Environmental Engineering Reference
In-Depth Information
results, the mobility of trace elements in street dust
follows the sequence: Cd
loss of accuracy - by
in vitro
tests (RIVN 2006).
These
in vitro
assays simulate the biochemical envi-
ronment, temperature, and duration of the different
stages in the process of ingestion: grinding and fi rst
dissolution in saliva in the mouth; release of metals
in the gastric juice of the stomach; and further release
(or removal from solution) and absorption with duo-
denal juice and bile in the intestine (Oomen
et al.
2002). Although undoubtedly easier to control than
in vivo
assays,
in vitro
experiments commonly
produce uncertain and little reproducible results
owing, among other diffi culties, to the very large
number and instability of reactants and solutions,
and the fact that concentrations in chyme can be near
or below quantifi cation limits.
Probably as a consequence, most
in vitro
studies
of urban particulate materials have focused on the
bioaccessibility of trace elements in soil and dust,
operationally defi ned as the maximum amount of
metal that is soluble in a synthetic gastric fl uid
(Hamel
et al.
1998). Most of these studies have used
hydrochloric acid (adjusted to pH 1.5) as a surrogate
for gastric juice, as in the European Standard Toy
Safety Protocol EN-71 (European Committee for
Standardization 1995; Rasmussen
et al.
2008), and
some have extracted the trace elements in the sample
with glycine, again adjusted to pH 1.5 with concen-
trated HCl (Ruby
et al
. 1999; Madrid
et al
. 2008).
The variability in the - sometimes contradictory -
results arrived at by different researchers refl ects the
complex and numerous factors that infl uence how
much of the total trace element load in a sample is
bioavailable (element investigated, granulometry and
mineralogy of the sample, organic carbon content,
mode of retention, anthropogenic or natural origin,
acid-to-sample ratio, etc.). Madrid
et al.
(2008)
report bioaccessibility values (relative to an aqua
regia extract) of up to 86% for Ni and 83% for Zn,
and as low as 1% for Cr in the less than 2
Cu, and the
concern about their environmental implications
should, perhaps, observe the same order. The car-
bonate fraction becomes less important in urban
soils and the relevance of the Fe-Mn oxides, organic
matter, and residual fraction increases (Zimdahl
& Skogerboe 1977; Harrison
et al
. 1981; Gibson &
Farmer 1984; Evans
et al
. 1992; Serrano-Belles &
Leharne 1997). This fact probably arises from several
causes, among them the lower abundance of calcite,
the lower pH, and higher concentration of organic
matter in urban soil relative to street dust. Trace
elements are consequently more tightly bound to soil
than to street dust particles, a fact that corroborates
the role of urban soil as a sink for pollutants.
However, changes in the environmental conditions
of the soil (pH, redox potential) might result in the
release of part of the load of trace elements that have
accumulated over time. However, there are many
sequential digestion protocols and little consensus
about which method is the most appropriate to use
(Perez-Santana
et al.
2007). It is also felt that eluci-
dating the binding sites on particulates does not give
the kind of information required when assessing
impacts on living tissue.
>
Pb/Zn
>
4.3.2 Bioaccessibility and bioavailability
The toxicity values used in risk assessments for the
route of ingestion are expressed in terms of absorbed
doses and are often derived from assays that employ
soluble salts or other easily available chemical forms
of trace elements. Consequently, human health risk
assessments implicitly assume that the concentration
term used in the standard equations to quantify
exposure represents the amount of trace elements in
the sample(s) that are available for absorption (i.e.
bioavailable) in the gastrointestinal tract.
Bioaccessibility is normally defi ned as the fraction
of the trace element content that is available in the
gastrointestinal tract for transport across the intesti-
nal lumen, whereas the term bioavailability usually
denotes the ingested contaminant fraction that actu-
ally reaches the systemic circulation (not all the mass
of metal released during its transit in the gastrointes-
tinal tract will be absorbed).
In vivo
assays have
evaluated bioavailability but they are expensive and
complicated. They can be substituted - without great
m frac-
tion of soils from two different urban environments,
and an order of bioaccessibility Ni = Zn
μ
>
Pb
Cr
in Turin. Rasmussen
et al.
(2008) analyzed the
HNO
3
+ H
2
O
2
“total” and the HCl bioaccessible
contents of the less than 150-
>
Cu
>
Cr for Seville and Pb = Cu = Zn
>
Ni
>
m fraction of samples
from urban gardens and indoor dust in Ottawa.
Their results suggest an order of bioaccessibility of
Cu = Zn
μ
Cu = Ni for dust.
Moreover, not only were concentrations of metals in
>
Ni for soil, and Zn
>
