Ind. Eng. Chem. Res. 1997, 36, 349-356
349
Kinetics of the Oxidative Degradation of Formaldehyde with Electrogenerated Hypochlorite Ion Jing-Shan Do,* Wen-Chin Yeh, and I-Ya Chao Department of Chemical Engineering, Tunghai University, Taichung, Taiwan 40704, Republic of China
The mechanisms and kinetics of the anodic oxidation of chloride ion on SnO2-PdO-RuO2TiO2/Ti (SPR) anode and the oxidation of formaldehyde with hypochlorite ion were studied, and the new kinetic data based on theoretical analysis were evaluated in this investigation. The reaction order of the anodic oxidation of chloride ion on SPR was unity. Also, the oxidation of formaldehyde with hypochlorite ion was second order in formaldehyde and first order in hypochlorite ion. Furthermore, the activation energy was evaluated as 37.9 kJ mol-1. Formaldehyde was degraded from 3000 to 279 ppm, and the degradation fraction was 90.7% when the electrolysis time was 111 min. A model calculation of the in situ oxidative degradation of formaldehyde with electrogenerated hypochlorite ion correlated well with experimental results. Introduction Aldehydes pose a potential problem in waste waters coming from a variety of process industry sources and must be treated before industrial waste waters can be discharged. Chemical treatment of aldehydes is a suitable pretreatment for a biological clarification plant. Formaldehyde is effectively degraded with hydrogen peroxide which was produced by the cathodic reduction of oxygen dissolved in the aqueous solution (Do and Chen, 1993, 1994a). The concentration of formaldehyde was effectively degraded from 1000 to 2 ppm. The kinetics of in situ degradation of formaldehyde with electrogenerated hydrogen peroxide was also explored (Do and Chen, 1994b). The oxidation of formaldehyde with hydrogen peroxide was second order in formaldehyde and first order in hydrogen peroxide. However, the oxidative degradation rate of formaldehyde with in situ electrogenerated hydrogen peroxide was small due to the smaller current density for the cathodic reduction of oxygen dissolved in the aqueous solution (Do and Chen, 1993, 1994a). Hypochlorite ion is a good oxidant for the treatment of waste waters containing toxic compounds (Pontius, 1990). The degradation of CN- with electrogenerated hypochlorite ion was investigated by Woolley (1976). The degradation fraction of phenol with electrogenerated ClO- was 95% (Masahiro and Yasuo, 1986). The oxidative degradation of formaldehyde with in situ electrogenerated hypochlorite ion was previously studied (Do and Yeh, 1994). The investigation revealed that the current efficiency of degradation of formaldehyde was 69.8% when 1500 C was charged into the reaction system and the initial concentration of formaldehyde was 3000 ppm (Do and Yeh, 1994). The kinetics of the oxidation of formaldehyde with hypochlorite ion in alkaline solution and the reaction order and activation energy still remain unclear. The production of hypochlorite ion with anodic oxidation of chloride ion in the aqueous solution has been widely studied (Slipchenko et al., 1988; Kovarskii et al., 1989; Bennett, 1974; Mikhailova et al., 1987; Krstajic et al., 1987). The anodic oxidation of chloride ion on graphite was first order in the concentration of chloride ion (Do and Chou, 1990). The mechanisms and kinetics of anodic oxidation of chloride ion on metal oxides have * Author to whom correspondence should be addressed. S0888-5885(95)00583-5 CCC: $14.00
