Role of H+, HF, SO42- and kaolin in fixing Hg of coal fire sponge

Abstract Coal fire sponges (CFS) are common in coal-fire areas. Due to the enrichment of Hg in CFS, large amounts of Hg is released by CFS into the atmosphere via natural weathering or solar radiation. Therefore, CFS should be of concern in Hg pollution management and control globally. In addition, CFS changes the Hg cycle path by capturing Hg from coal fires that would have entered the atmosphere. In this study, the concentration, distribution, species, and enrichment mechanism of CFS Hg were investigated. The results showed that the Hg concentration in CFS ranged from 1,008 to 35,310 ng/g, with an average of 8,932 ng/g (CFS number, n=153). The Hg concentration of CFS in different types of land was found to be significantly inhomogeneous. To determine the status of subterranean spontaneous combustion, the Hg concentration was added, which can improve the effect of coal-fire monitoring. Compared to the background area topsoil, CFS was enriched in Hg, acid, SO42–, and total fluoride. The Hg species in CFS was primarily HgSO4, followed by HgO. However, the primary Hg species in the surrounding topsoil were HgCl2 and HgO. By the simulation experiment, it was determined that hydrofluoric acid (HF) was beneficial to activate the stable species in the coal-fire areas. HgCl2, HgO, or Hg0 were ionized by acid liquor or HF, which can promote Hg migration and increase the adsorbed ratio; in the presence of SO42–, the primary Hg species was HgSO4. Ultimately, Hg was absorbed by clay minerals and organic matter. The high-efficiency activation of steady Hg species by the coal-fire HF should be studied further.