Agriculture Reference
In-Depth Information
TiO
2
is a photocatalyst capable of producing highly oxidizing ROS, and studies
have shown that it still exhibits a photocatalytic and antibacterial activity in the
absence of UV illumination (Sayes et al.
2006
). In a general manner, absorption of a
photon with sufficient energy (3.2 eV for anatase) is the condition required for
photochemical reactions to proceed at the photocatalyst surface (Czili and Horvath
2008
). According to Miller et al. (
2012
), “when TiO
2
reaches an electronically
excited state, an electron (e
) is promoted from the valence band to the conduction
band, generating a hole in the valence band (h
+
). The resulting electron-hole pair
can then recombine or migrate to the surface of the particle and may react with H
2
O
or OH
to form OH
•
or can directly oxidize adsorbed species. The electrons may
also react with adsorbed molecular oxygen to form O
2
•
ions.”
Some reports have dealt with the toxicity profile of nano-TiO
2
, showing that it is
toxic to
Escherichia coli
via ROS production, under sunlight (Wei et al.
1994
); it
can cause inflammatory responses in mammalian cells (Peters et al.
2004
) or even
be genotoxic (Carinci et al.
2003
). On the other hand, some studies concluded that
there were very low or even absent effects of TiO
2
nanoparticles (Rehn et al.
2003
;
Peters et al.
2004
). These contrasting conclusions arised from different dispersion
states in these experiments encouraged Planchon et al. (
2013
) to perform a com-
prehensive study coupling toxicity assessments with physicochemical studies,
using
E. coli
. Specifically, they aimed to explore the influence of the dispersion
state that nanoparticles would undergo in natural water on their potential toxicity to
the microorganism.
The authors used Seine River water (SRW) (pH between 7.8 and 8.2) as a natural
environmental medium to quantify the ecotoxicological impact of the three types of
manufactured titanium dioxide (TiO
2
) and also in synthetic waters at pH
5.0 and
8.0. The three nanoparticles were different, going from 100 % rutile (R) to a rutile/
anatase mixture (M and P25). R type were rod shaped, while M were rutile rods and
anatase spheres mixture, and P25 were an assemblage of anatase spheres and rutile
polygons (anatase spheres are smaller than rutile and thus P25 contained the largest
particles). They found the isoelectric points of the three materials to be between 6.0
and 6.2, close to that of bare TiO
2
surface (and thus the particles were not covered
by any stabilizing oxide or organic layer). All particles were negatively charged in
SRW and pH 8.0 water and positively in pH 5.0 water, the zeta potential being very
limited in amplitude, all below 15 mV (for colloidal stability, it must be at least
25 mV). This instability accounted for the aggregated state of TiO
2
nanoparticles in
the media used.
The impacts of the nanoparticles on
E. coli
survival were measured as function
of concentration and time. TiO
2
nanoparticle toxicity was found to start at 10 ppm
(the toxicity was found to be lower at pH 5 compared to Seine water and pH 8.0
water) and to depend slightly on the TiO
2
mineralogical phases, increasing from
pure rutile to assemblages of the rutile and anatase phases. The lethal effect was
effective from 1-h contact and did not amplify with time. Regarding composition,
P25 turned out to be more toxic than M and R types. The proportion of the rutile
phase affected the aggregation state of the nanoparticles and also their toxicity.
Indeed, the bacterial survival is all the more reduced when cells were exposed to
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