## 1 Introduction  
Dioxins, polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) negatively affect the environment and human health [1; 2]. They are considered nonvolatile chemicals and because of their characteristics of hydrophobicity and lipophilicity, their water-solubility is considerably low, meaning that they are often attached to soil and sediments [3]. Dioxins can accumulate in the food chain and may cause various cancers and mutations. PCDD/Fs have been reported to possibly be associated with sarcomas, lymphomas, chloracene, and other types of cancer and fatalities [4]. Toxic equivalent quantities (TEQs) are often employed to indicate the toxicity of dioxin compounds. The TEQ value of 2,3,7,8-TCDD (tetra chlorinated dibenzo dioxin) is designated to be 1.0 and for other congeners, TEQ values are expressed as fractions of the 2,3,7,8-TCDD's value. According to [PERSON] (1998), PCDDs and PCDFs have 75 and 135 possible congeners, respectively. However, only seven PCDD isomers and 10 PCDF isomers are considered as having a dioxin toxicity and assigned TEQ values. A variety of treatment methods have been proposed for the removal of organic compounds from contaminated soil, such as thermal desorption [5], soil washing [3, 6], advanced oxidation processes [7], photodegradation [8], bioremediation [9, 10, 11], etc. Among them, soil washing has been considered an effective method taking into account its low treatment cost, particularly when dealing with highly contaminated soil [6]. Several studies have been performed for assessing the capacity of soil washing in remediating different types of contaminants in soil. For example, [PERSON], et al. [12] used a commercial surfactant, Tween(r) 80, to enhance the efficiency of soil washing, which was coupled with an electro-Fenton process in treating soil contaminated with petroleum hydrocarbons. [PERSON], et al. [13] treated pentachlorophenol contaminated soil by the combination of soil washing with laboratory-made tetrapolyphosphate solvent and zero valent ion (ZVI)/Air treatment. In soil washing remediation, the power of ultrasonication is often employed with mechanical stirring because its sonophysical effects, which are induced by acoustic cavitation, can enhance the removal of the contaminants [14]. Ultrasound-assisted soil washing was reported to result in better removal efficiencies than the cases where only either mechanical stirring or ultrasonic power was used alone [15]. Ultrasound-assisted soil washing has been employed to treat different types of organic contaminants in soil, e.g., diesel [16] and polycyclic aromatic hydrocarbons (PAHs) [17]. It is reported that the combined effects of macro-scale mixing (mechanical) and micro-scale mixing (ultrasonic) could result in high removal efficiencies [14, 16]. Regarding the solvents used in soil washing, various types of solvents, both synthetic and natural, have been studied for different types of contaminated soils [3, 18]. [PERSON], et al. [19] employed four synthetic solvents, including HCl, H\({}_{2}\)SO\({}_{4}\), HNO\({}_{3}\), and Na\({}_{2}\)EDTA, for the washing of heavy metals from mining soils. [PERSON], et al. [20] used three types of surfactants (the nonionic: Tween20, Triton X-100; the cationic: CTAB and the anionic: Dodec-MNS, NPS-10) in their petroleum-contaminated soil remediation study. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [6] washed different concentrations of PCDD/Fs from aged-soils using ethanol as a solvent. In general, a good solvent should be capable of achieving a high removal efficiency, while being inexpensive and easily used without causing harm to humans and the environment, including soil bio-activity. Chemical solvents are highly effective in the washing of soil contaminated with organic pollutants [21, 22, 23], but they also remove micro-organisms, some of which may be beneficial for further bio-attenuation or cultivation [3, 18].