Reactions between some inorganic radicals and oxychlorides studied by pulse radiolysis and laser photolysis

Article abstract of DOI:10.1039/a700256d

Rate constants for the reactions of free radicals, including e_aq^-, ^.OH, SO₄^.-, NO₃^. and Cl₂^.- (Cl^.), with oxychloride compounds have been determi

Full text of DOI:10.1039/a700256d

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Reactions between some inorganic radicals and oxychlorides studied
by pulse radiolysis and laser photolysis
Zhihua Zuo, Yosuke Katsumura,*¤ Kiyotaka Ueda and Kenkichi Ishigure
Department of Quantum Engineering and Systems Science, Graduate School of Engineering, the
University of T okyo, 7-3-1 Hongo, Bunkyo-ku, T okyo 113, Japan
Rate constants for the reactions of free radicals, including e ~, ~OH, SO ~~, NO ~ and Cl ~~ (Cl~), with oxychloride compounds
aq
4
3
2
have been determined. The free radicals were generated by pulse radiolysis and laser photolysis. Most of the reactions have nearly
di†usion-controlled rate constants ([109 dm3 mol~1 s~1). ClO~ has relatively higher reactivity than its acid form. e ~ reacts
aq
with oxychloride compounds to form their electron adducts which dissociate easily. Electron transfer reactions occur between
~OH, SO ~~, NO ~ and Cl ~~ (Cl~) radicals and the oxychloride ions. The kinetics results obtained here can be used in simulation
4
3
2
work to understand the radiation-induced reactions taking place in groundwater which are relevant to the geological aspects of
radioactive waste storage and disposal.
studied in detail,14 there is little information on the rate con-
1
Introduction
stants for the reactions of SO ~~, NO ~ and Cl ~~ (Cl~) with
4
3
2
Free radicals participate as important intermediates in the
chemical changes that occur in the atmosphere. Since the early
1970s, it has become increasingly evident that manÏs release of
chloroÑuorocarbons and other halogen-containing com-
pounds has an impact on stratospheric ozone. The monoxides
of the halogens, XO~ (ClO~, BrO~, IO~ and possibly FO~) are
central to the chemistry that occurs. Participation of XO~ rad-
icals and related species in tropospheric chemistry has also
become recognized. Recently, the chemistry of halogen oxides
in the troposphere has been reviewed by Wayne et al.1 In
addition, since the review of Graedel and Weschler,2 model-
ling studies have shown that reactions of free radicals, includ-
ing ~OH, SO , NO ~, Cl
may play important roles in the atmospheric oxidation of SO
to sulfuric acid. Atmospheric aqueous phase chemistry, which
comprises a wide variety of inorganic and organic radical
reactions, also has impact on the gas-phase budget of stable
oxidants such as H O and O and on radicals such as ~OH,
the oxychlorides. In our previous paper,15 the electron trans-
fer reactions of SO ~~ and NO ~ with ClO ~ to yield ClO ~
4
3
3
3
radicals were studied by laser photolysis. Here, the rate con-
stants for the reactions of e ~, ~OH, SO ~~, NO ~, and Cl ~~
aq
4
3
2
(Cl~) with chloride ion and oxychloride compounds in aqueous
solutions have been determined systematically by pulse
radiolysis and laser photolysis. Various methods for the gener-
ation of free radicals were utilized, including the direct radi-
ation of concentrated solutions to produce SO ~~, NO ~ and
4
3
Cl ~~ radicals that has been studied previously.16h21 Some
results were compared with the literature data.
2
~~
~~
(Cl~) and CO ~~ in cloud water
4
3
2
3
2
Experimental
2
2.1 Pulse radiolysis and laser photolysis
Pulse radiolysis experiments were performed by using an elec-
tron pulse of 10 ns and 28 MeV from a linear accelerator at
the Nuclear Engineering Research Laboratory, the University
of Tokyo. Details have been given elsewhere.22 The absorbed
dosage was determined to be ca. 10 krad per pulse by using
2
2
3
HO ~ and NO ~.3h6
2
x
Additionally, these inorganic radicals have also been sug-
gested to be of potential importance in the chemistry of irradi-
ated groundwater, relevant to the storage of radioactive
waste.7 Chloride ion is assumed to be one of the predominant
solutes dissolved in groundwater, especially sea water. Re-
10~2 mol dm~3 KSCN solution saturated with N O and a G
2
e(SCN) ~ of 51 000 dm3 mol~1 cm~1 (100 eV)~1 at 475 nm.23
2
cently, in our unpublished work, ClO ~ was detected as one
Laser photolysis experiments were performed with an excimer
3
of the Ðnal products from the c-radiolysis of Cl~ solution. It
laser (LAMBDA PHYSIK LEXTRA 100) which provides 351
nm (XeF) or 248 nm (KrF) light with energy of 50 or 330 mJ
per pulse, respectively, for a duration of 20 ns. This system has
been described in detail, previously.15
can be suggested that ClO ~ ions and ClO ~ radicals are
x
x
involved in the oxidation processes from Cl~ to ClO ~. Thus
3
studies of the kinetics and the stable products formed in the
radiolysis of chloride ion and oxychloride compounds and
their redox reactions with radiation-induced primary radicals,
such as ~OH, e ~ and H~, and the secondary inorganic free
2.2 Chemicals
eq
radicals in aqueous solutions, will help us to understand the
Aqueous solutions of chlorine dioxide (ClO ~) were prepared
radiation-induced reactions taking place in groundwater.
