Fluorine And Acid Pollutant Within Soil Around Three Coal-rich Areas In China

Fluoride is an important environmental pollutant.Excessive fluoride in the environment is not only harmful to plants and animals,but also to human health.In view of the important status of coal in China's energy,the fluorine pollution caused by coal is very common in China,sometimes accompanied with acid pollution,and often appears in the coal-rich area.In this paper,coal,topsoil and plant samples from three typical coal areas in China,including the border region of Yunnan,Guizhou and Sichuan province,Helan mountain in Ningxia,Wuda district in Inner Mongolia are studied for the fluorine content and the acidity.The current situation of fluorine pollution and acid pollution in the surrounding environment caused by different forms of coal utilization,as well as the source are systematically investigated.This paper also discusses the formation mechanism of fluorine pollution and acid pollution,aiming at state the universality that human activities aggravate coal seam exposure and lead to the release of fluorine and other harmful elements into the environment.The major research results are as follows.1.The coal of longtan formation in the late Permian of the border region of Yunnan,Guizhou and Sichuan province are coals with high sulfur and low fluorine content.However,the coal sample is generally acidic,with average pH of 5.74.Thereinto,the natural outcrop coal?NOC?is generally highly acidic,with an average pH of 3.86.In contrast,underground mine coal?UMC?tends to be neutral or slightly alkaline,with an average p H of 7.26.NOC is rich in sulfate,with a average of 12136?g/g,while UMC was is poor of sulfate,with a total average of 2739?g/g.Accordingly,the efflorescent clay of coal-bearing strata in the longtan formation of late Permian in this area is acidic universally,with a average p H of 5.84.The content of sulfate was also significantly higher than that in the control area.The fluorine content is high as a whole,with a average of 751?g/g.The presence of acid in efflorescent clay may be acidic sulfate such as KHSO₄ or NaHSO₄.The surface soil and clay used for coal-burning Hehua village,Guizhou province,the area with typical coal-bearing strata exposed and severe endemic fluorosis were significantly acidified and the p H value was low to 4.90.The content of sulfates was significantly higher than that in other non-disease areas,with a average of 4465?g/g,which was positively correlated with the acidity,suggesting that the acid in the soil samples is likely to exist in the form of acidic sulfate.The fluorine content is also high,with a average of 1284?g/g.During the same period,the coal in coal-bearing strata of the Hehua village was also significantly acidic,and the clay in coal-bearing strata was also high in fluorine content and with high acidity.2.The fluorine,as well as the sulfate content and acidity in the surface soil of Helan mountain in Ningxia province was significantly higher than that in the control area.It reveals that the topsoil is relatively rich in acid,sulfate and even fluorine,which is likely to be interrelated.The source of pollution may be the long history of coal mining,coal using,accompanying coal seam spontaneous combustion.3.The coal seam fire and its high coal-consumption industrial park in Wuda,Inner Mongolia result in the elevation of the acidity,fluorine and sulfate content in surface soil and dust in a wide area of 270km²,including Wuda district and its peripheral regions.?1?The fluorine,sulfate content and acidity in the surface dust were higher than that in the regional background,and the coal mine area and industrial park were the most serious,and the urban areas and farms of the diffused area were also affected.The surface soil is also seriously polluted.Similar to that in the surface dust in the same place,the fluorine content,acidity and sulfate content are significantly higher than the regional background value,and the heavily polluted area is also concentrated in industrial parks and coal mining areas.The polluted area are adjacent to industrial parks,or under the direction of the coal mines,the general trend is that the farther away from the coal mines and industrial parks,the less polluted.?2?After further analysis,it was found that the property of the surface soil in the Wuda and industrial parks was fundamentally changed due to acid pollution,changing from the original type of CaCO₃to CaSO₄,and that in the urban areas and farms were less polluted,and the soil quality did not change significantly.4.It is found that the high fluorine geochemical background has obvious influence on the surrounding environment.?1?It was found the content of fluoride in the leaves,bark,and mushrooms collected in Hehua village,a typical high-fluoride area,was higher than that in the Leitun Village of the control area.The source of fluoride may be high-fluorine surface soil.?2?The fluoride content in pasture samples in sheep fluorosis area of Helan mountain in Ningxia province was higher than the standard of fluoride in China and reported by the previous people.The fluoride in goat feces samples was significantly higher than that in the typical fluorine-contaminated area in Baotou,indicating the presence of fluorine pollution in the area.?3?The average fluorine content in the leaves and stems of Bassia dasyphylla sampled immediately around the sites of undergoing coal fire are 687?g/g and 516?g/g,suggesting that the fire directly leads to air mercury pollution in limited area.Those in leaves and stems of B.dasyphylla collected at areas remote from the fire within the field are 146?g/g and 108?g/g,implying that the historic coal fire in the past 50 years may result in general deposition of fluorine.5.According to the actual measurement data in the three coal-rich areas,some instruments such as scanning electron microscope?SEM/EDS?and time-of-flight secondary ion mass spectrometry?TOF-SIMS?,combined with laboratory tests are conducted and some important conclusions are drawn.?1?Various forms of pyrite was detected in the outcropped coal of the Late Permian in the junction of Yunnan,Guizhou and Sichuan province by the analysis of coal petrology;Scanning electron microscope also observed a large amount of pyrite in clay of coal-bearing strata.TOF-SIMS analysis further confirmed that pyrite was indeed rich in clay.Pyrite weathering is widespread,for example,acid sulfate?HSO₄^-?was detected in clay sample DF2T-1.?2?HF was detected when the coal sample was heated at 400°C,and the concentration of HF released by NOC was the strongest,and the HF release concentration was found to be significantly positively correlated with the acidity of coal,indicating that acidity may promote HF emission.The indoor simulated combustion experiment is conducted and experimental conditions were close to the burning environment in rural Southwest China.The result verified the release of the sulfuric acid mist form coal.6.The synergistic mechanism between fluorine pollution and acid pollution is proposed according to the above four study results.The synergistic release mechanism can be concluded as follows.Sulfur in coal such as pyrite oxidizes to sulfuric acid or acidic sulfate.Under certain conditions,this acid will decompose the fluoride in the coal in the form of HF released.Therefore,fluorine pollution is accompanied by acid pollution and acid pollution causes fluorine pollution,and the chemical form of acid pollution is mainly sulfuric acid pollution,which is presented as sulfuric acid or acidic sulfate.The common form could be expressed as“S?1?H₂SO₄?1?H⁺?1?HF”.7.Fluorine and acid pollution patterns are different in different areas.?1?The coal seams of the Late-Permian in the Yunnan-Guizhou-Sichuan area are extensively exposed naturally affected by geological movements and long-term natural weathering and erosion,which results in the weathering of sulfur in the coal seam and clay layer into acid,then exacerbating fluoride release form coal-measure strata.This leads to the high fluorine content in large regional stream sediments,causing the abnormity of fluorine geochemical background in a large region.In terms of human health,it is caused by human activities.For example,the villagers in this area used to burn coal and clay in the open stove,which caused the epidemic of endemic fluorosis.?2?There is a long history of coal exploitation and domestic coal burning,along with spontaneous combustion of coal caused by coal exploitation.This may be the origin of fluorine pollution and acid pollution in this area,which is man-made.?3?The high-fluorine and high-acid in Wuda,Inner Mongolia,is caused by coal mining or the joint effect of the increased coal fire and industrial parks.There are natural causes?the nature of coal fires?,but mainly the result of people developing coal activities.