Experimental Study and Kinetic Modeling of Chloroform Decomposition in Aqueous Solutions under the Action of γ-Radiation

Article abstract of DOI:10.1134/S0036024419010126

Abstract: The radiolysis of aqueous chloroform solutions (10^–3–10^–2 M) in the presence of dissolved oxygen is studied to establish a chain mode for the decomposition of chloroform. CO₂, H₂, H₂O₂ and chlorinated hydrocarbons-dichloromethane and dichloroethane are identified as products of the radiolysis of aqueous chloroform solutions. Changes in the pH and COD from the absorbed dose for the radiolysis of an aqueous chloroform solution with a concentration of 4.2 × 10^?2 M are studied. It is found that as the dose is increased, the pH index and COD fall, indicating the decomposition of organic compounds and the formation of acids. It is shown that the radiation-chemical yields of chloroform decomposition depend on its initial concentration. At ≥4.2 × 10^?2 M, they lie in the range of 52–245 molec./100 eV. A formal kinetic scheme of the processes that occur during the γ-radiolysis of aqueous chloroform solutions is compiled using the obtained data and allowing for the available literature values of the rate constants of elementary reactions.

Full text of DOI:10.1134/S0036024419010126

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 1, pp. 102–105. © Pleiades Publishing, Ltd., 2019.
Russian Text © Z.I. Iskenderova, M.A. Kurbanov, 2019, published in Zhurnal Fizicheskoi Khimii, 2019, Vol. 93, No. 1, pp. 95–98.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Experimental Study and Kinetic Modeling of Chloroform
Decomposition in Aqueous Solutions under the Action of γ-Radiation
Z. I. Iskenderovaa,* and M. A. Kurbanova
aInstitute of Radiation Problems, Azerbaijan National Academy of Sciences, Baku, AZ1143 Azerbaijan
*е-mail: zenfira_iskenderova@mail.ru
Received March 30, 2018
Abstract—The radiolysis of aqueous chloroform solutions (10–3–10–2 M) in the presence of dissolved oxygen
is studied to establish a chain mode for the decomposition of chloroform. CO2, H2, H2O2 and chlorinated
hydrocarbons-dichloromethane and dichloroethane are identified as products of the radiolysis of aqueous
chloroform solutions. Changes in the pH and COD from the absorbed dose for the radiolysis of an aqueous
chloroform solution with a concentration of 4.2 × 10−2 M are studied. It is found that as the dose is increased,
the pH index and COD fall, indicating the decomposition of organic compounds and the formation of acids.
It is shown that the radiation-chemical yields of chloroform decomposition depend on its initial concentra-
tion. At ≥4.2 × 10−2 M, they lie in the range of 52–245 molec./100 eV. A formal kinetic scheme of the pro-
cesses that occur during the γ-radiolysis of aqueous chloroform solutions is compiled using the obtained data
and allowing for the available literature values of the rate constants of elementary reactions.
Keywords: chloroform, ionizing radiation, radiation-chemical yields, chain decomposition
DOI: 10.1134/S0036024419010126
INTRODUCTION
The pH was measured with a pH meter. Chemical
Oxygen Demand (COD) was measured via the per-
manganate oxidation, hydrocarbons and CO2 were
measured by chromatography. H2O2 was measured via
titration.
The use of chlorine for disinfecting drinking water
results in the formation of toxic chlorine-containing
hydrocarbon impurities, mainly chloroform. Various
approaches are currently used to remove chloroform
from aqueous media, including such physical means
as UV light, UV light in combination with ozone [1,
2], ultrasonic irradiation, and so on. The use of ioniz-
ing radiation to purify water of chloroform is a prom-
ising technique in which the decomposition of chloro-
form is initiated by primary particles of the radiolysis
of water: H atoms, OH radicals, and ehydr [3–5].
RESULTS AND DISCUSSION
CO2, H2, H2O2, and chlorinated hydrocarbons–
dichloromethane and dichloroethane were identified
as products of the radiolysis of aqueous chloroform
solutions.
Table 1 shows the data on the change in pH and
COD from an absorbed dose for the radiolysis of an
aqueous chloroform solution with a concentration of
4.2 × 10−2 M.
In this work, the radiolysis of aqueous solutions of
chloroform at concentrations of 10−3–10−2 M and in
the presence of soluble oxygen was studied to establish
the chain mode of chloroform decomposition. The
main prerequisite for this is the possibility of the chain
As can be seen, the pH and COD fell as the dose
decomposition of chloroform in the presence of oxy- grew, testifying to the decomposition of organic com-
gen, including reactions CCl3 + O2 → O2CCl3, pounds and the formation of acids. Using the kinetic
curve, changes in the concentration of CO2, H2, H2O2
O2CCl3 + CHCl3 → CCl3OOH + CCl3 [6].
EXPERIMENTAL
Table 1. Changes in pH and COD from the absorbed dose
for the radiolysis of an aqueous solution of chloroform at a
Aqueous solutions of chloroform (0.03, 0.5, 0.7,
and 1.0%) were irradiated with γ-radiation from a 60Co
isotope under static conditions in glass ampoules at
room temperature. The samples were irradiated in the
absorbed dose range of 4.3–43 kGy and a dose rate of
0.6 Gy/s.
concentration of 4.2 × 10−2
M
D, kGy
0
4.3
4.6
153
8.6
3.9
120
13
3.5
105
43
рН
COD, mgO/L
7
2.3
53
170
102

