**4. Behavior of surfactants at soil/liquid interface**

Surfactants at very low concentration can solubilize HOCs by reducing surface and interfacial tensions of the soil water solution. Surfactants will typically consist of a strongly hydrophobic group (water hating) referred to as the tail of the molecule and a strongly hydrophilic group (water loving), which is the head. Owing to the hydrophilic portion, surfactants can exhibit high solubility in water, while the hydrophobic portion causes part of the molecule to reside in an insoluble phase. Hydrogen bonding property and Van deer Waals forces between water molecules are the main reasons for preventing HOCs to form aqueous solutions in a soil system. Therefore, their mass distribution is primarily confined to the solid phase of the contaminated soil. However, at a specific, higher concentration of surfactant, commonly known as the critical micelle concentration (CMC), molecular aggregates are formed. The CMC is a specific property of a surfactant. In technical term, the CMC value represents the concentration of maximum solubility of a surfactant at 25°C in a particular aqueous soil solution. It should be noted that the effectiveness of CMC at a contaminated site may be affected by temporal and seasonal variations exhibited by the soil solution properties. It is through micellar solubilization, the process by which aggregations of surfactant monomers form micelles that HOCs canbecome solubilized. The solubilization process dictates the suitable approach in relation to remedial options and site-specific characteristics. The presence of surfactants in the soil solution will be accompanied by an interplay between the soil solution and concentration of surfactant. An adaptation of the interplay is depicted in **Figure 5**. Therefore, surface activity of surfactants should be viewed as a dynamic phenomenon. The solubilization of HOCs in the soil solution is accompanied by an increase in the Gibbs energy transfer which results in a decrease in entropy. This thermodynamic process is believed to to be the result of the breakdown of hydrogen bonding in the water molecule. Generally, the lower the CMC of a surfactant molecule in a soil system, the more stable will be the micelles and correspondingly the mass transfer process. The most commonly held view of key

**Figure 5.** *Illustration of various interplays at the soil water-interface and HOCs. (Reproduced with permission from [1].)*

factors affecting micellar solubilization of HOCs in soil by nonionic, ionic, and biosurfactants are the following: soil moisture, sorption, soil moisture, salinity, surfactant hydrophobic properties, texture, organic carbon, pH, and interfacial energy [1].

The effectiveness of a particular surfactant in solubilizing a specific HOC can be determined through the molar solubilization ratio (MSR) and micelle-water partition coefficient (Kmc). The MSR is the number of solute molecules solubilized per surfactant molecule. Namely the MSR can be calculated according to Eq. (1):

$$\text{MSR} = (\text{S-S}\_{\text{CMC}}) / (\text{C}\_{\text{s}} \text{-CMC}) \tag{1}$$

where

MSR = moles of organic contaminant solubilized per mole of surfactant added to the aqueous phase

S = apparent solubility of organic contaminant at a given surfactant concentration.

SCMC = CMC point of surfactant.

Cs = apparent solubility of organic contaminant at CMC (i.e., Cs > CMC).

CMC = critical micelle concentration

Studies on mixed surfactant systems competitive effects on hydrophobic contaminants solubilization has been investigated and reported elsewhere [21–23]**.** In mixed surfactants, the MSR for the HOC can be estimated using the MSR obtained in single-surfactant solutions assuming the ideal mixing rule [24] and can be represented by Eq. (2):

$$\mathbf{MSR}\_{\mathrm{m}} = \mathbf{Y\_1MSR\_1} + \mathbf{Y\_2MSR\_2} \tag{2}$$

where

MSRm = moles of surfactant solubilized in mixed surfactants

Y1 and Y2 = molar fractions of the two surfactants

MSR1 and MSR2 = molar solubilization ratios for the HOC

A plot of the aqueous HOC concentration solubility versus surfactant concentration, MSR and Kmc can be determined from the slope of the linearly fitted regression equation, respectively.

The Kmc can be obtained from Eq. (3):

$$K\_{mc} = \frac{\text{MSR}}{(\mathbf{1} + \text{MSR})V\_w \text{ } S\_{\text{CMC}}} \tag{3}$$

where the variables are as previously defined.

It is suggested that the greater the values of MSR and Kmc the larger the solubilization capacity of the surfactant in the soil micellar solution.

The micelle-aqueous phase partition coefficient (Km) is often used as another approach to quantify the solubilization capacity of a single surfactant [14]. Eq. (4) can be used to obtain Km:

$$\mathbf{K\_m} = \mathbf{X\_m}/\mathbf{X\_a} \tag{4}$$

where

Xm = the mole fraction of hydrophobic compounds encapsulated in the micellar phase given by {MSR / (1+ MSR)}.

Xa = the mole fraction of hydrophobic compounds in the aqueous phase

The soil-water partition coefficient Kd is a parameter commonly used to determine the relative affinity of a contaminant for the solid phase, Cs, and aqueous

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

phase, Cw. The greater the Kd value means that a contaminant tends to accumulate onto the soil matrix. Kd can be obtained from Eq. (5):

$$K\_d = \frac{\mathcal{C}\_t}{\mathcal{C}\_w} \tag{5}$$

The apparent soil-water partition,, *K*, *<sup>d</sup>*, can be determined from adsorption equilibrium and we get Eq. (6):

$$K\_d = \frac{K\_d + C\_{sorbel} \, K\_{pf}}{1 + C\_{micelle} \, k\_{mc}} \tag{6}$$

where

Csorbed = the amount of surfactant sorbed onto the soil

Kpsf = the partition coefficient of the HOCs in the sorbed surfactant

Cmicelle = concentration of micelle in soil solution

Kmc = micelle-water partition coefficient

For in-situ soil washing and surfactant-enhanced bioremediation, the solubilization potential of the HOC should be optimized. Basic information on the soil properties regarding range and distribution pattern of pH, texture, organic carbon, and salinity should be determined. Strategic adjustments in the delivery and concentration of the surfactant solution can be made.
