New processes in the environmental chemistry of nitrite: Nitration of phenol upon nitrite photoinduced oxidation

Article abstract of DOI:10.1021/es010101c

Nitrite is a relevant environmental factor in natural waters and in the atmosphere. The effects of dissolved Fe(III), α-Fe₂O₃, β-FeOOH, NO₃^-, and humic acid on phenol nitration in irradiated systems containing N

Full text of DOI:10.1021/es010101c

Environ. Sci. Technol. 2002, 36, 669-676
benzene (12) and various PAHs (13) is assisted by •OH. Other
New Processes in the
aromatic molecules of environmental concern that can be
nitrated are resorcinol, catechol, hydroquinone, biphenyl,
and some of its derivatives (2). However, reaction 2 becomes
significant only if [NO2-] is considerably higher than the usual
nitrite concentration in the environment. Nitration of phenol
(14) and various azaarenes (3) upon nitrite photolysis is
actually possible but only at high [NO2-] (up to 0.1 M).
Environmental Chemistry of Nitrite:
Nitration of Phenol upon Nitrite
Photoinduced Oxidation
Environmental factors have often been studied separately,
although most environmental processes are the result of very
complex interactions among several species. As a conse-
quence, the study of the interactions between the various
environmental factors is of foremost importance to the
understanding of real environmental processes.
D A V I D E V I O N E , V A L T E R M A U R I N O ,
C L A U D I O M I N E R O , A N D
E Z I O P E L I Z Z E T T I *
Dipartimento di Chimica Analitica, Universita` di Torino,
Via P. Giuria 5, 10125 Torino, Italy
We recently demonstrated that phenol photonitration in
the presence of nitrite is enhanced by TiO2 in aqueous
suspension (15). Titanium dioxide is both a promising
photocatalyst for water detoxification (16) and an environ-
mental factor (1). The reactive species in TiO2 photocatalysis
are surface adsorbed hydroxyl radicals, free or trapped
valence band holes (17), and conduction band electrons,
which promote the redox processes of many dissolved
molecules.
The role of nitrite as an environmental factor has been
widely recognized. Nitrite is a relevant source of •OH in the
atmosphere, both in the gas phase via photolysis of
gaseous HNO and in atmospheric hydrometeors by photolysis
2
•
of NO -. In aqueous systems, OH production through
2
nitrite photolysis can be negligible due to the competition
for light absorption by dissolved Fe(III), colloidal iron
oxides, and nitrate. These photoexcited oxidants interact
with NO - and HNO to form •NO , either directly or
The reaction causing the enhancement of phenol nitration
in the presence of TiO2 is very similar to reaction 2.
Photogenerated species on TiO2 particles (•OHads) perform
2
2
2
via formation of •OH. As a consequence, nitrite and nitrous
•
like homogeneous OH (15, 17). As a consequence, the rate
•
•
of light-induced •OH generation is no longer limited by NO2
-
acid may act as NO rather than OH sources. The
2
radical •NO is involved in the nitration of many aromatic
photolysis and thus by [NO2-] (see reaction 1). The con-
centration of nitrite only influences the formation rate of
2
compounds, of which phenol is a model in this work. Kinetic
•
•NO2. Thus, the generation rate of NO2 can be relevant at
•
measurements using 2-propanol as OH scavenger show
that the direct production of •OH by aqueous Fe(III) species
decreases as pH increases. At slightly acidic and neutral
pH values, oxidation of nitrite occurs by direct electron
transfer to photoexcited Fe(III)aq species or colloidal iron
nitrite concentrations lower than in the case of nitrite
photolysis. This has a potential environmental significance.
In this framework, an interesting point is whether titanium
dioxide is a unique case or also other substances present in
the environment can induce the same processes.
oxides, in addition to the OH-mediated oxidation of NO -.
•
2
In this paper, we report on the effects of dissolved Fe(III),
R-Fe2O3, â-FeOOH, NO3-, and humic acid on phenol nitration
in irradiated systems containing NO2-. Actually, the formation
of toxic nitroderivatives, in particular in the atmosphere, is
of concern for the consequences on human health (18) and
the environment (19). For instance, 3-nitrobenzanthrone and
1,8-dinitropyrene are the most powerful direct mutagens so
far detected in atmospheric particulate (20).
The reported findings suggest a completely new role of
nitrite in aquatic environments.
Introduction
Nitrite is a relevant environmental factor both in natural
waters (1-4) and in the atmosphere (5). Up to now, the
attention was mainly focused on nitrite UV photolysis, leading
to hydroxyl radical and nitrogen monoxide (6):
Phenol was chosen as a model aromatic molecule. Phenol
•
nitration takes place via reaction with NO2 or N2O4 ([N2O4]
[•NO2]2 (2, 7-10)) or the oxidation of nitrosoderivatives
(14). The first pathway, which has more general environ-
mental significance, involves the generation of nitrogen
dioxide by nitrite oxidation. Accordingly, the significance of
the second mechanism was assessed by evaluation of the
oxidation rate of 4-nitrosophenol to 4-nitrophenol under the
same experimental conditions used for phenol nitration.
NO2- + hv f •NO + •O- [Φ310 nm ≈ 0.06] (1a)
•O- + H+ / •OH [pKa,1b ) 11.9]
(1b)
As far as nitrite is the only photoactive species, it can play
a dual role. Besides being a source of •OH, nitrite can act as
a sink for the same radicals, reacting with them at a diffusion-
Experimental Section
•
controlled rate giving NO2 as product (6):
Reagents and Materials. Phenol (P), 2-nitrophenol (2-NP),
4-nitrophenol (4-NP), 4-nitrosophenol (4-NOP) (purity grade
>98%), and humic acid sodium salt were purchased from
Aldrich, NaNO2 (>97%) and FeSO4 ×7H2O (99.5%) from Carlo
Erba, Fe(ClO4)3 × 9H2O (>97%) from Fluka, NaNO3 (>99%),
HClO4 (70%), and HNO3 (65%) from Merck. All reagents were
used as received without further purification. Acetonitrile
and 2-propanol were LiChrosolv gradient grade, purchased
from Merck. R-Fe2O3 and â-FeOOH have been synthesized
following the procedure given by Leland and Bard (21).
