Ecotoxicology of Manufactured Nanoparticles - Environmental and Human Health Impacts of Nanotechnology - page 291

reached within 30 minutes. When exposed to metal contaminated water in the pres-
ence of TiO 2 nanoparticles metal accumulation in the fi sh rose sharply to 132% and
146% for arsenic(V) and cadmium, respectively, compared to metal contaminated
waters alone. Furthermore, Zhang et al. (2007) showed TiO 2 nanoparticles adsorbed
65% more cadmium than 19 m sediment particles and that the presence of sedi-

ment particles did not have a signifi cant infl uence on the accumulation of cadmium
in the fi sh over the 25-day exposure period.
A comparison of the toxic effects of TiO 2 and ZnO nanoparticles to D. magna
showed that ZnO was considerably more toxic than TiO 2 (Adams et al. , 2006a,
2006b). Only 40% mortality was observed in D. magna exposed to TiO 2 in spring
water for eight days but ZnO concentrations of 0.5 and 0.2 mg/l resulted in 100%
and 73% mortality, respectively. In this study no effects of primary particle size
were observed using particles nominally ranging from 66 to 44

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m and DLS and
optical microscopy characterisation confi rmed signifi cant aggregation of the par-
ticles in water. Franklin et al. (2007) compared the toxic effects of nanoparticulate
ZnO, bulk ZnO and ZnCl 2 to the freshwater alga Pseudokirchneriella subcapitata .
Particle characterisation using transmission electron microscopy and DLS tech-
niques showed that particle aggregation was signifi cant in a freshwater system,
resulting in fl ocs ranging from several hundred nanometers to several microns
(Figure 7.4). As discussed previously in Section 7.3.3.2, rapid dissolution of both
nanoparticulate and bulk ZnO was observed and all the observed toxicity could be
attributed solely to dissolved zinc.
As the primary agents driving biogeochemical change, microorganisms are
potential mediators of nanoparticle transformations which could affect their mobil-
ity and toxicity (Wiesner et al. , 2006 ). Furthermore, microbial ecotoxicology studies
may play an important role in elucidating cytotoxicity mechanisms that can be
extrapolated to eukaryotic cells (Wiesner et al. , 2006). Metal oxides are well known
for their antibacterial activity (Sawai, 2003). A number of investigators have con-
sidered the potential for enhanced bactericidal effects from nanoparticulate metal
oxides, with zinc oxide being the most widely studied. Yamamoto (2001) studied
the antibacterial effects of ZnO particles ranging from 100 to 800 nm (0.85- 26.0 m 2 /g)
to both Gram-negative and Gram-positive bacteria ( Escherichia coli and
Staphylococcus aureus respectively). Very high concentrations (400- 1000 mg/l) of
ZnO were used in these experiments; the aim being to assess their use as potential
biocides rather than any detrimental environmental effects, but nevertheless anti-
bacterial effects were shown to increase with both increasing ZnO concentration
and decreasing particle size. The effects were similar for both E. coli and S. aureus
with the most pronounced effect observed for E. coli . The mechanism of antibacte-
rial activity was assumed to be generation of hydrogen peroxide (H 2 O 2 ) from the
ZnO surface (Yamamoto et al. , 1998 ). Adams et al. (2006a, 2006b) also presented
evidence that the antibacterial effects of nanoparticulate metal oxides were due, at
least in part, to the generation of reactive oxygen species by UV activation. The
toxicity of TiO 2 and SiO 2 to both E. coli and Bacillus subtilus was greater in the
presence of light than in the dark; however, toxicity was observed under dark
conditions indicating that a mechanism other than ROS generation must contribute
to the antibacterial activity of these materials.

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