Catalytic degradation of CH2O and C6H5CH2OH in wastewaters

Article abstract of DOI:10.1016/S0043-1354(01)00458-4

The heterogeneous oxidation of benzyl alcohol and formaldehyde in aqueous medium has been investigated. The catalytic oxidation of these compounds is carried out in the presence of Ni-oxide system at ambient temperature. The results show that under studied conditions, a 90% conversion of CH₂O to CO₂ and a complete oxidation of C₆H₅CH₂OH to C₆H₅COOH is achieved. Addition of H₂SO₄ to adjust pH to 2 further precipitates benzoic acid. The precipitate is filtered and the resulting filtrate is free of organic substances (COD is below 10mg O₂dm^-3). Based on the results obtained, a technology for purification of wastewaters containing benzyl alcohol as well as a catalytic method for degradation of CH₂O in aqueous solutions have been developed.

Full text of DOI:10.1016/S0043-1354(01)00458-4

Water Research 36 (2002) 2297–2303
Catalytic degradation of CH O and C H CH OH in
2
6 5
2
wastewaters
S.T. Christoskova*, M. Stoyanova
Department of Physical Chemistry, University of Plovdiv, 24 Tzar Assen St, 40000 Plovdiv, Bulgaria
Received 15 March 2001; received in revised form 8 August 2001; accepted 1 October 2001
Abstract
The heterogeneous oxidation of benzyl alcohol and formaldehyde in aqueous medium has been investigated. The
catalytic oxidation of these compounds is carried out in the presence of Ni-oxide system at ambient temperature.
The results show that under studied conditions, a 90% conversion of CH O to CO and a complete oxidation
2
2
of C
precipitate is filtered and the resulting filtrate is free of organic substances (COD is below 10 mg O
results obtained, a technology for purification of wastewaters containing benzyl alcohol as well as a catalytic method for
degradation of CH O in aqueous solutions have been developed. r 2002 Elsevier Science Ltd. All rights reserved.
6 5 2 6 5 2 4
H CH OH to C H COOH is achieved. Addition of H SO to adjust pH to 2 further precipitates benzoic acid. The
ꢀ
3
2
dm ). Based on the
2
Keywords: Catalytic oxidation; Ni-oxide system; Detoxification of organic substances; Benzyl alcohol; Benzoic acid; Formaldehyde
1
. Introduction
provide complete mineralization of organic substances
while being environmental friendly.
The degradation of organic pollutants, such as
Transient reaction at 273 and 300 K has been used to
study the initial steps in the photocatalytic oxidation of
benzyl alcohol and benzaldehyde absorbed on a thin
film of TiO catalyst by O [3]. It was found that, in the
formaldehyde, benzaldehyde and benzyl alcohol, has
attracted considerable attention due to the worldwide
concern about the toxicity of these compounds. Waste-
waters from different industries (pharmacy, perfumery
and cosmetics, organic synthesis, manufacture of resin
and colours, etc.) contain formaldehyde and benzyl
alcohol in wide concentration rangesFfrom 0.5 to
2
2
course of oxidation reaction benzyl alcohol was first
photocatalytically oxidized to benzaldehyde and then to
CO and H O.
2
2
Selective oxidation of benzyl alcohol to benzaldehyde
was carried out over pumice-supported bimetallic and
monometallic Pd and Ag catalysts. Preliminary kinetic
studies were performed at 333 K in autoclave, at a
pressure of 2 atm in pure oxygen. Under these condi-
tions, small amounts of benzoic acid were detected [4].