•OH + NO2- f •NO2 + OH- [k2 ) 1.0 × 1010 M-1 s-1
]
(2)
Nitrogen dioxide promotes the nitration of a variety of
organic compounds. Phenol (7-11) and various azaarenes
(3) can be nitrated directly by •NO2, whereas the nitration of
* Corresponding author phone: +39-011-6707630; fax: +39-011-
6707615; e-mail: pelizzet@ch.unito.it.
9
10.1021/es010101c CCC: $22.00
Published on Web 01/15/2002
2002 Am erican Chem ical Society
VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 6 6 9

i
A
A
Ri(b) )
(1 - T)
(3b)
tot
i
where A , A , and T are the actual absorbance of the species
tot
i (A ) ꢀicib), and the (total) absorbance and transmittance
i
of the mixture, respectively. Equations 3a,b strictly hold for
monochromatic radiation. However, if the light beam
contains radiation in a narrow wavelength interval (which
is the case of light from the 312 and 360 nm emission sources
used in this work), and ꢀi may be considered almost constant
in the narrow wavelength interval considered, eqs 3a,b are
a good approximation for photochemical calculations. In
our case the optical path was b ) 0.4 cm.
Determ ination of the Initial Rates and the Relative
Uncertainty. The disappearance of phenol and the formation
of nitrophenols were followed for different irradiation times.
The time evolution of phenol concentration [Ph] follows with
good approximation a pseudo-first-order kinetics
FIGURE 1. Irradiance spectra of the lamps Philips TL 01 (max. at
312 nm) and Philips TL K 05 (max. at 360 nm); transmission spectrum
of the 430-nm filter; molar absorption coefficient E (M-1 cm-1) of
d
Ph‚t
[Ph] ) [Ph]0 ‚ e-k
Fe(ClO ) (pH 1.5, 3.0, and 4.0), HNO (pH 1.5), NaNO (pH 6.5), NaNO
4 3
2
2
3
(4a)
(pH 6); absorbance of humic acid 0.20 g/L (pH 6).
where [Ph]0 is the initial phenol concentration, and kd is
Ph
Irradiation Experim ents. Irradiation was carried out in
cylindrical Pyrex glass cells (4.0 cm diameter, 2.3 cm height),
containing 5 mL of aqueous solution (or suspension).
The following radiation sources were used: (i) a set of 3
× 40 W Philips TL K05 lamps with emission maximum at 360
the rate constant for phenol degradation. Equation 4a has
been used for curve fitting. At t f 0 the equation reduces to
[Ph] ) [Ph]0 ‚ (1 - kPhd‚t), and the initial phenol degradation
rate is kdPh[Ph]0.
The concentration of nitrophenols as a function of time
is described by eq 4b
nm. Total photon flux in the cells was 3.8 × 10-7 Ein s-1
,
actinometrically determined using the ferrioxalate method
(22); (ii) a 100 W Philips TL 01 lamp (emission maximum 312
nm), producing a photon flux of 1.3 × 10-7 Ein s-1 in the
cells; (iii) a Solarbox (CO.FO.ME.GRA., Milan, Italy) with a
1500 W Philips Xenon lamp, equipped with a 430 nm filter.
The lamp produces a photon flux of 4.4 × 10-6 Ein s-1 in the
cells in the wavelength interval 430-510 nm.
The emission spectra of the lamps, together with the
absorption spectra of the species of interest, are reproduced
in Figure 1 and will be discussed in the following section.
After irradiation, the solutions were directly analyzed,
while the suspensions were first filtered through 0.45 µm
filter membranes (cellulose acetate, Millipore).
kNPf‚[Ph]0
d
d
Ph‚t
NP‚t
-k
-
e
)
(4b)
[NP] )
‚ (e-k
d
d
kNP - kPh
where NP stands for either 2-NP or 4-NP, and kfNP and kd
NP
are the rate constants for nitrophenol formation and
degradation, respectively. The experimental data for nitro-
phenols have thus been fitted with eq 4b. At t f0 the equation
reduces to [NP]tf0 ) kNPf‚[Ph]0‚t, and the initial formation
rate is kfNP[Ph]0. The initial rates for phenol and nitrophenols
are reported in Table 1, together with the associated standard
errors.
Analytical Determ inations. The HPLC determinations
were carried out on a Hitachi HPLC chromatograph, using
a RP-C18 LichroCART column (Merck, length 125 mm,
diameter 4 mm) packed with LiChrospher 100 RP-18 (5 µm
diameter).
Results and Discussion
Dissolved Fe(III). Fe(III) hydroxo complexes absorb near-
UV radiation. Upon absorption, the complexes undergo
photolysis with a generation of •OH (2). The most photoactive
species is [Fe(OH)(H2O)5]2+ (25):
Phenol, nitrophenols, and 4-nitrosophenol were isocrati-
cally eluted with a 30/ 70 mixture of acetonitrile/ NaH2PO4 (2
× 10-2 M, pH 5.5) and detected at 210 nm. Under these
conditions the retention times were (min) as follows: P (3.75),
2-NP (9.30), 4-NP (5.10), and 4-NOP (2.05). The retention
time corresponding to the void volume was 0.89 min.
Absorbed Light Calculations. When several absorbing
species are present in the same solution, they compete for
light absorption. From the Beer’s law dIabs(x) ) - ∑iµi × I(x)
× dx, where µi ) 2.303 × ꢀi × ci, and the radiation balance
I0 ) I(x) + ∑iIabsi(x), it follows a set of i first-order ordinary
differential equations, which solved give the absorptance
Ri(b), that is the fraction of light absorbed by each single
species i in the mixture (23), as a function of the optical path
b:
[Fe(OH)(H2O)5]2+ + hν + H2O f
[Fe(H2O)6]2+ + •OH [Φ310 nm ) 0.19] (5)
This species is the main ferric monomeric hydroxo
complex in the pH interval 2.2-3.8, as calculated from
reported stability constants (26). However, a mM solution of
Fe(ClO4)3 at pH 3.0 is not stable, as the monomeric species
slowly convert to polynuclear complexes and Fe(III) hy-
droxide colloids (27, 28).
