Oxidation of Oxalic Acid in the Ozone–Chloride–Ion Reaction System in an Aqueous Solution

Article abstract of DOI:10.1134/S0036024420010173

Abstract: It is shown that the rate of oxidation of oxalic acid H₂C₂O₄ during the ozonation of its solutions grows considerably if sodium chloride is added to the solution. Based on the kinetics of evolution of molecular chlorine during ozone treatment of solutions of H₂C₂O₄ and NaCl, the mechanism of reaction of oxalic acid with chlorine is clarified, and the rate constant of this reaction is determined.

Full text of DOI:10.1134/S0036024420010173

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 1, pp. 71–76. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Fizicheskoi Khimii, 2020, Vol. 94, No. 1, pp. 65–70.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Oxidation of Oxalic Acid in the Ozone–Chloride–Ion Reaction
System in an Aqueous Solution
A. V. Levanova,*, O. Ya. Isaikinaa, P. Sh. Azizovab, A. N. Kharlanova, and V. V. Lunina
a Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
b Moscow State University, Baku Branch, Baku, AZ1144 Azerbaijan
*e-mail: levanov@kge.msu.ru
Received March 1, 2019; revised March 1, 2019; accepted April 9, 2019
Abstract—It is shown that the rate of oxidation of oxalic acid H2C2O4 during the ozonation of its solutions
grows considerably if sodium chloride is added to the solution. Based on the kinetics of evolution of molecular
chlorine during ozone treatment of solutions of H2C2O4 and NaCl, the mechanism of reaction of oxalic acid
with chlorine is clarified, and the rate constant of this reaction is determined.
Keywords: ozone, chloride ion, oxalic acid, molecular chlorine, kinetics, bubble column reactor
DOI: 10.1134/S0036024420010173
INTRODUCTION
the flow rate of the initial gas mixture (ozonized oxy-
gen) equal to = 21 L/h (standard conditions for tem-
v
Oxalic acid and its salts (i.e., oxalates) are water
pollutants and are present in the wastewater of differ-
ent industries [1]. Removing oxalates from solutions of
various compositions is of great importance in the
industrial processing of aluminum ores [2] and liquid
radioactive waste [3]. Ozone is widely used in human
activities in various types of water treatment [4], and
the final product of ozonolysis of many organic impu-
rities is oxalic acid or its anions [5]. They barely
undergo oxidative degradation. For these reasons,
perature and pressure), the concentration of ozone at
the reactor inlet equal to C°(O3) = 20 g/m3, and the
volume of the reaction solution equal to VL = 220 mL
in most experiments.
The gases leaving the reactor were passed through a
furnace for the destruction of ozone, and then the car-
bon dioxide CO2 and molecular chlorine Cl2 in them
were determined in the steady-state operating mode of
studying the oxidation of oxalic acid and oxalates in the reactor. The content of carbon dioxide was deter-
ozonation processes is an important task. The aim of
this was to study the kinetics of oxalic acid oxidation
during the ozonation of aqueous solutions containing
high concentrations of chloride ions, and to determine
the mechanism and rate constant of the reaction
between oxalic acid and chlorine.
mined via IR spectroscopy on an EQUINOX 55/S
infrared Fourier spectrometer (Bruker) with a resolu-
tion of 0.5 cm–1 and averaging over 50 scans. A 10-cm-
long optical cell with CaF2 windows was filled with
effluent gases and placed in the spectrometer’s cell
compartment. The device and cell compartment were
purged with nitrogen gas to eliminate the influence of
atmospheric CO2.
EXPERIMENTAL
Our experimental setup and technique were similar
to those described in [6–8]. Aqueous solutions of
oxalic acid with acidimetric concentrations of 0.02–
0.1 M oxalic acid, 0.02–0.1 M oxalic acid and 1 M
NaCl, and 0.06 M oxalic acid and 0.4–1 M NaCl were
ozonated. Once-distilled water, chemically pure
sodium chloride, and titrimetric standards of oxalic
acid were used for their preparation. The pH index
before and after ozonation was monitored using an
Expert-001 pH meter equipped with an ESK-10601/7
glass electrode and an EVL-1M3.1 silver chloride
electrode. The process was conducted in a bubble col-
Molecular chlorine was determined via photomet-
ric iodometry with the preliminary thermal destruc-
tion of ozone [9]. In this work, the furnace tempera-
ture was 500–550°C, which ensured the almost com-
plete removal of ozone and a constant concentration
of Cl2. After passing through the furnace, the gas mix-
ture was sent to a trap filled with 100 mL of an aqueous
solution of 50 g/L KI. A quantitative reaction between
chlorine and KI produced triiodide ion I3−, the con-
centration of which was determined using a KFK-3
photometer. Rate r(Cl2) of chlorine evolution was
umn reactor at room temperature (21 1°C), keeping found from the slope of the final linear segment of the
71

72
LEVANOV et al.