Bessona and Gallezot [5] have studied the oxidation of
aldehydes, alcohols or carbohydrate derivatives in
aqueous medium with air in the presence of palladium
and platinum catalysts under mild conditions (293–
353 K and atmospheric pressure).
ꢀ
3
1
0 g dm [1]. The treatment of hazardous wastewaters,
containing these pollutants in an environmentally
acceptable manner and at a reasonable cost is a topic
of great universal importance. The biological processes
which are preferably applied to treat wastewaters
containing organic compounds do not always give
satisfactory results, since many organic substances are
resistant to biological treatment [2]. Low-temperature
aqueous-phase catalytic oxidation is a promising alter-
native for the detoxification of wastewaters containing
toxic substances. Catalytic oxidation reactions could
The present investigation aims at studying the
possible development of an efficient catalytic method
for the purification of wastewaters containing organic
compounds using a low-temperature oxide catalyst.
*Corresponding author. Tel.: +359-32-261-534.
E-mail address: schrist@argon.acad.bg (S.T. Christoskova).
0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 4 5 8 - 4

2
298
S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
2.3. Analytical methods
2
. Experimental
2
.1. Catalyst
The changes in the concentrations of benzyl alcohol
and its oxidation productsFintermediate benzaldehyde,
and the final oxidation product benzoic acid, were
monitored by UV–VISspectroscopy and thin layer
chromatography. The corresponding absorbance max-
ima of these compounds are at 205, 248, and 225 nm,
respectively. Absorbance was measured with a Perkin
Elmer Lambda-15 UV–VISspectrophotometer.
A Ni-oxide system was used as a catalyst. It was
synthesized, according to the procedure described in
detail by Christoskova et al. [6]. The catalyst was
characterized by means of IR, XPS, ESR, X-ray
diffraction and chemical analyses. Analytical data
obtained gave us reasons to conclude that under the
applied conditions of synthesis of Ni-oxide with a high
active oxygen content (O*B8%), high degree of
oxidation and octahedral coordination of the metal
ions, and weak surface Ni–O bonds was obtained. The
following formula of the oxide was suggested: Nix
Thin layer chromatographic analysis was carried out
using standard chromatographic plates with silica gel 60
F
264 support (E. Merck) and trichloromethane as eluent.
The spots corresponding to benzyl alcohol, benzalde-
hyde and benzoic acid were developed with UV light.
The COD was determined according to standard
procedure [7].
(
OH)
surface being mainly in their highest oxidation state, i.e.
4. Due to these attractive properties, a high catalytic
y z 2
O * mH O, the metal cations on the catalyst
+
The initial and current concentrations of formalde-
hyde were checked by gas chromatography. GC analysis
was performed with a Carlo Erba Fractovap 2400 T gas
chromatograph equipped with an electrolytic conduc-
tivity detector. The operating parameters were as
follows: temperature of the detector 473 K, temperature
of the column (filled with 20% polyethyleneglycol on
Chromosorb W)F383 K, carrier gasFhydrogen, vo-
lume of the analysed sample 2 ml. The retention times of
activity of the catalyst was expected in reactions of
complete oxidation, carried out at low temperature.
2
.2. Treatment conditions
The heterogeneous catalytic oxidation of benzyl
alcohol and formaldehyde in aqueous solutions was
carried out in a thermostated reactor under continuous
stirring, thus providing an equal level of all parameters
describing the state of the system (temperature, con-
centration, pH). The experimental runs were carried out
as follows: the substrate solution was saturated with
oxygen by bubbling air under atmospheric pressure for
CO
were 30, 70, and 1624 s, respectively. The concentrations
of CH O, CO and HCOOH were calculated from the
2 2
, CH O, and HCOOH under the studied conditions
2
2
corresponding peak area, measured by means of an
integrator.
3
0 min prior to adding the catalyst. Then the desired
amount of catalyst (with particle size in the range
.6–1.0 nm) was suspended in the solution. The applica-
The IR spectra of the fresh and used catalyst as well as
the spectra of benzoic acid (product resulting in the
heterogeneous oxidation of benzyl alcohol and commer-
cially available reagent from E. Merck) were recorded
with an FTIR-1750 Perkin Elmer spectrophotometer in
KBr tablets.
