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THE EFFECT OF ACTIVATED CARBON SURFACE MOISTURE ON LOW TEMPERATURE MERCURY ADSORPTION
Li*, Y H., C W. Lee*, AND B K. Gullett*. THE EFFECT OF ACTIVATED CARBON SURFACE MOISTURE ON LOW TEMPERATURE MERCURY ADSORPTION. CARBON. Pergamon Press Ltd., New York, NY, 40(1):65-72, (2002).
Experiments with elemental mercury (Hg0) adsorption by activated carbons were performed using a bench-scale fixed-bed reactor at room temperature (27 degrees C) to determine the role of surface moisture in capturing Hg0. A bituminous-coal-based activated carbon (BPL) and an activated carbon fiber (ACN) were tested for Hg0 adsorption capacity. About 75-85% reduction in Hg0 adsorption was observed when both carbon samples' moisture (~2 wt.% as received) was removed by heating at 110 degrees C prior to the Hg0 adsorption experiments. These observations strongly suggest that the moisture contained in activated carbons plays a critical role in retaining Hg0 under these conditions. The common effect of moisture on Hg0 adsorption was observed for both carbons, despite extreme differences in their ash contents. Temperature programmed desorption (TPD) experiments performed on the two carbons after adsorption indicated that chemisorption of Hg0 is a dominant process over physisorption for the moisture-containing samples. The nature of the mercury bonding on carbon surface was examined by X-ray absorption fine structure (XAFS) spectroscopy. XAFS results provide evidence that mercury bonding on the carbon surface was associated with oxygen. The results of this study suggest that surface oxygen complexes provide the active sites for mercury bonding. The adsorbed H2O is closely associated with surface oxygen complexes and the removal of the H2O from the carbon surface by low-temperature heat treatment reduces the number of active sites that can chemically bond Hg0 or eliminates the reactive surface conditions that favor Hg0 adsorption.