Environmental Engineering Reference
In-Depth Information
V
V
e -
AlTCPc*
e - B
-0.6
Pc
Cr(V)
-0.42
Cr(VI)
CB
e -
AlTCPc •+
1.2
Pc •+
Pc
VB
AlTCPc
h + B
D
2.78
D •+
TiO 2
Pc
pH 2
figure 9.4
Visible light HP Cr(VI) reduction with TiO 2 modified with AlTCPc at pH 2.
with the generation of Cr(V) and O −I . According to the authors, CrO 4 2− can act as a photosensitizer promoting hole injection
to the TiO 2 VB. In this system, the generated TiO −I would oxidize TiO 2 , while Cr(V) would reduce O 2 as an external electron
acceptor.
Not many works have explored the photocatalytic reduction of Cr(VI) beyond the laboratory scale. In a very recent work,
HP technology for Cr(VI) and HA was applied in a pilot reactor [137], starting from the laboratory and scaling up the system
to a pilot reactor five times the volume and height of the laboratory reactor by maintaining geometric parameters and nominal
powers. The authors obtained very good results: after almost 4 h of reaction in a 15 l pilot reactor ([Cr(VI)] 0 = 10 mg l −1 ,
[HA] 0 = 15 mg l −1 , [TiO 2 ] = 1.5 g l −1 ), removal of Cr(VI) and HA was around 98 and 87%, respectively.
The scientific community has been successful in the generation of knowledge about fundamental mechanisms, mechanism
pathways, the role of O 2 and electron donors, and so on, related to a very rich reductive photocatalytic Cr(VI) system. More studies
and efforts should be undertaken to incorporate the knowledge of these promising technologies into practical applications.
9.5
urANium
Uranium is commonly found in its hexavalent form, U(VI), but it also occurs naturally in the +2, +3, +4, and +5 valence states.
The 238 U isotope constitutes more than 99% of naturally occurring uranium, with 235 U and 234 U isotopes comprising minor
percentages. Natural uranium can be found in granites and various other mineral deposits, and it is present in the environment
as a result of leaching from natural deposits, release in mill tailings, emissions from the nuclear industry, the combustion of coal
and other fuels, and the use of uranium-containing phosphate fertilizers [138]. The most common form of U(VI) in water is
uranyl ion, UO 2 2+ .
The health impact associated with high levels of naturally occurring uranium in drinking water is considered to result mainly
from its chemical toxicity rather than the risk from exposure to radioactivity. The primary chemically induced effect of uranium
in humans is nephritis; high levels of uranium have been associated with high blood pressure, bone dysfunction, and likely
reproductive impairment in human populations [139]. for human beings, a guideline value of 0.015 mg l −1 in drinking water has
been indicated, which serves only as a provisional guideline value due to limited information on the health effects [138].
Several methods are available for the removal of uranium from drinking water, such as ionic exchange, ultrafiltration [140],
adsorption on granular ferric hydroxide, iron oxides (ferrihydrite, hematite, magnetite, and goethite) [141-143], pine sawdust
[144], zeolites [145], activated carbon [146] or TiO 2 [147, 148], bioreduction [149], and zero-valent iron [150-153]. Normally,
the treatment of wastewater containing radioactive uranium includes the processes of concentration and solidification. Reductive
photocatalysis can be proposed for U(VI) reduction because two processes can be combined into one step, by reducing and
depositing uranium from aqueous solutions [154].
Search WWH ::




Custom Search