456
C. Ianni· N. Ruggieri
21.3
Selective Extractions
To date, it has generally been accepted that the most appropriate methods to evaluate
solid speciation are selective sequential extraction procedures (Kot and Namiesnik
2000), which are widely used to assess the long-term emission potential of pollutants
and to study the distribution of pollutants among the geochemical phases (Rauret
1998). One must keep in mind that selective sequential extractions give, as a result,
"operationally defined species" (Tessier et al. 1979), which depend on the different
procedures used. It is not possible to obtain specific chemical associations, because
the reagents utilized are often insufficiently specific to exclusively dissolve the "target" phase conditions (Davidson et al.1994) and results obtained can vary widely when
different experimental conditions are used (Tipping et al. 1985; Rauret et al. 1989).
Nevertheless, useful information can be gained from such studies; in fact, a large number of selective extraction methods have been studied and reported, many of which
are variants of the Tessier procedure (Tessier et al.1979). Selective extractions have also
been proven to be adequate for determining the metals associated wiili source constituents in sedimentary deposits (van der Sloot et al. 1997). Moreover, according to
Rubio et al. (1991), metals with an anthropogenic origin are mainly obtained in the first
extractions, while in the last stage of the process, the residual fraction is obtained,
corresponding to metals with lithogenic origins.
Recently, studies on selective extractions have received new emphasis with the availability of a sediment reference material, certified by the SM&T programme of the
European Union (Quevauviller et al.1997) following a less aggressive procedure. In fact,
the increasing performances of the analytical techniques used for element determination in an extract, together with the increasing evidence that exchangeable metals
better correlate with plant uptake, has led extraction methods to evolve towards the
use of less and less aggressive solutions (Gupta and Aten 1993).
21.3.1
Commonly Used Extraction Procedures
The fractions obtained when applying selective extraction schemes are related to exchangeable metals, metals mainly bound to carbonates, metals released in reducible
conditions such as those bound to hydrous oxides of Fe and Mn, metals bound to oxidizable components such as organic matter, and sulphides and residual fraction. The
reagents more commonly used in sequential extraction procedures are usually applied
according to the following order: unbuffered salts, weak acids, reducing agents, oxidizing agents and strong acids. Exchangeable fraction uses an electrolyte at pH 7 to
avoid oxide and carbonate solubilization and to prevent oxyhydroxide precipitation.
The carbonate fraction generally uses acetic acid or a buffer acetic acid- sodium acetate at pH 5. These reagents are not able to dissolve all the carbonates, nor can they
attack carbonate selectively, as they also remove labile organically bound metals
(Rauret 1998; Baffi et al.1998). The reducible fraction is mainly related to metals bound
to Fe and Mn oxides. Hydroxylamine in acid solution is the reducing agent most widely
used to solubilize these oxides, although iron oxide is not completely dissolved. Ammonium oxalate seems more effective if used in the dark, even if some precipitation
C. Ianni· N. Ruggieri
21.3
Selective Extractions
To date, it has generally been accepted that the most appropriate methods to evaluate
solid speciation are selective sequential extraction procedures (Kot and Namiesnik
2000), which are widely used to assess the long-term emission potential of pollutants
and to study the distribution of pollutants among the geochemical phases (Rauret
1998). One must keep in mind that selective sequential extractions give, as a result,
"operationally defined species" (Tessier et al. 1979), which depend on the different
procedures used. It is not possible to obtain specific chemical associations, because
the reagents utilized are often insufficiently specific to exclusively dissolve the "target" phase conditions (Davidson et al.1994) and results obtained can vary widely when
different experimental conditions are used (Tipping et al. 1985; Rauret et al. 1989).
Nevertheless, useful information can be gained from such studies; in fact, a large number of selective extraction methods have been studied and reported, many of which
are variants of the Tessier procedure (Tessier et al.1979). Selective extractions have also
been proven to be adequate for determining the metals associated wiili source constituents in sedimentary deposits (van der Sloot et al. 1997). Moreover, according to
Rubio et al. (1991), metals with an anthropogenic origin are mainly obtained in the first
extractions, while in the last stage of the process, the residual fraction is obtained,
corresponding to metals with lithogenic origins.
Recently, studies on selective extractions have received new emphasis with the availability of a sediment reference material, certified by the SM&T programme of the
European Union (Quevauviller et al.1997) following a less aggressive procedure. In fact,
the increasing performances of the analytical techniques used for element determination in an extract, together with the increasing evidence that exchangeable metals
better correlate with plant uptake, has led extraction methods to evolve towards the
use of less and less aggressive solutions (Gupta and Aten 1993).
21.3.1
Commonly Used Extraction Procedures
The fractions obtained when applying selective extraction schemes are related to exchangeable metals, metals mainly bound to carbonates, metals released in reducible
conditions such as those bound to hydrous oxides of Fe and Mn, metals bound to oxidizable components such as organic matter, and sulphides and residual fraction. The
reagents more commonly used in sequential extraction procedures are usually applied
according to the following order: unbuffered salts, weak acids, reducing agents, oxidizing agents and strong acids. Exchangeable fraction uses an electrolyte at pH 7 to
avoid oxide and carbonate solubilization and to prevent oxyhydroxide precipitation.
The carbonate fraction generally uses acetic acid or a buffer acetic acid- sodium acetate at pH 5. These reagents are not able to dissolve all the carbonates, nor can they
attack carbonate selectively, as they also remove labile organically bound metals
(Rauret 1998; Baffi et al.1998). The reducible fraction is mainly related to metals bound
to Fe and Mn oxides. Hydroxylamine in acid solution is the reducing agent most widely
used to solubilize these oxides, although iron oxide is not completely dissolved. Ammonium oxalate seems more effective if used in the dark, even if some precipitation