been also investigated (Kuhn and Mortimer, 1973; Evdokimov and Gorodetskii, 1986; Trasatti, 1981; Vetter, 1961). The various mechanisms and kinetics were found when the various metal oxides were used as anodes. In general, the Volmer-Tafel and VolmerHeyrovsky mechanisms of the evolution of chlorine were demonstrated on RuO2-TiO2 electrodes (Burrows et al., 1978; Janssen et al., 1983). However, the mechanisms and kinetics of the anodic oxidation of chloride ion on SPR anode seldom have been reported. Furthermore, the kinetics of the oxidative degradation of formaldehyde with in situ electrogenerated hypochlorite are not reported in the literature. For reactor design and practical applications, the study of the mechanisms and kinetics of anodic oxidation of chloride ion to hypochlorite ion and oxidative degradation of formaldehyde with electrogenerated hypochlorite ion is very important. The mechanisms and kinetics of in situ degradation of formaldehyde with electrogenerated hypochlorite ion were systematically studied in this work. The exchange current density, Tafel slope, and reaction order of chloride ion in the aqueous solution as well as the reaction rate constant, the reaction orders of formaldehyde and hypochlorite ion, and the activation energy were experimentally determined, respectively. A comparison was also made of the theoretical analysis of the reaction system with experimental results. Experimental Section Preparation of SnO2-PdO-RuO2-TiO2/Ti (SPR) Anode. An SPR electrode was prepared by thermal decomposition (Iwakura and Sakamoto, 1985; Janssen et al., 1977; Arikado et al., 1978). The procedures of the preparation of SPR was described previously (Do and Yeh, 1994, 1995). Anodic Oxidation of Chloride Ion on SPR Anode. The experiments were carried out in an undivided cell with prepared SPR as working electrode, a platinum wire as counter electrode, and a Ag/AgCl/3M NaCl aqueous solution as reference electrode. The currentpotential relationships were obtained by the steady state method. Oxidation of Formaldehyde with Hypochlorite Ion. Formaldehyde was oxidized with hypochlorite ion in a cylindrical glass reactor. A magnetic stirrer was used to agitate the solution. The reaction temperature was controlled with a water bath to within (0.1 °C. © 1997 American Chemical Society
350 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 Scheme 1
k1
Cl- y\ z Clads + e- (Volmer reaction) k
(1)
-1
Chlorine molecule is formed by a combination of one adsorbed chlorine atom and one chloride ion k2
Clads + Cl- y\ z Cl2 + e- (Heyrovsky reaction) (2) k -2
At the beginning of a run, the desired concentration of formaldehyde in aqueous solution (100 mL) was fed into the reactor, which was maintained at the desired pH value and temperature. The desired stirring rate was applied to the system. When the thermal equilibrium was achieved, 100 mL of the desired concentration of hypochlorite ion aqueous solution was introduced into the reaction system. The time was recorded as soon as the hypochlorite ion solution was fed. Samples were periodically taken from the reaction solution by use of a hypodermic syringe. The residual hypochlorite ion in the sample (1 mL) was destroyed by the addition of 99 mL of 0.02 M NaHSO3 aqueous solution. The concentration of formaldehyde was analyzed by measuring the light adsorption of chromotropic acid-formaldehyde colored complex at 575 nm (Do and Chen, 1993, 1994a,b; Altshuller et al., 1961). The concentration of hypochlorite anion was determined by the iodometry titration method. In Situ Degradation of Formaldehyde with Electrogenerated Hypochlorite Ion. The H-type dualcompartment glass reactor was used for the in situ degradation of formaldehyde with electrogenerated hypochlorite ion. The working electrode was the prepared SPR plate, and the counter electrode was a platinum wire. The experimental procedures and apparatus have been described elsewhere (Do and Yeh, 1994, 1995). Additionally, the concentrations of hypochlorite ion and formaldehyde were analyzed as described above. Theoretical Analysis