Pulse radiolysis and its complementary tool, laser photo-
lysis can be used for kinetics studies on reactions of free
radicals. There have been some reports of the rate constants of
the reaction of e ~ and ~OH with ClO~ and ClO ~,8h11 and
2
as follows:24 ClO ~ was prepared by reacting 0.1 mol dm~3
2
peroxosulfate with ca. 0.02È0.05 mol dm~3 of chlorite for 4 h
at 60 ¡C. The ClO ~ formed within hours, and was transferred
2
with a stream of argon into a washing bottle containing ice-
aq
2
cooled distilled water and then collected in water in an amber
bottle. The reaction was conducted away from all direct light
~OH with ClO ~.12,13 Although the rate constants for the reac-
2
~~
tions of ~OH, SO
and NO ~ with Cl~ have also been
4
3
and behind an explosion shield. The ClO ~ solution was
2
stored in the dark in a refrigerator. For reactions of ClO ~
2
¤ E-mail: katsu=quest.gen.u-tokyo.ac.jp
with hydroxyl radical or hydrated electron, it was transferred
J. Chem. Soc., Faraday T rans., 1997, 93(10), 1885È1891
1885

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from the deaerated stock solution into deaerated water in a 1
cm cell, by bubbling with N O or argon gas, respectively.
2
Before irradiation, the concentration of ClO ~ was determined
2
spectrophotometrically by using an e-value at 358 nm of 1250
dm3 mol~1 cm~1.24
Sodium hypochlorite solution was prepared by the method
of Taylor and Bostock25 by distilling a mixture of 35 g of
boric acid and 12 g of Ca(ClO) in 600 ml of water. The Ðrst
2
100 ml of distillate was collected in 200 ml containing 1 g
NaOH to give ca. 0.04 mol dm~3 ClO~ solution at pH ca. 12.
Three methods were used to determine the concentration of
ClO~ accurately. The Ðrst was the measurement of I ~ from
3
reaction of KI with ClO~ in the bu†ered solution of
C H COO~(H`), by spectrophotometer at 351 nm taking
6
4
e \ 26 400 dm3 mol~1 cm~1 at pH 5. The second was the
titration of I ~ with S O ~ after the reaction of ClO~ with
Fig. 1 Transient absorbance, observed at 575 nm, in the pulse
radiolysis of aqueous solutions of (a) 0, (b) 0.48 ] 10~4, (c) 1.5 ] 10~4,
3
2 3
KI had been completed and the third was the measurement of
ClO~ by spectrophotometry at 292 nm taking e \ 360 dm3
mol~1 cm~1. The results from the three methods agreed very
well.
Other chemicals were guaranteed reagents, solutions were
prepared with Millipore water and the measurements were
carried out at room temperature (ca. 20 ¡C).
(d) 2.9 ] 10~4 and (e) 5.6 ] 10~4 mol dm~3 ClO ~, and 10~4 mol
2
dm~3 of NaOH (pH 10), saturated with argon gas. Insert: pseudo-
Ðrst-order decay rate constants of e ~ at 575 nm vs. concentrations of
aq
ClO ~.
2
literature data, are listed in Table 2. Although there has been
no previous report of the rate constant for the reaction of
ClO ~ with e ~ , ClO ~ can be suggested to react with e ~
2.3 Kinetic analyses
2
aq
2
aq
rapidly because of its high reduction potential of 0.935 V vs.
Ideal conditions were chosen such that only one primary
radical was present and the concentration of the reactive
solute was high enough to ensure that pseudo-Ðrst-order
kinetics were applicable. The absolute rate constants were
obtained by direct observation of the decay of the transient or
the growth of its product, whichever had a suitable absorption
spectrum. When neither the primary radical nor the reaction
product could be observed directly, the relative rate constants
were deduced from measurements of product yields using the
competition method. A simulation method was used for
kinetic analysis of the complex reactions and the overlapping
absorbances of the transient species with the program FAC-
SIMILE (AEA Technology, Harwell).
NHE.12,29,30 Here, the rate constant for reactions of ClO ~
2
with e ~ was determined to be (2.1 ^ 0.1) ] 1010 dm3 mol~1
aq
s~1.
Table 1 Reduction potentials of inorganic couples (mV vs. NHE)
compound or couple X
aq/e
E(X/X~)
[2900
pH
ref.
