Thermal stability, chemical speciation and leaching characteristics of hazardous trace elements in FGD gypsum from coal-fired power plants

Hazardous trace elements (HTEs) may release from flue gas desulfurization (FGD) gypsum in the utilization concerning heat treatment or landfills, which cause great harm on the environment. Thermal stability, chemical speciation and leaching characteristics of seven HTEs (Hg, As, Cr, Cd, Ba, Mn and Pb) in the FGD gypsum from three power plants equipped with air pollution control devices (APCDs) in China were investigated in this work. Results show that concentration of Hg, As, Cd, Pb, Cr, Ba, and Mn in the three FGD gypsums is 0.20-3.85, 1.04-2.87, 0.02-0.69, 3.50-7.01, 9.19-21.32, 14.84-55.94, and 45.95-69.85 mg/kg, respectively. The evaporation rate (ER) of HTEs in the three FGD gypsums increases as the thermal treatment temperature rises. More attention should be given to Hg in the thermal disposal process of gypsum, because its thermal stability is worst with ER higher than 94% at the temperature higher than 350 °C. Hg and Ba in the three FGD gypsums are mainly existed in residual form with ratio of 73.11-86.69% and 72.97-82.46%, respectively. The easily soluble As in the three FGD gypsums has the minimum percentage of 2.75-4.78%. The sum of Cr in water/acid soluble and exchangeable fraction (F1), reducible fraction (F2), and oxidizable fraction (F3) in the three FGD gypsums has a considerable ratio of 37.88-53.84%. Compared with other HTEs in the three FGD gypsums, Mn in the easily soluble form occupies largest share of 40.64-55.43%, which will be leached out from gypsum and bioavailable to the environment under typical environmental condition. The HTEs in the three FGD gypsums are in different risk levels, namely, low risk (including Hg, As, Cr, Ba), medium risk (including Cd and Pb (except FGD-G1)), high or very high risk (Mn), which should take Mn in serious consideration once disposing FGD gypsum based on the RAC. Leaching characteristics test indicates the three FGD gypsums belong to non-hazardous wastes. Several suitable disposal measures to minimize the risk of HTEs release on the environment are needed considering the bulk production of gypsum and complex landfill conditions, etc.