Photolytic and Radiolytic Oxidation of Humic Acid†
Marcela V. Martin1, Gustavo T. Ruiz1, Mo ´nica C. Gonzalez1, Claudio D. Borsarelli2and Daniel O. Ma ´rtire1*
1Instituto de Investigaciones Fisicoquı ´micas Teo ´ricas y Aplicadas (INIFTA, CCT La Plata-CONICET⁄UNLP),
Facultad de Ciencias Exactas, Universidad Nacional de La Plata, La Plata, Argentina
2Laboratorio de Cine ´tica y Fotoquı ´mica. Instituto de Quı ´mica del Noroeste Argentino (INQUINOA),
Universidad Nacional de Santiago del Estero, Santiago del Estero, Argentina
Received 2 December 2011, accepted 14 February 2012, DOI: 10.1111⁄j.1751-1097.2012.01116.x
The reactions of Br2Æ), BrÆ, HOÆ and N3Æ with Aldrich humic
acid (AHA) were investigated. The Br⁄Br2Æ)radicals were
obtained in flash-photolysis experiments (kexc= 266 nm) per-
formed with NaS2O8solutions in the presence of bromide ions.
HOÆand N3Æ radicals were generated by pulse radiolysis of N2O-
saturated solutions. From the combination of a bilinear analysis
and computer simulations of the absorbance traces, it was
possible to obtain information on the rate constants for the
reactions of Br2Æ), BrÆ, HOÆ and N3Æ with AHA and on
the intermediate species involved in the mechanism. Evidence for
the participation of phenoxyl radicals (kmax= 410 nm) is given.
The hydroxyl radical (HOÆ) is a powerful oxidant known to
react with many organic compounds at nearly diffusion-
limited rates. In natural systems, the identified HOÆ radical
sources are (1) nitrate and nitrite photolysis (1,2), (2) direct
photolysis of dissolved organic matter (DOM) (3); and (3) the
photo-Fenton reaction (4).
Photodecomposition of DOM may proceed both via direct
photochemical reactions, involving energy and electron trans-
fer after light absorption by chromophoric DOM (CDOM)
(5,6), or via indirect (sensitized) processes, involving DOM
reactions with photochemically generated intermediates. Of
the various reactive intermediates produced in sunlit natural
waters, the HOÆ radical is one of the likeliest candidates to
have significant effects on dissolved organic matter (DOM)
all over the world as they have proved to be successful
alternatives to conventional treatment techniques of water,
waste-water, groundwater and potable water, because they can
AOPs rely on the action of in situ generated hydroxyl
radicals (HOÆ), which are nonselective, and degrade the
organic compounds (11–13).
The effectiveness of AOPs during water treatment is
hindered by the reaction between HOÆ and water matrix
components, including inorganic species and DOM. These
reactions compete with the target contaminants for HOÆ (11)
(see reaction 1 for the overall interaction of the hydroxyl
radicals with the inorganic anions), which results in a
reduction of the treatment efficiency.
HO?þ X?! HO?þ X?
X)and XÆ represent the inorganic anion and the corre-
sponding radical, respectively.
With this background and because humic substances (HS)
represent the main fraction of DOM present in natural waters,
we set out to investigate the early stages of the reactions of
HOÆ radicals and dibromide radical anions (Br⁄Br2Æ)) with
Aldrich humic acid (AHA). The BrÆBr2Æ)radicals are formed
through the HO-mediated oxidation of bromide ions, present
as impurities of the more abundant chloride ions, both in
natural and treatment waters (14). Here, we generated HOÆ
radicals by pulse radiolysis and monitored the absorbance of
the organic intermediate species formed after HOÆ decay.
Comparative pulse-radiolysis experiments with azide radicals
(N3Æ) were also performed. The Br2Æ)radicals were obtained in
flash-photolysis experiments of Na2S2O8 solutions in the
presence of bromide ions.
MATERIALS AND METHODS
Materials. Na2S2O8 from Merck (Darmstadt, Germany), NaBr
(Merck), NaN3(Sigma-Aldrich) and Aldrich humic acid sodium salt
(AHA) were used as received. Deionized water (>18 MW cm,
<20 ppb of organic carbon) was obtained from a Millipore system
(Bedford, MA). The temperature was controlled to ±0.1 K with a
Grant model GD 1200 thermostat (Chelmsford, UK).
