Soil heavy metal pollution status and technical specifications
How to repair soil contaminated with heavy metals? The goodness and fertility of the soil is one of the most concerned issues for farmers' friends. Today, Hui Nongwang Xiaobian will talk with everyone about soil heavy metal pollution and repair technology! 1 Heavy metal pollution sources in soil and their hazards 1.1 Cadmium 1.1.1 Main sources of cadmium pollution in soil Cadmium pollution in soil and natural environment is mainly from the discharge of human activities, which can be divided into three aspects. (1) In heavy metal production such as metal smelting, electronics industry, mining, etc., cadmium in the exhaust gas naturally settles into soil and rivers. (2) Chemicals and mining affect the sewage generated by urban life, which is used for agricultural irrigation and pollution. (3) In the process of phosphate fertilizer production, insoluble cadmium is converted into water-soluble state, and the effectiveness and toxicity are greatly improved, resulting in accumulation of cadmium in soil. 1.1.2 Mechanism and toxicity of cadmium Cadmium dissolves, aggregates, and chelate in the soil to form different forms of cadmium. Among them, the water-soluble exchange state is the most effective, most easily absorbed by plants, and then absorbed by the human body through the food chain. Cadmium poisoning causes serious damage to lung function and kidney function. Long-term living in cadmium-contaminated areas will lead to bone embrittlement, osteoporosis, lumbar vertebrae, and spinal deformity. 1.2 Lead 1.2.1 Main sources of lead pollution in soil The source of lead in the soil is the parent material and minerals, which release lead during weathering and sedimentation. There are three main sources of anthropogenic sources. (1) Lead-containing exhaust gas and particulate matter enter the soil with rainwater, causing pollution. (2) Waste lead products. For example, lead-acid batteries, cable wrappers and construction waste are not handled properly, causing pollution. (3) Lead-containing sewage is used for agricultural irrigation, resulting in a significant increase in lead accumulation in the ploughed soil. 1.2.2 Toxicity of lead After the lead in the soil is absorbed by the plant, it eventually affects the human body through the food chain. In addition to direct intake of lead-contaminated foods, lead can also enter the body through skin contact and respiratory pathways. Lead pollution is more harmful to human health. Its pollutants are highly toxic and a heavy metal element based on neurotoxicity. Heavy metal lead has varying degrees of damage to the human bone marrow hematopoietic system, nervous system, reproductive system and digestive system. Studies have shown that the blood lead level of pregnant women with long-term lead pollution is significantly higher than that of normal pregnant women, and the survival rate and health of the fetus are significantly lower than that of normal fetuses. 1.3 Mercury 1.3.1 Sources of soil mercury pollution The most basic source of mercury in the soil is the weathering of the parent material. Its mercury content directly determines the mercury content of the soil. The exogenous mercury entering the soil mainly comes from the “three wastes†emissions from industrial production and urban life. Among them, mercury-containing exhaust gas enters the soil through atmospheric deposition. Mercury-containing fertilizers and pesticides used in agricultural production are also important sources. 1.3.2 toxicity of mercury Elemental mercury cannot be absorbed by the human digestive system and can be quickly discharged even if it is accidentally taken. Mercury oxides cause mild diarrhea and most people have no clinical symptoms. 2 Remediation technology of heavy metal contaminated soil 2.1 Overview At present, heavy metal contaminated soil remediation technology is based on two ideas: (1) Activate heavy metal pollutants and remove them from the soil. (2) Stabilize heavy metal contaminants in the soil as much as possible to reduce migration. 2.2 Physical repair technology Physical techniques include measures such as dumping, soil, deep tillage, leaching, electrolysis, and heat treatment. Dumping, soil replacement, topsoil, and soil are generally considered to be the fundamental ways to control heavy metal pollution in soil. However, the amount of engineering is large. Deep ploughing is to reduce the heavy metal content in the topsoil by turning the soil up and down. (1) The leaching method is to extract heavy metals from the soil into the eluent to remove heavy metals from the soil. (2) Electrolysis is to insert an electrode into the contaminated soil to adjust the pH of the soil. Heavy metals are enriched in two stages through the soil solution, thereby removing heavy metal ions. The experimental results show that the removal rate of lead ions and chromium ions in the clay can reach more than 90%. (3) The heat treatment method removes heavy metal elements and their compounds by artificial temperature rise or pressure reduction. In the mercury pollution repair study of He Wei et al., the efficiency of heat removal to remove mercury is generally above 50%, and sometimes 90%. 