Implications Of Isotope For Sources,Migration And Transformation Of Antimony In The Water Environment Of The Xikuangshan,Hunan

Antimony（Sb）is a new type of toxic contaminant,and is ubiquitously present in the environment.Antimony compounds are potentially toxic to human immunity,nervous system,genes,and development.In recent years,the results of lake sedimentation and polar ice in remote areas show that antimony is also a global contaminant for long distance migration.Therefore,antimony pollution in the water environment is becoming an increasingly serious problem of global concern.As we all know,China has the largest Sb reserves and productions in the world,the atmosphere,water,soil and other ecological environment in China has been gradually affected by the long-term exploitation and application of antimony.Xikuangshan（XKS）Sb mine,which is located in Hunan province,is the largest Sb mine in the world,and is well known as the“World Capital of Sb”.Mining activities for over 110 years have caused serious water an soil environment pollution,in particular the pollution in groundwater.Groundwater from the Medium Shetianqiao aquifer（D₃s²）in Shetianqiao Formations and Magunao aquifer（D₃x⁴）in Xikuangshan Formations of Upper Devonian provides an important source of potable water.The elevated antimony concentrations in groundwater results from the mining activities and represents a potential threat to the local drinking water supplies.Thus,it is imperative to identify the sources,migration and transformation of antimony in the water environment of XKS Sb mine.Based on the analysis of the distribution of antimony in rainwater,surface water,groundwater and mine wastewater as well as their interrelationship in the study area,this paper focused on the research on the source,migration and transformation processes of antimony in groundwater.And based on the comprehensive analysis of physical geography,geology and hydrogeology,as well as the mining activities,after the collection and testing of typical water and ore samples,this paper firstly identified the distribution of antimony in the water environment of the XKS Sb mine,and preliminary analyzed the source,migration and transformation processes of antimony by hydrogeochemical methods.Then it used multiple isotopes(²H and ¹⁸O of water,³⁴S and ¹⁸O of sulfate,⁸⁷Sr)approach,and in combination of their fractionation mechanism and advantages,further elucidated the sources of antimony and decipher the migration and transformation processes of antimony in groundwater,as well as the influencing factors.Finally,based on the comprehensive analysis results with hydrogeochemical and multiple isotopes approaches,this paper summarized the model of sources,and migration and transformation of antimony in groundwater,respectively.From what has been discussed above,the major results and conclusions are as follows:（1）In general,the antimony in the water environment of the XKS Sb mine was characterized by wide distribution range,high antimony concentrations ratio and extremely uneven antimony concentrations.The antimony concentrations for rainwater ranged from 0.033 to 0.052 mg/L with an average of 0.044 mg/L,while that for surface water ranged from 0.18 to 8.12 mg/L with an average of 3.64 mg/L.The antimony concentrations for spring and well water ranged from 0.015 to 4.74 mg/L with an average of 4.557 mg/L that surpasses the China’s Drinking Water Quality Standard by900 times,whereas that for mine wastewater such as mine drainage,mine seepage,solid wastes leachate,smelting wastewater and tailing wastewater ranged from 0.078to 29.80 mg/L with an average of 7.014 mg/L that exceeds the China’s Emission Standards of Pollutants for stannum,antimony and mercury Industries by 23.4 times.Specially,groundwater samples can be divided into three types:low antimony groundwater（≤0.05 mg/L）,medium antimony groundwater（0.05⁰.5 mg/L）and high antimony groundwater（≥0.5 mg/L）,while the high antimony groundwater can be divided into three types:less high antimony groundwater（0.50⁵ mg/L）,medium high antimony groundwater（5¹0 mg/L）and anomalous high antimony groundwater（≥10mg/L）.The groundwater of the Medium Shetianqiao aquifer（D₃s²）and Magunao aquifer（D₃x⁴）was characterized by high antimony concentrations,wide concentrations range and more higher concentrations than their background values.（2）Waters at XKS Sb mine was neutral-alkaline,with pH ranging from 6.04 to 10.79,which seemed to be associated with carbonate mineral neutralization.The water chemistry type was complex and was mainly SO₄-HCO₃-Ca to HCO₃-SO₄-Ca type.The groundwaters were influenced by carbonate dissolution and silicate weathering,sulfide minerals oxidation and the mining activities,and were characterized by high concentrations of HCO₃^-,Ca²⁺,and SO₄^2-,as well as the anomalous high concentrations of Na⁺.The hydrochemical analysis preliminary indicated that antimony in groundwater might be influenced by mine solid wastes and mine wastewater in addition to the source of rock and ore weathering,and sulfide minerals oxidation.Additionally,hydrochemistry data also indicated that Sb was initially emitted as Sb（Ⅲ）（Sb₂O₃）into groundwater,then was oxidized to Sb（Ⅴ）（Sb（OH）₆^-）,which is more stable and can migrate long distance.Unlike the reduction and complexation,Fe-Mn oxides/hydroxides may affect the migration and transformation of antimony,but the influence was limited.