Heavy Metals in Surface Soils and Crops

Abstract

In the era of industrialization and technological progress, pollution has reduced the quality of life for humans. Heavy metal pollution is one of the main causes of environmental degradation. The underlying causes are natural as well as human. Heavy metal contamination of soil has become a worldwide environmental issue that has attracted considerable public attention, mainly due to increased concern for the safety of agricultural products. Heavy metals refer to some metals and metals with biological toxicity such as cadmium, mercury, arsenic, lead and chromium. These elements enter the soil agricultural ecosystem through natural processes resulting from raw materials and through human activities. Heavy metal pollution is a great threat to the health and well-being of animals and humans due to the risk of potential accumulation through the food chain. The main sources of heavy metal pollution are air pollution, river sediments, sewage sludge and municipal waste compost, agricultural chemicals such as fertilizers and pesticides, and industrial wastes such as factories that release chemicals. Heavy metals can enter the water supply through industrial and consumer wastes or even from acid rain that decomposes soils and releases heavy metals into streams, lakes, rivers and groundwater.

1. Introduction

The progress of industries and the growth of urban communities have caused an increase in man-made pollution caused by industrial and agricultural activities and many pollutants enter the environment [1]. One of the most important pollutants in the environment is soil pollution [2]. Soil pollution includes the entry of physical, biological, and chemical substances into this environment, which will eventually enter the life cycle of animals, plants, and as a result, humans, and will cause negative effects in the life of living organisms [3]. One of the effects of soil pollution is the reduction of plant growth and development, which causes the loss of vegetation and ultimately leads to soil erosion and desertification [4]. Most of the pollution created in the soil is caused by the discharge or leakage of organic substances. Petroleum substances and their derivatives cause soil pollution as a result of transportation or storage, while the deeper the petroleum substances penetrate into the soil, the more difficult it is to remove the pollution and the cost will be several times higher [5].

Soil pollution is very dangerous and due to the fact that this type of pollution, like air and water pollution is not directly related to human life, less attention has been paid to it. Soil is one of the valuable resources of nature, which provides about 96% of the food needed by humans [6]. Healthy and clean soil is very necessary and important for life on earth. Day by day, soil ecosystems become a place for harmful substances, scum, waste, and receiving harmful substances, and more than the weather, their pollution burden increases, and on the other hand, more and more Due to the construction of buildings, roads and urban and industrial facilities, a large amount of soil is taken out of the natural circulation and also from the agricultural area and becomes dead soil. Therefore, proper management to have a healthy soil is necessary for human survival [7]. Soils have a special advantage called self-purification and they are considered to be nature’s purifiers, but the self-purification power of soil is less than the self-purification power of water and air due to its low exchange with other regions and areas, for this reason soil pollution, It is considered one of the most important types of environmental pollution [8].

Heavy metals are a group of metals and quasi-metals, whose amounts and concentrations are toxic and dangerous. Mercury, lead, cadmium and arsenic can be mentioned from the group of toxic metals [9]. Because heavy metals enter the soil through anthropogenic activities or exist naturally in the soil texture, they can easily and very quickly cause soil pollution [10]. In addition to directly affecting the physical and chemical properties of the soil, reducing biological activity and reducing the bioavailability of soil nutrients, heavy metal pollution is also a serious threat to human health through entering the food chain and environmental security through penetration into They are considered underground waters [11].

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, 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, 49, 50, 51, 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 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].

5. Lead (Pb)

Lead is one of the most common and useful metals known to humans and can be detected in all environments and biological systems. The amount of lead in the environment has increased more than 1000 times during the last three centuries, which is the result of human activities, and between 1950 and 2000, the greatest increase in lead levels was observed [77, 78]. Lead is from the fourth group of the periodic table and has an atomic weight of 207.2. Lead is a gray heavy metal and is usually combined with two or more other elements [79]. Lead reaches aquatic ecosystems due to surface soil erosion and atmospheric sediments. The concentration of lead in the deep oceans is about 0.01–0.02 g/L, but it is about 0.3 g/L in the surface waters of the oceans [80, 81].

