## 3 Results
### Ultrasonic Regeneration  
The effect of ultrasonic intensity on IPA desorption from activated carbon is illustrated in Figure 5. An ultrasonic intensity of 32.4 W/cm\({}^{2}\) was found to be optimal, followed closely by an intensity of 24.5 W/cm\({}^{2}\). Experiments conducted with higher intensities should have more cavitation events occurring, leading to more IPA being desorbed. The intensity of cavitation is primarily dependent on the ultrasonic intensity, which leads to the enhanced breaking of more bonds between IPA and the activated carbon surface. However, a higher ultrasonic intensity can reduce regeneration efficiency as it can damage the activated carbon structure by inducing high-pressure shock waves and high-speed microjets. This effect was measured by comparing the average particle size of activated carbon before and after ultrasonic regeneration. Figure 6 demonstrates this for sonication at 24.5 and 56.4 W/cm\({}^{2}\), which showed that the reduction in average particle size of activated carbon increased for desorption conducted with increased ultrasonic intensity. This would explain the decreased regeneration efficiency for desorption experiments conduction with ultrasonic intensities higher than 40 W/cm\({}^{2}\). Considering that the desorption with ultrasonic intensities of 24.5 and 32.4 W/cm\({}^{2}\) had very similar regeneration efficiencies, it was assumed that an ultrasonic intensity of 24.5 W/cm\({}^{2}\) would cause less damage to activated carbon, leading to a higher regeneration efficiency over multiple regeneration cycles. Therefore, an ultrasonic intensity of 24.5 W/cm\({}^{2}\) was applied for the majority of ultrasonic regeneration experiments in this study. The regeneration of IPA-saturated activated carbon by ultrasound was examined at solution temperatures of 25, 35, and 45 \({}^{\circ}\)C with temperatures of over 50 \({}^{\circ}\)C being avoided to protect the ultrasonic probe. The regeneration efficiency was increased from 70% to 75% in experiments conducted at solution temperatures of 25 and 45 \({}^{\circ}\)C. This is likely due to an increase in the breaking of bonds between IPA molecules and the activated carbon surface at higher water temperature, as the desorption process is endothermic in nature [20; 30; 31; 32; 33]. Moreover, cavitation bubbles are more easily generated as the solution temperature is increased due to a decrease in surface tension and viscosity of the liquid medium. Figure 4: The effect of initial solution pH on the adsorption capacity by activated carbon. The effect of ethanol addition on the regeneration efficiency from IPA-saturated activated carbon is shown in Figure 7. The regeneration efficiency of activated carbon is higher for experiments conducted with the addition of 5 and 10% (_v/v_) ethanol. This can be explained due to the decreased tensile stress of aqueous solution by the presence of ethanol. In this study, the optimum ethanol concentration for ultrasonic regeneration of saturated activated carbon was 10% (_v/v_), at which the regeneration efficiency was 83%. However, the regeneration efficiency was decreased for ultrasonic desorption with ethanol concentrations of 15% and 20% (_v/v_), which may be due to a lowering of the cavitation threshold. This causes cavitation bubbles to coalesce into larger and more stable bubbles, thus decreasing the regeneration efficiency [21]. Figure 5: Effect of ultrasonic intensity on the regeneration efficiency of exhausted activated carbon. Figure 6: _Cont._  
In this study, it was determined that the optimal operating conditions were an ultrasonic intensity of 24.5 W/cm\({}^{2}\), a solution temperature of 45 \({}^{\circ}\)C and an ethanol concentration of 10% (\(\
icefrac{{\gamma}}{{\
u}}\)), with a regeneration efficiency of 82.9%.