~
27
28
37
38
12
50
51
38
38
52
53
29
30
12
54
15
aq
~OH/OH
1700
1890
2700
2430
2450
2410
2090
2200
1500È1800
934
neutral
neutral
acid
If both reactants were charged, the rate constants were cor-
rected for the primary kinetic salt e†ect by using the
BrÔnstedÈBjerrum equation:
SO ~~/SO 2~
4
4
NO ~/NO ~
3
3
Cl~/Cl~
log(k/k ) \ 1.02Z Z I1@2(1 ] I1@2)~1
(I)
Cl ~~/2Cl~
0
a b
2
1
where k and k are the rate constants at ionic strength I and
zero, Z and Z are the charges of reactants, respectively. Note
that, in the case of high ionic strength I, the corrected values
obtained by this equation were just rough estimations.
ClO~/ClO~
0
b
ClO ~/ClO ~
4È6
4È6
a
2
2
936
934
1277
2350
ClO ~, H`/HClO
0
2
2
ClO ~/ClO ~
3
3
3
Results and Discussion
3.1 Reactions with e —
aq
Table 2 Rate constants for reactions of oxychlorides with e ~ (109
aq
For reactions of oxychlorides with e ~, solutions were
dm3 mol~1 s~1)
aq
bubbled with argon before irradiation. Since the pK value of
a
reaction
pH
rate constant
comments
ref.
HClO is 7.52,26 the pH values of HClO solutions for irradia-
tion were adjusted to 5 with appropriate amounts of HClO
Cl~ ] e ~ ] products
\10~3
0.65 ^ 0.04
11 ^ 1
31
4
aq
HClO ] e ~ ] Cl~ ] ~OH
5
12.7
this work
this work
in which 99% of ClO~ existed in its acid form. For reactions
of other oxychlorides, the pH values of the solutions were
adjusted above 10 with NaOH in order to avoid e ~ decay-
ing rapidly with other substrates such as H`. The rate con-
aq
ClO~ ] e~ ] Cl~ ] O~
I \ 0.05
as
7.5 ^ 0.1
I ] 0, from k
at I \ 0.05
I \ 0.001
11
11
8.3 ^ 0.4
7.7 ^ 0.4
this work
aq
I ] 0, from k
at I \ 0.001
I ] 0
stants were determined from the decay kinetics of e ~ at 575,
7
53
8
9
aq
600 or 700 nm. For example, Fig. 1 shows the decay of e ~
aq
7
I ] 0
10
observed at 575 nm from the radiolysis of solutions of di†er-
ClO ~ ] e ~ ] products
10
11
21 ^ 1
4.4 ^ 0.3
4.0 ^ 0.3
this work
this work
2
aq
ent concentrations of ClO ~.
ClO ~ ] e ~ ] ClO~ ] O~
I \ 0.002
I ] 0, from k
at I \ 0.002
2
2
aq
As expected from its standard reduction potential of [2.9
V (as shown in Table 1),27,28 e ~ reacts rapidly with many
2.5
45
11
9
aq
species having more positive reduction potentials. Reactions
ClO ~ ] e
~
\10~3
9
3
aq
of oxychlorides with e ~ and the rate constants, including the
aq
1886
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
those determined by Buxton in 198710 and Anbar and Hart in
1968.8
There is no literature data for the reaction of HClO with
e ~. In this work, the reaction was attempted at pH 5. Since
aq
the decay of e ~ increased as the concentration of HClO
aq
increased, HClO was observed to react with e ~. The product
aq
of the reaction was suggested to be Cl~ and ~OH radical,
according to the reaction of ClO~ with e ~. However, the
aq
rate constant is one order less than that for the reaction of
ClO~ with e ~.
aq
Reaction of ClO ~ with e ~ to form ClO~ and O~ has
2
aq
been studied previously9,11. Here, the rate was determined to
be (4.0 ^ 0.3) ] 109 dm3 mol~1 s~1 at I ] 0, which is consis-
tent with that determined by Erikson et al.11 but one order of
magnitude less than that determined by Buxton and Subhani.9
The results reported here and by Erikson seem to be more
accurate than that reported earlier.
Fig. 2 Decay of ClO ~ observed at 400 nm after the pulse radiolysis
2
of solutions of (a) 3.8 ] 10~4 and (b) 5.6 ] 10~4 mol dm~3 ClO ~,
2
saturated with N O gas. The solid lines are the FACSIMILE curves.
2
Reactions of Cl~, ClO ~, ClO ~ with e ~ were too slow
3
4
aq
(\106 dm3 mol~1 s~1) to observe by pulse radiolysis.9,31
In general, the mode of reaction of oxychlorides with e ~
can be represented as the formation of an electron adduct fol-
aq
ClO~ reacts with e ~ to form Cl~ and O~.9 Although the
aq
rate constant was determined to be 5.3 ] 1010 dm3 mol~1 s~1
lowed by its dissociation:
in 1972 by Buxton and Subhani,9 another datum of 7.3 ] 109
dm3 mol~1 s~1 was reported by the authors.10 Before the
work of Buxton et al., Anbar and Hart had reported a rate
constant of 7.0 ] 109 dm3 mol~1 s~1.8 Here, we determined
the rate constant to be (1.1 ^ 0.1) ] 1010 dm3 mol~1 s~1 at
I \ 0.05, or (8.3 ^ 0.4) ] 109 dm3 mol~1 s~1 at I \ 0.001.
