APCBEE Procedia 5 ( 2013 ) 181 185 [619160]

APCBEE Procedia 5 ( 2013 ) 181 – 185
2212-6708 © 2013 The Authors. Published by Elsevier B.V.
Selection and peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society
doi: 10.1016/j.apcbee.2013.05.032
ICESD 2013: January 19-20, Dubai, UAE
Possible environmental risks of photocatalysis used for water
and air depollution case of phosgene generation
Traian Vasilache , Iuliana Lazar, Marius Stamate, Valentin Nedeff and Gabriel
Lazar

Abstract
This short review presents some deficiencies regarding photoc atalysis used for water and air depollution, because the
environmental risks have to be considered in order to minimize negative effects of the future applications. Since
photocatalysis was discovered a couple of de cades ago, it was intensively studied and many applications were developed.
In the environmental engineering depollution ex-situ and in-s itu of water and air, using pho tocatalysis, seems to be
revolutionary. Deficiencies of these proce sses are concerning in formation of undesirable secondary products. Some of the
processes involving photocatalysis could degrade efficient by-pro ducts, but there are situations when these products seem
to be more toxic than initia lly pollutant. One of these cases, phosgene gene ration during air depollution, is detailed in a
scenario, using related researches and simple calculus. The result proves th e environmental risk of organochlorine
compounds oxidation by photocatalytic processes.

© 2013 Published by Elsevier B.V. Selection and/ or peer review under responsibility of Asia-Pacific
Chemical, Biological & Environmental Engineering Society

Keywords: air depollution; environmental risks; photocataly sis; secondary products; water depollution
1. Introduction
Photocatalysis is an advanced oxidati ve process based on f ormation of high reactive species at the interface
of a semiconductor, when this is irradiated wi th EM-energy, especially UV and visible light.

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© 2013 The Authors. Published by Elsevier B.V.
Selection and peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society
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182 Traian Vasilache et al. / APCBEE Procedia 5 ( 2013 ) 181 – 185
Electrons in solids are situated on energy bands. An occupied band and an unoccupied one are separated by
a band gap. In semiconductors, the width of the band gap is lower compar ing with band gap in electrical
insulators but larger comparing with metals. If a photon from UV or visible range of electromagnetic
spectrum is absorbed, an electron (e-) could be promoted from the valence band (VB) into the conduction
band (CB), subsequent generating a hole in the valence band. The resulting electron is a reducing agent, while
the hole is an oxidizing one. In the m echanism of the photocatalytic process, the hole can react with water to
generate the hydroxyl radical and the electron can redu ce molecular oxygen, hydrogen peroxide or some other
oxidizing agent in the solution. These conduce to appearan ce of reactive radicals which are responsible for the
removal of hazardous components from the water and air [1 6].
In water depollution some medicines, solvents and dye s represent problems. Diclofenac is the disseminated
medicine in water, potentially toxic for aquatic life and photocatalysis degraded it successfully; some of
secondary products resulted are toxic, even more poisonous as primary po llutant. Degradation of 4-
clorophenol under microwave energy couplet with photocatal ysis in aqueous solutions occurs with formation
of chlorobenzene, phenol, hydroquinone, benzoquinone and 4-chlorocatechol which are undesirable products.
Textile dye Acid Red 88 was degraded by sonolysis and photocatalysis, but some intermediate products were
not mineralized. The same problem occurred on de gradation of diphenylamine from polluted water.
Regarding air depollution, photocatalysis was applying in nasty smells removal; 2-butanone and methyl-
ethyl-ketone were successfully oxidized, but acetaldeh yde results as secondary product. A pulse discharge
system combined with photocatalysis degraded phenols and the problem of sub products appears once again.
Some insecticides as malathion, isomalathion and ma laoxon were decomposed by photolysis coupled with
photocatalysis and some undesirab le secondary products formed. Also, trichoroethylene was decomposed
successfully by photocatalysis, but it results phosgene, chloroform and carbon tetrachloride.
All the methods need improvement and future research es will find those materi als and chemical reactions
which have to be eco-friendly.
2. Case studies presentation
2.1. Water depollution
Hai-Chao Liang et al. from Hong Kong, China studied the comparative homoge neous and heterogeneous
degradations of diphenamid (DPA) in aqueous solution by p hotolysis and photocatalysis [7]. Direct photolysis
performed with UVC (254 nm) and photocatalysis with TiO 2/UVA (350 nm). Direct homogeneous photolysis
was efficient in degradation of DPA up to 100% after 360 minutes, but was not very efficient to mineralize
the secondary products (only 8%). In contrast, photocatalysis evolved slowly (only 51% after 360 minutes),
but permitted a degradation of byproducts in a high er percent (11%). Analysis identified over 20
intermediates. The mechanism of reactions is quite sim ilar in those two processes, for example as regard
oxidation of N-methyl group of molecular chains. Opposi te, hydroxylation of aromatic ring was faster in
photocatalytic process.
The degradation of 4-chlorophenol (4CP) by a simu ltaneous microwave assisted UV electrodeless
discharge lamp photocatalysis technique (MW/UV/TiO2) was investigated by a Chinese collective [8]. The
process is affected by factors like the dosage of photoc atalysts, the initial pH va lue of the solutions, gas
bubbling, light intensity and addition of H 2O2 oxidant. The mechanism proposed is a pseudo-first-order one
relative to 4CP degradation. Synergistic effects between microwaves and photocatalysis were observed too.
Intermediates found were chlorobenzene, phenol, hydroquinone, benzoquinone and 4-chlorocatechol. All of
these are dangerous for human health, if they are not properly controlled. The photodegradation of 4CP
involved stages, as follow: (a) direct photolysis by UV light; (b) hydroxylation of the aromatic ring; (c)

