Environmental Geochemical Characteristics And Migration Mechanism Of Heavy Metals In Sulfide Pb-Zn Mining Areas In Carbonate Regions

The mining activities of sulfide Pb-Zn deposits can cause heavy metals and other hazardous substances to enter the surrounding environment,affecting the ecological safety of water resources and agricultural soils,and subsequently threatening human health.The mine drainage of lead-zinc sulfide deposits in carbonate areas was mostly neutral（p H≈7）and there was no significant enrichment in heavy metals,differing from those mine areas in non-carbonate areas,and easily be overlooked.However,the heavy metals and sulfur showed significant positive anomalies in crop soils and stream sediments around the mining area in the carbonate area.Due to the influence of soil acidification and acid rain,the heavy metals were highly susceptible to activation and migration,which would endanger ecological safety.Therefore,identifying the mechanisms of heavy metals release,migration,enrichment and geochemical processes associated with the mining activities of Pb-Zn sulfide deposits was a necessary prerequisite for effective environmental management and remediation.And it was the scientific problem that is the focus of this thesis.Although Pb-Zn mining activities were considered to be a most important source of heavy metals such as Pb,Zn,Cd,Hg,and As in the environment,it can’t be ignored that the rapid weathering of carbonate formations,which were the soil parent material,can also cause the accumulation of heavy metals in the environment.Consequently,the source identification and contribution quantification of heavy metals still need further exploration.Based on the geochemical characteristics of heavy metals in environmental mediums of Tangjiaxiang and Daliangzi Pb-Zn mining areas,the multivariate statistical method and stable isotope tracing method were used to identify the sources and discriminate the contribution.The dynamic leaching experiments of ore and tailings were used to inquire the release pattern,impact factors and mechanism of heavy metals,and the vertical mobility index（MI）and partition coefficients were used to research the migration and transformation modes of heavy metals and establish the prediction models of vertical migration and water-sediment partition coefficients of typical heavy metals.The entire study is to provide a scientific basis for the environmental remediation of mining areas.And this paper studied the following conclusions.The environmental geochemical characteristics of the sulfide Pb-Zn mining area in the carbonate region were refined through the study of these two mining areas.Weathering dissolution of dolomite was the most important cause of neutral-weak alkalinity of soils,water,and aqueous sediments in the study area.The soils were enriched in Cd,Hg,Pb,Zn and approximately 10%of Cd and Hg exceeded the risk control value,which has a high ecological risk.The waters showed Ca²⁺-Mg²⁺-HCO₃^--SO₄^2-type,affected by the dissolution or the oxidation of the dolomite,sulfide,and evaporite.The low concentration but high spatial dispersion degree of heavy metals in the water implied the intense interference from anthropogenic activity.The concentrations of Pb and SO₄^2-were mostly the worst categories in the evaluation index of water,which was not suitable for drinking.Spatially,the closer to the functional area of mining activities（e.g.,processing plant,tailings,and mining site）,the higher the concentration of heavy metals in the water,indicating that mining activities have impacted the hydrochemistry character.The stream sediments were enriched in As,Zn,Hg,Pb,Cd and approximately 10%of the sediment Cd exceeded the Cd risk control value.The multivariate statistical analysis and isotope tracing indicated the sources of the materials in the environmental mediums.The contributions of sulfide oxidation,evaporite dissolution,and atmospheric precipitation to the sulfur of the water in Tangjiaxiang Pb-Zn mining area were 69.59%,11.38%,and 19.03%,respectively,and44.5%,37.32%and 18.18%to the water in Daliangzi Pb-Zn mining area,respectively.It indicated that mining activities were the largest source of sulfur in the study area,followed by evaporite dissolution.Pb,Zn,As,and Cd would have been released into the environment with the dissolution and oxidation of sulfide minerals.The main source of soil heavy metals in Tangjiaxiang Pb-Zn mining area was mining activities,followed by natural sources and agricultural activities,etc.The contribution of mining activities and soil parent material to Pb in soil（mean value）was 61.42%and 38.58%,respectively.While the main sources of soil heavy metals in Daliangzi area were sulfide minerals and the surrounding rock strata,followed by agricultural activities and other anthropogenic sources.The Pb isotopic composition indicated that mining activities were the most important source of Pb in the soil of Daliangzi mining area,with some contribution from the soil parent material.Both multivariate statistics and Pb isotope analysis result well confirm that mining activities in the study area are the most important source of soil heavy metals,and soil parent material is also a contributor.The mechanisms of heavy metals release were investigated by leaching tests.The p H of the leachate from dissolved carbonate rocks is from 6 to 8,and the first 1/4 stage of leaching was the main stage of heavy metals release.The concentration of heavy metals leached out and their release rate was different at different initial p Hs,and the total concentration was the maximum at the neutral initial condition.The release mechanism of Cd,Co,Mn,and Zn from the ore at different initial p H were controlled by dissolution-wash off,while the release of Ni was controlled by diffusion-wash off.The release of As from the ore was mainly controlled by diffusion,wash off and diffusion,respectively,when the initial p H was located at 3,4.8,and 6.And the release of Pb from the ore was mainly controlled by diffusion,dissolution and diffusion,respectively,when the initial p H was located at 3,4.8,and 6.The release of Cd、Pb、Zn from the tailings was controlled by the mechanism of dissolution at different initial p H,and the release of As from the tailings was controlled by dissolution,dissolution,and diffusion,respectively,when the initial p H was located at 4,7 and 9,the others were mainly controlled by diffusion or wash off.The smaller the particle size of the ore was,the more complete the dissolution of carbonate minerals and metal sulfides would be.The release of Cd、Co、Mn、Ni、Zn from the ore was controlled by the mechanism of dissolution and dissolution-wash off,respectively,when the particle size was larger than 0.850mm and smaller than 0.850mm.The release of As was affected by wash off and diffusion,respectively,when the ore size was larger than 0.425mm and smaller than 0.425mm.The release of Pb was affected by wash off-diffusion and dissolution,respectively,when the ore size was relatively large and relatively smaller.In this paper,the migration activity of the elements was analyzed and a prediction model was developed.The order of vertical mobility of heavy metals in soils was Zn>As>Cd>Pb>Cu.As and Pb mainly existed as oxygen anion and lead carbonate precipitation,respectively.Zn,Cd,Hg,Cr,Cu,and Ba were mostly in acid-soluble and reducible fractions,and organic matter（OM）can block their migration.The predictive model of soil heavy metal vertical MI was well fitted using soil physicochemical properties（p H,OM,P,and depth H）as dependent variables.The mechanism of heavy metal migration in water bodies is mainly mechanical transport by water forces,and the magnitude of release capacity in sediments was As>Mn>Zn in order.Mostly,Pb and Cd were a part of the sediments as refractory carbonate precipitates,while Zn was mainly carbonate precipitates with a small portion in ionic form,and As was partially present as HAs O₄^2-and partially in the adsorbed state.These five physicochemical properties,p H_water,p H_sediment,OM,cation exchange capacity（CEC）,and P,as dependent variables were well fitted to the prediction model for the partition coefficients of As and Zn between water and surface sediment,while these six cations,Ca,Mg,Na,K,Fe,and Al,were used as dependent variables were fitted to the prediction model for the partition coefficient of Mn.