Mobility And Transformation Of Thallium In Contaminated Soil Profiles

As a typical,rare heavy metal,Thallium?Tl?has higher toxicity in human beings than mercury,lead,arsenic,and copper.With the exploitation and smelting of Tl-containing ore,a large amount of Tl is activated and released into soil,atmosphere and water.Then Tl continues to diffuse and mobilize,which greatly increases the harm to the environment.The Hg-Tl mining area in Lanmuchang,Xingren County,Guizhou Province is a typical independent Tl mine in the world and a typical high Tl polluted area.Therefore,two different pollution types of soil,Tl slag contaminated soil profile?profile A?and Tl slag leached rainwater contaminated soil profile?profile B?,were studied the characteristics of Tl migration,transformation and its influencing factors in the natural environment via a modified Institute for Reference Materials and Measurement?IRMM?sequential extraction?four-step?scheme.The purpose of this study is to provide basic theoretical support for the prevention and treatment of Tl contaminated soil.The results are as follows:?1?Tl content in soil profile A is 48.7?375 mg/kg,while Tl content in profile B is obviously lower,38.2?66.9 mg/kg,with the average content exceeding the background value of Chinese soil by 189 times and 83 times respectively.Also Tl may continue to mobilize downward and pollute groundwater.The characteristics of Tl migration behavior from different sources are obviously different,and it is related to the content of Fe/Mn oxides,silicate minerals,SO₃,TOC and pH.?2?The weak-acid-exchangeable fraction?F1?and oxidizable fraction?F3?of in soil profile A are both lower than 10%,mainly in reducible fraction?F2?and residual fraction?F4?.This part of Tl mainly comes from dissolution and release of Tl-containing slag or sulfide ore,and mainly transforms into reducible fraction and residual fraction during migration.Compared with profile A,Tl in profile B is derived from long-distance migration of slag after rainwater leaching,which is more labile.it is mainly transformed into F2?over 40%?and F4?over 30%?,and a small amount to F1?12.7%?and F3?6.23%?.The results showed that the chemical transformation of Tl in soil is"rapid"to reach equilibrium.?3?Tl released by leaching mainly exists in the form of Tl?I?,which can be adsorbed and oxidized by Fe/Mn oxides in soil,and then converted into F2.Tl?I?can also replace K?I?and enter the interlayer of illite and jarosite crystal lattice in soil, while Tl entering the illite mineral crystal lattice is transformed into F4.Furthermore,magnetite,hematite,birnessite and jarosite in soil are rich in Tl,and these minerals are important influencing factors of Tl migration and transformation in the soil.?4?The?²⁰⁵Tl ranges from?0.56 to 2.41 and?0.23 to 3.79 in soil profile A and profile B,respectively,indicating that Tl isotope fractionation occurs during Tl migration and transformation.During the downward migration of Tl in profile B,the underlying soil tends to enrich heavy Tl isotopes.At the same time,the proportion of F1 and F3 increase while F2 decrease,indicating that Tl may migrate in the form of soluble organic Tl?III?in addition to the form of Tl?I?.The possible major transformation forms of Tl in soil are F1?F2,F4 and F2?F3,F4.?5?The proportion of labile Tl(Tl_labile,Sum of F1,F2 and F3)in soil profile A and profile B is as high as 40.1%and 54.4%respectively,indicating that Tl in soil has high mobility and bioavailability.Adding alkaline materials such as lime to Tl contaminated soil can increase p H to reduce Tl mobility.Increasing the content of Fe/Mn oxides or clay minerals?such as illite,jarosite,montmorillonite,etc.?in soil is conducive to the transformation of Tllabile to residual fraction,and then fix Tl in soil.However,dissolved organic matter in soil will enhance the activity of Tl and make Tl migrate downward,thus increasing the risk of groundwater pollution.