Geochemical Characteristics Of Typical Elements In Guizhou Coal Mine Drainage And Their Environmental Significanc

Acid Mine Drainage（AMD）,produced by mining activities,is often characterized by low p H value,high Fe,high SO₄^2-and rich in various heavy metal elements,which has caused prominent problems in water environment.The water quality characteristics of mine drainage formed by different types of mineral development are significantly different.Sulfide minerals（mainly pyrite）are associated with coal and coal gangue.Guizhou is one of the provinces with the most abundant coal resources in China,and the center of the karst areas in southern China.In addition to high sulfur content,most of the coal in Guizhou is enriched in trace elements.A large number of beneficial or harmful elements are released into the environment during the formation and discharge of acidic mine water,resulting in ecosystem degradation and threatening the health of residents,which has become one of the most serious water environmental problems in Guizhou Province.Therefore,this study applied environmental geology,element geochemistry,isotope geochemistry and other methods to study the hydrochemical composition and regional pollution characteristics of coal mine drainage in Guizhou,the enrichment characteristics and control mechanism of trace elements,the isotopic composition of coal mine drainage and its environmental indicators.The geochemical process and main controlling factors of the formation and discharge of elements in mine drainage were revealed,and the feasibility of valuable element resources and recovery and utilization was preliminarily evaluated.The following understandings were obtained:（1）The basic hydro-chemical characteristics of coal mine drainage in Guizhou province are obviously different,which is closely related to geological background and the mine status（active mine or abandoned mine）.The chemical composition of drainage water in abandoned coal mines is mainly affected by the oxidation of sulfide minerals and the mixing of karst water,while the chemical composition of drainage water in active coal mines is mainly controlled by the dissolution of clastic rock.Most of coal mine drainage is acidic or weak acidic,and EC and TDS are obviously positive correlation.In addition to the higher concentrations of SO₄^2-,Ca²⁺and Mg²+in acid coal mine drainage,the content of soluble metal ions such as Fe,Mn and Al is also generally high.The p H value of coal mine drainage is negatively correlated with the content of metal elements.The change of p H value significantly affects the form of metal elements in the environment and the process of metal elements’migration and transformation.Most coal mine drainage,based on TDS,SO₄^2-,Fe and Mn,obviously exceeds WHO and China domestic water quality standards,and is not suitable for drinking or domestic use.Approximately 70%of samples are not met with irrigation water quality requirements（based on EC,SAR and Na%assessments）.The coal mine drainage pollution of Guizhou cannot be ignored.（2）The main characteristic elements in coal mine drainage of Guizhou are Fe,Al and Mn,with significantly higher contents,and the enrichment of heavy metal elements,pollution characteristics and environmental health risks are related to the background and distribution space of coal-forming era.Affected by different geological background and coal-forming environment,the content of metallic elements in drainage of different strata coal mines is slightly different.The content of Fe in the drainage from Xiangbai Formation is the highest,followed by Al,Zn,Ni and Pb.The drainage of Liangshan Formation is acidic,and the concentrations of Fe,Al,Cr,Cd,Pb and As are high,but the Mn and Cu are relatively low.The drainage of Liangshan Formation is acidic,and the concentrations of Fe,Al,Cr,Cd,Pb and As are high,but the Mn and Cu are relatively low.The coal seams of Heshan Formation are mainly sedimentation in Marine facies.In addition to high Fe content,other trace metal elements are low in the acidic coal mine wastewater,and some sample points have high As content.Longtan Formation is a series of coal-bearing rocks deposited in multi-cycle marine and continental alternative deposition.Influenced by a variety of geological factors,the content of metal elements in the drainage of coal mines varies greatly and the sample points are widely distributed,which mostly presents the obvious characteristics of strong acidity and high content of metal elements in abandoned coal mines.Especially in the Anshun-Pingba-Zhijin area,the content of Fe,Mn,Al and other metallic elements is generally high,and the average content of Mn is the highest.As is significantly enriched in AMD in Xingren high arsenic coal mine area.Due to the good quality of coal in Xuanwei Formation,the content of metal elements in coal drainage is generally low and As is relatively high.Although the drainage p H of Banan Group is acidic,Fe,Mn and Al are not enriched.Meanwhile,the contents