Once the soil is polluted by heavy metals, it is difficult to recover. Therefore, special attention should be paid to the pollution of soil by cadmium, mercury, chromium, lead, nickel, zinc and copper. Excessive amounts of these elements have great biological toxicity and can pose a threat to human health through the food chain.
1, soil chemical behavior of heavy metals
The fate of heavy metals after entering the soil will be controlled by a series of complex chemical reactions and physical and biological processes. Although some chemical behaviors of different heavy metals are similar, they are not completely consistent. When they are added to soil, the initial mobility will largely depend on the form of the added heavy metals, that is, it will depend on the source of the metals. In the process of sludge digestion, metals associated with organic matter account for a considerable proportion, and only a small part exists in the form of sulfide, phosphate and oxide. Particles discharged from smelters contain metal oxides; When burning petroleum, lead is discharged in the form of bromochloride, but it is easily converted into lead sulfate and oxygen-containing lead sulfate in the atmosphere and soil. Due to different forms, the forms and quantities of metal ions entering the soil are also very different, which directly affects the migration and transformation of heavy metals in the soil and the plant effect. Test environmental engineer, a good website worthy of your collection!
Under different soil conditions, including the types of heavy metals in soil, land use patterns (paddy field, dry land, orchard, woodland, grassland, etc. ), as well as the physical and chemical properties of soil (soil pH, redox conditions, adsorption, complexation, etc.). ), the forms of heavy metals in soil can be different, thus affecting the transformation of heavy metals and the absorption of heavy metals by crops.
1) redox conditions of soil and migration and transformation of heavy metals: soil is a redox system, and soil moisture status, organic matter and sulfur content in soil are in dynamic change. The redox system in soil is a complex system composed of many inorganic and organic redox systems. Among inorganic systems, there are important oxygen systems, iron systems, sulfur systems and hydrogen systems. Controlled by dominant determinant potential system. Among them, O2-H2O system and sulfur system play an obvious role in soil redox reaction and play an important role in the valence change of heavy metal elements.
(1) O2-H2O system: The oxygen in soil mainly comes from the atmosphere. Precipitation and irrigation water may also bring in some dissolved oxygen. In rice fields, oxygen secreted by rice roots and oxygen released by photosynthesis of some algae are also sources.
(2)H2 system: There is little hydrogen content in dry land soil, but H2 often accumulates in strongly reduced soil layer under flooding conditions.
O2-H2O system and H2 system are two extreme systems of soil redox system, while other redox systems in soil are between them. Therefore, these two systems constitute the upper and lower limits of soil redox potential.
(3) Sulfur system: Sulfur in soil exists in both inorganic and organic forms, and its content is generally 0.05%. It exists in the form of sulfate under oxidation conditions; Under reducing conditions, it exists in the form of hydrogen sulfide or metal sulfide.
Metal elements can be roughly divided into insoluble (oxidation-fixed) elements and reduced insoluble (reduction-fixed) elements according to their properties, such as iron and manganese. Cadmium, copper, zinc and chromium belong to the latter. Redox will not only change the valence state of heavy metals, but also change the form of heavy metals. For example, when the redox potential is low (about+1000 mv), iron arsenate can be reduced to ferrous form, and the potential is further reduced, thus reducing arsenic to arsenite and enhancing arsenic mobility. On the contrary, the increase of iron and aluminum components in soil may transform water-soluble arsenic into insoluble arsenic.
2) Migration and transformation of soil pH and heavy metals: Soil pH is closely related to the solubility of heavy metals. Under alkaline conditions, most of the heavy metals entering the soil are insoluble hydroxides, and may also exist in the form of carbonates and phosphates. Their solubility is relatively small, so the ion concentration of heavy metals in soil solution is also low. The solubility of heavy metal hydroxides such as copper, cadmium, zinc and lead is directly controlled by soil pH value.
According to the solubility product, the relationship between the concentration of heavy metal ions and pH value can be deduced theoretically. With the increase of pH value, the concentration of heavy metal ions decreases. But for amphoteric compounds, copper hydroxide and zinc hydroxide, when the pH value is high enough, they will dissolve again.
3) Adsorption of soil colloids and migration and transformation of heavy metals: the soil is rich in inorganic and organic colloids. It has obvious fixation effect on heavy metal elements entering the soil. Generally speaking, there are two forms of heavy metal elements in soil:
(1) Heavy metal elements are colloidal in soil solution. This mainly occurs in humid areas and acidic conditions rich in organic matter, such as iron, manganese, chromium, titanium, vanadium, arsenic and other elements can exist in colloidal form, and copper, lead and zinc also partially migrate in colloidal form;
(2) The adsorption and immobilization of metal ions by organic and inorganic colloids in soil is the main way for many metal ions and molecules to transfer from unsaturated solution to solid phase, and it is also an important reason for the accumulation and pollution of heavy metals in soil.
The amount of heavy metals adsorbed by soil colloid mainly depends on the substitution ability of soil colloid and the concentration and pH value of heavy metal ions in soil solution.
