**3. Solubilization of NAPLs by surfactants**

Chemical surfactants and natural surfactants (biosurfactants) are surface active agents. The first ones are manufactured by petrochemical plants, whereas the latter are produced by biological organisms. However, the majority of surfactants produced and utilized are chemicals because of economic factors. In their common form, surfactants are amphipathic molecules constituted by both a hydrophobic moiety (chain) and a polar or ionic moiety (head) of varying length in different surfactants. The chain can be linear or branched:

They tend to partition preferentially at the interface between fluid phases of different degrees of polarity and water bonding, consequently, making them the most versatile chemicals. Roy and Griffin [9] reported that the hydrophilic head group is the main factor responsible for the special chemistry of surfactants. Surfactants that are generated chemically are referred to as synthetic surfactants. They are generally grouped into various categories depending on the nature of the polar moiety (**Table 2**). The hydrophobic portion of these molecules are alkylbenzenes, alcohols, olefins, paraffin, or alkyl phenols, while the polar moiety will consist of either a sulfonate, sulfate, or a carboxylate group in the case of anionic surfactants. A quaternary ammonium group is found in cationic surfactants. The hydrophilic moiety of non-ionic surfactants is represented by sucrose, polypeptides, or polyoxyethylene groups. In contrast, biosurfactants are grouped according to the chemical composition of the different molecules representing the hydrophobic and hydrophilic moieties as well as microbial origin. Alternatives to petrochemicals and microbial generated surfactants are plant-based classified surfactants. As a natural solution for environmental remediation and daily common applications, plant-based surfactants offer the same very qualities and effectiveness that are found in a synthetic or biosurfactants.

**Table 2.** Summary of chemical surfactants classification.

*κ* = intrinsic permeability (1 darcy = 1 × 10−8 cm2

312 Soil Contamination - Current Consequences and Further Solutions

)

**3. Solubilization of NAPLs by surfactants**

length in different surfactants. The chain can be linear or branched:

)

p = density of NAPL (g/cm3

where

β = reference elevation

Q = atmospheric pressure.

g = force of gravity (980 cm/s2

ω = dynamic viscosity (cp) of NAPL

dh/dL = hydraulic gradient of NAPL mass

)

in Eq. (5), the hydraulic gradient is derived as described in Eq. (4), then Eq. (6) is expressed as:

Chemical surfactants and natural surfactants (biosurfactants) are surface active agents. The first ones are manufactured by petrochemical plants, whereas the latter are produced by biological organisms. However, the majority of surfactants produced and utilized are chemicals because of economic factors. In their common form, surfactants are amphipathic molecules constituted by both a hydrophobic moiety (chain) and a polar or ionic moiety (head) of varying

They tend to partition preferentially at the interface between fluid phases of different degrees of polarity and water bonding, consequently, making them the most versatile chemicals. Roy and Griffin [9] reported that the hydrophilic head group is the main factor responsible for the special chemistry of surfactants. Surfactants that are generated chemically are referred to as synthetic surfactants. They are generally grouped into various categories depending on the nature of the polar moiety (**Table 2**). The hydrophobic portion of these molecules are alkylbenzenes, alcohols, olefins, paraffin, or alkyl phenols, while the polar moiety will consist of either a sulfonate, sulfate, or a carboxylate group in the case of

(6)

It has also been suggested that biosurfactants can be conveniently divided into low-molecular mass molecules or high-molecular mass polymers. An adaptation of their classification is provided in **Table 3** [10, 11].


**Table 3.** Summary of biosurfactants classification (Adapted with permission from [10, 11]).

Hydrogen bonding property between water molecules is the primary factor responsible for NAPL insolubility in water. Surfactants can solubilize NAPL constituents by reducing surface and interfacial tensions of water (**Figure 3**). Reduction in the surface tension of water may range from 70 mN m−1 to less than 30 mN m−1 [12], thereby increasing the wetting ability of water. Surfactant molecule that is unable to form hydrogen bonding in an aqueous phase leads to an increase in the free energy of the system. This leads to an increase in NAPL solubilization in the water phase achieved through the formation of micelles. It has been reported that the aggregation number to form micelles is between 50 and 100 surfactant molecules [12]. Increasing surfactant concentration to above a critical micelle concentration (CMC) will lead to the formation of dynamic micelles by incorporating the hydrophobic solubilizates into the hydrophobic cores of the micelles [12]. Surfactant molecules that exist as monomers below the surfactant's CMC have minimal effects in the aqueous solubility of the system. As surfactant concentrations above the CMC threshold increase, the solubilization process of hydrophobic contaminants increases linearly with surfactant concentration. Invariably, micelle formation allows increased mobilization and partitioning of sorbed NAPL contaminants into the soil solution by lowering capillary forces. The lower the CMC value of a given surfactant in a system, the more stable will be the micelles and therefore the mass transfer process.

