Screening Of Pioneer Plants For Soil Chemistry And Ecological Restoration Of Tailings In High-altitude Copper Mining Area

Copper tailings are non-economic products produced after mineral processing,and are typical industrial solid wastes.Due to the low comprehensive utilization rate,only a small part has been recycled and utilized,and the remaining part is often stored in the tailings pond,which brings heavy metal pollution and environmental problems.Vegetation reconstruction technology is widely favored in tailings ecological restoration.However,the phytoremediation of copper tailings in high-altitude mining areas has many scientific problems,such as less accumulation of effective nutrients,lack of arable soil,low temperature and dry climate conditions,and the special physical and chemical properties of copper tailings are not conducive to vegetation growth.Therefore,the key to ecological remediation in this area is substrate improvement and screening of pioneer plants.Yak dung is a unique organic waste of high-altitude pasture with high nutrient content and high yield,which has the potential to improve copper tailings.Therefore,Tall fescue,Ryegrass and Elymus dahuricus were selected from the local shrubs and grasses in Tibet,and their drought resistance performance was evaluated.Finally,yak dung and copper tailings were mixed to obtain the ecological substrate to be used（also known as"artificial soil"or"technical soil"）.The physical and chemical properties,behavior changes of heavy metals,microbial community structure and mineral phase changes of copper tailings were studied after the pioneer plants were planted to explore whether they can promote the soilization of tailings and solve the fundamental problems in the ecological restoration of the area.The main conclusions of the study are as follows:（1）Yak dung has the material basis for improving copper tailings.Copper tailings mainly contain Si O₂,Ca O,Fe₂O₃,Al₂O₃ and other substances.The main minerals are quartz,calcite,wollastonite,calcaredot,grossulite olivine,grossular garnet,biotite.The nutrient contents of organic matter,total potassium,total phosphorus and available potassium are at a lower level compared with yak dung.The p H of copper tailings is slightly alkaline,and the potential ecological risk index is light pollution level.（2）When the concentration of polyethylene glycol（PEG-6000）was 15%,all the measured indexes of Tall fescue,Ryegrass and Elymus dahuricus were significantly lower than that of 10%stress concentration（P<0.05）.15%PEG-6000 was used as the appropriate concentration for drought resistance identification to comprehensively evaluate the drought resistance of the three plants.The order of drought resistance from strong to weak was Tall fescue>Ryegrass>Elymus dahuricus.The drought tolerance critical value(D₅₀)and limit value（D₀）of three plants were simulated by univariate linear regression.From the average D₀ of five indicators（relative germination rate,relative germination potential,germination drought resistance index,germination stress index,relative seedling weight）of three plants,the drought tolerance limit value of Tall fescue>Ryegrass>Elymus dahuricus.The average D₅₀ of the five indicators was consistent with the result of drought tolerance limit value.（3）The conductivity,cation exchange capacity,specific surface area,total phosphorus,total potassium,available phosphorus,available potassium,organic matter,alkali-hydrolyzable nitrogen,water stable aggregates（macroaggregates）of copper tailings increased significantly after adding yak dung and planting three plants（P<0.05）.The four treatments of T+YD（copper tailings+fermented yak dung）,T+YD+T（copper tailings+fermented yak dung+Tall fescue）,T+YD+R（copper tailings+fermented yak dung+Ryegrass）and T+YD+E（copper tailings+fermented yak dung+Elymus dahuricus）changed the morphology of Pb,Cu and Zn in copper tailings from T1 state（weak acid extraction state）,T2 state（reducible state）to T3 state（oxidizable state）and/or T4 state（residual state）..Tall fescue,Ryegrass and Elymus dahuricus mainly stabilized Cu,Zn and Pb in the roots,and the enrichment coefficient and transfer coefficient were all less than 1,which did not show the characteristics of super enrichment.（4）XRD,XPS and SEM results showed that the addition of yak dung and the planting of three plants could promote the transformation of primary minerals such as biotite,nepheline and calcium silicate in copper tailings into secondary minerals.Among them,zeolite appeared in the treatment of T+YD+T,kaolinite appeared in the treatment of T+YD+R,vermiculite appeared in the treatment of T+YD+E.The addition of yak dung and the planting of three plants at the same time can promote the weathering and/or oxidation of primary minerals such as pyrite,and produce secondary iron minerals such as goethite and hematite.（5）The diversity and richness of community species in copper tailings increased after yak dung addition and three plant colonization.The first five groups with higher abundance at the phylum level in each treatment group were Proteobacteria,Bacteroides,Planctomycotes,Cyanobacteria and Firmicutes.Among them,the abundance of Proteobacteria（except T+YD+T treatment）and Bacteroides decreased after treatment,while the abundance of Cyanobacteria and Firmicutes increased.（6）All three plants germinated normally in engineering practices at altitudes of4400 m and 5000 m.To sum up,yak dung addition and synergetic plant colonization can significantly improve the basic physical and chemical properties of copper tailings,reduce the mobility and effectiveness of heavy metals,increase the species abundance and diversity in copper tailings,and promote the soiling of copper tailings.Yak dung and three plants have the potential to repair copper tailings at high altitude.