Study On The Migration And Transformation Mechanism Of Heavy Metals In The Soil Of Ion-type Rare Earth Minerals

The exploitation and utilization of ionic rare earth has greatly enhanced the development of rare earth mining and the competitiveness of the international market.However,the mining process has also brought about geological environmental problems that cannot be ignored,especially the severe over-standard of heavy metals in rare earth mining areas and peripheral soils,which brings challenges to the green extraction and ecological environment protection of this type of rare earth minerals.But ionic rare earth ore extraction belongs to the chemical displacement reaction process.,while the rare earth elements are desorbed,the acid-base,salt-based and redox buffer systems in the soil are degraded,and heavy metal element activation and migration into the groundwater environment,causing catastrophic changes in the soil and groundwater systems of the rare earth mining area.Therefore,studying the migration and transformation mechanism of heavy metals in the leaching process has important guiding significance for the green mining of ionic rare earth and the ecological environment protection of mining areas.This paper takes a rare earth mining area in Longnan County,Ganzhou City,Jiangxi Province as the research object,selects the typical in-situ leaching profile through field research,and reveals the weight transfer characteristics of heavy metals migration and transformation and its influencing factors based on different period sampling analysis.Through the typical section remodeling simulation leaching experiment,controlling the leaching solution pH value,leaching solution concentration and deionized water and other experimental conditions,It reveals the migration and transformation mechanism of heavy metal elements in rare earth ore under different leaching conditions,and constructs the cumulative release model.of heavy metal elements.The main conclusions are as follows:(1)The in-situ leaching profile analysis showed that the background content of Pb,Zn and Cu in the ionic rare earth ore decreases gradually with increasing depth,and the average value exceeds the primary standard value of soil environmental quality(GB15618-1995).The soil pH and Eh values changed in different degrees in the leaching,the organic matter showed vertical migration and transient enrichment,and the feldspar content decreased significantly,the clay mineral content of each layer showed an increasing trend.In addition,Pb,Zn and Cu exhibit longitudinal migration,which is temporarily enriched in the third layer of soil during the mid-leaching period,and continues to migrate downward or migrate out of the soil in the later stage,while heavy metal Cu is still partially enriched in the third layer.In-situ leaching observation analysis shows that clay minerals and organic matter content are the main factors affecting the migration ability of heavy metal Pb.(2)Indoor simulated leaching experiments show that deionized water immersion has a weak effect on Pb migration and transformation.Under different pH and different concentration leaching conditions,the soil pH value of each layer first decreased and then increased and the Eh value showed the opposite trend.The clay mineral content of the upper layer decreased gradually with the leaching,while it shows a tendency to reduce fluctuations in the middle and lower soils.The main speciation of Pb in soil is weak acid soluble form and reducible form.After leaching,the weak acid soluble and reducible form of the upper and middle soils reduces,and the weak acid soluble and reducible form of the lower soil has a less change.(3)The migration and transformation of Pb under different leaching conditions is divided into two stages.The first stage is the rapid release stage,on the one hand,the weak acid-soluble Pb releases into the leaching solution;on the other hand,the micro-particles such as clay minerals and iron-manganese oxide in the upper layer and the Pb combined with them migrate to the middle and lower layers and temporarily enriches.In the above process,the conversion strength of Pb to the solution is inversely related to the pH value of the leaching solution,and is positively correlated with the concentration of the leaching solution.When the concentration of leaching is C=8%,the soil permeability coefficient is the highest.The second stage is a slow release phase,in which other forms of Pb are slowly converted into solution,and the lower the pH of the leaching solution,the more the release of Pb.Under different concentrations of leaching conditions,the soil permeability coefficient increases with the concentration of leaching.(4)The study on the cumulative release process of heavy metal Pb shows that it exhibits high-speed accumulation at the initial stage and slowly increases at the later stage.Using the kinetic equation model fitting,it can be known that the release reaction of Pb under simulated leaching is not a simple first-order reaction process,but a process controlled by multiple diffusion mechanisms and complicated reaction.There are also some differences in the equation model for the cumulative release of Pb under different leaching conditions.