Subcritical mineralization of sodium salt of dodecyl benzene sulfonate using sonication-wet oxidation (SONIWO) technique

Article abstract of DOI:10.1016/S0043-1354(00)00497-8

Subcritical mineralization of sodium salt of dodecyl benzene sulfonate via hybrid process-sonication followed by wet oxidation (SONIWO) has been investigated. Sonication of the compound enhanced the rates and % COD reduction during wet oxidation. In this

Full text of DOI:10.1016/S0043-1354(00)00497-8

Wat. Res. Vol. 35, No. 9, pp. 2300–2306, 2001
# 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(00)00497-8
0043-1354/01/$ - see front matter
SUBCRITICAL MINERALIZATION OF SODIUM SALT OF
DODECYL BENZENE SULFONATE USING
SONICATION}WET OXIDATION (SONIWO) TECHNIQUE
ATUL D. DHALE and VIJAYKUMAR V. MAHAJANI*
Department of Chemical Technology, Chemical Engineering Division, University of Mumbai, Matunga,
Mumbai – 400019, India
(First received 1 November 1999; accepted in revised form 28 September 2000)
Abstract}Subcritical mineralization of sodium salt of dodecyl benzene sulfonate via hybrid process}
sonication followed by wet oxidation (SONIWO) has been investigated. Sonication of the compound
enhanced the rates and % COD reduction during wet oxidation. In this process, homogenous CuSO4
catalyst was found to be effective. In wet oxidation studies, phenol, hydroquinone, maleic acid, oxalic acid,
propionic acid, and acetic acid were identified as intermediates. The global rate equations for wet
oxidation in terms of COD reduction were developed. # 2001 Elsevier Science Ltd. All rights reserved
Key words}sodium salt of dodecyl benzene sulfonate, sonication, wet oxidation, copper sulfate, COD
NOMENCLATURE
our earlier investigation (Dhale and Mahajani, 1999)
on wastewater treatment of the reactive dye house
BOD
COD
biological oxygen demand, g mꢀ3
waste by wet oxidation, it was observed that the
chemical oxygen demand, g mꢀ3
[COD]0 initial COD, g mꢀ3
waste containing surfactants like sodium salt of
dodecyl benzene sulfonate was difficult to oxidize.
Sonication as a pretreatment was found to make the
waste suitable for wet oxidation. Thus, the process
sonication followed by wet oxidation (SONIWO) has
been observed to be beneficial to treat compounds
that are difficult to degrade (Ingale and Mahajani,
1995). Therefore, a detailed investigation was under-
taken to treat the widely used surfactant sodium salt
of dodecyl benzene sulfonate by a similar process.
The high-power ultrasound application in waste-
water treatment has recently attracted many
researchers (Bhatnagar and Cheung, 1994; Kotro-
narou et al., 1995; Thoma and Gleason, 1999). When
a sound wave is transmitted through a fluid above the
frequency of ultrasound, i.e. 16 kHz very powerful
compression and rarefaction cycles are generated.
Cavitation occurs during these compression and
rarefaction cycles thereby resulting in collapse of
micro bubbles so that it is possible to achieve
temperatures as high as 5000 K and pressures as high
as 100 MPa at ambient conditions. The temperatures
generated are high enough for destruction of organics
through the various free radicals generated (Lorimer
and Mason, 1987).
[COD]f COD (g mꢀ3 ) at time t
i.d.
[O2]
r
internal diameter
oxygen concentration, kmol mꢀ3
rate of oxidation reaction with respect to COD
reduction, g mꢀ3 sꢀ1
t
T
time, s
temperature, K
INTRODUCTION
Various ionic and non-ionic surfactants such as
linear alkyl benzene sulfonates (LAB), alkyl phenol
exthoxylate, etc., are widely used in textile processing
such as dyeing, desizing, scouring and washing
agents, etc. The wastewater resulting from processing
contains the above surfactants which are harmful to
aquatic life. Other types of surfactants are also
present in domestic waste and many other industrial
effluents. The fate of these surfactants in the waste-
water and environment is reviewed by Naylor (1995)
and also cited by Westall et al. (1999). These auxiliary
compounds used in textile processing are poorly
biodegradable and can be the largest contributors to
COD and BOD of the waste (Correia et al., 1994). In
Sonication treatment was found to be effective
in treating toxic effluents and reducing the
toxicity. However, the treatment is highly energy
intensive and is not economically attractive or
*Author to whom all correspondence should be addressed.