0
tion of the catalyst in the form of a suspension is more
efficient because it leads to an increase in the catalyst
surface accessible to the reaction. The air was con-
tinuously bubbled during the runs, thus maintaining a
constant steady-state concentration of dissolved oxygen.
After exhausting the active oxygen of the catalyst, the
latter was regenerated with NaOCl.
The efficiency of the oxidation of benzyl alcohol and
formaldehyde with the participation of Ni-oxide system
under different conditions was evaluated through both
ꢀ
1
the rate constant (k; min ) and the overall degree of
conversion (a; %). The latter was calculated according
to the equation
The catalytic oxidation of formaldehyde was carried
out using 1 M solution of CH
2
O at pH 7.0, temperature
2
98 K and catalyst concentration of 0.5 or 2.0 g dmꢀ3
.
C0 ꢀ C
C0
The investigations concerning oxidation of benzyl
alcohol were carried out with both model solutions
a ¼
100;
ꢀ
3
where C0 is the initial substrate concentration, mg dmꢀ3,
containing 20 g dm benzyl alcohol and wastewaters
from ‘‘Hemus’’ plant, Bourgas, Bulgaria, containing
ꢀ
3
.
C is the current substrate concentration, mg dm
ꢀ
3
from 15 to 20 g dm benzyl alcohol. After complete
conversion of benzyl alcohol into benzoic acid, the
catalyst is filtered and 2 M H SO was added to the
The rate constant of the reaction of benzyl alcohol
oxidation was estimated according to the first-order
kinetic equation
2
4
filtrate and pH adjusted to 2.0. At this pH, white crystals
of benzoic acid were precipitated. After separation of the
crystals (by filtration) the liquid was neutralized with
NaOH. The resulting solution is free of organic
1
AN ꢀ A0
AN ꢀ At
k ¼ t ln
;
where A0; At; AN are the absorbances (determined from
the recorded UV spectra) of benzoic acid at the
ꢀ1
compounds and COD is below 10 mg O
2
l
.

S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
2299
beginning of the reaction (initial absorbance), at a fixed HCOOH produced in complete conversion of CH
2
O
moment of the reaction and at complete conversion of
benzyl alcohol into benzoic acid, respectively.
The activation energy (Ea; kJ molꢀ ) was calculated
from the rate constant vs. temperature plot according to
the Arrhenius equation.
into HCOOH under identical experimental conditions.
It was established that a complete oxidation of HCOOH
1
2
into CO was achieved. These results gave us reasons to
suggest that the catalytic oxidation of CH O proceeds
2
according to a consecutive scheme:
k
k
2
1
CH2O - HCOOH - CO2:
3
. Results and discussion
HCOOH was the only detected intermediate and its
measured concentration in the course of the oxidation
reaction was quite low. This is a reason to suggest that
the rate of consumption of HCOOH is higher than the
rate of its formation, i.e. the rate constant k2 has a
higher value than k1:
The results of kinetic investigations are illustrated in
Figs. 2 and 3. Data presented are average results of three
separate experiments and indicate that:
3
.1. Catalytic oxidation of formaldehyde
The change in formaldehyde concentration in the
course of oxidation is illustrated by gas chromatograms
represented in Fig. 1. The results of GC analysis show
that a high degree of conversion of CH
O is achieved (aB90%). The presence of HCOOH as
an intermediate oxidation product was recorded
Fig. 1b). A catalytic oxidation of a model solution of
2 2
O to CO and
H
2
(
*
HCOOH was carried out with a view to elucidating the
oxidation mechanism. The concentration of HCOOH in
the model solution was equivalent to the amount of
2
a low-temperature catalytic oxidation of CH O with
the participation of the synthesized catalyst proceeds
under the studied conditions;
Fig. 1. Typical products detected by GC analysis during formaldehyde oxidation over Ni-oxide system: (a) 0 min; (b) 30 min; (c)
50 min.
