Study On The Occurrence,Migration And Transformation Of Environmentally Sensitive Elements In Typical Metal Mining Area

Metal mining waste,a kind of typical bulk industrial solid waste generated during mining-metallurgical activities,usually contains substantial metal sulfides and environmentally sensitive heavy metal such as As,Pb,Cd,Cr,Cu,Zn,Ni,etc.During the long-term storage of mining wastes in the natural environment,environmentally sensitive elements can be released simultaneously with the formation of acid mining wastewater and migrate to the surrounding environmental media,threatening the ecological environment and life health.Thus,it is of great significance to comprehensively investigate the distribution,occurrences,as well as migration and transformation mechanism of environmentally sensitive elements in mining wastes and the surrounding environmental media,which could provide scientific support for the disposal and utilization of mining wastes,pollution control and remediation of mining areas,and green mine construction.In this thesis,the study area is located in the typical metal mining areas in Guichi District,Anhui Province,and the typical environmentally sensitive elements（As,Pb,Cd,Cr,Cu,Zn,and Ni）in different types of mining wastes and the surrounding environmental media were selected as the research objects.The mining wastes（waste ore,tailings and sludge sediments）as well as surface soil and surface water inside and outside the mining areas were systematically collected.Combined with interdisciplinary theories of element geochemistry,mineralogy,soil science,environmental chemistry,etc.,a series of techniques,including X-ray diffraction technique（XRD）,X-ray fluorescence spectroscopy（XRF）,Scanning Electron Microscopy-Energy Dispersive Spectrometry（SEM-EDS）,and X-ray photoelectron spectroscopy（XPS）were applied to comparatively investigate the distribution,enrichment,and occurrence of environmentally sensitive elements in various mining wastes and the surrounding surface environmental media（soil and surface water）.Furthermore,the spatial distribution characteristics of the selected elements were clarified,and the characteristic contaminating elements of different types of mining areas were distinguished.Consequently,the migration and transformation mechanism of the environmentally sensitive elements in the mining wastes-surface water-soil system was demonstrated,and the potential ecological and environmental risks of the elements in the surface environment of mining areas were evaluated.The main results are as follows:1.The physicochemical characteristics,substantial and mineral composition of mining wastes（waste ore,tailings,and sludge deposits）were studied.It was found that the mining wastes were primarily composed of Si,Al,Fe,Ca,Mg,K,Na,and other macro-elements.And the total content of SiO2,Al_<sub>2O₃,Fe_<sub>2O₃,CaO,MgO,K₂O,and Na₂O in the waste ore,tailings,and sludge sediments were 60.78-91.44%,70.86-91.26%,and 65.07-81.91%,respectively.Most of the mining wastes were of high sulfur content and enriched with environmentally sensitive elements such as As,Zn,Pb,Ni,Cu,and Cr to varying degrees.The mineral compositions of the mining wastes were mainly silicate ores（quartz,plagioclase,hornblende,etc.）,clay ores,and carbonate ores（calcite,dolomite,magnesite,siderite,iron dolomite,etc.）,accompanied by metal sulfide minerals（pyrite）and oxide minerals（hematite）.2.The distribution,enrichment,and occurrence characteristics of the environmentally sensitive elements（As,Cu,Zn,Pb,Cd,Cr,and Ni）in the mining wastes were revealed,and the characteristic contaminating elements of different types of mining areas were identified.Meanwhile,the potential ecological risks and environmental migration capacities of the environmentally sensitive elements in different types of mining wastes were evaluated.The results showed that the mining wastes were neutral to weakly alkaline,and majorly enriched with As,Cu,Zn,Pb,and Cd,while Cr and Ni were slightly enriched in few types of mining wastes.And the metallogenic elements for different mining areas were mostly enriched in the corresponding mining wastes.Additionally,Cu,Zn,Pb,and Cd in waste ore mainly existed in Fe-Mn oxidation state and organic-bound state,and the residue state of As is generally higher.The migration index（MI）of these environmentally sensitive elements followed the descending order of Cd>Zn>Cu>Pb>As.For mining tailings,Cu,Zn,Pb,Cd,and As were mainly in the Fe-Mn oxidation state,organic-bound state,and residue state,among which the MI values were:Cd>Pb≈Cu>Zn>As.For the