Geochemical Enrichment Characteristics And Mechanism Of Thallium In Soils Of Southwestern Guizhou,China

Thallium?Tl?is a dispersal element with high toxicity.In recent years,Tl pollution caused by human activity has become an extensive eco-environmental concern.The investigation observed that the carbonate weathering crust is relatively enriched for Tl,causing high contents of Tl in soils in the the karst terrain of Southern China.However,the migration and transformation process and mechanism of Tl enrichment during the carbonate weathering are still unclear.In order to explore the mechanism of Tl enrichment during the carbonate weathering crusts,and provide scientific data for the naturally occurring Tl contamination,two weathering crust with different lithologies in southwestern Guizhou,i.e.Shanlinjiao?SLJ?limestone profile and Ganlongdong?GLD?dolomite profile,were selected to study the geochemical enrichment of Tl in local soils,respectively,.The geochemical behavior and speciation of Tl in the weathering process of carbonate rocks was discussed in combination with mineralogy and geochemical analysis,the mass balance calculation,chemical index of alteration?CIA?and sedimentary environment conditions,and the results revealed the enrichment mechanism and influencing factors of Tl in supergene environment.The main findings are as below.Tl in the two-carbonate weathering crust presented the phenomenon of supergene enrichment.The Tl content of soil in each layer was higher than that of bedrock,UCC and Chinese soil background.For the fraction of Tl,the SLJ profile was mainly bound to residual,while the GLD profile was mainly bound to iron and manganese?hydr?oxides.And the fraction that Tl bound to iron and manganese?hydr?oxides of the two-carbonate weathing crust were higher than that of as usual surface soil.The overlying soils in the two-carbonate weathering crust were the productw of natural weathering in situ of the underlying carbonate bedrock.Specifically,the relationship of inactive element pairs of the underlying bedrock and the overlying soil,such as TFe₂O₃-Al₂O₃,Zr-Hf,and Nb-Ta,were extremely strong.The distribution model of REE had also confirmed the positive correlation.The Tl-rich karst soils are derived from two-stage model of carbonate weathering process.During the weathering process,carbonate minerals are dissolved in large quantities,and leaching loss occured for most elements?such as Ca,Mg,Na,Sr,U,Co,Pb,Cu,Mo,etc.?.However,obvious accumulation rather than leaching loss for Tl were developed with the reducing volume of insoluble residues.The enrichment peaks tended to occur at the rock/soil interface,which might be attributed for geochemical leaching and migration at the upper profile.Secondly,the pH value at the rock/soil interface was high,thus increasing the absorption ability of MnO₂ and clay minerals for Tl.For the GLD profile,the abnormally high content of Tl is partly due to the high Tl content of carbonate bedrock?dolomite?.In addition,the mass balance calculation revealed the mature and high degree of wearhering evolution in SLJ profile rather than the GLD profile.According to the correlation analysis,K,Na,Mn,Al,Ti,Fe and Si were significantly correlated with Tl.In particular,significant correction between Fe/Mn and Tl content in local natural soils were observed.Clay minerals,quartz,and amorphous oxides such as aluminum oxide and iron/manganese oxide may be the main carriers of Tl based on mineralogical characteristics.Tl may directly replace K⁺ at the edge of the illite crystal layer,and Na⁺in the interlayer structure of smectite to form a silicate material respectively.Tl may also be combined with Mn²⁺in chlorite and form complexations on the quartz surface.The pH value would indirectly affect the competitive adsorption of Tl.When the pH value of soil in the weathering profile was close to neutral,Tl would be preferentially bound to MnO₂,resulting in the proportion of amorphous and crystalline Fe and/or Mn?hydr?oxides in soil samples was abnormally higher.Overall,the geochemical enrichment of Tl in natural soil in the karst terrain of southwestern Guizhou was mainly controlled by Tl contents in the carbonate bedrock and the pedogenesis process,and the soil pH values indirectly affect the various fractions of Tl adsorption in soils.The present study provided basis data and new insight for the geochemical mechansiam of high Tl contents in natural soils of karst terrain.