## 1 Introduction
### Soil Flushing and Soil Washing for the Extraction of Hydrocarbons  
A potentially interesting approach for in situ mineral hydrocarbon removal is their extraction. The soil can be washed (flushed) with a suitable agent, or an adsorbent can be applied. Several authors apply the term \"soil flushing\" to the in situ application, whereas \"soil washing\" describes the use of a solvent in an on-site process [33]. In [34], polyurethane microparticles were used to capture PAHs from the ground. The enrichment factor was found to be 70. PAH concentrations in biodegradable polyurethane particles were 70 times higher than those in soil [34]. [PERSON] et al. [35] infiltrated 40 L of rapeseed oil in contaminated soil. Within 19 days, 17% of that plant oil could be captured, and it contained 5.4 g of PAH (EPA 16) [35]. Among the advantages of soil flushing over pump-and-treat methods are predominantly lower costs and reduced exposure to health hazards for workers. However, a major disadvantage is the risk that contaminants become mobilized and the contaminant plume spreads beyond the recovery zone [10]. In order to avoid said plume migration, groundwater flow must be controlled, e.g., through barriers and pumping off. Soil flushing is also only suitable for permeable soilssuch as sand or gravel. For the flushing solution, a non-toxic, biodegradable solvent is recommended [10]. Table 1 provides an overview. Organic contaminants such as NAPLs (non-aqueous phase liquids) can be flushed with surfactants or co-solvents [36]. The use of microemulsions allows the extraction of NAPLs in a single, low-viscosity phase [36]. One should try to recycle the flushing fluids as much as possible. The extraction of the contaminants from the flushing solution can be performed via air stripping, liquid/liquid extraction, precipitation, filtration, or distillation [36]. In 2007, the ASCE (American Society of Civil Engineers) stated: \"The costs of soil flushing are estimated to be between $75 and $300 per cubic yard of contamination plume. This estimate includes injection and recovery well and pump installation, operating and maintenance labor, sampling well installation, utilities, flushing solution preparation system installation, chemicals, flushing solution treatment, system installation, site supervision, site quality assurance and health and safety support, sampling and analysis for process control, and off-site disposal of sludge residual from flushing solution treatment. The estimate does not include project design and management, regulatory fees, site characterization, treatability, site pretreatment and contingencies.\"  
With 1 cubic yard being 0.764 m\({}^{3}\) and 1 USD in 2007 corresponding to 1.34 USD in 2022 (1.18 \(\upxi\)), it leaves the estimated costs at 116-463 \(\upxi\)/m\({}^{3}\). The flushing agent is obviously the main cost driver, which means recycling can yield strong cost reduction. Additionally, the type of flushing agent has a strong impact on costs. A soil-flushing setup is shown in Figure 1 below. \begin{table}
\begin{tabular}{c c} \hline \hline
**Flushing Agent** & **Contaminants Targeted** \\ \hline clean water & High-solubility organics; soluble inorganic salts \\ surfactants & Low-solubility organics; petroleum products \\ water/surfactants & Medium-solubility organics \\ cosolvents & Hydrophobic contaminants \\ acids & Basic organic contaminants, metals \\ bases & Phenols, metals \\ reductants/oxidants & Metals \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Types of flushing agents for soil. Source: [10]. Figure 1: Soil flushing. The first application was enhanced oil recovery. Reprinted with permission from [37]. The technique shown in Figure 1 is based on the surfactant-polymer flooding process [37], where the aim was to mobilize hydrocarbons in an enhanced oil-recovery process. It is also applicable to brownfield remediation. In soil washing, a common approach to reduce treatment costs is to separate the fine material from the relatively contaminant-free coarse materials in the excavated soil [10].