Environmental Engineering Reference
In-Depth Information
studies as this fraction provides an estimate of the absorbed dose. Tissue con-
centration comparison, after administration of different forms of a contaminant,
is useful for contaminants that preferentially accumulate in specific tissues. This
type of end-point measurement provides an estimate of relative bioavailability.
Additionally, the fraction of the dose excreted in the faeces may be measured; how-
ever this method reflects the amount of contaminant/dose that is not absorbed and
the actual dose is calculated by subtracting the excreted amount from the initial
amount dosed. As such, this method is subject to underestimating absorption if
the contaminant is absorbed and subsequently excreted via bile back into the GI
tract. For the organic contaminants and their metabolites, the database of available
results is smaller compared to the metals and metalloids. Furthermore the choice of
target organs/matrices for the metabolites is dependant on the metabolism process
experienced by the individual contaminants. The fate of highly metabolised contam-
inants may be assessed by analysing their metabolites in urine, whereas unaltered
contaminants may accumulate in adipose tissue.
7.1.3 Validation of Bioaccessibility Tests
In order to use a bioaccessibility test it is necessary to be able to demonstrate a
mathematical, fit-for-purpose, relationship between the bioaccessible concentration,
relative to the bioaccessibility of a soluble salt of the contaminant, in the test soil
(as measured by an in-vitro test) and the bioavailable concentration, relative to the
bioavailability of a soluble salt of the contaminant in the soil (as measured by an in-
vivo study), see Section 7.1.2 . In order to validate the in-vitro bioaccessibility test
with the bioavailability data it is important that comparable units of measurement
are used. The bioavailability of the element in the soil is almost always measured
relative a water soluble salt of the element. It is therefore important that the com-
parison is made with the bioaccessibility of the soil relative to the same soluble salt.
This may seem to be contradictory since, if the salt is completely soluble, the rela-
tive bioaccessibility will be the same as the absolute bioaccessibility. However, the
water soluble salts are not necessarily fully soluble in the simulated stomach and
intestine fluids particularly if the solubility of the element is reduced at the higher
pH values found in intestinal fluid phase. Under these conditions, if relative bioac-
cessibility is not used, the bioaccessibility will always under predict the true relative
bioavailability. For the test to be of pragmatic use, there are two major assumptions:
the soils used in the validation test are representative of the soils that will be
tested by the bioaccessibility test;
the soluble salt used to calculate the relative bioavailabilities and the rela-
tive bioaccessibilities have the same biological action as the salt used in the
toxicity test.
Previous studies (Basta et al. 2007 ; Drexler and Brattin 2007 ; Juhasz et al. 2007 ;
Rodriguez et al. 1999 , 2003 ; Schroder et al. 2003 , 2004a ) have shown that within
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