AHA’s shortcomings as a model for dissolved organic matter
present in natural waters has been well documented (15). However, the
reactivity of both the hydroxyl (16) on a per carbon basis is almost
independent of the nature of the DOM or HS. Sulfate radical
reactivity towards different HS including AHA was also shown to be
ruled by the carbon content of the HS (17). For these reasons, we
believe that the results obtained with AHA are expected to be useful
for evaluating the reactivity of the inorganic radicals with DOM of
Laser flash-photolysis (LFP) experiments. LFP experiments were
performed by excitation with the fourth harmonic of a Nd:YAG
Litron laser (2 ns FWHM and 6 mJ per pulse at 266 nm). The
experimental set-up was described elsewhere (18). A 10 mm-pathlength
cuvette was employed. Decays typically represented the average of 64
pulses and were taken by and stored in a 500 MHz Agilent Infiniium
†This paper is part of the Special Issue on the 21st Conference of the IAPS.
*Corresponding author e-mail: firstname.lastname@example.org (Daniel O. Ma ´ rtire)
? 2012 Wiley Periodicals, Inc.
Photochemistry andPhotobiology? 2012TheAmericanSocietyofPhotobiology 0031-8655/12
Photochemistry and Photobiology, 2012, 88: 810–815
The SO4Æ)radical was generated by excitation at kexc= 266 nm
(reaction 2, in Table 1) of 5 · 10)2M Na2S2O8solutions containing
10)2M of NaBr. To investigate the effect of AHA on the kinetic
behavior of Br2Æ), experiments in the presence of different amounts of
AHA in the concentration range from 4 to 12 mgÆL)1were performed.
Under these conditions, A266(AHA) << A266(S2O82)).
Bromide ions are oxidized by sulfate radicals to yield bromine
atoms (BrÆ, kmax= 275 nm, e275= 2900 M)1cm)1, (20)), reaction 3
in Table 1. Dibromide radical anions (Br2Æ)radicals, kmax= 360 nm,
e360= 9900 M)1cm)1, (20) are reversibly formed by reaction of
bromine atoms and bromide ions, reactions 4 and 5 in Table 1.
Pulse radiolysis (PR) experiments. Pulse radiolysis experiments
were carried out with a model TB-8⁄16–1ÆS electron linear accelerator.
The instrument and computerized data collection for time-resolved
UV–vis spectroscopy and reaction kinetics have been described
elsewhere (21). Thiocyanate dosimetry (22,23) was carried out at the
beginning of each experimental session. The procedure is based on the
production of (SCN)2Æ)radicals generated by the electron pulse in a
N2O saturated 10)2M SCN)solution. The calculations were made
with G = 6.13 and a molar absorption coefficient, e = 7.58 ·
103M)1cm)1at 472 nm, for the (SCN)2Æ)radicals (22).
The HOÆ and N3Æ radicals (reaction 1 with X)= N3)) were
generated by pulse radiolysis of N2O-saturated aqueous AHA solu-
tions in the absence and presence of 0.1 M NaN3, respectively. The
initial concentrations of HOÆ and N3Æ radicals generated were
1.33 · 10)5and 1.08 · 10)5M, respectively.
To obtain information on the kinetics and spectra of the absorbing
species, a bilinear regression analysis (24,25) was applied to the
experimental absorption matrix.
Kinetic computer simulations. To simulate the decay of the transient
traces a computer program based on component balances formulated
in terms of a differential algebraic equations system (DAE) was used.
The program employs the Runge Kutta method (17). The method was
validated by comparison with results obtained from a robust simula-
tion of the DAE with a modified version of the LSODI routine (25)
based on Gear’s Stiff method (26).
RESULTS AND DISCUSSION
Laser flash-photolysis (LFP) experiments
Photolysis experiments of S2O82), in the presence of NaBr
showed absorption traces at k > 250 nm, whose spectrum
immediately after the laser shot agreed with that of the Br2Æ)
radicals (kmax= 360 nm; 22; Fig. 1). The characteristic signal
of the sulfate radicals with maximum at 450 nm (20) was not
observed because the Br)ions are oxidized by the SO4Æ)
radicals (reaction 3) in the nanosecond time window.
The reaction mechanism is complex and thus computer
simulations of the absorbance profiles obtained at 360 nm
were performed. For this purpose, reactions 3–5 were consid-
ered. The initial concentration of SO4Æ)was obtained from
assays performed with solutions without NaBr taking the
value of 1650 M)1cm)1for the absorption coefficient of this
radical at 450 nm (27). The rate constants for reactions 3–12
were taken from the literature, whereas those of reactions
13 and 14 were varied to optimize the agreement between
simulated and experimental traces. The rate constant of
reaction 15 was obtained from pulse radiolysis experiments
performed with AHA (see below).
