Mercury emission from spontaneously ignited coal gangue hill in Wuda coalfield, Inner Mongolia, China

Coal mining waste material (coal gangue) is a serious problem in all coal-producing countries, consuming land resource and posing occupational and environmental hazards. In China alone, coal gangue inventory reaches 4.5-5 Gt, with an annual increase of 0.37-0.55 Gt. Coal gangue is commonly stored as gangue hills and known for the inclination to spontaneous combustion. This study targeted the largest gangue hill with 100 m in height, 2000 m in foot perimeter and a combustion history over 50 years in Wuda coalfield, Inner Mongolia of China-a global hot spot for underground coal fire. Lumex RA-915+ portable mercury analyzer was employed in the in-situ real-time measurement of exhaust gas at 46 accessible vents on the gangue hill slope, and a total of 2760 mercury concentration data showed an average mercury concentration of 5908 ng/m3 (1022-31,750 ng/m3, n = 2760) in the exhaust gas, which is 12.7 times that measured in underground coal fire in the same coalfield. Moreover, an elevated mercury content (43.2 ng/m3) was observed in the near-surface air in the vicinity of gangue hill, which is 15-30 times of the atmospheric background level. This findings suggest that the coal gangue may also be an important contributing factor to the global anthropogenic mercury inventory.