1. Introduction
1.1 Soil pollution
“Soil pollution” is one of the most important issues, which our Planet faces nowadays. It means the presence in the soil of a chemical or substance out of place and/or present at a higher than normal concentration (more than the critical concentration) that has as a result harmful effects on any non-targeted organism. Although the source of pollutants have human origins, some contaminants can occur naturally in soils as components of minerals and as a result of weathering and can be toxic at high concentrations. Soil pollution often cannot be directly noticed or visually seen, making it a hidden danger for the environment and for humans. (Eugenio et al 2018).

With a widespread, rapid dissemination of contaminants like heavy metals in agricultural land, the stress on terrestrial ecosystems and societies has substantially increased and they can be the reason for unsolved environmental problems over the wide range (Alloway 2013; Grimm et al. 2008; Nejad et al. 2016).

The diversity of contaminants is constantly increasing due to agrochemical and industrial processes. This diversity, and the transformation of organic compounds in soils by biological activity(bio degradation) into diverse metabolites, make soil surveys to identify the contaminants both difficult and expensive. The effects of soil contamination also depend on both soil properties and the chemical behavior of the contaminants since these control the mobility, bioavailability, and residence time of contaminants in the soil. (FAO and ITPS, 2015).

Industrialization, wars, mining and intensification in agriculture have left a legacy of contaminated soils around the world (Bundschuh et al., 2012; DEA, 2010; EEA, 2014; Luo et al., 2009; SSR, 2010; Eugenio et al 2018).

Soil pollution is an alarming and a global topic. It has been classified as the third most important problem, which threaten the soil functions in Europe and Eurasia, fourth in North Africa, fifth in Asia, seventh in the Northwest Pacific, eighth in North America, and ninth in sub-Saharan Africa and Latin America (FAO and ITPS, 2015; Eugenio et al. 2018).

Soils consist of a mixture of weathered minerals (heavy metals) and varying amounts of organic compounds and can be contaminated as a result of (anthropogenic activities) such as spills or direct contact with contaminated waste streams such as airborne emissions, process solid waste, sludge or leachate from waste materials. And only when the concentration of the elements, which the soil contains, more than the critical concentration. (Chirila and Carazeanu 2008)

There are thousands of contaminant sources and pollutant types, but it can mainly divided into
1. Natural sources:
Several soil materials are natural sources of heavy metals and other elements, such as radionuclides, and these can pose a risk to the environment and human health at elevated concentrations. Arsenic (As) contamination is one of the major environmental problems around the world. Natural sources of As include volcanic releases (Albanese et al., 2007; Eugenio et al. 2018)
Natural events such as volcanic eruptions or forest fires can also cause natural pollution when many toxic elements are released into the environment. These toxic elements include dioxin-like compounds (acutely toxic) . It can also cause soil, water and air pollution (Deardorff, Karch and Holm, 2008; Eugenio et al. 2018)

2. Anthropogenic sources:
Centuries of anthropogenic activities have led to a widespread problem of soil pollution around the world. The main anthropogenic sources of soil pollution are the chemicals (such as pesticides) used in or produced as by?products of industrial activities, domestic and municipal wastes, including wastewater, agrochemicals, and petrol?derived products .These chemicals are released to the environment accidentally, for example from oil spills or leaching from landfills, or intentionally, as is the case with the use of fertilizers and pesticides, irrigation with untreated wastewater, or land application of sewage sludge. (Eugenio et al. 2018)

Industrial activities release pollutants to the atmosphere, water and soil. Gaseous pollutants and radionuclides are released to the atmosphere and can enter the soil directly through acid rain or atmospheric deposition.

1.2 Toxic elements
The soil contaminants can be classified in two categories: organic (oil products,
pesticides, dioxins, PCBs) and inorganic (heavy metals or fertilizers).
Organic contaminants include herbicides, pesticides, and plant and animal tissues, and are usually expected to cause harmful impacts on the environment. Trace levels of organic contaminant residues present in the soil, water, air, and sometimes food may result in harmful effects for human and environmental health (Kookana et al., 1998). They dissolve and accumulate in organic phases of animals and plants. The net enrichment of contaminants relative to that in the environment is described by the process of bioaccumulation.