The simulated concentration profiles of Br2Æ)converted to
absorbance taking e360= 9900 M)1cm)1(20) are in good
agreement with the experimental ones (Fig. 1).
Pulse-radiolysis (PR) experiments
HOÆ radical experiments. The traces obtained in PR experi-
ments performed with N2O-saturated solutions of pH = 9
containing 21 mg L)1and 49 mg L)1AHA constitute a
complex system, as can be seen in Fig. 2. To analyze the
minimum number of intermediate species involved in the traces
a bilinear regression program was employed (24).
The reaction kinetics is complex and for this reason the
bilinear program was applied to obtain information on the
minimum number of species.
to 800 nm obtained with 21 mg L)1AHA solutions shows that
the minimum number of species necessary for reproducing the
experimental data is three. Two of them (species A and B) are
formed immediately after the radiation pulse and decay with a
pseudo first-order kinetics. The third species (C) is formed with
a rate coincident with the decay of B and presents a lifetime
longer than the time window of the experiment (see Fig. 3).
Table 1. Main reactions involved in the formation and decay of
S2O82)+ hm fi 2SO4Æ)
SO4Æ)+ Br)fi BrÆ + SO42)
BrÆ + Br)fi Br2Æ)
Br2Æ)fi BrÆ + Br)
Br2Æ)+ Br2Æ)fi Br3Æ)+ Br)
BrÆ + H2O ? BrOHÆ)+ H+
Br)+ HOÆ ? BrOHÆ)
BrOHÆ)+ H+? BrÆ + H2O
BrOH)+ BrÆ)? Br2Æ)+ HO)1 · 109M)1s)1
SO4Æ)+ OH)fi OHÆ + SO42)
S2O82)+ SO4Æ)fi S2O8Æ)
BrÆ + AHA fi Products
Br2Æ)+ AHA fi Products
OHÆ + AHA fi Products
3.5 · 109M)1s)1
1.2 · 1010M)1s)1
1.9 · 104s)1
2.4 · 109M)1s)1
1.1 · 1010M)1s)1
1.06 · 108M)1s)1
8.3 · 107M)1s)1
1.2 · 105M)1s)1
6.3 · 107MC)1s)1
5.6 · 106MC)1s)1
5.1 · 108MC)1s)1
13 This work
14 This work
15 This work
*The data taken from this reference were those measured at 21?C.
λ / nm
time / μs
Figure 1. Normalized absorption spectra obtained from LFP experi-
ments(T = 299 K)with:Ar-saturated
5.0 · 10)2mol L)1Na2S2O8and 0.01 mol L)1NaBr (s). The litera-
ture absorption spectrum (d) from ref. 20 is also shown. Inset:
Absorbance decay traces of the transient species obtained at
k = 360 nm with solutions containing (from top to bottom)
0.01 mol L)1Br)y 5.0 · 10)2mol L)1S2O82)and 0, 4.0, 8.0 and
12.0 mg L)1AHA.
Photochemistry and Photobiology, 2012, 88811
Because of the high absorption below 400 nm of the
49 mg L)1AHA solutions, signals for these solutions were
taken in the wavelength range from 400 to 800 nm. Thus,
formation of only one transient species with absorption
spectrum and growth kinetics coincident with those of species
C was detected with the bilinear program.
For the assignment of A, B, and C computer simulations
were performed. The traces obtained with the lower concen-
tration of AHA (21 mg L)1) were simulated with the mech-
anism shown in Table 2. The initial concentration of HOÆ
employed in the simulations (1.33 · 10)5M) was obtained
from the thiocyanate dosimetry (see Experimental).
The humic acid employed here contains 44.1% of carbon in
mass and from it 32.4% is aromatic. The phenolic groups
represent a 23.8% of its total acidic groups (31). For these
reasons, the proposed mechanism considers the addition of the
HOÆ radical to the phenolic groups of AHA (reaction 16),
followed by a sequence of reactions similar to that employed
by Bonin et al. (28) to interpret the reaction of HOÆ with
phenol (reactions 18–24). The reactivity of the HOÆ radical
with other reactive groups to yield an intermediate I which
decays in the time window of the experiment is considered in
reactions 17a and 17b. All the rate constants were taken from
the literature, except for those of reactions 16, 17a and 17b,
which were varied to optimize the agreement between exper-
imental and simulated traces.