183 Traian Vasilache et al. / APCBEE Procedia 5 ( 2013 ) 181 – 185
substitution of chlorine by *OH; (d) oxidation of ch lorinated hydroquinone to quinine; (e) oxidation to
aliphatic intermediates; (f) mineralization to carbon di oxide and water. Of course, in ideal situation,
mineralization would be total and undesirable byproducts are quantitatively quite zero, an aim for next
technologies.
Degradation of Acid Red 88 by combined sonolytic an d photocatalytic action was studied by Madhavan et
al. from Australia, India [9]. Acid Re d 88 (AR88) is a mono-azo dye used in the textile and food industries
like colorant. Sonolytic, photocatal ytic and sonophotocatalytic decomp osition of AR88 was performed in the
presence of Fe3+ and TiO 2 as photocatalyst. The effects of initial concentration of dye and of ultrasound power
were investigated and a first orde r-like kinetics was established. A synergetic effect was observed, so
sonolysis combined with photocatalysis was the effici ent process. Mineralization was studied analyzing total
organic carbon (TOC) and sonophotocatalysis method ex hibits better results. As byproducts mono and di-
hydroxylated compounds were identified using electro spray mass spectrometry (ESMS). Also a degradation
pathway was proposed. As an interesting fact to be signalized, byproducts appeared in all studied processes
(sonolysis, photocatalysis and sonophotocatalysis).
L. Rizzo et al. (Napoli, Italy an d Nicosia, Cyprus) focused their studies on degradation kinetics and
mineralization of diclofenac (DCF) by TiO 2 photocatalysis [10]. Like contro l tell-tale a set of bioassays
(Daphnia magna, Pseudokirchneriella subcapitata and Arte mia salina) was used to evaluate DCF toxicity and
obviously, degradation of DCF. The ki netic of reactions seems to be a pseudo-first-order one. Some
secondary products formed were more toxic than DCF. Best results were achieved for low concentrations of
DFC, so the process could be very efficient for post-treatment of wastewaters. The first two species noted as above were sensible at DFC and different conditions of experiment, while the third (Artemia salina) proved to
be not sensitive under the investigated conditions. UV absorbance could be also a fast indicator for
preliminary results as concern organic intermediates formation.
2.2. Air depollution
The reduction of nasty smells by photocatalytic oxidation was studied by G. Vincent and O. Zahraa from
Nancy Cedex, France [11]. 2-butanone and methyl-ethyl-ketone (MEK) were considered like volatile organic
compounds with moderated molecular mass (< 100 g/mo l) because these could cause a serious olfactory
discomfort. The photocatalytic degra dation occurred on commercial TiO
2 Degussa P25 deposited on glass
fibers. The compounds were disintegrated successfully, but as a secondary product acetaldehyde was formed.
It is known that acetaldehyde is undesi rable because has a cancerous poten tial. Also a mechanism of chemical
reaction was proposed, following Langmuir-Hinshelwood ki netics. This mechanism wants to be able to
predict other secondary products which could appear in the photochemical reactions and to point the path for
diminished the undesirable byproducts.
M. Bavcon Kralj et al., Sloven ia pointed to degradation by photo-catalysis of some toxic compounds
(malathion, radothion, isomalathion, malaoxon frequently used as insecticides in agriculture) [12]. Gas-
chromatography for analyses was used. Po sitive results achieved have to be enhanced because some toxic
byproducts were formed. For toxicity studies the enzyme ASChI (acetylthiocholine iodide) was the control
tell-tale.
Madjid Mohseni, Vancouver, Canada, studied photo-oxidation of trichloroethylene (TCE) by
photocatalysis with TiO 2 and photolysis [13]. Ultraviolet illuminati on was provided by lamps with outputs of
254 nm, 365 nm and 185/254 nm wave length. Even the re sults were satisfactory, especially when coupled
system was used, undesirable byproducts as phosgene, chloroform, carbon tetrachloride and dychloro-acetyle
chloride (DCAC) were produced too. Figure 1 pres ents time evolution for TCE, DCAC and phosgene
concentration.