When eq 2 is a rate-determining step (Janssen et al., 1977; Arikado et al., 1978; Janssen and Hoogland, 1970a), the current density of the anodic oxidation of chloride ion on SPR anode is obtained as
i ) 2Fk20 exp(RFη/RT)[Cl-]θCl
where k20, R, F, η, and θCl are the reaction rate constant at η ) 0, a charge-transfer coefficient, Faraday constant, the overpotential of anode, and the fraction of coverage with chlorine atoms on the anodic surface, respectively. Equation 3 can also be expressed as
i ) i0 exp(RFη/RT)
(4)
where i0 is the exchange current density of the anodic oxidation of chloride ion on the anodic surface
i0 ) 2Fk20[Cl-]θCl
(5)
Hypochlorite ion is formed by the hydrolysis of chlorine in the aqueous solution k3
z HOCl + H+ + ClCl2 + H2O y\ k
(6)
-3
k4
z H+ + OClHOCl y\ k
(7)
-4
II. Oxidation of Formaldehyde with Hypochlorite Ion. The oxidation of formaldehyde with hypochlorite ion occurs in alkaline solution. The equilibrium equations of formaldehyde in the alkaline solution are (Vaskelis and Norkus, 1991) k5
The reaction mechanism of the in situ oxidative degradation of formaldehyde with electrogenerated hypochlorite ion is described by Scheme 1. Formaldehyde in the aqueous phase is oxidized to formic acid with hypochlorite ion which is generated by the oxidation of chloride ion on the anode. Chloride ion is regenerated by the degradation of formaldehyde with hypochlorite ion. Chloride ion/hypochlorite ion functions in the role of redox mediator and shuttles between anodic surface and aqueous phase. I. Anodic Oxidation of Chloride Ion in the Aqueous Phase To Produce Hypochlorite Ion. The mechanisms of the anodic oxidation of chloride on the various anodes have been widely studied (Evdokimov and Gorodetskii, 1986; Vetter, 1961; Janssen and Hoogland, 1970a,b; Janssen et al., 1977, 1983; Denton et al., 1979; Arikado et al., 1978; Burrows et al., 1977, 1978). The mechanism of the anodic oxidation of chloride ion on a ruthenium oxide/titanium oxide electrode is found to be the Volmer-Heyrovsky mechanism (Janssen et al., 1977; Arikado et al., 1978). Therefore, the Volmer-Heyrovsky mechanism of the anodic oxidation of chloride ion on SPR anode is assumed. Chloride ion is oxidized on the SPR to chlorine atom which is adsorbed on the surface of SPR
(3)
z CH2(OH)2 CH2O + H2O y\ k
(8)
-5
k6
z CH2OHO- + H2O CH2(OH)2 + OH- y\ k
(9)
-6
In the alkaline solution OCl- reacts with CH2OHO- to become CH2OClOk7
z CH2OClO- + OH- (10) CH2OHO- + OCl- y\ k -7
CH2OClO-
further reacts with CH2O to form HCOO-, and the mechanisms are assumed to be
CH2OClO–
H
k8
+ CH2O
H
H
C O
C O– + OH–
OCl
H CH2ClO– + OH–
k–8 k9
H
H
H
C O
C O–
OCl
H
HCOO– + CH2ClO– + H2O
k–9 k10 k–10
HCOO– + Cl– + H2
(11)
(12)
(13)
The rate equations for the various rate-determining steps assumed are discussed as follows.
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 351
Case 1. If eq 10 is the rate-determining step, the reaction rate of the oxidation of formaldehyde with hypochlorite ion is
rh1 ) k7[CH2OHO-][OCl-]
(14)
The concentration of CH2OHO- was obtained from eqs 8 and 9 as
[CH2OHO-] ) K5K6[CH2O][OH-]
(15)
where K5 ) k5/k-5 and K6 ) k6/k-6. Substituting the concentration of CH2OHO- (eq 15) into eq 14, the rate equation of the oxidation of formaldehyde with hypochlorite ion is
rh1 ) k7K5K6[CH2O][OCl-][OH-]
(16)
Equation 16 shows that the reaction orders of the oxidation of formaldehyde are 1, 1, and 1 with respect to formaldehyde, hypochlorite ion, and hydroxyl ion. Case 2. If eq 11 is the rate-determining step, then the rate equation is
rh2 ) k8[CH2OClO-][CH2O]
(17)
The concentration of CH2OClO- is obtained from eqs 8-10
[CH2OClO-] ) K5K6K7[CH2O][OCl-]
(18)
where K7 ) k7/k-7. The rate equation of the oxidation of formaldehyde is obtained by the substitution of eq 18 into eq 17
