Microwave-boosted elemental mercury removal using natural low-grade pyrolusite

Utilizing natural minerals to remove Hg0 from flue gas was regarded as a preferable route to control mercury pollution. This paper purposed a novel microwave (MW)-boosted removal method using natural low-grade py- rolusite (NLP), by which over 85 % of Hg0 was removed and approximately 10-20 % of the increment of effi- ciency was obtained. Compared with thermo-catalysis, MW also empowered NLP with a wider active - temperature range (100-250 degrees C) and better H2O and SO2 resistances. Kinetic analysis showed that MW increased the rate constant and reduced the activation energy, Hg-TPD and material balance analyses demon- strated that major Hg0 was chemisorbed as HgO. Via the thermal treatment experiments, the regeneration performance of NLP was confirmed to be good. From the experiments and characterizations, the mechanism was preliminarily speculated: (i) the contribution order of the active components was MnO2 > Mn3O4 > Fe2O3; (ii) the redox pairs of Fe(II)/Fe(III) and Mn(III)/Mn(IV), as well as the O-species, involved in the conversion from Hg0 to HgO. Density functional theory calculations revealed the pathway and mechanism: (i) MnO2 exhibited higher oxidation activity, whereas Mn3O4 showed higher adsorption activity; (ii) Odis derived from Oads enhanced the removal of Hg0 by increasing the adsorption energies and amounts of transferred electron; (iii) Oads had advantages over Olat in term of desorbing HgO, which was the rate-determining step.