**1. Introduction**

Soil is the uppermost layer of Earth's crust, which is produced at the rate of a few centimeters per thousand years by the continuous transformation of solid crust material. According to FAO, the soil consists of mineral particles, organic matter, water, air, and living organisms [1]. It is one of the most essential, complex, and non-renewable natural resources. It provides humanity with a wide range of ecological, economical, and cultural services. These include provisional services: food, fiber, raw materials; regulating services: mitigation against flood, drought, carbon storage, support hydrological and nutrient cycle, recycling of wastes; cultural services: recreational, esthetic, heritage values, and cultural identity [2]. According to McBratney, 2017 soil provides around US\$ 11.4 trillion of ecosystem services [3].

Soil conditions underpin food security, habitat for various organisms, bioeconomies, and above-ground biodiversity. It is the major variable in regulating the climate, hydrological, and nutrient cycles. However, anthropogenic activities including industrialization and urbanization have polluted the environment extremely and deteriorating the quality of life for all living organisms. There is enormous pressure on this finite, non-renewable natural resource. Further, inappropriate land-use management severely impacts the functions of soil, which is amplified by climate change. These stresses lead to degradation processes of soil like erosion, contamination, and degradation [4].

#### **1.1 Soil pollution**

In the era of the Anthropocene, the imprudent discharge of waste, and chemicals in the ecosystem has led to the increase of concentration of contaminants to critical levels. According to FAO. "Soil pollution" refers to the presence of a chemical or substance out of place and/or present at a higher than the normal concentration that has adverse effects on any non-targeted organism [1]. Although there is the contribution of contaminants through natural sources like, volcanic, seepage from parental rock, biogenic, and forest emissions, the widespread soil contamination and degradation are caused by anthropogenic activities. The rapid and injudicious industrialization, intensive agricultural practices, faulty mining practices and waste disposals are the major causes of heavy metal contamination of soil.

The pollutants introduced in soil by anthropogenic activities can arise from a plethora of sources. These might be discrete point sources or diffuse sources. The emission of heavy metals from point sources includes thermal power plants, coal mines, gold mines, smelting, electroplating, textiles, leather, and e-waste processing; and non-point sources include soil erosion, agricultural run-off, vehicular emissions, ash fallout, combustion of fuel, acid deposition, mining tailings, heavy metal mining and smelting, mismanaged radionuclides waste, and open freight storage (**Figure 1**).

One of the major concerns is the contamination of heavy metals in agricultural soil. It has increased tremendously in the soil system since the last decade. Although most of the heavy metals exist geologically, the emission of them in the ecosystem through anthropogenic sources like increased chemical discharge through the indiscriminate usage of pesticides and fertilizers into the agricultural soil has led to the accumulation of heavy metals concentration to dangerous levels. As soil holds the largest terrestrial pool for carbon, thus degrading soil will only worsen the phenomenon of climate change. The conditions of soil also underpin various Sustainable Development Goals (SDGs) set by the United Nations (**Figure 2**).

In view of these facts, strategies for remediation of contaminated soil must be implemented. Various remediation techniques have been developed to solve or minimize the influences of contamination. These technologies include physical, chemical, and biological methods.

#### **1.2 Heavy metals**

Heavy metals and metalloids are generally referred to as a group of elements that have densities >5 g cm−3. These include lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), mercury (Hg), and nickel (Ni). They are naturally occurring elements, whose natural concentration in the soil ecosystem is primarily dependent on parent rock material [5]. Some heavy metals, like Zn, Cu, Fe, Ni, Mn, Mo, and Cr are essential for the functioning of structural and biochemical processes in living organisms and are required in trace concentrations, hence called micronutrients. They can cause harmful effects to plants if absorbed in higher concentrations. While non-essential heavy metals, including Pb, Hg, As have unknown biological functions but are used for various processes in modern industrial applications. The non-essential heavy metals are toxic to plants even at low concentrations [6]. However, the emission rate of pollutants through anthropogenic sources has increased the concentration of heavy metals in soil to hazardous amounts.

Heavy metals speciation plays an important role in their long-lasting presence in the environment, as mobile forms are easily leachable thus making them to spread ubiquitously in different media, and the bioavailable heavy metals are easily *Conventional and Contemporary Techniques for Removal of Heavy Metals from Soil DOI: http://dx.doi.org/10.5772/intechopen.98569*

#### **Figure 1.**

*Various anthropogenic sources of soil pollution.*

#### **Figure 2.** *The negative impacts of soil pollution on SDGs.*

absorbed by living organisms. They are non-biodegradable and non-thermodegradable so their accumulation in living organisms can cause biomagnification of heavy metals, that is they can affect organisms throughout all levels of the food chain. Particularly humans, as they are at the top of the food chain. The physicochemical properties of soil, like pH, cation exchange capacity and soil texture, also play a key role in the accumulation and availability of heavy metals [7]. Once heavy metals are exposed to humans, via inhalation, ingestion, or absorbed through the skin, they can accumulate in vital organs such as the kidney, brain, liver where they can be a threat to the health of humans [8]. Heavy metal contamination in agricultural soils may cause disturbance in the structure of soil, interfere with plant growth, and be harmful for human health via entering the food chain [9], posing health problems for all living organisms [10]. Furthermore, degradation of agricultural soil will impact crop yield and will put the most vulnerable people at higher risk of economic loss and malnutrition [4].

Soil Pollution by heavy metals is now a global concern. Europe has been found with 2.8 million sites that are potentially contaminated with heavy metal soil pollution, in China 19% of agricultural soil contain harmful pollutants exceeding the standards of environmental quality [11]. In India, heavy metal pollution in soil cover is approximately 80% by anthropogenic origin in the states of Maharashtra, Gujarat, and Telangana [12]. Therefore, the studies on agricultural soils which are contaminated with heavy metals are of much concern, especially due to two reasons. Firstly, ingestion is the main source of heavy metal exposure to humans and the agricultural food chain is the primary source of various food products for humans [8]. Secondly, densely accumulated heavy metals in agricultural soil can percolate through pore spaces and enter groundwater systems, consequently deteriorating the groundwater quality [13].
