**2. Sources and origins of heavy metals**

One of the basic problems and challenges of recent years has been the gradual accumulation of chemical pollution in the environment [12]. The most important soil pollutants include heavy metals, chemical compounds resulting from acid rain and organic materials [13]. The pollution whose importance has been realized for many years is soil pollution with heavy metals [14].

Spatial changes of heavy metal contents in surface soil may be influenced by parent soil materials and human resources [15]. In other words, these metals exist naturally in the soil, but they are added to the soil as a result of human activities [16]. Heavy metals have cytotoxic, carcinogenic and mutagenic effects on humans and other living organisms [17]. Pollution caused by heavy metals can spread in the air, water or soil. Among the aforementioned pollutions, more attention has been directed towards air and water pollution, and soil pollution has been neglected [18].

The rapid development of the industry and the increase in the release of chemicals used in industries and agriculture into the environment have led to increased concerns about the potential for the accumulation of heavy metals in the soils of big cities [19]. Heavy metal contamination in soils may lead to irregularity in the soil structure, negative impact on the growth of plants and animal ecosystems, and even damage to human health by entering the food chain [20].

Soil plays an important role in the cycle of mineral and organic elements and as a dynamic ecosystem provides the life of small and large living organisms, therefore monitoring and evaluating its pollution is of immense importance [21]. Heavy metals can have fatal effects on humans and animals that use contaminated plants

in the area. Therefore, determining the amount of these metals in soil environments has attracted the attention of many researchers [22].

Various industries such as metal mines to computers and electronics, chemical fertilizer production factories, dyeing, textile, weapons production and thermal power plants, oil and petrochemical industries, steel and pipe making industries, hospitals and livestock and poultry slaughterhouses cause pollution. The results of heavy metals play a role [23]. Heavy metals in the soil, such as lead, zinc, copper and cadmium, can originate from car parts, tire friction, grease and oily substances, the output of industrial factories (smoke) and furnaces [24]. Metallic elements enter the soil through the use of chemical fertilizers, fungicides, industries and sewage sludge [25]. In other words, human activities, especially agricultural effluents, industrial wastewaters, and pollution from the transportation industry, cause a significant amount of heavy metals to enter the surrounding environment [26].

Health risk assessment of surface soil in urban and industrial areas is widely used to quantify the carcinogenic and non-carcinogenic risks of potentially toxic elements for humans through three routes: ingestion, skin contact, and inhalation [27].

### **3. Heavy metal toxicity**

Heavy metals in the soil can enter the human body directly through swallowing and breathing [28] or enter the body through water and food after contamination of water and soil sources and entering the structure of plants [29]. The harmful effects of heavy metals on human health have been proven from various aspects, and exposure to these pollutants causes acute and chronic poisoning, as well as various diseases, including nervous disorders, hormone imbalances, and respiratory disorders and heart disease, memory loss, various types of cancer and eventually death [30, 31]. Arsenic, lead and cadmium are toxic elements that do not play a role in biological interactions in the human body and cause hemoglobin biosynthesis disorders and anemia, increased blood pressure, kidney damage, miscarriage and premature birth, nervous system disorders, and brain damage, male infertility, reduced learning ability and behavioral disorders in children [32]. Vanadium and nickel are heavy metals that have irreparable effects on humans. Vanadium causes respiratory abnormalities and has negative effects on the liver and kidneys, and increasing the accumulation of nickel in the body can cause lung, nose, larynx, prostate cancer, reduced reproductive ability, watery lungs, itching and skin problems [33]. Zinc and copper metals play a role in biological processes based on their amounts [18] and are among the essential elements in biological reactions, but the increase in biological accumulation in body tissues causes the high toxicity of these two metals [34].

Studies show that human industrial and agricultural activities are the only cause of soil pollution with toxic and dangerous compounds by heavy metals and other pollutants [35, 36]. In Iran, in many areas, the creation of petrochemical industries, the construction of refineries and the drilling of oil and gas extraction wells, steel industries, food and packaging industries, and agriculture have caused an increase in soil pollution in these areas. Factors such as improper disposal of wastes and wastes in industrial centers, pollutant spreading by refineries and power plants, pollutant leakage from underground oil tanks and gas stations, accidents of tankers and tankers contribute to this problem [33]. The adverse effects of heavy metals in soil and plants are determined when their concentration increases beyond a certain amount, and this increase depends on the type of metal, the type of soil, various human activities,

and the time of metal accumulation [37]. Research has shown that heavy metals are effective in reducing animal and plant populations [38]. In general, the accumulation of heavy metals in surface soils is more than in deep soils, which indicates the recent pollution in the region and indicates the impact of environmental pollutants such as industrial, urban activities and especially motor vehicles on the soils of the region [39].

