The Migration And Enrichment Mechanism Of Heavy Metals In The Process Of Carbonate Pedogenesis In Liupanshui,Guizhou

The southwestern region is a typical high geological background area in my country.The excess rate of heavy metals in the soil is relatively high,which is related to the large area of carbonate rocks developed in the region.The weathering process of carbonate rock will lead to the secondary enrichment of heavy metal elements in the soil.In the Liupanshui area of Guizhou,a systematic study was carried out on the enrichment characteristics and migration and transformation mechanisms of heavy metals in the process of carbonate weathering and soil formation.The rock and soil profile samples of the carbonate area in the study area were collected.By analyzing the changes in the main and trace elements,mineral composition,soil physical and chemical properties,and the occurrence of heavy metals in soil and rocks,the following understandings have been obtained:The process of carbonate rock weathering and soil formation is mainly manifested by the leaching of Ca O,the relative enrichment of Si O₂,Al₂O₃,and TFe₂O₃ in the soil.In terms of mineral composition,the soil is mainly composed of quartz,illite and goethite,and calcite is almost completely leached.Clay minerals and iron-manganese nodules are important factors in the accumulation of heavy metals in the soil.The weathering process of carbonate rock will lead to the enrichment of heavy metals in the soil to varying degrees,and the soil-to-rock ratio of Cd is as high as 5.48 times.The content of each heavy metal element on the profile gradually increases from bottom to top.The mass transfer coefficient of the element shows that Pb is in a state of depletion,and the other heavy metal elements are brought in in the profile as a whole.The p H of the soil profile is generally weakly alkaline.No strong leaching effect occurred in the profile.There are obvious differences in the form of heavy metals in the soil profile.Among them,the heavy metals Cu and Zn are mainly in the residue state,and the residue state accounts for more than 75%.Although the content is high,the risk is low.The heavy metals Cd and Pb are mainly in the oxidation state of iron and manganese,accounting for 49.9%and 52.7%respectively.The active state of Cd element accounts for 14.4%,and the bioavailability of the active state is relatively high,so Cd has high potential bioavailability.There is a significant positive correlation between the stable state of each heavy metal element and the total amount of the element,indicating that each heavy metal mainly exists in a stable state.The active state of heavy metals is affected by soil p H value and organic matter.With the decrease of surface soil p H value and the increase of organic matter content,the proportion of surface soil active state increases significantly.The enrichment of soil heavy metals in carbonate rock areas is mainly affected by the soil-forming parent material.During the weathering process,the heavy metal elements are relatively residual and enriched with the leaching of Si,K,and Na.The heavy metal elements are mainly present in clay minerals or Iron manganese oxides and other acid-insoluble mineral phases,or adsorbed on the surface of clay minerals.The enrichment of soil heavy metal elements is also affected by weathering.There is a significant negative correlation between each heavy metal element in the profile and the Si-Al-Fe index.As the degree of desiliconization and iron-rich aluminization of the soil in the profile increases,that is,the degree of weathering increases,heavy metal elements in acid-insoluble matter such as clay minerals will be further released,resulting in an increase in the content of heavy metals in the soil.