Table 1. Optical density at the absorption line maximum
near 2360 cm−1 in the IR spectrum of the gases leaving the
reactor (the volume of the reaction solution is 200–220 mL)
RESULTS AND DISCUSSION
When ozonating (C°(O3) = 10–57 g/m3) aqueous
solutions of 0.02–0.1 M oxalic acid (pH 1.3–1.9), the
characteristic vibrational–rotational structure of the
absorption band of CO2 with a center at about
Composition of reaction
C°(O3), g/m3
D2360
solution
2350 cm–1 was observed in the IR spectrum of effluent
gases, along with strong absorption bands of water
vapor; no contributions from absorption bands of
other substances to the spectrum were found. With a
constant flow rate of ozonized oxygen, the intensity of
the absorption bands of CO2 in the spectrum of efflu-
ent gases was proportional to the rate of its release
from the reactor. To measure changes in the concen-
tration of CO2, we compared the intensities of the line
of the vibrational–rotational structure with the maxi-
mum at 2360 cm−1. The intensities of this line under
different experimental conditions are given in Table 1.
The formation of CO2 is a result of the oxidation of
oxalic acid or its anions by ozone. This reaction is very
slow; the apparent rate constant determined from the
intensity of CO2 signals in the IR spectrum of effluent
gases and the concentrations of ozone and oxalic acid
in the solution was around 10−3–10−2 L mol−1 s−1 as an
estimate of their orders of magnitude.
When sodium chloride was added to the reaction
solution at a concentration of 1 M, the rate of carbon
dioxide evolution grew by 2–6 times (see Table 1). On
the other hand, rate r(Cl2) of the evolution of molecu-
lar chlorine, which formed as a result of the reaction
between chloride ions and ozone, became notably
lower in the presence of oxalic acid (Fig. 1). These
phenomena were due to the oxidation of oxalic acid
with active chlorine in solution.
57
57
10
10
20
20
20
20
0.1 М Н2С2О4
0.1 М Н2С2О4 + 1 M NaCl
0.02 М Н2С2О4
0.02 М Н2С2О4 + 1 M NaCl
0.02 М Н2С2О4
0.02 М Н2С2О4 + 1 M NaCl
0.06 М Н2С2О4
0.40
0.78
0.06
0.14
0.03
0.19
0.12
0.32
0.06 М Н2С2О4 + 1 M NaCl
time dependence of the amount of I3− using the for-
mula
dn(Cl2)
VL dt
Δn(I3–)
1
VL Δt
1
r(Cl2) ≡
=
,
where VL is the volume of the solution in the reactor. A
blank experiment was performed to consider the pos-
sible contribution from different side processes to the
oxidation of iodide in the trap. A 0.1 M solution of
oxalic acid was ozonized without sodium chloride (all
other conditions being similar to real experiments).
The rate of formation of triiodide was in this case a
negligible value of 0.03 μmol L−1 min−1 not exceeding
1% of the measured value of the rate of chlorine evo-
lution.
It should be noted that the primary products of the
reaction of O3 with Cl−(aq) are hypochlorous acid
15
HOCl and/or hypochlorite ion ClO−. They react in
reversible reactions
2
HOCl ꢀ ClO– + H+,
Cl2(aq) + H2O ꢀ HOCl + H+ + Cl–,
12
9
(1)
(2)
Cl2(aq) + Cl– ꢀ Cl3−,
(3)
1
in which equilibrium is established quickly. As a result,
chlorine compounds in the oxidation states of +1 and
0 (i.e., ClO–, HOCl, and Cl2) are present in the solu-
tion, the quantitative ratios between which are deter-
mined from the values of the corresponding equilib-
rium constants and the concentrations of H+ and Cl–
in the reaction solution. Particles of ClO–, HOCl, and
Cl2 can be considered different forms of a single set of
chemicals, the so-called “active chlorine.” If at least
one of these forms participates in any chemical reac-
tion, the concentrations of the other forms change
proportionally.
