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Adsorption kinetics of 4-chlorophenol onto granular activated carbon in the presence of high frequency ultrasound
Ultrasonics Sonochemistry 16 (2009) 15–22
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Adsorption kinetics of 4-chlorophenol onto granular activated carbon in the presence of high frequency ultrasound Oualid Hamdaoui a,1,*, Emmanuel Naffrechoux b,1 a b
Department of Process Engineering, Faculty of Engineering, University of Annaba, P.O. Box 12, 23000 Annaba, Algeria Laboratoire Chimie Moléculaire et Environment, Polytech Savoie, Université de Savoie, 73376 Le Bourget du Lac, France
a r t i c l e
i n f o
Article history: Received 21 January 2008 Received in revised form 12 May 2008 Accepted 16 May 2008 Available online 24 May 2008 Keywords: Ultrasound High frequency Adsorption 4-Chlorophenol Activated carbon Kinetics Modeling
a b s t r a c t This work describes the results of investigations carried out to examine the adsorption kinetics of 4-chlorophenol (4-CP) from aqueous solution containing tert-butyl alcohol (10%, v/v) onto granular activated carbon (GAC) in the presence of ultrasound of different high frequencies (516, 800 and 1660 kHz) and acoustic powers (15.2, 21.5, 31.1 and 38.3 W). The main objective of this study is to describe the mechanism of ultrasound-assisted adsorption rather than the enhancement of adsorption capacity. Sonochemical degradation of 4-CP was studied in the absence and presence of tert-butyl alcohol. The sonolysis of 4CP is effectively inhibited by the addition of tert-butyl alcohol (10%, v/v) and very little 4-CP degradation occurs, indicating that little or no pyrolysis of the compound occurs. Without addition of tert-butyl alcohol, after 300 min and at 1660 kHz, the removal of 4-CP in the presence of ultrasound for an acoustic power of 38.3 W was nearly total (99%), but in the conventional method only 60% was eliminated. In this case, the removal of 4-CP by GAC in the ultrasound-assisted technique is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption, which makes a direct comparison unacceptable. In order to distinguish sonochemical degradation and adsorption of 4-CP onto GAC and to make an exact and practical comparison of the adsorption in the absence and presence of ultrasound, kinetic adsorption experiments were conducted using aqueous solution containing 10% (v/v) tert-butyl alcohol. The obtained results show that both adsorption rate and adsorbed amount were significantly enhanced and improved in the presence of ultrasound for all the studied frequencies and powers. The enhancement of adsorption is favored by increasing ultrasonic power. Adsorption kinetic data were modeled using the liquid-film mass transfer equation and intraparticle diffusion model. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound are greater than that obtained in the absence of ultrasound. In the initial period of adsorption, where external mass transfer is assumed to predominate, liquid-film mass transfer coefficients significantly increased by the assistance of ultrasound. These results indicate that ultrasound enhances the mass transport in the pores as well as across the boundary layer. This effect increased with increasing ultrasonic power for the three studied frequencies. The average order for the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient is 516 kHz > 800 kHz > 1660 kHz. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Phenolic compounds which are generated by petroleum and petrochemical, coal conversion and phenol producing industries, are common contaminants in wastewater. Phenols are widely used for the commercial production of a wide variety of resins including phenolic resins, which are used as construction materials for automobiles and appliances, epoxy resins and adhesives, and polyamide for various applications [1,2]. Phenols are considered as priority pollutants since they are harmful to organisms at low * Tel.: +213 771598509; fax: +213 38876560. E-mail address: [email protected] (O. Hamdaoui). 1 The authors thank the Comité Mixte d’Evaluation et de Prospective de coopération interuniversitaire franco-algérienne (CMEP) for its financial support and for the grant for O.H. 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.05.008
concentrations and many of them have been classified as hazardous pollutants because of their potential to harm human health. Increasing concern for public health and environmental quality has led to the establishment of rigid limits on the acceptable environmental levels of specific pollutants. Because of their toxicity, phenols have been included in the US Environmental Protection Agency (EPA) list of priority pollutants [3]. Also the European Union (EU) has classified several phenols as priority contaminants and the 80/778/EC directive lays down a maximum concentration of 0.5 mg L1 for total phenols in drinking water [4,5]. Thus, the removal or destruction of phenols from process or waste streams becomes a major environmental problem. Adsorption is well-established technique for the removal of low concentrations of organic pollutants from potable water, wastewater and aqueous solutions. Activated carbon is one of the most
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effective adsorbents for organic compounds because of their extended surface area, high adsorption capacity, microporous structure and special surface reactivity. Ultrasound has been proven to be a very useful tool in intensifying the mass transfer process and breaking the affinity between adsorbate and adsorbent. Several researchers have studied the role of ultrasound on adsorption kinetics of organic compounds by various adsorbents [6–11]. Schueller and Yang [6] have explored the adsorption of aqueous phenol onto activated carbon and Dowex resin at 295 K at a frequency of 1 MHz and 60 W. They have showed from the comparison of uptake rates in a batch adsorber that mixing at 250 rpm provides the highest adsorption rate than ultrasound. Wang and Cheng [7], who studied the uptake of 4-(2pyridylazo) resorcinol (PAR) by Amberlite XAD-2 resin, also noted the trend of mixing having a greater effect than 20 kHz ultrasound on adsorption. Breitbach and Bathen [8] have investigated the adsorption kinetics of fructose on Amberlite CR1320 at 313 K using different powers input of ultrasound with frequencies 337 and 1158 kHz. The authors show the increase of pore diffusion coefficient with increasing ultrasonic powers for both frequencies. Hamdaoui et al. [9] have examined the effect of 21 kHz ultrasound on adsorption dynamics of p-chlorophenol on granular activated carbon. The obtained results show that low frequency ultrasound enhances pore diffusion and initial rate of adsorption. Ji et al. [10], investigated the effects of pulse 18 kHz ultrasound with different pulse parameter on the adsorption kinetics of Geniposide on Resin 1300. They have founded that pulse ultrasound can enhance both liquid-film diffusion and intraparticle diffusion, and the intensification of liquid-film diffusion with pulse ultrasound is stronger than that of intraparticle diffusion. Juang et al. [11] have studied the effect of 48 kHz ultrasound on the adsorption of phenol from aqueous solutions onto coconut shell-based granular activated carbons at 298 K. They have shown that the initial rate of adsorption was enhanced by sonication. Most of the previous studies were focused on the influence of low frequency ultrasound on adsorption dynamics. However, the adsorption kinetics in the presence of high frequency ultrasound using activated carbon as adsorbent has not been studied systematically yet. The objective of the present work was to investigate the adsorption kinetics of 4-CP from aqueous medium containing 10% (v/v) tert-butyl alcohol onto GAC in the presence of high frequency ultrasound (516, 800 and 1660 kHz) for different ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W). The removal of 4-CP without the addition of tert-butyl alcohol in the presence of ultrasound is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption. In order to suppress the ultrasonic degradation and make a correct and viable comparison of the adsorption in the absence and presence of ultrasound, tert-butyl alcohol was used during adsorption experiments. The goal of the present work is to clarify the effect of high frequency ultrasound on the adsorption of 4-CP by activated carbon rather than an enhancement of the removal of 4-CP from aqueous solutions. In order to gain insight into the dynamics of the process, the mechanism controlling the rate of adsorption was also studied. The sonochemical degradation of 4-CP and the scavenging effect of tert-butyl alcohol were investigated.
were prepared by dissolving the appropriate amount in ultra-high quality (UHQ) water (Elga elgastat 18.2 MX). 2.2. Activated carbon Granular activated carbon (GAC) employed as the adsorbent in the present study was supplied by Prolabo. Prior to use, the carbon was pretreated by boiling in UHQ water for 1 h and washed repeatedly with UHQ water until the electric conductivity and the UV absorbance were equal to zero, and the pH remains constant. Finally, the washed activated carbon was dried in an oven at 383 K to constant weight and stored in a desiccator until use. The BET (Brunauer–Emmett–Teller) surface area of the carbon (929 m2 g1) was obtained from N2 adsorption isotherms at 77 K. The GAC has a mean granulometry of 3 mm. 2.3. Ultrasonic reactor The ultrasonic irradiation was carried out with equipment operating at three different frequencies: 516, 800 and 1660 kHz. In the three cases, the ceramic transducer is located at the bottom of the vessel and connected to high frequency supply. The ultrasonic waves were introduced from the bottom of the solution. The cylindrical reactor was thermostated by a water jacket. The temperature inside the reactor was kept constant. Acoustic power dissipated in the reactor was measured using standard calorimetric method [12]. 2.4. Adsorption procedure
2. Materials and methods
In all adsorption experiments, 4-CP solutions of 100 mg L1 were prepared by dissolving the appropriate amount in 10% (v/v) tert-butyl alcohol aqueous solution. For the determination of adsorption kinetics onto GAC in the presence of high frequency ultrasound, an adsorbent weight of 0.09 g was transferred to the ultrasonic reactor containing a volume of 90 mL of 4-CP aqueous solution at a concentration of 100 mg L1. Ultrasonic waves of various frequencies (516, 800 and 1660 kHz) was used for sonication experiments at four different ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W) as measured by calorimetry. The temperature was maintained constant and equal to 294 K. All the kinetic experiments were carried out at pH 5.5. Strong convective currents occur within the reactor here and there along the transducer axis. These effects associated with hydrodynamic phenomenon due to cavitation are responsible for the perfect mixing of the reactor content. It was thereby established that under ultrasonic irradiation the used reactors are completely stirred tank reactor. Identical experiments were repeated in the absence of ultrasound (conventional method) using a glass reactor (with the same geometry as the sonication reactor) and a magnetic stirrer with a stirring speed of 400 rpm. The reactor provides uniform mixing conditions due to continuous agitation. All other conditions, such as temperature and pH, were the same as those used for ultrasound-assisted adsorption. After selected times of sonication (or stirring), the adsorption kinetics was determined by following the 4-CP concentration change in the reactor. The amount of 4-CP adsorbed (mg g1) at any time t was calculated by using Eq. (1)
2.1. 4-Chlorophenol
q¼
4-Chlorophenol (4-CP) supplied by Sigma (99.5%) was used as an adsorbate. 4-CP has a molecular weight of 128.56 g mol–1 and linear formula: ClC6H4OH. 4-CP solutions of desired concentration
where C0 is the initial concentration of 4-CP (mg L1), C is the concentration of 4-CP at any time t (mg L1), V is the solution volume (L) and W is the weight of the adsorbent (g) in the mixture.
