Geochemistry Characteristics Of Potentially Toxic Elements In Coal-bearing Strata Profiles Of Longtan Formation,Western Guizhou,China

The coal-bearing strata contain,As,Cd,Cr,Pb,Ni,and other potentially toxic elements（PTEs）.These elements not only have certain geological significance,but also the extremely high toxic and harmful elements in coal-bearing strata will cause harm to the environment.The aim of this work was to study the geochemical characteristics of PTEs in coal-bearing strata profiles in the west of Guizhou.To master the geochemistry characteristics of the PTEs in coal-bearing strata in the west of Guizhou,to improve the understanding of the pollution process of the PTEs,and to improve the evaluation of the ecological geochemistry in the southwest of China,to maintain the ecological and environmental safety in southwest China.The coal-bearing strata profiles of the Longtan Formation in the area southwest of Guizhou are studied in this paper.By means of inductively coupled plasma mass spectrometry（ICP-MS）,X-ray fluorescence spectroscopy（XRF）,X-ray diffraction（XRD）,BCR continuous extraction and leaching experiment,and combined with the knowledge of geochemistry and mineralogy,a systematic study was carried out on the macroelements,mineral composition,content,distribution and enrichment of PTEs,and the forms and leaching and release characteristics of PTEs in coal-bearing strata.Through the research,the following conclusions are drawn:（1）The coal-bearing strata profile of Si O₂,Al₂O₃,Fe₂O₃ are the main elements and Ti O₂,K₂O,Mg O,Na₂O are the next.quartz and clay minerals in coal-bearing strata are the main carriers of Si.The ratio of Si O₂/Al₂O₃ in most samples of coal-bearing strata profile in West Guizhou is higher than the mean value of Chinese coal.It shows that there are a lot of free Si O₂in coal-bearing strata profile in West Guizhou,this result is mainly due to the high content of quartz in the coal-bearing strata in the western part of Guizhou.There may be some potential environmental risks of PTEs Cr and As in coal-bearing strata in western Guizhou.The contents of Cr and As in the coal-bearing strata profile in western Guizhou are higher than the average value of Guizhou coal,and the average contents of Ni,Cd and Pb in the profile are close to the average value of Guizhou coal.The enrichment coefficient of PTEs shows that As is highly enriched,some samples are highly enriched,Cr is slightly enriched,and Ni,Cd and Pb are relatively low.（2）The content of PTEs in coal-bearing strata profile in western Guizhou is unevenly distributed.The content of PTEs in the upper and bottom mudstone of coal seam is relatively high,and the content of PTEs in the middle layer of coal seam is relatively low,and the distribution is uneven.The closer to mudstone,the higher the content of PTEs.The source and distribution of PTEs in coal-bearing strata profile in western Guizhou are influenced by many geological factors during the formation of coal-bearing strata.In this study,the uneven distribution of PTEs in coal-bearing strata profile and the relatively richer content in mudstone should be caused by a series of geological processes in geological history.（3）The content and mass fraction of As residue in coal-bearing strata profile is the highest,followed by reducible state,and the mass fraction and content of weak acid extracted state are the lowest.The main reason for the high reducible state is that pyrite is an important carrier of As.The content and mass fraction of Cr in residue state is the highest,followed by oxidizable state,and the content and mass fraction of weak acid extracted state is the lowest.The content and mass fraction of reducible state in mudstone is higher than that in coal seam,which may be caused by more pyrite in mudstone.（4）Through the leaching experiment,it is found that coal-bearing strata will release a certain amount of PTEs such as As and Cr,so it is necessary to pay attention to the potential ecological harm caused by their accumulation in the environment.The p H of the leaching solution is acidic as a whole,and the higher the acidity,the lower the p H,and the higher the leaching content.The leaching content of medium coal seam is relatively low and the p H is relatively high,while the leaching content of mudstone is relatively high and the p H is low.This phenomenon may be caused by the high content of PTEs in mudstone and the oxidation of pyrite to produce acid.