentration is very low, and their solubility product is not reached. Therefore,
their sorption onto the sediment is due to chemical and physical processes, which can
be summarised as follows (Tessier and Campbell 1988):
• Adsorption and ionic exchange of ions bonded to particle surface, particularly onto
clays, iron and manganese oxides and organic particles. This process is promoted
by an increase of the pH value and a decrease of the ionic strength.
• Incorporation of cations into the crystal reticulate of secondary minerals such as
carbonates and sulfides. This process is due to sulfate biological reduction and natural
carbonation; it is promoted by an increase of pH and by favourable environmental
conditions for biological activity.
• Coprecipitation with iron and manganese hydrous oxides. A decrease in the acidity
and the oxidating conditions increase this possibility because the most insoluble
oxides are the cations with an increased state of oxidation [Fe(III) and Mn(IV)].
• Incorporation in the crystal of the primary minerals, especially silicates created in
the diagenesis processes.
• Complexation with inorganic and organic particles of biological or aniliropical origin.