Ind. Eng. Chem. Res. 1999, 38, 4545-4551
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Kinetics of Anodic Oxidation of Nitrite Ion Using in Situ Electrogenerated HClO in a NaCl Aqueous Solution Chih-Cheng Sun† and Tse-Chuan Chou*,‡ Department of Chemical Engineering, Yung-Ta Industrial Commercial College, Pingtung, 909 Taiwan, Republic of China, and Department of Chemical Engineering, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China
The kinetics of anodic oxidation of nitrite ion in a 1-3 wt % NaCl aqueous solution using in situ electrogenerated HClO was studied. The experimental results indicate that the nitrite ion oxidation is mainly affected by both the concentrations of NaCl and NO2-. The derived reaction rate equation can be employed to describe the behavior of the nitrite and Cl- ions when the NaCl concentration is in the range of 0.17-0.51 M and the NO2- concentration is lower than 20 mg/L. Theoretical analysis correlates with the experimental results. The oxidation of NO2- was found to be an indirect oxidation reaction of NO2- with the active mediator oxidant, HClO, produced from the oxidation of Cl- on the anode. Introduction The detoxification of nitrite ion is significant in aquaculture.1-4 Aquaculture is an important business in many countries around the world. The toxicity of the nitrite ion threatens the lives of aquatic species, when the concentration is higher than the limiting level, i.e., 0.1 ppm.1-3 It is also important to remove the nitrite ion to maintain good quality breeding water. The biological decomposition of the nitrite ion is a traditional treatment procedure,1-2,5 but this method has several limitations such as temperature, NaCl concentration, and large land requirement. Chemical methods, e.g. ozone, H2O2/UV, and ozone/UV, are alternative techniques for nitrite ion oxidation.6,7 These chemical methods also have several limitations such as the high cost of instrumentation and operation. Of particular concern with the chemical method is the uneconomical and inefficient treatment of water containing an extremely low concentration of toxic nitrite ion. Recently, in situ oxidant generation, such as in the photoelectrochemical oxidation of pollutant in NaCl aqueous solution using generated OH free radicals8 and the electrochemical oxidation of pollutant in NaCl aqueous solution using generated HClO,9 may be alternative choices. In the photoelectrochemical oxidation process, the Cl- ion behaves as a supporting electrolyte,8,10,11 but the Cl- ion in the electrochemical oxidation process will be converted anodically into chlorine, which is further converted into hypochlorous acid, HClO.9,12-14 HClO is a strong oxidant. The indirect electrochemical decomposition of pollutants in NaCl aqueous solutions using HClO has been given more attention by several investigators.9,14-17 In addition, the anodic oxidation of organic compounds in the presence of Cl-/ClO- as a redox mediator has also been extensively studied.18-21 But the electrochemical oxidation process in a saline solution which contains 1-3 wt % NaCl has seldom been mentioned.9,22 * To whom correspondence should be addressed. Tel.: 8866-2757575, ext. 62639. Fax: 886-6-2366836. E-mail: tcchou@ mail.ncku.edu.tw. † Yung-Ta Industrial Commercial College. ‡ National Cheng Kung University.
The electrochemical oxidation process may be used as an auxiliary process of the photoelectrochemical oxidation process in a NaCl solution when a light source is unavailable.8,23 Therefore, it is essential to understand the kinetics and mechanism of electrochemical oxidation in a NaCl solution. Although low concentrations of nitrite ion can be decomposed using in situ generated HClO,9 the oxidation mechanism and kinetics of nitrite ion, especially in a 1-3 wt % NaCl solution, are somewhat unclear. Several kinetic models have been proposed for the oxidation of organic compounds in the presence of Cl-/ClO- as a redox mediator.18,19,21 Unfortunately, up to now, few papers have described the mechanism and kinetics of indirect anodic oxidation of trace nitrite ion in a 1-3 wt % NaCl aqueous solution. This kinetic model is very important in designing an electrolysis process using Cl-/ClO- as a redox mediator.21 In this work, the electrochemical oxidation of nitrite ion in a NaCl aqueous solution was systematically studied. The reaction mechanism of the electrochemical oxidation of nitrite ion is also proposed. The kinetic model was theoretically and experimentally studied as well. Experimental Section I-E Curve. The desired concentration and volume of electrolyte solution was prepared and added into a beaker-type undivided cell. An experimental apparatus was set up as shown in Figure 1. The working and counter electrodes were all graphite electrodes with a 4.90-cm2 projected surface area. The entire electrolysis system was placed in a temperature controlled water bath. An EG&G 273A potentialstat/galvanostat with a 270 electrochemical analysis system was used to control the potential of the working electrode and to calculate the amount of charge passed in this process. The potential was specified to a Ag/AgCl reference electrode, i.e., a laboratory produced Ag/AgCl/saturated KCl aqueous solution. The solution in the reactor was stirred until the temperature reached a steady state. Anodic Oxidation of Nitrite Ion in the Presence of NaCl. The desired nitrite ion concentration was
10.1021/ie990349z CCC: $18.00 © 1999 American Chemical Society Published on Web 11/12/1999
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Figure 2. I-E curve: anode, 4.90 cm2 graphite; cathode, 4.90 cm2 graphite; reference, Ag/AgCl; temperature, 25 °C; electrolyte, 0.2 M NaClO4, pH, 5.5: (a) 0.00 M NaCl, (b) 0.17 M NaCl, (c) 0.34 M NaCl, and (d) 0.51 M NaCl.
