Pressurized oxy-combustion is one of the most efficient emerging combustion systems for coal-based power generation with CO2 capture. Mercury reemission and the fate of mercury, arsenic, and selenium in the liquid phase during neutralization of a simulated wastewater from the direct contact cooler of a pressurized oxy-combustion process are investigated. The performance of selected commercial activated carbons (ACs) or modified ACs impregnated with sulfur or transition metals have been investigated and compared with a commercial additive for mercury reemission control. Sorbent addition, compared with the baseline case (i.e., no sorbent or additive), could increase or decrease mercury reemission during neutralization by a limestone slurry. The addition of selected commercial ACs to the solution was detrimental to mercury reemission control, as indicated by an increase in the cumulative mercury reemission by up to 5 times. In contrast, the addition of ACs impregnated with elemental sulfur, iron, or copper decreased mercury reemission by up to 90%, likely because of the adsorption of mercury by sulfur or metal species dispersed on the AC surface. Adsorption experiments showed that ACs with suitable properties could control mercury reemission and remove mercury and arsenic from a simulated wastewater, with some even outperforming the commercial additive used for mercury reemission control. However, none of the tested ACs or the commercial additive was effective in removing selenium. Overall, a combination of two mechanisms, namely, the adsorption of mercury onto AC adsorption sites and the reduction of the soluble ionic mercury to volatile elemental mercury by the AC, may control mercury reemission in the presence of an AC sorbent.