In general, lead is absorbed from food and air. Workers in smelting, casting and steel industries, battery manufacturing, plastic factories and printing industries are exposed to lead [82]. Lead is the most abundant and widely used heavy metal and its dispersion in the environment is wider [83]. It is easy to work with lead due to its low melting point, it can be easily made into various shapes. Due to the applications that have caused the uncontrollable dispersion of this element, its environmental concentration is increasing in most countries. Lead metal in car battery, ceramics, inside cans, cigarette ash, car exhaust fumes, leaded gasoline, hair dyes, insecticides, mascara, snow, soldering alloy, putties, paints, city water and or there are wells, alcoholic drinks, pipes [76].

Children are sensitive to the effects of lead, which is considered a primary environmental hazard. Metal poisoning in children causes sensitivity in the developing nervous system, which is due to the sensitivity to lead metal toxicity [84, 85]. Lead is classified in group 2B of IARC carcinogenic compounds, and its toxic effects in the body include the occurrence of disorders in four places, i.e. digestive system, central nervous system, peripheral nerves and hematopoietic system [86]. Lead may enter the human body through the intestine. It is also absorbed through the lungs, inhalation and skin or by direct ingestion and drinking [87]. Lead accumulates in high concentrations in bones, teeth, liver, lungs, kidneys, brain and spleen and passes through the blood–brain barrier and the fetal placenta. The symptoms of diseases caused by lead are completely different and unrecognizable in the first place. In the early stages, impatience, anorexia and lack of movement can be mentioned. Weight loss and blood loss are other symptoms of lead poisoning. Among women, monthly periods do not occur or are postponed. General changes also occur in the form of blood. Red blood cells change shape. As a result of lead deposition in the mouth, gums and teeth turn gray. This gray color can remain for some time even after the poisoning is removed [85, 88]. The biological half-life of lead may be much longer in children than in adults. Lead has a half-life of 35 days in blood, 40 days in soft tissue, and 20–30 years in bones [89]. The main route of excretion of absorbed lead is the urinary tract, which is usually done by glomerular filtration in the kidney. It can also be excreted through the digestive system through bile [90].

6. Zinc (Zn)

Zinc metal has been used for centuries due to its low boiling point. Like other metals, zinc reacts slowly. Zinc is a blue-white or silvery metal found in the earth’s crust. The abundance of zinc in nature changes depending on the place and season [91]. It combines with oxygen and other non-metals and reacts with dilute acid and releases hydrogen gas. Zinc is the fourth most common and used metal, after iron, aluminum and copper, it is the most produced metal. Features such as corrosion resistance, reactivity with iron and electrochemical properties of zinc metal have led to its use as a suitable coating against corrosion or galvanization. Galvanized steel is used in construction, power, construction of urban amenities (such as benches and tables), agriculture and transportation. This metal is used in the production of various alloys such as brass, aluminum alloys, and magnesium alloys, which are used in the construction industry and electric cars [92].

Also, this metal can enter the environment through textile and cotton, battery, rubber, paint, cosmetic, fertilizer and medical industries. In addition to metal smelting industries, the impact of acid rain on construction materials containing zinc are the main sources of this element entering the environment [76].

Zinc is one of the important components of some important biomolecules in the human body. There are more than three hundred important enzymes in the body, zinc is one of these enzymes [93]. These enzymes play an important role in maintaining body function and natural health, and some of these enzymes also play a role in the process of gene expression [94]. Zinc plays a role in regulating the synthesis of important biological molecules such as blood sugar balance, insulin hormone, glucose transport, body metabolism and its availability in the physiological system. The zinc element plays a very vital role in enzymes, so that if zinc is removed from their composition, the activity of the enzyme in question stops [95, 96].

Zinc is one of the rare elements of the body, poisoning with it leaves many effects in different organs of the body. Its acute toxic effect, which manifests itself in the form of fever, has been fully proven, but regarding the complications of chronic contact with this metal, various studies have put forward many opinions [97]. Some of the adverse effects of zinc accumulation in the body are: poisoning, fever, confusion, nausea, vomiting and diarrhea after consuming acidic drinks or foods that are prepared and stored in galvanized containers. Zinc is considered a low-risk element, but its toxicity increases in large quantities in the presence of arsenic, lead, cadmium, and antimony. Fever caused by zinc poisoning has symptoms of cold, fever and nausea. Zinc chloride vapor causes the lungs to dry [91].