Figure 1. Electrolysis set-up: (1) potentiostat/galvonostat; (2) pair stirrer thermostat; (3) undivided cell; (4) working electrode (graphite); (5) counter electrode (graphite); (6) reference electrode (Ag/AgCl); (7, 8) magnetic stirrers; (9) vent.
added into the cell containing 100 mL of a fixed concentration of NaCl electrolyte, which was deoxygenated using nitrogen gas introduced into the cell. The electrochemical oxidation was performed in an undivided cell similar to the I-E curve measurement. Both the agitation rate and temperature were controlled at the desired values. The nitrite ion solution was prepared by dissolving the sodium nitrite into the distilled water. The nitrite ion oxidation proceeded at a desired initial value. Samples were periodically taken with a pipet from the reactor; each sample volume was 1 mL. Then, the samples were analyzed according to the APAH testing.24 A color reagent was added to the sample by the NEDA (N-1-naphthyl ethylenediamine dihydrichloride) colorimetric method. The nitrite ion concentration was determined by a Jasco UV-vis spectrophotometer (Jasco) at a wavelength of 543 nm. In addition, the analysis of ClO- concentration in reaction solution was determined by using the iodimetric method.24 Results and Discussion Decomposition Potentials. Figure 2 shows the relationships between the current against the anodic potential in this system at different operating conditions. The decomposition potential can be determined from the I-E curve as shown in Figure 2. The decomposition potential of water occurred at potentials higher than about 1.50 V vs Ag/AgCl in a 0.20 M NaClO4 aqueous solution, as shown in curve (a) of Figure 2. When the applied potential is higher than 1.50 V, water is continuously electrolyzed to generate both oxygen and hydrogen at the working and counter electrodes, respectively, in the solution using a NaClO4 supporting electrolyte. The results from the addition of various amounts of NaCl into the NaClO4 solution are shown in curves (b)-(d) of Figure 2. The decomposition poten-
Figure 3. Time course of the HClO concentration in electrolysis of NaCl solution: electrode, the same as the captions in Figure 2; temperature, 25 °C; current density, 25 mA/cm2: (a) 0.17 M NaCl, (b) 0.34 M NaCl, and (c) 0.51 M NaCl.
tial of the NaCl aqueous solutions occurred at potentials lower than that in the NaClO4 aqueous solution. Comparison of the results of curve (a) with (b)-(d) in Figure 2, the current density increased from 2.04 to 8.16 mA/ cm2 at 1.50 V (vs Ag/AgCl) anodic potential when the NaCl load increased from 0.17 to 0.51 M. This result demonstrates that the Cl- ion is participating in the oxidation reaction on the working electrode. Figure 3 shows the change in the HClO concentration versus time in the electrolysis process using various concentrations of NaCl solutions. When the NaCl concentration increased from 0.17 to 0.51 M, the HClO concentration increased from 280 to 410 mg/L in a 20-min run. Effect of Potential on Reaction Rate. Figure 4 shows the effect of using selected operating potentials on the nitrite ion removal. Increasing the potential from 1.20 to 1.55 V increases the initial nitrite ion oxidation rate from 5.35 to 70.10 µM/min. When the anodic
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4547
Figure 4. Percentage of NO2- removal during the run: electrode, the same as the captions in Figure 2; temperature, 25 °C; NaCl concentration, 0.34 M; NO2- concentration, 2.17 × 10-4 M: (a) 1.20 V, (b) 1.30 V, (c) 1.45 V, (d) 1.50 V, (e) 1.55 V.