（3）Water isotopes(²H_H2O and ¹⁸O_H2O)are useful for identifying the source,recharge,migration and mixing of contaminants,as well as the evaporation process in the hydrological cycle.The water isotopes at XKS Sb mine revealed that groundwater originated from precipitation,and were influenced by evaporation.The good linear relationship between water isotopes of surface water,groundwater and mine wastewater suggested that they had a good hydraulic relationship.The relationship betweenδ¹⁸O_H2O2O values Sb and Na⁺concentrations indicated that the effect of antimony migration and transformation in groundwater by fast vertical recharge mine wastewater（including slag leachate,smelting wastewater and tailing wastewater）and mining activities underground,whereas evaporation was not the major factor for controlling the migration and transformation of antimony in groundwater.（4）In an effort to better understand the antimony source of sulfide minerals oxidation,the redox process,in particular the bacterial sulfate reduction（BSR）affecting Sb mobilization,isotope signatures(³⁴S_SO4,¹⁸O_SO4,³⁴S_sulfide)of dissolved sulfate and sulfide were analyzed.The results suggested that dissolved sulfate and antimony in the water environment were mainly from stibnite oxidation rather than precipitation,sulfate mineral such as gypsum,and sulfide minerals including pyrite and sphalerite,and groundwater has been influenced by waste rock and slags leachate and smelting wastewater.The relationships between sulfate isotopes and Sb and redox sensitive element such as arsenic further elucidated that reduction dissolution of Fe（Ⅲ）hydroxides was not the dominant process controlling Sb mobilization,whereas the sorption-desorption of Fe-Mn oxides/hydroxides affected the migration and transformation of antimony,and a competitive adsorption with As may occur.Additionally,the relationship betweenδ³⁴S_SO4 values andδ¹⁸O_H2O2O revealed that BSR may have not occurred or weak,and was not the major factor for controlling the migration and transformation of antimony in groundwater.（5）In order to better identify the antimony source of rock and ore weathering,and mining activities,in particular the pathway of Sb mobilization,the strontium isotope composition and its relationship with Sr and Sb concentrations as well as Na/Ca were analyzed.The results further demonstrated that antimony in groundwater were influenced by mine wastewater such as solid wastes leachate,smelting wastewater and tailing wastewater in addition to the sources of the weathering of carbonate and silicate,and the oxidation of sulfide minerals.More importantly,the results suggested that hydraulic relationship between Medium Shetianqiao aquifer（D₃s²）and Magunao aquifer（D₃x⁴）was not obvious,and Sb mobilization in the two aquifers was relatively independent.In addition,the relationship betweenδ³⁴Sr values andδ³⁴S_SO4 values further demonstrated the effect of mine wastewater,and Sb may be released by different hydrolysis and oxidation processes into groundwater after the leaching of waste rock and slags,which were piled up in the open air.（6）Finally,based on the above results of hydrochemistry and multiple isotopes(²H,¹⁸O,³⁴S,⁸⁷Sr),this study summarized the model of sources,and migration and transformation of antimony in groundwater at the XKS Sb mine,respectively.About the model of antimony sources,the elevated Sb concentrations in the low antimony groundwater samples were sourced from the non-ore bedrock weathering,while the elevated Sb concentrations in the medium antimony groundwater samples resulted from the ore-bearing bedrock（including waste rock）weathering.The elevated Sb concentrations in the high antimony groundwater samples were mainly related to oxidation of stibnite,particularly the influence of mine wastewater.About model of antimony migration and transformation,Sb in the rock and ore is initially emitted as Sb（Ⅲ）（Sb₂O₃）into groundwater,then some Sb（Ⅲ）is oxidized to Sb（Ⅴ）as the variation of redox in the aquifer,forming the groundwater including Sb（OH）₃ and Sb（OH）₆^-,which is affected by the adsorption and desorption of Fe-Mn oxide or hydroxide and the competitive adsorption of As,with the variation in antimony concentrations.It was proved that the multiple isotopes combined with hydrochemical methods,were effective in tracking sources,migration and transformation of Sb contamination in the water environment.This study can provide important scientific basis and strong technical support for the prevention of heavy mental pollution in XKS Sb mine area and its surrounding areas,and it may be applicable to other regions with similar conditions.This paper has two main innovations:（1）An effort was made for the first time to study on the sources,migration and transformation of antimony in the water environment using multiple isotopes,including ²H and ¹⁸O isotopes of water,³⁴S and¹⁸O isotopes of sulfate,and ⁸⁷Sr isotopes.（2）It was the first time this paper put forward the competitive adsorption with As during the Sb mobilization in groundwater,and summarized the model of sources,and migration and transformation of antimony in groundwater at the XKS Sb mine.