The addition of HOÆ radicals (32) to the phenolic groups of
AHA (reaction 16) should yield hydroxycyclohexadienyl
radicals (HCHD), as shown in Scheme 1 for phenol.
Conversion of the HCHD (D in Scheme 1) (kmax= 280–
330 nm) (33) into the radical cation (E in Scheme 1) is acid-
catalyzed and thus, although this process is very fast in acidic
solutions, it takes place in the microsecond range in basic
solutions as that employed here (34).
The radical cation is then deprotonated, a process with a
very low barrier (35), to yield the phenoxyl radical. In the
presence of small amounts of water, the radical cations of
0100 200 300 400
0 100 200 300 400
time / μs
0100 200 300 400
ΔA × 10
λ = 320 nm
λ = 410 nm
λ = 630 nm
Figure 2. Experimental traces obtained from pulse radiolysis experiments performed with N2O-saturated solutions of pH = 9 containing
21 mg L)1AHA ( ) and traces obtained from the bilinear analysis (s) at 320, 410 and 630 nm. The corresponding residuals are shown below.
λ / nm
Concentration / u.a.
time / μs
Figure 3. Absorption spectrum of species A (s), B (d) and C ( )
obtained from the bilinear analysis of the pulse radiolysis traces
obtained with N2O-saturated 21 mg L)1AHA solutions of pH = 9.
Inset: Kinetic profiles of species A, B and C.
Table 2. Main reactions involved in the decay of HOÆ radicals.
HOÆ + AHA fi HCHD
HOÆ + AHA fi I
I fi Product 1
HCHD fi Phenoxyl + H2O
2HCHD fi Product 2
2Phenoxyl fi Product 3
HCHD + Phenoxyl fi
OHÆ + OHÆ fi H2O2
OHÆ + OH)fi OÆ)+ H2O
OÆ)+ H2O fi OHÆ + OH)
4.1 · 108MC)1s)1
1.0 · 108MC)1s)1
1.2 · 105s)1
4.9 · 105M)1s)1
7.9 · 108M)1s)1
7.9 · 108M)1s)1
7.9 · 108M)1s)1
5.5 · 109M)1s)1
1.3 · 1010M)1s)1
9.4 · 107 M)1s)1
*MCrefers to moles of carbon per liter.
812Marcela V. Martin et al.
phenols are not detected and the phenoxyl radical is the only
observed transient (36). Thus, reaction 18 depicts the water
loss of the HCHD to yield the corresponding phenoxyl radical
(F in Scheme 1). Radical–radical recombination reactions 19–
21 are also included in the mechanism with rate constants
taken from the work by Bonin et al. (28).
The simulations show that the kinetic profiles of the HCHD
and phenoxyl radicals (see Fig. 4 for the phenoxyl radicals) are
coincident with those of species B and C obtained from the
bilinear analysis, respectively. The absorption spectra of B and
C are also those expected for HCHD (33) and phenoxyl
radicals (18), respectively, and thus were assigned to these
species. The species A is assigned to the intermediate I formed
by reaction of the HOÆ radical with non phenolic reactive
groups (reaction 17 a), which decays to yield product 1
(reaction 17 b).
To validate the proposed reaction mechanism the simulated
concentration profiles of the phenoxyl radical were converted
to absorbance at 410 nm taking the absorption coefficient
e410= 1360 M)1cm)1measured for the phenoxyl radical of
gallate (18). Only the contribution of the phenoxyl radicals
(species C) was considered because the absorption of species A
and B is much lower at this wavelength (see Fig. 3). A good
agreement between simulated and experimental profiles was
found (Fig. 4).
N3Æ radical experiments. The N3Æ radicals were generated in
pulse radiolysis experiments performed with N2O-saturated
0.1 M NaN3solutions of pH = 9 in the presence of 21 mg L)1
The reported absorption ma ´ ximum of the N3Æ radical is
?277 nm (20). At short times after the electron pulse (ca. 2 ls)
the contribution of this radical is observed (see Fig. 5).
The experimental traces at 280 and 410 nm (Fig. 6A,B)
were simulated with the simplified mechanism shown in
Table 3 and reactions 18–21 (Table 2).
