The density functional theory with the generalized gradient approximation has been used to determine the binding mechanism of Cl-2, HCl and Hg species on the ZnO(1 0 (1) over bar 0) surface, and the Hg-0 oxidation mechanism by Cl2 (or HCl) on the ZnO surface. Cl-2 and HCl are dissociatively adsorbed on the surface, and Cl atom bonds to two adjacent Zn atoms. Binding energy of Hg-0 is showing a physisorption mechanism, while HgCl2 is strongly adsorbed on the surface in the molecularly mode, which indicates that the oxidation of Hg is necessary for its removal from flue gas. HgCl is not an indispensable intermediate during the Hg oxidation to form HgCl2. Three Hg-0 oxidation mechanisms have been investigated, and the HgCl2 is easy to be formed via both the Eley-Rideal and Langmuir-Hinshelwood mechanisms, while Mars-Maessen mechanism is unfavorable since high activation energy is needed for Hg-0 reacting with the lattice oxygen of ZnO. The adsorbed H atoms by HCl dissociation inhibit the formation of HgCl2, and Cl-2 is the primary species being responsible for the Hg-0 oxidation on the ZnO surface. (C) 2014 Elsevier B.V. All rights reserved.