Study On The Effect Of Chemical Leaching And Lolium Perenne L.combined Remediation Of Rare Earth Tailings Polluted Soil

The ecological environment is related to the future of the nation and the well-being of the people,and it is of great and far-reaching significance to promote the construction of ecological civilization.Heavy metal pollution in mining areas poses a huge threat to the ecological environment,and its soil remediation cannot be delayed.Chemical leaching is a common remediation method for heavy metal contaminated soil,which can effectively reduce the content of heavy metals in soil,but will lead to nutrient loss,soil structure destruction and other problems.Phytoremediation is an environmentally friendly remediation method that can further remove the heavy metals left in soil after chemical leaching.Due to chemical leaching,it will cause the lack of nutrients in soil,which makes it difficult for plants to grow normally,thus limiting the remediation effect.Camellia oleifera Abel.is widely distributed in the south area,and its composted shell can compensate for the nutrient deficiency in soil,and combined with nitrification inhibitors(NIs)can further promote the nitrogen cycle in the soil,promote plant growth,and improve the remediation effect of heavy metal contaminated soil.Lolium perenne L.is one of the heavy metalenriched plants with rapid growth and strong adaptability,which is a pioneer plant commonly used in remediation of contaminated soil in mining areas.In this study,from the perspective of ecological remediation of mine soils,taking the soil contaminated by rare earth tailings in Gannan,Jiangxi Province as the object,distilled water leaching was used as a control,and different leaches of ethylenediaminetetraacetic acid disodium salt(EDTA-2Na),citric acid(CA)and 1:1 combination of both were used.The contents of lead(Pb),nitrogen,phosphorus and potassium and other elements in soil after leaching were measured.and the effects of different chemical leaching treatments on the removal of heavy metals Pb and on the chemical properties of the soil were investigated.Later,the phytoremediation experiments of L.perenne L.,C.oleifera Abel.shell compost and nitrification inhibitors were further used in combination in different chemically leached soils,and the effects of chemical leaching and L.perenne L.on the remediation of rare earth tailings contaminated soils were explored by measuring the biomass of L.perenne L.,the content and morphological changes of soil heavy metals Pb,enzyme activity and microbial community structure and diversity.In order to provide scientific basis and technical support for the improvement and ecological restoration of heavy metal contaminated soils in rare earth tailings.The results of the study are as follows:(1)EDTA-2Na,CA and combined leaching of both(EC treatment)can reduce the content of Pb in the soil,ranked by removal effect as EC treatment > EDTA-2Na treatment >CA treatment > CK treatment.The chemical leaching had changed chemical properties of the soil.After the leaching,the contents of total phosphorus and potassium in the soil decreased,the content of soluble organic carbon in the soil increased,and under EDTA-2Na and EC treatments,the content of total nitrogen in the soil increased.(2)L.perenne L.died in the EDTA-2Na and EC leached remediation of soil,and grew normally in the CK and CA leached remediation of soil.The growth of L.perenne L.consumed nutrients such as total nitrogen,phosphorus and potassium in the soil and showed some enrichment ability for heavy metals Pb.L.perenne L.can reduce the content of Pb in the soil,change the transformation direction of heavy metals and bacterial colony structure in the soil,while had less effect on soil enzyme activity.In distilled water leached soils,L.perenne L.showed high Pb removal rates of 12.88% and 36.35% respectively.(3)The addition of C.oleifera Abel.shell compost increased the content of total nutrient and soluble organic carbon in the soil,the p H,urease and β-Glucosidase activities of the soil were raised as well.The height,root length and biomass of L.perenne L.increased under the effect of C.oleifera Abel.shell compost,which in turn increased the accumulation of Pb by L.perenne L.,and the Pb content in the soil was further reduced.Among the four leached soils,the removal rates of Pb by C.oleifera Abel.shell compost were from high to low in distilled water leached soil > EDTA-2Na leached soil > CA leached soil > EC leached soil.(4)The addition of nitrification inhibitors increased the content of total N,P and K in the soil,increased the plant height,root length and biomass of L.perenne L..At the same time,L.perenne L.showed an increased enrichment capacity for Pb.Dicyandiamide(DCD)and 3,4-dimethylpyrazole phosphate(DMPP)increased the removal rate of Pb from the soil and affected the microbial community structure,and their effects on heavy metal removal were related to the chemical leach remediation.In the soil remediated by distilled water leach,DCD addition was more effective than DMPP in the removal of heavy metals Pb,while in EDTA-2Na and EDTA-2Na with CA,the removal effect of DCD addition on heavy metals Pb was lower than that of DMPP,and the remediation effect of DCD and DMPP was comparable in the soil remediated by CA leaching.(5)The multifunctionality of soil is closely related to soil nutrients and enzymatic activities.Among the four leached remediation soils,L.perenne L.alone had little effect on the multifunctionality index of soil,while the addition of C.oleifera Abel.shell compost and nitrification inhibitors significantly improved the multifunctionality index of soil.The maximum multifunctionality index was 0.82 in CA leached remediation soil with the combined effect of C.oleifera Abel.shell compost and DMPP,thus,the environmental quality of the soil was better improved with the combined remediation of CA leach,C.oleifera Abel.shell compost and DMPP fortified L.perenne L.phytoremediation.In conclusion,EDTA-2Na,CA and their combined leaching will reduce the content of heavy metals in soil to some extent,however,EDTA-2Na affected the necessary life activities of microorganisms,leaded to the reduction of the richness and diversity of microbial communities,disrupted the nutrient cycle in the soil,resulted in the stunted growth of L.perenne L.and the inability to complete the further restoration of L.perenne L..In CA leached soil,the combination of C.oleifera Abel.shell compost and nitrification inhibitors can improve the soil environment and strengthen the further enrichment ability of L.perenne L.to heavy metals in soil to achieve the purpose of remediation of heavy metal contaminated soil.Therefore,in the restoration of rare earth mines in Gannan,suitable chemical reagents such as citric acid can be used to passivate the soil and combine with phytoremediation techniques to further achieve mine regreening,soil improvement and governance.