Using sonochemical reactor for degradation of LAS from effluent of wastewater
M.H. Dehghania,⁎, A.A. Najafpoorb, K. Azamc
aTehran University of Medical Sciences, School of Public Health, Department of Environmental Health Engineering, Center for Environmental research, Tehran, Iran
bMashhad University of Medical Sciences, School of Public Health, Department of Environmental Health, Mashhad, Iran
cTehran University of Medical Sciences, School of Public Health, Department of Epidemiology and Biostatistics, Tehran, Iran
a b s t r a c ta r t i c l ei n f o
Accepted 16 May 2009
Linear alkylbenzene sulfonates (LAS) are anionic surfactants, which are found in relatively high amounts in
domestic and industrial wastewaters. The effectiveness of using sonochemical reactor for the degradation of
LAS from effluent of wastewater treatment plant has been investigated. In this study, experiments of LAS
solution were performed using methylene blue active substances (MBAS) method. The effectiveness of
sonochemical reactor for LAS degradation is evaluated with emphasis on the effect of sonication time and
initial LAS concentration. Experiments were carried out at initial concentrations of 0.2 mg/L, 0.5 mg/L,
0.8 mg/L and 1 mg/L, frequency of 130 kHz, acoustic power value of 400 W, temperature of 18–20 °C and pH
value of 6.8–7. This study showed that LAS degradation was found to increase with increasing sonication
time. In addition, as the concentration is increased, the LAS degradation rate decreases in the sonochemical
© 2009 Elsevier B.V. All rights reserved.
A surfactant combines in a single molecule a strongly hydrophobic
group with a strongly hydrophilic one. Such molecules tend to
congregate at the interfaces between the aqueous medium and the
other phases of the system such as air, oily liquids, and particles, thus
imparting properties such as foaming, emulsification, and particle
and its isomers together with other additives. LAS is a surface-active
material and is found in relatively high amounts in domestic and
industrial wastewaters, discharged mainly from the textile, cosmetic
and tanning industries [1–4].
LAS are anionic surfactants that contain an aromatic ring sulfonated
at the para position and attached to a linear alkyl chain at any position
except the terminal ones. The commercial product mainly consists of a
complex mixture of various homologues and isomers, representing
different alkyl chain lengths (ranging from 10 to 14 carbon atoms) and
aromatic ring positions along the linear alkyl chain [1,2].
Industries worldwide discharge a wide range of surfactant, or
surface-active agents, to their wastewater treatment facilities. Water
pollution caused by synthetic surfactants has been increasing during
the past few years due to their extensive use in household, agriculture
and other cleaning operations. Synthetic surfactants released into the
aquatic system have adversely affected ecosystems. Today the
detergent wastes constitute a major component of organic pollutants
thatare carriedbyvariousmeansin tolakes,rivers, andseas andcause
a great environmental problem [5–8].
Their presence in sewage works is variable depending on their use
in industrial processing in addition to domestic activities. An average
LAS concentration of 1–10 mg/L can be found in municipal wastewa-
ter treatment dealing only with domestic wastewater, but this range
is noticeably increased when industrial wastes from washing
processes are also treated [8–11].
In recent years, considerable interest has been shown on the ef-
fectiveness of ultrasonic reactor as a novel technology for the deg-
radation of contaminants from water and wastewater [12–36].
Irradiation by sonochemical reactor in a liquid leads to the acoustic
cavitation process, such as the formation, growth, and collapse of
bubbles, accompanied by the generation of local high temperature,
pressure, and reactive radical species [15,16]. Thus, the acoustic
cavitation in an aqueous solution results in chemical effects by the
acoustical reactor [24–26]. Three different reaction sites in the
cavitation bubble, i.e., the inside of the cavitation bubbles, the gas–
liquid interfacial region of the cavitation bubbles, and the bulk
solutions are present during the sonochemical process [26–29].
Surface-active molecules accumulate at interfaces, particularly at
the water–air interface of the cavitation bubbles, and introduce
Desalination 250 (2010) 82–86
⁎ Corresponding author. Tehran University of Medical Sciences, School of Public
Health, Center for Environmental Researches, Department of Environmental Health
Engineering, Tehran, Iran. Tel.: +98 21 66954237; fax: +98 21 88950188, +98 21 6641
E-mail address: email@example.com (M.H. Dehghani).
0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
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major physical and chemical changes to their sonochemical response
In this article, the major objective was to evaluate the effect of
sonochemical reactor as a novel treatment and to provide a greater
knowledge of the fundamentals of sonotreatment of surfactants so-
lution via acoustic cavitation. Since cavitation bubbles are the hot
spots where sonochemical reactions take place, surfactants should be
particularly a good target for this treatment. Also, in the present
research, the sonochemical degradation of linear alkylbenzene sul-
fonate is investigated with emphasis on the effect of sonication time
and initial concentration.
