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Mechanistic Studies in Ultrasound-Assisted Adsorption for Removal of Aromatic Pollutants Venkata Rao Midathana and Vijayanand S. Moholkar* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati-781 039, Assam, India
In this paper we try to discern the physical mechanism of enhancement of adsorption of organic pollutants with application of ultrasound. An attempt is made to discriminate between the contribution made by various physical effects of ultrasound and cavitation, viz., microstreaming, microturbulence, and acoustic (or shock) waves, which could generate convection in the medium and enhance the process of adsorption. A dual approach of coupling experimental results to the simulations of a bubble dynamics model has been adopted. Adsorption of three aromatic pollutants (viz., nitrobenzene, phenol, and p-nitrophenol) onto activated carbon has been chosen as a model process. Correlation of the experimental and simulation results reveals that the extent of adsorption in the presence of ultrasound shows an optimum with the intensity of convection generated in the medium by the cavitation bubbles. The microturbulence generated by cavitation bubbles makes a useful contribution to the enhancement of adsorption. This is attributed to the continuous nature of microturbulence with moderate liquid velocities. On the other hand, acoustic waves emitted by the cavitation bubbles render an adverse effect on the process. This is attributed to the discrete nature and high pressure amplitude of the waves, which create excessively high convection in the medium, causing desorption of the pollutant. The chemical nature of the pollutant is also found to influence the enhancement effect of ultrasound. For hydrophobic pollutants, the ultrasonic enhancement is more pronounced than for hydrophilic pollutants under otherwise similar conditions. 1. Introduction Wastewater discharge from the chemical and process industries contains many aromatic pollutants that are not easily degraded by conventional biological treatments. Extensive research has taken place in the past couple of decades for development of cheap and efficient methods for removal of these organic pollutants. One of them is adsorptive separation using adsorbents such as activated carbon or charcoal. The principal advantages of the adsorption process are better quality of treated wastewater effluent and toxicity reduction. In the recent past, ultrasound has been used as a means for enhancement of the adsorptive separation of pollutants. Similar to adsorption, the process of desorption (for regeneration of the adsorbent) is also enhanced by ultrasound. Ultrasonic enhancement of the adsorption/desorption is well studied and documented.1-8 Adsorption, being a mass transfer process, is limited by diffusion-convection in the system. The overall resistance to mass transfer can be reduced by increasing the convection in the medium or, in other words, making the system turbulent. Ultrasound, and its secondary effect, cavitation (which is nucleation, growth, and transient collapse of tiny gas bubbles driven by an ultrasound wave) can create convection in the medium through various physical phenomena such as microstreaming, microturbulence, acoustic (or shock) waves, and microjets. The exact nature of these phenomena is explained in the next section. It must be noted that ultrasound does not alter the equilibrium characteristics of the adsorption process, but only enhances the mass transfer rate in the direction of the concentration gradient, which is a sort of catalytic action. Although previous literature gives an experimental validation of the ultrasonic enhancement of the adsorption phenomena, no attempt has been made in these studies to deduce the physical mechanism of the ultrasoundenhanced adsorption process. Such an attempt would necessitate * To whom correspondence should be addressed. Tel.: 91-361-258 2258. Fax: 91-361-269 0762. E-mail:
[email protected].
