*2.1.2 Washing of soil*

Sediment washing is a simpler technique that is performed *ex-situ*. In this technique, a solution is utilized to wash the contaminated sediment for the transfer of pollutants from sediment to an aqueous solution. This is achieved by mixing the soil with an aqueous solution of alkalis, acids, and surfactants [14]. Washing includes (i) excavation of highly contaminated sediment from the bulk soil; (ii) washing of sediment is processed with the help of aqueous mixtures; (iii) the solubilized contaminants are removed from aqueous solution through various chemical processes. For performing this method more efficiently additives are added to the aqueous solution, depending upon the physicochemical nature of contaminated sediment. These additives should have high treatment efficiency and environmental compatibility. Common additives used are inorganic acids (sulfuric acid, nitric acid), organic acid (oxalic acid, ascorbic acids), and surfactants (sophorolipids

#### **Figure 3.**

*Capping technique for isolating contaminated sediment [18].*

and rhamnolipids) [16]. EDTA has been reported as the additive for the removal of heavy metals, due to its versatile chelating nature, however, the toxic effect of EDTA on the environment and its low biodegradability has reduced its application widely [19]. After washing, the sediment is considered contaminant depleted instead of contaminant free. Therefore, to make this technique successful, the number of contaminants treated should be quantified to be equivalent to the site-specific action limit. This technique is suitable for the contaminants which are weakly associated with sediments, and in coarse-grained sediments [14].

## *2.1.3 Excavation of soil*

This technique includes physical removal of majorly contaminated soil from the bulk soil. There are several ways to perform this technique. It can be divided into three methodologies (i) substitution of polluted sediment by removing the soil and putting it in another soil. This method is more suitable for land contaminated in small areas; (ii) the deep excavation of contaminated sediment for natural degradation of heavy metals; (iii) importing new soil and mixed with contaminated soil for dilution of heavy metals. This technique is expensive and is efficiently applicable only on land with small areas of contamination [20].

#### **2.2 Chemical remediation techniques**

This technique includes the utilization of chemical reagents, reactions, and principles for the removal of contaminants. Major methodologies used under this technique are solidification, immobilization, vitrification, and electro kinetics.

#### *2.2.1 Immobilization*

This methodology is used to stabilize heavy metals, can be applied *ex-situ* and *insitu*. It often uses organic and inorganic reagents for the reduction of heavy metals mobility, toxicity, and bioavailability in the soil. The primary objective of this technique is to alter the bioavailable phases of metals into more geo-chemically stable phases, with the immobilization of chemicals. It is achieved through combined mechanisms of adsorption, complexation, and precipitation. The stabilizing effect of amendments is dependent upon the physical, chemical, and biological characteristics of sediment, heavy metal type, remediation time, remediation method, and evaluation method. The most common inorganic reagents used for immobilization are silico-calcium reagents, phosphates, iron-containing materials, aluminum salts, and mineral-based amendments. Organic reagents for immobilization of heavy metals include manure, biochar, biosolids, bark, wood chips, sawdust, sewage sludge, and turf. A complex formulation of inorganic and organic amendments can also be applied to the contaminated sediments for more efficient stabilization [21].

#### *2.2.2 Solidification*

It is a technique applied by mixing contaminated sediments with materials that impart physical stability to encapsulate contaminants in a solid product. Solidification is the physical encapsulation of contaminants in a solid matrix, which are formed by cement, bitumen, asphalt, fly ash and thermoplastic binders. During *In-Situ* remediation, a binding agent is added to contaminated sediment which is followed by an auger spin mixing to transform the soil into a solid matrix [15]. The stabilization of heavy metals includes chemical reactions which reduce their mobility in the environment. The entrapped toxic metals are not leachable as

#### *Conventional and Contemporary Techniques for Removal of Heavy Metals from Soil DOI: http://dx.doi.org/10.5772/intechopen.98569*

the solid block is impermeable to water. A mixture of various salts can be used for the solidification or stabilization of contaminants in soil *ex-situ* or *in-situ*. Several economically effective and environmentally friendly waste resources have been reported for their application in contaminated sediment. These waste resources can also improve the quality of polluted soil, such as lime-based agents, calcined oyster shells, eggshells, waste mussel shells, and calcined cockle shells [20]. However, the process does not extract the pollutant. So, over the long term, if the integrity of solid matrix is deteriorated due to natural weathering or any uncontrolled physical disaster the contaminants which are trapped can mobilize into the environment. Therefore, this methodology is applied as a last option for remediation of soil. This technology is dependent on the concentration of contaminants present in the sediment, amount of water, and ambient temperature. These factors affect the binding reaction of contaminants to the solid material, it inhibits the binding and decreases the stability of the solid matrix [14].

