Environmental Engineering Reference
In-Depth Information
Cr(VI) reduction is enhanced in acid solutions, and the net reaction can be given as:
2
−
+ +
+ →+
3
0
2
Cr O
16
H
4
Cr
8
HO OG kJ
+
3
∆
=−
115 8
.
(9.25)
27
2
2
298
At neutral pH:
2
−
+ +
+ →+
3
4
Cr O
20
H
4
Cr
10
HO O
+
3
(9.26)
27
2
2
Although the process is thermodynamically feasible, Cr
2
O
7
2−
is stable at room temperature in acidic solutions due to the high
overpotential of the water oxidation conjugate reaction. Therefore, strategies for improving the reduction reaction should be
implemented.
A considerable number of works have been published on the photocatalytic reduction of Cr(VI) employing TiO
2
, fe/TiO
2
,
Pt/TiO
2
, and other SCs such as ZnO, CdS, ZnS, and wO
3
[68-74]. Many examples have already been described in our previous
reviews [7, 10] including reactors for technological applications, microparticles used in slurries or conveniently supported,
nanoparticles, and nanotubes [69, 71-112]. The major conclusions that emerge from past publications can be summarized in
the following points:
1. Cr(VI) HP reduction depends upon pH. Low pH favors the net reaction (eq. 9.19); however, neutral or alkaline conditions
favor precipitation and immobilization of Cr(III) as the hydroxide, facilitating further separation.
2. Cr(VI) reduction can be accelerated by the addition of organic compounds acting as holes or HO
∙
scavengers.
3. Molecular oxygen does not affect Cr(VI) HP reduction, at least at acidic pH, as will be explained in detail later.
from a thermodynamic point of view, e
C
−
have the appropriate potential to directly reduce Cr(VI), Cr(V), and Cr(IV)
(see figs. 9.1 and 9.3), and the HP mechanism for the reduction of Cr(VI) to Cr(III) is proposed to occur in three monoelec-
tronic steps [113-115]:
−
(9.27)
Cr VI
()
+ →
e
CrV
()
CB
−
(9.28)
Cr Ve Cr IV
CB
()
+ →
()
−
(9.29)
Cr IV
()
+ →
e
Cr III
()
CB
This was proven by our group through electron paramagnetic resonance (ePR) spectroscopy, which allowed the identification
of paramagnetic Cr(V) species [113-115].
detrimental reoxidation of reduced Cr species by holes or hydroxyl radicals, however, is possible:
(9.30)
+
•
Cr V V III hHO rV IV III
VB
(/ / ) (
+
)
→
(/ / )
An enhancement of the reaction takes place if suitable electron donors are present either by reducing the probability of
recombination or through an indirect reduction driven by radicals formed by hole/HO
∙
attack (eqs. 9.3 and 9.11), which, at high
concentrations, hinder reaction (9.30) and themselves contribute to Cr species reduction.
The role of O
2
in photocatalytic Cr(VI) systems is noteworthy due to its unique behavior compared with other metal cations,
because it appears to be independent of the presence of O
2
, at least at acidic pH. This process was explained by a very strong
association between Cr(VI) and TiO
2
through the formation of a charge transfer complex, identified by an absorption band at 380 nm
[117]. due to the fast capture of electrons by Cr(VI) as a result of the formation of this complex, no competition for O
2
by e
C
−
(eq.
9.5) is present, and, consequently, no inhibition by O
2
is observed either in the absence of organic donors or in the presence of edTA
and oxalic or citric acid [113-115, 117, 118]. experiments on platinized photocatalysts supported this independence, the same
temporal evolution of Cr(VI) concentration being obtained using Pt/TiO
2
or bare TiO
2
either in the presence or in the absence of O
2
[116-120]. If O
2
had exerted any effect, Pt would have enhanced the reaction by decreasing the overpotential for water oxidation.
Cr(VI) photocatalytic reduction continues to be a very rich and interesting study system, and extensive work has been carried
out in recent years, either with the aim of water treatment or as a practical system for the evaluation of the photocatalytic