ðC 0 CÞV W
ð1Þ
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2.5. Measurements Quantitative analysis of 4-CP was done by a high performance liquid chromatograph Waters (pump 515). Detection was made at 254 nm with a model 996 absorbance detector. The column used was a reverse phase column C18 (150 mm length, 3.9 mm inner diameter and 4 lm particle size). The mobile phase was an aqueous mixture of acetonitrile (40%) and acetic acid (1‰). The particle size analysis of the GAC was determined before and after ultrasonication. GAC was added to 90 mL of UHQ water, and the obtained suspension was exposed to ultrasound of different frequencies (516, 800 and 1660 kHz) and powers for 9 h. The suspension was then filtered (0.45 lm) and dried at 383 K for 48 h. The size distribution was determined using a laser granulometer (Mastersizer 2000 – Malvern instruments). Scanning electron microscopy (SEM) (Leo type Stereoscan 440) analysis was carried out on the GAC to study its surface texture before and after ultrasonication.
3. Results and discussion 3.1. Influence of sonication on GAC granulometry Particles characterization experiments were carried out to understand sonication effects on activated carbon properties. Additionally, changes in the size of the particles are important factors to understand the mechanisms of adsorption dynamics of 4-CP from aqueous phase onto GAC. Therefore, the resistance of the activated carbon granules in the presence of ultrasound of various frequencies (516, 800 and 1660 kHz) and different powers (15.2, 21.5, 31.1 and 38.3 W) was experimentally determined. The activated carbon was sonicated for 9 h in the same reactor employed for the adsorption experiments. GAC particle size distributions showed that no attrition (erosion) of the activated carbon was observed for all the tested frequencies and acoustic powers. Theoretically, when bubbles collapse in homogeneous solution, the shape of the bubbles remains spherical and symmetrical during their lifetime. However, in heterogeneous solution (i.e., when solid particles exist), the bubbles collapse near a solid surface; thus, dynamics of cavitation bubbles change significantly. In this situation, the bubbles collapse asymmetrically producing a high-speed jet of liquid, which passes through the interior of the cavitation bubble and toward the solid surface, called a microjet [13,14]. It has been reported that the speed of the liquid jet may reach speeds of more than 100 m s1 [13]. Such a powerful jet will produce dramatic effects on the solid surface, and is responsible for pitting and erosion of the solid surface. In addition, fracturing and melting of particles has been observed resulting from bubble implosion and shockwaves at the surface of particles [15]. In the present study, the particle analyses prove that the intensity threshold for particle breakup at all frequencies and powers had not been attained. This is due to cavitating bubbles size and the duration of the collapses that decrease with the increase of the frequency. The growth cycle of a cavitating bubble is dependent on the frequency of the applied ultrasound. When the frequency is increased, the growth time cycle decreases. This decrease in growth times results in smaller maximum sizes at higher frequencies and thus less violent collapses. Cavitation collapses are fewer but more violent at lower frequencies, and more frequent and less violent at higher frequencies.
A scanning electron microscope was used to look at the effect of ultrasound on the morphology of the activated carbon surface. It can be seen from the micrographs (not shown) that the activated carbon showed similar surface morphologies before and after sonication. 3.2. Destruction of 4-CP by ultrasound Sonochemical degradation of 100 mg L1 4-CP aqueous solutions was carried out in the cylindrical jacketed glass reactor described above operating at three different frequencies: 516, 800 and 1660 kHz and various powers: 15.2, 21.5, 31.1 and 38.3 W. Application of ultrasonic irradiation of different frequencies and acoustic powers to 90 mL of 4-CP solution resulted in the decline of the initial concentration. The disappearance of 4-CP follows a pseudo-first-order reaction kinetics. At a frequency of 516 kHz, for example, 4-CP was completely destroyed after 70, 80, 100 and 130 min of sonication for the ultrasonic powers of 38.3, 31.1, 21.5 and 15.2 W, respectively. Initial rate of degradation of 4-CP was calculated for the initial 10 min. The results presented in the form of initial rate of 4-CP degradation as a function of ultrasonic power are shown in Fig. 1. It was observed that the initial degradation rate increased with the increase of ultrasonic power. Additionally, the results of 4-CP sonochemical oxidation clearly show that the pseudo-first-order degradation rate constant increases with ultrasonic power (Fig. 2). An increase in power will thus result in greater sonochemical effects in the collapsing bubble. So because of the increasing acoustic amplitudes, oxidation of 4-CP increased. When power is increased, transmittance of ultrasonic energy into the reactor increases. Due to this energy, the pulsation and collapse of bubble occur more rapidly, the number of cavitation bubbles increase and realizing a higher concentration of OH radicals into the aqueous solution of organic pollutant. Thus, an increase in ultrasonic power results in an increase in acoustic amplitude, which favors more violent cavitation bubble collapse because the bubble collapse time, the transient temperature, and the internal pressure in the cavitation bubble during collapse are all dependent on the acoustic amplitude. That is, high enough acoustic power results in transient cavitation. Hence, the results of an increase in the sound power are greater sonochemical effects, resulting in higher 4-CP degradation rates. When ultrasound waves are introduced into an aqueous solution, cavitation occurs, as bubbles rapidly form and subsequently collapse inward from the build-up of pressure through rarefaction/compression cycles. Under these sonolytic conditions, high temperatures and pressures accompany the implosion of cavita-
10 9 Initial rate (µM min-1)
All experiments were conducted in triplicate and the mean values were reported. The maximum standard deviation obtained for triplicate measurements of the sorbed amount was ±1.5%.
8
516 kHz 800 kHz 1660 kHz
7 6 5 4 3 2 1 0 15.2
21.5 31.1 Ultrasonic power (W)
38.3
Fig. 1. Effect of ultrasonic power on the initial rate of 4-CP sonochemical degradation at different frequencies (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
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80
0.018 516 kHz 800 kHz 1660 kHz
0.014
70 Rate constant (nM min-1)
-1
Rate constant (min )
0.016
0.012 0.01 0.008 0.006 0.004
516 kHz 800 kHz 1660 kHz
60 50 40 30 20 10
0.002
0
0 15.2
21.5 31.1 Ultrasonic power (W)
15.2
38.3
21.5 31.1 Ultrasonic power (W)
38.3
Fig. 2. Sonochemical degradation rate constant at different ultrasound frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
Fig. 3. Effect of tert-butyl alcohol (10%, v/v) on the rate constant of 4-CP sonochemical degradation for different frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
tion bubbles. Thus, ultrasonic irradiation induces the formation of free radicals as a consequence of cavitation. Hydroxyl radical as well as homolytic oxygen (reactions 2 and 3) are produced in the bubble during the implosion. Radicals combine in the bubble (reactions 4–6) or escape from the bubble, which conducts to H2O2 release in the medium (reactions 7–9).
oxidized by hydroxyl radicals formed within the bubbles as a result of the sonolysis of water. Another factor that also affects the rate of 4-CP degradation is the formation of volatile products from tertbutyl alcohol degradation that accumulate inside the bubble. Such volatile products decrease the temperature inside the bubble, which, in turn, slow the sonolytic reactions [18]. From these results, we can infer that 4-CP does not undergo direct pyrolysis in the sonolytic bubbles but rather becomes oxidized at the interface by reacting with OH radicals. In view of the above and in order to distinguish sonochemical degradation and adsorption of 4-CP onto GAC, kinetic adsorption experiments were conducted using 100 mg L1 4-CP aqueous solution containing 10% (v/v) tert-butyl alcohol.
ð2Þ
O2 ! 2O
ð3Þ
O þ H2 O ! 2HO H þ HO ! H2 O
ð4Þ ð5Þ
2HO ! O þ H2 O
ð6Þ
H þ O2 ! HOO
ð7Þ
2HO ! H2 O2
ð8Þ
2HOO ! H2 O2 þ O2
ð9Þ
Degradation rate of 4-CP is higher at 516 and 800 kHz than at 1660 kHz. This difference can be explained by the size of the cavitating bubbles and the duration of the collapse. Diameter of cavitating bubbles is more important at 516 and 800 kHz than at 1660 kHz as a consequence the build-up of energy at the final stage of the collapse is more elevated at 516 and 800 kHz. At very high frequency (higher than the megahertz), rarefaction (and compression) cycles are becoming extremely short, the finite time required for the rarefaction cycle becomes too short to permit the molecules to be pulled apart sufficiently to generate a bubble. Consequently, the duration of implosion and the number of hydroxyl radicals formed decrease with the increase of frequency. In the present work, the scavenging effect of tert-butyl alcohol (10%, v/v) on the degradation of 4-CP (100 mg L1) for different frequencies and ultrasonic powers was investigated. As indicated in our previous works [16,17], the sonochemical degradation of aqueous 4-CP solutions in the presence of tert-butyl alcohol exhibits zero–order reaction kinetics. The zero-order rate constants obtained for different frequencies and acoustic powers were given in Fig. 3. Extensive work on the sonochemistry of tert-butyl alcohol has been reported by von Sonntag and co-workers [18]. They determined that the alcohol is pyrolized in the bubble via a freeradical-induced pyrolysis reaction mechanism. The tert-butyl alcohol is able to scavenge OH radicals in the bubble and prevent the accumulation of OH radicals at the bubble interface. From Fig. 3, it was observed that the degradation of 4-CP was effectively quenched, but not completely, by the addition of tert-butyl alcohol. This low degradation suggests that the destruction takes place at the interface of the cavitation bubbles where 4-CP molecules are
3.3. Removal of 4-CP by GAC without adding tert-butyl alcohol The removal of 4-CP by GAC in the absence (stirring 400 rpm) and presence of 1660 kHz ultrasonic irradiation of different acoustic powers was investigated using aqueous solution of 4-CP with an initial concentration of 100 mg L1. The adsorbent dosage and temperature were 0.09 g/90 mL and 294 K, respectively. The 4-CP solution was prepared by dissolving required amount of solute in UHQ water. The removal percentage of 4-CP obtained after 300 min in the conventional method (simple stirring) and in the ultrasoundassisted method was shown in Fig. 4. It was clearly shown that
100 90 Removal percentage
H2 O ! H þ HO
80 70 60 50 40 30 20 10 0 Stirring
38.3 W
31.1 W
21.5 W
Fig. 4. Removal of 4-CP by GAC after 300 min in the absence and presence of 1660 kHz ultrasonic irradiation (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K, contact time 300 min).
