372
v. Zelano . M. Gulmini
Hereafter we will deal with the last point, i.e. the formation of coordination bonds
at the solid-water interface, with particular attention to the interaction between ions
and protogenic groups on the sediment particles surface. The potentiometric approach
for the characterisation of acid-base and ligand properties of sediment particles will
be discussed.
20.2
Sediment: What is it?
Deposited sediments originate both from the settling of suspended particles and from
the chemical and biological processes that take place on the sediment surface. Although
sediment particles can be fairly different from one another, one can picture each particle as "onion-like": a matrix vehicle, generally siliceous, is covered with an array of
compounds, such as carbonates, amorphous aluminosilicates, organic matter and iron
and manganese oxides (Martin et al. 1987).
To simplify the discussion, it can be of great help to subdivide sediment components into inorganic and organic ones.
The main inorganic components, minerals or amorphous, are silicates (clay and
non-clay), carbonates, iron and manganese oxides, phosphates and sulfides. Silicates
derive mostly from fragmented rocks that are transported into the basin by wind and
rain. Other inorganic components, namely carbonates, iron and manganese oxides and
phosphates are formed by chemical (precipitation) and biological processes. Sulfides
form on the bottom, anoxic layer of the sediment.
The organic component of the sediment derives from living processes of benthic
organisms, mostly from accumulation and complex transformation of natural organic
matter from the overlaying water (NOM).
NOM is a very complex mixture of compounds; because of the diversity of the natural processes of synthesis and degradation, the number of constituents of NOM is
potentially infinite, and most of them have not been identified yet. Although there is
little hope of completely separating and identifying each component of the NOM, a
great amount of characterisation work has been completed. This work was reviewed
in an excellent book by Buffle (Buffle 1988).
After sedimentation, the NOM from the water column can undergo significant
modifications, since the chemical conditions in the sediment are very different from
those immediately above the solid. The biochemical conditions existing at the sediment-water interface can be extremely variable, therefore it is very difficult to establish general rules concerning the composition of sediment NOM. In general, proteins,
polysaccharides and amino-sugars are present, with a great variability in functional
groups and molecular weight distribution.
To investigate the nature and role of the organic components of the sediment, they
must be brought into solution by extraction from the solid, inorganic phase. The extract, that generally represents only a portion of the total organic content of the sediment, may be partitioned into fulvic and humic fractions, which are operationally
defined on the basis of their different solubility in acids and bases. Taking into account
the complexity of the initial mixture, humic and fulvic fractions are themselves heterogeneous and their composition may depend on the extraction condition used. Despite its complexity, a probable picture of the organic component of the sediment re-
v. Zelano . M. Gulmini
Hereafter we will deal with the last point, i.e. the formation of coordination bonds
at the solid-water interface, with particular attention to the interaction between ions
and protogenic groups on the sediment particles surface. The potentiometric approach
for the characterisation of acid-base and ligand properties of sediment particles will
be discussed.
20.2
Sediment: What is it?
Deposited sediments originate both from the settling of suspended particles and from
the chemical and biological processes that take place on the sediment surface. Although
sediment particles can be fairly different from one another, one can picture each particle as "onion-like": a matrix vehicle, generally siliceous, is covered with an array of
compounds, such as carbonates, amorphous aluminosilicates, organic matter and iron
and manganese oxides (Martin et al. 1987).
To simplify the discussion, it can be of great help to subdivide sediment components into inorganic and organic ones.
The main inorganic components, minerals or amorphous, are silicates (clay and
non-clay), carbonates, iron and manganese oxides, phosphates and sulfides. Silicates
derive mostly from fragmented rocks that are transported into the basin by wind and
rain. Other inorganic components, namely carbonates, iron and manganese oxides and
phosphates are formed by chemical (precipitation) and biological processes. Sulfides
form on the bottom, anoxic layer of the sediment.
The organic component of the sediment derives from living processes of benthic
organisms, mostly from accumulation and complex transformation of natural organic
matter from the overlaying water (NOM).
NOM is a very complex mixture of compounds; because of the diversity of the natural processes of synthesis and degradation, the number of constituents of NOM is
potentially infinite, and most of them have not been identified yet. Although there is
little hope of completely separating and identifying each component of the NOM, a
great amount of characterisation work has been completed. This work was reviewed
in an excellent book by Buffle (Buffle 1988).
After sedimentation, the NOM from the water column can undergo significant
modifications, since the chemical conditions in the sediment are very different from
those immediately above the solid. The biochemical conditions existing at the sediment-water interface can be extremely variable, therefore it is very difficult to establish general rules concerning the composition of sediment NOM. In general, proteins,
polysaccharides and amino-sugars are present, with a great variability in functional
groups and molecular weight distribution.
To investigate the nature and role of the organic components of the sediment, they
must be brought into solution by extraction from the solid, inorganic phase. The extract, that generally represents only a portion of the total organic content of the sediment, may be partitioned into fulvic and humic fractions, which are operationally
defined on the basis of their different solubility in acids and bases. Taking into account
the complexity of the initial mixture, humic and fulvic fractions are themselves heterogeneous and their composition may depend on the extraction condition used. Despite its complexity, a probable picture of the organic component of the sediment re-