Tel.: +91-22-4145-616; fax: +91-22-4145-614; e-mail:
vvm@udct.ernet.in
2300

Subcritical mineralization of sodium salt of dodecyl benzene sulfonate
2301
feasible alone. Further, sonication may destroy Oxygen with a minimum purity of 99.5% was used for
wet oxidation. AR-grade cupric sulfate was used as a
catalyst.
toxic components but will result into formation of
lower molecular weight compounds (Gonze et al.,
1999). Thus, the waste may exhibit a finite COD,
which subsequently needs to be treated by some
other techniques to meet the discharge standards.
Experimental
Sonication was carried out in a mechanically agitated
glass reactor having capacity 0.25 dm3. mounted in
Hence, it can be used to break bigger refractory
a
molecules into smaller ones or toxic to non-toxic
and then can suitably be discharged by the
conventional process. Earlier, Ingale and Mahajani
(1995) have already demonstrated the use of this
hybrid technology, sonication followed by wet
oxidation (SONIWO), to treat the refractory
waste. Dhale and Mahajani (1999) have also shown
the effectiveness of sonication followed by wet
oxidation.
The present study is focussed on treating the
pure compound surfactant, sodium salt of dodecyl
benzene sulfonate by sonication followed by
wet oxidation. The use of wet oxidation (WO)
technology is becoming very popular to treat
various industrial toxic and refractory wastes. Wet
oxidation is an attractive alternative because of
the reduced biological inhibition and has been
found to be an eco-friendly technology. The wet
oxidation technology can be successfully applied
to treat various types of waste streams including
hazardous and non-biodegradable wastes (Mishra
et al., 1995; Matatov-Meytal and Sheintuch, 1998;
conventional ultrasonic cleaning bath (manufactured by
Toshniwal Brothers, (Mumbai) Pvt. Ltd, India) equipped
with a thermostated jacket. The reactor had a four-bladed
impeller and was equipped with baffles and an air sparger.
The system was operated at 303 K having 150/350 W avg/
peak at a frequency of 40 kHz. External cooling was
provided through a cooling coil condenser, placed in a bath
to maintain the temperature in the reactor. Sonication was
carried out in the absence and presence of homogenous
CuSO4 as a catalyst.
The samples after sonication treatment were subjected
to wet oxidation. The wet oxidation experiments were
carried out in 0.3 dm3 SS-316 Parr High-Pressure
reactor having
a Parr 4842 temperature controller
(Parr Instruments Company, USA), in batch mode of
operation. The reactor (i.d. 65 mm) had a four-bladed
turbine type impeller (diameter 35 mm) and was
equipped with a pressure indicator, and a gas sparging
tube. The impeller speed was varied between
0 and
26.6 rps with a variable speed motor. The reactor was
also provided with a rupture disc as well as a non-return
valve at the gas inlet. Reaction mixture was then heated
to a desired temperature and once the temperature was
attained, the sample was withdrawn through a sample
condenser. This was deemed to be ‘zero-time’ for the
reaction. The liquid samples were collected through a
chilled condenser-cum-cooler mounted on the reactor.
As soon as the ‘zero-time’ sample of the reaction was
taken, oxygen was sparged into the reactor to a predeter-
mined pressure level and maintained at it while collecting
samples for analysis. In all experiments, it was ensured
that oxygen was available in far excess than that theoreti-
cally required. The total pressure is the sum of oxygen
pressure and vapor pressure. The experiments were
carried out at different temperatures, and oxygen pressure
and at various catalyst concentrations for the kinetic
study. The experimental variation in COD measurement
was less than 3%.
Imamura, 1999). Wet oxidation is
a process
of subcritical oxidation of organic material in
aqueous phase with oxygen (pure or air) at
elevated temperatures (373–623 K) and at pressures
ranging from 0.5 to 20 MPa. The products of
wet oxidation are usually intermediates like
low-molecular weight acids which are refractory
to further oxidation and subsequently harmless
products such as carbon dioxide, water and nitrogen.