7. Copper (Cu)

Copper is the first known element that is relatively red in color and has high electrical and thermal conductivity. Copper is one of the most widely used metals in industry. Copper in industries such as electronics (in wires, cathode ray lamps, in IC, vacuum lamps, switches and electronic amplifiers), military industries (production of weapons), metal industries (production of alloys and preparation of coins), tools Kitchen, water purification, is used as a reagent in chemistry and preparation of agricultural pesticides [98, 99]. Copper is one of the common elements in nature, which is found in abundance in the environment due to natural phenomena [100]. Many copper compounds are deposited in sediments or soil particles or stick to these particles. Soluble compounds of copper may be harmful to human health. Usually copper is released in the environment as water-soluble compounds after agricultural activities. Copper is generally found near mines, industrial sites and waste disposal sites. Copper does not decompose in the environment, and for this reason, when it is in the soil, it accumulates in plants and animals [101]. In copper-rich soils, a limited number of plants have a chance of survival. This is the reason why there is not much vegetation near the copper factories. Due to the effects of copper on plants, depending on the acidity of the soil and the amount of organic matter, this element is considered a serious threat to farms. When fields are contaminated with copper, animals absorb higher concentrations of copper, which harms their health [92, 102].

Copper is abundant in the human body and is important for several physiological functions. Copper is actually a mineral that is essential and important for maintaining natural health and for survival. Liver, brain, heart, kidneys and skeleton are important organs where copper is stored. Copper plays an important role in increasing the absorption of other metals such as iron. In addition, copper is also present in the collagen of the supporting tissue. Copper acts as a cofactor for several enzymes and thus are involved in the regulation of various physiological processes. Copper is part of hair and elastic tissues. This element is present in several important substances containing structural and functional proteins in the body [103]. Also, studies show that plants and animals need sufficient amounts of copper for normal growth and maintaining their health. The appropriateness of yield of crops and seeds is also related to copper concentration. Therefore, copper is an important mineral for producers (green plants) and therefore is very important for maintaining the food chain [96].

It is obvious that the element copper is one of the essential elements of the body of living organisms, but it should be noted that large amounts of copper can cause acute effects such as discomfort in the digestive system, damage to the circulatory system such as the liver and kidney systems, and anemia.. The most well-known metabolic disorder resulting from the accumulation of copper in the body is Wilson’s disease, in which the blood ceruloplasmin concentration is severely reduced [76].

8. Arsenic (As)

Arsenic is one of the natural elements and the source of global pollution that is found in rocks, soil, water, air and food [104]. Arsenic has a complex chemical structure and can be found in common inorganic forms, arsenite, arsenate, and ternary organic forms. Organic arsenic combines with carbon and hydrogen ions and forms. Organic arsenic compounds are found in fish and shellfish [105]. Inorganic arsenic found in soil and water has been classified by the US Environmental Protection Agency as a human carcinogenic pollutant [106]. High doses of organic arsenic can produce the same toxic effects as a lower amount of inorganic arsenic [107]. Mineral arsenic is present in some man-made resource industries, including waste of industrial products, coal, copper, lead and glass products [85]. The use of arsenic compounds as herbicides, pesticides and fungicides is another source of arsenic pollution [79].

