Envvironmentai Geochemistry Of Lead In Coal Mining Area

Lead (Pb) has gained much attention since the1970s because of its potential and cumulative toxicity. As one of the most hazardous elements in coal, Pb can be released into the environment during coal mining, storage, processing and utilization. This dissertation presents a synthesis on the abundance, distribution, modes of occurrence, enrichment mechanism and environmental impacts of Pb in Chinese coal and Huainan Coal Mining Area. Coal mining area is highly subjected to lead pollution from coal mining activities. Several decades of coal mining and processing practice in dozens of coal mines in the Huainan Coal Mining Area (HCMA) have led to the accumulation of massive amount of coal gangue, which is piled in dumps. During coal mining activities, Pb migrates with coal from underground coal seams to the surficial environment. In order to investigate the impact of coal gangue dumps on Pb level in the supergene media of the HCMA, a systematic sampling campaign comprising coal gangue, soil, water, wheat, paddy, soybean, fish and earthworm samples was conducted.Using the expected coal reserves as the weighting factor, and based on the previously published Pb content in4304coal samples (including results obtained in our laboratory) from main coalfields or coal mines in China, the weighted mean Pb content in Chinese coal is13.0μg/g, which is higher than that of American coal (11.0μg/g) and average world coal (7.8μg/g). With respect to regional distribution of Pb in Chinese coal, Pb content can be arbitrarily divided into three groups (<20μg/g,20-40μg/g,>40μg/g). Following this classification, coal from Tibet has the highest average Pb content (128.9μg/g). The abundance of Pb in coal varies with coal-forming period and coal rank, with the late Triassic and higher rank coal having the highest Pb content, which could be ascribed to regional geochemical difference and later geological evolution as well as magma hydrothermal activities.The enrichment of Pb in coal is influenced by several geological factors, including coal-forming plants, source rocks, hydrothermal fluid, and depositional environment. Lead dominantly associates with sulfide minerals, especially galena in coal. During coal combustion or pyrolysis, Pb is partly emitted into the atmosphere, and partly partitioned into solid residues (fly ash and bottom ash). Accumulation of Pb from coal utilization in the human body could lead to a range of health problems, and increase the risk of cancer.Coal mining areas are highly subject to lead pollution from coal mining activities. The average Pb content in the coal mining area soil was28.3mg/kg, which is comparable to the associated coal gangues (29.2mg/kg) and markedly higher than the China soil geochemical background value (12.4mg/kg), however within the maximum allowance concentration for Pb in agricultural soil (GB15618-2008). Soil in the HCMA presents a slight to moderate Pb contamination, which might be related to the weathering and leaching of coal gangue dumps. The elevated Pb in soil, especially in rainy months (June to August in Huainan) might relate to Pb leaching from ambient coal gangue piles. Lateral distribution of Pb in the HCMA soil differs among individual coal mines. The soil profile distribution of Pb depends on both natural and anthropogenic contributions. Earthworms in the selected area are significantly enriched in Pb, suggesting a higher bio-available Pb level in soil in the HCMA. The Pb content in water from subsidence lake ranges from nd to0.07mg/L, which is suitable for agriculture use and landscape requirement.Heavy metals accumulation in the agricultural soil is of great concern, as they can be finally transferred to the human body through the food chain. Excessive Pb concentration was found in the grains of food crops, which would pose a potential health risk to local inhabitants. Average Pb content is higher in roots than in stems, leaves and wheat husks, while the Pb level in seeds exceeds the maximum Pb allowance for foods (Maximum Levels of Contaminants in Foods of China, GB2762-2012). Major proportion of Pb was stocked in roots for paddy and soybean, followed by stems and husks. Therein, wheat showed higher Pb bioaccumulation ability than other crops. With regard to the Pb level in muscles, fish are considered to be safe for consumption. The calculation of daily intake and tolerable hazard quotient of Pb suggests that the potential health hazard posed by Pb is currently insignificant for the inhabitants in the HCMA.