2012). For anionic chemicals the binding to structural proteins is much weaker
compared to serum albumin (partition constants can differ by more than three
order of magnitude), while cations seem to show similar sorption to serum albumin
and structural protein (Henneberger et al. 2016b). Because binding to proteins can
be very specific, other protein classes might be important for some IOCs. For
example, perfluorinated acids show high affinities for serum albumin and the
fatty-acid-binding protein in the liver, explaining the preferential accumulation of
perfluorinated chemicals in blood and liver (Luebker et al. 2002; Ng and
Hungerbühler 2014; Woodcroft et al. 2010; Zhang et al. 2013).
Even hydrophilic IOCs that mainly reside in the water phase of the body may still
be enriched within organisms due to an ion trap effect when internal and external pH
differ (see also Sect. 6).
Bioaccumulation is defined as a steady-state distribution. Hence, not only the
equilibrium sorption is important, but also the kinetics of uptake, metabolism, and
excretion of IOCs. Uptake and excretion are influenced by the membrane permeability of the various chemical species (see below) but also the local pH (e.g., in the
gastrointestinal tract or of the gills) is important (Erickson et al. 2006). Hepatic
metabolism in fish has been investigated systematically for more than 50 ionic
species with an in vitro assay (Chen et al. 2016). For neutral chemicals such
information can be scaled up to the total organism and combined with physicochemical properties for an assessment of the bioconcentration potential of a chemical
(Nichols et al. 2013). However, it is not yet clear how the necessary quantitative
in vitro to in vivo extrapolation (QIVIVE) would work for ionic species because
their sorption must be understood for the QIVIVE procedure.
In general, we can expect bioaccumulation in fish to be less of a concern for
IOCs than for neutral chemicals but a case by case assessment remains necessary.
Biomagnification in fish and trophic magnification in aquatic food webs have
already been reported for perfluorinated acids which are almost completely ionized
at environmental pH values (Kelly et al. 2009; Martin et al. 2003; Ng and
Hungerbühler 2014). The available literature on the bioaccumulation potential of
IOCs in fish has been reviewed by Armitage et al. (2017). In two more recent papers
from our own group the tissue specific sorption of anions in fish and terrestrial
organisms was assessed (Goss et al. 2018) and a screening of the bioaccumulation
potential of almost 2000 IOCs was performed (Bittermann et al. 2018). For air
breathing animals a general statement in terms of biomagnification is not possible. In
this case neither neutral nor ionic chemicals are efficiently excreted via an aqueous
phase and much will depend on the chemicals metabolism (Goss et al. 2018).
4 Sorption of IOCs in Soil/Sediment and to Polymers
Soils are known to have a distinct cation exchange capacity which should be relevant
for the sorption of organic cations. And indeed, there are a number of studies
revealing that the cation exchange capacity of both, clay and humic substances in
50
L. Henneberger and K.-U. Goss
compared to serum albumin (partition constants can differ by more than three
order of magnitude), while cations seem to show similar sorption to serum albumin
and structural protein (Henneberger et al. 2016b). Because binding to proteins can
be very specific, other protein classes might be important for some IOCs. For
example, perfluorinated acids show high affinities for serum albumin and the
fatty-acid-binding protein in the liver, explaining the preferential accumulation of
perfluorinated chemicals in blood and liver (Luebker et al. 2002; Ng and
Hungerbühler 2014; Woodcroft et al. 2010; Zhang et al. 2013).
Even hydrophilic IOCs that mainly reside in the water phase of the body may still
be enriched within organisms due to an ion trap effect when internal and external pH
differ (see also Sect. 6).
Bioaccumulation is defined as a steady-state distribution. Hence, not only the
equilibrium sorption is important, but also the kinetics of uptake, metabolism, and
excretion of IOCs. Uptake and excretion are influenced by the membrane permeability of the various chemical species (see below) but also the local pH (e.g., in the
gastrointestinal tract or of the gills) is important (Erickson et al. 2006). Hepatic
metabolism in fish has been investigated systematically for more than 50 ionic
species with an in vitro assay (Chen et al. 2016). For neutral chemicals such
information can be scaled up to the total organism and combined with physicochemical properties for an assessment of the bioconcentration potential of a chemical
(Nichols et al. 2013). However, it is not yet clear how the necessary quantitative
in vitro to in vivo extrapolation (QIVIVE) would work for ionic species because
their sorption must be understood for the QIVIVE procedure.
In general, we can expect bioaccumulation in fish to be less of a concern for
IOCs than for neutral chemicals but a case by case assessment remains necessary.
Biomagnification in fish and trophic magnification in aquatic food webs have
already been reported for perfluorinated acids which are almost completely ionized
at environmental pH values (Kelly et al. 2009; Martin et al. 2003; Ng and
Hungerbühler 2014). The available literature on the bioaccumulation potential of
IOCs in fish has been reviewed by Armitage et al. (2017). In two more recent papers
from our own group the tissue specific sorption of anions in fish and terrestrial
organisms was assessed (Goss et al. 2018) and a screening of the bioaccumulation
potential of almost 2000 IOCs was performed (Bittermann et al. 2018). For air
breathing animals a general statement in terms of biomagnification is not possible. In
this case neither neutral nor ionic chemicals are efficiently excreted via an aqueous
phase and much will depend on the chemicals metabolism (Goss et al. 2018).
4 Sorption of IOCs in Soil/Sediment and to Polymers
Soils are known to have a distinct cation exchange capacity which should be relevant
for the sorption of organic cations. And indeed, there are a number of studies
revealing that the cation exchange capacity of both, clay and humic substances in
50
L. Henneberger and K.-U. Goss