376
V. Zelano . M. Gulmini
The exponential term may be considered as an activity coefficient (Dzombak and
Morel 1990 ) which takes into account the effects of the surface charge on the apparent
acidity constants.
Because of the very complex composition of sediments, the particle surfaces may
have a wide variety of sorption characteristics; however, most of the particles are covered with a limited number of compounds, namely amorphous iron hydroxides and
NOM (Lion et al. 1982; Luoma and Davis 1983; Tipping et al. 1990; De Wit et al. 1993a;
De Wit et al.1993b), which largely determine the complexing capacity of the solid, even
if their relative abundance in the total sediment composition is rather low.
This prompted the study of sorption properties of well-characterized solid phases,
in order to model the interaction between the sediment and the dissolved compounds
as a sum of discrete contributes of the solid components. Particular attention was given
to Fe and Mn oxides (Stumm et al. 1980; Benjamin and Leckie 1981; Farley et al. 1985)
and organic matter (Ephraim and Marinsky 1986; De Wit et al. 1990; PuIs et al. 1991;
Masini 1993; Lead et al. 1994). Studies on oxides were generally performed with synthetic material and the properties of organic matter were determined after their extraction from natural sediment and purification.
Such an approach however, does not consider that the properties of a single component may be influenced by the presence of other components (Honeyman and
Santschi 1982) and the extraction of a solid component from its natural matrix,
i.e. extraction of organic matter (Templeton and Chasteen 1980), may lead to its modification, hence the model may not be representative of the real sediment behaviour.
A better approach could be the direct study of natural sediment ligand properties
with as little manipulation as possible. For this reason, the interaction between ions
and protogenic ligand groups on the sediment surface can be studied by alkalimetrically titrating suspensions containing the water-sediment system, carried out with
potentiometric measurements. This technique still has a wide application in the study
of the acid-base and ligand properties of a vast range of soluble compounds and it
also appears promising for the study of heterogeneous systems (Gulmini et al. 1996;
Gulmini et al. 1997). Although the method is only qualitatively correct at a molecular
level, i.e. it is not sensitive to the detailed structure of the interfacial region and it does
not distinguish between structurally different groups of the same type, it is possible
to correctly describe proton-sediment interaction by curve-fitting, and, in some instances, it can be demonstrated that the sorption of metal ions is mainly due to interaction with the proto genic sites of the sediment surface.
20.4
Titration Procedure
Since the titration is performed on a heterogeneous system, we must take into consideration some procedural aspects in order to correctly carry out the titration.
As previously mentioned, the carbonates are a relevant component of the sediments,
and their presence interferes with the titration. It is for this reason that it is preferable
to eliminate them beforehand with a pH 4 acid treatment.
A second relevant aspect is the homogeneity of the sediment, upon which depends
the possibility of performing reproducible titration on different subsequent aliquots.
It is for this reason that it is better to use sediments with controlled granulometry, for
V. Zelano . M. Gulmini
The exponential term may be considered as an activity coefficient (Dzombak and
Morel 1990 ) which takes into account the effects of the surface charge on the apparent
acidity constants.
Because of the very complex composition of sediments, the particle surfaces may
have a wide variety of sorption characteristics; however, most of the particles are covered with a limited number of compounds, namely amorphous iron hydroxides and
NOM (Lion et al. 1982; Luoma and Davis 1983; Tipping et al. 1990; De Wit et al. 1993a;
De Wit et al.1993b), which largely determine the complexing capacity of the solid, even
if their relative abundance in the total sediment composition is rather low.
This prompted the study of sorption properties of well-characterized solid phases,
in order to model the interaction between the sediment and the dissolved compounds
as a sum of discrete contributes of the solid components. Particular attention was given
to Fe and Mn oxides (Stumm et al. 1980; Benjamin and Leckie 1981; Farley et al. 1985)
and organic matter (Ephraim and Marinsky 1986; De Wit et al. 1990; PuIs et al. 1991;
Masini 1993; Lead et al. 1994). Studies on oxides were generally performed with synthetic material and the properties of organic matter were determined after their extraction from natural sediment and purification.
Such an approach however, does not consider that the properties of a single component may be influenced by the presence of other components (Honeyman and
Santschi 1982) and the extraction of a solid component from its natural matrix,
i.e. extraction of organic matter (Templeton and Chasteen 1980), may lead to its modification, hence the model may not be representative of the real sediment behaviour.
A better approach could be the direct study of natural sediment ligand properties
with as little manipulation as possible. For this reason, the interaction between ions
and protogenic ligand groups on the sediment surface can be studied by alkalimetrically titrating suspensions containing the water-sediment system, carried out with
potentiometric measurements. This technique still has a wide application in the study
of the acid-base and ligand properties of a vast range of soluble compounds and it
also appears promising for the study of heterogeneous systems (Gulmini et al. 1996;
Gulmini et al. 1997). Although the method is only qualitatively correct at a molecular
level, i.e. it is not sensitive to the detailed structure of the interfacial region and it does
not distinguish between structurally different groups of the same type, it is possible
to correctly describe proton-sediment interaction by curve-fitting, and, in some instances, it can be demonstrated that the sorption of metal ions is mainly due to interaction with the proto genic sites of the sediment surface.
20.4
Titration Procedure
Since the titration is performed on a heterogeneous system, we must take into consideration some procedural aspects in order to correctly carry out the titration.
As previously mentioned, the carbonates are a relevant component of the sediments,
and their presence interferes with the titration. It is for this reason that it is preferable
to eliminate them beforehand with a pH 4 acid treatment.
A second relevant aspect is the homogeneity of the sediment, upon which depends
the possibility of performing reproducible titration on different subsequent aliquots.
It is for this reason that it is better to use sediments with controlled granulometry, for