Figure 5. Effect of potential on the reaction time for 50% NO2removal and the amount of NO2- removal per charge passed: electrode, the same as the captions in Figure 2; temperature, 25 °C; NaCl concentration, 0.34 M; NO2- concentration, 2.17 × 10-4 M; agitation rate, 450 rpm.
potential increased from 1.20 to 1.55 V vs Ag/AgCl, the reaction time for 50% NO2- removal decreased sharply from 24.00 to 2.00 min, as shown in Figure 5. The greater the overpotential, the shorter the time required to remove NO2-. The amount of NO2- removal per charge passed decreased from 4.16 to 1.06 µmol/C, as the anodic potential is increased from 1.20 to 1.45 V vs Ag/AgCl, as shown in Figure 5. In this anodic potential range, the oxidation of Cl- ion is low and oxygen may be generated. The oxidation rate of NO2- using the generated oxygen is low. With further increases in the potential from 1.45 to 1.55 V, the amount of NO2removal per charge passed slightly increases from 1.06 to 1.52 µmol/C, as shown in Figure 5. In this anodic potential range, the electrochemical oxidation of Cl- is obvious, but the loss of Cl2 from the solution may occur. Increasing the anodic potential promotes the generation of HClO oxidant.19,21,25 Therefore, the amount of NO2-
Figure 6. Effect of potential on the direct electrochemical oxidation rate of NO2-: electrode, the same as the captions in Figure 2; temperature, 25 °C; NaClO4 concentration, 0.20 M; NO2concentration, 2.17 × 10-4 M; agitation rate, 450 rpm.
removal per charge passed increases. When the loss of Cl2 from the solution is taken into account, an anodic potential lower than 1.55 V may be the best choice because both the generation and leakage of Cl2 are promoted at high applied potential. In addition, the direct electrochemical oxidation of pollutant was reported in the literature.26,27 Figure 6 shows the direct electrochemical oxidation of NO2- at the anode. When the anodic potential increases from 1.20 to 1.60 V in the NaClO4 solution, the electrochemical oxidation of NO2- is almost independent of the applying anodic potential, as shown in Figure 6. Effect of Nitrite Ion Concentration on Reaction Rate. Increasing the initial concentration of NO2- from 1.08 × 10-4 to 4.34 × 10-4 M increases the initial rate of NO2- removal from 24.70 to 30.06 µM/min, and the amount of NO2- removal per charge passed increases from 0.63 to 1.74 µmol/C, as shown in Figure 7. These results indicate that the reaction rate of NO2- oxidation in this indirect oxidation system depends on the concentration of NO2- in the bulk solution. The reciprocal plot of the initial rate of NO2- removal vs the nitrite ion concentration yields a straight line, as shown in Figure 8. The slope and intercept are 1.20 min and 2.98 × 104 min/M, respectively. Accordingly, the oxidation rate can be expressed as shown in eq 1:
(
kK[NO2-] d[NO2-] ) dt 1 + K[NO2-]
)
(1)
The paramters k and K are found to be 33.6 µM/min and 0.0025 µM-1, respectively. Effect of Concentration of NaCl on Reaction Rate. When the concentration of NaCl increases from 0.17 to 0.51 M, the initial rate of nitrite ion oxidation increases from 26.00 to 29.60 µM/min and the amount of charge passed increases from 0.77 to 2.25 µmol/C, respectively, as shown in Figure 9. In addition, as shown in Figure 2, the I-E curve indicates that the Clparticipates in the oxidation reactions on the working electrode and generates the HClO oxidant. The greater
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Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
Figure 7. Effect of NO2- concentration on the initial rate of NO2removal and the amount of NO2- removal per charge passed: electrode, the same as the captions in Figure 2; temperature, 25 °C; NaCl concentration, 0.34 M; applied potential, 1.50 V vs Ag/ AgCl; agitation rate, 450 rpm.