The initial concentration of the N3Æ radical (1.08 · 10)5M)
was obtained from the thiocyanate dosimetry experiment. The
mechanism considers the recombination of the azide radicals
(reaction 25 in Table 3), the electron transfer from AHA to the
azide radical (37) followed by a base-catalyzed hydrolysis of
the HCHD radical (see reactions 26 and 27), and reactions 18–
21 (Table 2), already taken into account in the hydroxyl
radical mechanism. Formation of the N6Æ)radical from the
reaction of N3Æ with N3)is irrelevant under our experimental
The rate constants of reactions 18–21, 25 and 27 were taken
from the literature, whereas that of reaction 26 was varied to
optimize the agreement between simulated and experimental
traces. To convert the simulated concentration profiles to
absorbance at 280 nm, the following species and molar
absorption coefficients (e) were considered: azide radical
[1400 M)1cm)1(20)], HCHD radical [1200 M)1cm)1(33)],
(630 MC)1cm)1); where MC refers to moles of carbon per
liter. Despite the complexity of the mechanism, a relatively
good agreement between experimental and simulated traces is
obtained (see Fig. 6A,B).
0 50100 150 200
time / μs
Figure 4. Experimental trace obtained at 410 nm by pulse-radiolysis
of N2O-saturated 21 mg L)1solutions of AHA at pH = 9 (d); kinetic
profile of species C ()). The grey solid line corresponds to the
simulation of the experimental data.
+ H+, -H2O
Scheme 1. Addition of hydroxyl radicals to phenol. D: HCHD radical derived from phenol; E: Radical cation of phenol; F: Phenoxyl radical of
λ / nm
Figure 5. Absorption spectrum of N3Æobtained obtained 2 ls after the
electron pulse from an experiment performed with an N2O-saturated
aqueous 0.1 M solution of NaN3 (pH = 9) containing 21 mg L)1
AHA (d). Literature spectrum of the N3Æradical (s) (20).
Photochemistry and Photobiology, 2012, 88 813
Comparison of the rate constants obtained here to literature data
The sum of k16+ k17obtained here from the simulation of the
experimental traces is 5.1 · 108MC)1s)1, in agreement on a
per carbon basis with data reported for other HS (39–42). It
was reported that for large macromolecules, such as AHA,
HOÆ radical rate constants are typically on the order of
108MC)1s)1. HOÆ radical reaction rate constants with reac-
tive sites at the center of large molecules approach diffusional
rates, even though some sites may be blocked as larger
molecules fold on themselves (39).
The bimolecular rate constant of the reaction of the Br2Æ)
radicals with AHA was found to be lower than that of the BrÆ
atom with the same substrate. A similar trend was observed for
the Cl2Æ-⁄ClÆ radicals (43). From the data obtained so far, it
seems that among the inorganic radicals whose reactivity
108MC)1s)1) is the only radical able to react with a rate
constant similar to that of the HOÆ radical.
(k = 1.8
From the combination of the bilinear analysis with computer
simulations of the traces it was possible to obtain information
on the rate constants for the reactions of Br2Æ), BrÆ, HOÆ and
N3Æ with AHA and on the mechanism involved in the reactions
with the hydroxyl and azide radicals. Evidence for the
participation of phenoxyl radicals, which are expected to
mainly decay by second-order reactions of self-recombination
and disproportionation, is given. These results agree with the
reported reaction preference of strong disinfectants as chlorine
and ozone on phenolic substituents of DOM. Such reactions
involving chlorine and bromine radicals might result in the
formation of trihalomethanes and other chlorinated and
brominated by-products (44).
These results will be useful in understanding the reactivity
between inorganic radicals with humic substances in engi-
neered and natural systems containing inorganic matter.
Acknowledgements—This workwas supportedbyGrant PICT 2007308
from Agencia Nacional de Promocio ´ n Cientı´fica y Tecnolo ´ gica,
(ANPCyT, Argentina). G.T.R., C.D.B. and M.C.G. are research
members ofCONICET(Consejo NacionaldeInvestigaciones Cientı´ficas
y Tecnolo ´ gicas de la Repu ´ blica Argentina). D.O.M. is a research
member of Comisio ´ n de Investigaciones Cientı´ficas de la Provincia de
hosting him in the Radiation Laboratory of Notre Dame University.
M.V.M. thanks CONICET (Consejo Nacional de Investigaciones
Cientı´ficas y Tecnolo ´ gicas de la Repu ´ blica Argentina) for a graduate
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