The corrected rate constants for I ] 0 were consistent with
e ~ ] ClOn ] ClOn~1 ] ClOn ] O~
(I)
aq
x
x
x~1
where n is the charge on the ion. Thus, reaction of ClO ~ with
2
e ~ can be suggested to form the hypochlorite radical (ClO~)
aq
and O~. The ClO~ radical then reacts with another ClO~ in
water to form ClO ~ and ClO~. This will be conÐrmed by
2
identiÐcation of the Ðnal products resulting from the c-
radiolysis of ClO ~ solution, in future work.
2
3.2 Reactions with ~OH radical
Since the ~OH radical has an absorption band below 300 nm,
reactions of the radical are usually observed from another
reactant or from the products. Reaction of ~OH with ClO ~
2
has been observed from the decay kinetics of ClO ~ in its irra-
2
diated solution of appropriate concentration.12 Fig. 2 shows
the fading decay of ClO ~ at 400 nm after the radiolysis of
2
N O-saturated solutions of 3.8 and 5.5 ] 10~4 mol dm~3 of
2
ClO ~. The rate constant determined here, (3.5 ^ 0.5) ] 109
2
dm3 mol~1 s~1, was consistent with the value of 4.0 ] 109
dm3 mol~1 s~1 determined by Klaning and Sehested.12
ClO ~ was also observed as the product from reaction of
2
ClO ~ with ~OH by pulse radiolysis.11 Here the ~OH radical
was produced by the KrF laser photolysis of H O solution,
2
2
2
Fig. 3 Transient absorption spectra of ClO ~ produced from the
because of the high quantum yield (U \ 1 for j [ 295 nm and
2
laser photolysis of solutions of 1 ] 10~2 mol dm~3 H O and
1.8 for j B 250 nm),32h34 and the slow reaction of H O with
2
2
5 ] 10~4 mol dm~3 ClO ~ at 2 ls after the pulse. Insert: build-up of
2 2
2
ClO ~ (less than 1% of ClO ~ was destroyed after 2 h9) that
ClO ~, observed at 358 nm.
2
2
2
can be ignored. About 2 ] 10~5 mol dm~3 of ~OH was pro-
duced from the photolysis of a solution of 1 ] 10~2 mol
dm~3 H O . Fig. 3 shows the absorption spectra of ClO ~
2
2
2
produced from the photolysis of solutions of H O in the
2
2
2
presence of 5 ] 10~4 mol dm~3 of ClO ~. The ClO ~ formed
was very stable and could be observed over a few ms, during
the Ñash time of the shutter. From the build-up kinetics of
2
ClO ~ at 358 nm the rate constant was determined to be
2
(7.9 ^ 0.4) ] 109 dm3 mol~1 s~1, which is consistent with that
determined by Erikson et al.11 and Buxton and Subhani.9
Since H O reacts with ClO~,9,35 pulse radiolysis was used
2
2
to produce ~OH radical for its reaction with ClO~ and HClO.
These reactions led to the formation of ClO~ radical which has
a weak absorption band below 300 nm.9 Thus the rate con-
stant of ClO~ with ~OH was determined by the carbonate
competition method.9 The sample solutions for irradiation
included 5 ] 10~3 mol dm~3 NaHCO , 5 ] 10~3 mol dm~3
3
Na CO and di†erent concentrations of NaClO under N O-
Fig. 4 Transient absorbance, observed at 600 nm, from the pulse
2
3
2
saturation conditions. The pK values of H CO and HCO ~
are 6.4 and 10.3, respectively; the pH value of the solution was
ca. 10.26 and CO 2~ and HCO ~ existed in the ratio 48 : 52.
radiolysis of solutions of 5 ] 10~3 mol dm~3 NaHCO , 5 ] 10~3
2
3
3
3
mol dm~3 Na CO and (a) 0, (b) 1 ] 10~3, (c) 2 ] 10~3 and (d)
2
3
4 ] 10~3 mol dm~3 of NaClO, saturated with N O
2
3
3
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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Table 3 Rate constants for reactions of oxychlorides with ~OH radical (109 dm3 mol~1 s~1)
reaction
pH
2
rate constant
comments
ref.
Cl~ ] ~OH ] ClOH~~
4.3
3.0
decay at 240 nm and build-up at 340 nm, PR
build-up at 340 nm, PR
41
42
HClO ] ~OH ] ClO~ ] H O
0
0.14 ^ 0.01
0.11 ^ 0.02
2.7 ^ 0.1
8.8
related to k(~OH ] HSO ~), PR
this work
this work
this work
2
4
\0
related to k(~OH ] HNO ), PR
3
ClO~ ] ~OH ] ClO~ ] OH~
ClO ] ~OH ] HClO ] O
10.3
related to k(~OH ] CO 2~), PR
3
11
7
7
7
7
related to k(~OH ] CO 2~), PR
9
13
13
3
1.4
2.6
4.0
2
2
ClO ] ~OH ] ClO ~ ] H`
2
3
total rate, PR
total rate, PR
12
this work
this work
9
3.5 ^ 0.5
7.9 ^ 0.4
6.1
ClO ~ ] ~OH ] ClO ~ ] OH~
build-up of ClO ~, FP
2
2
2
11
10
related to k(~OH ] CO 2~), PR
3
7.0
build-up of ClO ~, PR
11
2
PR: pulse radiolysis; FP: (laser) Ñash photolysis.
