**1. Introduction**

Human activities over time have left a legacy of contaminated soils around the world. The intense exploitation of soil and the inadequate disposal of hazardous wastes by urban expansion, industrial and transport activities, mining, military activities and armed conflicts, and even unsustainable agricultural practices are the main sources of soil pollution. These anthropogenic activities release various

chemicals into the environment that are often found to form a complex mixture of numerous contaminants. The different contaminants produce adverse effects on the health of ecosystems and all living beings that inhabit there. Moreover, the frequency and severity of extreme climatic events (droughts, floods, dust storms, and wildfires incidents) caused by climate change exacerbate soil contamination. Anthropogenic activities contribute to changes in the moisture and temperature regimes of soils and groundwater and can increase rates of movement of contaminants *via* soil erosion (wind or water), soil runoff, leaching, and volatilization [1]. In this sense, a detail of natural and anthropic sources of some elements can be seen in **Table 1**. For example, dust storms, volcanic eruptions, geothermal/ hydrothermal activity, forest fires increase the level of As and Hg in the environment. Climate change exacerbates these phenomena increasing the natural contribution of metal(loids).

The insufficient registration of contaminated areas in many regions of the world and the lack of regulations for their remediation accentuate this environmental conflict. About 3.5 million sites in the European Union (EU) were estimated to be potentially contaminated, with 0.5 million sites being highly contaminated and needing urgent remediation. There are 400,000 polluted sites in European countries, including Germany, England, Denmark, Spain, Italy, Netherlands, and Finland. Sweden, France, Hungary, Slovakia, and Austria count up to 200,000 contaminated sites. Greece and Poland reported 10,000 contaminated land areas, while Ireland and Portugal reported less than 10,000 contaminated sites. In America, approximately 600,000-ha brownfield sites are polluted with heavy metals [2]. Identification and assessment of potentially polluted sites are the essential first step in the management of soil pollution.

Among the persistent and potentially (eco)toxic heavy metal(loids)s (HMs) ubiquitous around polluted soils are arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), manganese (Mn), nickel (Ni), zinc (Zn), and radionuclides. Many of them are considered trace elements. The concentration of these HMs in soil has increased drastically over the last three decades, thus posing a risk to the environment and human health. A detailed description of the above-mentioned trace elements, including the natural and anthropogenic sources and uses, is given in **Table 1**. Identifying the sources of trace elements in the environment is of key importance to understanding the pollution patterns and natural global cycles, in addition to making decisions concerning soil pollution remediation.

The remediation methods are generally based on physical, chemical, and biological approaches, which may be used in combination with one another to clean-up HMs to an acceptable and safe level [3–5]. The physical and chemical conventional methods are usually expensive and can irreversibly affect the properties of soil, water and the living beings that inhabit them [6]. **Figure 1** resumes the soil remediation techniques based on chemical, physical, and biological processes developed during the last two decades [4, 7–9].

#### **1.1 Physical methods**

These methods consist of removing or reducing contaminants by physical methods such as dilution, heating, and solidification of contaminated soil. Some of the technologies involve the soil replacement or isolation, the thermal analytical method, vitrification, and the electric repair technique, which does not change the chemical properties of the pollutants.

*—Soil replacement and isolation method (1 y 2):* The soil replacement method reduces the concentration of contaminants by replacing the original contaminated



#### **Table 1.**

*Natural and anthropic sources of some elements and their industrial use (Source [1]).*

#### **Figure 1.**

*Comparison of physical, chemical and biological methods of remediation for polluted soils or contaminated substrate. Physical remediation methods include (1) soil replacement, (2) soil isolation, (3) vitrification, and (4) electrokinetic; biological methods generally include (5) phytovolatilization, (6) phytoextraction and (7) phytostabilization; chemical methods contain (8) immobilization and (9) soil washing. Biological and chemical methods can be applied jointly depending on the type of contaminant, soil, plant and chemical reagent. Moreover, the effectiveness of different phytoremediation techniques can be enhanced by microbial-, chelateand genetic-assisted remediation. Modified from [3].*

soil with fresh soil and transferring the contaminated soil to the surrounding environment. The method is simple and reduces the concentration of pollutants in a short time. It does not change the mobility and bioavailability of pollutants in the soil, so it is often required in engineering construction as prevention and control barriers to prevent secondary pollution to the environment.

*Scale-up of Mycorrhizal-Assisted Phytoremediation System from Technology Readiness… DOI: http://dx.doi.org/10.5772/intechopen.101584*

*—Vitrification method (3):* The soil vitrification consists of treating the contaminated soil with high temperature and pressure for a period, and then cooling it to form a vitreous substance. The result is a stable material where the contaminants are fixed.

*—Electrokinetic techniques (4):* Electrodes are placed into the soil and a direct electrical current is applied, which induces movements of contaminants to the cathode or anode through the electro-osmosis, electrophoresis, and electromigration [10, 11]. This technique has a short cycle and high efficiency but high energy consumption. It can be applied on-site, off-site, and *in situ* depending on the soil conditions.