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40 Stirring 15.2 W 21.5 W 31.1 W 38.3 W
35 30 q (mg g-1)
In order to make a practical evaluation of the adsorption of 4-CP by GAC by the conventional method (simple stirring) and ultrasound-assisted method, adsorption experiments were carried out using aqueous solution of 4-CP containing 10% (v/v) tert-butyl alcohol. Therefore, the effect of tert-butyl alcohol (10%, v/v) on the adsorption kinetics of 4-CP was investigated in the absence of ultrasound (conventional method). Adsorption kinetics was determined for an initial concentration of 100 mg L1, an adsorbent dosage of 0.09 g/90 mL and a stirring speed of 400 rpm. Fig. 5 shows the adsorption kinetics of 4-CP onto GAC in the absence and presence of tert-butyl alcohol. It was observed that the adsorption decreases significantly in the presence of tert-butyl alcohol. Adsorption capacity after a contact time of 9 h was 20.85 and 78.1 mg g1 with and without the addition of alcohol respectively. The significant decrease of the adsorbed amount (73%) in the presence of tert-butyl alcohol is due to the competitive effect between 4-CP and alcohol molecules for the sites available for the adsorption process. Additionally, alcohol screens the interaction between adsorbent and 4-CP molecules and the considerable increase of the solubility of 4-CP in the medium may be another reason for the decrease of the adsorbed amount. Utilization of tert-butanol decreases the adsorption amount, but the goal of the present work is to clarify the effect of high frequency ultrasound on the adsorption of 4-CP by activated carbon rather than an enhancement of the removal of 4-CP from aqueous solutions.
25
516 kHz
20 15 10 5 0 0
60
120
180
240
300
360
420
480
540
600
420
480
540
600
420
480
540
600
Time (min) 40 Stirring 15.2 W 21.5 W 31.1 W 38.3 W
35 30 -1
3.4. Effect of tert-butyl alcohol on the adsorption of 4-CP
sence and presence of ultrasound of different high frequencies (516, 800 and 1660 kHz) and various ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W) were shown in Fig. 6. Comparison of the obtained kinetic curves shows that adsorption rate was significantly enhanced and improved in the presence of ultrasonic irradiation for all the tested frequencies and powers. The enhanced adsorption rate by sonication may be attributed to the extreme conditions generated during the violent collapse of cavitation bubbles. When the bubble is collapsing near the solid surface symmetric cavitation is hindered and collapse occurs asymmetrically. The asymmetric collapse of bubbles in a heterogeneous system produces micro-jets with high velocity. Additionally, symmetric and asymmetric collapses generate shockwaves, which cause extremely turbulent flow at the liquid–solid interface, increasing the rate of mass transfer near the solid surface. Furthermore, the cavitation event also gives rise to acoustic microstreaming or formation of miniature eddies that enhance the mass and heat transfer at interfacial films surrounding nearby sorbent
q (mg g )
the removal of 4-CP in the presence of ultrasound is higher than that obtained by stirring. The removal increases with the increase of ultrasonic power from 21.5 to 38.3 W. After 300 min, the removal of 4-CP in the presence of ultrasound for an acoustic power of 38.3 W was nearly total (99%), but in the conventional method only 60% was eliminated. The removal of 4-CP by GAC in the ultrasound-assisted technique is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption, which makes a direct comparison erroneous. In order to suppress the ultrasonic degradation and make a correct and viable comparison of the adsorption in the absence and presence of ultrasound, tert-butyl alcohol was used during adsorption experiments.
25
800 kHz
20 15 10
3.5. Adsorption dynamics
5
Adsorption kinetic results of 4-CP from aqueous solutions containing 10% (v/v) tert-butyl alcohol onto GAC determined in the ab-
0 0
60
120
180
240
300
360
Time (min)
45
80 Stirring UHQ
70
1660 kHz
15.2 W
35
60
21.5 W 30
50
-1
q (mg g )
q (mg g-1)
Stirring
40
Stirring UHQ+10% t-Butanol
40 30
31.1 W
25
38.3 W
20 15
20
10
10
5 0
0 0
60
120
180
240 300 360 Time (min)
420
480
540
600
Fig. 5. Adsorption kinetics of 4-CP onto GAC in the absence and presence of 10% (v/ v) tert-butyl alcohol (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
0
60
120
180
240 300 360 Time (min)
Fig. 6. Adsorption kinetics of 4-CP onto GAC at different ultrasound frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
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particles and within the pores. As a result, sonication could produce not only high-speed micro-jets but also high-pressure shock waves and acoustic vortex microstreaming [9,13–17,19,20]. These actions lead to an improvement of the adsorption by an enhancement of mass transfer across the boundary layer as well as into the pores. On the other hand, acoustic streaming, which is the movement of the liquid induced by the ultrasonic wave, enhances and improves the mass transfer into bulk solution as well as at the boundary layer [20]. The adsorption increases with increasing acoustic power of ultrasound (Fig. 6), because with high powers more cavitation events occur and more molecules are adsorbed. An increase in ultrasonic power means an increase in the acoustic amplitude. The collapse time, the temperature, and the pressure on collapse are all dependent on acoustic amplitude; the cavitation bubble collapse will be more violent at higher acoustic amplitudes. An increase in power will thus result in greater ultrasonic effects in the collapsing bubble [19]. Additionally, when the acoustic power increases and simultaneously increases amplitude of vibration, the maximum radius of the cavity bubble also increases, as well as its time of collapse, and this bubble is not able to collapse within time equal half of the period. That is, before the sound field reverses itself and the rarefaction phase begins acting on the collapsing bubble [13,19]. Thus, it was concluded that high power of ultrasound leads to the enhancement of mass transfer by high-speed microjets, high-pressure shock waves and acoustic vortex microstreaming. Therefore, these effects of ultrasound could be reasons for the enhancement of adsorption at higher power. After 9 h of contact time, the amounts of 4-CP adsorbed onto GAC obtained by the conventional method (simple stirring) and with the assistance of high frequency ultrasonic irradiation of different frequencies and acoustic powers were presented in Fig. 7. The amount of adsorption were appreciably increased and improved in the presence of ultrasound. For all the studied frequencies, the adsorbed amount increased by increasing ultrasonic power. Sonication enhanced the removal of 4-CP through the extreme conditions generated by cavitation bubbles. Enhanced adsorption may be due to an altered equilibrium and improved kinetics of adsorption. Generally, differences between adsorbed quantities are not significant, whatever the frequency of ultrasound. The adsorption kinetics can be described by the Adam–Thomas relation:
dq ¼ K 1 Cðqm qÞ K 2 q dt
40
q540min (mg g-1)
35
dq V dðC C 0 Þ ¼ ¼ K 1 C 0 qm dt t!0 W dt t!0
ð11Þ
where C0 is the initial concentration (mg L1), V is the volume of solution (L) and W is the weight of activated carbon (g). It is then possible to calculate the initial adsorption kinetic coefficient c:
c ¼ K 1 qm ¼
V dC C 0 m dt t!0
ð12Þ
The values of c, calculated taking into account the initial slopes of the kinetic curves, were given in Fig. 8. The initial adsorption rate increased with increasing the power of ultrasound and was higher than that obtained in the absence of ultrasound by the conventional method. The order of the studied frequencies according to initial adsorption rate is: 516 kHz > 800 kHz > 1660 kHz. The observed variations between initial adsorption rates determined in the absence and presence of ultrasound may be explained by differences in diffusion phenomena. 3.6. Adsorption mechanism The kinetic studies help in predicting the progress of adsorption, but the determination of the adsorption mechanism is also important for design purposes. In a solid–liquid adsorption process, the transfer of the adsorbate is controlled by either boundary layer diffusion (external mass transfer) or intraparticle diffusion (mass transfer through the pores), or by both. It is generally accepted that the adsorption dynamics consists of three consecutive steps: Transport of adsorbate molecules from the bulk solution to the external surface of the adsorbent by diffusion through the liquid boundary layer. Diffusion of the adsorbate from the external surface and into the pores of the adsorbent. Adsorption of the adsorbate on the active sites on the internal surface of the pores. The last step, adsorption, is usually very rapid in comparison to the first two steps. Therefore, the overall rate of adsorption is controlled by either film or intraparticle diffusion, or a combination of
516 kHz 800 kHz 1660 kHz 3.7
30 25 20 15 10 5 0 Stirring
15.2 21.5 31.1 Ultrasonic power (W)
38.3
Initial rate (L mg-1 min-1)X106
45
ð10Þ
where q is the adsorption capacity (mg g1), C is the solution concentration (mg L1), K1 is the adsorption kinetic constant (L mg–1 min1), K2 is the desorption kinetic constant (min1), qm is the maximum surface concentration (mg g1) and t the time (min). At the initial stage of adsorption, when t0, then q0 and CC0. Then, Eq. (10) could be rewritten:
516 kHz 800 kHz 1660 kHz
3.2 2.7 2.2
Stirring
1.7 1.2 0.7 0
Fig. 7. Amount of 4-CP adsorbed onto GAC after a contact time of 540 min in the absence and presence of ultrasound of different frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
5
10
15 20 25 Ultrasonic power (W)
30
35
40
Fig. 8. Initial adsorption rates determined in the absence and presence of ultrasound of different frequencies and powers.
