Phenol photonitration upon UV irradiation of nitrite in aqueous solution II: Effects of pH and TiO2

Article abstract of DOI:10.1016/S0045-6535(01)00036-4

Phenol photonitration and photonitrosation were studied both in homogeneous and in heterogeneous phase in the presence of TiO₂ particles. The effect of pH as well as of the semiconductor particles on the kinetics and products of the reaction was observed. Formation of nitrophenols is enhanced at acidic pH, due to thermal processes initiated by nitrous acid, as well as in the presence of TiO₂, due to the photocatalytic oxidation of nitrite.

Full text of DOI:10.1016/S0045-6535(01)00036-4

Chemosphere 45 82001) 903±910
www.elsevier.com/locate/chemosphere
Phenol photonitration upon UV irradiation of nitrite in
aqueous solution II: e€ects of pH and TiO2
Davide Vione, Valter Maurino, Claudio Minero, Ezio Pelizzetti *
Dipartimento di Chimica Analitica, Universit aꢀ di Torino, Via P. Giuria 5, 10125 Torino, Italy
Received 26 September 2000; received in revised form 24 January 2001; accepted 29 January 2001
Abstract
Phenol photonitration and photonitrosation were studied both in homogeneous and in heterogeneous phase in the
presence of TiO particles. The e€ect of pH as well as of the semiconductor particles on the kinetics and products of the
reaction was observed. Formation of nitrophenols is enhanced at acidic pH, due to thermal processes initiated by
2
nitrous acid, as well as in the presence of TiO
Ltd. All rights reserved.
2
, due to the photocatalytic oxidation of nitrite. Ó 2001 Elsevier Science
Keywords: Nitrite; Nitrous acid; Photochemical transformations; Photolysis; Nitroaromatic compounds; Photocatalysis; Titanium
dioxide
1
. Introduction
2
Vione et al., 2001a), but never for nitrite. TiO inhibits
phenol photonitration in the presence of nitrate, very
probably because it inhibits nitrate photolysis, and it is
interesting to control if this e€ect is maintained also in
the case of nitrite.
Phenol photonitration upon nitrite UV photolysis
follows two paths 8Vione et al., 2001b). One is started by
Å
the reaction between phenol and NO
2
8or N
2
O
4
), the
other is due to the oxidation of the nitrosoderivatives.
This paper describes the e€ects of pH and of the
2
presence of TiO on phenol photonitration and photo-
2
. Experimental
nitrosation. The in¯uence of pH on nitrophenol for-
mation has been studied for the irradiation of nitrate
Extensive accounts of the experimental details have
8
Machado and Boule, 1995; Dzengel et al., 1999; Vione
been reported elsewhere 8Vione et al., 2001a; Vione et al.,
001b). The pH values of the solutions 8and suspen-
sions) were adjusted by adding NaOH 8purity grade
9%, Merck) or HClO 870%, Merck) as needed. They
were used as received, without further puri®cation. Use
of HNO was avoided to prevent interference, as nitrate
et al., 2001a), and all the authors concluded that nitro-
phenols form in larger amount at acidic pH. It is inter-
esting to control if this e€ect is linked only to the
presence of nitrate or if it is speci®c of the organic
substrate.
2
9
4
3
2
The e€ect of TiO aqueous suspensions has been
itself shows certain photoactivity 8Niessen et al., 19ꢀꢀ;
Guillaume et al., 19ꢀ9; Machado and Boule, 1995;
Vione, 199ꢀ; Dzengel et al., 1999; Vione et al., 2001a).
In addition to the already described 360 nm lamp, we
also used a 312 nm UV source 8100 W Philips TL 01
assessed for nitrate 8Vione, 199ꢀ; Dzengel et al., 1999;
*
Corresponding author. Tel.: +39-011-6707649; fax: +39-
11-6707615.
E-mail address: pelizzet@ch.unito.it 8E. Pelizzetti).
7
0
0
lamp), producing a photon ¯ux of 1:3 Â 10 Ein/s in the
cells. The optical path b and the volume V of sample in
045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 8 0 1 ) 0 0 0 3 6 - 4

904
D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
Fig. 1. Irradiance spectrum of the Philips TL 01 lamp 8irradi-
ance maximum at 312 nm), spectra of phenol, nitrite and ni-
trous acid.
the irradiation cells were 0.4 cm and 5 ml, respectively.
