Phenol photonitration upon UV irradiation of nitrite in aqueous solution I: Effects of oxygen and 2-propanol

Article abstract of DOI:10.1016/S0045-6535(01)00035-2

Nitrophenols are formed in aqueous solution upon UV irradiation of phenol and nitrite. The formation of nitrophenols is enhanced by dissolved oxygen and inhibited by the addition of 2-propanol. The mechanism of phenol photonitration involves both NO₂ (or N₂O₄), reacting with phenol, and 4-nitrosophenol, which is oxidised to 4-nitrophenol. A reaction scheme is proposed based on experimental results.

Full text of DOI:10.1016/S0045-6535(01)00035-2

Chemosphere 45 32001) 893±902
www.elsevier.com/locate/chemosphere
Phenol photonitration upon UV irradiation of nitrite in
aqueous solution I: E€ects of oxygen and 2-propanol
*
Davide Vione, Valter Maurino, Claudio Minero, Ezio Pelizzetti
Dipartimento di Chimica Analitica, Universitaꢀ di Torino, Via Pietro Giuria 5, 10125 Torino, Italy
Received 26 September 2000; received in revised form 24 January 2001; accepted 29 January 2001
Abstract
Nitrophenols are formed in aqueous solution upon UV irradiation of phenol and nitrite. The formation of nitro-
phenols is enhanced by dissolved oxygen and inhibited by the addition of 2-propanol. The mechanism of phenol
photonitration involves both ÅNO2 3or N2O4), reacting with phenol, and 4-nitrosophenol, which is oxidised to
4-nitrophenol. A reaction scheme is proposed based on experimental results. Ó 2001 Elsevier Science Ltd. All rights
reserved.
Keywords: Nitrite; Photochemical transformations; Photolysis; Nitroaromatic compounds
1. Introduction
for drinking-water quality is lower for nitrite than for
nitrate 3Guidelines, 1996).
Among nitrogen inorganic species, nitrite is less
common than nitrate in the environmental compart-
ments, as it is relatively unstable and can be involved in
many chemical and biological processes. The most im-
portant one is the oxidation to nitrate, nitrite being an
intermediate in the oxidation of nitrogen-containing
organic matter. As a consequence, the presence of nitrite
in water usually indicates recent microbial contamina-
tion. However, its higher reactivity and impact on hu-
man health balance its lower concentration. Both nitrate
and nitrite are involved in human methaemoglobina-
emia 3oxidation of haemoglobin to methaemoglobin,
which is unable to transport oxygen to the tissues), but
the toxicity of nitrate is thought to be solely the conse-
quence of its reduction to nitrite. The relative potency of
nitrite and nitrate with respect to their health impact was
assumed to be 10:1 on a molar basis. On this ground the
concentration level prescribed by the WHO guidelines
Nitrite can absorb in the UV region of solar radiation
to a greater extent than nitrate and the resulting
photodissociatioÀn leads to the formation of reactive
species 3ÅOH; ÅO2 , aquated eÀ; ÅNO2; ÅNO; Fischer and
Warneck, 1996), which can then promote transforma-
tion of water-dissolved organic molecules. As a conse-
quence, nitrite can act as an environmental factor as
ꢁ
nitrate does 3Hoigne, 1990), the lower concentration of
the former in environmental compartments being par-
tially counterbalanced by its higher reactivity.
Nitrite and nitrous acid are also an important source
Å
of radicals in the atmosphere. The radical OH photo-
formation, resulting from UV irradiation of N3III), has
been recently studied and its environmental signi®cance
assessed 3Arakaki et al., 1999). N3III) plays a relevant
role as an environmental factor in atmospheric acidic
water droplets.
Nitrite is also a nitrating agent that can be used in the
synthesis of nitroderivatives of aromatic compounds
3Uemura et al., 1978). The process used for synthetic
purposes is an electrophilic aromatic substitution in-
* Corresponding author. Tel.: +39-011-6707649; fax: +39-
011-6707615.
E-mail address: pelizzet@ch.unito.it 3E. Pelizzetti).
‡
volving NO2 , which takes place at strongly acidic pH,
the nitrating reagent being for instance a tri¯uoroacetic
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 3 0 1 ) 0 0 0 3 5 - 2

894
D. Vione et al. / Chemosphere 45 -2001) 893±902
acid solution of NaNO2. Moreover, referring to nitrite
Å
2.3. Kinetic simulations
photodissociation, one of its products is NO2, which is
involved in the nitration of aromatic molecules, though
the details of the mechanism of these reactions are often
unclear 3Boule et al., 1999). As an example, the relative
Concentration of nitrogen-containing radicals in ir-
radiated nitrite solutions were estimated with the CKS
package 3IBM, 2000), which runs simulation of complex
kinetic systems by using Monte Carlo techniques.
Å
Å
role of OH and NO2 in phenol photonitration is con-
troversial and has been the object of recent debate
3Louw and Santoro, 1999; Dzengel et al., 1999a; Dzen-
gel et al., 1999b). Phenol nitration upon nitrite irradia-
tion was excluded by Machado and Boule 31995).
Such considerations account for the importance to
assess the nitration of aromatic molecules in general and
of phenol in particular, in the presence of nitrite under
di€erent conditions, such as UV irradiation in aqueous
solution and in semiconductor suspension 3Vione, 1998).
Semiconductor oxides themselves are important envi-
3. Results and discussion
3.1. Kinetic analysis of photoinduced reactions in nitrite
solutions
UV irradiation of nitrite solutions between 300 and
400 nm leads to nitrite photodissociation followed by a
number of other processes. Data taken from published
ꢁ
ronmental factors 3Hoigne, 1990).