rh2 ) k8K5K6K7[CH2O]2[OCl-]
(19)
The results reveal that the reaction orders in case 2 are 2 and 1 with respect to formaldehyde and hypochlorite ion. Case 3. If eq 12 is the rate-determining step, the reaction rate is expressed as
rh3 ) k9[A][OH-]
(20)
in which A is
H
H
H
C O
C O–
OCl
H
The concentration of CH2ClO- based on the equilibrium equations, eqs 8-12, is obtained as
[CH2ClO-] ) K5K6K7K8K9[CH2O]2[OCl-][OH-]/[HCOO-] (24) where K9 ) k9/k-9. The reaction rate is obtained by the substitution of eq 24 into eq 23
rh4 ) k10K5K6K7K8K9[CH2O]2[OCl-][OH-]2/[HCOO-] (25) The reaction orders are 2, 1, 2, and -1 with respect to formaldehyde, hypochlorite ion, hydroxyl ion, and HCOO-, respectively, as shown in eq 25. III. Mass Balance of Oxidation of Formaldehyde with Electrogenerated Hypochlorite Ion. As illustrated in Scheme 1, hypochlorite ion in the aqueous solution is generated on the anodic surface and consumed due to the oxidation of formaldehyde in the bulk solution. The generation of hypochlorite ion from the anodic surface is expressed in eqs 3-7. The net equation of the oxidation of formaldehyde with hypochlorite ion in bulk solution is
2CH2O + OCl- + 2OH- f 2HCOO- + Cl- + H2 + H2O (26) As indicated in the above equation, 1 mol of hypochlorite ion reacts with 2 mol of formaldehyde. Hence, the mass balance of chloride ion in the bulk solution is
d[Cl-]/dt ) -iA/2FV + 0.5kh[CH2O]m[OCl-]n[OH-]p/[HCOO-]q (27) where A and V are the area of anodic surface and the volume of solution and kh, m, n, p, and q are the rate constant of the oxidation of formaldehyde with hypochlorite ion and the reaction orders of formaldehyde, hypochlorite ion, hydroxyl ion, and HCOO-, respectively. The initial concentration of chloride ion in the solution is
[Cl-]0 ) [Cl-] + [OCl-] + [Cl2] + [HOCl] (28) If eqs 6 and 7 are in equilibrium, then the concentration of chlorine is
According to the equilibrium equations, eqs 8-11, the concentration of A is obtained as
[Cl2] ) [H+]2[Cl-][OCl-]/K3K4
[A] ) K5K6K7K8[CH2O]2[OCl-]
where K3 ) k3/k-3 and K4 ) k4/k-4. At 25 °C, the equilibrium constants K3 and K4 are 4.48 × 10-4 M2 (Morris, 1946) and 3.2 × 10-8 M (Windholz et al., 1976), respectively. At pH 13 and upon substitution of K3 and K4 into eq 29, the concentration of chlorine is
(21)
where K8 ) k8/k-8. Substituting eq 21 into eq 20 gives
rh3 ) k9K5K6K7K8[CH2O]2[OCl-][OH-]
(22)
[Cl2] ) 7 × 10-16[Cl-][OCl-]
(29)
(30)
Equation 22 indicates that the reaction orders of oxidation formaldehyde with hypochlorite ion are 2, 1, and 1 with respect to formaldehyde, hypochlorite ion, and hydroxyl ion, respectively. Case 4. If eq 13 is the reaction rate-determining step, the reaction rate is expressed as
Equation 30 shows that the concentration of chlorine molecule is much less than chloride and hypochlorite concentrations in the aqueous solution. Hence, eq 28 becomes
rh4 ) k10[CH2ClO-][OH-]
[Cl-]0 ) [Cl-] + [OCl-] + [HOCl]
(23)
(31)
352 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997
On the basis of eq 7, the concentration of HOCl can be expressed as
[HOCl] ) [H+][OCl-]/K4
(32)
Combining eqs 31 and 32, the concentration of hypochlorite ion is
[OCl-] ) K4([Cl-]0 - [Cl-])/(K4 + [H+])
(33)
At pH 13, the value of concentration of H+ is much less than K4. On the basis of eq 33, the concentration of hypochlorite ion is
[OCl-] ) [Cl-]0 - [Cl-]
(34)
Substituting eq 34 into the left-hand side of eq 27, the following equation is obtained
d[OCl-]/dt ) iA/2FV - 0.5kh[CH2O]m[OCl-]n[OH-]p/[HCOO-]q (35) The dynamic concentration of formaldehyde in the bulk solution is expressed as
Figure 1. Steady-state polarization curves of the anodic oxidation of chloride ion. Temp ) 283 K, stirring rate ) 600 rpm, pH ) 13, anode ) SPR, and area of anode ) 3.0 cm2.
