Investigation of mercury adsorption and cyclic mercury retention over MnOx/γ-Al2O3 sorbent

In this study mercury sorbent based on manganese oxides impregnated γ-alumina was synthesized. Mercury retention characteristics were investigated by mercury speciation and thermal desorption experiments. No gaseous mercuric oxide was observed in mercury speciation experiments with a mercury mass balance ratio of 89.11%. Maximum mercury desorption peak at 480 °C indicated that mercury was adsorbed in the form of mercuric oxide. Three cycles of mercury retention were tested with different thermal treatment in-between to evaluate the cyclic performance. Changes in surface phase and manganese chemistry before and after thermal treatment were characterized by XRD and XPS. Deterioration in mercury retention capacity was observed after thermal desorption at 500 °C, which was interpreted with reduced initial adsorption rate calculated by Pseudo-second order kinetic model. XPS studies suggested that atomic ratios of Mn4+/(Mn4++Mn3+) decreased from 73.2% to 32.3% and 33.9% after thermal desorption in N2 and air, respectively. The reduction of MnO2 to Mn2O3 was irreversible thus the mercury retention capacity could not be restored by thermal desorption at high temperatures. Spent sorbents that were reactivated at 200 °C in air without thermal desorption at 500 °C possessed considerable cycling performance for mercury retention due to the preserved Mn4+.