**Abstract**

Soil contaminated with ubiquitous hydrophobic organic contaminants (HOCs) is a worldwide recurring concern arising from their indiscriminate disposal, improper management, and accidental spills. A wide range of traditional remedial strategies have been the common practice. However, these treatment methods have become cost prohibitive, not environmental friendly, and less accepted by society. Surfactant-enhanced remediation technology represents a cost-effective and green technology alternative to remediate such contaminated sites. Surfactant remediation technologies are conducted in-situ or ex-situ as two broad categories, or in combination. Among these technologies are soil flushing, washing, phytoremediation, and bioremediation. More applied research continues to quantify the efficiency of surfactant-enhanced mass transfer phase using a single surfactant solution while their binary blends to remove mixed HOCs in soils are also a focus of interest for research. There is a great potential to develop novel synthetic and biosurfactants that will exhibit higher biodegradability, less toxicity, higher removal efficiency, more economical and more recyclable. This work thus provides a review of the applications and importance of surfactant-enhanced remediation of soil contaminated with HOCs. Relevant environmental factors, soil properties, surfactant chemistry, mechanisms, mass transfer phase, and field designs are summarized and discussed with purposes of providing greater context and understanding of surfactant-enhanced remediation systems.

**Keywords:** Remediation, surfactants, soil, hydrophobic, contaminants

### **1. Introduction**

A major environmental concern around the world is soil contamination by ubiquitous hydrophobic organic contaminants (HOCs) due to their improper management. Such contaminants pose serious environmental and health risks to the public, and can be difficult to remediate due their intrinsic complexity and their weathering. Soils contaminated with HOCs not only can be deleterious to the ecosystem, it can lead to increasing economic loss and ecological insecurity. HOCs which are largely organic in nature, are characterized by relatively low solubility, a specific density that can be greater or less than 1, nonpolar compounds and have been shown to be toxic, mutagenic and/or carcinogenic even at trace concentrations in the soils. Example of HOCs include aromatic compounds in petroleum and fuel residue, chlorinated compounds in commercial solvents, pharmaceutical chemical

wastes like trichlorophenol, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) etc. Furthermore, many HOCs in the soil can be volatile and their behavior may engender vapor intrusions in various structures [1]. As previously reported [2] their availability for biochemical transformations is significantly affected by their large octanol–water partition coefficients (log Kow > 2). Even at very low concentrations, HOCs have shown to enter the food chain through various pathways and as bioaccumulating compounds, may ultimately threaten human life and other ecological receptors. Removal of HOC in soils can represent a significant challenge because such efforts can be site specific, costly, and often with limited success for its associated plumes [3]. Particular attentions to the ubiquitous deployment of surfactant-based remedial technologies indicate their ability to provide the means of great practical importance for implementing environmentally friendly remedial solutions, at low cost, and in a scientifically and engineering sound manner. Traditional framework in using surfactant remediation technologies are in-situ or ex-situ as two broad categories, or in combination. Among these technologies are soil flushing, washing, and bioremediation.

The in-situ remedial method involves remediation of the contaminated soil matrix without excavating the contaminated soil. This approach is generally considered less disruptive to the land ecosystem, may require multi-stage of operation, highly affected by soil physical properties and characteristics, and the time required to achieve the remediation effect may be substantial. The long treatment time associated with in-situ remediation may make the site unusable during the remediation period. Several in-situ remediation techniques have been developed which include surfactant aided flushing techniques. In conducting in-situ soil flushing (i.e., soil washing) remediation, a low concentration of surfactant solution is passed through an in-place contaminated soil using a vertical injection or infiltration process. The surfactant solution entrains the dissolved contaminants to an area where it can be collected and removed for treatment or disposal. However, the groundwater beneath the contaminated soil may serve as the discharge point for the extraction fluids. In such instance, the groundwater needs to be treated to adhere to environmental standards and maintain strict environmental quality at the site to protect public health and ecological receptors. Following HOCs in-situ surfactant-enhanced mass transfer phase into the soil solution, phytoremediation has been applied to extract, sequester, and detoxify the contaminants [4]. Since phytoremediation capacity is species specific, using a combination of plants as remedial agents will increase the efficacy of the remedial process. A notable advantage of phytoremediation, it is scientifically referred to as green technology and low cost. However, the time required to achieve the remedial target is typically longer compared to the other in-situ remedial approaches. Surfactants are also used in performing in-situ bioremediation of HOCs. The intended goal is to increase the bioavailability of the organic contaminants through mass transfer dissolution into the soil solution matrix and direct aqueous solubility. In this review, bioremediation is defined as a process, which relies on biological mechanisms to degrade, detoxify, mineralize or transform concentration of organic contaminants to an innocuous state. Often, nitrogen and phosphorous are limited as key soil nutrients and need to be added to biostimulate the soil natural microbial biodegraders. Both phytoremediation and bioremediation in-situ techniques will be affected by climatic conditions at the site.