EXPERIMENTAL STUDY AND KINETIC MODELING
Table 2. Initial parameters of calculation
103
О2CCl3 + 2Н2О → НО2 + СО2 + 3НСl,
О2CНCl2 + Н2О → ОН + СО2 + 2НСl.
J, Gy/s
0.6
GOH, molec./100 eV
GH, molec./100 eV
Ge, molec./100 eV
N, М
N(О2), М
T, K
2.7
0.6
The initial parameters of the calculations are given
in Table 2. The formal kinetic scheme is shown in
Table 3. The kinetics of formation of both active parti-
cles and the molecular products of radiolysis were cal-
culated (Table 4).
2.6
0.25 × 10–2–8.3 × 10–2
2.7 × 10–4
298
The results from our calculations are given in the
Fig. 1, where changes in the concentration of chloro-
form from the time of irradiation at its different initial
concentrations are presented.
were determined from their radiation-chemical yields,
depending on the absorbed dose. The results were 7.2,
1.3, and 3 molec./100 eV for CO2, H2, H2O2, respec-
tively. Depending on the absorbed dose, the formation
of H2O2 was of an extreme character with a maximum
concentration of H2O2 at a dose of 14 kGy, in contrast
to gaseous products.
A formal kinetic scheme of the processes that occur
during the γ-radiolysis of aqueous chloroform solu-
tions was compiled on the basis of the obtained data,
allowing for the available literature values of the rate
constants of elementary reactions. The formal kinetic
scheme includes elementary reactions with the partic-
ipation of primary water radiolysis particles (38 reac-
tions) [7] and their reactions with molecules of chloro-
form and the products of its transformation (24 reac-
tions), plus the hydrolysis reactions of chlorine-
containing products (intermediate and final 6 reac-
tions) with the formation of CO2 and HCl:
From a comparison of experiments and the calcu-
lated data, we established that the model was sensitive
to the rate constant of hydrated electrons with chloro-
form molecules, since the discrepancy between the
calculated and experimental values for the rate of
decomposition of chloroform was 37% at k = 3 ×
1010 M−1 s−1: at k = 1 × 1010 M−1 s−1, it fell to 4%.
The radiation-chemical yields of the decomposi-
tion of chloroform at different initial concentrations
were determined from the initial parts of the kinetic
curves and are presented in Table 5. As can be seen
from the table, the radiation-chemical yields of the
decomposition of chloroform depended on its initial
concentration.
At ≥4.2 × 10−2 M, they lay in the 52–
245 molec./100 eV range. This is much higher than the
initial yields of active particles in the radiolysis of water
(Gex/G0 ~ 40, where G0 = GOH + GH + Gehydr = 6) [6]. The
obtained values testify to the chain decomposition of
chloroform at concentrations of ≥4.2 × 10−2 M and a
CCl3
+ 2Н2О → СООН + 3НСl,
CНCl2 + Н2О → СНО + 2НСl,
0.003
(a)
(b)
0.04
0.03
0.02
0.01
0.002
0.001
0
2
2
4
4
6
8
8
0
2
4
6
8
t ꢀ 10ꢁ4, s
t ꢀ 10ꢁ4, s
0.08
0.06
0.04
0.02
(c)
(d)
0.06
0.04
0.02
0
6
0
2
4
6
8
t ꢀ 10ꢁ4, s
t ꢀ 10ꢁ4, s
−3
−2
−2
Fig. 1. Dependence of the chloroform concentration on the duration of irradiation at (a) 2.5×10 , (b) 4.2×10 , (c) 5.8×10
,
−2
and (d) 8.3×10 M. Points represent experimental data; the curve, calculations; J = 0.6 Gy/s; T = 300 K.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 1 2019

