**6. Mechanisms of surfactant-enhanced solubilization**

The typical soil system will consist of five distinct phases represented by solid, solution, organic, and gaseous. When HOCs are released in a soil system, the natural dynamic processes of immobilization and demobilization, and mobilization processes occur without solubilization enhancement (**Figure 6A**). Immobilization or sorption is the dominant process and implies the removal of HOCs from the soil solution and the soil gaseous phase leading to retention on the soil solid phase. When the system becomes surfactant-enhanced, demobilization or desorption dominates and as a result, the HOC is released from the solid phase into the solution phase (**Figure 6B**). Mobilization or migration which refers to transport of HOCs in the soil porous media is also significantly enhanced by increasing solubilization HOC. These processes interact to influence surfactant-enhanced soil remediation. Studies have shown that mobilization or emulsification and solubilization are the two main mechanisms by which surfactants enhance the mass transfer solubilization of hydrophobic organic contaminants sorbed onto the soil organic matter and sediments in the soil aqueous phase. Mobilization takes place at concentrations of surfactant below CMC, whereas emulsification allows for dispersion of one phase into the other. Surfactant monomers accumulate at the soil-HOC and soil-water interfaces. This has for effect to change the wettability of the soil system by maximizing the contact angle between the HOC and the soil. A repulsion effect between the hydrophobic groups of the surfactant moiety and the rest of the surfactant molecule is caused by surfactant molecules retained on the surface of the HOC, thereby further enhancing the desorption of the contaminant from the soil particles [33]. The solubilization process occurs at concentrations above the surfactant CMC. At the same time, more sorbed HOCs are partitioned in the soil solution phase leading to more contaminant being solubilized and bioavailable. It is inevitable that a certain amount of surfactant will be sorbed onto the soil system and will be ineffective. Sorbed surfactant does not contribute to the mobilization and solubilization dynamism. Mobilization effect results in enhancing soil flushing remediation through transport and leaching of the HOC in the soil porous media and increased

*Surfactants and Their Applications for Remediation of Hydrophobic Organic Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.100596*

#### **Figure 6.**

*Illustration of the enhancement effects of surfactant on the solubility of HOCs in a soil system: (A) no surfactant-enhancement, (B) surfactant-enhancement effects. (Arrows indicate intensity of equilibrium between HOC phases).*

bioavailability for biodegradation. Basically, solubilization effect of surfactant increases the apparent solubility of HOCs in a contaminated soil.

#### **7. Field strategy and design of surfactant-enhanced remediation**

It is important to characterize and delineate the HOC in the soil in order to successfully implement a surfactant-enhanced remedial program. In this review, a simplified overview of the main components at a specific contaminated site investigation approach is illustrated in **Figure 7**. The site investigation will begin with a site reconnaissance and inspection. Then, representative intrusive judgmental sampling as the primary approach, field screening, borehole logging as per USCS, sample collection, and analysis. A variety of field testing methods are often used by field investigators to aid in the preliminary site assessment delineation program and to facilitate selection of samples. Soil gas surveys are frequently used in the field as a means of detecting the presence of volatile organics (VOCs) in the soil. Headspace vapor analysis, this field testing method is commonly used for assessing conditions of the soil samples during a drilling and sampling program. The last stage of the soil

**Figure 7.** *General procedure of investigating HOC in soils.*

*Surfactants and Their Applications for Remediation of Hydrophobic Organic Contaminants… DOI: http://dx.doi.org/10.5772/intechopen.100596*

investigation and surfactant-enhanced remediation program is the collection of confirmatory samples to determine whether or not the target clean-up goal has been achieved.

The mostly widely remedial methods for surfactant-enhanced remediation are in-situ flushing (washing), phytoremediation, ex-situ soil washing and ex situ bioremediation. All methods require solubilization of the contaminant to be effective. However, in situ flushing solubilization must be accompanied with migration of the contaminant in the soil porous medium for collection, removal, and treatment. Each of these aforementioned methods is briefly discussed in the next sections.

### **7.1 In-situ flushing**

In-situ soil flushing remediation method is a process that uses a flushing aqueous solution of surfactant to extract HOC by flooding the surface of a contaminated site or injection through vertical wells into a contaminated zone. Through continuous injection of the surfactant solution via the injection wells, contaminants partition into the flushing solution and leached into the soil. The mobilized contaminantleaching solution flows through the contaminated zone and is extracted by downgradient extraction wells (**Figure 8**). The contaminant-flushing solution mixture is separated and treated or disposed of, or the treated effluent is reinjected. The physical and chemical properties of a soil, and the amount and type of surfactant solution are key factors in determining the efficiency of soil flushing [34]. However, some nonscientific factors including the cost of surfactant, dosage of surfactant solution, and the size of the contaminated site should be considered in order to ensure the economy of the remediation project.

**Figure 8.** *Schematic of an in-situ flushing system for soil remediation.*