The ex-situ approach can be conducted on-site or off-site. It involves excavating, storing and pre-treating the contaminated soil. Then, followed by treatment and redisposition of the clean soil. Treatment aided surfactant may take place in a variety of ways. Most common approaches involve biopile, windrow, and

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

bioreactor. Such ex-situ practices are more preferable compared to the popular dig and dump method in which the contaminated soil is excavated and dumped in an industrial landfill. Under this widely practiced conventional approach, the contaminants are not mineralized nor destroyed and represent long term threat to human health and ecological systems [5]. Environmentally friendly and cost saving features are among the major advantages of surfactant-enhanced bioremediation offer compared to landfilling, chemical and physical methods of remediation. However, the higher costs associated with transportation and associated liabilities of moving hazardous soil, and destruction of the soil ecosystem associated with excavation summarize the main disadvantages of soil ex-situ remediation over in-situ. In some instances, ex-situ treatment is preferred as a treatment as it offers more redelopment options of the land, treatment endpoint occurs faster and often the feasibility of being used with other treatment methods.

In their common form, surfactants are a group of amphiphilic chemicals constituted by both a hydrophobic moiety (chain) and a hydrophilic moiety (head) in the molecular structure of varying length in various surfactants. In fact, the unusual properties of aqueous surfactant solutions are best ascribed to the presence of the polar or ionic head group that interacts with an aqueous environment which leads to the solvation of the surfactant via ion-dipole or dipole–dipole interactions. Surfactants (short for "surface active agents" represent a unique class of compounds with distinct chemical and physical properties. Surfactants unique molecular structure give them the ability to dramatically alter interfacial and surface properties as well as to self-associate and solubilize themselves in micelles [6].

Surfactants manufactured by petrochemical plants are known as synthetic or chemical surfactants. Those produced from biological organisms are known as natural or biosurfactants. These lead to a vast array of their practical applications in terms of health, care products, food, petroleum processing etc. Irrespective of their source, the hydrophilic head group in the surfactant molecule is considered to be the main factor responsible for their special chemistry [7]. Historically, the costs of synthetic surfactants production remain comparatively less than biosurfactants. Several health and environmental concerns arise from using petroleum-based surfactants. In this regard, they are marginally biodegradable, can pollute soil and water, may bioaccumulate in the environment, and disruption of the endocrine system. On the other hand, biosurfactants being derived from biotechnology processes, are more environmentally friendly substance and often referred to as green technology. However, like petroleum-based surfactants, natural surfactants are associated with skin irritation and allergies. Considering the vast array of surfactants molecular structure and properties, one can anticipate an increase use in a myriad of environmental application for decontamination of soil matrices. This entails that surfactants with different properties and molecular structures can be strategically selected for different soil decontamination purposes. Importantly, particular consideration should be given to determine combining various surface agents for achieving greater remedial efficiency. This work provides an examination of surfactant-enhanced remediation of soil contaminated with hydrophobic organic contaminants as well as practical and general considerations involved in their implementation.

### **2. Classification of surfactants**

Surface active compounds are the most commonly used chemicals in everyday life. The number of different molecules of surfactants that have been manufactured must be in in the thousands and many have found practical use in society.

Unfortunately, it is somewhat surprising that surfactants, until only very recently, been explored for environmental remediation applications. Architecturally, a surfactant molecule contains a chain, the hydrophobic moiety, that can be linear or branched while the head is the polar or ionic moiety [1] (**Figure 1**). The hydrophobic is typically a hydrocarbon chain of an average of length of 12 to 18 carbon atoms and may involve an aromatic ring. For the purpose of this review, surfactants are divided into four main categories depending on the nature of the polar moiety as depicted in **Table 1** [1]. An in-depth discussions of surfactants chemistry and structure are presented elsewhere [6–8]. Furthermore, there are a number of review of publications available for surfactants use in specific industries [9]. A summary of basic information of various surfactants that have been used for the remediation of soil contaminated with HOCs is depicted in **Table 2**.

Biosurfactants are a group of surface active agent biomolecules produced by microorganisms. It has been suggested that surface active biomolecules can be best divided into low-molecular mass molecules or higher-molecular mass polymers. An adaptation of their classification is depicted in **Table 3**.

In recent years, scientists have been working diligently at evaluating the effectiveness of various types of surfactants to degrade organic contaminants in soils. In

**Figure 1.** *Structural parts of conventional surfactant molecule.*

**Table 1.** *Category of surfactants classification [1].*


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

#### **Table 2.**

*Basic information of surfactants used in soil remediation of HOCs.*

this section, the classification and discussion of surfactants will be more specifically focused on surfactants that have practical relevance in the remediation of soil contaminated with HOCs.