results 3#: Gratzel et al., 1969; §: Fischer and Warneck,
1996) are summarised below:
x
x
NO2À ‡ hm ! ÅNO ‡ ÅOÀ ‰U360
ꢀ 0:025Š
ꢁ1a†
ꢁ1b†
nm
2. Experimental
ÅOÀ ‡ H $ ÅOH ‰pKa;1b ˆ 11:9Š
ÅOH ‡ NO2À ! OHÀ ‡ ÅNO2
‡
2.1. Reagents and materials
x
x
x
x
‰k2 ˆ 1:0  1010 MÀ1sÀ1
Š
ꢁ2†
ꢁ3†
ꢁ4†
NaNO2 3purity grade > 97%) was purchased from
Carlo Erba. Nitrite concentration in the cells was 0.10
M, achieved by addition of NaNO2. Phenol concentra-
tion was 1:1 Â 10À3 M.
NO2À ‡ hm ! ÅNO2 ‡ eaÀq
‰U3 6 0:076U1aŠ
O2 ‡ eÀaq ! ÅOÀ2
‰k4 ˆ 1:9  1010 MÀ1sÀ1
Š
2.2. Irradiation experiments
ÅNO ‡ ÅOÀ2 ! ONOOÀ
‰k5 ˆ 6:7  109 MÀ1sÀ1
Š
ꢁ5†
ꢁ6†
ꢁ7†
ꢁ8†
The UV source 3irradiance maximum at 360 nm,
total photon ¯ux in the cells 3:6 Â 10À7 Ein/s) has al-
ready been described 3Vione et al., 2001a). The lamp
irradiance spectrum together with the absorption spectra
of nitrite and phenol are reproduced in Fig. 1.
Experimental details not included here are reported
elsewhere 3Vione et al., 2001a).
ONOOÀ ‡ H $ ONOOH ‰pKa;6 ˆ 6:6Š
‡
x
x
x
x
ONOOH ! NOÀ3 ‡ H
‰k7 ˆ 0:7 sÀ1
Š
‡
ONOOH ! ÅNO2 ‡ ÅOH ‰k8 6 0:3 sÀ1
Š
2ÅNO ‡ O2 ! 2ÅNO2
‰2k9 ˆ 4:2  106 MÀ2sÀ1
Š
ꢁ9†
ÅNO ‡ ÅNO2ꢁ‡H2O† ! 2NOÀ2 ‡ 2H
‡
x
‰k10 ˆ 1:6  108 MÀ1sÀ1
2ÅNO2 $ N2O4
Š
ꢁ10†
#
‰k11 ˆ 4:5  108 MÀ1sÀ1
;
k
ˆ 6:9  103 sÀ1
Š
ꢁ11†
ꢁ12†
ꢁ13†
ꢁ14†
À11
N2O4ꢁ‡H2O† ! NOÀ ‡ NOÀ ‡ 2H
‡
#
x
2
3
‰k12 ˆ 1  103 sÀ1
Š
ÅNO2 ‡ OÀ ! NOÀ ‡ O2
2
2
‰k13 ˆ 4:5  109 MÀ1sÀ1
Š
x
NO3À ‡ ÅNO ! ÅNO2 ‡ NO2À
‰k14 6 4  104 MÀ1sÀ1
Š
x
ÅOÀ2 ‡ NOÀ2 ! products
Fig. 1. Irradiance spectrum of the Philips TL K 05 lamp 3ir-
radiance maximum at 360 nm), phenol and nitrite spectra.
‰k15 ˆ 5  106 MÀ1sÀ1
Š
ꢁ15a†

D. Vione et al. / Chemosphere 45 -2001) 893±902
895
As to the ``products'' in reaction 315a), Fischer and
Warneck 31996) made the hypothesis that this reaction
steady-state approximation is no longer valid and in
order to include a higher number of reactions in the
kinetic analysis, further calculations have been carried
out with the CKS package 3IBM, 2000). Results from
both the exact solution of the system composed by re-
actions 31a), 31b), 32) and 310) 3Maple 6, Waterloo
Software) and the steady-state approximation exactly
match the output of CKS simulation.
Å
yields NO2, so a possible process might be:
ÅOÀ2 ‡ NO2À ‡ 2H ! H2O2 ‡ ÅNO2
‡
‰k15 ˆ 5  106 MÀ1sÀ1
Š
ꢁ15b†
The rate constant for reaction 314) is presumably much
lower than the upper limit. As a matter of fact, the 360
nm irradiation of a mixture of nitrite 0.1 Mand nitrate
0.1 Mdoes not yield a signi®cant increase in ‰ÅNO2Š, as
detected by the formation of phenol nitroderivatives.
Reaction 310) does not describe the complete mech-
anism of the hydrolysis of ÅNO and ÅNO2. Replacing Eq.
In the simulations both oxygen-free and aerated
systems were considered. For the former reactions 31a)±
33), 310)±312) were taken into account. For the latter, all
the reactions with the exception of reaction 314).
Moreover, the e€ect of substituting reaction 310) with
the couple 316) and 317) was assessed.