Absorption of arsenic through inhalation is strongly dependent on the solubility and size of the particles that enter the respiratory system, and arsenic is well absorbed from the digestive system, in other words, soluble arsenic compounds can be absorbed from the digestive system [108]. The biological half-life of absorbed inorganic arsenic is about 10 hours and about 50–80% is excreted in about 3 days, while the half-life of methyl arsenic is 30 hours [85, 88]. Absorbed arsenic can cross the placenta and lead to cord blood concentrations similar to maternal blood concentrations [109]. The mechanism of arsenic toxicity involves a number of sulfhydryl-containing proteins and enzyme systems that change with arsenic exposure [110]. The specialized food committee determined the acceptable weekly intake for inorganic arsenic at 0.015 mg/kg [111]. It seems that consumption of organic arsenic at about 0.05 mg/kg of body weight per day causes dangerous effects for humans [112]. This element has no role in biological interactions in the human body and affects the cardiovascular system and skin, central and peripheral nervous system, kidneys and hematopoietic system of the body and causes carcinogenesis. The health effects of ingested inorganic arsenic include skin cancer, internal cancer, and non-cancerous effects on the skin, vascular system, digestive system, and liver. In general, soluble inorganic arsenic compounds are more toxic than organic types, and trivalent forms of arsenic are more toxic than pentavalent forms of arsenic, and various body systems and organs, including the skin, respiratory, cardiovascular, immune, genetic, and reproductive systems., digestive and nervous systems are affected by this substance and combined side effects are observed.

9. Chromium (Cr)

Chromium is one of the elements of the periodic table with the symbol Cr and atomic number 24. Chrome or Chromium is a hard, shiny metallic gray metal with high polishability and high boiling point and remarkable resistance to rust and tarnishing. Chromium oxide was used to coat metal weapons in the Chinese Empire for more than 2000 years. Chromium was discovered as an element in 1761 and was first used as a pigment [113]. In 1797, chrome metal was separated from its ore for the first time. Since then, almost all the chromium in the world is obtained from the ore chromite. The value of this metal is mostly due to its high resistance to rust and erosion, especially when it was discovered that adding chromium to steel has a significant effect in preventing corrosion and tarnishing of steel. Today, about 85% of the world’s chrome consumption is used to make stainless steel, of which at least 10% of its volume is chrome, and chrome plating is also used [91]. Chromium metal is used in metallurgy to resist corrosion and in final polishing, as a component in alloys, for example in stainless steel, in chrome plating, in anodized aluminum, as a catalyst. Chromite is used to make molds for baking bricks. Chromium salts cause the glass to turn green, and chromates and oxides are used in hair color and ordinary colors [114, 115]. Chromium is used to harden steel and this element is used to make stainless steel products, these compounds have useful applications. This element is used for covering hard surfaces and for decorating and preventing corrosion and rusting. Chrome is glass-shaped and emerald green in color and widely used [76].

Surveys show that among heavy metals in the past, chromium was less important in agriculture and environment. While it can have severe destructive effects on plants and the environment, soil and water contamination with chromium is one of the major environmental concerns in recent decades [116]. Chromium has harmful effects on plant physiological processes such as photosynthesis, water relations and mineral nutrition, germination, growth and development of roots and leaves. The metabolic changes made by chromium in plants are either directly on enzymes and plant metabolites or through the creation of active oxygen species that cause oxidative stress [117, 118].

Chromium metal and chromium III compounds are usually not hazardous to health, but chromium VI compounds are toxic if swallowed. The amount of almost half a teaspoon of toxic chromium VI compounds is lethal, and it has been proven that non-lethal amounts of chromium VI are carcinogenic. Most chromium VI compounds are harmful to eyes, skin and mucous tissues. Permanent contact with these compounds can cause permanent damage to the eyes, except for cases where complete treatment is done [119, 120]. In 1958, the World Health Organization suggested the maximum allowable consumption of chromium VI from the health aspect of 0.05 mg per liter of drinking water. This proposal was reviewed many times and the announced amount did not change during this time [121, 122].

The amount of chromium in drinking water is very low, but it is possible that contaminated water contains some chromium IV and chromium VI, which are considered dangerous types of chromium. If the amount of chromium III consumed by humans is higher than usual, it affects human health and, for example, causes skin itching. Chromium IV has many effects on human health. There is usually chrome in leather products. This combination causes severe allergy such as skin itching in people. By breathing chromium IV, the nose is stimulated and nosebleeds occur. Other diseases caused by Chromium IV include skin itching, stomach ulcers, respiratory system problems, weakening of the body’s immune system, kidney and liver damage, genetic material changes, lung cancer and death.