The lamp irradiance spectrum, together with the ab-
sorption spectra of nitrite, nitrous acid and phenol, are
reproduced in Fig. 1.
3
. Results and discussion
Various reaction mechanisms have been proposed for
phenol photonitration 8Niessen et al., 19ꢀꢀ; Machado
and Boule, 1995; Dzengel et al., 1999; Louw and Sant-
oro, 1999; Vione et al., 2001a). An extensive review of
the literature concerning these mechanisms is reported in
Fig. 2. pH dependence for nitrophenol formation in homoge-
À3
Vione et al. 82001a). However, the results of a work on
Å
neous phase. Initial conditions: phenol 1:1 Â 10 M, NaNO
2
phenol nitration by NO
1
2
in the dark 8Coombes et al.,
994), and the demonstration of the contribution by 4-
0
4
.10 M, pH adjusted with the addition of HClO , irradiation at
3
60 nm. 8a) 2-nitrophenol; 8b) 4-nitrophenol.
nitrosophenol, brought us to propose also other reaction
pathways 8Vione et al., 2001b). The latter are the basis
for the discussion in this work.
increase with decreasing pH. Nitrophenol yields in the
presence of nitrite are quite high when compared to
those obtained with nitrate in the same irradiation
conditions 8Vione et al., 2001b), mostly due to the higher
absorption of nitrite at the irradiation wavelength 8360
nm).
While the relevant contribution of 4-nitrosophenol is
probably speci®c of nitrite photolysis, the mechanism
Å
involving NO
irradiation of both nitrite and nitrate.
2
8or N
2
O
4
) is probably the same for the
Å
3
.1. E€ect of pH
Production of NO upon nitrite photoexcitation is
2
pH-dependent in reaction 2 8Fischer and Warneck,
1996):
3
Phenol solutions 81:1 Â 10 M) were irradiated in the
presence of 0.10 M sodium nitrite at pH 3.0, 4.5, 6.5 and
0.5 in homogeneous phase. Concentration vs. time
1
À
Å
Å
À
NO ‡ hm ! NO ‡ O
2
curves for nitrophenols at di€erent pHs are shown in
Fig. 2 88a) 2-nitrophenol; 8b) 4-nitrophenol). Phenol
degradation curves are not shown: pseudo-®rst order
degradation constants for phenol in all performed ex-
periments are reported in Table 1.
‰
U
1
ꢀ360 nm† ꢁ 0:025Š
ꢀ1†
ꢀ2†
ꢀ3†
Å
À
‡
Å
O ‡ H $ OH
‰
pKa;2 ˆ 11:9Š
Å
À
À
Å
OH ‡ NO ! OH ‡ NO
2
2
The shapes of the curves in Fig. 2 indicate that the
rates of both nitrophenol formation and transformation
1
0
À1 À1
M s
‰
k
3
ˆ 1:0  10
Š

D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
905
Table 1
Pseudo-®rst order degradation constants for phenol 1:1 Â 10À3
a
M
À
À1
)
pH
Irradiation wavelength 8nm)
‰NO Š 8M)
TiO
2
8g/l)
k
D
8s
2
À4
À4
À5
À4
À5
À5
À4
À4
À5
À4
À4
À5
3
4
6
.0
360
360
360
360
dark
312
360
360
360
360
360
360
0.100
0.100
0.100
0.100
0.001
0.001
0.100
0.100
0.100
0.100
0.100
0.100
±
±
±
±
±
±
3:3 Â 10
3:0 Â 10
7:0 Â 10
1:6 Â 10
2:7 Â 10
2:ꢀ Â 10
1:3 Â 10
1:1 Â 10
5:3 Â 10
5:5 Â 10
1:5 Â 10
6:3 Â 10
.5
.5
10.5
2
2
6
6
6
3
4
.0
.0
.5
.5
.5
.0
.5
0.10
0.50
2.00
0.50
0.50
0.50
10.5
a
Conditions are reported in the table.
Å
À
However, protonation of O should be very fast in
In addition, N
8Durrant, 1964):
2 4
O is well known to react with alkali
aqueous solution. If so, on a mere thermodynamic basis,
it cannot explain the pH e€ect in the range 3.0±10.5. A
signi®cant variation in nitrophenol formation should
occur only around pH ˆ 12.