310) by the following couple of reactions 3Gratzel et al.,
1970) allows also the evaluation of N2O3 levels:
Fig. 2 reports the results for the simulations in
deoxygenated solution 33a): reaction 310); 3b): reactions
ÅNO ‡ ÅNO2 ! N2O3 ‰k16 ˆ 1:1  109 MÀ1sÀ1
Š
ꢁ16†
N2O3ꢁ‡H2O† ! 2NOÀ ‡ 2H
‰k17 ˆ 5:3  102 sÀ1
Š
‡
2
ꢁ17†
However, the values of k16 and k17 are not exactly
compatible with that of k10. In particular, compared to
reaction 310), reactions 316) and 317) are faster and have
Å
a more important relative role as a sink for NO. The
implications of this will be discussed later.
Å
Nitrite acts as an ecient OH scavenger, as can be
seen from reaction 32). The rate constant for the reaction
between ÅOH and NOÀ2 is very near that for the reaction
between OH and phenol 3k ˆ 1:4  1010 MÀ1
s
À1; Bux-
Å
ton et al., 1988). In our system the concentration of
nitrite is nearly 100 times that of phenol 3‰NOÀ2 Š ˆ 0:10
M, ‰PhenolŠ ˆ 1:1  10À3 M), thus, based on the kinetic
Å
constants reported, 98.5% of photoformed OH reacts
with nitrite and only the remaining 1.5% with phenol.
The high number of processes induced by nitrite
photocleavage makes desirable any possible simpli®ca-
tion when experimental results have to be interpreted. In
order to state which reactions are relevant and which
play a negligible role in the system, with particular at-
tention to the processes controlling ‰ÅNO2Š and ‰ÅNOŠ, an
attempt to perform a kinetic analysis applying the
steady-state approximation to the concentration of the
intermediates was made. The reactions 31a), 31b), 32) and
310) constitute a null cycle to which the steady-state
approximation can be easily applied. The calculations
give ‰ÅOHŠ ˆ 1:5  10À15 Mand
‰ÅNO2Š ˆ ‰ÅNOŠ ˆ
9:7 Â 10À8 M. However, the addition to this system of
reactions 311) and 312), which add to reaction 310) as a
sink for nitrogen dioxide, leads to a process with the
following equivalent stoichiometry:
3NOÀ2 ‡ H2O ! 2ÅNO ‡ NOÀ ‡ 2OHÀ
This means that ÅNO does not reach a steady-state
concentration, conversely it accumulates. Since the
ꢁ18†
3
Fig. 2. Simulation of the 360 nm irradiation of NaNO2 0.1 M
in the absence of oxygen. 3a) Reaction 310); 3b) reactions 316)
and 317) 3CKS package, IBM).

896
D. Vione et al. / Chemosphere 45 -2001) 893±902
316) and 317)), Fig. 3 the results in aerated systems 33a):
reaction 310); 3b): reactions 316) and 317)). The substi-
tution of reaction 310) with reactions 316) and 317) has
limited in¯uence on ‰ÅNO2Š, but a relevant one on ‰ÅNOŠ.
Speci®cally, reactions 316) and 317) lead to a faster hy-
involve OÀ2 , which is eciently scavenged by nitrite in
reaction 315b).
When nitrite alone is present in the solution, reaction
315b) can play a role only if U3 ˆ 0. If U3 ꢀ 0, the sys-
tem would be described only by reactions 31a)±32), 310)±
312). On the contrary, in the presence of organicÀcom-
Å
drolysis of NO than reaction 310). On the other hand,
Å
Å
the hydrolysis of NO2 via reactions 311) and 312) com-
pounds such as phenol in oxygenated solution, O2 and
pensates the di€erences in rates of reactions 310) and
316), 317), thus keeping ‰ÅNO2Š almost constant in both
cases.
In the presence of oxygen, ‰ÅNOŠ and ‰ÅNO2Š are
controlled by reactions 31a)±34), 310)±312) and 315b).
Reactions 35), 38), 39) and 313) play a negligible role.
Reaction 39) is a three-molecular reaction with relatively
low rate constant, while reactions 35), 38) and 313) all
HOÅ2 can also generate by reaction between molecular
oxygen and organic radicals 3see Scheme 1), thus reac-
tion 315b) may be relevant also if U3 ꢀ 0.
Å
The concentration of NO2 is enhanced by dissolved
O2 3compare Figs. 2 and 3). In the absence of oxygen,
‰ÅNO2Š ꢀ 3  10À8 M, while in oxygenated solution
‰ÅNO2Š ꢀ 4:5  10À8 M. The e€ect of oxygen is mainly
due to the contribution by reaction 315b). On the other
hand, the presence of oxygen depletes ‰ÅNOŠ, although
the actual value of its concentration varies according to
the model in use. This depletion is mainly due to the
enhancement of ‰ÅNO2Š, in¯uencing the mutual hydro-
lysis. The concentration of N2O3 is very little in¯uenced
by O2. In both cases, ‰N2O3Š ꢀ 3  10À9 M, unless de-
pletion processes with oxidants such as superoxide and
hydrogen peroxide, generated by other sources, take
place.
Fig. 3. Simulation of the 360 nm irradiation of NaNO2 0.1 M
in the presence of oxygen. 3a) Reaction 310); 3b) reactions 316)
and 317) 3CKS package, IBM).
Scheme 1. Reaction mechanisms proposed for phenol pho-
tonitration.

