soil during a test. But organic matter in soil may also result in a different availability
of nanoparticles compared to a sub-soil void of humus, which is far more representative of soils being remediated using such particles. This is a trade-off situation
where test organisms and test media should be selected as to be appropriate for the
purpose of the test.
Another example of exposure control concerns earthworms. Dermal exposure of
earthworms is usually measured by dissolving or suspending the material to be tested
in water that imbibes a filter paper lining a glass vial where worms are placed (OECD
1984). Exposure through ingestion, on the other hand, uses a soil matrix where the
material to be tested is mixed in. As in the example of exposing plant roots to
nanoparticles in water or soil, exposure conditions for earthworms also differ greatly
between water and soil. However, for worms the exposure matrix also determines
mode of contact: nanoparticles suspended in water mainly result in dermal exposure,
while nanoparticles mixed into soil or feed result in intestinal exposure plus dermal
contact (Lapied et al. 2010). To relate data from the rapid and inexpensive dermal
tests to test made with soil where bioavailability of nanoparticles is reduced by
interactions with the soil components, one may perform dermal contact tests in soil
by preventing worms from ingesting soil by gluing shut their mouths using super
glue. The contribution from intestinal exposure may then be found by comparing
worms with and without glued mouths.
For many test organisms, exposure through ingested material may differ widely
according to how nanoparticles are introduced into the test system. Here,
nanoparticles mixed into feed may result in far higher exposure than if directly
mixed into soil. While certain earthworms (epigeic and anecic worms) seek out
organic debris when they forage, other worms (endogeic worms) ingest soil and feed
on the evenly distributed organic matter therein. Thus, exposing earthworms, e.g., to
nanoparticles contained in organic feed or mixed homogeneously into soil that
represents a volume that would frequently be at least 50 times higher may result in
very different rates of uptake. Similarly, nematodes may be exposed to nanoparticles
adsorbed onto bacteria upon which they feed, or through a suspension of
nanoparticles where no prior association between bacteria and nanoparticles has
occurred (Kleiven et al. 2018). To maximize ingestion by nematodes or other
particle feeders (e.g., Daphnia), the test may omit the feed (e.g., bacteria and
algae, respectively), but this may cause constipation and blockage of the digestive
tract of the test organisms, and adverse outcomes that are caused by excessively high
availability of nanoparticles (Roberts et al. 2007). In a more realistic exposure
situation, the organisms would ingest mainly digestible particles that would ensure
normal gut passage.
Aggregation (including agglomeration) of nanoparticles during exposure in aqueous media is a major determinant of exposure when particle uptake is sizedependent. Both medium constituents, particularly divalent ions like Ca
2+ , excretions from test organisms and pH changes may cause this (Keller et al. 2012; Baker
et al. 2014). Benthic organisms typically feeding on larger particles may potentially
experience higher exposure due to aggregation, as discrete nanoparticles may be too
small to be perceived as food. As mentioned above, surface-active compounds may
28 Ecotoxicity of Nanomaterials Used for Remediation
577
of nanoparticles compared to a sub-soil void of humus, which is far more representative of soils being remediated using such particles. This is a trade-off situation
where test organisms and test media should be selected as to be appropriate for the
purpose of the test.
Another example of exposure control concerns earthworms. Dermal exposure of
earthworms is usually measured by dissolving or suspending the material to be tested
in water that imbibes a filter paper lining a glass vial where worms are placed (OECD
1984). Exposure through ingestion, on the other hand, uses a soil matrix where the
material to be tested is mixed in. As in the example of exposing plant roots to
nanoparticles in water or soil, exposure conditions for earthworms also differ greatly
between water and soil. However, for worms the exposure matrix also determines
mode of contact: nanoparticles suspended in water mainly result in dermal exposure,
while nanoparticles mixed into soil or feed result in intestinal exposure plus dermal
contact (Lapied et al. 2010). To relate data from the rapid and inexpensive dermal
tests to test made with soil where bioavailability of nanoparticles is reduced by
interactions with the soil components, one may perform dermal contact tests in soil
by preventing worms from ingesting soil by gluing shut their mouths using super
glue. The contribution from intestinal exposure may then be found by comparing
worms with and without glued mouths.
For many test organisms, exposure through ingested material may differ widely
according to how nanoparticles are introduced into the test system. Here,
nanoparticles mixed into feed may result in far higher exposure than if directly
mixed into soil. While certain earthworms (epigeic and anecic worms) seek out
organic debris when they forage, other worms (endogeic worms) ingest soil and feed
on the evenly distributed organic matter therein. Thus, exposing earthworms, e.g., to
nanoparticles contained in organic feed or mixed homogeneously into soil that
represents a volume that would frequently be at least 50 times higher may result in
very different rates of uptake. Similarly, nematodes may be exposed to nanoparticles
adsorbed onto bacteria upon which they feed, or through a suspension of
nanoparticles where no prior association between bacteria and nanoparticles has
occurred (Kleiven et al. 2018). To maximize ingestion by nematodes or other
particle feeders (e.g., Daphnia), the test may omit the feed (e.g., bacteria and
algae, respectively), but this may cause constipation and blockage of the digestive
tract of the test organisms, and adverse outcomes that are caused by excessively high
availability of nanoparticles (Roberts et al. 2007). In a more realistic exposure
situation, the organisms would ingest mainly digestible particles that would ensure
normal gut passage.
Aggregation (including agglomeration) of nanoparticles during exposure in aqueous media is a major determinant of exposure when particle uptake is sizedependent. Both medium constituents, particularly divalent ions like Ca
2+ , excretions from test organisms and pH changes may cause this (Keller et al. 2012; Baker
et al. 2014). Benthic organisms typically feeding on larger particles may potentially
experience higher exposure due to aggregation, as discrete nanoparticles may be too
small to be perceived as food. As mentioned above, surface-active compounds may
28 Ecotoxicity of Nanomaterials Used for Remediation
577