Unrefined low-grade pyrolusite for elemental mercury removal from flue gas: Experimental and DFT insights

Dual characteristics of oxidation capacity and economic advantage endow the unrefined low-grade pyrolusite (LGP) as a potential adsorbent/catalyst. This paper successfully applied it to remove Hg-0 from flue gas, by which 98.6% of removal efficiency was obtained without chemical modification or external oxidant. Effects of various flue gas components on the Hg-0 removal were investigated, and the synchronous improvement in efficiency and stability by HCl was revealed. Also, its reusability was evaluated and the decrease in removal efficiency after 5th cycles is less than 6%. Product analysis indicated that over 90% of Hg-0 was chemisorbed as HgO without HCl, but the presence of HCl reversed the pathway to form HgCl2 that easily escape from the surface, with a proportion of about 95%. By combing experiments, characterizations and theoretical calculations, the mechanism was preliminarily speculated: (i) without HCl, O-2 and the main components of LGP (MnOx, Fe2O3, etc.) involved in the catalytic oxidation according to Mars-Maessen mechanism; (ii) HCl tended to dissociate and adsorb on the MnO2 to form the chlorinated surface, and the mercury removal followed both L-H and E-R mechanism, although E-R was dominant; (iii) the formation of HgCl lowered the energy barrier of HgCl2 desorption, thereby expediting the oxidation-removal of Hg-0.