Environmental Engineering Reference
In-Depth Information
applications due to their low cost, good corrosion resistance, and high conductivity, but mechanical instability of their coatings
is a major disadvantage that has to be overcome. To achieve that goal, several efforts are being made, including the use of TiO
2
nanotubes prepared by the anodization of Ti plates, which provides a stake structure where the electroactive materials can
penetrate and combine more firmly, thereby increasing the stability of electrodes [85, 87]
.
Relatively few works dealing with TDN tests for water disinfection have been reported. Ng et al. synthesized TDNs via elec-
trochemical anodization followed by calcination and used these materials for
E. coli
inactivation in water [88]. They found that
over 95% of the initial
E. coli
viable counts were inactivated in only 50 min under UV-A radiation. Presence of ionic species
and organic compounds in water did not produce any observable effect in the inactivation rate.
2.8
Photoelectrocatalysis oN tdN
The idea of combining TiO
2
with other oxides in order to obtain an electrode that presents both PC and electrocatalytic prop-
erties is not new [89]
.
However, the possibility of using TDNs, prepared anodically on Ti plates, as both support and photoactive
material, makes it easier to grow or implant electroactive oxides to create an electrode material with photoelectrocatalytic
(PEC) properties. Thus, great attention has been paid recently to prepare electrodes of this type.
The use of naked TDN as photoelectrocatalysts has been reported for azo decolorization, for example, in the cases of methyl
orange (MeO) or acid orange 7. In the case of methyl orange, PEC and PC activities of the nanotube electrode were compared
for decolorization experiments where a 0.5-V potential was applied and a UV illumination was utilized. A 99% removal was
achieved under PEC conditions, while only 21.5% of the MeO was removed by the PC process for a total time of 90 min [80]
.
for acid orange 7, total removal (discoloration) was achieved after 45 min of PC decomposition under an applied potential of
1.0V versus the Ag/AgCl reference electrode [90]
.
According to this author, applying an electrochemical potential helps to
control the band bending, which can result in more efficient charge carrier separation.
One way to enhance the PEC properties of TDNs is to couple them with other materials like sb-doped snO
2
, siO
2
, Bi
2
O
3
,
fe
2
O
3
, ZnO, and Cds [87, 91, 92]
.
Thus, for example, a photocatalyst like Bi
2
O
3
that is capable of oxidizing water under a
visible light irradiation could be loaded onto a TiO
2
nanotube array electrode, and the composite electrode achieved higher
catalytic activities toward 2,4-dichlorophenol degradation than the individual Bi
2
O
3
for TiO
2
electrode materials did. An effec-
tive photocatalyst for the elimination of environmental pollutants should present high PC activity both in the UV and in the
visible light regions. This can be accomplished by a composite electrode like Bi
2
O
3
/TiO
2
, where Bi
2
O
3
is photoactive under
visible light and TiO
2
is active under UV irradiation [87]
.
PEC using TDN has also been briefly reported for application as a promising and powerful tool for bacteria inactivation
[93]
.
Current work deals with the development of TDN through electrochemical anodization in aqueous solution. The nanotube
electrode built by this procedure was compared with a mesoporous TiO
2
electrode for
E. coli
inactivation. High surface TiO
2
nanotubes resulted in high photocurrent and an extremely rapid
E. coli
inactivation rate ( ̴10
6
CfU mL
−1
killed in less than
10 min). Recently, a Ag/AgBr/TiO
2
nanotube array with enhanced visible light activity was synthesized and its PEC activity
tested for inactivation of
E. coli
under visible radiation (
λ
> 400 nm) resulting in complete sterilization highly superior than
with other reference photocatalysts [94]
.
However, other authors disagree with these results, and there is a controversy on the
real advantages of PEC for bacteria inactivation [95]
.
2.9
other NaNostructured Metal oxides
Metals have been used in water treatment for long. for example, silver has been in use for the treatment of infections and wounds
as well as for water disinfection due to its antimicrobial activity since Roman times. More recently, other metal derivatives, such
as metal oxides but also metal chalcogens, have drawn attention because of their potential applications in inhibiting microorganism
growth due to their high surface areas, unusual crystal morphologies, and high catalytic activity. The use of oxide nanomaterials
for environmental remediation has been reviewed recently [96, 97] and is a hot research field. They can be used for fast and
cost-effective cleaning procedures for contaminants in comparison to traditional methods [98, 99]
.
In particular, the use of nano-
materials for wastewater treatment has attracted the attention of some groups, as there are several reports on the specific biocide
action of some nanomaterials against different types of microorganisms that show low general toxic profiles and good stability
[100, 101]
.
This property of nanomaterials may be exploited for use as effective disinfectant agents. It remains a complex issue to
understand the several factors involved in toxicity, such as chemical composition, surface reactivity, size, distribution, cell type,
experimental setup, as it requires an interdisciplinary view. However, before their large-scale use for water disinfection, it is
necessary to understand the real environmental impact that engineered nanomaterials have [98]
.
