Nano-ZnS particles are newly developed alternatives to activated carbons for Hg-0 removal from flue gas. We investigated the Hg-0 oxidation mechanisms in the presence of chlorine (Cl-2) on the (110) surface of ZnS by first-principles calculations. The results show that the Cl-2 molecule can dissociate into two Cl ions on the ZnS(110) surface, forming chlorinated ZnS surfaces. Compared to the clean ZnS surface, the chlorinated ZnS surface with dissociated Cl ions exhibit enhanced binding affinity over Hg-0 with promoted Hg-0 oxidation efficiency (Hg-0 -> HgCl2). The active Cl ions on ZnS surface can directly interact with Hg-0 to form HgCl2, following the typical Eley-Rideal mechanism rather than the Langmuir-Hinshelwood mechanism.