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C ¼ C 0 K W t 1=2
C0 C q
ð15Þ
ð16Þ
where C is the bulk liquid-phase concentration of dye at a time t, CS the surface concentration of dye, kS the liquid-film mass transfer coefficient (m min1) and A is the surface area for external mass transfer (m2).
2.2 516 kHz 800 kHz 1660 kHz
KW (mg L-1 min-1/2)
1.8 1.6 1.4
Stirring
1.2 1 0.8 0.6 0
5
15 Stirring
13 11 9 7 0
5
10
15
20
25
30
35
40
Ultrasonic power (W)
ð14Þ
dC ¼ kS AðC C S Þ dt
2
17
Fig. 10. Liquid-film mass transfer coefficients determined in the absence and presence of ultrasound of different frequencies and powers.
where C0 is the adsorbate initial concentration (mg L1), C is the adsorbate concentration at any time t (mg L1), t is the time (min), q is the amount of 4-CP adsorbed at any time t (mg g1), w is the weight of adsorbent per solution volume (g L1), and KW is the intraparticle diffusion coefficient (mg L1 min–1/2). To determine the intraparticle diffusion coefficient, (wq) is fitted against t1/2. The values of KW determined from the slope of plots were shown in Fig. 9. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound were greater than that obtained in the absence of ultrasound by the conventional method. These results indicate that ultrasound enhances the mass transport in the pores. This behavior could be attributed to the induced turbulence and additional convective mass transport inside the pores caused by micro-jets [9]. The intraparticle diffusion coefficient increased by increasing ultrasonic power. The obtained results show that the intraparticle diffusion coefficients were in the order: 516 kHz > 800 kHz > 1660 kHz. The mass transport across the liquid-film can be described by the following expression [23]:
V
516 kHz 800 kHz 1660 kHz
19
ð13Þ
K W 1=2 or q ¼ t W with w ¼
21
kSA (m3 min-1)X108
both. Many studies have shown that the boundary layer diffusion is the rate controlling step in systems characterized by dilute concentrations of adsorbate, poor mixing, and small particle size of adsorbent. Whereas the intraparticle diffusion controls the rate of adsorption in systems characterized by high concentrations of adsorbate, good mixing, and big particle size of adsorbent [21]. Also, it has been noticed in many studies that boundary layer diffusion is dominant during the initial adsorbate uptake, then gradually the adsorption rate becomes controlled by intraparticle diffusion after the adsorbent external surface is loaded with the adsorbate. The intraparticle diffusion coefficient can be determined using the following equation [22]:
10
15
20
25
30
35
40
Ultrasonic power (W) Fig. 9. Intraparticle diffusion coefficients obtained in the absence and presence of ultrasound of different frequencies and powers.
During the initial stage of adsorption of an adsorbate onto fresh adsorbent, CS is very close to zero (CS being in equilibrium with q = 0). Integrating Eq. (16) for CS = 0 yields
ln
C A ¼ kS t C0 V
ð17Þ
The value of liquid-film mass transfer coefficient can be obtained from Eq. (17) and the concentration versus time data. Modeling of adsorption kinetics by Eq. (17) was applied to the first terms of the kinetic curves, when t ? 0. We will consider in this study the global external transport coefficient (kS A). Indeed, it is possible to standardize the liquid-film mass transfer coefficient by the total surface area of the adsorbent using (kS A) rather than kS. The extraparticle transfer coefficients obtained in the absence and presence of ultrasound were plotted in Fig. 10. It is observed that the rate in the initial period of adsorption, where external mass transfer is assumed to predominate, significantly increases by the assistance of ultrasound. Hydrodynamic effects induced by ultrasound promote a significant increase of the mass transfer across the boundary layer. This increase is a function of the applied ultrasonic power. The enhancement of the mass transport across the liquid-film may be due to the reduction of the thickness of the laminar boundary layer by the micro-scale turbulence created. In general, the liquid-film mass transfer coefficients were in the order: 516 kHz > 800 kHz > 1660 kHz. The average order of the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient shows that adsorption is inversely proportional to the frequency of ultrasound. This result can be explained by the size of cavitation bubbles as well as by the duration and intensity of the implosion of cavitating cavities. The growth cycle of a cavitation bubble depends on the frequency of ultrasound. With an increase of frequency, acoustic periods are shorter and the size of cavitating bubble decreases, which conduct to the decrease of implosion intensity. The duration of a growth cycle decreases by increasing ultrasound frequency. The decrease in the duration of growth involves the formation of cavitation bubbles of smaller sizes at high frequency and thus to less violent collapses. In addition, the number of acoustic cycle increases at high frequency and, consequently, cavitation events per time unit are more significant. Also, the cavitation threshold increases with increasing ultrasound frequency. 4. Conclusions Kinetic studies were carried out for the adsorption of 4-CP from aqueous medium onto activated carbon without and with the assistance of ultrasound of different high frequencies and powers.
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Comparison of the obtained kinetic curves shows that both adsorption rate and adsorbed amount were significantly enhanced and improved in the presence of ultrasonic irradiation for all the studied frequencies and powers. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound are greater than those obtained in the absence of ultrasound. Hydrodynamic effects induced by ultrasound promote a significant increase of the mass transfer across the boundary layer. These behaviors increased with increasing ultrasonic power. The average order of the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient shows that adsorption is inversely proportional to the frequency of ultrasound (516 kHz > 800 kHz > 1660 kHz). References [1] H.H. Fang, O. Chen, Toxicity of phenol towards aerobic biogranules, Water Res. 31 (1997) 2229–2242. [2] F.A. Banat, B. Al-Bashir, S. Al-Asheh, O. Hayajneh, Adsorption of phenol by bentonite, Environ. Pollut. 107 (2000) 391–398. [3] US EPA, Federal Register, vol. 52, 131, Washington, DC, 1987, pp. 25861– 25962. [4] EEC Directive 80/778/EEC 15-7-1990: Official Journal of the European Communities, 30-8-1990, European Community, Brussels, 1990. [5] I. Rodriguez, M.P. Llompart, R. Cela, Solid-phase extraction of phenols, J. Chromatogr. 885 (2000) 291–304. [6] B.S. Schueller, R.T. Yang, Ultrasound enhanced adsorption and desorption of phenol on activated carbon and polymeric resin, Ind. Eng. Chem. Res. 40 (2001) 4912–4918. [7] Z. Wang, K.L. Cheng, Effect of ultrasound and mechanical agitation on adsorption of PAR by Amberlite XAD-2, Ultrasonics 20 (1982) 215–216. [8] M. Breitbach, D. Bathen, Influence of ultrasound on adsorption processes, Ultrason. Sonochem. 8 (2001) 277–283.
[9] O. Hamdaoui, E. Naffrechoux, L. Tifouti, C. Pétrier, Effects of ultrasound on adsorption–desorption of p-chlorophenol on granular activated carbon, Ultrason. Sonochem. 10 (2003) 109–114. [10] J.-B. Ji, X.-H. Lu, Z.-C. Xu, Effect of ultrasound on adsorption of Geniposide on polymeric resin, Ultrason. Sonochem. 13 (2006) 463–470. [11] R.-S. Juang, S.-H. Lin, C.-H. Cheng, Liquid-phase adsorption and desorption of phenol onto activated carbons with ultrasound, Ultrason. Sonochem. 13 (2006) 251–260. [12] T.J. Mason, J.P. Lorimer, D.M. Bates, Quantifying sonochemistry: casting some light on a ‘black art’, Ultrasonics 30 (1992) 40–42. [13] T.J. Mason, Practical Sonochemistry: User’s Guide to Applications in Chemistry and Chemical Engineering, Ellis Horwood, Chichester, 1991. [14] L.H. Thompson, L.K. Doraiswamy, Sonochemistry: science and engineering, Ind. Eng. Chem. Res. 38 (1999) 1215–1249. [15] O. Hamdaoui, R. Djeribi, E. Naffrechoux, Desorption of metal ions from activated carbon in the presence of ultrasound, Ind. Eng. Chem. Res. 44 (2005) 4737–4744. [16] O. Hamdaoui, E. Naffrechoux, J. Suptil, C. Fachinger, Ultrasonic desorption of pchlorophenol from granular activated carbon, Chem. Eng. J. 106 (2005) 153– 161. [17] O. Hamdaoui, E. Naffrechoux, An investigation of the mechanisms of ultrasonically enhanced desorption, AIChE J. 53 (2007) 363–373. [18] A. Tauber, G. Mark, H.-P. Schuchmann, C. von Sonntag, Sonolysis of tert-butyl alcohol in aqueous solution, J. Chem. Soc. Perkin Trans. 2 (1999) 1129–1135. [19] Y.G. Adewuyi, Sonochemistry: environmental science and engineering applications, Ind. Eng. Chem. Res. 40 (2001) 4681–4715. [20] L. Nouri, O. Hamdaoui, Ultrasonication-assisted sorption of cadmium from aqueous phase by wheat bran, J. Phys. Chem. A 111 (2007) 8456–8463. [21] D. Mohan, K.P. Singh, Single and multicomponent adsorption of cadmium and zinc using activated carbon derived from bagasse – an agricultural waste, Water Res. 36 (2004) 2304–2318. [22] W.J. Weber Jr., J.C. Morris, Kinetics of adsorption on carbon from solution, J. Sanitary Eng. Div. Proceed. Am. Soc. Civil Eng. 89 (1963) 31–59. [23] H. Spahn, U. Schlunder, The scale–up of activated carbon columns for water purification based on results from batch test. I. Theoretical and experimental determination of adsorption rates of single organic solutes in batch tests, Chem. Eng. Sci. 30 (1975) 529–537.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Adsorption kinetics of 4-chlorophenol onto granular activated carbon in the presence of high frequency ultrasound Oualid Hamdaoui a,1,*, Emmanuel Naffrechoux b,1 a b
Department of Process Engineering, Faculty of Engineering, University of Annaba, P.O. Box 12, 23000 Annaba, Algeria Laboratoire Chimie Moléculaire et Environment, Polytech Savoie, Université de Savoie, 73376 Le Bourget du Lac, France
a r t i c l e
i n f o
Article history: Received 21 January 2008 Received in revised form 12 May 2008 Accepted 16 May 2008 Available online 24 May 2008 Keywords: Ultrasound High frequency Adsorption 4-Chlorophenol Activated carbon Kinetics Modeling
