| It has been 50 years since the discovery and mining of ionic rare earth in China.Mining has produced a series of ecological and environmental impacts,which are closely related to the mining technology.Nowadays,revival for the changed soil characteristics,serious soil erosion,and the destroyed vegetation relies on the high-cost manual reclamation,because it is tough to recover naturally.The location that is in the granite red soil ecologically fragile area,as well as the special deposit form and mining methods of ionic rare earth ore,which make the circumstance of the mining area with distinctive regional characteristics.Mining ionic rare earth not only wastes a lot of rare earth resources,but also leaches a large amount of rare earth into the surrounding environment.Under the dual effects of natural phenomena(eg.rainfall leaching,runoff)and human activities(eg.mining,smelting,and farming),rare earth elements are likely to migrate,accumulate,transform,and diffuse in the soil of the mining area.Then they enter the human body through the food chain and other means,which has an impact on the physical and mental health of the local residents.Based on the theories of soil and water conservation,soil science and geochemistry,this dissertation takes two mining technology of typical ionic rare earth minerals in southwestern Fujian as the research object,and uses methods by combination field investigation and experimental analysis with simulation to compare the impacts of two mining technology in soil-vegetation in mining areas systematically.The distribution and geochemical behavior of rare earth elements in the soil were analyzed and the law of element differentiation was expounded after mining rare earth in this dissertation.Simultaneously,the migration of rare earth elements in the soil of the mining area was simulated by rainfall leaching,and the pollution characteristics of the rare earth elements in the paddy field soil surrounding the mining area were analyzed.Only by studying the influence mechanism of different rare earth mining technologies on the ecological environment can we provide scientific basis for the environmental control,ecological restoration and the evaluation of the soil environment in the rare earth mining area.This study will support for above.The main research results are as follows:(1)Impacts on surface soil-vegetation in rare earth mining areas under two mining techniquesThe soil p H in the heap leaching mining technique area ranged 4.31-5.22,and the minimum value of different soil layers is the untreated site,with a strongly acidic value under 4.5.The soil acidity is increased by mining,and the soil structure is damaged.Because the content of sand particle is significantly higher than that of the unmined site,it has not been effectively improved by treatment.The maximum content of soil organic carbon is only 3.76 g·kg-1 after treatment for 8 years.Soil structure damages and nutrients loss are difficult to recover in the short term.In-situ leaching mining technique also led to the acidity of the surface soil to increase,and the p H range is between 4.31-4.71,but a less damage to the soil structure,and less loss of soil carbon and nitrogen nutrients compared with the heap leaching mining technique.In terms of soil eco-stoichiometric characteristics,the soil C:N in the heap leaching mine area was imbalanced ranged0.89-15.42,and the nitrogen shortage significantly restricted the soil recovery process.The plants formed a certain phosphorus shortage adaptation mechanism in the mining area.The relatively stable C:N ranged 8.87-14.3 state of the soil in-situ leaching mine area and a lower loss of phosphorus,which makes the soil in this area present a certain lack of soil organic carbon,and the soil nitrogen restriction is aggravated.The plants in the area of heap leaching area are herbs and shrubs,where it is difficult for trees to grow up in a short period.The changes in the vegetation restoration process took place in the herbaceous layer,the artificially planting herbs were gradually replaced by naturally invaded herbs(Dicranopteris dichotoma).While the changes of the community in abandoned in-situ leaching areas were less obvious.Mining mainly destroyed shrubs and herbs,but the herbs can recover quickly.(2)Differentiation of rare earth elements in the soil of rare earth mining areasIn the horizontal direction,15 rare earth elements in the surface soil were roughly the same,except for the difference in the order of Tb and Ho.The total content of rare earth elements(TREE)in the mining area varied from 327.63-653.64 mg·kg-1,with an average value of 425.50 mg·kg-1,which was higher than the Fujian Province's soil background values.Rare earth mining does not significantly change the distribution mode of rare earths,with the average of?Ce in the mining area and the unmined area were 0.64 and0.74,respectively.The average of?Eu in both the mining area and the unmined area was-1.24,presenting a strong anomaly.The light rare earth elements(LREE)of surface soil normally show moderately enrichment and slightly inclined to the right after being normalized by C1 chondrites,while Eu presented a negative anomaly distribution pattern,with some differentiation among each slope positions.The distribution curve was similar to rare earth elements in the soil of Fujian Province,which indicated that the properties of the parent rock formed by the soil were similar.It can be seen that the distribution difference was further manifested between each slope positions from the standardized distribution model of North American shale.The distribution curves of the middle and lower slope positions in the mining area show a relatively consistent trends except for La and Y,while the upper slope position showed a heavy rare earth leaching.In the vertical direction,the TREE in the mining area were in a large variation,263.99-1225.08 mg·kg-1,the average value was 667.90 mg·kg-1.The average value of light rare earth/heavy rare earth(LREE/HREE)was 4.47,and the average value of LREE/HREE in unmined areas is 6.24,which indicated the enrichment of LREE in two area,but LREE in unmined areas are more enriched.Rare earth mining has increased the enrichment of heavy rare earths in the lower layer,and it is further reflected by the rare earth differentiation parameters.