D. Vione et al. / Chemosphere 45 -2001) 893±902
897
To summarise, we propose that ‰ÅNO2Š and ‰ÅNOŠ be
As a consequence, both hydrogen atom abstraction and
direct electron transfer yield phenoxyl as reaction
product, and the two pathways for phenol oxidation are
equivalent with respect to nitration processes.
controlled by the following reactions:
NOÀ2 ‡ hm ! ÅNO ‡ ÅOÀ
ꢁ1a†
ꢁ1b†
ÅOÀ ‡ H $ ÅOH
‡
Another proposal 3b) concerns a molecular nitration
involving electrophilic attack by N2O4 on phenol
3Machado and Boule, 1995). Up to now, it has been
dicult to decide whether phenol nitration follows a
radical or an electrophilic path and whether it involves
ÅNO2 or N2O4 3Machado and Boule, 1995; Vione et al.,
2001a). As the purpose of the present work is to
demonstrate the possibility of phenol photonitration in
the presence of nitrite and to study the e€ects of oxy-
gen and 2-propanol on the process, detailed kinetic
analysis and discussion on the mechanism will be given
in a forthcoming paper 3Vione et al., 2001c).
ÅOH ‡ NO2À ! OHÀ ‡ ÅNO2
aq
ꢁ2†
ꢁ3†
NOÀ2 ‡ hm ! ÅNO2 ‡ eÀ
ÅNO ‡ ÅNO2 ‡ H2O ! 2NOÀ2 ‡ 2H
ꢁ10†
ꢁ11†
ꢁ12†
‡
2ÅNO2 $ N2O4
N2O4 ‡ H2O ! NOÀ ‡ NOÀ ‡ 2H
‡
2
3
O2 ‡ eÀaq ! ÅOÀ2
ꢁ4†
ÅOÀ2 ‡ NO2À ‡ 2H ! H2O2 ‡ ÅNO2
ꢁ15b†
‡
Another path 3c) involves ®rst phenol nitrosation,
followed by a fast oxidation of the nitrosoderivative to
the nitroderivative. This process is quite common in
aromatic nitration 3Ridd, 1998). In this work we have
found convincing evidence of a relevant role played by
mechanism 3c).
Finally, an ÅOH-mediated reaction has been pro-
posed 3Niessen et al., 1988; Louw and Santoro, 1999).
It implies a ®rst reaction between phenol and ÅOH
yielding phenoxyl radical. This radical can then react
The ®rst seven reactions describe the system in deoxy-
genated solution; the last two take place in the presence
of oxygen.
3.2. Mechanisms of phenol photonitration
The concentration trend with time has been followed
for phenol, 2- and 4-nitrophenol and 4-nitrosophenol. 3-
nitrophenol and dihydroxybenzenes, if present, are un-
der the detection limits of the analytical method 3about
5 Â 10À7 M).
Å
Å
either with NO2 to give nitrophenols or with OH to
give dihydroxybenzenes. Some studies in which nitrate
Å
was irradiated concluded that OH-induced phenol ni-
Di€erent paths have been proposed for phenol ni-
tration upon nitrate photoexcitation. They can serve as a
starting point for the interpretation of phenol transfor-
mation in the presence of nitrite and are summarised in
Scheme 1.
tration does not occur in the presence of dissolved
oxygen 3Machado and Boule, 1995; Vione et al.,
2001a). Such a mechanism may however have some
importance in the absence of oxygen 3Dzengel et al.,
1999b; Vione et al., 2001a).
A ®rst proposal concerns a radical mechanism in-
volving reaction between phenol and two ÅNO2 3or
N2O4). The radical intermediate might be hydroxyni-
trocyclohexadienyl 3Dzengel et al., 1999a; Vione et al.,
2001a) or phenoxyl 3Coombes et al., 1994). The two
proposed pathways are reported in Scheme 1 3mecha-
nism 3a)).
Å
Reaction between phenol and OH can also yield
dihydroxycyclohexadienyl. A nitration path was pos-
Å
tulated, requiring addition of NO2 to this radical and
formation of nitrophenols due to the elimination of a
water molecule. This reaction mechanism has been
proposed in analogy with nitration of benzene upon
radiolysis of aqueous nitrate 3Eberhardt, 1975) and
with nitration of atmospheric PAHs 3Pitts et al., 1985).
However, this path should lead to 3-nitrophenol as a
major intermediate, which is not the case.
Å
According to mechanism 3a), NO2 or N2O4 reacts
with phenol via hydrogen atom abstraction or via ad-
dition to the ring. Direct electron transfer between
phenol and nitrogen dioxide, yielding phenol radical
cation and nitrite, is thermodynamically allowed
In the system under study nitrite can eciently
scavenge ÅOH 3reaction 32)). In these conditions an
ÅOH-mediated nitration process is very probably e-
ciently inhibited, even in the absence of dissolved
oxygen. Furthermore, Fig. 4 reports the initial rates
for the formation of 2- and 4-nitrophenol in the
presence of phenol 1:1 Â 10À3 Mas a function of ni-
trite concentration. If the main pathway for phenol
Ň
0
À
0
Å
3E ꢁ NO2=NO2 † ˆ 1:03 V, E ꢁPhOH =PhOH† ˆ 0:97
V; Alfassi et al., 1990):
PhOH ‡ ÅNO2 ! PhOHŇ ‡ NOÀ2
ꢁ19†
However, phenol radical cation is a very strong acid
3pKa ˆ À1:9; Dixon and Murphy, 1976) and, in aqueous
solution, it almost instantaneously dissociates to phen-
oxyl:
Å
nitration was initiated by OH, the initial rate should
have a maximum for ‰NOÀ2 Š ꢀ 0:01 M, where the
steady-state concentration of ÅOH under such condi-
tions is maximum 3Vione et al., 2001b, Fig. 9). As the
PhOHŇ ! PhOÅ ‡ H
ꢁ20†
‡