Sulfur is converted to inorganic sulfates and phos-
phorus is converted to phosphates. Wet oxidation
has several advantages in that it can treat high
COD/BOD waste stream and can handle effluent
with high inorganic salt. Wet oxidation not only
treats the priority pollutants for suitable discharge
or makes them amenable to biodestruction but
also makes aqueous stream suitable for recycle or
reuse, thereby conserving the precious resource
‘water’ on this planet earth.
Analysis
Chemical oxygen demand (COD) content of the sample
was analyzed by the standard dichromate reflux method as
described by Snell and Ettre (1967). Acetic acid which is the
major intermediate formed during wet oxidation was
analyzed on a gas chromatography unit, Chemito 3865
(manufactured by Toshniwal Instruments Ltd, Mumbai).
A glass column packed with Carbopack BD-A 4% and
Carbowax 20 M was used with a flame ionization detector
and operated at an isothermal temperature, 413 K, to
analyze the acids formed. The N2 carrier gas flow was
In this study, the effect of ultrasound on wet
oxidation of sodium salt of dodecyl benzene sulfo-
nate was studied. The reaction pathways of wet 2.4 dm3 hꢀ1, temperature of detector and injector was kept
at 463 K.
oxidation were investigated and discussed.
The other intermediates and low molecular weight
acids formed during wet oxidation were analyzed by
high-pressure liquid chromatography (HPLC). A HPLC
METHODS
system (TOSOH, Japan) with
a UV detector, set at
230 nm alongwith RP18 (Merck Inc., Germany) column
was used for analysis. The separation was effected by
isocratic elution using a 50 : 50 mixture of methanol : water
at a flow rate of 0.060 dm3 hꢀ1. For the analysis, aqueous
samples were prepared in acetonitrile solution with suitable
Materials
Sodium salt of dodecyl benzene sulfonate was obtained as
a gift from Dai-ichi Karkaria Ltd., Mumbai, India. The
reagents used for analysis and other chemicals were A.R.
grades and obtained from S.D. Fine Chem Ltd, Mumbai. dilution.

2302
Atul D. Dhale and Vijaykumar V. Mahajani
RESULTS ANDDISCUSSION
The elimination of induction period is due to the fact
that the OH radicals as well as the strong oxidizing
agent, hydrogen peroxide, produced during cavita-
ꢂ
Sonication
An aqueous sample of sodium salt of dodecyl tion with oxygen (equations (1) and (3)) decompose
benzene sulfonate was prepared such that its COD the chemical substances during sonication, which can
was around 800–1000 g mꢀ3. It was subjected to then be well treated by wet oxidation. However, wet
sonication treatment in presence of air at a flow rate oxidation without sonication needs initiation, as the
of 0.0432 N/m3 hꢀ1 in a mechanically agitated glass compound is difficult to oxidize. The presence of
reactor mounted on a sonication bath for 90 min. oxygen leads to formation of ꢂO and ꢂOOH as
Temperature of bath was maintained at 303 ꢁ 2 K. discussed above. These radicals are mainly respon-
Cavitation in aqueous medium results in homolysis sible for decomposition of the substrate. The effect of
ꢂ
ꢂ
of water and produces OH and H:
sonication in presence of oxygen and nitrogen, on
wet oxidation, was studied and is shown in Fig. 3.
The results indicated that wet oxidation after
sonication with nitrogen resulted in a very low
COD reduction. The effects of oxygen and air are
ꢂ
ꢂ
H2O ! OH þ H
ð1Þ
ꢂ
The presence of oxygen leads to the formation of O
and ꢂOOH at the expense of ꢂH. This, in turn, would
lead to the formation of powerful oxidizing agent,
hydrogen peroxide (Lorimer and Mason, 1987):
O2 ! 2ꢂO
ð2Þ
ð3Þ
ð4Þ
ꢂ
ꢂH þ O2 ! OOH
ꢂOOH ! H2O2
These oxidative species from different sources are
used to degrade the organic compounds. Several
modes of reactivity have been proposed: pyrolysis,
decomposition, hydroxyl radical oxidation, plasma
chemistry and supercritical oxidation (Gonze et al.,
1999). Mankino et al. (1982) have confirmed that the
ꢂOH radicals play an important role in oxidizing
chemical substances during sonication. Henglein and
Gutierrez (1988) reported that chemical compounds
could be pyrolyzed within the cavitation bubble.