a b s t r a c t This work describes the results of investigations carried out to examine the adsorption kinetics of 4-chlorophenol (4-CP) from aqueous solution containing tert-butyl alcohol (10%, v/v) onto granular activated carbon (GAC) in the presence of ultrasound of different high frequencies (516, 800 and 1660 kHz) and acoustic powers (15.2, 21.5, 31.1 and 38.3 W). The main objective of this study is to describe the mechanism of ultrasound-assisted adsorption rather than the enhancement of adsorption capacity. Sonochemical degradation of 4-CP was studied in the absence and presence of tert-butyl alcohol. The sonolysis of 4CP is effectively inhibited by the addition of tert-butyl alcohol (10%, v/v) and very little 4-CP degradation occurs, indicating that little or no pyrolysis of the compound occurs. Without addition of tert-butyl alcohol, after 300 min and at 1660 kHz, the removal of 4-CP in the presence of ultrasound for an acoustic power of 38.3 W was nearly total (99%), but in the conventional method only 60% was eliminated. In this case, the removal of 4-CP by GAC in the ultrasound-assisted technique is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption, which makes a direct comparison unacceptable. In order to distinguish sonochemical degradation and adsorption of 4-CP onto GAC and to make an exact and practical comparison of the adsorption in the absence and presence of ultrasound, kinetic adsorption experiments were conducted using aqueous solution containing 10% (v/v) tert-butyl alcohol. The obtained results show that both adsorption rate and adsorbed amount were significantly enhanced and improved in the presence of ultrasound for all the studied frequencies and powers. The enhancement of adsorption is favored by increasing ultrasonic power. Adsorption kinetic data were modeled using the liquid-film mass transfer equation and intraparticle diffusion model. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound are greater than that obtained in the absence of ultrasound. In the initial period of adsorption, where external mass transfer is assumed to predominate, liquid-film mass transfer coefficients significantly increased by the assistance of ultrasound. These results indicate that ultrasound enhances the mass transport in the pores as well as across the boundary layer. This effect increased with increasing ultrasonic power for the three studied frequencies. The average order for the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient is 516 kHz > 800 kHz > 1660 kHz. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Phenolic compounds which are generated by petroleum and petrochemical, coal conversion and phenol producing industries, are common contaminants in wastewater. Phenols are widely used for the commercial production of a wide variety of resins including phenolic resins, which are used as construction materials for automobiles and appliances, epoxy resins and adhesives, and polyamide for various applications [1,2]. Phenols are considered as priority pollutants since they are harmful to organisms at low * Tel.: +213 771598509; fax: +213 38876560. E-mail address: [email protected] (O. Hamdaoui). 1 The authors thank the Comité Mixte d’Evaluation et de Prospective de coopération interuniversitaire franco-algérienne (CMEP) for its financial support and for the grant for O.H. 1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2008.05.008
concentrations and many of them have been classified as hazardous pollutants because of their potential to harm human health. Increasing concern for public health and environmental quality has led to the establishment of rigid limits on the acceptable environmental levels of specific pollutants. Because of their toxicity, phenols have been included in the US Environmental Protection Agency (EPA) list of priority pollutants [3]. Also the European Union (EU) has classified several phenols as priority contaminants and the 80/778/EC directive lays down a maximum concentration of 0.5 mg L1 for total phenols in drinking water [4,5]. Thus, the removal or destruction of phenols from process or waste streams becomes a major environmental problem. Adsorption is well-established technique for the removal of low concentrations of organic pollutants from potable water, wastewater and aqueous solutions. Activated carbon is one of the most
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effective adsorbents for organic compounds because of their extended surface area, high adsorption capacity, microporous structure and special surface reactivity. Ultrasound has been proven to be a very useful tool in intensifying the mass transfer process and breaking the affinity between adsorbate and adsorbent. Several researchers have studied the role of ultrasound on adsorption kinetics of organic compounds by various adsorbents [6–11]. Schueller and Yang [6] have explored the adsorption of aqueous phenol onto activated carbon and Dowex resin at 295 K at a frequency of 1 MHz and 60 W. They have showed from the comparison of uptake rates in a batch adsorber that mixing at 250 rpm provides the highest adsorption rate than ultrasound. Wang and Cheng [7], who studied the uptake of 4-(2pyridylazo) resorcinol (PAR) by Amberlite XAD-2 resin, also noted the trend of mixing having a greater effect than 20 kHz ultrasound on adsorption. Breitbach and Bathen [8] have investigated the adsorption kinetics of fructose on Amberlite CR1320 at 313 K using different powers input of ultrasound with frequencies 337 and 1158 kHz. The authors show the increase of pore diffusion coefficient with increasing ultrasonic powers for both frequencies. Hamdaoui et al. [9] have examined the effect of 21 kHz ultrasound on adsorption dynamics of p-chlorophenol on granular activated carbon. The obtained results show that low frequency ultrasound enhances pore diffusion and initial rate of adsorption. Ji et al. [10], investigated the effects of pulse 18 kHz ultrasound with different pulse parameter on the adsorption kinetics of Geniposide on Resin 1300. They have founded that pulse ultrasound can enhance both liquid-film diffusion and intraparticle diffusion, and the intensification of liquid-film diffusion with pulse ultrasound is stronger than that of intraparticle diffusion. Juang et al. [11] have studied the effect of 48 kHz ultrasound on the adsorption of phenol from aqueous solutions onto coconut shell-based granular activated carbons at 298 K. They have shown that the initial rate of adsorption was enhanced by sonication. Most of the previous studies were focused on the influence of low frequency ultrasound on adsorption dynamics. However, the adsorption kinetics in the presence of high frequency ultrasound using activated carbon as adsorbent has not been studied systematically yet. The objective of the present work was to investigate the adsorption kinetics of 4-CP from aqueous medium containing 10% (v/v) tert-butyl alcohol onto GAC in the presence of high frequency ultrasound (516, 800 and 1660 kHz) for different ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W). The removal of 4-CP without the addition of tert-butyl alcohol in the presence of ultrasound is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption. In order to suppress the ultrasonic degradation and make a correct and viable comparison of the adsorption in the absence and presence of ultrasound, tert-butyl alcohol was used during adsorption experiments. The goal of the present work is to clarify the effect of high frequency ultrasound on the adsorption of 4-CP by activated carbon rather than an enhancement of the removal of 4-CP from aqueous solutions. In order to gain insight into the dynamics of the process, the mechanism controlling the rate of adsorption was also studied. The sonochemical degradation of 4-CP and the scavenging effect of tert-butyl alcohol were investigated.
were prepared by dissolving the appropriate amount in ultra-high quality (UHQ) water (Elga elgastat 18.2 MX). 2.2. Activated carbon Granular activated carbon (GAC) employed as the adsorbent in the present study was supplied by Prolabo. Prior to use, the carbon was pretreated by boiling in UHQ water for 1 h and washed repeatedly with UHQ water until the electric conductivity and the UV absorbance were equal to zero, and the pH remains constant. Finally, the washed activated carbon was dried in an oven at 383 K to constant weight and stored in a desiccator until use. The BET (Brunauer–Emmett–Teller) surface area of the carbon (929 m2 g1) was obtained from N2 adsorption isotherms at 77 K. The GAC has a mean granulometry of 3 mm. 2.3. Ultrasonic reactor The ultrasonic irradiation was carried out with equipment operating at three different frequencies: 516, 800 and 1660 kHz. In the three cases, the ceramic transducer is located at the bottom of the vessel and connected to high frequency supply. The ultrasonic waves were introduced from the bottom of the solution. The cylindrical reactor was thermostated by a water jacket. The temperature inside the reactor was kept constant. Acoustic power dissipated in the reactor was measured using standard calorimetric method [12]. 2.4. Adsorption procedure
2. Materials and methods
In all adsorption experiments, 4-CP solutions of 100 mg L1 were prepared by dissolving the appropriate amount in 10% (v/v) tert-butyl alcohol aqueous solution. For the determination of adsorption kinetics onto GAC in the presence of high frequency ultrasound, an adsorbent weight of 0.09 g was transferred to the ultrasonic reactor containing a volume of 90 mL of 4-CP aqueous solution at a concentration of 100 mg L1. Ultrasonic waves of various frequencies (516, 800 and 1660 kHz) was used for sonication experiments at four different ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W) as measured by calorimetry. The temperature was maintained constant and equal to 294 K. All the kinetic experiments were carried out at pH 5.5. Strong convective currents occur within the reactor here and there along the transducer axis. These effects associated with hydrodynamic phenomenon due to cavitation are responsible for the perfect mixing of the reactor content. It was thereby established that under ultrasonic irradiation the used reactors are completely stirred tank reactor. Identical experiments were repeated in the absence of ultrasound (conventional method) using a glass reactor (with the same geometry as the sonication reactor) and a magnetic stirrer with a stirring speed of 400 rpm. The reactor provides uniform mixing conditions due to continuous agitation. All other conditions, such as temperature and pH, were the same as those used for ultrasound-assisted adsorption. After selected times of sonication (or stirring), the adsorption kinetics was determined by following the 4-CP concentration change in the reactor. The amount of 4-CP adsorbed (mg g1) at any time t was calculated by using Eq. (1)
2.1. 4-Chlorophenol
q¼
4-Chlorophenol (4-CP) supplied by Sigma (99.5%) was used as an adsorbate. 4-CP has a molecular weight of 128.56 g mol–1 and linear formula: ClC6H4OH. 4-CP solutions of desired concentration
where C0 is the initial concentration of 4-CP (mg L1), C is the concentration of 4-CP at any time t (mg L1), V is the solution volume (L) and W is the weight of the adsorbent (g) in the mixture.