898
D. Vione et al. / Chemosphere 45 -2001) 893±902
Fig. 4. E€ect of nitrite concentration on the initial rates of
nitrophenol formation in the presence of phenol 1:1 Â 10À3
Fig. 5. E€ect of dissolved oxygen on the formation of 2- and 4-
nitrophenol. Initial conditions: phenol, 1:1 Â 10À3 M, NaNO2
0.10 M, pH ˆ 6.5. Irradiation at 360 nm.
M
3aerated solution). Irradiation at 360 nm.
nitrophenol formation rate constantly increases with
increasing ‰NOÀ2 Š in the range 0.003±0.100 M, ÅOH-
mediated phenol nitration can be excluded as a major
pathway.
A possible explanation can be given considering the
processes following nitrite photolysis. In the absence of
dissolved oxygen, ‰ÅNO2Š ꢀ 3  10À8 M. In aerated so-
lutions, ‰ÅNO2Š ꢀ 4:5  10À8M. From Fig. 4 and from
the processes initiated by nitrite photolysis 3reactions
31a),31b),32),33), 310),311),312), and 34), 315b)) it can be
inferred that the rate of nitrophenol formation has a
second-order dependence with respect to ‰ÅNO2Š 3or a
®rst-order dependence with respect to ‰N2O4Š, since
‰N2O4Š / ‰ÅNO2Š2). Thus, a 50% increase in ‰ÅNO2Š
A previous study by Machado and Boule 31995)
indicated that no nitrophenols form upon 365 nm ir-
radiation of nitrite and phenol. On the contrary, we
have found relevant amount of nitrophenols under ir-
radiation with UV light around 360 nm. However, the
reactant concentrations they used were di€erent from
those utilised in the present work, and the di€erent
results can be easily accounted for. Machado and
Boule 31995) studied the photoinduced reactions in
systems containing phenol 5 Â 10À4 Mand nitrite
1 Â 10À3 M, while we have used quite higher concen-
trations of both 3phenol 1:1 Â 10À3 M, nitrite 0.1 M).
In addition, the ratio ‰NOÀ2 Š/[Phenol] was 2 in their
case, 90.9 in ours. When that ratio is low, phenol can
should result in
a 125% increase in nitrophenol
formation rate, only partially explaining the e€ect of
oxygen.
In deoxygenated solution the pseudo-®rst-order de-
gradation constant for phenol is higher than in the
presence of oxygen 3see Table 1). At the same time, the
formation rate of 4-nitrosophenol is higher in the ab-
sence of dissolved oxygen than in its presence 3see Fig.
6). Phenol nitrosation was proposed to be due to
electrophilic substitution by N2O3 3Pires et al., 1994).
N2O3 forms via reaction between ÅNO and ÅNO2, but its
concentration does not depend on the presence of O2
Å
eciently compete with nitrite for reaction with OH,
Å
thus lowering the yield in NO2.
Furthermore, when we illuminated phenol 1:1 Â 10À3
Min the presence of nitrite 1 Â 10À3 M3Vione et al.,
2001b), nitrophenols were below detection limit, thus
con®rming the results obtained by Machado and Boule
31995).
Table 1
Pseudo-®rst-order degradation constants for phenol 1:1 Â 10À3
M
3.3. E€ect of dissolved oxygen
‰NOÀ2 Š 3M)
pH
Notes
kD ꢁsÀ1
†
0.003
0.010
0.030
0.100
0.100
0.100
6.5
6.5
6.5
6.5
6.5
6.5
2:0 Â 10À5
2:5 Â 10À5
4:2 Â 10À5
7:0 Â 10À5
2:0 Â 10À4
2:2 Â 10À5
To perform experiments in the absence of oxygen,
solutions in the cells 3pH ˆ 6:5) were degassed by bub-
bling with a stream of high-purity nitrogen before irra-
diation. Results are shown in Fig. 5. It appears that
dissolved oxygen favours the formation of nitrophenols:
the ratio of the initial rates of formation is around 8.6 in
the conditions employed.
N2 purged
2-propanol
2.00 M

