Health Risk Assessment And Tracing Of Thallium Pollution In Farmland System Of Pyrite Mining Area

Thallium(Tl)is a highly toxic rare heavy metal,and its content in natural environment is generally lower than 0.50 mg/kg.Due to the development and utilization of thallium bearing minerals,Tl enters into water,atmosphere,soil,and human body through food chain,causing serious environmental pollution.In this paper,the farmland system within 10 km around a pyrite mine in Yunfu City were the major research area,and the contents of heavy metals such as Tl,Pb,Zn,Cd,Cr,Cu,Sb,Mn,Co and Ni in the soil and crop were measured systematically by ICP-MS..The contamination situation,bio-accumulation characteristics,health risks and pollution sources of heavy metals were studied by calculating the Geo-accumulation index,bio-accumulation factor,intake risk and pollution contribution rate.On this basis,the compositions of Pb isotopes in representative soil and crop samples were determined by multi-collector inductively coupled plasma mass spectrometry(MC-ICP MS).By using Pb isotope fingerprint tracing technology and two terminal mixed model,the source of heavy metal pollution such as thallium in farmland soil and crop system was quantitatively analyzed,which provided certain scientific basis and practical basis for the establishment of precise control scheme of thallium pollution in farmland system.(1)In the range of 0-5km from the mine,the contents of Tl in the farmland soils exceeded the maximum permissible limit(MPL)of Tl in soil,1 mg/kg,reaching 1.35-14.8mg/kg,presenting moderate heavy pollution.In addition to Tl pollution,the contents of Pb in78% of soil samples exceeded the MPL,50 mg/kg.The contents of Cr in 61% of soil samples exceeded the MPL,0.30 mg/kg.Most of the other heavy metals such as Zn,Cu,Cr,Mn,Ni,Sb and Co did not exceed the corresponding maximum permissible limit.A comprehensive analysis of the mineral composition characteristics of the soils confirmed that mining activities led to the entry of some sulfide minerals or secondary minerals into the cultivated soil in the form of micron scale fine particles.(2)There was a significant risk of Tl in crops consumption.The collected crops all appeared a moderate or higher intake risk of Tl.Tl pollution generally showed a trend ofdecreasing with the increase of distance from the center of the mine.In addition to Tl risk,there were also relatively obvious Cd and Cr intake risks in crops,while the risk assessment values of heavy metals Zn,Pb,Cu,Mn,Co,Ni and Sb were all within the allowable range.In addition,it was found that Brassica napus,Brassica juncea and Chrysanthemum coronarium L.may be the effective enrichment and transport of Tl.(3)The results of correlation analysis showed that Tl and Pb in soils and crops have significant positive correlation.It proved that Tl and Pb have some homology and similar geochemical migration process.By comparing the Pb isotopic information of samples,it was found that the Pb isotopic ratios of soils and crops in farmland system are similar to that of pyrite raw materials and waste residue,but obviously deviate from the Pb isotopic ratios of local unpolluted soil.It was calculated by the two end element mixed model that 63.9%-79.4% of Tl in the soils from sampling points B,C and D are from pyrite mining and utilization activities,and 64.3-71.3% of Tl in crops are from pyrite mining and utilization activities.Therefore,the main contribution source of farmland Tl pollution in the pyrite utilization area is the local mining activities.The results revealed the ecological risk,distribution characteristics and pollution sources of thallium in farmland system of pyrite affected area.It not only deepened the cognition of thallium pollution change in farmland system,but also had clarified the migration and diffusion path of thallium pollution through isotopic tracing,providing a important scientific basis for heavy metal pollution prevention and control and comprehensive treatment of farmland affected by mining area.