identification of the individual contributions made by the different physical phenomena related to ultrasound and cavitation mentioned above to the overall convection generated in the medium, which enhances the adsorption/desorption rates. In this paper we address this fundamental issue and attempt to establish the physical mechanism of ultrasound-induced enhancement of adsorption by coupling experiments to the simulations of the ultrasound and cavitation phenomena. It is also demonstrated how the adsorption characteristics show a close relationship with the physicochemical properties of the pollutant. The three most common pollutants, viz., phenol (Ph), p-nitrophenol (PNP), and nitrobenzene (NB), have been chosen as model adsorbates, and activated charcoal has been chosen as the model adsorbent. 2. Physical Effects of Ultrasound and Cavitation9,10 Prior to the main components of this study, we briefly describe various physical effects of ultrasound and cavitation, which could create convection in the medium that could be beneficial to the adsorption process. Ultrasound passes through the medium in the form of compression and rarefaction cycles, creating sinusoidal variation in the bulk pressure. Passage of ultrasound gives rise to the cavitation phenomenon in the medium. The nuclei for cavitation events are gas pockets trapped in the walls and crevices of the reactor wall, or they could be freely floating small bubbles in the medium. Cavitation bubbles grow from these nuclei during the rarefaction half-cycle of ultrasound, when the bulk pressure in the medium falls sufficiently below ambient or static pressure. During the compression half-cycle of ultrasound, the bubble contracts. The expansion of the bubble is accompanied by evaporation of the liquid medium into the bubble. During the compression phase, the radial motion of the bubble becomes extremely fast, and not all of the vapor evaporated into the bubble can escape by condensation into the bulk medium. This causes entrapment of the vapor molecules
10.1021/ie900049e CCC: $40.75 2009 American Chemical Society Published on Web 06/16/2009
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in the bubble. The spherical convergence of fluid elements in the ensuing compression phase imparts high energy to the bubble that results in energy concentration during collapse. The transient bubble collapse results in generation of very high temperatures (∼5000 K) and pressures (∼500 bar) inside the bubble, making the bubble a local hot spot.11 The extreme temperature and pressure conditions in the bubble result in generation of radicals due to dissociation of entrapped vapor molecules.12,13 With fragmentation of the bubble at the point of maximum compression, these radicals are released into the medium where they induce and accelerate chemical reactions, which are well-known as sonochemical reactions. The physical effects of ultrasound and cavitation, which give rise to strong convection in the medium through various physical phenomena, are as follows. 2.1. Microstreaming. During propagation of ultrasound waves through a liquid medium, the fluid elements undergo small-amplitude oscillatory motion around a mean position. This phenomenon is called microstreaming. The amplitude of this oscillatory motion varies directly with the pressure amplitude of the acoustic wave. 2.2. Microturbulence. During radial motion of the bubble, the fluid in the vicinity of the bubble is set into oscillatory motion, which is called microturbulence. The velocity of the microturbulence varies directly with the amplitude of the oscillations of the bubble. For large-amplitude bubble oscillations, the collapse is transient (with the bubble wall velocity reaching or even exceeding the sonic velocity), and accordingly, the velocity of the microturbulence generated is also quite intense. It should, however, be noted that the phenomenon of microturbulence is restricted only in the close vicinity of the bubble. The velocity of the microturbulence decreases very rapidly away from the bubble. 2.3. Acoustic Waves (or Shock Waves). During the transient collapse of the bubble, the fluid elements in the vicinity of the bubble wall spherically converge toward the bubble wall. If the bubble contains noncondensable gas, the pressure inside the bubble rises rapidly during the transient collapse. At the point of minimum radius during transient collapse, the bubble wall comes to a sudden halt and the converging fluid elements are reflected back. This reflection creates a high-pressure acoustic (or shock) wave that propagates through the medium. 2.4. Microjets. If the cavitation bubble is located close to a phase boundary, the motion of the liquid in its vicinity is hindered, which results in development of a pressure gradient around the bubble. The portion of the bubble exposed to higher pressure collapses faster than the rest of the bubble, which gives rise to formation of a high-speed liquid jet. The velocity of these microjets has been estimated in the range of 120-150 m/s.14,15 In the case of rigid boundaries, these jets can cause erosion at the point of impact. These jets can also cause particle size reduction. The microjet formation, however, also depends on the relative sizes of the solid boundary and the cavitation bubble. If the size of the solid present in the vicinity of the bubble is of the same order as the maximum radius attained during radial motion, the pressure field surrounding the bubble is not disrupted by the solid particles, and hence, microjet formation does not occur. For ultrasound frequencies of 20-50 kHz with pressure amplitude in the range 1-2 bar, the typical maximum radius attained by the bubble prior to transient collapse is about 100-150 µm.16 Therefore, solid boundaries (or solid particles) with dimensions of e100 µm cannot induce asymmetric bubble collapse and microjet formation.17
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Figure 1. Experimental setup: 1, ultrasound bath; 2, transducer; 3, adsorption mixture; 4, buret stand for holding the reaction flask in place; 5, timer/ regulator of the ultrasound bath; 6, rubber bulb with air flow control valve; 7, medium (water) for propagation of ultrasound waves in the ultrasound bath.