D. Vione et al. / Chemosphere 45 -2001) 893±902
899
In the presence of dissolved oxygen, 4-nitrosophenol
can be oxidised to 4-nitrophenol 3see Fig. 7; a similar
contribution by 2-nitrosophenol is reasonable). As 4-
nitrosophenol and 4-nitrophenol form in comparable
amounts upon phenol and nitrite irradiation 3compare
Figs. 5 and 6), the oxidation of 4-nitrosophenol gives a
relevant contribution to the formation of 4-nitrophe-
nol.
In the absence of oxygen, 4-nitrosophenol trans-
forms, but does not yield 4-nitrophenol 3Fig. 7). The fact
that 4-nitrophenol forms upon phenol and nitrite irra-
diation also in the absence of oxygen indicates that the
oxidation of 4-nitrosophenol is not the only path
yielding the nitroderivative, the other path being the
reaction between phenol and ÅNO2 3or N2O4). The latter
reaction path is favoured by oxygen as well, although to
a lesser extent 3‰ÅNO2Š is higher in oxygenated solution,
as already discussed).
Fig. 6. E€ect of dissolved oxygen on the formation of 4-ni-
trosophenol. Initial conditions: phenol 1:1 Â 10À3 M, NaNO2
0.10 M, pH ˆ 6.5. Irradiation at 360 nm.
In summary, in oxygenated solution there are two
paths leading to nitrophenols: one is phenol nitration by
ÅNO2 3mechanism 3a) in Scheme 1), the other is the ox-
idation of the nitrosoderivatives 3mechanism 3c) in
Scheme 1). In the absence of oxygen, mechanism 3a) is
inhibited to a certain extent due to the lower ‰ÅNO2Š,
while mechanism 3c) does not occur. These two e€ects
account for the lower formation of nitrophenols in the
absence of oxygen.
when reactions 31a),31b),32),33), 310),311),312) and 34),
315b) are taken into account. The lower formation rate
of 4-nitrosophenol in aerated solution is probably due to
Å
depletion of NO or N2O3, reacting with H2O2. Hydro-
gen peroxide can originate from nitrite photolysis 3re-
actions 33), 34) and 315b)) and from the interaction of
organic radical intermediates and O2 3see Scheme 1 for
the formation of superoxide, together with reaction
315b); for reaction 321), see Pietsch and Meyer, 1936):
3.4. E€ect of 2-propanol
2ÅNO ‡ H2O2 ! 2HNO2
ꢁ21†
2-propanol 2.00 Mwas added to the aerated system.
This molecule is an ÅOH scavenger through the following
process, ®nally yielding acetone 3Warneck and Wurzin-
ger, 1988):
The depletion of ÅNO and/or N2O3 can explain the lower
formation of 4-nitrosophenol and the lower reactivity of
phenol in the presence of oxygen.
ꢁCH3†2CHOH ‡ ÅOH ! ꢁCH3†2COHÅ ‡ H2O
ꢁCH3† COHÅ ‡ O2 ! HOÅ ‡ ꢁCH3† CO
ꢁ22†
ꢁ23†
2
2
2
In the absence of 2-propanol, 98.5% of photoformed
ÅOH reacts with nitrite and 1.5% with phenol. The rate
Å
constant for reaction between 2-propanol and OH is
3:0 Â 109 MÀ1 sÀ1 3Buxton et al., 1988). In the presence
Å
of 2-propanol 2.00 M, 85.5% of OH reacts with 2-pro-
panol, 14.3% with NOÀ2 and a residual 0.2% with phe-
nol. As a consequence, 2-propanol interferes with
Å
reaction 32) and lowers the production of NO2 through
this reaction. The initial formation rate of nitrophenols
is about halved in the presence of 2-propanol 2.00 M
3see Fig. 8). However, if reaction 32) is the main source
Å
of NO2 its inhibition should lower nitrophenol forma-
tion to a far larger extent, namely the ratio between
initial formation rates should be about 7. Thus, in this
system reaction 32) should have a minor in¯uence on
‰ÅNO2Š. This may happen because the sequence of reac-
tions 322), 323) and 315b) is operating 3pKa of superoxide
Fig. 7. Degradation of 4-nitrosophenol in aerated and N2-
purged solutions. Initial conditions: 4-nitrosophenol 1:7 Â 10À5
M, NaNO2 0.10 M, pH ˆ 6.5. Irradiation at 360 nm.

