Photocatalytic Treatment of Photoprocessing Effluents E'Exintaveloni1, K'Fytianos1, E'Darakas2, I'Po

1
Photocatalytic Treatment of Photoprocessing
EffluentsE.Exintaveloni1, K.Fytianos1,
E.Darakas2, I.Poulios11Department of Chemistry,
2Department of Civil Engineering Aristotle
University of Thessaloniki, 54124 Thessaloniki,
Greece
Aristotle University
Aristotle University
Introduction The elimination of toxic chemicals
from wastewater is presently one of the most
important subjects in pollution control. These
pollutants may originate from industrial
applications or from household and personal care
areas several of them are resistant to
conventional chemical and biological treatment
methods. Taking into account the composition and
the volume of the photoprocessing effluents, the
photofinishing laboratories are considered to be
very dangerous for the environment and their
effluents should be inactivated before their
deposition into the main drainage. The
photofinishing effluents are characterized by the
existence of numerous development reagents that
are refractory or toxic such as aromatic
compounds and EDTA, and fix-stabilizer
byproducts, primarily consisting of reduced
sulfur compounds such as thiosulfate and sulfite.
They also contain high silver ion concentrations
as a result of the photofinishing process. In the
present work, the efficiency of a heterogeneous
(TiO2) and a homogeneous (Photo-Fenton reagent)
photocatalytic method in the presence of
artificial light was evaluated concerning the
decolorization and organic content reduction
(DOC) of a simulated photo-processing effluent.















Heterogeneous and Homogeneous Photocatalysis
a
Heterogeneous photocatalysis By the irradiation
of an aqueous TiO2 suspension with artificial or
solar light with energy greater than the band gap
energy of the semiconductor, conduction band
electrons (e-) and valence band holes (h) are
generated. The photogenerated electrons react
with the adsorbed molecular O2 on the
Ti(III)-sites, reducing it to superoxide radical
anion O2.-, while the photogenerated holes can
oxidize either the organic molecules directly or
the OH- ions and the H2O molecules adsorbed at
the TiO2 surface to OH radicals, which act as
strong oxidizing agents (Fig.1). These can easily
attack the adsorbed organic molecules or those
located close to the surface of the catalyst,
thus leading finally to their complete
mineralization. Homogeneous photocatalysis The
Fenton reagent (mixture of Fe2 and H2O2) is
known for its capacity to oxidize a series of
organic impurities found in wastewater. This
happens because of the formation of OH. radicals
and the degradation that they cause to the
organic compounds of the effluents. The
disadvantages of this method are, the consumption
of great amounts of the needed reagents, the
formation of a great amount of sludge and the
partial oxidation of many organic compounds. The
effectiveness and yield of the specific method
can be increased impressively by the illumination
of the system with artificial or solar light. The
illumination results in the formation of more OH.
radicals, the reduction of the amount of the
sludge, due to the reuse of the catalyst and the
complete oxidation of most of the organic
compounds.
Fe2 H2O2 ? Fe3 HO? OH- (Fenton)
b Fe3 hv H2O ? Fe2 OH. H (
Photo Fenton) R-H OH ? ? ROO ? ? ? CO2 H2O
Figure 1a Heterogeneous photocatalytic oxidation
of organic compounds in the presence of TiO2.
b Homogeneous photocatalytic
oxidation of organic compounds in the presence of
Fe2/H2O2.
Organic Content Reduction in the presence of
TiO2 and the Photo-Fenton reagent
Results of the illumination of a 1500 mg l-1 TOC
(5000 mg l-1 COD) wastewater solution in the
presence of various semiconductor oxides as
photocatalysts are shown in Fig. 2. The
amount of the organic carbon reduction in the
supernatant, as determined from the DOC
measurements, is plotted as a function of
irradiation time. From the results of Fig. 2 it
is clear that the presence of the catalyst alone
leads after 6 h illumination to a small decrease
of DOC (15) in case of TiO2 P-25, probably due
to the high initial organic content of the
wastewater. The addition of H2O2 enhance
significantly the initial rate and after 6 h
illumination a 72, 65 and 50 DOC reduction has
been observed for TiO2 P25, TiO2 (A) and TiO2
UV-100 respectively.
As can be seen in Fig. 3, in the case of The
Photo-Fenton reagent, the increase of the Fe3
concentration from 0 to 112 mg l-1 leads
to an increase of the reaction rate, while after
six hours illumination in all cases an almost 70
DOC reduction has been observed. The approximate
results in absence of Fe3 are due to the fact
that in this type of wastewater there are various
complex compounds containing Fe3 in their
molecule. After some time the degradation of
these compounds gives ferric ions that
participate to the Photo-Fenton reaction.
Figure 2 Photocatalytic organic content
reduction of a simulated photoprocessing
waste-water as a function of illumination time in
the presence of (?)1 g l-1 TiO2 P-25, (?) 1 g
l-1 TiO2 P-25 1g l-1 H2O2 (?) TiO2 (A) 1g l-1
H2O2 (?) 1 g l-1 TiO2 UV-100 1g l-1 H2O2.
Figure 3 Influence of the Fe3 concentration on
the reduction of DOC of a simulated
photo-processing wastewater in the presence of 1
g l-1 H2O2.



SO42- release in the presence of TiO2 and the
Photo-Fenton reagent
In Fig. 4 are given some preliminary
experimental results concerning the homogeneous
and heterogeneous photocatalytic oxidation of the
sulfur reducing compounds (thiosulfate, sulfite,
etc) to SO42-. There is an important increase of
the SO42- concentration, as a result of the
oxidation of the organic and inorganic sulfur
compounds, which are present in this liquid
waste. The high efficiency in case of H2O2 is due
to the existing ferric ions in the solution, and
therefore the Photo-Fenton reagent, and due to
the possible direct oxidation of the sulfite and
thiosulfate anions by the H2O2 .
Figure 4 SO42- release as a function of
illumination time in the presence of 1 g l-1
H2O2, (?) without catalyst (Photo-Fenton
reagent), (?) in presence of 1 g l-1 TiO2 P25.
Conclusions From the results reported above it is
clear that, under the specific experimental
conditions of this work, (a) The synergetic
action of TiO2 with oxidants such as H2O2 leads
to a substantial increase of the initial reaction
rate and the extent of mineralization in
comparison to the TiO2 alone. (b) The
Photo-Fenton experiments were considerably faster
than those with TiO2 but a detailed experimental
and economical analysis has to be made in order
to arrive at a conclusion about the most
appropriate method for application. (c) After six
hours illumination in all cases an almost 70 DOC
reduction has been observed. An increase in the
efficiency of DOC reduction in this type of
wastewater could be reached by further
optimizing the experimental conditions.