To distinguish the effects of monomeric, dimeric, or
polymeric Fe(III) species as well as of iron hydroxide colloids,
first we carried out irradiation at pH 1.5. Under such
conditions, 95% of Fe(III) is present as [Fe(H2O)6]3+ and the
remaining 5% as [Fe(OH)(H2O)5]2+. The prevailing species
absi(b)
µi
I
j
[Fe(H2O)6]3+ can generate OH upon UV absorption (25):
•
Ri(b) )
)
(1 - e(-b∑ µ )
)
(3a)
j
I0
µj
[Fe(H2O)6]3+ + hν + H2O f
∑
j
[Fe(H2O)6]2+ + •OH + H+ [Φ310 nm < 0.05] (6)
Equation 3a shows the dependence of absorptance from
previously determined or known ꢀi, and it is equivalent to
the well-known eq 3b (24)
Taking into consideration the molar absorption coefficient
ꢀ of [Fe(H2O)6]3+ (ꢀ310 nm ) 48 M-1 cm-1) and [Fe(OH)(H2O)5]2+
9
6 7 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 4, 2002

TABLE 1. Initial Degradation Rates for Phenol and Initial Formation Rates for 2- and 4-Nitrophenola
λirr,
nm
phenol degr. rate,
M s-1
2-NP form. rate,
M s-1
4-NP form. rate,
M s-1
no.
conditions
pH
1
2
3
4
5
6
7
8
9
NaNO2 0.0010 M
NaNO2 0.0010 M + Fe(ClO4)3 0.0066 M
Fe(ClO4)3 0.0066 M
Fe(ClO4)3 0.0066 M + 2-propanol 0.10 M
NaNO2 0.0010 M
NaNO2 0.0010 M + Fe(ClO4)3 0.0066 M
Fe(ClO4)3 0.0066 M
Fe(ClO4)3 0.0066 M + 2-propanol 0.10 M
NaNO2 0.0010 M + Fe(ClO4)3 0.0066 M +
2-propanol 0.10 M
312 1.5 (2.2 ( 0.4) × 10-8
312 1.5 (1.1 ( 0.1) × 10-7
312 1.5 (1.5 ( 0.1) × 10-7
312 1.5 (3.2 ( 1.3) × 10-9
360 1.5 (5.7 ( 0.3) × 10-8
360 1.5 (1.2 ( 0.1) × 10-7
360 1.5 (1.8 ( 0.3) × 10-7
360 1.5 (6.7 ( 1.3) × 10-9
312 1.5 (2.5 ( 0.5) × 10-8
(8.7 ( 1.0) × 10-9
(3.0 ( 0.2) × 10-8
n/ac
(8.2 ( 1.3) × 10-9
(2.0 ( 0.1) × 10-8
n/ac
n/ac
n/ac
(1.4 ( 0.1) × 10-8
(3.7 ( 0.2) × 10-8
n/ac
(7.7 ( 0.5) × 10-9
(1.7 ( 0.2) × 10-8
n/ac
n/ac
n/ac
(1.0 ( 0.1) × 10-8
(8.1 ( 0.8) × 10-9
10 NaNO2 0.0010 M + 2-propanol 0.10 M
312 1.5 (3.4 ( 0.4) × 10-8
312 3.0 (5.2 ( 0.4) × 10-9
312 3.0 (2.2 ( 0.2) × 10-9
(1.4 ( 0.1) × 10-8
(9.1 ( 0.4) × 10-9
11 phenol 5.3 × 10-4 M + Fe(ClO4)3 1.0 × 10-4
M
n/ac
n/ac
n/ac
n/ac
12 phenol 5.3 × 10-4 M + Fe(ClO4)3 1.0 × 10-4 M +
2-propanol 0.10 M
13 phenol 5.3 × 10-4 M + NaNO2 0.001 M
14 phenol 5.3 × 10-4 M + NaNO2 0.001 M + Fe(ClO4)3
312 3.0 (1.6 ( 0.3) × 10-8
(5.9 ( 0.1) × 10-9
(4.0 ( 0.1) × 10-9
312 3.0 (3.9 ( 0.3) × 10-8
(7.4 ( 0.9) × 10-9
(5.4 ( 0.8) × 10-9
1.0 × 10-4
M
15 phenol 5.3 × 10-4 M + Fe(ClO4)3 1.0 × 10-4
M
312 4.0 (3.8 ( 0.3) × 10-9
312 4.0 (1.8 ( 0.1) × 10-9
n/ac
n/ac
n/ac
n/ac
16 phenol 5.3 × 10-4 M + Fe(ClO4)3 1.0 × 10-4 M +
2-propanol 0.10 M
17 phenol 5.3 × 10-4 M + NaNO2 0.001 M
18 phenol 5.3 × 10-4 M + NaNO2 0.001 M + Fe(ClO4)3
312 4.0 (3.4 ( 0.2) × 10-9
(7.6 ( 0.5) × 10-10 (5.6 ( 0.4) × 10-10
312 4.0 (5.8 ( 0.1) × 10-9
(1.4 ( 0.1) × 10-9
(9.8 ( 0.5) × 10-10
1.0 × 10-4
M
19 NaNO2 0.01 M
>430 6.0 (1.1 ( 0.2) × 10-8
>430 6.0 (3.5 ( 1.0) × 10-8
>430 6.0 (2.9 ( 0.4) × 10-8
>430 6.0 (3.5 ( 2.9) × 10-9
>430 6.0 (1.1 ( 0.8) × 10-9
>430 6.0 (1.1 ( 0.3) × 10-8
>430 6.0 (2.6 ( 0.7) × 10-9
>430 6.0 (6.0 ( 1.5) × 10-9
312 6.0 (2.0 ( 0.6) × 10-8
312 6.0 (8.6 ( 2.3) × 10-9
312 6.0 (1.3 ( 0.2) × 10-8
>430 6.0 (6.3 ( 0.5) × 10-9
>430 6.0 (4.8 ( 0.3) × 10-9
>430 3.0 (7.0 ( 5.8) × 10-10
>430 3.0 (5.0 ( 0.6) × 10-9
dark 3.0 (7.7 ( 0.3) × 10-8
dark 3.0 (7.2 ( 0.3) × 10-8
(1.9 ( 0.3) × 10-9
(1.8 ( 0.2) × 10-8
(1.4 ( 0.1) × 10-8
n/ac
(1.4 ( 0.2) × 10-9
(1.3 ( 0.1) × 10-8
(1.1 ( 0.1) × 10-8
n/ac
20 NaNO2 0.01 M + R-Fe2O3 0.20 g/L
21 NaNO2 0.01 M + â-FeOOH 0.20 g/L
22 R-Fe2O3 0.20 g/L
23 â-FeOOH 0.20 g/L
n/ac
n/ac
24 NaNO2 0.01 M, degas. N2
25 NaNO2 0.01 M + R-Fe2O3 0.20 g/L, degas. N2
26 NaNO2 0.01 M + â-FeOOH 0.20 g/L, degas. N2
27 NaNO3 0.1 M
(1.2 ( 0.2) × 10-10 (4.2 ( 0.6) × 10-11
(6.5 ( 0.4) × 10-10 (8.8 ( 2.3) × 10-10