10. Cadmium (Cd)

Cadmium is an industrial and environmental pollutant that affects a number of human body organs. Cadmium is a group IIB metal with an atomic weight of 112.41 [79, 105]. In general, exposure to cadmium occurs mainly through two sources. The first food route is through water and food contaminated with cadmium, especially vegetable leaves, seeds, grains, fruits and fish [123]. The second source is through the inhalation of cadmium particles in industrial or daily activities, among which the inhalation of cigarette smoke is considered as a very dangerous source, because cadmium is easily absorbed by the lungs [124, 125, 126].

Forest fires and volcanoes, human activities such as industrial waste leachates, production of synthetic phosphate fertilizers are important sources of cadmium emissions. The main use of this element is as a stabilizer and pigment in plastic and electrolysis industries, but its main part is used in soldering and as an alloy in nickel and cadmium batteries.. This element is used in the industry as an anti-friction, catalyst, anti-rust agent or in the composition of alloys. Cadmium is also used in rod protection semiconductors in nuclear reactors, metal plating, ceramic making, PVC factories and plastic industries, battery production, fungicide compounds, engine oil, rubber making and photography [76].

The amount of cadmium absorption in foods is caused by the way animals are fed, kidneys and liver are suitable places for cadmium accumulation. Sea shells also have a high concentration of cadmium. Absorption of cadmium through the skin is very limited [21]. The biological half-life of cadmium in humans, in soft tissues and bones, is 10 to 30 years. The speed of cadmium methylation is much lower compared to mercury, arsenic, and lead, and only two bacteria named Pseudomonas sp. and Staphylococcus aureus are able to methylate cadmium in water environments [121]. A serious disease caused by it in humans is a disease called itai itai (rheumatism disease or painful skeletal deformity). The main effects of cadmium toxicity are on the lungs, kidneys, and bones. Acute effects caused by its inhalation include bronchitis, pneumonia and liver poisoning. Chronic inhalation of cadmium compounds, in the form of vapors or dust, causes pulmonary edema, in which the small air sacs enlarge and are eventually destroyed due to the reduced lung volume. Both chronic inhalation and absorption of cadmium through the mouth affect kidney secretions, which is the first stage of protein excretion by the proximal tubules of the kidney. Acute poisoning with cadmium may cause the death of animals and birds and cause severe poisoning in aquatic animals. Absorption of cadmium from the lungs is more effective than the intestine, and 50% of the cadmium inhaled through cigarette smoke may be absorbed. On average, the concentration of cadmium in the blood of smokers is 4–5 times and in the kidneys 2–3 times more than that of non-smokers. It seems that cadmium reduces the body’s defense resistance, especially the host’s resistance against bacteria and viruses. Cadmium may cause demineralization of the skeleton and increase bone fragility and the risk of fracture [91].

11. Effects of heavy metals on humans

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 imbalance, respiratory and cardiac disorders, decrease memory, types of cancer and eventually death [30, 127]. The lethality of most heavy metals for humans is estimated in the range of 500–350 mg per day. Heavy metals cause various diseases such as infertility, poisoning, nervous system disorders, breaking chromosomes, premature aging and various cancers in humans [28, 128]. Cancer is the main cause of death in developed and developing countries of the world. The increase in cancer may be caused by the increase in environmental pollution [65]. Heavy metals are one of the most important environmental pollutants [129]. These metals have the greatest impact on the health of citizens due to the occurrence of health risks such as reduced growth of children, kidney diseases, cancer and other adverse effects [130].

Heavy metals accumulate in the vital organs of the human body due to their indestructible and stable nature and lead to serious health disorders. Heavy metals should be considered due to their indestructibility, stability, accumulation in living organs and damage to the health of living organisms [32]. Contact with heavy metals occurs chronically (contact over a long period of time) due to movement in the food chain, but acute poisoning by ingestion or skin contact with heavy metals is rare, but possible. There is [131].