ðC 0 CÞV W
ð1Þ
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2.5. Measurements Quantitative analysis of 4-CP was done by a high performance liquid chromatograph Waters (pump 515). Detection was made at 254 nm with a model 996 absorbance detector. The column used was a reverse phase column C18 (150 mm length, 3.9 mm inner diameter and 4 lm particle size). The mobile phase was an aqueous mixture of acetonitrile (40%) and acetic acid (1‰). The particle size analysis of the GAC was determined before and after ultrasonication. GAC was added to 90 mL of UHQ water, and the obtained suspension was exposed to ultrasound of different frequencies (516, 800 and 1660 kHz) and powers for 9 h. The suspension was then filtered (0.45 lm) and dried at 383 K for 48 h. The size distribution was determined using a laser granulometer (Mastersizer 2000 – Malvern instruments). Scanning electron microscopy (SEM) (Leo type Stereoscan 440) analysis was carried out on the GAC to study its surface texture before and after ultrasonication.
3. Results and discussion 3.1. Influence of sonication on GAC granulometry Particles characterization experiments were carried out to understand sonication effects on activated carbon properties. Additionally, changes in the size of the particles are important factors to understand the mechanisms of adsorption dynamics of 4-CP from aqueous phase onto GAC. Therefore, the resistance of the activated carbon granules in the presence of ultrasound of various frequencies (516, 800 and 1660 kHz) and different powers (15.2, 21.5, 31.1 and 38.3 W) was experimentally determined. The activated carbon was sonicated for 9 h in the same reactor employed for the adsorption experiments. GAC particle size distributions showed that no attrition (erosion) of the activated carbon was observed for all the tested frequencies and acoustic powers. Theoretically, when bubbles collapse in homogeneous solution, the shape of the bubbles remains spherical and symmetrical during their lifetime. However, in heterogeneous solution (i.e., when solid particles exist), the bubbles collapse near a solid surface; thus, dynamics of cavitation bubbles change significantly. In this situation, the bubbles collapse asymmetrically producing a high-speed jet of liquid, which passes through the interior of the cavitation bubble and toward the solid surface, called a microjet [13,14]. It has been reported that the speed of the liquid jet may reach speeds of more than 100 m s1 [13]. Such a powerful jet will produce dramatic effects on the solid surface, and is responsible for pitting and erosion of the solid surface. In addition, fracturing and melting of particles has been observed resulting from bubble implosion and shockwaves at the surface of particles [15]. In the present study, the particle analyses prove that the intensity threshold for particle breakup at all frequencies and powers had not been attained. This is due to cavitating bubbles size and the duration of the collapses that decrease with the increase of the frequency. The growth cycle of a cavitating bubble is dependent on the frequency of the applied ultrasound. When the frequency is increased, the growth time cycle decreases. This decrease in growth times results in smaller maximum sizes at higher frequencies and thus less violent collapses. Cavitation collapses are fewer but more violent at lower frequencies, and more frequent and less violent at higher frequencies.
A scanning electron microscope was used to look at the effect of ultrasound on the morphology of the activated carbon surface. It can be seen from the micrographs (not shown) that the activated carbon showed similar surface morphologies before and after sonication. 3.2. Destruction of 4-CP by ultrasound Sonochemical degradation of 100 mg L1 4-CP aqueous solutions was carried out in the cylindrical jacketed glass reactor described above operating at three different frequencies: 516, 800 and 1660 kHz and various powers: 15.2, 21.5, 31.1 and 38.3 W. Application of ultrasonic irradiation of different frequencies and acoustic powers to 90 mL of 4-CP solution resulted in the decline of the initial concentration. The disappearance of 4-CP follows a pseudo-first-order reaction kinetics. At a frequency of 516 kHz, for example, 4-CP was completely destroyed after 70, 80, 100 and 130 min of sonication for the ultrasonic powers of 38.3, 31.1, 21.5 and 15.2 W, respectively. Initial rate of degradation of 4-CP was calculated for the initial 10 min. The results presented in the form of initial rate of 4-CP degradation as a function of ultrasonic power are shown in Fig. 1. It was observed that the initial degradation rate increased with the increase of ultrasonic power. Additionally, the results of 4-CP sonochemical oxidation clearly show that the pseudo-first-order degradation rate constant increases with ultrasonic power (Fig. 2). An increase in power will thus result in greater sonochemical effects in the collapsing bubble. So because of the increasing acoustic amplitudes, oxidation of 4-CP increased. When power is increased, transmittance of ultrasonic energy into the reactor increases. Due to this energy, the pulsation and collapse of bubble occur more rapidly, the number of cavitation bubbles increase and realizing a higher concentration of OH radicals into the aqueous solution of organic pollutant. Thus, an increase in ultrasonic power results in an increase in acoustic amplitude, which favors more violent cavitation bubble collapse because the bubble collapse time, the transient temperature, and the internal pressure in the cavitation bubble during collapse are all dependent on the acoustic amplitude. That is, high enough acoustic power results in transient cavitation. Hence, the results of an increase in the sound power are greater sonochemical effects, resulting in higher 4-CP degradation rates. When ultrasound waves are introduced into an aqueous solution, cavitation occurs, as bubbles rapidly form and subsequently collapse inward from the build-up of pressure through rarefaction/compression cycles. Under these sonolytic conditions, high temperatures and pressures accompany the implosion of cavita-
10 9 Initial rate (µM min-1)
All experiments were conducted in triplicate and the mean values were reported. The maximum standard deviation obtained for triplicate measurements of the sorbed amount was ±1.5%.
8
516 kHz 800 kHz 1660 kHz
7 6 5 4 3 2 1 0 15.2
21.5 31.1 Ultrasonic power (W)
38.3
Fig. 1. Effect of ultrasonic power on the initial rate of 4-CP sonochemical degradation at different frequencies (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
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80
0.018 516 kHz 800 kHz 1660 kHz
0.014
70 Rate constant (nM min-1)
-1
Rate constant (min )
0.016
0.012 0.01 0.008 0.006 0.004
516 kHz 800 kHz 1660 kHz
60 50 40 30 20 10
0.002
0
0 15.2
21.5 31.1 Ultrasonic power (W)
15.2
38.3
21.5 31.1 Ultrasonic power (W)
38.3
Fig. 2. Sonochemical degradation rate constant at different ultrasound frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
Fig. 3. Effect of tert-butyl alcohol (10%, v/v) on the rate constant of 4-CP sonochemical degradation for different frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
tion bubbles. Thus, ultrasonic irradiation induces the formation of free radicals as a consequence of cavitation. Hydroxyl radical as well as homolytic oxygen (reactions 2 and 3) are produced in the bubble during the implosion. Radicals combine in the bubble (reactions 4–6) or escape from the bubble, which conducts to H2O2 release in the medium (reactions 7–9).
oxidized by hydroxyl radicals formed within the bubbles as a result of the sonolysis of water. Another factor that also affects the rate of 4-CP degradation is the formation of volatile products from tertbutyl alcohol degradation that accumulate inside the bubble. Such volatile products decrease the temperature inside the bubble, which, in turn, slow the sonolytic reactions [18]. From these results, we can infer that 4-CP does not undergo direct pyrolysis in the sonolytic bubbles but rather becomes oxidized at the interface by reacting with OH radicals. In view of the above and in order to distinguish sonochemical degradation and adsorption of 4-CP onto GAC, kinetic adsorption experiments were conducted using 100 mg L1 4-CP aqueous solution containing 10% (v/v) tert-butyl alcohol.
ð2Þ
O2 ! 2O
ð3Þ
O þ H2 O ! 2HO H þ HO ! H2 O
ð4Þ ð5Þ
2HO ! O þ H2 O
ð6Þ
H þ O2 ! HOO
ð7Þ
2HO ! H2 O2
ð8Þ
2HOO ! H2 O2 þ O2
ð9Þ
Degradation rate of 4-CP is higher at 516 and 800 kHz than at 1660 kHz. This difference can be explained by the size of the cavitating bubbles and the duration of the collapse. Diameter of cavitating bubbles is more important at 516 and 800 kHz than at 1660 kHz as a consequence the build-up of energy at the final stage of the collapse is more elevated at 516 and 800 kHz. At very high frequency (higher than the megahertz), rarefaction (and compression) cycles are becoming extremely short, the finite time required for the rarefaction cycle becomes too short to permit the molecules to be pulled apart sufficiently to generate a bubble. Consequently, the duration of implosion and the number of hydroxyl radicals formed decrease with the increase of frequency. In the present work, the scavenging effect of tert-butyl alcohol (10%, v/v) on the degradation of 4-CP (100 mg L1) for different frequencies and ultrasonic powers was investigated. As indicated in our previous works [16,17], the sonochemical degradation of aqueous 4-CP solutions in the presence of tert-butyl alcohol exhibits zero–order reaction kinetics. The zero-order rate constants obtained for different frequencies and acoustic powers were given in Fig. 3. Extensive work on the sonochemistry of tert-butyl alcohol has been reported by von Sonntag and co-workers [18]. They determined that the alcohol is pyrolized in the bubble via a freeradical-induced pyrolysis reaction mechanism. The tert-butyl alcohol is able to scavenge OH radicals in the bubble and prevent the accumulation of OH radicals at the bubble interface. From Fig. 3, it was observed that the degradation of 4-CP was effectively quenched, but not completely, by the addition of tert-butyl alcohol. This low degradation suggests that the destruction takes place at the interface of the cavitation bubbles where 4-CP molecules are
3.3. Removal of 4-CP by GAC without adding tert-butyl alcohol The removal of 4-CP by GAC in the absence (stirring 400 rpm) and presence of 1660 kHz ultrasonic irradiation of different acoustic powers was investigated using aqueous solution of 4-CP with an initial concentration of 100 mg L1. The adsorbent dosage and temperature were 0.09 g/90 mL and 294 K, respectively. The 4-CP solution was prepared by dissolving required amount of solute in UHQ water. The removal percentage of 4-CP obtained after 300 min in the conventional method (simple stirring) and in the ultrasoundassisted method was shown in Fig. 4. It was clearly shown that
100 90 Removal percentage
H2 O ! H þ HO
80 70 60 50 40 30 20 10 0 Stirring
38.3 W
31.1 W
21.5 W
Fig. 4. Removal of 4-CP by GAC after 300 min in the absence and presence of 1660 kHz ultrasonic irradiation (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K, contact time 300 min).