Unlike organic contaminants, metal(loid)s do not undergo microbial or chemical degradation, and the total concentration of these metal(loid)s persist for a long time after their introduction in soils (Adriano et al. 2004; Bolan et al. 2014). Therefore heavy metals over the wide range cause very big problems for the environment, which will not be easily solved without an early remediation.

1.3 Heavy metals
The term “heavy metals” refers to the group of metals and metalloids of relatively high atomic mass (>5 g/cm3) such as Pb, Cd, Cu, Hg, Sn, and Zn, that can cause toxicity problems. Other non-metals that are often considered together with heavy metals include As, antimony (Sb) and selenium (Se). They named as metalloids (Kemp, 1998). These elements naturally occur at low concentrations in soils. Many of them cause no problem to the soil and living things at these low concentrations. They are also essential micronutrients for plants, animals and humans, but at high concentrations may cause phytotoxicity and harm human health because of their non?biodegradable nature, which causes them to readily accumulate in tissues and living organisms. (Eugenio et al. 2018)

Heavy metals are the most persistent and complex kind of pollutants to remediate in nature. And they are able to stay unchangeable in the soil for a long time. Some of them cannot be degraded by microorganisms (non?biodegradable). They not only degrade the quality of the atmosphere, water bodies, and food crops, but also threaten the health and well?being of animals and human beings. Metals accumulate in the tissues of living organisms because unlike most organic compounds they are not subject to metabolic breakdown. Among the heavy metals, Zn, Ni, Co, and Cu are relatively more toxic to plants, and As, Cd, Pb, Cr and Hg are relatively more toxic to higher animals (McBride, 1994; Eugenio et al. 2018).

Heavy metal(loid)s reach the soil environment through both pedogenic and anthropogenic processes.

1. Pedogenic
Most heavy metal(loid)s occur naturally in soil parent materials, chiefly in forms that are not readily bioavailable, that means they aren’t in an accessible form for plants uptake. The main source of them is weathering of soil parent materials including igneous and sedimentary rocks, and coal. They are the main reason for the groundwater pollution in China and Bangladesh. Volcanic and geological activities mobilize natural Hg from deep reservoirs in the earth to the atmosphere. Annual emission of Hg from global mercuriferous belts, the zone along plate tectonic including western North America, central Europe, and southern China was estimated up to 500 Mg/year (Bolan et al. 2014)

2. Anthropogenic
Anthropogenic/ human activities, primarily associated with industrial processes, manufacturing, the disposal of domestic and industrial waste materials, and the application of P fertilizers are the major source of metal(loid) enrichment/accumulation in soils. Atmospheric pollution from Pb-based petrol was a major issue in many countries where there was no constraint on the usage of leaded gasoline. While biosolids is the major source of metal(loid) inputs in Europe and North America, P fertilizers are considered to be the major source of heavy metal(loid) input, especially Cd, in Australia and New Zealand. Phosphate compounds contain a range of metal(loid)s .Cadmium contamination of agricultural soils is of particular concern because it reaches the food chain through regular use of Cd-containing P fertilizers (Loganathan et al. 2008; Bolan et al. 2014)

Accumulation of Cu in agricultural soils resulting from continuous use of Cu fungicides and biosolids application has been reported in many countries .One of the main consequences of excessive accumulation of Cu in soils is its toxicity to plants and microbial communities, for instance, formation of bare sterile patches in orchards ( Van zwieten et al. 2007; Bolan et al. 2014)

Unlike pedogenic inputs, heavy metal(loid)s added throw anthropogenic activities typically have a high bioavailability, thus they are in an accessible form for plants uptake and they can reach and accumulate very fast in the food chain of humans.

The heavy metals essentially become contaminants in the soil environments because (1) their rates of generation via man-made cycles are more rapid relative to natural ones, (2) they become transferred from mines to random environmental locations with high potential for direct exposure, (3) the concentrations of the metals in discarded products are high compared to those in the receiving environment, and (4) the chemical form (species) in which a metal is found in the receiving environmental system may render it more bioavailable (Amore et al. 2005; Nejad et al. 2016)

Figure 1 summarizes the frequency of metals occurring commonly in all matrices at Superfund sites. The most common metals found at contaminated sites are in the following order: lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), cadmium (Cd), copper (Cu), and mercury (Hg).( Nejad et al. 2016)