Sources of cadmium in soil
When the Cd content in agricultural soil exceeds 1.0 mg/kg, we think it is polluted. There are many sources of cadmium pollution in soil, which can be divided into natural sources and man-made sources. Natural sources mainly refer to soil-forming parent materials, such as cadmium sulfide ore and galena cadmium sulfide ore. Man-made sources mainly include mining, smelting, fertilization, sewage irrigation, atmospheric deposition, urban garbage and so on. Mining and smelting are the main industrial sources of cadmium emission and affect the surrounding environment in the form of "three wastes". Cadmium in industrial waste gas diffuses with dust and enters the soil in the form of rainfall and natural sedimentation. Cadmium in industrial wastewater enters the soil in the form of sewage irrigation farmland or urban residents' water treatment is not up to standard. Long-term use of phosphate fertilizer and high amount of organic fertilizer will increase the cadmium content in soil. The fertilizers containing cadmium are mainly phosphate fertilizer and some domestic garbage containing cadmium. In addition, automobile exhaust emissions will cause heavy metal pollution in the soil on both sides of the road.
Migration and Accumulation Mechanism of Cadmium in Soil
1, the existing state of cadmium. Cadmium exists in soil in two states: water-soluble and insoluble. Different forms of pollutants have different environmental behaviors and biological effects. Water-soluble cadmium mainly exists in ionic state or complex state, which is easily absorbed by crops; Insoluble cadmium can exist in soil particles in exchange state (clay exchange and humus exchange), chemical precipitation state and insoluble chelate state, and is not easily absorbed by plants. Water-soluble cadmium and water-insoluble cadmium in soil can be transformed into each other under certain conditions, and the main influencing factors are soil pH, redox conditions and carbonate content.
2. Migration and transformation of cadmium. Due to the strong adsorption of soil, cadmium rarely migrates downward and accumulates on the soil surface. Under the influence of precipitation, the soluble part of cadmium on the soil surface may flow with water, leading to level of migration, entering the interface soil and nearby rivers or lakes, resulting in secondary pollution.
3. Influencing factors of cadmium activity in soil. There are many factors that affect the transformation of cadmium in soil, such as pH value and soil redox conditions. With the increase of pH value, the solubility of cadmium decreases. The redox conditions of soil are closely related to the activity of cadmium. Dry land is well ventilated, cadmium is mostly soluble and easy to be absorbed, and the cadmium content in crops and soil is positively correlated. Insoluble CdS produced by the combination of S2- and Cd2+ inhibited the migration of Cd2+ and the absorption of Cd2+ by crops, and inhibited the activity of cadmium in soil. In addition, soil clay composition, organic matter content and PO43- content will all affect cadmium migration ability. Among them, pH value is the main factor affecting the solubility of Cd2+ and the main reason for plants to absorb Cd2+. If other reactions are not considered, it can be written as the following equilibrium reaction formula:
The above formula shows that the theoretical concentration of Cd2+ decreases with the increase of soil pH value.
Remediation of cadmium pollution in soil
The principle of soil cadmium pollution control is "prevention first, combining prevention with control", and the specific means are as follows:
1, physical repair. Physical remediation methods include soil improvement, soil leaching, thermodynamic remediation and electrodynamics remediation. At present, the most widely used soil improvement methods include soil replacement, soil replacement, topsoil removal and deep ploughing. Soil leaching method is to transfer cadmium from soil solid phase to liquid phase, and then further recover cadmium-rich wastewater. At present, soil leaching solution includes organic acid or inorganic acid, alkali, salt and chelating agent. Thermodynamic remediation method is to use high-voltage variable-frequency voltage to generate electromagnetic waves and generate heat energy to heat soil and separate pollutants from soil particles. Electrokinetic remediation refers to inserting electrode pairs into polluted soil and applying low-intensity direct current to make cadmium ions in soil move directionally under the action of electric field and enrich near the electrodes, thus achieving the purpose of removing cadmium.
2. Chemical remediation. That is, chemical remediation agent is applied to soil to react with cadmium pollutants, which reduces the water solubility, diffusivity and bioavailability of cadmium pollutants in soil. Chemical remediation developed earlier and was relatively mature and effective, mainly including chemical leaching technology, chemical oxidation remediation technology, soil performance improvement technology and so on. At present, the widely used method is to apply modifiers, such as phosphate, silicate, zeolite and bentonite. By changing soil pH, precipitation, adsorption, ion exchange and complexation, the bioavailability of cadmium in soil was significantly reduced. It was found that the effective concentration of cadmium decreased by 94.6% when ammonium dihydrogen phosphate was used to repair cadmium contaminated soil in mining area.
3. Phytoremediation. Phytoremediation has the advantages of low cost, less labor, high treatment efficiency, no secondary pollution and easy recovery of metals. Phytoremediation refers to the removal of pollutants in soil through absorption, volatilization, transformation and degradation of plants and their rhizosphere microbial systems, which mainly includes two aspects: First, some hyperaccumulation plants are used to absorb, fix, transform and accumulate pollutants. The results of greenhouse cultivation experiments showed that the highest cadmium content in Indian mustard roots and leaves could reach 300 mg/kg and 160 mg/kg. In addition, ferns and sunflowers have strong tolerance and enrichment ability to cadmium. The second is to provide favorable conditions for microbial remediation and promote biodegradation and harmlessness. Yan Li and others found that after planting ramie in cadmium-contaminated soil, the number of viable bacteria in soil increased significantly, with the maximum increase of 65,438 0.33 times, and the maximum increase of soil fungi was 2.42 times. Planting ramie in cadmium contaminated soil can increase soil organic matter content, obviously reduce soil cadmium content, and play a role in repairing soil cadmium pollution.
4. Bioremediation. Bioremediation includes microbial remediation and animal remediation. Microbial repair mechanisms include cell metabolism, absorption, precipitation and redox reaction. Liu Hongjuan and others found that the studied Bacillus cereus RC grew well on the solid medium plate with cadmium concentration of 200 mg/L, which indicated that the strain had strong cadmium tolerance. Animal remediation refers to the method of reducing the proportion of heavy metals in polluted soil by using the metabolic activities of some lower animals in the soil. For example, Ramseier and others found that earthworms have strong cadmium enrichment ability. When the concentration of cadmium in soil is 3mg/kg, the accumulation of cadmium by earthworm can reach 120mg/kg. However, because the growth of lower animals is seriously restricted by environmental factors, the practical application of this technology is limited.