Migration And Transformation Of Typical Polymetallic And The Ecological Control Mechanism In Waste Of Mercury-thallium Mine In Southwest Guizhou

Lanmuchang mercury-thallium mine,a typical polymetallic mine is located in southwestern Guizhou,China,is a representative large polymetallic mining area in karst mountains of southwest China.The mining here has been going on for more than 300 years,and now stopped.A large quantity of untreated waste slags,tailings and waste rocks(hereinafter referred to as waste)were discharged during the mining and smelting process.They have been mixed open-air accumulation in the mining area in the past hundred years.Heavy metals such as Tl,Hg,As and Sb in the mercury-thallium mine waste usually co-exist with sulfides to form a Tl-As-Hg-Sb-S elements combination due to their sulphophile.In the weathering and leaching process of the supergene environment,the continuous oxidation and acid production of the mercury-thallium mine waste resulted in a harsh environment with acidification and poor nutrition,which made it difficult for vegetation to survive,thus forming a vicious cycle of"water and soil loss and aggravation of exposure oxidation and acid pollution".Thus,even though the mining had long been banned,environmental pollution continues to occur.Therefore,carrying out ecological restoration to prevent the migration,loss and diffusion of water and soil(pollution)in the waste site of mercury-thallium mine has become the key point to solve the special environmental problems in the mining area.However,there are many kinds,complex composition and high biological toxicity in the mercury-thallium mine waste,so it is difficult to control these heavy metals with different properties by carrying out ecological restoration.Which brings great difficulties to ecological in-situ control of mercury-thallium mine waste site.Based on the mercury-thallium mine waste rich in Tl,Hg,As,Sb and other typical composite metals was taken as the research object,through analyzing the effects different environmental media(e.g.pH,plant litter,root secretion and Fe(?))on the release,migration and biotoxicity of Tl,Hg,As,Sb in mercury-thallium mine waste,identify the biological and non-biological elements that play a key role in the migration and transformation of typical metals in the process of ecological restoration.Through the remediation of modifiers combined with pioneer plants and the continuous regulation of the decomposition and reduction of litter,we analyzed the physicochemical characteristics and typical metal changes in lixiviums of mercury-thallium mine waste,and the physicochemical characteristics,microbial diversity,acid production potential,Zeta potential,mineral composition and evolution of elements such as iron and sulfur of the mercury-thallium mine waste.In addition,the ecological restoration process under the remediation of modifiers combined with pioneer plants and the continuous regulation of the decomposition and reduction of litter were systematically expounded in the light of correlation analysis and redundancy analysis.The main results obtained are as follows:(1)The analysis of heavy metals in the leaching solution of mercury-thallium mine waste under different environmental conditions,the result showed that the release of Tl from mercury-thallium mine waste was significantly promoted by strong acidity(pH=2).Both strong acidity and alkaline solution environments promoted the release of Hg and As.The release of Sb increased with the increase of pH value.The strong oxidation environment mediated by FeCl₃ solution significantly inhibited the release of Tl,As and Sb from mercury-thallium mine waste,but promoted the release of Hg in the later stage of leaching due to its strong oxidation.The Fe₂(SO4₎₃ solution inhibited the release of As and Sb,but promoted the release of Tl and Hg.Litter decomposition solution and root exudates significantly reduced the release of Tl,but promoted the release of Hg,As and Sb,and their leaching solution had a higher pH and presented a neutral environment.(2)The remediation of modifiers combined with pioneer plants significantly improved the acidic and poor nutrient environment of mercury-thallium mine waste,and significantly increased the total microbial mass and microbial respiration intensity,while inhibiting the activity of the original acidophilic iron oxidizing microorganisms.The low molecular weight organic acids secreted by the roots of pioneer plants further increased the contents of available nitrogen,available phosphorus and organic matter in mercury-thallium mine waste,which resulted in the higher total microbial biomass and respiration intensity as well as the lower activity of acidophilic iron oxidizing microorganisms in the rhizosphere wastes.Based on the 16S rRNA gene high-throughput sequencing analysis,the remediation of modifiers combined with pioneer plants significantly increased the number of bacteria with special functions,such as Mesorhizobium,Rhizobium,Bradyrhizobium and other nitrogen fixing bacteria.The redundancy analysis of bacterial community characteristics and environmental factors showed that the improvement of acidic environment and the increase of nutrient content were the main reasons for the increase of bacterial community structure and diversity.