d[CH2O]/dt ) -kh[CH2O]m[OCl-]n[OH-]p/[HCOO-]q (36) The values of kh, m, n, p, and q are obtained experimentally. The concentration of chloride ion and formaldehyde in the bulk solution for the case of kinetic control are evaluated by solving eqs 3, 27, and 36 simultaneously when the anodic oxidation of chloride ion to hypochlorite ion is kept at potentiostate. When the constant current of the anodic oxidation of chloride ion is applied, the concentration of chloride ion and formaldehyde in the aqueous phase are evaluated by the solving eq 27 and 36 simultaneously. Results and Discussion I. Kinetics of the Anodic Oxidation of Chloride Ion on SPR. I.1. Effect of Concentration of Chloride Ion. The polarization curves of the anodic oxidation of chloride on SPR anode shown in Figure 1 indicated that the current density increased when the concentration of chloride ion in the aqueous phase increased at the same anodic potential. The current density increased from 1.65 to 4.85 mA cm-2 when the concentration of chloride ion in the bulk phase increased from 0.75 to 2.0 M and the anodic potential was kept at 0.75 V (vs. Ag/AgCl/3M NaCl aqueous solution). The results in the Figure 1 also revealed that the anodic oxidation of chloride ion was located in the region of reaction control when the anodic potential was less than 1.20 V. The plot of the logarithmic current density of anodic oxidation of chloride ion against the logarithmic concentration of chloride ion in the aqueous phase resulted in the straight lines for various applied potentials as illustrated in Figure 2. The slopes of the straight lines in Figure 2 were evaluated as 1.03 ( 0.09, 0.92 ( 0.07, 0.94 ( 0.10, and 0.98 ( 0.09 when the applied potentials were fixed at 0.75, 0.77, 0.80, and 0.82 V, respectively. The results indicated that the electrochemical reaction order of chloride ion was unity which correlated well with eq 3 in the theoretical section. The
Figure 2. Effect of concentration of chloride ion on the current density of anodic oxidation of chloride ion. Temp ) 283 K, stirring rate ) 600 rpm, pH ) 13, anode ) SPR, and area of anode ) 3.0 cm2.
value of coverage with chlorine atoms on the anodic surface (θCl) in eq 3 is approximately constant
i ) 2Fk20′ exp(RFη/RT)[Cl-]
(37)
where k20′ ) k20θCl. I.2. Effect of Temperature. The Tafel plot of the anodic oxidation of chloride ion on SPR for the various temperatures is shown in Figure 3. The experimental results revealed that the linear relationships were obtained in the overpotential range of 0.03-0.11 V. According to the equation of current density of the anodic oxidation of chloride ion (eq 37), the following relationship was obtained
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 353 Table 1. Effect of Temperature on the Anodic Oxidation of Chloride Iona T/K
i 0′ × 104/A cm-2
k20′ × 106/cm s-1
Tafel slope
R
283 293 298 303 308
2.09 2.77 3.55 4.29 5.48
1.08 1.43 1.84 2.22 2.84
119 119 117 117 110
0.49 0.49 0.51 0.51 0.57
a
pH ) 13, area of anode ) 3 cm2, and [NaCl] ) 1.0 M.
Table 2. Effect of Concentrations of Formaldehyde and Hypochlorite Ion on the Initial Reaction Ratea
Figure 3. Tafel plot for various temperatures. [NaCl] ) 1.0 M, stirring rate ) 600 rpm, pH ) 13, anode ) SPR, and area of anode ) 3.0 cm2.
η)
2.303RT (log i - log i0′) FR
a
[HCHO]/M
[OCl-]/M
ri/M min-1
0.04 0.06 0.07 0.08 0.1 0.15 0.15 0.15 0.15 0.15
0.3 0.3 0.3 0.3 0.14 0.02 0.04 0.06 0.08 0.10
0.0122 0.0240 0.0286 0.0444 0.0800 0.0100 0.0208 0.0300 0.0429 0.0533
pH ) 13, temp ) 198 K, and stirring rate ) 600 rpm.