(1.1 ( 0.2) × 10-9
(1.3 ( 0.3) × 10-9
(1.2 ( 0.1) × 10-10 (8.3 ( 2.6) × 10-11
28 NaNO2 0.004 M
29 NaNO3 0.1 M + NaNO2 0.004 M
30 hum ic acid 0.20 g/L
31 NaNO2 0.01 M + hum ic acid 0.20 g/L
32 NaNO3 0.1 M
33 NaNO3 0.1 M + hum ic acid 0.20 g/L
34 NaNO2 0.01 M
(2.4 ( 0.3) × 10-10 (4.1 ( 0.1) × 10-10
(4.0 ( 0.1) × 10-9
(2.1 ( 0.2) × 10-9
n/ac
n/ac
b
b
b
b
b
b
(6.5 ( 0.2) × 10-9
(3.0 ( 0.3) × 10-9
35 NaNO2 0.01 M + hum ic acid 0.20 g/L
(6.9 ( 0.2) × 10-9
(2.8 ( 0.1) × 10-9
a
b
Conditions are reported in the table. Initial phenol concentration ) 1.1 × 10-3 M, unless otherwise reported. Nitrophenol form ation was too
c
low to be detected. n/a: not applicable.
addition of Fe(ClO4)3 at pH 1.5, adjusted with the addition
of HClO4. In the presence of Fe(ClO4)3 the initial rate for
nitrophenol formation is higher than in the presence of
NaNO2 alone, indicating that phenol nitration is enhanced
by Fe(III) (see Table 1, entries #1 and #2).
In the presence of nitrite alone at pH 1.5, phenol nitration
is due to a thermal processes involving HNO2 (15), and
possibly to the nitrous acid thermal dismutation, which yields
•
•NO and NO2 (29). The photolysis of HNO2 at 312 nm, also
•
yielding NO2 (6), plays a minor role at pH 1.5 (15):
2HNO2 f •NO + •NO2 + H2O
[k7 ) 28.6 M-1 s-1 at 303 K] (7)
HNO2+ hv f •NO + •OH [Φ310 ≈ 0.35]
(8)
FIGURE 2. Phenol degradation and nitrophenol formation in the
presence and in the absence of Fe(III). Initial conditions: 1.1 × 10-3
M phenol, 1.0 × 10-3 M NaNO , and 6.6 × 10-3 M Fe(ClO ) when
•OH + HNO2 f •NO2 + H2O [k9 ) 2.6 × 109 M-1 s-1
]
2
4 3
present, pH 1.5, irradiation at 312 nm. Fitting curves drawn according
to eq 4.
(9)
The thermal processes are likely to occur at the same
extent both in the absence and in the presence of Fe(III), as
they are independent from irradiation. The net effect of Fe-
(III) photolysis on the initial formation rates of both 2-NP
and 4-NP can be evaluated as the difference between the
rates in the presence of Fe(III) (Table 1, entry #2) and those
in the absence of Fe(III) (Table 1, entry #1).
(ꢀ310 nm ) 1826 M-1 cm-1) (25), respectively, and the quantum
yields for •OH photoformation at 310 nm (see reaction 5 and
•
6), the main source of OH at pH 1.5 is [Fe(OH)(H2O)5]2+
.
Figure 2 shows the degradation of phenol and the
formation of 2- and 4-nitrophenol (2-NP, 4-NP) upon 312
nm irradiation of phenol and NaNO2, with and without the
9
VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 6 7 1

The enhanced nitrophenol formation rate in the presence
of Fe(III) is most probably due to reactions 5 and 9, yielding
•NO2. Reaction 6, followed by reaction 9, may give a secondary
contribution. Reaction 9 between •OH and HNO2 is in
competition with the reaction between •OH and phenol (rate
constant 1.4 × 1010 M-1 s-1 (30)). At phenol and HNO2
concentrations of 1.1 × 10-3 M and ≈ 1.0 × 10-3 M,
respectively, 86% of photoformed •OH should react with
phenol and 14% with HNO2, based on above given kinetic
constants. For this reason the initial degradation rate of
phenol is higher in the presence of Fe(III) + HNO2 than in
the presence of HNO2 alone (Table 1, entries #1 and #2). For
the same reason, degradation of nitrophenols is faster in the
presence of Fe(III) + HNO2 than in the case of HNO2 alone.
The time evolution of both 2-NP and 4-NP for the first 30
min irradiation is almost linear in the case of HNO2 alone,
indicating that the degradation rate is lower than the
formation rate (see Figure 2). On the contrary, the concen-
tration vs time curves of both 2-NP and 4-NP reach a plateau
after about 10 min of irradiation in the presence of Fe(III).
