Environmental Engineering Reference
In-Depth Information
1.46 V
-0.125 V
Pb
2+
α-PbO
2
Pb
1.70 V
0.356 V
PbSO
4
figure 9.6
Latimer diagram connecting the different Pb species [164].
figure 9.6 shows the Latimer diagram for Pb.
HP of lead systems has received scarce attention. In our previous reviews [7, 10] we cited a number of early papers [75, 78,
165-180], but information continues to be rather scant. The mechanisms of transformation of lead (II) in water by UV-TiO
2
are especially attractive because they depend very much on the reaction conditions, which are related to the nature of the
photocatalyst, the effect of oxygen, and the presence of electron donors.
Our group and others have investigated both the oxidative and the reductive route. In the first case, Pb(IV) formation through
hole or HO
∙
attack takes place through two consecutive monoelectronic steps. The one-electron reduction potential for the Pb(III)/
Pb(II) couple is not reported, but it is not unreasonable to assume that Pb(II) can be easily oxidized to this unstable intermediate.
+
•
(9.31)
Pb II hHOPb III
VB
() /
+
→
(
)
+
Pb(III) transforms into Pb(IV) by simple oxidation by O
2
or by stronger oxidants present in the system ( h
VB
, ROS, etc.);
disproportionation is also possible:
(9.32)
Pb III h OROS
() /
+
+
•
/
→
Pb IV
()
VB
(9.33)
2Pb III
() () ()
→+
Pb II
Pb IV
In the presence of O
2
, a dark brown PbO
2
deposit on TiO
2
was observed [19]. equation 9.5, that is, reduction of O
2
to
superoxide, is normally a very slow reaction due to its high overpotential. In this context, Pb(II) removal by oxidation is poor.
The use of platinized samples (Pt/TiO
2
) enhanced the reaction, because Pt decreases this overpotential, accelerating the reaction
in oxic systems [10, 19].
Ozone addition promotes HP oxidation of Pb(II), because of the photochemical formation of H
2
O
2
and other ROS, including
the ozonide radical HO O
3
(
/ [178, 181-183]; this was proven by x-ray diffraction (xRd), where the patterns revealed depo-
sition of PbO
2
and PbO
1.37
on the photocatalyst, the unstoichiometric oxide being formed by the action of the ozonide:
•
•−
3
•− •−
→ →
O
O
Pb
2
+
PbO
(9.34)
3
3
137
.
On the other hand, the one-electron reduction potential of the Pb(II)/Pb(I) couple is very negative (
E
0
= −1.0 V [184]),
preventing a direct reductive route by e
C
−
, as observed in experiments performed employing Pt/TiO
2
under N
2
[19, 165, 172].
In fact, direct reduction of Pb(II) to Pb(0) by a bielectronic process has been reported under laser irradiation, where, due to the
high photonic frequency, the authors propose that accumulation of electrons may allow multielectronic injection [176, 179].
However, the photocatalytic reduction of Pb(II) is possible under regular illumination via the indirect route in the presence
of electron donors (eqs. 9.3 and 9.11). different organic compounds have been used in the past to promote reductive lead HP
under N
2
and over pure TiO
2
[19, 20, 79, 114, 171].
Murruni et al. [20] obtained a high efficiency in Pb(II) reduction when using 2-propanol and formic acid as sacrificial
donors, the latter being a more environmentally friendly alternative as its degradation products are not toxic. Two monoelec-
tronic reduction steps were proposed:
+→ ()
(9.35)
Pb(II)
Pb I
0
(9.36)
Pb(I)
→
Pb()