‡ 2OHÀ ! NO ‡ NO ‡ H
À
À
O
ꢀꢀ†
N
2
O
4
2
2
3
Photolysis of HNO
2
is more ecient than nitrite
For instance, alkaline traps are used for its adsorp-
tion. The hypothesis is that N O is more stable in acidic
solution than in neutral to basic ones.
photolysis 8Fischer and Warneck, 1996):
2
4
Å
Å
HNO
2
‡ hm ! NO ‡ OH
Fig. 3 reports the initial rates for the formation of 2-
nitrophenol, 4-nitrophenol and 4-nitrosophenol as a
function of pH. In this case the irradiation was per-
formed at 312 nm instead of 360 nm, but the processes
following nitrite photolysis are the same at the two
wavelengths 8Fischer and Warneck, 1996). The only
‰
U
4
ꢀ360 nm† ꢁ 0:317Š
ꢀ4†
Å
Å
OH ‡ HNO
2
! NO
2
‡ H
2
O
9
À1 À1
s
‰
k
5
ꢁ 2:6 Â 10 M
Š
ꢀ5†
and the decadic absorption coecient for HNO at 360
À
nm is 1.7 times that of NO2 8Fischer and Warneck,
2
di€erences are U
1
, U
4
ꢀ312 nm† > U
1
À
; U
4
ꢀ360 nm† and
À
e
pensating each other. Nitrite concentration was 10 M.
312ꢀHNO
2
; NO † < e360ꢀHNO
2
; NO †, partially com-
1
996). At ionic strength 0.1 8NaNO
2
0.1 M),
2
2
À3
pKaꢀHNO2† ˆ 2:9 8Martell and Smith, 1974; however,
di€erent authors report slightly di€erent values), so at
À
pH ˆ 4.5 the ratio ‰NO Š=‰HNO
2
Š is 3ꢀ.9. This means
2
that the photolysis of nitrous acid should increase the
Å
rate of NO
2
production by 55% at pH ˆ 4.5, when
compared with pH ˆ 6.5. Kinetic calculations, per-
formed according to Vione et al. 82001b), show that
nitrophenol formation should be consequently increased
by at most 140%. As a consequence, it is not possible to
explain only on this ground the at least four-fold in-
crease in nitrophenol formation between pH 6.5 and pH
4
.5 8see Fig. 2).
Another explanation for the pH e€ect has been given
in the case of nitrate irradiation 8Vione et al., 2001a).
Å
After photoformation, NO
2
can undergo dimerisation
2 4
to yield N O , which disproportionates into nitrite and
nitrate 8Gr a tzel et al., 1969):
Å
ꢀ
À1 À1
2
NO
2
$ N
2
O
4
‰k
6
ˆ 4:5  10 M
s
3
À1
k
À6 ˆ 6:9  10 s
Š
ꢀ6†
ꢀ7†
Fig. 3. E€ect of pH on the formation of 2- and 4-nitrophenol
and of 4-nitrosophenol. Initial conditions: phenol 1:1 Â 10À3 M,
À
À
3
‡
N
2
O
4
‡ H
2
O ! NO ‡ NO ‡ 2H
3
2
2 4
NaNO 0.001 M, pH adjusted with the addition of HClO , ir-
À1
‰k
7
ˆ 1  10 s Š
radiation at 312 nm.

906
D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
Å
Å
The curves show a trend similar to an acid±base
2HNO
2
! NO
2
‡ NO ‡ H
2
O
ꢀ
9†
equilibrium for both nitration and nitrosation, with very
similar pH1=2s. This similarity is partially obvious, since
about 50% of nitrophenol formation is due to the oxi-
dation of the nitrosoderivatives 8Vione et al., 2001b).
However, also the concurrent mechanism for nitrophe-
nol formation needs having the same pH dependence,
otherwise the curves would not approach zero around
neutrality. This means that phenol nitration and nitro-
sation have a common pH-dependent step.
À1 À1
s
‰
k
9
ˆ 2ꢀ:6 M
at 303 KŠ
The value of pH1=2 given in Fig. 3 for 2- and 4-ni-
trophenol and for 4-nitrosophenol is compatible with
the pK
a
of nitrous acid, within experimental error. This
À
means that the acid/base equilibrium HNO
explain the e€ect of pH on phenol nitration and nitro-
2
=NO2 can
sation.
Å
Formation of NO
À3
2
upon photolysis of HNO
2
The combination of reaction 87) with reaction 8ꢀ),
with reaction 8ꢀ) much faster than reaction 87), can give
a pH dependence similar to that shown in Fig. 3.
