Impact of H₂O on the Adsorption of Hg⁰ on Activated Carbon

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.