Migration And Transformation Mechanisms Of Heavy Metals In Paddy Soils Affected By Acid Mine Drainage

The paddy soils around metal mining areas are seriously contaminated with heavy metals due to the long-term affected by acid mine drainage（AMD）,which poses a major threat to the ecological environment and human health.AMD is characterized by low p H,rich in sulfate and heavy metals,and its input changes the redox state,mineral composition and microbial community structure of paddy soils,which in turn affects the biogeochemical behavior of heavy metals.Therefore,it is particularly necessary to carry out the study of heavy metal migration and transformation processes by comprehensive environmental factor changes affected by AMD.In this study,we used a field study combined with laboratory soil column leaching experiments to introduce a contaminant transport model to analyze the changes of soil redox potential（Eh）,mineral composition,microbial community,pore water physicochemical properties and other environmental factors in the initial stage of AMD input,the continuous pollution stage of AMD and the clean water irrigation stage after the blockage of AMD input,and combined with the speciation transformation characteristics of Cu and Cd in the soil solid phase to systematically elucidate the heavy metal migration and transformation mechanisms affected by AMD.The main research results are summarized as follows:（1）The study of the gradient changes in heavy metal concentration and soil microbial community response caused by AMD-contaminated paddy soils irrigated by river water in the Hengshi River Basin found that:The soil heavy metal concentration of paddy soils in the Hengshi River Basin showed an overall decreasing trend along the river direction;Cu and Zn mainly existed in the residual fraction,and Pb mainly existed in the Fe/Mn oxides-bound fraction.Cd has a high concentration in the exchangeable fraction.Soil silt,clay and Eh play an important role in the fraction of heavy metals.River irrigation reduces the abundance and diversity of acidophilic communities in AMD-contaminated soils in the Hengshi River Basin and affects the spatial and temporal changes in the overall composition and structure of soil microorganisms.Principal component analysis showed that the main factors affecting changes in microbial community distribution were Cu,p H and soil particle composition.（2）Studies on the migration and transformation mechanisms of Cu and Cd in paddy soils at the initial stage of AMD input showed that the retention of Cu and Cd in paddy soils was as high as 70%-99%,with the maximum adsorption capacity of 658.04 mg/kg for Cu in paddy soils being much higher than that of Cd（335.20 mg/kg）.The results of Thomas and Yoon-Nelson model fitting showed that the mobility of Cd in paddy soils under the influence of AMD was higher than that of Cu.AMD flooding favors the formation of mackinawite by dissimilation and reduction of sulfate,which in turn affected the fraction of Cu and Cd,leading to Cu mainly in Fe/Mn oxides-bound fraction and organic matter-bound fraction,and exchangeable fraction and carbonate-bound fraction of Cd was significantly increased.（3）The study on the drivers of Cu and Cd speciation in paddy soils during the AMD persistent pollution stage showed that the mobility factor（MF）of Cu and Cd in soil increased significantly under the influence of low p H and high SO₄^2-in AMD,especially in 0 cm soils,the MF of Cu increased from 16.2%to 40.5%and that of Cd increased from 62.6%to 81.9%.After 60 days of continuous irrigation by AMD,mackinawite and pyrite formed in the soil of20 cm,which coincided with the results of the constructed Eh-p H mineral phase diagram analysis.Continuous AMD irrigation did not increase the Fe/Mn oxides-bound fraction and organic matter-bound fraction of Cu in the soil below 10 cm（～0.18 mg/g）,while the exchangeable fraction and carbonate-bound fraction of Cd increased in proportion by～5%.（4）The study on the changes of soil environmental factors and the speciation distribution characteristics of Cu and Cd in paddy soils at the stage of clean water irrigation after AMD input blockage showed that clean water irrigation led to an increase in the diversity of microbial communities,and the variability between groups was not significant,and the response of core microbial communities to soil environmental changes in paddy soils was relatively stable.With the extension of flooding time,the sum of Fe/Mn oxides-bound fraction and organic matter-bound fraction concentration of Cu decreased from 188.4 mg/kg to 30.8mg/kg,and the relative binding strength of Cu tends to stabilize.The sum of the the exchangeable fraction and carbonate-bound fraction of Cd in the soil decreased from 5.8mg/kg to 0.3 mg/kg,indicating that AMD-contaminated paddy soil under clean water irrigation would form a new source of contamination,which in turn would lead to the migration of heavy metals to deeper soils.This study is important for further understanding the changes of soil environmental factors and mechanisms of heavy metal migration and transformation in paddy soils at the initial stage of AMD input,the stage of continuous AMD contamination,and the stage of clean water irrigation after the blockage of AMD input,and the related results can provide theoretical references for the prediction of biogeochemical behavior of heavy metals.