However, the presence of an in¯ection point at pH ꢁ 3:4
1
:0 Â 10 M 8reactions 84) and 85)), combined with the
hydrolysis of nitrogen dioxide 8reactions 86), 87) and
8
10); Fischer and Warneck, 1996):
1
4
À1 À1
Å
Å
À
2
‡
requires k
ꢀ
ꢁ 3 Â 10
M
s
, which is not reasonable
NO ‡ NO ꢀ‡H O† ! 2NO ‡ 2H
2
2
ꢀ
10†
in aqueous solution.
ꢀ
À1 À1
‰
k
10 ˆ 1:6  10 M
s
Š
Dark experiments have been performed to assess the
weight of the dark processes at acidic pH, and the results
compared with those obtained under irradiation. Fig. 4
shows the results of the reaction between phenol
Å
Å
with the additional approximation ‰ NO
2
Š ˆ ‰ NOŠ 8in-
troduced to perform steady-state calculations), gives a
Å
Àꢀ
steady-state concentration ‰ NO
2
Š ˆ 5:2  10 M. The
À3
À3
1
:1 Â 10 M and HNO
2
1:0  10 M, at pH ˆ 2 for
thermal decomposition of nitrous acid 8reaction 89)),
together with reactions 86), 87) and 810), with the same
4
the addition of HClO , in the dark and upon 312 nm
irradiation. In the case of dark reactions, cells were
wrapped in double aluminium foil and were kept under
the same lamp and magnetic stirrer used for illumination
experiments, in order to obtain the same temperature
and stirring conditions.
Å
À7
approximation, gives ‰ NO
2
Š ˆ 3:0  10 M. The nu-
merical values are probably not very accurate, but their
ratio is and demonstrates that the thermal dismutation
of nitrous acid gives a much more important contribu-
Å
tion to ‰ NO
2
Š than its photolysis. This is further con-
The ®gure indicates that nitrophenols and 4-nitros-
ophenol form in practically the same amount both in the
dark and under illumination, in particular when focus-
ing on the initial rates, meaning that dark processes
prevail over photolytic ones at acidic pH.
®
rmed by the similarity of phenol pseudo-®rst order
degradation constants in the dark and under illumina-
tion 8Table 1): irradiation of the solutions has little or no
e€ect on the system.
Å
The formation of NO
2
in reaction 89) can explain the
Phenol nitration and nitrosation in the dark may be
due to nitrous acid thermal dismutation, which yields
e€ect of pH on phenol nitration. Furthermore, reaction
9) leads to enhanced formation of N , which prob-
ably is the reactive species for phenol nitrosation 8Pires
8
2 3
O
Å
2
NO among the products 8Park and Lee, 19ꢀꢀ):
et al., 1994). For the rate constant see Gr a tzel et al.
8
1969):
Å
Å
9
À1
s
À1Š ꢀ11†
NO ‡ NO
2
! N
2
O
3
‰k11 ˆ 1:1  10 M
2 3
The pH dependence for the formation of N O 8re-
actions 89) and 811)) thus explains the pH dependence
for the formation of 4-nitrosophenol.
In neutral solution, however, the thermal decompo-
sition of HNO
phenols form in the dark. The formation of 2- and
-nitrophenol at pH ˆ 6.5, in the presence of NaNO
under illumination 8see Fig. 2), is thus due to reactions
1)±83) and not to reaction 89).
2
plays a negligible role and no nitro-
4
2
8
3
.2. E€ect of TiO
2
TiO is a semiconductor oxide showing relevant
2
photochemical activity 8Bahnemann et al., 1994), as it
can eciently catalyse both oxidation and reduction
processes involving organic and inorganic molecules in
Fig. 4. Formation of nitrophenols and of 4-nitrosophenol in
the dark and under 312 nm irradiation. Initial conditions:
phenol 1:1 Â 10À3 M, NaNO
0.001 M, pH ˆ 2.0 8HClO
2
4
).

D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
aqueous solution. Among the possible applications, ti-
907
tanium dioxide heterogeneous photocatalysis is a very
promising advanced oxidation process for water detox-
i®cation.
In this work, we were interested in studying the in-
teraction between titanium dioxide and nitrite. A ®rst set
of irradiation experiments was performed in the pres-
ence of TiO
2
at pH ˆ 6.5, TiO
2
concentration being
varied in the range 0±2.00 g/l. Formation curves for
nitrophenols are shown in Fig. 5 88a) 2-nitrophenol; 8b)
4
-nitrophenol). When focusing on the initial rates, it can
be seen that the formation of nitrophenols increases
when increasing the concentration of the photocatalyst.
