Causes of heavy metal pollution in soil

Heavy metal pollution in soil means that the content of trace metal elements in soil exceeds the background value due to human activities and is too high due to excessive deposition, which is collectively called heavy metal pollution in soil. ? Heavy metals refer to metals with specific gravity equal to or greater than 5.0, such as iron, manganese, zinc, cadmium, mercury, nickel and cobalt. As is a metalloid, but its chemical properties and environmental behavior are similar to those of heavy metals, so arsenic is often included in the discussion of heavy metals, while others directly include it in the category of heavy metals. Because of the high content of iron and manganese in soil, it is generally considered that they are not soil pollution elements, but under the condition of strong reduction, the toxicity caused by iron and manganese has also attracted enough attention. Many rice produced in China were found to have excessive cadmium, and soil pollution became a "public hazard". The emergence of "cadmium rice crisis" once again sounded the alarm of soil pollution. ? Sources of heavy metal pollution in soil;

Natural sources:

1, the influence of parent material weathering process on the background content of heavy metals in soil;

2, the natural physical and chemical migration process of wind and water handling.

Man-made interference input:

1, additional input of heavy metals in soil by industrial production of different industrial and mining enterprises;

2. Heavy metal input into soil under the influence of agricultural production activities;

3. The influence of transportation on soil heavy metal pollution.

Heavy metals that pollute soil mainly include elements with obvious biological toxicity such As mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr) and metalloid arsenic (As), and elements with certain toxicity such as zinc (Zn), copper (Cu) and nickel (Ni). It mainly comes from pesticides, wastewater, sludge and atmospheric deposition. For example, mercury mainly comes from wastewater containing mercury, cadmium and lead pollution mainly comes from smelting emissions and automobile exhaust deposition, and arsenic is widely used as pesticides, fungicides, rodenticides and herbicides. Excessive heavy metals will cause plant physiological dysfunction and nutritional imbalance. The enrichment coefficient of cadmium, mercury and other elements in crop seeds is high, even if it exceeds the food hygiene standards, it will not affect the growth and yield of crops. In addition, mercury and arsenic can weaken and inhibit the activities of nitrifying bacteria and ammonifying bacteria in soil and affect nitrogen supply. Heavy metal pollutants have little mobility in soil, are not easy to leach with water and are not degraded by microorganisms. After entering the human body through the food chain, they are potentially harmful, so special attention should be paid to prevent heavy metals from polluting the soil. In some mines, stone yards and tailings ponds have not yet been established, and waste rocks and tailings are piled up at will, so that tailings rich in refractory heavy metals enter the soil, and the remaining metal residues after ore processing enter the groundwater system with rainwater, causing serious soil heavy metal pollution.

Only 10 metal elements are really classified as heavy metals in industry: copper, lead, zinc, tin, nickel, cobalt, antimony, mercury, cadmium and bismuth. From the perspective of soil-plant system, international academic circles believe that only elements such as cadmium, cobalt and selenium have the greatest risk of harm to animal and human health. Cadmium and selenium are the main factors that harm human health in the food chain. Although there are many heavy metals, their toxicity and geochemical behavior (the science of studying the chemical composition, chemical action and chemical evolution of the earth) are far from each other, and their effects on human beings are also different. From the point of view of food chain, there are only a few keys. [