6
3
0
0.02
0.04
0.06
0.08
0.10
C(H2C2O4), M
Fig. 1. Dependences of the rate of chlorine evolution
during the ozonation of a solution of 1 M NaCl and oxalic
acid on the acidimetric concentration of acid (C°(O ) =
3
3
20 g/m ). The points show experimental data: (1) results
calculated for a model that includes the oxidation of oxalic
acid with chlorine; (2) results calculated by assuming there
was no reaction between chlorine and oxalic acid.
It is well known that oxalic acid and oxalates react
quickly with active chlorine (but virtually do not inter-
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OXIDATION OF OXALIC ACID
73
act with chlorine compounds in higher oxidation
6
5
4
3
2
1
states, e.g.,
−, ClO , and
ClO2
−) [10]. The subse-
ClO3
quent sections of this w2ork are devoted to determining
the mechanism of this reaction based on experimental
data on the evolution of Cl2 during the ozonation of
solutions of oxalic acid and NaCl (Figs. 1 and 2). We
therefore constructed a mathematical model of the
kinetics of the process, the desired parameters of
which were the shape of the dependence and the rate
constant of the reaction between chlorine and oxalic
acid. We determined the parameters from the condi-
tion of minimal discrepancy between the calculated
and experimental dependences of rate r(Cl2) of chlo-
rine evolution on the concentrations of oxalic acid and
chloride ions.
We describe the bubble column reactor as an ideal
mixing reactor containing two phases, liquid and gas-
eous, and consider the steady-state mode of its opera-
tion. We write the material balance equation (the
equality of the rates of formation and consumption)
for active chlorine:
0
0.2
0.4
0.6
0.8
1.0
C(NaCl), M
Fig. 2. Dependence of the rate of chlorine evolution during
the ozonation of a solution of 0.06 M oxalic acid and
sodium chloride on the concentration of NaCl (C°(O ) =
3
3
kO +Cl−[Cl–][O3] = rOx
+
C(Cl2).
v
20 g/m ). The points show experimental data (for solu-
tions of 0.06 M H C O and 0.4 M NaCl + 0.6 M KNO ,
0.8 M NaCl, 1 M NaCl), and the line shows the results cal-
culated for models (solutions of 0.06 M H C O with an
ionic strength of 1 M and concentrations of chloride ions
in the range of 0–1 M).
(4)
3
2
2
4
3
VL
2
2 4
Here,
is the rate constant of the primary reac-
k
O3 +Cl−
tion between O3 and Cl−(aq) with the formation of
HOCl or ClO−, and rOx is the rate of the reaction
between one form of active chlorine (HOCl, ClO–,
Cl2, or Cl3−) and any form of oxalic acid (with acid
H2C2O4 itself, hydrooxalate HC2O4−, or oxalate
= 0.151 (at an ionic strength of 1 M and a tempera-
LO
3
ture of 21°C), as was determined in [6] for solutions of
sodium chloride in the same reactor under similar
experimental conditions.
C2O24− ). The reactions between hypochlorite ions and
ozone can be ignored, since using oxalic acid results in
fairly high acidity of the reaction solution (pH 1.1–1.7
in all experiments). The concentration of molecular
chlorine in the effluent gases is denoted C(Cl2). The
If we assume there is a chemical reaction between
only one of the forms of active chlorine and one form
of oxalic acid, we can write twelve different expres-
sions for rate rOx:
C(Cl )/V term describes the loss of active chlorine
v
due to2theLremoval of Cl2 from the reactor with a gas
rOx = kOx_A Cl H C O ,
2][
[
[
[
]
2
2
4
rOx = kOx_B Cl [HC O−],
(L mol−1 s−1) is determined by
]
flow. Constant
the expression
k
2
2
4
O3 +Cl−
rOx = kOx_C Cl [C O2−],
]
2
2
4
2.44 ×10−3 + 0.0212[H+]
1 + 0.0707[H+][Cl−]
rOx = kOx_D[HOCl][H2C2O4],
rOx = kOx_E[HOCl][HC2O4−],
(5)
kO +Cl−
=
3
under conditions where the ionic strength of the reac-
tion solution I = 1 M and the temperature is 21°C,
according to [6]. The presence of the concentration of
H+ in Eq. (5) reflects that the reaction of O3 with
Cl−(aq) is catalyzed by hydrogen ions. The concentra-
tion of ozone in solution, [O3] is determined by the
formula
rOx = kOx_F[HOCl][C2O42−],
rOx = kOx_G[ClO–][H2C2O4],
(7)
rOx = kOx_H[ClO–][HC2O4−],
rOx = kOx_I[ClO–][C2O42−],
rOx = kOx_J[Cl3−][H2C2O4],
rOx = kOx_K[Cl3−][HC2O4−],
rOx = kOx_L[Cl3−][C2O42−].
