Environmental Engineering Reference
In-Depth Information
Photocatalytic arsenite oxidation could also be carried out under visible irradiation. There, the mechanism is different from
the one that takes place under UV light, involving charge injection from an excited dye molecule into the CB of the SC as the
primary mechanism for the production of an oxidized dye radical (dye
•+
) [51]:
*
(9.17)
Dyeh visible ye
+
ν (
)
→
(
)
→+
(
)
−
•
+
−
Dye iO e ye
*+
TiOe
(9.18)
2
CB
2
CB
•+
+ →+
()
(9.19)
Dye s III
()
DyeAsIV
In the presence of oxygen, superoxide is produced through the cathodic pathway (eqs. 9.5-9.8), which can contribute to
As(III) oxidation (eq. 9.15).
Park et al. [52] used titania nanoparticles sensitized with an organic dye (Od, (e)-3-(5-(5-(4-(bis(4-((2-(2-methoxyethoxy)
ethoxy)methyl)phenyl)amino)phenyl)-thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid))) for the oxidation of arsenite and
compared the efficiency with those of bare TiO
2
and TiO
2
sensitized with a ruthenium bipyridyl complex (Ru
II
(4,4-bpy(COOH)
2
)
3
,
RuL
3
). The results under visible light (λ > 420 nm) showed better activities for Od/TiO
2
in comparison with other photocata-
lysts, with a constant photoactivity over a wide pH range. The superior stability of Od/TiO
2
over RuL
3
/TiO
2
was related to the
good chemical anchoring between the dye and the surface; in addition, this type of dye is better than Ru-containing dyes, which
are expensive and toxic.
In another work, a high degree of As(III) removal under visible light illumination was obtained using palladium-modified
nitrogen-doped titanium oxide (TiON/PdO) nanoparticles [53]. The strong adsorption (for both As(III) and As(V)) and photo-
oxidation by TiON/PdO led to efficient As removal. The system was good enough to reach As concentrations in agreement with
the wHO regulation limits at initial As(III) concentrations lower than 0.4 mg l
−1
. The enhanced photocatalytic activity under
visible light illumination was explained by a strong optoelectronic coupling between PdO and TiON.
In contrast, reductive photocatalytic systems for arsenic transformation are still scarce. Reduction of As(III) by TiO
2
e
C
−
is
possible either in the presence or in the absence of an electron donor in the aqueous system, but, in the case of As(V), direct
reduction does not seem to be feasible because the reported reduction potential of the As(V)/As(IV) couple is highly
negative (
E
0
≈ −1.2 V [37]). Therefore, the indirect pathway by using sacrificial electron donors like alcohols or carboxylic acids
(e.g., methanol, formic acid) is indispensable [21]. The indirect reductive mechanism for As(V) photocatalytic reduction was
first proven in anoxic conditions in the presence of methanol by yang et al. [34]. Later, our group [21] studied the reaction
starting from As(V) and also from As(III) in the presence of methanol; As(III) reaction in the absence of the organic compound
was also analyzed. A mechanism based on the formation of a hydroxymethyl radical produced by hole/HO
∙
attack on methanol
was proposed. This radical can donate electrons to the CB or itself be effective as an As(V) reductant, with formaldehyde
generation in both cases:
+
•
•
+
0
CH OH hHO HOHHHO
+
{}
→
+
{}
E
=
145
.
V
(9.20)
3
VB
2
2
(
•
CH OH CH OH
/
)
2
3
•
+
−
0
CH OH CH OH e
→+
+
E
(
≈
−
09
.
to V
−
118
.
(9.21)
2
2
CB
•
CH OH CH O
/
)
2
2
•
CH OH As V HO As IV H
2
()
+
(9.22)
+ →+
()
+
2
(
)
E
0
−
≈−
20
.
V
CH
2
O can be transformed to formic acid and finally mineralized to CO
2
with the generation of CO
•−
(
/
•
2
CO CO
2
2
[19], another strong radical that can contribute to the reducing process:
•−
+ →+
()
(9.23)
CO
As V OAsIV
()
2
2
In a further step, As(IV) is reduced by CB or trapped electrons, or by
∙
CH
2
OH or CO
•−
to As(III). As mentioned earlier,
unlike the case of As(V), Levy et al. [21] observed direct photocatalytic As(III) reduction by e
C
−
in the absence of MeOH; this
indicates that although the value for the monoelectronic couple As(III)/As(II) is not known, it should be below the CB level:
−
(9.24)
As III
()
+→
e sII
CB
()
Therefore, after As(III) formation, consecutive monoelectronic steps would lead to the formation of stable products such as
As(0) and AsH
3
, which were unambiguously identified by the authors in all cases through x-ray photoelectron spectroscopy
(xPS) and x-ray absorption near edge structure (xANeS) analyses of the solid residues formed on the TiO
2
surface. In this way,