#### **1.2 Biological method**

Phytoremediation technology:

Phytoremediation involves the use of plants to extract and remove chemical pollutants or to decrease their bioavailability in soil [12, 13]. In general, plants used to carry out phytoremediation are known as metallophytes. The main benefits reported for phytoremediation include less secondary waste generation and minimal-associated environmental disturbance *in situ*. However, the main constraint is the long period of remediation due to the growth cycles of plants. This technology can be improved with the inclusion of microorganisms such as filamentous fungi and bacteria with saprophytic or symbiotic nature. The mechanisms of phytoremediation used in the removal of HMs are phytovolatilization, phytoextraction, and phytostabilization.

*—Phytovolatilization (5)* plant roots absorb contaminants from soil and transport them through the xylem. Plants convert the contaminants into less toxic and volatile forms and release them into the atmosphere. Phytovolatilization has been widely used to remove metals such as mercury and selenium, as these metals have high volatility [14].

*—Phytoextraction (6)* is when plant roots absorb the contaminants from soil or water, and transport, and accumulate them in the aboveground biomass such as shoots and leaves. The hyperaccumulator species are the desirable plants to be used for phytoextraction as they have a high ability to accumulate different elements [15]. Plant biomass is comparatively very easy to recycle, dispose of, treat, or oxidize compared to contaminated soil. Phytoextraction guarantees a permanent removal of HMs from the contaminated sites. However, phytoextraction is suitable for those sites with low-moderate levels of HMs, because most plant species are not able to survive in heavily polluted habitats [9, 16]. However, some authors have mentioned the potential use of native hyperaccumulating plants with remarkable tolerance strategies facing polluted conditions [17]. Essential pre-requisites for successful phytoextraction include the following: a high uptake and translocation of HMs to aerial parts, an enhanced loading of HMs into the xylem, and an efficient detoxification in the plant [18, 19]. Physiological studies revealed that enhancement xylem loading of HMs and their transfer to the aerial plant parts are mediated by carrier proteins, generally found in the intracellular or plasma membranes (cation diffusion facilitator, CDF; zinc-regulated transporter proteins, ZRTP; iron-regulated transporter proteins, IRTP; heavy metal(loid) ATPase, HMA; natural resistance and macrophage protein, Nramp) [7, 20, 21]. Other natural chelators such as metallothioneins, phytochelatins, glutathione, thiol compounds, and organic acids are also involved in the improvement of HMs accumulation and translocation to the xylem, besides tolerance to stressful conditions. Secondly, there is also a need to pursue the role of plant growth regulators (indolebutyric acid, cytokinins,

gibberellic acid, naphthylacetic acid, and indole-3-acetic acid) to increase the potential biomass production of hyperaccumulating plants.

*—Phytostabilization (7)* is performed by plants that reduce the mobility and migration of HMs in soil by confining them in the vadose zone (the unsaturated strata above the water table) through the absorption and adsorption of these contaminants on the roots or the precipitation of toxic elements within the rhizosphere [22]. In this process, the plant that is being used to carry out phytostabilization induces changes in their rhizosphere, which has discrete physical-chemical, and biological conditions [23–25]. Metal excluder plants accumulate high levels of HMs from the soil into their roots with the limited transport to their aerial parts [20]. These plants have little potential for HMs extraction to be considered in a phytoremediation process, but are highly efficient for phytostabilization purposes. Phytostabilization can also be used in combination with other remediation approaches, such as the use of soil microorganisms and organic amendments to enhance HMs immobilization in soil. Soil microorganisms are reported to increase root metal contents *via* an increase in plant growth as well as the HMs immobilization in soil [26]. Besides soil organic matter comprises a wide range of organic molecules in different states of mineralization and complexation within the soil matrix, which will behave differently when interacting with contaminants. These organic macromolecules contain many functional groups (carboxylic acids, alcohols and phenols, or amines), dependent on pH- and redox potential, that play a major role in the adsorption of ionizable organic contaminants as well ionic forms of trace elements through covalent and hydrogen bonding, thus reducing accessibility to microbial interactions. Small organic compounds such as amino acids, sugar acids, short-chain aliphatic acids, and phenols can form stable chelates with trace elements, and contaminants can also be complex with Al and Fe oxides. Some substances excreted from microorganisms may contribute to the acidification of soil and increase the mobility of some contaminants. The buffering capacity of soils neutralizes excess anions in exchange for mobilizing cations (e.g., Mg2+, Ca2+, Na+ , K+ ) from the surface of soil particles, which results in cations leaching. But this capacity is limited, and if acid deposition exceeds the natural neutralizing capacity of the soil, other cations, such as Al3+ or Fe2+, can be mobilized from clay structures and organo-mineral complexes, entering the soil solution [1]. Once the sites are phytostabilized continuous monitoring is required to make sure that the stabilizing condition is maintained. Soil amendment used to reduce HMs mobility in soil may need to be occasionally reapplied to retain immobilizing conditions [22].
