## 3 Results and Discussion
### Proposed Mechanisms of PCDD/F Soil Washing  
#### 3.3.1 Mechanical Stirring and Ultrasonication  
There were two physical processes affecting the tested soil washing experiments--mechanical stirring and ultrasonication (Figure 1). Mechanical stirring is the cause of macro-scale mixing, which may facilitate the desorption of the contaminants from the surface of the soil [16; 35]. Ultrasonication, meanwhile, induces micro-scale mixing, which may then facilitate the desorption of the contaminants from either the soil surface or the pores [14; 36]. Additionally, it has been reported that it is less likely for the efficient particle-surface-damaging effect of ultrasonication to reach the inner region of the soil matrix, which could be attributed to large attenuation [14]. In other words, ultrasonication alone can only affect the contaminants on the soil particle surface. Therefore, for the contaminants residing deep inside the soil particles, ultrasonication alone cannot have a strong removing effect since its power is largely reduced (attenuated). To test this hypothesis, we conducted a set of washing experiments with DI water and the results are shown in Figure 5. From Figure 5, it can be clearly seen that the effect of ultrasonic power alone is far less effective than the effect of mechanical stirring (H value \(>\lambda_{\text{U}}^{2}\)), probably because ultrasonic power is unable to reach the inner areas of the soil particles without the macro-scale mixing effect of mechanical stirring. Further, the results of our sequential washings (ultrasonic power followed by mechanical stirring and vice versa) were similar to those of only mechanical stirring (H value \(<\lambda_{\text{U}}^{2}\)) and lower than those of the combined mechanical  
Figure 4: Changes in dioxin toxic equivalent quantities (ng-TEQ/kg dry wt.) and final removal percentages in a soil six-cycle washing experiment using deionized (DI) water.  
stirring and ultrasonic power (H value \(>\chi_{\rm U}^{2}\)), indicating that either the micro-scale mixing effect of ultrasonication followed by the macro-scale dispersing effect of mechanical stirring or the dispersing effect of mechanical stirring followed by the particle-surface-damaging effect of ultrasonic power is not as effective as the combined forces of both. As mechanical stirring helps to maintain stable slurry conditions (or the suspension of solid particles in the liquid), the micro-scale mixing effect of the ultrasonication can easily reach the contaminants residing deep inside the soil particles [16]. Therefore, in this study, the combination of both mechanical stirring and ultrasonication would be the combination of dispersing and surface-damaging effects, helping the soil washing process achieve its potentially highest efficiencies.  
The position of the propeller is also important for large and homogenous mechanical stirring. We positioned the propeller close to the bottom of the vessel so that the stirring of the double-blade was able to affect as much space within the vessel as possible. As the liquid level was close to the ridge of the cap, sonication was more likely to reach all particles inside. The double-blade propeller was designed to create a space between the two blades, where the particles collided with each other, resulting in the greater breakage and desorption of PCDD/Fs.