184 Traian Vasilache et al. / APCBEE Procedia 5 ( 2013 ) 181 – 185
Another study regarding photocatalysis of trichloroethylene (TCE) under TiO 2 was realized by Hsing-
Hung Ou et al. (Taiwan). They investigated the ef fects of the oxygen and relative humidity (RH) on the
formation of the dichloroacetyl chloride (DCAC) and phosgene [14]. Because the reaction rate constants of TCE are almost one order larger than that of DCAC, it results an accumulation of by-product. For optimal
conditions (oxygen and RH) this could be diminished. P hosgene also is formed like a secondary product, but
it seems to be generated from DC AC degradation more than from TC E destruction. An elevate level of
oxygen conduced to ph osgene inhibition.
3. Results and discussions
From the presented data it could be signalized th at secondary products of the reactions could not be
avoided. Taking as examples the last two related articl es [13,14] it will be discus sed the situation of phosgene
generation.
Phosgene concentration values exceeds maximum admiss ible limit during TCE degr adation [8]. A similar
result is proved by Madhavan et al. [9]. Even the TCE degradation is qu ite complete, phosgene concentration
exceeds 20 times the limits. For these reasons, a phot ocatalytic degradation of TCE could be performed only
in closed area reactors, for in-situ depollution appl ications (in industry).
Phosgene (CCl
2O) is a colourless gas used like reagen t in chemical industry and synthesis of
pharmaceuticals. Its odour brings to mind freshly cu t grass or hay. Molar mass of phosgene is 98.92 g/mol.
Phosgene could be generated by industrial product ion, but it could appear also from combustion of
organochlorine compounds. During the First World War, phosge ne was used like combat toxic gas, a fact that
emphasizes them dangerous potential [15]. The maximum admissible limit in the ambient atmosphere is 0.02
ppm for 8 hours (or 0.08 mg/m3) and 0.1 ppm for 15 minutes (0.4 mg/m3) [16].
Dichloracetyl chloride (DCAC, having compressed chemical formula C 2Cl3HO), is a chlorinated
hydrocarbon with molar mass 147.39, used mainly like organic solvent [17]. Maximum admissible limits are
3 mg/m3 for 8 hours and 5 mg/m3 for a short time (15 minutes) exposure.
Trichloroethylene (TCE, with formula C 2HCl 3), a chlorinated hydrocarbon is known also like
trichloroethene (the IUPAC name), and it is used like solvent. Molar ma ss of TCE is 131.39 g/mol. Maximum
acceptable limits for TCE are 100 mg/m3 for 8 hours, 150 mg/m3 for 15 min respectively, in atmospheric air.
But TCE could pollute also the groundwater.
A comparison of maximum acceptable limits between phos gene and TCE is interesting for a scenario. For
this purpose, data available from pu blic sources were compiled. A simple ratio (data for 15 minutes exposure)
discloses that if we have a half maximum concentration of TCE (75 mg/m3), that means 0.57 mol/m3 and
secondary product phosgene is generated at 1% molar ratio, the result gives 0.56 g/m3 phosgene, a value
higher than maximum admissible limit. Supposing the sa me value for TCE and th e same percent (1%) for
transformation in DCAC, after calculus a value of 0.84 mg/m3 was achieved. This value do not exceed
maximum acceptable limit, comparing with phosgene generation case.
4. Conclusions
Even photocatalytic processes seem to be revolutionary, there are aspects to be clarified and one of this is
represented by secondary undesirable products which a ppear in photochemical reactions. The mechanism of
photochemical reactions is complicated an d further studies have to establish the path to achieve complete
mineralization or even diminishing of these compo unds, to allow large scale industrial and commercial
applications. Many times there are secondary unknown products, in sense of unpredictable, so the research

185 Traian Vasilache et al. / APCBEE Procedia 5 ( 2013 ) 181 – 185
has to find them, to prevent environmental negative effects. Phosgene generation during oxidation of
organochlorine compounds represents a risk.
Acknowledgements
This paper was performed with the support of the -2013 founded by
the European Social Found and the Romanian Government.
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