In the presence of titanium dioxide and nitrite 0.1 M, 4-
nitrosophenol is quickly transformed, but only a very
small fraction yields 4-nitrophenol 8see Fig. 6). On the
contrary, in the presence of nitrite 0.1 M only, a relevant
fraction of 4-nitrosophenol transforms into 4-nitrophe-
nol 8Vione et al., 2001b): we conclude that phenol ni-
Fig. 6. Degradation of 4-nitrosophenol in the presence of ti-
5
tanium dioxide. Initial conditions: 4-nitrosophenol ꢀ:1 Â 10 M,
2 2
NaNO 0.10 M, TiO 0.50 g/l.
tration under photocatalytic conditions only involves
Å
2 2 4
NO or N O . A very limited formation of 4-nitrophe-
nol from 4-nitrosophenol in the presence of titanium
dioxide, but without nitrite, has already been observed
by Piccinini et al. 81997).
Titanium dioxide can interfere with various processes
linked with nitrite photolysis because of light absorption
Å
and scattering, and so also with light-induced NO
formation. This interference is of the same kind as that
on nitrate photolysis 8Dzengel et al., 1999; Vione et al.,
2
2
001a). Another e€ect to be expected in the presence of
the photocatalyst is enhanced nitrophenol degradation
with respect to homogeneous solution 8Maurino et al.,
1
997). This should result in further depletion of nitro-
phenol concentration. In particular, nitrophenol degra-
dation can become relevant at high irradiation times and
high photocatalyst concentration 8see Fig. 5). These
2
observations indicate that TiO must also have a rele-
vant positive e€ect on the formation of nitrophenols, as
its overall e€ect is positive. Such an e€ect can be a TiO
Å
2
-
, via the valence-band holes
induced formation of NO
or the surface-adsorbed hydroxyl radicals:
2
À
‡
Å
NO ‡ h ! NO
2
ꢀ12†
ꢀ13†
2
À
Å
Å
‡ OHÀ
NO ‡ OHads ! NO
2
2
This is con®rmed by the results obtained by Minero
et al. 81996), who found nitrate as the oxidation product
of nitrite under photocatalytic conditions. Nitrate forms
in these systems via hydrolysis of nitrogen dioxide 8re-
actions 86) and 87)).
Reaction 813) is similar to the process taking place in
Å
homogeneous solution, involving homogeneous OH
8
Fischer and Warneck, 1996; Vione et al., 2001b), and it
is thermodynamically allowed below pH ˆ ꢀ.5. In fact,
Fig. 5. E€ect of TiO
À3
2
concentration. Initial conditions: phenol
0.10 M, pH ˆ 6.5. 8a) 2-nitrophenol; 8b)
Å
À
0
1
:1 Â 10 M, NaNO
2
the redox couple NO
2
=NO has E ˆ 1:03 8Bard et al.,
2
Å
0
4-nitrophenol.
19ꢀ5), while the couple OHads=H
O has E ꢁ 1:5 V
2

90ꢀ
D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
8Lawless et al., 1991), this reduction potential decreasing
with increasing pH.
Reaction 812) is possible at any pH. In fact,
‡
0
E ꢀh † ˆ 3:05 V 8Pelizzetti and Serpone, 19ꢀ6) and, al-
though decreasing with increasing pH, it is always
À
0
Å
higher than E ꢀ NO
2
=NO †.
2
The di€erent processes, which can take place in the
2
presence of TiO , are summarised in Scheme 1 8Bahne-
mann et al., 1994).
The pH e€ect already seen in homogeneous phase is
maintained in the presence of TiO
mation curves for nitrophenols at pH ˆ 3.0, 4.5, 6.5
and 10.5 in the presence of 0.50 g/l TiO are shown in
2
suspensions. For-
2
Fig. 7 88a) 2-nitrophenol; 8b) 4-nitrophenol).
Phenol pseudo-®rst order degradation rate constant
is very little modi®ed by the addition of the photo-
catalyst 8Table 1): this probably means that titanium
dioxide inhibits phenol transformation via nitrite pho-
tolysis, but at the same time induces photocatalytic
phenol transformation, the two e€ects compensating
each other.