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40 Stirring 15.2 W 21.5 W 31.1 W 38.3 W
35 30 q (mg g-1)
In order to make a practical evaluation of the adsorption of 4-CP by GAC by the conventional method (simple stirring) and ultrasound-assisted method, adsorption experiments were carried out using aqueous solution of 4-CP containing 10% (v/v) tert-butyl alcohol. Therefore, the effect of tert-butyl alcohol (10%, v/v) on the adsorption kinetics of 4-CP was investigated in the absence of ultrasound (conventional method). Adsorption kinetics was determined for an initial concentration of 100 mg L1, an adsorbent dosage of 0.09 g/90 mL and a stirring speed of 400 rpm. Fig. 5 shows the adsorption kinetics of 4-CP onto GAC in the absence and presence of tert-butyl alcohol. It was observed that the adsorption decreases significantly in the presence of tert-butyl alcohol. Adsorption capacity after a contact time of 9 h was 20.85 and 78.1 mg g1 with and without the addition of alcohol respectively. The significant decrease of the adsorbed amount (73%) in the presence of tert-butyl alcohol is due to the competitive effect between 4-CP and alcohol molecules for the sites available for the adsorption process. Additionally, alcohol screens the interaction between adsorbent and 4-CP molecules and the considerable increase of the solubility of 4-CP in the medium may be another reason for the decrease of the adsorbed amount. Utilization of tert-butanol decreases the adsorption amount, but the goal of the present work is to clarify the effect of high frequency ultrasound on the adsorption of 4-CP by activated carbon rather than an enhancement of the removal of 4-CP from aqueous solutions.
25
516 kHz
20 15 10 5 0 0
60
120
180
240
300
360
420
480
540
600
420
480
540
600
420
480
540
600
Time (min) 40 Stirring 15.2 W 21.5 W 31.1 W 38.3 W
35 30 -1
3.4. Effect of tert-butyl alcohol on the adsorption of 4-CP
sence and presence of ultrasound of different high frequencies (516, 800 and 1660 kHz) and various ultrasonic powers (15.2, 21.5, 31.1 and 38.3 W) were shown in Fig. 6. Comparison of the obtained kinetic curves shows that adsorption rate was significantly enhanced and improved in the presence of ultrasonic irradiation for all the tested frequencies and powers. The enhanced adsorption rate by sonication may be attributed to the extreme conditions generated during the violent collapse of cavitation bubbles. When the bubble is collapsing near the solid surface symmetric cavitation is hindered and collapse occurs asymmetrically. The asymmetric collapse of bubbles in a heterogeneous system produces micro-jets with high velocity. Additionally, symmetric and asymmetric collapses generate shockwaves, which cause extremely turbulent flow at the liquid–solid interface, increasing the rate of mass transfer near the solid surface. Furthermore, the cavitation event also gives rise to acoustic microstreaming or formation of miniature eddies that enhance the mass and heat transfer at interfacial films surrounding nearby sorbent
q (mg g )
the removal of 4-CP in the presence of ultrasound is higher than that obtained by stirring. The removal increases with the increase of ultrasonic power from 21.5 to 38.3 W. After 300 min, the removal of 4-CP in the presence of ultrasound for an acoustic power of 38.3 W was nearly total (99%), but in the conventional method only 60% was eliminated. The removal of 4-CP by GAC in the ultrasound-assisted technique is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption, which makes a direct comparison erroneous. In order to suppress the ultrasonic degradation and make a correct and viable comparison of the adsorption in the absence and presence of ultrasound, tert-butyl alcohol was used during adsorption experiments.
25
800 kHz
20 15 10
3.5. Adsorption dynamics
5
Adsorption kinetic results of 4-CP from aqueous solutions containing 10% (v/v) tert-butyl alcohol onto GAC determined in the ab-
0 0
60
120
180
240
300
360
Time (min)
45
80 Stirring UHQ
70
1660 kHz
15.2 W
35
60
21.5 W 30
50
-1
q (mg g )
q (mg g-1)
Stirring
40
Stirring UHQ+10% t-Butanol
40 30
31.1 W
25
38.3 W
20 15
20
10
10
5 0
0 0
60
120
180
240 300 360 Time (min)
420
480
540
600
Fig. 5. Adsorption kinetics of 4-CP onto GAC in the absence and presence of 10% (v/ v) tert-butyl alcohol (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
0
60
120
180
240 300 360 Time (min)
Fig. 6. Adsorption kinetics of 4-CP onto GAC at different ultrasound frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
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O. Hamdaoui, E. Naffrechoux / Ultrasonics Sonochemistry 16 (2009) 15–22
particles and within the pores. As a result, sonication could produce not only high-speed micro-jets but also high-pressure shock waves and acoustic vortex microstreaming [9,13–17,19,20]. These actions lead to an improvement of the adsorption by an enhancement of mass transfer across the boundary layer as well as into the pores. On the other hand, acoustic streaming, which is the movement of the liquid induced by the ultrasonic wave, enhances and improves the mass transfer into bulk solution as well as at the boundary layer [20]. The adsorption increases with increasing acoustic power of ultrasound (Fig. 6), because with high powers more cavitation events occur and more molecules are adsorbed. An increase in ultrasonic power means an increase in the acoustic amplitude. The collapse time, the temperature, and the pressure on collapse are all dependent on acoustic amplitude; the cavitation bubble collapse will be more violent at higher acoustic amplitudes. An increase in power will thus result in greater ultrasonic effects in the collapsing bubble [19]. Additionally, when the acoustic power increases and simultaneously increases amplitude of vibration, the maximum radius of the cavity bubble also increases, as well as its time of collapse, and this bubble is not able to collapse within time equal half of the period. That is, before the sound field reverses itself and the rarefaction phase begins acting on the collapsing bubble [13,19]. Thus, it was concluded that high power of ultrasound leads to the enhancement of mass transfer by high-speed microjets, high-pressure shock waves and acoustic vortex microstreaming. Therefore, these effects of ultrasound could be reasons for the enhancement of adsorption at higher power. After 9 h of contact time, the amounts of 4-CP adsorbed onto GAC obtained by the conventional method (simple stirring) and with the assistance of high frequency ultrasonic irradiation of different frequencies and acoustic powers were presented in Fig. 7. The amount of adsorption were appreciably increased and improved in the presence of ultrasound. For all the studied frequencies, the adsorbed amount increased by increasing ultrasonic power. Sonication enhanced the removal of 4-CP through the extreme conditions generated by cavitation bubbles. Enhanced adsorption may be due to an altered equilibrium and improved kinetics of adsorption. Generally, differences between adsorbed quantities are not significant, whatever the frequency of ultrasound. The adsorption kinetics can be described by the Adam–Thomas relation:
dq ¼ K 1 Cðqm qÞ K 2 q dt
40
q540min (mg g-1)
35
dq V dðC C 0 Þ ¼ ¼ K 1 C 0 qm dt t!0 W dt t!0
ð11Þ
where C0 is the initial concentration (mg L1), V is the volume of solution (L) and W is the weight of activated carbon (g). It is then possible to calculate the initial adsorption kinetic coefficient c:
c ¼ K 1 qm ¼
V dC C 0 m dt t!0
ð12Þ
The values of c, calculated taking into account the initial slopes of the kinetic curves, were given in Fig. 8. The initial adsorption rate increased with increasing the power of ultrasound and was higher than that obtained in the absence of ultrasound by the conventional method. The order of the studied frequencies according to initial adsorption rate is: 516 kHz > 800 kHz > 1660 kHz. The observed variations between initial adsorption rates determined in the absence and presence of ultrasound may be explained by differences in diffusion phenomena. 3.6. Adsorption mechanism The kinetic studies help in predicting the progress of adsorption, but the determination of the adsorption mechanism is also important for design purposes. In a solid–liquid adsorption process, the transfer of the adsorbate is controlled by either boundary layer diffusion (external mass transfer) or intraparticle diffusion (mass transfer through the pores), or by both. It is generally accepted that the adsorption dynamics consists of three consecutive steps: Transport of adsorbate molecules from the bulk solution to the external surface of the adsorbent by diffusion through the liquid boundary layer. Diffusion of the adsorbate from the external surface and into the pores of the adsorbent. Adsorption of the adsorbate on the active sites on the internal surface of the pores. The last step, adsorption, is usually very rapid in comparison to the first two steps. Therefore, the overall rate of adsorption is controlled by either film or intraparticle diffusion, or a combination of
516 kHz 800 kHz 1660 kHz 3.7
30 25 20 15 10 5 0 Stirring
15.2 21.5 31.1 Ultrasonic power (W)
38.3
Initial rate (L mg-1 min-1)X106
45
ð10Þ
where q is the adsorption capacity (mg g1), C is the solution concentration (mg L1), K1 is the adsorption kinetic constant (L mg–1 min1), K2 is the desorption kinetic constant (min1), qm is the maximum surface concentration (mg g1) and t the time (min). At the initial stage of adsorption, when t0, then q0 and CC0. Then, Eq. (10) could be rewritten:
516 kHz 800 kHz 1660 kHz
3.2 2.7 2.2
Stirring
1.7 1.2 0.7 0
Fig. 7. Amount of 4-CP adsorbed onto GAC after a contact time of 540 min in the absence and presence of ultrasound of different frequencies and powers (conditions: 90 mL of 4-CP solution, initial concentration 100 mg L1, pH 5.5, temperature 294 K).
