###### Abstract  
Ultrasonic regeneration of activated carbon loaded with isopropyl alcohol (IPA) was studied. IPA adsorption was performed batchwise at varying solution pH. Adsorption was optimal at solution pH 7, which was closest to the point of zero charge of the activated carbon (6.7). Ultrasonic regeneration was performed on IPA-loaded activated carbon with three factors being tested: ultrasonic intensity, solution temperature and ethanol addition. Regeneration efficiency increased with ultrasonic intensity up to 32.4 W/cm\({}^{2}\). A higher intensity led to a higher desorption but damaged the activated carbon, shown by a decrease in the particle size of activated carbon. The regeneration efficiency increased with solution temperature primarily because desorption is endothermic and because the surface tension and viscosity of a solution are reduced with increasing temperature, promoting cavitation bubble production. Ethanol addition increased regeneration efficiency up to 10%, as ethanol reduces tensile stress, facilitating cavitation bubble generation. At 15% and above, regeneration decreases, possibly due to coalescence of bubbles into larger, more stable bubbles. Under optimal parameters, the regeneration efficiency was 83%, which dropped to 64% after four regeneration cycles. activated carbon; ultrasonic; regeneration; IPA; isopropyl alcohol; adsorption +
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a value of 8.1 billion USD by 2021 [4]. To produce activated carbon, a large amount of biomass is needed, with 6.7 tons of coconut shell required to produce 1 ton of activated carbon [5]. Considering this high biomass requirement and rising market demand, regeneration is economically and environmentally beneficial as it can greatly reduce the need for the manufacturing of new activated carbon. This regeneration is achieved through a variety of methods which can be categorized as desorption and decomposition. Desorption of activated carbon can be thermal (using steam, hot water, microwave, etc.) or non-thermal (using solvents, supercritical fluids, etc.) and works through principles that break the bonds between adsorbent and adsorbate. Decomposition of activated carbon works through the breaking down of adsorbate on the surface of adsorbent and is classified into electrochemical, microbiological, chemical and ultrasound (which is also a desorption method) [6]. Ease and practicality differ among techniques but high regeneration efficiencies can be accomplished for practically all methods. Taking phenol as an example, regeneration efficiencies of 90% and above were reached using pyrolysis [7] and bio-regeneration [8] and between 70-80% using electrochemistry [9,10], photocatalysis [11] and ultrasound [12]. Ultrasonic regeneration is a promising technique that offers advantages, such as being safe, clean and energy-conserving [13,14] and being able to desorb a wide range of chemicals. In one study, 64% of adsorbed trichloroethylene was desorbed from granular activated carbon after 1 h of ultrasonic at 20 kHz. [15]. Another study showed the potential in desorption of heavy metals, where 120 min of ultrasonic treatment led to a Cr\({}^{6+}\) desorption of 45.9%, as opposed to 10.7% desorption in distilled water [16]. One study showed its effectivity for organic solvents, where 70% of phenol was desorbed but BET surface area and total pore volume of the activated carbon was found to decrease by 25% [12]. Ultrasonic regeneration acts through cavitation, which is the formation, growth and implosive collapse of microbubbles. These collapses, which occur in the span of milliseconds, create extreme conditions, such as high temperatures and pressure, which are capable of breaking chemical bonds of water, generating highly active radicals, such as \({}^{\bullet}\)O, \({}^{\bullet}\)OH, \({}^{\bullet}\)H and \({}^{\bullet}\)HO\({}_{2}\)[17,18]. This makes ultrasonic regeneration a complicated process, as the effects of ultrasound are both thermal, mechanical and chemical [6].