sludge deposits,As were dominant in the residue state,while Cu,Zn,Pb,and Cd occupied a significant proportion in the other four states except in the residue state,with the MI value following the order of Cd>Pb>Zn>Cu>As.Cd possessed the highest environmental migration capacity in all the studied mining wastes,and the migration capacity of Cd in the waste ore and sludge deposits was greater than that in the tailings,followed by Pb,Cu,and Zn,and the migration capacity of As was comparatively weak.3.The distribution,enrichment,and occurrence characteristics of the studied environmentally sensitive elements in the surface environmental media（soil and surface water）of different mining areas were elucidated,and the spatial distribution patterns of the environmentally sensitive elements in the surface environment were described.Besides,the characteristic contaminating elements of different mining areas were identified.It was concluded that Zn,Cu,Pb,Cd,and As showed different enrichment degrees in the soils inside and outside the mining areas,among which As and Cd were the mostly enriched elements.As was detected in most surface water.Moreover,it was suggested that the enrichment degree of environmentally sensitive elements in the soils surrounding the mining areas was significantly correlated with the types of mines.It indicated that mining wastes were an important source of environmentally sensitive elements in the soils surrounding the mining areas.As for the occurrences of the selected elements,As,Zn,Cu,and Pb mainly appeared in Fe-Mn oxidation state,organic-bound state,and residual state in the soils,with relatively low migration capacity.In addition,Cd mainly existed in exchangeable state,carbonate-bound state,and Fe-Mn oxidation state in the soils surrounding some of the silver,iron,copper,and polymetallic composite ores.And Cd was more easily acidified and oxidized in the surface soil.Among the 17 mines in the study area,As was the characteristic contaminating element for all the mines,while Cd,Cu,Pb,and Zn were the characteristic contaminating elements for 16,15,12,and 9 mines,respectively.4.The mine of severe arsenic pollution,named Xincheng gold mine,was selected for the in-depth investigation.The physicochemical composition,micro-morphological characteristics,mineralogical characteristics,occurrences,chemical states,and fine chemical structure of the environmentally sensitive elements in the mining tailings,sludge deposits,and soils inside the mining areas and the nearby farmland were compared.The correlation between the distribution of environmentally sensitive elements in the mining wastes and the surrounding soils was demonstrated,and the influencing mechanism of environmental conditions on the release and migration behaviors of the studied elements was expounded.As a result,the migration and transformation mechanism of the characteristic contaminating element—As in mining wastes-water-soil system was interpreted.The result showed that the physicochemical composition of the soils in the mining area was similar to that of the tailings and sludge deposits,with the major chemical elements of Si,Al,Fe,Ca,Mg,and K,accompanied by environmentally sensitive elements of As,Ni,Cr,Cu,Zn,and Pb.The minerals were dominated by typical silicate minerals-clay minerals and quartz.The characteristic contaminating element As in the tailings,sludge deposits,and soils was mainly embedded in the Fe-Si-O phase as arsenate As（V）-O.The As-containing phases and the chemical forms of As observed in the soils were consistent with the weathering and oxidation products of the gold mining wastes.Specifically,rainwater leaching,surface runoff,and atmospheric transportation could contribute to the transformation of As from arsenic minerals in the mining wastes to the surrounding soils.5.By applying standard-value-based comparison,Single-factor index,Nemerow comprehensive index,Geo-accumulation index,and Risk assessment code（RAC）methods,the pollution level,environmental pollution and potential ecological and environment risks of environmentally sensitive elements in the soil and surface water for different mining areas were comprehensively evaluated.The results indicated that As was at a high environmental pollution risk level in the agricultural soil and mining land of all the mining areas,with the highest detection rate and over-standard rate in the surface water of the study area.Cd,Pb,Cu,and Zn presented relatively high environmental pollution risk mainly in agricultural soils,Cd and Pb presented environmental pollution risk in mining land soils,too.