2.3 Chemical repair technology Chemical repair techniques mainly use modifiers and chemical leaching methods. The mechanism of the modifier is to stabilize the solidified heavy metals. By adding a modifier to contaminated soil, adjusting the pH of the soil, causing adsorption, sequestration, precipitation and other redox reactions of heavy metals in the soil, thereby reducing the effectiveness of heavy metal elements or making them in a less toxic valence state, achieving curing Stabilization. The modifiers commonly used in production practice are lime, phosphate, calcium carbonate, silicate, and the like. Among them, the use of phosphate as a modifier is currently the most thorough method to reduce the effectiveness and solubility of mercury. Xi Huan et al. added honeycomb ash slag into lead-contaminated fluvo-aquic soil and analyzed changes in soil properties. It was found that the soil available phosphorus content increased, the effective lead content decreased, and the pH value increased. This may be due to the honeycomb coal ash slag itself. The pH is higher. The chemical leaching method uses a biological or chemical solvent as a leaching agent to dissolve the heavy metals contaminating the soil and then separate them, thereby achieving the purpose of repairing the soil. Tang Min et al. used citric acid as an arsenic eluent in the study of arsenic-contaminated soil. The results show that the removal rate of soil arsenic is up to 70%. 2.4 Bioremediation technology Bioremediation technologies include phytoremediation, animal remediation, and microbial remediation. 2.4.1 Phytoremediation Phytoremediation is the planting of specific heavy metal-tolerant plants on contaminated soil. The contaminated heavy metals are absorbed into the plants and harvested to restore the soil. Phytoremediation techniques can be further divided into plant stabilization, plant extraction and plant evaporation depending on the mechanism of action. (1) Plant stability is the adsorption of plant roots and the precipitation of root exudates, which can effectively reduce the effectiveness of heavy metals in contaminated soils and prevent them from harming the human body through groundwater and food chain pathways. (2) Plant extraction can absorb toxic metal pollutants, transport them to the stems and leaves of plants, and restore the soil after harvesting. At present, there are nearly 500 kinds of super-accumulated plants found in the world. There are many studies on the enrichment of arsenic by valerian, the enrichment of Zn by Sedum, the enrichment of copper by sputum, and the enrichment of cadmium by sunflower. In the study of Xing Weiqin et al., ryegrass was planted on lead-pollution phosphorus-poor soil and phosphate fertilizer was applied to soil restoration. The results showed that ryegrass increased the lead availability of the soil; applying phosphate fertilizer to promote plant growth and reduce soil lead content. (3) Plant volatilization is to promote the conversion of heavy metal contaminants in the soil into a volatile state by planting some plants. It is usually achieved by the reaction of plant root exudates, rhizosphere microorganisms and heavy metals. 2.4.2 Animal repair Some lower animals, especially soil small animals, are tolerant to heavy metals, and use this characteristic of animals to absorb heavy metals in the soil for the purpose of soil remediation. According to research by Gao Yan et al., for arsenic-contaminated soil, strontium has a certain ability to absorb and repair. 2.4.3 Microbial repair Microorganisms are used for the repair of heavy metal contaminated soils by their own biochemical reactions, specifically through adsorption, precipitation, methylation and other reactions. At present, microorganisms that can be used for heavy metal adsorption include bacteria, filamentous fungi, yeast, and algae. Bioprecipitation and methylation react with heavy metal ions to convert them into low-toxic or non-toxic valence states. The secretion of microorganisms can also react with heavy metal ions, and the product is a stable precipitate or complex, which effectively reduces the toxicity of heavy metals. Microorganisms affect the rhizosphere environment of plants, enhance the adsorption, transformation and accumulation of heavy metals by plant roots, and also achieve the purpose of soil restoration. Compared with other repair technologies, the advantages of microbial repair are: no damage to the soil environment, easy operation, low cost, and the like. However, the activity of microorganisms is easily affected by the environment of the soil, and the repairing effects of different microorganisms are different, which limits the promotion and application of microbial remediation technology. Today's content says that this is over. Hui Nongwang Xiaobian hopes that these contents will help others more or less!
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