Environmental Engineering Reference
In-Depth Information
exchangeable and carbonate fractions described
below should be released (Evans
et al
. 1992). Other
ì selectiveî extractions use ì mildî extracting agents,
such as EDTA, citric acid, acetic acid, ammonium,
or sodium acetate, etc.
The most comprehensive approach to the problem
of speciation involves a ì sequentialî extraction, in
which trace elements associated with the different
chemical phases present in the particulate material
are extracted separately. Almost all ì sequentialî pro-
cedures are based on the analytical protocol proposed
by Tessier
et al.
(1979) or modiÝ cations thereof
(Gibson & Farmer 1984). In it, the total amount of
a trace element is separated in Ý ve fractions: soluble
and exchangeable; carbonate bound; bound to
FeñMn hydrous oxides; bound to organic matter; and
residual fraction. The Ý rst two fractions (exchange-
able and carbonate-bound) are regarded as readily
bioavailable, whereas the fraction associated with
FeñMn hydrous oxides and organic matter should
only be available under severe environmental condi-
tions. The residual fraction is essentially unavailable.
The many analytical approaches used to assess the
bioavailability and transport mechanisms of particu-
late-associated contaminants has led to problems of
comparability between studies and also the assign-
ment of environmental relevance to the speciation
found. Some techniques appear to be more efÝ cient
than others (Agemian & Chau 1976) whereas others
encourage the redistribution of some elements during
the fractionation process (Ajayi & Vanloon 1989).
In fact, it has been suggested (Breward
et al.
1996)
that two or more schemes be used on the same
samples to elucidate metal binding sites better.
The speciation of urban deposits has elicited much
study. In summary, street and house dust shows a
fairly good agreement with the general trends in
trace-element partitioning among the different
phases. Lead is preferentially associated with the car-
bonate and FeñMn oxide fractions, and to a lesser
extent with the exchangeable fraction; Cu is pre-
dominantly bound to the organic fraction; Zn follows
the behavior of lead and seems to be bound to the
carbonate and FeñMn oxide fractions; and Cd is
associated with the Ý rst two fractions and shows the
highest afÝ nity of all these elements for the exchange-
able fraction (Harrison
et al.
1981; Gibson &
Farmer 1984; Evans
et al.
1992; Wang
et al
. 1998;
Charlesworth & Lees 1999). According to these
carcinogen that does not pose a small but Ý nite prob-
ability of generating a carcinogenic response. Risk to
the exposed individual is measured as the product of
the lifetime-average daily dose times a ì slope factorî ,
deÝ ned as the incremental probability of developing
cancer during a lifetime owing to chronic exposure
to a unit dose of contaminant. This probability must
not exceed a subjective level of risk (in the range
10
−4
ñ10
−6
) deemed acceptable by the corresponding
regulatory authorities. By jointly considering toxicity
and level of exposure, risk assessment allows the
identiÝ cation of the elements and pathways of most
concern during exposure in the environment.
However, it is a common practice to estimate the
concentration of trace elements in urban matrices as
the fraction extracted with aqua regia or similar
ì strongî digestion protocols. Because only a portion
of this pseudo-total content will be released in the
stomach and absorbed in the intestine, this approach
may lead to an overestimate of risk, particularly
when these elements in urban particulate materials
are strongly bound to their mineral matrix. As a
result, many digestion protocols have been devel-
oped that attempt to mimic conditions whereby the
metals can be assimilated into living organisms. The
following sections introduce these techniques.
4.3.1 Speciation
The different digestion procedures used in speciation
studies can be broadly divided in two groups: ì selec-
tiveî and ì sequentialî extractions. ì Selectiveî
extractions attempt to digest and analyze only that
fraction of the sample of environmental relevance,
i.e. the fraction of the total that would become
dissolved and therefore mobile and available under
realistic environmental conditions, or that could be
incorporated by the human organism upon exposure
to it. A common approach involves extraction with
hydrochloric acid at different concentrations, in
some cases attempting to simulate the conditions
existing in the human stomach (Day
et al.
1979;
Harrison 1979; Serrano-Belles & Leharne 1997). A
more sophisticated approach is mentioned by Evans
et al.
(1992), according to which the amount of bio-
available trace elements should be evaluated as for
foods by digesting the sample with synthetic gastric
juice, of pH 3.5, for 4 hours (Analytical Methods
Committee 1985). Under these conditions the