It has also been suggested that biosurfactants can be conveniently divided into low-molecular mass molecules or high-molecular mass polymers. An adaptation of their classification is

> **Polymeric biosurfactants:** Typically consists of three to four

*Most common biosurfactants:* emulsan, liposan, alasan *Producing microorganisms: acinetobacter calcoaceticus, candida*

**Particulate biosurfactants:** Can be extracellular vesicles and

*acinetobacter calcoaceticus, pseudomonas*

whole microbial cell. *Most common biosurfactants:* vesicles, whole microbial cells. *Producing microorganisms:*

*marginalis, cyanobacteria*

Hydrogen bonding property between water molecules is the primary factor responsible for NAPL insolubility in water. Surfactants can solubilize NAPL constituents by reducing surface and interfacial tensions of water (**Figure 3**). Reduction in the surface tension of water may range from 70 mN m−1 to less than 30 mN m−1 [12], thereby increasing the wetting ability of

sugars with fatty acids attached to

Saponins,lecithins, soy protein, lactonic, soybean oil, glycolipid,

Sunflower seed

**Microorganisms group Phytogenic group**

repeating

them.

*lipolytica*

**Table 3.** Summary of biosurfactants classification (Adapted with permission from [10, 11]).

provided in **Table 3** [10, 11].

**Low mass High mass**

314 Soil Contamination - Current Consequences and Further Solutions

Conjugates of fatty acids and carbohydrates.

trehalopids, Sophorolipids, rhamnolipids.

*Mycobacterrium, Arthrobacter spp, Pseudomonas*

Consist of a lipid attached to a polypeptide

**Phospholipids, fatty acids and neutral lipids:**

the hydrophilic and hydrophobic balance.

corynomycolic acid, phosphatidylethanolamine

*Rhodococcus erythropolis, corynebacterium lepus*

**Class type biosurfactants**

*Most common biosurfactants:*

**Lipopeptides and lipoproteins:**

*Most common biosurfactants:* surfactin and lichenysin *Producing microorganisms:*

Length of hydrocarbon chain in their structures determines

*Most common biosurfactants:*

*Producing microorganisms:*

Burkholderia plantarii, *Producing microorganisms:*

**Glycolipids:**

*aeruginosa*

chain.

*Bacillus sp.*

**Figure 3.** Interplay between hydrophobic contaminant solubility, surface tension, interfacial tension and micelle in the case of a specific surfactant at the core-water interface.

The capacity of surfactants to affect micellar solubilization of hydrophobic organic compounds is affected by the following factors:

**•** Temperature: CMC's typically increases with increase above a certain temperature as micelle formation is opposed by thermal agitation, termed the Krafft point. However, non-ionic surfactants do not show Krafft points. Consequently, increasing temperature tends to decrease their solubility. The temperature at which non-ionic surfactants begins to exhibit surface active properties loss is termed the cloud point.


The effectiveness of a particular surfactant in solubilizing a NAPL constituent may be represented by the molar solubilization ratio (MSR) [13] defined as expressed in Eq. (7):

$$\text{MSR} = (\text{S-S}\_{\text{CMC}}) / (\text{C}\_{\text{s}} \text{-CMC}) \tag{7}$$

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

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

CMC = critical micelle concentration

By plotting solute concentration as a function of surfactant concentration, MSR can be determined from the slope of the linearly fitted regression equation. The micelle aqueousphase partition coefficient (Km) is often used as another approach to quantify the solubilization capacity of a single surfactant [14]. Km can be defined according to Eq. (8):

$$\mathbf{K\_m = X\_m/X\_u} \tag{8}$$

where

ase their solubility. The temperature at which non-ionic surfactants begins to exhibit surface

**•** Salinity: presence of electrolytes tends to reduce repulsion forces between charged groups

**•** Surfactant hydrophobic property: As the hydrophobicity portion of a surfactant increases, this results in a decrease in the formation of CMC. Above C18. CMC appears constant. This

**•** Soil moisture content: Soil moisture level must be high enough to allow mass-transfer. Heavy soils relative to a coarse soil type will require a higher level of moisture in the system

**•** Presence of other organic molecules: May affect water structuring such as to create a shift in CMC. Structure makers such as sugars are known to lower CMC, while structure breakers like urea and formamide typically will increase surfactant solubility. In a mixed surfactant mixture system, CMC may synergistically occur at a lower level than any of the CMC's of

**•** Sorption: It reduces the concentration of surfactant monomers in the aqueous phase. Under such conditions will not aggregate to form micelles of colloidal-size until the sorption process is overcome through addition of more surfactant. CMC becomes more appreciable.

**•** pH: Depending on the nature of the surfactant and the degree of humification of the soil organic matter, CMC may be affected. Enhanced solubility of organic chemical may be observed at pH values at which soil humus and surfactant are found mostly ionized and at

**•** Interfacial energy: The interfacial tension of a given surfactant solution decreases with correspondingly increase in the surfactant monomers in a system. This leads to an attainment of a minimum free energy state. Enhanced micellar solubilization of hydrophobic

The effectiveness of a particular surfactant in solubilizing a NAPL constituent may be repre-

MSR = moles of organic contaminant solubilized per mole of surfactant added to the aque-

MSR=(S-S )/(C -CMC) CMC s (7)

sented by the molar solubilization ratio (MSR) [13] defined as expressed in Eq. (7):

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

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

is ascribed to coiling of the long hydrophobic moiety in the aqueous phase.

active properties loss is termed the cloud point.

316 Soil Contamination - Current Consequences and Further Solutions

the single pure surfactants.

opposite charged.

where

ous phase

organic compounds is favored.

CMC = critical micelle concentration

of the micelle and consequently inhibit CMC formation.

to enhance contaminant solubilization by a specific surfactant.

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.

Studies on mixed surfactant systems competitive effects on hydrophobic contaminants solubilization has been investigated and reported elsewhere [15–18].