(38)
where
i0′ ) 2Fk20′[Cl-]
(39)
According to the above equations, the slopes and intercepts of the linear relationships in Figure 3 are 2.303RT/ FR and -(2.303RT/FR) log i0′, respectively. Substituting the slopes and intercepts of linear relationships in Figure 3, the concentration of chloride ion in the aqueous phase, and the constants (R, F, and T) into the terms of slope and intercept of the relationship of η - i (eqs 38 and 39), the values of charge-transfer coefficients and k20′ for the various temperatures were obtained as illustrated in Table 1. The exchange current density (i0′) and electrochemical rate constant (k20′) increased from 2.09 × 10-4 A cm-2 and 1.08 × 10-6 cm s-1 to 5.48 × 10-4 A cm-2 and 2.84 × 10-6 cm s-1, respectively, when the temperature increased from 283 to 308 K. The value of charge-transfer coefficient (R) was located in the range of 0.49-0.57 (Table 1). A straight line with a slope of 3435 ( 242 was obtained from a plot of the logarithmic electrochemical rate constant against reciprocal of temperature as shown in Figure 4. The activation energy of the anodic oxidation of chloride ion on SPR anode was 28.6 kJ mol-1 as calculated from the slope of the straight line. The intercept of the straight line was -1.66 ( 0.82. The preexponential factor was evaluated from the intercept as 0.19 cm s-1. II. Kinetics of Oxidation of Formaldehyde with Hypochlorite Ion. II.1. Effect of Concentration of Formaldehyde. Increasing the concentration of formaldehyde in the aqueous phase from 0.04 to 0.10 M resulted in the increase of the initial rate of oxidation of formaldehyde from 1.22 × 10-2 to 8.00 × 10-2 M min-1 when the concentration of hypochlorite ion was 0.3 M as shown in Table 2. A straight line of slope 2.02 ( 0.21 was found when the logarithm of initial oxidation rate of formaldehyde was plotted against the logarithm
Figure 4. Effect of temperature on the rate constant of anodic oxidation of chloride ion. [NaCl] ) 1.0 M, stirring rate ) 600 rpm, pH ) 13, anode ) SPR, and area of anode ) 3.0 cm2.
of the concentration of formaldehyde in the solution (Figure 5). The experimental results revealed that the reaction order of the oxidation of formaldehyde with hypochlorite ion was 2 with respect to the concentration of formaldehyde. The reaction order was 1 with respect to the concentration of formaldehyde in the case 1 of the Theoretical Analysis section which did not agree with the experimental results. II.2. Effect of Concentration of Hypochlorite Ion. Experimental results demonstrated that the initial rate of oxidation of formaldehyde increased from 1.00 × 10-2 to 5.33 × 10-2 M min-1 when the concentration of hypochlorite ion in the aqueous solution increased from 0.02 to 0.10 M and the concentration of formaldehyde was kept at 0.15 M as illustrated in Table 2. A plot of the logarithm of the initial rate of oxidation of formaldehyde against the logarithm of the concentration
354 Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997
Figure 5. Effect of concentration of formaldehyde on the initial rate of oxidation of formaldehyde with hypochlorite ion. pH ) 13, [OCl-] ) 0.30 M, temp ) 298 K, and stirring rate ) 600 rpm.
Figure 7. Effect of concentration of formic acid on the initial rate of oxidation of formaldehyde with hypochlorite ion. pH ) 13, [HCHO] ) 0.10 M, [OCl-] ) 0.30 M, temp ) 298 K, and stirring rate ) 600 rpm. Table 3. Effect of Temperature on the Initial Reaction Rate and Rate Constant of the Oxidation of Formaldehyde with Hypochlorite Iona temp/K
ri/M min-1
kh/M-2 min-1
278 288 398 308 318
0.031 0.050 0.080 0.133 0.250
10.26 16.68 26.68 44.35 83.34
a pH ) 13, stirring rate ) 600 rpm, [CH O] ) 0.08 M, and [OCl-] 2 ) 0.14 M.
mechanism of the oxidation of formaldehyde with hypochlorite ion in the aqueous phase was case 2, and the rate-determining step was eq 11. The reaction rate equation was
rh ) kh[CH2O]2[OCl-]
Figure 6. Effect of concentration of hypochlorite ion on the initial rate of oxidation of formaldehyde with hypochlorite ion. pH ) 13, [HCHO] ) 0.15 M, temp ) 298 K, and stirring rate ) 600 rpm.