~OH reacts with HCO ~ and CO 2~ to form the CO ~~
reacts with HNO with a rate constant of 1.4 ] 108 dm3
3
3
3
3
radical anion with rate constants of 8.5 ] 106 and 3.9 ] 108
dm3 mol~1 s~1, respectively.36 Thus the total rate was
mol~1 s~1.16 The solutions for irradiation contained 3 mol
dm~3 of HClO , 0.015 mol dm~3 of HNO and di†erent con-
4
3
1.9 ] 106 s~1. Fig. 4 shows the absorption of CO ~~ observed
centrations of HClO. The rate constant for the reaction of
~OH with HClO was determined from the absorbance changes
3
at 600 nm in the presence of di†erent concentrations of ClO~.
From the changes in the yield of CO ~~, the rate constant was
of the NO ~ radical at 635 nm. The rate constant was deter-
3
3
determined to be (2.7 ^ 0.1) ] 109 dm3 mol~1 s~1, which is
mined to be (1.4 ^ 0.1) ] 108 or (1.1 ^ 0.2) ] 108 dm3 mol~1
smaller than that determined by Buxton and Subhani9 using
the same method.
s~1 from the HSO ~ or HNO competition method, respec-
4
3
tively. The two results are consistent. As can be seen, HClO
reacts with ~OH with a rate of one order less than that of
ClO~ with ~OH.
The rate constant for the reaction of ~OH with HClO was
also determined by the competition method. Since the pK
values of HClO and H CO are 7.52 and 6.4, respectively, at
Reactions of oxychlorides with ~OH and the rate constants
are listed in Table 3. The ~OH radical is a powerful oxidant,
having a standard reduction potential of 2.7 V in acidic solu-
tion and 1.8 V in neutral solution, as summarized in Table
1.12,37,38 The reaction of ~OH with oxychloride ions can be
thought of as a simple electron transfer,
2
3
pH 5 HClO exists stably, although H CO is unstable
2
3
because of its easy formation of CO especially under N O
bubbling conditions. Thus, instead of the carbonate ion,
2
2
HSO ~ was selected as the reactant for the competition. ~OH
4
reacts with HSO ~ with a rate constant of 4.7 ] 105 dm3
4
mol~1 s~1,17 to form the SO ~~ radical anion, which has a
peak absorption at 450 nm. The sample solution for irradia-
4
~OH ] ClOn ] ClOn~1 ] OH~
(II)
x
x
tion included 1 mol dm~3 of H SO and concentrations of
2
4
NaClO varied between 0 and 10 ] 10~3 mol dm~3. The rate
where n is the charge on the ion, although Erikson et al. sug-
constant was determined from the changes in the maximum
gested that, in the case of reaction of ~OH with ClO ~, both
2
absorption of the SO ~~ radical anion at 450 nm in the pres-
electron transfer and adduct formation occurs. The adduct
4
ence of di†erent concentrations of HClO. In addition, HNO
was probably a peroxide (HOClO ~~), which can react with
3
2
was used as a competitor instead of HSO ~. The ~OH radical
excess ClO ~ to form ClO ~.11
4
2
2
Table 4 Rate constants for reactions of oxychlorides with SO ~~ radical (109 dm3 mol~1 s~1)
4
reaction
pH
rate constant, k
comments
ref.
Cl~ ] SO ~~ ] Cl~ ] SO 2~
0.36 ^ 0.01
0.19 ^ 0.01
0.01
I \ 1.5, decay 500 nm, FP
I ] 0, from k at I \ 1.5
this work
4
4
2È3
build-up at 340 nm, FP
55
56
6.2
0.47
0.27
0.25
0.27
0.26
0.2
0.13
0.31
I \ 0.1, decay at 500 nm, FP
I ] 0, from k at I \ 0.1
I ] 0, decay at 500 nm, FP
I ] 0, decay at 480 nm, FP
I ] 0, decay at 510 nm, FP
I \ 0.2, decay at 480 nm, PR
57
44
58
59
18
43
1.5
1.4
I \ 0.056, build-up of Cl ~, PR
2
6.8
0
I \ 0.007, decay at 480 nm, PR
0.26
I ] 0, from k at I \ 0.007
PR of 1 mol dm~3 H SO
HClO ] SO ~~ ] ClO~ ] HSO ~
0.011 ^ 0.001
6.6 ^ 0.5
1.3 ^ 0.1
2.6 ^ 0.1
1.9 ^ 0.1
2.4 ^ 0.1
1.3 ^ 0.1
3.4 ^ 0.3
this work
this work
4
4
2
4
ClO~ ] SO ~~ ] ClO~ ] SO 2~
I \ 6, PR of 2 mol dm~3 Li SO
4
4
2
4
I ] 0, from k at I \ 6
ClO ~ ] SO ~~ ] ClO ~ ] SO 2~
I \ 0.03, PR
this work
this work
this work
2
4
2
4
I ] 0, from k at I \ 0.03
I \ 0.15, FP
I ] 0, from k at I \ 0.15
I \ 1.6, related to
k(SO ~~ ] NO ), FP
3.1
3.3
4
3
0.92 ^ 0.08
0.008 ^ 0.001
0.004 ^ 0.0005
I ] 0, from k at I \ 1.6
I \ 0.6, FP
ClO ~ ] SO ~~ ] ClO ~ ] SO 2~
15
3
4
3
4
I ] 0, from k at I \ 0.6
FP: (laser) Ñash photolysis; PR: pulse radiolysis.