The plateau indicates that the degradation rate equals the
formation rate, thus formation and transformation of ni-
trophenols are both enhanced by Fe(III).
very similar at 312 nm (9.2 × 10-9 Ein s-1) and 360 nm (8.8
×10-9 Ein s-1). As the observed rates of nitrophenol formation
at 360 nm are comparable to those observed at 312 nm (see
entries #2 and #6), reaction 10 can account for the effect of
Fe(III) photoexcitation only if the quantum yield is almost
equal at both the wavelengths and g 0.22. This is unlikely
because at higher wavelengths the quantum yield is normally
lower, as it is the case for example of reaction 11 (Φ350 nm
)
0.0015) (25). As a consequence, the reactions 5, 6, and 9
account for nitrophenol formation better than reaction 10.
In addition, phenol degradation experiments performed with
Fe(III) alone and with Fe(III) + 2-propanol indicate that the
•
rates of OH photoproduction are similar for irradiation at
312 nm (entries #3 and #4) and at 360 nm (entries #7 and #8).
As a consequence, the results are better explained by reactions
5, 6, and 9 rather than by reaction 10.
A second experiment, in which 2-propanol was added to
the phenol/ HNO2/ Fe(III) system under irradiation at 312
•
nm, was also carried out. As the alcohol is an OH scavenger,
it should inhibit reaction 9 without a relevant influence on
reaction 10. The addition of 2-propanol in the presence of
Fe(III) and HNO2 reduces the initial formation rates of 2-NP
and 4-NP (see Table 1, entry #9, and compare with entry #2),
which become very similar to those observed in the presence
of HNO2 alone (compare entries #9 and #1). The comparison
of entries #10 and #1 indicates that 2-propanol does not
inhibit the thermal nitration of phenol. Thus, in the presence
of Fe(III) + HNO2 + 2-propanol the reaction proceeds only
via the thermal decomposition of HNO2. As a consequence,
the enhancement of nitrophenol formation by Fe(III) is linked
The role of •OH, according to reactions 5 and 9, is further
supported by the degradation of phenol upon 312 nm
irradiation in the presence of 2-propanol. This alcohol is an
•OH scavenger (31), with a rate constant 3.0 × 109 M-1 s-1
with •OH radical (30). In the presence of 1.1 × 10-3 M phenol
and 0.10 M 2-propanol, 95% of photoformed •OH would react
with 2-propanol and 5% with phenol. This calculation is in
agreement with the ratio experimentally evaluated (see
entries #3 and #4 in Table 1). It can be concluded that a
relevant •OH photoproduction takes place in the system and
that the nitration occurs via reaction 9.
The addition of NaNO2 to an Fe(ClO4)3 solution induces
a small change in the solution absorption spectrum, which
cannot be explained by the mere overlapping between the
NaNO2 and Fe(ClO4)3 spectra. This change is most likely due
to the formation of the complex FeNO22+. Photolysis of
FeNO22+ might contribute to •NO2 photoformation according
to
•
to the photoproduction of OH, and reaction 10 plays a
negligible role in the system.
The significance of the reaction pathway involving the
oxidation of the nitrosoderivatives was also assessed. The
reactive species for phenol nitrosation is N2O3, which
transforms phenol into 4-NOP (33). Upon nitrite photolysis,
a fraction of 4-NOP is transformed into 4-NP (14). Control
experiments showed that dissolved Fe(III) under irradiation
in the presence of nitrite does not transform 4-NOP into
-
4-NP to a higher extent than NO2 alone. Furthermore, in
both cases, the production of 4-NP was very limited. This
suggests that the enhanced nitrophenol formation in the
presence of Fe(III) + nitrite with respect to nitrite alone does
not involve the nitrosoderivative pathway.
FeNO22+ + hν f Fe2+ + •NO2
(10)
A second set of experiments was carried out at pH 3.0 (pH
adjusted by the addition of HClO4). Although for experimental
convenience the concentration of Fe(ClO4)3 was reduced,
and [Fe(OH)(H2O)5]2+ would be the main Fe(III) species in
the system, the presence of possible dimeric and polymeric
iron species could not be excluded. In addition to reactions
5 and 6, which are characteristic of mononuclear Fe(III)
species, the following excitation and electron-transfer pro-
cesses for di- and polymeric species may be postulated for
phenol under these conditions, as suggested for 2,6-dim-
ethylphenol (28):
Reaction 10 may be postulated in analogy to the photolysis
of FeSO4 yielding SO4 (25):
+
•
-
-
FeSO4+ + hν f Fe2++ •SO4
[Φ310 nm ) 0.0035] (11)
Although the quantum yield of reaction 11 is very low, the
quantum yield for reaction 10 could be higher comparing
the monoelectronic reduction potentials of •NO2/ NO2- and
•SO4-/ SO42-, which are 1.0 and 2.4 V, respectively (32). As
FeNO22+ the complex FeSO4+ absorbs in the UV region (ꢀ310
nm )1826 M-1 cm-1) (25). Calculations using reported stability
constants (26) show that in a solution containing 0.0066 M
[FeIII] + hν f [FeIII]*
(12)
(13)
Fe(ClO4)3 and 0.0010 M NaNO2, at pH 1.5 for HClO4, [Fe3+
]
≈ 6.2 × 10-3 M, [FeOH2+] ≈ 4.2 × 10-4 M, and [FeNO22+] ≈
[FeIII]* + PhOH f Fe2+ + PhO•+ H+
1.7 × 10-5 M. Since [FeNO22+] is negligible with respect to
2+
•
the concentration of other Fe(III) species, the FeNO2
Although OH radicals are efficiently scavenged by 2-pro-
panol, no reaction has been detected between 2-propanol
and [FeIII]* (28). Thus, the effect of 2-propanol on the initial
phenol degradation rate at pH 3.0 still gives indication on
spectrum was obtained by spectra subtraction. It shows a
broad maximum around 298 nm, with a tail until 400 nm.