## *2.2.3 Vitrification*

This methodology of remediation is a type of stabilization/solidification technique. It requires high thermal energy in contaminated soil, at least 1400°C - 2000°C, for the removal of organic or volatile substances. It is achieved by mixing the contaminated sediments with glass-forming precursors, heating the mixture till its liquid solution is formed. The steam produced by introducing high thermal energy and the products of pyrolysis are collected from exhaust gas [21]. On the cooling of this solution, an amorphous homogenous glass is obtained. The contaminants can be stabilized by two ways of interactions with solid glass matrix, that is chemical bonding and encapsulation. For *in-situ* remediation, electrodes can be inserted directly into the contaminated sediments. This technique is efficient but expensive and complex to perform [20].

#### *2.2.4 Electrokinetic remediation*

In this technique, the electric field is applied to the wet contaminated sediments for the movement of ionized metals towards the cathode or anode. The pollutants are migrated towards electrodes through electro-migration (charged chemical movements), electro-osmotic flow (fluid movements), electrophoresis (charged particle movements), and electrolysis (chemical reaction due to electric field) procedures [21]. On the completion of the remediation process, the contaminant concentrated electrodes can be treated through several techniques for treating the heavy metals. This technique performs more efficiently in fine-grained clayey soil, where heavy metals are present as soluble ions, because of high electric conductivity and strong electric field [16]. To enhance the efficiency of this technique application of chelating agents can be performed, such as EDTA, nitrilinoacetic acid, succinic acid, citric acid. A schematic representation of this technique has been represented in (**Figure 4**).

#### **2.3 Biological remediation**

Biological remediation or bioremediation is a technique of transforming the heavy metals present in the contaminated soil, into a less toxic element. This technique uses biological phenomena that are intrinsic to plants and microorganisms, for the destruction, removal, or immobilization of hazardous contaminants from the polluted environment. Bioremediation is an eco-friendly and economically effective technique for heavy metal removal compared with the conventional chemical and physical methods, which are usually expensive and ineffective especially for sediments contaminated

#### **Figure 4.**

*A schematic representation of* in-situ *electrokinetic installation; B schematic representation of detailed electrokinetic remediation technique [22].*

with low metal concentrations, in addition to producing significant amounts of toxic sludge [20]. The main objective of the bioremediation technique is to stimulate a favorable condition for microflora or plants at the contaminated site by providing suitable growth conditions. So, they can grow at their full potential and produce enzymes as secondary metabolites for immobilizing the toxic metals. During the bioremediation process of the contaminant, chemical bonds are broken, and energy is released, which is further utilized by the microorganisms for their growth. Various investigations show that the total transformation percentage of various heavy metals by microbes are Cr (27%), Co (20%), Cd (31%), Pb (22%) [23]. Bioremediation technology is aided with several methodologies, such as bioventing, bioleaching, and land farming, bioreactor, composting, and bioaugmentation, rhizo-filtration, and biostimulation. Therefore diverse metabolic activity inherent to microbes can be exploited for degradation, removal, or transformation of heavy metals in contaminated soil [24]. Mostly bioremediation can be performed by utilizing microorganisms (algae, fungi, and bacteria), and plants (phytoremediation), or with the combinations of both.

#### *2.3.1 Phytoremediation*

This technique involves the use of various native, imported, or genetically modified plant species for the reduction, and removal of contaminants from soil,

#### *Conventional and Contemporary Techniques for Removal of Heavy Metals from Soil DOI: http://dx.doi.org/10.5772/intechopen.98569*

sludge, wastewater, sediments, and groundwater. This technique is best applicable when the contaminants are present around the rhizosphere and in a wide area of land. The basic principle in phytoremediation involves the disintegration through secondary metabolites or absorption by roots, and storing them in leaves of plants, of contaminants present in soil [20]. Hyperaccumulation and hyper tolerance are very important characteristic for a plant for their utilization in phytoremediation. Phytoremediation technique includes phytoextraction, Phytofiltration, Phytostabilization, Phytovolatilization, and Phytodegradation [19].

Phytoextraction/Photoabsorption/Phytosequestration/Phytoaccumulation refers to a biochemical process where the assimilation of heavy metal contaminants from the sediment or water is processed through roots and translocated to any harvestable part of the plant, based on the mechanism of hyperaccumulation (**Figure 5**). Hyperaccumulators can concentrate 100 to 1000 times higher than those found in non-hyperaccumulators without suffering any apparent phytotoxic effect. This method includes three steps (i) cultivation of suitable plant species in the contaminated land; (ii) harvesting of biomass concentrated with metal; (iii) post-harvest treatment for obtaining economic value [25]. The most used hyperaccumulators are from the family *Fabaceae, Brassicaceae, Lamiaceae, Cryophylaceae, Violaceae, Asteraceae, Cyperaceae, and Poaceae* [24].

Phytofiltration is the cleanup method for a contaminated environment with the use of plant roots. It could be performed in three forms of rhizofiltration (plant roots), blastofiltration (seedlings), caulofiltration (excited plant shoots) [19].