An anionic detergent, LAS, was chosen as model detergents. LAS
can be analyzed by nonspecific methods. The assay usually used is one
for substances that react with methylene blue, which responds to any
compound containing an anionic and hydrophobic group. It thus
suffers from analytical interference if used for environmental sam-
ples; furthermore, the sensitivity of this method is about 0.02 mg/L.
Although nonspecific alternatives to this method have been devel-
oped, they are not commonly used. Specific methods for environmen-
tal analysis are available only for LAS. An improved method based on
methylene blue reactivity and high-performance liquid chromatogra-
phy is available for analysis of LAS.
Methylene blue active substances (MBAS) bring about the transfer
of methylene blue, a cationic dye, from an aqueous solution into an
immiscible organic liquid upon equilibration. This occurs through ion
pair formation by the MBAS anion and the methylene blue cation. The
intensity of the resulting blue color in the organic phase is a measure
of MBAS. Anionic surfactants are among the most prominent of many
substances, natural and synthetic, showing methylene blue activity.
The MBAS method  is useful for estimating the anionic sur-
factant content of waters and wastewaters, but the possible presence
of other types of MBAS always must be kept in mind. This method is
relatively simple and precise. It comprises three successive extrac-
tions from acid aqueous medium containing excess methylene blue
into chloroform, followed by an aqueous backwash and measurement
of the blue color in the chloroform by spectrophotometry at 453 nm.
The method is applicable at MBAS concentrations down to about
2.1. Apparatus and procedures
Spectrophotometer: for use at 453 nm, providing a light path of 1 cm.
Separatory funnels: 500 mL, preferably with inert TFE stopcocks
Sonochemical reactor (Fig. 1): an acoustical reactor (Basin-Batch)
operating at a fixed frequency of 130 kHz and a power value of 400 W
wasused for sonochemical experiments(Table 1). Bothfrequency and
power were adjusted in a generator that is connected to the
transducer. Reactions were carried out in a stainless steel, which
was closed during sonication. The vessel was immersed in a water
bath. In all experiments, 200 mL of detergent aqueous solution was
prepared daily and exposed in sonochemical reactor. On the other
hand, 200 mL solution was sonicated with the sound waves emitting
from the bottom of the sonochemical reactor.
2.2. Chemicals and materials
Stock LAS solution was used. An amount of the reference material
equal to 1.00 g LAS on a 100% active basis was weighed. Then it was
dissolved in water and diluted to 1000 mL. Then, it was stored in a
refrigerator to minimize biodegradation.
− Standard LAS solution: 10 mL stock LAS solution was diluted to
1000 mL with water.
− Phenolphthalein as an indicator solution was used, alcoholic.
− Sodium hydroxide, NaOH, 1 N, sulfuric acid, 1 N and 6 N, and
chloroform were used.
− Methylene blue reagent was used: 100 mg methylene blue was
dissolved in 100 mL water. Then, 30 mL of the resulting solution
was transferred to 1000 ml flask. Then, 500 mL water, 41 mL 6 N
sulfuric acid, and 50 g sodium phosphate, monobasic, monohy-
drate were added.
− Washsolution: firstly,41 mL6 Nsulfuricacidwasadded to500 mL
water in a 1000 mL flask. Secondly, 50 g NaH2PO4∙H2O was added
and shaken until dissolved. Then, it was diluted to 1000 mL.
− Methanol and hydrogen peroxide were used.
− Glass wool was used. It was pre-extracted with CHCl3to remove
− For making all reagents and dilutions, water, reagent-grade, and
MBAS-free were used.
− For preparing the calibration curve a series of separatory funnels
with 0, 1.00, 3.00, 5.00, 7.00, 9.00, 11.00, 13.00, 15.00, and 20.00 mL
standard LAS solution were prepared. Then, sufficient water was
added to make the total volume 100 mL in each separatory funnel.
Then, the calibration curve of absorbance vs. micrograms LAS taken
which specified the molecular weight of the LAS used was plotted.
For direct analysis, a sample volume on the basis of expected MBAS
concentration was selected, as shown in Table 2.
− Peroxide treatment: to avoid decolorization of methylene blue by
sulfides, a few drops of 30% hydrogen peroxide were added.
a. Sample was added to a separatory funnel. By dropwise addition
of 1 N NaOH, and using phenolphthalein indicator alkaline was
made.Bydropwiseadditionof 1 Nsulfuric aciditdischarged pink
b. 10 ml CHCl3and 25 ml methylene blue reagent were added.
Then, funnel was rocked vigorously for 30 s and allowed phases
Fig. 1. Sonochemical reactor for LAS degradation.
