Agriculture Reference
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concentrations, the growth of algae decreases simultaneously. 86 % growth was
shown in the presence of TiO
2
, compared to the control.
Cl
ยด
ment et al. (
2013
) used commercial 15-, 32-, and 25-nm anatase
nanoparticles for assessing the TiO
2
nanoparticle toxicity. For the 15- and 32-nm
anatase particles, the calculated average particle sizes were, respectively, 16.3
1.9
and 32.6
3.8 nm. In the case of anatase, the results indicated that the smaller grain
size, the higher the specific surface area. There was a formation of aggregates in the
different particle suspensions (concentrations, 0.001-1,000 mg L
1
). They used
C. vulgaris
as test organisms, and the average toxicity obtained for a particle
concentration of 100 mg L
1
was 5.70
0.20 % for anatase (25 nm). They also
used
D. magna
, and for these microcrustaceans, the mobility was more inhibited in
the presence of TiO
2
nanoparticles compared to the micrometric particles. EC
50
values for 15-, 25-, and 32-nm anatase were 1.3, 3.15, and 3.44 mg L
1
, respec-
tively. The authors observed that the crystalline form of TiO
2
induced various toxic
responses. Anatase nanoparticles were more toxic than rutile compared to
D. magna
.
Manzo et al. (
2013
) evaluated the growth rate alterations of marine chlorophyte
D. tertiolecta
derived from the exposure to ZnO nanoparticles. The hydrodynamic
diameters of nano-ZnO and bulk ZnO particles, at 10 mg L
1
in seawater, were
900
200 nm, respectively. In the next 3 h, the mean size of the
nano-ZnO aggregates increased to 1,500
200 and 1,300
300 nm and up to a final detectable size
of around 2,600
700 nm on the third day of observation. The growth of the algal
population was clearly affected by the presence of nano-ZnO. The effects were not
significant at Zn concentrations below 0.08 mg L
1
(NOEC), while the LOEC was
obtained at 0.40 mg L
1
. The EC
50
was measured at 1.94 (0.78-2.31) mg L
1
.
Bulk ZnO presented as less toxic than nano-ZnO: the effects were not significant
at Zn concentrations below 0.8 mg L
1
(NOEC), while LOEC was obtained at
2.41 mg L
1
; the EC
50
value was found at 3.57 (2.77-4.80) mg L
1
. The nano-ZnO
and bulk ZnO aggregates showed very similar sizes, with nano-ZnO smaller than
bulk ZnO, and the polydispersity of the first system being much higher, i.e., nano-
ZnO had a wider particle size distribution centered at smaller dimensions. Thus, the
observed toxicity could not be attributed to a mechanical injury caused by large
aggregates onto the algal cells, but rather to the presence of smaller particles that
may have had an enhanced surface reactivity with respect to larger aggregates.
On the other hand, the enhanced solid-solid interfaces among nanoparticles can
result in increased surface energy transfer and reactivity. This is a critical issue to be
addressed when dealing with nanomaterials, and it is worth being further investi-
gated in the future/near future. Since the main differences in the designed experi-
ments relied on the original primary particle size (i.e., nano-ZnO against bulk ZnO),
the observed differences in the toxic effects are most likely to be attributed just to
this factor. Their results on the growth rate suggested that in their case, the toxic
action was likely to be exerted through a mechanical injury and hindrance to
diffusion processes as well (pristine size of the dispersed particles did affect the
overall toxicity).
