Pre-halogenated activated carbon (AC) sorbents are known to be effective in Hg capture when injected into the flue gas stream generated from the combustion of low-chlorine-containing sub-bituminous and lignite coals. In this study, virgin (unfunctionalized) and brominated AC powders were tested for Hg sorption in a low-HCl (4 ppm)-containing simulated Powder River Basin (PRB)-fired flue gas matrix before and after regeneration. X-ray photoelectron spectroscopy (XPS) was used to investigate changes in sorbent surface chemistries, which were then correlated to Hg breakthrough data obtained from bench-scale experiments. It was observed that Hg capture tests on brominated AC powders removed the surface-bound bromine, a known Hg-binding site, to levels below XPS detection limits. Further, XPS studies also revealed an increase in the chlorine content after adsorption on the tested samples and the samples regenerated at a lower temperature (204 degrees C). The decrease of bromine peaks coupled with the presence/increase of Cl, N, and S on the tested and some regenerated samples strongly indicates that the HCl, SO2, and NOx components in the flue gas play a collaborative role during Hg breakthrough. It is also observed from the breakthrough data that Hg adsorption on regenerated samples decreases significantly to 6.5-9.6% of the initial adsorption values. This indicates that regenerating AC sorbents for further Hg capture is not an effective process.