(3)The FTIR analysis of DOM combined with the physicochemical characteristics of mercury-thallium mine waste and the migration of typical heavy metals in the waste-leachate system showed that the remediation of modifiers combined with pioneer plants inhibited the dissociation of hydroxyl group in mercury-thallium mine waste.However,the addition of DOM with C-O and P-O groups could complex Hg,As and Sb to form soluble complexes and increase their migration,but it did not show a similar effect on Tl,and enhanced the adsorption of Tl.The remediation of modifiers combined with pioneer plants increased the concentration of Fe²⁺in the leachate.Combined with XPS analysis,we found that the remediation of modifiers combined with pioneer plants could reduce the Fe³⁺to the Fe²⁺,which has stronger migration and can be dissolved out.However,the structure of silicate minerals had not changed significantly,only the electronegativity of the waste of mercury-thallium mine waste had been enhanced,showing higher electronegativity.The reduced electronegativity tended to adsorb metal cations such as Hg and Tl to maintain electrical neutrality,but it was not conducive to the adsorption of oxygen-containing anions of negatively charged As and Sb,thus increasing the migration.Correlation analysis showed that As and Sb in mercury-thallium mine waste were also easily released with the reduction and dissolution of iron.(4)The decomposition of litter significantly increased the microbial biomass,pH and nutrient content in mercury-thallium mine waste.However,the decomposition of litter could reduce the activity of acidophilic iron oxidizing microorganisms.At the same time,because the activity of acidophilic iron oxidizing microorganisms was low,litter with a low supplemental level(1%)could reduce its activity,which had no significant difference with the influence of litter with a high supplemental level.The decomposition of litter significantly increased the diversity of bacterial community structure in mercury-thallium mine waste.Based on the 16S rRNA gene high-throughput sequencing analysis,there was no significant difference in the influence of the four litters on the richness of bacterial community structure,but there were differences in the influence on the diversity index.The redundancy analysis of bacterial community characteristics and environmental factors showed that organic matter was the main driving factor affecting the structure and diversity of bacterial communities.The effect of B.papyrifera and the other three litters was different.The reason was that the improvement of the acid environment and nutrient content of mercury-thallium mine waste by B.papyrifera was the most significant.(5)The improvement of acidic environment of mercury-thallium mine waste the litter was mainly related to the ammonification of organic nitrogen and protonation of amino acids.The nitro and nitroso groups produced by the decomposition of litter were adsorbed on the mercury-thallium mine waste,which provided electron transfer medium for iron reducing microorganisms such as geobacter,so that Fe³⁺in the mercury-thallium mine waste was reduced to Fe²⁺and released,enhancing its electronegativity and thus increasing the adsorption of metal cations such as Tl and Hg.However,under the condition of strong reduction,part of the released Hg(?)could combine with S^2-which produced by SO₄^2-reduction to form insoluble Hg S,so as to reduced its migration.The DOM of litter decomposition products promoted the release of Hg,and when the amount of litter increased,this effect was greater than the adsorption of Hg by the enhanced electronegativity in mercury-thallium mine waste,which increased its migration.The main reasons that litter increased the migration of As and Sb in mercury-thallium mine waste were as follows:1)The decomposition of litter reduced Fe(?)to Fe(?)in mercury-thallium mine waste and leaded to the migration of As and Sb with the dissolution of Fe(?).2)The decomposition of litters enhanced the electronegativity of mercury-thallium mine waste,which was not conducive to the adsorption of oxygen-containing anions of negatively charged As and Sb.3)The decomposition of litter increased phosphate had competitive adsorption with As and Sb.