(La/Yb)N and(La/Sm)Nshowed a synchronous change trend,exhibited LREE enrichment,and shallow layers were more enriched than that of deep layers,except for the anomaly at 50 cm depth.(Gd/Yb)N indicated the enrichment phenomenon of HREE,and the deep layers were richer than the shallow layers.The average value of?Ce and?Eu were 0.30 and-1.08 respectively,both showed obvious negative anomaly.The?Eu anomaly degree of each depth was no different.The TREE that were rarely changed in the unmined area ranged 148.99-743.91 mg·kg-1,the average value was 517.12 mg·kg-1.The average value of LREE/HREE was 6.24,relatively higher than that of in the mining area,indicating that the phenomenon enriching LREE in unmined areas was more obvious.The rare earth mining has exacerbated the enrichment of HREE in the deep layer.There is no significant change in the distribution mode of rare earths.The average values of?Ce in the mining area and the unmined area are 0.30 and0.81,respectively.Mining has increased the degree of negative Ce anomalies.The average values of?Eu in the mining areas and the unmined area are-1.08 and-1.13respectively,and both show strong abnormalities.(3)Migration of rare earth elements in the surface soil of mining area under leaching simulationUnder leaching simulation,the rare earth elements in the surface soil of the upper and middle slopes of the mining area were mainly changed in LREE,while the LREE and HREE in the soil of the downhill slope showed large change.Because TREE in the surface soil of unmined area is relatively low,the TREE changes obviously after leaching.From the distribution of the TREE at each depths after leaching,the maximum value was at the depth of 15cm,demonstrating that the enrichment of rare earth were consistent.Ce occupied a large proportion of the total rare earth elements.?Ce can be illustrate the difference in the distribution of rare earth elements in soil caused by mining.The rare earth elements migrated at different depths after leaching,and the migration of HREE accounted for a more proportion in the mining area than in the unmined area.During the soil leaching process,the content of rare earth in the leachate showed the same trend,and it decreased with the increasing leaching times.After 6 leaching times,it reached a low stable leaching state.The dissolved rare earth elements were higher in the middle and downhill position of the mining area,in the early stage was higher than the average value of the water in the small watersheds of southern Jiangxi Province.From the distribution curve of rare earth in the leachate is relatively consistent,the soil in the mining area was soaked with mineral leaching agents,and the more active rare earths have been exchanged and extracted,and the residual rare earths are relatively stable during the leaching process.But the leachate during the leaching process contains higher rare earth elements.(4)Characteristics and evaluation of rare earth elements in the surface soil of paddy fields surrounding the mining areaAccording to the calculation results of geological accumulation index method,the pollution level of the paddy soil in different locations varied.Zhuxi Watershed where scattered many abandoned heap leaching mining area,light pollution was in the east of the watershed,and the moderate pollution was in the mid-west of the watershed.In Liancheng County,the moderate pollution was in the valley of in-situ leaching area,and the mild moderate pollution surrounded the mining area.We calculated the rare earth element toxicity coefficients for the first time by simulating the methord of heavy metal potential ecological risk assessment,the results are as follows,La=1,Ce=1,Pr=5,Nd=2,Sm=5,Eu=10,Gd=5,Tb=10,Dy=5,Ho=10,Er=5,Tm=10,Yb=5,Lu=20,Y=2.With the result,we constructed a potential ecological risk assessment system for rare earth elements.According to the results of potential ecological risk evaluation method,the potential ecological risks of rare earth elements in the surface soil of the paddy in Zhuxi watershed:slight risk,moderate risk,strong risk were in the east,center,west of the watershed,respectively.In the valley of in-situ leaching area showed strong risk;and the surrounding mining area showed the moderate risk.Comparing the evaluation results of the two evaluation methods,there is a certain difference between the results of the two evaluation methods.The geological accumulation index method weakens the accumulation of single element pollution,and the potential ecological risk evaluation can more reflect the cumulative results of element evaluation.Combine the two evaluation methods organically to evaluate the pollution level of rare earth elements will more comprehensively and reasonably. |
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