When colloid adsorbs metal ions, the metal ions can be adsorbed in the lattice by isomorphic substitution. As adsorbed ions, metal ions remain in the colloidal lattice, so it is difficult to release them.
In a word, heavy metal elements are fixed by colloidal adsorption, which can be divided into two ways. For example, metal elements are adsorbed on the exchange points on the colloid surface and are easily released; If it remains in the crystal lattice of colloidal minerals, it is difficult to release, which is not conducive to the migration of metal elements.
4) Complexation-chelation of heavy metals in soil: In addition to adsorption, heavy metal elements also have complexation and chelation in soil. Generally speaking, when the concentration of metal ions is high, it is mainly adsorption and exchange, while when the concentration of heavy metal ions in soil solution is low, it is mainly complexation-chelation.
Among inorganic ligands, people pay more attention to the complexation of metals with hydroxyl and chloride ions. It is considered that these two factors are important factors affecting the solubility of some insoluble heavy metal salts.
The complexation of hydroxide ions with heavy metals is actually a hydrolysis reaction of heavy metal ions. Heavy metals can be hydrolyzed at low pH value. The hydrolysis experiments of mercury, cadmium, lead and zinc plasma show that the complexation of hydroxyl with heavy metals can greatly improve the solubility of heavy metal hydroxides.
Only when the concentration of chloride ion in saline soil is high, the form of heavy metal ions complexed with chlorine will appear. Generally, when the chloride ion concentration in soil is very low, the chloride complex of heavy metal ions will not form.
Humus in soil has strong chelating ability, and it has ligands that can chelate metal ions firmly, such as amino group, imino group, ketone group, hydroxyl group and thioether group. The stability of chelates in soil is affected by the properties of metal ions. When metal ions are combined with chelating groups through ionic bonds, the greater the ionization potential of the central ion is, the more favorable it is for the formation of coordination compounds.
5) immobilization and activation of heavy metals by soil microorganisms
There are many kinds and quantities of microorganisms in soil, which also play an important role in the fate of heavy metals. Experiments show that cadmium can be immobilized by complexing with microorganisms or their metabolites, which affects its bioavailability. Some microorganisms also change metals from high toxic state to low toxic state through biotransformation or physiological metabolism. The effects of microorganisms on heavy metal ions in soil can be summarized as follows:
(1) Extracellular recombination
Some microorganisms can produce extracellular polymers, such as polysaccharides, glycoproteins and lipopolysaccharides. Which has a large number of anionic groups and is combined with metal ions; Metabolites produced by some microorganisms, such as citric acid, are effective metal chelating agents, while oxalic acid forms insoluble oxalate precipitates with metals.
(2) Extracellular precipitation
Under anaerobic conditions, hydrogen sulfide produced by microorganisms such as sulfate reducing bacteria reacts with metal ions to form insoluble sulfide precipitates.
(3) Microbial transformation of metals
Microorganisms can transform heavy metals through oxidation, reduction, methylation and demethylation. A large number of studies have shown that the resistance of microorganisms to heavy metals is in many cases determined by the genetic material of chromosomes in cells-plasmids or transposon resistance genes. Metal detoxification enzymes encoded by resistance genes catalyze the transformation of highly toxic metals to low toxic States. Bacteria, actinomycetes and some fungi can reduce mercury ions to elemental mercury, making mercury volatilize from soil or exist in the form of precipitation. Organic mercury compounds are first decomposed into Hg+ and corresponding organic groups by organic mercury lyase, and then ionic mercury is reduced to elemental mercury. Mercury and other metals such as lead, selenium and arsenic can be methylated by microorganisms. The toxicity of methylation products of selenium is reduced, but the toxicity of methylation products of mercury is high. Hexavalent chromium can be reduced to trivalent chromium by bacteria, and highly toxic As+ can be oxidized to As5+ by microorganisms, which is more easily precipitated by Fe3+.
6) Enrichment and attenuation of soil rhizosphere
The rhizosphere is only about 0.1-4 mm. In this area, because of the existence of plant roots, its physical, chemical and biological characteristics are different from those of soil, which significantly affects the activity and bioavailability of heavy metals in soil.
(1) Formation of Redox Barrier in Rhizosphere
The solubility of many heavy metal elements is determined by redox state, and the solubility of reduced iron and manganese ions is higher than that of their oxidized ions. Therefore, when an oxidation microenvironment is generated in the rhizosphere of plants growing on a reducing substrate, when reducing ions in soil cross this oxidation zone and reach the root surface, the activity of free metal ions is obviously reduced, because they are oxidized into an oxidation state with low solubility, thus reducing their toxicity. On the contrary, the rhizosphere of plants growing on oxidizing substrates consumes oxygen due to the respiration of roots and rhizosphere microorganisms, and root exudates contain reducing substances. The reduction conditions of soil will affect the activity and effectiveness of variable valence metal elements, such as the reduction and removal of hexavalent chromium and the immobilization of microorganisms. The research shows that the anions in bacterial cell wall and plasma membrane can combine with cadmium in solution, but they will be released back into solution under aerobic conditions, and cadmium will not migrate under reducing conditions.