Hydrogen chloride (HCl) non-thermal plasma was applied to introduce Cl active sites on biochar prepared from sorghum straw in this study. Surface modified biochar was then placed in flue gas with typical components to investigate its elemental mercury (Hg-0) capture ability. To elucidate the adsorption mechanism & binding properties, samples were characterized by N-2 adsorption, scanning electron microscopy with energy dispersive spectrometer (SEM-EDS) and X-ray absorption near edge structure (XANES) analysis of Hg L-III-edge, Cl K-edge and S K-edge. Experimental results showed that HCl plasma modification successfully increased Cl active sites on biochar and greatly increased its mercury removal efficiency. Both HCl treatments (w/without plasma involvement) altered biochar's surface structure and layered structure generated. XANES spectra revealed that adsorbed-Hg on HCl-treated biochars mainly in the form of Hg+. Gaseous Hg-0 was believed to heterogeneously react with chlorinated sites through electron-transfer and formed Hg2Cl2 compounds. With the presence of NO or SO2 in the system, adsorbed mercury existed on biochar mainly as Hg+. SO2 competed and inhibited the adsorption of Hg-0; while NO promoted Hg-0 removal capacity by increasing the active sites and enhancing the adsorption kinetics of adjacent Cl-containing sites.