Soil Heavy Metal Pollution Risk Evaluation And Humic Acid Control Of Heavy Metals In A Mining Area Of Western Inner Mongolia

One of the important causes of heavy metal pollution in mining areas is tailings piling and The hazard cannot be ignored.In this experiment,soils around an abandoned copper tailings mine in western Inner Mongolia were the object of study.Investigated the geographic characteristics of the study area and determined the study area to be 1.4 km~2,including the tailings pond.A total of 75 soil samples were collected at 25 sampling points in this area,in stratified sampling(0～5 cm,5～10 cm,10～20 cm soil layers).The total contents of heavy metals Cu,Zn,As and Pb in soil were determined,and the fugitive forms of heavy metals were extracted using the modified Tessier method.The heavy metal contamination was evaluated by the Single Factor Index,Geoaccumulation Index,Potential Ecological Risk Index,Risk Assessment Code and Rations of Secondary Phase and Srimary Phase.Based on these,Humic acid was added to the soil to investigate the characteristics of heavy metal leaching release and the pattern of heavy metal forms transformation in the soil.Results show:(1)The individual sites in the study area are acidic due to mine dumping,the rest are alkaline and weakly alkaline soils.The organic matter content of the soil is low and the soil quality is poor.The soil Cu content in the study area was high,with the mean value 15.99times higher than the background value.The cumulative average contents of Zn,As and Pb were 1.84 times,2.71 times and 5.16 times of the background values,respectively.The heavy metal content of some sampling points exceeded the standard seriously.The overall contamination level in the study area was ranked as Cu>Pb>As>Zn,and the risk of contamination was significantly reduced in the 10～20 cm soil and the area outside 200 m from the tailings in the whole study area.(2)The residue state and Fe-Mn oxidation state are the main fugitive forms of heavy metals in the soil around the tailings.In addition,the contents of carbonate-bound Cu,primary sulfide and carbonate-bound Zn,organic-bound As,and carbonate-bound Pb in the soil were also at high levels.The biological effectiveness of heavy metals was higher in soils from 10～20 cm than in soils from 0～5 cm in the study area,and the biological effectiveness of heavy metals was lowest in soils from 5～10 cm.The migration capacity of three heavy metals,Cu,Zn and As,decreased with the deepening of the soil layer.(3)Soil p H and the content of residual state Zn were significantly and positively correlated.Soil organic matter was negatively correlated with the amount of residue state of heavy metals and positively correlated with carbonate bound state,Fe-Mn oxidation state and organic bound state.CEC showed a significant negative correlation(P<0.05)with the amount of carbonate bound state and organic bound state,and a significant positive correlation(P<0.05)with the amount of exchange state and residue state.CEC was not significantly correlated with the fugitive form of Pb.(4)Most of the heavy metals were released in large amounts within 0-72 h after humic acid was added to the soil to simulate natural rainfall leaching,and the release rate slowed down as the leaching time increased.The cumulative release of heavy metals was from Cu>As>Zn>Pb in descending order,and the amount of humic acid added to the soil had an important effect on the release rate of heavy metals.The release of heavy metals was lowest when the amount of humic acid added to the soil was 3%.(5)Exogenous addition of humic acid promoted the increase of adsorbable sites in the soil and enhanced the stability of each heavy metal in the soil.The effectiveness of heavy metals decreases continuously as the aging time increases.When the soil when is weakly acidic,the transformation between some active states is more active.And when the soil conditions are alkaline,heavy metals are more easily transformed into some more stable forms,such as organic bound state,primary sulfide state,and residue state.In summary,soil p H variability in the study area is high,organic matter content is low.The heavy metals copper and lead are high in the soil.The heavy metal form is composed of iron-manganese oxidation state and residue state.The heavy metal pollution reached a moderate level.After humic acid addition and drenching with simulated rainfall,a large amount of heavy metals had been drenched out after 72 h.The cumulative release of heavy metals increased slowly.At soil p H is 8 and 9,heavy metals are stably present in the soil medium,which facilitates the solidification of heavy metals.