Distribution Of Heavy Metals In Soils With Different Particle Sizes And Its Influencing Factors

With the rapid development of industrialization,the exploitation of mineral resources,the use of various chemical products,and the improper treatment of "three wastes",a large number of industrial wastewater,waste gas and waste residue enter the soil through various ways,resulting in serious pollution of heavy metals in the soil,especially in farMLand soils near industrial and living areas.Therefore,the remediation of heavy metal contamination in soil is needed to ensure that the soil can supply human health crops.It was found that the change of particle size of soil aggregates was an important factor affecting the distribution of heavy metals in soil.Many properties of particulate matter with different particle sizes,such as surface structure characteristics,specific surface area and surface active adsorption potential,could affect the morphology,content and distribution of heavy metals.Therefore,it is of great significance to identify the characteristics of heavy metals in soil aggregates with different particle sizes for guiding the remediation of soil pollution.Based on the project of "Research on the Technology and Method of Mineral Fertilizer Regulation and Heavy Metal Rehabilitation for FarMLand Soil",this paper takes farMLand soils in Shifang and Mianzhu areas polluted by heavy metals such as Cd,Pb and U as the research object.On the basis of identifying the distribution characteristics of heavy metals in soil aggregates with different particle sizes,the effect of pH change on the distribution of heavy metals in soil is discussed through dynamic leaching experiments.?The results showed that the content of Cd in soil aggregates exceeded the background value of surface soil in Chengdu Economic Zone.Among them,T2 soil far exceeded the soil standard value,which was 9.67 times of the background value.T0 exceeded the standard at least,but also reached 2.13 times.The content of Pb is less than the background value of surface soil in Chengdu Economic Zone,and the content of T0 and T2 is close to the background value,while the content of T1 is less than the background value,which is only 0.79 times of the background value.The U content in T0 soil is only 1.3 times higher than the background value of U in surface soil of Chengdu Economic Zone,while that in T2 soil is far less than the background value,which is only 0.68 times of the background value,and that in T1 soil is close to the background value.The contents of heavy metals in soil aggregates with different particle sizes were analyzed and compared.It was found that the contents of Cd,Pb and U increased with the decrease of particle size of soil aggregates.It is presumed that the smaller the particle size of soil aggregates,the larger the surface area,the more the organic matter content,and the higher the adsorption of heavy metals.The speciation content of heavy metals in soil aggregates with different particle sizes was measured.It was found that Cd in soil aggregates was mainly residual,accounting for 62.1%;Pb in soil aggregates was mainly residual,accounting for 63.3%,and weak acid-soluble was the least,accounting for 0.7%.The proportion of U in each form is residual state > oxidizable state > reducible state > weak acid soluble state.The leaching results showed that the leaching rules of heavy metals were different with the different kinds of heavy metals.The leaching process of Cd in soil can be divided into two distinct stages: rapid leaching stage and slow release stage.The leaching change of Pb is consistent in the early stage,that is,leaching amount increases first and then decreases with the increase of leaching days,and increases again after a period of stabilization.The daily leaching capacity of U was similar to that of Pb in the earlier stage,that is,with the increase of leaching days,the leaching capacity of U increased briefly,then decreased and finally stabilized.The leaching rate of Cd is higher than that of the other two heavy metals,which indicates that Cd is more active,mobile and easy to release in soil.Through the correlation analysis between the release rate of heavy metals and pH,particle size,it was found that the leaching rate of heavy metals Cd was positively correlated with particle size,indicating that with the increase of particle size,the leaching rate of Cd increased.The leaching rate of heavy metal Pb is negatively correlated with the particle size,indicating that with the increase of particle size,the leaching rate of Pb decreases,but the correlation is small.The particle size and the leaching rate of heavy metal U are negatively correlated,indicating that with the increase of particle size,the leaching rate of U decreases.There is a high negative correlation between the release rate of heavy metals and leachate pH,which indicates that acidity is conducive to the release of heavy metals,which can be converted into more active forms and thus released.