1888 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Table 5 Rate constants for reactions of oxychlorides with NO ~ radical (109 dm3 mol~1 s~1)
3
reaction
pH
rate constant, k
comments
ref.
Cl~ ] NO ~ ] Cl~ ] NO ~
0
(9.1 ^ 0.3) ] 10~3
9.3 ] 103
decay at 635 nm, FP
decay at 633 nm
this work
47
3
3
8.3È9.0
~~
conversion from SO , FP
4
0.071
0.1
build-up at 345 nm, decay at
60
18
640 nm, PR of 5 mol dm~3 NaNO
3
build-up at 345 nm,
PR of 2 mol dm~3 NO ~
3
HClO ] NO ~ ] ClO~ ] HNO
\0
0.043 ^ 0.001
3.6 ^ 0.2
decay at 635 nm,
this work
this work
this work
15
3
3
PR of 0.015 mol dm~3 HNO
3
ClO~ ] NO ~ ] ClO~ ] NO ~
decay at 635 nm
3
3
PR of 5 mol dm~3 NaNO
3
ClO ~ ] NO ~ ] ClO ~ ] NO ~
3.1
0
4.1 ^ 0.3
decay at 633 nm,
2
3
2
3
conversion from SO ~~, FP
4
ClO ~ ] NO ~ ] ClO ~ ] NO ~
(1.0 ^ 0.1) ] 10~5
decay at 633 nm, FP
3
3
3
3
FP: (laser) Ñash photolysis; PR: pulse radiolysis.
Such a simple electron transfer step is unlikely in the oxida-
tion of halide and pseudo-halide ions, because of the large
solvent reorganization energy involved in forming the
hydrated hydroxide ion. Instead, it is suggested39,40 that an
intermediate adduct is formed.
from the tail of the Cl ~~ radical anion. The results,
2
(3.6 ^ 0.1) ] 108 dm3 mol~1 s~1 at I \ 0.15 and
(1.9 ^ 0.1) ] 108 dm3 mol~1 s~1, corrected at I ] 0, were
consistent with most of the previously reported data.
Since S O 2~ was observed to react with HClO and ClO~,
2
8
for its reaction with HClO, SO ~~ was produced by reaction
4
~OH ] X~ ] HOX~~
(III)
of ~OH with 1 mol dm~3 of H SO ; and for its reaction with
2
4
ClO~, it was produced from the direct action of radiation in 2
mol dm~3 of Li SO , which has a G(wSO 2~) value of 3.0,
When X \ Cl, the rate constant was determined to be
4.3 ] 109 dm3 mol~1 s~1,41 or 3.0 ] 109 dm3 mol~1 s~1.42 In
acidic conditions, HOX~~ reacts with H` and the halide atom
leading to formation of the dihalide radical anion. Reaction of
2
4
4
determined previously in our laboratory.17 G(SO ~~) is related
4
to G(wSO 2~) and the electron fraction of the solute in solu-
4
tion. Rate constants were determined from the decay kinetics
~OH with ClO ~ is also complicated. Disproportionation of
2
of SO ~~ at 450 nm. SO ~~ reacts rapidly with ClO~
ClO ~ leads to the formation of HClO and ClO ~.12,13
4
4
2
3
(1.3 ] 109 dm3 mol~1 s~1 at I \ 0) but relative slowly with
HClO (1.1 ] 107 dm3 mol~1 s~1).
3.3 Reactions with SO ~—, NO ~ and Cl ~— (Cl~) radical
4
3
2
For reaction with ClO ~, SO ~~ was generated from the
2
4
photolysis of S O 2~ or pulse radiolysis via its reaction with
3.3.1 Reactions with SO ~—. The reactions of SO ~~ and
2
2
8
4
4
e ~ . The ClO ~ radical was observed, from the reaction of
aq
their rate constants are listed in Table 4. There have been
many reports of the rate constant for the reaction between
SO ~~ and Cl~ to yield Cl~ and then the Cl ~~ radical anion.
SO ~~ with ClO ~. The rate constant of the reaction was
4
2
determined from the decay kinetics at 500 nm; values of
1.3 ] 109 and 1.9 ] 109 dm3 mol~1 s~1 were deduced from
photolysis or radiolysis, respectively.
