**7.2 In-situ phytoremediation**

Phytoremediation remediation is a green technology technique that makes the use of plants as natural agents to absorb, degrade and/or sequester HOC over time in a contaminated soil. However, it can be slow and strategically should be used in a treatment train approach with in-situ flushing when feasible. Plants take up chemicals when their roots take dissolved chemicals and nutrients from the soil aqueous solution and additionally, HOC can be biodegraded by micro-organisms found in the plants rhizosphere. Efficacy of phytoremediation will depend on a combination multiple mechanisms in relation to specific plant species. The mechanisms may involve phytoremediation capacity, phytoaccumulation, phytovolatilization, rhyzodegradation, and phytodegradation**.** Crucial is optimizing surfactantenhanced mass transfer of sorbed HOC in the aqueous soil solution particularly in the presence of multiple contaminants. Various contaminants may have different affinity for the soil sorbing sites which in return will affect the strength and mechanism of retention. When choosing plant species for a phytoremediation project, several relevant factors should be examined including type of plant root system, above ground biomass, depth of roots penetration, toxicity tolerance to the contaminants and surfactant, plant hardiness, depth of vertical contamination, adaptability to prevailing climatic conditions, resistance to diseases and pests, plant growth rate, nutrients requirement, and time required to achieve the desired level of cleanliness.

#### **7.3 Ex-situ soil washing**

Ex-situ soil washing is a mechanical process that involves delineating the areal extent of contamination, excavating the contaminated soil, pretreat it as necessary and then treat it with a surfactant solution. The soil washing can be performed in batch or continuous modes. The main steps are schematically depicted in **Figure 9**.

**Figure 9.** *Schematic diagram of ex-situ soil washing.*

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

In practical term, ex-situ soil washing is considered a time-efficient and all-around technique, and a media transfer technology. It allows to treat a broad range of contaminant types and concentrations. Removal of coarse fractions is a key step and they can be reused on site as clean fill. However, there is a general held view that this technique is only cost efficient for coarse and granular soils where the clay and silt content make up less than 30% of the soil matrix. Factors that may limit the effectiveness and applicability of this method include effective removal of HOC sorbed onto clay-size particles by a surfactant, high soil humic content, and ambient temperature at treatment time.

### **7.4 Ex-situ surfactant enhanced bioremediation**

Ex-situ surfactant enhanced bioremediation method refers to the biostimulation of soil natural biodegraders and increasing contaminant bioavailability. Two main prerequisites for biodegradation to take place are carbon source as electron donors and nutrients, as amendment. HOC contaminants in soils exhibit no or very low solubility at all and thermodynamically tend to partition to the soil solid phase. The concomitant effect is the level of hydrophobicity displays limits dissolved mass transfer phase and bioavailability, thereby limiting its biotic degradation in the soil system. Optimizing nitrogen and phosphorous status in the contaminated soil can have direct impact on contaminants biodegradation and microbial activity. This technique can be performed in various configurations which include windrow and various types of bioreactors. The general procedure of an ex-situ soil bioremediation is illustrated in **Figure 10**. Regardless of the system configuration and design emphasized, the treatment process must be optimized. Aqueous slurry conditions range from 20 to 40% w/v and should be not toxic to the soil microbial population. The slurry bioreactor sometimes may operate in sequencing batch reactors to achieve a desired treatment train objective (**Figure 10**). In this regard, dehalogenation conducted under anaerobic conditions is a prerequisite prior to

**Figure 10.** *Illustration of a typical batch sequencing slurry bioreactor (adapted with permission from [1]).*

aerobic treatment. If dehalogenation is not required, the biodegradation treatment process can be performed under aerobic conditions only. Aerobism can be maintained during treatment by performing slurry mixing with mechanical or pneumatic devices in a rather intermittent than continuous mode. Mechanical mixing homogenizes the contaminant in the slurry bioreactor. A matrix summary of critical success factors for ex-situ surfactant enhanced bioremediation can be found elsewhere [1].

#### **8. Mixing surfactants for their enhancement effect**

Remediation of contaminated with mixed HOCs is generally very challenging and compounded due sorption on the soil matrix and different solubility properties. The strategy of mixing different classes of surfactants is to achieve a synergistic solubilization effect for the extracting solution. For example, when ionic and nonionic surfactants are combined, the mixed surfactants solution results in a stronger solubilization effect than single surfactant solution. The reason is that nonionic surfactants diffuse the ionic surfactants and to some degree, reduce the influence of electrostatic repulsion between affecting the ionic surfactant molecules [35]. It has been reported that appropriate combination of several surfactants could inhibit the respective sorption of individual surfactant onto the soil. So, the loss of surfactant resulting from sorption is reduced and thereby increases the capability of mixed surfactants for HOC desorption in soils [36–39]. Synergistic effects of mixed surfactants in the binary blends can be best attributed to a decrease of CMC of the surfactant solutions, larger amount of available micelles formation, increase of MSR, lower polarity and higher aggregation of number of the mixture micelles.

#### **9. The future of surfactants application for site clean-up**

The potential adverse impact of HOC in soil has been a significant concern around the world for the public, policy makers, environmental regulators, and scientists. Even at very low concentrations and low solubility, these contaminants are generally considered highly toxic, mutagenic as well as carcinogenic, or can pose some other harm to humans and other ecological receptors. Costly site-specific remediation strategies have often been employed and too often with limited success. In many instances, site-specific remediation strategies are designed towards partial mass removal, plumes containment, source zone stabilization, relative to a formulated acceptable risk-management objective. The use of surfactants-aided soil remediation represents a technically attractive, cost-effective, and promising technology for reclaiming and rehabilitating contaminated sites. As a remediation technology, it is becoming well established because of its effectiveness and its promising results to retain the original nature of soil. Ideally, the primary goal of surfactantaided remediation is to achieve 100% bioavailability and removal of contaminants with minimal xenobiotic effects and toxicity. Current research activities are very promising in this regard and continue to make more efficient synthetic and biosurfactants. However, there is an urgent need for both theoretical and empirical research on tertiary blends of surfactants-aided soil remediation and with additives mixed. More elaborative research works is also needed to elucidate the potential fate, characterization of soil and environmental interaction properties, health and ecological risks that may arise from surfactants entering the environment.

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