Geochemical Characteristics Of Trace Elements And Rare Earth Elements In The Late Paleozoic Coal From The Qinshui Basin

Coal,as China's primary energy,will occupy the largest proportion in the energy structure for a long time in the future.Therefore,the study of concentration and distribution of trace elements in coal is of great significance in environmental protection,coal resources utilization and analyzing coal-forming environment.Mineral composition,major elements and trace elements in the late Paleozoic coal,roof and floor rocks and pyrite vein from the Qinshui Basin were determined using X-ray diffraction,X-ray fluorescence spectrometry,inductively coupled plasma mass spectrometry,ICP-MS with collision cell technology(CCT),ion-selective electrode and DMA-80 Hg analyzer.Vertical and horizontal distribution of trace elements were analyzed.Occurrence modes of minerals in coal were identified under microscope and scanning electron microscope with X-ray energy dispersive spectrometer(EDS).Based on the distribution of trace elements and microscopic characteristics,occurrence modes of trace elements in coal were discussed,and the influence of coal-foming environment on the enrichment of trace elements in coal was discussed using geochemical parameters.The results show that minerals in coal from the Qinshui Basin are mainly composed of clay minerals(9.57%),followed by oxides(1.11%),sulfides(1.02%)and carbonate minerals(0.54%).The contents of clay minerals,oxides and sulfides in coal of the Taiyuan Formation are higher than those of the Shanxi Formation,whereas the content of carbonate minerals in coal of the Taiyuan Formation is lower.Organic sulfur is the main type of sulfur in coal,accounting for 78% of total sulfur.Pyritic sulfur only dominates in some high-sulfur coals of the Taiyuan Formation.Sulfur in coal of the Taiyuan Formation has diverse sources.Macroscopic occurrence modes of minerals in coal mainly include thin-layer calcite,disseminated pyrite,thin-layer pyrite,banded pyrite and nodular pyrite.Microscopic occurrences modes of minerals in coal mainly include thin-layer,banded,crystalline and disseminated kaolinite,pyrite,marcasite and calcite,etc.Hazardous trace elements in coal from the Qinshui Basin are relatively low,with the concentration coefficient of each element less than 2.The contents of trace elements in coal of the Taiyuan Formation are higher than those of the Shanxi Formation.Horizontal distribution of trace elements shows that the contents of trace elements in southern part of the Qinshui Basin are higher than those in the northern part.Arsenic in coal mainly occurs in pyrite and clay minerals and mercury mainly occurs in pyrite.Selenium mainly occurs in pyrite,but it has organic affinity at the same.Uranium and beryllium are mainly bound to organic matter,and organic sulfur determines the content of uranium in coal.Fluorine mainly occurs in inorganic minerals.Rare earth elements and yttrium(REY)in coal mainly occur in clay minerals,and veined pyrite is rich in REY,the content of REY in which is 225.94 ?g/g.Correlation analysis indicates that REY are highly and positively correlated to Si and Al,and the Pearson correlation coefficients are 0.833 and 0.878,respectively.Furthermore,almost fifty percent of REY in coal occur in organic matter.The distribution of trace elements in the Late Paleozoic coal from the Qinshui Basin is mainly controlled by sedimentary environment.The influence of seawater during and after peat swamp stage is the key reason for high total sulfur and minerals in coal of the Taiyuan Formation,which results in that REY and sulfophile elements such as arsenic,selenium,mercury and antimony in coal of the Taiyuan Formation are higher than those of the Shanxi Formation.There are more fine-grained sediments in the southern part of the Qinshui Basin,which can adsorb more REY than the northern Qinshui Basin.