Mercury emission from underground coal fires: a typical case in China

Mercury, a highly toxic environmental pollutant with a global circulation, must be controlled worldwide. Taking the Wuda underground coal fires, one of the most severe coal fire disaster areas in China, as a typical case, this paper systematically introduces the serious environmental output and strong environmental pollution of mercury from underground coal fires. Smoke with unusually high mercury concentrations was released from surface vents and cracks, resulting in significant enrichment of mercury in the air and surface sediments. A portion of mercury (particulate and reactive gaseous mercury) was deposited near the fire zones, but the positive high mercury fluxes of the surface soils indicated that mercury would again escape from the soil–air interface. The annual gaseous mercury emissions from the underground coal fires in China were estimated to reach 4.85 tonnes. Underground coal-fired mercury can be identified as an essential part of the global mercury cycle. Although some remediation measures were implemented, the development of coal fires proved difficult to control and was destined to be accompanied by the continuous release of mercury. Given the widespread distribution of coal fire cases worldwide, mercury pollution from underground coal fires deserves attention in the future.