Impact of H₂O on the Adsorption of Hg⁰ on Activated Carbon

Abstract

In this work, the influence of water on the adsorption of mercury is systematically investigated on basic and washed activated carbons. Breakthrough curves were measured and temperature-programmed desorption (TPD) experiments were performed with mercury and water. Both physisorptive and chemisorptive interactions are relevant in the adsorption of mercury. The experiments show that the presence of water in the pores promotes chemisorption of mercury on washed activated carbons while there is little influence on chemisorption on basic materials. Washing exposes or forms oxygen functional groups that are chemisorptive sites for mercury. Obviously, effective chemisorption of mercury requires both the presence of water and of oxygen functional groups. As mercury chemisorption is preceded by a physisorptive step, higher physisorptive mercury loading at lower temperature (30 °C) enhances chemisorption though the reaction rate constant is smaller than at higher temperature (100 °C). Sequential adsorption and partial desorption of water at lower temperature changes the surface chemistry without inhibiting mercury physisorption. Here, the highest chemisorption rates were found. The number of desorption peaks in the TPD experiments corresponds to the number of adsorption and desorption mechanisms with different oxygen functional groups in the presence of water. The results of the TPD experiments were simulated using a transport model extended by an approach for chemisorption. The simulation results provide reaction parameters (activation energy, frequency factor, and reaction order) of each mechanism. As in many heterogeneously catalyzed reactions, the activation energy and the frequency factor are independent of mercury loading and increase with increasing temperature.

Adsorption of Mercury on Chlorine-Modified Activated Carbon: Breakthrough Curves and Temperature-Programmed Desorption

“…This mechanism has been proposed in a previous publication and could also provide a small contribution to chemisorption of mercury in other modified materials. Compared to the loadings found in this work, the loadings described in ref . are significantly lower.…”

“…It is suggested that the increase in capacity here is due to an accumulation of mercury on oxygen functional groups formed or exposed during modification of the carbons. This mechanism has been proposed in a previous publication 27 and could also provide a small contribution to chemisorption of mercury in other modified materials. Compared to the loadings found in this work, the loadings described in ref.…”

“…Compared to the loadings found in this work, the loadings described in ref. 27 are significantly lower.…”

“…Because elemental mercury is present in higher concentrations in the gas phase due to its higher volatility and adsorbs worse than oxidized mercury, only the adsorption of elemental mercury is investigated in this publication. In previous publications, ,, the physisorptive single-component adsorption of Hg 0 , as well as the influence of the coadsorptives water and oxygen, have been presented in detail. As the physisorptive capacity of nonimpregnated activated carbons is very low, impregnated activated carbons are used for chemisorptive separation in technical applications.…”

Mercury Adsorption on Phosphoric Acid- and Nitric Acid-Modified Activated Carbon

“…In a previous publication, 25 we have investigated the coadsorption of mercury and water. It was shown, among other things, that adsorbed water on the surface of the activated carbon only leads to an increase in the chemisorption of Hg 0 in the presence of functional oxygen groups on the surface of the activated carbon.…”