Using calculated concentrations, the molar extinction coef-
ficient at 312 nm is 6.1 × 103 M-1 cm-1. The absorptances
at 312 nm as calculated from eq 3b are RFe(III) ) 0.60, RHNO2
) 0.0027, and RFe(NO2)2+ ) 0.058.
For discriminating the effect of reaction 10, one may use
the different absorption of FeNO22+ observed at 360 nm (ꢀ360
nm ≈ 0.2 × ꢀ312 nm). Taking into account the photon flux of the
two lamps, the rate of photon absorption by the complex is
•
the production of OH. Table 1 reports the initial phenol
degradation rate in the absence (entry #11) and in the
presence (entry #12) of 0.10 M 2-propanol. The alcohol lowers
the rate of about 2.4 times. This indicates that a moderate
•OH photoproduction takes place in the system along with
direct electron transfer to [FeIII]* (reaction 13). This process
should have a role also in the production of •NO2 via reaction
9
6 7 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 4, 2002

14, since E° (•NO2/ NO2-) ) 1.0 V(32) and E° (PhOH•+/ PhOH)
) 0.97 (34).
[FeIII]* + NO2- f Fe2+ + •NO2
(14)
In addition to reaction 14, a further contribution to phenol
nitration may come from the phenoxyl radical via reaction
15 (8, 10).
PhO• + •NO2 f O2N-Ph-OH
(15)
Experiments have been carried out also in the presence
of 1.0 × 10-3 M nitrite. Table 1 reports the initial rates of
phenol degradation and nitrophenol formation for nitrite
alone (entry #13) and for Fe(III) + nitrite (entry #14). At pH
-
3.0, 60% of nitrite is present as HNO2 and 40% as NO2 as
calculated using reported stability constants (26). The
absorption spectra of Fe(III), HNO2, and NO2- are reported
in Figure 1. Samples were irradiated at 312 nm. Light
absorption calculations, performed according to eq 3b,
showed that in a solution containing 1.0 × 10-4 M Fe(III) and
FIGURE 3. Phenol degradation and nitrophenol formation in the
presence and in the absence of Fe(III). Initial conditions: 5.3 × 10-4
M phenol, 1.0 × 10-3 M NaNO , and 1.0 × 10-4 M Fe(ClO ) when
2
4 3
present, pH 4.0, irradiation at 312 nm. Fitting curves drawn according
to eq 4.
1.0 × 10-3 M nitrite at pH 3.0, the absorptances are RFe
)
0.057 and RNO2 ) 0.005 for Fe(III) and nitrite + HNO2,
respectively. This means that also at pH 3.0 the reaction 5
prevails over reactions 1 and 8.
14 and 15). The role of these reactions increases as pH
increases. At environmental acidic pH values, these reactions
play a significant role.
The initial rate of nitrophenol formation is slightly higher
in the Fe(III) + nitrite case, consistently with the role played
by reactions 5, 2, and 9, but the difference is limited. The
non-negligible rate of nitrophenol formation in the presence
of nitrite alone at pH 3.0, contributing for almost 80% of the
NP formation rate observed in the presence of Fe(III) species,
may be ascribed to the thermal process depicted in reaction
7.
R-Fe2O3 and â-FeOOH. These oxides are semiconductors
with indirect band gap absorption in the visible region.
Hematite absorbs radiation at λ < 530 nm (band gap 2.3 eV
(35)). The band gap for â-FeOOH was reported to be 2.12 eV
(21), corresponding to an absorption at λ < 575 nm. The
photocatalytic behavior of R-Fe2O3 (hematite) has been widely
studied and the mechanism of some of its photoreactions
elucidated (21, 35-38). Upon absorption, electrons are
promoted from the valence (VB) to the conduction band
(CB), leaving holes in the valence band:
To limit the role of reaction 7, a third set of experiments
-
was carried out at pH 4.0. Under such conditions, [NO2
]
(88% of total nitrite) prevails over [HNO2] (12%), [Fe(OH)2-
(H2O)4]+ (61% of the total mononuclear Fe(III)) prevails over
[Fe(OH)(H2O)5]2+ (38%) and [Fe(H2O)6]3+(1%), and the ab-
sorption spectrum of Fe(III) slightly changes (see Figure 1).
Absorbed light calculations give RFe ) 0.047 and RNO2 ) 0.008,
comparable to those previously calculated at pH 3.0. To assess
the extent of •OH generation (reaction 5), phenol was
irradiated in the presence of Fe(III) at pH 4.0, with and without
2-propanol 0.10 M. Table 1 reports the initial phenol
degradation rate in the absence (entry #15) and in the
presence (entry #16) of 0.10 M 2-propanol. The alcohol lowers
the rate of about 2.1 times. The comparison of the effect of
2-propanol on the initial phenol degradation rate at pH 1.5,
3.0, and 4.0 indicates that the role of reactions 5 and 6
decreases as pH increases.
The degradation of phenol and the formation of 2- and
4-nitrophenol upon 312 nm illumination at pH 4.0, with and
without addition of 1.0 × 10-4 M Fe(ClO4)3, are reported in
Figure 3. Initial rates are listed in Table 1 (entries #17 and
#18). The initial rates of nitrophenol formation are almost
double in the presence of iron species. The formation of
nitrophenols accounts in both cases for about 40% of the
initial phenol transformation. Conclusions
R-Fe2O3 + hν f e-CB + h+
(16)
VB
Electrons and holes can quickly recombine or react with
dissolved molecules. Radiative excitation can also result in
charge-transfer reactions, involving surficial species (37):
tFeIII-OH + hv f Fe2+aq + •OH
(17)
Reaction 17 can induce oxidation of natural or xenobiotic
compounds via photoformed •OH and causes photocorrosion
of R-Fe2O3.
Since nitrite absorbance has a maximum at 354 nm and
becomes negligible above 420 nm, to excite iron oxides almost
selectively, the suspensions were illuminated with a Xenon
lamp equipped with a 430 nm filter. Under these conditions
reaction 17 is expected to play a minor role, as it takes place
upon excitation of a charge-transfer band with band gap )
3.3 eV (λ ) 375 nm (37)).
