Enrichment Behavior Of Heavy Metal Elements And Environmental Risk Assessment During Weathering And Soil Formation Of Carbonate Rocks

Carbonate rocks?extremely?poor in heavy metals during weathering and soil formation,on the one hand,heavy metal elements are leached and released in large quantities,which flow into the external environment?water system?with weathering solution,on the other hand,heavy metal elements in weathering residues are significantly enriched,and there are serious hidden dangers to the soil ecological environment.The rock-soil interface is a very important place for the release/enrichment of heavy metals during the weathering and soil formation of carbonate rocks,but the study on leaching,migration and enrichment of heavy metals on it is still relatively scarce.In this paper,four weathering profiles of carbonate rocks in Guizhou karst area are studied.The enrichment behavior of eight heavy metals,such as Cr,Ni,Cu,Zn,Cd,Pb,Th and U,is studied.The geochemical mechanism of heavy metals enrichment during weathering and soil formation of carbonate rocks is revealed.The following conclusions and understandings are obtained:?1?The homogeneity and in-situ property of parent rocks are demonstrated by correlation analysis of conserved elements in carbonate weathering profiles,which is the fundamental premise for discussing the development and evolution characteristics of the profiles.?2?In the process of weathering and soil formation of carbonate rocks,heavy metal elements generally present the geochemical characteristics of low background,strong loss and high enrichment,that is,the content of heavy metal elements in bedrock is generally very low.During the process of rock-soil interface interaction,heavy metal elements are continuously leached and enriched.At the same time,the further release of residual heavy metal elements in weathering crust is a long process.Long process:On the one hand,acid-soluble phase in weathering crust is strongly leached,showing obvious activity.On the other hand,due to the large reduction of carbonate rock mass,residual heavy metals are significantly enriched at the rock-soil interface.It also shows that the rock-soil interface is a very important geochemical site where heavy metals are strongly depleted and enriched.At the same time,the alkaline barrier in the rock-soil interface leads to the leaching and deposition of heavy metal elements from weathering solution,which results in the introduction of rock powder layer and conceals the leaching characteristics of rock powder layer,and the leaching characteristics of powdered rock are partly concealed.?3?In the process of weathering and soil formation of carbonate bedrock,accompanied by rapid dissolution of carbonate components,the loss of Cr,Ni,Cu,Cd,U and other heavy metals occurs,which indicates that these heavy metals have strong activity,and mainly occur in acid-soluble phase,while Zn,Pb and Th show both active side and overall entrance under certain weathering medium conditions.The energy is due to the transverse migration of elements in adjacent longitudinal profiles with the weathering solution.In addition,different types of parent rocks?limestone or dolomite?have no obvious influence on the migration,release and enrichment of heavy metals,and there is no necessary relationship between the depletion degree of heavy metals and the content of their corresponding parts.?4?In the initial stage of weathering,with the dissolution of carbonate,the acid-insoluble phases in carbonate rocks decompose simultaneously,and the dissolution rates of different mineral phases are different.The total leaching amount of heavy metals in carbonate rock system is mainly due to the dissolution and release of acid-soluble phase,and partly comes from the contribution of acid-insoluble decomposition.The lower the acid-insoluble phase content,the higher the acid-soluble phase content.In the process of weathering and soil formation of carbonate rock,the larger the volume of carbonate available for dissolution,the higher the concentration of residual heavy metals.?5?In the four weathering profiles of carbonate rocks,heavy metals mainly occur in relatively soluble acid-soluble phase components such as CaO,MgO,K₂O,and relatively stable acid-insoluble phase components such as SiO₂ and Al₂O₃.Organic matter?OM?is not the main carrier of heavy metals,which also shows that its influence on the geochemical behavior of heavy metals is relatively weak.With the decrease of pH value,heavy metals mainly occur in acid-insoluble phase components such as SiO₂ and Al₂O₃.Geochemical behavior has little influence,showing obvious inertia,while other elements show obvious activity,of which Cd is most sensitive to the change of pH value.?6?Among the carbonate weathering profiles studied in this paper,Cr,Ni,Cu,Zn,Cd and Pb all exceed the screening value of soil pollution risk of agricultural land in varying degrees,especially Cd is the most serious,which is likely to become a potential area of heavy metal pollution in soil under natural background,and cause adverse effects on soil ecological environment quality,which should be paid attention by scholars.?7?In the process of weathering and soil formation of carbonate rocks,the reason for the obvious enrichment of heavy metals at T1 of overlying soil layer in the rock-soil interface is that on the one hand,the huge loss of carbonate phase leads to the significant relative enrichment of residual heavy metals in acid-insoluble phase;on the other hand,the weathering residues formed by early weathering of upper weathering crust,along with the weathering profile.Further weathering has been accompanied by intense leaching and deposition,in which Cd in T1 of the overlying soil layer on the rock-soil interface of XP profile has been super-enriched.