The lack of a fundamental understanding of microcosmic reaction mechanisms for elemental mercury (Hg-0) accommodation over a mineral selenide significantly impedes evaluations of their performances and potential applications for Hg-0 adsorption from coal combustion flue gas. Hence, in this work, Hg-0 adsorption profiles and conversion pathways were established for heterogeneous Hg-0 conversion over an efficient and cost-effective mineral selenide, i.e., copper selenide (CuSe). Hg-0 was found to be first physiosorbed by Cu-top sites over an intact CuSe(001) surface to form a Hg-Cu amalgam, which was then converted into stably chemisorbed mercury selenide (HgSe) when encountering surface active ligands such as Se monomer. The reaction pathway for Hg-0 adsorption and transformation over CuSe(001) surface was Hg-0 -> Hg-Cu -> HgSe. This proposed road map for Hg-0 conversion was further proven by experimental results, in which the formation of Hg-Cu amalgam over CuSe surface was observed. The influences of typical coal combustion flue gas such as oxygen (O-2), sulfur dioxide (SO2), and water vapor (H2O) on Hg(0 )capture over the CuSe(001) surface were also investigated. O-2 was found to exhibit negligible influence on Hg-0 removal, while SO2 and H2O had slight detrimental impacts on the physisorption stage of Hg-0 on the Cu-top site. These results were also cross-checked by experimental observations to fully justify the accuracy of the predictions. This work thus gives in-depth microcosmic understandings on Hg-0 removal over CuSe and guides further design of efficient CuSe based sorbent for Hg-0 capture from coal combustion flue gas.