Vol. 25, No. 1 (2013)
Kinetics of Chlorine Dioxide Oxidation of Benzidine in Wastewater 495
Effect of temperature on reaction rate constant: Under
condition of [BD]0 = 0.25 mmol L-1, [ClO2]0 = 2.5 mmol L-1
and pH 6.5, second order reaction rate constants k were 2.833
× 10-4, 3.883 × 10-4, 5.850 × 10-4 and 7.817 × 10-4 L (mmol s)-1
with water temperature T = 288, 293, 298 and 303 K, respec-
tively. As shown in Fig. 4, a linear equation of ln k = 17.245-
6146.6 (1/T) (R2 = 0.9902) was obtained, which means the
relation between reaction rate constant k and absolute tempe-
rature is in good accordance withArrhenius empirical equation.
The slope of the equation is -Ea/R and intercept is ln A. The
reaction activation energy was calculated as Ea = 51102.8 J
mol-1 ≈ 51.1 kJ mol-1, A = 5.143 × 105 L(mmol s)-1. Apparently,
ClO2 oxidation of benzidine could proceed under common
water treatment conditions.
Fig. 5. Relation between -ln(Ct/C0) and time under different reaction pH;
298K; [BD]0 = 0.25 mmol L-1; [ClO2]0 = 2.5 mmol L-1; pH = 3.0
( ), 4.66 ( ), 6.5 ( ), 8.0 ( ), 9.5 (
)
0, 5, 18 and 60 min, were chosen to reveal the intermediates.
Typical GC patterns of the reaction solution intermediates at
different time, containing the residual benzidine and products
formed, are provided as Fig. 6(a) to (d), respectively, where
compounds related to the starting pollutant show retention time
in the range from 6.27 to 21.98 min. Prior to the commence-
ment of oxidation, the first GC chromatogram of the benzidine
solution was collected [Fig. 6(a)]. This shows a clear trace at
the retention time of 21.21 min. After reacted with ClO2 for
5 min [Fig. 6(b)], the GC analysis revealed that the concen-
tration of benzidine in the solution decreased dramatically and
two new peaks appeared with retention time at 9.14 and 12.45
min, respectively. At 18 min of the reaction proceeded
[Fig. 6(c)], The peak of the benzidine further decreased and
the peak of the product at 9.14 min increased with the decrease
at 12.45 min. At the termination of the procedure, 60 min, the
peak of benzidine decreased continuously with time, while
another new strong peak appeared with retention time at
6.42 min, but there were no other peaks that appeared
[Fig. 6(d)].
Fig. 4. Relation between rate constant and temperature. pH: 6.5; [BD]0 = 0.25
mmol L-1; [ClO2]0 = 2.5 mmol L-1; Temp. 288 K, 293 K, 298 K, 303 K
Effect of pH value on reaction rate constant: The
solution pH changes not only the oxidation potential of ClO2,
but also presence state of benzidine in water as well. Thus, pH
is a key factor affecting reaction rate. Under condition of [BD]0
= 0.25 mmol L-1, [ClO2]0 = 2.5 mmol L-1 and 298 K, second
order reaction rate constants k were 1.383 × 10-4, 3.667 × 10-4,
5.850 × 10-4, 5.167 × 10-4 and 3.283 × 10-4 L(mmol s)-1 with
pH value of 3.0, 4.66, 6.5, 8.0 and 9.5, respectively.
As shown in Fig. 5, second order reaction rate constant k
was increased with the increase of pH value to pH 9.5 and
then decreased down. The reaction rate constant k was affected
by oxidation potential of ClO2 as well as presence state of
benzidine. The oxidation potential of ClO2 was the predominant
factor affecting the reaction rate constant k while presence
state of benzidine remained unchanged. With pH value higher
than 9, oxidation potential was decreased due to dispropor-
tionation reaction of ClO24.
It is evident that there are three intermediates mainly
formed during the reaction and they were identified by EI mass
spectra through above 95 % match with the NIST 02L MS
library. The results reveals that the three intermediates are
hydrazobenzene, azobenzene and para-benzoquinone. Further-
more, the identification of the products were established from
a comparison of the GC-MS data with that of an authentic
sample.
Conclusion
The reaction between ClO2 and benzidine was first-order
with respect to both ClO2 and benzidine and the entire reaction
was of second-order. Under condition of pH 6.5 and 298 K,
the second-order reaction rate constant k was 5.721 × 10-4
L(mmol s)-1. The rate constant k had the same change with the
reaction temperature. The reaction activation energy was 51.1
kJ mol-1, revealing that the reaction could take place under
usual water treatment conditions. The rate constant k increased
with the reaction pH increasing, then decreased. Sequences
Reaction intermediates identification of ClO2 oxidation
of benzidine: The colourless benzidine solution turned to light
red gradually, then pale yellow slowly and almost colourless
finally during the oxidation reaction at pH 6.5, which meant a
series coloured intermediates generated during the oxidation
reaction5,6.
Since it was found that the reaction proceeded quickly
and finished in about 60 min, some sampling time, such as