Figure 8. Reciprocal plot of initial rate of NO2- removal vs the NO2- concentration: electrode, the same as the captions in Figure 2; temperature, 25 °C; NaCl concentration, 0.34 M; applied potential, 1.50 V vs Ag/AgCl; agitation rate, 450 rpm.
the concentration of Cl- ion, the greater the amount of HClO generated. Therefore, the indirect oxidation of NO2- depends on the concentration of NaCl in the bulk solution and is promoted by increasing the Cl- concentration. Accordingly, when the NaCl load increases, the hypochlorous acid concentration increases and then the amount of NO2- removal per charge passed increases. The reciprocal plot of the initial rate of NO2- oxidation vs the concentration of NaCl yields a straight line, as shown in Figure 10. The slope and intercept are 0.12 × 104 min and 3.19 × 104 min/M, respectively. Accordingly, the oxidation rate of NO2- oxidation can be expressed as shown in eq 2:
(
d[NO2-] k′K′[Cl-] ) dt 1 + K′[Cl-]
)
Figure 9. Effect of NaCl concentration on the initial rate of NO2removal and the amount of NO2- removal per charge passed: electrode, the same as the captions in Figure 2; temperature, 25 °C; NO2- concentration, 2.17 × 10-4 M; applied potential, 1.50 V vs Ag/AgCl; agitation rate, 450 rpm.
Figure 10. Reciprocal plot of initial rate of NO2- removal vs the NaCl concentration: electrode, the same as the captions in Figure 2; temperature, 25 °C; NO2- concentration, 2.17 × 10-4 M; applied potential, 1.50 V vs Ag/AgCl; agitation rate, 450 rpm.
The paramters k′ and K′ are found to be 31.30 µM/min and 26.62 M-1, respectively. Mechanism and Kinetics of Nitrite Ion Oxidation. According to the experimental results, the mechanism of the electrochemical process on the electrode in the presence of Cl- was proposed as follows:9,22
Working electrode: (Anode, graphite) k3
Cl-(aq) 98 Clad + ek4
Clad + Cl-(aq) 98 Cl2(g) + e-
(3) (4)
Bulk solution: (2)
k5
Cl2(g) + H2O 79 8 HCl(aq) + HClO(aq) k -5
(5)
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4549
Side reaction on the working electrode: 2H2O f O2 + 4H+ + 4e-
Combining eqs 13, 16, and 18, and assuming
2H+(aq) + 2e- f H2(g)
(7)
2H2O + 2e- f H2(g) + 2OH-
(8)
-
H + OH f H2O
NO2
then eq 19 is obtained. Because eq 4 is a fast electrontransfer reaction and the reverse reaction of eq 5 is decreased by the oxidation of nitrite ion using HClO,
[ ( )]
R3Fη [Cl-] RT [HClO] ) k10[NO2-] + k-5[HCl] k3 exp
+ HClO(aq) 98 NO3-(aq) + Cl-(aq) + H+(aq) (10)
d[NO2-] ) k10[NO2-][HClO] dt
(11)
-
d[NO2-] ) k10[NO2-][HClO] dt
According to the Butler-Volmer equation and at the constant overpotential, the reaction rate of chlorine, which adsorbed at the surface of the anode, was obtained.28,29 It is assumed that the species of Clad, Cl2, and HClO is the medium product with short lift time in the reaction mechanism. Then, eqs 12-14 were derived from the reaction equations of the reaction mechanism, i.e., eqs 3-10:
[ ]
R3Fη d[Clad] ) k3 exp [Cl-] dt RT k4 exp
(20)
Combining eqs 11 and 20, the reaction rate is expressed as
The reaction rate is expressed as
-
(19)
Combining eqs 14, 17, and 19, the concentration of hypochlorous acid is
k10
(aq)
[ ]
R3Fη k3[Cl-] exp k5 RT
[Cl2] )
(9)
where the subscript “ad” denotes the species adsorbed on the electrode. The hypochlorous acid oxidizes nitrite ion to nitrate ion according to eq 10:9 -
R4Fη [Clad][Cl-] . k-5[HClO][HCl] RT
k4 exp
Counter electrode: (Cathode, graphite)
+
[ ( )]
(6)
[ ]
R4Fη [Clad][Cl-] (12) RT
[ ]
R4Fη d[Cl2] [Clad][Cl-] ) k4 exp dt RT k5[Cl2] + k-5[HClO][HCl] (13) d[HClO] ) k5[Cl2] - k-5[HClO][HCl] dt k10[NO2-][HClO] (14)
) k10
(
)
[ ( )]