The experimental techniques adopted in this study are capable of selectively altering (either intensifying or diminishing) the relative magnitudes of these effects. Thus, these techniques enable us to identify the individual contributions of the abovementioned phenomena to the intensification of the adsorption process. 3. Experimental Section 3.1. Experimental Setup. A schematic diagram of the experimental setup is shown in Figure 1. A standard 250 mL conical flask was used for all experiments. An ultrasound bath (Transsonic T-460, Germany; capacity 2 L) operating at a frequency of 35 kHz with a 35 W power was used for sonication of the adsorption mixture. For each experiment, two-thirds of the bath was filled with water (the medium for ultrasound propagation) at 25 °C, which would immerse the flask in the medium to about 75% of its height. The flask was placed at the center of the sonication bath for all the experiments. The intensity of the ultrasound field in the bath shows significant spatial variation.18,19 Any change in the location of the flask in the bath in consecutive experiments can result in generation of artifacts due to the changing intensity of the ultrasound field. To avoid this, the position of the flask in the bath was carefully maintained during all experiments with the help of a burette stand with a clamp attached to it. The water in the bath was replaced every 15 min to avoid a significant temperature rise during the experiment. With this procedure, the average temperature of the water in the bath varied by less than 2 °C. The temperature inside the flask was the same as that of the surrounding water. A simple arrangement was made to raise the static pressure in the flask. The flask was air-sealed using a rubber cork with a metal tube pierced centrally in it. A rubber bulb with an air flow control valve was attached to the outer end of the tube, which could be pressed to raise the static pressure inside the flask. Prior to the main experiments, the rise in the static pressure in the flask that could be achieved with this arrangement was measured using a pressure gauge (make, Oasis; range, 0-6 kg cm-2 gauge). It was revealed that a pressure rise to 1.5 bar could be achieved. Moreover, the reproducibility of this value with successive release and
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restoration of pressure was also assessed. The restoration of pressure was within (100 Pa, which was within acceptable limits. 3.2. Chemicals. The chemicals used in the experiment are as follows: Ph (synthesis grade, Merck), PNP (synthesis grade, Merck), NB (synthesis grade, Merck), and activated charcoal (Merck, solubility in water PNP > Ph.
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in parts A and B of Figure 3 for initial concentrations of 250 and 500 ppm, respectively. The results of adsorption of NB are shown in parts A and B of Figure 4 for 250 and 500 ppm initial concentrations, respectively, while the results of adsorption of PNP are shown in parts A and B of Figure 5 for 100 and 250 ppm initial concentrations, respectively. As mentioned in section 3.3, adsorption of each pollutant was studied in eight sets of experiments. In each of these sets, three experimental trials were done, and the mean of these trials has been reported in Figures 3-5. The reproducibility of the results in the consecutive trials was satisfactory (with variation of (10% or less). The peculiar trends in the adsorption rates of the three pollutants are summarized in Table 1A. In addition, it can be seen that the absolute amount of pollutant adsorbed for any specific experimental condition (for the same duration of ultrasound irradiation) varies considerably with the initial concentrations. Sample calculations for conditions of 1 bar of static pressure and saturated medium are given as follows for adsorption of Ph, NB, and PNP for 5 min of ultrasound irradiation: (Ph) (1) initial concentration (co) 250 ppm, concentration after 5 min (c5 min) 179 ppm, adsorption 250 - 179 ) 71 ppm; (2) co ) 500 ppm, c5 min ) 429 ppm, adsorption 71 ppm; (NB) (1) co ) 250 ppm, c5 min ) 67 ppm, adsorption 183 ppm; (2) co ) 500 ppm, c5 min ) 261 ppm, adsorption 239 ppm; (PNP) (1) co ) 100 ppm, c5 min ) 10 ppm, adsorption 90 ppm; (2) co ) 250 ppm, c5 min ) 118 ppm, adsorption 132 ppm. The absolute adsorption is constant for Ph for both initial concentrations, while it varies directly with the initial concentration in the case of PNP and NB. Comparing NB and PNP, we find that this effect is more pronounced for NB. This result is in accordance with the contemplations presented earlier. NB, being the most hydrophobic among the three pollutants, shows the highest tendency for adsorption onto a solid surface, while Ph shows the least tendency for adsorptionsthanks to the -OH group, which makes it hydrophilic. 5.1.5. Simulation Results. Representative simulations of the radial motion of a cavitation bubble in an unsaturated solution of the pollutant at 1 and 1.5 bar of static pressure are shown in Figures 6 and 7, respectively. The summary of the simulation results for 5 and 10 µm cavitation bubbles is given in Table 1B. It could be perceived that the overall level of convection
Figure 5. Trends in the ultrasound-assisted adsorption of p-nitrophenol (PNP) on activated charcoal for different experimental conditions: (A) results for an initial concentration of 100 ppm, (B) results for an initial concentration of 250 ppm.
With this preamble, the experimental and simulations results are presented in the following paragraphs. 5.1.4. Adsorption of the Pollutants. The results of adsorption of Ph under different experimental conditions are shown
Table 1. (A) Trends in the Adsorption Rates of the Pollutants and (B) Summary of the Simulation Results (A) Trends in the Adsorption Rates of the Pollutants pollutant
initial concn (ppm)
phenol
trend in adsorption standstill