Phytostimulation enhances the conditions of the rhizosphere for the efficient growth of microbes. It is performed for the removal of organic pollutants in the sediment.

Phytostabilization aims to the reduction of mobility and bioavailability of heavy metals in the environment by stabilizing the contaminants in the rhizosphere of plant species. It is performed by reducing the accessibility and mobility of heavy metals through precipitation, root sorption, metal valence reduction, and complexation. The efficiency of this technique can be enhanced by changing the pH and organic matter content in the sediment [25].

Phytodegradation is a technique utilized for degrading organic matter into non-hazardous chemicals through secondary metabolites or enzymes secreted by plants. Enzymes like nitroreductase and dehalogenases are used by plants for the degradation of organic matter. These enzymes are used only in optimal conditions (temperature, pH). This process can be performed more efficiently with the introduction of microorganisms in the contaminated soil, this technique is called Rhizodegradation [26].

Rhizofiltration is the process in which plants absorb and precipitate organic and inorganic contaminants through roots from contaminated wastewater, groundwater, and surface water. Major characteristic features of plants are hypoxia tolerant, and large absorption surface area for a suitable application of this technique. Terrestrial plants are more efficient for this purpose than aquatic plants [27].

#### *2.3.2 Microbial remediation*

Microorganisms can absorb or adsorb the heavy metals present in the soil to transform its chemical nature and reduce its mobility, bioavailability, and solubility. This remediation technique by microbes can be carried out in two ways, through mobilization or immobilization. These processes are accomplished by mechanisms, like bio-precipitation, biosorption, bioaccumulation, bio-assimilation, bioleaching, biodegradation, and biotransformation (**Figure 6**). Commonly microbial species used for remediation methodology are *Bacillus, Arthrobacter, Pseudomonas, Enterobacter, Aspergillus, Penicillium, Rhizopus, Rhodotorula, Candida utilis* [23].

Biosorption is a mechanism where microbes either absorb or adsorb the inorganic contaminants on the cell surface or into the cell. While adsorption is performed on the surface of the cell, absorption involves an entire volume of material. Several mechanisms involved in biosorption are precipitation, the formation of stable complexes with organic ligands, and redox reaction. The process of adsorption involves forming a complex of the heavy metals and functional groups on the cell surface, from where they can be absorbed into the cell. Adsorption is executed by binding heavy metals to the cell surface through electrostatic interaction, complexation, and ion exchange. According to Jin et al. [28], microbes perform adsorption predominantly in comparison to absorption.

Bioleaching is the mobilization of heavy metals from contaminated soil through biological dissolution, complexation, or bio-oxidation by microbial activity. The best-known microbes for bioleaching are *Thiobacillus* and *Leptospirillum ferrooxidans*. Various mechanisms of microbial metabolism produce several secretions, like low molecular organic acids. These organic acids have shown to effectively dissolve heavy metals and soil particles containing toxic heavy metals [28].

Bioaccumulation includes the agglomeration of contaminants into the microbe where it is concentrated, where metal is sequestered.

Bio-assimilation of heavy metals includes the active transport of microbial cell's siderophore for the chelation of toxic metals. Siderophores are biomolecules that are produced when microbes are present in iron-deficient media/environment. These

*Conventional and Contemporary Techniques for Removal of Heavy Metals from Soil DOI: http://dx.doi.org/10.5772/intechopen.98569*

#### **Figure 6.**

*Schematic representation of various mechanisms involved in microbial remediation of heavy metal contaminated soil [3].*



#### **Table 1.**

*Mechanisms, advantages and disadvantages of the available remediation techniques for heavy metal contaminated soil [19].*

*Conventional and Contemporary Techniques for Removal of Heavy Metals from Soil DOI: http://dx.doi.org/10.5772/intechopen.98569*

biomolecules are specifically iron (Fe III) chelators which are finally transported into microbes by various uptake proteins. Many reports have suggested that if siderophores are bonded with other metals, they can still be recognized by uptake protein for its transportation into the microbial cell [16, 24].

Bioprecipitation is a method that uses the mechanism of immobilization for the reduction of mobility and bioavailability of heavy metals in soil. It involves converting soluble heavy metals into insoluble hydroxides, carbonates, sulfides, and phosphates.

Biotransformation changes the chemical nature of heavy metals, altering their toxicity, mobility, and bioavailability. This methodology includes methylation, reduction, dealkylation, and oxidation of heavy metals for altering their soluble form into an insoluble form [16].

The applicability of these individual techniques in any specific soil remediation project is determined primarily by contamination site geography, characteristics of contaminants, the goal of remediation, cost-effectiveness, financial budget, readiness in implementing the technique, the time provided, and public acceptability (**Table 1**). Integration of more than one technique has been experimentally proved to be more efficient, such as application of chemical remediation in highly heavy metal contaminated sediment, which can be followed by phytoremediation for further removal of remaining contaminants [15].
