Environmental Engineering Reference
In-Depth Information
Magnesium oxide (MgO) nanoparticles showed biocidal activity against certain Gram-positive and Gram-negative bacteria,
as well as spores, in particular against
E. coli
,
B. megaterium
,
Streptococcus pneumoniae
, and
Bacillus subtilis
. [102, 103]
.
Alumina (aluminum oxide, Al
2
O
3
) nanoparticles have also shown mild antibacterial activity against
E. coli
,
S. pneumoniae
,
P.
aeruginosa
, and
B. subtilis
cultures [104-106]
.
Bulk versus nanoalumina comparison showed that the nanosized Al
2
O
3
had higher
toxicity at the same concentration [107, 108]
.
Copper (II) oxide (CuO) and copper (I) oxide (Cu
2
O) are both semiconducting compounds that are cytotoxic to several types
of cells [109, 110]
.
In fact, they are considered among the most toxic nanomaterials based on several studies. In the case of envi-
ronmental effects, it has been reported than CuO nanoparticles with sizes lesser than 50 nm are toxic when tested against
Nitellopsis obtusa
,
Chlorella
, shrimp
Thamnocephalus platyurus
, and rotifer
Brachionus calyciflorus
[111]
.
In that sense, copper
oxides may be useful for applications in water disinfection. for example, CuO nanoparticles were effective in killing several
pathogenic bacteria (
P. aeruginosa
,
Proteus
spp.,
B. subtilis
) when used in high concentrations [112]
.
several reports relate the
toxicity to copper ions more than to the presence of nanoparticles, but sometimes the nature of the specific toxic agent is unclear.
Coordinative interactions with chemical moieties with great affinity to copper ions such as carboxylic or amine groups, as well
as the probability of intercalation into DNA structures to form complexes with nucleotides, may cause disruption of biochemical
process and thus be the reason for their toxicity [113]
.
Cu
2
O nanoparticles supported onto natural clinoptilolite showed 100%
antiprotozoan activity against
Paramecium caudatum
and
Eurytemora affinis
after 1 h of contact and inhibitory growth effects on
E. coli
and
S. aureus
in tests specifically designed to measure the effectiveness of these materials for disinfection of secondary
effluent water and removal of pathogenic microorganisms in the tertiary stage of wastewater treatment [114]
.
Zinc oxide (ZnO), on the other hand, has been used in several commercial applications such as in sunscreens, cosmetics,
pharmaceutical, and the plastic and food industry due to its semiconducting properties, stability, and antibacterial activity. It has
been tested against bacteria such as
B. subtilis
,
E. coli
,
Pseudomonas fluorescens
,
Listeria monocytogenes
,
Salmonella
enteritidis
,
Salmonella typhimurium
, and
S. aureus
, showing from mild to strong microbicidal activity, although the
studies were more oriented toward finding novel food and agricultural applications than toward finding a utility for wastewater
treatment [107, 115-118]
.
The mechanism of antibacterial activity is suggested to proceed through generation of hydrogen per-
oxide on the ZnO surface. It has also been suggested that the release of Zn(II) ions may damage the cell membrane, and they
may interact with specific biomolecules [119, 120]
.
Cobalt-doped ZnO antibacterial activity was tested against
E. coli
,
Klebsiella
pneumonia
,
Shigella dysenteriae
,
Salmonella typhi
,
P. aeruginosa
,
B. subtilis, S. aureus
, and the fungi
Fusarium
sp., showing
significant results [121, 122]
.
The use of thin films of ZnO is also being considered to avoid the growth of biofilms, which may
be very useful for wastewater treatment technologies involving large active surfaces or membranes [123]
.
Cerium oxide (CeO
2
) nanoparticles have shown antimicrobial activity by electrostatically binding the cell membrane of
Gram-negative cells [124]
.
They have been detected by monitoring the bacterial growth on a solid medium of
S. aureus
,
P. aeruginosa
,
Klebsiella
sp.,
S. pneumoniae
, and
S. aureus
[104, 125]
.
When compared with bulk and nano CeO
2
, bulk CeO
2
showed less toxicity
than their nanosized counterparts [126]
.
Nitrogen-doped ZrO
2
has been reported to effectively inhibit the growth of
E. coli
and biofilm heterotrophic bacteria under
solar light illumination [68]
.
Iron oxides (feO, α-fe
2
O
3
, β-fe
2
O
3
, γ-fe
2
O
3
, fe
3
O
4
) are also of chemical interest as they are bio-
compatible and some of them are magnetically active. They are able to participate in light-driven or pH-induced redox changes
that are responsible for the generation of ROs and other highly reactive chemical species (such as primarily hydroxyl radicals),
following fenton-like mechanisms. Just as in the case of TiO
2
, the use of such reactive species for water disinfection is attractive
as they are produced by the illumination of a suspension of contaminated water and nanoparticles for a short time, and solar
light may be used when the shifting of the optical response from UV to visible light is to be achieved. In that sense, the growth
of
S. aureus
cultures was inhibited significantly compared to control samples when 3 mg/ml of iron oxide dose was used [127]
.
It has also been reported than fe
3
O
4
nanoparticles have an inhibitory effect on
E. coli
in a concentration-dependent manner;
although no correlation to light exposition was made, the authors suggested than ROs may be responsible for growth inhibition
[128]
.
However, a recent study warns that superparamagnetic iron oxide nanoparticles (sPIONs) may have a negative effect for
wastewater remediation as they induce an increase of
P. aeruginosa
biofilm biomass [129]
.
2.10
coNclusioNs
The specific biocide activity of some nanomaterials against microorganisms present in wastewater is a very attractive property
for their eventual incorporation in large-scale methods for water disinfection. However, several important challenges remain
with respect to their commercial use. first, it is necessary to remove particles from water as they may negatively interfere in
secondary treatment processes or, due to their inherent toxicity, may present a risk for the environment (biomass) or water
consumers. second, electrostatic or van der Waals interactions among nanoparticles to form larger aggregates may be useful
