Environmental Risks of Nanomaterials - Nanotechnologies for Water Environment Applications - page 603

Hdroxylated C 60 forms stable aggregates ~100 nm in pure water. The structure and
charge characteristics of the n-C 60 aggregates have been found to vary considerably as a
function of the manufacturing method and surrounding conditions (Brant et al., 2005b).
In some cases, aggregation of NMs may affect both exposure and toxicity.
17.4.1.3 Biodegradation, Bioavailability and Bioaccumulation of NMs
Many NMs are composed of inherently nonbiodegradable inorganic chemicals,
such as ceramics, metals and metal oxides, and are not expected to biodegrade. However,
a recent preliminary study found that C 60 and C 70 fullerenes were taken up by wood
decay fungi after 12 weeks, suggesting that the fullerene carbon were biodegradable
(Filley et al., 2005). Some biodegradable polymer NMs are being investigated for use in
drug delivery (Brzoska et al., 2004).
Bioavailability and bioactivity of NMs in the environment eventually occur
because of the environmental release from the manufacturing industries and the
consumer products that contain them, and the waste products. Bacteria and living cells
can take up NMs, providing the basis for potential bioaccumulation in the food chain
(Biswass and Wu, 2005). The bioavailability of specific NMs in the environment will
depend on the particle, complexation and its surroundings. For example, the
bioavailability of iron depends on its free ion concentrations in water, which are affected
by complexation. Complexation reduces bioavailability by reducing free metal ion
concentrations, and dissolved iron is quantitatively complexed by organic ligands. Iron
and iron oxides can participate in enzymatic redox reactions that change the oxidation
state, physical chemical properties and bioavailability of the metal (Reguera et al., 2005).
Kashiwada (2006) investigated the distribution of water-suspended fluorescence
monodispersed NPs made of latex (polystyrene) in the body of the medaka ( Oryzias
latipes ) and how these materials reach the organs, eventually exerting their effects. The
NPs pass through the membranes of the gills and/or intestine and enter the circulation.
Particles 39.4 - 42,000 nm in diameter were adsorbed on the chorion of medaka eggs and
accumulated in the oil droplets. Particles 474 nm in diameter showed the highest
bioavailability to eggs, and 39.4 nm particles shifted into the yolk and gallbladder along
with embryonic development. Adult medaka were exposed to 39.4-nm NPs at 10 mg/L,
and these particles were detected at high levels in the gills and intestine.
17.4.1.4 Computer Simulation and Models
Carbon-nanostructures are the only class of NMs that have received focused
toxicological characterization. For example, many different forms of water-soluble
fullerenes have been screened in vitro and in animal studies for toxicology (Colvin,
2003). However, the toxicological studies for most NMs are insufficient. In developing a
risk management strategy for emerging NMs, much can probably be learned from the
past management experience with conventional hazardous materials and pollutants.