1

2300
S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
ꢀ
1
00
11
of HO
2
ion-radicals was detected in the liquid phase
2
as it has been observed by Andreev et al. [8] by means
of UV spectral analysis.
80
60
40
20
0
1
9
8
7
0
1
3.2. Catalytic oxidation of benzyl alcohol
The changes in the concentration of benzyl alcohol
and its oxidation products during the process are
illustrated by the UV spectra represented in Fig. 4. An
isobestic point is evident which points out to the
formation of a single intermediate (benzaldehyde), the
latter being further oxidized to benzoic acid. Data
obtained by thin-layer chromatography provide addi-
0
30
60
90
120
Time, min
Fig. 2. Kinetics of formaldehyde oxidation: 1 ꢀ ln C ¼ f ðtÞ;
ꢀ a ¼ f ðtÞ:
2
tional evidence that the transitions: C
CHO-C COOH take place.
The kinetic curves illustrating changes in the concen-
6 5 2
H CH OH-
C
H
6 5
6 5
H
tration of the starting compound and of the reaction
products during the reaction are shown in Fig. 5.
Calculations based on these data indicate that a
heterogeneous catalytic oxidation of benzyl alcohol
proceeds as a first-order reaction with respect to the
alcohol concentrationFthe ln AC H CH OH vs. time plot
6
5
2
is a straight line (Fig. 6).
Data concerning the impact of reaction parameters
such as temperature and mass of the catalyst, on the rate
constant and on the time for complete degradation of
benzyl alcohol are presented in Table 1 and Fig. 7. The
results reveal that the studied factors influence signifi-
cantly the efficiency of the catalytic process. It is seen
that at 313 K and catalyst amount of 0.75 g, a complete
degradation is achieved within 13 min. The value of the
activation energy of the reaction was found to be
Fig. 3. The effect of the catalyst amount on the efficiency of the
formaldehyde oxidation; catalyst concentration: 1–0.5 g dm
ꢀ3
;
ꢀ3
2–2.0 g dm .
ꢀ
1
9
8 kJ mol .
The results from the low-temperature catalytic oxida-
*
*
a very high degree of degradation of formaldehyde to
CO has been achieved as proved by means of GC
analysis. Fig. 1c shows that the conversion is approx.
2
tion of benzyl alcohol in an alkaline medium (pH 10–11)
show that benzyl alcohol was selectively oxidized, its
9
0%;
the catalytic oxidation of formaldehyde proceeds
according to pseudo-first-order kinetics with
a
respect to substrate concentration, which is shown
by the linear ln C vs. time plot (Fig. 2). Zero-order
dependence with respect to oxygen was found under
studied conditions due to the fact that all experiments
were performed with continuous air bubbling
through the liquid phase;
*
the amount of catalyst influences the rate of
formaldehyde degradation (Fig. 3). Evidently, the
increase in the catalyst amount causes an increase in
the oxidation efficiency. The rate constants of the
ꢀ
1
oxidation process are 0.015 min with m ¼ 2:0 g and
ꢀ
1
0
.007 min with m ¼ 0:5 g, respectively. This is a
reason to suppose that the reaction proceeds only on
the catalyst surface and is not homogeneous. An
additional evidence for this conclusion is the fact that
after removing the catalyst from the reaction mixture
the oxidation process ceased. Moreover, no presence
Fig. 4. UV spectrum profiles during the oxidation of benzyl
alcohol on the Ni-oxide system: a–eFabsorbance maxima
0
0
0
0
0
corresponding to C H COOH; a , b , c , d , e Fabsorbance
6
5
6 5
maxima corresponding to C H CHO.

S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
2301
C H CH OH
0.25
0.20
6
5
2
3
13 K
C H CHO
6
5
0
.8
.6
293 K
C H5COOH
6
0
0.15
0
0
.4
.2
0.10
0
.05
.00
0
0
0
50
100
150
200
0
0.5
1.0
1.5
2.0
Time, min
Time, h
Fig. 5. Concentration change of benzyl alcohol and reaction
products during oxidation process.
