The abundance of chromium in chromium shell is 200μg/g, and the average background value of chromium in soil is100μ g/g. Chromium in soil exists in four forms, namely trivalent chromium ion Cr3+, CrO2- and hexavalent anions CrO42- and Cr2O72-, among which trivalent chromium is stable. Soluble chromium in soil only accounts for 0.0 1% ~ 0.4% of total chromium. The migration and transformation of chromium is related to soil pH value, redox potential, organic matter content and other factors. After trivalent chromium enters the soil, more than 90% of it is quickly adsorbed and fixed by the soil, and it is a mixture of chromium and iron hydroxide or wrapped by iron oxide, so trivalent chromium in the soil is difficult to migrate. The solubility of trivalent chromium in soil solution depends on pH, and when pH is greater than 4, the solubility of trivalent chromium decreases. When pH is 5.5, all precipitate; The polyhydroxy compound of chromium is formed in alkaline solution. In addition, at low pH value, chromium can form organic complex, and its migration ability is enhanced. The strong adsorption of trivalent chromium by soil colloid is positively correlated with pH, and Cr3+ can even exchange Al3+ in the lattice of clay minerals, and the adsorption capacity of clay minerals is about 30 ~ 300 times that of hexavalent chromium. After hexavalent chromium enters the soil, most of it is free in the soil solution, and only 8.5% ~ 36.2% is adsorbed and fixed by soil colloid. Different types of soils or clay minerals have different adsorption capacities for hexavalent chromium. The adsorption capacity is roughly as follows: red soil > yellow brown soil > black soil > yellow soil; Kaolinite > illite > vermiculite > montmorillonite. The more organic matter in the soil, the stronger the electronegativity and the weaker the adsorption of hexavalent chromium anion. Redox conditions have great influence on the migration and transformation of chromium in soil. In the common range of soil pH and pE, Cr(ⅵ) can be rapidly reduced to Cr (Ⅲ) by organic matter. In different paddy fields, the reduction rate of Cr (ⅵ) is positively correlated with the content of organic carbon. When the organic carbon content in latosol is 65438 0.56% or 65438 0.33%, the reduction rates of Cr (ⅵ) are 89.6% and 77.2% respectively. Overall, the soil organic carbon increased by 65438 0%, and the reduction rate of Cr (ⅵ) increased by about 30%. The reduction of Cr (Ⅵ) by organic matter is negatively correlated with soil pH value. When the content of organic matter in soil is extremely low, the effect of pH on the reduction rate of Cr (Ⅵ) is more obvious. For example, when the soil pH is 3.35 or 7.89, the reduction rate of Cr (ⅵ) is 54% and 20% respectively.

After the wastewater containing chromium enters the farmland, the Cr (Ⅲ) in it is adsorbed and fixed by soil colloid. Cr (Ⅵ) is rapidly reduced to Cr (Ⅲ) by organic matter, and then adsorbed by soil colloid. The migration ability and bioavailability of chromium decrease, and chromium accumulates in soil. But under certain conditions, Cr (Ⅲ) can be transformed into Cr (ⅵ); For example, when the pH is 6.5 ~ 8.5, Cr (Ⅲ) in soil can be oxidized to Cr (ⅵ), and the reaction is as follows:

4Cr(OH)2 ++ 3 O2+2H2O→4 cro 42-+ 12H+

In addition, manganese oxide in soil can also transform Cr (Ⅲ) into Cr (ⅵ). Therefore, Cr (Ⅲ) is potentially harmful. In the process of plant growth and development, chromium can be absorbed from the external environment, and can also enter the plant through roots and leaves. The content of chromium in plants varies with plant species and soil types, and the residue of chromium in plants is positively correlated with the content of chromium in soil. Most of the chromium absorbed by plants from soil is accumulated in roots, followed by stems and leaves, and the amount of chromium accumulated in grains is the least. Trace element chromium is essential for plants. Chromium deficiency will affect the normal development of plants. Low concentration of chromium can stimulate the growth of plants, but excessive chromium accumulation in plants will produce toxic effects, which will directly or indirectly bring harm to human health. For example, when Cr (Ⅲ) in soil is 20 ~ 40μ g/g, it can obviously stimulate the growth of maize seedlings. When Cr (Ⅲ) is 320μg/g, it has inhibitory effect. For another example, when the concentration of Cr(ⅵ) in soil is 20μg/g, it can stimulate the growth of maize seedlings. When Cr (Ⅵ) is 80 μg/g, it has obvious inhibitory effect. High concentration of chromium is not only harmful to plants, but also affects the absorption of other nutrients by plants. For example, when the chromium content in the soil exceeds 5μg/g, it will interfere with the absorption of calcium, potassium, phosphorus, boron and copper by the aboveground part of the plant, and the damaged soybean will eventually wither severely at the top of the plant. The toxicity of chromium in soil to plants is related to the following factors: (1) Chemical forms of chromium. For example, Cr (Ⅵ) is more toxic than Cr (Ⅲ). (2) Soil properties. Soil colloid has a strong adsorption and fixation effect on Cr (Ⅲ), and also has a strong adsorption effect on Cr (ⅵ) under acidic or neutral conditions. Soil organic matter can adsorb or chelate, and can also reduce soluble Cr (ⅵ) to insoluble Cr (Ⅲ). Therefore, the contents of clay and organic matter in soil will affect the toxicity of chromium to plants. (3) Soil redox potential. For example, at the same concentration of Cr (Ⅲ), the available Cr in dryland soil is much higher than that in paddy field. (4) The toxicity of soil pH Cr (ⅵ) in neutral and alkaline soils is greater than that in acidic soils. Cr (Ⅲ) is more toxic to plants in acidic soil.

Generally speaking, the inhibition of chromium on plant growth is weak, because the migration of chromium in plants is very low. The results of rice cultivation experiments showed that the migration order of heavy metals in plants was CD > Zn > Ni > Cu > Cr. It can be seen that chromium is one of the most difficult elements to be absorbed, and the possible reason is that the process of (1) trivalent chromium being reduced to divalent chromium and then being absorbed by plants is difficult to occur in soil-plant systems. (2) Hexavalent chromium is available, but the absorption of hexavalent chromium by plants is strongly inhibited by anions such as sulfate.