Copper sulfides (CuSx) and iron sulfides (FeSx) have been developed to capture gaseous elemental mercury (Hg-0) originating from the smelting flue gas. However, these compounds exhibit different Hg-0 adsorption characteristics and Hg species adsorbed on CuSx can be spontaneously released as gaseous Hg-0. Following these findings, the adsorption/desorption kinetics of Hg-0 onto and from FeSx and CuSx were determined. After comparing the kinetic parameters, the mechanisms behind some of the remarkable differences between FeSx and CuSx with respect to Hg-0 adsorption were discovered. The Cu-S bond in CuSx was not completely broken during Hg-0 oxidation, but the S-S bond in FeSx was. Hence, the activation energy for the oxidation of Hg-0 physically adsorbed on CuSx was much lower than that for FeSx, resulting in a much higher efficiency of CuSx to oxidize Hg-0 than FeSx. However, the bond strength of Hg-S for HgS on CuSx was weaker due to the sharing of S2- in HgS with Cu+, resulting in a decrease in the thermal stability of HgS on CuSx. Therefore, HgS adsorbed on Cu-based sulfides was metastable, and could be spontaneously decomposed to release moderate concentrations of gaseous Hg-0, which was not preferable for capturing high concentrations of Hg-0. (C) 2020 Elsevier Inc. All rights reserved.