(6)
[O3] = LO C°(O3),
3
where C°(O3) is the concentration of ozone in the gas
flow at the inlet to the reactor. The value of the solu-
bility coefficient (the apparent Henry constant) is
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74
LEVANOV et al.
To derive formulas for calculating concentration
In the range of oxalic acid concentrations used in
C(Cl2) of Cl2 in the effluent gases and rate r(Cl2) of the the experiments of this work (C(H2C2O4) = 0.02–
evolution of chlorine from the reactor using Eq. (4), 0.1 M), the ratio of the concentrations of hydroxalate
anions and H+ ions is almost constant: [HC2O4−]/[H+] =
we must express the concentrations of Cl2, HOCl,
ClO−, and Cl3 in Eqs. (7) through C(Cl2), and the
−
0.97–0.99. It is therefore convenient to express rates
concentrations of H+, H2C2O4, HC2O4−, and C2O24−
(7) through this ratio, and through the concentration
of H+; the [H+] values are determined by solving
through the acidimetric concentration of oxalic acid,
Eqs. (9).
C(H2C2O4). Using the quasi-equilibrium approxima-
tion for different forms of active chlorine, we obtain
It turns out that the theoretical expressions for the
chlorine evolution rate obtained from Eq. (4) corre-
spond to the experimental dependences shown in
Figs. 1 and 2 only when rate rOx does not depend on
[Cl2] = HCl C(Cl2),
2
KCl HCl
2
2
[HOCl] =
C(Cl2),
the concentration of H+ (only the [HC2O4−]/[H+] ratio
[H+][Cl−]
(8)
is included in the expression for rOx) and is inversely
KHOClKCl HCl
[ClO−] =
C(Cl2),
2
2
proportional to the concentration of Cl–. This can be
only in two cases that correspond to when the reaction
between oxalic acid and chlorine is the chemical inter-
action between either a hypochlorous acid molecule
and a hydroxalate ion according to the scheme
[H+]2[Cl−]
[Cl3−] = K [Cl–]H C(Cl2).
Cl3−
Cl2
Here,
is the Henry constant of molecular chlo-
HCl
2
rine, KHOCl is the constant of hypochlorous acid disso-
ciation (reaction (1)), is the constant of Cl2
HOCl + HC2O4− → H2O + Cl– + 2CO2
(11)
KCl
2
or a hypochlorite ion with a molecule of undissociated
oxalic acid by scheme
hydrolysis (reaction (2)), and
Cl3− ion stability (reaction (3)).
is the constant of
KCl−
3
ClO– + H2C2O4 → H2O + Cl– + 2CO2.
(12)
The concentrations of H+ ions, undissociated
oxalic acid molecules H2C2O4, hydroxalate anions
In addition, rate rOx is determined by the correspond-
−, and oxalate anions
in the reaction
С2О24−
ing equations
НС2О4
rOx = kOx_E[HOCl][HC2O4−]
solution were calculated by solving a system of alge-
braic equations that included expressions of the equi-
librium constants of acid dissociation in an aqueous
solution, and the material-balance and charge-con-
servation equations
[HC2O–4 ]
[H+]
(13)
(14)
KCl H
Cl2
[Cl−]
2
= kOx_E
C(Cl2),
rOx = kOx_G[ClO–][H2C2O4]
[H+][HC2O–4 ]
[H2C2O4]
[H+][C2O24–]
Ka1
=
,
Ka2
=
,
[HC2O–4 ]
[H+]
[HC2O–4 ]
K
HOClKCl HCl
2
2
= kOx_G
C(Cl2).
Ka1[Cl−]
С(Н2С2О4) = [H2C2O4] + [HC2O4−] + [C2O42−],
[H+] = [HC2O4−] + 2[C2O24−].
