Soil contaminated with mixtures of organic and inorganic contaminants such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals is a worldwide problem and poses a threat to human health and ecosystems. Remediation of these soils is challenging because heavy metals and PAHs are persistent, have different chemical and physical properties and behave differently in the soil. Different remediation techniques have been developed for remediating one type of contaminants, while only few have been tested for soil with mixed contamination. So far excavation and landfilling are still the most common practices for dealing with contaminated soil. However, these practices do not remediate the contaminated soil but only remove it to another place. Applying remediation techniques that can extract or degrade the contaminants is a more sustainable and appealing solution. Soil washing applying chemical extraction is a promising and fast technique with high potential to extract heavy metals and PAHs. The efficiency of soil washing is affected by the type of washing agent. But most of the widely tested agents are effective for one type of contaminants and some of them are toxic, non-degradable or may destroy soil structure. Therefore, finding sustainable soil washing agents capable of extracting heavy and PAHs simultaneously is needed. The overall aim of this PhD study was to advance the current knowledge about remediating soil contaminated with mixtures of heavy metals and PAHs with a special focus on soil washing. Towards this end, the following sub aims were addressed: (1) testing the efficiency of different sustainable soil washing agents; (2) testing the most efficient washing agent on a number of soils with aged contamination; (3) investigating the effect of soil characteristics on the efficiency of the optimized soil washing procedure; (4) testing the optimized soil washing procedure on another group of organic contaminants (pesticides) and mercury; (5) investigating how PAHs interact with the least and the most effective soil washing agents to understand the factors affecting washing agents extraction capabilities. Understanding these mechanisms of interactions can be used to predict the solubilization properties of these compounds which in turn will help in selecting efficient agents. The potential of five sustainable soil washing agents to extract heavy metals and PAHs simultaneously were tested (Greish et al., 2018 Paper II). Five heavy metals including Cd, Cu, Ni, Pb, Zn and the 16 PAHs identified by the U.S. Environmental Protection Agency as priority contaminants (PAHs16) were addressed. The tested washing agents included two types of dissolved organic matter (F-DOM and CRC-DOM), two bio-based surfactants (BBE-1000 and Supersolv) and one bio-surfactant (Rhamnolipid). The two DOMs were used at their maximum soluble concentrations. They showed low extraction efficiency <15% for heavy metals and were inefficient for PAHs extraction. Rhamnolipid extracted 7 and 36% of the Ʃheavy metals and ƩPAHs16. Foaming of the rhamnolipid made it difficult to increase its concentration, consequently, further optimization was unlikely. The extraction efficiency of the bio-based surfactants was concentration dependent, especially for Supersolv. At the lowest tested concentrations, BBE-1000 extracted 2% Σheavy metals and 13% ΣPAHs16 and Supersolv extracted 3% Σheavy metals and 0% PAHs. Since it was possible to increase their concentrations, optimizing the concentrations was further investigated. At the highest concentration, the extraction efficiency of Supersolv increased to approximately 30 and 68% for Σheavy metals and ΣPAHs16, respectively, proving to be more efficient than BBE-1000 which extracted 19 and 31% of the Σheavy metals and ΣPAHs16, respectively. The extraction efficiency of Supersolv was further enhanced up to 38% for heavy metals and 88% for PAHs after three soil washing steps. At the highest applied concentration, Supersolv showed the highest extraction efficiencies for both heavy metals and PAHs. The optimal concentration of Supersolv was tested on 13 aged soils with different soil characteristics. Supersolv showed high potential for extracting 9-33% of Σheavy metals and 31-86% of ΣPAHs16 from the 13 soil samples. However, its extraction efficiency decreased as the soil content of organic carbon, clay and CaCO3 increased (Greish et al., 2018 Paper II). The efficiency of Supersolv to treat other types of mixed contamination was also tested. Soil highly contaminated with organophosphate pesticides and mercury was washed with the optimal concentration of Supersolv (Greish et al., 2018 Paper III). Results showed high extraction efficiencies of 61% and 66% for the two organophosphate pesticides malathion and ethyl-parathion, respectively. Meanwhile, no extraction was observed for mercury. Understanding interaction mechanisms between PAHs and the soil washing agents will help in selecting efficient washing agents. Greish et al. (2018 Paper I) used pyrene as a model PAH and its interaction with F-DOM, CRC-DOM, BBE-1000 and Supersolv was investigated using fluorescence spectroscopy combined with multivariate curve resolution-alternating regression (MCR-AR). The studied washing agents were selected based on their efficiencies in extracting PAHs from the soil as presented in Greish et al. (2018 Paper II), where the two DOMs showed the lowest efficiencies and the two bio-based surfactants showed the highest efficiencies. Pyrene showed π-π interactions with FDOM and no interaction with CRC-DOM (Greish et al., 2018 Paper I). This could be attributed to the more aromatic structures in F-DOM compared to CRC-DOM. For the interaction mechanisms between pyrene and the bio-based surfactants, the MCR-AR model resolved three spectroscopically active species from pyrene emission spectra as a function of pyrene and bio-based surfactants concentrations. These species resembled pyrene emission in a polar and nonpolar microenvironment, respectively and of an excimer (Greish et al., 2018 Paper I). Concentration profiles retrieved by the model for the three species showed that, below the critical micelle concentration (CMC), Supersolv created more nonpolar interactions with pyrene compared to BBE-1000. Above the CMC first the nonpolar interactions help in desorbing PAHs, and then micelles will increase the solubilization in the washing solution (Greish et al., 2018 Paper I). In conclusion, this PhD study showed that the two DOMs at the studied concentrations showed low efficiency for extracting heavy metals and PAHs. The studied rhamnolipid showed moderate efficiency and the bio-based surfactants showed high efficiencies at their highest concentrations. A soil washing method was developed for the simultaneous remediation of heavy metals and PAHs using Supersolv. The developed method showed potential for extracting heavy metals and PAHs from 13 soils with different soil characteristics. Soil washing efficiency decreased as the soil content of organic carbon, clay and CaCO3 increased. The developed method showed also high efficiency for the extraction of organophosphate pesticides malathion and ethyl parathion, but the method showed low efficiency in extracting mercury. Furthermore, this PhD study highlighted the importance of nonpolar interactions in enhancing the extraction of PAHs from the soil. Below the CMC, the nonpolar interactions of bio-based surfactants with soil constituents and PAHs will affect desorption rate and bioavailability of PAHs. Nonpolar interactions above the CMC would help in desorbing PAHs and micelles would increase PAHs solubility in the washing solution