104
ISKENDEROVA, KURBANOV
Table 3. Formal kinetic scheme [8–10]
Elementary reactions
H2O =0> OH
k, 1/s, L/(mol s)
Elementary reactions
k, 1/s, L/(mol s)
3.02e–07
6.7e–08
2.91e–08
5.03e–09
7.8e–09
3.02e–08
1.4e+11
2.52e–05
3.74e+07
5.3e+09
1.4e+10
3.82e+07
5e+09
H3O++ HO2− = H2O2
H + OH– = e + H2O
2e+10
3e+10
0.3
H2O =0> H
H2O =0> e–
2O2− + H2O = O2 + HO2− + OH–
O2− + H2O2 = O2 + OH– + OH
OH + CHCl3 = H2O + CCl3
H + CHCl3 = H2 + CCl3
e– + CHCl3 = CHCl2 + Cl–
OH + C2H2Cl4 = CCl3CHCl + H2O
H + C2H2Cl4 = CCl3CHCl + H2
e– + C2H2Cl4 = C2H2Cl3 + Cl–
OH + CH2Cl2 = CHCl2 + H2O
H + CH2Cl2 = CHCl2 + H2
e– + CH2Cl2 = CH2Cl + Cl–
CCl3 + O2 = CCl3O2
H2O =0> H2
16
H2O =0> H2O2
H2O =0> H3O+
H3O++ OH– = H2O
H2O = H3O++ OH–
OH + H2 = H2O + H
2OH = H2O2
5е+06
7.8e+07
1e+10
1e+06
1e+07
1e+10
9e+07
4е+06
6e+09
4.7e+09
1e+04
3.7e+08
1e+08
1e+08
4.7e+09
1e+04
1e+08
1e+05
1e+06
1e+06
1e+10
1e–04
1e–04
1e–05
1e–05
1e–05
1e–05
5e+08
1e+07
1e+05
OH + HO2 = O2 + H2O
OH + H2O2 = HO2 + H2O
OH + HO2− = HO2 + OH–
OH + O2− = O2 + OH–
H + OH = H2O
9.96e+09
7e+09
CCl3O2 + CHCl3 = CCl3OOH + CCl3
2CCl3 = C2Cl6
OH + e = OH–
3e+10
H + O2 = HO2
H + O2− = HO2−
2e+10
2CHCl2 = C2H2Cl4
2e+10
CHCl2 + CHCl3 = CH2Cl2 + CCl3
CHCl2 + O2 = HClO + COCl
COCl + CHCl3 = COCl2 + CHCl2
2CCl3O2 = C2Cl6 + O2 + O2
COCl2 + H2O = 2HCl + CO2
CCl3OOH + H2O = 2HCl + HClO + CO2
H3O++ Cl– = HCl
H + HO2 = H2O2
2H = H2
2e+10
7.9e+09
3.44e+07
1.94e+10
1.3e+10
2e+10
H + H2O2 = OH + H2O
e– + O2 = O2−
e– + O2− = HO2− + OH–
e– + HO2 = HO2−
e– + H = H2 + OH–
e– + H2O2 = OH + OH–
e– + H3O+= H
2e– = H2 + OH– + OH–
HO2 + O2− = O2 + HO2−
2HO2 = O2 + H2O2
HO2 + H2O2 = O2 + OH + H2O
HO2 = H3O++ O2−
H3O++ O2− = HO2
H2O2 = H3O++ HO2−
2.5e+10
1.14e+10
2.3e+10
5.6e+09
9.5e+07
8.1e+05
3.7
HCl = H3O++ Cl–
CHCl2 + H2O = CHO + 2HCl
CHO + H2O = HCOOH + H
CCl3O2 + H2O = OH + PrA + HCl
*CCl2O2 + H2O = 2HCl + CO2
*CCl3 + H2O + H2O = COOH + 3HCl
*2COOH = HOOCCOOH
*2COOH = HCOOH + CO2
*CHCl2 + O2 = CHCl2O2
7e+05
4.5e+10
0.0356
*CHCl2O2 + H2O = OH + CO2 + 2HCl
The values of reaction rate constants (k) marked with asterisks were chosen in analogy with the literature data [10].
=0> symbol denotes the radiation-chemical reaction of zeroth order.
PrA is an unidentified product.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 93
No. 1
2019

EXPERIMENTAL STUDY AND KINETIC MODELING
105
Table 4. Products of the radiolysis of aqueous chloroform
solutions included in the kinetic model
constant concentration of adsorbed oxygen (~2.7 ×
10−4 M).
Active particles
Molecular products
CONCLUSIONS
OH
CHCl3
CH2Cl2
H2
The proposed kinetic model matches the experi-
mental values of the radiation-chemical yield. The
deviation of their values with the chosen scheme of
elementary reactions lies within the range of 7–40%
and diminishes as the chloroform concentration grows
in the chain reaction mode.
H
e–
H3O+
OH–
H2O2
C2H6
O2−
CO2
CCl3
HCl
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Gcal
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9
Translated by M. Aladina
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 93 No. 1 2019