900
D. Vione et al. / Chemosphere 45 -2001) 893±902
enhanced formation of HOÅ2=ÅO2À through reactions
322),323). In the absence of 2-propanol, the formation of
ÅOÀ2 depends on reactions 33) and 34), in the presence of
2-propanol it depends on reactions 31a/b) and 322),323),
with U1a P 13U3.
The depletion of 4-nitrosophenol inhibits the for-
mation of 4-nitrophenol through mechanism 3c). A
similar e€ect probably takes place with the ortho de-
rivative, thus explaining why nitrophenols formation is
inhibited by 2-propanol. If this scenario is correct, Fig. 8
indicates that mechanism 3a) and mechanism 3c) for ni-
trophenol formation 3see Scheme 1) have a comparable
weight in the absence of 2-propanol, since the inhibition
of mechanism 3c) halves the formation rate of 2- and 4-
nitrophenol.
Fig. 8. E€ect of 2-propanol 2.00 Mon nitrophenol formation
3aerated solution). Initial conditions: phenol, 1:1 Â 10À3 M,
NaNO2 0.10 M, pH ˆ 6.5. Irradiation at 360 nm.
4. Conclusions
Irradiation of nitrite 0.1 Min aqueous solution leads
to the formation of ÅNO2 in relevant concentration,
mainly via reactions 32), 33) and, in aerated systems,
Å
is 4.8), regenerating NO2 and compensating for the in-
hibition of reaction 32).
At the same time, the formation of 4-nitrosophenol is
inhibited by 2-propanol to a far larger extent than the
formation of nitrophenols 3see Fig. 9 and compare with
Fig. 8). This di€erence is dicult to explain in the hy-
Å
315b). The radical NO2 3or the dimer N2O4) can be in-
volved in the photoinduced nitration of organic matter,
as demonstrated by phenol nitration. Nitrite photolysis
Å
Å
also yields NO, which reacts with NO2 to give N2O3,
Å
pothesis of the depletion of NO2, which contributes
both to phenol nitration and to its nitrosation via N2O3.
Our hypothesis is that the addition of 2-propanol de-
Å
Å
pletes NO rather than NO2. This probably happens
Å
since hydrogen peroxide and/or superoxide oxidise NO
3reactions 35), 321)) and/or N2O3. The formation of
H2O2 in the presence of 2-propanol is higher than in
aerated solutions without the alcohol, because of an
Fig. 9. E€ect of 2-propanol 2.00 Mon the formation of 4-ni-
trosophenol 3aerated solution). Initial conditions: phenol,
1:1  10À3 M, NaNO2 0.10 M, pH ˆ 6.5. Irradiation at 360
nm.
Scheme 2. Early steps in reaction pathways proposed for the
transformation of phenol in the presence of nitrite.

D. Vione et al. / Chemosphere 45 -2001) 893±902
901
Dzengel, J., Theurich, J., Bahnemann, D., 1999a. Formation of
nitroaromatic compounds in advanced oxidation processes:
photolysis versus photocatalysis. Environ. Sci. Technol. 33,
294±300.
probably responsible for the formation of 4-nitros-
ophenol.
The formation of nitrophenols can also occur via the
oxidation of the nitrosoderivatives, which takes place in
the presence of nitrite, light and oxygen. This process
probably has a role comparable with that of the nitra-
Dzengel, J., Theurich, J., Bahnemann, D., 1999b. Response to
comment on formation of nitroaromatic compounds in
advanced oxidation processes: photolysis versus photoca-
talysis. Environ. Sci. Technol. 33, 3282.
Å
tion via NO2.
2-Propanol acts as a scavenger of ÅOH, but very
Eberhardt, M.K., 1975. Radiation-induced homolytic aromatic
substitution. III. Hydroxylation and nitration of benzene. J.
Phys. Chem. 79, 1067±1069.
Å
probably it does not deplete NO2. On the contrary, it
Å
can lower NO and N2O3 concentration levels through
Fischer, M., Warneck, P., 1996. Photodecomposition of nitrite
and undissociated nitrous acid in aqueous solution. J. Phys.
Chem. 100, 18749±18756.
generation of active oxygen species. The consequence is
an almost complete inhibition of the formation of 4-ni-
trosophenol, together with a partial depletion of nitro-
phenol formation.

Gratzel, M., Henglein, A., Lilie, J., Beck, G., 1969. Pulsradio-
lytische Untersuchung einiger Elementarprozesse der Oxy-
dation und Reduktion des Nitritions. Ber. Bunsenges. Phys.
Chem. 73, 646±653.
The early steps in reaction pathways proposed for the
transformation of phenol in the presence of nitrite are
reported in Scheme 2. Studies on the detailed kinetic
aspects of the system 3dependence on the type of sub-
strate and its concentration as well as on O2 and NO2À
concentration) are under way in order to gain more in-
sight about the ®ner details of the overall photonitration
and nitrosation process 3Vione et al., 2001c).