Characteristics of sonochemical reactor used in the experiments.
L=30 cm; W=25 cm; H=32 cm
M.H. Dehghani et al. / Desalination 250 (2010) 82–86
separation. Alternatively, a magnetic stirring bar in the
separatory funnel was placed; then, funnel was laid on its side
on a magnetic mixer and speed of stirring was adjusted to
produce a rocking motion. Excessive agitation may cause
emulsion formation. To break persistent emulsions, a small
volume of isopropyl alcohol and also a small volume of
isopropyl alcohol were used for all standards. Some samples
required a longer period of phase separation than others. Before
draining the CHCl3layer, the sample was swirled gently, and
then was allowed to settle.
c. CHCl3layer was drowned off into a second separatory funnel.
Then, the delivery tube of the first separatory funnel was rinsed
with a small amount of CHCl3. Extraction was repeated two
additionaltimes and each time10 mlCHCl3wasused.Whenever,
the blue color in the water phase became faint or disappears, it
was discarded and repeated by using a smaller sample.
d. All CHCl3 extracts were combined in the second separatory
30 s. Emulsions did not form at this stage. Then, the result was
allowed to settle, swirl, and CHCl3layer was drawn off through a
funnel containing a plug of glass wool into a 100 ml volumetric
flask; filtrate must be clear. Wash solution was extracted twice
with 10 ml CHCl3each and it was added to the flask through the
glass wool. Then, the glass wool and funnel were rinsed with
CHCl3. Next, washings were collected in volumetric flask, diluted
to mark with CHCl3, and mixed well.
e. Measurement was done by the determination of absorbance at
453 nm against a blank of CHCl3.
− Calculation was done from the calibration curve by reading micro-
grams of apparent LAS corresponding to the measured absorbance.
mg MBAS= L = μg apparent LAS= mL original sample
All the analyses were performed according to the procedures
outlined in standard methods for the examination of water and
2.3. Statistical methods
The potential of using a sonochemical reactor on the degradation
of LAS was analyzed statistically by using One-way test (ANOVA),
Post-hoc test (Multiple comparisons), Pearson correlation test and
multiple regression. The variables were sonication time, degradation
percentage and initial concentration. Frequency and power were
fixing parameters. The data gathered was analyzed statistically by
SPSS 11.5 and Excel software.
3. Results and discussion
In this research, sonodegradation of LAS was applied for different
periods of time; 0, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 min.
Also, sonodegradation experiments of LAS were carried out in the
presence of various concentrations to observe if there was any effect
on the degradation of LAS. Sonodegradation of LAS was performed
at initial concentrations of 0.2 mg/L, 0.5 mg/L, 0.8 mg/L and 1 mg/L,
acoustic frequency of 130 kHz, pH 6.8–7.0 and applied power of
400 W.Thetemperaturewasmaintainedat18–20 °Cwiththeaidofan
ice bath, surrounding the water sample.
3.1. Effect of initial concentration
Experiments were conducted in various times to see if there was
any synergistic effect on the degradation of LAS. Increasing the
concentration from 0.2 mg/L to 1 mg/L showed a decrease in degra-
dation of LAS. Experiments showed that with sonochemical reactor,
about 83.30%, 72.28%, 65.69% and 51.70% degradation of LAS occurred
after 120 min but only 37.84%, 24.27%, 20.25% and 5% degradation of
LAS was observed within 20 min as shown in Figs. 2–5. Therefore,
results obtained from the sonochemical degradation of LAS at various
concentrations indicated that removal rates were found to decrease
with increasing LAS concentration.
Using one-way ANOVA, we found statistically significant differ-
ences in different concentrations (P value=0.001), as shown in Table 3.
Also, statistical analysis using Post-hoc test showed that there was a
significant difference between 0.2 mg/L and 1 mg/L (P valueb0.001).
But there are no significant differences for other concentrations, such as
Expected MBAS concentration.
Expected MBAS concentration (mg/L)Sample taken (mL)
Fig. 2. Degradation percentage of LAS vs. sonication time for 0.2 mg/L.
Fig. 3. Degradation percentage of LAS vs. sonication time for 0.5 mg/L.
Fig. 4. Degradation percentage of LAS vs. sonication time for 0.8 mg/L.
M.H. Dehghani et al. / Desalination 250 (2010) 82–86
0.2 mg/L–0.5 mg/L, 0.5–0.8 mg/L and 0.8 mg/L–1 mg/L (P valueN0.001)
as shown in Table 4. Using multiple regression indicated that there were
the other hand, the linear relationship equations for degradation
percentage (Table 5) are as follows: Degradation=10.348+0.459 Time.