R3Fη [Cl-] RT [NO2-] k10[NO2 ] + k-5[HCl] k3 exp
[ ( )](
) exp
R3Fη RT
k10k3[Cl-][NO2-]
k10[NO2-] + k-5[Cl-]
)
(21)
When the Cl- concentration remains constant, eq 21 can be simplified to eq 22:
( )](
[
R3Fη d[NO2-] ) k3[Cl-] exp dt RT )
(
kK(NO2-]
1 + K[NO2-]
)
K[NO2-]
1 + K[NO2-]
)
(22)
where
K)
at steady state:
k10 -
( )
, k ) k3[Cl-] exp
k-5′[Cl ]
R3Fη RT
d[Clad] )0 dt
(15)
When the NO2- concentration remains constant at the initial stage, eq 21 can be simplified to eq 23:
d[Cl2] )0 dt
(16)
d[NO2-] k10k3[NO2-] R3Fη exp ) dt k-5′ RT
d[HClO] )0 dt
(17)
Combining eqs 12 and 15, the following equation is obtained:
[
]
k3 (R3 - R4)Fη [Clad] ) exp k4 RT
)
( (
where
1 + K′[Cl-]
(
K′[Cl-]
1 + K′[Cl-]
)
(23)
)[ ( )]
k10k3[NO2-] R3Fη , k′ ) exp K′ ) k-5′ RT k10[NO2-] k-5′
(18)
k′K′[Cl-]
)[ )
( )](
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Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
the reaction rate could be expressed with the derived rate equation. During the electrolysis, Cl-/ClO- behaved as a redox mediator, and the effect of nitrite and Clions on the oxidation rate can be expressed with the derived reaction rate equation. Acknowledgment The support of the National Science Council of the Republic of China (NSC 85-2214-E006-018), National Chung Kung University, and Yung-Ta Industrial Commercial College are acknowledged. Nomenclature
Figure 11. Experimental data and prediction results from derived equation. Table 1. Comparison of the Experimental and Theoretical Analysis Results reaction rate material NO2-
theoretical
(
experimental
kK[NO2-]
1 + K[NO2-]
NaCl
(
k′K′[Cl-]
) (
1 + 2.48 × 104[NO2-]
)
(
1 + 26.62[Cl-]
Where
K) b
)
8.33 × 10-4[Cl-]
b
1 + K′[Cl-]
a
)
0.89[NO2-]
a
k8 -
,
k-5[Cl ]
k ) k3[Cl-] exp
Where
K′ )
k-5′ k8[NO2-]
,
k′ )
(
( )
R3Fη . RT
)[ ( )]
k8k3[NO2-] R3Fη exp . k-5′ RT
Comparison of Experimental and Theoretical Analysis Results. A comparison of eqs 22 and 23 with the experimental data indicates that the experimental results correlate with the theoretical analysis well, as shown in Table 1. Figure 11 shows the experimental data and prediction results. The derived reaction rate equation can be employed to describe the behavior of the nitrite and Cl- ions when the NaCl concentration is in the range of 0.17-0.51 M and the NO2- concentration is lower than 20 mg/L. Conclusions The indirect oxidation of NO2- in 1-3 wt % NaCl solution was carried out at a suitable potential of about 1.45 V vs Ag/AgCl. A reaction mechanism, including the reactions of both the anodic and cathodic electrodes, was proposed. A kinetic model of nitrite ion oxidation was obtained. The nitrite ion was oxidized by HClO, which was generated from the anodic oxidation of chloride ion. Both experimental and theoretical results indicated that
k3 ) forward reaction rate constant defined in eq 3, min-1 k4 ) forward reaction rate constant defined in eq 4, mol-1 L min-1 k5 ) forward reaction rate constant defined in eq 5, min-1 k-5 ) backward reaction rate constant defined in eq 5, mol-1 L min-1 k10 ) forward reaction rate constant defined in eq 10, mol-1 L min-1 Clad ) chloride atom which adsorbed upon the surface of the anode R3 ) transfer coefficient defined in eq 3 R4 ) transfer coefficient defined in eq 4 F ) Faraday’s constant, 96485 C mol-1 R ) gas constant, (J/(K mol)) T ) absolute temperature in Kelvin, K η ) overpotential of the working electrode, V Ri ) initial rate of nitrite ion removal, µmol L-1 min-1
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Received for review May 19, 1999 Revised manuscript received September 29, 1999 Accepted September 30, 1999 IE990349Z