Gratzel, M., Taniguchi, S., Henglein, A., 1970. Pulsradiolyti-
sche Untersuchung der NO-Oxydation und des Gle-


ichgewichts N2O3 $ NO ‡ NO2 in waûriger Losung. Ber.
Bunsenges. Phys. Chem. 74, 488.
Guidelines for Drinking-Water Quality, 1996. second ed. World
Health Organisation, Geneva, pp. 313±323.
ꢁ
Hoigne, J., 1990. Formulation and calibration of environmental
reaction kinetics: oxidations by aqueous photooxidants as
an example. In: Stumm, W., 3Ed.), Aquatic Chemical
Kinetics, Wiley 3Chapter 2) pp. 43±70.
Acknowledgements
IBM, 2000 www.almaden.ibm.com/st/msim/ckspage.html, last
accessed July 2000.
Financial support of CNR, MURST and PNRA
Progetto Antartide is kindly appreciated.
Louw, R., Santoro, D., 1999. Comment on formation of
nitroaromatic compounds in advanced oxidation processes:
photolysis versus photocatalysis. Environ. Sci. Technol. 33,
3281.
References
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. 86, 73±80.
Alfassi, Z.B., Huie, R.E., Neta, P., Shoute, C.T., 1990.
Temperature dependence of the rate constants for reaction
of inorganic radicals with organic reductants. J. Phys.
Chem. 94, 8800±8805.
Niessen, R., Lenoir, D., Boule, P., 1988. Phototransformations
of phenol induced by excitation of nitrate ions. Chemo-
sphere 17, 1977±1984.
Arakaki, T., Miyake, T., Hirakawa, T., Sakugawa, H., 1999.
pH dependent photoformation of hydroxyl radical and
absorbance of aqueous-phase N3III) 3HNO2 and NOÀ2 ).
Environ. Sci. Technol. 33, 2561±2565.
Pietsch, E., Meyer, R.J., 1936. Gmelins Handbuch der Anor-
ganischen Chemie, vol. 4 3Sticksto€), Verlag Chemie, Berlin,
p. 725.
Boule, P., Bolte, M., Richard, C., 1999. Phototransformations
induced in aquatic media by NOÀ3 NO2À, FeIII and humic
substances. In: Boule, P., 3Ed.), The Handbook of Envi-
ronmental Chemistry, vol. 2.L 3Environmental Photochem-
istry), Springer, Berlin, pp. 181±215.
Pires, M., Rossi, M.J., Ross, S.R., 1994. Kinetic and mecha-
nistic aspects of the NO oxidation by O2 in aqueous phase.
Int. J. Chem. Kinetics 26, 1207±1227.
Pitts, J.N., Arey, J.S., Zielinska, B., Winer, A.M., Atkinson,
R., 1985. Determination of 2-nitro¯uoranthene and 2-
nitropyrene in ambient particulate organic matter: evi-
dence for atmospheric reactions. Atmos. Environ. 19,
1601±1608.
Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B.,
1988. Critical review of rate constants for reactions of
hydratedÀelectrons, hydrogen atoms and hydroxyl radicals
Å
ꢁ OH=ÅO † in aqueous solution. J. Phys. Chem. Ref. Data
17, 1027±1284.
Ridd, J.H., 1998. Some unconventional pathways in aromatic
nitration. Acta Chem. Scand. 52, 11±22.
Coombes, R.G., Diggle, A.W., Kempsell, S.P., 1994. The
mechanism of nitration of phenol and 4-methylphenol by
nitrogen dioxide in solution. Tetrahedron Lett. 35, 6373±
6376.
Uemura, S., Toshimitsu, A., Okano, M., 1978. Nitration of
aromatic hydrocarbons and ipso-nitrosodemetallation of
arylmetal compounds in sodium nitrite-tri¯uoroacetic acid.
J. Chem. Soc. Perkin Trans. I, 1076±1079.
Dixon, W.T., Murphy, D., 1976. Determination of the acidity
constants of some phenol radical cations by means of
electron spin resonance. J. Chem. Soc., Faraday Trans. II,
1221±1230.
Vione, D., 1998. Trasformazioni del fenolo indotte dalla fotolisi
UV di ioni nitrato e nitrito in soluzione acquosa. Thesis,
University of Torino.

902
D. Vione et al. / Chemosphere 45 -2001) 893±902
Vione, D., Maurino, V., Minero, C., Vincenti, M., Pelizzetti, E.,
2001a. Formation of nitrophenols upon UV irradiation
of phenol and nitrate in aqueous solutions and in
TiO2 aqueous suspensions. Chemosphere 44 32), 237±
248.
in aqueous solution. II: E€ects of pH and TiO2. Chemo-
sphere 45 35), 907±914.
Vione, D., Maurino, V., Minero, C., Pelizzetti, E., 2001c.
Phenol photonitration: a mechanistic study, in preparation.
Warneck, P., Wurzinger, C., 1988. Product quantum yields for
the 305-nm photodecomposition of NOÀ3 in aqueous solu-
tion. J. Phys. Chem. 92, 6278±6283.
Vione, D., Maurino, V., Minero, C., Vincenti, M., Pelizzetti, E.,
2001b. Phenol Photonitration upon UV irradiation of nitrite