Kotronarou et al. (1991) have reported that sono-
chemical degradation mechanism of p-nitrophenol
was initiated by cleavage of the C–O bonding and
then followed by ꢂOH radical attack. However,
Nagata et al. (1996) suggested that the hydrophobic
ethylene chloride was decomposed by pyrolysis
within cavitation bubbles rather than dechlorination
Fig. 1. Effect of sonication on wet oxidation (temperature,
493 K, O2 pressure, 0.69 MPa).
ꢂ
by OH radicals.
Under typical conditions, wet oxidation was
performed before and after sonication. Wet oxida-
tion of the compound without sonication at 493 K
and 0.69 MPa O2 pressure resulted in about 58%
COD reduction in 120 min and an induction period
was observed. Induction period is the time required
for the generation of a sufficient number of free
radicals to initiate the reaction. However, sonication
treatment prior to wet oxidation showed a marked
enhancement in COD reduction and it was possible
to achieve about 75% COD reduction at similar
conditions. The effect of sonication followed by wet
oxidation on COD reduction is shown in Fig. 1. Also,
with sonication treatment, it was possible to elim-
inate the induction period, and hence enhance the
initial rate of oxidation. The effect of sonication on
induction period of wet oxidation is shown in Fig. 2.
Fig. 2. Effect of sonication on induction period of wet
oxidation (temperature, 493 K, O2 pressure, 0.69 MPa).

Subcritical mineralization of sodium salt of dodecyl benzene sulfonate
2303
neglected due to very low concentration of the
substrate in water.
The destruction of organics via wet-oxidation
technique is known to be a combination of various
free radical reactions (Tufano, 1993; Li et al., 1991;
Thomsen 1998). The wet-oxidation reaction scheme
for any organic compound can be represented as
ð5Þ
From the design point of view, slowest step will be
rate determining. Further, we consider COD to be
the right parameter rather than total organic carbon
content (TOC) because relationship between TOC
and COD is required to draw meaningful inferences
from TOC measurements. Attempts have been made
in the past to model kinetics on the above reaction
scheme. It is our experience that kinetics of acetic
acid mineralization to CO2 and H2O is also
influenced by other species (COD contributing)
present in the system (Shende and Mahajani,
1994a,b), so also was the formation of acid. These
free radical reactions can be deemed to be auto-
catalytic. We have, therefore, decided to take a
process engineering friendly approach to make
modeling more meaningful, and express the entire
COD destruction scheme as
Fig. 3. Wet oxidation after sonication in different environ-
ment (temperature, 503 K, O2 pressure, 0.69 MPa).
comparable. The fact that wet oxidation was more
effective when sonication was under O2 (air or O2)
atmosphere than that without O2 presents indirect
evidence to formation of H2O2 along with ꢂOH
radicals as explained earlier through reactions (1)–
(4). Since the effect of air and oxygen was similar, in
all further experiments, air was used during sonica-
tion. Thus, it is seen that at near ambient tempera-
ture, we can have sonication resulting in reforming of
the refractory substrate into components, which can
then be easily oxidized.
k1
ðCODÞwaste stream ! ðCODÞlower mol wt: compounds
k2
! CO2 þ H2O
ð6Þ
Wet oxidation
Thus in wet oxidation, the reaction would exhibit
different kinetic behaviors, namely the fast oxidation
of organic substrate to CO2, and acids, in parallel
followed by the slower oxidation of low molecular
weight compounds formed such as acetic acid to
CO2. The kinetic data for catalytic and non-catalytic
wet oxidation reactions were interpreted similarly as
studied earlier (Shende and Mahajani, 1997) and the
global rate equations were obtained.