Fig. ꢀ shows the formation curves for 4-nitrosophe-
nol at pH ˆ 6.5, both in homogeneous phase and in the
2
presence of 0.50 g/l TiO . The main e€ect of the phot-
ocatalyst seems to be the degradation of 4-nitrosophe-
nol, as its formation rate seems very similar in both
2
homogeneous phase and TiO suspension. Indeed, the
pseudo-®rst order degradation rate constant for 4-ni-
À4 À1
trosophenol is 1:1 Â 10
pH ˆ 6.5, NaNO
.50 g/l TiO
hancement in 4-nitrosophenol degradation by TiO
s
in homogeneous phase
À3 À1
8
2
0.1 M) and 3:75 Â 10
s
when
0
2
is added to the system. A similar en-
has
2
already been observed by Piccinini et al. 81997), that
work being carried out in the absence of nitrite.
Fig. 7. pH dependence for nitrophenol formation in the pres-
À3
ence of TiO
.10 M, TiO
2
. Initial conditions: phenol 1:1 Â 10 M, NaNO
2
0
2
0.50 g/l. 8a) 2-nitrophenol; 8b) 4-nitrophenol.
Fig. ꢀ. Formation of 4-nitrosophenol. Initial conditions: phe-
nol 1:1 Â 10À3 M, NaNO
0.10 M, pH ˆ 6.5.
Scheme 1. Reactions taking place on TiO
2
surface.
2

D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
2 3
N O , the reactive species for phenol nitrosation,
909
Å
Å
2
forms via reaction between NO and NO 8Fischer and
Warneck, 1996). Another conceivable nitrosation path,
Å
namely, the reaction between phenoxyl radical and NO
8
phenoxyl forms via reaction between phenol and OH.
Machado and Boule, 1995), is inhibited in our system as
Å
Å
Here OH is eciently scavenged by nitrite.
TiO interferes with nitrite photolysis, depleting NO,
Å
2
Å
but at the same time favours the formation of NO
2
. The
two e€ects probably compensate each other, so that the
formation rate of 4-nitrosophenol is very similar in ho-
mogeneous solution and in semiconductor suspension.
In addition, the photocatalyst degrades 4-nitrosophenol,
and this seems to be its main e€ect.
Å
3
.3. Nitrite as OH source
Å
À
2
Fig. 9. ‰ OHŠ vs. ‰NO Š, irradiation at 360 nm, phenol
ss
À3
1
:1 Â 10 M.
As already cited, nitrite photolysis generates nitric
oxide and hydroxyl radical. The photolysis and the
Å
subsequent reaction between nitrite and OH control the
hydroxyl steady-state concentration 8reactions 81)±83)).
An important consequence of these considerations is
that nitrite can control the concentration of hydroxyl in
many environmental compartments. In this respect, the
behaviour of nitrite is very di€erent from that of nitrate,
Å
When only nitrite is present in the solution, ‰ OHŠ is
given by:
Å
Àeb‰NOÀ
which can generate OH upon photolysis but is not a
Å
U
1
I
0
ꢀ1 À 10
2
Š†
‰ÅOHŠss
ˆ
;
ꢀ14†
sink for it, so that ‰ OHŠ increases with increasing ni-
À
ss
k
3
V ‰NO Š
2
trate concentration 8Zepp et al., 19ꢀ7).
In acid solution, photoformation of OH occurs via
Å
0
where I is the irradiation intensity, e the molar ab-
sorbivity of nitrite, b the optical path of the solution and
V its volume. In the presence of OH scavengers other
than nitrite, such as phenol, other reactions are to be
considered 8Buxton et al., 19ꢀꢀ):
photolysis of nitrous acid. In this case, reactions 84) and
8
5) substitute reactions 81)±83), and similar consider-
ations can be made. Nitrous acid can also undergo
thermal decomposition 8reaction 89)), yielding
Å
Å
Å
NO ‡ NO
2 2
. This work demonstrates that HNO is a
Å
relevant source of nitric oxide and nitrogen dioxide via
dark processes, which under certain conditions can have
higher importance than UV photolysis.
Phenol ‡ OH ! products
1
0
À1 À1
M s
‰
k
15 ˆ 1:4  10
Š
ꢀ15†
Catechol and hydroquinone are two of the products
of reaction 815).
