| Soil plays an important role in the global geochemical cycle of elements as it’s an important product during continental weathering.However,in the process of weathering,the migration and transformation of elements are affected by many geological processes,such as mineral dissolution,adsorption,precipitation and biological processes.It is difficult to understand the influence and contribution of the single process when directly analyzing the content of metals in soil.Therefore,it is necessary to study the speciation distribution characteristics of metals in soil in order to better understand the behavior of elements in weathering,migration and transformation.In this work,five widely distributed typical soils(latosol,yellow brown earth,brown soil,red soil,paddy soil)and a stream sediment reference from mining area are selected to carry out comparative study on the speciation distribution characteristics of metals in soils and sediments.1)Based to the properties of the samples and the comparison of the results of the two most commonly used extraction methods Tessier and modified BCR method,we choose the modified BCR method for speciation extracted for collecting four fractions of elements,which are the acid soluble,reducible,oxidizable and residual,respectively.The modified BCR method is then applied to five types of soils and one stream sediment,the speciation distribution characteristics of potentially toxic elements(chromium,nickel,copper,zinc,lead)and metals with important scientific research and economic value(vanadium,cobalt,zirconium,strontium,cesium,gallium and rare earth elements)are carefully investigated.The recovery of many elements is between 94% and 112%.2)The results show that the speciation distribution of different metals in the sediment and different types of soils are different due to the influence of geochemical properties of metals themselves and formation conditions of soils(such as parent rock type and climatic conditions).In particular,latosol shows completely different metal speciation distribution characteristics from other soil types,for example,the acid soluble fraction of strontium,reducible fractions of cobalt,cesium and rare earth elements as well as residual fractions of nickel,copper and zinc are significantly higher than those in the same speciation in other soil types and stream sediment.Based on the above results,3)a complete latosol profile formed through long-term weathering of basalt is chosen and analyzed to determine the speciation distribution characteristics of metals and further explore the mechanism of metal speciation transformation during weathering.It is showed that vanadium,chromium,nickel,copper,zinc,zirconium and cesium have relatively consistent distribution throughout the profile,mainly concentrated in residual fraction(> 80%).The speciation distributions of rare earth elements(except cerium)in the profile are similar,while the speciation of lead,strontium and cobalt in different depth of the profile are different.4)Comprehensive analysis indicates that the speciation distribution of metals in different soil types and stream sediment are affected not only by the physical and chemical properties of metals,but also by the parent rock type and climatic conditions(rainfall,p H,etc.).In latosol profile,the content and speciation distribution of cobalt and rare earth elements(except cerium)are obviously affected by manganese minerals,and the content of lead and cesium are largely controlled by total organic carbon.In addition,5)the bioavailability factor of rare earth elements in latosol are significantly higher than those of other metals,which may be caused by the specific adsorption of rare earth elements on iron manganese oxides and clay minerals. |