#### **2.1 Ionic surfactants**

The family of ionic surfactants is comprised of cationic, anionic and zwitterionic surfactants. They have been applied successfully for the mass transfer solubilization and removal of a variety of HOCs such as PCBs, dense nonaqueous phase liquid (DNAPLs), light NAPLs, BTEX in different soil types. Many literature documents their success in laboratory scale testing and from site-specific soils at pilot or full scale [8–12]. Interest in developing more effective (higher performance/cost ratio) and less toxic surfactants formulation has led to the emergence of Gemini surfactants. It has been reported that the surface active of Gemini surfactants could be of a several order of magnitude greater than conventional surfactants [13]. They are


#### **Table 3.**

*Biosurfactants classification (adapted with permission from [1]).*

used to describe dimeric surfactants which are surfactants that have two hydrophilic (chiefly ionic) polar groups and two hydrophobic chains on each surfactant molecule (**Figure 2**). These twin parts of the surfactant are linked through a spacer of varying link [14]. Gemini surfactants offer a number of superior properties when compared to conventional ionic surfactants. These advantages can be best

**Figure 2.** *Illustration of a gemini surfactant.*

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

summarized as lower concentration requirement for solubilizing HOCs, higher aggregation at significantly lower concentration, superior wetting agent, surmount hard-water tolerance effect on mass transfer into soil solution and increased surface activity (C20).

#### **2.2 Nonionic surfactants**

Nonionic surfactants are a group of surfactants that hardly dissolve in water, are neutral, and do not have any charge on their hydrophilic end. Their polar portions are typically made up of oxygen-containing groups. Nonionic surfactants solubilize in aqueous phase through hydrogen bonds formation of hydrophilic moieties with water. As the temperature is raised, it reaches the point at which large aggregates of the nonionic surfactant separate out into a distinct phase. There are several properties of nonionic surfactants that make them more suitable candidates to use in soil remediation of HOCs compared to ionic surfactants. Nonionic surfactants tend to have low toxcity, more biodegradable, more cost-effectiveness, low susceptibility to aggregate clay minerals, and low CMC. In the context of this review, toxicity is the measurable adverse effect that a surfactant will have on the soil microorganisms, while biodegradability refers to the ability of the soil microorganisms to destroy the surfactant. The literature abounds with scientific reports that document the wide application of nonionic surfactants for site-specific contaminated by a variety of HOCs [15–18].

#### **2.3 Biosurfactants**

Recently, there has been significant research interest in developing and investigating cost-effectiveness production of biosurfactants with unique properties and potential wide applications. One germane challenge that environmental scientists faces in the application of synthetic surfactants-enhanced soil remediation is their toxicity and biodegradability in the environment. It is noted that the environmental applications of biosurfactants has been gaining rapid interest and acceptance in the field of surfactant-enhanced soil remediation. This is due to their attractive physicochemical properties, low toxicity, high biodegradability and relative ease of preparation make these surface active biomolecules suitable candidates for soil remediation.

#### **3. Classification of soil matrices**

#### **3.1 Laboratory method**

Investigation of contaminated soils requires determining their physical properties for their classification. To this effect, soil classification can be approached from the perspective of the soil texture and organic matter content. The co-influence of both characteristics will have significant impact on the behavior of contaminants and surfactants when performing surfactant-enhanced soil remediation. Such impact is demonstrated through sorption and desorption, bioavailability, mechanism of interactions, contaminants leaching and fate in the soil. Most soils consist of a combination of sand, silt and clay and their range in size is reported in **Table 4**. Depending how much clay, sand, and silt that are present, the soil is given a name. The textural class of a soil is determined by the percentage of sand, silt, and clay. Soil texture determination begins by segregating the fine earth from the rock fragments. Fine earth refers to soil fraction that passes through a #10 sieve. It includes


#### **Table 4.**

*Size range of soil particles.*

all particles smaller than 2 mm in diameter. Sand, silt, and clay particles are components of fine earth. These three are called the separates of the fine earth. The soil textural triangle (**Figure 3**) is a representation of the mineral content of a soil in various combinations of clay, silt, and sand. The most common method for determining soil texture is the hydrometer method. According to this method, the soil separates are dispersed with solution of sodium metaphosphate (Calgon), blended and the density of the suspension measured at various time intervals. After dispersion, the amount of each particle group (sand, silt, clay) are determined by using a hydrometer. Once these percentages have been determined by the hydrometer method, the triangle can then be used to determine the soil textural class name.

**Figure 3.** *USDA triangle representation of textural soil classes.*

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