of hypochlorite ion resulted a straight line of slope 1.04 ( 0.02 (Figure 6). II.3. Effect of Concentration of Formic Acid. The reaction order is obtained as 1 with respect to the concentration of formic acid as indicated from case 4 of the Theoretical Analysis section. When the concentrations of formaldehyde and hypochlorite ion were fixed at 0.10 and 0.30 M, respectively, and the concentration of formic acid increased from 0 to 0.08 M, the initial rate of oxidation of formaldehyde with hypochlorite altered slightly (Figure 7). The experimental results indicated that eq 13 was not the rate-determining step. The effect of pH on the reaction rate of oxidation of formaldehyde with hypochlorite ion in the alkaline solution was insignificant (Suzuki et al., 1975) when the pH was greater than 10. Therefore, the reasonable
(40)
where kh ) k8K5K6K7. II.4. Effect of Temperature. The initial reaction rate of oxidation of formaldehyde increased from 0.031 to 0.250 M min-1 when the reaction temperature increased from 278 to 318 K (Table 3). The reaction rate constant (kh) was evaluated by the substitution of the concentrations of formaldehyde and hypochlorite ion as well as the initial reaction rate for various temperatures into eq 40. Increasing the temperature from 278 to 318 K resulted in the increase of rate constant from 10.26 to 83.34 M-2 min-1 (Table 3). A plot of the logarithmic rate constant of the oxidation of formaldehyde with hypochlorite ion against -1/T yielded a straight line as shown in Figure 8. The slope of the straight line was 4561 ( 255, which corresponded to an activation energy of 37.9 kJ mol-1. The preexponetial factor was 1.27 × 108 M-2 min-1 evaluated from the intercept of the straight line. III. In Situ Degradation of Formaldehyde with Electrogenerated Hypochlorite Ion. Upon substituting the reaction orders and rate constants obtained in the above discussion into the eqs 27 and 36, the concentration of formaldehyde in the aqueous phase was
Ind. Eng. Chem. Res., Vol. 36, No. 2, 1997 355
Conclusions Experimental results obtained in this study confirmed that the reaction order of the anodic oxidation of chloride ion on SPR anode was unity with respect to chloride ion. The reaction orders of the oxidation of formaldehyde with hypochlorite ion were found to be 2 and 1 with respect to formaldehyde and hypochlorite ion, respectively. The activation energies of the anodic oxidation of chloride ion and the oxidation of formaldehyde with hypochlorite ion were 28.6 and 37.9 kJ mol-1, respectively. The concentration of formaldehyde was degraded from 3000 to 279 ppm and the degradation fraction was 90.7% when the electrolysis time was 111 min. The theoretical calculations of the oxidative degradation of formaldehyde with electrogenerated hypochlorite ion correlated sufficiently with the experimental results. The deviation between the model calculations and experimental results might be due to the oxidative degradation of formaldehyde with dissolved oxygen in the aqueous solution. Acknowledgment Figure 8. Effect of temperature on the rate constant of the oxidation of formaldehyde with hypochlorite ion. pH ) 13, [HCHO] ) 0.1 M, [OCl-] ) 0.3 M, and stirring rate ) 600 rpm.
The support of National Science Council of the Republic of China (NSC 82-0421-P-029-001-Z) and Tunghai University is acknowledged. Nomenclature A ) surface area of anode E ) anodic potential F ) Faraday’s constant i ) current density i0 ) exchange current density k ) rate constant K ) equilibrium constant k0 ) electrochemical rate constant at η ) 0 r ) reaction rate t ) time T ) temperature V ) volume of solution Subscripts ads ) adsorption h ) homogeneous i ) initial state Superscripts m, n, p, q ) reaction orders
Figure 9. Effect of electrolysis time on the concentration of formaldehyde. Temp ) 318 K, i ) 75 mA cm-2, [NaCl] ) 1.0 M, pH ) 13, area of anode ) 3.0 cm2, volume of electrolyte ) 120 mL, anode ) SPR, and stirring rate ) 600 rpm.
Greek Symbols
evaluated when the constant current density was applied to the reaction system as shown in Figure 9. The concentration of formaldehyde decreased from 3000 to 279 ppm when the initial concentration of formaldehyde was 3000 ppm, and the electrolysis time increased from 0 to 111 min. The degradation fraction of formaldehyde was 90.7%. The experimental results closely correlated with the model calculations when the initial concentration of formaldehyde in the aqueous solution was located in the range of 1000-3000 ppm (Figure 9). The deviation of model calculations from the experimental might be due to the degradation of formaldehyde with dissolved oxygen in the aqueous solution (Do and Chen, 1993).
Literature Cited
R ) charge-transfer coefficient η ) overpotential θ ) degree of coverage with chlorine atoms
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Received for review September 21, 1995 Revised manuscript received October 30, 1996 Accepted November 8, 1996X IE950583Z
X Abstract published in Advance ACS Abstracts, January 1, 1997.