Heavy metals are one of the environmental pollutants whose amounts have been widely and differently reported [33] and the toxicity of heavy metals in humans, animals and plants has been investigated and proven in numerous studies [34]. Also, in other reports, researchers found that heavy metals can be transferred to sediments in water, which increases the possibility of environmental risk and poisoning of aquatic organisms [35, 36]. A very small amount of some heavy metals such as copper, zinc, iron and magnesium are necessary for all living organisms, but a large amount of them can cause fatal poisoning of living organisms [37]. Some plant species are able to absorb a large amount of heavy metals from soil and water, as a result, by consuming contaminated plants, heavy metals enter the human body [38]. As much as possible, the body's immune system tries to remove heavy metals through sweat, urine and feces, but some types of these metals are deposited in the tissues before the body can remove them and the effects They leave their mark [39, 40]. In general, neurological disorders (Parkinson's, Alzheimer's, depression, schizophrenia), various types of cancer, nutrient deficiency, hormonal imbalance, obesity, abortion, respiratory and cardiovascular disorders, damage to the liver, kidneys and brain. Allergy and asthma, endocrine disorders, chronic viral infections, lowering the body's tolerance threshold, dysfunction of enzymes, changes in metabolism, infertility, anemia, fatigue, nausea and vomiting, headache and dizziness, irritability, weakening of the body's immune system, destruction Genes, premature aging, skin disorders, loss of memory, anorexia, joint inflammation, hair loss, osteoporosis and in severe cases death are the results of the effects of heavy metals entering the human body. On the other hand, the accumulation property of heavy metals in plants and their entry into the food chain doubles the risks caused by them. With the growth of industry and the increase in the consumption of chemicals, their entry into water, soil and air and the pollution of the environment, the possibility of humans facing the dangers caused by them has increased [41, 42].

### **4. Soil heavy metals and effects of corps**

Increasing the concentration of heavy metals is very dangerous because heavy metals are toxic, stable and non-degradable [43]. Physical and chemical processes (saturation and oxidation) can release the heavy metals accumulated in the soil, which means that the metals can be transferred to the surrounding waters and be consumed by crops and as a result from water supply and food chain to affect public health [44]. As a result of various human activities such as agriculture, mines, industries, the passage of vehicles and urban ecology, the soil becomes polluted [45]. Some of these pollutions are caused by the exploration and extraction of petroleum materials from the ground. Accidents involving vehicles that transport pollutants are another example of soil contamination by human activities. Other polluters that cause soil pollution include cars, trucks, and airplanes that carry materials such as fuel, and soil pollution occurs due to their spilling and exiting the vehicle. Spilling toxic substances such as solvents, dyes and detergents spread the pollution of the earth and soil [21]. Also, heavy metals can enter the soil through chemical processes of minerals,

### *Heavy Metals in Surface Soils and Crops DOI: http://dx.doi.org/10.5772/intechopen.108824*

mother rocks and natural activities. Also, through other ways such as urban sewage, agricultural runoff is also absorbed into the soil [46, 47]. It has been reported in many researches that heavy metals can also affect the soils of wetland areas and when heavy metals enter the soil environment, the self-remediation function of the soil alone in the short term cannot effectively prevent its impact. Reduce the impact on the environment [48–52]. High concentration of heavy metals in soil can be a threat to human health and wildlife and should be investigated in agricultural soil [53, 54].

Microbial activity and soil enzyme activity can reflect soil quality sensitively [55]. Aceves et al. (1999) believed that soil microbial biomass is an important indicator in determining the degree of soil contamination. Microbial activity is significantly inhibited in soil contaminated with heavy metals [56]. Kandeler et al. (1997) showed that microbial biomass is strongly inhibited in soil contaminated with Cu, Zn, Pb and other heavy metals. The soil microbial biomass near the mine was significantly lower than the mine, and the effect of different concentrations of different heavy metals on the soil microbial biomass was different [57]. Chander et al. (1995) by studying the effect of different concentrations of heavy metals on soil microbial biomass, found that only if the concentration of heavy metals in the soil is three times higher than the EU environmental standard, it can inhibit the microbial biomass [58]. Fliebbach et al. (1994) found that low concentrations of heavy metals can stimulate microbial growth and increase microbial biomass, while high concentrations can significantly reduce soil microbial biomass. In addition, the enzymes in the soil play an important role in the decomposition of organic matter and the cycle of nutrients. Studies have shown that the activity of enzymes in soil is related to heavy metal contamination [59]. Chander et al. (1995) found that with the increase in the concentration of heavy metals, the activity of enzymes in the soil decreases significantly and approximately 10 to 50 times [58].