Figure 4 shows the degradation of phenol and the
formation of 2- and 4-NP when 1.1 × 10-3 M phenol is
illuminated in the presence of 0.01 M NaNO2 (pH 6), in
homogeneous phase and in the presence of the colloidal
ferric oxides (0.20 g/ L). The initial rates of nitrophenol
formation are significantly higher in the presence of the oxides
than in homogeneous phase (see entries #19-21). The
formation of nitrophenols in homogeneous phase is due to
the residual nitrite absorption (see Figure 1). Moreover, the
formation of nitrophenols accounts for 30% of initial phenol
transformation in the case of nitrite alone, 90% in the presence
of R-Fe2O3, and 85% in the presence of â-FeOOH (see Table
1, entries #19-21). Thus, photoexcitation of colloidal iron
oxides in the presence of nitrite and at environmentally
significant conditions leads to a sustained production of
In summary, illumination of aqueous Fe(III) solutions
•
-
produces OH, which can then oxidize NO2 and HNO2 to
nitrogen dioxide (reaction 5, 6, 2, and 9). The Fe(III)-
•
photoinduced production of NO2 largely dominates pho-
tolysis of both nitrite and nitrous acid (reactions 1, 2, 8, and
9) as well as the thermal dismutation of nitrous acid (reaction
7). However, measurements carried out in the presence of
•
an •OH scavenger show that the production of OH upon
Fe(III) illumination (reactions 5 and 6) decreases as pH
increases. The fraction of photoinduced processes not
•
accounted for by OH production is attributed to direct
electron transfer to [FeIII]* (reactions 12 and 13 and possibly
9
VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 6 7 3

FIGURE 4. Phenol degradation and nitrophenol formation in the
presence and in the absence of iron oxides. Initial conditions: 1.1
FIGURE 5. Phenol degradation and nitrophenol formation in the
presence and in the absence of nitrate and nitrite. Initial condi-
tions: 1.1 × 10-3 M phenol, 0.1 M NaNO , and 4.0 × 10-3 M NaNO
× 10-3 M phenol, 0.01 M NaNO (pH 6), irradiation under Solarbox
2
3
2
+ 430-nm filter. Fitting curves drawn according to eq 4.
(pH 6), irradiation at 312 nm. Fitting curves drawn according to eq
4.
nitrophenols. In the presence of the iron oxides alone, without
nitrite, very low photoinduced transformation of phenol was
observed, as the measured initial degradation rates show
(Table 1, entries #22 and #23).
the photocatalytic oxidation of sulfur dioxide to sulfate in
R-Fe2O3 slurries (35). Surface-bound oxygen was shown to
react with surficial sulfur-containing radicals, acting as an
indirect electron scavenger. As in our case, the photocatalytic
process was favored in the presence of oxygen.
As already done in the case of dissolved Fe(III), the role
of the pathway of nitrophenol formation upon oxidation of
the nitrosoderivatives (14) had to be assessed. 4-NOP was
not detected under the present experimental conditions,
neither in the presence of NaNO2 alone nor in the slurries
iron oxide + nitrite. This could be due to a slow formation
rate or to a fast degradation rate. Control experiments on 2
×10-5 M 4-NOP in these systems showed that the degradation
rate of 4-NOP is not significantly different from that observed
in the presence of 0.1 M NaNO2 (pH 6.5, irradiation at 360
nm (14)). The irradiation of 1.1 × 10-3 M phenol + 0.1 M
NaNO2 at 360 nm gave relevant amounts of 4-NOP, differently
from the systems containing NaNO2 alone or with iron oxides,
irradiated at >430 nm. The lack of detection of this compound
in the presence of NaNO2 alone or iron oxides suggests that
the 4-NOP formation rate is low. Thus, a conceivable
nitrosoderivative pathway cannot explain the effect of iron
oxides.
In conclusion, iron oxides and nitrite show mutual
interaction, as indicated by the enhancement of both the
nitrophenol formation and the overall phenol transformation
rates, if compared with systems in which they are present
separately. Most likely, the interaction is due to the oxidation
of nitrite to nitrogen dioxide by electron vacancies on the
irradiated oxides.
-
Nitrate. The ion NO3 absorbs near UV radiation (ab-
sorption maximum at 302 nm). Absorption causes photolysis
according to reaction 19, followed by reaction 1b (31):
NO3- + hv f •NO2 + •O- [Φ310 nm ≈ 0.01] (19)
•
In the presence of nitrite, the radical OH formed upon
nitrate photolysis can be involved in reaction 2, oxidizing
nitrite to nitrogen dioxide. This should add to the •NO2
produced by nitrate photolysis, enhancing phenol nitration.
To gain information about the role of nitrate photolysis,
nitrite should not be excited. The illumination of 0.1 M nitrate
in the presence of 0.004 M nitrite at 312 nm partially fulfills
this requirement (see Figure 1). Calculations performed
according to eq 3b under the used experimental conditions
indicate that the absorptance of 0.1 M nitrate is RNO3 ) 0.450
when it is alone, and RNO3 ) 0.443 in the presence of 0.004
M nitrite. The absorptance of 0.004 M nitrite is inhibited in
the presence of nitrate (RNO2 ) 0.036 when nitrite is alone
and RNO2 ) 0.027 in the presence of 0.1 M nitrate).
Figure 5 reports the degradation of phenol and the
formation of 2-NP and 4-NP in the presence of 0.004 M nitrite
alone, 0.1 M nitrate alone, and the mixture nitrate + nitrite.