4
2
SO ~~ and Cl ~~ radical anions have molar absorption coeffi-
4
2
cients of 1600 dm3 mol~1 cm~1 at 450 nm17,43,44 and 8800
dm3 mol~1 cm~1 at 340 nm,41 respectively. In this work,
In our previous paper15 reaction of SO ~~ with ClO ~ to
4
3
form ClO ~ radical, was observed by laser photolysis. The rate
SO ~~ was produced from the KrF laser photolysis of per-
3
4
constant was determined to be (4.0 ^ 0.5) ] 106 dm3 mol~1
oxodisulfate ion (S O 2~). Both the build-up of Cl ~~ and the
2
8
2
~~
s~1 from simulation.
decay of SO were observed in the presence of Cl~. The rate
4
constant was determined from the decay kinetics of SO ~~ at
4
500 nm, to avoid any contribution to the absorption decay
3.3.2 Reactions with NO ~. The reactions of the NO ~
3
3
radical and their rate constants are listed in Table 5. For reac-
tion with Cl~, the NO ~ radical was produced conveniently by
3
the XeF laser photolysis of the complex of cerium(IV) and
nitrate in the presence of nitric acid.45,46 The rate constant
was determined from the decay kinetics of the radical at
around 635 nm. There has been considerable disagreement
about the rate constant for this reaction. It has been
suggested47 that this di†erence might be due to a secondary
ionic strength e†ect which is known to inÑuence the reaction
rate of ionÈdipole reactions.
Since HClO and ClO~ were observed to react with
cerium(IV) ions, NO ~ radical was produced by pulse
3
radiolysis via reaction of ~OH with nitric acid for its reaction
with HClO, and by the direct radiation of concentrated solu-
tion of NaNO for its reaction with ClO~. For radiation of
3
nitrate, G(wNO ~) \ 4.8 has been determined previously in
3
our laboratory.16 Like SO ~~, the NO ~ radical reacts rapidly
4
3
with ClO~ (3.6 ] 109 dm3 mol~1 s~1) but relatively slowly
with HClO (4.3 ] 107 dm3 mol~1 s~1).
Fig. 5 Transient absorbance of NO ~, observed at 635 nm, from the
3
laser photolysis of solutions of 0.2 mol dm~3 (NH )S O , 1 mol
4
2 8
For reaction of ClO ~, because of its instability in strong
acid solution (the pK value of HClO is ca. 2.5), NO ~ was
produced by reaction of SO ~~ with 1 mol dm~3 of NO ~.
dm~3 NaNO and (a) 0, (b) 1.1 ] 10~5, (c) 2.3 ] 10~5 and (d)
2
3
4.6 ] 10~5 mol dm~3 NaClO . The solid lines are the FACSIMILE
2
3
2
curves.
4
3
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1889

View Article Online
Fig. 5 shows the absorbance of NO ~ at 635 nm in the pres-
Klaning and Wol†48 have reported the rate constants for
the reactions of Cl~ with HClO and ClO~. The radical was
produced by the photolysis of solutions of HClO and ClO~;
however, a detailed description of the calculation of the rate
constants was not given in that paper. It can be seen, from
Table 6, that Cl~ reacts with HCl and ClO~ more rapidly than
3
ence of di†erent concentrations of ClO ~. As can be seen, the
2
decay of NO ~ increased, but its maximum absorbance
3
decreased, as the concentration of ClO ~ increased. It was
2
concluded that reaction of SO ~~ with NO ~ and ClO ~, and
4
3
2
reaction of NO ~ with ClO ~ had occurred simultaneously.
3
2
From the decay kinetics at 635 nm, the rate constant of the
Cl ~~ owing to the relative higher reduction potential of the
2
reaction of NO ~ with ClO ~ was determined to be
former radical. Recently, a new value of K \ 4.7 ] 103 dm3
3
2
(4.1 ^ 0.3) ] 109 dm3 mol~1 s~1. The rate constant of SO ~~
mol~1 for Cl ~~ was reported by Adams et al.,49 which is 40
4
with ClO ~ was deduced to be (3.4 ^ 0.3) ] 109 dm3 mol~1
2
2
times lower than the generally accepted value. This low value
s~1 at I \ 1.6 from simulation in which the rate constant of
of K makes it possible to measure the rates of reaction of Cl~
using pulse radiolysis.
SO ~~ with NO ~ was set to be 2.0 ] 105 dm3 mol~1 s~1 at
4
3
I \ 1.6, deduced from the value of 5.0 ] 104 dm3 mol~1 s~1
at I ] 0 by eqn. (1).47
In general, free radicals R~, including ~OH, SO ~~, NO ~
4
3
and Cl ~~ (Cl~), react with oxychloride ions via an electron-
2
In the previous paper,15 the equilibrium reaction of NO ~
transfer mechanism, leading to the formation of oxychloride
3
with ClO ~ was observed by laser photolysis. From the rate
3
radicals,
constants determined for the forward and reverse reaction, the
R~ ] ClO ~ ] R~ ] ClO ~
(IV)
reduction potential of ClO ~ radical was measured to be 2.35
x
x
3
V vs. NHE.