In the present investigation, we have demonstrated
the usefulness of the hybrid technology (SONIWO)
to destroy the refractory compounds. Hence, the
kinetics of WO after sonication was studied in detail
to aid the process design engineers. The wet
oxidation of sodium salt of dodecyl benzene sulfo-
nate after sonication treatment was studied in the
temperature range of 473–503 K and oxygen partial
pressure 0.69–1.138 MPa at near neutral pH. The pH
after sonication was observed to be 6.6 as compared
to initial pH 7.3. It is well known that in wet
oxidation, formation of highly refractory lower
molecular weight carboxylic acids like formic, acetic,
glyoxalic, oxalic etc., takes place. These intermediates
Wet oxidation is a gas–liquid reaction involving
various transport processes that take place in series
like transfer of oxygen from bulk of gas phase to gas–
liquid interface and transfer of oxygen from interface
to the bulk of liquid. The gas–liquid interface is
assumed to saturate instantaneously. The chemical
reaction takes place between substrate and dissolved
oxygen followed by desorption of products. Since
oxygen has higher diffusivity in gas phase and lower
solubility in water, and higher partial pressures of O2
were used, the resistance associated with transfer of
O2 from bulk gas phase to the gas–liquid interface
can be neglected under our experimental conditions.
In the preliminary experiments at temperature 503 K,
the rate of oxidation was found to be independent of
the impeller speed between 6.7 and 25.6 rps, indicat-
ing the absence of liquid-side mass transfer resistance
(Doraiswamy and Sharma, 1984). All subsequent
experiments were carried at an impeller speed of
25 rps. The oxygen concentration in aqueous solution
was estimated from the solubility data published by
Crammer (1980) considering aqueous phase as water.
The effect of substrate on oxygen solubility was

2304
Atul D. Dhale and Vijaykumar V. Mahajani
Over the entire temperature range studied, the
exhibit finite COD and BOD and severe oxidation
conditions are required to completely oxidize these reaction was found to obey first-order kinetics with
acids. Copper forms complex with ‘O’ in RC¼¼O and respect to COD though other orders were also tried.
not in RCH2OH. It is through this complex it The reaction kinetics could be well explained by
mineralizes acid to CO2. We have found that though order with respect to substrate as one. Kinetic plots
copper is a good catalyst in oxidizing acetic, formic for non-catalytic and catalytic oxidation are shown in
and oxalic acid (Shende and Mahajani, 1994a,b, Figs 5 and 6, respectively, indicating the two-step
1997), it is not that effective in destroying diethylene mechanism. The energy of activation for the first
glycol (Pandit, 1998). Copper is also good catalyst in and second step was found to be 68.05 and
methanol manufacture and in shift converters. While 60.19 kJ molꢀ1, respectively, for non-catalytic oxida-
chloride ion would result in heavy pitting corrosion tion. In case of catalytic oxidation, similar kinetics
and nitrate ion would lead to stress corrosion, the was observed and activation energy was found to be
sulfate ion would be less hostile. We have also 33.06 and 59.80 kJ molꢀ1 for the first and second
observed copper sulfate was effective in mineraliza- steps, respectively. Based on the above experimental
tion of complex molecule of reactive dye (Dhale and findings, the global rate equation for non-catalytic
Mahajani, 1999). Therefore, a readily and cheaply wet oxidation was developed and given below
available copper sulfate was used in as homogeneous
catalyst. The catalyst, CuSO4, loading was varied in
.
the range, 2.002 Â 10ꢀ4 to 1.001 Â 10ꢀ3 kmol mꢀ3
The catalyst was also added during sonication
because the catalyst CuSO4, was also effective during
sonication to break the substrate and achieving a
better yield in wet oxidation (Ingale and Mahajani,
1995).
It was observed that in non-catalytic oxidation at
503 K and at 0.69 MPa oxygen pressure; about 73%
COD reduction was achieved within 120 min. For
catalytic wet oxidation at 1.001 Â 10ꢀ3 kmol mꢀ3
CuSO4, COD reduction of 84% was achieved in
120 min with otherwise identical conditions.
A
comparative study of catalytic and non-catalytic
oxidation can be seen in Fig. 4. It clearly indicates
that the performance of wet oxidation is enhanced in
the presence of catalyst. At lower temperatures below
483 K reaction, exhibits induction period. Oxidation
efficiency increases with increase in temperature and
oxygen pressure.