5
10
15 20 25 Ultrasonic power (W)
30
35
40
Fig. 8. Initial adsorption rates determined in the absence and presence of ultrasound of different frequencies and powers.
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O. Hamdaoui, E. Naffrechoux / Ultrasonics Sonochemistry 16 (2009) 15–22
C ¼ C 0 K W t 1=2
C0 C q
ð15Þ
ð16Þ
where C is the bulk liquid-phase concentration of dye at a time t, CS the surface concentration of dye, kS the liquid-film mass transfer coefficient (m min1) and A is the surface area for external mass transfer (m2).
2.2 516 kHz 800 kHz 1660 kHz
KW (mg L-1 min-1/2)
1.8 1.6 1.4
Stirring
1.2 1 0.8 0.6 0
5
15 Stirring
13 11 9 7 0
5
10
15
20
25
30
35
40
Ultrasonic power (W)
ð14Þ
dC ¼ kS AðC C S Þ dt
2
17
Fig. 10. Liquid-film mass transfer coefficients determined in the absence and presence of ultrasound of different frequencies and powers.
where C0 is the adsorbate initial concentration (mg L1), C is the adsorbate concentration at any time t (mg L1), t is the time (min), q is the amount of 4-CP adsorbed at any time t (mg g1), w is the weight of adsorbent per solution volume (g L1), and KW is the intraparticle diffusion coefficient (mg L1 min–1/2). To determine the intraparticle diffusion coefficient, (wq) is fitted against t1/2. The values of KW determined from the slope of plots were shown in Fig. 9. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound were greater than that obtained in the absence of ultrasound by the conventional method. These results indicate that ultrasound enhances the mass transport in the pores. This behavior could be attributed to the induced turbulence and additional convective mass transport inside the pores caused by micro-jets [9]. The intraparticle diffusion coefficient increased by increasing ultrasonic power. The obtained results show that the intraparticle diffusion coefficients were in the order: 516 kHz > 800 kHz > 1660 kHz. The mass transport across the liquid-film can be described by the following expression [23]:
V
516 kHz 800 kHz 1660 kHz
19
ð13Þ
K W 1=2 or q ¼ t W with w ¼
21
kSA (m3 min-1)X108
both. Many studies have shown that the boundary layer diffusion is the rate controlling step in systems characterized by dilute concentrations of adsorbate, poor mixing, and small particle size of adsorbent. Whereas the intraparticle diffusion controls the rate of adsorption in systems characterized by high concentrations of adsorbate, good mixing, and big particle size of adsorbent [21]. Also, it has been noticed in many studies that boundary layer diffusion is dominant during the initial adsorbate uptake, then gradually the adsorption rate becomes controlled by intraparticle diffusion after the adsorbent external surface is loaded with the adsorbate. The intraparticle diffusion coefficient can be determined using the following equation [22]:
10
15
20
25
30
35
40
Ultrasonic power (W) Fig. 9. Intraparticle diffusion coefficients obtained in the absence and presence of ultrasound of different frequencies and powers.
During the initial stage of adsorption of an adsorbate onto fresh adsorbent, CS is very close to zero (CS being in equilibrium with q = 0). Integrating Eq. (16) for CS = 0 yields
ln
C A ¼ kS t C0 V
ð17Þ
The value of liquid-film mass transfer coefficient can be obtained from Eq. (17) and the concentration versus time data. Modeling of adsorption kinetics by Eq. (17) was applied to the first terms of the kinetic curves, when t ? 0. We will consider in this study the global external transport coefficient (kS A). Indeed, it is possible to standardize the liquid-film mass transfer coefficient by the total surface area of the adsorbent using (kS A) rather than kS. The extraparticle transfer coefficients obtained in the absence and presence of ultrasound were plotted in Fig. 10. It is observed that the rate in the initial period of adsorption, where external mass transfer is assumed to predominate, significantly increases by the assistance of ultrasound. Hydrodynamic effects induced by ultrasound promote a significant increase of the mass transfer across the boundary layer. This increase is a function of the applied ultrasonic power. The enhancement of the mass transport across the liquid-film may be due to the reduction of the thickness of the laminar boundary layer by the micro-scale turbulence created. In general, the liquid-film mass transfer coefficients were in the order: 516 kHz > 800 kHz > 1660 kHz. The average order of the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient shows that adsorption is inversely proportional to the frequency of ultrasound. This result can be explained by the size of cavitation bubbles as well as by the duration and intensity of the implosion of cavitating cavities. The growth cycle of a cavitation bubble depends on the frequency of ultrasound. With an increase of frequency, acoustic periods are shorter and the size of cavitating bubble decreases, which conduct to the decrease of implosion intensity. The duration of a growth cycle decreases by increasing ultrasound frequency. The decrease in the duration of growth involves the formation of cavitation bubbles of smaller sizes at high frequency and thus to less violent collapses. In addition, the number of acoustic cycle increases at high frequency and, consequently, cavitation events per time unit are more significant. Also, the cavitation threshold increases with increasing ultrasound frequency. 4. Conclusions Kinetic studies were carried out for the adsorption of 4-CP from aqueous medium onto activated carbon without and with the assistance of ultrasound of different high frequencies and powers.
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Comparison of the obtained kinetic curves shows that both adsorption rate and adsorbed amount were significantly enhanced and improved in the presence of ultrasonic irradiation for all the studied frequencies and powers. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound are greater than those obtained in the absence of ultrasound. Hydrodynamic effects induced by ultrasound promote a significant increase of the mass transfer across the boundary layer. These behaviors increased with increasing ultrasonic power. The average order of the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient shows that adsorption is inversely proportional to the frequency of ultrasound (516 kHz > 800 kHz > 1660 kHz). References [1] H.H. Fang, O. Chen, Toxicity of phenol towards aerobic biogranules, Water Res. 31 (1997) 2229–2242. [2] F.A. Banat, B. Al-Bashir, S. Al-Asheh, O. Hayajneh, Adsorption of phenol by bentonite, Environ. Pollut. 107 (2000) 391–398. [3] US EPA, Federal Register, vol. 52, 131, Washington, DC, 1987, pp. 25861– 25962. [4] EEC Directive 80/778/EEC 15-7-1990: Official Journal of the European Communities, 30-8-1990, European Community, Brussels, 1990. [5] I. Rodriguez, M.P. Llompart, R. Cela, Solid-phase extraction of phenols, J. Chromatogr. 885 (2000) 291–304. [6] B.S. Schueller, R.T. Yang, Ultrasound enhanced adsorption and desorption of phenol on activated carbon and polymeric resin, Ind. Eng. Chem. Res. 40 (2001) 4912–4918. [7] Z. Wang, K.L. Cheng, Effect of ultrasound and mechanical agitation on adsorption of PAR by Amberlite XAD-2, Ultrasonics 20 (1982) 215–216. [8] M. Breitbach, D. Bathen, Influence of ultrasound on adsorption processes, Ultrason. Sonochem. 8 (2001) 277–283.
[9] O. Hamdaoui, E. Naffrechoux, L. Tifouti, C. Pétrier, Effects of ultrasound on adsorption–desorption of p-chlorophenol on granular activated carbon, Ultrason. Sonochem. 10 (2003) 109–114. [10] J.-B. Ji, X.-H. Lu, Z.-C. Xu, Effect of ultrasound on adsorption of Geniposide on polymeric resin, Ultrason. Sonochem. 13 (2006) 463–470. [11] R.-S. Juang, S.-H. Lin, C.-H. Cheng, Liquid-phase adsorption and desorption of phenol onto activated carbons with ultrasound, Ultrason. Sonochem. 13 (2006) 251–260. [12] T.J. Mason, J.P. Lorimer, D.M. Bates, Quantifying sonochemistry: casting some light on a ‘black art’, Ultrasonics 30 (1992) 40–42. [13] T.J. Mason, Practical Sonochemistry: User’s Guide to Applications in Chemistry and Chemical Engineering, Ellis Horwood, Chichester, 1991. [14] L.H. Thompson, L.K. Doraiswamy, Sonochemistry: science and engineering, Ind. Eng. Chem. Res. 38 (1999) 1215–1249. [15] O. Hamdaoui, R. Djeribi, E. Naffrechoux, Desorption of metal ions from activated carbon in the presence of ultrasound, Ind. Eng. Chem. Res. 44 (2005) 4737–4744. [16] O. Hamdaoui, E. Naffrechoux, J. Suptil, C. Fachinger, Ultrasonic desorption of pchlorophenol from granular activated carbon, Chem. Eng. J. 106 (2005) 153– 161. [17] O. Hamdaoui, E. Naffrechoux, An investigation of the mechanisms of ultrasonically enhanced desorption, AIChE J. 53 (2007) 363–373. [18] A. Tauber, G. Mark, H.-P. Schuchmann, C. von Sonntag, Sonolysis of tert-butyl alcohol in aqueous solution, J. Chem. Soc. Perkin Trans. 2 (1999) 1129–1135. [19] Y.G. Adewuyi, Sonochemistry: environmental science and engineering applications, Ind. Eng. Chem. Res. 40 (2001) 4681–4715. [20] L. Nouri, O. Hamdaoui, Ultrasonication-assisted sorption of cadmium from aqueous phase by wheat bran, J. Phys. Chem. A 111 (2007) 8456–8463. [21] D. Mohan, K.P. Singh, Single and multicomponent adsorption of cadmium and zinc using activated carbon derived from bagasse – an agricultural waste, Water Res. 36 (2004) 2304–2318. [22] W.J. Weber Jr., J.C. Morris, Kinetics of adsorption on carbon from solution, J. Sanitary Eng. Div. Proceed. Am. Soc. Civil Eng. 89 (1963) 31–59. [23] H. Spahn, U. Schlunder, The scale–up of activated carbon columns for water purification based on results from batch test. I. Theoretical and experimental determination of adsorption rates of single organic solutes in batch tests, Chem. Eng. Sci. 30 (1975) 529–537.