where x \ 0, 1, 2, or 3. As can be seen from Table 1, since *E
between R~ and ClO ~ (x \ 1, 2) is more positive than 0.4 V,
x
the electron transfer will be complete. Furthermore, these
3.3.3 Reactions with Cl (Cl~). The reactions of Cl ~~ (Cl~)
2
2
reactions occur with nearly di†usion-controlled rate constants
([109 dm3 mol~1 s~1).
and their constants are listed in Table 6. Cl ~~ is formed from
2
reaction of Cl~ with Cl~ with
a
stability constant of
1.9 ] 105 dm3 mol~1.41 Such a high value implies that Cl~ are
unlikely to be chemically signiÐcant, except in very dilute
solutions of chloride ion in water. Thus only a few rate con-
stants for the reactions of Cl~ in water have been obtained,
compared with the wealth of data that is available on the
4
Conclusion
Free radicals, including e ~, ~OH, SO ~~, NO ~ and Cl ~~
aq
4
3
2
(Cl~) were generated by various pulse radiolysis and laser pho-
tolysis methods in order to determine the rate constants for
their reactions with oxychloride compounds. Most of the reac-
tions have nearly di†usion-controlled rate constants ([109
dm3 mol~1 s~1). ClO~ has relatively higher reactivity than its
reactions of Cl ~~. Cl~ can be produced through the formation
2
of an adduct of ~OH with Cl~ followed by reaction of the
adduct (HOCl~~) with H` in acidic conditions. Since, in acid
solution, HClO reacts with HCl to reform Cl , for its reaction
acid form, HClO. e ~ reacts with the oxychlorides to form
their electron adducts which dissociate easily. Interconversion
reactions occur between ~OH, SO ~~, NO ~, and Cl ~~ (Cl~)
radicals and the oxychloride ions, via an electron-transfer
mechanism. The kinetics results obtained here will be used in
our future simulation work to understand the radiation-
induced reactions taking place in groundwater, which is rele-
vant to geological studies on radioactive waste disposal. These
reactions may also be suggested to play potential roles in the
chemistry of the atmospheric aqueous phase.
2
with ClO~, Cl ~~ was produced by direct radiation in concen-
2
aq
trated solutions of LiCl. Many investigations have been
4
3
2
carried out on the production of dihalide radical anions in the
radiolysis of concentrated solutions of halide ions.18h21 From
the decay at 340 nm, the rate constant of Cl ~~ with ClO~
2
was determined to be (5.4 ^ 0.3) ] 108 dm3 mol~1 s~1 at
I \ 5.
The same method was used to study the reaction of Cl ~~
2
with ClO ~. In addition, the reaction was also observed using
2
KrF photolysis to generate Cl~ and then Cl ~~, by reaction of
SO ~~ with Cl~. Absorption due to ClO ~ was observed after
the reaction was completed. The rate constant for the reaction
was determined to be (1.3 ^ 0.1) ] 109 dm3 mol~1 s~1 at
I \ 5, from pulse radiolysis, or (3.5 ^ 0.2) ] 108 dm3 mol~1
s~1 at I \ 0.25, from laser photolysis. Their corrected values
at I ] 0 are comparable.
2
We are grateful to Mr. N. Chitose, Mr. K. Dohi, Mr. D.
Hiroishi and Mr. C. Matsuura for their assistance with experi-
ments. This work is performed as part of a study on radiation-
induced reactions in groundwater for the performance
assessment of the high-level radioactive waste disposal system
at the Power Reactor and Nuclear Fuel Development Corpor-
4
2
Table 6 Rate constants for reactions of oxychlorides with Cl ~~ (Cl~) radical (109 dm3 mol~1 s~1)
2
reaction
pH
rate constant, k
comments
ref.
Cl~ ] Cl~ H Cl ~~
6.5
8
build-up at 360 nm, FP of ClO~
48
61
2
3.5
2
build-up of Cl ~~,
2
conversion from SO ~~, FP
4
21
K \ 1.9 ] 105 dm3 mol~1
41
Ðtting at 340 nm, PR
K \ 4.7 ] 103 dm3 mol~1
no detail, FP of HClO
no detail, FP of ClO~
I \ 5, decay at 340 nm
PR of 5 mol dm~3 LiCl
I ] 0, from k at I \ 5
I \ 5, decay at 340 nm,
PR of 5 mol dm~3 LiCl
I ] 0, from k at I \ 5
I \ 0.25, decay at 340 nm,
49
48
48
HClO ] Cl~ ] ClO~ ] HCl
ClO~ ] Cl~ ] ClO~ ] Cl~
3
8.2
0.54 ^ 0.03
ClO~ ] Cl ~~ ] ClO~ ] 2Cl~
this work
2
0.11 ^ 0.01
1.3 ^ 0.1
this work
this work
ClO ~ ] Cl ~~ ] ClO ~ ] 2Cl~
2
2
2
0.25 ^ 0.02
0.35 ^ 0.02
this work
this work
conversion from SO ~~, FP
4
0.16 ^ 0.01
I ] 0, from k at I \ 0.25
this work
FP: (laser) Ñash photolysis; PR: pulse radioysis.
1890 J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1891