of Zn and Ni are relatively high,and As and Sb are the lowest.In addition to the characteristic elements such As Fe,Al and Mn,the contents of other nine typical metal elements（Zn,Ni,Co,Cu,Cr,As,Pb,Cd,Sb）are also significantly higher than those of rivers or natural water bodies in this region.The comprehensive evaluation results show that the pollution of Fe,Mn,Ni,Al and Co in coal mine drainage in Guizhou is the most serious.Heavy metal pollution index（HPI）showed significant regional differences,with the provincial average of 576.62,which was 5.76 times of the critical value.The health risks of chemical non-carcinogens Cu,Pb,Zn,Ni,Al,Fe,Mn,Co,Sb in coal mine drainage are small,which are all lower than the maximum acceptable level recommended by EPA of 1.0×10^-6 a^-1.However,the health risks of chemical carcinogens Cr,As and Cd are higher,especially the pollution of heavy metals Cr and As in southwestern Guizhou.（3）The variation and enrichment characteristics of∑REY content in mine drainage of different coal-bearing strata in Guizhou are different,and it has a good positive correlation with the plasma content of SO₄^2-,Fe,Al and Mn in mine drainage.Comparatively,its content is obviously high,and it has potential recycling value.The average content of∑REY in coal mine drainage in Guizhou varies widely.The average content of coal mine drainage in different strata is Liangshan Formation>Longtan Formation>Xiangbai Formation>He Shan Formation>Xuanwei Formation and Bainan Formation.The content of rare earth elements（1590μg/L）in acid mine drainage（p H≤5.5）is much higher than that in weak acid or neutral drainage（p H>5.5）.In acid coal mine drainage（p H≤5.5）,SO₄^2-is the most important ligand controlling REY form,and Ln（SO₄）⁺and Ln（SO₄）^2-are the two most important existing forms of REY.For the drainage of acidic coal mine with p H≤5.5,except for some sample points of the Xiangbai Formation,which show the enrichment characteristics of heavy rare earth elements,the drainage of other strata coal mines show obvious enrichment characteristics of middle rare earth elements,with the mean value of（La/Sm）_N is 0.413 and（Gd/Yb）_N is 5.298.The drainage of most acid coal mines does not show obviousδCe anomaly andδEu anomaly.Compared with the drainage of acid coal mine,the enrichment characteristics of REY are not obvious in weak acid or neutral（p H>5.5）conditions,but it is still mainly enriched in LREE or MREE.Most of the samples show strong positiveδEu anomalies,especially in the Longtan Formation and Xuanwei Formation.According to the comprehensive evaluation of critical rare earth elements’proportion and application prospect coefficient,the resource utilization prospect of rare earth elements in drainage of acidic coal mines in Guizhou is generally good.In particular,liangshan formation,Xiangbai Formation and Longtan Formation can be used as the target areas of priority development.The annual AMD emission contains a total dissolved REY of about 635.89 t（the amount of rare earth oxides is about 747.81 t）,and the resource recovery potential is huge.（4）The sulfur and oxygen isotopes of sulfate in the drainage of Guizhou coal mines are relatively enriched in light isotopes,and the sulfur and oxygen isotopes of sulfate in the drainage of coal-bearing strata in different geological ages have a wide range,which is closely related to different coal-forming ages and coal-forming geological environments.The sulfur isotopic composition of sulfate in coal mine drainage in different geological ages is similar to that in atmospheric precipitation,karst groundwater and major rivers in Guizhou Province.Among them,the coal mine drainage of the Permian Liangshan Formation has the most negative sulfur isotope composition（about-25‰）,and the sulfur isotope composition is relatively concentrated mainly in the Liangshan Formation（about-25‰）,Xuanwei Formation（about+5‰）and Bannan Formation（about+8‰）.The oxygen isotopic composition of sulfate also has a wide range,but it is more negative than that of precipitation,karst groundwater and main rivers in Guizhou.Among them,the coal mine drainage in the Permian Xuanwei Formation is the most negative（-5.68～-3.73‰）,followed by the coal mine drainage in the Triassic Banan Formation（-4.56～-3.48‰）,and the coal mine drainage in the Permian Heshan Formation（-3.3～+0.48‰）.The oxygen isotopic composition of the Permian Longtan Formation and the Triassic Xiangbai Formation is the largest.Due to the high superposition of coal mining area and karst geological background in Guizhou,AMD is affected by the mixing of karst water and the dissolution of calcite and dolomite.So,theδ³⁴S-SO₄^2-shifts to negative value.As a result,the range of sulfur isotopic composition of mine water in the region becomes wider.The combination ofδ³⁴S-SO₄^2-,δ¹⁸O-SO₄^2-and the concentration of SO₄^2-in mine water can be used as an important tool to identify the source of SO₄^2-in acidic mine environment and the evolution process and path of sulfide in mine environment.