Environmental Engineering Reference
In-Depth Information
activity of several SC samples in the aqueous phase [121-132]. Most of the recent papers use the HP Cr(VI) reduction reaction
as a model system for testing new photocatalysts; this is the case of immobilized nanotubular TiO
2
electrodes [130], wO
3
-doped
long TiO
2
nanotube arrays [133], and Au/TiO
2
heterojunction nanotube composites [126], among other materials. Other papers
shed more light on some specific features of the Cr(VI) photocatalytic system. for example, Mu et al. [127] explored the HP
Cr(VI) reduction using TiO
2
nanofibers prepared via an alkaline hydrothermal reaction and TiO
2
nanoparticles with varied
structural properties. The results showed that Cr(VI) reduction is affected mostly by the specific surface area of the catalyst
rather than by the crystalline-phase composition. This agrees with the hypothesis of the formation of the strong complex bet-
ween Cr(VI) and TiO
2
, mentioned earlier, which would be more favorable at a higher surface area of the photocatalyst.
Pandikumar et al. [131] evaluated TiO
2
-Au nanocomposites for UV light Cr(VI) reduction and simultaneous methylene blue
oxidation; the authors observed a synergy between these two processes due to the presence of Au nanoparticles on the TiO
2
surface. In another work, TiO
2
coupled to nonfunctionalized carbon nanotubes (CNTs) was found more active than
OH-functionalized CNTs because of the higher Cr(VI) adsorption on the first samples [128]. Similarly, Au/TiO
2
heterojunction
composite nanotube arrays showed a higher efficiency than unmodified TiO
2
nanotube arrays for the simultaneous transforma-
tion of Cr(VI) and acid orange 7 (AO7) [126]. In another paper, Cr(VI) reduction in the presence of edTA [124] was employed
to evaluate the activity of anatase TiO
2
films deposited by cathodic arc (CA) on glass substrates and compared with the activity
of P-25 films obtained by dip-coating (dC) immersion. despite dC films showing higher photoactivity for Cr(VI) reduction,
its adhesion properties are very poor compared to CA films, making this last type of films a promising material for photocata-
lytic applications that require immobilized catalysts.
Photocatalytic Cr(VI) reduction is also possible under visible irradiation. Kim and Choi [123] investigated the formation of
surface complexes between TiO
2
and electron donors such as methanol, formic acid, acetic acid, triethanolamine, or edTA,
which can absorb visible light through a ligand-to-metal charge transfer (LMCT) mechanism; the TiO
2
-edTA complex, espe-
cially, showed an outstanding visible light activity for Cr(VI) reduction. A similar approach was presented by wang et al. [129]
using low molecular weight organic acids (SOAs) such as tartaric, citric, and lactic acids with TiO
2
under light irradiation above
420 nm; the authors support a charge transfer complex mechanism in which the photogenerated electrons are transferred from
SOAs to TiO
2
and then accepted by the adsorbed Cr(VI). Another example is simultaneous Cr(VI) removal and naphthalenesul-
fonate (NS) oxidation in textile wastewater by the combination of visible light photocatalysis and ion exchange membrane
processes (electrodialysis) [122]. while NS acts as a hole scavenger attenuating electron-hole recombination, the ion exchange
membrane prevents Cr(III) reoxidation. A CuAl
2
O
4
/TiO
2
heterosystem irradiated with visible light was used to achieve Cr(VI)
reduction, the reaction being enhanced in the presence of salicylic acid as a sacrificial agent [134]. Also, Cr(VI) was effectively
reduced under visible light irradiation employing TiO
2
-coated CdS nanowires (Nws) [125]. Other modified TiO
2
materials
showed activity for visible light-driven photocatalytic Cr(VI) reduction such as SnS
2
/TiO
2
nanocomposites or N-doped and
N-f codoped TiO
2
[121, 132].
dye-photosensitized TiO
2
samples were proven to reduce Cr(VI) under visible light; some examples follow. di Iorio et al.
[116] studied Alizarin red chelated to TiO
2
. Results indicate a high efficiency for Cr(VI) reduction, almost independent of the
photon flux and of the irradiation wavelength, and slightly dependent on Cr(VI) concentration in the explored range (40-
200 μM), with similar results in air and nitrogen atmosphere. ePR experiments confirmed Cr(V) formation. In another work
[135], TiO
2
coated with hydroxylaluminumtricarboxymonoamide phthalocyanine (AlTCPc) irradiated under visible light in the
presence of 4-chlorophenol (4-CP) as the sacrificial donor showed a very rapid Cr(VI) reduction to Cr(III). As discussed in the
section on Arsenic, in the presence of dyes, the photocatalytic mechanism is different from that taking place under UV light,
according to the scheme shown in figure 9.4: after excitation of the dye, electron injection into the CB promotes Cr(VI)
reduction [51]. A very good protective effect of 4-CP preventing AlTCPc photobleaching was observed, as reactive oxygen
species (ROS, e.g., OHO
2
, ), which could oxidize the dye, could be formed due to the fast reaction of e
C
−
with Cr(VI). In
another paper, Park et al. [52] tested the visible light activity of the Od-sensitized TiO
2
catalyst described earlier (Section 9.3)
for the reduction of Cr(VI). As in the case of As(III), Cr(VI) reduction with Od/TiO
2
showed better results than with both RuL
3
/
TiO
2
and bare TiO
2
. The addition of an electron acceptor such as fe
3+
significantly retarded Cr(VI) reduction because of com-
petition for e
C
−
, confirming that Cr(VI) reduction proceeds by accepting electrons from the dye-sensitized TiO
2
. In a very recent
work, Park et al. continued studies on this system [136] and tested the conversion of Cr(VI) to Cr(III) using a very similar dye,
that is, (e)-3-(5-(5-(4-(bis(4-((2-(2-methoxyethoxy)methyl)phenyl)amino)phenyl)thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic
acid), agglomerated with bare TiO
2
nanoparticles; the authors indicate an enhancement in the reduction rate of Cr(VI) attrib-
uted to the retardation of the charge recombination between the oxidized dye and the injected electron by the agglomeration with
bare TiO
2
.
A very unusual mode of titania photosensitization was presented by Kuncewicz et al. [120]. The authors proposed that as
the broad LMCT band of CrO
4
2−
in aqueous solutions (with a maximum at 373 nm at pH 7) extends to visible light, excitation
of the CrO
4
2−
species adsorbed on the titania surface at 440 nm (2.8 eV) should result in electron transfer from O
−II
to Cr(VI)
•−
•
2
