| Heavy metal pollution in farmland is a key issue facing global pollution prevention and control,which has a significant impact on environmental safety,human health,and sustainable agricultural development.This study is based on the climate ba ckground of high latitude cold regions.Through indoor simulation of mixed cadmium lead(Cd,Pb)contaminated soil,different biochar application amounts(0%,1%,2%)and initial soil moisture content(15%,20%,25%)were set,and the changes in heavy meta l forms in soil and their response to changes in soil physicochemical properties were explored.The adsorption capacity changes of soil and biochar after freeze-thaw cycles were verified,And the effects of freeze-thaw cycles and the application of biochar on the transport of heavy metals in saturated soil were verified through soil column leaching experiments.The main research findings are as follows:(1)The improvement effect of biochar application on farmland freeze-thaw soil.During the freeze-thaw cycle,soil p H decreases,electrical conductivity increases,and cation exchange capacity increases.Freezing and thawing destroy soil structure,and bio char reduces the formation of small particle aggregates by stabilizing the composition of soil aggregates,thus reducing the adsorption capacity of soil to Cd and Pb.High soil moisture content(25%)aggravates the effect of freeze-thaw cycles on soil,promotes more obvious changes in soil p H,stabilizes the electrical conductivity in soil,increases the cation exchange capacity of soil,and severely damages the soil structure.Moreover,the more biochar was applied,the greater the increase of soil p H and conductivity,and the soil cation exchange capacity also showed an upward trend,which also stabilized the soil structure.The freeze-thaw process increases the adsorption sites and functional group abundance of biochar,increases the specific surface area of biochar,reduces the pore size and volume of biochar,and strengthens the adsorption and regulation effect of biochar on heavy metals in multiple aspects.(2)The effect of biochar application on the environmental behavior of heavy metals in farmland freeze-thaw soil.During the freeze-thaw process,the proportion of soluble and reducible Cd and Pb increased by 0.28-56.19%.Biochar reduced the migration of Cd and Pb in the soil,reducing the availability of heavy metals.The number of reducible,oxidiz able,and other forms of heavy metals that cannot be utilized by crops increased with the increase of biochar application,It can be seen that the application of biochar significantly reduces the environmental stress tendency of heavy metals during the fre eze-thaw cycle.The increase in soil moisture content leads to a decrease in the availability of Cd and Pb in the soil.When the soil mass moisture content changes from 15% to 25%,the soluble proportion of heavy metals Cd and Pb decreases by about 6.78% a nd 5.41%.(3)Seeking the optimal soil regulation model for farmland freeze-thaw soil.A method based on dry wet sieving method for determining heavy metals in soil aggregates was used to accurately classify the sources of heavy metals in soil.By analyzin g the heavy metal carrying capacity of different particle size aggregates,it was found that the carrying capacity of large particle size aggregates was weaker than that of small particle size aggregates.Based on the distribution characteristics of heavy metals in soil,it is concluded that applying 1% biochar to soil with a moisture content of 20% before freeze-thaw cycles plays the best regulatory role in seasonal frozen soil remediation.(4)The impact of freeze-thaw soil migration and transformation o n farmland.In the penetration test,the application of biochar prolonged the water leaching time and increased the soil blocking effect.The destruction of soil structure by freeze-thaw cycles increases the dispersion coefficient of soil,the maximum buff er capacity of Cd and Pb and the penetration rate of soil.The simulation effect of solute using the chemical non equilibrium two point model of HYDRAS-1D is good,and the adsorption parameters of soil and carbon soil mixture are within the available range of actual values,which can achieve the effect of supplementing parameters.In summary,biochar,as a soil amendment,can reduce the negative impact of freeze-thaw cycles on soil,while also serving as a solidifying agent to reduce heavy metal pollution i n soil.The freeze-thaw cycle significantly enhances the effectiveness of biochar in regulating soil,increases the prevention and bearing capacity of carbon soil complexes for pollution,and thus enhances the crop planting potential of farmland soil.The research results can provide theoretical basis and technical support for the protection of agricultural ecological environment in seasonally frozen soil areas,soil heavy metal pollution control,biochar improvement of soil adsorption of pollutants,and an alysis of heavy metal behavior in saturated soil. |
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