A low concentration of heavy metals in the soil, regardless of whether they are essential or unnecessary for plants, will not affect the growth of plants in a certain range, but if the concentration is too high, the content of heavy metals enriched by the plant will exceed its tolerance threshold. And so the plant is poisoned and even leads to the death of the plant. In a research, it was found that if the amount of copper in the soil is more than 50 mg/kg, it affects citrus seedlings. If the amount of soil copper reaches 200 mg/kg, wheat wilts [60]. The research showed that the growth of cabbage plant and bean plant is inhibited under the concentration of 30 micromol/ liter of cadmium metal, root length is reduced and plant height and leaf area are reduced [61]. Cadmium may disturb the photosynthesis of crops and protein synthesis and cause membrane damage etc. [62, 63]. However, through a series of environmental effects, nature returns bad results to humans. The most important issue is to control the excessive discharge of pollution sources from the main stream, and source monitoring should correct the technical path. The ultimate goal of preventing soil erosion and pollution is sustainable human development, while heavy metal pollution seriously threatens human health. Its biological effectiveness is not only an important basis for evaluating soil pollution status, but also a theoretical basis for pollution control and a key parameter for evaluating its treatment results. It is of great importance in guiding the production of agricultural products and protecting the environment. By analyzing the sources of heavy metals in soil, it is possible to address the main pollution process and then find a logical way to intervene as soon as possible from the source to reduce the toxicity damage to the soil. Heavy metal pollution is a complex process involving metal elements that changes over time and space with the interaction of biological system forces and environmental system forces. It is necessary to

systematically study the dynamic process of interaction between the environmental system and the biological system contaminated with heavy metals in order to increase the value of its application and the importance of its guidance in environmental governance. At present, there are still some deficiencies in the methods of analyzing the source of heavy metal pollution in soil, so it is necessary to conduct more systematic and comprehensive research in the next stage. It can also be combined with various analysis methods to make the heavy metal pollution source analysis method more complete, comprehensive and scientific. In the future, according to the actual situation of the contaminated soil, along with environmental factors such as crop planting, biological community, and weather conditions in the contaminated area, it is possible to select a targeted and suitable extracting for investigation. The chemical form of heavy metals in the soil, the bioavailability data of heavy metals in the human body can be obtained by using it in the simulated human absorption test. It can make the assessment of pollution and heavy metal remediation effect more objective, fast, accurate and efficient [64].

Soil contamination with heavy metals has become a serious environmental problem in many parts of the world [2, 65]. The most important soil pollutants include heavy metals, acid precipitation, and organic substances, among them, heavy metals have been receiving a lot of attention in recent years due to their polluting characteristics in the soil [66]. Spatial changes of heavy metal contents in surface soil may be influenced by parent soil materials and human resources. In other words, these metals exist naturally in the soil, but they are added to the soil as a result of human activities. In fact, human activities may lead to more accumulation of heavy metals in the soil [67]. The background concentration of elements in the soil depends on the mineralogical composition of the parent material and weathering processes affecting the formation of the soil, as well as characteristics such as particle size, amounts of clay and soil organic matter [68]. As a result, the natural concentration of elements in soils is widely variable, and the use of the background levels of other countries and the global average to identify the extent and risks of heavy metal pollution in the soils of areas where environmental limits have not been defined is incorrect [69]. Therefore, although the natural background concentrations of heavy metals in the soil have been studied in many countries such as Poland and many other European countries, the United States and China, and the basis for understanding the natural changes of elements and identifying soil pollution has been laid. It is also necessary to estimate background concentrations in Iran [42].

Heavy metals are naturally present in the parent materials of the soil, but the main human sources of metals in the soil and environment are mining and smelting of metals, agricultural activities, sewage sludge and combustion of fossil fuels, factories and metal industries, waste disposal, use and disposal of metal and electrical materials, electricity production, chemical production industries [70, 71]. There are two sources for soil heavy metal contamination; Natural resources and human resources. Natural resources include the entry of heavy metals through the erosion of parent soil materials and are therefore related to the geology of the region. Iron and steel industries, mining, road transportation, waste burning, and especially the use of fertilizers and chemicals in agriculture are very important human sources of heavy metals entering the soil and water in surface ecosystems [72, 73]. The use of fertilizers and chemicals in agricultural lands leads to an increase in the concentration of heavy metals such as chromium, cobalt, chromium, cadmium, lead, copper and zinc and causes an increase in the concentration of heavy metals in these areas [74, 75]. Considering the concern about heavy metals as environmental pollutants and also because of their stability in

### *Heavy Metals in Surface Soils and Crops DOI: http://dx.doi.org/10.5772/intechopen.108824*

the ecosystem, biological monitoring can be a suitable method to measure the concentration of these metals and their bioavailability. For example, large fish and birds are useful indicators for measuring heavy metal pollution due to being at the high levels of food chains in an ecosystem and also their sensitivity to toxic substances. Bioaccumulation refers to the ability to accumulate a chemical substance from the surrounding environment, which occurs directly from water, soil and sediments by different organs or indirectly through eaten food [76].
