Environmental Engineering Reference
In-Depth Information
is often poorly representative for the metal concentrations to which the organism
is exposed. In general, DGT will deplete the pore water of metals and replenish-
ment of metals towards the solution phase is a slow process. However, organisms
move around and are likely exposed to concentrations that are controlled by equi-
librium between pore water and the solid phase, while metal concentrations are not
depleted.
16.4.1.2 Organic Contaminants
Organic contaminants have been recognised as contaminants in soils for a shorter
period of time than inorganic contaminants. This is due to the historic importance
of metals, the relative ease of analysis of metals compared to organic contami-
nants, and the mere fact that the spectrum of chemical structures is more diverse for
organic contaminants than for metals and metalloids. Although the list of potential
approaches to simulate bioavailability of organic contaminants is long, consequently
far fewer chemical extraction procedures have been proposed as proxies for the
availability of organic contaminants compared to metals. Paradoxically, this makes
extraction procedures for organic contaminants harder to summarise and review,
because there has been less time for the establishment of one or several standardized
procedures. Additionally, compared to metal extractions, a far higher proportion of
extractions have been validated to organisms other than plants. A good review of
different approaches is given by Ehlers and Loibner ( 2006 ) and readers are referred
to that paper for details.
As with metals a number of physical-chemical (extraction) procedures have been
developed to aid in the prediction of the bioavailable concentration of organic con-
taminants. These procedures provide knowledge about the extent of contaminant
retention in soils, within shorter periods and for lower budgets and give more precise
information on soil constituents being responsible for the sequestration of hydropho-
bic organic contaminants in soil compared to bioassays. Table 16.3 provides an
outline of some of the principle studies reported in the literature in which chemical
solvents were used to evaluate bioavailability/bioaccessibility of organic contam-
inants. The most straightforward extractions involve shaking soil with a weakly
polar (e.g., methanol-water, n butanol, ethanol) or non-polar (e.g., hexane) liquid.
Kelsey and Alexander ( 1997 ) and Tang and Alexander ( 1999 ) report good corre-
lations between extractable concentrations of atrazine, phenanthrene, anthracene,
fluoranthene and pyrene and uptake by earthworms and plants and breakdown by
bacteria, but found no universal extractant that produced good correlations for all
chemicals and organisms tested.
In recent years, biomimetic extractions have become increasingly common to
assess bioavailability of organic contaminants in soils. Among biomimetic extrac-
tions of hydrophobic organic contaminants, two distinctive lines of approaches can
be distinguished, i.e. those that are equilibrium-based and negligibly change the par-
titioning of a contaminant between the solid phase and the pore water, and those that
deplete all contaminants released from the solid phase into the pore water, within a
certain time period.
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