Regeneration of granular activated carbon using ultrasound
Introduction
Removal of volatile organic compounds (VOCs) by adsorption on activated carbon (AC) has long been recognized as an effective means for water and air purification. Depending on its adsorption capacity, however, AC saturates after some time. Moreover, the spent AC itself brings about additional pollution to the environment. Thus, the spent carbon should be regenerated for wider application of carbon adsorption processes.
The common techniques for regeneration are thermal volatilization, chemical extraction, and bio-regeneration [1], [2], [3], [4]. Thermal regeneration is the most commonly applied method but has several disadvantages such as loss of carbon and treatment of exhaust gases [1], [3]. Chemical regenerations using organic solvents or inorganic chemicals are not necessarily acceptable because additional separation or decomposition steps are necessary. Supercritical regeneration to extract adsorbate into a supercritical fluid such as CO2 is very expensive [3]. Although bioregeneration may be the most economical process, it requires long reaction time for regeneration [4]. In recent years, catalytic regeneration technique such as photocatalytic oxidation and direct oxidation on AC/metal oxides catalyst were suggested [5].
One of promising methods as an alternative to conventional regeneration of AC is ultrasonic regeneration. Volatile organic carbons (VOCs) adsorbed physically or chemically on AC can be either desorbed ultrasonically followed by decomposition of the desorbed species in liquid phase or degraded directly on AC by ultrasound. However, there are few studies on ultrasonic regeneration of AC. Rege and Yang [6] examined the feasibility for the use of ultrasound on desorption of phenol from AC. However, they did not mention the degradation of phenol in liquid phase by ultrasound.
Ultrasound has known well as cleaning method removing unwanted particles from an object in environmental field [7], [8]. Since 1990s, many studies on ultrasonics and sonochemistry for decomposition of toxic organic pollutants such as pesticide, solvent and dye have been carried out in aqueous solution [9], [10], [11], [12], [13], [14]. More recently, ultrasound has been applied for decontamination of soil and sediments. Feng and Aldrich [15] and Meegoda and Perera [16] reported that organic and inorganic matters adsorbed on adsorbent can be significantly released by sonication. Facilitating the processes of cleaning, destruction and extraction by ultrasound are possible due to physical and chemical phenomena taking place within the aqueous solution. That is propagation of an ultrasound wave in aqueous solution causes high energy acoustic cavitation; the formation, growth and implosive collapse of bubbles in liquid. The collapse of these bubbles creates extreme conditions such as very high local temperatures and pressures at the inside and interface of the collapsing bubble. This phenomenon leads to the dissociation of H2O, the production of radical species such as OH radical, and the production of high speed micro-jets and high pressure shock waves causing the high-velocity inter-particle collisions [18], [19], [20]. And also, ultrasound produces acoustic vortex micro-streaming with the pores of the solid particles as well as the solid–liquid interface. Thus, organic pollutants can be destroyed effectively by thermal decomposition as well as oxidation and the breaking of physical bonds between the adsorbate and the adsorbent can be enhanced by ultrasonic irradiation [15], [18], [21], [22].
On the basis of ultrasonic effects mentioned earlier, ultrasonic method shows promise as a means of enhancing the regeneration of AC. However, its effect is still not understood well at present. The purposes of this study were to evaluate the feasibility of ultrasonic regeneration of AC saturated with TCE which is widely known as an important contaminant of soil and groundwater and to suggest the likely mechanism of ultrasonic regeneration.
Section snippets
Experimental methods
The activated carbon used in this work was the F400 granular activated carbon (Calgon Corp., Pittsburgh, PA). Carbon received from manufacturer was sieved using US standard sieve, No. 60 (opening size: 250 m) to remove powdered activated carbon. And then, the sieved GAC was washed with deionized water, oven dried at 105 °C to constant weigh, and stored in a desiccator until use. According to the experimental purpose, the GAC samples which adsorbed various concentration of TCE were prepared and
TCE desorption and degradation by ultrasound
To evaluate the feasibility of desorption of TCE, 5 g GAC adsorbed 6.5 mg TCE was treated in 100 ml pure water by 20 kHz ultrasound of 20 W power. During ultrasonic irradiation, GAC was completely dispersed in solution due to acoustic streaming. As shown in Fig. 2, the desorption efficiency of TCE increased with increasing ultrasonic irradiation time. About 64% of TCE adsorbed on GAC was desorbed for 1 h. In liquid phase, TCE concentration increased for first 10 min and then decreased. If TCE
Conclusion
In this work ultrasonic desorption from GAC loaded with TCE was conducted to evaluate the feasibility of regeneration by ultrasound. In conclusion, the ultrasonic regeneration showed a possibility as alternative to chemical and thermal regenerations of GAC. Specific conclusions obtained from this study are as follows.
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TCE can be desorbed from activated carbon in the presence of ultrasound at 20 kHz. However, desorption of TCE by ultrasound occurred only at a nearby surface layer of GAC.
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Although
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