Performance of BiO1.5-xIx and composite quasi-photocatalysts for the removal of gaseous elemental Hg0 from coal combustion flue gases: A review

The atmospheric emission of mercury occurs primarily in its elemental form (Hg0), although it can also proceed in an oxidized or particulate-bound form. Remediation of elemental Hg (Hg0) in coal combustion flue gas is extremely challenging due to its low affinity toward common adsorbents and low partial pressure in flue gas (e.g., < 3.0E-04 Pa). Herein, the photocatalytic removal potential of BiOI photocatalysts is assessed in its pristine (BiO1.5-xIx) and binary/ternary composite forms against gaseous Hg0 in light of their advantageous properties (e.g., desirable band structure for electron/hole generation, low band gap for visible light capture, abundant active sites, remarkable optical properties, and structural tunability). Accordingly, Ag (1%)/BiOI/ZnFe2O4 and BiOI microspheres are identified as the best performers in terms of figure of merit (FOM: 95 μg∙g−1∙L ∙h−1∙W−1) and quantum efficiency (QE: 3 × 10−6 molecule∙photon−1), respectively. In summary, BiO1.5-xIx photooxidation of Hg0 is not yet market-ready (e.g., in terms of 10% breakthrough volume and/or its negligible treatment capacity, i.e., <10 μg∙Hg∙(g catalyst)−1) when their potential is evaluated in terms of several key performance metrics (e.g., QE, FOM, space-time yield (STY), turnover number (TON), and 10% breakthrough). Especially in terms of FOM, there is still a gap of 7 orders (i.e., relative to ideal FOM) to treat Hg0 from the flue gases. In light of such limitations, several suggestions are proposed to help improve their mass processing performance against elemental Hg.