Fig. 7. The impact of the temperature on the kinetics of the
benzoic acid formation; catalyst concentration 0.25 g.
Fig. 6. Linear transformation ln AC6H CH OH ¼ f ðtÞ of the
kinetic curves of benzyl alcohol disappearance.
5
2
oxidation running practically completely to benzoic
acid. In the course of the oxidation process, benzalde-
hyde is formed as an intermediate which is further
oxidized to benzoic acid.
Fig. 8. IR-spectra of the obtained benzoic acid (1) and the
commercial product of E. Merck (2).
Fig. 8 shows the IR spectra of benzoic acid obtained
after catalytic oxidation of wastewater from ‘‘Hemus’’
plant, Bulgaria (1) and of commercially available
product from E. Merck (2). The comparison of the
spectra shows that catalytic oxidation of benzyl alcohol
on the Ni-oxide system results in benzoic acid of high
purity. The analysis of the remaining solutions after
removal of the benzoic acid shows that they do not
contain organic compounds (COD is below 10 mg
3
2
ꢀ
O
dm ).
The low-temperature complete oxidation of CH
2
O as
well as oxidation of benzyl alcohol to benzoic acid with
the participation of Ni-oxide system can be ascribed to
the high content of weakly bonded surface-active oxygen
on the catalyst which is responsible for the catalytic
oxidation.
Table 1
The influence of the temperature and mass of the catalyst on the efficiency of the benzyl alcohol oxidation reaction
Temperature
T (K)
Mass of the
catalyst m (g)
Time for complete
degradation t (min)
Rate constant
)
Activation energy
ꢀ1
k (minꢀ
1
Ea (kJ mol )
2
2
3
3
93
93
13
13
0.25
0.75
0.25
0.75
230
63
0.010
0.048
0.132
0.450
98
28
13

2302
S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
the corresponding intermediates, or are further oxidized
to carbonate complexes, which can subsequently pro-
duce CO . The a-H atom released by the organic
2
1
1
2
molecule forms OH radicals with the surface-active
oxygen of the catalyst surface. These radicals possess
very high reactivity and take part in the next stages of
oxidation. In support of this statement, is the fact that
during the depletive oxidation of the substrate the
amount of active oxygen in the catalyst is continuously
decreased as proved by chemical and IR spectral
analysis. In the course of the oxidation process, the
2
4
000 3500 3000 2500 2000
1500
1000
500
ꢀ
1
band at 586 cm , characteristic of the valent vibrations
of metal–oxygen bond on the surface of the Ni-oxide
system, disappears from the spectrum and a new band at
−1
ν, cm
Fig. 9. IR spectra of the Ni-oxide system : (1) fresh catalyst; (2)
after depletive oxidation of benzyl alcohol.
ꢀ
1
3641 cm appears (Fig. 9). The latter band is attributed
to surface OH groups (formed between active oxygen of
the catalyst and abstracted hydrogen atom). On
regeneration of the reduced catalyst with NaOCl, the
The present data are in agreement with both the
results concerning activity and selectivity of the Ni-oxide
system with respect to oxidation of other organic
compounds as well as with the results of the character-
ization of the catalyst before and after oxidation
reactions by means of different physical and chemical
methods reported in a previous paper of ours [9]. Based
on the summarized results some assumptions about the
main steps in the oxidation of oxygen-containing
organic compounds in the presence of Ni-oxide system
can be made.
ꢀ
1
ꢀ1
band at 3641 cm disappears while that at 573 cm
reappears, i.e. these bands are generically related [6].
Based on these results and their interpretation, a
probable mechanism of the catalytic oxidation of
formaldehyde and benzyl alcohol with the participation
of Ni-oxide system can be proposed, as shown in
Fig. 10.
