Sequential Chemical Extraction Study On Mobility Of Elements During Weathering Of Black Shales In Central Hunan, China

On the basis of the existing work,this study studies on the activity characteristics and migration and transformation rules of elements in the weathering process of Black Shale in Central Hunan Province,and associated with past through mass balance equation of element active regularity,this paper compares and analyzes further explore and examine black shales weathering elements active characteristics.The results show that:（1）By analyzing the proportion of residue state and active state in weathering process,it is concluded that there is certain activity in weathering process of Al₂O₃,MgO,Fe₂O₃,MnO,P₂O₅,CaO and other major components in weathered black shale.And the activity size may increase in turn.In addition,the occurrence characteristic of the main component TiO₂ is mainly the residue state,and there is no obvious change in the spatial distribution,which indicates that the chemical property of the component is very stable in the weathering process.During weathering,the change of chemical activity characteristics of major components is related to the change of major minerals,and the adsorption of organic matter has little influence on its activity.（2）In the weathering process,the heavy metal elements such as Sn,Tl,Cr,V exist in the residual state obviously,and are not easily leached and released in the weathering process,and have little influence on the environment.Heavy metal elements such as Ni,Cu,Bi,Cd,Zn,Pb and Co show similar leaching mechanism in the weathering process.In the middle section,the oxidation state is dominant and the chemical activity is complex.（3）About lithophile elements,Ga,Ge,Cs,Rb,W and other elements are mainly residue state in the weathering process,and their chemical properties are stable.The initial weathering layer of Th,Sc,and U elements mainly exists in weathering resistant sulfide minerals with small activity.In the subsequent weathering process,the above elements are decomposed from sulfide minerals and adsorbed on organic matters or other substances with strong activity.In the weathering process,the Sr,Ba elements mainly exist in carbonate-bound state,and their chemicalpropertiesaremoreactive.（4）The high field strength element（Zr,Hf,Nb,Ta）accounts for the majority of residues in the weathering profile and maintains a stable chemical form and weak activity in the weathering process.（5）On the weathering profile of black shale in central Hunan,rare earth elements mainly occur in carbonate,phosphate and other secondary minerals,or are adsorbed on the surface of clay minerals.In the process of black shale weathering,the rare earth element has certain activity and the rare earth element is fractionated after the initial weathering,and the selective leaching of the rare earth in the subsequent weathering process is easy to occur.（6）Three chemical reactions in the process of black shale weathering are confirmed:the oxidation of the sulfide,the dissolution of the refractory minerals and the change of the clay minerals.（7）According to the migration regularity and activity characteristics of elements in black shale,the elements in black shale can be divided into four categories:Ti,Zr,Hf,Nb,Ta,Cs,V,Rb,Ga,Sn,W,Tl and Cr which are stable elements with stable chemical properties and are basically not leached out;Ca,P,Ba,Sr and REE are active elements;Co,Ni,Zn,Cd,Pb and other elements are related to Mn oxides;elements such as Fe,Al,Mg,Bi and Cu constitute elementsrelatedtoFeandAloxides.（8）In comparison with previous studies,it is found that the results of element activity obtained by mass balance equation are different from those obtained from successive extraction experiments.Among them,the activity discrimination of some major elements（such as Al,Fe）and trace elements（such as Mn,Zn,V,Cr,Ga,etc.）is quite different.It shows that the mass balance equation is used to judge the migration release rate of elements in the black shale weathering process.The activity judgment of many heavy metal elements and lithophile elements is not consistent with the actual situation.