(293c) Mercury Adsorption on Activated Carbon

Emissions from coal combustion processes constitute a significant amount of the elemental mercury released into the atmosphere today. Current technologies only allow for the capture of oxidized forms of mercury, while gaseous elemental mercury passes on to the environment freely. This investigation looked into the ways that activated carbon, with adsorbed halogens, can attract and adsorb mercury. Understanding the way in which halogen adsorbs to the surface of activated carbon, and subsequently mercury, can lead to the development of new technologies for the capture of mercury from the flue gases of coal combustion.

For the theoretical model it was assumed that the activated carbon molecular framework is similar to that of graphite. Both four-fused and seven-fused benzene rings were examined to serve as representative cluster species to model the activated carbon surface. Ab initio energetics calculations were performed using HF and B3LYP methods with relativistic pseudopotentials for mercury using Gaussian03 software. The quantum mechanical level of theory chosen was based upon a previous detailed analysis comparing theoretical and experimental geometries and heats of reaction for mercury-containing compounds. Energetics associated with the adsorption of chlorine and bromine at different sites on the graphite were calculated to determine a stable halogen-embedded activated carbon cluster. Using this cluster model, further ab initio calculations were performed to find the lowest energy interactions between mercury and the halogen-embedded graphite surface. Modeling graphite surface with different halogens such as fluorine, chlorine, bromine and iodine allowed determining the best halogen yielding the highest binding energy of mercury to the surface. To simulate an activated carbon surface with more accuracy, oxygen functional groups such as carbonyl, lactone, carboxyl and phenol groups were also considered on the cluster. The binding energies of mercury with each type of complex were considered to determine a mechanism by which mercury adsorbs to an activated carbon surface. The study will be extended to include the interactions with other functional groups like sulfur and nitrogen groups embedded in the cluster as well. Overall, this work will allow for a better understanding of interactions between mercury and halogens as well as different functional groups on activated carbon, which will provide direction for further experiments that will aid in the development of a novel sorbent for effective mercury capture.