2. Soil pollution in industrial and mining areas

stage of cultural development of humanity left behind its pollution with metals, mainly stored

Heavy metal pollution is a major public health concern, and although efforts have been made to limit the population exposure, the problem persists due to the accumulation of these substances in the environment [2]. Especially long-term industrial and mining activities are the preponderent sources for heavy metal environmental contamination worldwide. Unlike other pollutants (organic compounds and radionuclides), heavy metals are considered to be the most persistent contaminants in soil because these elements tend to accumulate in the soil and then, through the plant and animal food chain, the population is exposed to their toxic effects [3, 4].

Heavy metals are defined as elements with metallic properties (conductivity, ductility, cation stability), atomic number greater than 20, and density greater than 5 kg/dm<sup>3</sup> [5, 6]. Recently, the term "heavy metal" is used as a general term for those metals and semi-metals with toxic potential on the human body or the environment [7]. The most common elements in heavy metal contamination of environment are Cd, Cr, Cu, Hg, Pb, and Zn. They occur naturally in the soil in relatively small concentrations but can occur in much higher quantities as a result of

Some heavy metals, in small amounts, have a physiologically beneficial role for plants or in the human body (e.g., Zn, Mn, and Se), and others are potentially toxic to organisms and humans

Expansion of areas affected by mining and industrial activities contaminating the environment with heavy metals makes the application of traditional technologies inappropriate due to the high costs associated with soil remediation. The majority of conventional methods such as incineration, vitrification, or land replacement are extremely expensive. Also, the potential impact on the environment must be considered, in particular the change of agricultural soil properties and damage to the landscape [9]. In historically polluted areas, the challenge is to decontaminate soils in order to resume agricultural practices and protect the population health. Thus, in addition to soil remediation, it is necessary to remedy the water or wastewater

The importance of biodiversity, both below and above ground, is currently increasing for the cleaning of metal-contaminated ecosystems [11]. The concept of ecosystem services may be integrated in this field, having implications for the practice of soil remediation [12, 13]. There is a close connection between soil, plants, and other ecosystem components. In fact, ecosystem services include the services provided by air, water, soil, and biota. Of these, the soil functions are important for the ecosystem good functioning and refer to some valuable properties: the capacity of storing, filtering, or transforming nutrients, substances, and water; biomass production (crops and forestry); host of biodiversity (habitats, species, etc.); source of raw materials; physical and cultural environment for humans; and human activities [14]. Ecological consequences of soil pollution apply not only to soil functions, such as its biological activity, but have also negative effects on soil-crop-animal-human system [5]. On the other hand, plants and microorganisms play a crucial role in restoring the specific soil functions and also other

ecosystem components, being considered as ecological or ecosystem engineers [15].

in soil, sediments, and ice [1].

196 Ecosystem Services and Global Ecology

human activities.

regardless the concentration [8].

used for crop irrigation [10].

Soil is the most important compartment for all terrestrial ecosystems, providing essential nutrients for plant growth, plant degradation, and transport of biomass. A significant role of the soil is also as natural buffer within the transport of chemical elements and substances in the atmosphere, hydrosphere, and biota [3].

The persistence of contaminants in soil is much higher than in other compartments of the biosphere, and soil pollution by heavy metals appears to be permanent in soils [23]. Heavy metals are native components of the earth crust, existing in different concentrations in all ecosystems [24].

The period of existence of metals in soil in temperate climatic conditions can be estimated for the metal elements, as follows: Cd between 75 and 380 years, Hg between 500 and 1000 years, and between 1000 and 3000 years for Ag, Cu, Ni, Pb, Se, and Zn [3, 25].

The sources of heavy metals in the environment are very diverse and can be of both natural and anthropogenic origins. The main natural sources are rocks and soils [26], and the anthropogenic sources are represented by socioeconomic activities; some of these are illustrated in Table 1. The problem of this type of pollution derives in particular from the exploitation of minerals and the use of metals by the human population.

Historically contaminated areas by heavy metals are found all over the world, especially caused by mining and ore processing activities. Consequently, the metal pollution is not


Phytoremediation is defined as a phenomenon of polluting substances extraction by using plants. With all these, there are many types of phytoremediation, so we can state that phytoremediation represents a much broader defined term [39, 40]. Phytoremediation of soils, waters, and sediments is not a new concept; for decades it has been found that some plants can degrade or extract heavy metals and other pollutants from these environmental compartments. Plants have been used for the decontamination of wastewater about 300 years ago. Thlaspi caerulescens and Viola calaminaria were the first species of plants used in the nineteenth

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A strong motivation to apply phytoremediation in historically contaminated sites, in addition to other advantages, is the particularly low cost of this method compared to conventional ones. Table 2 highlights the costs of different soil remediation techniques. Nevertheless, the most frequently applied remediation techniques for contaminated soil in Europe include land exca-

The metal extraction or accumulation by plants involves a variety of biological mechanisms

Through the rhizosphere (the interface between plant roots and soil), the water is absorbed by the roots to replace the evaporated water from the leaves. The metals in the soil solution (free ions or organometallic complexes) can move together with water (by convection or mass transfer) as the plant absorbs the water needed for vital processes. Absorption of water from the rhizosphere creates a hydraulic gradient directly from the ground to the surface of the roots. This concentration gradient or hydraulic control ensures the diffusion of ions from the

The elimination by plants of exudates and metabolites play an important role in the phytoremediation process. Thus, enzymes such as dehydrogenase, hydrolase, peroxidase, and phosphatase are released at the plant-soil interface and contribute to the degradation of

Remediation method Remediation costs (in US dollars/m<sup>3</sup> soil)

century and found to accumulate large concentrations of metals [4].

and requires direct knowledge of plant physiology and soil science.

soil particles to the deficient layer surrounding the roots [45, 46].

Excavation and disposal 140–720 Vitrification 360–1.370 Soil washing, ex situ 80–860 Soil washing, in situ 20–270 Solidification and stabilization 40–200 Electrokinetic methods 30–290 Bioremediation 10–310 Phytoremediation 1–150

Table 2. Costs of different soil remediation methods [42–44].

vation or disposal [41].

3.1. Phytoremediation process and techniques

Table 1. Industrial sources of the most important heavy metals in the soil [27].

attributable exclusively to mining activities, although these are preponderent in many regions [28–30]. Most of these activities are currently closed, remaining behind enormous quantities of heavy metals that have been deposited in the soil. The volume of tailing dumps discharged has exceeded 10 billion tonnes per year [31]. Usually, these mine tailings are not covered by vegetation caused by a poorly structured soil, being potential sources of heavy metal spreading through water infiltration or wind [32].
