Enhanced removal of elemental mercury using MnO2-modified molecular sieve under microwave irradiation

Abstract

Developing efficient catalysis-adsorption method for elemental mercury (Hg0) removal with wide temperature window and good sulfur/water resistance is a hot topic. In this study, a novel method of microwave (MW) catalysis of MnO2 modified molecular sieve (Mn@MOS) composite for Hg0 removal was developed. Results indicated that Hg0 removal efficiency of MOS was poor, only 18% at 150 °C, while the efficiency was significantly increased to 97.8% when MW/Mn@MOS was used. Compared with thermo-catalysis way, MW irradiation method achieved better Hg0 removal efficiency (97.8% vs 86.4%) under a wider temperature window (150–450 °C), and sulfur/water resistance were greatly strengthened under MW irradiation. The dynamical adsorption capacity of Hg0 was calculated as 1.52 mg/g when the efficiency was below 80%. Kinetic analysis indicated that MW field significantly increased the rate constant and reduced the activation energy. The fate of Hg0 was unfolded: almost 96% of Hg0 was chemisorbed as HgO, less than 4% was converted to gaseous Hg2+. The mechanism was speculated as followed: (i) excited active site of *MnO2 was first generated under MW irradiation; (ii) then the oxidizing species including Mn(IV)/Mn(III), Oads, Olatt and O* dominated the Hg0 removal process; (iii) the reactive oxygen species of Oads, Olatt and O* were replenished by the interaction between oxygen vacancies and free O2. This study first demonstrates the superiority of MW catalysis towards Hg0 removal, and provides a new thought on the development of the novel Hg0 removal technologies.