The results indicate that under almost selective photoexci-
tation of nitrate nitrophenols form in much larger amounts
in the presence of the mixture NaNO3 + NaNO2 than in the
other cases (see Table 1, entries #27-29). Formation of
nitrophenols accounts for 1% of initial phenol transformation
in the presence of nitrate alone, 8% in the presence of nitrite
alone, and 45% in the mixture. It can be inferred that nitrite
enhances the formation of nitrophenols under conditions of
nitrate photolysis. The results are consistent with the
hypothesis that the photolysis of nitrate in the presence of
nitrite leads to the generation of •NO2 according to reactions
19, 1b, and 2. The scavenging of •OH by nitrite (reaction 2)
makes clear why the initial phenol degradation rate in the
The main reaction mechanism in semiconductor slurries
should involve oxidation of surface adsorbed nitrite by
photoformed holes:
h+VB + NO2- f •NO2
(18)
This process is thermodynamically allowed because at
pH 7 E (•NO2/ NO2-) ) 1.0 V (32). Faust et al. (35) reported
EVB ) 2.3 V for R-Fe2O3. The data by Leland and Bard (21)
allow for roughly evaluating EVB ) 2.6-2.9 V for â-FeOOH.
For reaction 18 to take place, a parallel process of
scavenging of e-CB has to occur. No reduction of Fe(III) was
detected as Fe(II)/ 1,10-phenathroline complex in aqueous
solution under illumination >430 nm. A considerable (>90%)
inhibition in nitrophenol formation is detected when oxygen
is purged with a stream of high purity nitrogen from the
oxide slurry before irradiation. Moreover, the initial degra-
dation rates for phenol in the absence of oxygen are lower
than in aerated solution in the case of the slurries (see Table
1, entries #25 and #26 compared with entries #20 and #21),
while practically no difference was observed in the presence
of nitrite alone. Thus, oxygen should have a role in the
photocatalytic system. Direct electron scavenging by free
dissolved oxygen is not allowed (at pH 7, ECB ) 0.0 V for
R-Fe2O3 (35) and 0.5-0.7 V for â-FeOOH (21), while E (O2/
•O2-) ) - 0.33 V (32)). However, conduction band electrons
could still reduce surface-bound dioxygen, as is the case for
9
6 7 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 4, 2002

presence of nitrate + nitrite (1.3 × 10-8 M s-1) is lower than
in the case of nitrate alone (2.0 × 10-8 M s-1).
due to the thermal decomposition of HNO2. Since this process
occurs in the dark, competition for light absorption by humic
acid is not an issue. A negligible effect on the rate of
nitrophenol formation in the dark due to the presence of
humic acids was observed in the presence of NaNO2 0.01 M,
at pH 3.0 (see Table 1, entries #34 and #35). This result allows
for the exclusion of the hypothesis that humic acids react
with nitrogen dioxide.
Environm ental Significance. The interactions between
different environmental factors are of paramount importance
in the understanding of environmental processes. Photo-
reactions of environmentally ubiquitous dissolved iron(III),
iron oxides, and nitrate lead to the oxidation of NO2-/ HNO2
to •NO2 and give a new pathway for the •NO2-promoted
nitration of organic matter, either natural or xenobiotic. These
processes are quite different from those involving direct
photolysis of nitrite, which mainly generates •OH leading to
oxidation of organic molecules (2, 5, 7), and add new relevant
pathways to the complex chemistry of environmental com-
partments, such as natural aquifers and atmospheric aerosols,
in the presence of sunlight.
Since a faster conversion of 4-NOP into 4-NP was observed
in the presence of nitrate + nitrite (55% conversion after 2
h irradiation) than in the presence of nitrite alone (20%
conversion after the same irradiation time), the pathway
involving the oxidation of the nitrosoderivatives may have
a more important role in the process of phenol nitration in
the presence of nitrate + nitrite than in the presence of nitrite
alone. However, no 4-NOP was detected when phenol was
irradiated in the presence of 0.004 M NaNO2 and of 0.004 M
NaNO2 + 0.1 M NaNO3. The transformation rate of 4-NOP
observed in these systems is not significantly different from
that observed in systems in which relevant formation of
4-NOP occurs (14). Thus, the lack of detection of 4-NOP in
the presence of NaNO2 and of the mixture NaNO2 + NaNO3
is due to a slow formation rate rather than to a fast
transformation rate. As a consequence, the fairly high yield
of oxidation of 4-NOP to 4-NP in the presence of nitrate +
nitrite cannot give a significant contribution to the overall
formation of 4-NP, due to the negligible formation of 4-NOP.
These findings suggest that, under conditions of nitrate
photoexcitation, the nitration of phenol in the mixture nitrate
+ nitrite is enhanced by the oxidation of nitrite to nitrogen
Acknowledgments
Financial support of CNR, MURST, Inter-University Con-
sortium “Chemistry for the Environment” (INCA), PNRA
Progetto Antartide and Universita` di TorinosProgetto Gio-
vani Ricercatori is kindly appreciated.
•
dioxide due to OH radicals generated by nitrate photolysis.
Hum ic Acids. Humic substances constitute a relevant
fraction (30-60%) of dissolved organic matter. They are a
biologically refractory, reddish-brown material and originate
from the decomposition of vegetal organic matter (39). Their
ubiquitous diffusion in natural waters, together with their
ability to absorb a consistent fraction of sunlight, makes them
a potentially important environmental factor when photo-
induced processes are considered (2, 40, 41). Humic acids
(HS) are involved in the iron redox cycling in surface waters
(42). The absorption of radiation by HS promotes the
transition to the first excited singlet state, which partially
converts to the reactive excited triplet state. The latter can
induce production of solvated electrons and singlet oxygen
and conversion of the ground singlet states of some organic
substances into excited triplets (2). Humic acids thus play
the role of photosensitizers for many reactions of environ-
mental importance.
Nitrite, humic acids, and their mixture have been il-
luminated under the Xenon lamp, equipped with 430 nm
filter (see entries #30 and #31). The illumination of a mixture
of nitrite and humic acids in the presence of phenol does not
produce nitrophenols. On the contrary, the photolysis of
nitrite alone under the lamp yields some nitrophenols (see
entry #19). These results indicate that humic acids inhibit
nitrite photolysis, thus impeding nitrogen dioxide and
nitrophenol formation under the adopted experimental
conditions.
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•
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ES010101C
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