Migration And Distribution Characteristics Of Heavy Metals In Contaminated Soil And Their Ecotoxicity Assessment In Ionic Rare Earth Mining Areas

The extensive and unregulated mining activities in the ionic rare earth mining area leads to the widespread migration and accumulation of heavy metals in the soil,results in the serious pollution of surrounding water and soil ecosystems,which seriously threatens human being.In this study,the distribution characteristics of heavy metals and their ecological risk assessment were analyzed for the contaminated soil of a typical ionic rare earth mine in southern Jiangxi province.Furtherly,the inhibition of toxicity on microorganisms and soil animals by different concentrations of lead（Pb）and cadmium（Cd）were investigated.Finally,we used the measured data obtained from experiments and reported data collected from literature to co-construct the species sensitivity distribution（SSD）curves,and deduce the ecological safety thresholds for Pb and Cd in mining soil conditions,respectively.Our results provided the basic database and theoretical basis for the ecological remediation and carrying capacity assessment of ionic rare earth mining areas.The main research results are as follows:（1）There is an obvious accumulation of heavy metals in the soil of the typical ionic rare earth mining area in southern Jiangxi province,and the ecological risk of heavy metals is high.In particularly,the highest contents of Pb and Cd in the topsoil of the mine area exceed the background value of Jiangxi soil by more than 13 and 40times,respectively.The results of the ground accumulation index and RAC risk evaluation indicate that Pb is at high contamination and ecological risk throughout the mine area,while Cd,copper（Cu）,zinc（Zn）,and arsenic（As）have moderate ecological activity.（2）Pb and Cd addition inhibited the soil potential ammonia oxidation rate（PNR）,soil enzyme activity,and soil fauna and microorganisms to some extent.The correlations between soil PNR,soil acid phosphatase and dehydrogenase activities,alpha diversity indices（ACE,Chao,Shannon,and Simpson）of bacteria and fungi,survival rate of Folsomia candida and heavy metal concentrations were conformed to logistic dose-effect relationships,and the correlations with the effective state of Pb and Cd were tighter.According to the dose-effect curves,we deduced that the 10%and median effect concentration(EC₁₀ and EC₅₀,respectively)of Pb were in the range of0.55-394.95 mg·kg^-1 and 61.03-3008.52 mg·kg^-1,respectively,the EC₁₀ and EC₅₀ of Cd were in the range of 0.10-126.62 mg·kg^-1 and 4.49-344.91 mg·kg^-1,respectively.（3）The normalized toxicity values were used to construct the species sensitivity distribution（SSD）and deduce the ecological safety thresholds that can protect 95%of the species（HC₅）for Pb and Cd.The HC₅ of Pb and Cd in the ionic rare earth mining area were 15.15 and 0.02 mg·kg^-1,respectively.The survival rate of F.candida and the biomass of Capsella bursa-pastoris were at the bottom of the Pb and Cd SSD curves,respectively,indicating that these two indices are more sensitive to Pb and Cd,and can be used as one of the early judgment indicators of Pb and Cd contamination in soil.