Environmental Engineering Reference
In-Depth Information
Oxidative TiO
2
HP has been shown to be a good alternative for As(III) oxidation. In an effort to explain the mechanism of
oxidation, several authors have suggested monoelectronic steps with formation of As(IV) [30, 33, 35] that could involve the
action of HO
∙
(eq. 9.13), reaction with VB holes (eq. 9.14), or reaction with O
•−
(eq. 9.15):
(
)
→+
•
•
−
As III HO
()
+
or HO
As IV OH
()
(9.13)
ads
free
+
(9.14)
As III
()
+→
h sIV
VB
()
•
•
−
−
−
(9.15)
As III HO O sIVHOO
() /
+ →+
(
)
/
2
2
2
2
The reduction potential of the As(IV)/As(III) couple has been reported as
E
0
≈ +2.4V [37]. Therefore, the formation of
As(IV) by the attack on holes, HO
∙
, or HO O
2
•
•
−
is thermodynamically possible. Regardless of the first oxidation step, As(IV)
/
2
transforms into As(V), the stable form:
•
−+
(9.16)
As IV OO/ sV
VB
() /
+
h
→
(
)
2
2
In recent years, some examples of further work on oxidative As(III) removal using HP under UV light were reported, in
addition to those described in our previous review [10], and are detailed in the following paragraphs.
The mechanism of photocatalytic oxidation of As(III) has been a matter of discussion for many years, the controversy being
centered on determining the major oxidant in the system, O
•−
[32, 33, 38, 39], HO
∙
, or h
V
+
[30, 31, 35, 40-43]. However,
whatever the oxidant, it is not possible to deny the efficiency of the HP process to transform As(III) into As(V). The real
problem, however, is to remove the later dissolved As(V).
In this line, an interesting review on As HP removal has been published recently, and most of the articles therein are also
reviewed in the following texts [44]. The authors focused on the application of TiO
2
and TiO
2
-based materials for removing
inorganic and organic arsenic, highlighting that TiO
2
-based arsenic removal methods developed to date focus on the photocata-
lytic oxidation of arsenite to arsenate and on the adsorption of inorganic and organic arsenic (monomethylarsonic acid (MMA)
and dimethylarsinic acid (dMA)) on TiO
2
. They observed that improvements in the photocatalytic process should focus on the
combination with adsorbents (e.g., slag-iron oxide-TiO
2
, meso-TiO
2
/α-fe
2
O
3
composites) to achieve higher photocatalytic
abilities and adsorption capacity and also to immobilize TiO
2
to facilitate the ulterior separation of the solid from the aqueous
phase. Another important point highlighted in the chapter, which is also a problem in real groundwater and wastewaters, is the
influence of the coexisting solutes (silicate, phosphate, carbonate oxyanions, and humic acid (HA)) on HP of arsenic systems
because of the competition of these species with As for adsorption on TiO
2
. In this context, Tsimas et al. [45] studied the simul-
taneous HP oxidation of As(III) and HA—usually found together in groundwater—in a ternary As(III)/HA/TiO
2
system. It was
shown that the efficiency of both As(III) and HA oxidation decreased in the ternary system compared with that of the
corresponding binary systems (As(III)/TiO
2
and HA/TiO
2
), indicating that one species inhibits the oxidation of the other. In
another work [46], a hybrid system combining TiO
2
with nanoscale zero-valent iron (nZVI) in a tubular photoreactor with recir-
culation was used for As(III) photooxidation. The addition of nZVI significantly enhanced arsenite removal, reducing the
required mass of TiO
2
by five times and achieving As values below the wHO-recommended concentrations (from 500 µg l
−1
to
<10 µg l
−1
). xu and Meng [47] studied the effect of the crystalline size (6.6 and 30.1 nm) of TiO
2
on As(III) and As(V) adsorption
and photocatalytic oxidation employing single-phase anatase nanoparticles. The adsorption capacity of TiO
2
for As(III) and
As(V) increased linearly with the BeT surface area of the particles, and no significant difference in the rate of As(III) photoox-
idation was found between the nanoparticles of 6.6 and 14.8 nm diameter, but a clear decrease was obtained with the 30.1 nm
particles, indicating the importance of the size of the nanoparticles to be used.
Regarding low-cost applications, several papers describe the use of very simple photocatalytic systems to remove As from
water at household levels. Several works in our laboratory [22, 27, 28, 48] report results of an application to remove arsenic from
natural waters in rural areas; to perform HP experiments, walls of PeT plastic bottles were impregnated with TiO
2
by a very
simple technique [49]. As(III) solutions ([As(III)]
0
= 1000 µg l
−1
, pH 7.8) were added in these bottles, which were put in a
horizontal position and irradiated for 6 h by UV light (366 nm, 800 μw cm
−2
). After irradiation, nongalvanized packing iron wire
pieces (usually employed in agricultural applications) were added. After 24 h settlement in the dark, As removals ranging
80-86% were obtained. The same bottle could be reused at least three times without loss of efficiency. The HP procedure was
proposed to remove As (500-1800 µg l
−1
, unidentified speciation) from well water samples of Las Hermanas (Santiago del estero
Province, Argentina). In this case, a feCl
3
solution was added at the end of solar irradiation, and more than 94% As removal took
place, attaining concentrations below the wHO limits. fostier et al. [50] proposed a similar low-cost application for As(III)
oxidation using TiO
2
immobilized on PeT bottles combined with fe(II) addition to the solution before exposure to sunlight.
