| Thallium（Tl）is a typical dispersed and highly toxic element and is usually enriched in various sulfide minerals.As a result,severe pollution in water bodies,soil/sediment,and crops occurs during industrial activities such as coal combustion,mining,metallurgy,sulfuric acid/cement production,and rock’s natural weathering associated with these thallium-enriched minerals.Due to its extreme toxicity over Hg,Cd,As,and other heavy metals,thallium’s environmental impact has continued to be a concern by countries around the world.Insights into the occurrence,isotopic fractionation,speciation,adsorption/desorption,and redox behavior in various environmental media have been widely studied.However,the constrained mechanism on Tl’s mobility and transportation in super-genetic conditions remains to be further elucidated.Lanmuchang Hg-Tl ore deposit in Guizhou Province of China is the first reported independent Tl deposit globally,and chronic thallium poison cases around the mines have attracted focused concerns for decades.In this dissertation,various analytical methods are applied to investigate Tl’s mobility and transformation processes under supergene circumstances from Lanmuchang mercury-thallium mine of Xingren,Guizhou Province of China.Lanmuchang Hg-Tl ore deposit is hosted by the upper part of the Permian Longtan Formation,Changxing Formation,Dalong Formation,and the first section of Lower Triassic Yelang Formation.The wall rocks are dominated by siltstone and claystone.Lorandite（Tl As S2）,christite（Tl Hg As S3）,imhofite(Tl6Cu As16S40),raguinite（Tl Fe S2）,and pyrite enriched in thallium have been recognized as the primary thallium-bearing minerals and lanmuchangite（Tl Al（SO4）2·12H2O）as the secondary thallium-bearing mineral in the deposit.The mineralogy in mineralized rocks and mining residue is predominated by quartz and kaolinite with fewer illite,smectite,and pyrite.Concentrations of Tl in mineralized rocks and mining residues are ranged from 23.2–427μg/g,with an average of 146.3±118.2μg/g,which is three orders of magnitude higher than the 0.27μg/g in the background area.In situ LA-ICP-MS analyses for pyrite in mineralized rocks and mining wastes show that pyrite is the chief Tl host phase with fewer Tl in cinnabar and christite.The thallium content in the pyrite has an apparent bimodal distribution,with thallium content in the low content group ranging from 0.08–35.12 ppm with an average of 3.14±4.97 ppm,and thallium content in the high content group ranging from 91.21–2437.34 ppm with an average of 927.14±492.83 ppm.Tl’s bimodal distribution is attributed to two types of pyrite genesis with lower contents in diagenetic pyrite and higher contents in hydrothermal pyrite,reflecting that the thallium mineralization process in the mine area has an essential control on the distribution of thallium in the mineralized rocks.The secondary sulfate minerals associated with the weathering of mineralized host rock were analyzed,and it is found that the mineralogy is characterized by melanterite,halotrichite,and fibroferrite.The contents of these sulfate minerals are ranged from 0.05–0.15μg/g of Tl with an average content of 0.11±0.04μg/g,which is only 0.03%of the average thallium content of 349.0±85.3μg/g of pyrite.It indicates that most of the Tl derived from pyrite oxidation are not scavenged by these sulfate minerals.Both thermodynamic prediction and laboratory tests suggest that these secondary sulfate minerals formed in low p H environments.It is consistent with pyrite’s finer particle size in the rocks and the weak acid-neutralizing mineral composition（quartz,kaolinite dominated,lack of carbonate minerals）.Higher solubility of the sulfate minerals could discharge acid to the downstream environment in a later wet season.The low p H conditions in which the sulfate minerals precipitated keep soluble Fe3+in the solution,accelerating pyrite oxidation and Tl release from pyrite,enhancing Tl transportation to the surrounding environment.The soil in the mine area is the oxisols with low maturity,and the thallium content in the topsoil ranges from 5.79–270μg/g with an average content of65.74±62.32μg/g（n=21）.The main minerals of the soil are quartz,kaolinite,and illite.Analysis from a soil profile about 100 cm deep affected by underlying slag shows that the variation trend of Tl is similar to Mn and As and that there is a significant discrepancy between the mean Tl content of 30.4±11.5μg/g in the topsoil layer and that of 454.5±163.2μg/g in the slag layer.XRD,XPS,SEM-EDS,and oxalate extraction for the soil samples show that both Mn oxides and jarosite are very important Tl-bearing phases in the studied soil profile.Redox processes and mineralogical transformations related to manganese oxides,therefore,exert an important control on Tl’s mobility and transformation in soils,thus governing the bioavailability of Tl in the soil.It is suggested that the partitioning of Tl among different minerals in the soils and subsequent uptake by plants in the study area is a complex dynamic process regulated by mineralogical transformation. |
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