Fig. 5. Kinetic plot for non-catalytic wet oxidation after
sonication.
Fig. 4. Effect of catalyst on wet oxidation after sonication
(temperature, 503 K, O2 pressure, 0.69 MPa, catalyst, Fig. 6. Kinetic plot for catalytic wet oxidation after
CuSO4, 1.001 Â 10ꢀ3 kmol mꢀ3).
sonication (catalyst, CuSO4, 1.001 Â 10ꢀ3 kmol mꢀ3).

Subcritical mineralization of sodium salt of dodecyl benzene sulfonate
2305
Non-catalytic wet oxidation
first step:
dðCODÞ
r ¼ ꢀ
dt
0:72
¼ 8:718Â104 exp½ꢀ8223=Tꢃ½CODꢃ½O2ꢃ
second step:
ð7aÞ
ð7bÞ
dðCODÞ
r ¼ ꢀ
dt
0:67
¼ 5:32 exp½ꢀ7293=Tꢃ½CODꢃ½O2ꢃ
Catalytic wet oxidation
first step:
dðCODÞ
r ¼ ꢀ
dt
Fig. 7. Acetic acid formation in non-catalytic wet oxidation
at different temperature (O2 pressure, 0.69 MPa).
0:56
¼ 19:25 exp½ꢀ3355=Tꢃ½CODꢃ½O2ꢃ
 ½CuSO4ꢃ
0:22
ð8aÞ
ð8bÞ
second step:
(55 ppm) as compared to non-catalytic oxidation
(160 ppm). It was observed that acetic acid formation
in wet oxidation was dependent on temperature. The
acid concentration increased with increasing tem-
perature as shown in Fig. 7. But, the partial pressure
of oxygen had little influence. The above study
proves that wet oxidation of sodium salt of dodecyl
benzene sulfonate proceeds via formation of phenol.
The salt is first converted into phenol and further
formation of low molecular weight compounds and
acids takes place due to free radical mechanism.
dðCODÞ
r ¼ ꢀ
dt
0:67
¼ 1:28Â104 exp½ꢀ7245=Tꢃ½CODꢃ½O2ꢃ
 ½CuSO4ꢃ
0:11
Reaction products
Wet oxidation proceeds via formation of various
low molecular weight compounds as intermediates.
At typical operating conditions viz. temperature
493 K and 0.69 MPa oxygen pressure, wet oxidation
of sodium salt of dodecyl benzene were carried out
and, the intermediate products were identified.
HPLC analysis of the samples initially showed the
formation of phenol as a major intermediate product
along with maleic acid. This can be explained by the
fact that breaking of alkyl chain by sonication and
desulfonation of benzene ring at higher temperature
results in phenol formation. The formation and
disappearance of these intermediates depends on
the reaction conditions. In case of non-catalytic
oxidation phenol lasted till 20 min. However, during
CONCLUSIONS
The hybrid system, sonication followed by wet
oxidation (SONIWO) is found to be effective to treat
the surfactant, sodium salt of dodecyl benzene
sulfonate at temperatures 483 K and above. Homo-
genous copper sulfate catalyst enhances COD reduc-
tion. The mineralization of sodium salt of dodecyl
benzene proceeds through phenol formation and
subsequently harmless products like CO2 and H2O
are formed.
Acknowledgements}The author Atul D. Dhale thanks the
University Grants Commission, Government of India,
New Delhi, for awarding fellowship and financial support
for this work.
catalytic
wet
oxidation
(CuSO4
1.001 Â
10ꢀ3 kmol mꢀ3), it disappeared after 10 min. The
mechanism of phenol oxidation is well known
(Devlin and Harris, 1984). Phenol is first converted
to quinones and dihydroxy compounds and finally to
low molecular weight acids. The some of the major
intermediate products identified were hydroquinone,
maleic acid, oxalic acid, and propionic acid. Acetic
acid, which is a major intermediate, was identified by
gas chromatography. Hydroquinone was also found
to disappear early in the reaction. At the end of
reaction after 120 min, more refractory compounds
like acetic acid and maleic acid were found to remain
in the reaction mixture. In catalytic wet oxidation,
the quantity of acetic acid remaining was far less,
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