Also, coefficient of multiple determination (R square) is equal to
0.628, as shown in Table 6.
The effect of initial concentration has been studied by researches
[15,16,23,24,36]. It has been reported in sonochemical reactors that
for increasing concentration of a pollutant, the apparent pseudo first-
order rate constant decreases . At high concentrations when
micelles (micelle is an aggregate of surfactant molecules dispersed in
a liquid colloid) are present, the rate constant increases slightly over a
slightly lower concentration. In addition, at low concentration,
surfactants preferentially accumulate on cavitation bubble surfaces,
reducing the surface tension of the solution .
3.2. Effect of sonication time
In order to observe the effect of sonication time on the LAS degra-
dation rate during treatment, sonodegradation or sonication time for
aqueous LAS concentrations was performed in twelve intervals. As
clearly seen, by increasing the sonication time, considerable levels of
LAS degradation can beexpectedafter120 min.It wasobservedthatthe
degradation efficiency of acoustic frequency was increased when
sonication time was increased, as shown in Figs. 2–5. Therefore, the
statistical study using Pearson correlation tests indicated that when
sonication time is increased, there is an increase in removal percentage
(P valueb0.001, r=0.792), as shown in Table 7. This effect is due to the
cavitation process as the time of sonication is increased [15,16].
3.3. Effect of acoustic power
The effect of acoustic power on the sonodegradation of LAS may be
explained in terms of cavitational activity. High levels of acoustic
power increase the number of cavitational events and consequently
the opportunities for free radicals to be generated enhancing deg-
radation [15,16,25,31,35]. Suzuki et al.  confirmed that the deg-
radation rate is dependent on the acoustic power, because acoustic
power may lead to more extensive acoustic cavitation.
3.4. Effect of initial pH value
The experiments were performed at pH 6.8–7. Experiments
showed that pH did not have an effect on degradation of LAS. The
effect of pH on rates of sonodegradation has been reported previously.
These studies indicated that a surfactant will accumulate on a surface
independent of whether it is protonated or deprotonated, the effect of
pH is minimal on degradation [20,25,29]. Also, this conclusion is in
accordance to the studies of Dehgani et al.  and Mahvi  who
investigated the sonochemical degradation of synthetic surfactants
and found that the pH value was not a key parameter affecting the
sonodegradation of surfactant.
3.5. Effect of temperature
With an increase in the sonication time, temperature increases.
For example, after 120 min temperature was within 50 °C and it is due
to acousticbubble process.In this study, the reaction temperature was
controlled with the help of condensation water surrounding the
reactor bath. Therefore, experiments showed that temperature in-
crease of LAS samples during sonication had no considerable effect on
degradation of LAS.
The cavitation threshold has been found to decrease with in-
creasing temperature. In general, increased temperatures are likely to
facilitate bubble formation due to an increase of the equilibrium vapor
pressure. However, the sonochemical effect of such bubbles may be
reduced. If during the bubble growth some gas or vapor has diffused
into the bubble, complete collapse may not occur and the bubble may
in fact oscillate in the applied field. Overall, larger sonochemical
effects are observed at lower temperatures at which most of the
bubbles consist of gas rather than vapor; the latter may cushion the
Fig. 5. Degradation percentage of LAS vs. sonication time for 1 mg/L.
Sum of squares
(a) Correlation is significant at the 0.01 level (2-tailed).
Post-hoc tests (multiple comparisons).
Sig.95% confidence interval
aThe mean difference is significant at the 0.05 level.
Linear relationships equation [coefficients (a)].
(a) Dependent variable: degradation.
Coefficient of multiple determination (R square).
RR squareAdjusted R square Std. error of the estimate
aPredictors: (constant), time.
M.H. Dehghani et al. / Desalination 250 (2010) 82–86
implosion as well as use enthalpy for condensation [16,21–23,33– Download full-text
Results obtained from this research demonstrate that sonochem-
ical reactor at a frequency of 130 kHz and a power of 400 W is capable
to some degree of LAS degradation in aqueous synthetic solutions. The
potential of sonochemical reactors for LAS degradation is evaluated
with emphasis on the effect of sonication time and initial concentra-
tion. Experiments showed that sonication time is one of the most
important parameters for LAS degradation. Also, this study indicates
that the overall treatment efficiency rises with the decreasing
Sonochemical reactors alone may not be useful for reducing com-
pletely complex wastewaters of high surfactant load. In this respect,
effectiveness may be improved coupling acoustical reactors with
other treatment processes including ozone, UV, chlorination and
H2O2. Alternatively, acoustical processor reactor could be used as a
pre-treatment stage in a sequential chemical and biological treatment
This research has been supported by the Tehran University of
Medical Sciences (Grant 85-01-46-3401).
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