4
. Conclusions
The corresponding steady-state concentration for
OH in the presence of phenol thus becomes:
Å
UV irradiation of nitrite in aqueous solution leads to
Å
Àeb‰NOÀ
Š
U
1
I
0
ꢀ1 À 10
2
†
the formation of NO
tration. Nitrite photolysis also yields NO, which reacts
2
, which is involved in phenol ni-
Å
‰ÅOHŠss
ˆ
:
ꢀ16†
À
V ꢀk
3
‰NO Š ‡ k15‰PhenolŠ†
2
Å
with NO
2
to give N
2
O
3
, responsible for the formation of
Å
Å
As
this function has a maximum for a value of [NO ], de-
lim‰NOÀŠ!0ꢀ‰ OHŠ † ˆ lim‰NOÀŠ!1ꢀ‰ OHŠ † ˆ 0,
4-nitrosophenol. The oxidation of 4-nitrosophenol 8and
possibly 2-nitrosophenol) in the presence of oxygen
gives a further contribution to nitrophenol formation
8Vione et al., 2001b).
2
ss
2
ss
À
2
pending on the irradiation wavelength and on phenol
À
concentration. For low values of [NO ], nitrite photol-
ysis is negligible, while for very high values the reaction
2
Phenol photonitration and photonitrosation are pH-
dependent and are higher at low pH. This e€ect of pH is
Å
between nitrite and hydroxyl keeps ‰ OHŠ low.
ss
Å
Fig. 9 shows the trend of ‰ OHŠ with nitrite con-
2
due to thermal processes involving HNO , possibly to its
ss
centration, under the 360 nm light source 8photon ¯ux of
À7
thermal dismutation. Under the studied conditions, this
thermal reaction largely prevails over nitrous acid pho-
tolysis.
3
:6 Â 10 Ein/s in the cells), in the presence of phenol
À3
1
:1 Â 10
M. The curve has a maximum for
À
‰
NO Š ˆ 0:012 M in the present experimental condi-
Titanium dioxide in suspension partially interferes
with nitrite photolysis through light absorption and
2
tions.

910
D. Vione et al. / Chemosphere 45 ꢀ2001) 903±910
scattering. At the same time, the holes and the adsorbed
hydroxyls on the surface of the catalyst can react with
Guillaume, D., Morvan, J., Martin, G., 19ꢀ9. In¯uence of light
on phenol compound reactivity in the presence of nitrates in
abiotic solution. Environ. Technol. Lett. 10, 491±500.
Å
nitrite to yield NO
mation is positive.
The e€ect of TiO
2
. The net e€ect on nitrophenol for-
Å
Lawless, D., Serpone, N., Meisel, D., 1991. Role of OH
radicals and trapped holes in photocatalysis. A pulse
radiolysis study. J. Phys. Chem. A 95, 5166±5170.
Louw, R., Santoro, D., 1999. Comment on formation of
nitroaromatic compounds in advanced oxidation processes:
photolysis versus photocatalysis. Environ. Sci. Technol. 33,
32ꢀ1.
2
on the formation of 4-nitros-
ophenol indicates a good level of compensation be-
Å
tween NO depletion, via inhibition of nitrite
Å
photolysis, and photocatalytic formation of NO
2
. The
two radicals react to give the nitrosating agent, N
In addition, TiO degrades the organic compound. In
the presence of TiO , the light induced transformation
2 3
O .
2
Machado, F., Boule, P., 1995. Photonitration and photonitro-
sation of phenolic derivatives induced in aqueous solution
by excitation of nitrite and nitrate ions. J. Photochem.
Photobiol. A: Chem. ꢀ6, 73±ꢀ0.
2
of 4-nitrosophenol gives 4-nitrophenol in negligible
amount.
Martell, A.E., Smith, R.M., 1974. Critical Stability Constants.
Plenum Press, New York.
Maurino, V., Minero, C., Pelizzetti, E., Piccinini, P., Serpone,
N., Hidaka, H., 1997. The fate of organic nitrogen under
photocatalytic conditions: degradation of nitrophenols and
Acknowledgements
Financial support of CNR, MURST and PNRA
Progetto Antartide is kindly appreciated.
aminophenols on irradiated TiO
A: Chem. 109, 171±176.
2
. J. Photochem. Photobiol.
Minero, C., Piccinini, P., Calza, P., Pelizzetti, E., 1996.
Photocatalytic reduction/oxidation processes occurring at
the carbon and nitrogen of tetranitromethane. New J.
Chem. 20, 1159±1164.
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