Heavy metals in soils are harmful to human health, especially children [132, 133]. Children have a high rate of heavy metal absorption due to their active digestive system, small body size, developing nervous system, swallowing dust, soil or suspended particles, weak immune system and excessive use of hands [134]. Heavy metals are very harmful to the human body because they do not have any effective elimination mechanism in the body [135]. These metals affect health indirectly by consuming plants that grow in contaminated soil and directly by inhaling and consuming contaminated water [136]. Heavy metals may become a problem for human health and have adverse environmental effects [137]. Heavy metals can enter the human body directly through swallowing and breathing, or reach the earth’s surface through atmospheric fallout, and enter the body through water and food after polluting water and soil sources and entering the structure of plants [29].

Some heavy metals are necessary for the continuation of life and activities of animals and they play a significant role in the body. Unnecessary and toxic heavy metals without having a role in the physiological activities of animals, even in low concentrations, cause disturbances in the body system of animals [27]. Among the pollutants in the environment, toxic metals in high concentrations cause poisoning for living organisms. Some metals, especially heavy metals, are of high environmental importance due to their toxic properties and accumulation in living organisms, even in relatively low concentrations [138]. Unlike some organic substances, these toxic metals are not biodegradable and their accumulation in living tissues can lead to death or serious threats to health [139]. Due to the fact that these metals are not decomposed by conventional biological processes, as a result, by accumulating in the tissues of living organisms, they are easily moved through the food chain, hence, by increasing their amounts in the soil over time, to It significantly damages plants. For example, copper and zinc in very low concentrations are essential trace elements for the survival of plant and animal life [140]. Long-term biological durability and remaining in the soil causes the accumulation of these metals in food chains and as a result, potential negative effects for human health. The amount of access to these metals depends on the type of plant and their required amount as micronutrients and the ability of plants to efficiently regulate their metabolism through the secretion of organic acids or protons into the root environment. In addition, the soil properties are effective on the mobility of these metals and therefore regulate their release rate in the soil solution. The ability of plants to absorb metals from the soil, their internal use and detoxification mechanisms have met with increasing popularity [141].

Physiological stages in plants are affected by increasing the concentration of heavy metals around the target plant. These metals cause oxidative stress in the plant, which is one of the harmful effects of this stress in the production of free radicals. In high concentrations of metals, substitution with essential metals occurs and since essential metals play an important role in the formation of pigments and enzymes, therefore the formation of pigments is disturbed and hence the elements It makes the soil unsuitable for plant growth and destroys biodiversity [142, 143]. For example, cadmium metal is one of the most toxic elements for plants and has no biological role. Cadmium metal mainly enters the environment and food chain through industrial processes and phosphate fertilizers. This toxic metal is easily absorbed by the roots and by forming complex complexes with organic compounds such as proteins, it prevents the necessary activity of cells. Cadmium, by increasing the peroxidation of lipids and the production of reactive oxygen species, provides membrane deterioration [100]. Since this metal has two positive charges (bivalent) and competes with elements such as magnesium in chlorophyll and with iron ions which are divalent and replaces them and the chlorophyll molecule in the plant is thus destroys Therefore, photosynthesis is very sensitive to cadmium. The high concentration of essential metals such as copper and zinc also harms the plant. Copper metal reduces plant growth by preventing the absorption of other elements such as calcium, iron and potassium, which are essential plant elements [144, 145].

12. Conclusions

Dietary exposure to toxic metals is a public health concern. As a result, food safety is an issue that threatens human health and agricultural business. The principles, advantages, and disadvantages of immobilization, soil washing, and phytoremediation techniques, often cited as among the best available technologies for cleaning up heavy metal-contaminated sites, are presented. Remediation of soils contaminated with heavy metals is necessary to reduce related risks, make land resources available for agricultural production, increase food security, and reduce land ownership problems caused by changes in land use patterns. Also, washing rice reduces some toxic and essential elements in rice. The distribution of elements in cereals from different regions helps countries make informed decisions about importing cereals such as rice. Rice and other cereal producers can develop strategies to reduce significant metal uptake from soil. Identification of appropriate rice treatment processes such as washing will provide information on reducing metal exposure to the rice consuming population while preserving essential elements in the grain.