(9)
In light of Eqs. (4), (6), (8), (13), and (14), the con-
centration of chlorine in the gas phase at the outlet of
the reactor is determined by the equations
The calculations using Eqs. (4)–(9) were per-
formed for a temperature of 21°C and a reaction solu-
tion ionic strength of 1 M, which corresponded to the
experimental conditions. The equilibrium constants
under these conditions had the values
kO +Cl−[Cl−]L C°(O3)
O3
3
C(Cl2) =
,
[HC2O–4 ]
KCl
H
Cl2
v
2
+ kOx_E
[Cl−]
[H+]
VL
HCl = 1.51 [11], KCl = 7.50 ×10–4 M2 [12],
kO +Cl−[Cl−]L C°(O3)
2
2
O3
3
C(Cl2) =
.
=0.18 М–1 [13], KHOCl =3.92×10–8 M[14],
[HC2O–4 ]
K
Cl3−
v
KHOClK H
Cl2 Cl2
+ kOx_G
(10)
Ka1 = 0.151 M [15], Ka2 = 3.18 × 10–4 M [15].
Ka1[Cl−]
[H+]
VL
Rate r(Cl2) of the evolution of chlorine from the
reactor is associated with concentration C(Cl2) by the
formula
It was believed that the acidimetric concentration of
oxalic acid, C(H2C2O4), and the concentration of
chloride ions, [Cl–], in the reaction solution did not
change during ozonation, since the real drop in these
concentrations during our experiments was negligible.
v
r(Cl2) = C(Cl2),
VL
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OXIDATION OF OXALIC ACID
75
where v is the volumetric flow rate of the gas mixture is therefore extremely improbable. We may therefore
and VL is the volume of the reaction solution. The the- conclude that the reaction between oxalic acid and
oretical expressions of the rate of chlorine evolution in active chlorine is the chemical interaction between
our model, in which the chemical reactions between hydroxalate ions hypochlorous acid molecules accord-
chloride ions and ozone and oxalic acid with chlorine ing to scheme (11), with a rate constant of kOx_E
=
30.2 L mol−1 s−1 (21°C).
take place, are thus
kO +Cl−[Cl−]L C°(O3)
Let us determine the error of the obtained kOx_E
value. The relative error of the experimental values of
rate r(Cl2) of chlorine evolution is less than 5%. This
means a relative error of coefficient κ equal to 9%. If we
O3
3
(15)
(16)
(17)
r(Cl2) =
[HC2O–4 ]VL
v
KCl H
Cl2
[Cl−]
2
1 + kOx_E
[H+]
assume that equilibrium constants
and
given
KCl
HCl
or
2
2
in Eq. (10) are known with an accuracy of 5%, the
estimate of the relative error for kOx_E = 30.2 L mol−1 s−1
is 20%:
kO +Cl−[Cl−]L C°(O3)
O3
3
r(Cl2) =
.
[HC2O–4 ]VL
v
KHOClKCl HCl
2
2
1 + kOx_G
Ka1[Cl−]
[H+]
kOx_E = 30 6 mol L–1 s–1.
These formulas can be rewritten as one:
The reaction between oxalic acid and active chlo-
rine was studied earlier only in [17] (1932). The
authors of [17] came to the conclusion that reaction
(11) takes place in the range of 10–20°C with the rate
constant equal to k = 3.659 × 1012exp(−7787/T) at
21°C, i.e., k = 12 L mol−1 s−1. The correctness of the
rate constant values found in [17] is doubtful, since
kO +Cl−[Cl−]L C°(O3)
O3
3
r(Cl2) =
,
[HC2O–4 ] VL
1 + κ
−
+
v
[Cl ][H ]
where
(18)
(19)
κ = kOx_EKCl HCl
2
2
the values of equilibrium constants KHOCl
,
, and
KCl
2
for reaction (11) or
, which differ substantially from the current
KCl3−
κ = kOx_EKHOClKCl HCl /Ka1
data, were used to calculate it. Our value of the rate
constant of reaction (11) would seem to be more reli-
able and accurate.
2
2
for reaction (12). Coefficient κ is an unknown param-
eter of expression (17), since rate constants kOx_E and
kOx_G of reactions (11) and (12) are unknown.
The value of the κ parameter was determined from
the condition of the minimum discrepancy between
the r(Cl2) values obtained in the experiment (Figs. 1
and 2) and calculated with formula (17):
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