4. Conclusion
Most probably, the process is initiated by a dissocia-
tive adsorption of the substrate through a-H atom
abstraction and formation of surface complexes. The
latter, depending on the conditions, are decomposed to
The results of the present study give us reasons to
conclude that it is possible to realize the purification of
wastewaters containing CH O and benzyl alcohol by
2
.
H
H
OH
HO
H
Ni-oxide
.
C
O
O
C
C
O
C
(O*)
H
Ni-oxide
O*)
(
(a)
.
HO
HO
H O HO
OH
.
O
CO2
C
O
H
-
2
OH
H
.
Ni-oxide
.
OH
H C C
H
H C C
5 6
H C
5 6
C
H
5
6
(
O*)
OH
OH
OH
(
b)
-
H O
2
.
H C
5 6
H C
Ni-oxide 5 6
H C
5
6
OH
.
C
O
C
O
O
C
(
O*)
H
HO
Fig. 10. Simplified reaction pathway of formaldehyde (a) and benzyl alcohol (b) oxidation reactions.

S.T. Christoskova, M. Stoyanova / Water Research 36 (2002) 2297–2303
2303
their catalytic oxidation over Ni-oxide system under
mild conditions. A 90% conversion of formaldehyde to
[2] Mantzavinos D, Sahibzada M, Livingston A, Metcalfe I,
Hellgardt K. Wastewater treatment: wet air oxidation as a
precursor to biological treatment. Catal Today
1999;53:93–106.
CO can be achieved for 150 min at pH ¼ 7:0; 298 K and
2
ꢀ
3
concentration of the catalyst 2 g dm . The complete
degradation of benzyl alcohol to benzoic acid is achieved
within 13 min at 313 K and catalyst amount of 0.75 g.
The results of the present investigation were used to
develop a technological scheme, which was applied to
purification of wastewaters from ‘‘Hemus’’ plant,
Bourgas, Bulgaria [10]. The main advantage of the
proposed catalytic method for purification of waste-
water containing highly toxic organic compounds is that
a very significant cleaning effect is achieved for a short
time. This fact allows the treatment of wastewater of
high flow at low temperatures (25–361C).
[
3] Larson S, Falconer J. Initial reaction steps in photocata-
lytic oxidation of aromatics. Catal Today 1997;44:
5
7–65.
4] Yang J, Li D, Zhang Z, Li Q, Wang H. A study of the
catalytic oxidation of formaldehyde on Pt/Fe /TiO . J
Photochem Photobiol A: Chem 2000;137:197–202.
[
2
O
3
2
[5] Bessona M, Gallezot P. Selective oxidation of alcohols and
aldehydes on metal catalysts. Catal Today 2000;57:
127–41.
[
6] Christoskova St, Danova N, Georgieva M, Argirov O,
Mehandjiev D. Investigation on nickel oxide system for
heterogeneous oxidation of organic compounds. Appl
Catal A General 1995;128:219–29.
[
[
[
7] Eaton A, Clesceri L, Greenberg A. Standard methods for
the examination of water and wastewater. Washington:
American Public Health Association, 1995.
Acknowledgements
8] Andreev A, Khristov P, Losev A. Catalytic oxidation of
sulfide ions over nickel oxide hydroxides. Appl Catal B
Environmental 1996;7:225.
The authors gratefully acknowledge the financial
support from the University of Plovdiv Research Fund
through Project 01-X-03.
9] Christoskova St, Stoyanova M. Degradation of phenolic
wastewaters over Ni-oxide. Water Res 2000;3096:1–5.
[
10] Christoskova St. Investigations on the catalytic oxidation
of organic compoundsFalcohols, phenols, aldehydes etc.
with the aim of development of technological decisions for
the purification of wastewaters. Final Report, Contract X-
123, National Science Fund, Ministry of Education and
Science, Republic of Bulgaria, 1993.
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1] Grushko Y. Harmful organic compounds in industrial
wastewaters. Peterburg: Himia, 1982.